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AN INTENSIVE COURSE
BLAST FURNACE.'
IRONMAKING
Volume One
PRINCIPLES, ,DESIGN.AND RAW MATERliAlS
" IIMcMASTER UNIVERSITY 'lHam i Iton,Ontario, Canada f
JUNE,,1999
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AN INTENSIVE COURSE
BLAST FURNACE IRONMAKING
JUNE 7-11, 1999
VOLUME ONE
PRINCIPLES, DESIGN AND RAW MATERIALS
COORDINATING COMMITTEE:
A.J. Fischer, Dofasco Inc. (Chairman)G.A. Irons, McMaster University (Secretary)
R. Brown, Stelco Inc.P. Kuuskman, Algoma Steel Inc.
J.J. Poveromo, Quebec Cartier Mining Co.F.e. Rorick, Bethlehem Steel Corp.
S. Sostar, Lake Erie Steel Co.
Copyright 1999
Department of Materials Science and EngineeringMcMaster UniversityHamilton, Ontario, CanadaL8S 4L 7
No part of this book may be reproduced in any form, except with the consent of anindividual author concerning his own lecture or with permission from the Department ofMaterials Science and Engineering, McMaster University, or the Coordinating Committeeof this Course.
Printed in Canadaat McMaster University
PREFACE
The efficient operation of the iron blast furnace is essential to the economic well-being of any integrated steel plant; any improvement in operation usually has a signifcantimpact upon the entire company.
Today's ironmaking technology has evolved over many years through innovationsin raw materials preparation, blast furnace design, refractories improvements, and blastfurnace practice. Much remains to be done; significant gains remain to be realized. Muchis being done.
This course on Blast Furnace Ironmaking was organized in response to a felt need;the response has been overwhelming. It is an intensive, in-depth course covering everyaspect of blast furnace ironmaking, which should make it useful to many people - managers,operators, engineers, researchers, and suppliers of equipment, refractories and rawmaterials.
The 1999 course was organized by a Coordinating Committee consisting of:
Randy Fischer, Dofasco Inc. (Chairman)Gord Irons, McMaster University (Secretary)
Rick Brown, Stelco Inc.Peter Kuuskman, Algoma Steel Inc.
Joe Poveromo, Quebec Cartier Mining Co.Fred Rorick, Bethlehem Steel Corp.
Steve Sostar, Lake Erie Steel Co.
In developing this course, we adhered to two criteria; the lecturers would beacknowledged experts in their fields and the contents would be practical, with onlysufficient theory to understand the process.
We, the Committee, hope that this course has satisfied your present needs, that youwil have made some valuable and lasting "contacts", and that these notes wil continue tobe a valuable reference for you in years to come.
Randy Fischer, ChairmanCoordinating Committee1999 Blast Furnace Ironmaking Course
FOREWORD
The first Blast Furnace Ironmaking Course was initiated in 1977 under theleadership of John Holditch and Don George. The course has been offered 14 times (1977,1978,1980,1981,1982,1984,1985,1987,1989,1990, 1992, 1994, 1996 and 1998) and owesits success to the excellent reputations and efforts of the lecturers and of the CoordinatingCommittees. This, the 15th course, is being offered at McMaster University in June 1999.
Since 1984 the course has been officially recognized by the American Iron & SteelInstitute, and is jointly supported by the AISI and McMaster University. The overwhelmingresponse every year to this course has been not only in the number of registrants but alsoin their diversifed industrial backgrounds. Another notable fact is that among theregistrants, many are well-known experts in their own right, in certain aspects ofiron making. We would like to take this opportunity to express our sincere appreciation toall the lecturers who have contributed to this course, and to their employers for allowingthem to take time off from their busy schedules and for defraying their travel expenses.
Gord Irons, SecretaryCoordinating Committee1999 Blast Furnace Ironmaking Course
1999 BLAST FURNACE IRONMAKING COURSE
CONTENTS
VOLUME ONE: PRINCIPLES, DESIGN AND RAW MATERIALS
Lecture 1 Historical Development and Principles of the Iron Blast Furnace
J.A. Ricketts, Ispat Inland Inc.
Lecture 2 Blast Furnace Slag
J. L. Blattner, AK Steel Corp.
Lecture 3 Blast Furnace Reactions
A. McLean, University of Toronto
Lecture 4 Blast Furnace Energy Balance and Recovery: Rules of Thumb
and Other Useful Information (Computer Game)J.W. Busser, Stelco Inc.
Lecture 5 Blast Furnace Design I
J. Carpenter, Paul Wurth Inc.
Lecture 6 Blast Furnace Design II
N. Goodman, Kvaerner Metals
Lecture 7 Blast Furnace Design IIIS. Sostar, Lake Erie Steel Co.
Lecture 8 Ironmaking Refractories: Considerations for CreatingSuccessful Refractory "Systems"A.J. Dzermejko, Hoogovens Technical Services Inc.
Lecture 9 Iron-Bearing Burden Materials
M.G. Ranade, Ispat Inland Inc.
Lecture 10 Blast Furnace Control- Measurement Data and Strategy
R.J. Donaldson and B. J. Parker, Dofasco Inc.
Lecture 11 Maintenance Relial?i1ty Strategies in an Ironmaking FaciltyG. DeGrow, Dofasco Inc.
1999 BLAST FURNACE IRONMAKING COURSE
CONTENTS
VOLUME TWO: OPERATIONS
Lecture 12 Coke Production for Blast Furnace Ironmaking
H.S. Valia, Ispat Inland, Inc.
Lecture 13 Day to Day Blast Furnace Operation
A. Cheng, National Steel Corp.
Lecture 14 Challenging Blast Furnace Operations
F.e. Rorick, Bethlehem Steel
Lecture 15 Burden Distribution and Aerodynamics
J.J. Poveromo, Quebec Cartier Mining Co.
Lecture 16 Casthouse Practice and Blast Furnace Casthouse Rebuild
J.B. Hyde, Stelco Inc.
Lecture 17 Environment, Health and Safety Issues in Blast Furnace IronmakingE. Cocchiarella and D. Foebel, Dofasco Inc.
Lecture 18 Fuel Injection in the Blast FurnaceF.W. Hyle, USX Corp.
Lecture 19 Ironmaking/Steelmaking Interface
C. Howey and R. Brown, Stelco Inc.
Lecture 20 European Blast Furnace PracticeD. Sert, IRSID
Lecture 21 Japanese Blast Furnace PracticeK. Yoshida, Kawasaki Steel Corp.
Lecture 22 Future Trends in Ironmaking
W-K. Lu, McMaster University
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LECTURE #1
HISTORICAL DEVELOPMENT AND PRINCIPLES
OF THE IRON BLAST FURNACE
John A. RickettsManager of Operating Technology, Iron Production
Inland Steel Company
FOREWORDThis lecture is essentially a blending of the
material prepared for the previous McMaster Blast FurnaceIronmaking Courses, by R. W. Bouman on the HistoricalDevelopment of the Blast Furnace and by John F. Elliotton Principles of the Iron Blast Furnace. A section onModern Aspects of Blast Furnace Theory has been updatedby A. McLean with material drawn from the 1978 HoweMemorial Lecture by E. T. Turkdogan and also two recentpapers by W-K. Lu which discuss the behavior of siliconand alkali metals in the blast furnace. A new section oniron making 100 years ago has also been added by thecurrent author.
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INTRODUCTION
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The contents of this lecture have been arranged inthe following sections:
EARLY IRONMAKING
The First IronmakersIronmaking in the Middle Ages
DEVELOPMENT OF THE BLAST FURNACE
Pre-Industrial RevolutionEarly Industrial Revolution
Late Nineteenth CenturyEarly Twentieth Century
DEVLOPMENT OF BLAST FUACE FUDAMNTALS
Early ScientistsGas-Solid Contact
Solution Loss
MODERN BLAST FUACES
Raw Material PreparationCombined Blast
Large Blast FurnacesTop Pressure
Burden and Gas Distribution
MODERN ASPECTS OF BLAST FUACE THEORY
Reduction of Iron OxidesFluxesSlags
Reactions in the Bosh and HearthEnergy Considerations
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CONCLUDING REMA
SOURCES OF ADDITIONAL INFORMTION
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INTRODUCTION
The ironmaking blast furnace has played an important role in thedevelopment of our industrialized civilization. This furnace has beena means of producing metallic iron, which has been and continues to bea major building block of heavy industry. The principal aim of theiron blast furnace is to smelt iron ores and prepared agglomerates oriron ore concentrates to produce a liquid crude iron. When liquid,the crude iron is called hot metal or pig iron, and when solidified,it usually is termed pig iron. The composition of the product dependsto a considerable degree on the use to be made of the metal. Theprincipal use is as a raw material for oxygen steelmaking for which atypical composition is approximately 4.2% carbon, 0.8% manganese, 0.7%silicon, less than 0.035% sulphur, and from 0.15 to 0.01% phosphorus.The concentrations of manganese and phosphorus depend primarily on thecomposi tions 0 f the iron ores and agglomerates charged to the furnace.
The raw materials consumed in the smelting operation in additionto the iron-bearing materials, i.e., the ores and agglomerates, are:coke which is the principal fuel; limestone and dolomite which act toflux the earthy constituents, or gangue, in the iron-bearing materialsand ash in the coke to form a slag; and hot air and oxygen which areneeded to burn the coke; and minor fuels such as heavy oil, tar andna tural gas.
The blast furnace produces a slag resulting from the union ofthe fluxes with silica (Si02), alumina (A1203) and some of the manganousoxide (MnO) which are obtained from the coke ash and gangue of the iron-bearing raw materials. A nominal composition of the slag is 45% CaO,5% MgO, 35% Si02, 12% A1203, a few percent MnO, and 1 to 2% sulphur.A large volume of low-grade gas is produced as well. The compositionof this gas varies somewhat with different furnaces and with rawmaterials and fuels, but it will be approximately 56% nitrogen, 25% CO,17% CO2, and 2% H2 on a dry basis. It will also contain some watervapour. The heating value (low) of the gas is relatively poor, beingin the range of 0.8 to 1-.1 M cal/m3 (90 to 125 BTU/ft3). On leavingthe furnace shaft, these gases will contain considerable quanti ties ofdust, a major portion of which is removed in auxiliary facilities.
The furnace in which the process of smelting occurs is a tall,refractory-lined steel shell having a circular cross-section. Duringoperation of the furnace, this shaft is filled with a carefully con-trolled mixture of the iron-bearing materials, coke and fluxes which arecoarsely granular in form. It is to be noted that in many modernopera tions some, or in some cases all, of the fluxes are incorporatedin the iron-bearing portion of the charge. Hot air for combustionof the coke in the èharge is injected into the lower portion of thefurnace through water-cooled nozzles, or tuyeres. The coke andauxiliary fuels that may be injected into the tuyeres are burned inthe region just in front of the tuyeres to produce a very hot gas thatconsists principally of CO and nitrogen. This gas passes up throughthe charge in the shaft and heats and alters the charge chemically inits passage. As a result of burning of the coke at the tuyeres and
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mel ting of the iron and formation of the liquid slag inthe lowerregion of the shaft, the solids in the shaft descend slowly and passthrough the furnace in approximately 8 hours. Accordingly, new chargesof iron-bearing materials, fluxes and coke are added at regular inter-vals to the top of the furnace, and the liquid slag and hot metal aredrawn off at the bottom periodically.
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The lower end of the shaft below thetuyeres is -a crucible inwhich the liquid slag and hot metal is collected. This crucible islined with carbon brick or with high quality refractory brick.
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The contour of the shaft is designed very carefully and will varyin subtle ways depending on the type of raw materials being smelted,furnace size, etc. From the top or throat section where the solidmaterials are placed on the bed, the shaft widens at a very low angleto allow the bed to expand slightly as it descends. There is a cylin-drical section, or belt, approximately two-thirds the distance downthe shaft which joins the upper tapered section to the lower taperedsection, or bosh. The bosh is a short, tapered section which restrictsthe cross-section to compensate for the sintering and fusion of the bedas its temperature rises. The barrel-shaped section below the boshcontains the tuyeres and the crucible.
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IFacilities at the top of the furnace shaft seal it to permit
operation at pressures of 1 to 3 atmospheres, gage. These facilitiesprovide for collection of the gases after they leave the shaft and forregular and controlled additions of the raw materials and coke. Thefurnace is also serviced by facilities for removing the hot iron andslag. The system for supplying the hot air blast for the tuyeresincludes very large air compressors, three or four stoves for preheatingthe air, and duct-work to distribute the air to the tuyeres. Most fur-naces also include equipment by which the auxiliary fuels may beinjected into the tuyeres.
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In the following sections the history of ironmaking is brieflyreviewed. Particular emphasis is given to the major structural andmechanical developments as well as the evolution of blast furnacetheory. The aim of this lecture is to cover the most basic fundamentalsof the ironmaking blast furnace process and show how these fundamentalshave resulted in furnaces that today are capable of producing over10,000 tons of pig iron per day.
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EARLY IRONMAING
The First Ironmakers
The first reduction of iron ore to iron probably took placeduring the bronze age and was accomplished by using smelting holes ofthe type illustrated in Figure 1. By the time of the Romans, ironsmelting was practiced throughout most of the known world. At thisstage the process was a batch operation in which charcoal was ignitedand, when sufficiently hot, produced hot carbon monoxide that ascended
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IFigure 1. Early Ironmaking Smelting Hole
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Figure 2. Early Bowl or Shaft Furnace for Smelting Iron Ore
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to reduce and smelt the ore. Bellows were apparently used qui te earlyto provide the air for combustion. These operations were very ineffi-cient in the use of both the ore and the reductant. Much of the ironoxide in the ore was not reduced, and since mel ting temperatures werenot reached, this unreduced iron and impurities such as silica andalumina were surrounded by metallic iron at the end of the smeltingoperation. The spongy mass, or bloom, was removed from the smeltinghole when the charcoal was spent and formed into tools and weapons.The forming and shaping operations also served the very importantfunction of removing most 0 f the iron oxides and other impuri tiestrapped in the bloom. Analyses of some of these early iron blooms andimplements indicate that their average composition before surface car-burizing was:
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Percent ~
CarbonSiliconManganeseSulphurPhosphorus
0.03 - 0.10nil- 0.05nil - 0.150.005 - 0.0500.05 - 0.50
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IThis implies that the iron content of these materials was greater
than 99% and that some of these early irons were relatively pure. Thesefirst attempts at ironmaking produced mostly wrought iron, but some ofthe material would today be classified as steel.
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As the demand for iron increased, ironmakers began looking forbigger and better methods of producing their blooms. Bowl furnacesor short shaft furnaces similar to the one shown in Figure 2 cameinto use. The shafts were probably no more than 6 feet in height andwere lined with clays. The advantages of this type of smelter werethat they could hold a larger charge of ore and charcoal, and eventuallyhad an opening in the bottom for the removal of the mol ten slag thatformed during the smelting operation. These slags contained the oreimpuri ties such as silica, alumina and lime, and unreduced iron oxide.Air was introduced into these furnaces through one or more openingslocated above the slag hole by natural draft and by mechanical blowingdevices. The early shaft smel ters were still batch operations and theiron product was still a bloom or spongy mass. After each batch wasprocessed, the shaft was at least partially dismantled to remove thebloom. Some of these furnaces were constructed or excavated on theside of a hill and others were free-standing on level ground.
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Another type of early iron smelting furnace is shown in Figure 3.This furnace resembles a beehive coke oven and was constructed withal terna te layers 0 f charcoal and iron ore. The charcoal and ore moundwas then covered with a thick layer of clay, the bottom charcoal layerswere ignited, and the smelting operation was started. Near the end ofthe smelting operation, the clay dome undoubtedly collapsed around theiron bloom.
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The early Japanese smelters produced iron f~oW iron sands andcharcoal on an elaborately constructed hearth. This operation,called the Tatara process, was practiced in Japan as late as the 19thcentury. The Tatara furnace was large by early ironmaking standardsand apparently produced as much as four tons of spongy metal in onebatch. By comparison, it is doubtful that the early ironmaking opera-tions shown in Figures 2 and 3 produced blooms much larger than 500pounds.J
JThe earliest cast of liquid iron was probably produced in China.
There is evidence that cast iron was made in China during the firstcenturies of the Christian era, much before any such activity in Europe.
~ Ironmaking in the Middle Ages
IThe art of ironmaking spread rather rapidly throughout Europe
and the Medi terranean area during the Roman era. Roman shaft smelterssimilar to that shown in Figure 2 and dating back to the second centuryA.D. have been found in Britain. with the decline of the Roman Empire,ironmaking seemed to decline in importance. At the beginning of the14th century, ironmaking was being practiced as i thad been 2000 yearspreviously . However, the 14th century marks the start of ironmakingdevelopmen ts that continue today.
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IIn addition to shaft furnaces, European iron smel ters in the
Middle Ages used hearth furnaces. This type of smelter was eventuallyexpanded in size and equipped with a mechanical air blowing device, asshown in Figure 4. Smelters of this type were used in Spain andFrance, and were known as Catalan forges. The air blowing equipmentused with the Catalan forges was a large air aspirator and apparentlycould develop as much as 1.5 to 2 psig of air pressure - considerablymore than could be achieved with the hand or foot powered bellows thatwere used during the previous centuries. The Catalan forge did notchange the basic ironmaking practice that had previously developed butdid significantly increase the size of the blooms produced.
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was the enlargement of the shaft smelter. A larger shaft smelter,named the Stückofen, came into use in Germany during the ,early 14thcentury. This development is now generally recognized as the earliestblast furnace. At first the Stückofen was a batch operation and pro-duced a bloom as in early shaft furnaces. However, the Stückofen waseventually made taller, probably as a result of the availability ofthe higher blast pressures made possible by water-powered bellows.The Stückofen was constructed as two truncated cones with one on topof the other as shown in Figure 5, and was made up to 15 feet high and5 feet in diameter at the widest section.
As a direct result of water-powered bellows to produce higherblast pressures and the larger Stückofen furnace with reduced heatlosses, mol ten iron started to be produced in Germany during the verylate Middle Ages. The formation of liquid iron in the smelter
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~Figure 3. Beehi ve Furnace for Smelting Iron Ore
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Figure 4. Catalan Forge wi th Air Aspirator
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I Figure 5. stückofen or Bloom Furnace
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Figure 6. Early Charcoal Blast Furnace
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undoubtedly presented problems for the ironmaker. First, be was facedwith a containment problem, and secondly, the liquid product was notof the same composition as the previously produced blooms. It appearsthat the most common solution to the containment problem was to allowthe mol ten iron to flow from the hearth of the shaft into a forehearth.Here the mol ten iron was allowed to solidify and form what is nowcalled pig iron. The second problem wi thpig iron was its high carboncontent. This problem was solved by the development of a two-stageprocess that produced wrought iron. The first stage was the productionof pig iron in the Stückofen, and the second stage was the mel ting anddecarburizing of the pig iron in a small hearth furnace, or bloomery.The two-stage operation then resulted in a product that was similar tothe blooms that were first produced in shaft furnaces. This two-stageoperation, developed well before the Industrial Revolution, is analogousto present day steelmaking in blast furnaces and oxygen blown converters.One result of the two-stage process was that the smelting of iron oreina blast furnace could be separated from the product-making operation.This separation of functions eventually played a major role in theenlargement of shaft smelters.
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One other notable ironmaking event that took place in the MiddleAges was the passing from a batch operation to a continuous operation.This event has apparently not been noted by historians, but it must beconsidered significant in the development of blast furnaces. Continuousblast furnace operation probably started shortly after liquid iron wasproduced in the Stückofen. Once the iron smelters realized they didnot have to drag a bloom from the bottom of their shaft, it was alogical step to continue the charging of raw materials and the castingof liquid iron.
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DEVELOPMENT OF THE BLAST FURNACE IPre-Industrial Revolution
IDuring the 17th century, Britain was beginning to emerge as a
leading ironmaking country. Up to this time, other European countries,notably Germany, France and Sweden, had been the leaders in ironmakingdevelopments. The ironmaking operations of this era were producing atbest 1 to 2 tons per day ,and were dependent on the essential rawmaterials of iron ore, wood to make charcoal, and water power.Because of this dependence, ironmaking operations were required tomove frequently as the local supplies of wood and ore were exhaustedand new sources were discovered. In Britain, and to a lesser extent,in other ironmaking countries, the availability of wood became aproblem in the 17th century. The ironmaking operations consumed vastquanti ties of wood, and concern about the availability of wood forironmaking and ship-building was increasing. This supply problem wasrecognized by the British iron smelters, and to a lesser extent, inother ironmaking countries. Attempts to use coal in place of charcoalwere made in the late 17th century. These attempts were largelyunsuccessful due to the high sulphurcontent of the coal and its inability tosupport the ore in the blast furnaces without a large pressure drop.
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1The ironmaker i sunderstanding of his blast furnace increased
significantly in the 18th century. In the early 18th century, afterunsuccessful attempts at using coal, a British ironmaker by the nameof Abraham Darby tried to use coke in his blast furnaces. Coke wasbeing produced near Darby's ironmaking operations for use in maltingkilns, and after some experimenting with this new ironmaking fuel,Darby established an ironmaking operation based on coke in 1713. Thisevent must be considered one of the most important blast furnacedevelopments of all time. In view of the serious wood shortageproblems then facing the country, this development was to eventuallysave the British ironmaking industry. In 1740 there were 50 blastfurnaces operating in Britain. The average production of a furnacewas 6 tons per week, and only Darby i s furnace was using coke. By 1790there were 106 blast furnaces and 81 of these were using coke. Thefurnaces using coke averaged about 17 tons per week.
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IOther blast furnace developments that occurred in this Pre-
Industrial Revolution period were the changing shape of the lowersections of the shaft and improved methods of blowing. The charcoalfurnace used prior to the use of coke had a small hearth and a flat,almost horizontal bosh just above the hearth as shown in Figure 6.The purpose of the bosh was to support the raw materials in the shaftabove. Because liquid iron and slag dropped to this surface and raninto the hearth, the bosh eroded rapidly and was probably where theseearly furnaces failed most frequently. With the use of coke insteadof charcoal, the ironmakers soon found the flat bosh was not requiredbecause the coke was much stronger and could support the raw materialsin the shaft without crushing. Furnacemen also found that with cokethe shafts could be built taller and thus produce more iron.
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I wi th taller furnaces made possible with the use of coke, airblowing requirements increased. At first this was achieved with morewater for the water wheels; horses were also used to produce blast forthe furnaces. However, late in the 18th century, steam engines cameinto use for blowing blast furnaces. At the same time as the intro-duction of steam engines, piston and cylinder blowing machines beganto replace the bellows that were used with the earlier water wheels.These developments significantly increased the blowing and productioncapabili ties of exis ting furnaces and, with coke as a fuel, permittedthe furnaces to be increased in size.
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In the very early 19th century various grades and quality of ironhad already been established for trade. The ironmaker of this erahad learned how to control the reduction of silica in his furnace andhad apparently long since learned how to make fluid slags with theaddi tion of limestone to the charge. The blast furnaces of thisperiod were still no more than about 30 feet high and were constructedentirely of stone and fireclay. The largest of the circular furnaces
(many were rectangular in cross section) were two to three feet indiameter at the top, up to nine or ten feet in diameter at the top ofthe bosh, and had a hearth three to five feet in diameter. Theproduction from these furnaces was only a few tons per day, and thecoke consumption was, at the very best, two tons per ton of iron. Thefurnace tops were open and belched great quantities of fire and smoke.
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Significant developments in methods of refining iron into usefulproducts were also made in this period. The use of cupolas for themel ting of pig iron was developed in the 18th century. More importantly,the puddling furnace was invented by Henry Cort at the start of theIndustrial Revolution. The puddling furnace removed carbon and othermetalloids from remelted pig iron with an oxidizing flame and theadditions of ore, the result being a spongy mass of wrought iron thatcould be formed. This operation was a type of early open hearth furnaceand further permitted the separation of the ironsmel ting and ironrefining steps.
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Thus, at the start of the Industrial Revolution, the ironmakersin Britain were in a strong position to provide the building blocks ofheavy industry as a result of the development of coke and steam powerfor blowing. The further developments of the two-stage ironmakingprocess as a result of the puddling furnace invention also opened theway for the yet-to-come two-stage steelmaking processes.
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IEarly Industrial Revolution
IDuring the early part of the Industrial Revolution, the basicprinciples of iron smelting blast furnaces did not change from theearlier 18th century technology. However, significant mechanicaldevelopments were incorporated into iron blast furnaces in this period.These mechanical improvements were prompted by the tremendous increasein the demand for iron and iron products. In Britain for instance,pig iron production increased from about 125,000 tons at the beginningof the 19th century to about 400,000 tons in 1820 and again to about2.5 million tons by 1850.
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The most significant ironmaking development in the first half ofthe 19th century was the invention of preheated blast air in 1828 byJames Neilson, a Scotsman. Up to this time, ironmakers believed thathot blast would not help their blast furnaces. This belief was basedon their observation that the furnaces seemed to operate moreefficiently and produce more iron during the colder winter months.The early ironmakers did not recognize that this seasonal fluctuationwas due to changes in the moisture content of the air. Neilsonapparently made a chance observation that blast furnace air that wasonly slightly elevated in temperature made a remarkable improvement inthe performance of the furnace. He further developed the idea andreceived a patent for his preheated blast concept. The technique wasquickl y adopted by furnacemen in Scotland and the res t 0 f Bri tain . Thefirst hot blast systems consisted of an iron pipe enclosed in a refrac-tory tunnel, with either coal or blast furnace off-gas being burned inthe annular space. These early systems were limited in hot blast tem-perature; however, the effects on furnace operations were quitenoticeable. The production on the largest furnaces of that day wentfrom 30 to 40 tons per day. Because of the importance of high hot blasttemperatures in modern blast furnace technology, the development of pre-heated blast must rank in importance with the use of coke in the histor-ical development of the blast furnace process.
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By about 1840, blast furnaces were being built up to 60 feet highwith an internal diameter of 16 feet at the top of the bosh. The hearthsof these furnaces were up to 8 feet, and the internal reactor volume wasas much as 7,000 cubic feet. One of these furnaces is illustrated inFigure 7. It was also apparently in the early 19th century in Scotlandwhen iron pipes and water-cooled tuyeres were first used to introducethe air into blast furnaces. Previously, leather and canvas tubingcarried the blast air to the furnaces and clay tuyeres were used tointroduce the blast into the furnace.
By the middle of the 19th century Britain had become the leadingiron producer in the world and pig iron production by the largestfurnaces was up to 30 tons per day. Coke was the most common fuel andreductant for blast furnaces in Britain at this time and coke consump-tion was abut two tons per ton of pig iron. However, there were atleast two significant ironmaking operations based on the direct use ofcoal. Scottish ironmakers were successfully using a hard splint coalin their blast furnaces during this period, and American ironmakershad developed an anthracite blast furnace practice.
By 1870, blast furnaces were producing up to 60 tons per day. Theincentive to produce more iron and build larger blast furnaces increasedwith the development of steelmaking by Bessemer, Siemens and Thomas.The processes developed by these individuals allowed the conversion ofpig iron into steel and, as a result, started the modern steelmakingera. The effect of these developments on iron production in the late19th century was dramatic. Blast furnace iron production in Britainrose from 2.5 million tons in 1850 to 8 million tons in 1895. Theproduction of steel in Britain rose from about 200,000 tons in 1865 to3.3 million tons in 1895. However, the growth of the young steelindustry was most dramatic in the United States. In 1871 blastfurnaces in the U. S. produced about 1.7 million tons of pig iron peryear, but by 1890 the production of U.S. furnaces was over 9 milliontons per year and greater than that of the British industry. As inBritain, the production of blast furnace iron was driven by the increas-ing demand for steel and steel products, and by 1910 U. s. furnaces wereproducing more than 27 million tons of pig iron per year. As a result,a new leader in iron producing capability and technology was established.
The American blast furnace in the early 1870 decade was for themost part still a stone and masonry structure lined with refractorybrick. The furnaces were hand-filled through open tops; however, somefurnaces were using a single bell and hopper arrangement to seal thefurnace between charges. Some furnaces also had facilities for direct-ing the off-gases to a boiler for steam generation. steam-poweredblowing machines were fairly common, but some furnaces, particularlycharcoal operations, were still blown by water-powered equipment. Hotblast, when used, was typically produced in iron pipe stoves. A produc-tion 0 f 30 tons per day was cons idered good in 1870. A productionrecord of 100 tons per day by the Lucy furnace located near Pittsburghin 1874 received world-wide publicity. In 1870 half of the pig ironproduced in the U.S. was made in anthracite furnaces, 30% in furnacesusing coke and 20% in charcoal furnaces.
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Figure 7. Mid-19th Century Blast Furnace
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CHACOAL IRON MAING1860 TO 1890
1870 BLAST FURNACE DESCRIPTION
The typical shape of a blast furnace is a verticalshaft formed by two truncated cones joined at theirbases. The upper, taller cone stands upright and isknown as the "STACK". The lower, shorter cone is in-verted and is known as the "BOSH". Below the bosh is abottom-sealed vessel where liquids accumulate called the"CRUCIBLE" or "HEATH" (Figure 8).
The top of the furnace is open and is called the"THROAT". The platform on the top of the furnace sup-ports a short chimney with an opening for raw materialcharging called the "TUEL HEAD". Gases from the ironmaking process are captured at the throat by a ¡'GASPORT" and are transported to a boiler or hot blast ovenby the "DOWN COMER".
1'1 LLING HO LI'
WHI TIWORI
GAl PORT
---TUN" IL HIAD
.ID WORIt
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THROAT
aoH11l,
CRU~I.LI _
aLA S TIPI
aLAITMAl H
TU",..I HOUII
TYM"
HI.RTH I,,OMI
Figure 8 - Charcoal Blast Furnace
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Figure 9 - Furnace Stack Overview
The massive construction of the tapered rectangularblast furnace is known collectively as the "STACK PIL-LA". These stack pillars form a four sided block ofmasonry that is braced with iron tie rods and united bycylindrical arches on each side which form the "TUYEREARCHES". The tuyere arches allow an opening for the"BLAST MAIN" to feed hot air through the "BLAST PIPE"and into the "TUERE" which fits into the furnace (Fig-ure 9). The inside bosh and stack is lined with a firebrick called "WHITE WORK". This brick is 15 to 18inches thick and withstands the high iron making tempera-ture. The outside masonry that supports the fire brickis ei ther brick or rough stone and is known as "REDWORK". A small space, filled with loose sand or slag ismaintained between the white work and red work for expan-sion as the fire brick heats.
The crucible or hearth of the furnace has severalparts. The bottom is a solid stone called the "HEATHSTONE". Liquid iron and slag sit on top of this stone.The front of the furnace where the iron and slag is re-moved is called the "FOREPART". The liquid productsmust flow over the "DAM" and under the "TYMP". Thefurnace is constantly filled with raw materials throughthe tunnel head but is only cast by knocking out a por-tion of the dam when iron fills the hearth. The slag isdrained continually into "SLAG PITS" , but the iron isonly cast every few hours into a ditch called a"TROUGH" which leads to small runners called "SOWS"which have numerous cavities attached called "PIGS".These iron pigs weigh between 70 and 100 pounds. Thiswhole process takes place in the "CASTHOUSE".
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other maj or parts of the blast furnace include a"BOILER" which produces steam for a "BLOWING ENGINE"that supplies air for burning the fuel in the furnace. A"HOT BLAST OVEN" is a rectangular brick structure withmany pipes. Gas collected from the furnace stack isburned in the oven and heats the pipes. As the "COLDBLAST" from the blowing engine passes through theseheated pipes , it becomes "HOT BLAST" which flows intothe furnace.
Charcoal which is the fuel in the blast furnace isproduced by partially burning wood in a "CHACOALKILN". Other raw materials charged into the blast fur-nace are "IRON ORE" which becomes the pig iron and"FLUX" which forms the slag. All of these raw materi-als are stored in a "STOCKHOUSE" . In the stockhouse,they are weighed to specific proportions. The raw materi-al s are then lifted to the furnace top by a "HOISTHOUSE" elevator and charged into the furnace (Figure10) .
BLAST FURNACE PLANT LAYOUTCOLD BLAST
...."i R.
BOILER ANDBLOWING ENGINE
HOUSE I
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BOILER ANDBLOWING ENGINE
HOUSEHOT
BLASTOVEN
HOTBLASTOVEN
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MA.RekKILN
HOT BLAST
ICASTHOUS '
STACK HOT BLAST~L:
STOCKHOUSE
DOCK
Figure 10 - Plant Layout
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RAW MATERIALS
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Charcoal was the chosen fuel for blast furnace opera-tion in early industrialized America because there werevast forests of hardwood in most unsettled areas. Char-coal is simply partially burned wood, which is a form ofcarbon. Wood normally burns in three stages. First,moisture in the wood is driven out as steam. Then thevolatile matter, sap, oils, and pitches, is burned offwhich creates gases and smoke. Finally, with only thecarbon remaining, flames and smoke disappear and charcoalembers glow releasing great energy in the form of heat.The production of charcoal for blast furnaces was accom-plished by allowing only the first two steps of thisprocess which resulted in the final product of high car-bon charcoal.
The preferred wood for charcoal production was hard-woods, such as maple, oak and birch. The wood was cutinto four feet lengths with a diameter of four to sixinches.
The averageand pile fourwood choppersthe 1860's.was about 50required 100wood.
production of a two man crew was to cut(4) cords of wood in a ten hour day. The
were paid approximately $0.80 per cord inThe charcoal yield from a cord of hardwoodbushels. On the average, one ton of ironbushels of charcoal which is two cords of
Once the wood was cut, the charcoal could be producedby two methods: pi t and Kiln. The pit method could beused in any open location since it did not require apermanent structure. The kiln method was performed instationary stone structures that were originally locatedin close proximity to the blast furnace. As forests werecut down and wood supplies were exhausted, the kilns werebuil t farther from the iron plants. A number of blastfurnaces were permanently shut down due to lack of char-coal since charcoal transport costs from distant loca-tions resulted in iron prices that were too high to re-main competitive. This same issue has resurfaced onehundred years later because many steel companies cannotinternally support coke requirements and their iron pro-duction costs increase with the purchasing and shippingof coke from distant production locations.
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The first step in producing charcoal by the pit meth-od was to clean off a 30 to 40 foot circle of flat,packed ground. Then 25 to 30 cords of wood were piled toform a mound. The wood was positioned standing on endand leaning toward the middle so that the mound lookedlike an igloo. Once the cord wood had been put in place,
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small dry branches, called lapwood, were placed over themound of cord wood. This lapwood was the kindling woodfor the cord wood. Then a layer of wet leaves was placedon top of the lapwood and over the entire mound. Final-ly, a 4 to 6 inch layer of earth covered the mound toreduce the amount of oxygen entering into the wood core(Photo 1).
Once the mound was complete, the pi twas lit andallowed to burn for seven to eight days. At no time wasa live fire allowed to burn freely. Remember, only themoisture and volatile matter were to be removed from thewood, so a slow, low heat, smoldering fire was neces-sary. Slowly the mound decreased to one-third of itsoriginal size as the moisture and volatile matter burnedoff. Finally, the charred wood was carefully raked fromthe mound without exposing the remaining wood that wasnot fully charcoal. The finished charcoal cooled whilethe remainder of the mound was allowed to complete theprocess.
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Photo 1 - Charcoal Pit(Courtesy Marquette County Historical Society)
The cooled charcoal was sacked and loaded into wagonswhich were drawn by horses or mules. The finished char-coal was then delivered to the blast furnace plant. Theaverage pit of 25 to 30 cords of wood would yield 1,000to 1,500 bushels of charcoal.
Charcoal kilns were hollow, beehive shaped structuresmade from local stone or brick (Photo 2).
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Wherever possible, the kilns were built along hill-sides to allow loading the cord wood from the top. Ifthis hillside location was not available, then a loadingplatform was constructed. Each kiln was 14 to 28 feet inheight. There were two large openings in each kiln; oneat top center and the other on the side at the bottom.The top hole was 4 to 5 feet in diameter and was thecharging hole used to stack the cord wood. The bottomopening was slightly larger, in the shape of a door, andwas used to start the fire and later to remove the char-coal. There were also approximately 15 to 30,four-inch-square openings, called "air vents", locatedroughly two feet apart all around the kiln about threefeet from the ground.
Photo 2 - Fayette Kiln(Photo by Author)
The four foot lengths of cord wood were brought inthrough the top charging hole. Each piece was piledparallel to the ground in two concentric circles. The 8foot diameter center remained vacant and was later filledwith dry kindling wood. A small tunnel was made to theside door to be used for an ignition channel. Anywherefrom 40 to 75 cords of wood could be placed in a kilndepending on its size. Once the kiln was filled andready, an oil saturated rag was lit and pushed in throughthe ignition channel. The kindl ing wood was lit andallowed to burn until flames were visible through thecharging hole. At this time, the door at the base of thekiln was sealed and the charging hole diameter reduced byusing stone and plaster. The smouldering fire within thekiln slowly worked its way from top to bottom. When thekiln man saw glowing, red coals at the air vents, hewould seal these openings and the remainder of the tophole. The kiln was now completely sealed and the woodwas allowed to char for eight (8) days.
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When the charring was complete, the large side door wasopened and the charcoal removed with 15-tine forks andshoveled into "scuttle-baskets". Each man would carry 2to 3 bushels of charcoal in his basket to a wagon orrailroad car. Each kiln would produce 2,000 to 3,750bushels of charcoal which would support 200 to 375 tonsof pig iron production.
The charcoal produced in both the pit and kiln methoddid not have all the volatile matter fully removed. Insome samples gathered around an old furnace, the charcoalstill contained almost 18% volatile matter resulting in a75% fixed carbon. It should also be noted that charcoalsamples had 0.5% K20, an alkal i, which is high comparedto coke and would result in accelerated furnace refrac-tory lining wear. However, the sulfur content of char-coal is very low at approximately 0.05% which yields alow sulfur, high quality pig iron. A full comparison ofcharcoal to coke analysis can be seen below:
Parameter Charcoal Coke
Carbon (% ) 75.40 90.90Volatile Matter (% ) 17.90 0.90Ash (% ) 6.70 8.20S (% ) 0.04 0.72CaO (% ) 3.70 0.28MgO (% ) 0.30 0.09Si02 (%) 1. 50 4.13Al203 (%) 0.20 2.24P (%) 0.03 0.03K20 (% ) 0.50 0.16
r Most nineteenth century blast furnaces were builtadj acent to iron ore deposits.
The mines were originally open pits or "cuts". Theore was mined by blasting solid rock into pieces of orethat could be lifted by miners onto carts. Once the pitsreached depths of approximately 200 feet, then tunnelsbecame necessary to follow the veins of rich ore. Ironore removal was done by strong men with hand drills,sledge hammers, pick axes and explosives. Tram carscarried the ore to the surface. Miners were paid$2.00jDay for 10 hours of work in 1865.
since the ironnew rich deposits,materials used intable below:
ore mined in the late 1800' s was fromthe iron content is better than rawtoday's blast furnaces as seen in the
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ParameterFe (% )Mn (%)P (%)CaO ( % )MgO (% )Si02 (%)Al203 (%)
MichiganOre
67.800.070.050.290.053.400.95
AcidPellets63.300.100.020.200.225.610.33
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FluxedPellets59.800.060.014.331. 455.310.39
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Acid pellets used by iron makers today contain only 63% -65% iron and fluxed pellets contain 59% - 61% iron. Thesilica content of Michigan pellets is 5.5% to 6.0%. Itwas the depletion of the high-iron content raw ore thatforced the development of concentrating low-iron contentores with 30% - 35% iron into pellets with the 60% plusiron content.
Another raw material required in ironmaking is lime-stone. High calcium and dolomitic limestone are bothsuitable as fluxes for the blast furnace. Fluxes areused in the ironmaking process to form slag of a properchemistry to remove sul fur from the iron. sul fur causescast iron to be brittle and break easier, therefore, thehighest quality and highest priced iron has the lowestsulfur. Most blast furnaces were built in the immediatevicini ty of limestone deposits. Enough flux should becharged to remove sulfur from the iron, but too much fluxcan result in a thick, gummy slag that will not run outof the blast furnace. Therefore, iron masters moni taredflux additions, slag properties and iron chemistry to getthe right balance.
A good blast furnace flux should have large percentsof calcia (CaO) and magnesia (MgO) since they remove thesulfur and low quantities of silica (Si02) and alumina(Al203) since they do not remove sulfur but increasethe quantity of slag produced.
BLAST FURNACE OPERATIONRAW MATERIAL CHARGING
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Once all of the _ raw materials had arrived at theblast furnace plant, they were usually stored in a build-ing or at least under a roof to keep them dry. Thisstorage area was known as the stockhouse. The stockhousenot only contained the various ore types, charcoal andflux but also included a crusher and a scale. The crush-er was driven by a steam engine and was used to crush oreand flux to a smaller, nugget sized material to improvefurnace permeability and efficiency. The scale was usedto weigh the ore, charcoal and flux to the right propor-tions to make the desired iron and slag qual i ty.
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Once the materials had been weighed, they were takento the top of the furnace. If the furnace was built atthe base of a bluff, a platform called the "stock bridge"connected the flat top of the bluff where the stockhousewas located with the furnace top platform. If the fur-nace was not built at the base of a bluff, an elevatorwas constructed (Figure 11). These elevators were called"hoist houses" and consisted of a hollow, roofed towerwith two adjacent lift platforms. The tower also con-tained a flight of stairs to the furnace top in case theelevator malfunctioned. The elevator platforms werehoisted by small stearn engines.
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Figure 11 - Hoist House
After the wheelbarrows reached the furnace top, theywere dumped into a charging hole by pushing the wheelsagainst a charging ring and lifting the back handles _ ofthe wheelbarrow. Charcoal and ore/flux were dumped inal ternating layers.
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It was this concern and the inability to increaseproduction with this method of charging that resulted inthe installation of the first inclined skip hoist on aPennsylvania blast furnace in 1883.
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Originally, raw materials were dumped into an openmouthed stack through a tunnel head. This could be dan-gerous to the individual charging the furnace since hecould fall into the furnace. Early blast furnace opera-tors and technicians realized that an open top furnacehad two disadvantages. First, the flammable gas from thestack could not be captured to fire boilers or heat hotblast ovens and second, that the distribution of the rawmaterials was causing furnace inefficiency. When rawmaterials are dumped directly in the center of the fur-nace, they form a conical heap. The fine material staysat the center of the heap while the coarse particles rolldown and deposit at the furnace wall. This resulted inthe wall area having higher permeability, and, therefore,most of the gas and heat ran up the furnace walls. Thiswas detrimental to the furnace operation since the materi-al at the center of the furnace arrived unprepared formelting in the bosh area and the excessive gas flow atthe wall would wear out the lining.
The first device placed into the open mouth of thefurnace to allow the capture of all the gas and in anattempt to help the distribution of raw materials was the"cup and cone". It consists of an inverted conicalcast- iron funnel fixed to the top of the furnace. This"cup" was approximately one-half the diameter of thethroat. Inside of this cup would sit a cast-iron conewhich was suspended from a fulcrum beam with a counter-weight. In first design, the cone sat on the top of thecup, and was raised by a hand winch (Figure 12). Thissystem worked in closing the top to capture gas, but allthe raw materials still were piled at the center so theburden distribution problem was not resol ved. In thesecond design, the cone sat below the cup suspended by achain to a counterweight fulcrum that pulled the cone upagainst the rim of the cup (Figure 13). r
Figure 12 - Cup and Cone Charg ing
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Figure 13 - Bell Type charging
When the charge was placed on the cone, the cone would belowered and the raw materials would slide off the conetowards the wall of the furnace. In this case, the mate-rial peaked in the form of a ring at the furnace walls.The fine material stayed at the wall and the coarse mate-rial rolled toward the center. This resulted in a changeof gas flow patterns from the heavy wall flow with theopen top or top opening cone system to a heavy centralflow wi th the bottom opening cone system. This change ingas flow patterns resulted in less wall wear and improvedstability of the operation and a more consistent ironquality. It is interesting that one hundred years later,devices are still being developed to measure and controlgas flow in the blast furnace.
If' Once the cup and cone was installed, all the gas,except that released when charging, was captured. Thegas was collected below the cup and in openings at theside of the furnace that led to a large cast iron pipecalled a downcomer. This large pipe left the top of thefurnace and was then split into smaller pipes. Some ofthe gas was diverted to boilers which provided steam tothe blowing engine, hoist engine or crusher engine andthe some of the gas was diverted to the hot blast ovenand burned to heat the cold blast.
This description of furnace charging gives an ideaof the numerous steps and equipment, but it also indi-cates that blast furnace iron masters understood what washappening inside their furnaces. Many of their improve-ments were the basis of our current day equipment on a-typical North American furnace.
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BLAST FURNACE STACK
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The four main parts of the furnace stack from top tobottom were the throat, stack, bosh and hearth. Thecharcoal furnace stood 40 to 50 feet high. The throatwas a vertical cylinder that was 4 to 5 feet in diame-ter. Therefore, the top of the stack region was the samediameter, but it tapered outward as it descended to adiameter of 8 . 0 to 9 ~ 5 feet. At this point, the stackregion met the bosh. The top of the bosh was its widestpoint and its diameter decreased as it descended. Thebosh bottom diameter was usually 3.5 to 4.0 feet. Thediameter of the lower stack and upper bosh was determinedby the type of fuel, type of ore and quantity of airblown into the bottom of the furnace. If the bosh wastoo narrow, the passage of hot gases moved more quicklyand smelting occurred higher in the stack. If the boshwas too wide, the hot gases moved more slowly and smel t-ing occurred lower in the furnace. The optimum boshdiameter would yield the best balance between the chemi-cal reactions occurring between gases and the lumpy parti-cles in the stack and the physical reactions when liquidslag and iron forms. This balance was critical to maxi-mize production and minimize fuel requirements.
The hearth of the furnace may be a vertical cylinderor slightly tapered truncated cone toward the furnacebottom. The hearth diameter was 3. 5 to 4. a feet. Thehearth was smaller than the bosh because it held onlyliquid which was denser than the raw materials, there-fore, required less space.
Approximately 30 to 40 inches above the hearth bot-tom was where the tuyeres were located. The tuyeres werethe openings where the blast was introduced to the fur-nace. Charcoal furnaces normally have two (2) tuyereseach at 900 from the front of the furnace or three (3)tuyeres with two (2) at 900 and one (1) at 1800 from thefront of the furnace (Photo 3). The inside of thethroat, stack, bosh, and hearth was lined with firebrick. This brick was wedge shaped to give a tight fitat the desired diameter.
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There were usually two to three rows of brick makingthe lining 16 to 24 inches thick. One main wear mecha-nism of this brick was the high alkali content of char-coal which dissolved the refractory. Immediately behindthe fire brick was a layer of sand or crushed brick.This material was loosely packed and granular to allowthe fire brick to expand as the furnace was heated.Behind the loose layer was the outer layer of the fur-nace. This was usually made of large stones held togeth- -er by mortar. Horizontal iron tie rods were placed
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along the outer stack on all four sides to add support(Photo 4). These tie rods were usually wrought iron barsfrom 1-1/2 to 2 inches thick. At each end of the bar wasa large cast iron washer and a nut.
Photo 3 - Tuyere Arch - Fayette, Michigan(Photo by Author)
Photo 4 - Tie Rod Support - Fayette, Michigan(Photo by Author)
At the front of the hearth were two sections re-quired for iron and slag removal: the dam and the tymp.The dam rose from the hearth bottom to a height of 15 to25 inches. The dam held the liquid iron and slag in thehearth. The tymp hung down from the upper hearth anddirectly over the dam. A small gap was left between thetymp and dam for liquid slag to run out of the furnace.A hole was made in the dam for removing the iron from thefurnace. The complete casting operation will be dis-cussed in a future section.
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Furnace DimensionsTypicalCharcoalFurnace
FurnaceToda V
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The tympjdam and tuyeres were located under archesthat were built into the outer stone stack column. Thesearch roofs were formed by keyed brick and tapered towardthe furnace. These arches could be 8 to 16 feet wide and8 to 12 feet high at the outside. The arch met the innerlining of the furnace where the lining was supported by ahorizontal cast iron beam. The arches over the tuyereswere called "tuyere arches" and the arch over thetympjdam was called the "casting arch". In many cases,the casting arch was bigger to allow better access to thedam.
The total inner volume of the furnace was 1400 to1500 cubic feet. As time went on into the 1890' sand1900' s, the blast furnaces were constructed to have big-ger volumes, more tuyeres, and, therefore, higher produc-tion rates. A typical blast furnace of today has a53,000 cubic feet working volume.
Volume 1464 Ft. 3 53,000 Ft.3Tuyeres 2 20Hearth Diameter 3 Ft. 8 In. 28 Ft.Bosh Diameter 9 Ft. 4 In. 30 Ft.Throat Diameter 4 Ft. 6 In. 22 Ft.Total Height 45 Ft. 100 Ft.Tuyere Height 32 In. 12.5 Ft.
from Hearth
BOILERS AND BLOWING ENGINES
The key difference between an ordinary furnace and ablast furnace is the blast of air forced into the furnacethrough the tuyeres. Originally, blast furnaces were fedair by a water wheel connected to an eccentric shaft thatpumped a leather bellow. The blast furnaces in the LakeSuperior region of 1870 used steam engines to deliver airor "wind" to the furnace.
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The first step in running a steam engine is generat-ing the steam within a boiler. Blast furnaces were builtnear water sources not only for shipping purposes butalso for water supplies to generate steam. The boilerswere usually located in a stone building adj acent to thefurnace stack. The boilers were fired by the gas collect-ed at the top of the blast furnace stack. These boilers,approximately 30 feet long, were positioned verticallyabove a boiler chamber. There were no internal flues-inside the boilers. Gas was burned in the boiler chamber
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and flames directly contacted the bottom of the boiler toheat the water. Each boiler chamber had its own stack todraw the flames across the boiler which formed a draft.The waste gases were then exhausted from this stack. Thesteam generated in the boiler was then piped to theblowing engine.
BL(IING ENGINE
FLYWHEEL
COlDBLT
BLOWING CYllHDER
COlDBLAT
Figure 14 - Blowing Engine
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The blowing engine was mounted horizontally on atimber frame. It consisted of a single steam cylinderthat was 18 to 72 inches in diameter with a stroke rang-ing from 28 inches to 48 inches. The steam cylinderpiston rod was connected via a crank to a heavy, largediameter flywheel. This flywheel was then connected toeither two cylindrical blowing tubes that compressed airin one direction or one blowing cylinder that compressedair in both directions (Figure 14). These blowing cylin-ders were 4 a to 50 inches in diameter and had a stroke of3 to 5 feet. The cold blast pipes were connected to theend of these blowing cylinders and wind called "coldblast" was then piped to the hot blast ovens. The blastpressure was 2 to 3 psi, but the volume of cold blast wasnot measured. This system remained in use on all blastfurnaces until 1910 when the first turbo blower was usedat the Empire steel Company in Oxford, New Jersey. Cur-rently, turbo blowers deliver 80,000 to 120,000 Cu. Ft.per minute of air to the blast furnace at 25 to 30 psipressure.
HOT BLAST OVENS AND WIND DELIVERY TO FURNACE
The first use of hot blast was in 1829 in Glasgow,Scotland. In 1831, a New York blast furnace engineerapplied for a patent on "heated air blast". The idea didnot become a reality until 1836 and its success was mini-mal. Other operators tried various methods to heat thecold blast, but the first success came in 1840 at Dan--ville, Pennsylvania.
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Throughout this developmental period, a controversystill raged over which is better; hot blast or coldblast. Theoretically, hot blast should deliver a higherenergy blast to the furnace with a heat value that shouldoffset some of the charcoal consumption, but actual blastfurnace operation showed that cold blast furnaces operat-ed in the winter with a lower blast temperature used lesscharcoal than the same furnace run in the summer with ahigher blast temperature. Therefore, the colder theblast, the better the fuel rate. What the operators didnot know, and was explained later, was that the moisturecontent of air is lower in winter than in summer, andthat it was the high humidity in summer blast that causedfuel rates to increase not the temperature difference.Once this was explained to the blast furnace operators,further development of the hot blast oven and hot blaststove continued.
The hot blast oven used on most charcoal blast fur-naces was a simple heat exchanger. The oven was a rectan-gular brick structure (Photo 5). The cold blast pipe wasfed into one end of the oven. The pipe was than connect-ed to several rows of hair pin shaped pipes that stoodupright in the oven similar to radiator coils. Thesehair pin shaped pipes reached to the top of the oven andwere connected in series. In the bottom of the oven wasa combustion chamber. The gas collected from the blastfurnace stack was brought through the downcomer into agas flue in the combustion chamber. This gas flue con-tained numerous sl its where the gas was burned. Theburning gas heated the inside of the oven and all of thecold blast hair pin pipes. The exhaust gas was suckedout of the opposite side of the combustion chamber by adraft created by an external ~tack.
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Photo 5 - Hot Blast Oven - Fayette Circa 1880(Courtesy Marquette County Historical Society)
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As the cold blast passed through the numerous pipes, itbecame progressively hotter (Figures 15 & 16).
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The blast left the oven as "hot blast" and continuedunderground through the hot blast main. The hot blastmain went to the base of the furnace where it was splitto feed the two or three tuyeres (Figure 17). The pipesthen turned upward out of the ground, came up to tuyerelevel and turned 900 toward the furnace. The right anglepipe is currently called the bootleg. The blast thencontinued through the blast pipe, now known as the blowpipe, and into the tuyere. Since the volume of air beinggenerated by the steam blower engine could not be easilycontrolled, a valve was placed on each blast pipe tocontrol the quanti ty of hot blast going into the fur-nace. This valve allowed wind volume adjustments duringstart-ups, shutdowns or during cast. This valve wasusually a slide type orifice (Figure 18).
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Figure 15 - Hot Blast
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Figure 16 - Hot Blast Oven End View
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Figure 17 - Hot Blast Main to Tuyere
,- -;. .',~ ,-- 4l '~ rr \~\~\-~-- -" ;-'--' ~ .::-:~ \
~~Figure 18 - Hot Blast Slide Valve
The tuyere design was different for a cold blast or hotblast furnace. A cold blast furnace used a sol id conicalcopper nozzle for a tuyere and the blast pipe was connect-ed to the bootleg with a flexible leather tube. A hotblast furnace required a water cooled tuyere with a solidball-and-socket joint at the blast pipe. These watercooled tuyeres were double walled, liD" shaped tubes thatwere tapered into the furnace. Clay was used to helpseal connections between the tuyere and blast pipe.steam dr i ven pumps forced water through the tuyeres. Atthe bend of the bootleg was a small hole with a shuttercontaining a piece of glass or mica. This allowed theoperator to look at the heat intensity inside the furnacein front of the tuyere. This opening was also used tointroduce various fusible metal samples to determine thetemperature of the blast when they fused at their knownmelting point. Originally, cast or wrought iron tuyereswere used followed by bronze tuyeres. These metals wouldnot conduct the heat away from the tuyeres so theyburned. Copper, an excellent conductor, carried the heat
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quickly to the water and the standard tuyere became cop-per as it is today. Leaky tuyeres were a constantproblem causing a great waste of fuel since water cooledthe furnace. It also effected iron quality because morewhi te rather than gray cast iron was made due to thecooling effect of the water in the furnace. In the ex-treme case of water leaking tuyeres, explosions and break-outs were caused in the hearth. The normal tuyere diame-ter was 3 to 5 inches, and this depended on the number oftuyeres and the volume of blast delivered to the furnace.
The first regenerative type of stove in the unitedStates, similar to those used on today' s blast furnaces,were erected at the Cedar Point Iron Company, Port Henry,New York and at the Rising Fawn Furnace in Dade County,Georgia in 1875. Hot blast temperatures now average from17000 F to 19000 F on a typical blast furnace of today.
CASTHOUSE AND CASTING OPERATION
The casthouseoperation. Thefurnace and wasfeet long. Theto allow smokedoors to allowslag to be taken
was the very heart of the furnacebuilding extended from the front of theapproximately 30-40 feet wide and 50-70roof was slightly raised above the wallsand fumes to escape. There were numeroussand to be brought in and pig iron and
out (Photo 6).
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The casthouse contains areas for iron casting andslag casting (Figure 19). The side for iron removalconsisted of a large ditch called a trough that slopedfrom the front of the furnace to the casthouse floor. Itthen split into two runner systems. A main runner oneach system ran parallel with the length of the cast-house. As this runner sloped down hill, a series of damswere made at regular intervals. At a right angle beforeeach dam, a smaller runner called a sow was produced.Then off of this sow were numerous cavities called pigs.This system looked like a series of piglets sucklingtheir mother. There were several parallel rows of sows.These were produced by pushing "D" shaped wooden formsinto moist beach sand on the casthouse floor. During thecast, as each sow and its pigs were filled, the sand damon the main runner was knocked out with a bar and theiron ran downhill to the next sow and pig bed. Therewere two such complete systems so that as one side hadits pigs removed and beds reformed, the other side couldbe cast. This allowed an uninterrupted furnace opera--tion.
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DAM
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Photo 6 - Casthouse - Fayette, Michigan(Photo by Author)
~SLAGFURNACE RUNNERS
~ SLAG JPIT
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Figure 19 - Casthouse Layout
The other side of the casthouse was used for slag re-moval. Slag was constantly running over the front of thedam down a runner toward a pit. The dam was divided intotwo halves, each one feeding a separate slag runner andslag pit. The slag pit was a large depression in the sandwith sand ridges. These ridges would act as cracking
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points when it was time to remove the slag. In some cast-houses, a jib type wood crane is used to remove thickpieces of slag. If the casthouse man saw the slag layergetting too thick, he would place a bar in the center ofthe liquid slag. Then when the slag froze around thebar, a rope or chain could be wrapped around the bar andhoisted by the crane. Once again, there were two completeslag systems so that as one was being used, the othercould be cleaned and made ready.
What is the origin of the word "casting"? It proba-bly was originated when the first furnace men saw theiron being "cast" or thrown from the furnace. The cast-ing operation was in two parts. As mentioned earlier,while liquid slag was formed and its level reached thedam, it flowed between the dam and tymp, down the runnerand into the pit. The other part of the casting opera-tion was the removal of the iron. First, the blast wasshut off the furnace using the valves at each tuyere.This was done for the casthouse man's safety. Then thetaphole in the middle of the iron side of the dam wasopened. The taphole was opened as one man held a wroughtiron bar in the taphole and another man drove the barthrough the dam with a sledge hammer. The iron ran downthe trough, into one of the runner systems and into thesow/pigs closest to the furnace. When this pig bed wasfilled, a dam in the main runner was knocked down andiron ran into the next pig bed (Photo 7). This fillingof pig beds continued until iron stopped running from thetaphole. The furnace men then replaced the taphole witha moist mixture of sand-and-fire clay or sand-and-coal.The blast valves were then reopened and wind put backinto the furnace.
Photo 7 - Casthouse During Cast(Courtesy Marquette County Historical Society)
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Iron Slaq
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When the cast was complete, the men removed the ironfrom the sow and pigs. This was done while the iron wasstill hot since the pigs broke loose more easily when theiron was red and slightly mushy. This was accomplishedwi th sledge hammers and pry bars. Once the pigs had beenloosened from the bed, they were allowed to cool. Thecooled pigs were then loaded on to carts or railroadcars. This was a hard job since each pig weighed between70 and 100 pounds. The casthouse men also wore woodenclogs on their shoes to protect their feet from the heatof the pig beds.
The blast furnace was cast approximately six times aday and produced 4 to 6 tons per cast. The iron producedwas classified into No.1, No. 2 or No. 3 grade. Exactspecifications for each were not discovered but the bestiron was a gray cast, low sulfur, low phosphorous ironmainly used for railroad car wheels and rails. Charcoalfurnaces could produce this low sulfur iron due to thesmall quantity of sulfur in wood versus the large quanti-ties of sul fur carried by anthracite coal or coke inother blast furnace operations. The iron also had only3 . 5% carbon versus 4. 3 % carbon in the iron of today.This was due to the fact that iron making temperature wasmuch lower in these old furnaces, and, therefore, did nothave as much carbon in solution. It should be noted thatthese furnaces used an acid slag practice since the sul-fur was so low and the slag volume was 400-500 Lbs./Tonof iron.
Some samples of iron and slag found around a char-coal blast furnace were analyzed and the chemical analy-sis is presented here:
CsiSMnpTiCrTemp.
3.54%3.37%
0.013%0.27%0.14 %0.06%0.02%2300°F
CaOMgOSi02Al203SK20FeOB/SB/A
ll.16%5.94%
53.87%10.42%0.014%
5.08%5.2%
0.320.27
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The slag samples found around the furnace are blue,purple, green, black and white. (Note: B/S = CaO +MgO/Si02 & B/A = CaO + MgO/Si02 + Al203) .
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Today a much lower silicon iron is required in thesteel making process and iron sulfur is controlled with ahigher basicity slag. Typical iron and slag chemistriesof today are listed below:
Iron Slaq
CsisMnPTiCrTemp.
4.30%0.60%
0.040%0.50%0.04%0.03%0.01%
2670°F
CaOMgOSi02Al203SK20FeOB/SB/A
40.25%11. 45%37.50%
7.72%1.37%0.47%0.26%1.38%1. 13%
BLAST FURNACE OPERATING RESULTS
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The blast furnace iron master both of 100 years agoand of today looks at several key indicators to gauge hissuccess. These indicators are production, fuel rate,yield and cost. The charcoal blast furnaces of the UpperPeninsula of Michigan showed progress in production andfuel rate from 1865 to 1890, but the cost and marketvalue of their iron finally shut them down. In the1860's, the furnaces produced 15 to 20 tons per day whichwas increased to 40 to 50 tons per day by the mid1880' s. Today, the typical furnace makes 3,000 NT/Day.The fuel rate was approximately 115 bushels (2,000 Lbs.)per ton of iron in the 1860' s, which was decreased to 96bushels (1,920 Lbs.) per ton by the mid 1880's. Today atypical fuel rate is 1,000 Lbs./NT iron. The productivi-ty, measured by net tons of iron per 100 cubic feet offurnace working volume, ranged from 2. 1 in the 186 a's to3.5 in the 1880' s. Today productivity ranges from 6.5 to8.5 NT/lOa Cu. Ft.. The yield for these furnaces whichis the ratio of the quantity of metallic iron put in thefurnace top to the quantity of iron sold was 90%. Thisis lower than modern day standards of 97% since a largeamount of iron went into the slag 100 years ago.
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These results show maj or improvements not only in the20 years from 1865 to 1885 but from 1885 to today. Thereare some world class blast furnaces today with 4 a tuyeresand 4 tapholes that are producing 9600 NT/Day at 910 Lbs.Fuel/NT of iron.
The table belowlife of a charcoalfurnace of today.
compares an average operation in thefurnace in 1880 to a typical blast-
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Production (Ton/Day)Productivity (Ton/100 Cu. Ft. Vol.)Blast Pressure (PSI)Blast Temperature (0 F)Charges/DayFuel Rate (Lbs ./Ton Iron),Flux Rate (Lbs./Ton IronOre Rate (Lbs . /Ton Iron)Pig Yield (%)
SAFETY ISSUES ON THE CHARCOAL BLAST FURNACES
188033
2.252.63
750107
2280130
343090
FurnaceToda v
30006.5
251750
1501000250
300097
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Theous.furnacefurnace
charcoal blast furnace operation was very hazard-One of the main concerns was fire. Many blastplants had fires caused by charcoal kilns or
breakouts.The men who filled the blast furnace were also in
constant jeopardy. One furnace man fell into the furnaceduring charging and another man, who became fatiguedwhile pushing his wheelbarrow, lost his balance and diedwhen he fell off the furnace top. The worst incidentoccurred when a man charg ing the furnace opened the coneto dump material just as the stock in the furnace"slipped". This slip sent flames shooting out of thefurnace top and engulfed the man. He died within a weekfrom severe burns.
The other maj or hazardous area was the casthouse.Mol ten iron and slag shooting out of the furnace were aconstant threat. Here is the account of a furnace fore-man's near miss, as told in the March 13, 1874, MiningJournal of Marquette, Michigan.
The furnace was acting like an animal does aftertaking a large dose of caster oil and was pretty softinside, but rather hard at the forebed: and as he wastrying to ease her of her burden, she flew at himlike a fiend. He succeeded in getting a hole in herand after pulling the bar out, the cinders flew likewater from a hose, striking him on the shoulder, backand legs, burning his pantaloons badly. But he wasquick in getting them off, he escaped with little orno inj ury .
The casthouse also contained other hazards such aswet sand. When the casthouse men would prepare the sowand pig beds for casting, the sand had to be moist enoughto retain the molded shape. If the sand was too wet,-molten iron would trap the water and quickly convert it
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to super heated steam. This would cause a severe erup-tion sending liquid iron high into the air and sprayingthe whole casthouse.
NEW TECHNOLOGY
Cornelius Donkersley, the Iron Master, of the Morganblast furnace in the Upper Peninsula of Michigan wastruly ahead of his time. There are two very interestingstories of his ingenuity that are forerunners to modernoperating practices.
First, in May of 1871, the furnace took a big "slip"and cold raw material fell into the hearth, causing it tochill. In fact, all the iron froze, filled both tuyeres,resul ting in a 45 inch thick mass known as a "sala-mander". The Mining Journal of May 31, 1871 explainswhat was done in this seemingly hopeless situation.
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Mr. Donkersley not being inclined to give it up andalways fertile in expedients, went deliberately towork to save the stack if possible. By his directionthe arch was broken through, a large tuyere insertedabove the chilled mass and coal oil was then forcedinto the stack through a pipe leading from thetop-house to the tuyere. The effect of this experi-ment was most satisfactory, for in a short time af-ter, iron and cinder were running out above the dam.The oil has been used steadily ever since and hasgradually cut the iron away until the present writ-ing. The mass has been reduced to wi thin eight inch-es of the top of the hearth; and the prospects arethat the furnace will soon be making iron as usual.
Seventeen days later, the Mining Journal printed thisfollow-up.
The salamander has been entirely removed with buttrifling damage to the hearth structure. The furnaceis now running as smoothly and as successfully as ifno accident had occurred.
This whole salamander removal process required sixdays and seven barrels of oil. This is the first record-ed case of using oil as a inj ectant to heat a blast fur-nace. oil injection is now commonplace as an auxiliaryfuel and as an operating variable to control furnace heat-levels, but it was not fully developed until the 1960' s.
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Charging white pine in connection with charcoal forfuel, in a small percent, we find the furnace worksadmirably by being supplied in this way with hydrogenwhich serves as a lubricant for the stock, giving atougher fibrin to the iron and effecting a saving ofover ten percent in fuel.
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The second bi t of technology used by Mr. Donkersleyto improve the furnace operation was also amaz ing forthat era. The fuel in the blast furnace was charcoal.However, this iron master would also charge raw wood.The Mining Journal of August 24, 1972 gives Mr. Donker-sley's reasonings:
It is a well known fact that today's blast furnaceoperators add hydrogen in the form of moisture or fuelinj ection and get the same results. The increase ofhydrogen gas allows smoother furnace raw material de-scent, reacts wi th iron ore to remove the oxides andresults in a more efficient operation. Maybe Mr. Donker-sley didn' t exactly understand the mechanism was for thisphenomenon, but it is amazing that he used this techniqueover 100 years ago.
The charcoal ironmaking era laid the foundation fornew theory and practice to be further developed in thetwentieth century.
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1Early Twentieth Century
IBy 1910 radical changes had occurred to blast furnace equipment.
The new furnaces of this era would be recognized by a modern ironmaker,whereas many of the 1870 operations might have been mistaken for alarge house. The furnace lines had changed from the low, flat boshesthat came from the previous charcoal and anthracite eras to steeplysloping boshesas shown in Figure 20. Furnace hearths had been increasedin diameter to 17 feet, and the height had reached 100 feet. Thisheight was of considerable concern to furnace-men; some in fact feltthat 100 feet was too high for a blast furnace and 75 to 90 feet wasmuch more reasonable. The internal volume of these furnaces was about25,000 cubic feet.
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iJ Furnace construction had improved with ,the use of steel plates and
beams, and water cooling plates had been introduced to protect the steelstructure and extend the furnace campaign life. Prior to 1890, a typicalfurnace campaign was two years, but with the introduction of steel struc-tures and water cooling, furnace campaigns were increased to over eightyears. A cross-sectional view of a furnace representing the latest 1910technology is shown in Figure 21. While the furnace shown in this figureis much smaller in diameter than the newer furnaces in use today, it haslines and external features similar to those of modern furnaces. Thedistance from the tuyeres to the stock line in the 1910 furnace wasabout 70 feet, whereas the largest furnaces in operation today have atuyere to stock line distance of about 85 feet.
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I Regenerative hot blast stoves had replaced the iron pipe stovesand blowing equipment had become much more powerful by the early twen-tieth century. The numer of tuyeres in the furnaces had increasedfrom two or three to as many as twenty, but more typically eight totwelve. Internal gas combustion engines using blast furnace off-gaswere introduced for blowing in 1902, and turbo blowers were first beingconsidered. Since the furnace off-gas was commonly being used in boilersor combustion engines, dust catchers and gas cleaning devices were beingdeveloped and used.
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The type of raw materials used in U.S. blast furnaces also changedmarkedly from the late 19th century to the early 20th century. Cokemade from bituminous coal had become the standard blast furnace fueland reductant. The rich Mesabi iron ores had been discovered and werebeing used in the blast furnaces located in Pittsburgh, Chicago, andCleveland. The beneficial effect of sized and washed ores was appre-ciated, .and certain operations were practicing some form of raw materialpreparation. The equipment for delivering raw materials to the furnacesis probably the area that improved the most in this era. Most notably,skip hoists replaced wheelbarrows as the most common method of chargingthe furnaces. Also, large bulk carriers of all types had appreciablyal tered the way in which ores and coals were mined and transported tosteel plants.
Blast furnaces in the early 20th century were equipped with avariety of fairly sophisticated top charging and raw material distribu-~
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Tuyeres --t------1 _ _--~
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Melfingzone --
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Figure 20. Late 19th Century Blast Furnace
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Figure 21. Early 20th Century Blast Furnace
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tion devices. The double bell and hopper arrangement that later becamethe industry standard had been introduced but had not yet become themost popular top charging and sealing device. A particularly interest-ing historical note for the modern ironmaker is that blast furnace in-strumentation had begun by 1903. The temperature and pressure of gasesentering and leaving the newest furnaces of this era were being moni-tored, and J.E. Johnson, Jr. had developed .an automatic stocklinerecorder that greatly improved the furnaceman' s knowledge of his opera-tion. These simple measuring devices were the start of bIas t furnacecontrol developments that have continued through to today.
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The production of the largest blast furnaces in the early 20thcentury had increased from 60 tons per day in 1870 to as much as 500tons per day. The coke required to produce one ton of iron ranged from1750 to 2100 pounds and, as today, depended a great deal on the type ofore and the blast temperature.
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The blast furnace developments described to this point havecentered on the furnace structure and the amount of iron produced.The next section will show how the ironmaker' s understanding of thephysical and chemical phenomena occurring in ironmaking smel ters hasevolved. This evolution started with the first ironmaking smeltinghole and continues today. But, because of its importance in modernblast furnace operations, the portion of this evolution that hasoccurred in the past 100 years will be emphasized.
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DEVELOPMENT OF BLAST FURNACE FUNDAMNTALS IEarly Scientists
IOne of the earliest researchers of the chemical and physicalphenomena occurring in a blast furnace was Charles Schinz. Schinzstudied "the art of measuring heat and applying it rationally in thevarious branches of industry" in his native Germany, and was struck withthe lack of understanding of blast furnace phenomena that existed in themid-19th century. Because his early studies of "heat" convinced himthat many accepted theories of blast furnace operations were incorrect,he embarked on an extensive study of the blast furnace. The results ofhis work were compiled in a book that was published in 1868. Schinzattempted to make quantitative mass and energy blances of blast furnaceoperations but was severely limited by the lack of accurate thermodynamicdata. He conducted laboratory experiments to determine heat capacity andheats of formation and apparently was the first to determine the reduci-bili ty of iron ore in the laboratory. More importantly, Schinz defineddifferent zones of the blast furnace and major chemical reactions takingplace in each zone. In comparison to the present understanding of blastfurnace phenomena, Schinz' s theories were incomplete and in some casesinaccurate. However, he was one of the first to attempt to change theart of ironmaking into the science of ironmaking.
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Many of the principles recognized today by ironmakers were firstpostulated by Sir Lothian Bell, a well educated scientist and an
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Iironmaker entrepreneur who worked diiring the midd:L~ and late 19thcentury in England. His book, "Chemical Phenomena of Iron Smelting",published in 1872, is recognized as the first text on blast furnaceironmaking. While Bell had many ironmaking "firsts" during his career,only a few of the more important ones will be mentioned here. In 1884,he was apparently the first to document the function of different con-sti tuentsinblast furnace slags and note that the melting temperatureof the slags was important. He also observed that there was a range ofslag compositions which resulted in good fluid properties and good de-sulphurizing capability and that blast furnace slags were complexstructures.
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IProbably the most important of Bell's many contributions was his
understanding of the chemical reactions in the blast furnace. He deter-mined that certain concentrations of CO and CO2 could be oxidizing orreducing to iron or iron oxide depending on the temperature of thesystem. He made these observations under carefully controlled condi-tions in the laboratory and was therefore apparently the first to startdefining equilibrium in the Fe-O-C system. Bell also recognized that:"a considerable excess of carbonic oxide (CO) is indispensable for thereduction of the oxides of iron", and felt that, ideally, the best thatcould be achieved at the top of a blast furnace was a CO :C02 ratio of 2.But, in addition to being an experimentalist, Bell was a practicingironmaker and felt that blast furnaces would not be able to achieve the"minimum" CO:C02 ratio because of the need for a "little margin" toallow for upsets in the furnace. It will be shown later that Co: C02ratios of 1 and lower are achieved frequently in modern blast furnaceoperations.
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IBell also recognized that the stack ofa blast furnace was impor-
tant for the preheating and pre-reduction of ores prior to entry intothe higher temperature zones of the furnace. He had observed blackslags and off-grade iron quality as a result of poorly prepared ironoxides dropping into the bosh and hearth of his furnaces, and herelated these experiences to his laboratory work. These observationsand experiences led Bell to a concern for the influence of furnaceheight on iron production and fuel requirements as shown by the consid-erable space devoted to this subject in his second book. It was Bellwho first stated that there is an optimum height for each furnace: ashorter furnace would not properly pre-reduce and prepare the ore, anda taller furnace would be a waste of capi tal resources .
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Sir Lothian Bell made carbon, oxygen and nitrogen balances of hisblast furnace operations and showed that some of the charged carbonwas consumed in the stack by carbon dioxide. A legacy left by Bell tomodern ironmakers is the use of the term "solution loss" to designatethe carbon consumed by carbon dioxide in the blast furnace stack. Belland other earlier ironmaking theoreticians did not fully understandthe role of this blast furnace reaction and felt that they could achievethe ideal blast furnace operation only when this reaction was eliminated.
Another well-known late 19th century scientist-ironmaker wasM.L. Gruner, a professor of metallurgy in France. Gruner expanded
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Bell's method of determining blast furnace heat balances by comparingmany different furnace operations. Gruner observed large differencesin heat requirements among furnace operations and related these differ-énces to furnace volume and height. The most quoted statement byGruner concerns the "ideal working furnace" and what eventually becameknown as Gruner i s theorem. The theorem states that the "ideally perfectworking" of a blast furnace will be achieved when "the reduction of ironore is made as far as possible by the transformation of CO into CO2,that is, without any consumption of solid carbon". Gruner, like Bell,believed that the minimum fuel rate for bIas t furnaces would be reachedwhen the "solution loss" was eliminated. Gruner and Bell believed thisbecause they felt that the solution loss reduced the total amount ofheat produced in the combustion zone. As will be discussed later thismisconception remained a part of ironmaking art until the middle ofthe 20th century.
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One of the blast furnace mysteries of the Bell and Gruner era waswhy hot blast had such a large and dramatic effect on furnace productionand fuel rates. Bell incorrectly explained the effect of hot blast asthe result of increased residence time of both solids and gases in thefurnace. In making this explanation, he failed to recognize that theavailability of energy above certain temperatures, that is the SecondLaw of Thermodynamics, was important in the process. The Second Law ofThermodynamics had been stated in 1850 and ironmakers were probablyquite familiar with steam engines and the important role of "steamquality". However, at this point in time the implications of theSecond Law with respect to ironmaking were not yet understood.
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The first to apply the Second Law to the blast furnace process wasJ.E. Johnson, Jr., an American ironmaker in the late 19th and 20thcenturies and the author of two well-known and often quoted books onblast furnaces. As related by Johnson in his second book, he was oftenbothered by the explanation offered by Bell for the effect of blasttempera ture on blast furnace production and fuel rates. As a resul t,Johnson postulated that the fuel rate of blast furnaces was determinedby two thermal equations, these being the First and Second Laws ofThermodynamics. With these principles Johnson was able to explain theeffect of blast temperature on furnace performance, and in so doing hemade a major breakthrough in the Understanding of blast furnace opera-tions. This line of reasoning eventually lead him to postulate thatthere is a critical furnace temperature above which a minimum amount ofheat is required. This minimum amount of heat he called "hearth heat".He used this principle to explain the high fuel rates experienced withthe production of ferromanganese in a blast furnace and to explain theeffect of dry blast, as proposed and practiced by Gayley. Possiblymore important than the specific explanations provided by Johnson'sthermal equations is the fact that the application of his criticaltemperature and hearth heat concepts further convinced furnacemen thattheir process was rational and, as a result, predictable. The thermalequations were not Johnson i s only contribution to blast furnace iron-making. He was a very active engineer and responsible for many equip-ment innovations made in blast furnace plants during his lifetime.
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J Gas-Solid Contact
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During the period 1920 to 1930, the flow of solids and gases inblast furnaces was studied extensively by a group of workers at theu.s. Bureau of Mines. This group, composed of P.H. Royster, S.P. Kinney,C.C. Furnas and T.L. Joseph, was interested in the physical and chemicalphenomena occurring in blast furnaces, and in,.order to understand thesephenomena they felt it was necessary to sample and probe operatingfurnaces. Their work started with a small experimental furnace atMinneapolis, and eventually lead to studies in commercial furnaces andto studies in laboratory cold models. The initial work of this groupon an experimental blast furnace showed that the flow of gases andsolids was not uniform across any horizontal plane in a blast furnace.This observation was confirmed in a large commercial furnace in a classicwork by S.P. Kinney. The most important result of this work was thegroup realization that the efficiency of the ironmaking blast furnacecould be significantly increased by improving gas-solid contact in thestack of the furnace. Before this time, furnacemen apparently thoughtthat the ultimate in blast furnace efficiency was represented by a topgas CO:C02 ratio of 2 as stated by Bell. However, Kinney's work showedthat much lower ratios were reached in certain areas of operating fur-naces. This finding was particularly significant in 1929 becauseequilibrium in the Fe-O-C system was not well understood until the workof Darken and Gurry in 1945-46. This observation by Kinney lead to anintense interest in raw material and gas distribution in blast furnacesthat has occupied the time of many ironmaking investigators for the past50 years.
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I As a result of their belief in the prospect of improved blastfurnace efficiency, Furnas and Joseph conducted a series of blastfurnace cold model tests in an effort to determine methods of improvinggas-solid contact. This work resulted in a reasonable understanding ofthe furnace charging parameters that affected raw material distributionin the furnace stack. But, more importantly, Furnas and Joseph realizedfrom this work that raw material size was a critical parameter in deter-mining both raw material and gas distribution in the furnace stack andthat raw material size was therefore important in determining furnaceefficiency. They observed that large pieces of raw material rolled tothe centre of the furnace after charging and provided a minimum path ofresistance for the gases. Such segregation allowed relatively unusedhot reducing gases to leave the furnace and thus decrease the efficiencyof the operation. They also observed that very small pieces of rawmaterial restrict gas flow and caused channeling of gases. These obser-vations led Furnas and Joseph to make thé important suggestions thatiron ore be crushed to a maximum size of two inches and that undersizedmaterials be agglomerated.
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Furnas and Joseph were not the first to recognize the benefits ofclosely sized raw materials and furnace performance. Frank Firmstone,an operator of anthracite blast furnaces during the late 19th century,reported the benefits of sized ore used in his furnaces during theperiod 1882-1886. Firmstone also noted that others before him hadrecognized the possibilities of improving furnace performance with
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sized raw materials. 'However, Furnas and Joseph started the era ofprepared blast furnace raw materials when they clearly demonstrated theeffects of raw material sizing. Fortunately, the technique of iron oresin tering had been developed before they started their work and wasavailable for the agglomeration of undersized ore.
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Along with their concern for the effect of raw material size ongas-solid contact, Furnas and Joseph were concerned about the effect ofiron ore reducibility on furnace efficiency and about the effect of oresize on pressure drop and permeability in the furnace stack. The lattereffect is illustrated in Figure 22, where the resistance to gas flow ina packed bed is shown as a function of the size of particles in the bed.This finding led Furnas and Joseph to speculate that the optimum sizeof iron ore in blast furnaces would be a compromise between permeabilityand reducibility considerations. They were apparently the first tostate this basic conflict in blast furnace technology.
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Another major contribution to the understanding of the interactionof gases and solids in ironmaking blast furnaces is the ability to pre-dict the pressure drop in the stack region of the furnace. A quanti ta-tive expression of pressure loss in a blast furnace is difficult toderive, because first, the stack is a non-homogeneous packed colum andthen, lower in the furnace, the flow phenomena are complicated by themel ting and trickling of iron and slag. The most accurate expressionfor quantifying pressure loss in the stack region of a blast furnace isan equation developed by Sabri Ergun. This equation was developed in1952 and has been widely used by blast furnace engineers ever since.Based on his study of fluid dynamics, Ergun speculated that productionof current blast furnaces might increase four-fold with proper sizing ofraw materials and the use of furnace top pressure. This speculation hasproven to be remarkably accurate, because the largest furnaces in 1952were producing about 1000 tons of iron per day whereas these samefurnaces are now capable of producing three to four times as much.
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Solution LossI
A review of blast furnace fundamentals would not be completewithout putting the "solution loss" reaction and Gruner's theorem inproper perspective. As stated earlier, Bell and Gruner believed thatthe ideal working blast furnace would be achieved if the solution loss,that is the oxidation of coke by carbon dioxide:
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CO 2 + C -+ 2 CO
could be eliminated from the furnace. The elimination of solutionloss was a goal of many ironmaking researchers and furnacemen into the1950s. The realization that solution loss played a beneficial ratherthan a detrimental role in the blast furnace was apparently firstrecognized in the late 50s. This recognition was made possible by thecomplete definition of equilibrium in the Fe-O-C system and the detailedmass and energy calculations that were being made for the first time inthis period. The simplest statement of the role of solution loss was
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tREISTANCE
TOGA FLOW
114 112 3/4 1 1Y4PARTICLE SIZE, inches
Figure 22. Relationship Between Particle Sizeand Resistance to Gas Flow
tCARBON
RATE
100"0INDIRECT
REDUCTIONSOLUTION LOSS'"
1004fDIRECT
REDUCTION
Figure 23. Relationship Among Indirect Reduction, Direct Reduction,Solution Loss and Carbon Rate in a Blast Furnace
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made by R.L. Stephenson in 1962. Stephenson pointed out that ironoxide reduction in a blast furnace is a combination of the followingreactions:
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FeO + CO 7 Fe + CO2, i. e. Indirect Reduction J
and FeO + C 7 Fe + CO, i. e. Direct ReductionJThe point to note is that indirect reduction followed by solution loss
is direct reduction. Stephenson pointed out that the two iron oxidereduction routes shown above have quite di fferent chemical and thermalrequiremen ts. Indirect reduction requires about three moles of CO foreach mole of FeO reduced because of equilibrium considerations, butdirect reduction requires only one mole of carbon to reduce a mole ofFeO. On the other hand, direct reduction is highly endothermic whereasindirect reduction is only slightly endothermic. Using these considera-tions to determine carbon rates for all combinations of these two reduc-tion routes as a function of solution loss results in the plot shown inFigure 23. This plot first of all shows that the total carbon requiredin a blast furnace is determined by either chemical or thermal require-ments, whichever is greater. It also shows that some amount of solutionloss actually reduces total carbon requirements for reduction. Thesolution lO$s reaction is thus seen to be a critical balancing reactionthat regenerates reducing gas and cools the hot gases rising from thecombustion zone. Since the reaction is very temperature dependent, itregenerates reducing gas only at high temperatures and has for the mostpart stopped by the time temperatures are near l600oF. The chemical andthermal requirement lines shown in Figure 11 are different for differentblast furnaces and are dependent on blast temperature and iron orereducibili ty, among other variables.
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Although it is not known with certainty when this situation wascompletely understood or who first explained it, Stephenson was one ofthe earliest and he explained it very well.
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ITo sum up the development of blast furnaces to this point, at the
beginning of the 1960 decade the important principles governing furnaceoperations had been discovered and stated. From the time of the lateMiddle Ages to the early 20th century, ironmakers had learned mostlyby trial and error how to build large blast furnaces and what combina-tion of operating variaples seemed to maximize performance. The effectof preheated blast was demonstrated by British ironmakers and explainedby Johnson. The importance of fluid dynamics was shown by the U.s.Bureau of Mines Group, and as a result, furnacemen knew that closelysized raw materials dramatically improved furnace performance. As willbe discussed next, many of the furnaces operating both here and abroadin the early 60s took advantage of these principles. However , it hasbeen the aggressive Japanese steel industry that has taken full advantageof this technology in the past 10 years.
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IMODERN BLAST YURNAÇES
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The best blast furnaces operating in the early 20th century wereproducing up to 500 tons per day with a coke consumption of about oneton per ton of product. Furnacemen in the U. S. did not believe thiswas the ultimate and began designing a blast furnace capable of produc-ing 1000 tons per day. A special committee of the Blast Furnace andCoke Association in the Chicago district was formed to design such afurnace, and their report was presented in April 1930. The generalarrangement of this furnace is shown in Figure 24. This furnace hasa working volume (stockline to tuyeres) of about 35,000 cubic feet anda hearth diameter of 25.3 feet. Many of these furnaces were constructedin the U.S. and at the start of World War II this type of furnace wasthe most modern being used in the world.
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IThere were many mechanical and structural improvements in the 1000
ton furnaces in comparison wi th the earlier 500 ton furnaces, but as achemical reactor, the larger furnaces were a direct scale-up of thesmaller. Ma terials handling equipment improved markedly in this period,and furnace construction was much more substantial by 1940. Improvedhot blast stoves had been developed, and large turbo blowers were beingused with the newest furnaces. Furnace tops had been improved for rawmaterial charging and for containment of the furnace off-gas. TheMcKee double bell hopper arrangemetn was accepted as the best furnacegas sealing and raw material charging device by 1940. However, theblast temperature and types of raw materials used in the 1910 and 1940models were about the same, and the increased production with the 1940version was obtained by blowing twice as much air into a furnace withabout twice the volume.
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IThe next step in blast furnace development was the design and
construction of the classic 28-foot hearth diameter furnace. Many ofthese furnaces were built after World War II in the U. S. and later inEurope and Japan. These furnaces were originally designed to produce1200 to 1500 tons per day and were an extension of know-how developedwi th the 1000 ton per day furnaces. The basic 28-foot furnaces havebeen modified many times, and today the f~rnaces range from 28 to 31feet in hearth diameter with working volume of 50,000 to 55,000 cubicfeet. The soundness of this furnace design is indicated by the factthat after 30 years they are still the workhorse of the U. S. steelindustry. The producti vi ty of these furnaces has improved three-foldin this 30 year period of time due mostly to improved iron bearing rawmaterials. This raw material improvement is discussed in the nextsection.
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Raw Material Preparation
The most important development in blast furnace technology andpractice in the' past 25 years has been the use of beneficiated andsized raw materials. This breakthrough started with the development ofthe sintering process and was given a solid basis with the u.s. Bureauof Mines work in the 20s and 30s. However, more significant for the use_of
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IPYROMETER
IPLA 'FORM
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Figure 24 . Blast Furnace Designed for 1000 Tons/Day
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1beneficiated ores in the u. S. was the depletion. of _thE!direct shippingMesabi ores after World War II. This led to the development and use ofthe pelletizing process, which in turn marked the beginning of dramaticimprovements in blast furnace productivity and efficiency. These im-provements are illustrated in Figure 25 , where changes in the averageU.S. blast furnace coke rate and the use of sinter and pellets areshown for the period 1957-1966. The improvements in this period weredue to closer sizing of agglomerate materials versus direct ores, lowergangue content and thus lower slag production with beneficiated ores,and a moderate increase in blast preheat temperature. Another step inthe development of high productivity blast furnace operations was therealization of the beneficial effects of very close sizing of sinter,pellets, and coke on furnace performance. It was also found that therewere important relationships between the size of coke and the size offerrous raw materials, and that these relationships were important inthe optimization of blast furnace operations. A relatively smallincrease in coke size can improve the permeability of the furnaceburden and thus permt a higher wind rate and production.
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While the above information confirmed on an industrial scale theprinciples formulated by the Bureau of Mines Group earlier in thecentury, the most dramatic demonstration of these principles was madein Japan. New steel plants were being constructed in Japan during thepost-war period and these plants were designed to take full advantageof raw material preparation and sizing. Large blending and beddingfacili ties for coals and ores were built to produce homogeneous rawmaterials for coke plants and sintering operations. Because Japandoes not have indigenous supplies of iron ore and metallurgical coal,these new steel plants were constructed for high efficiency and thecapability of handling a wide variety of imported raw materials.Close sizing of all raw materials charged to the blast furnace was aprimary objective of these facilities. The results of applying thelatest raw material and ironmaking technology in the new Japanesesteelplants started to be realized in 1965. An example is shown belowfor the Sakai No.1 furnace of Nippon Steel Corporation.
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IFurnace Sakai No. 1
r DateFurnace ,Size
Hearth Diameter, ft.Working Volume, CF
Production, NT/dayCoke Rate, lb/NTBlast Temperature, 0 FOre Burden, %
sinterPelletsSized Ore
July 1966
32.863,100442710221890
651421
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80
60
PECENTPELLETS 40
ANDSINTER
20
o 1957 58 59 60 61 62 63 64 65 66
YEAR
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1600
AVERAGE u.S.14 COKE RATE,
Ibsl NT
1200
100
Figure 25. Trend of Coke, sinter and Pellet Use in U.s.Blast Furnaces During 1957-66 Period
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IThis operation was achieved with the following raw materials sizeranges:
JSize Range, inches
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CokesinterPelletsOre
0.4 - 3.0O. 4 - 3.0
0.2 - 0.80.4 - 1.0
i Current material preparation practices in some operations in NorthAmerica and Japan have reduced the size range of ferrous materials evenfurther than that shown above. It is standard practice in some plantsto size sinter and ore to the range of 0.25 - 1 ~O inches and screenpellets to 0.25 - 0.60 inches. In many operations coke is screenedinto two size fractions and charged separately to the furnaces in aneffort to minimize pressure drop and improve efficiency.
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~ Another improvement in blast furnace raw material preparation hasbeen the production of fluxed and superfluxed sinter. Fluxed sinternot only removes the thermal load of limestone calcination from thefurnace but also produces a smaller, stronger and narrower size rangeof raw material as compared with acid or unfluxed sinter. Thus, theproduction and use of fluxed sinter has affected furnace performancefor thermal, chemical and physical reasons.
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IThus far, the discussion of improvements in blast furnace perfor-
mance during the post war era has been concerned with raw material prep-aration and gas-solid contact. Another development of great significancein modern blast furnace operations is the use of blast additives and veryhigh blast temperatures. Blast addi ti ves include steam, hydro-carbonfuels and oxygen. The use of these with high blast temperatures iscalled combined blast. The historical development of these aspects isoutlined below. A more detailed discussion of their influence on opera-tions is given in Lecture 14 by R. W. Bouman.
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Combined Blast
It was probably some time in the early 20th century when blastfurnace operators first noticed that their furnaces would not respondto higher and higher blast temperatures. This phenomenon has beendescribed in many ways, but it is typically referred to as a "tight",or hanging, furnace. In the period between 1910 and 1950, mostfurnacemen believed blast temperatures higher than l200-1400°F couldnot be used, particularly with the Mesabi or "lake" ores. In the1950s it was found that steam additions to the blast relieved a tightfurnace, thus permitting the use of higher blast temperatures andhigher wind rates.
In 1957 it occurred to a new group at the u.s. Bureau of Minesthat hydrocarbon fuels injected in the blast might improve furnaceoperations even better than steam. This group was composed of
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N . B . Melcher, J.P. Morris, E. J . Ostrowski andP. L . Woolf. They wereinterested in hydrocarbon injection not only as a method of improvingthe flow of gases and solids in the furnace, but also as a method ofsubs ti tuting low cost hydrocarbons for expensive metallurgical coke.
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JSoon after initial experiments with natural gas, industrial adoption
of hydrocarbon injection followed quickly in the u.s. By the end o£ 1963,67 of the 134 blast furnaces operating in the U.S. were equipped forhydrocarbon inj ection. In addi tion to natural gas and coke oven gas,some operations were using fuel oil, and trials with powdered coal werebeing conducted. Fuel oil was tested in a low-shaft furnace in Belgi uras early as 1958, and European operations quickly adopted this form ofhydrocarbon inj ection. More recently, Japanese and North American blastfurnace operations have also made extensive use of fuel oil injection.A recent development that increases the maximum amount of oil that canbe injected is the use of oil-water emulsions. The emulsion helpsatomize the oil stream and thus improves burning characteristics.
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At the present time, fuel oil is the most common hydrocarbon usedfor injection into blast furnaces. Except in Russia, natural gas isnot commonly used because of the availability problem and its poorercoke replacement characteristics compared to other hydrocarbons. Coaltar is used in many North American and Japanese operations with resultssimilar to those obtained with fuel oil. However, coal tar is usuallymore di fficul t to handle, and as a result coal tar inj ection rates havebeen lower than oil injection rates.
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Oxygen enrichment of blast air has been used in many furnacesthroughout the world. The first industrial trial of oxygen enrichedblast was carried out by National Steel in 1951. The benefits ofoxygen enrichment are increased furnace production due to increasedfuel burning capability and an ability to use more hydrocarbon tuyereinj ectants. There are heat transfer and heat capacity limits to theamount of oxygen enrichment that can be used in an ironmaking blastfurnace, and these limits would be typically reached in North Americanoperations with air enriched to about 25% oxygen. However, this limitis not normally reached in practice because of the high cost of oxygen.The justification for oxygen enrichment is in the need for iron produc-tion that could not otherwise be obtained, or in the replacement of veryexpensive coke by fuel injection.
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Overall, the use of high blast temperature with various types oftuyere additives has played an important role in the development ofmodern blast furnaces. Combined blast has provided the furnaceman witha process tool that permits much flexibility in the establishment of agood operation. This tool and the raw material preparation techniquesdiscussed previously have been combined by the Japanese steel industryto produce blast furnace operations that, with a few exceptions, areunmatched in the world today. As of the middle 1960s, Japan became thenew leader in blast furnace technology.
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J Large Blast Furnaces
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Around 1951 Japan literally made the expansion of their steelindustry a national goal and today Japan is clearly the leader in theefficiency and size of primary steelplant operations. Japan's successin blast furnace operations is due to the application of ironmakingprinciples established earlier in North America and Europe and to theimplementation of their own technical and practice developments . Theresul t of the Japanese developments are operating furnaces with workingvolumes 2.5 times larger than the nominal 28 foot hearth diameterfurnaces that were being constructed in the late 1940s. In addition,these furnaces are capable of producing more with a unit of workingvolume due to an intensification of the process. Improvements in furnaceperformance have been achieved by:
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o Increased use of agglomerated and closely sized rawmaterials.
~ o Higher blast temperatures with oil injection and oxygenenrichment.
I o The application of top pressure.
Io Control of burden and gas distribution in the furnace stack.
IThe importance of rawdiscussed previously.construction of largeand will be discussed
material preparation and combined blast have beenThe last two items listed above along with the
blast furnaces have been more recent developmentsbelow.
I Top Pressure
IThe pressurization of a chemical reactor is usually an advantage
because it intensifies the process and reduces the critical size of thevessel required for a specified output. This is true in the case of theironmaking blast furnace because increased pressure increases the resi-dence time of gases in the furnace and, as a result, increases gas-solidcontact. In addi tion, from a fluid flow standpoint, increased pressurewill decrease the pressure drop experienced in a packed bed reactor at aconstant mass rate of gas. This is illustrated in Figure 26 fromFurnas' work and can also be demons tra ted with the Ergun Equation. Theuse of top pressure in blast furnace operations began in Russia and theu. s. at about the same time during the 1940s. The initial efforts inthe u. s. were limi ted to 5 - 10 psig by the double bell and hopper charg-ing equipment. Later charging and sealing equipment developments inJapan and Europe have led to furnace top pressures as high as 2.5 atmos-pheres (gage). These equipment developments have included the use of3 or 4 bell and hopper arrangements, the use of sealing valves with thenormal double bell and hopper, and the use of sealing valves with arotating shute inside the furnace for raw materials distribution. Thelast mentioned arrangement is called the "bell-less" top and has beenone of the most revolutionary blast furnace developments in modern times.
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fPRESSURE
DROP
Figure 26 .
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SYSTEM PRESSURE. Atm.1.01.52.0
GAS FLOW, SCFM --
Rela tionship Among Gas Flow, System Pressureand Pressure Loss in a Packed Bed
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IThis developmènt was made in Luxembourg and first used on a commercialfurnace in Germany. Not only is the bell-less top an important innova-tion for the use of high furnace pressure, it also has significantlyincreased the flexibility of raw materials charging and distribution.
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IBurden and Gas Distribution
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The great importance of raw materials and gas distribution in theblast furnace has been appreciated by furnacemen for about 100 years.Many different types of top charging and materials distribution deviceswere designed and used in the early 20th century, but the usefulnessof these devices for con trolled distribution was limited. The use ofsized raw materials improved the distribution of solids and gases in thefurnace as discussed earlier. However, as larger blast furnaces werebuilt, it became apparent that additional measures were needed to controlthe movement of solids and gases. The basic problem in the distributionof raw materials in a blast furnace is the large difference in densityand angle of repose between iron ores and coke as shown below:
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ISinterPelletsCoke
120-140130-150
24-28
32-3628-3236-44
IThese differences cause the ferrous materials and coke to radiallydistribute qui te differently, and since coke provides the least resis-tance to gas flow, the furnace gas will preferentially flow up throughthe thickest part of the coke layers. This phenomenon is accentuated asthe furnace diameter increases, and since increased furnace capacityhas been achieved mostly by increasing furnace diameter, burden and gasdistribution has received more attention in the last 20 years.
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I The first attempts to mechanically alter the distribution of rawmaterials inside the furnace were made in Germany in the late 1960s.This was accomplished by installing movable panels at the throat of thefurnace that could be set at different angles for ores and coke. Thisbasic technique with several mechanical variations is being used on mostof the very large furnaces in operation today. The control of burdenand gas distribution has received a large industrial and research effortin the past few years and has resulted in significant improvements infurnace performance. The more recent bell-less top has been used onseveral 50,000 - 55,000 cubic foot furnaces and is now being installedon some of the larger furnaces. The result of using a bell-less top ona large furnace is of considerable interest to furnacemen.
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Another consideration that has a very important effect on gas flowin blast furnaces is coke quality, in particular, coke strength. Fur-nacemen have said, probably from the time of Abraham Darby, that cokequality is crucial in the operation of a blast furnace. Sometimes thesestatements have been true and sometimes they have been a convenientalibi for some difficul t-to-explain event. However, it has become clear
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wi th the construction and operation of large blast furnaces that cokestrength requirements increase as the size of the furnace incrèases. Acomprehensive investigation by Sumi tomo Metal Industries on the effectof coke strength was recently reported. Interest in coke quality con-siderations will be increasing in the future because of the limitedsupply of good metallurgical coking coals.
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To sum up, the use of prepared burdens, combined blast, toppressure, and raw materials distribution techniques have all had an impor-tant role in the development of large blast furnaces. These have had theeffect of intensifying the process, that is, producing more iron for auni t of internal reactor volume. A commonly used method of expressingthis productivity is to compute the tons of iron produced per day per100 cubic feet of working volume, or NT/day/100 CFWV. When this produc-tivi ty factor is plotted chronologically for some of the monthly recordblast furnace performances in the past 17 years, a curve as shown inFigure 15 is obtained. This shows that in the early 1960s the classic28- foot furnaces were producing at the rate of 5.5 - 6.5 NT/day /100 CFWand, more recently, that the high producti vi ty furnaces in Japan havereached 8.7 - 8.8 NT/day /100 CFWV. These figures represent a remarkableimprovement.
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A simple method of sumarizing the development of the modern blastfurnace is to review how 20th century furnace performance has evolved.Furnace operations for different periods during the past 70 yearspresent just such a sumary:
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r1910 1940 1963 1974
Production, NT/day 500 1000 3000 10000Fuel Rate, lb/NT 2000 1800 1200 950Blast Temperature,oF 1000 1200 1400 2100Furnace SizeHearth Diameter, ft 17 26 29 45Working Vol lie, CF 25,000 47 , 000 50,000 130,000
Productivi tyNT/day/lOO CFW 2.0 2.1 6.0 7.8
These are not record performances as shown in Figure 27 but ratherrepresent what the typically good operations were doing in each period.The increase in furnace size and the improvement in productivity shownabove are the result of monumental efforts by blast furnace designersand operators. The results of their efforts must rank with the best ofmodern engineering achievements.
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FUKUYAMA 2
8
PRODUCTIVITY,NT / DAY /100 CF
WORKING 7VOLUME
ø SAI 1
6ø PORT KEMBLA 4
MIDDLETOWN 3
;961 62 63 64 65 66 õT 68 69 70 71 72
YEAR
EACH POINT IS AONE MONTHOPERATION
7! 74
Figure 27. Change in Record BIas t Furnace Producti vi tyPerformance During 1961-74 Period
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MODERN ASPECTS OF BLAST FURNACE THEORYJ
Reduction of Iron Oxides
with few exceptions, the iron-bearing components in the charge tothe furnace are the simple oxides of iron, Fe203 and Fe304. The naturalores usually are hematites (Fe203) or magnetites (Fe304). Pellets areprincipally Fe203. Sintered ores can range in composition from Fe203and Fe304 to fused mixtures containing magnetite, fayalite, 2FeO.Si02'and dicalci um ferrite. The reduction of iron oxides generally takesplace in steps. The reactions with carbon monoxide (CO) are:
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J3Fe203(s) + CO(g) 2Fe304(s) + C02(g); ßH -11,537 cal ( 1)
Fe304 (s) + CO(g) 3FeO(s) + C02 (g); Lm -5,200 cal (2) IFeO ( s) + CO ( g) Fe(s) + C02 (g) ; Lm -2,620 cal ( 3)
IThese reactions are accomplished at successively higher temperatures,and farther down the furnace. As shown in Figure 16, successivelyhigher percentages of carbon monoxide are required to complete reactions(1), (2) and (3) by the rising gases. It is to be recognized that it isnot possible for all of the CO in the gases to be converted to C02 foreach reaction. For example, there is an equilibrium ratio as given bythe constant for Equation (3) and from Figure 16:
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Iat each temperature. At 800°C, the equilibrium gas mixture containsabout 65% CO and 35% CO2. If the C02 con tent exceeds this value in thegases in contact with FeO and solid iron at this temperature, ironpresent will tend to be oxidized back to FeO. Accordingly, to forcethese reactions to occur, there must be a considerable concentration ofCO in the gases at each step as indicated in Figure 28 , and it is notpossible to convert CO completely to CO2 by the reactions.
Because of hydrogen in the auxiliary fuels and moisture from thefuels and the air blast, the gases leaving the tuyeres may also containup to 2 or 3% hydrogen. Steam may be added to the blast as an aid incontrolling the furnace. The reduction of steam by carbon in thecoke and fuels proceeds by the overall reaction:
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H20(g) + C(s) = CO(g) + H2(g); åH = 31,380 cal ( 4)
This reaction is endothermic whereas the oxidation of carbon by oxygenin the blast to form carbon monoxide is exothermic:
C(s) + 1/2 0 (g) = CO(g); åH = -26,420 cal (5 )
The reduction of iron oxides by hydrogen also proceeds by steps:
3Fe203(s) + H2(g) = 2Fe304(s) + H20(g); åH = -1,698 cal ( 6.)
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o20 40 EO eo ioTemperature .C
12
l1Figure 28 . The Fe-C-O System Showing the Fields of
Stabili ty of Iron and Various Iron Oxides
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Fe304(s) + H2( g) = 3FeO ( s) + H20(g); L1H 15,040 cal ( 7)
FeO(s) + H2 (g) Fe (s) + H20(g); Ll 7,220 cal ( 8)
The effect of temperature on the equilibria of these reactions is shownin Figure 29 .
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The water gas shift reaction can take place among the variousspecies in the gas phase to redistribute the oxygen and bring thehydrogen-bearing and carbon-bearing gas species into equilibrium:
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CO2 (g) + H2 (g) = H20(g) + CO(g); L1H = 9,840 cal ( 9)J
This reaction requires very little heat and the equilibrium constant~
(PH 0 . Pc ) / (p2 0 H2 PCO ),2
Iis unity at 8250C.
The gases in the shaft will react with the carbon of the coke aswell as with the oxides of iron in the charge (Eqs. 1, 2 and 3). Theoverall reaction of carbon monoxide and carbon dioxide wi th carbon asgraphite is the "solution loss" or Boudouard reaction:
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C02 (g) + C(s) = 2CO(g); L1H 41,200 cal (10) IThe equilibrium of the reaction is shifted strongly to the right attemperatures above 750oC. Below 6000C the equilibrium is strongly tothe left, resulting in the deposition of carbon as soot in the furnaceburden:
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2CO(g) =C(s) +C02(g); L1H=-4l,200cal (lOa) IThe "s" shaped curve leading from the lower left to the top center ofFigure 16 represents the equilibrium of Eqs. (10) and (lOa). A gaswhose temperature and composition place it above the line will tend todeposit carbon by reaction (lOa), and one whose composition and tempera-ture place it below the line will oxidize carbon in accordance withreaction (10). The principal effects of the carbon solution reaction athigh temperatures are a relative reduction of heat generated at thetuyeres where it is needed and an increase in the concentration of coin the gases at regions of the furnace above 700oC. This latter condi-tion is particularly desirable as it increases the volume of the gasesand aids in heat transfer, a point that will be treated in greater detaillater. It is to be noted that the combination of Eq. (10) with Eq. (3)corresponds to the "direct" reduction of FeO by carbon:
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FeO(s) + C(s) = Fe(s) + CO(g); L1H = 31,380 cal (11)
It will be evident from Figure 16 that the gases passing up thestack cannot generally be in equilibrium with carbon in the coke andthe iron oxides in the descending burden. Measurements of the tempera-tures and compositions of gases in operating furnaces show that they do
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0 Iron 0Q) N
f6 ~~ ~00 0....:i :i
0 £~
0 0N Q)
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o20 140040 ti BO 100 1200Temperature °C
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Figure 29. The Fe-H-O System Showing the Fields ofStabili ty of Iron and Various Iron Oxides
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not follow either set of equilibrium curves, those for the iron oxidesor that for carbon. They tend to fall between the CO/C02-C line andthe wustitela-iron line above 8000C, touch the wustite/a-iron line atbetween 6500 to 8000C, and then remain at or just above the Fe304/a-ironline as shown in Figure 30 :
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The actual relationship between gas Gomposition and temperature inthe blast furnace stack will depend to a great extent on the actualpractice employed. This aspect is illustrated in Figure 31 , which istaken from E. T. Turkdogan i s Howe Memorial Lecture. The lower curve inFigure 31 is for regular blast furnaces operating with acid sinter orpellet and lump ore, and a high coke rate of about 800 kg/tonne of hotmetal. The upper curve is for high-pressure furnaces operating withbasic sinter, oxygen-enriched high-temperature airblast, and a low cokerate of less than 400 kg/tonne of hot metal. In older type blast furnaceoperations, the gas composition is reducing to wusti te at all levels inthe stack. In modern blast furnace operations however6 the gas composi-tion is oxidizing with respect to iron below about 950 C.
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Fluxes ILimestone charged to the furnace will calcine by the following
reaction at approximately 8000C (14720F): ICaC03(s) = CaO(s) + C02 (g) ; ~H = 42,500 cal ( 12)
Magnesi ur carbonate in dolomitic limestone in the charge calcinesby a similar reaction at 500 to 1000C (900 to l800F) lower temperatures:
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MgC03(s) = MgO(s) + C02(g); ~H = 40,000 cal (13) IThese reactions result in several undesirable conditions in the
furnaces. The first is that they require considerable heat and thesecond is that CO2 is released in the furnace. The additional C02raises the oxygen potential of the gases which inhibits the final stepin the reduction of the iron ore, i. e., FeO to Fe. It also favours"solution" of carbon from the coke by Eq. (10). A significant improve-ment in furnace operations is obtained when "self-fluxing" agglomeratesof iron-ore concentrates are the principal iron-bearing charge to thefurnace. Limestone and dolomite may be added to the feed of sinteringmachines and pelletizing furnaces. When the sinter is fired and thepellets are indurated, the fluxes are calcined and reacted with ironoxides to form calciur-ferri tes and other more complex compounds. TheCaO and MgO carried into the blast furnace by these agglomerates arethen free of CO2.
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Slags
The oxide system that forms the basis for blast furnace slags isthe lime-silica-alurina (CaO-Si02-A1203) system as shown in Figure 32 .Slags with compositions in the region of 40% Si02, 48% CaO and 12% A1203
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I 100dP \ \ Japanese
80 \( German') N0
60ull,+ FeO
0 40 i~- ..u J. ..ll ..
Fe 3°4 / J .-'- 20
-- ---0 /U ,-
ll ."0200 400 600 800 1000 1200
Temperature, °C
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I Figure 30. CO Content og Gas Samples from Operating Furnaces
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0.1
rNo(.
+
ôo(. U+ +00.. Nx: x:
+N
x:
80 80 100 1100 120 130
, ~EMPERATUAE, .c '
Figure 31. Gas Compositions in Blast FurnacesWi th Different Operating Practices
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Figure 32. The CaO-Si02-A1203 Phase Diagram
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1have low melting points, i.e., l3000C (23750F) ,and are appropriate forcontrol of sulphur and silicon in the metal. Often 6 to 10% MgO is usedin place of an equivalent amount of CaO to lower the viscosity of theslag. Small amounts of MnO, FeO, Na20, K20 etc. help to lower the melt-ing point of the slag.J
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Essentially there are two slags in the furnace. The first is the"early" slag that is formed principally from the gangue constituents inthe ores and agglomerates and CaO and MgO from the calcined fluxes, orthe self-fluxing portions of the agglomerates. This slag is relativelybasic compared to the final slag and would contain some iron oxide.The "final" slag is formed by the union of the early slag with consti tu-ents of the coke ash that are freed from the coke when it is burnedbefore the tuyeres. This final slag continues to have its compositionmodified as it passes down into the hearth and mingles with liquid ironthat also is flowing down into the crucible. There is an adjustment inthe silica con tent of the slag, iron oxide may be reduced from it and itmay absorb sulphur from the coke and liquid iron.
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I Reactions in the Bosh and Hearth
ISulphur is a troublesome element in blast furnace operations
because hot metal for steelmaking must be Hlowinsulphur ¡ levels of0.035 to 0.02% are usual. The reaction by which sulphur is removedfrom liquid iron into the slag is often represented by the reaction:
I S + (CaO) + C = (CaS) + CO(g) ( 14)
Iwhere sulphur and carbon in the metal react with limeslag to form calcium sulphide in the slag and CO gas.of sulphur between slag and metal, (S) /~, is stronglynumer of factors:
dissolved in theThe distributioninfluenced by a
~ (a) Increasing the basicity of the slag (lime/silica ratio) tends toraise the thermodynamic activity of lime in the slag which pushesreaction (14) to the right.
(b) An increased oxygen potential in the system pushes the reaction tothe left. This is shown by rewriting the reaction as follows:
S + (CaO) (CaS) + 1/2 02(g) (15)
The effect is very strong, and the presence of a small concentrationof FeO in the slag will seriously limit the sulphur ratio, (S) /~.
(c) Fortunately both silicon and carbon in hot metal raise the thermo-dynamic activity of sulphur in the metal at a given concentrationlevel. Accordingly, sulphur at 0.02 to 0.035% in ordinary hotmetal for steelmaking is 5 to 7 times easier to remove than it wouldbe in liquid steel that contains relatively little carbon andsilicon.
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The sulphur distribution ratios found in the blast furnace gener-ally vary between 20 and 120. On the other hand experiments have shownthat when metal and slag samples from the blast furnace are remelted ingraphite crucibles at 1 atm CO, the distribution ratio increases tobetween 120 and 220, depending on the slag basicity. This suggests thatthe oxygen potential of the system is higher than might be expected forC-CO equilibrium in the furnace hearth. Thus while thermodynamic condi-tions favour sulphur removal from hot metal wi thin the blast furnace,kinetic considerations imply that the reaction can be more readilyaccomplished outside the furnace by external desulphurization. Theimplications of this approach are discussed by A .M. Smillie in thelecture on External Treatment of Hot Metal (Lecture 18).
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For many years it was considered that silica and manganese oxidewere reduced directly from the slag by reaction with carbon in ironaccording to the reactions:
~
Si02 (slag) + 2C si + 2CO (g) (16) IMnO (slag) + C Mn + CO (g) (17)
IIt was thought that mol ten iron droplets picked up silicon as theypassed through the slag phase and on into the hearth. Research duringthe last decade however, has shed new light on these reactions and alsothose involving sulphur. Several laboratory studies together with plantdata from Japan have shown that at the temperature of the combustionzone, about 20000C, silicon monoxide gas is produced during the combus-tion of coke by the reaction:
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ISi02 (coke ash) + CO + SiO(gas) + CO2 (18)
ICombining Eq. (18) with the coke oxidation reaction:
CO2 + C (coke) + 2CO
Iyields the overall reaction:
Si02 (coke ash) + C (coke) + SiO (gas) + CO ( 19) rWhile the presence of FeO in slag is likely to make SiO formation fromslag very difficult, an additional source of silica would be reducedsilica-rich slag adhering to coke particles. Following these reactions,silicon is transferred to iron droplets by reaction with silicon monoxidein the gas phase:
SiO(gas) + C + Si + CO ( 20)
As iron droplets containing silicon pass through the slag layer, some ofthe silicon is oxidized by iron oxide and manganese oxide, and taken upby the slag:
2 (FeO) 1 + Sis ag (Si02) slag + 2Fe ( 21-)
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2(MnO) 1 + Si = (Si02'..) 1 . + 2Mns ag s agAssuming sulphur in coke ash is present as CaS, the following
reaction can occur with SiO in the combustion zone to form volatile sis:
(22)
CaS (k ) + SiO ( )-+co e ash gas CaO + SiS ( )gas (23)
To a lesser extent, some CS gas may form by the reaction:
CaS (k h) + CO -+ CaO + CS ( )co e as gas (24 )
Sulphur transfer from these volatile species to molten iron dropletsthen takes place with the bosh zone. Turkdogan has shown that when irondroplets containing Si and S are allowed to fall through mol ten slag, inthe absence of MnO, the Si content of the metal actually increases, andthere is no transfer of S. In the presence of MnO, Si is removed fromthe metal by reaction( 22) and Mn transfers from slag to metal togetherwith S transfer from metal to slag. Based on the various results avail-able, Turkdogan suggests the following sequence of reactions in the boshand hearth:
1. The formation of sio and SiS in the combustion zone.
2. The transfer of silicon and sulphur to metal and slagdroplets in the bosh.
3. The oxidation of silicon by FeO and MnO in the slag as theiron droplets pass through the slag layer.
4. The desulphurization of metal droplets as they pass throughthe slag layer.
other reactions involving the formation of volatile species arethose associated with so-called rogue elements such as sodium, potassiumand zinc. These elements have adverse effects on furnace operation dueto refractory attack, generation of fines, accretion formation anddecreased burden permeability. Problems of this type were accentuatedduring the 60s as more furnaces began to operate with higher drivingrates, increased flame temperatures, lower slag volumes and relativelyhigh basicities. During the 70s, our understanding of these phenomenahas been greatly enhanced both by laboratory studies and results fromplant operations. A leading contributor to this field has been W-K. Luand co-workers.
The alkali metals, sodium and potassium, generally enter thefurnace with the raw materials in the form of very stable alumino-silicates. Certain amounts of sodium and potassium leave the furnacewith the top gas and in the slag phase, while the remainder accumulatesin the furnace in the form of cyanides, carbonates and intercalationcompounds in coke, e. g., C6K and C8K. These compounds decompose in thehigher temperature regions of the blast furnace to form metallic andcyanide vapours, e. g. :
K2Si03 + C -+ 2K(gas) + Si02 + co (25j
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These vapoursare carried by massive gas flow to lower temperatureregions where condensation reactions occur and compounds are reformedto be transported back down the s tack wi th the burden materials, e. g. :
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2K(gas) + CO + K20 + C
Decomposition reactions of the type indicated bYEq. (25) are stronglyendothermic and bring cooling to the hearth zone. During condensationreactions in the cooler regions of the furnace, heat is released.
( 26) J
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Since the alkali metals form basic oxides , they are readily neutral-ized by the use of acidic slags. Their removal with the slag is there-fore enhanced when the slag has a low basicity (Figure 33) and tempera-tures in the hearth are relatively low. These conditions do not favourthe production of low pulphur hot metal and the behaviour of alkaliesconsti tutes another reason, why hot metal should be desulphurizedoutside the furnace. The reactions of sodium are similar to those of
potassium except that sodium is more difficult to gasify and thus itcan be more easily removed in the slag phase.
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IThe main source of zinc in the blast furnace is from sinter which
contains dust from steel furnaces in which a high proportion of galvan-ized scrap has been melted. Following reduction of ZnO, zinc vapourwill recirculate through the furnace with the subsequent formation ofZnO and ZnC03 in the regions of lower temperature and higher oxygenpotential. Unlike the alkali elements, zinc does not form stablesilicates and cannot be removed in the slag phase.
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IEnergy Considerations
The counter-flow of gases and solids in the shaft of the blastfurnace provides for highly efficient use of the heat and reducingpower of the gases leaving the tuyere region. Heat transfer from gasto solids is accompanied by oxygen transfer from solids to gas. It willbe recalled that the strongest reducing gases are employed first incarrying out the most difficult reduction step, that of converting FeOto iron. Similarly, the gases when hottest complete the highest temper-ature work, that of melting and superheating the slag and metal, andproviding the heat required for reactions in the bosh and hearth zones.There is a gradual transfer of heat from the gases ascending the furnaceto the solids that are descending. At steady state operations, the tem-perature profiles of gases and solids do not alter their positions inthe furnace. The temperature of the gas, T, is always higher than thatof the solids, 8, and the difference (T-8) is the driving force for thetransfer of heat from the gases to the solids.
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Application of the concept of thermal flows in heat and massexchange in a counter-current, gas-solids reactor to the operation of ablast furnace can be useful and illuminating. The thermal flows ofgases and solids may be represented in terms of the products of theirrespect thermal capacities (G and S) and velocities (U and V). Thermalflow for the gas phase is given by the product I UG I while that for the
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2 D
SLAGK10..
0 cc Cc
D D I If cD 0 c cc c
11'1 i c ::
0.80 c:
.90 1.0 1.1 1.2
BASICITY
,- Figure 33. Relatiqnship Between K20 Content of Blast Furnace Slagand the Slag Basicity Ratio (CaO + MgO/Si02 + A1203)
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l 400
;"ë..~t' 800
rThermal capaityof or, coe ash
and ftuaCi
1200
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Thrmal capacity15 200
keal/hr .CI
lO
Figure 34. Contributions to the Effective Thermal Capacity ofSolids in the Stack of a Blast Furnace
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I ! r,: I \ I i I---'--f--- ,-- ,-_.. --I 1'£ ¡ ',I Ii I~,Tuyër~ ,¡ : "' ìICVI£Î: i ! I ""i400 800 1200 160
Tcmp~raturc, °C200
r..X
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Figure -J§. Temperature Profiles in a Blast Furnace Stack
1 - Temperature of Solids 2 - Temperature of GasA - Direct Reduction B - Indirect Reduction
1-74 I
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1solids is Ivs I. 2 0The products may be expressed in terms of J/S. m . F.
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The heating of the charge in the blast furnace can be expected tobe most uniform if IUGI ~ Ivs I. However, both UG and VS will change asthe streams pass through the shaft. Generally the heat capacities ofmany substances, particularly coke and gas species increase with increas-ing temperature (Figure 34,). Accordingly,_both G and S tend to increasewith temperature. Loss of combined water, drying of solids, and calcin-ation of limestone and dolomite will all increase UG and decrease VS.These reactions also absorb heat which in effect results in an increasein the value of VS.
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The net effect of the changing thermal flows of the gases andsolids on the temperature profile in a furnace is shown in Figure 35 .In the region Hi, I UG I ~ I vs I and the temperature profile is pushedhigh in the shaft. The region H2 is present in most furnaces where
I UG I ~ I vs I. This region is often termed the thermal reserve zone orthermal pinch point, because there is little heat transfer and the tem-peratures of the two streams change very little. Depending on the typeof burden and the blast furnace practice, the temperature level for thethermal reserve zone varies from about 8500 to 10500C, and the lengthof this zone varies from about 1 to 4 m. with burdens containinghydra ted ore and carbonates, addi tional thermal pinch points may occurat lower temperatures where the hydrates and carbonates dissociate.The region H3 is in the bosh just above the tuyere level and the thermalflow of the gases tends to be lower than that of the solids principallybecause of the heat required to melt the slag and iron and because ofreduction reactions in this location.
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If operating conditions are established to give a good balance ofthermal flows, the descent of the solids is uniform and the permeabilityof the solids is also uniform, it is to be expected that the solids willbe heated reI a ti vely uni formly as they pass down the shaft. If, however,the gas channels locally through the bed because of non-uniform packing,there will tend to be very hot colums of gas reaching very high in thefurnace. On the other hand, portions of the bed will be starved of gaswhich will result in cold, partially reduced material arriving in thetuyere region. Such a condition leads to serious operating problems.
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It will also be evident that it is not possible to replace all ofthe air in the blast with oxygen. with the substitution, the amount ofni trogen in the gas stream decreases and the thermal flow of the gasstream decreases dramatically. As a consequence, the.heating of thematerials in the stack is impaired. steps have been taken to reducethe thermal flow of solids per unit of production of pig iron. Byincreasing the blast temperature, it is necessary to burn less coke.Thus there is less coke in the solids and the value of VS is decreasedso that the value of UG may be decreased somewhat by a replacement of asmall amount of the air blast by oxygen. The use of self-fluxingsintering also allows a reduction in UG because of the elimination ofthe need for the supply of heat in the furnace for calcination of thefluxes.
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This concept of thermal flows in heat and mass exchange helps toillustrate why a blast furnace operates so successfully on raw materialsthat are uniform in size and composition. As mentioned in a previoussection, because of the differences in the physical characteristics ofpellets, sinter and coke, the placing of materials in the furnace toobtain a uniformly permeable bed is extremely important. Similarly, itis essential that the materials in the burden retain their size andshape and do not degenerate into fines as they pass through the furnace.
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JIn concluding this discussion on energy aspects, it it worth noting,
as pointed out by W-K. Lu, that the major difference between Japaneseand North American blast furnace practice, may be characterized in termsof the temperature and silicon content of the hot metal. In NorthAmerica an increase in hot metal temperature is usually associated withan increase in silicon level and vice versa. At the present time hotmetal silicon contents are about 0.8% or higher. In Japanese plantshowever, silicon concentrations are 0.4% or less. In spite of these lowsilicon levels, the hot metal temperature is about 500 to 1000C higherthan in North America. From the standpoint of melting scrap in the BOF,Lu has indicated that the beneficial effect of 0.1% silicon in hot metalis equivalent to raising the hot metal temperature by 12. 30C. In theprocess of using the chemical heat provided by silicon oxidation, oxygenmust be supplied and a basic slag formed in the converter. Theserequirements are decreased when low-silicon, high-temperature hot metalis used. From an overall energy conservation standpoint, it would appearlikely that increasing use will be made of this approach in the 80s. Toa~complish this objective, however, further improvements will be requiredin the quantity, reducibility and high temperature characteristics ofthe burden materials.
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CONCLUDING REMARKS IThe modern blast furnace operating with a low coke rate is an
efficient processing unit primarily because of the intrinsic character-istics of a counter-current gas-solids reactor. A successful use ofthis concept requires that each of the materials charged to the furnacebe of uniform physical character, and have a uniform composition. Inaddi tion, each material must retain this good physical character as itpasses down through the furnace to where melting occurs. It is importantto note that much of the improvement in furnace operations that has beenachieved in recent years has resulted from improvements in the physicaland chemical characteristics of the materials charged to the furnace andin procedures for distributing the charge wi thin the furnace. Othercrucial developments have been the use of high-temperature blast, tuyereinjection processes, high-top pressure and external desulphurization ofhot metal.
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During the 60s and 70s substantial progress has been made in ourunderstanding of the physical and chemical aspects of blast furnaceironmaking. This has been accomplished by an appropriate blending oflaboratory experiments, plant trials and production experience. Signif-icant advances have been achieved in the areas of burden reducibility,
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1 fluxed charge materials, coke properties, slag-metal reactions, alkalibehaviour, and heat and mass transfer aspects. A schematic representa-tion of current thinking on the behaviour of materials within the blastfurnace is shown in Figure 36. A detailed discussion of the reactionswhich take place wi thin the various zones indicated on this diagram, isgiven in the lecture on Blast Furnace Reactions (Lecture #3) byC.M. Sciulli.
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LUMPY ZONE
I
I SOFTENINGFROT
IMELTINGFRONT
· ~~:tETSSLAGo DRPLETS
I COKESLIT
I ¡SOFTENING/MELTING ZONE
rRACEWAY
Figure 3.6. Schematic Representation of ReactionZones in a Modern Blast Furnace
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SOURCES OF ADDITIONAL INFORMTION J1. Aitchinson, L., A History of Metals, Vol. 1, Interscience
Publishers, Inc., 1960.J
2. Tylecote, R.F., "Roman Shaft Furnaces in Norfold", JISI, Vol. 200,January 1962, p.19.
J3. Maddin, R., "Early Iron Metallurgy in the Near East", Transactions
ISIJ, Vol. 15, 1975, p.59.
~4. Matsushita, Y., "Restoration of the Tatara Ironmaking Process, anAncient Ironmaking Process of Japan", Supplemental TransactionsISIJ, Vol. 11, 1971, p..2l2.
I5. Morton, G.R. and W.A. Smith, "The Bradley Ironworks of John
Wilkinson", JISI, Vol. 204, July 1966, p.66l.
I6. Morton, G. R., "The Furnace at Duddon Bridge", JISI, Vol. 200,
June 1962, p. 444.
7. Murray, D., "Who Invented the Hot Blast?", Steel Times, April 1965,p.597.
I
8. Firms tone , F., "Development in the Size and Shape of Blast Furnacesin the Lehigh Valley, as Shown by the Glendon Iron Works",Transactions AlME, Vol. XL, 1909, p.459.
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9. Gramer, F. L., "A Decade in American Blast-Furnace Practice",Transactions AlME, Vol. XXXV, 1905, p.124.
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10. Birkinbine, J., "The United States Iron Industry from 1871 to 1910",Transactions AlME, Vol. XLII, 1912, p.222.
I
11. Johnson, J.E., Jr., "An Automatic Stock-Line Recorder for IronBlast-Furnaces", Transactions AlME, Vol. XXXVI, 1906, p.79.
r
12. Bell, I.L., Principles of the Manufacture of Iron and Steel,George Routledge & Sons, 1884.
13. Howe, H.M., "Biographical Notice of Sir Lothian Bell, Baronet",Transactions AlME, Vol. XXXVI, 1906, p. 412.
14. Bell, I. L., Chemical Phenomena of Iron Smelting, Iron and SteelInstitute, 1872.
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115. Gruner, M.L., Studies of Blast Furnace Phenomena, translated by
L.D.B. Gordon, Henry Carey Baird, Publisher, 1874.
i 16. Johnson, J.E., Jr., Blast-Furnace Construction in America,McGraw-Hill Book Company, 1917.
i17. Johnson, J.E., Jr., The Principles, Operation and Products of the
Blast Furnace, McGraw-Hill Book Company, 1918.
i18. Gayley, J., "The Application of Dry-Air Blast to the Manufacture
of Iron", Transactions AlME, Vol. XXXV, 1905, p.746.
I19. Royster, P.H., T.L. Joseph and S.P. Kinney, "(a) Reduction of
Iron Ore in the Blast Furnace; (b) Significance of Hearth Tempera-tures; (c) Heat Balance of the Bureau of Mines Experimental Furnace;(d) Time Element in Iron Ore Reduction, (e) Influence of Ore Sizeon Reduction", Blast Furnace and Steel Plant, VoL. 12, 1924,pp. 35-38; 97-101; 200-204; 246-250; 274-280.I
I20. Kinney, S.P., "The Blast-Furnace Stock Column", u.S. Bureau of
Mines Technical Paper 442, 1929.
I21. Furnas, C.C. and T.L. Joseph, "Stock Distribution and Gas-Solid
Contact in the Blast Furnace", U.S. Bureau of Mines TechnicalPaper 476, 1930.
I22. Darken, L.S. and R. Gurry, "The System Iron-Oxygen", Journal
American Chemical Society, Vol. 67, 1945, p.1398 and Vol. 68,1946,p.798.
I23. Ergun, S., "Pressure Drop in Blast Furnace and in Cupola",
Industrial and Engineering Chemistry, VoL. 45, No.2, 1953, p.477.
r24. Stephenson, R.L., "Improved Productivity and Fuel Economy Through
Analysis of Blast-Furnace Process", Iron and Steel Engineer, 1962,p. 601.
25. Sweetser, R. H., Blast Furnace Practice, McGraw-Hill Book Company,1938, p.ll.
26. Strassburger, J.H. (Editor), Blast Furnace Theory and Practice,Vol. 1, Gordon and Breach Science Publishers, 1969.
27. Melcher, N.B. et aI., "Use of Natural Gas in an Experimental BlastFurnace", U.S. Bureau of Mines Report of Investigation 5261, 1960.
28. British Iron and Steel Institute Special Report 72, Part I -Injection Processes, 1962, pp.1-7l.
29. Ashton, J.D. and J.E.R. Holditch, "Homogenized Oil Injection atDOFASCO", AlME Ironmaking Proceedings, VoL. 34, 1975, p.261.
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30. Strassburger, J.E., et al., "Solid Fuel Injection of the HannaFurnace Corporation", AlME Blast Furnace, Coke Oven and Raw MaterialsConference Proceedings, 1962, p. 157.
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31. Bell, S.A., J.L. Pugh and B.J. Snyder, "Coal Injection - BellefonteFurnace", AlME Ironmaking Proceedings, VoL. 26, 1967, p. 180. J
32. Strassburger, J .H., "Blast Furnace Oxygen Operations", AISIYearbook, 1956. J
33. Higuchi, M., et al., "High Top Pressure Operation of Blast Furnacesat Nippon Kokan K.K.", Journal of the Iron and Steel Institute,September, 1973, p.605.
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34. Furnas, C.C., "Flow of Gases Through Beds of Solids", u.S. Bureauof Mines Bulletin 307, 1929.
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35. Legille, E. and K.H. Peters, "Operation of a Blast FurnaceIncorporating a Paul Wurth Bell-Less Top Charging System and itsApplication to Large Blast Furnaces", AlME Ironmaking proceedings,Vol. 32, 1973, p.144.
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36. Hatano, M. and M. Fukuda, "The Effect of Coke Properties on theBlast Furnace Operation", AlME Ironmaking Proceedings, VoL. 35,1976, p.2.
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I37. Elliott, J.F., M. Gleiser and V. Ramakrishna, Thermochemistry for
Steelmaking, Vol. II, Addison-Wesley Press, N. Reading, Mass., 1963.
I38. Levin, E.M., C.R. Robbins and H.F. McMurdie, Phase Diagrams forCeramists, The American Ceramics Society, Inc., Columus, Ohio,1964, p.2l9.
I39. Turkdogan, E.T., "Blast Furnace Reactions", Met Trans. B, AlME,
Vol. 9B, No.2, 1978, p.163.
I40. Lu, W-K., and J.E. Holditch, "Alkali Control in the Blast Furnace:Theory and Practice", Blast Furnace Conference Proceedings, ArIes,France, June, 1980.
r41. Kitaev, B.I., Yu.G. Yaroshenko and V.D. Suchkovi, Heat Exchange in
Shaft Furnaces, Pergamon Press, London, 1967.
42. Elliott, J.F., and J.C. Humert, "Heat Transfer from a Gas Stream toGranular Solids - An Idealized Analysis", Proceedings, Blast Furnace,Coke Oven and Raw Materials Committee, AlME, Vol. 20, 1961, p.130.
43. Elliott, J.F., "Some Problems in Macroscopic Transport", Trans.Met. Soc. AlME, Vol. 227,1963, p.802.
44. Elliott, J.F., R.A. Buchanan and J.B. Wagstaff, "physical Conditionsin the Combustion and Smelting Zones of a Blast Furnace, Trans.AlME, Vol. 194, 1952, p. 1168.
45. Lu, W-K., "Silicon in the Blast Furnace and Basic Oxygen Furnace", -Iron and Steelmaker, VoL. 6, No. 12, 1979, P .19.
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BIBLIOGRAPHY
1. Bartholomew, Craig L. and Metz, Lance E., The Anthra-cite Iron Industry of Lehiqh Valley, Center for CanalHistory and Technology, Easton, PA., 1988.
2. Beard, Directory & History of Marquette Country -Early History of Lake Superior - Mines and Furnaces,Detroi t, MI., 1873.
3. Benison, Saul, Railroads. Land and Iron: A Phase inthe Career ofLewis Henry Morgan, University Microfilms Internation-al, Ann Harbor, MI., 1954.
4. Blast Furnaces, Article from Marquette County Histori-cal Society.
5. Boyer, Kenyon, Historical Highliqhts, Radio Manu-script from Marquette Country Historical society,Marquette, Michigan, 1958.
6. Claney, Thomas, "Charcoal Humor". Michiqan HistoricalMaqazine, Volume 5, 1921, Page 410.
7. The Daily1890-1900.
Journal, Marquette, Michigan,Mininq
8. Data on Furnace of Jackson Iron Company, Article fromMarquette County Historical Society.
9. Directory to the Iron and Steel Works of the unitedStates, The American Iron and Steel Association,Philadelphia, PA., 1884.
10. The Iron and Steel Works of the united States, Ameri-can Iron and Steel Institute, 1880.
11. Johnson, J . E., The Principles. Operation and Productsof the Blast Furnaces, McGraw-Hill Book Company, NewYork, 1918.
12. King, C.D., Seventy-Five Years of Proqress in Ironand Steel, AIME, New York, 1948.
13. Lake Superior1855-1865.
Michigan,Journal, Marquette,
14. Lake Superior Mining Institute, Published by theInstitute, Printed byD. Thorp, Lansing, Michigan.
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15. The Lake Superior Mininq and Manufacturinq News,Negaunee, Michigan, 1867-1868.
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20. Strassburger, Julius,Practice, Gordon andYork, 1969.
H., Blast Furnace - Theory andBreach Science Publishers, New
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16. Letter to William Mather from CVR Townsend, MarquetteCounty Historical Society.
17. The Mininq Journal, 1869-1891.
18. Rist,Iron1974.
DonaldReqion,
E. , Iron Furnaces of the Hanqinq RockHanging Rock Press, Ashland, Kentucky,
19. Schallenberg, Richard H., Innovation in the AmericanCharcoal Industry 1830-1930, 1970.
21. Swank, James M., The Manufacture of Iron in All Aqes,The American Iron and Steel Association, Philadel-phia, PA., 1892.
22. Weale's Rudimentary Series, Metallurqy of Iron, Cros-by Lockwood & Company, London.
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LECTURE #2
Blast Furnace Slag
J. L. BlattnerPrincipal Research EngineerPrimar Process Research
AK Steel CorporationMiddletown, Ohio
INTRODUCTIONThe importance of blast furnace slag in achieving good furnace operation is illustratedby the old saying that goes something like "If you take care of the slag, the furnace willtake care of the rest". There has been a tremendous amount of work done studying theproperties, formation mechansms, and impacts on furnace operations of blast fuaceslag. The purose of this paper is to sumarize the concepts of this previous work toanswer the following questions on blast furnace slag:
1. What is it;2. Why do I care;3. How do I manage it; and4. What do I do with it when I'm done with it.
The fudamentals ofblast furnace slag are complex. At approximately 40 weightpercent, oxygen is the largest single element in slag. Slag is, therefore, a oxide systemand ionic in nature. Due to the_nature of the blast furnace process, slag formation is amulti-step process involving signficant changes in composition and temperature.Slag's four primar components form numerous compounds which result in a widerange of chemical and physical properties. The lesser components of slag are ofparticular interest with respect for hot metal chemistr and fuace control, and add tothe complexity of the physicochemical properties of slag.
Fortunately, there are general relationships which provide a more practical view of thenature of slags which can be used on a daily basis. It is important, however, to have abasic understanding of the fundamental nature of blast fuace slag to understand these
general relationships.
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SLAG FUNDAMENTALSThe following is a brief discussion of some fundamental issues of the blast fuaceprocess and blast furnace slag. The issues include the slag formation, flow in thehearh, the molecular strcture of slag and how the strctue relates the chemical indices
known as basicity, slag solidification, and the impact of changes of the thermal state ofthe fuace on slag composition.
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Slag FormationThe iron blast fuace is a pressurized, counter-current heat exchanging, refluxing, gas-
solid-liquid, packed bed reactor. The iron blast furnace has 3 primar fuctions:1. Reduction of Fe oxides to metallic Fe;2. Fusion of the metallic Fe and oxides; which provides for the3. Separation of the impurities ofthe burden and fuel from the molten Fe.
These characteristics of the process lead to the division of the furnace into 3 verticalzones with respect to slags; Granular, Slag-Formation, and Hearh Zones. These zonesand some specific reactions for each zone are given in Figures 1 and 2.
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The granular zone is located in the upper part of the fuace where all chargedcomponents are in solid phases. The granular zone is bounded by the stockline on thetop and by the start of the formation of liquid phases, the cohesive zone, on the bottom.As the burden descends through the granular zone it is heated by gases from the lowerpart of the furnace and a portion ofthe reduction of the iron oxides is performed. Theamount of reduction that occurs in the granular zone is a fuction of the nature of theiron bearing materials, burden distrbution, and the gas composition and flow patterns.
The slag-formation zone begins at the cohesive zone, where softening of burden begins,and continues down to below the tuyere elevation. The slag-formation zone thusincludes the cohesive zone, active coke zone, deadman, and raceway. The slag formedin the upper par of the slag-formation zone is call the 'Bosh' or 'Primar' slag, and theslag leaving the zone at the bottom is the 'Hearth' slag. The Primary slag is generallyassumed to be made up of all burden slag components including the iron oxides notreduced in the granular zone, but does not include the ash from the coke or inj ectedcoaL. The slag composition changes as it descends in the furnace due to the absorptionof the coke and coal ash, sulfur and silicon from the gas, and the reduction of the ironoxide. The temperature of the slag increases of the order of500 °C (1,000 OF) as itdescends to the tuyere elevation. These changes in composition and temperature cansignficantly impact the physical properties of the slag, specifically the liquidustemperature and the viscosity.
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The third zone is the slag layer in the hearh of the fuace. The slag produced in slag-
formation zone collects in the slag layer, filling the voids in the hearh coke and'floating' on the hot metallayer. The hot metal passes through the slag layer to reachthe hot metal layer. The high surface area between the hot metal and slag as the hotmetal passes through the slag layer enhances the kinetics of the chemical reactions. -These reactions result in signficant changes in the hot metal chemistry. In paricular
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the (Si) and (S) contents prior to entering the slag layer are much higher than the thosein the hot metal layer.
The formation of slags in the slag-formation zone is very furnace specific due to theimpact of burden properties and fuace operation, and is not discussed further in thispaper. The remainder of this paper is directed primarly at the properties of the hearthslag.
Slag Flow In The HearthThe control of the slag level in the hearth is important for maintaining stable fuaceoperation, especially as the hot metal production rates have been increased. High slaglevels result in increasing blast pressure and bosh wall working, and disrupting theuniform descent of the burden.
One of the issues in controlling slag level is slag flow in the hearh during casting. Inthe hearh, slag flow to the taphole is more diffcult than the flow of hot metal to thetaphole. Hot metal flow has a larger driving force due the higher density of hot metalcompared to slag. The hot metal flow path is thought to be primarily through 'cokefree' regions below and/or around the deadman coke. The slag flow path to the tapholeis through deadman coke.
Figure 3 is an illustration of the configuration of the hearth and a possible sequence ofstages of the hearth durng casting that lead to a false dr-hearth condition at the end ofthe cast. The surface of the hot metal is thought to remain relatively flat across theentire hearh area throughout the cast due to the high density of hot metal and the 'cokefree' path to the taphole. The slag surface maybe signficantly lower in the region aboutthe taphole than at other regions ofthe hearth. When the slag cast rate is greater thanthe slag flow rate across the hearth to the taphole region, a depletion of slag occurs inthe taphole region and the slag surface begins to curve down towards the taphole, Step 4of Figure 3. The slag depletion continues until there is no slag at the taph6le and thefurnace appears to be dry when there is still signficant slag remaining in the hearth,Step 5 of Figure 3.
Minimizing the resistance to slag flow in the hearth minimizes the slag remaining in thehearth at the end of a cast. Resistance to slag flow in the hearth is reduced as theporosity of the hearth coke bed is increased and the slag viscosity is reduced.
Slag StructureThe conceptualization of slag structure is based upon the structure formed by silica,Si021. On the molecular level, the silicon atom is located in the center of a tetrahedronsurounded by 4 oxygen atoms, one oxygen atom at each comer of the tetrahedron asillustrated in Figue 4. Each oxygen atom is bonded to two silicon atoms, thus eachoxygen is a comer of two tetrahedrons. The sharing of oxygen atoms results in apolymer or network in three dimensions in the crystalline state where all comers are
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shared, Figure 5. As silica is heated, some of the comer bonds are broken but thepolymer nature of the structue is maintained even when molten as illustrated in Figue6.
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The addition of metallic oxides, such as CaO and MgO breaks down the polymerstrcture. These oxides act as oxygen donors, replacing an oxygen atom in one comerof a tetrahedron and breakng the tetrahedron-to-tetrahedron comer bond, Figure 7. Thebreakdown of the polymer structure continues with the addition of more metal oxidesuntil the molar ratio of metal oxides to silica equals two, at which point all tetrahedron-to-tetrahedron comer bonds are broken, Figure 8. The molar ratio of2 is theorthosilicate composition, 2CaO-SiOi, 2MgO-SiOi, and CaO-MgO-SiOi. Ah03 actsin a similar fashion as SiOi in forming polymers and accepting oxygen atoms frombasic oxides.
Oxides that accept oxygen, SiOi and Ah03, are termed acid oxides. Oxides that donateoxygen, CaO and MgO are termed basic oxides.
Slag BasicityIt is very useful when relating the properties of a multi-component system to itscomposition to develop an index based upon the composition. The problem indeveloping an index is how to reflect the signficance of each component of the systemin the index.
The different natue of the acid and basic oxides has been used in the development ofslag composition indices, generally termed basicities. Examples of basicity indices thathave been developed are given below in equations 1 to 4.
Excess Bases = r (CaO)+ (MgO) ) - r (SiOi) + (Ah03) ) (1)
Basicity = r (CaO)+ (MgO) ) / r (SiOi) + (Ah03) ) (2)
Bell's Ratioi, = r (CaO) + 0.7*(MgO) ) / r 0.94*(SiOi)+ 0.18*(Ah03) ) (3)
Optical Basicitl = (CaO) + 1.11 *(MgO) + 0.915*(Si02) + 1. 03 * (Alz.;Ù (4)
(CaO) + 1.42*(MgO) + 1.91 *(SiOi) + 1.69*(Ah03)
Basicity indices can be grouped into general catagories:a) Differences between the amount of bases and acids, equation 1;b) Bases to acids ratios based upon the weight percentages, equation 2;c) Bases to acids ratios based upon the molar concentrations, equation 3; and
d) Sum of the basicity of each component and its molar concentration, equation 4.
As would be expected based on the previous description of slag structure, those indiceswhich reflect the molecular natue of the slag composition, equations 3 and 4, tend to b_ebetter predictors of slag properties. However, as the index defined by Equation 2 is '
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probably the most commonly used definition, it is used throughout the remainder of thispaper as B/ A.
Temperature Impact -ISil, Basicity, and Slag VolumeThe (Si) increases with increasing hot metal temperatue for all blast fuaces asillustrated in Figure 9. The amount of (Si) increase for a given temperature increasevaries from furnace to furnace, but the trend is the same for all furnaces. As the (Si)increases, the (SiOi) decreases and therefore the basicity increases and the slag volumedecreases. The amount of increase in the basicity for a specific increase in (Si) is afuction of the slag volume.
Shown on Figure 9 is the change in BfA for initial slag volumes of 200 and 300 kg /THM and for the (Si) andhot metal temperature relationship given on the figure. Thegeneral trend demonstrated here is that the larger the slag volume the smaller the changein BfA for the same change in (Si) or hot metal temperature.
Slag SolidificationThe common definition of melting temperatue only applies to a single componentsystem such as water, where only liquid water exists above the melting temperature andonly solid water exists below the melting temperature. Slags are a multi-componentsystem and, therefore, do not have the common definition of melting temperature exceptat specific compositions. Most slag compositions have both solid and liquid phasespresent over a range of temperatures. The lowest temperature at which only the liquidphase exists for a specific composition is called the liquidus temperature.
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The solidification path of a slag is ilustrated on the simplified phase diagram shown inFigure 10. Star with slag of composition Cstart at temperatues where only liquid slagexists. As the slag cools, moving down vertically on the diagram, the composition ofthe liquid slag does not change until the intersection with the Liquidus Line. Theintersection with the Liquidus Line is the liquidus temperature for the composition Cstart.A very small amount of the solid compound on the left forms at the liquidustemperature. Three changes continue as the temperature is further reduced below theliquidus temperatue:a) More of the solid compound is formed;b) The amount of liquid slag decreases; andc) The composition of the liquid slag changes, moving towards the right along the
Liquidus Line.In the example, where the compound formed is 2CaO.SiOi, the basicity of the liquidslag decreases as the slag is cooled because 2CaO.SiOicontains approximately twice asmuch CaO as SiOi.
The solidification path illustrates how a compound can be formed even when the liquidslag composition is significantly different than the composition of the compound. Theweight ratio of CaO to SiOi = 1.86 for the compound dicalcium silicate, 2CaO.SiOi. -Whle no blast fuace has ever been successfully operated using slags with a CaO to
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(CaO) Less Than 0.9 * (Si02) + 0.6 * (Ah03) + 1.75 * (S) (5)
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Si02 approaching 1.86, significant amounts of dicalcium silicate can be formed in theslags of operating blast furnaces. The formation of suffcient dicalcium silicate resultsin a solid slag that breaks down into dust upon cooling, know as a 'Falling' or 'Dusting'slag. The breakdown is caused by the 10% volume expansion of dicalcium silicate as itgoes through a phase change at 675°C. The following guideline for avoiding a fallngslag has been reported4:
It is important to remember that phase diagrams are based upon equilibrium conditions.Equilibrium conditions imply that the cooling rate is slow relative to the rate of thereactions, such as the formation of dicalcium silicate. The solidification path describedabove is 'bypassed' if the cooling rate is very high as in slag granulation and, to a lesserextent, slag pelletization. The rapid cooling locks the composition in a solid glassphase, where the kinetics of the reactions are too slow for the compounds to form.
SLAG PROPERTIESThe physical and chemical properties of slags are primarly a fuction of the slag
composition and temperature. The following describes these relationships for thepurose of developing general trends.
Liquidus TemperaturesThe definition of liquidus temperatue was described previously in the section onsolidification. The relationships of liquidus temperature and composition for the fourprimary components of slag are represented on a quaternary phase diagram. Figures 11and 12 were generated from ternar planes of the quaternar phase diagram. Note thatFigures 11 and 12 are not phase diagrams.
There are two general trends derived from these figures. First, the liquidustemperatures increase with increases in BfA and (Ah03)' Second, (MgO) in the rangeof 8 to 14% tend to minimize the increase in liquidus caused by the increase in eitherBfA or (Ah03).
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ViscosityViscosity is a measure of the amount of force required to change the form of a materialand is reported in units called Poise. The higher the viscosity, the more force requiredto cause a liquid to flow. For comparison purposes consider that at 20°C the viscosityof water is 0.01002 poise, while a typical acceptable slag viscosity is 2 to 5 poise, andthe viscosity of molten Si02 is ofthe order of 100,000 poise.
The high viscosity of liquid Si02 is caused by the polymer strcture discussedpreviously. The breakdown ofthe polymer strctue by the basic oxides, lowers theviscosity. The decrease in the viscosity of all liquid slags with increasing the BfA isshown Figure 13.
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In general the viscosity of any liquid/solid mixtue increases as the amount ofsuspended solids increases. The impact of temperature on slag viscosity is significantlygreater at temperatures below the liquidus temperature than above the liquidustemperatue, Figure 14.
There are two general trends that for viscosity. The viscosity of liquid slags, above theliquidus temperature, decreases with increasing B/ A and temperature. At temperaturesbelow the liquidus temperatue, the viscosity decreases with decreasing B/ A andincreasing temperature.
Sulfur Partition RatioThe iron blast furnace is a very good desulfurizing process compared to the steelmakingprocess because of the difference in the oxygen potential of the slags of the processes.The effect of the oxygen potential on desulfurization can be illustrated using Equation6, where the oxygen potential is indicated by the (FeO). The higher the (FeO) the morethe reaction is driven to the left and the higher the (S). Steelmaking slags with (FeO) of15 to 25 % are, therefore, weaker desulfurizing slags than the blast fuace hearth slagswith (FeO) of less than 1 %.
(CaO) + (S) = (CaS) + (FeO) (6)
Essentially all the sulfur into the blast furnace leaves the fuace in the hot metal andslag. A relationship for the prediction of (S) can be developed based upon a massbalance of sulfur for one ton of hot metal, Equation 7, and the defined term sulfurpartition, Equation 8. The prediction of (S), Equation 9, is derived by substitution of(S) from Equation 7 into Equation 8 and then solving for (S).
ST = (S) / 100 * 1,010 + (S) 1100 * SVol (7)where1,010 is the kg of hot metal in a ton of hot metal including a 1 % yield loss.The remaining terms are defined in the Nomenclature section.
(S) = ST*100 / ( SP * SVol + 1,010 1
(8)
(9)
SP = (S)/(S)
The slag SP can be predicted based upon Equations 10 and 11. Note that thecoefficients in Equation 10 were developed from regression analysis of a specificfurnace.
SP = 147.7 * BB + 37.7 * (Si) - 190
BB5 = ( (CaO) + 0.7*(MgO) 11 ( 0.94*(SiOz)+ 0.18*(Ali03) 1
(10)
(11)
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Equations 10 and 11 were used to construct Figure 15, and Equations 9, 10, and 11 wereused in the constrction of Figue 16.
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The general trends that can be derived from the above equations and figures are:a) (S) decreases with decreasing ST and increasing SP and SVoi;
b) SP generally increases with B/ A; howeverc) CaO is a better desulfurizer than MgO; andd) Ah03 has a smaller effect on SP than SiOz.
Alkali CapacityA 'refluxing' or 'recycling' phenomena occurs in the furnace due to the counter-curentflow of gases versus solids/liquids, paricularly for sulfu, zinc, and alkalis. The
recycling of the alkali potassium, K, is illustrated on Figure 17. The recycling'phenomena is when an element travels down the fuace in a solid or liquid phase,reacts to form gas species in the higher temperatue regions of the fuace, then travelsback up the furnace as gases, where it reacts and is absorbed by the solid/liquid phasesin the lower temperature region of the furnace. The recycling results in much higherinternal concentrations of the recycled element than the concentration going in or out ofthe fuace. For example the internal loading ofK may be 10 kg / THM when thematerials being charged contain only 2 kg / THM.
Alkalis have no beneficial, but many deleterious effects on the blast furnace. Alkalisare absorbed by refractories, coke, and ore causing degradation of the refractories andcoke, and ore swelling. Alkalis can also form scabs which can peal off upsetting thethermal condition of the fuace, or build up and constrict burden and gas flow.
Alkalis cannot be avoided as they are contained in all coals, cokes, and to a lesser extentores. The alkali loading should be minimized whenever possible.
A portion of the alkalis leave the fuace in the top gas, the amount being a function ofthe top temperature profile. The remaining alkalis must be removed in the slag. Theability of slag to remove alkalis from the furnace is referred to as the alkali capacity ofthe slag. The relationships of alkali capacity to slag composition and temperature areshown in Figure 186. In general the alkali capacity increases with lower B/ A andtemperature.
Silica ActivityThe (Si) produced is dependent upon the burden materials, furnace operation, and slagchemistry. The impact of the slag chemistry is shown in Equation 16. Equation 16 isdeveloped from the equilibrium constant, Equation 13, for the reaction given inEquation 12, the definitions of the activities of(SiOz) and (Si), Equations 14 and 15,and assuming that the activity of the carbon in the hearh equals one.
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(Si02) + 2 C = (SiJ + 2 COgas
Keq = t ASi * p2 co ) / t Asi02 * Ac )
ASi02 = (Si02) * YSi02
ASi = (SiJ * YSi (15)
(SiJ = (Si02) * YSi02 / YSi * Keq / p2 co
(12)
(13)
(14)
(16)
The reader is referred to the work by Chaubal and Ricketts9 for details of the aboveequations. The trend implied by Equation 16 is that the (SiJ decreases as the (Si02)decreases.
SLAG DESIGN FACTORSIn North America, a typical slag composition that would be formed from the gangue inthe ore and ash from the coke is 9% CaO, 5% MgO, 75% Si02, and 10% Ah03. A slagof this composition would have a liquidus temperature of the order of 1,600 °C (2,900OF ) and would not flow well even above it's liquidus temperature. CaO and MgO are
added to the burden to 'flux' the gangue and ash resulting in acceptable liquidustemperatures and flow characteristics.
Basic slag design is the selection of the tyes and amounts of fluxes to be used with aburden and coke to produce a slag of acceptable properties. Burden and coke selectionsare largely drven by economic issues such as local verses foreign sources and degree ofbeneficiation. These economic driving forces have resulted in a wide range of slagcompositions throughout the world, Table 1.
The following are the general factors to be considered in designng a slag for normaloperation:1. Liquidus Temperature - the slag must be completely liquid in the hearth and
casthouse;2. Viscosity - the slag must have a low viscosity, high fluidity, so as to drain from the
hearh and down the casthouse runners;3. Sulfu Capacity - the SP must be suffcient to produce hot metal with sulfu
contents within specifications;4. Alkali Capacity - the slag alkali capacity must be suffcient to prevent alkali build
up in the furnace;5. Hot Metal Silicon Control- the effect of the slag chemistry on the (SiJ must be
considered;6. Slag Volume - the slag volume should be high enough to contrbute to the stability
of the slag properties and hot metal quality, but not so large as to require excessivefuel or contribute to furnace instability;
7. Robust Properties - the slag properties should be as insensitive to variations innormal variations in fuace operation as possible, specifically hot metaltemperatue; and
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8. End Use - the requirements of the end use of the slag must be considered.
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Slag design must recognze that the above factors are not independent and that thedesign always involves a balancing of the above factors to resolve the conflictingtrends, Table 2. Two examples of the slag design are given below.
SLAG AFTER THE BLAST FURNACE
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In the first example, the problem was to increase the alkali removal without increasingthe (S). The resolution ofthe problem was to increase the slag volume through the useof additional SiOz in the burden, while decreasing the slag basicity.
The problem in the second example was to lower the (Si) without negatively impactingthe other properties of slag and furnace operation. This problem was resolveddecreasing the (SiOz) by increasing the (Ah03) using diaspore, a high (Ah03) burdenmaterial, while holding the (CaO) and (MgO) constant. Note that the change in slagchemistry resulted in a decrease of both (Si) and (S).
The use ofblast furnace slag is driven by the economics of processing and marketdemand. In the past, when the processing and marketing was performed by thecompany producing the slag, the markets tended to be local in nature with minimalprocessing. The trend to use independent companies that take ownership of the liquidslag at the end of the slag runner, has lead to wider markets with more extensiveprocessing. The product slag can be classified by the rate of cooling.
Air-cooled slags are those produced with low cooling rates. These are slags that aresolidified in pits and frequently cooled with water sprays. The largest uses for air-cooled slag are in road construction, railroad ballast, and aggregate. Air-cooled slag hasalso been used in the production of cement, mineral wool insulation, roofing, and glass.
Pelletized and granulated slags are those produced with high cooling rates. Pelletizedslag is produced by pouring liquid slag onto a rotating drm, sometimes with water.Granulated slags are produced by either pourng the liquid slag directly into a large pitof water or through the use of high pressure water sprays which breaks the slag up intodroplets. Rapidly cooled slags have been used for the same applications as air-cooledslags. The high glass content of rapidly cooled slags makes it particularly sui tab Ie forportland cement production.
ACKNOWLEDGEMENTSThe objective of this review is to summarize the work done by others. Due to themagnitude of the work that has been done, it is difficult to give the personal recognitiondue. The author would like to recognze the previous authors of this section,
paricularly R.L. Shultz, who provided the foundation of the strcture and contents of -=
this paper.
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Table 1 - Examples of Typical Blast Furnace Slags?
NorthAmerica Japan Europe India Australia
__ÇQ.IlQositiQn__ --------------- ~-------------------- ------------ ------------- --------------------%SiOi 37-41 34 36 33 35-38
------%-AI;Õ;------------------------ ------------- ------------- --------------------- -----------------
7-10 13-15 11-13 21- 25 15-17--------------- ------------ ------------ ---------- ------------ ------------------%CaO 37-41 41 37-43 33 37-42--------------- ------- -------- --------- ------------------ --------------%MgO 10-12 7 6-11 7-10 3-7
V olume*175 - 280 310-320 300-320 500-600 300-420
(350-560) ( 620-640) (600-640) (1000-1200) (600-840)Alkali 2-4 2-3 2.5-5 7-10** 2.5-3.5
Loading* (4-8) ( 4-6) (5-10) (14-20) (5-7)
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* Units are kg/metric ton of hot metal (lb/short ton of hot metal)* * estimated
Table 2 - General Conflcting Trends
Basicity (Ah03)Lower Liquidus Temperature Lower LowerLower Viscosity HigherHigher K Removal Lower LowerLower (S) Higher HigherLower (Si) Higher Higher
Table 3 - Example of Designng Slag for Increased KiO Remova18
Slag K20 SBasicity Volume (K20) Removed (S) Removed
0 (kg/THM) (wt% ) (kg/THM) (wt% ) (kglTHM)1.10 225 0.47 1.30 1.82 5
1.05 282 0.55 1.55 1.77 5
1.00 290 0.63 1.85 1.72 5
0.95 298 0.71 2.10 1.68 5
Table 4 Example of Designng Slags for Lower (Si)9
Period Base No 1 No2 No3Basicity 1.12 1.13 1.13 1.12
(MgO) 11.8 11.5 11.7 11.5
(Ah03) 7.8 10.2 10.3 11.7
(Si) 0.76 0.53 0.54 0.49
(S) 0.043 0.031 0.029 0.026
2-11
Nomenclature(X) = weight percent of X in the slag(XJ = weight percent of X in the hot metalSP = Sulfu Partition Ratio = (S) / (SJST = Sulfur Loading = total weight of sulfur per unit weight of hot metal. Units
are kg per metric ton or pounds per short ton of hot metalAlkali Loading = total weight of alkali per unit weight of hot metal. Units are kg
per metric ton or pounds per short ton of hot metal.SV 01 = Slag Volume = weight of slag per unit weight of hot metal. Units are kg
per metrc ton or pounds per short ton of hot metal.B/ A = basicity as defined by Equation 2BB = basicity as defined by Equation 3Kea = Equilibrium constant
ASi = Activity of Si in hot metalASiOZ = activity of SiOz in slag
Ae = Activity of carbon in the hearth cokeYSiOZ = Activity coeffcient of SiOz in slag
YSi = Activity coeffcient of Si in hot metal
Pea = Partial pressure of CO in the hearh
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References
1 Richardson, F.D., Physical Chemistry of Steelmaking, Edt. J.F. Elliott, MIT, Mass.,1958, pp. 55-62.z Kalyanram, M.R, Macfarlane, T.g. and Bell, H.B., " The acitivity of Calcium Oxide
in Slags in the Systems CaO-MgO-SiOz, CaO-Ah03-SiOz and CaO-MgO-Ah03-SiOzat 1500 °C," Joural of the Iron and Steel Institute, 1960, pp 58-64.3 Sommervile, LD., and Sosinsky, D.J., "The application of the Optical BasicityConcept to metallurgical Slags," Second International Symposium on Metallurgy Slagsand fluxes, edited Fine,H.A., and Gaskell, D.R, published by the Metallurgical societyof AIME, 1984, pp. 1015-1026.4 BisWas, A.K. Principles of Blast Furnace Ironmaking, Cootha Publishing House,Australia, 1981, pp. 347.5 Kalyanam, M.R, Macfarlane, T.G. and Bell, H.B., "The acitivity of Calcium Oxidein Slags in the Systems CaO-MgO-SiOz, CaO-Ah03-SiOz and CaO-MgO-Ah03-SiOzat 1500 °C," Journal ofthe Iron and Steel Institute, 1960, pp 58-64.6 Poos, a., and Vidal, R, "Slag Volume and Composition for Optimal Blast fuace
Operation," 12th McMaster Symposium on burden Design for the Blast Furace, Ed. W-K Lu, May 1984, pp 67-89.7 Shultz, RL., "Blast Furnace Slag," Blast Furnace Ironmaking, published by McMaster
University, 1990.8 Sciulli, C. M., and Ravasio, D., "Alkalies in Raw Materials and Their Effect on the
Blast Furnace," 51st Anual Meeting, Minnesota Section AIME, and 39th AnualMining Symposium, Duluth, Minnesota, Januar 1978.9 Chaubal, P.C. and Ricketts, J.A., "Slag Properties Optimization Program at Inland's _
Eight Meter Blast Furnaces," Ironmaking Conference Proceedings, 1991, pp 445-455..-
rc ~
2-12
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J Figure 1 - Blast Furnace Zones
~I Granular Zone
i Cohesive Zone J
Active Coke Zoneand Deadman
Raceway
SlagFormation
Zone
~ Hearth
I
IFigure 3 - Ilustrtion of Slag Flow in Heart
I.. Coke above Slag Layer
I Taphole
I ~fJ
Figure 5 - Crystallne Silica Strcture
Figure 2 - Blast Furnace Slag Zones - Reactions
Granular Zone ¥"Fe,O, =0. FeD
FeO =0. Fe
JSlag
FormationZone
FeO + Gangue + Fluxes=0. Bosh Slag
(FeD) =o.IFelSiO." ~o.ISil or (SiO,)
SiOiCoke =;: SiOgas
AshCoke =:; Slag
eartb .- (SiO" MoO, S) = ISi, Mn, Si
Figure 4 - Silica Atomic Strcture
Atomic structure of (SiO,)-4 is Tetrahedral structure witb:
t~ . = Si atom in the center
. = Oxygen atoms at the corners.
Figure 6 - Molten Silica Strcture
b
2-13
Figure 7 - Addition of Bases to Molten Silica
.. :: Base Oxide
Figure 9 - Temperature & Slag Volume Impact
..Ø/A. Slag Vol- 200 -oB/A - Slag Vol- 300 (Sll
1.20 1.00%
1.5 0.95%
~ 1.10 0.90% ~el
1.05 0.85%
1.00 0.80%
1,350 1,400 1,450 1,500
HM Temperature
Figure 11 - Liquidus Temperature ~ BfA = 1.0
2,00
1,9
~ 1,l~e 1,70=
f 1,(ß'"i:~ 1;0¡.
!Ap,10
1,40
is5-10
1,3o 10 ~ 20 30
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-,l-v-.l--v- ,l-~-~-V-~ ~ ~'WÄ-Ä.Ý..Á: Ý-Ä-Y-Ä-Ý-- i-v- ;-v- ;.v- ;.v-~ ~ ~ AA- ~~- ~A- V;. A- ~.. :.YI- :.'i- ,.,. .*l(*_..,~, .. ,~,........ ..
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Figure 10 - Ilustration of Slag Solidification IStart with only Liquid Slag, Composition = Cstar
ILiquidusTem, -
I
I'"..
Hi:e'"¡.
LiquidusK- Line
Compound
(ex. 2CaO' 8i02)
Chqud -
I
rFigure 12 - Liquidus Temperature (f 10% (Ali03)
rr2,(0
i,8 l, BlA
1.3'" 1,70..3 1.2..
1,60.. 1.0'"i:e 1,'"¡.
1,40
1,0 10 () 20 30
2-14
IUJ
J Figure 13 - Viscosity Verses BfA
7
At i,500C
~J 6
i~
5'õS-
f 4
;;3
2
0.7 0.8 0.9 1.0 1.1
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IFigure 15 - Sulfur Partition
IWhere (CaO) I (MgO) = 4; ISil = 0.8 %
60
50
1.00
40
BfA
1.10I ~re 30 1.05
I20
0.9510
3.0 5.03.5 4.0 4.5
~(SiO,) f (A1,0,)
Figure 17 - Alkali Recycling
In As Out inSolid Gas
K~O Reduction
K Condensation~.. + Si0i,llId + COi=;: KSi0.i,s,lId + CO
K¡OlllId + C =)02~..+CO
(KiO,)+ (CaO) + C ='"
~.. + (CaOSiO,) + CO
Figure 14 - Viscosity Vs Temperature
35
30A B
25~'õ.e 20
~ 150;i;; 10
0
1,31.
cLlauidus. BfA
A) 1,250 C - Low
B) 1,:345 C . Mlddl~
q 1.390 C - High
1,350 1,45 1,501,4Tempratu (C)
Figure 16 - HM Sulfur Prediction
Where (CaO) f (MgO) = 4; fSi) = 0,8 %; ST ~ 3 kgfHM; SlagVol ~ 200 kgflHM
BfA
0.950.08
0.0
0.06
f2 0,051.00
0.04 1.05
0.031.10
0.02
3.0 3.5 4.0 4.5 5.0
(SiO,lf(AI,O,l
Figure 18 - Alkali Capacity
4.0
3.5
;t 3.0
I 2.5.ia 2.0-¡~ 1.5
lJ 1.0
¡; 0.5
0.0
0.85
2-15
0.90 0.95 1.05 1.51.00 1.10
BIA
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LECTURE #3
BLAST FURNACE REACTIONS
Alex McLean
Deparent of Metallurgy and Materials ScienceUniversity of Toronto
Toronto, Ontao M5S 3E4Canada
Abstract :- Durng the latter half of this centuy, in parallel with developments
pertaining to greater productivity, there have been increasing demands for improved hotmeta quality. To a large extent, these demands have been met by advances in ourknowledge of the chemical, physical and thermal interactions between gas, solid andliquid phases that tae place within the fuace and during external treatment of hot
metal. In this lectue, the reactions discussed include those involving carbon and
oxygen, the reduction of iron and other oxides, the behaviour of alkalies and sulphur,and interactions with slag. The concept of optical basicity is described and examples arepresented of how it can be used to design slags with appropriate characteristics forspecific operations.
1-0 INTRODUCTION
Over one hundred years ago in 1890, Hemy Maron Howe, a distinguished steelmaker,an eminent professor of metallurgy and President of AIME, published his classic textentitled "The Metallurgy of Steel"(l). In spite of the time difference, much of thematerial contaned in this volume is stil worthy of study today. Howe had the great giftof being able to express in vivid terms, some of the basic truths of iron and steelmakngtechnology. An example of this may be found in his use of the term "The Treachery ofSteel", which he employed in a very graphic maner to discuss what we today wouldcall, "The Management of Quality":
"Owing to the very nature of the processes by which steel and wrought-iron aremade, carelessness and ignorance, whether in selecting materials, in conducting theprocesses, or examining the product, is likely to lead to the making and sellng oftreacherous steel, treacherous simply because it is unsuited to the purpose_ f~r
which it is sold."
3-1
Major technological changes have transformed the iron and steel industry during thepast centu. These changes have had a profound effect on process intensification,energy Utilization, metal yield and product quality. They include: high productivity blastfuaces, external treatment of hot metal, oxygen steelmakng, alternate ironmaking
processes, ultra high power arc fuaces, ladle metallurgy, vacuum processing,
continuous casting, thermo-mechanical processing and novel coating technologies.
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Coupled with the implementation of advanced production technologies there have beenever increasing demands for improved steel performance which in tur have stronglyinfuenced changes in steel chemistry and steel quality. For example, in 1911 at the timeof the launch of the Titanic, the steel plates used for construction of the hull met all ofthe required stadards. The ship was built by Harland and Wolff at their Belfastshipyard in Northern Ireland, Figue 1. The steel was manufactued at the Motherwellworks of David Colvile & Company in Scotland. This is the same company whichtwenty five years later provided steel for the construction of the world's largest
passenger liners, the Queen Mar and the Queen Elizabeth.
On Sunday April 14th, 1912 at 11.40 PM, the Titanic struck an iceberg and san twohours and forty minutes later, with a loss of over fifteen hundred men, women andchildren, Figure 2. Six years ago, metallurgists at the Metals Technology Laboratories,CANMET, in Ottwa, Canada, published a report on their investigation of a number ofcast iron, wrought iron and steel samples recovered from the Titanic wreck site, on thebed of the Atlantic, over 4,000 meters below the surace of the ocean. (2) Chemical
analysis of a section of hull plate, which was approximately 25 mi thick, Figure 3,indicated that the steel contained 0.2%C, 0.52%Mn, 0.025%Si, 0.065%S, O.OLO%P,o:O.005%Al and 0.004%N. From ths analysis, it is evident that the hull was constrctedfrom low carbon, semi-killed steel, produced by the open hear process. The highsulphur content is of paricular significance. The micro-strctue shown in Figure 4,indicates extensive carbon banding, typical of hot rolled O.2%C steel and moreimporttly, elongated in the rolling direction, long MnS stringers, some of which
exceed 25 mi in length. In Figure 5, the results of Chary tests performed on samplestaken in the longitudinal direction are compared with data for a semi-killed steel ofsimilar composition but with considerably lower sulphur content which would be typicalof steels produced in the early fifties. With a seawater temperature of approximatelyzero degrees Celsius, the hull plates had essentially no ductility.
A major thrst in curent steelmakng technology is the production of steel with lowerresidual concentrations of sulfu. This element has a profound infuence on the qualityof the final steel product because of the effects on mechanical properties. Today, high-quaity steels are produced for demanding applications such as Arctic pipelines, offshoreplatforms, ice-breaker vessels and ships for the transporttion of liquid natual gas.
These steels are produced with extremely low inclusion contents and the residualsulphur levels can be less than 10 ppm.
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THE RELATIVE STABILITY OF OXIDES
As iron oxide, coke and slag-makng materials move down through the stack of thefuace, several important exchange processes take place. Heat is removed from the
ascending fuace gases that consist mainly of carbon monoxide, carbon dioxide andnitrogen and transferred to the descending burden materials. Oxygen is removed fromthe descending iron oxides and transferred to the ascending reducing gases. Thus withinthis very efficient counter-curent reactor, chemical reactions take place as the chargedescends, the temperatue of the 'burden materials increases, fusion of the reduced iron,iron oxide and slag-makng materials begins and finally liquid metal and slag collect inthe hear of the fuace. Much of the coke charged to the fuace is bured with
oxygen in the hot air blast at the tuyeres to provide both heat and the reducing agentcarbon monoxide.
The relative stabilty of varous oxides is plotted against temperature in Figure 6 whichis adapted from Gaske1i3. This is known as an Ellngham Diagram and is extremelyuseful for understanding the behaviour of oxides in the blast furnace. The relativestability is measured in terms of the free energy of formation of the oxides. The greaterthe negative free energy of formation of the oxide, the greater is the oxide stability. Thismeans that oxides that are located in the upper part of the diagram have a relatively lowstabilty, while oxides located in the lower portion of the diagram have a high stabilty.Oxides located in the center of the diagram have a moderate stability.
. Oxides with a relatively low stability include potassium oxide, sodium oxide,
phosphorus oxide and iron oxide.
. Oxides with a moderate stability include manganese oxide, chromium oxide,
silica and titanum oxide.
. Oxides with a high stabilty include, alumina, magnesia and lime.
r-oo It is also useful to consider this diagram in terms of the affinity of an element foroxygen. For example, elements that are located at the top of the diagram have a lowaffnity for oxygen, while elements located towards the bottom of the diagram, have ahigh affinity for oxygen. Ths means that oxides at the top are relatively easy to reduce,while those at the bottom, are diffcult to reduce.
Ths is ilustrated by the line for the formation of phosphorus oxide which lies above theline for formation of iron oxide at temperatues corresponding to those found in thehearh of the blast fuace. This implies that phosphorus oxide has a lower stability than
iron oxide and consequently, since reducing conditions in the fuace are sufficient toreduce iron oxide, essentially all of the phosphorus entering the fuace wil end up inthe hot metal.
On the other hand, stable oxides such as alumina, magnesia and lime are not reducedunder blast fuace conditions, and end up in the slag phase. Oxides with a moderate
3-3
stability such as manganese oxide, chromium oxide, silca and titaum oxide areparially reduced to give some manganese, chromium, silcon and titaum dissolved inthe hot metal, while the remaining uneduced oxide constitutes par of the slag.
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The Ellngham Diagram is constructed on the basis that a pure element at unit activityreacts with one of mole of oxygen gas to form pure oxide at unt activity. Thethermodynamic term "activity" is a paricularly useful concept for discussing thebehavior of elements dissolved in molten iron, or oxides dissolved in molten slag. Forexample, when small concentrations of elements such as oxygen or sulphur aredissolved in molten steel, their activity can frequently be taken as equal to theirconcentration in weight percent. However, in the presence of high concentrations ofother elements, for example, carbon in hot metal, the activity of sulphur is greater thanthe concentration, while the activity of oxygen is less than the concentration. In suchcases it is importt to distinguish between activity and concentration.
· The concentration of a component in solution is a measure of how much of the
component is present.
· The activity of a component in solution is a measure of how the componentactually behaves.
All the lines on the Ellingham Diagram except those involving carbon, have a positiveslope, indicating that the oxide stability decreases with increasing temperatue. The linesfor the oxides of potasium oxide, sodium oxide, magnesia and lime, each show a sharincrease in slope at the temperatues corresponding to the boiling points of the
respective metals.
The line for the formation of carbon dioxide from carbon and oxygen has almost zeroslope indicating little change in stability with increasing temperatue, while that forcarbon monoxide has a strong negative slope which means that the stability of carbonmonoxide actually increases as the temperatue increases. The lines for the two oxidesof carbon cross at about 700 C. Above this temperatue, carbon monoxide is more stablethan carbon dioxide while at lower temperatues, carbon dioxide is more stable thancarbon monoxide.
CARON-OXYGEN REACTIONS
The pre-heated air blast injected through the tuyeres at a temperatue of about 1000 Cand two to three atmosphere's pressure, produces a pear shaped reaction zone in front of
each tuyere. The temperatue in this region is about 2000 C and rapid reaction firstoccurs between excess oxygen and coke to give carbon dioxide. This is an exothermicreaction.
C + O2 = CO2 (1)
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Immediately outside this zone, there is no longer free oxygen available and the carbondioxide reacts with excess coke to give carbon monoxide. This is known as theBoudouard reaction and is endothermic.
CO2 + C = 2CO (2)
Combining reactions 1 and 2 gives the reaction for partial combustion of carbon withoxygen to provide carbon monoxide.
2C + O2 = 2Ca (3)
The heat evolved in the formation of one mole of carbon dioxide is about three and onehalf times that for the formation of one mole of carbon monoxide and one measure ofthe efficiency of the blast fuace is the degree of conversion of carbon in the coke tocarbon dioxide.
Below 700 C, carbon dioxide is more stable than carbon monoxide and reaction 2proceeds to the left:
2CO = C + CO2 (4)
Ths reaction is often referred to as the carbon deposition reaction and wil be mentionedagain later.
F-'
Above 700 C, carbon monoxide is more stable than carbon dioxide and reaction 2proceeds to the right. Ths is sometimes called the carbon solution loss reaction and inthis sense implies a negative behavior. On the other hand the reaction represents aregeneration of reducing gas within regions of the fuace above 700 C. This is one ofthe important fuctions of coke within the blast fuace and is paricularly desirable asit increases the volume of the gases and helps in heat transfer. However this reaction isendothermic and when it occurs within the tuyere zone it creates a cooling effect withina location where high temperatues are important.
The effect of temperatue on the equilibrium reaction between coke and a gas mixtuecontaining carbon monoxide and carbon dioxide at one atmosphere pressure and alsothree atmospheres pressure, which is more typical of modem blast fuace practice, isshown in Figure 7. To the right of the graph, carbon monoxide is more stable thancarbon dioxide, while at lower temperatures, to the left of the graph, carbon dioxide ismore stable than carbon monoxide. From this figure it is evident that above 1000 C, thepercentage of carbon dioxide in equilibrium with coke is essentially zero . On the otherhand, at temperatures below 400 C, the concentration of carbon monoxide is smalL.
Thus as the temperatue decreases between 1000 and 400 C, the stability of carbonmonoxide decreases while the stability of carbon dioxide increases and the partialpressure of both gases in equilibrium with coke is significant.
3-5
THE CARON DEPOSITION REACTION
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The gases leavíng the top of the furnace are usually about 200 C and íf equílíbríum wasobtaíned wíth coke, the ratío of carbon monoxíde to carbon díoxíde would be about 10-5.In fact, the ratío ís usually between 1 and 3, Le. the gas ís very much more reducíngthan that predícted from equílbríum consíderatíons and full use ís not beíng made of thereducíng potentíal of the gas. Tils ímplíes that the coke rate ís ín excess of theoretícalrequírements.
Thís lack of equílbríum between the gases and coke can be attríbuted maínly to the ilghgas velocíty ín the stack. The gas retentíon tíme ín the fuace ís only about 10 seconds,and extremely hígh velocítíes can occur, parícularly ín loosely packed, coke ríchregíons. Another factor ís that the gas temperatue drops by about 1800 C as ít rísesthrough the fuace and so there ís líttle opportíty for equílbríum to be maíntaíned.
Sínce the carbon monoxíde content of the gas wíthín the stack of the blast fuace at
temperatues below 1000 C ís consíderably ilgher than ít should be, there exísts adrvíng force for the carbon deposítíon, or sootíng, reactíon to proceed. Thís dríving
force ís partícularly strong between 500 and 700 C. A gas with a temperature andcomposítíon above the líne ín Fígure 7 wíll tend to deposit carbon by reactíon 4, and onewíth a composítíon and temperatue below the líne wíll oxídíze carbon ín accordancewíth reaction 2. Fortately reactíon 4 ís sluggísh and equílbríum ís never attíned,
otherwíse seríous cloggíng of the spaces wíthín the burden at the top of stack could
occur. Ths ín tu could lead to írregular flow of the reducíng gases and uneven
descent of the burden. Even for paríal reaction, a suítable catalytíc surface ís required,upon whích the carbon can nucleate and grow. Iron parícles, partíal reduced íron oreand íron carbíde have all been suggested as possíble catalysts. The reactíon appears tobe enhanced by hydrogen and water vapor whíle nítrogen and sulphur compounds, forexample, amorua, hydrogen sulphíde and carbon dísulphíde act as ínhbítors. Zíncoxíde and alkalíne compounds oppose the ínhíbítíng effect of sulphur, and although theconcentratíon of these compounds ín the furnace ís generally small, they volatílze athígh temperatues ín the hearh and condense agaín ín the cooler regíons of the stack.The cumulatíve effect ís that such compounds can offset the ínfluence of sulphur.
The carbon deposíted by the reactíon ís ín a very finely dívíded form and some may beaccommodated wíthín the pores of the íron ore parícles and cared back down the stackagaín. Tils can affect the reductíon process ín several ways.
,-
. Because of the actíve natue of the carbon and íts close assocíatíon wíth the ore,
reductíon by solíd carbon can take place at lower temperatue than that requíred for
reductíon by coke, parícularly sínce coke canot penetrate the pores and reductíon
can only take place at poínts of contact between the solíd parícles. The rate of suchreductíon wíl depend upon the rate of díffusíon of oxygen from the ínteríor of of theparícle to the poínt of contact. In the upper par of the fuace, the reductíon bycoke is neglígíble, compared wíth gaseous reductíon. It becomes sígníficant onlyabove about 1000 degrees C. when the gaseous reactíons are ímpeded by slag
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formation. In contrast, reduction by precipitated carbon may occur at temperatuesas low as 800 C.
. The formation of carbon monoxide during reaction within the pores tends to open updeep fissures within the paricle, thus increasing the gas-solid contact area, and
increasing the efficiency of gaseous reduction.
. When carbon dioxide is produced within the pores of a paricle by the gaseousreduction reaction, it can be rapidly regenerated to carbon monoxide by reactionwith the carbon in the pores, thus allowing the reaction to continue.
Unfortately, the carbon deposition reaction can also have certain adverse effects.
. The reaction can cause splitting of refractories by deposition on active iron spots, inregions where the temperatue is about 500-550 C., for example in the outer shells atlower levels in the stack, or within the inner shells at the upper levels.
. If excessive, carbon deposition can cause ore pellets or sinter to cruble into powderand this can cause irregular gas flow and uneven descent of the burden.
. Since the reaction is exothermic, the temperature of the exit gases is increased.
Although the overall effect of the carbon deposition reaction may be debatable, certainfacts remain.
. The reaction does decrease the CO/C02 ratio of the exit gases.
. The reaction recirculates a certain amount of carbon, which otherwise would be
cared out of the fuace, thus increasing the time available for reaction with carbon
and increasing the chemical effciency of the reduction process.
REDUCTION OF IRON OXIDES
The reduction of iron oxides by carbon monoxide can be represented by the followingreactions:
3Fe203 + CO = 2Fe304 + C02 (5)
Fe304 + CO = 3FeO + CO2 (6)
FeO + CO = Fe + CO2 (7)
These reactions are accomplished at increasingly higher temperatues and as shown inFigue 8, with increasingly greater percentages of carbon monoxide. Ths means th~treactions 5 and 6, which are relatively easy to achieve, can take place within the upper
3-7
regions of the fuace. Reaction 7 which entails the removal of the last amount of
oxygen from the iron, is in fact the most diffcult to achieve and therefore takes placefuher down the fuace where the temperatues are higher and the carbon monoxide
content of the reducing gases is greater. Below 570 C, the non-stoichiometric wustitephase (FexO) is unstable and it is possible to reduce magnetite directly to iron.
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FeO + C = Fe + CO (8)
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At any paricular temperature, there is a minimum carbon monoxide content in the gasmixtue required for reduction of a specific oxide. This means that it is not possible forall the carbon monoxide in the gases to be converted to carbon dioxide if the reductionreactions are to continue. For example, at 800 C the equilibrium gas mixtue in contactwith FeO and solid iron contains about 65%CO and 35%C02. If the CO2 content of thegases exceeds this value at this temperature, iron wil tend to be oxidized back to FeO.Accordingly, for these reactions to occur, there must be a minimum concentration ofcarbon monoxide in the gases at each step as indicated in Figure 8, and it is not possibleto convert CO completely to CO2 by these reactions. Fortately at these temperatures
the carbon dioxide produced by the reduction reactions is unstable in the presence ofcoke and carbon monoxide is regenerated based on reaction 2 so that the reductionreactions can continue.
It is wort noting that the combination of reaction 2 with reaction 7 corresponds to the"direct" reduction ofFeO by carbon and this is a strongly endothermic reaction:
The reduction of iron oxides may also take place by hydrogen which is generated byparial combustion of auxiliar fuels injected through the tuyeres to produce two
reducing gases, carbon monoxide and hydrogen. Hydrogen is also produced whensteam is added to the blast as an aid in controllng the fuace. Excellent discussions ontuyere additives and their effects on blast furnace operation are presented in the twochapters" Blast Furace Energy Balance and Recovery", and "Fuel Injection in the BlastFurace" .
Whle the oxidation of carbon by oxygen in the air-blast to form carbon monoxide isexothermic, the reduction of moistue by coke to form carbon monoxide and hydrogen isstrongly endothermic:
H20 + C = CO + H2 (9)
The reduction of iron oxides by hydrogen again proceeds in a sequential maner:
3Fe203 + H2 = 2Fe304 + H20
FeO + H2 = Fe + H20
(10)
(11)
(12)
Fe304 + H2 = 3FeO + H20
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The effect of temperatue on these reaction equilbria is shown in Figue 9. Whlereaction 10 is slightly exothermic reactions 11 and 12 are endothermic. The presence ofhydrogen, which because of its small size has a high diffsivity, markedly reduces thedensity and viscosity of the blast fuace gases and, paricularly at high temperatures,enhances the reduction of low reducibility raw materials.
The water gas shift reaction can take place between the different components in the gasphase to bring the hydrogen-bearing and carbon-bearing gases into equilibrium:
C02 + H2 = H20 + CO (13)
It wil be evident from Figure 8, that the gases passing up the furnace canot be inequilibrium with carbon in the coke and at the same time in equilbrium with iron oxidesin the descending burden. Above about 800 C the reaction of the gases with carbon ismore rapid than with oxides and the equilibrium between coke and the gas phase isprobably approached fairly closely. As shown in Figue 10, measurements of thetemperatues and compositions of gases in operating fuaces indicate that they tend tofall between the CO/C02-C line and the FeO/Fe line above 800 C, cut the FeO/Fe linebetween 600 and 800 C and then remain at or just above the Fe30JFe line. Attemperatues below 600 C, the very rapid gas flow allows little time for reaction withsolids and the CO content of the gas is far in excess of that which would be inequilibrium with coke.
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If the iron oxide is chemically associated with other oxides, its activity in the blastfuace will be decreased. This means the iron oxide will be more difficult to reduceand the CO/C02 ratios required will be greater than those considered here. For examplewith ferrous silicate, the minimum CO/C02 ratio required for reduction at 700 C wouldbe increased from about 1.5 to about 22, i.e. from about 60%CO to almost 96%CO on acarbonaceous gas basis. Since combined oxides are more diffcult to reduce, highertemperatues are required for reduction and thus the amount of reduction obtained withCO before slag formation occurs will be decreased. This implies an increase in cokerate since the amount of reduction required in the lower par of the fuace wil be
increased.
REACTIONS IN THE BOSH AND HEARTH
Reduction of Other Oxides
The reduction of oxides more stable than iron oxide such as manganese oxide and silicawould not take place in the blast fuace if the products were pure metals since the
reaction:MnO + CO = Mn + CO2 (14)
would have, at equilibrium, a percentage of CO very close to 100 percent. That is, theeffciency of reduction is extremely low and enormous quantities of gas would berequired for very small amounts of manganese reduced. The situation with silca is even
3-9
more extreme since it is a very stable oxide. However, by dissolving the manganese andsilicon in iron, the reactions:
Si02 + 2CO = Si (dissolved in iron) + 2C02 (16)
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MnO + co = Mn (dissolved in iron) + CO2 (15)
and
are moved somewhat to the right so that there is a distribution of manganese and siliconbetween metal and slag which is a fuction of the slag composition and of the
temperatue. Since the reduction of both of these elements is endothermic, the amountof each in the hot metal increases with temperatue and the extent of the reactions wil tosome degree be controlled by controlling the temperature in the hearh of the fuace.
Of greater importance is the fact that the CO2 produced by these reactions will react bythe Boudouard reaction and wil cause an increase in the coke consumption.
The amount of manganese reduced clearly also depends on the amount in the chargedore. Ores such as Wabush from the Labrador trough with up to 2 percent manganesegive much higher than normal manganese contents in hot metal with consequent highercoke rates per tonne of iron produced. Silicon "swings" caused by erratic burdening ofthe fuace or by temperatue variations can also have another serious effect: as thesilicon is reduced into the hot metal it is depleted from the slag, increasing the basicityratio and changing the melting point and fluidity of the slag sometimes dramatically.
Effects of Silcon Monoxide Formation
For many years it was considered that silca and manganese oxide were reduced directlyfrom the slag by reaction with carbon in iron according to the reactions:
Si02(slag) + 2C = Si + 2CO(g) (17)
MnO(slag) + C = Mn + CO(g) (18)
It was thought that molten iron droplets picked up silicon as they passed through theslag phase and on into the hearh. Research however, has shed new light on these
reactions and also those involving sulphur..(4) Several laboratory studies together withplant data from Japan have shown that at the temperature of the combustion zone, about2000C, silicon monoxide gas is produced during the combustion of coke by the reaction:
Si02(coke ash) + CO = SiO(gas) + C02 (19)
Combining Eq. (19) with the reaction for coke oxidation:
cO2 + C(coke) = 2CO (2)
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yields the overall reaction:
Si02(coke ash) + C(coke) = SiD(gas) + CO (20)
While the presence of FeD in slag is likely to make SiO formation from slag verydiffcult, an additional source of silica would be reduced silica-rich slag adhering tocoke paricles. Following these reactions, silcon is transferred to iron droplets byreaction with silicon monoxide in the gas phase:
SiO(gas) + C = Si + CO (21)
As iron droplets containing silicon pass through the slag layer, some of the silicon isoxidized by iron oxide and manganese oxide, and taken up by the slag:
2(FeO) slag + Si = (Si02) slag + 2Fe (22)
(23)2 (MnO) slag + Si = (Si02) slag + 2Mn
Behaviour of Sulphur
Sulphur is a troublesome element in blast fuace operations because hot metal for
steelmakng must be low in sulphur; levels of 0.035 to 0.02% are usuaL. The reactionby which sulphur is removed from liquid iron into the slag is often represented by thereaction:
~ + (CaO) + C = (CaS) + CO(g) (24)
Where sulphur and carbon in the metal react with lime dissolved in the slag to formcalcium sulphide in the slag and CO gas. The distribution of sulphur between slag andmetal, (S) /~, is strongly infuenced by a number of factors:
r . Increasing the basicity of the slag (lime/silica ratio) tends to raise thethermodynamic activity of lime in the slag which pushes reaction (24) to the right.
. An increased oxygen potential in the system pushes the reaction to the left. This isshown by rewriting the reaction as follows:
~ + (CaO) = (CaS) + 0 (25)
This effect is very strong, and the presence of even small concentrations of FeO Inthe slag wil seriously limit the sulphur ratio. (S) /~.
. Fortately both silicon and carbon raise the thermodynamic activity of sulphur in
hot metal by 5 to 7 times. Accordingly, sulphur in hot metal is 5 to 7 times easier toremove than it would be from liquid steel that contains relatively little carbon andsilicon.
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CaS (coke ash) + CO = CaO + CS (gas) (27)
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Assuming sulphur in coke ash is present as CaS, the following reaction can occur withSiO in the combustion zone to form volatile SiS:
CaS (coke ash) + SiO (gas) = CaO + SiS (gas) (26)
To a lesser extent, some CS gas may form by the reaction:
Sulphur transfer from these volatile species to molten iron droplets then takes placewithin the bosh zone. Turkdogan has shown that when iron droplets containing siliconand sulphur are allowed to fall through molten slag, in the absence of MoO, the siliconcontent of the metal actually increases, and there is no transfer of sulphur. (5) In thepresence of MoO, silicon is removed from the metal by reaction (23) and manganesetransfers from slag to metal together with sulphur transfer from metal to slag. Based onthe various results available, Turkdogan suggests the following sequence of reactions inthe bosh and hearth:
~
. The formation of SiO and SiS in the combustion zone.
. The transfer of silcon and sulphur to metal and slag droplets in the bosh.
. The oxidation of silcon by FeO and MoO in the slag as the iron droplets passthough the slag layer.
. The desulphurzation of metal droplets as they pass through the slag layer.
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The sulphur distribution ratios found in the blast fuace generally var between 20 and120. On the other hand experiments have shown that when metal and slag samples fromthe blast fuace are remelted in graphite crucibles at 1 atm CO, the distribution ratioincreases to between 120 and 220, depending on the slag basicity. This suggests that theoxygen potential of the system is higher than might be expected for C-CO equilibrium inthe fuace hearh. Thus while thermodynamic conditions favour sulphur removal from
hot metal within the blast fuace, kinetic considerations imply that the reaction can be
more readily accomplished outside the furnace by external desulphurzation.
Alkalies and Zinc
Sodium, potassium and zinc, often called the "rogue elements", can cause seriousoperating problems in the blast fuace and must be monitored and carefully controlledif stable conditions are to be maintained. The alkali metals enter the blast fuace asconstituents of the gangue in the ore and also as a part of the coke ash, generally as
silicates. In the stack ofthe fuace, the silicates react by the formulas:
KzSi03 + CO = 2K + SiOz + COz (28)
(29)NazSi03 + CO = 2Na + SiOz + COz
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In the blast fuace, the potassium reaction can take place above 500 C. While the
sodium reaction occurs at about 600 C. At temperatues of about 900 C, the alkalimetals are above their boilng point so they join the gas phase. However, as these gasessta to rise up the fuace, the metal becomes unstable with respect to other compounds
that can form and cyandes, oxides and carbonates all sta to precipitate from the gasphase as very fine fues or mists, since the cyanides are liquid over a wide temperatuerange. These fine paricles of solid and liquid can deposit on the iron ore paricles, thecoke, and the fuace wall, with some, of course, being swept out with the fuace gasand being captued in the dust catching system. Paricularly the liquid alkali compoundscan penetrate the brick lining of the fuace and cause serious deterioration and spalling.As well, these compounds can build up on the wall and cause scaffolding, hanging andslipping.
The alkalies which land on the iron and coke are cared to the lower par of the furnace.
There, they are again reduced to the metal which rises up the stack as a gas, forms thesame alkali compounds, and repeats the cycle, joining new material in the process. Thereduction requires carbon, increasing the coke rate and cooling the fuace, and therecycling material can build up to the point where it degrades the coke in the fuace,causing it to break into small pieces and increasing the reactivity of the coke to C02.This increased reactivity can again reduce the temperatue of the furnace and decreasethe heat efficiency of the whole system. The high concentration of alkalies in thefuace also effects the strength and reduction characteristics of the iron bearngmaterials, causing dramatic swellng and catalyzing carbon deposition on the pellets.These deleterious reactions with both the coke and the ore can have serious impacts onthe gas permeabilty in the fuace and on the stability of the blast fuace operation.
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Fortately, the alkali oxides are very basic oxides and can be fluxed with SiOi in acid
slags and removed from the furnace. Generally, decreasing the slag basicity can carincreasing amounts of alkali away in the slag. This is in direct contrast to sulphurremoval, where increasing the slag basicity increases the sulphur removaL. When mostde sulphurizing took place in the blast fuace, there was a confict between theattinment of low sulphur and removal of alkalies and the basicity of the fuace wascarefully controlled to balance both problems. With external desulphurization, this is nolonger a problem and the fuace can generally be burdened to minimize alkali attck.
Zinc normally originates in steelmakng off-gas dust from furnaces using galvanzedscrap which in some fashion has been recycled to the blast fuace. Occasionally, the
zinc content of iron ores or coal ash may also be a signficant source. Behaving notunlike sodium, zinc is reduced from the oxide or ferrte at about 600 C, forms a vapourthat subsequently forms oxides or carbonates that can react with the sidewalls or hecaried down the furnace on coke or ore to be reduced and fuher cycled, consumingcoke at each tu. Zinc that escapes as a fue in the gas stream enters the blast fuacefilter cake, makng it unsuitable to recycle if present in a high enough percentage.Unlike the alkalies, zinc is not captued to any extent in the slag and can only effectively
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be removed by decreasing the input and allowing the recycling vapour to slowly leavevia the gas phase.
Clearly, the best protection against alkali metals and zinc is to ensure that the absoluteminimum are par of the blast fuace feed. Because of the tendencies of these elementsto circulate in the fuace, they are unseen and unown consumers of coke and causerefractory, ore and coke problems. Unfortately, the symptoms of the problem are notalways evident until the problem is of fairly major proportions and then requires fairlydraconian measures, such as eliminating certin feed materials, to effect a solution.
Titanium and Lead
Lead is seldom a problem in blast fuaces but occasionally enough can enter a blastfuace through the ore or sinter to cause a problem. Lead is very easily reduced in theiron blast fuace and falls to the bottom of the hearh which normally has a chilled hot
meta11ayer which protects the hearh refractories. Lead has virtally no solubility in thehot metal so it forms a low melting point liquid pool on which the hot metal floats, andthus promotes more rapid hearh attack. In certin fuaces where this problem is
known to occur, a second tap-hole, deeper than the iron notch, can be used toperiodically tap the lead.
Titanum is an even more stable oxide than silica but in the blast fuace it can formextremely stable carbides and nitrides. These titanium compounds, if present in smallquantities can be effective in forming a light protective layer on the hearh surfaces andprolonging hearh life. For this reason, especially in Japan, titaiferrous ores are addedjudiciously to sinter mixes. However, at high concentrations, these same compoundscan stiffen the slag while building up a heavy hearh layer, reducing the hearh capacityof the fuace. As with zinc, the best solution is to reduce the input and slowlyeliminate the titaum from the fuace.
CORRLATION OF SULPHIDE AND ALKALI CAPACITIES WITH OPTICALBASICITY OF BLAST FURNACE SLAGS
Optical basicity is a relatively new concept which provides a good foundation for abetter understading of the behaviour of molten slags than the conventional basicityratios. The simple (CaO/SiOi) ratio ignores the effects of other oxides and, as indicatedin the chapter devoted to blast fuace slags, the relationship (CaO + MgO/ Ah03 +SiOi) implies that lime and magnesia behave as equivalent basic oxides and thatalumina and silica have the same degree of acidity, neither of which is the case.
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The concept of optical basicity was developed by glass scientists(6) and introduced to themetallurgical community by Sommervile and co-workers in the late seventies. Thisapproach has proved to be a valuable tool for designing slags or fluxes which wil havethe required characteristics in terms of, for example, sulphide capacity, phosphoruscapacity, magnesia capacity and even viscosity. (7-1i). Details of the method used tocalculate the optical basicity of molten slags are provided in the appendix.
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The relationships between the composition of lime-silica and lime-alumina slags withrespect to optical basicity are shown in Figure 11. From this diagram it is clear thatlime-alumina slags have a greater basicity than lime-silica slags and therefore would beexpected to have a higher sulphide capacity. As can be seen from Figue 12, wheresulphide capacity data for several slags systems are plotted against the mole percent ofbasic oxide, this is indeed the case. This type of information is often plotted againstthe lime-silca ratio. With either method of plotting, there is a separate line for each slagsystem.
On the other hand, if the sulphide capacity of slags is plotted against the optical basicity,the behaviour can be represented by a single line, Figure 13. This is because the opticalbasicity parameter is a more fudamental measure of the slag behavior.
Figue i 4, shows lines of iso-sulphide capacity for lime-alumina-silca slags at 1600 Ci.e. 2910 F. From this diagram it can be deduced that a binary slag consisting of 50percent lime and 50 percent silica has a sulphide capacity of approximately 5xl0-4, Onthe other hand, a binar slag consisting of 50 percent lime and 50 percent alumina, has asulphide capacity of approximately 8xlO-3, which is sixteen times greater than thesulphide capacity of the equivalent lime-silica slag. Thus, a lime-alumina slag is a moreeffective desulphurzing agent than a lime-silica slag. However, to enhance alkaliabsorption, a lime-silica slag would be more effective than a lime-alumina slag. Thsaspect is discussed in more detail below.
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As discussed previously, the formation of volatile species associated with sodium andpotassium have adverse effects on the fuace operation due to refractory attack,generation of fines, accretion formation and decreased burden permeability. Problemsof this type are accentuated when fuaces operate with higher driving rates, increasedflame temperatues, lower slag volumes and relatively high basicities. Ourunderstanding of these phenonmena has been greatly enhanced both by laboratorystudies and results from plant operations. Major contributions to this field have beenmade by W-K. Lu and his co-workers.(13,14)
Figure 15 and Figue 16 show the relation between the KiO solubility in slags of blastfuace composition and slag basicity defined in terms of the (CaO/SiOi) ratio andoptical basicity, respectively. The data were derived from equilibrium experiments onthe KiO solubility in slags of blast fuace composition caried out at 1500° C byKarsrudYS), In these Figures, there are two data points representing slags with a high
basicity containing 50% CaO, 49% Ah03 and 0.35-0.40% SiOi. With slags of thesecompositions it is clearly not appropriate to use the ratio of CaO to SiOi to characterizethe basicity. Using the ratio of CaO to Ah03 to express basicity is possible, however itis really not accurate to assume that Ah03 is equivalent to SiOi in terms of acidicbehaviour. Comparing Figure 15 with Figure 16, it is evident that the optical basicityapproach provides a more reasonable expression of slag basicity than the simpleCaO/SiOi ratio.
3-15
From the experiments conducted by Karsrud. (IS) it is possible to calculate the K20capacity of the slag, defined as follows:
aK - K,OI (p~.p~:) (31)
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12K(g)+-02 (g) = KiO(slag )
2 (30)
C _(Wt%K20)_~KiO (i 05) +PK'PO~ j KiO
(32)
As shown in Figure 17, the alkali capacity decreases, with increasing optical basicity.This behaviour can be represented by the following equation which is valid for a slagtemperatue of 1500 C:
Log CK20 = -11.57 A + 13.43 (33)
Included in Figure 17, is a line showing the dependence of sulphide capacity on slag
optical basicity obtained from the work of Sosinsky and Sommervile (7). In contrast withthe behaviour of alkalies, the sulphide capacity increases, with increasing opticalbasicity:
Log Cs2- = 12.60 A - 12.30 (34)
It will be evident from this discussion, that with the optical basicity model, an optimumslag composition can be designed in order to meet paricular operating requirements in
terms of alkali removal and/or sulphur removaL. It should also be noted, that since thestability of oxides increase with decreasing temperatue, operating at a lowertemperatue rather than at a higher temperatue, will improve the recovery of alkalioxides within the slag phase.
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CONCLUDING COMMENT
In his 1987 Extractive Metallurgy Lectue, Professor Julian Szekely reviewed the stateof extractive metallurgy and its important place within the national economy.(16) In this
excellent paper, he emphasized the fact:
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"Both process optimization and process control require a quantitativerepresentation of the process."
He also stressed the concept:
"Calculations and measurements are not alternatives, but most often must bepursued in a complementary fashion. "
Professor Szekely went on to say:
"The main barrier to the implementation of these concepts tends to be thenonavailabilty of suitably trained personneL."
The continued existence of this Blast Furace Course at McMaster University
represents a major contribution to the training of individuals equipped with the
knowledge and understanding of the importance of measurements and process modelsboth of which are essential for the control and optimization of ironmaking operations.
ACKNOWLEDGEMENTS
Acknowledgements are due to Professor T.R Meadowcroft, Deparment of Metals andMaterials Engineering, University of British Columbia who presented the lecture onBlast Furnace Reactions at the 1998 Ironmakng Course. In the present lectue, thesections: "Reduction of Other Oxides", "Alkalies and Zinc", as well as "Titaum andLead" have been reproduced in their entirety from Professor Meadowcroft's 1998lectue. In addition, some material originally prepared by the late Professor J.F. Elliottfor the lecture on "Principles of the Iron Blast Furace" when ths course was firstoffered has been incorporated within the text of this chapter.
REFERENCES
I (1) H.M.Howe, "The Metallurgy of Steel," The Scientific Publishing Company, NewYork, 1890.
(2) RJ.Brigham and Y.A.Lafreniere, "Titaic Specimens," Metals Technology Labor-atories, CANMET, Ottwa, Canada, Report No.92-32 (TR), 14 pages.
(3) D.RGaskell, "Introduction to Metallurgical Thermodynamics," 2nd ed.,Hemisphere Publishing, New York, 1981.
(4) W-K.Lu, "Silicon in the Blast Furace and Basic Oxygen Furace," Iron andSteelmaker, VoL. 6, No. 12, 1979, p.19.
(5) E.T.Turkdogan, "Blast Furace Reactions," Met. Trans B, Vol.9B, 1978, p.163.(6) J.A.Duffy and M.D.Ingram: "Establishment of
an Optical Scale for Lew's Basicityin Inorganc Oxyacids, Molten Salts and Glass," J. American Chemical Society,December 1971, pp. 6448-6454.
(7) D.J.Sosinsky and LD.Sommervile, "The Composition and TemperatueDependence of the Sulphide Capacity of Metallurgical Slags," Met. Trans. B,Vol.17B, 1986, pp.331-337.
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Sul!l!ested bv Professor T.R. Meadowcroft
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(8) D.J.Sosinsky, I.n.Sommervile and A.McLean, "Sulphide, Phosphate, Carbonateand Water Capacities of Metallurgical Slags," Fifth International Iron and SteelCongress. Process Technology Proc., ISS-AIME, Vol.6, 1986, pp.697-703.
(9) A. Bergman, "Some Aspects on MgO Solubility in Complex Slags," SteelResearch, Vol.60, No.5, 1989, pp. 191-195.
(10) I.D.Sommervile, "Optical Basicity as a Control Parameter for MetalurgicalSlags," Advanced Materials-Application of Mineral and Metallurgical ProcessingPrinciples, SME-AIME, 1990, pp.147-159.
(11) R.W.Young, J.A.Duffy, G.J.Hassall and Z.Xu, "Use of Optical Basicity Conceptfor Determining Phosphorus and Sulphur Slag-Metal Paritions," Ironmaking &Steelmaking, VoL. 19, No.3, 1992, pp. 201-219.
(12) Y.Yang, A.R.McKague, I.D.Sommervile and A.McLean, "Phosphate andSulphide Capacities of CaO-CaCh-CaF2 Slags," Canadian MetallurgicalQuarerly, VoL. 36, No.5, 1997, pp. 347-354.
(13) W-K.Lu, "Fundamentals of Alkali-Containing Compounds," Proceedings ofSymposium on Alkalies in the Blast Furnace, McMaster University, 1973, pp. 2-1to 2-18.
(14) W-K.Lu, and J.E.Holditch, "Alkali Control in the Blast Furace: Theory andPractice," Blast Furace Conference Proceedings, ArIes, France, June, 1980.
(15) K.Karsrud, "Alkali Capacities of Synthetic Blast Furace Slags at 1500°C,"Scandinavian Joural of Metallurgy, VoU3, 1984, pp. 98-106.
(16) J.Szekely, "The Mathematical Modeling Revolution in Extractive Metallurgy,"Met. Trans B, VoL. 1 9B, 1988, pp.525-540.
ADDITIONAL SOURCES OF INFORMTION
1. Ellott, J.F., Gleiser, M., Ramakshna, V., "Thermochemistry for Steelmakng,"
Volumes 1 and 11, Addison-Wesley Publishing Co, Reading, Mass. U. S. A.,1963, now out of print but many copies in various steel companes. Stil anexcellent source of data in very comprehensible form.
2. Thompson, W.T.*, Pelton, A.D.o, Bale, C. W.o, "Facility for the Analysis ofChemical Thermodynamics," (The FACT System), Interactive ComputerSoftare available through the authors at *Royal Milita College, Kingston,
Ont., and ° Ecole Polytechnque, Montreal, Quebec.
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3. "HSC Chemistry," vers 1. 10, Outokumpu research Oy, Pori, Finland, a veryeasy to use softare package for PC's with an excellent data base for iron andsteelmaking.
4. Stadish, N., Lu, W-K., "Alkalies in the Blast Furace," Proceedings of the 1973
McMaster Symposium, stil the best collection of aricles on this subject.
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APPENDIX
CALCULATION OF OPTICAL BASICITY OF BLAST FURNACE SLAG
Optical basicity of the molten slag is calculated using the following equation:
n
A = " A ,N,L. i Ii=1
(1)
N, = XinOii n¿XinOii=1
(2)
Here A: Optical basicity of the slagAi: Optical basicity value of component "i "N¡: Compositional fractionXi: Mole fraction of component "i " in the slaglli: Number of oxygen atoms in component "i"
Optical basicity of the slag is calculated by the following procedure:
1) Select the optical basicity value Ai for each component of the slag. The opticalbasicity values for several oxides are given in Table 1.
Table 1. Optical Basicity ValDes of Varioos Oxides (6)
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Oxides K20 Na20 CaO FeO MgO MnO Ah03 Si02 Ti02
Ai 1.40 1.15 1.00 0.51 0.78 0.69 0.61 0.48 0.61
2) Calculate compositional fraction N¡ using equation (2).
3) Calculate the compositional optical basicity value for each component of the slagusing the relation: Ai Ni.
4) Calculate the sum of the compositional optical basicity values using equation (1).
An example of the calculation process is outlined in Table 2.
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Table 2. Calculation of Slag Optical Basicity
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Items CaO MgO Ali03 Si02 SumOptical basicity of oxide, Ai 1.00 0.78 0.61 0.48 -
Number of oxygen atoms, 1 1 3 2 -
IliMolecular wt. of oxide, M¡ 56 40 102 60 -
Composition, wt% i 47.52 2.90 12.07 37.51 100
Number of moles, 0.8486 0.0720 0.1183 0.6252 1.6641n¡ = (wt% i)/Mi
Mole fraction, Xi=n¡ /¿ni 0.5099 0.0432 0.0711 0.3757 1.0000Compositional parameter, 0.5099 0.0432 0.2133 0.7514 1.5178X¡.IliCompositional fraction, 0.3359 0.0285 0.1405 0.4951 1.0000N¡= X¡Il¡/¿X¡noi
Compositional optical 0.3359 0.0222 0.0857 0.2376 0.6814basicity, Ai N¡
Example: l1aO = (wt% CaO)/Mcao= 47.52/56 = 0.8486
XCaO = l1aol(l1ao+nMgo+nA103+ns¡02) = 0.8486/1.664 = 0.5099
XcaO.Il in CaO = CaO mole fraction. the number of oxygen atoms in CaO
= 0.5099xl = 0.5099
XS¡02.nOinSi02 = 0.3757x2 = 0.7514
NCao= 0.5099/1.5178 = 0.3359
NSi02 = 0.7514/1.5178 = 0.4951
Acao .Ncao =lx0.3359 = 0.3359
ASi02 NSi02 = 0.48x0.4951 = 0.2376
A = Ncao. ACao + NMgO.AMgO + NAl203.AAl203 + NSi02. ASio2
= 0.3359xl + 0.0285xO.78 + 0.1405xO.61 + 0.4951x0.48
= 0.3359+0.0222+0.0857+0.2376
= 0.6814
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Fig. 1 The Titanic prior to departure on her maiden voyage.
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Fig. 2 The sinking of the Titanic in the North Atlantic offNewfoundland as depicted by artist W. Stoewer.
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Fig. 3Sample of Titanic hull plateshowing location and orient-ation of test specimens. (2)
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Fig. 4Microstructure of section from thehull plate showing carbon bandingand MnS stringers elongated in therollng direction. (2)
80,
!: 50~(! ¡a: :W 40'"Z IW '.. 30r)- !g 20~W '~ 10r
- 0,18% C
- 0,54% Mn
- 0,07% Si
'ã 70 r Mild steel / Acier doux(ljo 60 ~3, i
("-
~itaniC0'-40 -20 o 20
TEMPERATURE ( C)40 60
Fig. 5 A comparison of the results of Charpy tests onspecimens of plate steel from the Titanic witha similar steel produced in the early 1950'sY)
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Hz/HzOratia
CO/COz ralio10-5
0"
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- .10' 10' 10" 10" 10-40
7 70
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4: -,0." ~.... v 0 ifeO1- ¿ -/I ife" C+Oz' COzI~.. ~--~?~~V 7__ __ ;r.'!04.. V .o~. ~c ' V mo \f.:.~ Ai.. /v 7..¡-,¿.. .. 1
~ ~~ ~~ /~V m~~~~~~rui ß.; ..~c~~",I\o __,," ..~ J:: i/"..~ / V./ ~~ illI\ · ...- .. I.. ~~ ,I i..c¡'~ ", /.. ~ .~ ..\o~.."/ c¡\ ~.. ~ ') .- / ..:; ;. ~/ i
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'l"'~"'V V ? m-.. °- "" it 01. 'lCO'l"'~ 0;: em melting point of metal Ii. 'lC~V l! b boiling poini of metol
V .M melting poinl of oxideI
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J-20
~-30
I-400
o 200 400 Temperoture,.CPaz (atm) 10.100
CO/COz rolio
Hz 1HzO rolio
Z
10310
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-600 10410'
-50 103104
I -700
I 106-800
O.
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~ -1000
r-1100
-1200
Fig.6 Ellngham Diagram (adapted from Gaskeli3) for the effect of temperatureon the standard free energy of formation of oxides, including thenomographic scales produced by Richardson. The diagram has been
modified to include the behaviour of phosphorus, potassium and sodium. ~
3-23
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~100 20 300 .c, 50 800 700 10 10 1000 1100 12001:, 140
Teil"re (deg C~)
Fig. 7 The effect of temperature on the CO content of aCO/C02 gas mixture in equilbrium with carbonfor total pressures of 1 and 3 atmospheres.
B
T"-
is
II:2
:Iou~'
~g
2
(:20fife
&0 80 io 12 14T emprëlure .C
40
Fig. 8 The effect of temperature on the CO and CO2contents of gas mixtures in equilbrium withcarbon and various iron oxides.
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~ 40 eo eo 100 IZO 1400lemperature .C
Fig. 9 The effect of temperature on the H2 and H20contents of gas mixtures in equilbriumwith various iron oxides.
100tI \ \ Japanese
80 y\ GermanN0
60uø.+0 40 -u ~-
ø.- 20..0uø.
0200
rFeO
J ..J ..-Fei04 /
/,,/.. ,
.. ...-.. .... ---400600 800
Temperature, °C1000 1200
Fig.l0 Actual gas compositions at various temperatures
based on samples taken from operating furnaces,
in comparison with the conditions for equilbriumeither with carbon or with different iron oxides.
3-25
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Fig. 11 Optical basicity values for lime-silca and lime-alumina slag systems.
3-26
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Fig. 12 The effect of basic oxide content on the sulphide capacity of different ,slags at 1500C.
3-27
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. CaO-AI203
. CaO-Si02o CaO-Ali03 -SiOi
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.$ .60 ;6 .70 .15 .eo" . Oøtical Baicity
.85~n
Fig. 13 The relationship between the sulphide capacities ofdifferent slag systems and optical basicity at 1500C
3-28
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Fig. 14 Iso-sulphide capacity lines derived from optical, basicity calculations for liquid slags in the
lime-alumina-silca system at 1600C.
3-29
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~Equilbrium
7 T= 1500 °c- 6?F~- 5 (ò..~r:Q) 4..r:0() 30N~ 2
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r
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Fig.15 Dependence of K20 solubility in SF slag on CaO/Si02 ratio at 1500°C.
8
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~8EquilibriumT = 1500 °c-
?F 6~- 5 ((~r:Q) 4..r:0() 30
N~ 2
1
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0.64 0.66 0.68 0.70 0.72 0.74 0.76 0.78
Optical basicityFig.16, Dependence of K20 solubility in SF slag on optical basicity at 1500oC.
3-30
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Fig. 17 The alkali and sulphide capacities of slagsat 1500C as a function of optical basicity.
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LECTUR #4
BLAST FURACE ENERGYBALCE AN RECOVERY
RULES OF THUMB AN OTHERUSEFUL INFORMTION
John W. BusserSupervisor
Ironmking SystemSteleo Ine.
Haml ton, Ontario
Abstract: Simplified mass and energy balances are outlinedfor the purpose of optimising blast furnace operations. Asumary of useful blast furnace related data from numeroussources is presented. Tuyere zone, stack and general blastfurnace reactions are reviewed from an energy standpoint. Theimpact of variability in blast furnace input parameters isdiscussed. 'Rules of Thum' relating furnace raw material andpractice changes to energy consumption are reviewed. Theseprinciples are demonstrated through a computer simulationmodel "The Blast Furnace Game" that uses mass, energy,chemical and cost balances to assess means of improving theblast furnace process.
~;INTRODUCTION
In order to make changes to the blast furnace process thatwill meet their intended goals, the Ironmaker must have anunderstanding of his process, his facilities, and the costsassociated with each of the process changes. Although rulesof thum can be applied, each situation is somewhat differentdue to the variety of constraints and physical limitationsthat apply to individual furnaces. The purpose of this paperis to promote an understanding of the process. In thisregard, many of the concepts presented have been simplified,and some liberties have been taken in estimating data.
Models, in general i do not have to beuseful. What is most important is thatmagnitude of change can be predicted.why rules of thum have been developed.
exact in order to bethe direction and theThis, in essence, iB
4-1
EXHIBIT 1 BLAST FURNACE ENERGY BALANCE J
(MMBTU PER NET TON OF HOT METAL):J
ENERGY RECYCLED ENERGYINPUTS ENERGY OUTPUTS
~JBFG is USED FOR STEAM BFG EXPORTED 1.0
FURACE COKE 12.7 TO DRIVE BLOWING ENGINS IRON PRODUCED 7.5INJECTED FUEL 1. AN PUMPS FOR COOLING TOTAL LOSSES 5.9
J--- WATER AN ALSO TO HEAT ---TOTAL INPUT 14.4 HOT BLAST STOVES TOTAL OUTPUT 14.4
EFFICIENCIES LOSSES CONSUMD ~FUACE FUL 63% TOP TEMP LOSS 0.3 TOTAL OUTUT 14.4
STOVE HEATING 71% DUST BTU LOSS 0.1 LESS BFG~BLOWER STEAM 9% TOP BFG LOSS 0.1 EXPORTED 1.0
FE YllLD 96% STOVE ENERGY 2.1 ---STEAM ENERGY 2.1 CONSUMED 13.4
IPROCESS 56% FUACE COOLING 0.3
SLAG BTU LOSS 0.3
CALCINATION 0.3 IOTHR LOSSES 0.3---
TOTAL LOSSES 5.9
I
INPUTS OUTPUTSITOP TEMP 0.3
DUST LOSS 0.1 EXCESSBFG LOSS 0.1 BFG TO
IPLA NT 1.0
STOVES 2.1
ISTEAM 2.1
FE BURDENCONVERSION TOTAL ENERGYCOKE 12.7 LOSSSES AT FOR CONVERSION
irFL UXES STOVES 0.6 TOP LOSSES;BLOWERS 1.9 COOLING LOSS;
CALCINA nON;TUYERE HEA T FROM SENSIBLE HEA T
INJECTED 1.7 jlh .. STOVES 1.5 IN SLAG;¡ , BLOWERS 0.2 OTHER LOSSES;
FUEL TOT AL 5.9
COOLING 0.3 IRON 7.5SLAG TEMP 0.3CALCINE 0.3
TOT AL 14.4 OTHER 0.3 TOTAL 14.4
4-2
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An Energy Balance
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The blast furnace process is a significant consumer ofenergy, consuming about two-thirds of the total energyrequired for an integrated steel plant. The blast furnacetypically consumes all the coke produced by the coke ovens,as well as some additional inj ected fuels, in the productionof hot metal for steelmaking.
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As shown in Exhibit 1 about 14.4 million BTU of energy isrequired to make a ton of hot metal. This energy is providedmainly by coke and supplemented by inj ected fuels such asnatural gas, oil, tar or pulverised coal. All of these fuelsare burned in the raceway of the furnace with a limitedamount of air to provide the reducing gases for the iron oresmel ting process. Since these fuels are not burned tocompletion, significant amounts of by-product top gas orBlast Furnace Gas (BFG) is produced.
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Of the total top gas energy produced of 5.3 million BTU/NTHM,about 2.1 million BTU/NTHM or 40 percent is used in the blastfurnace stoves to preheat air for the blast furnace. Thestoves are fairly efficient recycling more than 70 percent ofthe energy or about 1.5 million BTU/NTHM to the blast furnaceprocess.
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Another 40 percent or 2.1 million BTU/NTHM is used to makesteam to drive blast furnace blowing engines and coolingwater pumps. Of this amount, only about 10 percent or 0.2million BTU/NTHM is recovered in the heat of compression ofthe cold blast air that is returned to the blast furnace.
~ì
Only about 20 percent of the total topexport to the rest of the steelworks.than ten percent of the energy thatprocess.
gas is available forThis represents lesswas provided to the
There isthe fuelfurnace,charging
some fuel energy loss from the process, includingenergy in the dust that is carried out of the
and the BFG that escapes during the raw materialopera tion .
Finally, there is also a significant (about 4 percent) lossof iron from the process. Iron is lost through poor iron/slagseparation, through runner and other scrap losses, and in theform of iron bearing dust exiting the top of the furnace.Iron yield loss has an impact on furnace energy performance,increasing the energy required per net ton of hot metalproduced.
Of the total energy input of 14.4 million BTU/NTHM that isprovided to the blast furnace process, only the BFG that isexported to the plant (1.0 million BTU/NTHM) is for non blast
4-3
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HEMTITE Fe203 (s) + 294 BTU /LB Fe -- Fe304 (s)
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furnace use. The remaining 13.4 million BTU/NTHM is alldirected in some manner toward the heating and reduction ofiron. Since the actual iron reduction process requires only7.5 million BTU/NTHM of energy i the difference of 5.9 millionBTU/NTHM is lost in the conversion process. Since the energyrequired to reduce iron of consistent chemistry is constant,this loss becomes directly related to fuel rate. As aprocess i the blast furnace shown in the model is only about56 per cent energy efficient. (i. e. 7.5 million BTU /NTHMdivided by 13.4 million BTU/NTHM)
Due to the tremendous amount of energy required to make hotmetali much attention has been given to reducing blastfurnace fuel rates i reducing heat losses, and to recoveringenergy from the blast furnace process. Furnace fuel ratesare the result of the sum total of the energy demands of theprocess, and can be viewed from several perspectives.
The Reduction Process
The purpose of the blast furnace is to reduce oxides of ironand to melt them for subsequent refining in steelmaking. Themain reactions are reviewed here from an energy perspective.The energy requirements have been developed from StandardHeat of Formation data.
MAGNETITE Fe304 (s) + 821 BTU /LB Fe -- FeO (s)
WUSTITE Fe ° (s) + 2056 BTU/LB Fe -- Fe (s)
IRON Fe ( s) + 600 BTU/LB Fe -- Fe (l)
TOTAL Fe203 (s) + 3771 BTU/LB Fe -- Fe (l)
In total about 7.5 million BTU/ton Fe is required to converthemati te to liquid iron. The actual amount of energy requiredto make hot metal will differ somewhat based on incoming rawmaterial and resultant hot metal chemistries. A typical hotmetal chemistry is shown below.
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Composition of Hot Metal
IronCarbonManganeseSiliconPhosphorusSulphur
93. °3.92. °
1. °
o. i %0.04 %
%
%
%
g,o
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The main blast furnace reduction reactions (plus the carbonreaction) are shown below. The carbon content of hot metal isdetermined by solubility of carbon in hot metal and isconsistent at a given temperature. The reduction energyrequirement for manganese is similar to iron but far moreenergy is required to reduce silica than iron ore.
IRON Fe203 + 317l BTU/LB Fe -7 Fe
CARON CO2 + 14093 BTU/LB C -7 C
MAGANSE Mn°2 + 4077 BTU/LB Mn -7 Mn
SILICON Si02 + 13490 BTU / LB Si -7 Si
PHOSPHOROUS P20S + l0452 BTU/LB P -7 P
SULPHUR S02 + 3991 BTU /LB S -7 S
Energy Inputs
The energy inputs to the process are twofold. The first isthe chemical energy content of the fuels and the second isthe sensible heat of the hot blast. Energy contents ofvarious fuels are shown below:
CoalCokeTarOilNatural Gas
l3,60012,8 ° °
16,80018,70023,800
BTU /LBBTU / LBBTU / LBBTU /LBBTU /LB
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The sensible heat of the hot blast airwhich are about 70 percent efficientfuel energy into hot blast energy.specific heats and densities for solidare shown in the following tables. 1,2
is provided by stoves,in converting top gas
Rough estimates ofand gaseous materials
Specific HeatBTU/LB/oF
DensityLB/ SCF
SOLIDMATERIALS
Wa terSinterPelletsIronSlagSiliconOilTarCoalCoke
1. °
0.20.20.20.30.20.40.40.40.4
62100145424206l45
60756535
4-5
(79% Nz in Air / 55% Nz in BFG) = 1.44
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GASEOUS Specific Heat DensityMATERIALS BTU/LB¡OF LB / SCF
Air 0.26 0.076
Ni trogen 0.26 0.074Oxygen 0.26 0.085
Carbon Monoxide 0.25 0.074Carbon Dioxide 0.23 0.117
Tuyere Reducing Gas 0.27 0.069Blast Fce Gas (Dry) 0.26 0.081
Hydrogen 3.50 0.005Natural Gas 1. 2 0.042
Stearn o . 6 * for comparison 0.044 *** g 212 degrees F 0.037 **
Energy au t:Du t s
Fuel energy inputs to the blast furnace not consumed in theprocess exit in the form of top gas. The BTU heating valueand volume of the top gas decrease as the furnace becomesmore fuel-efficient. The volume of top gas produced can becalculated from the specific wind rate (SCF wind /NTHM) byusing a nitrogen balance. Nitrogen is inert in the process;hence the volume of nitrogen remains constant. A typicalBFG/Wind ratio is calculated as follows:
Energy Losses
Losses from the process result primarily from losses in theconversion of energy i. e. wind generation, cooling waterpumping i and hot blast stove heating. Il
Blast Furnace Gas generated by the blast furnace process istypically used to make stearn to drive turbines for blowingengines and water cooling pumps. Most of the energy, however,is lost along with the condensate from these turbines. Theonly heat recovered from these two processes is in the formof the sensible heat of compression of the hot blast air iwhich is in the order of 300 degrees F depending on the blastpressure. This represents only about 10 percent of the fuelenergy input to the boilers i an area of significant energyloss.
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Blast Furnace Gas is typically enriched with Coke Oven Gas orNatural Gas to fire the blast furnace stoves. Theused to further heat the compressed blast air tohot blast temperature of around 1900 degrees F.stoves are reasonably combustion efficient i thecooling losses are limited to about 30 percent.loss of energy is due to the stack loss associatedgas exiting the stoves at about 600 degrees F.
stoves arethe normalSince thestack andThe majorwi th was te
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Cooling losses result mainly from water cooling members inthe hot blast stream in the walls of the furnace. Coolingmembers in the hot blast stream are devices such as hot blastvalves and tuyeres. Cooling members in the furnace walls canbe either stack plates or staves, or external water sprays,depending on the design of the cooling system. All of thesedevices can remove a great deal of heat from the process.
The next maj or group of lossessensible heat lost in the slag,from the casting process etc..
are intop gas,
the form of theand radiant heat
Other losses include the heat of reaction required forcalcination, the chemical energy lost in flue dust and filtercake, and top gas losses from top filling equipment.
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It should be noted that Higher Heating Values arecalculations to avoid double counting moisture(The HH measures the total heat release cooling theof combustion to 60 of that includes the latentvaporization of water.)
used inlosses.
productshea t 0 f
Sumry of Losses MMTU/NTHM
Top Temperature Loss 0.3
Dust BTU loss O. 1
Top BFG loss 0.1
Stove Fuels 2.1
Blowing /Pumping Fuels 2.1
Furnace Cooling Loss 0.3
Slag BTU Loss 0.3
Calcination Reaction Loss 0.3
Total Losses 5.9
4-7
Hot Blast Temperature
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Prior to the 19th century, all blast furnaces operated withair at ambient temperature with the tuyere zone of thefurnace serving a dual purpose. Firstly, it provided thereducing gases for the reduction process, and secondly, itprovided the heat to drive the reduction process and melt thehot metal and slag.In 1828, James Nielsen found that an elevated blasttemperature had a remarkable effect on furnace performance.Energy introduced to the process via the hot blast was energythat did not have to be provided by coke. As the fuel rateswere reduced, the volume of hot blast required was alsoreduced, generating diminishing returns, but quite favourablein any event.
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As better designed stoves were developed to produce evenhigher hot blast temperatures i blast furnaces began tooperate erratically due to too much heat being generated atthe tuyeres and further increases were achieved only afterthis problem was resolved with the injection of steam intothe hot blast. It was later found that hydrocarbons inj ectedthrough the tuyeres had the same stabilising effect andseemed to save even more coke than expected.
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IThe mechanism explaining these developments can be understoodby reviewing the reactions at the tuyeres.
Tuyere Zone Material Heat ContentI
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All materials entering the tuyere zone, both from inside andoutside the furnace must be heated up to the flametemperature as part of the combustion process at the tuyeres.
Generally burden materials are preheated by the ascendinggases and do not play a maj or part in determining flametemperature. Hot Blast air and steam inj ected prior to thestoves temperature must be increased from that provided bythe stoves up to flame temperature. Inj ected fuels must beheated from the injection temperature, usually ambienttemperature, up to the flame temperature.
If
Exhibit 2 displays the heat content of hearth zone materialsand Exhibit 3 displays the heat content of tuyere zonematerials.
4-8
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EX 2
LL
,.,. 1(0W~om-lOc: mi- ::Z~ 1mW r-i- mZ_o 40()i-~ 20i:
HE ZC ~ HF a:140
120
o 'o 50 1(0 150 :i 2S :D~IN~~
EX 3
r' Q~\ 6000=!-Q
'l 2' MM HE C'12000
10000H2
8000
4000
MET HAE
PLUS ENERGY
DE~pæES01 ::C+2H2
C+2H2
2000 H20CAIR
CH4
o
o 2000 300025001000 1500500
TEMPERATURE IN DEGREES F
4-9
Carbon C + O2 -- CO2 + 14093 BTU/LB C
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Tuyere Zone Reactions
Energy is introduced into the blast furnace process via twomaj or elements by the following general reactions:
C + Y: 02 -- CO + 3935 BTU/LB C
Hydrogen H2 + Y: O2 -- H20 + 61095 BTU/LB H2
However, in the tuyere zone, other reactions occur as windand inj ected fuels are blown into the furnace. Initiallythere are a numer of step reactions, which absorb heat priorto the above reactions to produce CO and H2 reducing gas. Theenergy absorbed by these reactions serves to moderate theheat (i.e. flame temperature) generated by the combustion ofcoke with hot blast oxygen. The energy shown is the heatrequired to break up the molecules into reducing gases anddoes not include the energy required to heat the injectedmaterials up to flame temperatures.
F1ame Temperature Moderating Reactions:
SteamMethaneEthanePropaneButane
H20 +CH4 +C2 H6 +
C3H8 +C 4 Hio +
5800 BTU/LB -- H2 + l/ O2
2010 BTU /LB -- C + 2 H2l206 BTU/LB -- 2C + 3H21028 BTU/LB -- 3C + 4H2936 BTU /LB -- 4C + 5H2
F1ame Temerature
The temperature of the gas leaving the tuyere zone can becalculated by assuming that the chemical energy of whateverreactions occur in the raceway (complete or incomplete) istotally converted to heat energy. Thus, the temperature ofthe flame in the raceway assuming no heat losses (adiabaticcondi tions) is called the Raceway Adiabatic FlameTemperature.
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RAT = Heatinq ValueSum of combustion product weights x mean specific heats
The American Iron and Steel Institute Technical Committee onBlast Furnace Practice has adopted the following formuladeveloped by Naren Sheth of Bethlehem Steel Corporation.
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RAT = 2686 + 0.82 (BT) - 23.5 (BM) + 95 (OE)
-124 (Oil) - 80.7 (Tar) - 8.7 (HW) - 7.0 (AS)
-29.9 (Coal) - (68 + 0.034 (GHV)) (NG)
The definitions of terms are:
RATBTBMOEOilTarCoalHW
ASNGGHV
Raceway Adiabatic Flame Temperature, FBlast Temperature, FBlast Moisture, grains/SCF dry blastOxygen Enrichment, (% O2 in blast - 21)Dry Oil Injection Rate, lb/1000 SCF dry blastDry Tar Injection Rate, lb/1000 SCF dry blastDry Coal Injection Rate, lb/1000 SCF dry blastHomogenizing Water, lb/100 lb dry oil or tarAtomizing Steam, lb/100 lb dry oil or dry tarNatural Gas Injection, SCF/lOO SCF dry blastGross Heating Value of Natural Gas, Btu/SCF
The equation shows that increased blast temperature or oxygenenrichment will increase the RAFT while greater use of anyhydrocarbon or water will decrease the RAFT. The aboveequation can be simplified by assuming a constant wind rate,e. g. 42,000 SCF /NTHM. (Note: 7000 grains = 1 pound and theGHV of Natural Gas is lOOO BTU/SCF). The resultant formulacan be broken into components as follows:
RAT (OF) = 2686 Constant
+0.82 * Blast Temp (OF)
+95 * Oxygen Enr i chmen t (% )
-3.9 * LB/NTHM Steam/Water
-5.8 * LB/NTHM Natural Gas
-2.9 * LB/NTHM Oil
-1. 9 * LB/NTHM Tar
-0.7 * LB/NTHM CoalReducing Gas Volume and RAT
in
Natural Gas has a greater moderating effect on flametemperature than steam because it is inj ected at ambienttemperatures and requires in the order of 4000 BTU/lb to be
4-11
heated to flame temperature. Natural Gas also completelydissociates in the raceway requiring another 2010 BTU/lb.
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The Reducing Gas to Wind Ratio exiting the tuyere zones willbe lower than the BFG/Wind Ratio because the Carbon, Oxygen,Ni trogen, and Hydrogen in the burden has not been liberatedto form CO, CO2, or H2 gases. If only H2, N2, and CO gases arepresent at the tuyeres, the Reducing Gas to Wind Ratio isabout 1.33. The reducing gas has a specific heat of about0.27 and a density of about 0.68 lb/SCF.
Considering a wind rate of 42,000 scf/NTHM and the reducinggas parameters above, about 1,050 BTU is required to changethe RAFT of the reducing gas by one degree F. Since NaturalGas, for example, requires about 6,000 BTU/lb for heating anddissociation, it will reduce the flame temperature by justless than 6 degrees.
It should be noted that the AISI coefficient for coal appearsto be low relative to the other hydrocarbon based injectedfuels. The overall flame temperature effect should be thesum of the effect of dissociating the hydrocarbon and thenheating the hydrogen and carbon components. To have a flametemperature effect less than the heating of pure carbon wouldappear to be incorrect. As shown previously, the moderatingeffect of hydrocarbons are related to the size of moleculeinvolved (i. e. hydrogen to carbon ratio) in terms of theenergy required for dissociation.1 (See Exhibit 4)
Hydrocarbon H2 (Wt%) C (Wt%) N¡ (Wt% ) lli- / N¡
Natural Gas 22.5 69.4 8.1 0.32
Bunker "C" Oil 9.3 88.6 0.3 0.10
Tar 7.1 91. 4 1. 1 0.08
Bi twninous Coal 5.0 80.1 0.0 0.06
Anthracite 2.8 80.6 0.0 0.03 IT
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EXHIBIT 4r- 1400Zc:l-~ 1200..Z~ 1000 -
oo,.a: 800Wc.Wen 600"c:wa:o 400WCc.:æ 200Wl-
o
0.00
FLAME TEMPERATURE VS H2 TO C RATIO
UJl-t)-:CC::I-;;-:
AISI
EFFECT Cf PURE HYDRCGN
-i-:()uf,::o;;,~::~-m
(J((~-i0:CC::I-((;;
0.05
-iÕ
EFFECT Cf PURE CARBQ\
EFFECT Cf DISSO:IATlO\
0.10 0.15 0.20 0.25 0.30 0.35
HYDROGEN TO CARBON RATIO
DISCUSSION OF TUYRE ZONE RECTIONS AN RAT
The coefficient for Hot Blast Temperature is 0.82 because thesensible heat in the DRY hot blast excludes 18 percent ofother materials, (including natural humidity and steam) iinjected at the tuyeres.
The explanation for the benefit associated with hydrocarbonfuel inj ection versus steam can be found by comparing theoverall reactions occurring at the tuyeres.
When steam is inj ected to moderate the flame temperature itunavoidably reacts with coke absorbing a great deal of energyat tuyere level. It also generates a significant coke penaltybecause i t involves the carbon in coke in the reaction.
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The reaction of steam with the carbon in coke is:
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H20 + C + 3138 BTU/LB Hp -7 CO + Hi I
Wi th hydrocarbon fuels less energy is consumed when comparedwith water and the reaction does not involve carbon in coke. I
CH4 + 1¡ 02 + 959 BTU/LB CH4 -7 CO + 2 Hi I
IFurthermore, the energy required to dissociate thehydrocarbon is not a penalty because it is already consideredin calculating the Higher Heating Value of the fuel. Forexample, the reactions considering one pound of methane are: I
CH4 + 2010 BTU -7 C + 2HirL
C + O2 -7 CO2 + 10570 BTU
2Hi + °i -7 2HiO + 15270 BTU
CH4 + 202 -7 CO2 + 2HiO + 23830 BTU
Using hydrocarbons to control flame temperature then has adouble barrel effect. Firstly, it replaces water in thetuyere zone and eliminates the associated coke penalty.
Secondly, it acts as a replacement for coke because itprovides addi tional reducing gas to the process.
If hydrocarbons are injected at a level greater than requiredto control flame temperature then they act only as a cokereplacement.
4-14
EXHIBIT 5 ici FURNACE ENERGY CONSUMPTION
, ¡
18
16
:æ 14J: -l-Z-~ 12l-II:æ:æ 10 ~ . .
8
6
1977 1978 1979 1980 1981 1982 1983
-- Net Energy -- Coke Energy -. Total Energy
Exhibi t 5 shows the changes that occur wi th varying fuelinjection practices on 'C' Furnace at Hilton Works. Thehigher quanti ties of fuel inj ection replace coke on an energybasis and also increase the top gas recoveries. interestinglythe net energy required to make a ton of hot metal ofconsistent analysis remains about the same.
Blast Furnace Efficiency
Since the blast furnace is a reduction process, the objectiveat tuyer~ level is to generate reducing gas (i. e. CO and H2 ).From a combustion standpoint, the objective at the top of thefurnace is for the exiting gases to contain only the productsof complete combustion (i. e. CO2 and H20). Although theseconflicting obj ecti ves make it impossible for the blastfurnace to be efficient from a combustion standpoint, stepscan be taken to make it more efficient. Since the majorityof the fuel is carbon based and hydrogen efficiency parallelscarbon efficiency, only the carbon-based reactions will bediscussed.
The common measure of furnace efficiency is the percentage ofcarbon burned to completion in the furnace top gas.
% CO2% CO- + % CO2
x 100 = % Efficiency
4-15
The main reactions being dealt with, considering one pound ofcarbon being burned to completion in step reactions are:
C + Vi02 -- CO + 3,935 BTU/LB C
CO+ Vi02 -- CO2 + 10,158 BTU /LB C
C + O2 -- CO2 + 14,093 BTU/LB C
Considering a furnace that is 40 percent efficient atconverting carbon to CO2, this means that for each pound ofcarbon in the process 40 percent burns to CO2 and 60 percentburns to CO. (It is important that the CO2 produced by thecalcination of limestone/dolomite be deducted prior to thesecalculations) .The energy available to the process is:
o . 40 LB C x 14, 093 BTU = 5, 637
0.60 LB C x 3,935 BTU = 2,361
Total = 7,998 BTU/LB C
This constitutes about 56.8 percent of the energy into thefurnace if burned to completion. Hence, the furnace is 56.8percent fuel-efficient. A one percent improvement in gasefficiency would provide more energy to the process:
o . 41 LB C x 14, 093 BTU = 5 , 778
0.59 LB C x 3,935 BTU = 2,322Total = 8, LOO BTU/LB C
This constitutes about 57.5 percent of the energy into thefurnace, an improvement of about 1.28 percent. Consideringan 1100 lb/NTHM fuel rate this is equivalent to 14 lb/NTHM.
4-16
Variation In Furnace Efficiency
There are a number of factors, which affect furnaceefficiency, and consequently the effective fuel rate of theblast furnace. Factors in this category include burdendistribution, coke stability and raw material fines. Forease of analysis the effects of variations in these factorswill be discussed in total.Variations in furnace efficiency are exhibited as variationsin top gas BTU content specifically the CO/C02 ratio. Ablast furnace that becomes inefficient will generate more COin the top gas, robbing the reduction process of energy,lowering the hot metal silicon.
The "Rule of Thumb" for these changes in efficiency is anincrease of 14 lb coke rate for each 1 per cent decrease ingas utilisation.
Effect Of Efficiency Variation On Top Gas Energy Content
Top gas BTU content can be calculated from the analysis ofblast furnace gas. The main gases with a fuel value are COand H2. (C02 and N2 are inert).
BTU/SCF = 3.25 BTU/% ~ + 3.23 BTU/% CO
In general, H2 and C fuel efficiencies tend to follow oneanother (i. e. a more efficient furnace will burn more of bothof these fuels). It should be noted that H2 does not sufferincomplete combustion. It either burns to form H20 vapour orexi ts the furnace as H2 in the top gas. Hydrogen forms onlya small part of the fuel to the furnace and its efficiency isaffected by many of the same factors that affect carbonefficiency.
Tyical Top Gas AnalysisHeating Value
Component Percentage BTU / SCF
CO 22.7% 73.32CO2 18.5% 0.00H2 3.8% 12.35N2 55.0% 0.00
Total 100.0% 85.67
4-17
A 1% increase in furnace efficiency will change both thepercent CO and CO2 in the top gas by 1 percent. Assumingthat the total volume of CO + CO2 remains about the same at41% of the top gas i the percent CO2 in the top gas willincrease by 0.41%.
Component Before After
CO 22.70 22.29CO2 18.50 18.91CO + CO2 41.20 41.20CO2 / (CO + CO2) 0.449 0.459
BTU Contribution 73.32 72.00
BTU % Change 1. 8%
Assuming that hydrogen efficiency changes at the same rate ascarbon efficiency a one percent increase in CO2 utilisationwill reduce the top gas BTU content by 1.8% x 85 BTU/SCF orabout i. 5 BTU/SCF.
4-18
Effect Of Efficiency Variation On Hot Metal Silicon
The objective of applying the "Rules of Thum" hastradi tionally been to obtain answers, which are expressed interms of "coke" or "carbon in coke". It can, however, alsobe used to determine the change in hot metal silicon, given aconstant ore to coke ratio, for a step change in rawmaterials or operating practice. For example, the reductionin hot metal silicon associated with a loss of hot blasttemperature can be both calculated and readily observed.
Using the coke rate formula in this manner, the step cast-to-cast changes in raw materials and operating practice can beassociated with step cast-to-cast changes in hot metalchemistry. Furthermore, the effect of blast furnace inputvariability on hot metal chemistry can be statisticallyquantified. Two measures of variability used in thisdiscussion are the standard deviation (S. D.) and the variance(VAR) where VAR = (S.D. )2.
Since the fuel rate change due to a one percent change inefficiency is about the same as the fuel rate change for a0.1% change in hot metal silicon, a "rule of thum" that canbe used is that a loss of 1% in furnace efficiency will lowerhot metal silicon by 0.1% (Si J and will raise the top gas BTUvalue by 1.5 BTU/SCF.
A review of blast furnace operating data at Stelco indicatesthat the BTU value of individual top gas samples is typically83 :! 4.3 BTU/SCF. A review of continuous top gas analysisdata indicates that a standard deviation of :! 2.5 BTU/SCFreasonably represents cast-to-cast variations due to changesin furnace efficiency only. (Both the mean and standarddeviation of BTU content can change as the result of varyingfuel inj ection practices, blast moisture, etc.)The variation in furnace efficiency can be calculateddirectly or indirectly as follows:
S. D. (Efficiency) = Top Gas BTU Std Dev.1.5 BTU/% Efficiency
= :t 2.5/1.5= :! 1. 67 % (1 S.D.)
This variation in Gas utilisation (Efficiency) as shown abovecauses a corresponding variation in hot metal silicon contentof:!O.167% (SiJ (1S.D.).The relative importance of efficiency variation on hot metalsilicon can be calculated using the coefficient ofdetermination.
4-19
COEFFICIENT OF DETERMINATION = S.D.ll (EFFICIENCY) 2
S. D. (SiJ (TOTAL) 2
= (0.167)2(0.245)2
r2 = 0 . 4 6
Consequently, about half of the variability in the processresults from factors such as coke stability and burdendistribution which affect furnace gas utilisation or fuelefficiency.
Discussion of Efficiency Variation
Due to the relative importance of efficiency variation in theoperation of a stable blast furnace, the factors that affectfurnace efficiency will be significant with regard to theireffect on hot metal quality.
The primary factors affecting furnace efficiency are burdendistribution, raw material size distribution and cokestabili ty. These are the same factors that have allowed thetremendous improvements in furnace fuel rates.
Other factors such as alkalis, high temperaturebreakdown, scaffolding, etc. which also affectcontact are much harder to quantify.
ma terialgas / solid
Discussion of Other Variation
All factors that have an impact onthe furnace have an effect onchemistry. Cast-to-cast variationweights, coke moisture, hot blastmoisture, natural humidity, have afurnace operation.
the charged fuel rate ofcast-to-cast hot metalin ore weights, coketemperature, hot blastcast-to-cast impact on
Operating practices and irregularities can also have a cast-to-cast effect on hot metal chemistry. One example is castingpractice, particularly on one-taphole furnaces. Variation incasting time and residual hearth hot metal will changeparameters such as the burden descent rate. Late casting forexample will fill the hearth with hot metal and float morecoke into the tuyere zone generating more heat on late casts.
4-20
OTHER ARAS TO BE CONSIDERED
Raw Materials Preparation
The quality of the ferrous material charged to the blastfurnace improved dramatically in the mid nineteen sixtieswith the introduction of pelletized ore of an optimum size topromote good gas/solid contact.
Improving the gas/solid contact decreases the amount ofcarbon required to reduce the ore since the reaction to CO2reduces twice as much ore as the reactions to CO. Thisimproves the furnace fuel efficiency, which in turn reducesthe furnace coke rate.
Raw Flux
The calcination of raw flux (dolomite or limestone) in theblast furnace consumes energy and releases CO2 which servesto dilute the top gas produced by the furnace. The reactionproceeds as follows:)
Ca CO) + 766 BTU/LB = CaO + CO2 (g)
The energy requirements to make a ton of hot metal can bereduced if this reaction does not take place in the furnace.(i. e., if previously calcined flux can be charged to thefurnace. ) The most commonly used method of providing acalcined flux to the blast furnace is by using fluxed sinteror pellets, or by charging BOF slag.
Coke Properties
Coke is the main reductant for the blast furnace processsince it serves to support the burden and provide a means forgas to flow through the burden. Increasing the stability ofcoke improves the gas/solid contact and makes the furnacemore fuel-efficient since more carbon can be converted tocarbon dioxide.
Increasing coke ash has two effects. Firstly, the carboncontent of the coke is reduced. Secondly, the ash must bemel ted and drained from the furnace as slag, robbing theprocess of energy.
4-21
Hot Metal Chemistry
The two main specifications for hot metal chemistry concernsilicon content and sulphur content.
The silicon content is determined by how much silica isreduced in the process. Since it takes more energy to reducesilica than iron oxide, there is an energy penalty associatedwi th increasing the silicon content of hot metal.
The sulphur content of iron is determined mainly by slagbasici ty. Higher basicities are usually achieved by addingflux, which requires a calcination reaction and contributesto slag volume. There is an associated energy penalty foreach of these factors.The effect of other elements making up the hot metalchemistry can be determined in a similar fashion.
Scrap
The benefit of charging scrap to the blast furnace can befound by reviewing the energy required to reduce Fe203 to hotmetal versus the energy required to only mel t iron. Themelting component constitutes only about 16 percent of thetotal energy requirement. The average coke rate for thefurnace can be significantly reduced with only a smalladdi tion of scrap to the burden.
Any partial reduction to Fe304 or FeO will also reduce thetotal furnace energy requirements mainly through increasedtop gas recoveries. This is because the reducing gas in theupper stack can exit the furnace without having to performthese steps in the reduction process and will consequentlyhave a higher BTU value.
4-22
Producti vi ty
The productivity of the blast furnace is proportional to twobasic factors. The first is the amount of wind (oxygen)blown and the second the furnace fuel rate.
In the past, significant increases in productivity were madewith the conversion from raw to pelletized ore, mainly due tothe higher wind rates allowed by the more permeable burdenand the lower slag volumes allowed by the lower ganguecontent.
For a given bed of materials, there is a maximum wind ratefor the furnace beyond which the furnace becomes unstable.At this point, the producti vi ty of the furnace depends mainlyon the furnace fuel rate since the volume of wind to thefurnace is fixed. In this situation the relationship thatapplies is:
New Production = Old Production x Old Fuel RateNew Fuel Rate
OxYgen Enrichment
In cases wherelimiting factor,made by increasing21 percent.
the volume of wind to the furnace is afurther increases in producti vi ty can bethe oxygen content of the hot blast above
Oxygen enrichment reduces the amount of inert nitrogen in thesystem, thereby concentrating the process. With oxygenenrichment there is more oxygen available to form reducinggases at tuyere level. In addition, more injected fuel canbe introduced at tuyere level to replace coke whilemaintaining the same flame temperature. These gains,however, are partially offset by the reduced sensible heat ofhot blast per ton of hot metal.
Some discussion of the use of Oxygen enrichment is in ordersince this the use of oxygen enrichment can have a profoundeffect on the Blast Furnace process.
4-23
OxYgen Enrichment (continued)
The amount of oxygen required to make a ton of hot metal isdetermined by combustion requirements in the raceway.
Generally the amount ofmetal is calculated aswi th a specific windenrichment, the amounthot metal is
wind required to make a ton of hotthe Specific Wind Rate. For examplerate of 42,000 SCF/NTHM with no
of oxygen required to make a ton of
42,000 SCF * 21 percent Oxygen in air = 8,820 SCF Oxygen/NTHM
Oxygen is typically added to the cold blast after the blowersand the total amount of oxygen added can be calculated
Percent Enrichment * Wind Rate / 0.79 = Oxygen SCFM
Percent Oxygen in Wind * Wind Rate = Oxygen SCFM
Considering that the total amount of oxygen required to makea ton of hot metal will remain the same for a constantfurnace fuel rate, the specific wind rate will decrease withthe use of oxygen enrichment.
Oxygen SpecificPercent wind Rate (SCF/NTHM)
21 % 42,00022 % 40,09023 % 38,34724 % 36,75025 % 35,28026 % 33,923
The reduction in specific wind rate for the furnace willsignificantly reduce the amount of wind required from theblowers and will also significantly reduce the amount of fuelrequired for heating that wind in the stoves. Appliancesusing BFG such as the stoves and Boilers will also becomemore efficient due to the reduced N2 content.
4-24
Since the use of oxygen enrichment has the effect ofconcentrating the process by eliminating nitrogen, the topgas BTU value will rise about 2.8 BTU per percent enrichment.This value will decrease slightly as the level of oxygenenrichment is increased.
Tyical Top Gas Analysis
BFG with NO enricluent BFG with 1 % enricluent
Gas Percent Volume MMTU Percent Volume BTU
CO 22.7 13 f 701 4.43 23.4 13 f 701 4.43CO2 18.5 11, 116 0.00 19.1 II f 116 0.00H2 3.8 2 f 294 0.74 3.9 2 f 294 0.74N2 55.0 33,180 0.00 53.5 31, 270 0.00
Total 60,358 5. l7 58,431 5.17
BTU / SCF 85.7 88.5
wind SCF02 %02 SCFN2 SCF
42 f 00021 %
8 f 82033 f 180
40,09022 %
8 f 82031,270
Rules Of Thwr
The Rules of Thumb for blast furnace operations are based onthe work of many people. Special mention should be given toR. V. Flint and his Flint Carbon Rate Formula. (4).
The Rules of Thum are the result of multiple regressionanalysis of blast furnace operating data. Most steelcompanies to suit their individual operating conditions haveestablished similar data.Table 1 presents the most common or accepted "Rules Of Thumb"and describes their application to blast furnace practice.
4-25
Overview Of Rules Of Thum
The data in Table 1A outlines Hilton Works Blast FurnaceOperations since 1964. Although a strict comparison cannot bemade due to the magni tude and numer of changes, severalobservations of step-wise improvements are worthy of note.
A) The introduction of sized low gangue pelletizedore in 1964 significantly improved the efficiencyof i A i Fce. The reduction in coke rate caused acorresponding reduction in slag volume, whichprovided further energy savings. The introductionof pellets provided a more stable operation, whichallowed the hot metal silicon content to belowered.
B) Higher hot blast temperatures on i B i furnacesignificantly lowered coke rates in spite of theamount of steam injected. The addition of fluxedsinter to the burden (not shown) combined with thelower coke rates allowed a large reduction in rawflux used and the slag volumes generated.
C) The introduction of Burden Distribution on i D iFurnace significantly increased top gas efficiencyby about 3.5 percent. Combined with a furtherincrease in hot blast temperature, minimum rawflux charge, and minimum hot blast moisture, fuelrates were cut by one third over this period.(1964 to 1988)
D) The net energy required to make a ton ofof similar chemistry has remained aboutNote that no corrections have beenvariations in hot metal chemistry.
hot metalthe same.made for
E) The majority of energy savings over this 25 yearperiod were achieved through reduced conversioncosts, mainly in wind and stove heating.
4-26
A Furnace A Furnace B Furnace D Furnace
Parameter 1964 1964 1970 1988
Raw Ore % 75 40 0 0
Flux lb/NTHM 671 544 104 17
Hot Metal (Si J % 2.14 1.20 1. 03 1.1l
Slag Volume 680 630 406 388
H.B.T. of 1450 1500 1750 1900
Moisture gr / SCF lO 9 19 8
Coke lb /NTHM 1520 1270 1084 890
N.G. lb/NTHM 69 45 75 68
Total Fuel Rate l599 1315 1159 958
Wind SCF /NTHM l20,OOO 80,000 65,000 44,000
RAFT of 3500 3570 3394 3680
Top Gas
% CO 22 21 24 22
% CO2 14 l7 17 20
% H2 5 5 5 4
CO2/ (CO+C02) % 39 45 42 48
Energy Balance MMTU /NTHM
Hot Blast In 4 3 3 2
Energy In 21 17 16 13Out 13 9 8 5
Net 12 11 11 10
4-27
Discussion Of "Rules Of Thum"
Some of the "Rules of Thumb" can be verified by reviewing theamount of energy involved for the variable concerned anddetermining the energy input required considering the fuelefficiency of the furnace.
For example, the addition of 1 lb/NTHM of raw flux requires766 BTU for the calcination reaction. If the furnace is 55percent fuel efficient, this means that about 1392 BTU willhave to be added. Since 1 lb of coke contains 12,800 BTU,about 0.1 lbs of coke will be required. This happens tocorrespond exactly with the empirical "Rule of Thumb".
New "Rules of Thum" can be estimated on the same basis. Forexample, if a more efficient cooling system removes more heatfrom the process that heat can be quantified in BTU/NTHM.Again, considering a furnace fuel efficiency of 55 percent,the amount of additional energy required for the process canbe calculated.
100, 000 BTU/NTHM / (0.55 * 12,800 BTU/LB Coke ) = 14 LB/NTHM
Other "Rules of Thum" can not be so easily developed due tothe number of variables involved. For example, an increasein hot metal silicon is usually accompanied by other changessuch as a change in hot metal temperature. Increasing thesilicon content of hot metal requires more energy not onlyfor the reduction of silicon, but also for the increasedheating of the hot metal and slag. Accompanied by minorchanges in furnace efficiency, the effect of a change in hotmetal silicon content is difficult to estimate and is bestevaluated from experimental data and the empirical "Rule ofThumb" .
COMPARING TWO PERIODS OF OPERATION USING THE RULES OF THU
An example of how the Rules of Thum are used to compare twoperiods of blast furnace operation is shown in Table 2. TheTable is constructed by listing all the parameters that havechanged that will affect the furnace fuel rate. Thecorresponding fuel rate correction can be calculated for eachvariable. The total amount of these adjustments willgenerally explain the change in fuel rate between the twoperiods of operation.
4-28
PARTER
Blast Moisture( grains/scf )Blast Temp( degrees F )
Hot Metal %Si
Hot Metal 'YoM
ASTM CokeStabili ty
Coke % Ash
Slag Volume( lb/NTHM )
Flux Rate( lb/NTHM )
Nat Gas Rate( lb/NTHM )
Tar Rate( lb/NTHM )
TABLE 2 COKE RATE ASSESSMENT
BASE CURRNT
8. a 11.1
1875 1864
1.00 0.8
1.00 0.9
57.0 54.1
7.5 7.8
350 382
40 24
29 28
85 84
+3.1 * 4
COKE RATE ADJUSTMNTS
= + 12.4
-11 * 22/100
- 0 . 2 * 13/0.1 =
-0.1 * a
-2.9 * -16
+0.3 * 30
+32 * .25
-16 * .15
-1 * -2
-1 * -1
Total Adjustments ( lb/NTHM )
BASE CURRNT
Actual Fuel Rate 985 1034
Dry Coke Rate 871 922
Coke Rate Adjustment 0 53
Adjusted Coke Rate 871 869
4-29
= + 2.4
26.0
= 0.0
= + 46.4
= + 9.0
= + 8.0
= 24.0
= + 2.0
= + 1.0
= + 52.8
( lb/NTHM )
lb/NTHM )
( lb/NTHM )
( lb/NTHM )
Model Blast Furnace Examle
To further illustrate the comparison between periods as shownabove, the effect of step wise practice changes on a modelblast furnace are shown in Table 3. The model Blast Furnaceshown in Case 1 is operating on a burden of lump ore with lowblast temperatures and no injected fuels. The furnace isinefficient due to poor gas/solids contact and has a tendencyto slip, limiting the wind rate.
Swi tching to a low gangue pellet burden with no other changeswould result in excessively high slag basicity. Accordingly,the raw flux consumption must be cut in half as in Case 2. Atremendous producti vi ty gain is made due to the increasedwind rate allowed by the improved burden materials. Makingthese changes serves to reduce coke rate in three ways, fluxrate, slag volume, and furnace efficiency changes.Case 3 shows further coke savings can be made by swi tchingpart of the burden to sinter and eliminating the raw flux.
Increasing the blast temperature alone results in excessiveRAFT, hence, the blast moisture must be increased as well asshown in Case 4. The BTU value of the top gas increases dueto the higher Hydrogen content. Using oil instead of moistureto control RAFT has a tremendous effect on coke rate as shownin Case 5. Lowering the hot metal Silicon content as in Case6 lowers the coke rate and has a secondary effect on slagbasici ty since more silica from ore is diverted to the slag.
Case 7 outlines the effect of burden distribution equipmenton the furnace, generating a large reduction in coke rate andalso the amount of top gas produced.
The use of Oxygen enrichment to increase productivitycombined with an increased oil inj ection rate to maintainRAFT is shown in Case 8. Note the higher BTU value of the BFGdue to the lower ni trogen content.
Finally, the addition of scrap to the burden displacingpellets is shown in Case 9. The significant reduction in fuelrate has a significant effect on producti vi ty.OveralL, the producti vi ty ofmore than doubled and thereimprovemen t .
this model Blast Furnaceis still an opportunity
hasfor
4-30
TABLE 3 Case Case Case Case Case Case Case Case Case1 2 3 4 5 6 7 8 9
NTHM per day 2477 4351 4464 4525 4723 5976 5241 5688 6100
Wind Rate MSCFM 100.0 160.0 160.0 160.0 160.0 160.0 160.0 160.0 160.0Wind MSCFM/NTHM 58.1 53.0 51. 6 50.9 48.5 46.3 44.0 40.5 37.8
Scrap lb/NTHM 0 0 0 0 0 0 0 0 ~Lump Ore lb/NTHM 3280
1300~11 300~1
0 0 0 0 0 0 0
Pellets lb/NTHM 0 1960 1960 1960 1974 1974 1974 ~Sinter lb/NTHM 0 0 1300 1300 1300 1300 1300 1300 1300Raw Flux lb/NTHM 700 ~ 0 0 0 0 0 0 0
Blast 02 % 21 21 21 21 21 21 21 ~ 23Blast Temp. F 1500 1500 1500 118001~ 1800 1800 1800 1800 1800Moist gr/scf 6 6 6 17 ~ 6 6 6 6
RAT deg F 3775 3775 3775 4021 3763 3767 3753 3739 3752 3719
Coke lb/NTHM 1395 1287 1254 1238 1086 1025 969 918 846Oil lb/NTHM 0 0 0 0 ~ 100 100 ~ 150Total lb/NTHM 1395 1287 1254 1238 1186 1125 1069 1068 996
Hot Metal (Si) % 1. 00 1. 00 1. 00 1. 00 1. 00 0.501 0.50 0.50 0.50Hot Metal (S) % 0.029 0.029 0.026 0.026 0.026 0.03 0.028 0.028 0.018
Slag lb/NTHM 742 432 511 509 500 519 515 512 488Slag B/A 1.14 2.02 1.13 1.13 1.13 1.17 1. 08 1.10 1.11 1. 20
BFG BTU/ scf 83.7 83.1 84.4 87.5 89.0 85.5 79.3 87.1 88.1BFG MMTU /NTHM 6.84 6.38 6.19 6.44 6.34 5.59 4.86 5.13 4.89
Efficiency % 38.0 42.0 42.0 42.0 42.0 42.0 ~ 46.0 46.0
The Blast Furnace Game
The blast furnace computer model used to demonstrate theeffects of practice changes was originally constructed withthe assistance of Mr. Duncan Ma of McMaster Uni versi ty andhas been revised several times since then.This model incorporates nearly all of the principles involvedin the use of the "Rules of Thum". The model involves thesimul taneous solution of mass i energy, chemical and costbalances and reasonably reflects changes in operatingpractice and changes to the process via equipmentmodifications.
i
i
.. I
The challenge that is presented in the Blast Furnace Game isto optimise this model blast furnace by judiciouslypurchasing equipment and improving on the furnace operatingpractice. The obj ecti ve is to ...... in the words of BillTaylor, a retired Blast Furnace Operator,
"Keep the wheels turning and the costs down".
4-31
REFERENCES
1. "North American Combustion Handbook" , NorthAmerican Manufacturing Company, Second Edition,1978, Page 356.
2. "Perry i s Chemical Engineering Handbook", McGrawHill Chemical Engineering Series Fourth Edition,Page 9-43.
3. Strassburger et al.,Practice" . Gordon1969, Page 697.
"Blast Furnace -& Breach Science
Theory andPublishers
4. R. V. Flint, Blast Furnace and Steel Plant, 50, 1,1962.
5. I.N. Gibra "Probability and Statistical Inferencefor Scientists and Engineers" Prentice Hall Inc.,Englewood Cliffs, N. J. 1973 Page 110.
6. A. J. Duncan "Quality Control andStatistics" Richard D. Irwin Inc. ,Illinois 60430,1974, Page 767,768.
IndustrialHomewood,
7. M. R. Spiegel "Theory and Problems of Statistics"Schaum Publishing Company, New York 1961, Page253.
8. J. B. Hyde and J. W. Busser "Use of a ChargeControl and Coke Moisture Gauge System at Stelco 's'E' Blast Furnace" 46th Ironmaking Conference,pittsburgh, P.A., 1987.
4-32
LECTU #5
BLAST FUACE DESIGN i
John A. Carenter
Paul Wur Inc.600 Nort Bell Avenue
Buidig 1, Suite 230Caregie, Pennylvana i 5 i 06
Abstract: - 1bs paper is of a general natue, coverig the blast fuace proper and
those ancilar components imedately upstream and downstream of the fuace. It
will focus on the stockhouse. the chaging equipment, the fuace top, the fuace
proper, the cooling system, and the casouse area of typical blast fuaces.
Blast fuace ironmakg is a system comprised of many components
fuctionig in hanony. Proper application and operation of these components is
necessar to support the ironmakg process. Selection of specific components isdependent upon such factors as existing conditions, physical constraits, productionrequiements, cost, schedule. reliabilty, and maitaability. Interdependence ofcomponents is as importt to the system's operation as their individua capability.
1bs paper will illuste the major requirements and "usua" practices for each
area or component. It \\ ill also explore some alternative technologies which arecommercially available. The inerent advantages and disadvantaes of those
alternatives will be discussed.
In the overal ironmakg course, other disserttions providing more detal onspecific components and the process will be presented. 1bs paper is complementa tothose more specific presentations.
5-1
INODUCTION
Ths paper is organed in the followig maner:
i
,i
(1) The Introduction pro\ides a genera description of blast fuace ironmakg.(2) There are eight sections which describe in more detal a blast fuace's
components and equipment.
(3) A short design exercise which is provided to demonstrate component sizig,equipment selection, and the interaction between equipment and process.
Thè bl~ fuace (Figu 1), converts iron bearg ores and revert into moltenmeta. Associated with blas fuces are coke plants which convert coal into coke and
pellet plants, which prepare iron ore for the blast fuace. The blast fuace convertthese prepared raw materials into a product of greater value. Iron from some blastfuace operations will be made dictly into saleable cast iron products in a foundr.Other operations produce a lower in silicon, "hot meta", which is converted into steel.Blast fuace by-products are slag, off gas, flue dust, and fiter cake. These by-productsmay have either positive or negative economic impact, depending on the localpossibilities for utiliztion.
Blast fuace ironmakg is a four hundred year old technology. Even so, theint~grated mill using blas fuce hot meta is still the most common method used forthe production of steel. Today' s integrated steel plant process relies upon the blast
fuace to provide on schedule. predictable quatities of molten iron of consistentquaity. Varation in any of the ascts of the supply of molten meta has a serious
impact on the rest of the steel production processes. Therefore, the blast fuce is a
key component in the modem integrted steel mill.
There are some who say tht the blast fuace is at the end of its useful life.Ths is not so. Consider that twenty thee years of operating data from a tyical pair ofmedium size blast fuaces shows an average increase in productivity of thee percentper year (Figue 47). At the sae time, the average reduction in fuel rate was one
percent per year (Figue 48)18. Also, the productive time between relines, "thecampaign", has been extended though improvements in equipment, materials, anddesignl8. As a result, the overal cost of makg iron, corrected for ination, haimproved even more than the opetig data indicates. The Blast Furace is not dead; itis a four hundred year old technology which is stil progressing in every area at asignficant rate. The Blas Fure is a dynamc science supported by constatly
improving technologies.
Some of the many aspects afecting every consideration for blast fuace designor re-design are profit, employee health, safety, environmenta protection, governentaregulation, market requirements, downstream processing, available workforce,_
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constrction, maitenance resoures, changing technologies, equipment obsolescence,
raw materials, utilities, and so on. Any serious constrait in one of these factors couldjeopardize the viability of an exig uit (or even a steel plant) or preclude or
necessitate the consction of a nev..' blast fuace.
Blas fuaces are generly grouped by size. "Small" - under five thousand nettons of hot meta per day (Nflday), "Medium" - six thousand to eight thousandNTIday, and "Large" - nie thousd to twelve thousand NTHday. A givenintegrted steel plant will operate the number and size of blast fuaces requied toprovide the hot meta needed. \-iulti-fuace mills are less afected by individuafuace repai relines or control prblems. Small fuaces have shorter relines thanlarge fuaces and are considere to be easier to operate. However, the hot meta fromsmall fuaces is higher in cost. An individua mill will operate the mium numberof cost effective fuaces. In some cases, upgrades are made in order to reduce the
number of fuaces in operation.
Blast fuces are "relined" periodically. In the past, ths involved thereplacement of the internal brick lig of the mai vesseL. In recent times, extensive
component rebuilding, replacement, and general maitenance was performed at thesame time. With ths practice. the more effcient plant with fewer "large" blast fuaceswill lose a higher percent of it' 5 production durng a reline than the plant with moresmall fuaces. In order to have both the low operating cost and the miniuminterference from relines, the indus has worked to maxe the blast fuacecampaign (time between relies) and to reduce the reline duration.. The clear trendtoday is for mils to operate tèwer large fuaces and to utilize technques and designwhich will extend their campaign indefitely.
At the same tie, Ùle reduction in product varability has become more
importt so investments have ben made which improve monitorig and control of theprocess. Blast fuace operators, researchers, maitenance personnel, and designers
have applied modern technology and analytical methods to the process in order to bettermonitor and control the process. As a consequence, the stadard deviation of the hotmeta quaities have been reduced. Improved data collection systems also provide moreinormation for suppliers and manufacruers. Tbs improves the materials selection andthe design of fuaces and equipment. Campaign lengts have increased from thee orfour years to more than eight year.
BLAST Fù~ACE PLAN LAYOUT
The layout of a blas fuace plant is essentially an exercise in integrating theequipment requied to handle the varous materials required to make iron and theresulting product and by-products. The most effcient design will properlyaccommodate the process and will be judged effective from the stadpoint of both_
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intial capita investment and ongoing operating costs. Plant layout is dependent uponmany factors such as terr raw marial delivery method, in-plant raw materialprocessing systems, climatic conditions. downeam processing systems and locations,quatity/flow requirements for "hot meta", hot meta delivery "fleet" size, and so on.
A blast fuace plant (Figu 2) typically comprises the followig elements:
(a) Raw Material Storae, Handlg and Reclai(b) Stockhouse(c) Charging System(d)' Furce Proper(e) Casthouse(f) Slag Handling
(g) Hot Meta Handling(h) Stoves and Hot Blas System(i) Gas Plant
(j) Utilities
(k) Control Systems(1) Maitenance Facilities(m)Personnel Support Facilities
RAW MATERI STORAGE AN HALING
Bulk materials such as ore, pellets, fluxes, and coal are normally delivered bybulk carers (ship or barge) or by ra car. Coke could be delivered in the same fashion
or be produced in-plant by coke ovens. Sinter can be delivered to the plant or can beproduced in-plant from ore and in-plant generated materials (mill scale, B.O.F. scrap,pellet fines, coke breeze, etc.). These raw materials, whether purchaed or produced in-plant, requie sufcient controlled storae to support the blast fuace plant operations.
Storage capacity is required in the event of predictable delivery disruptions (i.e.normal cessation of seaway shipping due to witer ice conditions) or unpredictabledisruptions (such as possible late delivery due to ship mechancal problems).
Additional storage capacity can be required due to possible changes in the
source of cert raw materials. Separ storage locations are requied due to differentphysical or chemical characteristics in simar materials. Mixig of "simlar" materialscould cause process control/metaurgical problems.
The storage piles must be searted to prevent intermxig of dissimilarmaterials. The piles must be placed on prepared beds to enable the raw material reclai
equipment operators to distinguish bet\\"een prie and tramp material. Piles are laid out
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to mie material degrtion and to prevent wid pick-up of fies. Water sprays
and cocoonig agents may be used to mi dust pick-up/car-offby wids.
Many different technques ar avaiable for raw material laydown and retreval.Laydown: self-unoadig ships, ore bridges, stackig conveyors, scrapers, etc.Retreval: bucket wheel reclaiers, frnt-end loaders, scrapers, diectly from bin orpile bottoms, etc.
Obviously, the lay down and retreval systems must be sized to ensure thethoughput requied for the blas :fe plant.
,i. .
STOCKOUSE
The stockhouse is the blas :fe operator's storage unt for direct feed of theburden to the fuace. Storage bin ar provided for each of the burden materials for theblast fuace. Individua bin are provided for simlar materials (i.e., pellets) havingdifferent metalurgical propertes.
The stockhouse provides adequate capacity for the varous burden materials inthe event of short term disruption of supply from the raw material storage areas.Typical stockhouse bin thoughput caacities, in the event of loss of raw material feed,are:
(a) Coke(b) Pellets and Sinter(c) (c) Miscellaneous Materials
2 to 8 Hours4 to 16 Hours8 to 24 Hours (fluxes, scrap, etc.)
These capacities are basd on rated fuce production and var dependingupon the reliability and the access tie for their replacement from inventory or from asupplier.
Burden materials tend to degre due to climatic conditions and repeated
handling. The greater the number of ties that the material is handled (stockpiling,reclaig, dumping, conveyor chutes, ore bridge buckets, etc.), the greater the percentof fines in the burden. The blas fue process requies controlled permeability and
hence controlled burden. The chagig of excessive fines, either generally thoughoutthe charge or concentrted over spifc short charging periods, can be disruptive to theprocess and daaging to the fue equipment. The stockhouse provides the lastreasonable opportty for removal of fies prior to charging into the fuace. Wherepossible, vibrating screens are ined afer the coke, sinter, and pellet storage bin to
elimate the major portion of the fies. The removed fies are collected forreprocessing or sale. Some blas fu operators charge fies to specific areas in thefuace to adjust local fuace permeailty and control heat loads on the fuace walls. _
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Moistue gauges are often provided in the stockhouse to monitor the actuwater quatities charged to the fwe. Ths inormation permts adjustments to thecharging quatities to compens for varing ambient conditions (i.e. higher cokemoistue due to rai fall).
Since different tys and varing amounts of burden material are requied to
support the continuous operation of the blast fuace, the burden materials must beprovided in a specific sequence (which itself can be changed frequently to supportvaring fuace operatig parete). Hence the stockhouse must be provided with
reliable equipment for extctig and feeding accurte quatities of specific burden
materials tò meet a specific schedule.
The most common type of stockhouse has been the highine tye (Figue 3).Ths type of stockhouse is located diectly adjacent to the fuace. Ral cars or bridgecraes feed the storage bin: the storae bins feed directly to a trveling scale car. Ascale car operator manualy controls the bin discharge gate to feed specific amounts ofmaterial into the scale-equipped hopper located in the scale car. Afer collecting the
proper tyes and amounts of maerial, he moves the scale car to a position above the"skip pit" and dumps the burden. \ ia a chute, into a waiting skip car. The skip car willthen be hoisted to the fuace top.
Placement of the stockhouse adjacent to the fuace often results in layout
congestion and restrcts flexibilty for futue modifications.
Many stockhouse have been modified to accommodate automatic cokehadling. Coke is often fed to the storage bins via conveyor. Upon demand, the coke isdischarged from the storae bin, over vibrating screens (for fies removal), anddiected into weigh hoppers in the skip pit. When the fuace charging sequencedictates, the coke is discharged into the skip car.
Improvements to the highe tye of stockhouse have primarly focused upon
automation of the bin gates (prmision ofpnewnatic or hydraulic actutors on each gate)and the scale car. A trckig system is usually provided to ensure that the automatic
scale car is selecting material and amount from the correct bin.
The highline tye of stockhouse, in conjunction with a scale car, has presentedfew options for the provision of ferous charge (pellets and sinter) screenig.
As knowledge of the blas fuace process has increased, more strgentrequiements for the burden have developed. The concept of "engineered burden" iswell recogned in the indus. It is generally accepted that there are limts to theflexibilty and adaptability of the highine stockhouse to support ths requiement.Where circumstaces have permtted (i.e. major fuds available for rebuilding or for_
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new intalations), automatconveyorized stockhouses have been implemented
(Figue 4).
Provision of an automated stockhouse can provide more effcient feed of raw
material to the stockhous and more effcient selection, screenig, weighg, anddelivery of the burden to the fue.
The automated stackhous can be located directly adjacent to the fuace
feeding skip car (i.e. conversion of an existig highline type stockhouse), or can be
located re~ote to the fue for chagig via a conveyor belt..~. p
HOISTING SYSTEM
Modem blast fues ar chaged with skip cars or by conveyor belt.
Skip Car Hoisting
The use of skip car (Figue 3) for blast fuaces evolved from the migindustr .
Blast fuace skip c.arare sized to suit the fuace thoughput (small
fuace/low thoughput/smal skips; large fuacelhgh thoughput/large skips).Obviously, many factors such as hoist capacity, skip bridge design, and so on, havetheir own inuences or consts upon the skip size.
Generally, two skips operate in opposing fashion (to reduce hoisting powerrequiements) on a common hoist. Skips travel on rails on a skip bridge, usualyintaled at an approxiate inclie between 60° and 80° to horionta. The ful skip
accelerates slowly as it leaves the skip pit, accelerates as quickly as possible reachigand traveling at maxum sp for most of the lift. The hoist slows the skip down asit approaches the top of the skip bridge. Dumping and horn rals gude the wheels of theskip as it is overted into the fuace top charging eqrupment. As the hoisting skip
reaches and stops at the fi dumping position, the empty skip (descending at the same
speeds) is just reachig the bottom of its travel into the skip pit, awaiting filling.
The skip charging system is a very reliable, effective technque for deliverigthe burden to the fuace top. However, it lacks flexibility for the operator in tht theskips can only hold a specifc amount of material (overloading results in overflling orexcessive hoist loads) or beomes ineffcient if small volumes of specific burden arerequired.
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For a 5000 THday fuce, a tyical skip hoisting system would comprise:
(a) Two skips, each with 375 cu.ft active capacity.(b) One hoist 50,000 lb capacity, two drves at 400 hp, 600 fpm maxum rope
speed.
(c) Vertcal hoisting height, 200 ft.
Furce Charging Conveyor
With the conveyorition of the stockhouse has come the conveyorization of thehoisting system?. It is now common tor stockhouses to be located remote from thefuce and one large conveyor belt (Figu 5) will car the burden to the fuace top.
If the fuace top is about 200 feet high, then a conveyor belt (inled at 80)
inclination to horionta will position a stackhouse at least 1,135 feet from the fuace.Steeper belt inclinations are usualy a\"oided to mize pellet roll back. It is commonto charge miscellaneous materials ditly over and afer the end of a pellet charge on
the conveyor belt in order to hold the pellets in place until they reach the fuace top.
For a 5000 THday fuace, a typical feed conveyor would comprise:
(a) Two drves (including one stdby)(b) Belt speed(c) Belt width(d) Belt lengt
500 hp each
350 fpm54 il.
-2700 ft. tota
CHAGING SYSTEM / FURACE TOP
The fuce proper is operated with some amount of positive (gauge) toppressure. Blast fuace gas consistg priarly of carbon monoxide, carbon dioxide
and rutrogen is generated by the fuace process along with large amounts of entraineddust. The blast fuce operator wants to maita the top pressure due to process
benefits and to conta the gases and dus (bth for fuel value and environmenta control
puroses). However, he mus reguarly place burden material inide the top of thefuace in order to replerush the internal process, without losing the fuace top
pressure.
Bell Type Top
For many years, the most common tye of fuace top has been the two-bell top(Figure 6). As the burden reaches the fuace top (by skip or conveyor), it falls into areceiving hopper and into the smal bell hopper. The small bell (corucal shaped steelcasting about 8Y: feet in diameter and 412 feet high for a 5,000 THday fuace)~
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lowers and permts the burden to fal into the large bell hopper. The small bell is liftedand seals agai a fixed seat on the smal bell hopper. Depending on the volume of the
large bell hopper, additiona loads of burden are sequenced into the large bell hopper bythe small bell. Thoughout ths process the large bell has remaied closed, sealing thefuace. When the correct number of load of burden have been collected, the large bell(conical shaped steel casg about 18~-S feet in diameter and 11 Yi feet high for a 5,000THday fuace) lowers and alO\\'s the burden to slide down the bell into the top ofthe fuace proper. Afer the burden discharges the large bell is raised and seals agaitthe underside of the large bell hopper.
Ooviousy, the burden distbuton control with the fuace for ths style of topis limted by how evenly the burden is placed on the large bell (skip dumping results inuneven placement of burden into th style of top) and the falling cures of the specificburden materials (i.e. coke or pellets) as they slide and falloff the large bell.
The two-bell top is susceptible to loss of sealing of the large and small bells andof the packig between the large bell rod and small bell tube. Bell leakge results fromabrasion by the burden material sliding oyer the bell sealing suraces. The rod packigleakage is a result of abrasion from fies either from with the fuace or fromcollecting on the large bell rod afer burden is dinped in the receiving hopper.
In an effort to mie wear of the large bell sealing surace, blast fuace gas
taen from with the fuace is introduced between the bells to equaize the space(reducing the pressure differential across the large bell sealing suiace). 1bs gas isrelieved to atmosphere prior to openig the small bell to permt introduction of moreburden.
The followig are some options ayailable to improve the limtations of the two-bell type top system.
The McKEE Distrbutor
The McKEE Distrbutor (Figue 7) for many years was the mai burdendistrbution improvement available for the two-bell tye top. It is stil in operation on
some fuaces today. However, it is quickly being replaced by other technologies.
1bs design incorporates the abilty to rotate the small bell and small bell hoppertogether while the skip car is dischagig. Burden is evenly distrbuted into the smallbell hopper, thus improving the even placement of burden onto the large bell.
1bs style of top is stil prone to small and large bell wear and subsequent lossof sealing effect.
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The CRM Universal Rota Distbutor Top
The CRM (Centr Recherhes Metalurgiques - Belgium) Universal RotaDistrbutor (Figue 8) was develope to elimte the loss of small bell sealing effect.Two bells (a sealing bell and a marial bell) are intaled in place of the normal smallbell. A revolving burden hoppe is mounted on the material bell. The sealing bell islocated beneath the material bell and seals agait a fixed seat. Durg skip dischage,the burden hopper and the close material bell are rotated to evenly fill the hopper.When filling is complete, the hoppe rotation stops. When it is time to dump onto thelarge bell, ,the revolving hoppe, material bell and sealing bell lower. The sealing belllowers below the- fied seat. Pan way though the lowerig process, the hopper descent
is stopped and the material bell and sealing bell contiue to descend until they reachtheir stop position. As the gap opens between the material bell and the hopper, theburden dischages evenly into the large bell hopper. As the burden leaves the gap, itdoes not come into contact v.ith the sealing valve seating sUDace, thus maitag thetop sealing capability. TIs style of top is capable of maitag two atmospheres ofintern pressure.
The CRM Top! improves me sealing capabilty and longevity of the two-belltop. It does not provide howewr a dratic fuace burden distrbution improvementover the McKEE Distrbutor and does not eliminate the vulnerabilty of the large bellsealing sUDace.
The GI- Lockhopper Top
The "Lockhopper Top" (Figu 9) is marketed by MA-GI- (Germanyl TIsmodification to the two-bell top reuces dependency on the large bell to maita a gasseal. The addition of lockhoppers \\ith separate seal valves for each skip dump locationprovides an additional capacity for sealing the top. The large bell can be operated withno differential pressure across its sealg surace (i.e., fuace top pressure equas largebell hopper pressure).
The operation is as follows:
(a) A skip dumps the load of burden into the lockhopper via a receiving hopperand open seal valve. The burden is placed on the rotating small bell andunfonny fills the rotatig distbutor hopper above the small belL. A sealbetween the lockhopper and the rotating small bell hopper is open while therotation is underway.
(b) When the burden dischage from the skip is fished, the seal valve and theseal between the lockhopper and small bell hopper are closed. Equaiziggas is introduced and the lockhopper is pressurzed to fuce top pressure.
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(c) The small bell is then lowered to introduce the burden into the large bellhopper.
(d) The small bell closes and the pressure from the lockhopper is relieved toatmosphere.
(e) The seal valve on the opposite side (i.e. at the other skip dumping position)is opened.
(1) The seal between the lockhopper and the small bell hopper is opened.(g) Rotation of the smal bell and hopper commences.(h) The top is now able to accept burden from the other skip.
-Ths style" of top improves the sealing capability and the longevity of the two-
bell top. The "Lockhopper Top" however, does not provide a dratic fuace burden
distbution improvement over either the McKEE or CRM Top. Although the large bellno longer is requied to perform a sealing fiction, the small bell sealing effectlongevity is stil critical.
Movable Arour
The major step taen to improve the burden distrbution of the bell tye top wasthe development of movable anour (Figure 10). Adjustable deflectors are installed inthe thoat area of the fuace to deflect the burden after it slides off the large belL.
The movable anour is adjused depending upon the specific burden materialbeing discharged and where the operator wants to place that burden with the fuace.
Several manufactuers pro\ide alternate styles of movable anour,3,4. Individuaarour segments can be moved unformy (simultaeously and equaly) inside thefuce to place the burden in an anular pattern.
Other styles of movable anour are available to provide individua control ofthe anour plates in order to achieve non-circular distrbution pattern.
Some disadvantaes assoiated with movable anour are:
(a) Most mechancal and wear components lie with the harsh envionment of
the fuace top cone.
(b) Some loss of internal workig volume is required to provide clearancebetween the movable anour and the design stockline level (albeit ths areaof the fuace canot be classified as a rugh productivity zone for fuaceworkig volume consideration).
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(c) Limted capability to deflect burden to the very center of the fuace,paricularly when the stockline level is aleady high. The rollingchacteristc of pellets often negates the limted displacement of the
movable anour.
The PAUL WUTH Bell-Less Top
In the early 1970,s PA.ll WUTI S.A. of Luxembourg developed the Bell-Less Top Charging System (Figu 11). lbs style oftop5,6 is a radical depare fromthe bell ~e top.
.t, .
Burden can be placed \\ iùi the fuace in any pattern requied by the fuace
operator. Anular rigs, spir, segment and point placement are common pattern
achievable by synchroni or independent tilting and rotation of a burden distrbutionchute located with the top cone of the fuace.
Furace top sealing is maitaed thoughout the campaign of the fuace.Maitenance activities are simple and of short duration.
Generally, the bell-less top consists of a receiving chute or hopper (receivingburden from the skips or from a conveyor belt), a lockhopper with upper and lower sealvalves, a material flow control gate, a mai chute drve gearbox (a water or gas-cooledunt used for chute rotation and titig), and the burden distrbution chute.
There are three mai styles (Figue 12) of bell-less tops available today, namely:
(a) Parallel Hopper(b) Central Feed(c) Compact Style
Typically, the parel style incorporates two lockhoppers (thee hoppers have
been intaled on some fuces for thoughput and backup puroses; a one "eccentrc"hopper style has been ined for an application with restrcted clearce). Since theearly 1980's, many fuces haw selected the "central feed" single lockhopper style forits improvements in burden segregation and burden distrbution control resulting inenhced fuace operation.
A "compact" stle of bell-less top has been developed for small to mid-sizedfuaces to permt the introduction of the bell-less top (and its advantages) to fuaceswhere the other larger types of bell-less tops canot be used due to cost or physicalconstrts.
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Bell-less top operation for a central feed tye (Figue 13) is as follows:
(a) Burden is dischaged frm a skip or conveyor belt though a receiving chuteor hopper pas an open se valve into the lockhopper.
(b) Afer the burden is reeived in the lockhopper, the upper seal valve is closed
and equaizg gas is introduced to pressurze the lockhopper to fuepressure.
(c) The lower seal valve opens.(d) The burden discharges from the lockhopper. The material gate ha been set
to the preselected openig to suit the specific burden material to bedischaged.
(e) The burden drops vercaly though the feeder spout with the maitrmission geabox and falls onto the burden distrbution chute.
(f) The burden distbuton chute directs the burden to the requied point(s)with the fuce (Figu 14).
(g) When the lockhopper is fuly discharged (monitored by load cells and/oracoustic monitorig), the lower seal valve is closed.
(h) A relief valve is opened to exhaust the lockhopper to atmosphere (orthough an energy reovery unt).
(i) The upper seal val\'e opens and the sequence can repeat.
Users regularly repon bell-less top advantages over other top charging systems,such as:
(a) Higher top pressur capability (i.e. 2.5 atmospheres).(b) Furace fuel sa'\ ings.(c) Increased fuace production.(d) More stable operation.(e) Reduced maitenace in terms of cost and tie.(f) Increased fuace campaign life.(g) Improved fuce opeonal control when employing high coal injection
rates at the tuyeres.
Fl"RACE PROPER
The fuace proper is the mai reactor vessel for the blast fuace ironmakgprocess.
Its internal lines ar designed to support the internal process. Its external linesare designed to provide the necessa systems to conta, mainta, monitor, supportand adjus the internal process.
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The blast fuce is a counterfow process:
(a) Burden at ambient conditions is placed in the fuace top onto the columof burden with the fue.
(b) As the burden descends \lith the burden colum, it is heated, chemicallymodified and fily melted.
( c) Furer chemical modicatons occur with the molten material.(d) The molten products ar extted near the bottom.
(e) Melting of the burden materal and extaction result in the descent of the_ burden colum and the nee for replenishment of the burden at the top.
(£) Hot blat ai is introduced thugh tuyeres near the bottom.
(g) Blas fuace gases ar genered in front of the tuyeres and ascend thoughthe burden. They chemicaly modify the descendig burden and theythemselves are chemicaly modified and cooled.
(h) Blast fuace gas (and dus) is extcted near the top of the fuace.(i) Heat is extacted from the vessel in all directions (priarly though the
ling cooling system) and along with the blast fuace gas, molten iron and
molten slag.
Furace Stvle
Furaces are constrcted to be mantle supported or free stading (Figure 15).
Mantle type fuaces (most ~ort American fuaces) characteristically have arig girder (mantle) located at the bottom of the lower stack of the fuace. The mantleis supported in tu by colum \vhich re on the mai fuace foundation. The hear,tuyere breast and bosh are also supported by the foundation. Furaces with mantlesupport colum tend to have restcted access and reduced flexibilty for improvementsin the mantle, bosh and tuyere breas aras.
Since thermal expanion is a major consideration in fuace shell design, the
mantle style of fuace provides an interestig design consideration. The mantle
support colum are relatively cooL. The mantle tends to maita a constat height,
thoughout the fuace campaign \\ith respect to the fuce foundation. Thermalexpanion of the stack due to process heat is considered to be based at the "fixed"mantle (i.e. the top of the fuace rases with respect to the mantle). The effectiveheight of the bosh, tuyere breas and hear wall shells (supported on the fuacefoundation) increases due to the theral expanion of the shell caused by the processheat. The lower portion of the fuace li upwards towards the fixed mantle; therefore
the provision of an expanion joint of some type is required at the bosh/mantle
connection or somewhere appropriately located in the lower portion of the fuace.
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Free stadig fues wer developed to elimte the colum and permt the
inlation of major equipment and fuace cooling improvements. Ths fuace style
has a thcker shell for stctu support. Instalation and maitenance of a reliable
cooling and ling system is esstial in order to susta the strctu longevity of theshell.
Two varations of the fr stdig fuace have been employed. One stle
provides for a separte stctu support tower to car the fuace off-gas system and
chagigloistig system load. The other style (while it does employ a separatesupport tower for shell replacement puroses durg relines) uses the fuace proper tosupport th: off-gas system and chagloistig system loads.
.~. R
Special consideration to the fuace shell design mus be made regardless of thefuace stle. The vessel is subjected to internal pressures from the blast and gas,
burden, molten iron and slag. De and live load durg all operating, maitenanceand reline states mus be consider as weif.
Furace Zones
The major fuce prope zones (Figue 16) are as follows:
(a) Top Cone(b) Thoat(c) Stack
(d) Mantle/Belly(e) Bosh(f) Tuyere Breas
(g) Hear Walls
(h) Hear Bottom(i) F oiidation
Top Cone
The top cone or dome is the uppermost par of the fuce proper. It support
the fuace top charging equipment, and the off-gas collection system (tyically inNort America). Stock rods (stockle recorders or gauges) are usualy placed here tomonitor the upper level of the buren in the fuace. These devices are the unts whichprovide the permssive or indication signals to charge the next scheduled burden inputto the fuace.
Typically, they are weights lowered by special wiches, or microwave unts.Some fuaces incorporate raioactive isotope emitters and detectors moiited in thefuace thoat to monitor the burden leveL. Infared camera can be instaled in the top
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cone to monitor the off-gas tempetue distrbution as it escapes the fuace burdenstockline.
The top cone is the coolest zone of the fuace proper but can be exposed toextemely high temperatues if burden "slips" (rapid, uncontrolled burden descent afera period of unusua lack of descent). The newly charged burden falls though ths zone;off-gas is cared away from th setion.
Thoat
Stèel wea plates or arour are instled in ths zone. Here, abrasion of the
fuace ling from the charged burden is the prie cause of deterioration. Furace
operators work to maita the upper level of the burden (the stockline) in ths region.As noted earlier, movable anour can be intaled in ths area in order to deflect theburden falling from a large bell.
With the advent of the PAlL WUTH bell-less top, wear of the stockline areacan be greatly dimshed. Some users have elected to elimate the anour plates anduse an abraion resistat refrctory ling intead.
Stack
The stack (someties caled the "in-wall") is the zone between the mantle (orbelly on a free stading fuce) and the stockline area. Smooth, unform lines (the
process "workig surace") of the stck are essential for unform and predictable burdendescent, blast fuace gas ascent and stable process control thoughout the fucecampaign. Process considerations dictate a larger diameter at the base of the stack thanat the top. Typical stack angles ar -850 from the horizontal.
Mantle/Bellv
The mantle or belly (free stding fuace) area provides the tranition betweenthe expanded stack and bosh setions. Maitenance of the effectiveness of thecoolingling system is parcularly importt for the mantle tye fuace in order toprotect the mantle strctue. Ob\ iously thermal protection is importt for the free
stading fuace stle as well; however, the free stading design is less complicated and
more accessible in ths area
Bosh
The bosh area lies between the tuyere breast and the mantle/ belly of thefuace. The bosh diameter incres from bottom to top. The inclination of the boshpennts the effcient ascent of the process gases and has been found to be essential in_
5-16
order to provide the necessar zone seice life (the process gases are extemely hot andinternal chemical attck conditions ar severe). Typical bosh angles are ~80° from the
horizonta.
Boshes are constrcted in two baic styles, banded and sealed (Figue 17). Theycan be cooled by varous technques.
Banded boshes are found in older mantle supported fuaces (they canot be
applied to free stding fuces). A number of steel bands are placed in incrementaly
increasing diameters (smallest at the bottom of the bosh; largest at the top) and are tiedtogether With cenecting strps. Gas between the bands pemit the introduction ofcopper cooling plates. Ceramc brick lig must be used as ai inltration would result
in oxidation of carbon-based lings. Ga leakage though the banded bosh can be high.Ths style is not suitable for fues with high blast pressurelhgh top pressure
requiements. Banded boshes pro\ ide sucient flexibility to elimate the requiementfor a shell expanion joint in the lower porton of the fuace.
Sealed boshes, using contiuous steel shell plate instead of separate bands, areemployed to pemit the use of improved cooling/ing systems, elevated fuaceoperating pressures, and the free stdig fuace style. Sealed boshes retain valuablegases with the fuace, thus imprO\ ing the metalurgical process. As well, the sealbosh, since it precludes ai entry into the linng, supports the use of carbon basedrefractories.
Tuvere Breast
Hot blas ai is introduced to the fuace though tuyeres (water-cooled copper
unts) located with the tuyere brea The munber of tuyeres requied depends upon
the size (production capacity) of the fuce.
The tuyere breast diameter, tuyere spacing and number of tuyeres are inuencedby the expected raceway zone sizg in front of each tuyere.
Tuyere stocks (Figure 18) convey the hot blast ai from the bustle pipe to thetuyeres. The tuyeres are supported by tuyere coolers (water-cooled copper unts) whichare in tu supported by steel tuyere cooler holders (either welded or bolted to the
fuace shell). Special consideration mus be made in the tuyere breast shell and ling
design in order to maita effective sealing of the varous components in order to
prevent escape and loss of the fue gases8.
5-17
Hear
The hear (Figue 19) is the crucible of the fuce. Here, iron and slag arecollected and held unti the fuce is tapped. The hear wall is penetrated by tap holes(often called iron notches) for the removal of the collected iron and slag. The number oftap holes is dependent upon the size of the fuce, hot meta and slag handlingrequirements, physical and capita constrts, etc.
Many fuaces are equipped with a slag or cinder notch (usualy one perfuce, although some fuaces could have two). The slag notch openig elevation isusualy seyeral feet higher th the iron notch elevation. In earlier days, when slag
volumes were high the slag was flushed from the slag notch periodically. Thssimplified the iron/slag separtion process in the casthouse. More commonly now,however, the slag notch is retaed solely for intial fuace st-up procedures or foremergency use in case of irn notch or other fuace operating problems.
Hear Bottom
The hear bottom support the hear walls and is flooded by the iron withthe fuace. As the campaign progresses, the hear bottom ling wears away to a
fixed (hopefully) equilibrium point.
The remaig refrctory contas the process and with sufcient cooling orinerent insulation value protects the fuace pad and foundation.
Cooling System
Little reference ha ben made in ths paper so far in the provision of specificling technologies to the varous zones and areas of the fuace proper. Specific ling
technologies will be covered in grater depth by other authors in the course.
The application of spifc cooling technques (if at all) to individua fuacezones is dependent upon many factors such as campaign life expectacy, fuace
operation philosophy, burden types, refrctories, cost constraits, physical constraints,available cooling media, preference, etc. Different cooling technques can be providedfor different zones to assist the lig to resist the specific zone deterioration factors.
Generally speakg, the provision of adequate cooling capacity is essential ineach of the applicable fuce zones if the linig system located there is to surive.Where the thermal, chemical and to some extent the abrasive conditions of the processare exteme, suffcient cooling mus be provided to maitan the necessar unforminterior lines of the fuace and to protect the fuace shelL.
5-18
Typically, the top cone and droat areas of the fuace are tucooled. The hearbottom can be "actively" cooled by underhear cooling (ai, water or oil media) or"passively" cooled by heat conduction though the hear bottom ling to the hear
walL.
The basic cooling options for the balance of the fuace are:
(a) No Cooling (tyicaly the upper portion of the stack is uncooled in many
fuaces)(bl Shower or Spray Coolig(c) Jaclæt or Chanel Coolig(d) Plate Cooling(e) Stave Cooling
Shower Cooling
Water is directed by sprays or by overfow troughs and descends in a film overthe shell plate. Effective spray nozze design, numbers and positionig are importtfor proper coverage and to mie rebound. Proper deflector plate design is essentialto ensure effcient cooling water distbution and to mize splashig. Showercoolig is often employed in the bosh and hear wall areas. Spray cooling is
commonly applied for emergency or back-up cooling, primarly in the stack area.Exterior shell plate corrosion or organc fouling are common problems which candisrupt water flow or ÌnlÙate the shell from the cooling effect of the surace appliedcooling. Water treatment is an importt consideration to reta effective cooling.
Jacket or Chanel Cooling:
Fabricated cooling chabers or indeed strctual steel chanels or angles are
welded directly to the outside of the shell plate. Water flows at low velocity though thecooling elements in order to cool the shell and ling. Jacket or chanel cooling is often
applied to the hear walls, niyere breas and bosh areas. Scale build-up on the fuaceshell and debris collection in the bottoms of the extema cooling elements cancompromise their effectiveness. Hence periodic cleanng of the cooling elements isessential.
, ,I
The critical area of concern in the cooling schemes mentioned so far is thenecessity for the shell plate to act as a cooling element. If exteme heat loads are actingupon the inide face of the shelL there will be an extemely high thermal gradient acrossthe shelL. Ths effect reslÙts in high thermally induced shell stresses and eventulcrackig. The cracks \\ill st from the inide of the fuace and propagate to the
outside. The cracks will remai invisible (other than a "hot spot") until they can fullypenetrate the shell plate. Thoug crackig of the shell plate results in blast fuace gas ~
5-19
leake, exposed shell carburon and disruption of the cooling effect (parcularlyspray or shower cooling). Shell crackig into a sealed cooling jacket or chanel isdiffcult to locate and can resut in long fuace outae time for repai. Entr of waterinto the fuace (often when the fuace is off-line and intern fuace gas pressurecanot prevent entr of coolig \'\-ater though shell cracks) can have detrenta effectupon the fuace ling. Water in the fuace could be potentially dangerous due to
explosion risk (steam or hydrogen).
Shower and jacket coolig rely on the shell plate to conduct the process heat tothe cooling media; plate and stve cooling are confgued to isolate the shell fromprocess. ~. .
Plate Cooling
Instalation of coolig elements though the shell of the fuace (Figue 20) hasbeen a major fuace design improvement resulting in effective cooling of the fuaceling and protection of the shell plate. Cooling is provided along the lengt of the
cooling element penetration into the linig. The inserted elements provide positive
mechancal support for the refrctory ling.
Typical cooling plate manufactue is cast high conductivity copper. Single ormultiple passes of cooling water can be incorporated.
Cooling boxes (Figue: 1 J \\ith larger vertical section have been produced fromcast steel, iron or copper.
Cigar tye (cylindrcal) coolers of steel and/or copper have also been
successfully employed5.9.
The philosophy of dens plate cooling (i.e. vertical spacing of 14" to 16" center-to-center, and horizonta spacing of 24" center-to-center) has enhanced the coolingeffect and increased ling lie5.
Copper cooling plates haw traditionally been anchored in the shell plate withretainer bars or bolted connections to permt ready replacement if plate leakage occurs.More recently, plates have ben designed with steel sections at the rear of the plate forwelding directly to the steel shell. Whle sometimes tag longer to replace, ths stleprovides a positive seal agait blast fuace gas leakage.
Plate coolers are tyicaly intaled in areas above where the molten iron
collects in the fuace. Hence the mid point of the tuyere breast, right up to theunderside of the thoat anour is the rage of application.
5-20
Stave Cooling
Cast iron cooling elements (Shaon plates or staves) have been used for manyyears in the bosh and hear wal ar. These castings have cored cooling passages of
large cross-section. Whle their service lie was not remarkable in the bosh, multiplecampaign were common for the hea wcù1.
These staves often sufered frm low flow rates of marginal quaity coolingwater (scaling and debris depositioIlbuid-up) and someties casting porosity. Waterleak into !he hear wall can be a signcant problem.
~, .
In the 1950's, the USSR develope a new style of stave cooler (Figue 22) and"natual evaporative stave cooling"lo. For ths design castings were of gry cast iron
contag steel pipes for water pases. The pipes were coated prior to casting to
prevent carburzation of the cooling pipe and metalurgical contact with the stave bodymaterial. The staves are instaled in horionta rows with the fuace and the coolingpipes project though the shell. Vertcal colum of staves are formed by theinterconnection of the projecting pipes from one stave up to the corresponding stave inthe next row.
Staves can be applied to al the wals in the zones below the anour (Figure 23).Staves in the hear wall and tuyere bre are supplied with smooth faces. Staves in the
bosh, mantlelbelly and stack usualy haw rib recesses for the instalation of refrctory.
Evolution of the stave cooler design has been dramatic. Staves in the higherheat load areas are now typicaly cas from ductile iron for improved thermal
conductivity and crack resistace.
Whle early stave design usd castable refractory (intaled afer staveinlation with the fuace), ribs now normally incorporate refractory bricks, either
cast in place (with the stave body at the foundr) or slid and morted in place prior toinlation in the fuace.
Staves are normally expected to reta a refractory ling in front for some tie.
Afer loss (expected) of the ling the st\"es are designed to resist the abrasive effects ofdescending burden and ascending dirty gas. As well, they must absorb the expectedprocess heat load and resist thermal load cyclig and shock.
Four generations of stves (Figue 24) are commonly recognzed In theindustr
1 i .
5-21
First Generation (no longer commonly used):
(a) Four cooling body ciruits (with long radius bends which did not effectively
cool the stave comer).
(b) Gry iron casgs.(c) Castable rib refrctory.
Second Generation:
(a), Four cooling body ciruits with short radius bends for improved comercoolhi.
(b) Ductile iron casgs.(c) Cast-in or glued-in rib bricks.
Thd Generation:
(a) Two-layer body coolig incorporating four or six cooling body circuits(stave hot face) and one or two serpentine cold face circuits (stave cold face)for additional or back-up cooling in the event of hot face circuit loss.
(b) Additional edge coolig (top and bottom).
(c) More frequent use of cooled ledges to support a refractory ling.
(d) Cast-in or morted-in rib bricks.
Four Generation:
(a) Two-layer cooling (simar to thrd generation).(b) Cooled ledges.(c) Cast-in wall brick lig elimatig the need for a manualy placed interior
brick ling.
Staves incorporating hot tàce ledges are more effective in retag a brick
ling than the smoother rib faced bricks. However, once the brick ling disappears,
the ledges are very exposed \\-ith the fuace. The ledges disrupt burden descent and
gas ascent. Exposed ledges tend to fail quickly. They are often servced by coolingwater separate from the mai stye cooling circuit(s). In ths way leakg ledge circuitscan be more easily located or isolated. Some stave suppliers are now providing separateledge castings so that ledge crakig and loss will not damage the parent staves. Aswell, there is some curent change in philosophy to abandon the application of ledgesentirely.
Varations of the basic stve generation styles are common5. For example,staves of four generation style utg a refractory castable for the wall ling have
5-22
been employed successfuy. Alternely, brick lings have been anchored to the stave
bodies. Such approaches can be used to substitute for brick support ledges.
A "fift" generation of stves design ha been the developed. It is the copperstve (Figue 42). Rolled copper plates are drlled to form cooling passages. Rib
recesses are machied to permt the ination of low conductivity refrctory bricks.Test intalations of ths stve ty have ben successfui12.13.
Natual EVaporative stve Coolig (NVC) (Figue 25) is a technque whereboiler quaity water is introduced into the bottom row of staves and flows by natualmean up the vètcal cooling circuits. As the process heat conducts though the staveand cooling pipe into the water, the water in tu heats up. As the water wans, itexpands. Since cooler water is being intruced below, the wan water tends to moveupwards. At some point in the vertcal cooling circuit, the water will be at the boilingpoint. As the water changes phae to steam, due to the latent heat of vaporization,additional heat is absorbed (drving the phase change). Afer boiling begin, two-phaseflow (water and steam mie) ascends the cooling pipes to the top of the fuace.Usualy located on the fuace top platíòrm are steam separator drs used to extact
and vent the steam to atmosphere. Make-up water is introduced to the dr (to replace
the discharged steam). The water is piped back by gravity to the fuace bottom and isfed once more to the staves.
Ths cooling technque is very effcient and has low operating costs; there is nopumping equipment.
More recent improvements ha'-e been to boost the flow of the cooling waterwith recirculating pumps (Forced EVaporative Cooling -FEVC) in order to ensureunform cooling water flow and to cool the recirculating water (Forced Cold WaterCooling - FCWC)14. Both of these approaches have resulted in improved stave andling life.
Russian stves and natu evaprative cooling were first used in Nort Americaat STELCQ's Hilton Works 'D' Blas Furce in 197410.
Staves provide an excellent protection for the shell plate thoughout their servicelife (which is extended while the interior brick ling remai in place). Staveapplication has been implemented in al areas of the fuace from hear wall up to andincluding the upper stack.
Whle some people (priary non-stave users) maitan that stave leakdetection and stave cooler replacement is complicated, in fact simple and effectivemeans have been developed.
5-23
¡
One drwback for conversion of an existig plate cooled fuace to stavecooling could be the cost of a new shell. However, if the existing shell is aleady indistress and must be replaced in any event, the conversion cost is not a major factor.
CASTHOUSE
The casthouse (Figue 26) is the area or areas at the blast fuace whereequipment is placed to safely extct the hot metal and slag from the fuace, separatethem and direct them to the appropriate hadling equipment or facilities.
-
As mentÍoned earlier, the iron and slag are removed from the fuace thoughthe tap hole (iron notch). Only inuently today is slag flushed from the slag or cindernotch.
Tap Hole Equipment
Tap hole equipment mus be reliable and require mium maitenance.Furaces typically cast eight to eleven ties per day.
Mud Gun
The mud gu (Figue 27) is us to close the tap hole afer casing is complete.A quatity of clay is pushed by the mud gu to fill the worn hole and to maintain anamount of clay ("the mushroom"') \\itb the hear. The mud gu is usualy held in
place on the tap hole until the tap hole clay cures and the tap hole is securely plugged.
A "hydrulic" mud gun uses hydrulic power to swig, hold and pus the clay.Typical clay injection pressure is in the order of 3,000 to 4,000 psi, permtting it to pushmodem, viscous clays into fuaces operatig at high pressures. The hydrulic gun is
held agait the fuace with the equivalent of 15 to 35 tons of force. Ths style of mud
gu can be swug into place in one motions.
An "electromechacal" gu ha thee separate electrc drves for unt swing,barel positionig and ramg. Hence several separate motions are required toaccurately position the gu at the tap hole. Clay injection pressure is in the range ofonly 600 to 1150 psi. The electromechacal clay gu is latched to the fuace to keepit in place durg plugging!5.
Tap Hole Drill
A tap hole drll (Figue 28) is used to bore a hole though the tap hole clay into
the hear of the fuace. A dr unt is swug into place hydraulically and held
hydraulically in the workig position. A pneumatic motor feeds the hamer drll unt ~
5-24
(with an attched drll rod and bit) into the hole. Compressed ai is fed down the centerof the drll rod and the drll bit to cool the bit and blowout the removed tap hole clay.When the tap hole ha penetrted into the hear the drll rod is retrcted and the drllswigs clear of the hot meta steam.
Soakg Bar Technque
In recent year, the application of the soakg bar practice has improved thecasting process. Whle the tap hole clay is stil pliable afer plugging, a steel bar isdrven int~ the tap hole by the tap hole dr. Wlle the bar sits in place durg the time
between casts, it, heats up via conduction from the hear iron. Ths permts curng ofthe tap hole clay along its entir lengt (as opposed to curg with the fuace andsetting at the outside near the fue cooling elements). The cured tap hole clay ismore resistat to erosion durg taping, thus improving cast flow rate control. Less
clay is requied to replug the hole. When the tap hole is to be opened, a clamping
device and back hamerig device on the tap hole drll extact the rod. The timg fortap hole openig can be more easily controlled (predicted) than by conventional drlling.Ths featue is importt for smooth fuce operation and for scheduling of hot meta
delivery to downstream facilities5.16,¡-.
Same Side Tap Hole Equipment
Mud gun and drlls have normly been instaled on opposite sides of the taphole. More recently, design development has permtted instalation of the unts on one
side of the tap hole. The drll S\\ings over the mud gu or vice versa. Ths type ofintalation faciltates improved access for tap hole and trough maitenance and theimproved application of trough and ta hole area fue collection.
With the advent of tuyere access platforms to facilitate tuyere and tuyere stockinspection and replacement, the headoom available for the tap hole equipment hasdimshed. However, same side ta hole equipment intalations (Figure 29) can beachieved with low headroom (for exaple 7.25 feetr
Trough and Runer System
Typical hot meta and slag taping rates are in the range of four to six and theeto five tons per miute, respectively. The trough and ruer systems must be designedto properly separate the iron and slag and to convey them away from the fuace forflow rates with the normal flow rate rage and for unusua peak flow rates.
5-25
Trough
The iron trough (Figue 30) is a refrtory lined tudish located in the casthouse
floor and designed to collect irn and slag afer discharge from the fuace. The ironflows down the trough, under a skier and over a da into the iron ruer system.
The iron level in the trough is dictaed by the da. Proper dam design submerges thelowest porton of the skier in the irn pooL. The slag, being lighter than the iron,floats down the trough on top of the irn pooL. Since it canot sin into the iron and
though the skier openig, it pols on top of the iron until sufcient volume collectsto overfo~ a slag da and ru do\'\TI the slag ruer. At the end of the cast, the slagruer da height is lowered to dr off most of the slag. The residua iron is retaedin the trough to prevent oxidation and thermal shock of the trough refractory ling.
When maitenance of the troug lig is requied, the iron pool can be dumped byremoving the iron da (Baker da t: -p), or by openig a trough drai gate, or bydrllng into the side of the trough (at its lowest point) with a drai drlL.
The trough bottom is usuay designed with a 2% (minum) slope for effectivedrainig. Trough cross-section and lengt design are important for effective iron andslag flow pattern, retention and separtion. A "good trough" design results in hot metalyield improvements17.
Effective trough ling and coolig technques are importt for ling life, hotmeta temperatue, and casous Stctual steel and concrete heat protectionconsiderations.
Trough traditionally were contaed in steel boxes "bured in sand" in thecasouse floor system. ImprO\'ed trough design incorporate forced or natual aiconvection or water-cooling.
Runers
Modem practice requis tht the ruers be as short as possible. Ths
mies iron temperatue loss and reuces ruer maitenance and fue generation.As well, shorter ruers can resut in reduced capita outlay for casouse building
intalation or modification.
Since the ruers mus slope away from the fuace, the casthouse floorgenerally follows the same slope as the ruers. Steep floor slopes can result in diffcult
access and workig conditions. Now, where possible, operators and designers tr toincorporate relatively flat floors to enhance casthouse operation and improve safeworkig conditions.
5-26
'I
Slag Runers/Handling
Slag ruers are usuay designed with a 7% (minium) slope. Slag can bediected to:
(a) Slag pots for raway or mobile equipment haulage to a remote site for
dumping.
(b) Slag pits adjacent to the fue for air cooling and water quenchig prior toexcavation by mobile equipment. Pit ru slag is used for backfll or can be
_ cruhed for use as aggrgat.(c) Pelletzig or grulation facilities adjacent to the fuace for conversion of
the slag to material sutale for backfll, aggregate or Portland cement
replacement. Graulation unts (Figue 31), in paricular, can be providedwith systems to elimte fue emissions associated with envionmenta or
industral hygiene problems.
Iron Runers/Handling
Iron ruers are usualy designed with a 3% (minum) slope. Iron is usualydirected to hot metal tranfer ladles (torpdo cars/ bottles/ etc.) for movement to thesteel shop (or iron foundr, pig caser, iron granulation unt, etc.). Whle normalpractice used to have one iron ruer system with diverter gates directing the iron todifferent pourng positions, each \'\ ith a ladle, more recent application of the tilting ironruer practice has been beneficial (paricularly in the "shortened ruer" benefits
mentioned earlier).
A tilting ruer (Figue 32) diyerts molten iron to either of two torpedo cars
afer collecting it from the iron ruer. Often provided with an electrcal motor-drvenactuator (with a manua handwheel back-up), the ruer is tilted at about 5° to divert theiron. A pool of iron is held in the titig ruer to minmize splashig and refractorywear. When one torpedo car is fied. the ruer is tilted to the opposite side to fill theother ladle. If required, a 10comotIye removes the full ladle and spots an empty ladle inits place. Ths operation can be done \\1thout plugging the fuace. When the cast isfished, the tilting ruer is tilted an additional 5° to dump its pool of iron into a
torpedo car.
Fume Collection
Fume collection requirements and applications appear to var signficantly in
Nort America. Furaces curently have ful, parial or even no casthouse fuecollection.
5-27
Exhaust fan and bagouse capacity in the order of 325,000 to 400,000 acfI(depending upon operation and design prctices) is tyical for ful fue captue of a two
tap hole casthouse intalation (for troug. ruers and tilting ruers).
Proper design and application of fue collection ruer covers canfaçilitate
casthouse access (i.e. flat floor confguon using steel slabs or plates) for personneland mobile equipment crossover. As well, ruer covers can reduce hot meta
temperatue loss and improve ruer refrctory longevity.
So-ie fuaces employ flame supression which elimiates the oxygen in theai diectly over-lhe trough and iron ruers. Products of combustion prevent oxidation
of the iron surace reducing visible parculate and fues.
DESIG~ EXALE
Introduction
The eight sections above are a cataog of the blast fuace equipment anddesigns. Followig is a shortened and simplified example of the application of some ofthat equipment in the development of a proposed blast fuace modification. Thsexample is limited to the fuace proper. For a ful study every element in the process
chai, from raw materials delivery to hot meta consumption must be checked to seethat there are no "bottle necks" in the system which will prevent the fuace frommeeting the goals set out for the modifcation. Ths example will use an existing NortAmerican blast fuace. It's lines and cooling are typical of the blast fuces builtaround the Great Lakes. Ths is a short yersion of one of the many thought process thatcould be used to develop the improvements and some alternatives that would beconsidered for an upgrde of ths fuce. The goal of ths study is to search out theoperation and equipment that will best meet the futue needs of ths operation.
Financial justification is the oyerrding consideration for all improvements.Depending on the size of improvement the economic study can become very extensive.Some of the larger fiancial stdies reach from the ore mine to the customer. Sincecosts are confdential and site speifc they will not be presented here. There is one cost
aspect to keep in mid, that is the effect of the fuce reline on the cost ofimprovements. Relines are a major capita expense. At the same time they present anopportty to improve profitabilty thugh facility improvements. Ths opporttycomes first from the obvious fact tht the improvement's cost will not have to include adown time penalty. Also, facility maitag costs, that money which would havebeen spent to repai or replace the less cost effective materials, equipment or fuacedesigns can be deducted from the cost of the improvement. The benefit resulting fromthe upgrade need justify only the differnce in cost between repair and improvement.
5-28
For ths reason studies which seach out those opportties should be performed wellin advance of any planed relie.
Background
Ths example if bas on one of the tyical small Nort American blasfues with 30,000 cubic feet of workig volume. (Figue 33) Ths fuce'sperformance has been very good (Table 1).
Example Blas Furace~. .
Present Opration
Smelting 2,491 NTHdayProduction 2,428 NTHdayPercent delays 2.5 %Operations 1 ,404 mi./day
Coke 835 #/NTHOil 115 #/NTHPellets 3,096 #/NTHMSlag 53 #/NTHSlag Volume 410 #/NTH
Cu. ft./NTH 41,119 SCFMWind 71,143 SCFMOxygen 21. 04 %Temperatue 1,729 OF
Grais 2.79RAT Calculation 3,706 OF
Hot Metas Si 0.44 %Sul 0.062 %Mn 0.43 %
Table 1
5-29
However the reline history leaves somethg to be desired.. Campaign havebeen short and uneliable (Table 2).
1. CAMPAIGN TOTAL Tons 2.2MM2. Tons 2.4 MM" Tons 2.4 MM~.
4. Tons 2.8MM5. Tons 3.2MM6. Tons 1.6 MM7. Tons 2.0MM8. Tons 1.5 MM
Averae Campaign 2.2 MM
Average Hear 4,623,791
Table 2
The causes for the fuce' s ling life problems are hardware specific. The
cooling is inadequate for today' s operation and the fuace lines are not good.
The post reline production requiement is 3,000 NTHMday which is 120% ofthat achieved by today's operation (Table 3).
Production
. Production Daiy Intaeous. . . . . . . . . . . . . . .Scheduled Do\\TI Time .... ....................Unscheduled Do\\TI Time.....................
Production, Daiy A \"erae ....................Anua Production . . . . . . . . . . . . . . . . . . . . . . . . . . ...
3,000 ton/day
2 %1.5 %
2.895 ton/day
1,000,000 ton
.
.
.
.
Campaign Life
. Campaign Lengt. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. Interi Stop At . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. Duration of Interi Repai....................
Hot Blast Delivered
8 years
4 years
30 days
. Hot Blast Flow Rate (Max.) ..................
. Hot Blas Tempenr ........................80,000 scfi1,850 0
Table 3. Rebuild Objectives
5-30
With some fuce upgres and chages in the operation ths production goalis achievable.
The more diffcult problem wil be to assure that our fuace can achieve thedesired eight year campaign. Furce campaign must be looked at for both lengt oftie and tota tonne The expted campaign tonnage is 8.8 millon tons when the
fuace is ru for eight year at the new production rate. Ths is approxiately four
times the average campaign tonnage previously achieved. In light of the fuaces
previous performance an eight year campaign with the existig fuce design fuace
would be ~ikely.,to .
Taken together, the performance improvements needed are quite large. If
today's technology can be implemented they should be achievable.
Comparson
How do the key pareters for the present operation of the example fuacecompare with the rest of Nort America? If the proposed operating goals are imposedon the existig facility how would it compare?
Stack Productivity
The fist measure to be looked at is stack productivity. Ths is rated in tons perday per hundred cubic feet of \vorkig volume. Working volume is the fuace'sinternal volume calculated ben:veen the tuyeres and the stocldine. Workig volume is ameasure of the volume of materials actuly in process. Therefore, tons per day perhundred cubic feet of workig volume is a measure of the specific productivity of ablast fuace (production rate per unt volume). Ths measure makes it possible tocompare the workig rate of diferent size fuaces.
As shown in (Figure 34) the present production is with the normal range ofoperating fuace. The proposed 3,000 NlHday production imposes a very high rateof productivity, on ths older fuace, when it is compared to other fuces. With thsin mind we will now look at other pareters which will afect the modification of thefuce proper.
Hear Productivity
Hear productivity is rated in tons per day per hundred cubic feet of activehear volume. Active hear volume is the fuace's internal volume calculatedbetween the tuyeres and the ta hole. Active hear volume is a measure of the
fuace's holding capacity for the liquids melted in the workig volume (above thetuyeres). Therefore, the tons per day per hundred cubic feet of active hear volume is a
5-31
measure of the specific capacity (thoughput per unt volume) of a blast fuace'shear.
At the present operatig rae. the existing hear is at the highest productivity ofany of the fuaces sureyed in ~ort America (Figue 35). It may be second in the
world. At 3,000 NHTMday. me existg hear could be operating at a world recordrate (Figue 36). The existg hear design with internal staves (Figue 37) tyicallywears well but, it is way too smal. At 3,000 NTHMday, the fuace would beuntable. It would be diffcult to operate because the hear liquid levels will change
rapidly wtich would cause varatons in gas flow pattern, gas utilzation and blastpressure. Also/because of mes rapid changes in liquid level, the fuce would notcome otIwid easily and, it ''iould probably be a tuyere burer.
The best opportty tòr improvement on ths fuace is to remove the presenthear thoughput limitation by increasing it's volume. It may be considered to be arequiement for production of 3.000 NTIday. To provide the additional volumeneeded a shower cooled carbon hear. could be intalled inside the existing shell(Figue 38). Hear volume "il increase from 2,965 ft to 3,467 ft. At 3000 tons perday specific productivity is deceased to the point of being manageable (Figure 39).Ths hear design has been extemely successfu in Nort America, having no elephantfoot and litte or no penetrtion beyond the cup. The size increase makes the hearmore manageable at 3,000 t/d. However, it is stil in the upper range of normal fuaceoperation, so the operators mus exercise very good hearh liquid is level control. Thshigh thoughput would requi around 90% time spent casting. To make ths timecasting possible, cast floor modcations will be needed. They are not included in thsexample.
Campaign Life
Looking at the previous campaign life (Table 3) and the fuace's wear lines
(Figure 40) presents another opportty to make signficant improvements on thsfuace. The bosh and mantle ara are badly worn and are too smalL. On a mantle
fuace, the mantle's protection and stability is crucial for long campaign. Thsfuace's lines violate Bercz:nski's 4 x 4 rule, British Steel's 12 x 5 rule andCarenter's 21.5 degree aw-shIt lie (Figue 41). Since the geometr is bad It is
essential that the mantle protection be able to handle intense activity, varability, and ahigh heat load. A row of copper stves at the mantle is capable of dealing with theseconditions. It is also the thest option, thereby improving the geometr and openigup ths area for better process operation (Figue 42). Copper staves are expensive butthey are very economical in comparson to the alternative which, in ths case, would beto enlarge the mantle.
5-32
Production
The production needed usy deteres a facility's size. However, in sizga blast fuace, raw materials, product chemistr and even operating philosophy enter
into the determation of specifc prouctivity for a fuace and therefore the sizeneeded to produce a given amount of hot meta. In Nort America specific productivity(Figue 34 J vares from six tons per hundred cubic feet per day to twelve tons perhundred cubic feet per day. From th wide range of possible operating rates theworkig volume of the fuace mus be caculated. Productivity and therefore fuacesize will be based on the fuel rate. The expected fuel rate is determed by compargthe proposed operation to a knO\\T ba operation and adjusting it's fuel rate fordifferences between the two opetions in raw materials, hot blast pareters, iron
quaity and even the operatig phiosophy. One example comparson of the operatingpareters afected by these modifcations is shown in Table 4.
OprationPresent I Proposed Difference Coke Rate Effect
Smelting 2A9 iI
3,000Production 2A28 2,924Delays - Minute 1.056Percent 2.53% 2.53%Operation - Minute/Day i A04
Coke 835 782 -53.4Oil 1 15 150 35 -28.3
950 932
Pellets 3,096Slag 53
Scrap or DR Slag Volume 410 375 -35 -7.1
Cu. ft./NTH 41.19 38,403Wind 71.-B 80,000 8,857Oxygen 21.0 22.06 1
Temperature 1. 729 1,850 121 -18.7Grains 2.79 4.00 1.21 3.6Horn. H2O 0.02 0.263 0.8
RAT Calculation 3)06 3,726
Hot Metal Si 0.+4 0.4 -0.04 -3.7Sulf. 0.062 0.062 0.00Mn. OA3 0.43 0.00
Table 4
5-33
Increasing production on an existing facilty nearly always requies that
somethg be chaged. Opors would produce more if there were not constrtsimposed by some existg condition. Decisions as to how to increase production arefudaenta. Either incras the fuce size (workig volume) at the same
productivity, or increase the fue's productivity, or depending upon individuacircumces both.
INCREASE PRODUCTnTY
,t.-
1. MORE OXYGEN
(a) Wind(b) Enrchment
2. LO\VERFULRATE
(a) Increa.-: Hot Blast Temperatue(b) Increa.-: Injectat(c) Impro\'ed Burden
- Scrap or HBI
- Decree Burden Moistue
- Impro\'e Coke Quaity
(d) Hot :\leta
- Lower Silcon
- Higher Sul
(e) Impro\'e Opration
- Dry Hean- Burden Distbution
Table 5
Increase size is a capita cost. Increasing productivity increases operatig cost.Increasing both increases both capita and operatig costs.
5-34
Lookig to a comparson \\ith others (Figue 34), it seems that there are 3 tiersof blast fuace operation. In gener the first tier are rug above 9 tons per day per100 cubic feet of workig volume. These are high cost operations using a highpercentae of scrap in the burden and signficant amounts of oxygen in the hot blast.Furace's operating at the very highest productivity also need more robust equipment.Hence, high capita and high COSL The second tier of fuaces operate between 7 and8.5 tons per day per 100 cubic feet of workig volume per day. These fuaces willhave either high cost or high capita In general, the cost of operation goes down asmore capita is spent. The best option for most operators will be in ths area. Blast
fuaces operating in the thd tier, at less than 7 tons per day per 100 cubic feet ofworkig volumè' are being ru at ths low rate for reasons specific to their operation.Perhaps a niche market is smaler th the fuace capacity, or there is a supply of verylow cost raw materials that makes low productivity cost effective.
With these examples of productivity in mid, look at the stack refractory linesfor the example fuace (Figue 33). It is, at best, marginal in refractory thckness andthe shell is likely to be distessed durg its present campaign. Solutions designed tokeep the existing stack shell and make the next campaign longer by as adding to theexisting coolers or inertg st\"es between the existing cooling plates are likely to
result in a costly intallation. Alost all options which keep the same shell will result
in the same workig volume so the fuace must be ru at high productivity in order toreach the desired 3,000 NTf/day. Ths would be a high cost, high capital operation.Another solution might be bener. With the normal wear lines (Figure 39), shelldamaged is expected. Since the shell should be replaced, enlarging the workig volumebecomes practical. Ths option also fits in with the mantle stave scenaro (above) whichis necessar to achieve the spifed ling life. With a shell change there are a number
of options all which are aied at reachig a specific productivity which allows the
:face to produce 3,000 ~-llday goal without exceeding 8.5 tons per day per 100
cubic feet of workig volume. In Figures 43, 44, and 45, five options are presented.The options will be subjected to operatig and economic analysis to determne which isthe best fit for ths paricular location.
Operation
Only one option to increase hear volume was developed. Ths is becauseeither an increase in diameter or height will require changing the whole :face. Evenwith the increased hear volume, ths fuace's hear has the greatest constraint onincreasing production. To operate reliably at 3,000 NTHMday, the operators must begiven equipment to allow a very high percent time casting.
On the other hancL chagig the stack with the limts of the mantle rig and
the fuace top lip rig afects only the stack. Since the example's stack has been
distressed in every previous campaign, it will require extensive repais. So, five options~
5-35
(Figues 43, 44, and 45), al of \vhich fit with the above consaits have beendeveloped. Each of the stck options have a different capita cost and a differentworkig volume. Each option wi operate at a different rate when producing therequied 3,000 NTHday (Figu.l. The different operatig rates will requiedifferent operating practices (Table 5) and their operating cost will var. From theoperating and capita cost, the renr on investent (ROI) for each case will becalculated. From ths data the fi decisions regarding capita investments are made.
CO~CLUSION-
Ths paVèr began with a gener description of the blast fuace followed with amore detaled look at the blast fuce proper, it's components and it's ancilares. Thscataog of components is followed \\ith a short design example which demonstrates onemethod for selectig from the many blas fuace options available. Ths paper isintended to ilustrate the requirements and the "usua" practices for each area orcomponent and explore some of the alternative technologies which are available today.More detaled discussion of specific components and their effect on process will begiven in another presentation.
The modem blast fuace ba dewloped over the past four centues. It existswith a system of changing consts and technologies. The blast fuace has shown
itself to be a process that can be readily modified and improved to suit changingrequiements. It will continue to chage and improve in the futue. It provides achallenging field of endeavor to all those people involved in the ironmakg field.
ACI00\VLEDGMENTS
I wish to acknowledge tht ths presentation is a continuation of the previousDesign II paper by Mr. Robert G. Goff.
REFERENCES
(1) Author unown "Univers Rota Distrbutor", PAUL WUTH-CRMpromotional document, 1984, pp. 2-3.
(2) Author unown, "Components for B.F. Top", MA-GHH promotionaldocument, undated, pp. 2-3.
(3) Author unown, ''NPPON STEEL Blast Furace Charging System", NSCpromotional document, 1987.
(4) Author unown, "NKK Typ ::Im"able Arour", NKK promotional document,1979.
5-36
(5) Bernard, G et al, "Modern Blast Furace Design by PAUL WUTH S.A.",PAUL WUTH promotiona docinent, 1991, varous pages.
(6) Author unown "The Bell-Less Top Charging System", PAUL WUTHpromotiona docinent 1985.
(7) AISE Sub-Commttee No. 27, New Steel Pressure - Contag Components forBlast Furace Inaton. AISE Techncal Report No. 27, Association of Ironand Steel Engineers, 1984.
(8) Goff, R.G., "Six Yea of Maitenance Experience on STELCO's Lake Erie,~. ,
Blast Furce", Iron and Steel Engineer, July 1987, pp. 17-22.
(9) Dzennejko, AJ., G. Hoelpes et aI, "Design Considerations for UtilzigCylindrcal Cooling Elements in the Blast Furace", PAUL WUTH, 1988.
(10) Blackbur H.W., "Evaporave Stave Cooling in a Modern Blast Furace",presented at the American Irn and Steel Institute, 1976, pp. 1 -8.
(11) Wak, S. et al, "Report on );SC Stave-Cooled Blast Furaces", 4th InternationalStave Conference, i 986, Hamton, Ontao, pp. 1-41.
(12) Bachofen, 1.1. et al. "Copper B.F. Staves developed for Multi-Campaign Use",presented at the AISE i 991 Exposition.
(13) Author unoWI "Coppe Stave for B.F. Cooling", MA-Gll promotionaldocinent, 1991.
(14) Dercycke, 1. and M. Sohi. "Characteristics and Pedonnance of SIDMA'sStave-Cooled Blas Furce 'A"', 4th International Stave Conference, 1986,Hamlton, Ontao, p. I-I.,l
(15) Author unoWI "High Pressure Clay Gun", BAILEY ENGINERSpromotional docinent (Puct Cataog No. CG-2- 1 3A), undated.
(16) Author unown "Compact Tap Hole Guns and Drills", PAUL WUTHpromotional docinent 1980. pp. 2-5.
(17) Ruther H.P. and H.B. Lungen et al, "Refractory Technology and OperationalExperience with Tap Holes and Troughs of Blast Furaces in the FederalRepublic of Germany", Metaurgical Plant and Technology, V oline 3, 1980, pp.12-29.
(18) Carenter, J.A. and D.E. Swanon, "Burs Harbor 'D-4' Reline Improvementsand Results", Ironmakg Conference Proceedings, Voline 53, 1994 pp. 351-361.
5-37
FIGURE 1BLAST FURNACE PLANT
PUliPHOUSE~Al.P
SlAG PIT
BOURSAND
TURBOBLOWERS
r-CAS"" OU sc S
HOT i.ET AL TRACKS
RAW MATE~lAI.REClilo
OMAID
ljALJ 00FIGURE 2
PLAN OF BLAST FURNACE PLANT
5-38
!Q
DD
~ OFGAS SYSTE\
FUNAce TOPClRGI SYSTD
SKIP 8R1GI
HOPER CARS
o 0 0IIGH UtSTOCKHOSK
SCALa CAR
FIGURE 3SECTION THROUGH
BLAST FURNACE PLANT
5-39
-Sca_ouaCOIC.
COLLCTIG AN CHGIG CONVEYOR
STORA_BI WrTFEERS
_RATIGSCRDN
wmGHHOPSWIFES
FIGURE 4FLOWSHEET FOR
AUTOMATED STOCKHOUSE
BLASTFURNACe
BLAST FUNACE DRI HOE l."fOR ft~-8TOCHOUSE --G eOl~~R~
FIGURE 5A CONVEYOR FED BLAST FURNACE
5-40
FURNACE
SULLnuROO
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SMALL BILlHOPPR
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LARGI BILlHOPPER
FIGURE 6CONVENTIONALTWO BELL TOP
5-41
FIGURE 7McKEE DISTRIBUTOR TOP
5-42
SKI CAR
I
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FIGURE 8CRM UNIVERSAL ROTARY
DISTRIBUTOR TOP
5-43
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LARGI BilROD
SEAL VALVE
SMALl BILLROD
LOWIR SIAL
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LAROEBELHOPIR
FIGURE 9GHH LOCK HOPPER TOP
5-44
SH
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FIXED ARMOUR
MOV ABARMOURPLA ,.
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FIGURE 10STOCKLINE ARMOUR
5-45
FIGURE 11PAUL WURTH
TWO HOPPER BELL LESS TOP
5-46
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FIGURE 13SECTION THROUGH PAUL WURTHCENTRAL FEED BELL LESS TOP
5-48
./,'..
FIGURE 14BURDEN DISCHARGE FROM PAUL WURTH
BELL LESS TOP DISTRIBUTION CHUTE
MAH£
S~RTCOlUMN
--IIANT SUPPORTED FURNACE FR£! aT ANDINO FURNAC!
FIGURE 15MAIN FURNACE STYLES
5-49
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CINDER NOTCHIRON NOTCH
IRON NOTCH- - ELEVAnON
HEARTH WALL
UNR HEARTH
FUC! PADOR FOA nON
FIGURE 16FURNACE NOMENCLATURE
5-50
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FIGURE 18TUYERE BREAST
5-52
aT AVE
TUVERE COOERRaT AIMIG BAR
TUveRE COOERHOLDeR
TUVERe COOLER
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5-54
FIGURE 20DENSE COPPERCOOLING PLATEINSTALLATION
FIGURE 21COPPER COOLING
BOXES ANDCYLINDRICAL
(CIGAR) COOLERS
RI~
_ IICDaFACTORY
~ BODY CAS,.., ¡
COa~G pp "/PRoncnoNPt
COLD FAC! 8l HOT FACB
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FIGURE 22ST AVE COOLER
5-55,
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FIGURE 23LOOKING UP
INSIDE ASTAVE COOLED
FURNACE
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FIGURE 24GENERATIONS OF
STAVE DEVELOPMENT
5-56
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tit:~:t;:::~;1~:m~:
5
MULTIPLE TAP-HOLE FCR.VACE CASHOUSE LAYOUT USING SLAG PITS.
1) Trough2) Iron R:u..3) Tiiing R;vU
4) Slag RUlner5) SLag Piis
FIGURE 26CASTHOUSE LAYOUTS
5-58
-I
FIGURE 27MUD GUN
FIGURE 28TAP HOLE DRILL
5-59
FIGURE 29SINGLE SIDE T APHOLE EQUIPMENT
INSTALLATION
J 5 4::. ;::::::''::::''
~t~~m~~t~::::..t;~:~r~:~:~;~~:t\:::::;::;::;~~:::;:::;m:;~:::;~:;:::: ~~::;;:::'~:::::::;:.::~.: ..:.". .' ~ . .
1) Taphol~2) Iron Poal3) Siag Lan
4) Skimmr5) Slag Runnr Takoff6) Iron RUJr Takeoff
FIGURE 30TYPICAL IRON TROUGH
5-60
lSlag Granulatior
~Hot Metal Cars
FIGURE 31PAUL WURTH -INBA SLAG GRANULATION
FACILITY
5-61
ru~OUGH
DRAINRUNNER
CASTHOUSE FLOOR
~
nL nNG RUNNR
SUPPORT CRADLE
TORPIDO
I LADLZ
GRADE
FIGURE 32TIL TING
IRON RUNNERINSTALLATION
5-62
SHE" :!"J _"E
SHE" ei..;: _"E
3 l -g. 10. SHELL
MATLE
::...:;~~
1423
FIGURE 33EXAMPLE FURNACE LINES
5-63
11.0
10.5
10.0
9.5 f-
9.0
8.5
8.0
7.5
7.0
6.5
16.0
Tons per 100 C.D ft Working Volume
.10
~3,OOO· 9 · T/d
.8
. 5 . 6 · Exam pie.4.3
.2
Data Labels are Furnace I.D.
FIGURE 34
5-64
Tons per Day per 100 cu ft Hearth Volume
98.0 + .13
88.0 -. Exam pie
78.0 .4.568.0 -
. 12. 7
58.0 - .1.6
48.0
38.0 · 2
Data Labels are Furnace I.D.
FIGURE 35
5-65
Tons per Day per 100 cu ft Hearth Volume
98.0 -- .13.3,000T/d
88.0 -
. Exam pie
78.0 .4.5
68.0 -.12.7
58.0 - .1.6
48.0
38.0 ~ 2
Data Labels are Furnace I.D.
FIGURE 36
5-66
\J
i
I
21.. 6N DIA.
-_.. ..-
~ .._.., '_e_
--FIGURE 37
EXAMPLE FURNACE HEARTH
5-67
- f -
~'\ '~"'\"Eo __I,' --" 1 r :'./.. ÃI ¡p ~ .:a õl
ii ,.; .X~""~_!! ~~-. ! i /,.-Il!~ ',;,\, ~~.;_.._--. '..: : -_~-~ ~"'-i"ó.. t-=.~': - ~.- - - --
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s- ~NC~~: wiL~
FIGURE 38EXAMPLE FURNACEPROPOSED HEARTH
5-68
98.0
88.0
78.0
68.0
58.0
48.0
Tons per Day perIOO cu ft Hearth Volume
.13.3,000111
. Proposed. Exam pre
.4.5
1,.7. -
.1.6
38.0 ; 2
Data Labels are Furnace I.D.
FIGURE 39
5-69
FIGURE 40EXAMPLE FURNACE WEAR LINES
5-70
i
R'\i \ I'~'l~~~.~~,,'~ ~',I
\
\
\\
\ MATLEBOSH STAVES
12'xS' POINT
4'x4' POINT
T-lYERES
HEATH STAVES
CONE 14
CONE 23
IRON NQICH_
FIGURE 41CRITICAL WEAR POINT
APPLIED TO EXAMPLE FURNACE
5-71,/
i
FIGURE 42COPPER STAVE
5-72 '
il....--
OI'r
r ..__
-~
BASE CAS::EXISTING FL.~\.AC=-
-WORKING VCL~MEJ1.948 CU.FT.
-USEABLE HEARTH VOL:.i.E2,965 CU.F;.
,r
JI'.. a. --
~
.~
ALT. "A"Stack Integrated Lining\Cooling
Bosh Stove Cooled-WORKING VOLUME
.:5.64.: CU.FT.-U5EABLE HEARTH VOLUME
.:.199 CU. FT.
FIGURE 43
5-73
...; ..
..
I¡
, '..
~
I ~.
...; .. IT-,°i-
-,.
~.. . il-r Go St.. "-J~OI
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..
..
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;. :~~ ~
'I-. S:cvesALT. "c"
Generation 3 Stoveswith Movable Armor
(Lower Stockline 3' -0")-WORKING VOLUME
3.3,.318 cU.FT.-USEABLE HEARTH VOLUME
FIGURE 44 .3,199 CU.FT.
ALT.Generatior
II_It:=
-WORKING VCL;Ji.E34.575 C~.r.
-USEABLE HEAR7~ VCL~ME3,199 C:".r7,
5-74
r-, \~~---. //;/~'(///';'~---
l!.ç Q. tI: 1I
ri.. G. HU
..
.......~
is.r ra
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ALT. "OnGeneration 3 Steveswith Paul Wurth ic::
-WORKING VOLUME.:5,424 cU.FT.
-USEABLE HEARTH VOLUi.E.:,199 cU.FT.
ALT. nEttGeneration 2 & Coooer Staves
with Paul Wurth Too
FIGURE 45
-WORKING VOLUi.E.:6,723 cU.FT. (1 157.)
-USEABLE HEARTH VOLUME.:,199 CU. FT. (10a7.)
5-75
11.0
10.0 -
9.0
8.0
7.0
6.0
Tons per Day per 100 cu ft Working Volume
.10
.9. Ex ~3,000 T/d
. Alt C
. 8. Alt B
. A~Att D
. AU E
. Example.5.6.4
.3.2
.1
Data Labels are Furnace 1.0.
FIGURE 46
5-76
BLAST FURNACE PRODUCTION
7500
6500~=e:E==~ 5500 i
C1Z l--i
~ i
o. I- n=;.~~~
4500
3500 :
70 80 90
YEAR
FIGURE 47, i
5-77
1190
BLAST FUR~ACE FUEL RATE
1090
~==~ ~C~i: -D-i..
990
890
70 80 90
YEAR
FIGURE 48
5-78
LECTURE #6
BLAST FURACE DESIGN II
Neil J. Goodman
K vaerner Metals
Pittsburgh, Pennsylvania
Abstract: - Blast Furnace Design II covers air (blast) and gas system designs for modem blastfurnace operations. Increases in hot blast pressure and temperature during the past thirt years,
together with the need to improve operating and maintenance effciencies, and corresponding costreductions, have resulted in design improvements in the air and gas system designs. The subjectwil be covered in the following areas:
Functional Layout and Design of Hot and Cold Blast SystemsHot Blast Stove and Ancilary DesignOptimization of Stove Operation and ControlFunctional Layout and Design of Gas Cleaning SystemsOptimization of Gas Cleaning System Water UsageTop Pressure Control and Energy Recovery Turbines
INTRODUCTION
Among the major inventions and progress achieved in blast furnace technology in the 19th centurywere the production and use of coke, and heating of the blast air. In 1828 James Beaumont Neilsonintroduced blast air heating in recuperative form at Clyde Iron Works, Scotland. In the course ofonly a few years the equipment used for heating blast air developed from makeshift installations towell thought-out heating apparatus. It was possible to achieve the blast temperature up to 930°Fusing recuperative iron cylindrical hot blast stoves.
This was the state of blast heating when Eduard Alfred Cowper made his patent application for abrick-type hot blast stove in 1857. From this point in time onwards there has been a steady furtherdevelopment of the "Cowper" design, lasting through to the present period.
The development of the "Cowper" hot blast stove has been a function of advances in combustiontechnology, refractories qualities, etc. and not least the evolution of the blast furnace process. In thelast 30 years, the progress of blast furnace technology and support ancilary plant has beenparicularly rapid.
6-1
Hot Blast Stove Operation
Overview
The operation of hot blast stoves involves the sequential heating and cooling of a regenerativemass of refractory. The regenerative heating is performed by the combustion of gasses and thepassage of the waste gas products of combustion through the refractory (gassing). Figure 1shows the gassing operation.
The cooling of the regenerative refractory is performed when the blast air is introduced into thestove (i.e. the stove is on blast, see Figure 2).
The intermediate position with the gas system and blast system isolated, the stove is "bottled"or "boxed" (see Figure 3).
Gassing Control
Typically, a stove can provide hot blast at 100°F less than the flame temperature, and the
typical flame temperature available from 100% blast furnace gas is 2000°F. Therefore anyblast temperatue greater than 1900°F wil usually require an enriching gas to be mixed withthe blast furnace gas (natural gas or coke oven gas are usually used).
During the gassing cycle, the refractory in the stove is heated and the temperature at the topdome of the stoves rises. Eventually the dome temperature wil achieve a target temperature(typically 50°F below the flame temperature) and wil be kept at this value by the addition ofexcess combustion air. With the dome temperature controlled to a constant level, excess heatfrom the combustion gasses wil heat up the refractories lower down the stove and increase thetemperature of the waste gas. Eventually, the waste gas temperature could exceed a pre-setmaximum (typically 650°F) and the gassing wil be automatically stopped to protect the wastegas system from thermal damage to the mechanical valves and duct work..
Ideally, the gassing should be stopped just before the stove is switched over to "blast".Typically, stove switchovers are performed either at pre-set times, upon operator initiation orhigh waste gas trips. In modern stove installations, computer models are used to match theheat input from the gas with the heat output from the blast. These models therefore reduce theenergy losses associated with bottling and excess waste gas temperatures.
Blast Control
The cold blast (typically at 300°F) is heated by the refractory and exits the hot blast stove atalmost the same temperature of the dome refractory. As the refractory cools down, thetemperature of the hot blast also cools down until it approaches the temperature required by theblast furnace. At this point the next hot stove wil be taken "off-gas" and then put "on-blast".When the second stove is "on-blast", the cold stove wil be put "on-gas" to be reheated.
6-2
,i
. I
HOTBLAST
GAS
AIR
Figure i - Stove On-Gas
CHECKERS
COLDBLAST
WASTEGAS
6-3
Figure 2 Stove On-Blast
GAS
AIR
CH ECKERS
COLDBLAST
WASTEGAS
6-4
HOTBLAST
I
, !
GAS
AIR
Figure 3 Stove Bottled or Boxed
6-5
CH ECKERS
COLDBLAST
WASTEGAS
,.
ON BLAST
,.
ON BLAST
""
ON GAS
Figure 4 Prior to Changeover
'"
BOTTIED
..
ON GAS
,.
ON GAS
Figure 5 Stove 2 "Gas" to "Bottled"
6-6
,.
ON BLAST
AIR
BOTTLED
AIR
ON BLAST
Figure 6 Stove 2 "On Blast"
AIR
ON BLAST
,.
ON GAS
..
ON GAS
Figure 7 Stove i "Bottled" from "Blast" prior to "On Gas"
6-7
COLD BLAST SYSTEM
For a modern blast furnace practice the air requirement at the tuyeres broadly ranges from30-40,000 scf/NTHM. This value must be supplemented by a margin suffcient to allow forlosses in the blast system, particularly stoves pressurization. The actual cold blast (blower)demand depends on the levels of oxygen and tuyere injectant being practiced, together with theamount of scrap or DR! in the burden.
The blower specification for a blast furnace producing an average of 5000 NTPD, to meet therange of operating practices currently adopted in North America would incorporate anoperating window which accommodates blowing rates (including losses) of 100,000-160,000 scfm, at the design furnace top pressure.
Blower pressure requirements are generally set by the following guidelines:
Loss thru the Cold & Hot Blast SystemLoss thru the FurnaceFurnace Top Pressure
2 psig max.28 psig max.6-35 psig
Turbo Blowers
Figure No.8 is a schematic diagram of the cold and hot blast system from the blower station tothe bustle main.
VENTURI UHEA
STOVE PRESSURIZING LINE
REGULATING VALVE
COLD BLAST WAIN
DRAfTCONTROL
ÐACKORAfrSTACK
SHUT -OFFVAlVE
BUSTLEPIPE
Figure 8 - Typical Cold and Hot Blast System
The cold blast system starts at the air inlet to the blowers and ends at the entrance to the hotblast stoves and blast mixing chambers. A two blower configuration, as it applies to a singleblast furnace plant is shown. Inlet fiters are shown for protection of the blower intemals,
venturi meters for machine control and relief (anti-surge) valves in the discharge lines for backpressure control. Check valves and isolation valves are not shown.
The centrifugal blower has been the predominant means of delivering wind to the blast furnacein North America. Figure 9 is a cross section thru a centrifugal blower. The machine shownhas five stages, each stage compressing air to a higher pressure.
6-8
The air enters the eye of each impeller and leaves at the periphery; travels thru all five stages ofcompression in a series manner. The most blowers of this type stil in service have a designpressure rating of 40 psig (actual 37-38 psig). A number of machines are being upgraded to45-46 psig conditions.
ioii PRESSUREAIR IN
HI~H PRESSUREAIR OUT
Figure 9 - Centrifugal Blower Cross Section
Traditionally, a centrifugal blower is coupled to a steam turbine, hence the term turbo blower.The volume of air delivered is controlled by the rotor rpm which in turn is regulated by thesteam flow to the turbine.
Since the middle 60's, axial blowers have been more widely used due for their greatereffciency and lower horse power requirements. They are more suited for compressing largevolumes of air at the pressures (60 psig) required for modem high production blasts. Axialblowers are also smaller and lighter than centrifugal machines for the same capacity andpressure ratings. As shown in Figure 10, the air in an axial machine travels longitudinally,parallel with the rotor shaft.
HIGH PRESSUREAIR OUT
LOLL PRESSUREAIR IN
Figure 10 - Axial Blower Cross Section
Axial blowers are coupled to either steam turbines or electric motors. Wind volume iscontrolled by rotor speed, or stator blade setting.
6-9
COLD BLAST MAIN
Cold blast temperatures and pressures vary greatly, 300°F and 38 psig for a "typical" low toppressure operation, to 550°F and 65 psig for a high production furnace. Facilities operating athigher temperatures generally insulate the cold blast main to conserve energy in the system.The cost of cold blast main insulation is harder to justify at lower temperatures particularlywhen the blowers are remote from the blast furnace.
Figure i i - Snort Valve
In Figure 8, the first component in the coldblast system after the blowers is the "snort"valve, which is used to regulate blastpressure and flow rate to the furnace duringchecking or "on -blast/off-blast" procedures.The snort valve is actually a combination oftwo valves (Figure i i).
The discharge valve which opens to theatmosphere is linked to a butterfly valve inthe cold blast main. As the discharge valveopens to atmosphere, the valve in the maincloses, thus diverting cold blast from thefurnace.
The snort valve is fitted with a silencer tocontrol the noise level of the discharge.
From a process standpoint, the snort valveshould be located upstream of the blastmetering device and any subsequentconditioning of the blast.
Located downstream of the snort valve are two lines in paralleL. The larger of the two is acontinuation of the cold blast main, while the smaller is used to pressurize the stoves. Locatedin each is a venturi meter. Ideally each time a stove is pressurized, the total flow of cold blastis increased to meet stove pressurization requirements without upsetting blast air flow to thefurnace. However, this is difficult to achieve even with a modern installation. Stovepressurization (or fillng) requires 2 to 4 minutes and up to 8,000 scfm depending upon stovedesign.
There are also relief or blow-off valves included in the system. These valves are intended todepressurize the stove from blast pressure to atmospheric, prior to putting the stove on gas.Depressurizing time can be up to 4 minutes depending on the level of blast pressure. The reliefvalve exhausts to atmosphere and is fitted with a silencer.
Another branch line is taken from the cold blast main downstream of the stove pressurizingline to supply mixer air. Mixer air is added to the hot blast air that comes from the stove tocontrol the hot blast temperature.
6-10
There are three concepts for mixing of hot and cold blast:
· Individual mixing via the lower combustion chamber.
· Individual mixing between the stove and hot blast valve.· Central mixing in the hot blast main.
Individual mixing where the cold blast entry port is sited in the lower combustion chamber isgradually being replaced. The thermal cycling in that area of the combustion chamber resultsin high refractory maintenance.
Where "mushroom" type hot blast valves are used (Figure 12), the mixer connection is at thebase of the valve. This system eliminates the temperature variations in the stove and ensures
little temperature variation in the hot blast main. Unfortnately the "mushroom" type valvesare not generally applicable to hot blast systems supplying temperatures greater than 2000°F.
~ VALVE STE/1/' (WATER COOLEOI
VALVE DISC
(WATER COOLED)HOT BLASTCONNECTON
VALVE SEAT(WATER COOLED)
STOVE
REFRACTORYLINING
o /1IXERCONNECTION
Figure 12. Stove Mixer Connection via a Mushroom Valve
When "gate" type hot blast valves are used, the mixer connection is usually located betweenthe stove and the hot blast valve in the trunk connection (Figure 13A).
The alternate to individual mixing is to install a central mixing station located in the hot blastmain just before the bustle pipe. There are a number of design variations for this mixer.However, good mixing can be achieved with a ring mixer as shown in Figure 13B.
6-11
This arrangement subjects the major portion of the hot blast main to temperatures up to thelevel of the dome temperature which must be accounted for in the refractory design.
i
1 Stove
-1-,"
,
I
I,
I
I
1---- -,
I,
I
r:::""..+ ~
.",J¡Ò HOT BLAST HAIM
HIXER
Figure 13A - Mixer- Connectionto Hot Blast Outlet
Figure 13B - Mixer Connection to Hot
Blast Main via Ring Mixer
Cold Blast Conditioning
The cold blast system includes the facilties to condition the blast air with moisture andoxygen. Current blast furnace practices are based on high injection rates of pulverized coal, ornatural gas at the tuyeres. The loss in raceway adiabatic flame temperature is corrected byoxygen enrichment of the blast. Twelve percent (12%) oxygen enrichment of the blast hasbeen safely practiced for a number of years. The safe handling of oxygen calls for the use ofstainless steel fittings and seal tight isolation valves when the furnace is "off-blast".
Blast moisture additions are only used as a secondary control of flame temperature. Steam isadded to cold blast on the basis of dew-cell measurements of the ambient moisture in the air.
HOT BLAST SYSTEM
The hot blast stoves are a regenerative heat exchange system used to preheat blast to the blastfurnace. The hot blast stoves utilize the top gas from the blast furnace as their source ofenergy. The blast furnace gas used to fire the stoves is often enriched with natural gas or cokeoven gas to attain the flame temperature required to meet the specified blast temperature. Theflame temperature is normally 125°F higher than the dome temperature.
The hot blast system shown in Figure I starts at the entrance of the hot blast stoves and ends atthe blast furnace tuyeres. The main components of the system include the hot blast stoves, hotblast main, bustle pipe, tuyere stocks, tuyeres, back-draft stack and auxiliar fuel injection
system. Most hot blast systems include three hot blast stoves with some plants having theavailability of a fourth stove.
6-12
Internal and External Combustion Chamber Hot Blast Stoves
Two types of stoves are in use in North America; i.e. the internal and external combustionchamber designs. As hot blast temperatures increased above 1700°F so did the incidence ofmajor problems with the internal combustion chamber stove design of the 1950's.
The major area of failure was the dividing wall between the combustion chamber and thechecker chamber, which is the most critical part of the refractory construction. Due to theuniform temperature in the combustion chamber and the decreasing temperature in the checkerwork, the dividing wall, particularly at the lower level of the stove, is subject to thermalstressing and differences of movement in individual layers.
As the flame temperature was raised, the thermal stresses and the differential expansion in thedividing wall increased, resulting in bending and destruction of the wall, short circuiting of thecombustion gases, and damage to the checker work.
Other problems included:
· Dome refractory failures.· Failures at nozzle connections.· Checker system failures (subsidence, flue misalignment & bottom checker crushing).· Checker support failures caused by above problems.
In the early 1960's, the solution to the problems experienced with older internal combustionchamber stove design lay in the development of the external combustion chamber stove. Whenthe combustion chamber and the checker chamber are completely separated, the foregoingdamages can be avoided. The popular approach in the late 60's and early 70's for designing fora blast temperature of 2500°F (1350°C) and dome temperature of 2825°F (1550°C) was toadopt the external combustion chamber stove.
There are currently three designs of external combustion chamber stove:
The Davy Krpp Koppers (DKK) design shown in Figure 14 is basically two separatechambers each with its own dome. The two domes are connected with a pipe incorporatingtwo expansion joints. Differential movement is taken up in the expansion joints. The twodomes are tied together with I-beam links to contain the internal pressure force.
The M&P design (Figure 15) is similar to the DKK design except that the dome connectingpipe does not contain an expansion joint. Differential expansion is catered for by pre-stressingthe vessel before installation of' refractory. During initial warm up the stresses are relieved bythe differential expansion between the two chambers. As the shell temperatures increase abovethe average, the vessel stresses increase, but stay within permissible levels at maximumtemperature and pressure.
The Didier design (Figure 16), incorporates a heavy dome steelwork arrangement which iscarried by the checker chamber. The combustion chamber including the refractories andburner are parially suspended in a cantilever arrangement from the dome steelwork. The baseof the combustion chamber is mounted on a "hydraulic foot", which also partially supports thecombustion chamber and absorbs differential expansion.
6-13, r
¡
"l'kOfC;fl~
"MOlu,,, SwC4i
Figure 14.
Davy Krupp KoppersHot Blast Stove with
External Combustion Stove
Figure 15.
Martin & PagenstecherHot Blast Stove with
External Combustion Stove
Figure 16.
DidierHot Blast Stove with
External Combustion Stove
The external combustion chamber stove has not generated great momentum in North America.Inland Steel BF 7 is the only installation of this design of stove.
The internal combustion stove design was not abandoned. In the late 1960's the super highduty (2825°F dome temperature) design became an alternative to the external combustionchamber stove.
The survival of the internal combustion stove design was based on improvements to thepartition wall design, which included:
· The introduction of an insulating layer in the partition wall with dense refractory on eitherside. This concept minimized the temperature gradient across the dividing walL.
· The use of "sliding joints" which allowed individual layers of refractory to expandvertically, independently of adjacent layers, thus avoiding wall bending.
· The adoption of gas sealing concepts by stainless steel sheets or concrete panels in thedividing walL.
International competition between external and internal combustion chamber stoves hasbecome a commercial rather than a technical issue.
6-14
The North American hot blast requirements lie in the range 2000°F to 2200°F withcorresponding dome temperature of2250°F and 2450°F.
Figures 17 and 18 ilustrate the styles of internal combustion stoves seen in North America.
Air +
Combustion Chamber
Hemispherical Dome DesignJr Conispherlcal Dome Design
Silica Brick
Combuslion Chamber
Iniertocklno Hlugonil Ch_i"
Hol BiasI +-
Ceramic Burner ~
Gas+
.- Allo Iron Gri Support
-- Wasie Gas.-old Biai
Alloy Iron Support Columns
Figure 17 Hot Blast Stove withInternal Combustion ChamberNew Dome
Figure 18 - Hot Blast Stove with
Internal Combustion ChamberRebuilt Inside Existing Shell
Figure 17 represents the design adopted when a new dome shell, or a new dome and vesselshell, is included. In both cases, the dome refractory is independently supported from the stoveshell.
Figure 18 represents a stove re-built within an existing shell. In this case, dome refractory issupported by the refractory ring wall.
The important features to consider in the construction of a hot blast stove are:
· The stove structure must be designed to withstand stresses due to thermal expansion andcontraction.
· There must be sufficient mass of bricks to deliver required stove duty and the brickmaterials must be of correct quality.
· The grid at the bottom of the stoves must be able to withstand the weight of the checkerwork, and misuse due to overheating.
· Stove materials must be able to withstand chemical attack from the gases used for heating.
· The burner gives good effcient burning characteristics in order to save energy. Also, theflame must not impinge on the dome or the checker work in order to avoid damage torefractory brick or lining.
6-15
The following aspects have to be taken into consideration when choosing materials for
refractory bricks for a hot blast stove:
· The maximum temperature that the materials can withstand.· The mechanical strength to withstand required loads.· The need for resistance to chemical attack.· Creep characteristics of bricks.· The cost of bricks.
A typical stove construction wil have 4 or 5 differing grades of refractory (Figures 17 and 18).The use of thin wall checkers gives a high heating surface/mass relationship, which providesfor high effciency of heat transfer, and in many situations, permits an upgraded stove to bebuilt within an existing shelL. However, physical characteristics have to be considered due totemperature and load conditions within a stove. For example, creep resistance is required -creep being deformation of a material with load and temperature over a period of time.
Silica checkers are chosen for the higher temperature areas due to their excellent creepresistance, and near zero expansion at temperatures above 1380°F. Care must be taken whenheating and cooling silica below this temperature to absorb volume changes associated withcrystalline phase transformations in the temperature range 480°F to 1 070°F.
Checkers used in both stove designs are a hexagonal shape with circular or hexagonal flues.Each checker is interlocked with the course below by a series of male and female connectionsand are laid up in an overlapping pattern as shown in Figures 17 and 18. The interlock designprovides structural integrity to the checker mass, making it unnecessary to rely on the ringwalland combustion chamber to maintain checker positioning. By providing clearance allowancesbetween the individual checkers, expansion due to heat-up takes place between them.
The checker work is supported by anumber of iron columns, girders andgrids. Figure 19 shows a typicalchecker support system used in bothinternal/external combustion chamberstove designs. All components are of
a low alloy cast iron suitable fortemperatures of 850°F. The systemshown is designed to maintain a flatsupport beneath the checkers thereby
preventing checker deterioration dueto an uneven supporting platform.The column, girder and grid supportare interlocked with each other toprevent movement of girders, and gridsupports in relation to one another
during operation.Figure 19 Modem Hot Blast Stove
Checker Support System
6-16
The system can also include a series of tie rods between columns to improve rigidity. When theinternal and external combustion chamber stove designs were put into service for dometemperatures in excess of 2460°F (1500°C) and operated for a number of years, a newunexpected problem with the shells of the stoves occurred; i.e. intercrystalline stress corrosioncracking or, for short, stress corrosion cracking.
Stress corrosion cracking is the failure of a metal resulting from chemical attach in thepresence of tensile stresses. The essential ingredients necessary to promote stress corrosion
cracking are:
Tensile stress.Acidic environment.Susceptible metal.
The following methods have been employed to control stress corrosion cracking:
· Application of a protective coating of the inner surfaces of the steel shell to prevent theacidic environment from coming in contact with the susceptible metal under stress.
· Dew point control by applying insulation to the external surfaces of the shell therebyeliminating condensed water vapor from combining with NOx to form the acidicenvironment.
· Limiting stove dome temperatures to below 2460°F thereby, eliminating NOx formation
resulting in a lack of the acidic environment.
Hot Blast Stove Ancilaries
While all hot blast stove ancilaries are important to the effective performance of the stove, thetwo most important are the stove burner and the hot blast valve.
i
Iì
A typical stove combustion air and gas flow diagram is shown in Figure 20. Many hot blaststoves in North America are equipped with mechanical burners, external to the stove itself.Figure 2lA ilustrates a design which consists of two concentric tubes separating air and gaswhich mixes in the stove combustion chamber. This type of burner pedormed adequately onlow effciency stove operations, i.e. dome temperatures up to 2 100°F. Improvements in thedesign of mechanical burners Figure 21B have extended the dome temperature range whichcan be attained by this type of burner to 2350°F.
COMBUSTION CHAMBER
BLAST ISOLATION VALVE
VENTURI METER
Figure 20. Combustion Gas and Air Flow Diagram
6-17
Burner No:i:ile
Conventional low-EnergyWith Shutoff Valve
Figure 21a Typical Mechanical Stove
Burner Designs
Water Cooled Gas Burner Valve
High Energy
Figure 21 b Typical Mechanical StoveBurner Designs
To overcome the problem associated with mechanical burers, vertical firing ceramic burnershave been developed and are in use successfully throughout the world. Ceramic burners havethe following advantages:
The limit in dome temperature of2350°F is eliminated (increased to 2825°F).Combustion chamber target wall failures due to thermal shock are eliminated.The need for a high temperature burner isolation valve is eliminated.
The basic design features which should be incorporated in a ceramic burner system are:
· Air and gas chambers function as plenums to provide uniform gas and air entry at the pointof mixing. The gas chamber should also act as a low velocity separator to drop out anysubstantial portion of entrained moisture, which should be drained on a periodic basis.
· Gas and air- should enter their respective chambers at the lowest elevation of the burner.This will reduce temperatures in the gas and air inlet ports to the lowest possible leveL.
· At the point of burner exit, the air and gas should be mixed while flowing at velocities inthe turbulent flow range to insure a uniform mixture.
In the burner shown in Figure 22, the uniformly distributed alternating parallel streams ofturbulent fluids provide for effective gas and air mixing as they are blended into each otherwhen rising through a three level ceramic grid configuration. This ceramic grid is placedabove the slots and functions like many individual nozzles. Each nozzle is served by aminimum of one pair of parallel slots. Therefore, gas and air are thoroughly and uniformlymixed prior to entering the stove combustion chamber.
By having a completely combustible mixture prior to entrance to the combustion chamber, theflame wil be stable and short. This wil prevent the combustion chamber from being subjectedto severe differential temperatures or the effects of incomplete combustion.
6-18
A pilot burner should be provided to assure ignitionimmediately after the gas and air combustion
mixture exits from the burner.
A low pressure drop style of ceramic burner isshown in Figure 23. The flame produced by thisburner is several times longer than that produced bythe burner represented in Figure 22.
The design of ceramic and mechanical burners must
lead to complete combustion over a range of gascalorific values. Incomplete combustion gives riseto pulsations which result in refractory damage. Theprincipal cause of pulsations is related to theharmonics of the combustion chamber and externalgas and air main systems. When combustion doesnot take place uniformly, low frequency pulsations
are initiated which can be amplified by aninterrelationship between combustion chamber andgas and air main harmonics.
Figure 22 Internal Combustion ChamberCeramic Burner Design
A physical device which creates a pressure drop across the burner system is often designed intothe system to act as a decoupler of combustion chamber and gas and air main system
harmonics.
The hot blast valve is the most critical valve in theentire hot blast system since it is exposed to the
highest temperature. In North America themushroom type valve has been the standard formany years for hot blast temperature applications upto 2000°F (1 ioO°C). See Figure 12. However, ashot blast temperatures have been increased beyondthis level, more and more interest has been directedtoward gate type hot blast valves.
Gate type hot blast valves have undergone radical
changes since their inception. Early designs were
made of cast iron and later of cast steel with watercooled seat insert rings of electrolytic cast copper.
This design had the disadvantage that under certainoperating conditions, particularly in the case ofincreased temperatures, leaks caused by distortion ofuncooled valve components develop. This problemled to failures requiring repair or replacement of hotblast after only a few weeks of operation.
6-19
hr
Figure 23 Internal Combustion ChamberLower Pressure DropCeramic Burner Design'
To alleviate these problems, a fabricated hot blast valve was developed utilizing a water cooledbody with integral steel seat rings instead of insert rings, thus avoiding the disadvantages ofindependent copper seats. Also, the HEV cooling passages were redesigned to increasecooling water velocity and eliminate "dead spots".
Figure 24 ilustrates a current hot blast valvefor use up to 2800'F. A summary of the hotblast valve specification is:
· Hot blast valves are water cooled,refractory lined gate valves. The valvesare suitable for working temperature andpressures of 1500°C and 4.5 bar g(70 psig).
· The paddle is faced with refractory onboth sides and is water cooled. Coolingwater flow is arranged in a spiralarrangement to minimize differentialtemperature and consequent distortionacross the paddle.
· The seat, body and bonnet are also watercooled and refractory lined.
--'"+-
Figure 24 Hot Blast Valve
Figure 25 shows a cutaway section of body and paddle showing water passages and refractorylining.
Figure 25 Hot Blast Valve BodyfPaddle(showing Cooling Water Passages and Refractory Lining)
6-20
Hot Blast Main
Most operating, problems with hot blast mains are the result of inappropriate design for theexpansion of the refractory lining and the steel shell. The design basis of mains includesprovisions to:
· Allow movement due to thermal expansion and pressure forces in steel and brickwork.
· Keep loading on supports to a minimum by designing mains to prevent high pressureforces being transmitted into structures, and incorporating slide bearings into the supports toreduce friction loads.
· Keep loading at branch connections and through valves to a minimum.
When designing, a main, the route and location of supports are established. The system is thenanalyzed to determine the optimum location of fixed points and expansion joints.
Figure 26 shows a typical hot blast system layout. It indicates the location of fixed points inthe system, location of valves, expansion joints, tie rods and supports.
PRESSURE BALANCE
Figure 26. Hot Blast Main AnchorÆxpansion System for Gate Valves
In this case, the centerline of the BF is effectively a fixed point as the bustle pipe is heldconcentric with the furnace. The centerline of the stove/hot blast branch is fixed in plan andtherefore the main is restrained axially at the intersection of main and each branch centerline.
Expansion joints are required between each fixed point to accommodate thermal expansion ineach section. Expansion joints must be restrained to prevent them "blowing out" orstraightening under the integral pressure force.
This can be done by either making the anchors suffciently robust to resist the pressure endforce, or by installing a tie rod system. In case of large diameter mains at relatively highpressure, the forces become too great to economically restrain them with support brackets andsupport structure. Tie rods are therefore used to contain this force.
A pressure balance expansion joint is required in the main to cater for the movement of the tierod bracket adjacent to the bustle pipe and also extension of the tie rods due to temperaturefluctuation and tensile loading due to pressure forces in the main.
6-21
This expansion joint ensures that tie rods always remain in tension and that pressure forces arenot transferred into the fixed or anchor points.
The hot blast branches are equipped with twin expansion joints. Twin joints are necessary in
this location as lateral movement is required in the joint. The stove branch moves upwards dueto expansion of the stove shell when it is heated up. The hot blast main elevation is relativelystable as it is supported on structural steelwork which is subject only to fluctuations in ambienttemperatures. The branch expansion joints also give flexibility for the valve flanges to beseparated to facilitate changing of hot blast valves.
Figure 27 shows a hot blast restraint! expansion system for a set of stoves with McKee blastvalves. The older hot blast mains designs use fabricated, diaphragm type expansion joints
located between stoves and between the stoves and the bustle pipe. These hot blast mains areoften anchored to the cast house frame and to the stove furthest away from the furnace. Visibletwisting of the mushroom hot blast valves at their seals ilustrates the inadequacy of thisdesign.
2-HANGERS PER STOVE
I.USHROOI. TYPE H.B.VAL VE UPPER TRUNK1.0TION CONTROLLED
2-BELLOWS EXP. JOINTSW/2-CAST HOUSE COLS.SUPPORTED HANGERS
Anchor to C.H. Structure
iI
!
Figure 27. Hot Blast Main AnchorÆxpansion System for Mushroom Valves
Figure 28 shows the refractory configuration of the hot blast main, together with a tyical hot
blast main expansion joint. The lining should consist of "hard" refractory and insulating brick.The use of compressible insulating layers has been discontinued. The weight of the workinglining caused this material in the lower section of the main to compress, leaving a gap in theupper section. This resulted in hot spots on the shell, and distortion of the shell if close to anexpansion joint.
2300' F Insula lion
2600' F Insula lion
60% AI ,03
Figure 28. Hot Blast Main Expansion Bellows
6-22
Common practice is to control the hot spot by either grouting the area and/or use water spraysboth these methods provide only temporary benefits. Grouting wil eliminate any futureexpansion capabilty and can result in further refractory problems. While water sprays inducevery high stress levels in the steel often resulting in cracking of the steel.
The relative position of hot blast valve to the branch expansion joint is determined for eachstove installation when the layout of stove vessel, hot blast valve and hot blast valve changinghoist is established. The ideal location of the hot blast expansion joint is on the hot blast mainside of the hot blast valve. In this position there is minimal thermal load changes.
Tuyere stocks (Figure 29) make the connection between the bustle main and the blast furnace.Centering of the bustle main is important for reaching optimum working conditions for thetuyere stocks. The double Cardan units compensate for all relative movements between thebustle main and the blast furnace due to thermal expansion. Movements are controlled byrestraining straps that form a gimbel-type joint. The assembly is gas tight from the bustle pipenozzle to the tuyere. Changing the blow pipe is accomplished by unbolting the flange joint atthe top of the elbow.
BullltP'øe
furnac.W..II
Figure 29. Tuyere Stock Arrangement
Tuveres
The ability to maintain long tuyere life results in significant reductions of furnace downtime.The primary reason for losing a tuyere is bum out of the nose of the tuyere. There has been agreat deal of improvements in both copper casting quality and cooling systems to help reducethe problem of nose burnout. The addition of hard surfacing to the tuyere nose has furtherimproved tuyere life.
The use of high quality castings with a "high velocity" tuyere design wil provide optimumtuyere life. There are many "high velocity" designs available some with separate nose circuit
cooling, they all maintain minimum velocity of 50 fts in the nose circuit. Tuyere burnoutproblems can be identified before adding excessive mounts of water to the furnace bymonitoring the tuyere nose with thermocouples.
Attention should be paid to the water system supplying the tuyeres so each tuyere is suppliedwith the proper amount of water. Many piping systems do not provide any control of waterquantity. Variations in supply pipe length can result in wide variations at individual tuyeres.Some operators are utilizing refractory tuyere liners to reduce heat loss in the hot blast,improving overall system effciency.
6-23
Backdraft Stack
The primary purpose of backdrafting is to ensure safe working conditions around furnaceopenings made as a result of changing tuyeres, coolers, blow pipes, etc.
The backdraft stack is connected to the hot blast main between the first stove and the bustlemain. The connection varies between 30" and 48". Backdrafting is started by opening a gatevalve (identical in design to a hot blast valve ). Various devices are used to control the draftfrom the stack. Too much draft can draw coke as far as the bustle main. A butterfyarrangement at the base of the stack (not exposed to heat) is the principal means of controllingthe draft. After the butterfy valve, the stack extends upwards to a point nearly level with the
bleeders. In some areas steam injection is used rather than an air draft.
Backdrafting places the hearth and bosh region of the furnace under a small negative pressureensuring that any carbon monoxide and hydrogen formed after the blast has been taken off thefurnace is drawn out of the furnace.
At the start of the backdrafting temperatures can reach 3,000°F in the bustle pipelbackdraft
connection as any carbon monoxide coming from the furnace wil combust as air is also drawninto the system. Sufficient excess air should be admitted to dilute the combustion gases andreduce the flame temperature.
Energy Efficiency
During the past decade increasing effort has been made to improve the effciency of the hotblast system. Typically the improvements are based on the following actions:
· Accurate metering of blast furnace gas and combustion air.
· Analysis of the blast furnace gas plus any enrichment gas. Subsequent determination ofthe correct proportion of the gasses for the required flame temperature.
· Measurement of the excess oxygen in the stove waste gas. Note: Some operators are nowusing both CO and O2 to trim the fuel/air ratio.
Gassing management models are available which allow combustion conditions to be controlledto blast heat requirements and can develop either maximum stove effciency or minimumenrichment gas usage for a given hot blast temperature. The use of such automation avoids"bottling" of stoves and minimizes stove changeover time contributing to improvements inoverall stove effciency.
The sensible heat remaining in this stove waste gas can be used via a heat exchanger to preheateither combustion air or blast furnace gas. The energy saved by this type of technology can beused to reduce the dependence on enrichment gas or to increase stove dome temperatures
(Figure 30). There are four primary methods in use:
Fixed Plate TypeHood Rotation Type (Rothemule)
Element Rotation Type (Ljungstrom)Heat Medium Recirculation Type
6-24
3.5
1.5
3.0
2.5
2,0
1.0
100 200 300PREHEAl TEnPERATURE 'F
Figure 30. Effect of Gas and Air Preheat on % Enriching Gas Requiredto Maintain 2350°F Dome Temperature
The first three can only be used to preheat the combustion air as they do not completelyseparate the two gases, whereas the heat medium recirculation type can be safely used topreheat both combustion air and blast furnace gas. A schematic arrangement of the
recirculation system is shown in Figure 31.
('m......_¡¡¡l¡¡¡..~:::n. .,
1........._._...-1 j ~:~..-
Figure 31. Hot Blast Stove Heat Recovery System Using a Heat Transfer Medium
An installation of this type can increase stove effciency by up to 3%. However, to date, thelow energy costs in North America rarely allow a waste heat recovery system to beeconomically viable.
6-25
BLAST FURACE GAS CLEANING SYSTEM
The gas produced in the blast furnace is an important energy source in the effcient operation ofan integrated steel milL. The gas is used to preheat the blast air in the stoves and as theprincipal fuel in the boiler house.
Efficient operation of the furnace will produce gas within the following range of analysis:
Chemical Analysis (dry) COCO2H2
N2
20-25% by Volume20-25% by Volume4-12% by VolumeRemainder
Calorific Value (net)TemperatureDust Content
80-105 BTU/scf250°F to 350°F5-10 gr/scf
The size range of the dust is a function of the screening effciency of the stockhouse and themoisture content is dependent on the water content of the material charged to the furnace.
The primary purpose of the gas cleaning system is to produce a clean gas that can be burned inthe stoves without causing the stove effciency to deteriorate over time. This requires a dustcontent in the clean gas of not more than 0.005 gr/scf. Since most furnaces utilze wetscrubbing as the gas cleaning method, then the gas must also be cooled to as Iowa temperatureas the water supply wil allow, to minimize the level of saturated water of the gas, thusimproving the net CV of the gas.
Dustcatcher
The first element of the gas cleaning system is the dustcatcher. The dustcatcher is a largechamber which reverses the direction of the gas flow while simultaneously reducing itsvelocity. This results in dust particles greater than 50 microns being removed from the gasstream. As much as 60% of the total dust content wil be removed in an effcient dustcatcher,which should be emptied daily to assure continued effcient operations. Dust is removedthrough a variety of systems which typically wet the dust to reduce the generation of a dustcloud during the dumping operation.
Gas Cleaning SvstemISO
130""
'- ..Ì' '" .."'" ~ ..
With the introduction of high top pressure, avariety of gas cleaning techniques, based onutilzation of the pressure energy of the gas,
have been engineered.
110~.ooC) II~
The level of top pressure required to cleanblast furnace gas from a gas cleaning plant
inlet level of 5 grlscfd is shown in Figure 32.
~ 7D
oQ)=i soInInGlõ: ..
:i0.00 0.00 O.oo 0.00 0.00 0.008 0.01 0,02
Outlet Dust Loading ( gr/SCF)
Figure 32
6-26
The most common type of scrubber in North America is the variable throat type as shown inFigure 33. The variable throat venturi is normally sited prior to the gas cooler, Figure 34. Theunits give excellent performance up to a top pressure of 8-10 psig. Above that level of toppressure, maintenance costs have risen and gradually this type of gas cleaning system is beingovertaken by the annular gap scrubber.
The current generation of annular gap scrubbers, Figure 35, are based on two stage scrubbing.This figure ilustrates the gas passing through a conditioning unit where it is contacted with
water from centrally located sprays, causing cooling, saturation and partial cleaning of the gas.
Brick Lining
Figure 33 Adjustable Gas Venturi
Scrubber Cross Section
COtuIITIO_INO VEIU£l
DlIMlaT1!I1l ,.iCIlIJlO
MYDIlAULIt. AC1'UA TOfl
OIRTV_GAS
CLEAN-GAS
M'ñillE! WATERINlElX 6PI1
lOWERPACKING
Figure 34 Variable VenturiFollowed By Cooler
Figure 35 Annular Gap Scrubber Cross Section
6-27
The annular gap scrubbing section consists basically of a fixed cone within which a movablecone operates. Raising and lowering of the movable cone decreases or increases the annulargap between the two cones through which the gas passes for final cleaning. A hydraulicsystem controlled by signals from the furnace top pressure controller adjusts the gap to create agas pressure drop required for turbulent gas scrubbing and for furnace top pressure control.Water for the gas cleaning is applied through radial and tangential sprays positioned just abovethe annular gap. A good "rule of thumb" to estimate the water requirements for gas cleaning is10 gall 1,000 scf of blast. This does not allow for gas cooling requirements.
Effciency of dust removal is dependent principally on the degree of turbulence created by thescrubbing section. The scrubber is able to achieve the required level of gas cleanliness over awide range of gas flow rates and furnace top pressure by controllng the cross-sectional area ofthe annular gap between the inner and outer cones of the scrubbing section.
Moisture carrover is inherent in gas cleaning/cooling systems and a mist eliminator isnormally installed at the outlet of the system.
Gas Conditioning Svstem Water Treatment
Most blast furnace gas conditioning plant water systems are ofthe closed circuit type as shownin Figure 36. By their nature, closed circuit systems reduce the amount of blowdown and thequantity of contaminants discharged. However, this must be balanced with increased
contaminant concentration. In some cases, contaminant concentration may be self-limiting,which is the case with suspended solids. Additionally, closed circuit systems reduce theamount of blowdown requiring treatment and makeup, the cost of waste treatment facilities andoperation is minimized, and in some cases, the actual need for any waste treatment faciltiesmay be eliminated. Some plants are limited in the amount of makeup water that is availableeither because the water is scarce or the cost of purchasing water from a municipal authority isprohibitive. Water reuse is helpful in reducing actual water need.
Overflow
Recirculation1900 USGPM
Cool Water8300 USGPM
CoolingTower
RecirculationPump
Slowdown700 USGPM
Figure 36. Closed Circuit Variable Annular Gap Scrubber Gas Cleaning System
Efficient operation of the solids removal equipment is probably the most important part of asuccessful closed circuit system. Suspended solids in the recycled water should be reduced toat least 25 ppm to prevent problems of deposition in low flow areas such as cooling tower_sumps and pipe manifolds. -
6-28
The recovery turbine is usually placed after the final scrubbing units as shown in Figure 37 andit may be arranged to recover a fixed quantity of energy, the balance of pressure being losteither over the scrubber or a suitably positioned septum valve, or it may be aranged to recoverthe maximum amount of energy available in which case there must be certain provision on theoutput side of the turbine to cope with variations in output.
FURNACE
GAS SYSTEMSEPTUM VALVEFOR ALTERNATIVEPRESSURE CONTROL
\----..--- TO DISTRIBUTIOSYSTEM
GENERATOR
'V
SCBBR RECVERYTURBINE
ElCTRICAL POER
DUSTCATCHER
Figure 37. Energy Recovery Electrical Power Generation
The provision of a bypass to the turbine is always made to maintain the independence offurnace operation. However, availability of these turbines usually exceeds that of other furnaceequipment.
The most common method of utilizing recovered energy is by generation of electrical powerusing a generator directly coupled to the shaft of the recovery turbine.
With the advent of top pressure recovery systems, the type of gas cleaning system to be usedneeds to be re-evaluated. As stated earlier, a good scrubbing system wil utilize 80-100 ins.H20 of pressure. Japanese operators are utilizing electrostatic precipitators and bag fiters asthe primary cleaning device in order to save this energy for the recovery turbines.
Top Gas Recoverv
High top pressure furnaces are equipped with a gas lock chamber through which raw materialsare charged into the furnace. With each charge of raw material, the pressure in the gas lockchamber is equalized with furnace top pressure using blast furnace gas and then reduced downto atmospheric pressure. During this process the gas in the gas lock chamber is discharged toatmosphere. The volume of gas discharge depends on the chamber volume, charging cycle andmagnitude of top pressure. In the Pacific Basin, where the majority of furnaces are operated attop pressures in excess of 30 psig, the total quantity of gas released is roughly 2% of the totaltop gas volume generated.
With fuel costs escalating, means to recover the heat value in top gas released is attractingmore and more attention. Systems have been developed to recover the gas from furnacesoperated at elevated top pressure.
6-29
The gas released from the gas lock chamber is effused naturally by action of the pressuredifference between the gas lock chamber inner pressure and the gas main pressure when therelief valve is opened, permitting recovery of the gas in the main. When the gas lock chamberand the gas main pressures are close to being equal, the primary relief valve closes and thesecondary relief valve is opened to discharge residual gas in the gas lock chamber toatmosphere.
CONCLUSION
In summary, even though dramatic improvements have already been made in North AmericanIronmaking facilities, opportunities stil exist for improvement in overall energy effciency.Unquestionably, furnaces that are to remain in operation wil be those employing advancedtechnological features to reduce iron production costs. Methods such as hot blast stovesystems designed to generate higher blast temperatures, charging systems which wil permithigher pressure and more effective burden distribution, and effective means of handling thehigh volumes of high pressure gas generated can all contribute to more effcient operation.
The implementation of these new technologies wil be dictated by escalating costs of energy.Blast furnace operators and designers wil have to bring about effective ways of saving energyby not only being aware of the fuel management program within the blast furnace plant, butalso knowledgeable in the disposition of fuel and energy on a plant-wide scale. Since this is anever changing scene, only effective, forward planning will reduce energy consumption to apractical minimum.
6-30
LECTURE #7
BLAST FURACE DESIGN III
Steve Sostar, General ForemanBlast Furnace
Lake Erie Steel CompanyNanticoke, Ontario, Canada
Abstract: This paper will cover the basic features of anideal blast furnace and ancillary equipment. Considerationwill be given to equipment which will operate consistentlyand reliably, thus providing the operator with a minimumoperating cost per ton of hot metal.
Recognising current trends in the Iron and Steelindustry, it is highly unlikely that a furnace as describedwould be builtin the near future in North America11.Consequently, also covered in this paper will be some of theactual improvements incorporated into the last reline atHilton Works "E" Blast Furnace and some recent improvementsat Lake Erie Steel #1 Blast Furnace.
INTRODUCTION
Many hours can be spent debating the merits of variousdesigns of the "ideal" blast furnace based on:
(a) Operator preference as a result of pastexperiences i both good and bad, with existingequipment and processes.
(b) Types of raw materials used in an individual'sblast furnace plant.
(c) Specific preferences for types of equipment basedon detailed skills of those personnel who willmaintain the equipment.
(d) Geographic location of the plant which may affectthe use of various raw materials, especiallyfuels, based on transportation costs.
7-1
The "ideal" blast furnace as described by this operatorwill reflect the most current equipment available to providethe plant with an efficient, consistent operating furnace.One of the aims of this furnace design will be to provide aninstallation which can be maintained by a series of shortmaintenance stops i. e. (less than 24 hrs.) i in combinationwith periodic interim repairs (3-5 week duration) to replaceor overhaul specific areas of the furnace every 8 to 10years.
When designing detailed layouts of blast furnaces, manyengineering hours are spent putting the most economicalequipment into the smallest possible space for the leastcost.2,3 In many cases this saving in initial capitalinvestment is lost in maintenance costs, furnace productiondelays and costly changes required at the next reline orinterim repair. To avoid some of these costs it is wise toinstall spare, critical equipment as an "on line" running orstandby spare rather than having this equipment sitting anddeteriorating on a shelf in a warehouse. Equipment does wearout and fail, so one of the tasks of a good design engineeris to provide methods to isolate and remove equipment in atimely, safe fashion for maintenance while having little orno effect on the balance of the blast furnace plant.
Legislation, either currently in place or beingconsidered, also has a part to play in our "ideal" furnacedesign. Safety of workers, short term and long term healtheffects on workers and environmental restrictions for air,water and noise must all be considered when selectingequipment.
In an attempt to satisfy all of the above concerns wewill now discuss both our "ideal" blast furnace and recentrepairs, improvements and changes at Stelco and Lake ErieSteel under the following topics:
StockhouseTop Charging EquipmentFurnace DesignFurnace Cooling SystemsFurnace RefrpctoryStoves and Hot Blast SystemGas Cleaning PlantCasthouseInstrumentation and Control Equipment
7-2
I STOCKHOUSE
Handling of raw materials for the blast furnace processhas changed dramatically over the past 20 years. Traditionalscale cars and skip cars are being replaced with vibro-feeders, belts and weigh hoppers.
Ouridenticalconveyor.contain:
ideal stockhouse will be configured in twohalves on either side of the main furnace feed(Figure 1) Each half of the stockhouse will
( a) Two sets of two ferrous material bins.
(b) One set of 5 miscellaneous bins.
(c) One large coke bin.
Each set of ferrous material bins will have two vibro-feeders per bin discharging onto a collector belt, over ascreen and then into a weigh hopper located over the mainfurnace feed belt.
Material from each miscellaneous bin will be dischargedinto its own weigh hopper located directly below the bin.These bins require their own weigh hoppers because when smallquantities per charge are required, they cannot be accuratelyweighed in a central holding hopper. Material from eachweigh hopper is then transferred to one holding hopper overthe main feed belt, where total weight for all miscellaneousmaterials per charge are checked.
The coke bin will have 5 vibro-feeders feeding onto abel t, over a screen and into its own weigh hopper.
All belts in our stockhouse system will be covered anda dust collection system will be installed to capture dustgenerated at transfer points. It is very important to ensurethat well designed covers and side skirts are installed overthe full length of all inclined belts. This will stop therollback of individual material particles, particularlypellets, thus keeping walkways clean and safe to use. In 1997a new belt fed stockhouse was installed at Rouge Steel 12which incorporated a circular conveyor gallery for thefurnace feed conveyors. This unique design incorporates alarge area for spillage accumulation, off the walkways and aseries of drop legs for cleanup. This concept will beincorporated into our furnace.
7-3
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7-4
Moisture gauges will be installed in the four pelletweigh hoppers and the two coke weigh hoppers. Belts andscreens will run continuously while the starting and stoppingof the vibro-feeders will be done by two PLC i S (programmablelogic controllers), one operating and one standby. ThesePLC i s will sequence material, and compensate for belt runoff, material discharge speeds and moisture content. Timingof our stockhouse will be such that we will have 140% feedrate to the furnace to ensure that any minor problems, whichmay cause delays in the stockhouse system, do not affectfurnace filling.
Two optional improvements were investigated for "E"Furnace, a conventional skip-fed furnace. One was toeliminate the scale car and install vibro-feeders and beltsystems to existing ore holding hoppers. This was ruled outdue to economic considerations. Another option was acombination of replacing pneumatic cylinders with a hydraulicsystem and also making the scale car a remotely operatedunit. This was considered in an attempt to improve theworking environment for the car operator. This was alsoruled out because of the concern that a hydraulic systemwould not be fully reliable with the existing bin gatearrangement. Consequently, there were no maj or changes orimprovements to the existing stockhouse on this reline.
The Lake Erie Steel #1 Furnace was started in 1980 withhalf of the ideal stockhouse because of low start-upproj ected production rates and cost considerations at time ofconstruction. Since then this existing stockhouse has beenable to sustain production rates of over 6600 NT/day. Thiswas accomplished by:
(a) Manipulating vibro-feeder rates to optimisefilling of weigh hoppers.
(b) Altering equipment-sequencing logic to movematerial quicker.
(c) Setting the stockhouse to be proactive and look atburden level and burden decent rate rather thanjust the operation of top filling equipment.
7-5
II TOP CHAGING EOUIPMENT
Our choice of furnace filling equipment will be a duallockhopper bell-less top.4 (Figure 2) There are a number ofreasons for this choice:
(a) Flexibility in burden distribution is far greaterwith this system as opposed to standard two bell systemswi th or without movable armour.
(b) Individual components are much smaller and lighterthan conventional two bell systems, thus reducing furnacetop loading.
(c) Maintenance of individual components can beplanned and carried out in short stops.
(d) Lockhoppers will be fitted with load cells to actas check weights on all of the various weigh hoppers in thestockhouse as well as to control accurate burdendistribution.
(e) Twin lockhoppers will provide increased fillingcapacity, as well as continued filling availability in thecase of a failure in one lockhopper.
At the last reline "E" furnace was fitted with a singlelockhopper bell-less top capable of holding 3 skips ofmaterial. A receiving hopper was constructed above thelockhopper, capable of holding 2 skips of material. Thisdesign was chosen because it had less structural impact onthe furnace top than a two-lockhopper system. (Figure 3)
7-6
DUAL LOCK H O.PPE R
BE LL-LESS TOP
FLOW ,CONTROL __GATE
TRAVELLINGCHUTE
SEAL VALVE
SEALVALVE
~ HOUSING
SEALVALVE
MAIN DRIVEGEARBOX
REVOLVINGCHUTE
...::''';..: .,~;::: ... . 'f'
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Figure 2
7-7
SINGLE LOCKHOPPERBELL - LESS TOP
RECEIVINGHOPPER
UPPER SEALVALVE
LOWER SEALVALVE
ROTATINGCHUTE
Figure 3
7-8
SKIP CAR
HOPPER
r- GEAR BOX
I I I FURNACE DESIGN
Our II ideal II furnace would be a free standing furnacewith a 33 ft diameter hearth. This hearth diameter has beenchosen because of:
(a) Personal experience with two furnaces of thissize.
(b) Ability to control low production levels ( as lowas 1700 NT/day) and high levels (up to over 6000NT/day) depending on plant requirements and marketconditions.
(c) Much of the equipment used will have beenproduced, tested and used on existinginstallations.
A free standing furnace design has many advantages overthe traditional mantle supported furnaces.
(a) Access to the tuyere breast and bosh area of thefurnace is far less restricted.
(b) Top equipment has its own support and is totallyindependent of the furnace shell.
(c) Piping configuration in the furnace area can besimplified.
(d) Maintenance walkways around the stack of thefurnace can be enlarged for ease of access andmaintenance.
(e) The complicated designs necessary to cool themantle area are no longer required.
7-9
iv FURACE COOLING
In this section we will look at three areas of thefurnace: shell cooling, tuyeres and tuyere coolers, andunderhearth cool ing .
Shell Cooling
Our furnace will be completely stave cooled from hearthto the underside of the stockline armour. The cooling mediumwiii be forced recirculated, boiler quality, treatedfeedwater. Boiler quality water with 02 scavenger additionsand corrosion inhibitors is necessary to prevent oxidation orbuild-ups inside the cooling pipes, which ultimately causereduction in cooling efficiency. (Figure 4)
Stave design will incorporate some of the followingfeatures: (Figure 5)
(a) Ductile iron will be used in low heat load areasof the hearth, tuyere breast and upper stack ofthe furnace. Ductile iron has better hightemperature crack resistance than grey iron. Inthe high heat load areas of the bosh, lower andmid stack of the furnace we will use copperstaves13,14.
(b) Intense corner cooling pipes will be incorporatedin the bosh, lower and mid stack staves.
(c) A serpentine pipe will be cast behind the fourbody pipes, as a backup to protect the cast ironin case of a body pipe failure.
(d) Periodic spacing of water cooled ledge staves willbe installed to hold the initial brick lining inplace for as long as possible.
(e) Alumina/Silicon Carbide brick will be embedded inthe ribs of the bosh and stack staves to protectthe staves after the loss of the refractory liningin front of the staves.
Supply of water to the staves will be done by fourseparate pumping systems. Each system will feed one of themain body pipes in each stave. This has been done to providefor cooling in all staves, even if one system fails for anyreason. Ledge and corner pipes will be staggered and balancedamong the four systems to equalize cooling requirements.
7-10
STAVE COOLED FURNACE
TUYEREJACKETHEARTHWALL
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7-11
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7-12
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Flow meters will be used to monitor feed and dischargevolumes to each of the four systems as a primary means ofleak detection. Thermo-couples in the same location as theflow meters will also be used to measure total heat flux onthis portion of the furnace.
With a cooling system of the design as described, ourfurnace should run gas tight and trouble free for at least 10years and probably up to 20 years if required.
On the last rebuild of "E" furnace, 9 rows of staveswith some corner cooling were installed. Three rows of boshstaves replaced the existing shower cooled bosh. Six rows ofstack staves replaced the existing plate cooled stack. Allstaves were made of ductile iron. The existing tuyere breastand shower cooled hearth walls were not altered as they havebeen proven to supply adequate cooling in these portions ofthe furnace.
Tuyeres and Tuyere Coolers
Tuyeres on our "ideal" furnace will be dual chamber,with high intensity nose cooling. This is required toensure that the most vulnerable area of the tuyere gets thebest cooling possible. If a tuyere nose does fail duringoperation and the body circuit is still intact, the water tothe nose circuit can be turned off and the tuyere changed atthe next regular maintenance stop. Tuyeres will be externallycoated to protect them from liquid iron splashes andinternally insulated for energy efficiency.
As with the stave system, the water supply to thetuyeres and coolers will be recirculated boiler qualitytreated feed water. There will be two independent systems ione very high pressure system for the nose circuits and oneof slightly lower pressure for tuyere body, tuyere coolersand stove valves. Accurate feed and discharge flowmeasurement will be installed on each circuit to monitorleaks.
This system was contemplated for "E" blast furnace butwas ultimately put on the back burner because of overalleconomic restrictions.
7-13
Under Hearth Cool ing
Our furnace will have an induced draft under hearth aircooling system installed in the hearth refractory. Thiscooling has proven successful in reducing erosion and thusextending the campaign life of the hearth bottom. This typeof cooling is currently in use on all of Stelco i s furnacehearths.
V FURACE REFRACTORIES
Having chosen our basic furnace design and coolingsystem, we will now look at the refractories for inside thefurnace.5 Above the under hearth cooling tubes, which arealready embedded in high conductivity carbon, we will installfour 28" thick layers of carbon beams. (Figure 6) Ourhearth wall will be a 27" thick layer of hot pressed carbonbrick, using alternate layers of 3 - 9" and 2 - 13.5" bricks.Of prime importance is to ensure good contact between thestaves and the hearth carbon, by using a tar based, highconducti vi ty rammed refractory in the gap between the hearthbeams and the staves. Carbon in the hearth wall will be laidtight to the staves. The hearth wall will be corbelled to54" in the area of the iron notches to account for the excesswear expected in this area of the hearth. Refractory aroundthe tuyere coolers will be hot pressed carbon bricks, pre-cutand pre-glued for ease of installation.
To extend the life of the staves we will install acarbon lining in the bosh and tuyere breast of the furnaceand a high alumina lining in the stack.
Stockline armour will be installed in the top 6 feet ofthe furnace to protect the shell from the abrasion of theburden. This armour will be installed in a concrete fill andwill be supported by a steel ring and tie rods back to theshell behind the armour.
VI STOVES AN HOT BLAST SYSTEM
Our "ideal" blast furnace will be built to provide theoperator with the capability of running with a straight line,2200 degrees F hot blast temperature at a maximum wind rateof 200,000 SCFM.
7-14
.-UNDERRnRTH-- ,COLING
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SLOW INUNING
CARBO
CARBON
, CARBON
CARBON .' CARB
; 'I.; RE BRICK
, FIRE BRICK-
SECTION - FURNACE HEARTH
Figure 6
7-15
To achieve this temperature requirement we will install 3stoves with the following features: (Figure 7)
(a) Internal ceramic burners because of their highefficiency and trouble free operation as opposedto conventional external burners.
(b) A waste heat recovery system on the stove stack topreheat the combustion air.
(c) High silica checkers capable of providing a dometemperature of 2400 degrees F.
(d) Provision for enrichment of blast furnace gas witheither natural gas or coke oven gas, depending oncurrent plant needs for fuels.
(e) Gate type hot blast valves because of theirtrouble free operation and their reliability athigh temperature and high pressure operation.
(f) A back draft stack system capable of burning thegas at the bottom, prior to emission to theatmosphere. This stack is absolutely necessary toprotect the stoves from being exposed to verydirty and sometimes very hot products ofcombustion during back drafting.
The design of the hot blast main and bustle pipe isvery critical to the operation of this system. Because ofthe volume and temperature of hot blast anticipated theremust be sufficient allowance for expansion and movement ofthe stoves, hot blast main, bustle pipe and pent stock .Double cardan tuyere stock will be provided to allow formovement between the bustle pipe and tuyeres.
Provision will be made for oxygen inj ection, of up to15% of the wind, into the blast system so that if economicsdictate, oxygen enriched hot blast can be provided to theoperator. Steam inj ection for blast humidity control willalso be provided.
7-16
, 'i INTERNAL COM-
BUSTION CHAMBER
~
::'STOVE 'GAS MAIN
HIGH TEMPERATURE STOVE
Figure 7
7-17
Our blowpipes will be designed to provide the operatorwith the ability to inject natural gas, oil or coal or acombination of these fuels. Because of the environmentalrestrictions on coke ovens, inj ected fuels are a necessity.Very critical to the efficiency of high volume inj ection ofoil, gas or coal is the need to know the oxygen availabilityat each tuyere so that fuel rates can be adjusted accordinglyto provide for proper combustion of these fuels. Venturituyere stock will be provided to give this information.
A hot blast main isolation valve will be providedto allow stove maintenance without having to drop blowpipesand seal all the tuyeres.
In an effort to get at least another 2 a years out ofthe stoves at "E" furnace, all three stoves were completelyrebuil t during the last reline. In addition, a back draftvalve and stack was installed to protect the stoves duringfurnace stops. No other major changes to the existingequipment were done. E furnace currently has the capabilityto inj ect natural gas and pulverized coal.
VI I GAS CLEANING PLAN
Our "ideal" furnace will have a gas cleaning plantcapable of handling 280, 000 SCFM of blast furnace gas atthree atmospheres top pressure.
Primary dust removal will be done in a dustcatcher.Dust removal will be done on a continuos basis using a pairof seal valves and a surge hopper. A spherical shutoff valvewill be installed at the top of the dustcatcher. This will beused to isolate the furnace from the gas cleaning systemduring furnace stops. Because of the valve design, it can beutilized to provide maintenance access to all of the gascleaning equipment.
Following the dustcatcher will be a variable annulargap scrubber. This unit will provide for final gas cleaningand cooling and for top pressure control. To give theoperator the most flexibility based on the required wind rateand top pressure the vessel will be designed with 3 variablegap cones in parallel. (Figure 8)
To ensure that as much moisture as possible is removedfrom the gas a mist eliminator will be installed downstreamof the scrubber.
7-18
INLET l
i
PRE-WASHSPRAYS
GASOUTLET"
HIGH PRESSURESIDE
~R.S. SPRAYS
R.S.ELEMENHOUSING
R.S. ELEMENT
LOW PRESSURESIDE
ANNULAR GAPSCRUBBER
Figure 8
7-19
Supporting the gas cleaning plant will be arecirculating water system comprised of thickeners, coolingtower and lagoons to remove the dirt particles from theprocess water.
On the last "E" reline, the gas cooler, septum valveand precipitators were replaced with a single unit annulargap scrubber and a packed bed cooler. The cost of thisinstallation was approximately equal to the cost ofoverhauling the existing equipment. This new design operateswith a much lower maintenance cost and still provides goodtop pressure control, and cool, clean gas for the stoves andcentral power station.
VIII CASTHOUSE
The final maj or area to deal wi th is the cas thouse . Inour "ideal" blast furnace, this is the area that is the mostdynamic and can create one of the most disastrous situationsif not handled properly. Our aim in designing a cast houseis to create a truly continuous process by giving theoperator the ability to remove the slag and iron from thefurnace as it is made. This philosophy has many advantages:
(a) The most stable operating blast furnaces run witha dry hearth practice.
(b) With a dry hearth practice the furnace can be shutdown safely at any time for maintenance.
(c) The metal generated is stored in torpedo cars, notin the hearth and is thus available to thesteelmaker when he needs it.
The best way to accomplish the above requirements iswi th a furnace that has 4 tapholes, 90 degrees apart. Eachtaphole will have its own convection air-cooled trough,feeding a tilting runner. Slag from each pair of tapholeswill be run to a slag granulation plant, with slag pits as abackup to the granulator.
The taphole drill and mudgun will be located on thesame side of the trough so that mobile equipment can beutilized for cleanup and tearout when trough maintenance isrequired. Our mudgun will be a hydraulic powered unit,capable of handling the best taphole clays. The tapholedrill will have a reverse jackhammer capability so that a hotbar practice can be utilized if the operator so wishes.
7-20
Each taphole with have remote cameras to moni torcasting from a central control room. Continuos irontemperature measurement will be done using either immersionthermocouples of infrared monitors. Each ladle position willhave automatic ladle fill monitors as well as automaticprobes for iron sample and ladle temperature measurement.
There are a number of labour-saving devices which willalso be included on the cast house floor. One is a rodchanger to install a new drill rod or soaking bar on thetaphole drill boom and also to remove the old, hot i spent rodafter opening of the taphole. 6 Another device is an oxygenopener to allow for remote oxygen lancing of the tapholewi thout having to place a person adj acent to the hot trough. 7
Access to the casthouse will be via ramps from groundlevel - one positioned for each pair of tapholes. A secondmezzanine level will be located above the casthouse floorbelow the tuyeres. This will be accessible by ramp from thecasthouse floor. With this arrangement, mobile equipment canbe utilized for tuyere, cooler and penstock changes.
The overall casthouse floor will be large enough tostore spare equipment and spare casthouse refractories yetstill allow for full access by mobile equipment such asbackhoes, small front end loaders and fork lift trucks.Because of this mobile equipment the casthouse floor will bemade as level as possible.
To eliminate casthouse emissions and improve theenvironment for the casthouse crew, all runners will becovered. A baghouse will be constructed to draw the fumesfrom the runners, both during casting and during rebuilds ofthe runner-work.
A remote control crane, large enough for tilting runnerchanges will be available on each casthouse.
Currently "E" Blast Furnace has two tapholes, each withits own casthouse. During the last reline this area wascompletely revamped. Included in the revamp was:
(a) New air-cooled, carbon lined troughs.
(b) New hydraulic mudguns.
(c) New taphole drills.
(d) Drills and guns mounted on the same side ofthe trough.
~21
(e) Installation of a tuyere mezzanine level,accessible by forklift from the casthouse viaa ramp from the one casthouse.
(f) A ramp from grade level to the casthousefloor.
(g) A complete rebuild of the existing casthousefloor to make it level for equipment access.
(h) Runner covers designed for fume capture andalso to carry mobile equipment in criticallocations.
The above improvements resulted in a reduction inrefractory costs, improved casthouse availability and areduction in physical "bullwork" required by casthouse andmaintenance crews.
IX INSTRUMENTATION AN CONTROL EOUIPMENT
The final requirement of our "ideal" blast furnace is tobe able to monitor, analyse and control both the iron makingprocess itself and all the various mechanical systemspreviously described. When choosing control equipment, keepin mind that we have three requirements:
(a) To know instantaneously the status of theprocess;
(b) To collect and display data in a format whichwill be useful to the operator for decisionmaking and also to indicate to him subtlechanges or trends in the process;
(c) To collect sufficient data to analyse pastperformance in an attempt to improve futureperformance.
To monitor and control the various mechanical systems wewill select modular units. Each unit will control its ownspecific area such as stove system, stave cooling system, gascleaning plant, stockhouse and bell -less top. 8
7-22
Because this is an "ideal" blast furnace we would putin all of the available equipment to allow us to monitor theprocess variables including: (Figure 9)
(a) Top gas analyzer complete with a good samplepreparation unit to measure CO, CO2, H2, N2, 02and thermal value of the top gas. (Note: 02would only be used during maintenance stopsor a complete furnace blowdown.)
(b) Four stockrods to monitor filling. (Threemicro wave and one mechanical rod as backup.)
(c) A profile meter to allow for a periodicmeasurement of the burden profile.
(d) Four above-burden probe for temperature andgas analysis across the furnace radius, 900apart.
(e) In-burden gas and temperature probes at theupper stack and mid stack.
(f) Vertical probe.
(g) Refractory thermocouples in the bosh andstack to monitor rate of wear and ultimateloss of this refractory.
(h) Thermocouples in the staves to monitor theirperformance.
(i) Hearth wall and under hearth thermocouples.
(j ) Pressure taps on the stack to measure stackpressure drop.
(k) Top pressure and temperature measurement.
(l) Hot blast pressure, temperature and humiditymeasurement.
(m) Cold blast volume measurement. (i. e. Turboblower flow minus snort valve bleed)
(n) Flow measurement of wind to each tuyere.
(0) Flow measurement of inj ected fuel to eachtuyere.
7-23
PROFILEMETER
THERMOCOUPLES
PRESSURETAPPINGS
BLASTINSTRUMENTS
STOCKRODSVERTICAL PROBE
GAS ANAL YSER
INFRA REDCAMERA
FURNACE PROCESS SENSORS
Figure 9
7-24
(p) Flow, inlet temperature and outlettemperature measurement to all furnacecooling elements to determine the wall heatflux.
As one can see, with all of the above equipment tomonitor and analyze we will have to develop a supervisorycomputer system to monitor and display the data in a formatthat is useful to the operator.
Once we have collected sufficient information for a gooddata base, and have gained some confidence in the reliabilityof our equipment, we will look to incorporate some on- linecomputer control of our process. These systems have beendeveloped and proven in operation and are currently beingused on some furnaces in Japan and Finland. (Figure 10) 9,10.
During the last reline on "E" Blast Furnace thefollowing equipment was installed:
(a) PLC with on line backup for stockhouse andbell - less top control.
(b) PLC with basic relay logic for stave systemcontrol.
(c) Data logger with computer display of thevarious thermocouples in the stave coolingsystem.
(d) Provision for above-burden probes.
x SUMYI have approached this topic with the same perspective
as "the kid in the candy store". As one can see it would beimpractical to build an "ideal II blast furnace with all of therecommended "goodies
II . This becomes very evident when weconsider the amount of equipment and changes we initially puton our "wish list" for "E" Blast Furnace and then compare itto the final list of equipment selected based on our economicrationalisation.
As good blast furnace operators, we must each look at ourown operation, available capital for new equipment and ouroperating budgets before we can justify each individualexpenditure for new equipment. i would hope that thislecture will help you in the future when you look atimproving your blast furnace operation.
7-25
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7-26
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REFERENCES
1. HILL, R. N., "Blow-in and Low Level Operation ofStelco Lake Erie Works No.1 Blast Furnace" - A.M.I.E.Ironmaking Proceedings Vol.40 Toronto, Ontario. 1981(pages 300-307)
2. HOLDITCH, J. E. "Blast Furnace Design III II - AnIntensi ve Course - Blast Furnace Ironmaking Vol IMcMaster University, Hamilton, Ontario. May 1989
3. BERCZYNSKI, IIBlast Furnace Design 11111 - An IntensiveCourse - Blast Furnace Ironmaking Vol I - McMasterUni versi ty, Hamil ton, Ontario. May 1985
4 . BERNARD, G. i & CALMES, M., II Modern Blast FurnaceDesign" - Paul Wurth CY Luxembourg publication.
5. VAN LA, J., II Ironmaking Refractories" - An IntensiveCourse - Blast Furnace Ironmaking Vol I McMasterUniversity, Hamilton, Ontario. May 1989
6. Nippon Steel "Rod Changer" - Nippon Steel Corporationpublication
7. Nippon Steel "Oxygen Opener" - Nippon Steel Corporationpublication
8. BEST, C. L., & CARTER, G. C., "Application of moderntechnology to the design of a large blast furnace"Davy McKee publication.
9. Nippon Steel "Outline of Ironmaking Division" - NipponSteel Corporation, Oita Works publication
10 . Rautaruukki OY "The Rautaruukki Blast FurnaceSupervision and Control System" - Rautaruukki OYEngineering publication.
11. Jo Isenberg-O'Loughlin , "Banking on Blast Furnaces" -33 Metal Producing 11/97
i
¡I
12. Thomas A. Obrecht, David M. Armstrong, Anthony Bridges,David V. Walnoha, John A. Carpenter, "Automated RawMaterial Handling System and Blast Furnace ChargingSystem at Rouge Steel - AISE Annual Convention andMini-Expo, Pittsburgh, September 1998
7-27
13. Luc Bonte, Heli Delanghe, Maarten Depamelaere, BertSpeleers, "Installing Copper Staves and OperationalPractice at Sidmar" - AISE Annual Convention and Mini-Expo, Pittsburgh, September 1998
14. Robert G. Helenbrook, Paul F. Roy, Hartmut Hille"Correlation of Experimental Data with AnalyticalPredictions for Blast Furnace Copper Staves" - AISEAnnual Convention and Mini-Expo, Pittsburgh, September1998
7~8
LECTURE #8
IRONMAKIG REFRACTORIES: CONSIDERATIONSFOR CREATING SUCCESSFUL REFRACTORY "SYSTEMS"
Albert 1. DzermejkoHoogovens Technical Services Inc.
Pittsburgh, Pennsylvania, USA
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Abstract: Successful lifetimes of refractories utilzed in the blast furnace aredependent upon a variety of factors. The factors that directly influence performanceand lifetime can be categorized as external or internal to the refractories. "External"factors are those influences that have nothing to do with the refractories themselvessuch as furnace productivity, operating practices, burden material type and quality,furnace availability, furnace geometry and cooling capability, yet tremendously affectpedormance potentiaL. "Internal" factors are those influences that have a directbearing on refractory pedormance such as configuration, wear mechanisms, stressesand thermal movements, heat transfer capabilities, material type, characteristics andproperties. The success or failure of refractories wil be determined by how theseexternal and internal factors are addressed or ignored. The paper reviews thesesignificant factors, with the intention of providing guidelines for creating successfulrefractory "systems" in the blast furnace.
INTRODUCTION
Optimizing blast furnace productivity and effciency demands high rates of tuyereinjected fuels, oxygen injection and higher hot blast temperatures. Profitabilityoptimization often requires rationalization of facilities and concentration of productionin fewer, highly productive furnaces. These factors result in increased thermalloading, more frequent and intense temperature "peaks" and higher potential fordestructive effects on blast furnace refractories.
8-1
The concentration of production in fewer blast furnaces often in single-furnace plants,increases the need for reliable, uninterrpted operation. Unscheduled stops to repairor replace damaged linings or reduction of production intensity to "nurse" sick liningsto permit continued, albeit reduced production levels, are simply ttnacceptable intoday's competitive environment. Furthermore, capital intensive relines often result incrippling production interrption and adversely affect profitability. Consequently,
long campaigns with minimum reline periods are essentiaL.
Today, it is possible to design and configure blast furnace lining/cooling "systems"that provide the potential for continuous, uninterrpted service and allow for the so-called "endless" campaign. The cost of relining and providing the requiredcomponents can represent a large proportion of the capital available for the entireplant. The capital available is often limited because of the cyclic nature of thebusiness and by higher priorities such as the finishing end. These capital investmentlimitations often dictate compromises in design, configuration and materials withconsequential pedormance penalties. However, to achieve the endless campaignrequires cooling capability and protection for it, utilizing an appropriate refractorysystem.
Pedormance and lifetimes of refractories are dependent upon a variety of factors, bothexternal and internal to the lining/cooling "system". The success or failure ofrefractories wil be determined by how these external and internal factors areaddressed or ignored. The actual refractory product comprises only one par of a
complex, interrelated system of components and features affected by these externaland internal factors. External factors are those influences that have nothing to do withthe refractories themselves such as furnace productivity, operating practices, burdenmaterial tye and quality, burden distribution capability, furnace availabilty, furnacegeometry and cooling capability, yet can adversely affect performance potentiaL.Internal factors are those influences that have a direct affect on refractory performancesuch as configuration, wear mechanisms, stresses, thermal movements, heat transfercapabilities and refractory material type, characteristics and properties.
There is no ideal or "pedect" refractory which possesses magical powers to guaranteelong life. The very best refractory material for a paricular application wil failmiserably if consideration is not given to these external and internal factors.Refractory selection based solely on properties wil not assure successtul pedormanceor long life. It is imperative that expected operating conditions be identified, wear
mechanisms evaluated, and a comprehensive analysis conducted of all of the externaland internal factors which wil impact refractory performance. Only then can the"system" be properly configured and refractory materials selected, appropriate for theconfiguration and application.
8-2
BLAST FURNACE HEARTH
One of the largest users of refractory materials is the blast furnace hear. Worldwide,the 'configuration and design of this large volume refractory system variesconsiderably, with major differences in performance. This zone of the blast furnaceprobably exhibits more varied designs, conflicting practices and vastly differentperformance histories than any other. Technical aricles from certain countriescontinually describe the hear as the zone of the blast furnace most responsible forthe termination or interrption of the campaign. Contrasting this experience are the
apparent success stories from other countries of trouble-free, long campaign lives ofblast furnace hearhs (1). Figure 1 depicts these historical wear pattern differences.There are many reasons for this different pedormance history, especially when thedesigner analyzes the internal and external factors which affect the hearh refractory"system". Especially important are behavioral differences of the various refractorymaterials utilized, resulting from these factors.
Refractories
Traditional hear refractories have been carbonaceous in nature. Various grades of
carbons, graphite containing carbons, semigraphites and graphites are utilized. Often,various grades of ceramic refractories are combined with these carbonaceous materialsto form composite linings. It is also common to utilize several types and grades ofcarbonaceous refractories in these composite lining configurations to utilize specificproperties or characteristics of each type to their best advantage.
The words "carbon" and "graphite" are often used interchangeably in the literature,but the two are not synonymous. Additionally, the words "semigraphite" and"semigraphitic" are also misused. Compounding the problem is the fact that there areno industry-wide standards or specifications to define carbonaceous products. Eachworldwide producer manufactures unique products exhibiting unique properties andcharacteristics. This is the result of raw material differences, proprietar productingredients, additives, manufacturing methods and techniques. This is important torecognize because the behavior of these unique products can be very different in thesame application. These behavioral differences can result in major systemperformance differences, despite experiencing identical external and internal wearfactors. This is especially true when refractory configuration is not compatible withmaterial characteristics, properties and behavior resulting from these factors.
I
The following describes the major differences and characteristics of the carbonaceousmaterial types used as refractories in the blast furnace. Please remember that thegeneral nomenclature of these material types represent an entire family of materials,from a variety of manufacturers exhibiting unique compositions, characteristics andproperties.
8-3
Carbon
The terms carbon, formed carbon, manufactured carbon, amorphous carbon and bakedcarbon, refer to products that result from the process of mixing carbonaceous fillermaterials such as calcined anthracite coal, petroleum coke or carbon black with bindermaterials such as petroleum pitch or coal tar. These mixtures are formed by moldingor extrusion, and the formed pieces conventionally baked in furnaces to carbonize thebinder at temperatures from 800° to 1400°C (1500° to 25500P). The resulting productis comprised of carbon paricles with a carbon binder.
Typically, conventionally baked carbon is manufactured in relatively large blocks. Asthe binders carbonize and the liquids volatilize they escape through the block,resulting in porosity. This porosity results in a permeable material that can absorbelements from the blast furnace environment such as alkalies. These contaminants usethe same passages that the volatilizing binders used to escape the block to enter thecarbon and chemically attack the structure.
Conventionally baked carbon can be densified and thus permeability improved andpore sizes reduced. This can be accomplished by the introduction of additionalbinders impregnated into the baked carbon under a vacuum and the resultant productrebaked to carbonize the impregnation. Multiple impregnations are also possible todouble or triple densifY the end product. Each densification however, adds additionalcost and results in a higher priced product.
Some manufacturers also add special raw materials to the carbonaceous mix prior tobaking to improve the end products' properties. Silicon carbide, alumina powders, orsilicon metal can be added to improve permeability, reduce pore sizes and improveabrasion resistance. Arificial or natural graphite can also be added to improvethermal conductivity. Some manufacturers also impregnate the baked carbon toimprove thermal conductivity. However, each of these steps also results in a higherpriced product.
Conventional carbon is manufactured in large blocks and can be machined to precisetolerances. Grain structure however, can be different depending on the manufacturer,which can result in moderate paricle pull-out at sharp comers when paricle sizes arevery large.
Hot-Dressed Carbon
A North American manufacturer developed a unique proprietar method ofmanufacturing carbon which is called the BP process or hot pressing. In this methodof manufacturing carbon which, as previously described, is a product comprised ofcarbon paricles with a carbon binder, a special pressing/carbonizing operation is
utilized.
8-4
In this process, carbon paricles and binders are mixed as with conventional carbon,but are then introduced into a special mold. A hydraulic ram then pressurizes themixture while simultaneously an electric current passes through the mold, carbonizingthe binders. Unlike conventionally baked carbons that take several weeks to properlybake the binders, this proprietar process carbonizes the binders in minutes. Moreimportantly, as the liquids volatilize, the hydraulic ram squeezes the mixture together,closing off the pores formed by escaping gases. This forms an impermeable carboncompared to conventionally baked carbon, usually at least 100 times less permeable.This low permeability makes it diffcult for blast furnace contaminants such as alkalisto enter hot-pressed brick.
Special silica and quar additions are also added to improve alkali attack resistance.These additions are made because sodium or potassium in the blast furnace reactpreferentially with silica, forming compounds that do not swell in the carbon.Normally, the reaction of these alkalis with carbon would form lamellar compoundswhich do swell, causing volume expansion spalling of carbon. However, thecombination of hot pressing and raw material composition results in an improvedalkali-resistant carbon.
Hot pressing also results in a higher thermal conductivity than conventional carbonwhich helps promote the formation of a protective skull of frozen materials on thelining hot face. High conductivity linings have the ability to maintain a hot face
temperature that is below the solidification temperature of iron and slag. The resultingskull protects the wall from chemical attack and erosion from molten material flow.
Because of the special manufacturing process required for hot pressing, the product islimited to sizes not exceeding approximately 500 x 250 x 120mm (20 x 10 x 5 in).
Graphite
The term graphite, also called synthetic, artificial or electrographite, refers to a carbonproduct that has been additionally heat treated at a temperature between 2400° and3000°C (4350° and 54000P). This process of graphitization changes thecrystallographic structure of carbon and also changes the physical and chemicalproperties.
Graphite is also found in nature in flake form as a mined mineraL. It can be added tovarious carbonaceous or ceramic refractory products to enhance thermal conductivity.It is also utilized as the major component of graphitic ramming materials. The soft,flake form of natural graphite is unsuitable as a refractory lining material however.
Arificial or synthetic graphite refractories begin as a baked carbon material, similar inmanufacture to the carbon refractory material described previously. However, aftercarbonizing of the binder is completed, this baked carbon is then loaded into anotherfurnace to be graphitized at a high temperature. Graphitization changes the structurç;
8-5
not only of the carbon paricles, but also the binder. The resulting product iscomprised of graphitized paricles as well as graphitized binder.
There is no industry-wide system for designating the various grades of graphite thatare commercially available. Each manufacturer has a method and nomenclature todescribe the available grades and varieties which are made for specific purposes orproperties. These grades differ with regard to raw materials, grain sizes, purity,density, etc. For denser versions, the porosity of the material can be filled with
additional binder materials such as tar or pitch by impregnation under a vacuum. Thenthe impregnated material is regraphitized, forming a less porous product. Multiplereimpregnations and graphitizations can be pedormed to provide additionaldensification and higher thermal conductivity.
Purification can be utilized to reduce the ash levels of graphite for special
requirements. In addition, proprietar manufacturing methods and techniques can also
be used to minimize ash or iron contamination of graphites. Since iron is a catalyst forcertain chemical attack of graphite in a blast furnace, graphites intended for use as arefractory should contain relatively low iron.
Graphite products are manufactured in large blocks or rounds and must be cut andmachined into shapes for use as a refractory. Precise tolerances can be maintainedwith machined graphite components due to its easy machinability.
Semie:raDhite
The term semigraphite is used to describe a product that is composed of arificialgraphite paricles mixed with carbonaceous binders such as pitch or tar and baked atcarbonization temperatures of 800° to 1400°C (1500° to 2550°F). The resultingproduct is comprised of carbon bonded graphite particles in which the graphiteparicles had previously been manufactured at temperatures close to 3000°C (5400°F),but with binders that have only been baked in the 800° to 1400°C (1500° to 2550°F)range. The resulting product, a true carbon bonded graphite exhibits higher thermalconductivity than the carbons but, because of the carbon binder, not as high as 100%graphite. Thermal conductivities wil var with baking temperature and can beincreased by baking at higher temperatures.
These products are also conventionally baked (as described for carbon), which resultsin a relatively porous materiaL. However, these conventionally baked semigraphitescan also be densified and rebaked to carbonize the impregnated binder. Thus porosityand consequently, permeability can be reduced. Some conventionally bakedsemigraphites are also impregnated with or combined with silicon metal and siliconcarbide for greater abrasion resistance and lower permeability. These productshowever, are usually intended for use in the bosh and stack.
8-6
Semigraphite products are manufactured in large blocks or rounds and must be cut andmachined into shapes for use as a refractory. Precise tolerances can be maintainedwith machined semi graphite components, but the carbon binder makes it a hardermaterial than graphite, which can affect machining pricing.
Hot-Dressed Semi2raDhite
One North American manufacturer also utilizes its proprietar hot-pressing method tomake a true semigraphite refractory product. The resultant product is considerablyless penneable and has a higher thennal conductivity than conventionally baked
semigraphites.
Two distinct products are available for a variety of applications. One grade iscomposed of crushed graphite paricles, which were previously processed atgraphitization temperatures, with a carbonaceous binder and the addition of silica andquarz materials for alkali resistance (as previously described for hot-pressed carbon).
The other grade is a silicon carbide containing hot-pressed semigraphite refractory. Itis composed of the same graphite component as the first product and the samecarbonaceous binder. However, silicon carbide is substituted for the silica and quar.The resultant product is more abrasion resistant and even less penneable than the firstproduct. It has proven especially resistant to thennal shock and cyclic operation.
Because of the special manufacturing process required for hot pressing, the resultantproducts are limited to sizes not exceeding approximately 500 x 250 x 125 mm (20 x10 x 5 in.).
Semi2raDhitized
The tenn semigraphitized material refers to a carbon product that has been baked at atemperature between 1600° and 2400°C (2900° and 4350°F). This high bakingtemperature begins to change the crystallographic structure of the carbon and alters itsphysical and chemical properties. However, because this heat treating occurs attemperatures below graphitization temperatures, the product is considered to be onlysemigraphitized. It is comprised of carbon paricles with a carbon binder, which areboth semigraphitized during baking. (This is different than a semigraphite productwhich is composed of true graphite paricles with a carbon binder). Semi graphitizedcarbon has a higher thennal conductivity and resistance to chemical attack (alkali oroxidation) than carbon or semigraphite. This is because the binder is usually
preferentially attacked and the semi graphitized binder is more resistant to attack thanthe carbon binder of a semigraphite.
These semigraphitized products are manufactured in large blocks or rounds and mustbe cut and machined into shapes for use as a refractory. However, because of theirsemigraphitized bonding, they are more difficult to machine than a true graphite.
8-7. ¡
Discussion
These groups of carbonaceous materials form the basis for a full range of specializedproducts that are intended to enhance performance in the blast furnace. As discussed,various additives such as graphite paricles, alumina, silicon carbide or other ceramicsare included by some manufacturers to improve properties, or multiple impregnationsare used to improve permeability or reduce pore sizes. However, the general
description of each material classification does not change. For convenience, theseclassifications are summarized in Table i.
TABLE I
Classifvinl! Carbonaceous Materials
Product BakingClassification Temnerature. °C Particles Binder
Carbon 800 - 1400 Carbon Carbon
Hot-pressed carbon , 1000 Carbon Carbon
Graphite 2400 - 3000 Graphite Graphite
Semi graphite 800 - 1400 Graphite Carbon
Hot-pressed , 1000 Graphite Carbonsemigraphite
Semigraphitized 1600 - 2200 Semigraphitized Semigraphitizedcarbon carbon
Currently, there is a large variety of carbonaceous refractory material on the market,produced by different manufacturing techniques, exhibiting unique properties. It isdiffcult to provide material properties for these products without referring to specificmanufacturer's grade designations because as was noted before, each manufacturerproduces products that are unique to that manufacturer and thus exhibit unique
properties. However, a representative listing of some of these materials' properties are
summarized in Table II.
In general, carbon or semi graphite materials are used for the hot face lining materials
that wil be in contact with molten iron. Usually, graphite materials are reserved for a
backup lining to take advantage of their high thermal conductivities and because theyare more easily dissolved by the iron. In addition, many ceramic materials such as
8-8
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high alumina, mullite and chrome corundum are used in the hear pad as a wearingsudace to minimize exposure of the carbonaceous materials of the hearh to moltenmaterials. Some designers also configure a lining of ceramic materials on the face ofthe hearh walls for wear protection and to minimize heat losses, mainly because ofpoor historical performance with some large, conventionally baked, carbon blockdesigns.
Ceramic materials used for the hear pad are normally inexpensive super-duty
fireclays of 40 to 50% alumina or a variety of high alumina products in the 60% range.The objective is to provide a lining that wil melt and vitrify (or fuse together) on itshot face in the presence of liquid iron, effectively sealing the surface to penetrationand preventing potential brick dislodging and flotation.
In another philosophy, refractory materials such as arificial mullites or chromecorundum are chosen which are resistant to melting. These materials however, requirejoining techniques such as interlocking, tongue and groove or roll-lock interfacing toprevent joint penetration by molten materials and resultant flotation of bricks.
Whichever ceramic materials are utilized in the pad, the effect is that the iron remainsin contact with the ceramic, which is more resistant to abrasion from moving liquids.The carbonaceous material in the pad thus forms a cooling member instead of a
crucible, until late in the campaign when the ceramic may totally wear away byabrasion. The high conductivity of carbonaceous materials, especially if underhearhcooling or a graphite cooling course is utilized, enables penetration of the iron into thepad to be arested in the ceramic layer. This provides a long-wearing hearth design,
combining the properties of two or more different refractory materials to optimize theperformance of each, in the zone to which they are most suited.
There is also a growing belief that the incorporation of a ceramic hearh pad in highproductivity blast furnaces can often result in accelerated hear wall wear. This canbe especially true if the hear well volume is less than desirable and if poor cokequality is utilized. These conditions tend to result in higher peripheral flow of hotmetal. This problem is intensified by a high melting point ceramic pad layer, whichprevents formation of a bowl shaped "salamander" penetration and its consequentialwell volume increase which reduces hot metal velocities. Alternatively, a carbonhearh pad would quickly form a bowl-shaped "salamander" depression from
dissolution by the iron, increasing active well volume and consequently increasing theiron buoyancy effect on the coke deadman and decreasing hot metal velocities. Figure2 depicts the effects on hearh well volume of an all-carbon pad versus a high meltingpoint ceramic pad. An explanation of the resulting damaging effects is described later.
8-10
Representative properties of ceramic materials used in the hearh are shown in TableIII. They can be combined in various layers such that the more economical materialsare located on the hot face, where they wil be consumed more easily until thermal. equilibrium is reached. The more expensive, hot metal resistant materials are thenlocated next to the carbonaceous materials where they can be more easily cooled forlongevity. The tendency is to utilize specific grades of refractories in each hearh zonethat can best withstand the attack mechanisms prevalent in that zone. The result is ahearh lining composed of not just one grade of refractory, but sometimes even six oreight different tyes of materials, both carbonaceous and ceramic.
TABLE III!
¡_I ReDresentative Ceramic Hearth Materials
MaterialHard-burned
superduty 60% Artificial ChromeProDertv fireclav Alumina mullte Corundum
Density, glcc 2.24 2.40 2.45 3.43Crushing 31 35 85 78strength, MPaPorosity, % 13 22 19 8
Thermalconductivity ,W/moK
at 5000 C 1.9 2.0 N.A. N.A.at 10000 C 0.9 1.7 1.8 2.3
r Wear Mechanisms
In the hear, refractory survivability depends upon proper uninterrpted cooling. Thehearh bottom pad and walls are cooled on their cold face and almost exclusivelyutilize various conductive refractory materials such as carbon, semigraphite,
semigraphitized carbon and arificial graphite alone or in combination with each other,or combined with ceramic materials. The pedormance of the hearh lining system istotally dependent upon effective and uninterrpted heat transfer through the refractoryconfiguration because it is cooled on its cold face. All chemical attack mechanismsthat affect hearh refractories are temperature dependent chemical reactions. Thismeans that if refractory temperatures can be maintained below the temperature atwhich a particular chemical reaction begins, attack by that mechanism cannot occur.This threshold temperature at which the chemical reaction begins is called the "criticalreaction temperature". The only way that the refractory hot face temperature can bçmaintained below the critical reaction temperature for the various wear mechanisms
8-11
encountered is to provide an effcient and unchanging heat transfer path from the hotface to the cold face (1,i).
If conductive refractory hot face temperatures are allowed to exceed approximately1150°C (21000P), these carbonaceous materials wil be chemically attacked by
dissolution by the iron and wil be subjected to erosion and wear by the movement ofmolten materials. This is because the refractory hot face temperature would be abovethe solidification temperature of the iron and thus would be in constant contact withthe molten materials. Consequently, these molten materials may also be forced intothe pores of the refractories due to ferrostatic head and high furnace operatingpressures.
If conductive refractory temperatures are allowed to exceed approximately 870°C(16000P), these carbonaceous materials wil be chemically attacked by alkalis and zincwhich preferentially destroy the refractory binder system. As the binder system isattacked, material strengt and properties are destroyed, most notably thermalconductivity. Thus, as chemical attack progresses, the ability of the refractory totransmit heat is lost, which then results in even higher hot face refractory temperaturesand intensified attck.
If conductive refractory temperatures are allowed to exceed approximately 450°C(8400P) for carbon and approximately 500°C (950°F) for graphite, steam oxidationfrom cooling water leaks wil occur from the chemical reaction:
C + HiO-+ CO + Hz
which results in carbon loss and disappearance as it dissociates to form the two gassescarbon monoxide. and hydrogen. The resulting carbon loss can form irregular"ratholes", tunnels, chambers or similar cavities in the lining from the flow of thesegasses as they escape into the furnace.
If conductive refractory temperatures are allowed to exceed approximately 450°C(840°F) for carbon and approximately 650°C (l200°F) for graphite, carbon monoxidedegradation wil occur. This reaction is catalyzed by iron contamination in thecarbonaceous materials and intensifies as iron content increases. The presence ofsteam and hydrogen from cooling water leaks wil also dramatically intensify carbonmonoxide degradation as shown in Figure 3(3. This degradation results in depositionof carbon within the molecular structure of the refractory formed during the chemicalreaction:
2 CO -+ C + COz
As this carbon deposit increases with time, it causes a volumetric expansion whichresults in swelling, cracking of the refractory and destruction of strengt. Thecracking also interrpts the heat transfer path from hot face to cold face, resulting in
8-12
increased hot face temperatures and consequential intensified chemical attack fromalkali and zinc and carbon dissolution by the iron.
I
Blast furnace designers generally utilize computer modeling techniques to locate thecritical reaction temperature isotherms in the hearh lining. Table iv summarizesthese critical reaction temperatures for various hear refractory attack mechanisms.The location of these isotherms permits an evaluation of the potential chemical attackzones and "salamander" penetration into the hearth due to thermal considerations. Asnoted earlier, the 1150°C (21000P) isotherm wil define the star of dissolution of thecarbon by iron, the 450° or 650°C (840° or 12000P) isotherms will define the start ofcarbon monoxide degradation depending on the material, and the 870°C (16000P)isotherm wil define the star of alkali and zinc attack. Additionally, the 1250°C
(22500P) isotherm wil define the softening point of some ceramics that would beexpected to be eroded away by molten material movement. This computer modelingtool can therefore be utilzed to provide an estimate of chemical attack zones and ironpenetration, once the hear refractory mass reaches thermal equilibrium.
TABLE IV
Critical Reaction TemDeratures for Hearth Refractorv Chemical Attack
Chemical Attack Refractorv TVDe Critical ReactionMechanism TemDerature. °C
Dissolution in Iron Carbonaceous 1150Alkali / Zinc Carbonaceous Stars (c 870, stops (c 1100
Alkali / Zinc All Ceramics / Ceramic 560, intensifies as temp.Additives increases
Alkali / Zinc Silicon Carbide Additives 870, intensifies as temp.increases
CO Degradation Carbons, High Iron Graphites / stars ê 450, stops ê 750Semigraohites
CO Degradation Low Iron Graphites / stars ê 650, stops ê 750Semigraphites
CO Degradation Ceramics 400Steam Oxidation Carbons / Semigraphites 450Steam Oxidation Graphites 500
It should be noted that the major causes of chemical attack in carbonaceous materials,notably alkali / zinc attack and CO degradation, only occur within a specifictemperature range. The "critical reaction temperature" defines the temperature atwhich the chemical attack mechanism begins. Attack severity is dependent upontemperature, increasing, then gradually decreasing until the chemical reaction ceasesas it reaches its upper temperature limit. This attack behavior often results in the
8-13
critical reaction temperature and the corresponding reaction temperature upper limitboth being located within a specific refractory zone. The material located betweenthese two isotherms wil be chemically attacked. This zone of chemically attacked
carbon could be located between two unaffected zones of carbon, at the hot and coldfaces of the affected zone. This chemically afected zone, located between twounafected zones, is referred to as the "brittle" zone or "mushy" zone. This is shownin Figure 4.
Actual hear pad penetration and wall deterioration is a function of more than justthermal effects from temperature dependent chemical reactions. Mechanical stress,lack of thermal expansion provisions and erosion from molten material movement alsocontribute to hear wear. For this reason, many computer models utilize historicalwear line data to establish boundary conditions.
These boundary conditions allow the computer model to simulate hearh wear in the
profie that historically results from that particular lining concept. However, aparicular hearh model may have no significance for a different hearh concept orconfiguration, or for a furnace exhibiting a different historical wear pattern. Forexample, hearh computer models that are developed to estimate the invertedmushroom shaped wear pattern or "elephant's foot", with severe wall material loss atthe wall/pad interface, wil not accurately predict the expected wear pattern of afurnace that exhibits a historical bowl-shaped wear pattern, with little or no wallmaterial loss. Therefore, the designer must consider the concept, historical wear
performance and other "internal" and "external" factors which affect refractoryperformance when designing a heart(1.
External Factors: ODerations Effects
The blast furnace hearh lining can be adversely affected by furnace productivity,operating practices, burden material quality especially the coke and furnaceavailability. The effects from these external factors can often be intensified by otherfactors external to the refractory system such as furnace geometry and coolingcapability. Another external factor which can dramatically affect hearth refractoryperformance is leaking cooling water from cooling plates, tuyeres or staves. Nocarbonaceous refractory can survive steam oxidation caused by cooler water leaks.These leaks can also result in sudden loss of protective skull accretions as theyexplosively separate from the wall hot face due to pressure forces from steamformation. Therefore, cooling system maintenance practices can also play an
important role in refractory lining longevity and survivaL.
The following summarizes critical operations effects which are major external factorsin blast furnace hear performance:
8-14
Productivitv
High productivity increases the amount of molten materials flowing through the hearhand out of the tapholes. Hearh productivity is normally expressed as an index in
tonnes of hot metal per unit heart "well" volume, per day. The increased throughputof molten materials may accelerate hearh wear because of increased hear activity(4).There are many interacting factors involved including the effect of geometry. Hearthactivity intensity can be reduced by increasing the holding capacity (well volume) andincreasing the taphole-to-pad distance. Figure 5 ilustrates the effect on molten
material velocity by increasing hearh well volume. Multiple tapholes can also beutilized to decrease throughput and distribute potential erosive wear.
Coke Quality
Coke must be stable and strong to support the burden weight without mechanicalfailure or degradation. Coke eventually forms a "deadman", an inactive zone in thefurnace center from the hearh upwards, to above the tuyeres in the bosh. Strong,
properly sized coke tends to result in a permeable deadman with suffcient voidagebetween the individual coke pieces to permit molten metal flow. If the deadman ispermeable, it allows the liquids to flow completely across the hearh diameter. If cokequality and sizing is poor, deadman permeability is decreased as the voidage betweenpieces is reduced or disappears. This loss of permeability forces molten metal flowaround the perimeter of the deadman, in an anulus created between the refractoriesand the impermeable coke mass. This peripheral flow can intensify erosive loss andheat flux. Additionally, if the hear well volume is too small and cannot provide abuoyancy effect by the molten iron, the coke deadman wil "sit" and rest on the hear
pad instead of "floating" and providing additional flow area for the metal under thedeadman. This is depicted in Figure 6.
Iniected Fuels
High levels of tuyere injected fuels especially coal, reduce the proportion of cokecharged into the furnace. Consequently, coke quality becomes extremely important tohearh wear as high rates of injected fuels are utilized. It is also theorized that highrates of injected coal have a deleterious effect on deadman permeability because cokevoidage is blocked by the by-products of combusting coaL. Operating practices mustthen be adjusted to increase the proportion of coke in the furnace center. Center cokecharging practice requires furnace top burden distribution capabilities. This center-charged coke gradually works its way downward and replenishes the coke in thedeadman increasing permeability and thus decreasing molten material flow velocities.
Availabiltv
¡
I
'I
Whenever the furnace is off-wind, furnace stability is interrpted. Refractory damagecan occur from the results of coming on and off blast, refractory temperature cyclingand consequential fatigue and erratic operation that might occur during recovery from
8-15
the stoppage. Long campaigns are most likely with virtally continuous operationwithout lengthy planed or many, short duration, unplaned shutdowns. However,some operators theorize that off-wind periods during planned maintenance outagesencourage the formation of protective accretions (skulls) on the refractories, which arebeneficial to achieving long life. This is especially true if titania bearing materials arecharged or injected to assist with protective skull formation.
Hot Metal Chemistry
Higher hot metal silcon levels decrease the probability of hearth carbon dissolution(4).This is because the carbon saturation level decreases with increasing silicon content.Additionally, an increase in hot metal silicon increases the hot metal liquidus
temperature and reduces its fluidity. This results in increased metal viscosity, reducesflow velocity and encourages accretion (skull) formation. At lower hot metaltemperatures, the carbon saturation level of the iron is lower and is more easily
achieved. Conversely, loss of hearh carbon from dissolution wil be more likely assilicon levels are reduced.
TaDhole Lell~th and Practice
Long tapholes allow withdrawal of metal from deeper in the hearh and reduce the
probability of wall flow as the molten materials flow towards the open hole(4).Taphole clay quality and clay gun capability play important roles in determiningtaphole length. The taphole clay forms a "button" or "mushroom" where it exits thetap hole at the wall hot face and can be progressively increased in size as the number oftaps increases. Increased taphole lengths generally result in lower refractory wall
temperatures. Short taphole lengths, especially in single taphole furnaces, generallyresult in more intense sidewall activity, higher wall temperatures and increasedprobability of skull loss and erosion damage. Poor taphole clay or inadequate clay guncapability can prevent the achievement of long tapholes and their benefits. Multipletap holes often can be utilized to spread tap hole wear more evenly and allow for longerclay curing times between taphole uses. Alternate casts from tapholes located onopposite sides of a furnace also result in more effective hearth drainage. Lowercasting rates also decrease hot metal flow velocities in the hearh but increase thecasting time. Decreasing the number of casts per day also increases casting duration,which can have an adverse effect on wall wear if taphole clay quality is lacking.
External Factors: Geometrv
The greater the volume of the hear holding capacity (well volume), the more likelythe hot metal buoyancy effect can "lift" the coke deadman. Consequently, theavailable flow area for the molten material increases, which decreases their velocityand destructive effects. Deep well hearhs provide a taphole-to-pad distance of 2m(6.6 ft) or greater which also allows longer tapholes and decreased flow activity at thewalls. However, the positive benefits of a deep well hearth can be negated by poor
8-16
coke quality and consequential deadman impermeability. The effects on velocity ofwell volume and coke quality is depicted in Figure 6.
External Factors: Coolinf! Capabilty
All hearh refractories must be cooled on their cold face which requires anuninterrpted heat flow path. Any disruption of this heat flow path or loss of coolingefficiency wil result in elevated refractory temperatures. The most commondisruption of cooling capability results from loss of cooling effectiveness from
sediment or mineral deposits or corrosion of the cooling elements. These effectivelyinsulate the water from the heat source and can result in refractory degradation andloss. Another potential cooling capability loss can occur because of separation of thecooling element from contact with the refractories. This most often results from hightemperature differentials across the cooling element and consequential differentialthermal expansion between the cooled element and the refractories. The resulting "airgap" wil reduce heat transfer, significantly increasing refractory temperatures andincreasing the probability of chemical attack, skull loss and material loss. Pressuregrouting a conductive material into the separation anulus formed between the coolingelement and the refractories is a proven corrective action that reestablishes the heattransfer path.
Desif!n Considerations
Hearh walls comprised of large carbon blocks exhibit problems that can be traced to acombination of factors: lack of thermal expansion relief, high thermal gradients acrossthe wall block and the inability to accommodate differential thermal expansion. All ofthese factors promote cracks with subsequent hot metal and chemical attack (5). Attackof the wall by hot metal and chemicals most often is a result of the cracking problem.
Proper wall design requires a high thermal conductivity refractory that minimizesthermal gradients through the wall and consequently promotes the formation of aprotective accretion of solidified materials on its hot face. Proper wall design alsoincorporates provisions for radial thermal expansion of the wall but more importantly,incorporates provisions to accommodate differential thermal expansion of the wallthickness(6) .
Differential expansion occurs because the wall hot face temperature is higher than thewall cold face temperature. This differential is at least 1450°C (2650°F), especiallywhen an accretion of solidified materials is absent. As a result, the hot face of the wallgrows at a faster rate than the cold face. The differential growth induces high stressesin the blocks which are restrained from bending or bowing. Cracks result, parallelwith the hot face.
8-17
Thermal spalling and cracking of the hot face can also be induced by the rigors of ablow-in, especially when the wall design canot accommodate radial expansion andthe refractory thermal conductivity is low. This type of cracking also occurs parallelto the refractory hot face.
Cracks interrpt the ability of individual blocks to convey heat and facilitate coolingbecause each crack acts as an air gap which is a barier to effective heat transfer.Once the ability to convey heat is lost, the protective accretion may no longer formand therefore, carbon could be attacked by the hot metal and chemicals. This isbecause the carbon temperature wil be above the critical reaction temperature forattack by these mechanisms.
Additionally, the ramed layer required between the shell ( or stave) and the cold faceof a large block carbon wall also insulates the lining from the cooling system. This isbecause ramming materials shrink when cured and possess thermal conductivities thatare significantly lower than baked carbon. The lower conductivity and shrinkagecombine to provide additional bariers to heat transfer and result in high hot facetemperatures, often higher than the solidification temperature, so that skulls canotform on block walls.
Proper wall design not only accommodates thermal growth, expected differentialmovements and utilizes a carbon refractory with high thermal conductivity, but alsouses a carbon refractory possessing an extremely low permeability. The lowpermeabilty minimizes chemical and hot metal attack by preventing penetration intothe refractory.
It has also been demonstrated that a hearh refractory that possesses a low elasticmodulus, combined with a low coeffcient of thermal expansion, results in lowmechanical stress at the important pad/wall intedace. American big beam blocks andhot-pressed carbon as well as graphite and semigraphite, fulfill these requirements.Because of the elastic properties of these materials, expansion stresses are easilyaccommodated which prevents cracks from occurring in the wall. The opposite is truefor the strong, large blocks that typically are used in Europe and Asia in an attempt toincrease the life of the hearh wall.
Because of differential expansion and bending and the tight fit due to precisionmachining and the lack of thermal expansion provisions, these stronger blocks areprone to stress cracking, pinch spalling and thermal shock. Thermal shock isparicularly size dependent so that the larger the exposed hot face cross-section, themore likely thermal shock wil occur. Walls composed of smaller cross-section piecesare usually unaffected by thermal shock.
Expansion relief is also a requirement for preventing pinch spalling and stresscracking. This relief can be provided by specially designed expansion joints betweenblocks or by the use of special heat setting, carbonaceous cements. Ideally, thesecements should be installed in a suffciently thick layer to provide expansion relief
8-18
before curing. Afer curing, they should provide a strong carbonaceous bond to seal
the joint. Multiple layers and rings provided by small brick also permit differentialexpansion without cracking.
High thermal conductivity hot-pressed carbon and semigraphite refractories promotethe formation of a protective skull of frozen material on the hot face of hearh walls.This protective skull prevents wear of refractories due to erosion from gases or moltenmaterials. Additionally, rammed layers should not be utilized to maximize heattransfer to the stave or shell.
A single, full-thickness block canot accommodate the differential growthexperienced and consequently it cracks, thus interrpting heat transfer. The crack
prevents the hot face of the block from being cooled below solidification temperatureso a protective skull canot form. Thus, the large block carbon is continually exposedto molten materials at high ferrostatic pressure and high gas pressure. These highpressures tend to force the molten materials into the pores of the big block materials.Hot metal impregnation results in damage to the carbon and additional cracking andspalling.
In an attempt to prevent hot metal impregnation of large carbon blocks, many
manufacturers have introduced densified or reimpregnated carbon blocks with lowporosity and minimal pore size. These "micropore" carbon refractories are designed tolimit the amount of molten materials that can enter the structure of the materialthrough its porosity. This solution is contrar to that employed with hot-pressed
carbon or semigraphite concepts which utilize high thermal conductivity and theprevention of cracking to promote a hot face temperature that is maintained belowsolidification temperature. Thus, in the case of the latter, cooler wall concept, a skullquickly forms on the wall hot face and impregnation by molten materials is prevented.The resulting skull thickens over time to form an insulating layer once thermal
equilibrium is achieved. Wall hot face temperatures at the back of the skull in thesesystems are typically in the range of 200° to 300°C (400° to 570°F). Anotheradvantage that this cooler wall provides is that other temperature dependent reactionssuch as carbon monoxide degradation, alkali and zinc attack canot occur as long asthe wall temperatures remain below their critical reaction temperatures. Typically,these critical reaction temperatures are between 450° and 1100°C (840° and 2000°F)as previously discussed. As long as wall temperatures can be maintained below thesecritical reaction temperatures, attack by these mechanisms canot occur. However, ifstress-induced cracking, deterioration of ram layers or any other disruption of heattransfer occurs, wall temperatures wil increase, usually above these critical reactiontemperatures. This results in chemical attack of the wall material in the zone of thewall that exceeds these critical temperatures. As was also previously discussed, somechemical reactions do not occur above 11 OO°C (2000°F). Consequently, a deterioratedband of material can be formed within the wall thickness. This brittle zone is usuallysandwiched between sound carbon on both the hot and cold faces which is defined bythe critical reaction temperature isotherm locations.
8-19
As was also previously mentioned, some designers are utilizing a ceramic hot facelayer on the carbon walls to prevent wall erosion. In addition, because of the low
thermal conductivity of these materials, it is believed that wa11 heat losses wil bereduced. Several furnaces in Europe and Asia have been lined using this concept. Thelongevity of the ceramic is dependent upon good thermal contact with the carbon andmaintaining uninterrpted heat transfer capability through the large block carbon forthe life of the ceramic. For reasons previously discussed, large block carbon walls are
prone to cracking and loss of heat transfer capability. Thus, if cracking does occur,high temperatures result in the ceramic, hastening their demise.
In Europe and especially Japan, it is a1so common practice to create hearh protectionby adding significant amounts oftitania bearing ores in the burden or directly injectingthrough the tuyeres. This addition allows the formation of a protective layer oftitaium carbide to temporarily form on the hear walls, as long as the titania ischarged into the furnace. Once injection is stopped, the protective layer quickly wearsaway. This is an expensive method of heart preservation since the titania bearing oreis expensive and the furnace coke rate increases since energy is required to release thetitania from the ore(7. However, this operating cost penalty is often accepted becauseof the high financial pena1ty that would be incurred if the protection layer were notinduced and the hearh refractory failed. Thus, some operators are forced to add thesetitania ores to artificially induce accretions on the linings, as a result of the failure ofthe lining "system" to naturally provide the ability to freeze process materials on thewall hot face.
All carbonaceous products are resistant to chemical attck as long as they are properlycooled. However, because all hearh wall cooling is dependent upon heat transferthrough the entire wall thickness and then to a stave or furnace shell on the wall coldface, it is imperative that contact be maintained with the cooling system at all times.Properly designed and configured hearh staves are unaffected by differential thermalmovements between the furnace shell and refractories. Therefore, staves provide amore certain cooling contact with the wall, with virtually no separation. Externallycooled steel shells, especially the sprayed tye, often experience high differentialtemperatures and are prone to separation from the refractories. Often, highconductivity grouting materials must be injected between the shell and wa11 to re-
establish contact with the refractories, thus assuring heat transfer. Otherwise, thesmall air gap that forms between the shell and wall wil result in high walltemperatures and consequently, chemical and hot metal attack wil occur.
When rammed layers are used between the cooling elements and refractories, caremust be taken to insure that ram materials are utilized that exhibit little or no shrinkageand are installed utilizing the highest densities possible. Preferably, no rams should beused because a rammed layer is never as good as the refractory material adjacent to it.This is because the density, porosity and thermal conductivity of the ramming materialwil always be inferior when compared to the carbonaceous refractory product.Shrinkage of the rammed layer over time wil also result in a loss of heat transfercapability of the wall, shortening its life. When combined with other problems such as
8-20
block cracking or low thermal conductivity, this combination of problems results insevere hearh wall deterioration, cutback in an inverted mushroom shape and ultimatefailure.
Summary - Blast Furnace Hearth
Blast furnace hearh design concepts, materials, configuration and wear patterns varygreatly throughout the world. Hearhs are generally composed of varing grades ofcarbonaceous and ceramic materials, zoned to take advantage of the properties of eachgrade, to minimize wear.
Historically, severe hearh wall erosion problems are minimal in North America, butare a major source of downtime and termination of campaigns in Europe and Asia.
Solutions to hearh wall problems must consider all of the external and internal factorsresponsible for refractory wear such as operations effects, burden materials, thermalshock and stress, mechanical stress, differential thermal expansion as well astraditional mechanisms such as chemical attack and erosion.
Creating the successful hear refractory "system" requires comprehensive analysis.
A variety of design concepts, configurations and materials are available that enable theblast furnace operator the capability to extend hearh campaign life. Survival dependsupon recognition of and reaction to all of the external factors that can destroy anyrefractory system, even one that properly addresses all of the internal factors in itsexecution. With the proper combination of operating practices and expertise,performance monitoring and control and the appropriate lining/cooling system, the"endless" hearh campaign can truly become a realistic goal.
8-21
BOSH. BELLY AND STACK
The refractory systems comprising the bosh, belly and stack of the blast furnaceproper probably are the most critical in terms of their impact on the operating
capability of the ironmakng complex. No other refractory systems in the ironmakingplant have a greater effect on the day-to-day survivability and integrity of their processcontainment vesseL. Indeed, if the furnace must shut down because of bosh, belly orstack lining problems or failures, iron production ceases and the need for any otherrefractory lined system in the complex ends.
In North America, this is especially true because of the scarcity of major heart wallproblems. Many other worldwide blast furnaces suffer a myriad of hear wearproblems. These problems are often more critical to campaign termination than thoseexperienced in the bosh, belly and stack, because there are no easy ways to correctthem once they appear. However, once hear refractory problems are minimized oreliminated by adopting proper operating practices, raw materials, lining concepts,configurations and refractories, the bosh, belly and stack becomes the critical systemfor attention.
The cooling aspect of the lining/cooling "system" is a most critical factor which candetermine the success or failure of a refractory product. If refractory temperatures riseabove their "critical reaction temperature" for chemical attack, refractory failure andloss are inevitable(8). Additionally, severe blast furnace gas flow pattern changes canthermally shock certain types of refractories, even if they are properly cooled. Thus, itis imperative to consider not only the cooling method, but how to configure a"system" which incorporates refractory materials that are appropriate for the expectedwear factors. The properties and characteristics of these refractories must work incombination with the cooling, to achieve the intended performance.
Refractory Materials
Bosh linings are comprised of various conductive refractories such as carbon,semi graphite and graphites, varing tyes of ceramics or sometimes combinations ofboth. Historically, belly and stack linings were comprised of various types ofceramics. Lately however, conductive refractories such as graphite and semigraphitehave proven superior, used alone for their excellent chemical attack and thermal shockresistance, or for their cooling capability in combination with various ceramics.
Carbonaceous (Conductive )Refractories
Table I in the preceeding BLAST FURNACE HEARTH Section lists theclassifications of the various types of conductive carbonaceous refractories. It isdiffcult to present material properties of these products, without referring to specific
manufacturer's grade designations. That is because each manufacturer produces
unique products that exhibit unique properties. Tables V and VI however, present arepresentative listing of conductive carbonaceous materials that are used' as
8-22
refractories in the bosh, belly and stack and typical properties. The iron content ofcarbonaceous refractories is critical because iron catalyzes carbon monoxidedegradation. Therefore, the lower the iron content of the material, the greater thepotential to resist CO degradation.
Ceramic Refractories
The properties and characteristics of all ceramic refractories depend upon the rawmaterials utilized and their size consist. The fine paricles in the mix form the ceramicbonding of the larger paricles as the material is fired at high temperature. The firedrefractory contains larger crystallne paricles bonded together with glass or othersmaller crystalline paricles that have fused together during firing.
Crystals composed of silica or alumina form strong bonds in materials such as fireclayor high alumina. Glass bonded refractories tend to have good strengt but soften anddeform under load. Additionally, impurities such as iron oxide or lime promote theformation of glass. Therefore, manufacturers try to limit the amount of impurities inthese types of products.
Table VGraDhite Material ProDerties
Pro er
GraStandarddensity,Std ash
hite Material Descri tionStandard Mediumdensity, density,Low ash Low ash
Highdensity,Low ash
Bulk densi cc 1.63 1.67 1.72 1.80
Porosi , % 21 16 14 12
Cold crushingStren h, MPa 20 28 40 51
Thermal conductivity,W/moK 0) 20°C
1000°C15070
14070
1575
16080
Ash, % 0.5 0.2 0.2 0.2
8-23
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When designing ceramic lining "systems", significant refractory properties are thebulk density, which reflects the heat caring capacity of the refractory and its porosityor permeability which provides a means to determine the ability to resist penetrationby molten materials and gases. Bulk density also influences thermal conductivity andthe chemical resistance of the brick to such wear mechanisms as alkali, carbonmonoxide degradation and slag or hot metal attack.
Refractory service temperature is an important issue in any ceramic refractory system.For each chemical attack wear mechanism, there exists a "critical reactiontemperature" for that refractory grade, which defines the point at which chemicalattack commences. If refractories can be cooled below this critical reactiontemperature, chemical attack could be prevented. This is because all chemical attackmechanisms are thermochemical reactions and as such, the rate of reactions istemperature dependent. Therefore, the refractory designer must provide a "system"that provides refractory temperatures consistently below the critical reactiontemperature for each grade of refractory in the system. Cooling enhancement orhighly conductive refractory components can assist with this effort.
Thermal shock resistance is also a critical issue when investigating refractory systemdesign. Thermal shock or "spallng" is caused by thermal stresses which develop fromuneven rates of expansion and contraction within the refractory, caused by rapidtemperature changes. There are no standard tests for evaluating thermal shockresistance because shock is also a fuction of size and shape. A qualitative predictionof the resistance of materials to fracture by thermal shock can be expressed by thefactor:
ks/aE, where:
k = Thermal conductivitys = Tensile strengtha= Coeffcient of thermal expansionE = Modulus of elasticity
The higher the value of this factor, the higher the predicted thermal shock resistance ofthe materiaL.
Erosion and abrasion are important issues, especially in the top of the furnace andareas of high gas flow. Erosion of refractory particles results when the bond of therefractory is destroyed by impact or impingement of process materials or dust-ladengasses. The high-density materials exhibit higher resistance to abrasion or erosion.
Fireclav
Fireclay refractories consist of hydrated aluminum silicates and minor proportions ofother materials. Examples of fireclay refractories are super duty, high duty or mediumduty. These materials contain between 18 to 50% alumina and 50 to 80% silica,depending on the grade.
8-25
Super duty fireclay refractories exhibit good strengt and volume stability and have analumina content of approximately 40 to 50%. Often, super duty fireclays can be hightemperature fired to enhance the high temperature strength of the brick, stabilizevolume and prevent damage by carbon deposition. These materials are often used aseconomical bosh, belly and stack linings in low production facilities or as a sacrificiallining material on the hot face of the refractory wall. Tar impregnation can be used toreduce porosity and permeability to improve resistance to chemical attack.
High duty or medium duty fireclays are normally utilized in areas subjected tomoderate attack mechanisms. In the bosh, belly and stack, they are often used as ablow-in protection lining and as a low cost sacrificial hot face lining materiaL.
Hi2h Alumina
High alumina refractories are available with alumina contents of 45 to 99+ %. Theyare limited to a maximum service temperature of approximately 18000e (3300°F).They exhibit high refractoriness and chemical resistance at high temperatures. Mulliteand corundum materials are also considered as high alumina refractories. Thesematerials are often used as a bosh, belly and stack refractory in low to moderateproduction facilities or where budgets are limited. They can also be utilzed as the hotface or cold face lining layers in "sandwich" lining configurations. These materialsalso can be tar impregnated to improve permeability and thus improve chemical attackresistance.
Silcon Carbide
Refractories comprised of silicon carbide are used in the bosh, belly and stack due totheir higher resistance to chemical attack, abrasion and thermal shock as compared tofireclay or high alumina refractories. Silicon carbide refractories can utilize severaldifferent bond types which change the physical properties of the refractory.
In general, silicon nitride (ShN4) bonded silicon carbide has proven to be preferredover various direct bonded, self-bonded or carbon silicon bonded materials. Recently,sialon (SkxAIOxNs-x) bonded silicon carbide (as well as sialon bonded high aluminas)have been used in the bosh, belly and stack for their improved alkali resistance.
The bonding system used in silicon carbide refractories can be affected by the variouswear mechanisms encountered in the blast furnace. For example, oxidation can be aproblem to the self-bonded or direct bonded silicon carbides, which causes a"swelling" of the materiaL. For this reason, most ironmakers utilize either siliconnitride bonded or sialon bonded silicon carbide for the bosh, belly and stack.
8-26
Silicon carbide refractories, because of their abrasion resistance, can also be utilized inthe stockline area where impact from falling charge materials is severe. Phosphatebonded high alumina materials also are successfully utilized in this erosion pronezone.
Ceramic Properties
Historically, many different grades of ceramic refractories have been used in the bosh,belly and stack with varing degrees of success. Often, capital restraints or theexisting cooling system and its capabilities limit the choice of refractories.Sometimes, very economical grades of refractories are chosen for their "sacrificial"use as a blow-in protection lining or for a stave cooling system that is intended tooperate "naked", that is, without a refractory lining. Sometimes, budgetar limitationspreclude the use of exotic ceramics or silicon carbide refractories that could improveperformance. However, there are a large and varied group of ceramic refractoriesavailable, at varing price levels, to achieve the intended lifetime goals. Arepresentative listing of some of these materials' properties are summarized in TableVII.
TABLE VII
Representative Bosh. Bellv and Stack Ceramic Materials
Superduty High Alumina High Alumina Silcon Carbide Silcon CarbideProperty ("'48%Ali03) (",60%Ali03) ( ",90%Ali03) (SbN4 Bonded) (Sialon Bonded)
Density, wcc 2.4 2.55 2.95 2.58 2.72Crushig strengt,Mpa , 60 80 120 140 180Porosity, % 11 15 15 15 14
Thermal conductivity,W/moK (t 1000°C 1.7 1.9 2.9 13 12
i I!
The many permutations possible from the range of different materials available todayoffer the designer challenging opportunities for optimizing the lining design. The keyof course, is to recognize that the success of the "system" wil depend upon how themany internal and external factors which affect the refractory system are addressed.Additionally, lifetime improvements can also occur if various grades and types ofrefractories are combined in one system to take advantage the best properties orcharacteristics of each of the products used.
8-27
Refractory Performance Factors - External and Internal
Bosh, belly and stack refractory performance is a function of many factors, most ofwhich are determined by the presence of chemicals and high temperatures in the blastfurnace process. A major contributing factor is the location, direction and velocity ofthe counter-current gas flow as it permeates through the burden materials. Alsocontributing are the abrasion affects of the descending burden materials and
ascending, dust laden gasses. Review of historical furnace refractory "wear lines" wilshow evidence that the areas that experience the most intense gas flow patterns andhigh heat load are the bosh, belly and lower stack.
The location of this severe "wear" zone can var slightly from furnace to furnace andis dependent upon many operating variables. These variables can include type andquality of the burden materials, burden distribution capability, quality and quantity oftuyere injected fuels, quantity of wind blown, furnace availability and many otheroperations related factors.
The purpose of this discussion is to review all of the external and internal factorswhich can affect performance with the intention of optimizing refractory life. Thiswil permit the configuration of an initial lining with the best chance of survival andthus wil postpone inevitable repair until very late into the furnace campaign.
External Wear Factors
Blast furnace operations can destroy any refractory system, even one that is comprisedof the most appropriate refractory material for the application. The intense chemicalreactions that occur in the blast furnace, coupled with high velocity, high temperature,dust laden gasses often impinging directly against refractories, results in relentlessattack.
If the geometry of the lining, the so called "furnace lines" or the refractory
configuration are incorrect for the application, refractory loss wil be hastened. Inparticular, the cooling system type and effciency is a most critical external factorwhich can determine the success or failure of any refractory product. Some expertshave even claimed that "cooling water is the best refractory". However, it should berecognized that cooling water removes heat energy from the blast furnace process andthat properly engineered lining/cooling systems provide a way to optimize liningperformance and minimize wall heat losses over the full campaign.
The wear mechanisms encountered in the furnace vary by type and intensity, by zone.Recognizing which mechanisms of wear wil be encountered and gauging theirintensity allows the designer to configure a lining system best able to resist the attackmechanisms for the longest time. Each of these external factors must be consideredindividually to properly create a successful refractory system.
8-28
External Factors: ODerations Effects
The goal of any blast furnace operator is to make a good profit for the owner. Eachblast furnace wil have productivity, fuel rate and performance goals unique to theowner's paricular situation. This means that in some cases, furnaces must be operated
at intense levels for maximum production, even at the expense of fuel rate orrefractory life. Others must try to achieve a balance of high productivity with aminimum fuel rate to keep costs low. Stil others must operate their furnacesconservatively at moderate production rates, especially if total ironmaking capacity isout of balance with steelmaking capacity. Others are limited by raw materials tye or
quality or physical limitations such as blowing capacity. Stil others are able toachieve very high production rates at minimum fuel consumption and high effciencybecause of the type and high quality of the burden materials and modem physicalplant.
It is because of this broad variation in furnace performance and capabilities, which arefunctions of external influences unique to each furnace owner, that it is impossible to"standardize" refractory configurations. What might work for one furnace might wellbe a total failure when applied to a similar furnace operating with different variables.
Productivity
High productivity intensifies the destructive factors which affect lining life. Windrates are high which results in high process gas volume and velocity. High wind ratesrequire high tonnages of charged raw materials, increasing abrasion. Highproductivity also intensifies heat loads on refractory walls and can often intensifyvariations in wall heat loads resulting in severe temperature "peaks".
Conversely, low or moderate productivity can result in minimal wall gas flow, smallertemperature "peaks" and low wall heat load. Thus, these conditions would be morefriendly to refractory life and may permit a much different lining configuration andquality than that required for a high productivity furnace.
, \
!
Injected Fuels
The type and quality of tuyere injected fuels can also play an important role inrefractory life. This is especially true if high rates of oxygen are injected. Thechemical reactions from tuyere injected fuels can be endothermic (where heat isabsorbed) or exothermic (where heat is released). Operators try to control racewayflame temperatures, depending upon whether endothermic reactions or exothermicreactions wil occur. This can result in different raceway conditions and consequentialgas flow pattern differences.
8-29
Burden Distribution
Burden materials are deposited in predetermined, premeasured layers. The coke layersare configured to allow gas flow through the mass, because they are reasonably
permeable compared to the ore component of the charge. This is graphically depictedin Figure 7. There is a "science" of burden distribution techniques utilizing a varietyof furnace charging hardware which permits the operator to control gas flow patterns.This is done by changing the ore-to-coke ratio at various locations across the burdensurface and by physical placement of ore or coke at specific locations to either retardor enhance gas flow. Thus, it is possible to control productivity, fuel rate and wall gasflow to achieve the goals established for maximum profitability. Conversely, ifburden distribution capabilities and techniques are unavailable or minimal, wall gasflow, heat load and temperatures would be erratic. This would adversely affect longterm refractory performance.
Burden Materials
High quality burden materials especially coke, can improve furnace performance andeffciency and thus have a positive influence on refractory life. This is especiallyimportant in the lower stack, belly and bosh where the combusting coke must allowunimpeded gas flow, and yet have suffcient strengt to support the great weight of theburden column above. It has been proven time and again that poor coke quality wiladversely affect furnace permeability and thus performance, with consequential lininglife penalties.
A vailabiltv
Blast furnaces pedorm best when they are operated continuously with a minimum ofstoppages or disruptions to driving rate. Every reduction of wind volume, materialcharge delay, casting delay or maintenance stop disrupts "smooth" operation and gasflow. Thus, furnaces which are plagued with maintenance problems which force
shutdowns or productivity reductions, wil be paricularly hard on refractories.Conversely, blast furnaces which operate smoothly and at a reasonably constant
production rate with a minimum of shutdowns and delays, wil be easier onrefractories.
These operations effects can result in changes to the shape and location of the"cohesive zone" where the liquid metal droplets form and changes in the intensity anddirection of the hot process gasses, as they pass through the permeable coke "slits"that are layered in the burden mass. The shape of the burden profie can vary from a"Y" to a "W", depending upon burden distribution capability and practice. The "Y"shaped profile results in more central gas flow and reduced wall working, and the "W"shaped profie results in less central flow and more wall working.
8-30
There are many operating philosophies regarding the "optimum" burden distribution,gas flow patterns and cohesive zone shape and location that maximize productivityand minimize fuel rate. However, many of these practices are paricularly severe onrefractory survivaL. Many operators are also familiar with burdening practices that canminimize wall working and protect refractories. The problem in this age ofcompetitiveness is that most producers must optimize production and minimize fuelrate at the expense of the refractories. This compromise results in shorter lining lifeand requires periodic furnace stoppages to repair linings by the injection of groutingmaterials or by the application of sprayed-on, "gunned" refractory, both of which self-set in the furnace to form a "consumable" lining. However, this practice requiresperiodic re-application to be effective. The goal should be to provide an initialrefractory system which can optimize life and postpone repair.
External Factors: Geometrv Effects
If the physical geometry or so called "furnace lines" are improperly configured, severerefractory wear can result. This is especially true in the bosh where raceway actioncan result in impingement of high velocity gasses and entrained paricles on therefractory wall. An example of this is shown in Figure 8. Proper geometry wilconsider expected gas flow patterns and directions as well as historical performance toeliminate potential problems. Even the most appropriate refractory for the applicationcannot survive the relentless and never ending actions of the raceway if the geometryresults in impingement. Furnace designers the world over can provide the properrelationship required when determining furnace lines (geometry) for a new or rebuiltvesseL. However, the problem most users must address is how to incorporate propergeometry into existing facilities if there is a shortage of capital. However, norefractories can correct the problem of bad geometry.
External Factors: Confiiwration Effects
The actual refractory configuration can affect refractory life. For example, arefractory wall cooled on its cold face such as with sprays or staves, can have a veryhigh hot face temperature ifthe wall is configured excessively thick. Accretion (skull)formation could prove diffcult or the resulting accretion might be very thin. This isbecause the heat must travel completely from hot face to cold face and if the thermalresistance of the wall is very high (thick wall, low thermal conductivity), it would bediffcult to remove the heat fast enough to prevent a high hot face temperature. Figure9 depicts a comparison of hot face temperature and skull thickness for a thin versusthick wall configuration, utilizing the same refractory materiaL. You could improvethe situation by increasing the thermal conductivity of the wall or reducing the wallthickness (or both) to lower the thermal resistance and thus lower wall hot facetemperature. Another way to decrease the thermal resistance of a thicker wall is toutilize a composite construction of a very high conductivity cold face material todecrease the thermal resistance of the entire wall. This is depicted in Figure 10. Theobject is to obtain wall hot face temperatures that are low enough to condense vaporsand form thick protective accretions of solidified process materials. -
8-31
External Factors: Coolin!! Considerations
Cooling system capability is affected by the type of cooling employed. Typically,bosh, belly and stack refractories are either cooled externally on their cold face orinternally cooled by a multiplicity of copper elements installed in rows, which areinserted radially within the wall. Figure 1 1 ilustrates arangements of the variouscooling system types used in the bosh, belly and stack.
Coolin!! Tv De - External Water Film (SDrav Coolin!!)
External cooling is accomplished by several methods, each of which has good and badfeatures. The earliest application of external cooling is merely the introduction of aseries of water nozzles, aranged circumferentially around the furnace jacket, whichprovide cascading water film on the jacket surface. The water is then collected in atrough at the bottom of the jacket. This arangement is often called "spray" cooling,but in fact is really "fim cooling" since a thin fim of water actually performs the heatremoval. The advantages are simplicity, low cost, efficient heat removal and thepressure containing jacket remains visible for easy "hot spot" or crack detection.
Disadvantages are that the open water collection system is easily contaminated by dustand debris, water flow can be disrupted or inadvertently stopped by obstructions in thewater flow path and instrumentation or other vessel connections disrupt water flowand are hard to install during furnace operation. External cooling also results inadding thermal stresses to the pressure containing jacket and can result in differentialthermal expansion between shell and refractories, causing a loss of cooling contactwith the refractories.
Coolin!! TVDe - External Panels
Another form of external shell cooling is the use of water containment jackets,
chanels, angles or other steel weldments to form water flow passages on the shellcold face. The advantage is the ability to totally close the water system to preventcontamination by dust or debris. The disadvantages of this type of jacket cooling orpanel cooling as it is called are many. First, it is very difficult to arange the flowpassages to achieve high water velocity throughout the flow path. There can be areasof low velocity or eddy currents which impede heat removaL. Additionally, ifuntreated river or lake water is utilized, organic, mineral and sediment build-ups caninsulate the water from the jacket, interrpting heat transfer. Another problem is thatthe panels completely hide the sudace of the pressure containing vessel, which canprevent the discovery of shell hot spots or cracks until damage occurs. This type ofcooling also adds thermal stresses to the pressure containing jacket.
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Coolin!! TVDe - Staves
A third type of external cooling utilizes cast iron or copper cooling elements calledstaves. Water flow passages are integrated into the cast iron elements using steelpipes, or in the copper elements by using integrated pipes or machined or cast flowpassages. The advantage of stave cooling is primarily the ability to cool therefractories from within the pressure vessel, thus eliminating thermal stress of thejacket. Additionally, the jacket is totally exposed for inspection purposes and sincethe cooling system intercepts heat before it reaches the pressure containing jacket,thermal stresses in the jacket are low. The disadvantages of staves are their high cost,inability to be easily changed in case of wear or damage and they generally requirechemically treated water to prevent mineral build-ups and maintain effectiveness.
Cast iron staves also require various types of "insert" materials, installed withinhorizontal grooves located on the stave hot face. The iron stave "ribs" which formthese grooves, contain and support the insert materiaL. These insert materials can beconductive refractories for best heat removal capability, or be insulating materials incase the stave is intended to operate "naked", that is without a refractory lining in frontof the stave.
All external cooling systems are sensitive to any interrption of the heat flow path
from the hot face of the refractory to the water. Any degradation, disruption or loss ofcontact in this heat flow path wil adversely affect refractory temperature and hastendegradation. These factors can include poor water quality and low velocity, whichresult in corrosion, mineral build-ups, sediment deposits and organic build-ups on thecooling element. Once these deposits form, they provide an insulating layer betweenthe cooled surface and the water, preventing heat removaL. Consequently, refractorycooling is adversely affected and chemical attack results.
Another serious potential problem with any externally cooled refractory system is thatseparation or loss of cooling contact can occur from differential movements. Oncerefractory cold face contact is lost, the resulting "air gap" provides a very effectivebarier to heat transfer, causing a disruption of cooling and high refractorytemperatures. All externally cooled refractory systems should be equipped withprovisions for injecting conductive grouts between the refractory cold face and thecooling surface. Periodic injections of this grout can fill these air gaps and re-establishheat transfer, thus improving refractory pedormance.
Coolin!! TVDe - Inserted Coolers
Another type of cooling system utilized in the bosh, belly and stack is the use ofinserted cooling elements. Most often, especially in high heat load zones of thefurnace, these inserted elements are comprised of cast or forged copper. Water flowpaths are formed by cast-in-place or machined passages, tubes or combinations ofthese methods in the same element.
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The elements are generally arranged in rows, radially inserted into the lining andconfigured such that the number of elements per row and the row spacing aresufficient to provide the required cooling effect on the refractories. The "density" ofthese cooling elements, that is, the number of elements per row and the spacing of therows can be such that intense refractory cooling can be provided. Generally, the hotface of the cooling element (the nose) should be located at or very close to therefractory hot face.
Heat transfer to the cooling elements is effected through the use of a conductive
rammed anulus between the cooler and the refractories or by intimate contact withthe refractories. Care should be taken to carefully configure this critical detail tominimize interrption of the heat flow path.
The advantages of inserted cooler elements are primarily the ability to cool therefractory wall internally from hot face to cold face and the ability to changeindividual cooling elements if they are damaged in service. They also providephysical support of the refractory mass, which is very important for refractory wallintegrity.
For optimum pedormance, the connection where the cooling elements penetrate thepressure vessel must be gas-tight. This can be a problem on older furnaces wherecapital constraints prevent gas-tight connections from being incorporated. This canresult in "gas tracking", consequential loss of heat transfer capability, high shelltemperatures and of course, a safety hazard to personneL.
The disadvantages of the inserted cooling elements are mainly related to economicconsiderations. In order to incorporate a modem, densely spaced cooler pattern on anexisting furnace, a new steel jacket may be required. Additionally, water quality andcooling element design are important considerations that should be incorporated foroptimum pedormance. However, inserted cooling elements offer the highest coolingeffciency, longest life potential and easiest maintenance capabilty of all of theavailable cooling systems.
Often, bosh, belly and stack linings are cooled with both internal and external coolingtypes, especially the combination of staves with inserted coolers. This can be done byzone, such that the bosh might be stave cooled and the belly and stack cooled byinserted plates. Another concept utilizes stave coolers between rows of insertedcoolers, when the existing cooler rows are too far apart for proper refractory cooling.The object of any cooling system configuration is to provide the desired coolingeffect, while recognizing the strengts and weakesses of each, and makingallowances for them in the refractory configuration.
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External Factors: Wear Mechanisms
The severity of attack by the mechanisms of wear in the bosh, belly and stack can bedifferent from furnace to furnace even in the same plant, due to variations in furnacegeometry, burden materials and distribution and furnace operation. The lower zonesof the blast furnace, from the bosh to the mid stack are most affected by thermalshock, high head load and chemical attack. These zones are the real "trouble areas" onvirtually all blast furnaces and are most responsible for termination of furnacecampaigns or lengthy repair downtime.
From the upper middle stack to the stockline, mechanical wear and impact fromcharging become the main contributors of wear, along with chemical attack.
Thermal attack includes exposure to high temperatures over time, severe temperaturefluctuations and fatigue. Chemical attack includes attack by alkali vapor andcondensate, carbon monoxide degradation (carbon deposition), oxidation and attack byslag or molten metal. Mechanical wear includes erosion from ascending dust laden
gases, abrasive wear of descending burden materials and impact loads from fallingburden materials.
A summar of these wear mechanisms, by severity and by furnace zone, is listed inTable VIII and is graphically portrayed in Figure 12.
Wear Mechanisms - Thermal Shock
It is universally agreed that the predominantly pellet charged, typical North Americanblast furnace wil be subject to more intense high temperature fluctuations at the wallthan experienced by predominantly sinter charged, European and Japanese blastfurnaces.
It has been demonstrated by Hoogovens, an ironmaker in the Netherlands, that thesetemperature fluctuations increase dramatically as the pellet charge exceeds
approximately 15 to 20% of the total metallics charged. Actual temperature peaksexperienced by a 50% pellet - 50% sinter charged furnace, have been shown to betypically up to 1000°C (1850°F) over a 6 to 7 minute period, or approximately 150°C(300°F) per minute temperature change. However, the predominately sinter chargedfurnaces consistently experience wall temperature fluctuations of only approximately40°C (100°F) over the same 6 to 7 minute period, or approximately 7°C (20°F) perminute temperature change (9).
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Wear Mechanism
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This means that whatever refractory is chosen, it can experience exposure totemperature changes of up to 150°C (300°F) per minute if pellets are charged and onlyapproximately 7°C (20°F) per minute if predominantly sinter is charged. Typical NorthAmerican operation utilizing predominantly pellet burdens, thus exposes wallrefractories to the more severe temperature fluctuations due to gas flow changes in thepellet burdens.
It has been demonstrated that all ceramic refractories, including silicon carbidematerials, wil spall and thermally crack if they experience temperature fluctuations ofthis severe magnitude. Critical spalling rates have been discussed in several technicalpapers by Hoogovens of the Netherlands (9). A list of typical critical spallng rates for avariety of materials is shown in Table ix.
TABLE IX
Critical Spallng Rates for Various Materials(9)
Material °C/Min. OF IMin.
High Duty 4 7High Alumina 5 9Chrome Corundum 5 9Cast Iron 50 90Silicon Carbide 50 90Carbon 200 400Semigraphite 250 450Graphite 500 900
These critical spalling rates define the maximum temperature variations (heating orcooling) that the hot face of the refractory materials can survive without cracking.
Beyond these rates, cracking and spalling wil occur. As can be seen from the table, theonly materials which can withstand the normally occurring 150°C (300°F) per minutetemperature excursions of a typical pellet charged furnace are carbonaceous materials,the so called "conductive refractories".
The thermal shock failure effect is most severe when refractories are cooled from oneside, like with staves or externally cooled jackets. This is because thermal shock cracksoccur parallel to the refractory hot face, which result in three problems. First, thesecracks permit the alkali vapors and condensate to be exposed to a greater refractorysurface area including the interior of the refractories, hastening chemical attack.
Second, because these cracks occur parallel to the hot face, air gaps form whichinterrpt heat transfer to the cooling system, thus increasing refractory hot face
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temperature. Consequently, since chemical attack is temperature dependent, thisincrease in refractory temperature at the hot face wil assure that the hot face is
chemically attacked as temperatures exceed approximately 600°C (1100°F) for highaluminas and fireclays and 800°C (1500°F) for silicon carbides(8).
Third, as' accretions (skulls) form and fall off as wall temperatures increase, the fallngskulls pull away this cracked layer of refractory which adheres to the skull, thus againexposing a new hot face to be thermally shocked, repeating the cycle. This sequence isgraphically depicted in Figure 13.
As the thermal shock/chemical attack/scab pull-out of material cycle repeats itself overtime, refractory lining thickness is reduced continuously until the stave or furnace jacketis completely exposed to the furnace environment. In the case of the cast iron stavecooled wall, exposing the staves to the same temperature fluctuations as the refractorybefore it wil cause cracking and spalling of the cast iron surface, shortening stave lifedramatically. Wall temperatures can sometimes by controlled by burden distributionand charging techniques. However, these measures usually result in production and fuelrate penalties, which may prove unacceptable to plant goals.
Virtally all refractory/cooling system design improvements have historicallyconcentrated on finding refractories which were resistant to chemical attack. Theeffects of thermal shock were either unkown or ignored, until failures or minimallifetime improvements were experienced, even with effciently cooled silicon carbonlinings. The Japanese and Europeans in paricular began to study the thermal shockphenomenon in detail and many technical papers have been published on the subject.What was leared is that it was not enough to have a chemically resistant lining whensevere thermal shock wil be experienced.
In stave cooled or other externally cooled boshes, this is especially important becausethe continuous vast expanse of hot face refractory sudace is exposed to manytemperature differentials over this surface. This can result in severe localized spallingand subsequent loss of refractory support. Once support is lost, entire "panels" ofrefractory can fall out in "sheets". This destroys the integrity of the wall unit and is thereason many stave designers include refractory support "shelves" integral to the stave,to support the refractories at various levels. The inserted copper cooler system alsoprovides this valuable support function.
Wear Mechanisms - Chemical Attack
The chemical attack mechanisms in the bosh and stack are identified as oxidation,carbon deposition, alkali, slag and hot metal attack. Oxidation can occur by steamformed from burden moisture, hot blast moisture or leaking coolers. Oxidation can alsooccur from carbon dioxide formation, leaking outside air during backdrafting or from a"lazy" raceway which is too close to the refractory hot face.
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Carbon deposition can occur especially when iron is present in the refractories, whichbreaks down the CO to CO2 and C. The carbon builds up within the refractory, causingcracking.
Alkalies, most notably potassium and sodium, attack the refractory by destroying thebonding mechanisms which hold the refractories together. This attack causes refractoryswelling and cracking.
As was previously discussed, the temperature at which a paricular refractory is attackedby a paricular mechanism is called its "critical reaction temperature". Critical reactiontemperatures can be different for each refractory type and are different for each attackmechanism. Many factors can affect the actual value of a critical reaction temperaturefor a paricular refractory, such as the presence of a tramp element or contaminantwhich catalyzes the chemical reaction. In general, Table X lists the typicallyrecognized critical reaction temperatures for various attck mechanisms and refractorytypes.
TABLE X
Critical Reaction Temperatures
Attack Alumina/ Silcon Hot Pressed Low IronMechanism Fireclay Carbide Semigraphite Graphite
Alkali 590°C 870°C ~900°C ~900°C
Oxidation None 800°C 400°C 500°C
CO 480°C 600°C 450°C 650°C
As was previously discussed, all of these chemical reactions are temperature dependentreactions. This means that if the refractory can be maintained at a temperature which isbelow the "critical reaction temperature" for chemical attack of that refractory, thechemical reactions canot occur. One of the diffculties of trying to maintain a low hotface temperature of a stave or other outside cooled refractory wall is that all heat musttravel through the wall to the cooling medium. Any interrption of the heat transfersuch as an air gap between the stave or brick due to differential growth or a stress crackparallel to the refractory hot face, assures that the refractory wil be chemically attackedbecause it cannot be cooled below its critical reaction temperature.
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Additionally, in the event the refractory is effectively cooled, the formation ofaccretions is accelerated. However, as furnace temperatures fluctuate, these scabs falloff exposing cool refractory to the hot gases, thermally shocking them and cracking therefra:cory as previously described.
The important point to consider is that if the refractory configuration is able to becooled below its critical reaction temperature, the material chosen must be compatiblewith the cooling capabilities. In the case of externally cooled refractories, it must berecognized that periodically, protective accretions wil fall off the refractory hot face.Consequently, if insulating ceramic refractories are thus exposed, the cooling effect wilbe slow and thermal shock effects wil occur to the refractory hot face. Therefore, therefractory lifetime wil be shortened, unless shock resistant refractories are utilized.
It is also important to provide a refractory configuration that is also compatible with therefractory materials that are considered. Refractory walls cooled from one side such aswith staves wil be diffcult to maintain below their critical reaction temperature, if theyare configured excessively thick or if material conductivity is too low. The key tosuccess is to configure the refractories to withstand the expected wear mechanisms andto select the best available materials to do the job. This sometimes requires compositelinings of two or more materials.
Finite element computer modeling can be utilized to locate critical reaction temperatureisotherms and identify zones of potential chemical attack in the refractory. Figure 14shows examples of an externally (stave) cooled lining and an inserted plate cooledlining. Two critical reaction temperature isotherms are located in each example. The590°C (11000P) isotherms define the star of alkali attack of alumina and the 870°C(1600°F) isotherms define the star of alkali attack of silicon carbide.
If the linings were alumina, all of the material from the 590°C (1100°F) isotherm to thehot face wil be chemically attacked. The situation could be improved by either
intensifying the cooling (which is impossible when the lining is cooled from its coldface) or by choosing a material that exhibits a higher critical reaction temperature, inthis case, 870°C (1600°F) for silicon carbide. Thus, potential refractory loss fromchemical !attack would be reduced as shown in the figure.
Wear Mechanisms - Abrasion
Mechanical abrasion and erosion also contribute to bosh, belly and stack wear but at amuch smaller magnitude than thermal shock or chemical attack in the lower zones.Most abrasion in these zones is the result of dust laden ascending gasses and descendingburden materials. As was previously discussed, furnace operations and geometry cangreatly affect erosion of refractories. If burden distribution results in excessive wall gasflow or if furnace wind is often reduced so that tuyere velocity is low or if the furnace is"fanned" for extended periods or the bosh is allowed to "flood" due to improper casting,severe wall working can contribute to excessive wall wear. Also, if furnace geometry_is
not appropriate for the intended productivity and operations intensity, long..teim
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impingement effects wil destroy any refractory, even the most appropriate type for theapplication. The important point is that erosion or abrasion effects canot be stoppedonly by refractory properties.
Internal Wear Factors
Afer review and consideration of the external factors which affect refractoryperformance, a review of those wear factors internal to the refractory system must beconducted. These internal factors can be critical in determining the success or failure ofa bosh, belly and stack refractory system. Even the most appropriate refractory for theapplication wil fail if these factors are ignored or improperly taken into account. Itshould be remembered that merely selecting appropriate refractory properties is notenough to assure survival or long life.
Internal Factors - Accommodatin2 Expansion
One of the most critical internal factors in any refractory system is to assure that thearangement and configuration of the individual components allows for thermalexpansion without damage as temperatures increase. This means not only must
refractory thermal expansion provisions be included, but an examination of the effectson the pressure containing vessel must also be conducted. Failure to properly allow forthermal expansion compensation can result in destructive cracking of refractories andthe vessel, deformation of the vessel or the lining and premature failure. The use ofheat setting cement, installed in joints with suffcient thickness to compensate for theexpected movements, is one way to provide for thermal expansion. Another is to utilizecompressible layers of refractory fibers or layers of organic materials that wil bumaway to compensate for expected movements. This is especially important when therefractory configuration wil encounter abrupt changes in diameter or shape and atnozzle projections.
Internal Factors - Accommodatin2 Differential Movements
It is also important to recognize situations which wil result in differential thermalmovements in a refractory system. These can be caused by utilizing refractorymaterials with different coeffcients of thermal expansion in the same lining thickness.These differential movements can also result from configurations with excessively thickwalls with high hot face and low cold face temperatures. This high temperature
differential across the wall can cause cracking if the wall if comprised of a one-piecematerial thickness. Accommodating differential movements is thus mandatory toprevent cracking and displacement of the refractory components, which interrptscooling.
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Internal Factors - Accommodatim! Stresses
The successful refractory system wil properly compensate for the expected thermalexpansion of components. It wil also consider all expected differential thermalexpansion from all sources and provide required movement compensation. If properlydone, accommodating these movements wil result in a refractory system free ofdamaging mechanical and thermal stresses. These stresses can result in "pinch"spalling of the refractory hot face, caused by two adjacent components squeezing tightlytogether until the contact surfaces literally explode, displacing large hot face pieces ofthe refractories. Insuffcient compensation can also result in highly stressed refractoriesthat are restrained by rigid structural weldments such as a furnace mantle (lintel) ringgirder. The successful refractory system wil minimize stresses on the refractorycomponents, which prevents cracking and consequential loss of heat transfer capability.
Internal Factors - Effective Heat Transfer
Proper refractory system configuration requires that the heat transfer path from the hotface to the water be as effcient as possible. This means that the more direct is thecontact between components, the more effective the heat transfer path wil be.However, practical considerations require compromise to this direct path philosophy.The object is to minimize bariers to effective heat transfer. Thermal resistance shouldbe optimized by utilizing highly conductive materials, eliminating or minimizingrammed anuli, preventing cracks and the resulting "air gaps" and by taking steps toperiodically grout the gaps which occur at externally cooled refractory contact surfaces.Another important factor is to correct sources of cooling system ineffectiveness frommineral or organic deposits and sediment build-ups, as well as separation from
refractory contact. As cooling effectiveness deteriorates, refractory temperaturesincrease, intensifying chemical attack and preventing the formation of protectiveaccretions.
Internal Factors - Refractorv Properties
Afer considering all of the external and internal factors which affect refractory systempedormance, the last internal factor that must be considered is the refractory itself. Insome ways, refractory properties are the least important of all the factors considered sofar. As was mentioned several times previously, even the most appropriate refractoryfor the application wil fail if the other external and internal factors are not properly
accommodated. You can't overcome with refractory properties, the effects of pooroperations, poor quality burden materials, improper geometry or configuration, poorcooling, lack of thermal expansion provisions, high thermal and mechanical stresses andbariers to effective heat transfer. However, you can stil have failure even if you doproperly consider all of these other external and internal factors, if you choose aninappropriate refractory material for the application.
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The important point is that the refractory material chosen be appropriate for theexpected wear mechanisms. They must also be compatible with the type and effciencyof the cooling system employed. And most importantly, the properties of the materialsselected must be compatible with the refractory configuration being considered. Norefractory properties can overcome the effects of excessive wall thickness when cooledfrom one side or resist continual exposure to impingement by high temperature gasseswith entrained solids. However, proper selection of refractory type, possessing thecharacteristics and properties desired, wil provide the best opportnity for achievingthe intended service life.
Desie:n Considerations
Bosh and lower stack wear is a combination of factors, primarily of thermal shockinduced cracking, which accelerates chemical attack by exposing more refractorysurface area to alkali and by increasing refractory temperature by interrpting heattransfer.
Upper stack wear is a combination of different factors, primarly chemical attack andabrasion, especially at the stockline working zone. Thus, when analyzing the zones todetermine suitable refractory materials, often the best potential for success will be tocombine several different grades or types of refractories in each "system". Thus, thebest properties or characteristics from each type used wil contribute to the overallsuccess of the system.
For example, in a stave cooled system, the refractory "inserts" in the stave face canutilize highly conductive semigraphite or graphite to optimize cooling effciency. Thispermits one hundred percent of the stave face to effciently cool the refractories, thuslowering their temperature and consequently lowering the rate of chemical attack.Insulating tyes of refractory stave inserts reduce the stave's ability to remove heat bylimiting the heat pick-up area to the exposed cast iron rib sudaces only. This results inhigher refractory temperatures and increased chemical attack.
Another case would be the use of a refractory "sandwich" consisting of three differentgrades of refractory in the same wall thickness. For example, a silicon carbide layer ofrefractories could be "sandwiched" between a cold face lining of lower cost highalumina or highly conductive semigraphite and an economical fireclay on the hot faceto absorb 'the rigors of blow-in and initial thermal shock damage.
Another example would be to utilize lintel blocks of highly conductive graphite orsemigraphite to form the bridged opening for copper cooling plates or to form "passive"cooling bands or rings to enhance the cooling effect of widely spaced cooling plates.
The possibilities are endless but the important point is to remember to consider all ofthe important internal and external factors that wil affect the "system" pedormancesuch as expansion provisions, differential movement, mechanical stresses, integrationwith the cooling elements and analysis of the wear mechanisms to be encountered.' -
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Bosh ADDlIcations of Conductive Refractories
The application of carbonaceous materials as "conductive" blast furnace refractories forthe bosh and lower stack are many and varied. The most common usage of carbon andsemigraphite has been as a bosh lining, primarily cooled on its cold face by staves or byexternal shell cooling such as sprays or enclosed panel type cooling. External coolingwas originally preferred to eliminate any possibility of water leaks that would result inoxidation of the carbonaceous refractories.
These traditional arangements share a common requirement for satisfactoryperformance. That is, cooling effectiveness is totally dependent upon good surfacecontact between the refractory cold face and the stave or steel shelL. Some users preferto utilize a high conductivity ram between the refractory and the shelL. However, a rammaterial wil always possess lower thermal conductivity, lower density and higherporosity than the refractory materials and thus results in a weak point in the heattransfer capability. Therefore, it is usually best to design these lining systems so thatminimal or better yet, no rammed joints are utilized. Instead, heat curing cement orexpansion joints can be utilized to accommodate the thermal expansion of the lining,while maintaining a tight fit between brick and shell or stave, which is coated with alayer of high conductivity, heat setting cement. However, even with this arangement,it is often desirable to inject a high conductivity carbonaceous grout between therefractory and the shell as the furnace campaign progresses, to fill-in any gaps whichmay develop due to differential thermal expansion between the shell and the lining orfrom localized shell heating.
Conventionally baked carbon blocks or hot pressed carbon bricks are usually used forlow cost linings of this type. The benefits include good thermal shock resistance and inthe case of hot pressed carbon, excellent resistance to alkali attack. These materials canreadily promote an accretion of solidified slag and iron because of their good thermalconductivity and thus achieve reasonable life at minimum capital cost.
Conventionally baked semigraphite, semigraphitized blocks or hot pressed semigraphitebrick are also utilized as improved materials in linings of this type. They offer evenhigher thermal conductivity, resistance to thermal shock and in the case of hot pressedsemi graphite, excellent resistance to alkali attack. Various additions such as siliconcarbide can also be incorporated to increase abrasion resistance and lower permeabilityand thus improve resistance to chemical attack.
The main drawbacks of these lining/cooling configurations are the total dependency ongood contact with the shell or the stave to maintain cooling and the lack of periodicrefractory support along the bosh height to prevent the loss of refractories above, if alocalized failure occurs. These drawbacks can and often do result in premature loss ofrefractory due to insuffcient cooling or sudden loss of entire sections of wall due to lossof wall integrity in a small, localized wear area. However, many furnaces worldwidehave had excellent success with bosh linings of this type for moderate campaign lifegoals. -
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Cast iron stave users are especially concerned with the refractory/stave interface.Water cooled shelves or even inserted cooling plates are utilized to provide physicalsupport of the refractories at various levels in the lining. Thus, a localized loss of
material below such supported linings would not cause the collapse of the lining above,as would be the case without the supports.
Another concept rapidly finding favor is to attach lining materials directly to the stave,either by casting them in place or by the use of special cements. This arangementallows the simultaneous installation of stave and lining in prebricked assemblies.
These configurations also permit improved cooling effciency when conductive
refractory stave inserts are utilized with either a cold face conductive refractory liningcombined with a hot face layer of silicon carbide or an all conductive refractory lining.These conductive materials permit low hot face temperatures for good skull formationand excellent thermal shock and chemical attack resistance.
Another improved bosh design utilizes semi graphite and/or graphite linings, sometimescombined with silicon carbide, in combination with densely spaced, copper coolingplates, which offer solutions to the weakesses of the traditional configurationsdescribed previously. This improved design concept provides intensified cooling of thevery high conductivity linings throughout their wall thickness, while providing thecritical physical support of the wall. Thus, the cooling of the wall is no longerdependent upon tenuous contact of the vast expanse of shell or staves with the equallyvast surface area of the refractory cold face. Instead, individual "fingers" of copper
coolers penetrate into the wall in a densely spaced pattern, thus maintaining lower
overall refractory temperatures.
The heat removal capabilities of such systems are dependent upon the contact of thelining material with the copper coolers. Two methods are used to maintain contact withthe linings. The most commonly used method is to provide an anulus between therefractory and the cooler, which is filled with a high thermal conductivity rammingmaterial. However, another proprietar design has also been adopted, utilizingmachined refractories in contact with machined copper coolers, which provides intimatecontact between cooler and refractory for good heat transfer. This method eliminates
the possibility that a poor ramming job wil adversely affect heat transfer to the coolers,assuming of course, that a good machining job and consequently no air gaps areallowed to exist at installation or during operation.
The lining materials used in these plate cooled designs can be all graphite materials or acombination of semigraphite and graphite or all semi graphite, or sometimes combinedwith silicon carbide depending upon the intended campaign life and anticipated wearmechanisms. Some blast furnaces exhibit a history of minimal bosh wear and canutilize more economical ceramic materials. Others, especially if high rates of injectedfuels such as pulverized coal are used, wil be affected by a lowered cohesive zone andthus intensified bosh wear mechanisms, requiring intensified cooling and higher quality
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materials. As with any lining/cooling system design, the individual furnace operationcharacteristics and campaign life goals would dictate which combinations are required.
Bellv and Stack Applications of Conductive Refractories
The application of graphitic and semigraphitic materials to the belly, lower and mid-stack has been gaining in acceptance. It was previously feared that the use ofcarbonactous materials above the bosh was a risky proposition because of thepropensity of water leaks from low quality coolers and the possibility of contact with airfrom non-gas-tight cooler holders during backdrafting. Advancements in cooler platedesign and manufacturing as well as modem cooling system leak detection and gas tightfurnace jackets, have eliminated potential risks and offer new opportunities forextending campaign life in these critical areas.
Graphite and semigraphite, because of their high shock resistance, resistance tochemical attack and high thermal conductivity, can be combined with intensifiedcooling from densely spaced, copper cooling plates, to provide a solution to severe wearareas of the belly and lower stack. As was described for the bosh, contact between thecooling plates and the refractories can be achieved with high conductivity rams or withthe proprietar machined contact system. Because chemical attack of all materials istemperature dependent, the high conductivity of the refractory and the intensifiedcooling combine to provide refractory walls that are too cool to be chemically attackedand thus readily form protective accretions on their hot face.
The materials utilized can be combinations of various grades of graphites, varing in
density and properties depending on the zones where they wil be utilized, orcombinations with semi graphite or even ceramic materials and silicon carbide, as waspreviously discussed.
Whenever the cooler plate spacing canot be optimized, conductive carbonaceous
refractories can be utilized to help conventional refractories work better in the blastfurnace. These conductive refractories can cool the hot face ceramic refractories muchmore effectively than if the entire lining was composed of the lower conductivityceramic material, by directing heat to the back of the cooling plates, which are normallyunder-utilized at the back side. Another benefit of this arrangement is that wheninsulated from the steel shell, this conductive layer directs heat effectively to thecoolers, preventing overheating of the shell as the hot face ceramic lining becomesthinner over time. Three dimensional, finite element analysis can be utilized todetermine the improvement of the location of the critical reaction temperature isothermsin the ceramic material by the inclusion of the conductive zone. Additionally, if thevertical cooler plate spacing is exceptionally great, "coolers" of graphite can be locatedbetween the existing copper cooler rows to act as "passive" coolers, by directing heat tothe copper coolers.
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Cooler plate spacing wil var depending upon furnace zone and expected heat load.
Typically, vertical spacing of between 250 to 380mm (10 to 15 in) is utilized for mosteffective cooling and horizontal spacing is aranged so that "overlapping" of coolersfrom row to row is achieved. Thermal modeling is especially effective for predictingisotherm locations for optimizing cooler spacing. However, often other external factorssuch as required capital costs or available reline time dictate design parameters thatresult in less than optimum spacing. It is in these situations that arangements usingconductive refractories to enhance cooling effectiveness can provide compromisesolutions.
It should also be mentioned that the use of conductive refractories, especially when theyare used alone in a wall, do not necessarily result in excessive process heat loss. This isbecause their high thermal conductivity provides a cold refractory hot face, whichpromotes a build-up of an insulting accretion. This skull protects both the cooler andthe refractory from abrasion and reduces the total heat loss through the walL.
This is especially true when compared to the situation when cooler plates or staves arecompletely exposed in the furnace due to the loss of a ceramic lining. The resultingheat losses are much higher during this situation than when the coolers are covered byeven a high conductivity graphite materiaL. The concept is to utilize materials whichwil remain in place for long periods of time, to maximize life. Carbonaceous materialscan provide the means to achieve this end, alone or in combination with ceramic
materials, when combined with an effective cooling system.
Summary - Bosh. Bellv and Stack
Blast furnace bosh, belly and stack lining/cooling concepts are many and varied. Wearmechanisms differ from furnace to furnace and zone to zone and must be thoroughlyanalyzed before any refractory selection can be made.
The lining is only one par of a complex, interrelated system of components and
features, ~nc1uding influences by internal and external factors. Wear mechanisms suchas thermil shock, high heat loads, chemical attack, abrasion, erosion and impact, are
some of these factors. Another important factor is the cooling type and effciency,which when combined with the proper refractory products, can significantly improvecampaign life.
The materials available are many and varied, with a full range of desirable propertiesand characteristics. Proprietar design concept systems are available to the user, as wellas more conventional designs, utilizing various cooling methods.
Carbonaceous refractories can be combined with a variety of other refractories toachieve optimum performance of each product used in the system or to minimize theeffect of a cooling deficiency.
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HOT BLAST SYSTEM
A very large user of refractory materials in the ironmaking complex are the hot blaststoves and the related hot blast delivery system. The technology and design features ofthese complex, refractory lined entities are often comprised of proprietar, sometimespatented know-how. An entire volume can be written on proper stove design andconfiguration including combustion chamber concepts, internal ceramic burnertechnologies, metallc gridwork and checker supporting systems, dome configuration,concepts of differential expansion accommodation, checkerwork and flue design as wellas nozzle lining concepts and configuration. However, some general comments can bemade in regard to proper refractory "system" design.
Stove refractories must be designed to accommodate the expected differential thermalgrowth that cycles continuously as long as the stove operates. This heating and coolingcycling, especially in the combustion chamber and checker mass, results in the constant"moving" of refractories. During operation, this cycling can destroy refractories andinsulation and open up joints to allow hot gas short-circuiting and hot spots in the steelshell. Expanding refractories can "grab" the steel shell and by the force of friction,actually lift the shell from its foundation.
Insulation in critical areas can be crushed, abraded away or destroyed by short circuitinghot gasses. Lack of thermal expansion provisions can dislodge nozzle brick, whichallows gasses to penetrate into the insulating layers.
Improper stove firing, incomplete or non-working instrumentation, gas explosions,entrained moisture in combustion air or gas, backdrafing of the blast furnace throughthe stove proper, dirt blast furnace gas and a myriad of similar occurrences, can
dramatically affect refractory life and pedormance.
Additionally, stresses from thermally expanding steel mains and shells can result indeformation and/or cracking of the steel containment vessels, which can adverselyafect the refractory contained within. Therefore, it is imperative that stove shell andmain configurations, support and anchoring systems be analyzed and engineered bycompetent stove and hot blast system designers.
Hot blast stove and the related hot blast delivery system refractory designs and
concepts, as well as material selection, is a specialized field, best conducted incollaboration with professional hot blast system engineers and suppliers.
8-48
TROUGH AND RUNER SYSTEMS
The casthouse trough and runner systems represent a large consumable refractorydemand that requires periodic maintenance and replacement. Additionally, a portion ofthe refractory lining materials may be semi-permanent, such as insulating, back-up orsafety linings.
Trough and runner lining life is usually determined by operating practices such asnumber of casts per day and in the case of the trough, how often it is drained and if it iscooled. Severe thermal shock of refractories is experienced whenever a lining system isallowed to cool down between casts.
Lining life also can be extended by remedial repairs between casts using gunite
application, ramming or hot patching techniques. Often, maintenance contracts are letby the furnace operator, whereby all responsibility for material selection, installationand maintenance of the linings are assigned to a subcontractor. However, operating andmaintenance practices of the furnace operator can have a major effect on lining
performance. For example, water sprayed onto hot refractory surfaces wil result inthermal spalling and cracking. Taphole driling angles also can adversely affect impactwear in the trough and taphole practices and poor clay quality can result in high castingrates as the taphole erodes.
Refractory life in the trough can also be prolonged by the cooling of the exterior of thetrough enclosure. This can be forced or induced draft air cooling, water cooling ornatural convection.
Refractory life is also affected by the physical layout of the trough and runner system.Flow velocities, impingement areas, turbulence, "eddy" currents and the like canquickly cause erosion of even the best refractories.
Additionally, thermal expansion of long runs of refractory linings can result indisplaced runner offakes and similar connections, causing cracking and breakouts. Thesystem designer must take care that proper anchors are used and provisions made forthermal expansion and differential movement of the branch connections whenever therefractory containment "boxes" or forms are configured.
i
Trough and runner refractories can consist of a variety of different materials includinglow moisture castables, dry vibratables, rams, precast shapes, carbon and graphiteblocks and many combinations of each other. It is beyond the scope of this paper to beable to consider all of the possible combinations and configurations for discussion. It isbest to consult with experts in the field regarding proper trough and runner designconfiguration before embarking on any refractory design or selection. Proper systemconfiguration can eliminate many of the wear points due to impact, impingement, highvelocities and turbulence, which can destroy even the best refractory available for theintended application.
8-49
SUMMARY
Successful refractory systems are dependent upon consideration of a variety of externaland internal factors, which can affect wear. Furnace operation, geometry, liningconfiguration, cooling type and capability and the wear mechanisms encountered, allcan adversely affect refractory life. Improper refractory configurations which canotaccommodate expected thermal movements, differential expansion and stresses,ineffective heat transfer or inappropriate properties, wil not survive long.
The refractory systems designer wil also recognize that properties alone cannot assurelong life and that refractory survival depends upon utilization of the most appropriateconcept and configuration for the application. This often requires the utilization of twoor more different materials in the same configuration, to take advantage of the bestproperties and characteristics of each. There is no "perfect" refractory that canovercome the effects of poor cooling, rough operation or abuse. Nor is there a
"standard" refractory system that is appropriate for every worldwide blast furnace. Thesuccessful refractory system is one that considers each furnace as a unique problem,demanding a unique solution, by examining its particular external and internal factorswhich affect refractory performance.
It should also be remembered that refractory survival is totally dependent uponrecognition of factors external to the lining/cooling system. How these factors areaddressed or ignored wil determine whether or not the refractory system that wascreated can be considered truly "successful".
8-50
REFERENCES
1. Dzermejko, Albert 1., "Blast Furnace Hear Design Theory, Materials and
Practice", Iron and Steel Engineer, December, 1991, pp. 23-31.
2. Dzermejko, Albert J., "Design Considerations for Utilizing Graphitic Materialsas Blast Furnace Refractories", Ironmaking Conference Proceedings, ISS/AIME,VoL. 49, 1990, pp. 361-377.
3. van Stein Callenfels, Egenolf, et.al., "Intermediate Repairs for Hearh, Bosh and
Lower Stack", Iron and Steel Society Svmoosium on Blast Furnace CampaignLife Extension, Myrtle Beach, North Carolina, November, 1997.
4. Jameson, D., et.al, "Prolonging Blast Furnace Campaign Life", Technical Study
into the Means of Prolonging Blast Furnace Camoaign Life, EuropeanCommission on Technical Steel Research Final Report, pp. 5-16, 1997.
5. Robinson, G.c., et.al., "Alkali Attack of Carbon Refractories", CeramicBulletin, Vol. 58, No.7, 1979, pp. 668-675.
6. Bongers, Uwe, "Improving the Lifetime for Furnace and Runner Linings with
Carbon and Graphite Products", Sorechsaal, Vol. 117, No.4, 1984, pp. 332-340.
7. Higuchi, Masaaki, "Life of Large Blast Furnaces", Ironmaking Conference
Proceedings, ISS/AIME, Vol. 37, 1978, pp. 492-505.
8., R. M. Bucha, A. 1. Dzermejko and 1. G. Stuar, "Combining Equilibrium Theory
with Three Dimensional Heat Transfer Analysis, To Predict Blast Furnace StackCooling and Refractory Performance", Ironmaking Conference Proceedings,ISS/AIE, VoL. 42, 1983, pp. 673-679.
9. DeBoer, J., et.al., "History and Actual State of Lining and Cooling Systems atthe Estel Hoogovens IJmuiden Blast Furnaces", International Ceramic Review,VoL. 32, 1983, pp. 16-18.
8-51
I
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8-52
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8-59
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LECTURE #9
IRON-BEARING BURDEN MATERIALS
Madhu G. Ranade
Inland Steel CompanyEast Chicago, Indiana, 46312 USA
INTRODUCTION
Iron-bearing materials are natural and synthetic materials with asufficient iron (Fe) content to be economically usable in an iron making blastfurnace. Iron-bearing materials may come from a mine, from steel-millwastes (fines and dust), or from discarded metallc products of iron (Le.,scrap). This chapter wil focus on iron-bearing materials originating from a.mine, Le., "virgin" or "natural" Fe units in iron ore, which are processed intovarious forms, such as lumps, pellets, and sinter. The use of steel-mil wasteoxides in the blast furnace will be discussed briefly. Since it is mosteconomical to recycle metallic scrap directly to pneumatic or electricsteelmaking processes, it will not be discussed in this chapter.
Pellets are roughly spherical, thermally- and/or chemically-bondedagglomerates 5 to 15 mm in diameter. Sinter consists of irregularly shaped,partially fused agglomerates in the size range of 5 to 30 mm. Lump oreconsists of irregularly shaped, large ore particles in the size range of 5 to 30mm. Briquettes are mechanically- and/or chemically-bonded pilow-shapedor cylindrical agglomerates, in the size range of 20 to 75 mm. Other
9-1
miscellaneous materials include processed steelmaking slag and siliceousore trim. These materials are irregularly shaped and sized 5 to 30 mm.
Iron-bearing materials should enable the reliable production of hotmetal (also called pig iron) from the blast furnace in the desired quality andquantity and at a minimum cost. While these requirements seem simpleenough, differences in geographical, geological, and commercial factorshave required the customization for each iron-bearing material supplier andblast furnace user.
Iron ore mines and blast furnace operations are often located severalthousand kilometers from each other. The transportation of iron orerepresents about 15% of the dry cargo trade in the world. In 1990, thisamounted to 350 milion tonnes. The major iron producers and consumersare listed in Table 1.(1) Typically, 95 millon tonnes of iron ore products areproduced in North America each year, with more than 80% coming from theMinnesota, Michigan, Quebec, and Labrador regions.
Iron-bearing materials should be able to survive transportation andhandling from the mine into the blast furnace. Since iron-bearing materialsare charged downward from the top of the blast furnace through gasesmoving in an upward direction, they should be free from fines that can becarried out of the furnace by the gases. Also, the materials should not be solarge as to cause difficulties with conveyor transfers, bins, and chargingequipment. In general, materials originating from iron ore should contain aminimum of 50% Fe, with particles mostly in the size range of 5 to 30 mm tomeet these basic requirements.
Iron ore mined from the ground does not meet the basic requirementsstated above. Invariably, further processing is required. A few high-grade(;: 50% Fe) ores can be easily converted into blast furnace feed throughsimple crushing, washing, and screening. Most iron ores require finercrushing, grinding, and mineral dressing to separate the impurities (gangue)from Fe minerals. The extent of crushing and grinding depends on the"liberation size," Le., the size to which an ore particle must be crushed inorder to break apart iron minerals from the gangue minerals, such asquart, silicates, carbonates, and aluminates. This concept of liberation sizeis schematically shown in Figure 1. When the gangue minerals are fine andintimately mixed with iron-bearing minerals, the ore particle must be crushedto a very fine size in order to "un-lock" the iron minerals from the gangue.Further processing may stil be required to physically separate iron mineralsfrom the finely crushed mixture. These beneficiation (up-grading) operationsoften produce iron mineral particles that are too fine to meet the sizerequirements mentioned earlier. Therefore, agglomeration techniques, suchas pelletizing, briquetting, and sintering, are used to increase the apparent _
9-2
size. Depending on the as-mined ore grade and liberation size, appropriateprocessing schemes (Figure 2) are practiced to produce a suitable blastfurnace feed.
The above discussion makes it clear that liberation size plays a veryimportant role in determining whether an iron ore can simply be sized as alump ore, or would require sophisticated processing to produce pellets andsinter. In addition to the simple transportation and size requirementsmentioned above, there are a number of metallurgical aspects that governthe suitabilty of an iron-bearing material as a blast furnace feed. After brieflyreviewing these aspects, the production of various iron-bearing materialswil be examined.
IRON BURDEN PROPERTIES
In the blast furnace, iron-bearing materials are subjected to reductionand smelting reactions. Reduction reactions involve the removal of oxygencontained in the iron oxide minerals in order to produce metallc iron. Atypical reduction sequence entails:
Fe203 (Hematite) --). Fe304 (Magnetite) --). FeO (Wustite) --). Fe
The smelting reactions consist of the melting of metallic iron and thereactions with other non-iron-bearing minerals to produce liquid slag andhot metal. All of these reactions are dependent on the temperature and gascompositions prevalent in the blast furnace. The dissection of quenchedblast furnaces in Japan led to a new understanding of the internal state ofthe blast furnace above the hearth region in terms of the five zones shown inFigure 3. (2-4) Also indicated in Figure 4 are the reactions occurring in thesezones.(5) Various laboratory tests are used to estimate the likely behavior of
iron-bearing materials given the time, temperature, gas composition, andstress prevalent in these zones, as shown in Figure 4.
Perhaps the single most important finding from the dissection studieswas the existence of the cohesive zone. It was deduced that most gas flowin the cohesive zone occurs through the coke layers or "slits." The cohesivezone represents the zone in which iron-bearing materials undergo solid toliquid transformation. The location, shape, and size of this conical zone,consisting of alternate layers of "cohesive" or fused iron-bearing materials
and coke, had a profound effect on hot metal productivity, composition,operating stability, and lining wear.(2-5) The properties of iron-bearingmaterials and coke, as well as blast furnace operating practices, such astuyere variables and burden distribution, determine the configuration of thecohesive zone. (2-5)
9-3
In reviewing the burden properties, it is essential to remember that theblast furnace is a moving bed reactor in which gases move in a directioncounter-current to the movement of solids and liquids. Thus permeabilty ofthe bed to ascending gases is extremely important to ensure good gas-solidand gas-liquid contact so that the necessary reactions can take place. It isalso important to consider that reaction occurring in one zone of the furnacecan cause changes in burden characteristics that can affect behavior insucceeding zones.
The iron-burden properties shown in Figure 4 can be divided into fourgroups according to the testing temperature:
· Ambient Temperature: Chemical composition, sizeconsistency, compressionstrength, and the tumble index.
· Low Temperature: Low TemperatureDisintegration (L TD) orReduction Degradation Index
(RDI).
. Intermediate Temperature: Swelling, reducibility,compression strength afterreduction (CSAR).
. High Temperature: Contraction, softening, andmelting characteristics.
Previously, an extensive analysis was made of the data reported in theliterature concerning the effect of various burden properties on blast furnaceperformance.(6) Table 2 summarizes the results of this analysis. It isapparent that, although iron burden properties can have a significant effecton blast furnace productivity and fuel consumption, the extent can differconsiderably from one furnace to another, as evidenced by the 95%confidence interval.
Among the ambient properties, chemical composition directly affectshot metal and slag compositions, and indirectly affects many otherproperties. Some of the direct effects are summarized in Table 3. Asindicated, the chemical composition can affect hot metal composition aswell as blast furnace operation. The Si02 content of iron-bearing materials isparticularly important as it determines the slag "volume" (actually, mass ofthe slag) produced in the blast furnace. Some slag is necessary in the blastfurnace for removing impurities, such as S, K20, and Na20. Excessive slagvolumes, however, represent unnecessary thermal load on the furnace and_
9-4
lead to a greater fuel consumption. To ensure a fluid slag with sufficientdesulfurization and alkali removal capabilty at ironmaking temperatures, itsSi02, CaO, MgO, and AI20S contents are controlled through properburdening of the furnace. The Si02 content also affects physical andmetallurgical properties of iron-bearing materials. The CaO/Si02 ratio ofpellets and sinter is often designed to yield the desired combination ofphysical strength, LTD/RDI, and the intermediate and high temperatureproperties.
The compression strength and tumble index primarily indicate thegeneration of fines during the stockpilng, transportation, and handling ofmaterials. When the materials are screened in the stockhouse, the finesrepresent a loss. If screening is not employed, the fines affect permeabiltyin the granular zone. The effect of the tumble strength of sinter on blastfurnace permeabilty is shown in Figure 5. (7)
Size distribution can affect the void fraction contained in a packed bedof particles; fine particles tend to occupy voids between large particles,thereby reducing the overall void fraction. Thus, size distribution can affectblast furnace permeabilty. Size distribution also reflects the surface area tovolume ratio, and can affect the rate of reduction in the furnace.
The LTD/RDI and the CSAR indicate the tendency of the materials tobreakdown and generate fines in the granular zone. Consequently, they canaffect permeabilty, gas distribution, and the flue dust generation rate. TheL TD value represents the tendency of pellets to disintegrate due to stressesgenerated in the iron oxide lattice during the reduction of hematite tomagnetite. The effect of sinter RDI on blast furnace permeabilty is shown inFigure 6. (7)
The intermediate temperature properties, reducibilty and swelling, areimportant in the lower part of the granular zone. Reducibility can affect theutilzation of the reducing potential of CO and H2 gases in the furnace. Ahigh reducibility also leads to less FeO reduction in the high temperaturezone, and, therefore, improved softening and melting properties. Swellngcan affect burden movement, permeabilty, and gas distribution. The effectof pellet reducibility on the blast furnace coke rate is shown in Figure 7. (8)
The high temperature properties, such as the softening and meltingtemperatures, influence the location and geometry of the cohesive zone. Itis important to remember that burden materials in the lower zones aresubjected to the weight of the materials in the zones above. In the upperpart of the cohesive zone, where liquids begin to form inside the particles,the incident load leads to plastic deformation blocking voids and restrictinggas flow within the particle. This aspect is characterized by the contraction _
9-5
test. The effect of the contraction of pellets on the blast furnace operation isshown in Figure 8.(9)
With the exception of contraction and softening-melting, all burdenpropert evaluation procedures have been standardized by the InternationalOrganization for Standardization (ISO) and adopted by the AmericanSociety for Testing of Materials (ASTM). The test details can be obtained byreferring to the appropriate documents and will not be discussed here. Abrief summary is provided in Appendix i. The typical equipment used inthese tests, as well as in the contraction and softening-melting tests, isshown in Figures 9 and 10. The testing conditions used in Kobe Steel'scontraction test and in a softening-melting test in use in North America areshown in Table 4.
The burden distribution characteristics of iron-bearing materials arealso important. Pellets, being spherical, tend to roll, whereas sinter and lumpore, which are angular and irregularly shaped, tend to remain wheredeposited. This is evidenced by the angle of repose of pellets (28-32°),which is shallower than sinter (32-36°). Therefore, sinter and lump ore havemore predictable distribution characteristics when charged usingconventional equipment, such as multiple bell tops with movable armor. Atone time, this was considered to be a serious disadvantage for pellets, andit was doubtful that a large blast furnace with a significant proportion ofpellets in the burden could be operated successfully. However, thedevelopment of a bell-less top which employs a series of lockhoppers and arotating chute for distributing materials has enabled effective burdendistribution control with pellets. As a result, the distribution characteristics ofpellets are no longer considered to be a major technical disadvantage.
IRON BURDEN COMPOSITION
Typical burden compositions for North American, European, andJapanese blast furnaces are shown in Table 5; pellets predominate in NorthAmerica and sinter predominates in Europe and Japan. A brief historicalperspective is helpful in understanding the reasons for this difference.
In the early 1900's, iron ore producing mines, wood/charcoal sources,and ironmaking operations were located relatively close-by. The UnitedStates, United Kingdom, France, and Germany were the major producers.Generally, ores with ;: 50% Fe content ("high-grade" or "direct shipping")were mined and used with minimal processing. As the demand increased,spurred by the industrial revolution and wars, high grade ores weredepleted and new ore sources had to be found. Also, with the advent ofcoke-based iron making, operations often had to be located away from ore _
9-6
mines, closer to steel customers and sources of other raw materials, suchas metallurgical coaL. During 1930-1945, many steel plants were constructedin the Lower Great Lakes area relying on the supply of high-grade,hematitic, "direct shipping" lump ore mined in Minnesota, Michigan, Ontario,and Quebec. With the advent of the Bessemer steelmaking process,phosphorus content in the ore had to be maintained below 0.045%. Highgrade ores in Australia, Asia, Africa, and South America were known to existat the time, but with a few exceptions, their use in North American andEuropean iron making operations was not economical at the time. TheJapanese steel industry did import ores from Asia and South America.
In the United States, high-grade ores in Minnesota and Michigan weredepleted towards the end of the second world war. A totally new processingtechnique had to be developed during the 1950's to make use of low-grade(25-30% Fe) Taconite ore, which was considered as a "waste rock" earlier.The liberation size for this ore was very fine (80% -325M or -45lL). Newmining and grinding techniques were developed to process this hard rock,and magnetic separation and flotation techniques were applied to produce amagnetite concentrate. Since the concentrate was too fine to be transportedor used in the blast furnace, pelletizing and thermal induration equipmentand processes were invented to produce pellets containing more than 60%Fe. Figure 11 shows the transition from direct-shipping lump ore to pelletsfor the Minnesota region. (10) In this time period, sintering was employed inNorth American steel plants to recycle accumulated stocks of blast furnaceflue dust, which was becoming a major storage nuisance. With theconstruction of major pelletizing facilties in the 1960's, North American blastfurnace operations transitioned from high-grade lump ore to pellets as themajor iron-bearing burden materiaL. Some low-grade hematite ores in theQuebec-Labrador area could also be upgraded through fine crushing andhydraulic separation techniques. The liberation size for these ores issomewhat coarser than Taconite. Thus, these mines can produce pelletfeed or coarse concentrates which can be used in limited quantities forsintering. The majority of production is pelletized and indurated.
In Europe, high-grade ores were exhausted between 1955-1965 and,with the exception of Sweden, no other major iron ore reserves wereavailable. Therefore, iron ore imports were inevitable. As ocean shippingbecame feasible and economical, ore reserves in South America, Australia,and Africa were developed. As these ores could be liberated at a relativelycoarse size (-10 mm), they were suitable for sintering. Therefore, steelplants built or re-built after the second world war utilized sintering facilties.Thus, European and Japanese blast furnaces transitioned in the 1960'sfrom high-grade lump ore to sinter as the major iron-bearing burdenmateriaL. During the upgrading of South American, Australian, and African
9-7
ores, a small portion could be liberated at the size of lump ores. Therefore,European and Japanese furnaces do use 5-20% lump ore in the burden.
As explained above, pellets have become the major iron burdenmaterial for North American blast furnaces while sinter predominates inEuropean and Japanese blast furnace burdens, however there are someinteresting exceptions. During the 1970's, some steel plants in the UnitedStates were built or expanded in anticipation of a great surge in steeldemand. Some of these plants were specifically designed to use oreimported from South America in the form of lumps, pellets, or sinter feed.Bethlehem Steel's Sparrows Point plant is one such example. The presenceof an ore similar to Taconite has also been the basis for pellet plants and apellet-based blast furnace operation in Sweden. During the 1970's, there
was a great debate on the optimum feed for the large, high temperature,high top pressure blast furnaces built at the time. After evaluating economicand technical factors, Hoogovens in Netherlands and Kobe Steel in Japandecided to employ pelletizing and sintering operations on-site, usingimported ores.
In light of this background, the data in Table 5 can be interpreted asfollows: (1) the majority of hot metal produced in North America is fromdomestically produced iron-bearing materials, (2) the majority of hot metalproduced in Europe and Japan is from imported iron-bearing materials, and(3) South America, Asia, and Australia export most of their iron-bearingmaterials to Europe and Japan.
At present, there are 20 iron ore mines, 13 pelletizing plants, and 12sintering plants (Table 6) in North America.(11,12) The pelletizing capacity of
87.1 milion tonnes far exceeds the sintering capacity of 17 milion tonnes.The production of lump ore in North America is practically non-existent.
This historical perspective serves as a background for the followingdescription of the production of pellets, lump ore, sinter, and miscellaneousmaterials. In producing iron-bearing feed, it is necessary to upgrade itthrough removal of gangue and to impart the physical and metallurgicalproperties desirable to the blast furnace in an economical fashion.
PRODUCTION OF LUMP ORE
Lump ore is produced mainly in South America, Africa, Australia, andIndia. These ores are typically high-grade hematite and often are readilyaccessible and relatively uncontaminated outcrops. The productionprocess is, therefore, straightforward. Stripping, drilling and blastingoperations are similar to those described later in the production of pellets, _
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I
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albeit easier. Coarse crushing and fine crushing operations are also similar.At this stage, the ore is screened to produce lumps sized in the range of 10to 50 mm. Water is sprayed on the screens to eliminate "piggy-back" finesadhering to the ore lumps. The "clean" or "washed" lump ore containing
;: 60% Fe is railed to the shipping docks, stockpiled, and loaded on largeocean-going vessels for transport to Europe and Japan.
Lump ore recovered as described above usually consists of a smallportion of the ore mined (5 to 20%). A large amount of -10 mm ore isproduced in the process. This fraction can be easily upgraded using simplehydraulic separation techniques to ;: 60% Fe content and a size range of 1to 10 mm. After dewatering (and sometimes thermal drying in rotatingdrums), this fraction becomes suitable for sintering. This fraction of ore isoften referred to as "fine ore," "sintering fines," or "sinter feed." If a largequantity of -1 mm ore particles are produced in the process, they are furtherground to -100 M or -150 ¡i, upgraded if necessary, and used in productionof pellets. For most South American and Australian mines, the production oflump ore and sinter feed is sufficient for profitable mine operation as theproportion of -1 mm particles produced in the process is relatively smalL.
Product characterization for lump ore and fine ore involves systematicsampling and testing according to standard (ISO/JIS/Proprietary)procedures specified in the customer supplier agreement. These teststypically involve only size, moisture, and chemical analysis and bulk densitycharacterization. Typical properties of lump ore are listed in Table 7.
PRODUCTION OF PELLETS
Pellets are commonly produced and used in North America. Theproduction of iron ore pellets consists of a sequence of operations involvingthe removal of ore from the ground, size reduction, upgrading,agglomeration to produce spherical pellets, and thermal induration to impartthe necessary physical and metallurgical properties. A typical sequence ofsteps is described below. .Stripping
The process of converting ore in the ground with 25-30% Fe tonarrowly sized pellets with 60-65% Fe begins with ground preparation formining. "Overburden," the earth covering the ore body, is removed withshovels and trucks, creating the ore pit. For each tonne of crude ore, 3 to 4tonnes of overburden may have to be removed.
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Driling and Blasting
A large rotary dril is used to blast holes in a precisely engineeredpattern. Each hole can be 0.4 m diameter X 15 m deep and spaced 6 m.Explosives are pumped into the holes and detonated by a blasting cord. Aslight delay between the detonation of successive rows of holes causes theprogressive fracturing of ore into crude ore chunks that can be easilyscooped up by an electric powered shoveL. For each tonne of pellets, asmuch as 3 to 4 tonnes of ore have to be processed. The broken crude oreis loaded into trucks or rail cars and delivered to a crushing plant.
Crushing
A large gyratory crusher reduces ore to 150-200 mm chunks and,subsequently, additional gyratory crushers reduce the ore to gravel-sizepieces.
Grinding
In this operation, ore is reduced to its liberation size to faciltate thesubsequent separation of gangue from iron minerals. The grinding processtakes place in large rotating mils. Grinding can be autogenous--the ore isbroken up as it tumbles against itself, or exogenous--grinding media, suchas steel rods or balls are used. At this stage, the ore consists of a mixture ofliberated iron mineral particles, gangue particles, and still locked gangue-iron mineral particles in a water slurry.
Concentrating
For Taconite ores, magnetic properties are exploited to achieve thephysical separation of liberated iron ore minerals from the rest of the ore.For hematite ores, the difference in density between iron ore minerals andgangue is often exploited to achieve the separation. In both cases, thedifferences in their settling velocity in a fluid is exploited by using spiralclassifiers, hydrocyclones, and hydroseparators. Other techniques includefine screening and flotation. Often a large proportion of silica-bearingparticles is present in a relatively coarse size fraction of the feed from thefinal stage of grinding. Wet screening can effectively remove this fractionwithout overgrinding the ore. In flotation, differences in surfacecharacteristics of iron and silica-bearing minerals are exploited to "float" thelatter by attaching an air bubble while the iron ore concentrate slurry isdrawn-off from the bottom of the flotation celL. These operations areuniquely coupled for each plant, depending on ore characteristics andequipment. Two different flowsheets are presented in Figures 12 and 13
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which highlight the wide variations in processing schemes and equipmentused.(13,14)
Flux Preparation
Often a mixture of fine limestone and dolomite is used to produce"fluxed pellets." In most cases, coarse flux in the size range of 5 to 50 mm isreceived from limestone and dolomite quarries. It is blended in the desiredproportion and then crushed to about the same size as the concentrate bydry crushing in a roll or gyratory crusher, followed by wet grinding in a ballmill circuit incorporating fine screens (Figure 12). The flux slurry is stored ina slurry storage tank.
Dewatering and Filtration
Iron ore concentrate slurry from the concentrator is partially dewateredin thickeners and pumped to a slurry tank. A limestone-dolomite slurry isadded at this stage if "fluxed" pellets are to be produced. The finaldewatering stage is in the disc filters where water is removed from theconcentrate-flux mixture by a vacuum through a series of cloth covereddiscs. Remaining on the disc is the filter cake containing about 9% waterand 60-65% Fe with a particle size of roughly 80% -325 M or -45lL.
Balling
Balling is the process in which the filter cake with a proper moisturecontent is mixed with a binder and rolled into spherical pellets. Uniformmixing of the binder with the concentrate is important for a stable ballngoperation. Bentonite clay or an organic binder is used to assist in enhancingpellet growth and strength. Bentonite does contaminate the concentratewith silca and alumina. To compensate, the ore has to be upgraded a littlemore than is necessary, strictly based on the final pellet compositionspecification. This represents an additional cost and also represents asource of variability in pellet chemistry. Organic binders avoid thiscontamination, but may be expensive or unavailable. The ballng operationis performed using rotating drums or discs. Green pellets discharging fromthe disc or drum are screened prior to their being loaded on the induratingmachine.
Induration
An indurating system may consist of a travellng grate alone (the"straight grate" system) or a linked travellng grate-rotary kiln-circular coolersystem (the "grate-kiln" system). In rare cases, shaft furnace modules areused as the indurating system. The green (unfired) pellets are loaded onto a _
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travelling grate which is a moving metal conveyor. In the straight gratesystem, it contains a series of pallets. The pellets are dried, preheated, andheat-hardened, as they pass through the furnace heated by a series ofburners fired by natural gas, pulverized coal, or oiL. Sometimes externalcombustion chambers replace some or all of the individual burnersarranged along the side of the indurating system. In a system employing arotating kiln, a burner is provided at the discharge end. Typically, pelletsexperience a peak temperature of 1280° to 1320°C for a pre-determinedtime. Following induration, pellets are air-cooled to an ambient temperatureon the travellng grate itself or in a circular cooler. The air, thus pre-heated,is used for combustion in other parts of the indurating system. Cooledpellets are screened; fines are discarded or recycled to a re-grind mill andadded back to the concentrate slurry tank. The coarse size fractionrepresents product pellets for shipping to steel mils.
Some product pellets are re-sized on a coarser bottom screen for useas a hearth layer. This layer is deposited on the strand prior to thedeposition of green pellets. Thus, the grate bars in the pallets used on thestraight grate system are protected from experiencing excessivetemperatures.
The induration process has a significant and irreversible effect on thephysical and metallurgical properties of pellets. Until this stage, all changesundergone by an ore particle are physical in nature. During induration,chemical changes involving a series of phase transformations take place.These include the exothermic oxidation of magnetite to hematite, thecalcination of limestone and dolomite fluxes, a reaction between iron oxides,gangue, binder, and fluxes which produces silicates and aluminates of iron,calcium, and/or magnesium, and the sintering and recrystallzation of iron-bearing phases. The proper selection and control of the temperature-timecycle experienced by pellets in the indurating machine is, therefore, criticalfor producing pellets with the desired physical and metallurgical properties.
Product Characterization and Shipping
Fired pellets are systematically sampled and tested using standard(ASTM/ISO/Proprietary) procedures specified by the customer-supplieragreement. These tests typically include size, moisture, and chemicalanalysis, bulk density, physical strength, and a variety of metallurgicalproperties. These tests are used as a cross-check on the pellet productionprocess and as an aid to the blast furnace operator. Typical properties ofpellets are shown in Table 8.
Pellets are loaded onto railroad cars for short- or long-term storage atthe shipping docks and subsequent transport on ships to the steel mills _
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located in the Great Lakes region. At the steel mills, pellets are directlyunloaded and stored in the ore field for subsequent use in the blast furnace.In some instances, additional barge, rail, and/or truck transfer may beinvolved. A few steel mils without convenient waterway access receive pelletshipments by rail directly from the mine.
Recent Developments
The development of fluxed pellets has been the most significantchange in pelletizing practice in North America in recent years. It was foundthat although "acid" pellets, produced without the addition oflimestone/dolomite fluxes, have good ambient and low temperatureproperties, their intermediate and high temperature properties are relativelypoor. This is because at elevated temperatures, wustite combines withsilicious gangue in the pellet, forming a low-melting fayalite. This leads to ahigh contraction, low melting temperature, and a large softening-meltingtemperature range. The addition of appropriate fluxes to achieve aCaO /Si02 ratio of 0.9 to 1.2 with an MgO content of 1.5 to 2.0% yields avast improvement in pellet properties as shown in Table 8. As a result, itbecomes possible to achieve a more favorable cohesive zone configurationwith fluxed pellets than with acid pellets as shown schematically in Figure14. A number of blast furnace trials in North America have conclusivelyproven that significant productivity, hot metal composition, and fuel rateimprovements can be achieved by using fluxed pellets. As a result, theproduction of fluxed pellets has been rapidly rising in North America asshown in Figure 15,(15)
Another related development has been the use of synthetic, organicbinders to replace bentonite used in forming green pellets. The mainjustification appears to be a lower and less variable silica content in thepellets and improved reducibilty in the case of acid pellets. However, thephysical strength of these pellets appears to be weaker. Therefore,
significant changes in indurating conditions and/or the addition of limestoneare used as countermeasures. The growth of these organic binder-based,partially fluxed pellets can also be seen in Figure 15. The relatively high costof synthetic binders appears to be a limiting factor in their wider application.
PRODUCTION OF SINTER
Sinter is the most commonly used burden material in Europe andJapan. In North America, it usually serves as a supplemental iron-bearingmaterial to pellets.
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As mentioned before, sintering operations are usually located at thesteel milL. Raw materials used in sintering consist of virgin ore ("fine ore"), aswell as a number of waste products from iron and steelmaking operations.Sintering operations thus serve as an important and profitable method forrecovering valuable iron, manganese, magnesium, calcium, and carbonunits from these wastes, while minimizing the environmental liabilties ofwaste disposaL. In many North American steel plants, sintering operationsare based exclusively on recycled steel mil wastes, such as mil-scale, fluedust, coke breeze, flux fines, fine steel slag, and pellet fines (blast furnacestockhouse screenings). As these waste materials are heterogeneous with abulk composition that tends to vary over time, considerable efforts have tobe invested in order to produce a relatively homogenous sinter feed of aknown composition, through extensive bedding, blending, and mixingsteps. Sinter plants using fine ores tend to purchase several brands of oresand also require a similar treatment.
Typically, limestone and dolomite are used as fluxes in sintering to yieldthe desired product sinter composition. Sinter is often classified on the basisof its basicity, B/A = (CaO + MgO)/(Si02 + AI203):
AcidFluxedSuper-fluxed
B/A -= 1.0B/A = 1 to 2.5B/A :: 2.5
Acid sinter is now rarely used. Blast furnaces using sinter as the majorburden component use fluxed sinter. Blast furnaces using sinter as asupplemental feed to pellets use either fluxed sinter (for fluxed pellets) orsuper-fluxed sinter (for acid pellets) in order to achieve a chemically-balanced blast furnace burden.
The sintering operation is schematically shown in Figure 16.(16) In atypical sinter plant, raw materials are received by ship, rail, and truck. Abedding and blending yard is used to prepare and reclaim a relativelyhomogenous feed. Other materials are added to this feed in the blendingyard and in the sinter plant, and composition adjustments are made asnecessary. Coke breeze is added as a fueL. Hot and cold in-plant sinterreturn fines are also added at this stage. A drum or a pug mixer is used toincrease feed uniformity. This is often followed by a drum or a disc toachieve the micro-pelletization of the mix. Water is added at this stage topromote the adhering of fine particles in the mix to coarse particles called"nuclei." This ensures that the fine particles wil neither "plug-up" the bed onthe sinter strand, thereby interfering with the sintering process, nor be lostto the off-gases exiting from the strand. There is an optimum mix moisturefor maximum pre-ignition permeability (Figure 17).(17) The optimum value
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has to be experimentally determined for each mix and micro-pelletizationconditions.
The sinter mix is then carefully deposited on the sinter strand, which isessentially an open travelling grate machine with a series of pallets. Anignition furnace is provided at the feed end. The ignition furnace employs amatrix of burners fired with coke oven gas, natural gas, oil, or pulverizedcoaL. When the mix enters the ignition furnace, coke breeze in the top layersis ignited. The quantity and size distribution of coke breeze play animportant role in achieving proper ignition. Subsequently, suction is appliedunder the strand and air entering from the top of the bed sustains thecombustion of coke breeze within a narrow layer.
While the flame front formed in the top layer of the bed movesdownwards in the bed, the air pre-heated above the flame front ignites thecoke breeze in the lower portions of the bed. At the discharge end of thesinter strand, the flame front should be near the bottom of the bed,indicating the completion of sintering. The exhaust gas temperature in thelast few wind boxes is measured to predict this "burn-thru" point, peak gastemperature. Strand speed and bed height are controlled to maintain the"burn-thru" point just before the last wind box.
After being discharged from the strand, the sinter passes through asinter breaker, which breaks large chunks of sinter, and moves on toscreens capable of handling hot sinter (called "hot screens"). The coarsehot sinter is discharged into a circular cooler. Hot fines are recycled back tothe sinter feed. Sinter is cooled by an updraft flow of air. The air, thus pre-heated, can be used in the ignition furnace for combustion. After cooling,sinter is screened and sized for transport to blast furnace. Fines generatedduring screening are recycled to the feed.
When the sinter bed comes out of the ignition furnace, it contacts withambient air and undergoes rapid cooling. This results in a glassy, friablesinter in the top region of the bed. In some plants, a hood is placed on thestrand after it comes out of the ignition furnace, to reduce the cooling rate,thereby improving sinter yield.
Coke breeze additions to the mix are controlled to maintain a relativelystable circulating load of hot and cold sinter fines in the plant. The quantityof the coke breeze used determines this circulating load. Too little cokebreeze can cause a large fines circulating load and low productivity. Toomuch coke breeze can produce a rock-hard sinter with very low reducibilty;also, bed slagging problems could curtail productivity. Any exothermicreactions that may take place in the mix during sintering must be consideredin determining the coke breeze addition rate. This is particularly true of _
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mixes containing a significant quantity of mil-scale, which oxidizes liberatingheat during the sintering process.
A portion of the product sinter is re-sized on a coarser bottom screenfor use as a hearth layer. This layer is deposited on the strand prior to thedeposition of the sinter mix. As the flame front does not enter the hearth
layer, grate bars in the pallets used on the strand are thus protected fromexcessive temperatures. The hearth layer also helps in minimizing the lossof sinter mix into the windbox through the openings between grate bars.
Product sinter is systematically sampled and tested according tostandard (ASTM/ISO/JIS/Proprietary) procedures specified in theagreements between sinter plant and blast furnace operations. In NorthAmerica, this characterization is generally limited to size, moisture, andchemical analysis and to physical strength. In Europe and Japan, wheresinter is the major iron burden component, several metallurgical propertiesof sinter are also measured. Typical sinter properties are shown in Table 9.
Recent Developments
Major developments have taken place recently in the selection ofsintering ore blends, micro-pelletization technology, energy recovery, andthe use of sensors and control techniques at the sinter plant. The mostremarkable development, however, is the development of Hybrid PelletSinter (HPS) at NKK.(18) This is an attempt to combine the desirable burdendistribution characteristics of sinter with the desirable metallurgical
characteristics of pellets. The HPS process uses a mixture of sintering orethat provides "nuclei" and ore concentrates that provide "adhering" fines.This mixture is pelletized on a disc pelletizer and coated with fine cokebreeze in a rotating drum to yield mini-pellets, about 5 mm in diameter,which are then sintered on a travelling grate. A commercial plant has beenrecently put into operation by NKK.
Another significant change has been the use of olivine instead ofdolomite at some sinter plants to obtain MgO. This has the advantage oflower coke breeze consumption, and increased strength and productivity.Some plants are also employing burnt lime additions to act as a binder atthe micro-pelletization stage. The resultant improvements in mixpermeabilty lead to increased productivity.
In North America, a number of sinter plants are being operated solelyfor the recycling of steel-mill wastes, such as mill-scale, flue dust, and finesteel slag. This provides a low cost feed material to the blast furnace whileminimizing waste disposal costs.
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MISCELLANEOUS IRON-BEARING MATERIALS
For reasons of economy and environmental protection, a growingemphasis is being placed on recycling the waste products of iron andsteelmaking operations. In general, when little processing is involved andthe material can be recycled to downstream operations (e.g., primary orsecondary steelmaking), economic benefits are maximized. This isexemplified by processing and recycling of steelmaking slag as shown inFigure 18. Through this processing, materials for direct use in the BOF,blast furnace, and sinter plant are produced. However, not all materials canbe recycled in this fashion. Chemical bonding (or "cold bonding") is thenemployed through briquetting (Figure 19) or cold pelletizing (Figure 20).(19)The binders used in these processes are often proprietary, however, mostinvolve materials containing lime, silica, and cement.
Briquetting can handle relatively coarse materials. The processinvolves mixing feed materials and binder in proper proportions and passingthem through rolls containing briquette molds. Pressure applied on the rollsresults in dense, compact pilow-shaped briquettes. Green briquettesusually have to be cured for 12 to 48 hours to develop the strength sufficientto withstand further handling. While relatively strong briquettes can be
produced, there is insufficient experience in their handling behavior usingconventional ore field and stockhouse equipment. Recent trials at U. S.Steel have shown that the briquettes may be used in up to 10% of theburden. (20)
When fine, wet dusts that cannot be used directly in sintering areinvolved, cold pelletizing can be used to produce pellets with physical andmetallurgical properties comparable to conventional pellets. The processused by NSC is shown in Figure 20.(19) In this process, binder consisting offinely crushed blast furnace slag and cement is employed. The process hasbeen in operation for several years at the Nagoya Works. MichiganTechnological University (MTU) developed a hydrothermal agglomerationprocess in the late 1970's. In this process, a calcium-hydrosilcate bond isformed in a steam autoclave.(21) The process is now in use in a pilot plant inMichigan.
While cold pelletizing and briquetting appear to be technically feasible,their high cost relative to sintering has prevented wider applications.However, the situation may change if stricter environmental regulations leadto a shutdown of sintering operations or if fine, wet blast furnace and BOFdust disposal costs escalate.
Some blast furnaces using high basicity pellets and/or fluxed sinterrequire silica additions in order to achieve a chemically balanced burden _
9-17
commensurate with the aim hearth slag basicity. In the past, silca gravelwas used as a burden trim for this purpose. Recently, sized Taconite ore isbeing used instead. While the quantities used are very small (1-3% of theburden), this silicious ore does bring in relatively inexpensive iron unitswithout alkali or phosphorus contamination. Taconite, after the fine crushingstage (Figure 12), is re-sized to 5 X 25 mm for this purpose.
DISCUSSION
Thus far, we have covered raw materials, processing techniques, andthe important physical and metallurgical characteristics of iron-bearingmaterials used in the blast furnace. While applying this information in real-life, either to select an iron-bearing material, or to identify whether aparticular burden material propert is affecting blast furnace operation, threeimportant aspects must be kept in mind.
Firstly, a definite hierarchy exists among the various physical andmetallurgical properties. This is pictorialized in Figure 21. When faced with aburden material that does not meet several physical and metallurgicalproperty requirements, it is important to focus first on improving thoseproperties towards the base of the pyramid shown in Figure 21. There ishardly any merit to being concerned about low reducibility if a material hashigh levels of undesirable impurities, e.g., alkali, zinc, or phosphorous.Furthermore, when more than one iron-bearing material is used in theburden, the compatibility of high temperature properties becomesimportant. If the softening and melting characteristics of the materials aresignificantly different and each material is charged as a nearly separateburden layer, the cohesive zone configuration may be worse than if anymaterial was used alone. It is good practice to ensure that the majority of theiron burden constituents have similar high temperature properties. lron-bearing materials with different high temperature properties should be mixedprior to (or during) charging in the blast furnace to minimize adverse effects.
Secondly, it should be noted that properties are measured underconstant and "idealized" conditions in the laboratory. Re-circulating speciesinside the furnace, such as sulfur and alkali, could greatly affect the actualbehavior of burden materials inside the furnace.
Thirdly, blast furnace performance is a composite of interactionsbetween iron burden, coke, and operating variables, as shown in Figure 22.To utilze the full capability of high-temperature hot blast stoves, hightemperature properties of iron-bearing materials must be adequate tosustain a high flame temperature operation. Coke provides permeabilitybelow the cohesive zone and provides CO for reduction through the _
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solution loss reaction (Figure 4). If coke properties are inadequate or ifburden distribution is poor, iron burden reducibilty may not play any role inaffecting blast furnace performance. Similarly, when the blowing rate (Le.,wind rate) is increased for production, the furnace is likely to be moresensitive to the burden material that generates fines inside the furnace. Onthe other hand, the same material may perform adequately at lower wind
rates.
ACKNOWLEDGEMENTS
In preparing this chapter, I have liberally used material from mypredecessors, Messrs. Gladysz, Limons, and Cheplick. My colleagues atInland Steel and in the industry have also provided valuable information andinsights on the subject. I would like to thank Inland Steel for givingpermission to present this lecture.
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REFERENCES
1. "World Steel in Figures," 1991, International Iron and Steel Institute.
2. Kanabara, K., Hagiwara, T., Shigemi, A., Kondo, S., Kanayama, Y.,Wakabayashi, K., and Hiramoto, N., Trans. Iron & Steel Institute ofJapan, Vol. 17, 1977,371-380.
3. Shimomura, Y., Nishikawa, K., Arino, S., Katayama, T., Hida, Y., andIsoyama, T., ibid., 381-390.
4. Sasaki, M., Ono, K., Suzuki, A., Okuno, Y., and Yoshizawa, K., ibid.,391-400.
5. Ishikawa, Y. and Yoshimoto, H., Proceedings of the Metal Bulletin'sFirst International Iron Ore Symposium, Amsterdam, March 1979,142-155.
6. Ranade, M. G., Proceedings of the 57th Annual Meeting of theMinnesota Section AIME. and 45th Annual Mining Symposium, 5-1 to5-32.
7. Nishio, H., Yamaoka, Y., Nakano, K., Yanaka, H., and Shiohara, K.,
lronmaking Proceedings, ISS-AIME, Vol. 41,1982,90-97.
8. Blattner, J. L., Ranade, M. G., and Ricketts, J. A., IronmakingProceedings, ISS-AIME, Vol. 43, 1984,267-271.
9. Saeki, 0., Taguchi, K., Nishida, I., Fujita, I., Onoda, M., and Tuchiya,0., Agglomeration 77, AIME, Vol. 2, 803-815.
10. Minnesota Mining Tax Guide, October 1991,4.
11. 33 Metal Producing, May 1991, 23-24.
12. Personal Communications with Mining and Steel Plant personneL.
13. Minorca Mine, Brochure, Inland Steel.
14. MinnTac Mine, Brochure, U.S. Steel.
15. Minnesota Mining Tax Guide, October 1991, 22.
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16. Ball, D. F., Dartnell, J., Davison, A., Grieve, A., and Wild, R.,
Agglomeration of Iron Ores, Heinemann Educational Books Limited,1973.
17. Balajee, S. R., and Wilson, G. S., lronmaking Proceedings, ISS-AIME,Vol. 43, 1984,59-71.
18. Niwa, Y., Komatsu, 0., Noda, H., Sakamoto, N., and Ogawa, S.,lronmaking Proceedings, ISS-AIME, Vol. 29, 683-690.
19. "Recycling of Dust and Sludge," Nippon Steel Brochure, 1984.
20. Wargo, R. T., Bogdan, E. A., and Myklebust, K. L., lronmakingProceedings, ISS-AIME, Vol. 50,1991,69-87.
21. Goksel, A., Coburn, J., and Kohut, J., ibid., 97-112.
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LIST OF FIGURES
Figure 1: The Concept of Liberation Size
Figure 2: Processing Routes for Iron are
Figure 3: Internal State of the Blast Furnace as Deduced FromDissection Studies(2,5)
Figure 4: Blast Furnace Reactions and the Relevant Raw Material
Properties(5)
Figure 5: The Effect of Tumble Index of Sinter on Blast FurnacePermeabilty (NKK)(7)
Figure 6: The Effect of the Sinter RDI on Blast Furnace Permeability(N KK) (7)
Figure 7: The Effect of the Pellet Reducibility on Blast Furnace FuelRate (Inland Steel)(8)
Figure 8: The Effect of the Pellet Contraction on Blast Furnace FuelRate (Kobe Steel) (9)
Figure 9: Schematic of the Low and Intermediate Testing Equipment
for Iron-Bearing Materials
Figure 10: Schematic of the High Temperature Testing Equipment for
Iron-Bearing Materials
Figure 11: Transition from Lump are to Pellets Shipped fromMinnesota(10)
Figure 12: Flowsheet for Processing the Minorca Pit Ore(13)
Figure 13: Simplified Flowsheet of the MinnTac Plant(14)
Figure 14: Conceptual Sketch of the Effect of Acid and Fluxed Pelletson the Blast Furnace Cohesive Zone and Performance
Figure 15: Trends in Pellet Production in Minnesota(15)
Figure 16: Schematic of the Sinter Plant Operation(16)
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Figure 17: The Concept of Optimum Sinter Mix Moisture (InlandSteel) (17)
Figure 18: An Example of Processing Scheme for Basic OxygenFurnace (BOF) Steelmaking Slag (Inland Steel)
Figure 19: A Schematic Flow Diagram of A Briquetting Operating
Figure 20: Cold-Bonded Pellet Plant of Nippon Steel(19)
Figure 21: A Hierarchy in Iron-Bearing Material Properties
Figure 22: Inter-relationships Between Iron-Bearing MaterialProperties, Coke Properties, Operating Conditions, andBlast Furnace Performance
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APPENIX I: standard Testinq Methods for Iron-Bearinq Materials
DETERMINATION OF CRUSHING STRENGTH OF IRON ORE PELLETS
ReferenceDocument
ISO/DIS 4700ASTM E382-72/1978 (Revised)
Sample
Particle Size*Number of PelletsConfiguration
1 0 x 1 2.5/9 .5 x 12. 5 mm)60Load appl ied to a single pellet
Testing Conditions
Loading MethodPlaten Speed
Constan t speed15+5 mm/min
Test Measurement Max imum compressive load atwhich each pellet breakscompl etel y
Test Repor t** ' 1) Crushing Stength = Arithmaticmean of the test measurements
2) Standard deviation of thetest measurements
· Al ternately, particle siz~ as agreed upon between theinterested parties may be used.
*-Relative frequency of pellets which break at less than aspecific compressive load (e.g., 80 or 100 kgf or daN) isalso reported in some cases.
N: G,., Ranade2/15/83Inland Steel Company
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APPENIX I: standard Testing Methods for Iron-Bearing Materials(cont. )
TULER TESl FOR IRON ORE, PELET, AN SINlR
Reference iso ASDocument 3271 E279-69 (1979)
SaleParticle Size (nu x nu)
i, Pellets 6.3 x 40 6.3 x 38.1i
! Ore/Sinter 10 x 40 9 . 51 x 50.8
Weight (kg) 15 :! 0.15 11.3 :! 0.23
Testing Conditions
OIU
10 x Width (nu x nm) 1000 x 500 914 x 457
Shell Thickness (mm) 5 6.3Lifters 2 ~ SO x 50 x 5 DI 2 8 SO.8x50.8x6.3S nm
~er of Reolutions 200 E! 25 :! 1 rp 200 8 24 :! 1 rp
Treatmnt after React ion Screen Anlysis Screen Anys is
Test Report
1. Tumler Index (Tl) i +6.3 nu \ +6.3 nu
2. Abrasion Index (AI) \ -0 . 5 nu \ -0.6 mm (30 Mesh)
M. G. RaeInlan Steel Compy2/23/83
9-25
STANDA~n PROCEOURES OEVELOPEO BY INTERNATIONAL ORGANIZATION FOR STANDAROIZATION (I.S.O)
FOR TESTING, IRON ORE. PELLETS, ANO SINTER
Tes
t Rep
Qrt
(-)
Averige ROI
Rounded off
to one dec1mil
plac
e
Averige of
Two tests (dlscird
mInImum ind maxIm..
Vi i
ues
from
4 te
sts)
rounded off to
two declmii places
*Generaiery 10 mIn.
*:Averige of the pilred results 1s found to, be sufflc1ently preciSe.
++Usuilly N2.
+++ Tumble Drum: 130 in. x ZOO in, Z lifters ZOO x ZO x Zin; 10 mln ~ 30 rpm.
Optlonii for measurIng CSAR.
Ipermitted virlltlon In compositIon,. +o.~i ibsolute.
ZHaxlmum o.ozi HZ' ii ternitely zoi co-"žoicoz-zi HZ-5BiNZ gas mixture miy be used.
Ave
rige
of
Two or Four
tests rounded
off to In
1 nt
eger
(' ~
o '"
;; ~ H X H
Averige Swe 111 ny
ind Reduction
rounded off to
one declmil place
tf rt II :: p, II 1' p, i- CD tI rt ... :: \Q :i CD rt =r o p, tI Hi o 1' H 1' o :: i ti C
D II 1' ... :: \Q :i II rt CD
1' ... II I- tI
H. G. Ranade
Inland Steel Company
2/23
/82
TABLE 1: MAJOR IRON ORE PRODUCERS AND CONSUMERS(1)(1989 Milion Metric Tons)
ApparentProduction - Exports + Imports = Consumption
North America 105.7 34.1 27.6 99.2South America 185.7 138.4 4.8 52.1Western Europe 50.7 25.0 150.6 176.3Africa 60.6 38.5 1.5 23.6Asia (inc!. Japan) 52.5 38.3 172.3 186.4Australia & New Zealand 111.6 109.6 1.0 3.0
9-27
TABLE 2: ANALYSIS OF THE REPORTED EFFECTS OF IRON BURDENPROPERTIES ON BLAST FURNACE PRODUCTION AND COKE RA TES(6)
Property
RelativeChange
(%)
Increase inProduction Rate
95% C.I. *(%)
Fines -1 ** 0.8 to 2.3
Low TemperatureDisintegration +10 1.3 to 5.9
Reducibility +10 NR
Softening and Melting *** 3.8 to 33.8 + +
* Calculated 95% confidence interval** Absolute change in the fines content of burden materials
*** Not quantified due to differences in measured test parameters+ Indicates that no change in coke rate is possible
+ + Reported absolute range of effectsNR Not Reported
9-28
Decrease inCoke Rate,95% C.I.*
(%)
0.5 to 1.3
-1.4 to 2.0+
2.2 to 4.3
3.2 to 14.1 + +
TABLE 3: EFFECT OF CHEMICAL COMPOSITION OF IRON-BEARINGMATERIAL
CompositionDescriptor Effect
Fe
p
Reports to hot metal (95-97%)
Reports to hot metal (90-95%)
Reports to hot metal slag; and contribution to hotmetal (60-80%) affects steelmaking
Mn
Si02 Reports primarily to slag, but contribution to hot metalaffects steelmaking
AI203
CaO
Reports to slag (90-95%)
Reports to slag (90-95%)
Reports to slag (90-95%)MgO
S
Zn
Reports primarily to slag but contribution to hot metalaffects steelmaking
Re-circulate causing scaffolds, report primarily to slag
Reports to flue dust, but can penetrate the furnacelining
Na20, K20
Ti02 Reports primarily to slag affecting viscosity; controlledaccumulation in the hearth decreases wear
As, Cu, Sn, Ni
Cr
Report to hot metal (90-95%); not desirable forcarbon steel production
Reports primarily to hot metal; not desirable forcarbon steel production
H20 Reports to off-gas; represents additional thermal loadon the furnace
9-29
TABLE 4: CONTRACTION AND SOFTENING-MELT DOWN TEST
SofteningContraction Melt-Down
Sample 500 g, 9.5 x 12.7 mm 500 g, 9.5 x 12.7 mm
Reactor 75 or 78 mm 10, Stainless Steel 75 mm 10, Graphite
Packing None Coke Breeze (Top & Bottom)
Load 2 kgj cm2 0.5 kgjcm2
Gas Flow 15 LPM 20 LPM
ReductionProgram
Gas Composition (%)Temperature(CO) C02 CO N2
Up to 800800-1100
Gas Composition (%)Temperature(CO) C02 CO N2
a a 100 Up to 400a 30 70 400-600
600-800800-1000Above 1000
Heating Rate -8-9°Cjmin to 800°C1.67"Cjmin from 800-1100°C
Bed HeightSample Weight after Cool-down
Measurements
9-30
a a 10022 18 6016 24 6012 28 60a 40 60
5°Cjmin
Differential PressureExhaust Gas CompositionWeight of Drippings
TABLE 5: BLAST FURNACE BURDEN COMPOSITIONS EXAMPLES
North America
Europe*
Japan
*Excluding Sweden and Netherlands
% PelletsIRON BURDEN
% Sinter
65 25
15
10
65
70
9-31
% Other
10
20
20
TABLE 6: PELLET AND SINTER PLANTS IN NORTH AMERICA(11,12)
PELLET PLANTS
Capacity*Plant Location Millon tonnes/year
Cyprus Northshore Silver Bay, Minnesota 4.0Empire Palmer, Michigan 8.0Eveleth Forbes, Minnesota 5.4HibTac Hibbing, Minnesota 8.5IOC-Carol Lake Labrador City, Newfoundland 10.3L TV-Erie Hoyt Lake, Minnesota 8.0MinnTac Mountain Iron, Minnesota 15.0Minorca Virginia, Minnesota 2.5National Keewatin, Minnesota 4.7Pea Ridge Missouri 1.2Tilden Tilden, Michigan 6.7QCM Quebec 8.3Wabush Point Noire, Quebec 4.5
Total 87.1
SINTER PLANTS
Capacity*Company Location Millon tonnes/year
Armco Ashland, Kentucky 0.8Middletown, Ohio 0.8
Bethlehem Burns Harbor, Indiana 2.6Sparrows Point, Maryland 3.6
Geneva Orem, Utah 0.6Inland East Chicago, Indiana 1.0LTV East Chicago, Indiana 1.2Stelco Hamilton, Ontario 0.5USSteel Gary, Indiana 4.0Warren Consolidated Youngstown, Ohio 0.5Wierton Wierton, West Virginia 1.0Wheeling-Pittsburgh Steubenvile, West Virginia 0.4
Total 17.0
* The reported capacity numbers could vary from year to year.
9-32
TABLE 7: EXAMPLES OF LUMP ORE PROPERTIES
Chemical Analysis (Wt%)Fep
Mn
Si02AI203CaOMgO
S
Australia64.60.0540.073.61.50.150.150.004
Brazil67.50.050.300.500.900.100.100.005
Size Analysis (%)+ 10 mm-5mm
678
758
), ¡
i¡
9-33
TA
BL
E 8
: EX
AM
PLE
S O
F PE
LL
ET
PR
OPE
RT
IES
MIN
EE
MPI
RE
MIN
NT
AC
CAROL LAKE
WA
BU
SH
Pelle
t Typ
eA
cid
Flu
x I
Flu
x II
Aci
dFl
uxA
cid
Aci
d
Che
mic
al A
naly
sis
(W~
k)Fe
65.4
59.5
61.4
65.5
63.8
66.4
65.6
Si02
5.6
5.5
5.5
5.4
4.0
5.0
3.7
AI2
030.
40.
40.
40.
20.
20.
20.
4C
aO0.
37.
14.
90.
43.
50.
40.
1M
gO0.
32.
11.
60.
31.
10.
20.
2M
n0.
070.
10.
10.
10.
10.
11.
9P
0.01
60.
013
0.01
50.
014
0.01
40.
010.
01S
0.00
20.
002
0.00
20.
001
0.00
20.
003
0.00
4CaD /SiD2
0.05
1.30
0.9
0.07
0.9
0.09
0.02
\DS
ize
Ana
lysi
s (%
)i
+ 1
2.7
mm
4.5
1.8
5.0
7.5
10.0
1.5
14v. .t
-6.3
mm
0.7
1.0
0.8
1.0
.15
1.5
0.5
Pro
pert
ies
(IS
O /
AS
TM
)T
umbl
e In
dex
(%)
96.5
97.2
9797
9695
.596
Com
pres
sion
(kg
)23
222
820
524
521
020
122
2LT
D (
+6.
3 m
m)
8690
8593
8994
92Swellng (%)
229
12-
-16
17R
educ
ibilt
y l%
/min
)0.
71.
31.
10.
81.
20.
71.
0Contraction (%)
257.
612
Softening (OC)
1220
*14
10*
1400
*-
-11
90**
1245
**So
ften
ing-
Mel
ting
270
150
110
Range CO C)
1 K
obe
Ste
el M
etho
d* Inland test
** Dofasco Test
TABLE 9: EXAMPLES OF SINTER PROPERTIES
Chemical Analysis (Wt%)
Total FeFeOSi02
AI203CaOMgO
Size Analysis (%)
-5mm
Properties
Tumble Index (%)LTD/RDI (-3 mm %)
Reducibilty
* JIS
** ISO
*** ASTM
Japan
57.06.05.41.99.61.4
5
75*35*65*
9-35
Europe USA
51.59.06.251.2
13.52.5
56.57.06.01.59.71.5
8 12
74*25**1.4**
80***25**1.0**
LIST OF FIGURES
Figure 1: The Concept of Liberation Size
Figure 2: Processing Routes for Iron Ore
Figure 3: Internal State of the Blast Furnace as Deduced FromDissection Studies(2.5)
Figure 4: Blast Furnace Reactions and the Relevant Raw Material
Properties(5)
Figure 5: The Effect of Tumble Index of Sinter on Blast FurnacePermeabilty (N KK) (7)
Figure 6: The Effect of the Sinter RDI on Blast Furnace Permeability
(N KK) (7)
Figure 7: The Effect of the Pellet Reducibilty on Blast Furnace FuelRate (Inland Steel)(8)
Figure 8: The Effect of the Pellet Contraction on Blast Furnace FuelRate (Kobe Steel)(9)
Figure 9: Schematic of the Low and Intermediate Testing Equipment
for Iron-Bearing Materials
Figure 10: Schematic of the High Temperature Testing Equipment for
Iron-Bearing Materials
Figure 11: Transition from Lump Ore to Pellets Shipped fromMinnesota(10)
Figure 12: Flowsheet for Processing the Minorca Pit Ore(13)
Figure 13: Simplified Flowsheet of the MinnTac Plant(14)
Figure 14: Conceptual Sketch of the Effect of Acid and Fluxed Pelletson the Blast Furnace Cohesive Zone and Performance
Figure 15: Trends in Pellet Production in Minnesota(15)
Figure 16: Schematic of the Sinter Plant Operation(16)
9-36
Figure 17: The Concept of Optimum Sinter Mix Moisture (InlandSteel) (17)
Figure 18: An Example of Processing Scheme for Basic OxygenFurnace (BOF) Steelmaking Slag (Inland Steel)
Figure 19: A Schematic Flow Diagram of A Briquetting Operating
Figure 20: Cold-Bonded Pellet Plant of Nippon Steel(19)
Figure 21: A Hierarchy in Iron-Bearing Material Properties
Figure 22: Inter-relationships Between Iron-Bearing MaterialProperties, Coke Properties, Operating Conditions, andBlast Furnace Performance
9-37
MR
5320
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MR
5320
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Pro
pert
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Preh
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Com
pres
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Granular I 3Fe203 + CO ~ 2Fe304 + CO2
* Lo
w te
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eact
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* C
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afte
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SR)
Act
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* H
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MR
5320
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992
MR5 3 2012 . VR Sat May 31 08: 2 6 : 56 1980
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LECTURE #10
BLAST FURNACE CONTROL - MEASUREMENT DATAand STRATEGY
R. J. DonaldsonB. J. Parker
DOFASCO INC.Hamilton Ontario
Canada
Abstract: - The most desired operation of a blast furnace is through the use of qualityraw materials and dependable control strategies. This offers a dilemma for operators ascosts increase when trying to satisfy both criteria. Consequently, Ironmakers must walka tightrope between obtaining adequate raw materials while ensuring their controlstrategy is predictive enough to eliminate process upsets that may affect corporate.profitability.
This paper wil deal with the measurement systems and control strategies that arecurently in use in today's blast furnace operations. It wil make special note of thefinancial impact of model based control strategies and the measurement systemsrequired to successfully operate these systems. Emphasis wil also be placed on "whatmakes sense" for a facility in designing and implementing a furnace control strategy.In providing this summar, the fundamentals of model design and operation wil alsobe discussed.
MEASURMENT SYSTEMS
Blast furnace measurement systems can be broken into two areas; Process Controland Monitoring and Process Optimization. The former is the measurement and controlsystems that wil:
. Ensure plant safety.
. Ensure plant operation is within specifications
· Ensure that plant remains in control.
10-1
Process optimization measurements provide data for higher level models thatdeliver feedback that will;
. Validate sensor measurements.
· Ensure plant economics are on target.· Assist in developing new control strategies and strategic direction.
To discuss either of these areas in detail would require a book and is outside thescope of this paper. What will be presented, are some details on the most crucialmeasurements for supporting a dependable blast furnace control strategy.
Process Control and Monitoring
Process control and monitoring sensors consist of "Discrete" and "Continuous"Measurements. Discrete measurements are "on/off' readings such as pump star and
stops, proximity switches, and zero speed switches. Continuous sensors are those thatprovide an ongoing measurement of a process variable. Items such as temperature,pressure, and flow are the most common in this category. A modem blast furnace relieson both types of measurement systems to safely operate the plant and equipment.
Discrete Sensors:
The most obvious discrete sensors are limit and proximity switches as shown inFigures i and 2. The proximity switch produces an electrical flux that wil detect thepresence of a magnetic object as it moves into its field. The limit switch on the otherhand has a mechanical striker that wil move when struck by a moving piece ofequipment. These sensors are usually used for logic applications were safety of theplant is of importance. For instance, valve position, piston locations or sequential
control.
Some discrete sensors take a continuous measurement of a process variable andconvert them to a digital output. These include pressure, temperature and level
switches, vibration sensors and resolvers. The switches and vibration sensors areusually tied to primar measurement devices that continuously monitor the relatedparameter of the body or fluid. When a preset limit is obtained, a digital output isgenerated that is then used to initiate some action (i.e. alar or shutdown). Resolverswil take the circular or angular movement of an object and convert it to a digital outputat a prefixed setting;
10-2
In all the above, discrete sensors are generally used to control a sequential
operation or to protect the plant. They are tied into a PLC (Programable LogicController) or DCS (Distributed Control System) to ensure rudimentary control isalways available. These systems form the "level zero" control structure of a facilityand are typically found in stove control and furnace filling applications. In thiscapacity, these sensors must be simple, repeatable and reliable. High maintenance isnot acceptable and if it is required, the sensor application should be re-examined.
Continuous Measurement Systems:
These sensors are not only important for safe operation but are critical for thesuccess of many level 2 control systems. Most process calculations depend on the
basic measurements of Flows, Temperature, Pressure, and Composition,
Flows:
The measurement of gases, water and auxiliar fuels is of extreme importance tothe Blast Furnace process. Gas flows (air too) are required for the calculation of processmodels and the proper control of injected fuel rates, water flows for plant safety orenvironmental concerns and injected fuel flows for thermal control.
There are many types of flow meters available for gas measurement as shown intable I, and to cover each one would require a book and a good cigar. However, themeter of choice is the venturi (See BF Control - Two Stage Heat and Mass balance).Based on the Bournelli principle of operation, the venturi correlates the pressure dropthrough the device with the flow measurement. The venturi is preferred due to itsaveraging of the entire flow stream to produce a very accurate measurement. Its designalso provides less of an unrecoverable pressure drop. The downside of the venturi is itscost. Whereas an orifice plate could cost $5000 installed, a comparable Ventui caneasily run i 0 times this figure.
The preferred flow measurement for water is the magnetic flowmeter (See BFControl - Two Stage Heat and Mass Balance). The meter, based on Faradays principle,is simply a wire coil that surrounds a pipe. The coil has a known field power and asshown in Figure 3 and in equation onel. The flow of the fluid through the field wildevelop a proportional electrical output called the electro-motive force (emf). This typeof flow meter is very desirable for water or slur applications as there is no pressure
drop, (i.e. no increase in pumping costs) and the accuracy of the device is extremelygood. The only drawback is that the pipe must be full and the fluid electricallyconductive.
emf = - BDV * 10-8 (1)
10-3
One of the most important flow measurements in the Blast Furace is that of
auxiliar fuel injection. Whether it is natural gas, bwier C oil or coal, it is extremelyimportant that this measurement is as accurate and as repeatable as possible. From aprocess control standpoint, it is preferred that this measurement is mass flow and notvolumetrically based. To do this with gas is a matter of adding pressure and
temperature compensation and, as long as the gas composition is known, the mass flowcorrection can be made. The principles get slightly more diffcult for higher specificgravity substances such coal and oiL. In this case, load cell based injection systems ormass flow meters are used.
One of the most repeatable, linear, and accurate mass flow devices is the Corio lismeter. Figure 4 illustrates the operation of this meter 1. To summarize, as the materialgoes through the oscillating pipe, the pipe wil twist, this measured twist will beproportional to the mass flow rate of the materiaL.
Temperature:
There are thee common methods of temperature measurement in the blast furnacearea - Thermocouples, Resistive Thermal Devices, and Optical sensors (See BFControl- Two Stage Heat and Mass Balance, Burden Distrbution, Auxilar Systems).
The basis of the thermocouple is the Seebeck effect and is ilustrated in figure 52.If two dissimilar metals are heated at the same time an electron flow (emf) is produced.This voltage versus temperature relationship can then be used to establish a calibration
curve. There are several types of thermocouples available as shown in table 2.
A Resistance Thermal Device (RTD) relies on the principle of the Wheatstonebridge. Graphically this is shown in figure 6 2. If resistors are placed in thisarangement, the resulting voltage is proportional to the temperature increase. Thesame principle is used in strain gauges for weight and force measurements. RTDs aremore sensitive and more accurate than thermocouples (+/- O.3°C versus +/- 2°C).However, they are limited in temperature range (-260° to 630° C), more expensive(platinum based) and do require an external power source. In the iroruaking facility,R TDs are predominantly used for water measurement while thermocouples are used forhigher temperature applications such as hot blast or fuace gas streams.
The third most common blast fuace temperature measurement method is usingoptical pyrometry. There are two basic methods of measurement, infrared and spectralradiation. Infra red systems, which include optical pyrometers and fibre optic systems,have an infrared source that is compared to the infra radiation of the body being tested.The difference in the readings is then used to determine temperature.
10-4
Radiation type pyrometers depend on the radiant heat transfer principles of thetested body. This measurement is based on the simple formula 3;
E+R+T=l (2)where:
E =Emittance - Ratio of energy released by a body relative to a black body.R= Reflectance - Percentage of total radiant energy falling on a body that isreflected without entering the body.T= Transmittance - - Percentage of total radiant energy falling on a body thatpasses through.
A black body is defined as an item with an Emittance of one (see figure 7) 3. Astove dome is a perfect example. The optical sensor reads the energy level of the bodyand comparing this energy level with that of a black body (using an emissivity factor)determines the temperature.
Both infra red and radiation systems are somewhat slower to respond totemperature changes due to their non-contact nature (i.e RTD's and thermocouples sitright in the gas stream). On the other hand, their life expectancy is far superior whenproperly maintained.
Level and Pressure:
Level and Pressure measurements are extremely important in the daily operationof a Blast Furnace. The reason that these are mentioned together is that in severalapplications the same principles of measurement are used (i.e. in many cases fluid headpressure is converted to level).
I
Pressure sensors (See BF Control- Cohesive Zone Model) come in severalvarieties and full list of these devices would take forever to review. However, the mostcommon is the capsule based pressure transmitter (Figure 8). Typically thesetransmitters are connected to the process vessel via a stainless steel pipe or impulseline. One pipe is connected to the high-pressure side or low-pressure side or both sidesof a vessel to get an absolute pressure or a differential pressure. In either case, thepressure can then be used to either indicate the system pressure or converted to flow orlevel using basic mathematical relationships.
In the level measurement area, bubbler devices are very common for blast furnacewater and slurr systems. In this application, the level of a body of fluid is indicated bythe amount of air pressure required to allow a bubble to be transmitted. This isproportional to the level of the liquid based on its specific gravity.
10-5
In the area of stockline and torpedo car measurement, the use of microwavesystems and in some cases laser systems is gaining more and more popularity. In manyinstallations, the ever-reliable mechanical gauge system (Figure 9) is stil in service.
Analyzing
Moisture
There are two basic measurements in the blast furnace for moisture measurement,the nuclear moisture analyser for coke and pellet moisture and the measurement of gasand air stream moisture in the blast furnace (See BF Control - Heat and Mass balance,Coke Rate).
The nuclear moisture gauge is based on the principle that neutrons wil bethermalized or slowdown by hitting hydrogen atoms. The number of slow movingneutrons is directly proportional to the number of Hydrogen atoms that are present.Consequently, the moisture content of a coke or pellet bed can be determined from theresulting rate count. In many cases, density compensation is also added to this controlsystem by using gamma sources. The combination of the two readings is then used tocontrol the dry coke unit of the blast furnace.
Blast moisture is very important in the determination of furnace hydrogenutilization and therefore the identification of leaks. There are two basic types ofmeasurements the dewcell and the chiled mirror. The dewcell operates on the basisthat a lithium chloride salt solution wil create an ionic curent as it gets wet. Theresulting current heats the probe, and this temperature is an indication of dewpointtemperature. From this dewpoint temperature, the relative humidity can then bedetermined. The chilled mirror works on the principle that a mirror's temperature iscontrolled until a slight fim of water vapour is seen on the lens. This temperature isthe dewpoint of the water and from this, the humidity level can be determined.
Carbon Monoxide, Carbon Dioxide and Hydrogen.
Although there are other gases present in the blast furnace, these three (and bydifference Nitrogen) are the most important for blast furnace control and monitoring(See BF Control - Two stage Heat and Mass Balance, Gas Distribution). As explainedlater they typically form the basis of all heat and mass balance developments and areessential components for thermal control and leak detection.
There are two preferred analysis techniques in blast furnaces today for themeasurement of these gases, the mass spec and a combination of both infra red andthermal conductivity measurement systems. The mass spec is based on the principle ofexcitation of atoms and the resulting spectral emission. From this emission, theconcentration of the chemical species can be determined.
10-6
In the case of Infrared technology, common for both CO and C02 speciesdetermination, the absorption rate of infra red radiation is proportional to thecomposition of the element. This type of measurement is also common for the COsafety alar systems that are in place in most facilities. Infra Red detection is not assensitive for hydrogen and consequently, thermal conductivity measurements are usedfor this element. In this case, the gas cools a heating element that is maintained at aconstant reference temperature. The voltage maintaining the reference temperature isthen used to determine the hydrogen content.
i-¡
Oxygen
Oxygen sensors are most commonly found in the blast furnace stove area (See BFControl - Auxiliary Systems). They are typically based on the Nerst equation (seebelow) due to the higher temperature application 3. The output of the reading is alogarithmic function of the difference in oxygen content of the sample and referencesources. These units are very repeatable and relatively easy to maintain.
E = RT lnr P(02RiJnF L P(02sl
(3)
E = VoltageR =Ideal Gas ConstantF = Faraday Constant.n = Number of electrons in the electrode reactionT = Absolute Temperature - Reference TemperaturePOiR = Oxygen Partial Pressure of Reference Gas.POiS = Oxygen Partial Pressure of Sample Gas
Weight:
One of the essential elements for furnace control is the measurement of rawmaterial arid hot metal weights (See BF Control - Two Stage Heat and Mass Balance).The method of choice is the load celL. Based again on the Wheatstone bridge theory,the R TD wil measure the strain put on the cell from changes in the structure weight.Simple in construction (figure 10 ) the load cell is very reliable 3. The most commonproblem comes with mechanical binds of the weighing structure.
10-7
PROCESS OPTIMIZATION
The sensors discussed thus far are really those related to fundamental operationand control of the blast furnace. However to improve the furnace, more intelligentmonitoring systems must be pressed into action. This includes what we would call thesmar sensor arrangement. For the sake of argument, we wil include in this categorythe following sensors:
· Above Burden Gas Probe
. Profilometer
· In Burden Probes - Shaft, Vertical and Bosh or Belly.· Tuyere Probe.
Above Burden Gas Probe
This probe can be of the permanent or the retractable design (figure 11). Severalpoints can be placed throughout the probe for temperature and gas measurement.
Depending on the arangement, water cooling may be required. Typically, these probesare used on a periodic basis to sample the furnace top gas. The gas from the differentsample points is stored in pressurized bottles, and then analyzed to determine eachpoint's composition. This information is then used by level two models to developedgas utilization profies and gas flow models for the furnace.
The drawback with the above burden gas probe is that there is a fair amount ofmixing on the top of the burden. As well plugging of the sample ports of these probesis an ongoing problem.
Profiometers
These probes are used to periodically monitor the burden level of the fuaceacross a radius or diameter (See BF Control - Gas Distribution). In many cases theyare combined with the above burden gas probes. They typically consist of severalmechanical probes that are allowed to settle on the burden surface. From these readingsand associated top gas or in-burden measurements, very good modelling strategies canbe developed.
In Burden Probes
The most common of these probes is the horizontal shaft probe, which can beeither fixed or retractable. As in the above burden gas probe, there are severalmeasurement points for gas and temperatures. In some cases, mechanical sampling
systems and cameras are used to determine size distribution of the burden materials.~Magnetometers are also used to detect material movement 4.
10-8
Typically they are located about 10 meters below the burden's surface and areused in gas and burden distribution models. Fixed probes (figure 12) of this naturemustbe of tough construction to ensure long service life 4. Retractable probes requiresturdy external support to ensure trouble free movement in and out of the furnace. Ineither case, cost for these probes versus their above burden counter parts is muchhigher. On the other hand, it is felt that the information provided under the burden ismuch more reliable from a furnace modelling perspective. As always, there is a tradeoff between cost and performance.
The vertical shaft probe (figure 13) is a more rare probe largely due to its cost,headroom and auxiliary equipment requirements (figure 14). These probes have beentried in Japan, Australia, and Germany. The Japanese design, which has gone to a depthof 23 meters, has an on board fibre scope that feeds a high sensitivity colour TV usedfor particle size distribution 5. As well, samples can be taken when the probe iswithdrawn from the furnace. Information from this probe can be used in thedevelopment of furnace shaft gas flow and cohesive zone models.
Belly or Bosh probes have also been tried by the Japanese as shown in Figure 155.
As in the case of the vertical shaft probe, cameras are used to determine size
distribution. When it is withdrawn from the fuace, a hole wil develop in front of theprobe that is the positive indication of the cohesive zone location.
Tuyere Probes
Tuyere probes have ranged from pipes pushed through the tuyeres to high techcamera systems (Figure 16) 4. The purose of these probes is to determine the size,temperature and activity of the raceway (See BF Control - Raceway). Typically, high-speed cameras are used to monitor the raceway through the probe. From the periodicbrightness changes, the raceway size and temperature can be determined. Thisinformation is especially useful for coal injected furnace, where raceway collapse andrestructuring is crucial for successful high coal injection.
10-9
BLAST FURNACE CONTROL STRATGEY
Today's control strategies are possible because of development in five areas. Figure17 shows that optimal blast furnace performance can be visualised as a pyramid. Mostoperations strive to have their operation as efficient as possible. However, there is anincreasing cost associated with this goal. Similarly, unstable operation is undesirable
since operating costs (fuel rate) will increase. The challenge to Ironmakers is to findtheir optimal performance level given the measurement systems and modelling
capability that is available today.
Model Types
Today's models can be broadly classed as follows:
Mass and Energy BalancesBurden Distribution and Gas FlowPredictive ControlAuxiliar Systems
Many of these models include the same data sets and often the outputs of onemodel are important inputs to another. What follows is a brief discussion of the basics ofthese models.
Mass and Energy Balances
Global Mass and Energy Balances
The original attempts at a furnace mass and energy balance were quite simple innature. The evolution of the global heat and mass balance is summarised very well byPoos and wil not be covered 6. However, the operation of the blast furnace is mosteasily understood through the examination of an overall furnace mass and energybalance.
Figure 18 ilustrates the basic assumptions of some early mass balance models 9. Asomewhat more detailed representation is shown in figure 19 ~°. Quite simply, thefigures translate to the following equations 9.
ninPe = noulFe
ninC = nOUIC
nino = noutO
(1)(2)(3)
where:nin = number of moles of each component entering the furnace.nOUI = number of moles of each component exiting the furnace.
1 0-1 0
All three of these chemicals enter the furnace in various forms but leave in limitedpaths. The iron leaves as hot metal and trace amounts of iron oxide (FeO) in the slag.The oxygen exits in the top gas as a carbon bonded gas and the carbon leaves as gas andas about 4.5% of the hot metal. These simple reactions are the basis of blast furnaceoperation. For a summary of common chemical balances often considered in thesemodels refer to Table 3.
An overall heat balance output is shown in Table 4. In summary, heat in the blastfurnace comes primarily from two sources; the combustion of carbon (coke andinjectants), and the sensible heat from the hot blast. The use of the heat can generally beclassified as; the sensible heat of the liquids, the reduction requirements, the solutionloss reaction (i.e. the combustion of coke), heatlosses, and the top gas heat.
When reviewing the outputs some generalizations on these models are possible:
i) There is no indication of the effciency of the operation.ii) The use of heat is concentrated on the melting of materials.iii) An error term is required.
These models are convenient tools for assessing various operating scenarios from aglobal perspective but are not robust enough to determine actual impact of burden orprocess changes on the efficiency of the furnace operation.
Two Stage Heat and Mass Balances
Akerman (1866) introduced the idea of staged mass and energy balances bydifferentiating between reduction and heating carbon. In the 1920's Reichhardt took thisidea and made it useful by dividing the blast furnace into temperature rather thangeometric regions. The typically accepted isothermal lines of 950, 1200 and 1500° C arestill in use today 6.
To understand the theory behind the staged mass and energy balance, we must firstintroduce the zoned blast fuace. Although two stage balances make sense from amathematical modelling standpoint, there are at least 3 and perhaps 4 very distinctoperations in the ironmaking blast fuace 9. 11.
1) An upper zone where the burden is heated to 950° C. This zone is located from theburden surface to 3 meters below it. In this area the gas entering the zone is asmuch as 450° C hotter than the burden. This is also called the preheating area ofthe furnace.
10-11
2) An intermediate or preparation zone (also called the thermal reserve zone) wherethe gas and solid's temperature remain the same (approximately 9500 C). In thisarea the hematite (Fe203 ) and magnetite (Fe304 ) components of the burden arereduced to Wustite (FeO). As well the water/gas shift reaction (H20 + C = CO +H2) occurs in this region.
3) The lower zone of the furnace called the elaboration or processing zone where theFeO is reduced to iron and both the iron and slag are superheated to temperaturesfrom 9500 C to over 15000 C. In some cases this is considered two zones, meltingand superheating. The solution loss (C02 + CCoke = 2CO) and direct reduction (FeO
+ C = Fe +CO) reactions also occur in this area. This area's size and location arelargely driven by coke quality.
In all modem two-stage heat and mass balances, the furnace is divided at a point inthe intermediate zone. Typically this is done at a point where the gas and solids are atapproximately the same temperature 950 0 C. The oxidation potentials of CO and Hz are0.295 and 0.39 respectively, and only Wustite is present. This is represented by point Won the Rist diagram, shown on figure 20 1 i. Figure 21 shows this point as a relativefurnace position and figure 22 gives a quick overview ofthe various reactions that occurin the different regions 9,12.
The function of the two-stage model is to close the heat balance between thebottom and top regions of the furnace and the furnace globally. This is accomplished bycalculating the heat generated by the sensible heat of the hot blast and by the
combustion of coke at the tuyeres. From this, the temperature difference between the gasand solids can be calculated. This calculation is then made for the upper section of thefurnace. As shown by Table 5, some assumptions are made as to the locations of variousreactions. These assumptions can be validated by ensuring that the second law ofthermal dynamics is followed (i.e., gas temperature must be higher than solids).
Several of these models are available with varying degrees of output complexity.One example is the Carbon Direct Reduction Rate (CDRR) model (figure 23) whichprovides an estimate of the optimal furnace operating point 13 . In this model the fuel rateis plotted against the direct reduction rate. The heat boundary (left-hand side) is basedon the heat transfer requirements from the gas and the chemical boundary (right side) isdetermined by the amount of gas required for Wustite reduction. The point ofintersection ofthe two lines is the minimum fuel rate required to support the operationl4.
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To place the operating point on the diagram, a two-stage heat and mass balance iscalculated 16 (See Measurement Systems - Mass Spec, Moisture, Load Cells, Venturi).As with many two-stage balances, more than one set of assumptions is used to calculatethe heat and mass balance. The CDRR model takes three variables; top gas analysis,coke rate, and wind rate and uses two of these to calculate the third 15 . It does this foreach combination and then will either plot one selected or all three points from eachcalculation on the diagram. If no instrumentation problems exist, all three calculations
wil yield the same results and will lie close to each other. Table 6 provides a balancedoutput example for one operating day. If instruentation problems exist then the threecalculations will yield different results. This cross check is a good tool fortroubleshooting the process, and establishing the required level of data quality. Figure24 shows the misalignment in operating points due to data quality.
Another common feature of these models, is the calculation of an excess heat term.This term typically represents the amount of heat required to superheat the hot metal.An example is the IRSID calculation of "Wu" (thermal condition of furnacesuperheating required) i I. As shown in figure 25, "Wu" when used as a control variableto predict the silicon content of the hot metal was very successfuL. Despite this success,use of calculated values such as "Wu" and IRM's "Ec" must be used with other models,such as gas flow and kinetic models, to obtain full benefit 16 .
Still others, use the output of the mass and energy balance in a more direct format.BHP's model "HBM2" produces a predicted hot metal temperature from its mass andenergy balance. A much more tangible term for most operators 17.
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Another major characteristic of staged models is the calculation of the directreduction rate of the furnace. In most cases, the calculation is a variation of the followingequation from US Steel's carbon direct reduction equation ~°.
C DR = CCoke - CFlue Dust - CMetalIoid Reaction - CBumed - C Hot Metal (4)
Two-stage energy and mass balance models are primarily used in evaluatingfurnace performance and identifying faulty measurement systems. They are standardmodels now for all blast furnace operations and are the basis for many higher level on-line control systems. In all of the above cases, the two stage model began as an off-linetool and was later rewritten to offer on-line furnace functionality.
Burden Distribution and Gas Flow Models
Burden Distribution
As early as 1850 Ironmakers recognized that certain fillng methods and materialtypes placed fines in the centre and promoted wall working 7. It is well known thatsuccessful furnace operation depends on intimate contact of the reducing gas and burdensolids. The objective of all operators is to get ideal gas flow that includes 12,18 ;
1) High central gas flow to ensure furnace movement.2) Ideal side wall flow to both reduce wall accretions and heat losses.3) Good intermediate gas flow to maximize furnace efficiency.
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Today, most facilities have either developed or purchased models that simulate thefilling methods used for their fuaces.
In all burden distribution models, whether bell or bell-less type, severalassumptions are made about the raw materials. These include properties such as bulkdensity, angle of repose, size fraction, discharge velocities, and shape factor. The goal ofthese models is to establish the optimal ore to coke thickness that allows maximum gasutilization and uninterrpted burden movement with minimum pressure drop.
Figure 26 is a good representation of a burden distribution model output 19. These
models give an indication of the amount of material located in the furnace relative to thewall and centre. As the angle of trajectory, burden type and discharge times are
changed, a prediction as to the resulting burden profile is generated. Many of today'smodels predict burden profiles that are remarkably close to measured values as shown infigure 27 20 . The power of these models is greatly enhanced if validation is possible byactual measurement of the burden profile (See Measurement Systems - Profilometer).
The importance of these models cannot be underestimated. When used with gasdistribution models, the impact of raw material changes can be predicted beforeimplementation. As all blast furnace operators know, if burden distribution is impaired,production rates wil decrease, furnace operating problems wil occur, andenvironmental problems may also be generated.
Gas Distribution Models
The most important process function of the blast furnace is to efficiently get thereducing gas to contact the solids. To optimize furnace performance, researchers havefor several years probed and simulated the flow of gases and several models have beencreated. Included in this category are stack gas distribution, cohesive zone, and racewaymodels.
Stack gas distribution models predict a furnace's behaviour when either rawmaterial and/or furnace tuyere parameters are adjusted. Good gas flow models considerthe output from the burden distribution model in their assessment. In most cases, the gasflow model determines the axial gas flow characteristics of the fuace. Most gasdistribution models are based on the Ergun equation for packed bed reactors, whichreads as follows 21.22 :
ML
1. 75 * ( ~ - & J * G2& DeØ Pg
(5 )
I
1
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where:
dP = Pressure DropL = Length of the column or packed bed.a = Void FractionDe = Equivalent paricle diameter.
N = Shape FactorG = Gas Flow RateDg = Gas Density
If the burden properties are kept constant, the equation could take the form:
M = k* G2
Pg(6)
As ilustrated by equation 6, gas density is inversely proportional to the pressuredrop. As top pressure increases, differential pressure decreases and the bosh gas wil getmore dense. Because of this, the mass flow rate of the gas wil increase and productivitywill increase.
Researchers have used the basis of this equation to model the entire furnace bybreaking it into small patches or meshes on several levels and then calculating the gasflows and heat transfer between meshes 23. A graphical output of one of these models isshown in figure 2824. These models are extremely computer intensive and were first usedin an off-line capacity. However, with today's computer technology, these models arebeginning to see more on-line application.
Other researchers have also included to a certain extent the radial aspects of bothgas and heat transfer results in their modellng. By analyzing the radial gas profile (SeeMeasurement Systems - Above Burden and In Burden Gas Probe, ) either in the burdenor above it, an estimate of the gas distribution can be made and the radial heat and massflows can be estimated. One example is the "Model Super", which divides the furnaceinto six circumferential or parial furnaces and calculates the inter-furnace reactions ofgas and heat transfer 25.
Top gas temperature distributions have also been used to track the evolution of thegas distribution in the shaft 26. Polynomial approximations of top temperatures are usedto measure changes in gas flow patterns in the centre of the burden and at the walls.These indices, centre flow and wall flow, can be plotted over time (see figure 29) tohelp detect process disturbances and / or validate burden distribution control actions.
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Cohesive zone models have also been developed by several companies. One
example is that of figure 30 which gives reasonable correlation between actual pressurereadings (See Measurement Systems - Level and Pressure J and predicted mathematicalresults 25. The prediction algorithm is based on the amount of coke charged to thecentre of the furnace.
This has also been shown by BHP , who use their RABIT model (originallydeveloped by NIPPON steel) with their cohesive zone model to predict the impact ofchanges in burden distribution on gas flow 27,28. In these simulations, all other variablesother than burden distribution are held constant. The model divides the furnace into 14grids with 65 levels. As in the other model previously discussed, the gas mass flow andheat transfer are then calculated between meshes. Figure 31 a shows the theorized burdendistribution and figure 31 b its effect on cohesive zone properties such as temperatureand CO utilization as calculated by the modeL. Figure 32a shows the predicted COutilization and temperature profiles for both a "V" and "W" profile. The furnace wasactually operating in a region between the two profiles and the measured process datacan be found in figure 32b. Figure 33 shows the results of the model versus the verticalprobe results. In all cases good correlation was experienced.
Most cohesive zone models assume that softening begins at 1200° C, melting at1400° C, and superheating is done up to 1500° C. These temperatures are used to
develop isotherms in the model to provide a "melting line" and an indication of furnacethermal changes to the operator.
Raceway models are another important gas flow model used in fuace
assessment. Raceway modellng was first attempted in 1952 by Ellot et al who usedwood to simulate the raceway response and high speed photography to record the results30. Poveromo et al combined theoretical work and actual blast furnace results to produce
a very good mathematical model of the raceway penetration 31.
D(38.9 Q2 TbJ %0APr WHN
(7 )
where;D= depth of the raceway in inches.Q = wind rate in SCFM.Tb = blast temperature in ° RPr = raceway pressure = 2/3 (Pb1as' -Piop) + Piop
A = cross sectional area of the tuyere opening in square inches.W = average bulk density of burden in pounds per cubic foot.
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H = vertical distance from tuyere to stockline level in feet.N = number of tuyeres.(See Measurement Systems - Tuyere Probes)
As shown in equation 7, raceway penetration is dependent on the kinetic energy ofthe blast as defined by Q and Tb. As shown in figure 34, good correlation was obtainedbetween several different operating blast furnaces and predicted model results. Laterresearch examined the impact of tuyere parameter changes on raceway profie andpenetration. In this testing dry ice was used to represent gas flow and inert particles suchas beans and sand were used to simulate particle flow 32, 33, 34.
By combining the raceway, cohesive zone, gas flow, and burden distributionmodels it is possible to develop a fully dynamic model of the modern blast furnace. Thiscan be a valuable analysis tool when evaluating the impact of proposed raw material orinjection practice changes on the blast furnace operation. However, the developer mustbe aware of the assumptions and data used to validate the models to ensure they are notused outside their valid limits.
Predictive Control
For many years the goal of researchers has been to develop reliable models thataccurately predict the thermal state of the fuace. Operators would prefer a model that
was real time and could predict actual furnace performance. Today there are essentiallythree types of models that do this fuction, statistical, thermal dynamic, and reactionkinetics.
Statistical
Since the 1950's, several attempts have been made at furnace control usingstatistical based techniques. Probably one of the most famous, was Flint's multipleregression work in the 1950's that forms the basis of many quick furnace calculationsthat are used today 8. However, Flint's factors were empirical relationships that neededtuning for each installation. Due to the limitation of the available computer technology,they were not ready to be used in an on-line capacity.
Attempts at using on line mass balances to predict blast furnace thermal conditionswere made as early as 194235. In the 1960's the first successful control systems wereintroduced using this type of technology i 1,12. Some of these balances used multi variableregression techniques on such parameters as silicon, sulphur, hot metal temperature, andwind rate to predict the movement in furnace thermal condition 35. An example ofcontrol predictions based on these models is shown in figure 35. However, due to thetime delay of the process, the confdence in these models was limited.
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In the 1970's, other statistical techniques were attempted that considered the timedependency or auto correlation of the process. Time related regression analysistechniques such as the ARIMA (auto regressive integrated moving average) and ARV(auto regressive vector) were employed 36, 37 . These models considered process autocorrelàtion by using historical readings in the preparation of the predicted varable. Inmost cases, readings 3 to 4 hours old were used to help calculate the next predictedvalue. Some techniques such as the DDS (Dynamic Data System) method use transferfunctions to adjust discrete measurements, such as silicon, manganese and hot metaltemperature, into the continuous domain enabling discrete control of the continuousblast furace process 37 .
These early technologies have been furthered developed and are stil in use today.Figure 36 illustrates the results of an ARV model in use at Rautaruukki, whichcalculates every 5 minutes and stays 25 calculations ahead of the actual processmeasurements. The plot of actual (solid lines) versus predicted (boxes) silicon shows verygood correlation 38.
Presently, newer techniques such as Principal Component Analysis (PCA) andParial Least Squares have been developed and used in several industries. Thesestatistical techniques condense several data points into one or two variables that can becontrolled with standard SPC techniques. The use of this technique has been limited inblast furnace applications, but as computers become more powerful, more uses of thistechnique wil develop 39.
It should be stressed that statistical models are only as powerful as the datasupplied and outside the range of the analyzed data no longer valid. As pointed out byThompson and Bowman, regression models can only be applicable in good qualityburden situations where the fuace operation can stay within historical values 40. If rawmaterial changes are made, tuning of the model is required to maintain its integrity.
Thermodynamic Prediction Models
Thermodynamic models use inferences between chemical reactions to predict hotmetal chemistry and temperature. One example is the model by Ponghis et al that usesthe chemical activities of various hot metal and slag components to predict the hot metalmanganese, silicon, sulphur, and carbon 41. As shown in figure 37, predicted valuesmirrored the actual hot metal composition quite welL.
Reaction Kinetic Prediction Models
Kinetic models try to predict fuace parameters such as the furnace melting line
(cohesive zone). They use selected process data inputs and apply control loop strategiesusing Kalman fiters (an electronic technique that compensates for both measurementand model error). In essence they try to control the furnace like an analog control loop42.:
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In many cases certain assumptions such as axial gas flow only are used to allow quickercalculations. Over the last few years these models have seen greater use as an on-linetool with the improvement in computer technology.
All these types of predictive models, whether statistical, thermodynamic, or
kinetically based, use inferences from other measurements or process calculations topredict the desired control parameters.
Auxilary Systems
There are several other models used today to help operators in the evaluation ofwhat could be called auxiliary furnace systems.
Hearth area monitoring includes drainage or tapping models that determine the bestmethod and timing for maintaining optimal furnace liquid levels for both slag and hotmetal 43, 44. Most of these models are tied into operator guidance systems for feedbackcontrol. Other Hearth models monitor refractory condition by using either one or twodimensional heat transfer calculations to determine hearth lining thickness and skullbuildup 45, 46, 47.
Most facilities have a fuace heatloss model either based on inwall thermocouples
or water flow and temperatue measurements (See Measurement Systems - Mass flowmeters, thermocouples). From this data, changes in burden distribution patterns orscaffolding/peeling problems can be identified.
Several models have been developed to simulate stove operation. In most casesthese models break-up the stove along its height and calculate the heat transfer betweenequal size layers (See Measurement Systems - Temperature, Optical Pyrometer). Themodel goes through several iterations to reach steady state and then determine the finalprofile. These models are used to evaluate fuel efficiency (See Measurement Systems -Oxygen sensors) and hot blast strategy changes on stove performance.
In several cases, energy management models have also been produced thatdetermine and monitor plant wide energy consumption. These models are extremely
valuable in determining the economic impact of fuel changes on the plant.
CONTROL STRATEGIES
As shown in the aforementioned, there are several models, calculations,measurements, and predictions available for today's furnace operator. Consequently,information overload for the operator is a major concern. To address this problem,engineers have developed control strategies that can be placed in two categories,"Strategic" and "Process".
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Strategic Control
Strategic control is a long or medium term method of establishing the mosteconomical scenario to operate the ironmaking plant. This could be considered thesetpoint or deterministic control of the overall process of making iron. Generallyspeaking, these control strategies are centred on fuel rate and material quality. Forinstance, a new pellet type is proposed by purchasing to replace an existing materiaL. Aheat and energy balance, burden distribution, and gas flow model would be executed todetermine the impact of the new material on the fuel rate and furnace operation. Ifacceptable, the new material would be purchased and become part of the plant operatingstrategy.
Usually, this type of analysis is validated by a plant test of the materiaL. However,most operations want models that are robust enough to give an accurate assessment of amaterial's impact without the undue cost and time involved with a detailed triaL. Thiscan only be done if the model can freely and accurately associate material qualityaspects with its outcomes. Furnace control at this level is normally the responsibility ofthe plant managers.
Perhaps one of the most comprehensive examples of this type of system is theNICE system (Nippon Steel Ironmaking Control and Data Exchange System) 49. Asshown in figure 38, data from all plants is fed into a control computer system. At thecorporate level, daily data is available for analysis in proposed strategic changes. Thistype of structure allows senior management quick access to models and results that wilhelp reduce costs.
Most plants have some sort of evaluation model for management. However, thesuccess of the control strategy is as good as the data provided to the model from boththe field sensors and the management team. Large overview models like this can besubject to some very inaccurate conclusions if input data is not rigorously screened.
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Process Control
Process control is really the random error portion or stochastic control of the blastfurace. Once the plant "setpoint" is established, operators are then responsible to runthe plant at the most cost effective rate possible. To accomplish this, today's processcontrol methods can be classified as "operating practice" based and "knowledgesystems" based.
Operating Practice Based
Figure 39 is an example of a simple operating practice based control system. In thismethod, the control variable is the measured hot metal temperature. Standard statisticalprocess control rules are applied to the reading when it goes outside the control limits. :The operator actions are predetermined and shown on an accompanying decision
making flow char. Several items in the control chart would have their own standardpractices associated with them. Similar control strategies are applied to other keyprocess variables (KPV's) such as slag basicity, manganese, and sulphur.
Similar variations of this theme are common in North America. Although simple,they do not limit blast furnace production capability. Arco Middletown works, one ofthe most productive plants in the world, uses a simple steam control strategy based onhot metal temperature 50.
The big disadvantage of this method of control is that it targets only a few of theprocess parameters involved. They also are reactive in nature and do not anticipatefurnace cooling or heating trends. As a result, the overall impact is greater processvariability with that comes higher fuel rates (i.e. most operators carr more insurancewith their setpoint aims) and on average, higher processing costs for a given operatingpoint.
Knowledge Based Control Systems
As process models developed, different control strategies based on .Jess tangibleparameters such as calculated fuel rate were developed. By the mid sixties,.these controlsystems were common in both Europe and Asia. The original Nippon Kokan controlsystem, which used the output of calculated carbon rate as the control variable andadjusted steam injection to control the total heat demand, is but one example 51.
However, as previously stated with the increase in complexity, a need for enhanceddecision making capability became apparent.
As a result, Operator Guidance Systems (OGS) were developed using KnowledgeBased or Artificial Intelligence (AI) control systems. AI systems consist of Expertsystems, Fuzzy Logic Controllers and Neural networks. These systems have been inexistence since the late seventies and are radically expanding in several industries.Knowledge based systems provide the operator with control recommendations based onprogrammed response to measured process data.
The purpose of a Knowledge Based system is to 52,53:
I) Preserve the experience base of the plant.
2) Allow higher level decision making at a lower level of process expertise.3) Optimize the process.
4) Enable decision making to be more automatic and consistent.
5) To reason heuristically (by discovery), to allow qualitative assessment of empiricaldata.
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Before developing a Knowledge Based system, certain criteria should be met 54,55 :
1) The problem is complicated enough to warant a heuristic control system.2) A real expert is available.3) The end users are interested in the system. Is there "BUY IN" ?4) The system is cost effective.5) The system fits in with the current computer structure.6) System development time is reasonable.
Once these questions are answered, then picking the correct AI tool is needed.
Expert Systems (ES) are one of the most basic AI type systems available today.Essentially they are written to try to mimic the thought process of the best possibleoperators "the expert". They consist of "IF - THEN" logic programmed in such a way tolead an operator through a series of operating data points to a solution. Expert systemshave four basic components 56:
Knowledge Base - this is a data file of previous experiences and what theoutcomes were following reactions to situations. These experiences can becaptured on-line as process knowledge develops or by triaL. The trial methodrequires that the time, magnitude and source of a known process disturbance becorrelated with the effect of the disturbance. This dynamic testing method has beenused by British Steel to reduce thermal and production variation 57 .
Rule Base - this is the logic pattern that the operator followed in developing hiscourse of actions when troubleshooting the problems.
Inference Engine - this is the software that connects the rule and knowledge basestogether.
Operator Interface - the medium by which the information is communicatedbetween the operator and the expert system.
The development of an expert system is extremely tricky and requires theassembly of the correct people:
· Knowledge engineer - to develop the software.· Process expert - understands both the long and short-term control aspects of the
process.· Operations expert - knows the effect of process moves and their success rate.· Systems engineer - knows the computer system structure and capability.
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Once the correct team is assembled the fun has just begun. In an effort to captureall related knowledge a rigorous process must be followed. Sollac reported that a totalof 6100 hours of knowledge capitalization was required to develop the SACHEM 58system from documentation of experience to tuning the data-base with plant data.
An example of a simple expert system is shown below. In this system, a Socraticapproach known as "forward chaining" is applied. The stove operator answers a senes ofquestions that have been designed based on the normal operating charactenstics of thestove system. The expert system examines the input data to decide if the stoves areoperating normally. In this case, the time on blast is judged to be too low, and remedialaction is given.
Input:
What is the time on blast of the three stoves? - 55 minutes.What is the range of the time on blast? 3 minutes.What is the average wind rate? 75, 000 scfm.
What is the aim hot blast temperature? 1750 0 FWhat is the present cold blast temperature? 300 0 FHas the mixer positon been checked? Yes.
What is the diferential temperature between check and control ? 18 0 F
Output:
The control temperature is reading lower than the check temperature. The lowertime on blast indicates that the check thermocouple is correct. Change control tothe check thermocouple and re-evaluate operation in three stove cycles.
Perhaps one of the first and most famous ES is the Kawasaki GO-STOP system.Developed in 1977, the first prototype features the recommendation to operators toadjust furnace operating conditions based on eight indices 59:
· Total pressure drop.· Pressure drop in the furnace shaft.· Change in burden descent speed.. Top gas temperature
· Shaft gas efficiency.. Shaft wall temperatures.
· Thermal state of the furnace.
· Slag residual heat in the hearth.
By analyzing the absolute values, variation, and the combined effects of thesevariables, an evaluation of the furnace is made. Based on this analysis, arecommendation is made to the operator that typically involves a wind cut or a change:
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in the ore to coke ratio. In the mid-eighties, the original system was upgraded toincrease the robustness of this control strategy by allowing the operators to adjust evenmore parameters.
There are several other expert systems available and all have varied on the sametheme 60 to 68. Their application range from suggesting appropriate measures for furnacecooling trends to identifying or predicting furnace disturbances such as slips andchanges in furnace gas flow.
Fuzzy logic is used in cases where simple "IF - THEN" logic is insufficient toadequately control the furnace. Some conditions such at looking into the tuyere andjudging the reaction of the coke by its brightness is operator based and can't be
numerically identified. As a result, when a mathematical model assigns a value to this"judgement" a certain amount offuzziness or error will be associated. To understand thetheory of fuzzy logic a brief example is required 69.
If given a set of numbers such that:
A = 0,2,3,4,5,6, 7, 8, 9, 10, 11, 12)
and we were then asked to identify all the prime numbers in this set then;
B = ~ 2,3,5, 7, 11)
Clearly a precise solution exists to this condition. Ifit was asked that we define theset of "small numbers" from set "A", such a clear solution would not exist. What couldbe stated is that the number 1 is definitely the smallest number and therefore is amember of the sub set "small numbers". The certainty value (CV) of one could beassigned to this condition. Similarly, number 12 is the largest number and clearly not amember of the subset and is assigned the CV of zero. The remaining numbers can bearbitrarily assigned any value between zero and one based on the programer's orexpert's experience. The resulting values identified with each number produce what iscalled a "membership" fuction. Graphically, this is shown figure 40.
In practice, several different parameters are evaluated and assigned their own CV's.From these, an overall certainty value of an event or condition can be concluded. Oneexample is shown in figure 41 that depicts the slip index certainty based on severalother "fuzy logic" determinations 70. This type of logic has been used for things likestove control, furnace heat control, sensor evaluation, and burden distributiondiagnoses7!. Success rates as high as 97%, have been achieved using this type of controllogic in predicting the effects of furnace changes 72.
When the number of parameters becomes too excessive, evaluation by both Fuzzlogic and ES becomes unmanageable. Consider for example the application of a 6-point:radial gas probe, along with a vertical probe with 14 measuring points all measuring
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CO, COl, Hi, and temperature. This would give the possibility of over 2500 outcomesalone for the vertical probing. When this is combined with the outcome of the radial gasprofile, interpretation of the data becomes very complex. To handle this volume of data,neural networks have been developed.
Figure 42 shows a neuron and a neural network 73. Several inputs go into theneuron, each with its own weighting from 0 to 1.0 (fuzz application), a transferfunction (equation 8 75 typical example) is performed on the inputs and an outputproduced. There can be several layers in such a network, each feeding a higher level ofreasoning.
f ( I) = 1
l+e-I ( 8 )
To tune these models two constants are typically used, the learning rate andmomentum constant. The learning rate is dependent upon the amount of error that isacceptable for the model output and is gauged by the number of iterations required tocome to convergence. The momentum constant indicates the amount of weighting that isapplied to the previous calculated result. When the tuning parameters are selected, alearning set based on actual operating data is used to teach the modeL. Upon "training"completion, the model can then be used in the operating environment.
These types of control strategies have been applied off-line in silicon control loopsand have been developed for on-line control strategies such as burden distributionpattern recognition and control 74. Once again the output of these systems arerecommendations to the operator of the condition present and the remedial course ofaction to be executed. Figure 43 shows the logic flow of one such network used toevaluate gas flow parameters.
Economic Considerations for Control Strategies
, ¡
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Obviously far more options exist for blast furnace control than ever before. Tohelp determine the most cost effective control solution, we can again refer to figure one.While reviewing the figure, it becomes apparent that selection of a control strategy isplant specific. Such factors as cost of currency, governent subsidies, raw material cost,and training requirements all figure into the equation. If a plant has very good rawmaterials, good process measurement, and well-trained operators it probably makessense for them to spend less time and money in model development and advancedprocess control techniques. If, however, a plant has an ageing workforce and wilexperience a "brain drain" of talent, investment in on-line process models and expertsystems would probably make sense.
Investment in expert systems, from implementation of primar models, to:development of the expert shell, will probably cost between two and five milion
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dollars, depending on the software and sensor requirements. Although this may soundsteep, a fuel savings of about 18 to 45 pounds of carbon per net ton of hot metal, for a5000-ton operation, would pay this offin one year.
The challenge to all furnace operators is to establish a process control strategy thatmaintains consistent operation but is also cost effective. By manipulating all the
parameters found in figure 1, it is possible to devise a control strategy based on thestrengths and weaknesses of a paricular facility.
CONCLUSIONS
The improvements in both measurement systems and modelling are driven by theneed to understand what is happening inside the blast furnace to improve processstability and product quality.
As improvements to measurement system accuracy and robustness develop so doesthe ability to model and control the blast furnace. However, regardless of the controlstrategy, the biggest potential gain is through the application of what we learn throughmeasurement and modellng. If we canot apply what we lear in a consistent andrepeatable maner there is no true measurable gain, for the company or for the operator.
Increased computing power has made it possible to provide huge amounts ofsensor and modelled information to the operator on-line. The need to manage themassive amount of information has resulted in advanced control techniques such asExpert Systems. It is important to note two things; these systems are only as good as thedata provided by the measurement systems and the information provided by theoperators, and that at the end of the day the people are the experts - not the system !Wisdom, common sense, and attention to detail will always be the operator's greatestasset when taming the giant reducing machine called the blast furnace.
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triangle Park, NC, 1984, pp 158,211,,213,214,228.
2) Omega Energy Corporation, "Volume 29 - Complete Temperature Measurement Handbook andEncyclopaedia", Stamford CT, 1995, pp Z13, 226
3) B. G. Liptak et aI, "Instrumentation Engineering Handbook", Chilton Book Company, RadnorPenn's., 1982, pp 504.
4) N. Konno et aI, "A Study of Phenomena Inside Blast Furnaces Using Vertical Probes", AIME_lronmaking Proceedings, Volume 45, 1986, pp 43 I to 439.
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5) K. Tamura et ai, "Studies on the Inside Phenomena of Blast Furnace by Newly DevelopedSensors", AIME Ironmaking Proceedings, Volume 43, 1984, pp 407 to 414.
6) A. Poos, "Blast Furnace Control - Strategy". Blast Furnace Ironmaking Course, Lu, W. K.,
editor, McMaster University, Hamilton, Canada, 1994, pp 12- I to 12-41.
7) J. A. Ricketts, "In search of Ancient Iron: Ironmaking in 1870's vrs. 1980's", AI MEIronmaking Proceedings, Volume 50, 1991, pp 39 to 60.
8) V. Flint, "Effect of Burden Mass and Practices on Blast Furnace Coke Rate", AISI Regional
Technology Meeting, Chicago, 1961.
9) 1. G. Peacey and W. G. Davenport, "The Iron Blast Furnace", Pergamon Press, Elmsfort New
York and Oxford U.K., 1979.
10) T. W. Oshnock et ai, "Utilzation of a Daily Material and Energy Balance Program toEvaluate and Improve Blast Furnace Performance", AIME Ironmaking Proceedings, Volume
42, 1983, pp 427 t0435.
1 I) C. Staib et ai, "Blast Furnace Dynamic Behaviour and Automatic Control", AIME IronmakingProceedings, Volume 26, 1967, pp66t088.
12) A. K. Biswas, "Principles of Blast Furnace Ironmaking", Cootha Publishing House, Brisbane
Australia, 1981, page 8 and page 170.
13) S. Haimi et ai, "Advances in the Application of Blast Furnace Control Models at
Rautaruukki's Raahe Blast Furnaces", AIME Ironmaking Proceedings, Volume 51, 1992, pp
149to 158.
14) K. H. Peters et ai, "Influencing the Fuel Consumption and Productivity of Blast Furnaces bythe Use of Models", AIME Ironmaking Proceedings, Volume 47, 1988, pp 545 to 564.
15) R. 1. Donaldson et ai, "Blast Furnace Process Modellng and Sensor Applications at Dofasco",AIME Ironmaking Proceedings, Volume 50, 1991, pp 733 to 736.
16) J. M. van Langren et ai, "Burden Preparation and Computer Control of the Blast Furnace",AIME lronmaking Proceedings, Volume 26, 1967, pp326t0335.
17) P. Goldsworthy et ai, "Application of an On-line Mass and Energy Balance to Blast FurnaceThermal Control", AIME Ironmaking Proceedings, Volume 5 I, 1992, pp 159to 162.
18) A. S. Harshaw and R. R. Schat, "Two Bell Charge Flexibilty at Dofasco", AIME Ironmaking
Proceedings, Volume 42, 1983, pp 511 to 521.
19) J. J Poveromo, "Blast Furnace Burden Distribution Fundamentals", Iron and Steelmaker,
November 1995.
20) S. Inaba, "The Control Techniques of Burden Distribution in Blast Furnaces at Kobe Steel,LTD.", AIME Ironmaking Proceedings, Volume 42,1983, pp 503 to 509.
21) S. Ergun, "Fluid Flow Through Packed Columns", Chemical Engineering Program, Volume 48,
1952, p 59.
10-27
22) J. J. Poveromo, "Blast Furnace Burden Distribution Fundamentals", Iron and Steelmaker,
December 1995.
23) Y. Sawa et aI, "Mathematical Modellng of Blast Furnace Characteristics by the PreciseLayer Structure in Stock Column", AIME Ironmaking Proceedings, Volume 50, 199 I, pp 4 I 7 to423.
24) Y. Otsuka, "A Two-Dimensional Simulation Program to Analyze the Inner State of BlastFurnaces", AIME lronmaking Proceedings, Volume 47, 1988, pp 305 to 310.
25) G. Danloy and C. Stolz, "Shape and Position of the Cohesive Zone in the Blast Furnace",AIME lronmaking Proceedings, Volume 50, 199 i, pp 395 t0401.
26) H. Saxen, "Interpretation of Probe Temperatures in the Blast Furnace using Polynomial
Approximations", Steel Research, Volume 67, No.3, 1996.
27) P. D. Burke and J. M. Burgess, " A Coupled Gas and Solid Flow, Heat Transfer and Chemical
Reaction Rate Model for the Ironmaking Blast Furnace", AIME lronmaking Proceedings,
Volume 48, 1989, pp 773 to 78 I.
28) K. L. Hockings et aI, "Application of RABIT Burden Distribution Model to BHP Blast
Furnaces", AIME lronmaking Proceedings, Volume 47,1988, pp289t0296.
29) S. Wakuri et aI, "Development of Cohesive Zone Control System for #2 Blast Furnace at Oita
Works", AIME lronmaking Proceedings, Volume 40, 1981, pp i 12 to 120.
30) Ellot et aI, "Physical Conditions in the Combustion and Smelting Zones of a Blast Furnace",
Transactions AIME, Journal of Metals pp 709 to 7 i 7, July 1952.
3 i) 1. 1. Poveromo et aI, "An Experimental Measurement of Raceway Dimensions in BethlehemSteel Corporations Bethlehem, PA. Plant", AIME Ironmaking Proceedings, Volume 35, 1975,
pp 383 to 65.
32) K. A. Fenech, " Attempts to Model Physically the Formation and Structure of the Raceway of
the Iron Blast Furnace", AIME lronmaking Proceedings, Volume 47, 1988, pp 26 i to 267.
33) M. Hatano et aI, "Aerodynamic Study on Raceway in Blast Furnace", AIME Ironmaking
Proceedings, Volume 42, 1983, pp 577 to 586.
34) C. Yamagata et aI, "Fundamental Study on the Internal State of the Blast Furnace at HighPulverized Coal Injection", Blast Furnace Ironmaking Symposium #19, Raceway Control forOptimum Blast Furnace Performance, W. K. Lu, editor, McMaster University, Hamilton, Canada,1991, pp 47 to 59.
35) A. N. Pokhevisnev et aI, "Blast Furnace Process Automation Control", AIME Ironmaking
Proceedings, Volume 3 I, 1972, pp 403 to 4 I I.
36) S. M. Pandit et aI, "Modellng, Prediction and Control of Blast Furnace Operation from
Observed Data by Multivariable Time Series", AIME Ironmaking Proceedings, Volume 34,
1975, pp403t041 i.
, I
37) A. A. Nayeb Hashemi et aI, " Blast Furnace Modelling and Control by the DDS Method",
AIME lronmaking Proceedings, Volume 36, 1977, pp 2 to 297.
I
10-28
38) H. Saxen and L. Karilainen, "Model for Short-Term Prediction of Silcon Content in the BlastFurnace Process", AIME Ironmaking Proceedings, Volume 51, 1992, pp 185 to 191.
39) 1. V. Kresta et ai, "Multivariable Statistical Monitoring of Process Operating Performance",The Canadian Journal of Chemical Engineering, Volume 69, February, 1991,pp 35to 47.
40) C. D. Thompson and R. W. Bowman, "Multiple Regression Analysis of Blast FurnaceOperating Data", AIME Ironmaking Proceedings, Volume 41, 1982, pp 372 to 509.
41) N. Ponghis et ai, "Limits and Constraints for the Production of Hot Metal Contents in Silcon,
Sulphur and Nitrogen", AIME Ironmaking Proceedings, Volume 50, 1991, pp 457t0464.
42) A. 1. Kilpinen, "On-line Estimation of the Melting Line", AIME lronmaking Proceedings,
Volume 48, 1989, pp 793 to 796.
43) K. Shibata et aI, "Control of Hot Metal Flow in Blast Furnace Hearth by Blasting and
Drainage", Blast Furnace Ironmaking Symposium #19, Raceway Control for Optimum BlastFurnace Performance, W. K. Lu, editor, McMaster University, Hamilton, Canada, 1991, pp 118 to140.
44) T. Fukutake, "The Flow of Slag and Metal in the Blast Furnace Hearth During Tapping",
AIME Ironmaking Proceedings, Volume 50, 1983, pp 567 to 574.
45) G. Kiniel and G. Kolb, "Hearth Lining - the Crucial Factor for the Campaign of Blast Furnace
"A", AIME lronmaking Proceedings, Volume 50, 1991, pp 287 to 292.
46) G. Leprince et ai, "P28 Hearth Brick Erosion Measurement and Modelling", Second European
lronmaking Congress.
47) G. Leprince et ai, "Blast Furnace Hearth Life: Models for Assessing the Hearth and
Understanding the Transient Thermal State", AIME Ironmaking Proceedings, Volume 52,
1993,pp 123to132.
48) 1. G. Mathieson et ai, "The Use of Sensing Techniques and Mathematical Models to ImproveBlast Furnace Performance", AIME Ironmaking Proceedings, Volume 48, 1989, pp 587 to 595.
49) H. Kanoshima et ai, "Nippon Steel Iron making Control and Data Exchange System", Nippon
Steel Technical Report, Number 29, April 1986, pp i to iI.
50) D. A. Wise and J. F. Blattner, "The Process Control Philosophy and Strategy of ARMCO SteelCompany", AIME Ironmaking Proceedings, Volume 50, 1991, pp 435 to 443.
51) Y. Fujii et ai, "Application of Automatic Blast Furnace Control at Nippon Kokan Mizue",AIME lronmaking Proceedings, Volume 26,1967, pp 58t065.
52) K. Noderer and H. Henein, "A Survey on the Use of Expert Systems in the Iron and Steel
Industry", AIME lronmaking Proceedings, Volume 49, 1990, pp 497 to 501 T.53) C. Forsberg et ai, "Real-Time Expert System for Blast Furnace Control", AIME Ironmaking
Proceedings, Volume 50,1991, pp 739to 745.
54) J. Walters, "Developing Knowledge-Based Applications: Factors for Success and Failure",_
AIME Ironmaking Proceedings, Volume 49, 1990, pp 491 to 495.
10-29
55) H. J. Bachhofen et aI, "The Application of Modern Process Control Technology in Hot Metal
Production", AIME Ironmaking Proceedings, Volume 50, i 99 I, pp 703 to 708
56) R. J. Thierauf, "Effective Management Information Systems", Merrill Publishing Company,
Columbus Ohio, 1987, pp 58 to 65.
57) M.J. Hague, "Dynamic Testing and Modellng for Improved Thermal and Production Control
of a Blast Furnace", ICSTI / Ironmaking Conference Proceedings, 1998, pp 237 to 234.
58) J.M. Libralesso et aI, "The Blast Furnace under High Supervision", ICSTI / Ironmaking
Conference Proceedings, 1998, pp 231 to 236.
59) Nagai et ai, "Go-Stop System Applied to Blast Furnace Computer of CHIBA Works,Kawasaki Steel Corporation", AIME lronmaking Proceedings, Volume 36, 1977, pp 326 to 335.
60) P. W. Warren, "Recent Developments in Blast Furnace Control Within British Steel", AIME
Ironmaking Proceedings, Volume 54, 1995, pp 28 I to 284.
6 I) W. Kowalski et aI, "Process Control Techniques at the Blast Furnaces of Thyssen Stahl
AG",AIME Ironmaking Proceedings, Volume 54, 1995, pp 23 I to239.
62) L. G. Lock Lee and A. R. McNamara, "A Review of Expert System Developments for Primary
Processing within BHP Australia",AIME Ironmaking Proceedings, Volume 49, 1990, pp 523 to528.
63) L. Karilainen et aI, "Interactive Control of Blast Furnaces" , Steel Technology International,
1995/96, pp 54 to 60.
64) P. Inkala et aI, "Computer System for Controllng Blast Furnace Operations at Rautaruukki",Iron and Steel Engineer, Vol 72, #8, August 1995, pp 44-48.
65) L. Karilainen et aI, " An Interactive System for Real Time Operator Aided Control of Blast
Furnace",AIME Ironmaking Proceedings, Volume 53,1994, pp 365to 370.
66) N. Ponghis et aI, "ACCES - Model for Blast Furnace Control", AIME Ironmaking Proceedings,
Volume 48,1989, pp 523t0527.
67) H. Rusila et aI, "Control Strategy of Rhetoric's Blast Furnaces", AIME Ironmaking
Proceedings, Volume 48, 1989, pp 529 to 532.
68) D. Vanderberghe et aI, "Process Control Techniques for the Sidmar Blast Furnaces", AIME
Ironmaking Proceedings, Volume 54,1995, pp 281 to 284
69) M. Stachowicz, "The Application of Fuzzy Modellng in Real Time Expert Systems forControl",AIME Ironmaking Proceedings, Volume 49, 1990, pp 503 to 51 i.
70) S. Hirose et aI, "Application of AI Systems to Mizushima No.3 Blast Furnace", AIMEIron making Proceedings, Volume 5 I, 1992, pp 163 to 170.
7 I) R. Nakajama et aI, "Operation Control System of Blast Furnace by Artificial Intellgence",AIME Ironmaking Proceedings, Volume 46, 1987, pp 209 to 216.
10-30
72) Y. Tsunozaki, "An Expert System for Blast Furnace Control at Fukugama Works", NipponKokan Technical Report, Overseas, No.5 I, 1987, pp I to 10.
73) M. Bramming et aI, " Development and Application of New Techniques for Blast FurnaceProcess Control at SSAB Tunnplat, Lulea Works", AIME lronmaking Proceedings, Volume 54,
1995, pp 271 to 279.
74) O. Iida, "Application of AI Techniques to Blast Furnace Operations", Iron and Steel Engineer,
Oct 1995, pp 24 to 28.
75) Z. Guangquing, et ai, "A Neural Network Model for Predicting the Silcon Content of Hot
Metal at No.2 Blast Furnace of SSAB Lulea", AIME Ironmaking Proceedings, Volume 55,
1996, pp 21 I to 221.
10-31
..Sqiiare~edge,prlf~e '
'Ecê
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c pr
ifce
"!¡:'!Se"" riiáortiee
. ,_,_ ,000'" ,'.' .." '
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ô~(é
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eter
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rgy
Add
itive
Spelal Techniques
P I Tàgglng
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-flig
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óppl
er=
1R :tlS
:tV.
:t V
.Fl
uidi
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AP
:!1R
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5:t
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.:t
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. Üse
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pler
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:iJ9l
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, ,i2
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AP
O~
n, C
hinn
elFl
uid
Dyn
amic
A =
sys
tem
acc
urac
y/un
cert
aint
y
S .. percent of full scale accuracy
'Wei
rs.,A
umes
:t3R
:5R
:t:i.
:t:i.
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tex
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cess
ing
vort
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uidi
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tor
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l
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'I.R .. percent of actual flow rate accuracy
p, '"
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epea
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lity
Table 1: Types of
Flow
Met
ers.
TABLE 2
Thermocouple Types and Their Range
ANSI ALLOY COMBINATION MAXIMUM TEMPERATURE LIMITS W ERRORCODE RANGE (Whichever is Greater
+ Lead - Lead Standard SpecialJ Fe Cu-Ni -21010 1200OL. -346 10 2193° 2.2OL orO.75% 1. OL or 0.4%
(Magnetic)K Ni-Cr Ni-AI -27010 1372"' . -454 to 2501 ° 2,2OL or 0.75% 1. OL- or 0.4%
(Magnetic)V Cu Cu-Ni o to 80 32 10 176°'
T Cu Cu-Ni -270 to 400~ . -454 to 752° 1.0 or 0.75% 0.5°C or 0.4%
E Ni-Cr Cu-Ni -270 to I OOOOL . -454 10 1832° i. 70L or 0.5% 1,0= or 0,4%
N Ni-Cr-Si Ni-Si-Mg -270 to 1300OL. -450 to 2372 2,2"' or 0.75% I. ¡= or 0.4%
R Pt-13% Rh Pi -50101768"'. -58103214° I.5OL or 0.25% 0,6 orO.I%
S Pt-IO% Rh Pt -50 to 1768~. -58 10 3214° !.5OL or 0,25% 0.6"' orO.I%
U. Cu Cu-Ni o to 50~32 10 122°
B Pt-30% Rh Pt-6% Rh o to I 8200L . 32 to 3308° Not establishedW W W-26% Re o to 2320~ , 32 to 4208°' 4,5 to 4250L Not established.
1.0% to 23200CWS W-5% Re W-26% Re o to 2320~. 32 to 420lF- 4.5 to 425~ Not established
i .0% to 23200CW3 W-3% Re W-25% Re o to 23200L . 32 to 4208'" 4.5 to 425"' Not established
i .0% to 23200C
Reference - Volume 29 Ome~a Complete Temperature Measurement Handbook 1995 Stamford CT.
10-33
TABLE 3: TYPICAL MATERIAL BALANCE DATA
Fe C O2 Si02 CaO MgO AhOJ Ti02 S K20 Na20 Mn "2
In 1896 837,8 1546 172.9 178.0 53. 38,0 7.8 7,8 1.9 1.4 14.5 1.0Ibs/nt
Out 1790 841.6 1532 1720 177.9 53.0 38,0 8,) 8.2 2.2 2,1 14,5 1.0Ibs/nt
Yield 94.4 100,5 99.1 99.5 99.9 99.3 100 104.2 105.2 116,6 151.4 100 100%
Table 4: GLOBAL HEAT BALANCE
INPUT: ( MEGACALIMTHM) VALUE
Combustion of Carbon 587.
Sensible Heat of Blast 351.8
Slag Formation Heat 15. I
Reduction by CO 51.6
TOTAL 1005.8
OUTPUT: (MEGACAL/MTHM) VALUE
Sensible Heat of Top Gas 62.1
Sensible Heat of Hot Metal 300.9
Sensible Heat of Slag 99.5
Reduction of Si 21.
Reduction Mn 5.7
Reduction of P .9
Solution Loss 333.5
Fuel Decomposition .3
H20 Decomposition 4,0
Reduction by H2 15.3
Calcination Reactions 6.3
Carbon Solution in Hot Metal 22.3
Unaccounted Heat Loss (UHL) 134.5
TOTAL 1006.5
10-34
Table 5: Two STAGED ENERGY BALANCE
PREP ARA TION ZONEHEAT SUPPLY
(MEGACALITHM)Gases from Procssing Zone 550.4
Heat from Burden -22
Heat from Coke -0.6
TOTAL 547.6
PREPARA TION ZONEHEAT DEMAND
(MEGACALITHM)Heating of Solids 381.
Reduction of Ore Oxides 11.9
Flue Dust 0.4
Decomposition of Fe and Mg -0.4Carbonates
Heat of Vaporization of Moii.1ure 352
Top Gas Humidity 4.3
Reduction ofCO¡and CaCO~ -0.2
Top Gas Enthalpy 61.2
Heat Losses 53.9
TOTAL 547.6
PROCESSING ZONEHEAT SUPPLY
(MEGACAL/THM)Blast Enthalpy 388.0
Solids from Preparation Zone 381.
Injected Fuel Oil 0.9
Combustion at Tuyeres 594.8
TOTAL 1364.8
PROCESSING ZONEREA T DEMAND
(MEGACAL/THM)Hot Meta Heating and Melting 2812,
Slag Formation and Heating 72.1
Si, Mn, & P in Slag 303
Wustite Reduction 221.8
Decmposition of Carbonates -0.5
Si and P in Hot metal 5.0
Nees iit Tuyeres 323
Reduction ofCO¡ and CaCO" -02
Limestone Calcination 5.8
Gases to Preparation Zone 550.4
Heat Losses 172.4
TOTAL 1364.8
TABLE 6: MATERIAL BALANCE DATA
VARIABLE COKE BLAST CO¡ CO H¡ ET ACO ETAH
Input Data 784.2 35386.3 21.8 22.5 2.8 49.2 52.0
ETA+CKE 784.2 34692.0 22.6 23.4 2.7 49.2 52.4
CKE + BLT 784,2 35032.5 22,8 22.9 2.7 49.9 52.0
BLT+ETA 788.1 35032.5 22.5 23.3 2,7 49.2 52,0
,¡I
10-35
Figure 1- Mechanical Limit Switch
Figure 2 - Electrical Proximity Switch
10-36
'" magnetic~coi
. FI w Meter" f Magnetic 03 Principle 0Figure -
-- _.-:-.: _:l'
l ..fe, ..jì¡it -:.._..~......,
--.;:......9.... ....~1;:..- , FC1
~i
-,..
-: -.: - - :: - ::::1)
Fe.
o
w
. i" Meter. f the Corio iS. 4 - Principle 0Figure
10-37
Figure 9 - Mechanical Stockrod Installation
HERMtTlCALLYSEAtEDGAUGE
C~AliaER
LOAO BUTTON
LOAO SUPPORtCOLUMN WITH
BONO€.O STRAtNGAUG E 5
CAeL£: CONtECTlONP'1p, fOR POWER
SOUflGE AMOOUTPUT CONNE C T I Of
800Y
1f;l8ASE-..,
Figure 10 - Load Cell Construction
10-40
Figure i 1- Above or In-Burden Retractable Probe - Paul Wurth Design.
8
Col í ng vvter
~
Magnetomet e~
i
i
i
i I
I
i I
i
Thormocooplc and Gas sampl jog pipe
61: "Trmocple,. k: Gas samplíng piPß~ : Mågntómc to,-
Figure 12 - Fixed In-Burden Probe
10-41
Optimum SF Perormance Requirements
Incing Co of impleg~nnce Steg
--~~'!~~-~- - -- -- - -- -- - -- --- -- - - - -- -- - - ----
- --- - - - - - -- - - - -- - ----MIniumReulnt foStbl Openin
:-:::':~7.:~õdelS:';.::. ~;::::
-:--:ontl,:-: -.~DM-i Oper-ng Range
111lng Coof UnatabkOpenron ~1Í~;~5~~~~g~tr~~¡
Fact lnfncng Owraa Furnce~Figure 17: Blast furnace performance and its contributing factors
The fron Blat FurnacefoiD, + C
(298K)TOP GAS (298K)
CO, COi ,Ni
AIR (296 Kl--(Oi,Ni) L
\Far, C
(IBOOK)
Figure 18: Simplified Representation of Material and Energy Balance Concepts (9)
10-44
Material Balance
Raw Materials i Top GasFlue Dust
Iron BeargCoke
Fluxes
Ivsc.
\ll
Blast ConditionsWindHot BlastMoistueFuelO:x-ygen
Molten Material
Hot MetalSlag
Figure 19: More Detailed Material Balance Concepts (10)
oy= Fe
u-r
-lPi
t E
2 x=Qc
T
1000
800
Fe600
400
COzCO + COz
Figure 20: Operating Line (Rist Diagram) of the Blast Furnace (11)
10-45
I 1,0",,1, I¡ \j L~~O!~~I:Or°lSKJ
\
\(i
I!--
f
I ._",
l, ,,:::~..':~:~ l
~\-
I
nlt'li-- "UI('Æ'UtotNt
:K
\. J" a..~
\ ".:~~~:.
iiw 210:iTe:t.P'R~rUR€. I(
\"~'--i/i c..oc:w:.:: iU.HlI..
r-" ()~.I,')~O..¡il aci)l
i
I./
(j ¡ iI ¡-'-",=~liJ J~ ~ÒOft RAn:; IN GAS
Figure 21: Conceptual Division of the Blast Furnace - Physical Position (9)
20
11
16
14
12
DútaeAbov 10TUN...
8
6
4
2
T UN..
iap G"~: 100 - !sa.c.. 1"- -l(1 '4. f;ai" ~o ~l.:"J. C:J .. ~s.r hz
ii=_~ ,,iäs ê;:'à; " ..
_ _ ~ ,_ _ _~:; 'lJ:,'~o~._(;~'_¡:"~': :0,~ i F~J()\ 1" CO: JFoi(l" ::1,1
- -:~-E:l~A-L - - M . :I::~~~; - i:ll~ - - - _. -- 1______ r------- - ., --¡ ,., i~ ,'w I ii i Š I t
.. io ~:t I r'fO .. cO = F~ -+ CO ilI Z I¡ Q ..~ 2 i ~ ~ UlIflECUCEO F'tOa: -i 0..~ ~ ~ ~
¡- ~ ~; :
~ ~ i~-iL_m_ ;".-.;:l-.;;;u--
OIRECT _ AEDUc.lICM '\ ctC-9! ., t~Q 4. tOi
..0 MElfi NÇ lONE, ~I. '\ C(\ . e, . ''0ÅND lP~ 1r1',~O .. ç"" N,'I . CO, "P1C$ ~ 5Ç . it,. s~.
~i(l:: . 2; :I ii . -ieo .. ..5.C~O"C 5"CO
500 1000 1500 2000
Tenmeratu °C
Figure 22: Furnace Zones - temperature and reactions (12)
10-46
H., t
(l flAuHi:OUllo(KI :)y BLAST FURNACE f..'eek 4 f 199 rA.""!;h.,IW.,i-:
(~ ' DRR DII;GRAM .~a,2 ïor.J ..~ II HlJ ,ii~5.1i Gi/IHI1'-~:i k9/11111ci7.2 i-g/I K.. I""- ..-----,' -.'.'..'. -'.---
flUS' VOLUME " 1. i(" )1 h !02- EIII1CHM.EIlT ....., km'/h ¡Sìi: 1thn:WPEAnlRi; ioa~' C/w OISTUni::U3 lj.l rn JHEATiOSS£S itJl GJI¡"
4~)CO~E:OiLFUR MlEopn1lUM
:E1:::4C'"..
",~'..
"~c"e 3S-:J
3CZS .J M
Ojrc-cl;o rcd.JdiOlFak- ~
liar r.ET.\L 2112 iId,Si-% 0,44 'ISl.~.il!c". 0.12 ~Hm""..tu.. 14 'C
Figure 23: CDRR (Carbon Direct Reduction Rate) Diagram (13)
-E.i..""C).:-Q)-lcoc:i:a..'-cou t
opmum operating po1nt(minImum fuel roe)
% Direct Reduction
I (1 oper.ting point ...uaing top g.. uti UzaUon andI
coke r.te ere correct(2) operating point ...uaing cok. rate end bl..t
conauaptlon .ra correct(3) operating point ...uaing top ga. ut i 1 ization .nd
bl..t coneu-øtion ar. correct
Figure 24: Ilustration of CDRR model to detect Measurement Error (15)
10-47
Wu1lrmrron Fe
%Si r~')¡-30
L,c,
(:L:o)
%S
Fuel Oil Rate t 2 ~
IN 3 :ig iniS
Blat Moistu3
glNin
f ic
Q
-\Ç
/------_.\/'-,/-_.V... ,- '...'\..v
~-~._~--._~./-, - ,.,...,/' .. ~. --../\. ",'" "~,_/'..__.-i _.-..__.__._~
r '0
20
'30" 5"~"1 ~.
12'Ó.67 1:ó'ó7 . ',-\'ó7
Figure 25: Blast Furace Control Results using "Wu" Heat Index as Control Variable andSteam Injection as Control Strategy (11)
0;
l-,
Ê-!i..GI
=-..-DIiII~C....
.:¡. ..
i)~
-l,
RGmler Wil;
Loo!
so!- Sloifr +- l'i.kisi
-t-=-Cl:
"Eo:t-M~· 0Cc..lo8&: o
CeiierRadial Positionin Furnace
WaJl
Figure 26: Burden Profile from Mathematical Model (19)
10-48
o
-u~"
~c'"~ 2'"
..
C 2ui,~5o ni; ~f7
I,IL-Obs-e r-ied
,i
¿.Est;mCl1f:d
Dis1Qni;~ from ient~r (rnlQ
Figure 27: Comparison of Estimated and Actual Surface Profie (20)
a
Inti mesh
b
Clunologiallis an fllis of soli
c:
Mesh Generatedby use ofc:lunologiallis of soli
Figure 28: Mesh Generation for Gas Flow Models (24)
10-49
t-556h60
40
20A.
0
-20
a)
-400 50
t-570h60
40
20A.
0
-20
b)-40
0 50
--_~_.-il' - .....~..' .
....--
./~:.l++'- /".r .. . ./~'O00 __.f;
IV"".
B100 150
Ac
.,,.+'" t,~'
.+ '~,~" . _.~/,fil.;:.B.._.Aèt .B
100 150
,,$
Ac
Figure 29: Evolution of the Indices after Changes in the Charging Program (26)
;l~i~i.i l/l ~~ojM:n1 .~ì;lllrl
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10-51
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10-53
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10-54
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10-55
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10-57
M
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Figure 40: A schematic representation of a "Fuzzy Logic" membership function. A valueof 1 = full membership (69)
Slip warnìrHJ
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Figure 41: Slip Index model- evaluation results (71)
10-58
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10-59
LECTURE #11
MAINTENANCE RELIABILITY STRATEGIESIN AN IRONMAKING FACILITY
Gary De Grow
Dofasco Inc.P.O. Box 2460, Hamilton, Ontario, Canada L8N 3J5
ABSTRACT
The approach to maintenance in the 1970's was to "Fix it when it Breaks" or
commonly known as Reactive Maintenance. One of the main reasons for this was due to
the focus on productivity. This approach was costly due to ineffective use of
maintenance resources and high inventory of spares. In most integrated plants, this could
be tolerated because there were multiple facilities or process streams so production losses
would be minimaL. At the same time, maintenance resources became great "Fire
Fighters".
In the 1980's the focus was now moved to quality aspects, but maintenance costs
could no longer be tolerated. Maintenance costs were escalating exponentially with no
end in sight. Work began to examine maintenance costs and develop plans to control
costs by improving equipment availability. Planning and scheduling maintenance
activities were thought to be the ultimate solution, however this alone was not the answer.
The focus in the 1990's moved toward the total equipment concept, targeting
reliability as the key to long-term success. This paper is an example of an equipment
reliability program developed to meet the challenges in an ironmakng facility.
11-1
INTRODUCTION
In the 1970-80' s, Dofasco Hamilton fit the typical mould of the integrated steel mill
with two generations of facilities, i.e., four blast furnaces, two steelmaking facilities and
two hot mills. In the early 1990's, a decision was made to shut down our older Stream 1
facilities and optimize our newer Stream 2 facilities. This meant shutting down two blast
furnaces, No.1 Steelmaking consisting of three basic oxygen vessels, and direct coupling
our two remaining blast furnaces to one oxygen KOBM vessel at our No.2 Steelmaking
facility. The Ironmaking facility must now provide a continuous supply of iron without
interruption to Steelmaking. This meant that the blast furnace/hot metal areas in
Ironmaking must extend shutdown intervals to coincide with steelmaking vessel bottom
changes/relines. In other words, there would be 12-14 week intervals between alternate
blast furnace shutdowns.
To meet this challenge, maintenance had to move from a Reactive to a "Pro Active"
organization anticipating what wil happen in the future and planning and scheduling
corrective action ahead of time.
Activities were aimed at three key elements:
· Maintenance Process
· People
· Organizational Structure
11-2
1. MAINTENANCE PROCESS
There are many processes in a steel plant, i.e., Chemical, Metallurgical, etc. In
any good maintenance program, there is a maintenance process. This process consists
of six basic critical elements:
. Work Identification
. Planning
. Scheduling
. Execution
. Follow up
. Analysis
a) Work Identification
One of the keys to a successful equipment maintenance program is knowing the
condition of your equipment at any time, anticipating what wil happen in the future
and planning/scheduling corrective actions in advance. The work identification stage
is critical to the program.
There are a number of tools to be used in the work identification. These tools are:
. Preventative Maintenance Activities
. Predictive Maintenance Activities
. Reliability Centred Maintenance
. Root Cause Failure Analysis
. Process Information Systems
. Computerized Maintenance ManagemenUIntelligent Condition Monitoring
System. Regular Communication Meetings
11-3
Preventative Maintenance Activities
Preventative Maintenance inspection wil involve the senses, i.e., visual, touch,
sound. Information gathered through inspections is recorded via check sheets or hand
held data loggers (HHL). The HHL information is preferred as the data can be
downloaded directly into the computerized database.
The PM Program will also include:
· Routine Lubrication
· Cleaning· Minor Adjustments, servicing
· Varous assessments/audits (environmental) etc.
Predictive Maintenance (PdM)
Predictive Maintenance itself does not prevent anything. It does however give
information on condition changes that wil result in a breakdown.
Predictive Maintenance consists of:
· Non Destructive Testing. Liquid Penetrant
. Magnetic Particle
. Ultra Sonic
. Radiography
. Eddy Current
. Acoustics
. Fibre Optics
. Vibration/Alignment
. Stress Analysis
. Pump Performance/Flow Monitoring
· Lubrication Analysis
. Trace Metals
. Particle Counting
. Wear Particle Analysis/Ferrography
11-4
. Infrared Thermography
. Electric Circuit Monitoring
. Refractory Insulation Monitoring
· Motor Circuit Analysis. Insulation Testing
· Merger Testing
· Dielectric AbsorptionlPolarization Index· DC High Pot Test
. Stator WindinglPower Circuit Testing
· Surge Testing
· Surge Comparison Testing
· Rotor Fault Diagnosis
Reliabiltv Centred Maintenance (RCM)
Reliability Centred Maintenance is a process used to determine the maintenance
requirements of any physical asset in its operating context.
This process involves addressing seven key questions about the asset selected:
· What are the functions and associated performance standards of the asset in itspresent operating context?
· In what ways does it fail to fulfill its functions?· What causes each functional failure?· What happens when each failure occurs?. In what way does each failure matter?
· What can be done to prevent each failure?· What should be done if a suitable preventative task cannot be found?
Utilization of this process wil deliver the benefits of:
. Improved safety and environmental protection
· Improved operating performance including output, product quality and service· Increased maintenance cost effectiveness· Longer equipment life
· A comprehensive equipment maintenance database
· Motivation and ownership by the team
· Improved teamwork between Operation, Maintenance and Technology
11-5
Root Cause Failure Analysis (RCFA)
Root cause failure analysis is designed to scrutinize every component of the failed
system. Based on scientific/engineering data, and the RCFA Group expertise,
possible failure modes are systematically eliminated leaving the remaining modes as
the causes of the failure. RCFA is initiated:
· After a costly breakdown
· For an in-depth look at a specific equipment failure. Events leading up to andsurrounding equipment failure are known
· To determine root cause failure and revise existing Standard Operating Practices,create PM Tasks, or re-design equipment
A typical RCFA will require approximately 12 hours of group meetings tocomplete.
Process Information System
Many facilities utilize on-line data collection systems to capture process
information. This process information, in many instances, is valuable equipment
condition data that can be used to monitor and trend equipment deterioration and plan
correcti ve acti on.
Computerized Maintenance Management (CMMS)
Intellgent Condition Monitoring System (lCMS)
A Computerized Maintenance Management System (CMMS) is a key tool
required moving to world class maintenance in any facility, but it alone will not
achieve the desired result. What is needed is a system that wil support a planned
maintenance process and have the ability to input, track, and trend equipment specific
data.
11-6
Dofasco uses a CMMS system with a fully integrated component called the
Intelligent Condition Monitoring System (ICMS). The major components of this
system are:
· Reliability Centred Maintenance
. Equipment Maintenance Program
· Mathematical calculation
· Rules based failure model
. Route planning
. Data collection
· Graphical data
ICMS in conjunction with maintenance components of CMMS, i.e.,
planning/scheduling and spare parts inventory support Dofasco's planned
maintenance process.
The Equipment Maintenance Program (EMP) is the critical element of the system.
The EMP is the list of work activities to be performed to maintain a piece of
equipment at its required level of performance. These activities wil be a combination
of PM, PdM, RCM or RCFA Tasks.
Data points can then be used as condition indicators to give information on
equipment conditions. The indicators can be:
· Numeric. Alphanumeric
. Boolean
Alarms can be established based on severity limits and rules set up to identify
when equipment is progressing toward failure. Data collection can be accomplished
in three modes:
· Operator check sheet
. Plant System Signals (PI)
· Hand Held Data Loggers (IlL)
11-7
Once input into ICMS, searches can be run on non-normal condition and move
down to the data points in question for action. The system will allow a spin-off work
request, but action/follow-up must occur, as the alarm does not disappear until
corrected.
Re2ular Communication Meetin2s
One of the simplest tools used for work identification is regular communication
meetings. A daily manufacturing meeting allows operators, maintenance, trades and
technical personnel to review the past 24 hours and identify potential trends in
equipment condition. This is, in part, total productive maintenance activities
peiformed during the course of their shift. This information is gathered during their
inspection rounds and through the PI system monitoring the process.
Information reviewed in these meetings may then either require immediate
follow-up or allow for work requests to be issued to plan and schedule corrective
work.
Work Prioritization
With these various methods of work identification, a strategy must be deployed to
prioritize the facility assets, and then the best tools required must be identified to
determine equipment condition.
In the Ironmaking facility, two key steps were utilized to get started:
· Equipment Criticality Assessment· Predictive Maintenance Needs Assessment
11-8
· Equipment Criticality Assessment
The equipment criticality assessment tool was developed to focus on equipment
reliability improvement as a means to improve manufacturing results. The
consequences of equipment failure are assessed in key areas of business performance,
i.e.,
. SafetyEnvironmental impact
Product qualityThroughputCustomer ServiceOperating Costs
.
.
.
.
.
For each consequence area, a consistent criteria has been defined and a weighting
factor assigned for performing the assessment. The criticality risk number is
determined by multiplying the total consequences of failure by the
frequency/probability of failure.
This assessment was performed on all major equipment within the Ironmaking
Business Unit including the supporting hot metal facilities. The resulting risk
numbers provided this prioritization of assets to focus on the critical equipment that
has the highest impact on performance in the Iron Business Unit.
· Predicitive Maintenance Needs Assessment
In any facility a maintenance program of sorts has been established to minimize
unplanned outages. In many instances, a time-based program was set up to plan and
schedule equipment rebuilds/replacements and inspections. This method, however
may not be the most cost-effective approach to equipment reliability.
11-9
¡
Once the equipment criticality has been established a predictive maintenance
needs assessment should be applied. This involves a team of PdM Technology
experts to conduct an audit of the existing maintenance program. This audit is
conducted at the operating plant level and involves interviews with maintenance and a
review of inspections on critical equipment.
An in-depth evaluation looking at applicable predictive technologies is applied to
the equipment taking into consideration the existing program, process requirements
and common cases of failures.
The resulting recommendations are documented and submitted to the Business
Unit. The end result will reduce maintenance costs by utilizing the PdM activities
and redirecting efforts of Tradespeople toward more value added maintenance
activities.
b) Plannin2
Planning is identifying a road map of where you are going with your equipment
maintenance program in order to achieve equipment reliability. There must be a short
term and long term plan for a successful reliability plan. The long term plan is vital,
as reliability cannot be reached in one year.
The short term consists of tasks that are required to perform the work now on the
critical equipment. The short-term tasks include:
· Equipment Hierarchy
· Bills of Materials· Equipment Spare Parts Inventory· Procedures
· Backlog of Work
· Type of Work
· Repair/Rebuild Programs
11-10
· Equipment Hierarchy
In the Computerized Maintenance Management System, an equipment hierarchy
is needed to break down the equipment to the level where maintenance is performed.
The hierarchy is also based on physical/geographical locations within the plant. This
hierarchy will allow for accurate cost allocation for equipment maintenance.
· Bil of Materials
The Bil of Materials is of importance to ensure that the Master Parts Catalogue in
the Computerized Maintenance Management System accurately reflects the
equipment in the plant. It is important to ensure parts inventory is correct.
· Equipment Spare Parts Inventory
The Spare Parts Inventory is tracked and monitored via the CMM System. Spare
parts must be identified, stocked and available when needed for planned work. Just-
in-time delivery plays a larger part in reducing parts inventory and relies on the
suppliers/manufacturers to provide the parts where and when needed. Single sourcing
of certain commodities is also playing a larger part in the purchasing /partnering
strategy to further reduce inventory.
· Procedures
Procedures are the instructions of the "what and how" to complete the work.
These instructions may take the form of Job ProcedureslPurge Procedures and must
identify all safety requirements including such items as confined space entry. A
database of procedures has been developed for all the major work to be performed.
Before the procedures are used, they must be reviewed, updated and revised to ensure
content accuracy.
11-11
. Backlog of Work
The CMMS is required to develop and continually update the outstanding work.
This work will come through the various methods listed in the work identification.
The backlog is used by the Planners to develop packages of work to be ready for
scheduling and execution.
. Type of Work
The type of work can be categorized into three areas:
· Daily
· PM· Shutdown
The daily work requests may require little planning preparation and can be
planned by the First Line Supervisor. Other work requests may involve considerable
detail involving parts, procedures, etc. and would be passed on to the Trades
Planners.
PM work requests for the most part wil require minimal planning, as they
become pait of regular inspection routes.
Shutdown work requests are generated from the CMMS work backlog. Because
shutdown planning is the most in-depth and detailed planning function in our
Ironmaking facility, a guideline document was developed for shutdown planning and
scheduling. This document identifies a standardized approach to planned and
scheduled outages or shutdowns to:
· Eliminate or minimize start-up delays.· Improve safety and efficiency of the workforce.· Improve understanding of the amount and scope of work.
· Provide a basis for monitoring and controlling tasks during shutdown.
11-12
The resulting document defines the roles and responsibilities of the shutdown
team, requirements of a good CMMS work plan and scope, schedule-building
techniques, downloading requirements for scheduling various critical meetings and
follow-up audit.
· Repair/Rebuild Program
A repair/rebuild program must be an integral part of the planning process in order
to improve maintenance. Approximately 70% of all failures can be classified as
"Maintenance Induced". These failures are in part caused by:
· Lack of skills to do the work,· No job procedures. No documented specifications
· Lack of job planning
The repair/rebuild program wil require equipment experts within your
organization working with equipment builders to develop accurate rebuild
procedures. These procedures will contain all pertinent specifications, materials and
measurements to repair the pars or equipment back to original equipment
manufacturer standards.
The benefits of having such a program will reduce maintenance cost by
eliminating re-work, improving equipment life and minimizing unplanned/emergency
maintenance.
c) Scheduling
The scheduling for the routine PM and minor type repair/corrective work is
performed by the First Line Supervisor.
The more complicated jobs are planned by the Planners and scheduled by the First
Line Supervisors with input from the Planner.
11-13
Shutdown Planning requires a much larger project management tool to accurately
define the planning/scheduling needs for our blast furnaces. A scheduling tool "Open
Plan" is utilized in Ironmaking. All backlog work orders are to have accurate task
durations, estimated labour requirements (trades, contractors, and operations) and the
work centre/area. This information, as well as the available Tradespeople is
downloaded in this scheduling software tool.
The Scheduler will add the logic to the plan for each job and the logic/pert
diagram developed for the shutdown.
The First Line Supervisors and Coaches of both Maintenance and Operations
review the logic to develop the final approved logic diagrams.
From this point on, meetings will be conducted to develop crew assignments for
the work using their own internal resources, field service resources and contractors.
d) Work Execution
In the work execution step the resources required to complete the assigned work
are analyzed. In order to optimize our trades resources needed to perform the work,
several coordination groups have been established:
· Field Coordination Team
· Shop Coordination Team· Manpower Sharing Team
In the Shop and Field Coordination Teams, Planners from the Primary Business
Units meet with Operating Services Supervision. Together, these people prioritize
the work sent to Operating Services and develop a plan for the work. Any work that
is entered on a rush basis is usually sent to a contractor or an outside shop.
11-14
The Manpower Sharing Coordination Team was established to provide a resource
sharing pool of Tradespeople to accommodate peak loads, i.e., shutdown, in the
primary facilities of Cokemaking, Ironmaking, Steelmaking and Hotmili. The
purpose of this group is to maximize the use of internal plant personnel and minimize
contractor requirements for major shutdowns.
In a similar way, the Business Unit also allows for flexibility within their
manufacturing facilities. Manpower sharing wil occur between facility teams to
accommodate variation in workloads and work schedules.
Shift maintenance staffing has been reduced to two Tradesmen. This is possible
due to the maturity of the Equipment Reliability Program. Focus should be to
maximize Tradespeople on days to perform planned/scheduled work and reduce shift
size and possibly eliminate supervision.
e) Follow-up
The follow-up step in the maintenance process consists of a number of activities
that immediately take place once maintenance has been executed. These activities
are to:
· Record historical data· Record future work identified during execution
· Revise parts and procedures
· Upgrade equipment
· Record Historical Data
¡i
The work order completion comments must contain key points if they are to be
useful for analysis and improvements in the Equipment Maintenance Program. The
information must be thorough, accurate and identify the problem, explanation of
repairs performed, determine the cause of failure and time required to complete the
work.
11-15
· Record Future Work Identified During Execution
This future work could pertain to other work identified during job execution. It
could also refer to follow-up work - PM, PdM activities that need to be performed to
base line the as installed condition of the equipment. Examples of these PMldM
activities could be vibration testing on rotating equipment, lubrication sample
analysis and on-line motor testing. These activities would be performed to confirm
acceptable limits and base line as installed record data for future monitoring and
trending. Visual inspections may also be part of the follow-up to ensure the
equipment is operating at its desired level of performance.
· Revise Parts and Procedures
During the executions, problems may be encountered with the parts used or
procedures followed to complete the repair. It is imperative that these issues are
corrected to ensure future repairs will not encounter the same pitfalls.
· Equipment Upgrades/Redesign
Follow-up may also include equipment upgrades/redesign to improve equipment
reliability and reduce maintenance and, ultimately costs. This may involve
Technology/Maintenance/Operations personnel in conjunction with outside suppliers
and manufacturers to redesign equipment. A recent example in lronmaking would be
the replacement of the high maintenance cost double drum stockrod winches with the
microwave systems to measure blast furnace burden stockline levels.
It may require a higher initial cost, however, when considering the return on the
investment, the cost of maintenance would far outweigh the initial capital outlay.
11-16
In the planned and scheduled shutdown approach to performing maintenance, the
post audit meetings are a fundamental step for continuous improvement in the
maintenance process. The post audit meeting closes the gap by identifying the
problems encountered during the shutdown, lists solutions, identifies the persons
responsible for the actions, expected completion date and any additional comments.
A database is regularly updated until all actions are completed.
f) Analvsis
Performance measures are the means to identify gaps from your target or
benchmarks and trigger actions to close the gap. These measures should be as close
to the action as possible to motivate people to trigger corrective action in order to get
back on track.
The performance measures for maintenance can be separated into three
categories:
· Functional Maintenance Metrics
· Functional Operations Metrics
· Business Metrics
· Functional Maintenance Metrics
Functional Maintenance Metrics are associated with measuring the best practices
of sound maintenance programs/processes. These best practices are the activities
related to the maintenance process. As a result, a number of projects will be
implemented and measured on the maintenance process steps.
These measures should keep everyone thinking about how well they are executing
best practices and what impact they will have on your business.
11-17
· Functional Operations Metrics
The Functional Operations Metrics are performance measures surrounding the
operating process. These measures involve quality availability and operating rate. At
this level, pareto analysis may be performed to identify the various key factors, i.e.,
equipment, process, raw material, and/or utilities that caused the unscheduled
shutdown, operating rate, or quality defect.
· Business Metrics
The Business Metrics consists of a core set of measures that result from the
functional measures or best practices. These measures are:
· Maintenance Costs
· Equipment Availability· Failure Rate
· Overall Equipment Effectiveness
· Failure Rate
Failure rate is the measure of all equipment failures in the facility. The failures
can be categorized from pareto diagrams to identify areas of opportunity over time.
This number is not as important as the trending of the failures.
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· Overall Equipment Effectiveness
, i
Overall Equipment Effectiveness (OEE) is a measure that reflects how the
equipment is performing overall while it is being operated. This measure takes into
account equipment availability, equipment efficiency and product quality or, in other
words is it operating at its desired rate. The product of these three factors yields a
number that quantifies equipment performance relative to design performance for the
net available time a machine was scheduled to run. The calculated number has no
meaning when comparing to dissimilar processes. It is important however, to
evaluate the trend in GEE measure over time to monitor improvement.
This measure is important to communicate to all the individuals in the Business
Unit who can impact the equipment/facility performance. The GEE measure is a
common measure that includes both the production and maintenance components of a
facility.
In Iron, the measures consist of:
OEE% = Q% xPE% xA%
%Q The appraisal of quality which is the measure of the % of product in
control on hot metal temperatures
%PE The performance efficiency that is the measure of the rate of operation of
the equipment or the percent of time the blast furnace is operating at full wind.
, I
A % The percentage of time that the furnace was not shutdown excluding
inventory stops.
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· External Maintenance Assessment
In addition to the internal performance analysis, an external consultant was
contracted to benchmark Dofasco's capabilities in the areas of maintenance and
reliability.
The consultant has benchmarked numerous companies in North America and
developed a good baseline comparison. The team of consultants worked closely with
our own personnel to evaluate our strengths and improvement areas, and compared
them to world class organizations. From this, the consultant identified opportunities
and assisted in the development of a plan to improve our practices, systems and
procedures. Each Business Unit in Dofasco was evaluated in the areas of:
. Management CommitmentWork IdentificationSafetyInformationScheduling
. QualityCoordinationPlanningPeople
. .
. .
. .
.
11-20
2. PEOPLE
The key to moving the equipment reliability program ahead is the people. Our
Tradesmen are the experts in the trade and equipment knowledge so we must harness
the knowledge and focus everyone on continuous improvement. Everyone must
clearly understand the goals and be committed to continuous improvement toward
them. This requires a change in the culture from a reactive approach to a pro-active
approach in maintenance. It must be supported by Operations, Technology and
Maintenance people within all levels of the Business Unit and the Company.
. Team Development
To demonstrate the commitment to changing the culture, all employees were senI
to various sessions on team development and empowerment. Training sessions such
as Pecos River and Athenium training were used to help execute the change. This
provided opportunities to breakdown the traditional trade/department bamers and
refocus on equipment reliability using a team approach.
Manufacturing trios or teams in the Ironmaking Business Unit were developed
consisting of Maintenance, Operations and Technical Supervisors for each operation
facility including No.3 Blast Furnace, No.4 Blast Furnace and the Hot Metal areas.
This developed a sense of ownership by the cross-functional groups down to and
including the floor leveL.
· Trade/Technology Training
The Tradespeople's skills must be continually upgraded to keep in tune with
technological changes. It requires attendance at Ironmaking conferences to
benchmark other steel plants. Similarly, the Tradesmen must also keep ahead of the
predictive maintenance developments in technology, equipment and techniques.
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. Communications
Communication is an important factor in order to develop and maintain
commitment to change. Regular lunchroom meetings, equipment reliability
newsletters, performance measurement graphs are tools used to identify the
improvements and the achievements. Remember, celebrate the successes as they
were accomplished by your people. This will help to heighten awareness, increase
participation and the sense of ownership.
3. STRUCTUR
The last piece of the puzzle for equipment reliability is the Business Unit
maintenance organizational structure. Equipment reliability must be a separate
function within maintenance, divorced from the day-to-day planned corrective work.
The Equipment Reliability Group is a headed by a Maintenance Coach and together
the group's mandate is:
· PM Inspection· Lubrication· Data Analysis· PreventativelPredictive/Technology Application and Analysis
· Shutdown Planning and Scheduling
. PM Inspection
A group of Tradespeople of the various trade disciplines perform pre-identified
inspections and record findings on check sheets or input to Hand Held Data Loggers.
They wil make minor adjustments and repairs. However, their primary responsibility
is the PM Program. Anything found during the inspection wil be put on the work
requests and sent to the appropriate Trade Planner/Supervisor to perform the
necessary repairs. Each Tradesperson wil have a facility responsibility to assist in
the development of ownership.
11-22
. Lubrication
The lubrication team is also assigned to individual facilities. They are responsible
for daily, weekly, monthly time-based lubrication. In addition to lubrication, they
perform sampling on critical hydraulic and oil lubrication systems for analysis. A
First Line Supervisor/Teamleader will work with the team to identify, interpret and
trend lubrication laboratory analysis and develop action plans to identify root causes
and eliminate the problem. He wil also work closely with the various trades and the
vibration analysis to monitor unusual vibration and oil related issues.
· Data Analysis
The data analysis is involved with reviewing the various information collected
and working with the numerous reliability specialists to identify areas for opportunity
to improve the equipment reliability program. From the various information collected
through inspection, the reliability specialists can analyze and trend critical equipment
measurements and work with Tradespeople/Planners to develop corrective action
plans before failures occur. They wil use the different tools, i.e., reliability centre
maintenance, and root cause failure analysis to develop an appropriate equipment
maintenance program.
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· Preventative/Predictive Technology Application Analysis
This group consists of specialized Tradespeople/Technicians in the various trade
areas whose primary purpose are to investigate analyze and implement Predictive
Maintenance Technologies and to continuously improve the Equipment Reliability
Program. This group consists of:
. Vibration Specialist
. Electrical Specialist
. Alignment Specialist
. Instrumentation Specialist
. Hydraulic Team
. Repair/Rebuild Team
. ICMS Development Team
The ICMS Development Team consists of a full time equipment specialist
working with various Tradespeople from the Floor and Reliability Teams. This team
is responsible for implementing the ICMS Program to the various equipment in the
facilities.
· Shutdown Planning and Scheduling
Two Schedulers work with the Equipment Reliability Group, daily Planners and
Supervisors to plan and schedule all the work on backlog. For a shutdown, a logic
diagram and critical path items are identified to determine the duration of the
shutdown. The group monitors work at the furnace during the shutdown execution
and continually update the logic diagram as work is completed. This ensures all work
is completed and assists in identifying the importance of adherence to the scheduled
sequencing.
11-24
CONCLUSION
The implementation of an Equipment Reliability Program wil positively impact
the bottom line at your facilities. You must, however view maintenance as a process
and ensure that you have the dedicated personnel and structure in place to support the
program.
The end result wil change the perception of maintenance from an expense to
being recognized as an integral part of the manufacturing organization. Results will
be:
· Throughput increase caused by equipment reliability· Safety Program Improvement
. Personnel development and improved efficiency
· Quality improvements
· Cost reductions on parts inventory, contracts and maintenance
Effective implementation of an Equipment Reliability Program has resulted in an
increase in intervals between blast furnace maintenance stoppages from two weeks to
eight weeks to align with steelmaking KOBM bottom change/vessel relines.
Opportunities for further increasing shutdown intervals are realistic as the Equipment
Reliability Program continues to mature and the steelmaking shop continues to
implement new technology to increase vessel life.
11-25
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