iron ore pellets and pelletizing processes

8/17/2019 Iron Ore Pellets and Pelletizing Processes

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Iron ore pellets and Pelletizing processes

Pelletizing is a process that involves mixing very finely ground particles of iron ore fines of sizeless than 200 mesh with additives like bentonite and then shaping them into oval/spherical lumps

of 8-16 mm in diameter by a pelletizer and hardening the balls by firing with a fuel. It is the

process of converting iron ore fines into “Uniformed Sized Iron Ore Pellets” that can be chargeddirectly into a blast furnace or into a furnace used in the production of Direct Reduced Iron(DRI). The pellets are shown in Fig 1

Fig 1 Iron ore pellets

The typical properties of the pellets is given in Table 1

Table 1 Properties of Pellets

Chemical analysis(On dry basis) Unit Value Tolerance Fe % 65 Minimum

FeO % 0.3 Maximum

SiO2 + Al2O3 % 5 MaximumCaO % 0.03 ± 0.01MgO % 0.06 ± 0.01

Basicity % 65 MaximumPhosphorus % 0.05 Maximum

Physical properties Bulk Density t/Cum 2.2 ± 0.2

Tumbler Index % 93 MinimumAbrasion Index % 3 Min ± 0.5 %

Cold Crushing Strength (Avg) Kg/P 250 Minimum

Size analysis

8 – 16 mm 94 Minimum-5 mm % 2 Maximum

+16 mm % 4 Maximum

Metallurgical properties Porosity % 18 MinimumReducibility % 62 Minimum\

Process technology


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There are four stages involved in the production of pellets. They are:

1. Raw material preparation.2. Formation of green balls or pellets

3. Induration of the pellets

4.

Cooling, storage and transport of pellets

During the process for pelletization iron ore concentrate from iron ore beneficiation plant is dried

and heated to about 120 deg C. The dried material is fed to the ball mill for grinding. Concentrate/ ground iron ore of typical size 80% sub 45 microns is required to be at 9% moisture. Suitable

binder (Bentonite) is added to the concentrate and is thoroughly mixed in high intensity mixer.The wet material is fed to the disc pelletizer which is rotated at a pre determined rpm and at an

inclination. Additional water is sprayed on to the disc in which the material is coagulated and bycontinuous rotary motion forms into nodules/ pellets. These pellets are called green pellets as

they do not have the required strength. The process of making of pellets consists of:

Ball Formation – Surface tension of water & gravitational force creates pressure on particles, sothey coalesce together & form nuclei which grow in size into ball. The forces responsible for theagglomeration of iron ore fines are surface tension and capillary action of water and gravitational

forces of particles due their rotation in balling unit. When the solid particles come in contact withwater, the ore surface is wetted and coated with water film. Due to the surface tension of water

film, liquid bridges are formed. As result of the movement of particles inside the balling unit andof the combination of the individual water droplets containing ore grains, first agglomerates,

called seeds, are formed. The liquid bridges in the interior of these seeds hold them together as ina network. With the further supply of water, the agglomerates condense and become denser.

Capillary forces of liquid bridges are more active in this stage of green ball formation. Theoptimum of this ball formation phase is attended when all the ports inside the balls are filled with

liquid. When the solid particles are fully coated with water, the surface tension of water droplets becomes fully active dominating the capillary forces. Besides this effect, the rolling movement

of grains and movement or shifting of particles relative to each other plays an important role.

Pelletizing in discs – Green pellets with a size range of 8-16 mm are prepared in balling drum ordiscs. Discs are preferred to produce quality green pellets as these are easy to control operation

with minimum foot space. The disc is an inclined pan of around 5 to 7.5 meters diameter rotatingat around 6 to 8 rpm. The inclination of disc is around 45 Deg. and it can be adjusted in the off-

line between 45 deg. to 49 deg. The pre wetted mix is fed into the disc at a controlled rate. Orefines are lifted upwards until the friction is overcome by gravity and the material rolls down to

the bottom of the disc. This rolling action first forms small granules called seeds. Growth occursin the subsequent revolutions of the disc by the addition of more fresh feeds and by collision

between small pellets. As the pellet grows in size, they migrate to the periphery and to the top ofthe bed in the discs, until they overflow the rim. Pellet growth is controlled by the small amount

of water sprayed in the disc and the adjustment in the disc rotational speed. The green pellets arethen screened in a roller screen and the required size material is fed to the traveling grate.

Induration (Heat Hardening) – Pellet induration consists of three main steps:


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1. Drying of green pellets.2. Firing of pellets at around 1300 Deg. C to sinter the iron oxide particles.

3. Cooling of hot pellets before discharging.

During drying (180-350 deg C), moisture content in the green pellet is evaporated. Surface and

interstitial moisture evaporates at lower temperatures where as chemically combined water (asgoethite or limonite) or any hydrate or hydroxide combinations lose their water at slightly highertemperature. During pre-heating stage (500–1100 deg C), decomposition of carbonates and

hydrates takes place. Gasification of solid fuels like coal or coke and conversion of iron oxideslike goethite, siderite to higher oxide state, hematite, also takes place. Commencement of solid

oxide bonding and grain growth are the important steps in this stage. During firing stage (1250 -1340 deg C), the temperature is below the melting temperature of major oxide phase but within

the reactivity range of gangue components and additives. Formation of oxides and slag bonds isdecisive in this stage. Bonding of mineral grains developed during induration of pellets is

affected by the following factors:

1.

Solid oxide bonding: Oxidation of ferrous iron oxides to ferric iron oxides results in bonding and bridging, but only to limited amount.

2. Re-crystallization of iron oxides: Essentially a physical process in which smaller particlesconsolidate into larger ones with the loss of surface energy. Continued growth of iron

oxide crystals imparts sufficient strength. Grain growth for hematite starts at around 1100deg C.

3. Slag bonding: Gangue by forming melt transport medium for ferrous or ferric oxidesfacilitate grain growth and crystallization of oxide grains. It also enables the mechanism

to proceed at lower temperatures than would be required in its absence.

This treatment (Induration) causes certain chemical reactions to occur that change pellet’s

specific metallurgical properties. These reactions may include the oxidation of magnetite anddehydration of earthy hematite. For BF grade “fluxed pellets” are produced by additions of

limestone, dolomite, silica, etc. to the balling feed. These additions react with the gangue in theiron ore to enhance the performance of the pellets in certain downstream processing steps.

Pellet cooling and handling – The pellets are then cooled and screened. The over size is crushed

and along with the undersized is sent to the stock house bins where they are reprocessed.

Pelletization processes

There are several iron ore pelletizing processes/technologies available. Some of them are Shaft

Furnace Process, Straight Traveling Grate Process, Grate Kiln Process, Cement Bonded Process(Grangcold Process, MIS Grangcold Process, Char process etc. and Hydrothermal Processes,

(COBO Process, MTU Process, INDESCO Process) etc. However, currently, Straight TravelingGrate (STG) Process and Grate Kiln (GK) Process are more popular processes

The Straight Travelling Grate process (Fig 2) developed by former Lurgi Metallurgie accountsfor world’s major installed capacities. In this process a double deck roller screen ensures right

size of green pellet (8-16 mm) is evenly distributed across the width of the travelling grate. The


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grate carries the green pellets on a bed 300 mm to 550 mm thick through a furnace withupdrafting, downdraft drying, preheating, firing, after firing and heating zones.

Fig 2 Straight Travelling Grate Process- Typical flow sheet

The Grate Kiln Process was developed by Allis Chalmer and the first plant on this technologywas constructed in 1960. In the Grate-Kiln process (Fig 3) the traveling grate is used to dry and

preheat the pellets. Material moves on straight travelling grate till it attains the temperature of800°C to 1000°C. Thereafter it is transferred to refractory lined rotary kiln for induration where

the temperature is further raised up-to 1250 – 1300°C. At 800°C, the Magnetite iron Ore getsconverted into Fe2O3 in an exothermic reaction. The liberated heat hardens the green balls which

is helpful to withstand the tumbling impact in the rotary kiln. A circular cooler is used forcooling of the fired pellets.


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Fig 3 Grate Kiln process – Typical flow sheet

A comparison of the two processes is given in the Table 1

Table 1-Comarison between Straight Traveling Grate (STG) and Grate Kiln (GK) ProcessesSl No. Straight Traveling Grate (STG) process Grate Kiln (GK) Process 1 Drying , preheating, Induration and cooling

cycle is carried out in a single unitDrying , preheating, Induration and coolingcycle is carried out in different units

2Green pellets remain undisturbed during the

process

Entire process takes place in three equipmentsnamely travelling grate, rotary kiln and

circular cooler hence pellets transfer takes place.

3 Grate cars moves at the same speed in thedrying , induration and cooling zones. Any

disturbance in one zone affects the other zones

Independent control of the three zones hencehave better operational flexibility

4 Fines generation is negligible since there is no

transfer of materials

Since material transfer takes place at several

places hence higher generation of fines5 There is no strength requirement of

intermediate product

Before transfer to the Kiln the green pellets are

to be sufficiently hardened6 Process availability is higher Lower process availability

7 Higher specific energy consumption Lower specific energy consumption8 Lesser Maintenance Higher Maintenance


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9 Lower dust generation Higher dust generation10 Higher investment cost Lower investment cost

11 Suited both for hematite and magnetite ores Process is more suited for magnetite ores..

A comparison of temperature distribution during the two processes is shown in Fig. 4

Fig 4 Temperature distribution during the two proce

Advantages of Pellets

Iron ore pellet is a kind of agglomerated fines which has better tumbling index when comparedwith the iron ore and it can be used as a substitute for the same both in the blast furnace and for

DR production.

1. Pellets have good reducibility since they have high porosity (25-30%). Normally pelletsare reduced considerably faster than sinter as well as iron ore lumps. High porosity also

helps in better metallization in DRI production.2. Pellets have a uniform size range generally within a range of 8 -16 mm.

3. Pellets have spherical shape and open pores which give them good bed permeability.4. Pellets have low angle of repose which is a drawback for pellet since it creates uneven

binder distribution.5. The chemical analysis is uniform since it gets controlled during the beneficiation process.

Fe content varying from 63% to 68% depending on the Fe content of Ore fines. Absence

of LOI is another advantage of the pellets.6. Pellets have high and uniform mechanical strength and can be transported to longdistances without generation of fines. Further it has got resistance to disintegration. High

mechanical and uniform strength of pellets is even under thermal stress in reducingatmosphere


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Understanding Pellets and Pellet Plant Operations

Pelletizing is an agglomeration process which converts very fine grained iron ore into balls of acertain diameter range (normally 8mm to 20 mm, also known as pellets. These pellets are

suitable for blast furnace and direct reduction processes. Pelletizing differs from sintering in thata green unbaked pellet or ball is formed and then hardened by heating.

Iron ore pellets can be made from beneficiated or run of mine iron ore fines. Lean iron ores are

normally upgraded to a higher iron ore content through beneficiation. This process generatesiron ore filter cake which needs to be pelletized so that it can be used in an iron making process.

Also during the processing of high grade iron ores which do not need beneficiation, generatedfines can be pelletized and used instead of being disposed of.

Pellet plants can be located at mines, near ports or can be attached to steel plants. Equipped withadvanced environmental technology, they are virtually pollution free, generating no solid or

liquid residues.

History of pelletization

The history of pellets began in 1912 when A.G.Andersson, a Swede, invented a pelletizingmethod. The commercial use of pellets, however, began in the USA after World War. Various

studies were conducted in USA with the aim of developing the vast reserves of taconite (a lowgrade iron ore) in the area around the Great Lakes. The process of enriching taconite ore

involved grinding the ore to remove gangues and upgrading the iron ore (i.e., an ore beneficiation process). The resultant high grade ore is in the form of fine particles, as small as

0.1 mm or less, which are not suitable for sintering. This issue led to the development of the pelletizing process.

In 1943, Dr. Davis, a professor at the University of Minnesota, Mines Experiment Station, andhis associates invented a method for processing taconite containing low grade iron ore. Their

invention showed that it was possible to ball or pelletize fine magnetite concentrate in a ballingdrum and that if the balls were fired at sufficiently high temperature (usually below the point of

incipient fusion) a hard, indurated pellet well adapted for use in the blast furnace, could be made.Consequently, despite the unquestioned benefits of sinter on blast furnace (BF) performance,

intense interest in the pelletizing process had developed because of the outstanding performanceachieved by steel plants in extended operations with pellets as the principal iron bearing material

in the blast furnace burden.

Pelletizing plants are expected to play an important role in an era when the global reserve of highgrade lump ore is shrinking. The plants promote the concentrating of low grade iron ores into

upgraded pellets, which will be increasingly used by blast furnaces and direct reduction furnacesin coming years.

Iron ore pellets


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The iron ore pellets may be acid or basic pellets. Acid pellets are also called as DRI (directreduced iron) grade pellets while basic pellets are also known as BF grade or fluxed pellets.

DRI grade pellets – Basicity of these pellets is usually less than 0.1. The fired pellet

strength is, to a certain degree, due to the hematite bridges of polycrystalline structure.

These pellets normally have large volume of open pores. The reduction gas quickly penetrates through these pores into the pellet core and simultaneously attacks thestructure in many places. This results into an early structural change which begins at low

temperatures over the entire pellet volume. BF grade pellets – Basicity of these pellets is greater than 0.1 and can vary. Basicity of

normal basic pellets range from 0.1 to 0.6 and have low CaO percentage. During thefiring of these pellets, a glassy slag phase consisting of SiO2, CaO, and Fe2O3 of varying

percentage is formed. Due to increased flux addition, there is formation of some slag anddue to it, there is to a certain extent slag bonding with iron ore crystals. High basicity

pellets have a basicity level greater than 0.6. These pellets contain higher level of CaO.These pellets not only have glassy phase consisting mainly of SiO2, CaO, and Fe2O3, but

also calcium ferrites (CaO.Fe2O3). During firing of these pellets, the availability of CaOconsiderably favours the crystal growth of hematite. These pellets normally have a high

mechanical strength after pellet firing. Fluxed pellets exhibits good strength, improvedreducibility, swelling and softening melting characteristics. Because of these properties

these pellets give better performance in the blast furnace.

Quality of the pellets is influenced by the nature of the ore or concentrate, associated gangue,type and amount of fluxes added. These factors in turn result in the variation of physicochemical

properties of the coexisting phases and their distribution during the pellet induration. Hence properties of the pellets are largely governed by the form and degree of bonding achieved

between the ore particles and the stability of these bonding phases during reduction of ironoxides. Since the formation of phases and microstructure during induration depends on the type

and amount of fluxes added, there is an effect of fluxing agents in terms of CaO/SiO2 ratio andMgO content on the pellet quality.

Mineralogically pellets comprise essentially hematite (original surviving) particles of iron ore,crystalline silica (quartz, cristobalite and tridymite) and forsterite (Mg2SiO4). The principle

variation in pellet mineralogy is in the proportion of gangue phases present in the product. Thesewill vary depending upon the pellet feed material and the type and the amount of any additives to

feed such as limestone, dolomite, olivine and bentonite etc.

The strength of iron ore pellets is important in minimizing degradation by breakage and abrasionduring handling and shipping, and in the blast furnace. Strong bonding in pellets is believed to be

due to grain growth from the accompanying oxidation of magnetite to hematite, orrecrystallization of hematite. Although slag bonding may promote more rapid strengthening at

slightly lower firing temperatures, pellet strength is normally decreased, especially resistance tothermal shock. Pellet strength is most commonly determined by compression and tumble tests.

Compressive strengths of individual pellets depend upon the mineralogical composition and physical properties of the concentrate, the additives used, the balling method, pellet size, firing

technique and temperature, and testing procedure. The compressive strengths of commercially


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acceptable pellets are usually in the range of 200 to 350 kg for pellets in the size range of 9 mmto 18 mm. In the tumbler test 11.4 kg of +6 mm pellets are tumbled for 200 revolutions at 25 rpm

in a drum tumbler(ASTM E279-65T) and then screened. A satisfactory commercial pellet shouldcontain no more than about 5 % of minus 0.6 mm (minus 28 mesh) fines, and 94 % or more of

plus 6 mm size, after tumbler testing. A minimum of broken pellets between 6 mm and 0.6 mm

in size is also desirable. Other important properties of the pellets to be used for blast furnace feedare reducibility, porosity, and bulk density. With some concentrates these can be varied withincertain limits.

Palletization process

A pelletizing plant normally has four process steps namely (i) receipt of raw materials, (ii)

pretreatment, (iii) balling, and (iv) induration and cooling. These process steps are described below.

Receipt of raw materials

The location of a pelletizing plant affects the method of receiving raw materials such as iron ore,additives and binders. Many pelletizing plants are located near iron ore mines. This is because

these plants are installed to pelletize the iron ores which are beneficiated at these mines. Such plants receive the iron ore by rail and/or slurry pipelines. Many other pelletizing plants are

installed away from the iron ore mines. These plants are independent of iron ore mines. These plants receive iron ore mostly by rails. some plant may receive by long distance slurry pipeline.

In pelletizing plants located at port which are dependent on imported iron ore, the receivingmethod involves the transportation of the ore in a dedicated ship, unloading the ore at a quay and

stockpiling it in a yard. Iron ore is usually shipped for such plants in bulk for maximumeconomy.

Pretreatment process

In the pretreatment process, the iron ore is ground into fines having sizes required for thesubsequent balling process. The pretreatment includes concentrating, dewatering, grinding,

drying and pre-wetting. Generally low grade iron ores are ground into fines for enriching thequality of the ore, for removing gangues containing sulfur and phosphorus, and for controlling

the size of the grains. In the case of magnetite ores, magnetic separators are employed forupgrading and gangue removal. On the other hand , with hematite ores, these operations are

accomplished by gravity beneficiation, flotation, and/or wet-type, high intensity magneticseparators. The grinding methods can be categorized roughly as per the following three aspects.

Dry grinding or wet grinding

Closed circuit grinding or open circuit grinding Grinding in single stage or grinding in multiple stages

These methods are used in combination depending on the types and characteristics of the ironores and the mixing ratio, as well as taking into account the economic factors. Wet grinding

systems need dewatering units with a thickener and filter, while dry grinding systems requires


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pre-wetting units. Pre-wetting is usually associated with dry grinding. Pre-wetting includesaddition of an adequate amount of water homogeneously into the dry-ground material to prepare

pre-wetted material suitable for balling. This is a process for adjusting the characteristics of thematerial that significantly affect pellet quality. Occasionally, the chemical composition of the

product pellets is also adjusted in this process to produce high quality pellets.

Binders, such as bentonite, clay, hydrated lime or an organic binder, are generally used to raisethe wet strength of green balls to more acceptable levels for handling. Bentonite consumption at

the rate of 6.3–10 kg per ton of feed is a significant cost element and adds to the silica content ofthe final product.

Addition of lime and/or dolomite to the ore adjusts the pellets so as to have the target chemical

composition.

Considerable efforts have been made for the reduction of the bentonite usage and for the

development of cheaper substitutes. The ballability and strength of green balls are influenced by

the additives and by the moisture content and particle size distribution of the concentrates.Optimum moisture content for good balling is usually in the range of 9 % to 12 %. It appears that balling characteristics are relatively independent of the chemical composition of a concentrate,

but are strongly affected by its physical properties. For example, specular hematites are moredifficult to ball than magnetite concentrates because of the plate like structure of the specular

hematite particles. In any case, satisfactory pellet formation is usually achieved by grinding toabout 80 % to 90 % minus 43 micro meter ( minus 325 mesh). Normally, any material

considered for pelletizing should contain at least 70 % minus 43 micro meter (minus 325 mesh)and have a specific surface area (Blaine) greater than 1200 sq cm/gram for proper balling

characteristics.

Balling process

In this process, balling equipment produces green balls from the pre-wetted material prepared inthe previous process. The balling drum and the disc pelletizer are the most widely used devices

for forming green balls. Both of the units utilize centrifugal force to form the fine materials intospheroids.

The green balls produced by a drum are not uniform in diameter. A significant portion of thedischarge (about 70 %) is smaller than target size and are usually returned to the drum after

screening. It is difficult to adjust the drum operation for varying raw material conditions. Theoperation, however, is stable for uniform raw material conditions (chemical composition, particle

size, moisture, etc.).

Compared with the balling drum, the disc pelletizer has the advantages of lighter weight andgreater possibility for adjustment. Its inherent design averages out the effect of instantaneous

fluctuations in the feed, whereas the drum cannot. The disc pelletizer classifies green balls byitself, reducing the amount of pellets returned. The classifying action of the disc promotes

discharge of balls of more uniform size, which simplifies screening of the product. The operationof the disc pelletizer can easily be adjusted for varying raw material conditions by changing the


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revolution, inclined angle and depth of the disc. However, the capacity of the discs is low anddiscs generally require closer control than drums.

Best control of ball size is achieved when the balling device is in closed circuit with a screen to

remove and recycle the undersize material. Both the drop and compressive strengths of green

pellets are important.

Induration process

The firing of pellets establishes the binding of hematite particles at an elevated temperatureranging from 1250 deg C to 1350 deg C in oxidizing condition. Slag with a low melting point

may form in the pellets during this firing step, if the raw material contains fluxed gangue, or iflimestone is added to it. In these cases, the product may have an intermediate structure with both

hematite binding and slag binding. The firing process is characterized by process temperatureslower than those required by sintering which requires partially melting and sintering fine ore

mixed with coke breeze, a fuel which generates combustion heat.

Three systems are normally used for the induration of pellets. They are namely (i) vertical shaftfurnace system, (ii) straight grate or travelling grate system, and (iii) grate – kiln cooler system.

Each system has been used commercially to make acceptable quality pellets and thus, capital andoperating cost factors are usually involved in choosing one or the other system.

Oxidation of magnetite to hematite during pelletizing will provide a significant proportion,

around 100 M cal per ton of the heat requirement in all of the systems. For pelletizing ofhematites, the use of coke breeze (or some carbon source) in the pellet feed mixture has become

a common practice to provide the additional indurating energy normally provided by magnetiteoxidation.

Vertical shaft furnace system is the most traditional facility. However, vertical shaft furnaces arenot as common as the traveling grate or grate kiln systems. There are several variations in shaft

furnace design but the most common is the Erie type, shown in Fig. 1. Green balls are charged atthe top and descend through the furnace at a rate of 25 to 40 mm per minute countercurrent to the

flow of hot gases. About 25 % of the total air enters the furnace through the hot gas inlet attemperatures from 1280 deg C to 1300 deg C. Pellets in this zone of the furnace reach

temperatures of 1315 deg C or higher because exothermic heat is released when the magnetiteoxidizes to hematite, increasing the temperature. The remaining 75 % of the furnace air enters

via the cooling air inlet. Pellets discharge at about 370 deg C, and the top gas temperature isaround 200 deg C. Typical furnace capacities are 1000 to 2000 tons per day.

Shaft furnaces are more energy efficient than the traveling grate or grate-kiln systems. The shaft

furnace is well suited for pelletizing magnetite, but not hematitic or limonitic ore materials.Disadvantages of shaft furnaces are low unit productivity and difficulty in maintaining uniform

temperature in the combustion zone. Hot spots may occur which cause pellets to fuse togetherinto large masses, producing discharge problems. It is also very difficult to produce fluxed

pellets in a shaft furnace. Typical schematic diagram of vertical shaft furnace system is shown inFig 1.


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Fig 1 Typical schematic diagram of vertical shaft and grate-kiln systems

A straight grate system emerged in the industry soon after the shaft furnaces. It is essentially amodification of the sintering process. The green balls are fed onto the grate continuously to give

a bed depth of around 300 mm to 400 mm and are dried in the first few wind boxes by updraft airrecuperated from the firing zone, followed by downdraft drying using recuperated air from the

cooler. This arrangement of hot air flows limits pellet damage resulting from condensation ofmoisture in the bed. Following drying, the pellets are preheated by downdraft air from the

cooling zone. Firing is done downdraft in the combustion zone by burning fuel oil or natural gaswith hot air from the cooling zone. The cooling zone follows the combustion zone and uses

updraft fresh air.

The traveling grate system for producing pellets consists of a single unit which moves a staticlayer of pellets. The system has a simple structure for drying, preheating, firing and cooling

pellets. Due to its relative ease of operation, along with ease of scaling-up, makes the systemused by many plants.

Fuel consumption in the traveling grate system is about 85-140 M cal per ton of pellets producedfrom magnetite and up to 240 M cal per ton when pelletizing hematite. The system offers good

temperature control in the firing zone. Pellet consistency throughout the bed may be achieved byrecirculating some fired pellets to form hearth and side layers on the grate. The large grate

machines are 4 m wide and are capable of producing more than 3 million tons of pellets per year.Circular grate machines have also been designed and are in operation. A typical schematic

diagram of the straight grate system is shown in Fig 2.


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Fig 2 Typical schematic diagram of straight grate system

The grate-kiln system depicted in Fig 1 consists of a traveling grate for drying and preheating the

pellets to about 1040 deg C, a rotary kiln for uniformly heating the throughput to the finalinduration temperature of 1315 deg C, and an annular cooler for cooling the product and heat

recuperation. Heat for firing is supplied by a central oil, gas, coal or waste wood burner at thedischarge end of the kiln. Hot gases produced in the kiln are used for downdraft preheating of the

pellets. Hot air from the cooler is used to support combustion in the kiln and is also recuperatedto the traveling grate for drying and tempering preheat.

The grate-kiln system offers excellent temperature control in all stages of the process and produces a consistently uniform quality pellet. Fuel consumption is 75 M cal to 100 M cal per

ton of standard pellets produced when using magnetite ore, and up to 170 M cal per ton ofstandard pellets produced when the feed is hematite. These fuel consumption numbers increase

by 60 M cal per ton when producing fluxed pellets. Power consumption, from balling to pelletload out, is around 23 kWh per ton.


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The grate-kiln system is easy to control, and the product pellets have a uniform quality. It canalso be scaled up to a fairly large degree. Grate-kiln systems can be designed for production

capacities up to 6 million tons per year per line. These systems are used by many plants.

Pelletizing processes are being improved constantly. The production of self-fluxing pellets is an

example of an innovation that has been accepted on a commercial scale and has led to majoradvances in blast furnace performance. Other articles on pellets and pelletization process areavailable under following links.

Use of Iron Ore Pellets in Blast Furnace Burden

Pelletizing is a process that involves mixing very finely ground particles of iron ore fines of sizeless than 200 mesh with additives like bentonite and then shaping them into oval/spherical lumps

of 8-20 mm in diameter by a pelletizer and hardening the balls by firing with a fuel. It is the process of converting iron ore fines into ‘uniformed sized iron ore pellets’ that can be charged

directly into a blast furnace. Fig 1 shows iron ore pellets.

Fig 1 Iron ore pellets

There are several iron ore pelletizing processes/technologies available. However, currently,

straight traveling grate (STG) process and grate kiln (GK) process are more popular processes.

The physical properties of iron ore pellets are given below.

• Size – 8-20 mm

• pH (40 gm/L, 20 deg C; slurry in water) – 5.0 – 8.0

• Melting point – 1500-1600 deg C

• Bulk density – 2.0 -2.2 t/Cum


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• Tumbler index (+6.3 mm) – 93-94 %

• Abrasion index (-0.5 mm) – 5-6 %

• Compression strength (daN/p) – Around 250

• Porosity – > 18 %

The chemical analysis of iron ore pellets is given below.

BF grade DRI grade

Fe % 63 – 65.5 65 -67.8

SiO2 + Al2O3 % < 5


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3. Bed shrinkage at 80 %reduced

Quality of the pellets is influenced by the nature of the ore or concentrate, associated gangue,

type and amount of fluxes added. These factors in turn result in the variation of physicochemical

properties of the coexisting phases and their distribution during the pellet induration. Hence properties of the pellets are largely governed by the form and degree of bonding achieved between the ore particles and the stability of these bonding phases during reduction of iron

oxides in the blast furnace. Since the formation of phases and micro structure during indurationdepends on the type and amount of fluxes added, there is an effect of fluxing agents in terms of

CaO/SiO2 ratio and MgO content on the pellet quality.

History of use of iron ore pellet in blast furnace

The history of pellets began in 1912 when A.G. Andersson, a Swede, invented a pelletizing

method.

The commercial use of pellets, however, began in the USA after World War 2. Various studieswere conducted with the aim of developing the vast reserves of taconite in the area around the

Great Lakes. In 1943, Dr. Davis, a professor at the University of Minnesota, Mines ExperimentStation, invented a method for processing taconite containing low grade iron ore. His process

involved grinding taconite to remove gangues and upgrading the iron ore (i.e., an ore beneficiation process). The resultant high grade ore is in the form of fine particles, as small as

0.1 mm or less, which are not suitable for sintering. This issue led to the development of pelletizing of these fine particles. Pelletizing plants today play an important role in an era when

the global reserve of high grade lump ore is shrinking. The plants promote the concentrating oflow grade ore into upgraded pellets, which are increasingly being used by blast furnaces and

direct reduction furnaces.

The US iron making has historically been based to a large extent on pellets primarily because allthe local iron ores needed beneficiation (up grading) by grinding it to fine particles (< 0.1 mm)

and agglomerating these fines into pellets but also because sinter plants are not used due toenvironmental reasons.

Iron ore pellets are now the single largest source of iron for North American blast furnaces.Pellets constitute about 70 % of the blast furnace burden. Initially acid pellets (DRI grades) were

produced and used in blast furnaces.

In the mid 1980s, a number of pelletizing and blast furnace trials were conducted to evaluate the benefits of limestone dolomite fluxed pellets. Towards the end of the decade, fluxed pellets had

been firmly established as a major product, accounting for about 30 % of North America’s pellet production.

The transition to fluxed pellets involved many changes in plant equipment (e.g., flux grindingmills, pre-heat burners) and practices. Each pellet plant has customized the fluxed pellet

chemistry to meet its customer’s blast furnace operation. As a result, there are pellet plants


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producing as many as four grades of pellets. In North America, pellets have changed from acommodity in the early 1970s to a custom made product, meeting a customer’s specific most

demanding specifications in the 1990s. North American fluxed pellets are now equivalent to the best sinter in terms of reducibility and softening meltdown properties and are superior in terms of

strength and low temperature breakdown (LTD/RDI).

Advantages of pellets

Iron ore pellet can be used as a substitute to sinter and calibrated lump ore in the blast furnace burden due to the following properties.

Spherical shape and open pores of pellets gives better and uniform permeability resultingin smoother furnace operation. Pellets have a uniform size range generally within a range

of 8 – 20 mm. Pellets have very high cold crushing strength resulting in negligible generation of fines in

stock house.

High porosity (greater than 18 %) leads to faster reduction High strength of pellet (around 250 daN/p ) provides good resistance to disintegration

during the descent of burden. It has better tumbling index when compared with the

calibrated iron ore. Uniform chemical composition compared to calibrated lump ore. Absence of LOI is

another advantage of the pellets.

The swelling index of pellets is an Important of metallurgical property. Swelling indicatesvolume change of pellets during reduction. The volume expansion of pellets during the reduction

results in lower compressive strength of pellets. High swelling inside the furnace causes increasein volume of the pallet which in turn decreases voids in charge. This Impedes gas flow in the

furnace and results into pressure drop. This in turn causes burden hanging and slipping inside the blast furnace. The addition of dolomite is favorable for the improvement in swelling property of

pellets. Maximum allowable swelling of pellets for the blast furnace ranges from 16 % to 18 %.Acid pellets (DRI pellets) and MgO free pellets exhibit higher swelling.

Fluxed pellets can be produced as equivalent to the best sinter in terms of reducibility and

softening meltdown properties and are superior in terms of strength and low temperature breakdown (LTD/RDI). Fluxed pellets exhibit good strength, improved reducibility, swelling and

softening melting characteristics. Because of these properties fluxed pellets give better performance in the blast furnace.

Metallics in BF burden

Sinter, pellet and the calibrated lump ores are the three iron bearing metallics normally used inthe blast furnace burden. Uses of all the three metallics in blast furnace burden can vary from

zero to hundred percent with adjustment of the furnace parameters. These three metallics can beused in any combination of two metallics or three metallics. There is no standard formula for the

choice of metallics. The choice of metallics depends on several factors varying from plant to plant. Some of the factors influencing the choice of metallics are given below.


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Availability of the metallic material of correct specification Metallurgical properties of metallic materials such as the reducibility, thermal

decrepitation properties and softening properties Relative cost of the metallic material

Effect of the use of metallic on the overall production cost of hot metal

Possibility of adjustment of various furnace parameters such as distribution pattern, fuelrate etc. Possibility of adjustment in process control parameters

Facilities available in furnace stock house Availability of captive sinter plant

Type of iron ore available in the captive mine or nearby area of the blast furnace Issues related with the smooth operation of the BF without hanging and slipping

Issues related to environment

Pellets in blast furnace can be used from 0 % to 100 %. There is no standard solution forincreasing the pellet content in the blast furnace burden. Every location and each furnace will

have different issues which need to be identified, analyzed and solutions are to be found for toarrive at the maximum amount of the pellet content which can be used in the BF burden. The

objective should always be to have trouble free blast furnace operation at the lowest possible costof production for the hot metal.

Iron ore pellets

Iron ore pellet is a type of agglomerated iron ore fines which has better tumbler index whencompared with that of parent iron ore and can be used as a substitute of lump ore for the

production of direct reduced iron (DRI) and in blast furnaces for the production of hot metal. Theterm iron ore pellets refers to the thermally agglomerated substance formed by heating a variable

mixture of iron ore, limestone, olivine, bentonite, dolomite and miscellaneous iron bearingmaterials in the range of 1250 deg C to 1350 deg C. Iron ore pellets are normally produced in

two types of grades namely DRI grade and BF grade. BF grade pellets have higher basicity thanthe DRI grade. The general identification details of iron ore pellets are given Tab 1.

Tab 1 Identification details of Iron ore pellets Chemical name Iron ores, agglomerates

Other names Iron ore pellets, iron oxide pelletsCAS No. 65996-65-8

EINECS No. 265-996-3Molecular formula Fe2O3

Molecular weight (gram/mole) 159.7Synonyms Di iron trioxide

Mineral of identical or similarcomposition

Hematite

Other identity code: Related CAS No. Hematite (Fe2O3) 1317-60-8REACH (Registration, Evaluation,

Authorization, and restriction ofChemicals) registration No.

01- 2119474335-36-0013


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DRI pellets donot contain CaO while BF grade pellets are fluxing pellets containing CaO. ForBF grade pellets reducibility and swelling index are important properties while for DRI grade

disintegration is an important property. The properties of pallets are given in Tab 2.

Tab 2 Properties of pellets

Size 8-20 mmAppearance GranularColour Dark grey

Odour Odourless pH (40 gm/L,20Deg C; slurry in

water)

5.0 – 8.0

Melting point 1500-1600 deg C

Bulk density 2.0 -2.2 t/CumWater solubility Insoluble

Oil solubility InsolubleTumbler index (+6.3 mm) 93-94 %

Abrasion index (-0.5 mm) 5-6 %Compression strength (daN/p) 250 min

Size distribution (+8mm -18mm)

95 % min

Size distribution (-5 mm) 1.50%Porosity 18%

The chemical analysis of pellets are given in Tab 3

Tab 3 Chemical analysis of pellets BF grade DRI grade

Fe % 63 – 65.5 65 -67.8SiO2 + Al2O3 % < 5


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and amount of fluxes added, there is an effect of fluxing agents in terms of CaO/SiO2 ratio andMgO content on the pellet quality. Other factors which have effect on pellet quality are given

below.

Cold crushing strength of pellets found to increase with increasing firing temperature.

Decrease of mean particle size (MPS) does not have a relationship with the pelletstrength, but it reduces the porosity of the pellet. Fluxed pellets exhibits good strength, improved reducibility, swelling and softening

melting characteristics. Because of these properties these pellets give better performancein the blast furnace.

Swelling indicates volume change of pellets during reduction. Higher swelling reducesthe strength of the pellets after their reduction thereby resulting in high resistance to gas

flow, burden hanging and slipping indiside the blast furnace. Maximum allowableswelling of pellets for the blast furnace ranges from 16 -18 %.

Acid pellets (DRI pellets) and MgO free pellets exhibit higher swelling at 0.6 basicityand decreased thereafter.

Mineralogically iron ore pellets comprise essentially hematite (original surviving) particles of

iron ore, crystalline silica (quartz, cristobalite and tridymite) and forsterite (Mg2SiO4). The principle variation in pellet mineralogy is in the proportion of gangue phases present in the

product. These will vary depending upon the pellet feed material and the type and the amount ofany additives to feed such as limestone, dolomite, olivine and bentonite etc.

The mosaic image of cross section of a pellet is shown in Fig 1. The radial variation of phases,especially pores, is readily visible in the image.


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Fig. 1 Mosaic image of pellet cross section

Advantages of Pellets

Iron ore pellet has the following advantages.

1. Pellets have good reducibility since they have high porosity (25-30%). Normally pellets

are reduced considerably faster than sinter as well as iron ore lumps. High porosity alsohelps in better metallization in DRI production.

2. Pellets have a uniform size range generally within a range of 8 -18 mm.3. Pellets have spherical shape and open pores which give them good bed permeability.

4. Pellets have low angle of repose which is a drawback for pellet since it creates uneven binder distribution.

5. The chemical analysis is uniform since it gets controlled during the beneficiation process.Fe content varying from 63% to 68% depending on the Fe content of Ore fines. Absence

of LOI is another advantage of the pellets.

6.

Pellets have high and uniform mechanical strength and can be transported to longdistances without generation of fines. Further it has got resistance to disintegration. Highmechanical and uniform strength of pellets is even under thermal stress in reducing

atmosphere.


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Iron Ore Pellets and Pelletization Process

Pelletizing of iron ore was started in the 1950s to facilitate the utilization of finely ground ironore concentrates in steel production. For the pelletizing of iron ore there are two main types of

processes namely, the straight travelling grate (STG) process and the grate kiln (GK) process. In

the STG process, a stationary bed of pellets is transported on an endless travelling grate throughthe drying, oxidation, sintering and cooling zones. In the GK process, drying and most of theoxidation is accomplished in a stationary pellet bed transported on a travelling grate. Thereafter,

the pellets are loaded in a rotary kiln for sintering and then on a circular cooler for cooling.

The pelletizing processes are discussed in the article under the link http://ispatguru.com/iron-ore- pellets-and-pelletizing-processes/.

The pellets may be acid or fluxed pellets.

Acid pellets – Basicity of acid pellets is usually less than 0.1. The fired pellet strength is,

to a certain degree, due to hematite bridges of polycrystalline structure. These pelletsnormally have large volume of open pores. The reduction gas quickly penetrates throughthese pores into the pellet core and simultaneously attacks the structure in many places.

This results into an early structural change which begins at low temperatures over theentire pellet volume.

Fluxed pellets – These are also known as basic pellets. Basicity of fluxed pellets isgreater than 0.1 and can vary. Basicity of normal basic pellets range from 0.1 to 0.6 and

have low CaO percentage. During the firing of these pellets, a glassy slag phaseconsisting of SiO2, CaO, and Fe2O3 of varying percentage is formed. Due to increased

flux addition, there is formation of some slag and due to it, there is to a certain extent slag bonding with iron ore crystals. High basicity pellets have a basicity level greater than 0.6.

These pellets contain higher level of CaO. These pellets not only have glassy phaseconsisting mainly of SiO2, CaO, and Fe2O3, but also calcium ferrites (CaO.Fe2O3).

During firing of these pellets, the availability of CaO considerably favours the crystalgrowth of hematite. These pellets normally have a high mechanical strength after pellet

firing.

Pellets plants are normally integrated with the iron ore beneficiation plants. In case they are notintegrated then concentrated iron ore in slurry form is usually pumped to the pellet plant by

slurry pipelines. Pellets plants based on rich iron ore fines are rare and not very economical. Theentire pelletizing process can be divided into two main segments of sub processes namely (i) the

segment of the cold sub processes and (ii) the segment of the hot sub processes.

The segment of the cold processes has the following sub processes.

Slurry tank for storage of concentrated iron ore slurry from beneficiation plant. The

particle size of iron ore in the ore slurry is less than 45 micro meter for around 80 % ofthe particles.

Additive materials (such as dolomite, limestone, lime or olivine etc. depending on thequality of the pellet to be produced) is added to the slurry.


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Majority of the water is removed from the slurry by use of filters. The water content ofthe iron ore after filtering is around 9 %.

A mixer is used for the addition of the binder (bentonite or organic binder) for obtainingsufficient mechanical strength of the green pellets

Pelletizing discs or balling drums are used for the production of green pellets. Pelletizing

discs are more popular for the production of green pellets. Vibrating screens are normally used for separating different fractions of green pellets.

Undersized pellets are recycled back to the process, while the oversized pellets are

crushed before their recycling. The rest is on sized (8 mm to 16 mm) pellets which are transported on a conveyor to the

drying process in the segment of hot processes.

The segment of the hot processes has the following sub processes.

Drying furnace to remove most of the water content of the green pellets by flow of hot airthrough the bed. Dried pellets then enters the pelletizing furnace.

The dried pellets are fired (1250 deg C to 1300 deg C) in the pelletizing furnace forconverting the green pellets into the final product

Cooling of the hot hardened pellets to around 200 deg C is carried out in cooler by blowing cold air.

Most of the above sub processes are operated these days with the help of control techniquesusing automatic controls. Some of the sub processes depend highly on a well functioning of the

preceding sub process. For example, if the particle size is too large, or the fines are too dry, orthe drying is not working, then no pellets can be produced. A less critical situation is when some

segments are working sub optimally. As an example If the process in which binder is added themixing of binder is not uniform, then the process for the formation of green pellet suffers. Hence

the iron ore pelletizing process is a chain of several sub-processes which depends highly uponeach other’s performance.

The addition of binder to the ore serves two main objectives They are (i) to make the ore plastic

so that it can nucleate seeds which grows into well formed pellets, and (ii) to hold the pellettogether during handling, drying and preheating or until it has been sufficiently strengthened by

hardening during firing. The optimum binder should produce high quality pellets at a minimumcost and introduce as little contaminants as possible. The binder is also to be non-toxic, easy to

handle and should not require an advanced feeding system. Small additions of bentonite promote bonding by the formation of ceramic bonds and by greater compaction of the particles during the

rolling of green balls.

The quality of green pellets depends on input parameters like mineralogy, chemistry andgranulometry of ore fines, balling parameters like feed particle size, amount of water addedduring pelletization, disc rotating speed, inclination angle of disc bottom and residence time of

materials in the disc etc.

In green pellets, water plays an important role. It agglomerates the ore and performs the functionof binding liquid. Wet agglomerates can exist in a number of different states depending on the


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amount of the water present. These are shown schematically in Fig. 1. Binding liquid fillingdegree or liquid saturation describes the portion of the pore volume which is filled with binding

liquid.

Fig 1 Schematic presentation of different state of wet agglomerate

At low saturations, the particles are held together by liquid bridges (pendular bonds, pendularstate). In the state of tension (funicular), both liquid filled capillaries and liquid bridges co-exist.

In the capillary state, all capillaries are filled with liquid and concave surfaces are formed in the pore openings due to the capillary forces.

The droplet state occurs when the agglomerate is kept together by the cohesive force of the

liquid. In the pseudo-droplet state unfilled voids remain trapped inside the droplet. A commonfeature is that

in the capillary and droplet states, either concave capillary openings or free superficial water,

over the whole agglomerate outer surface are expected. The capillary theory for wet agglomeratestrength is well established now.

Additives have been employed to improve both the operation and the economics of the pelletization process. Lime (CaO) and hydrated lime [Ca(OH)2] proved to be beneficial additives

during pellet production. The green and dry as well as fired pellet properties were significantlyimproved with the addition of lime or hydrated lime

The physical and metallurgical quality of the product pellets broadly depends on green pellet

quality, type and amount of binders, fluxes and additives used, and induration parameters such asfiring conditions (temperatures and time etc.). Ingredients of the green pellets react together,

during firing, to form in the product pellets different phases and microstructures.

The type and amount of these phases, their chemistry and distribution plays a vital role in

deciding the metallurgical properties of the product pellets during reduction in the subsequentiron making process. Studies of green pellets and induration of magnetite and high grade

hematite iron ore fines with low alumina have been carried out in different parts of the world.But the results of these studies cannot be directly interpreted to iron ores with higher amount of

alumina (Al2O3) due to the difference in chemistry and mineralogy. Iron ore fines withcomparatively high Al2O3 content exhibit different pelletizing characteristics.


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As for the firing of hematite iron ores, more heat is required to be supplied from external sourcesdue to the absence of the following exothermic reaction of oxidation of magnetite.

4FeO + O2 = 2Fe2O3

The energy consumption needed for the pellet production from hematite ore fines is greater thanthat needed for pelletizing the magnetite ore fines. Moreover, pellets made from hematite oreshave poor roasting properties and do not achieve adequate physical strength until the roasting

temperature is higher than 1300 deg C. It is observed that the hematite particles and pelletstructure keep their original shapes if the temperature is below 1200 deg C. The size of hematite

particles do not get enlarged, nor the Fe2O3 crystal lattice defects are eliminated until thetemperature is higher than 1300 deg C. At high temperatures, initial connecting bridges are

formed between crystal grains and recrystallization of Fe2O3 occurs. However, if the roastingtemperature is of greater than 1350 deg C, then it is detrimental as Fe2O3 decomposes to Fe3O4

as expressed by the following reaction and this adversely results in the loss of pellet quality.

6Fe2O3= 4Fe3O4+O2

The thermodynamic of this reaction indicates that decomposition temperature of Fe2O3

increases with increasing oxygen partial pressure. Hence excessive high firing temperature andlow oxygen partial pressure is to be avoided to prevent the decomposition of Fe2O3. Thus, it is

necessary to maintain a higher roasting temperature for hematite pellets as well as narrowerfiring temperature range. This makes difficult the operation of firing equipments.

To enhance the induration of hematite pellets, both magnetite-addition and carbon burdened

methods are found to be the favourable techniques in practice.

In case of pelletization of the magnetite ores, when a pellet starts to oxidize, a shell of hematite isformed while the pellet core is still magnetite. Thermal volume changes in these two phasesindicates that sintering in the magnetite phase starts earlier (950 deg C) compared to the hematite

phase (1100 deg C). The difference in sintering rates between the magnetite and hematite phasesis more at around 1100 deg C. The sintering rate increases in both the phases with increasing

fineness in the magnetite concentrate. A finer grind in the raw material, therefore, promotes theformation of the unwanted duplex structures with a more heavily sintered core pulling off from

the shell. At constant original porosity in green pellets, the oxidation rate decreases as themagnetite concentrate becomes finer, because of the enhanced sintering. However, in practical

balling, finer raw materials necessitates the use of more water in balling, which results in anincrease in green pellet porosity. These two opposite effects level out and the oxidation time

becomes constant. Under industrial process conditions, differences in the duplex structure is stillexpected. This is because only partial oxidation takes place before induration.

The addition of lime (CaO) to the iron ore fines slightly decreases the productivity of green

pellets, but increases the drop resistance and the compressive strength of green pellets. Furtherthe strength of the pellets is directly proportion with the shrinkage that takes place during firing

of the pellets. In case of fluxed pellets, the addition of lime increases pellet shrinkage and henceincreases pellet strength. This is due to the interaction between lime, silica and iron oxide


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forming calcium silicate and calcium ferrites. The addition of lime results into the sintering offine hematite particles supported by very localized secondary mineral bonding near original lime

particles sites. With the increasing of both the temperature and the lime concentration more slagis formed which consolidates the structure and lowers the accessible porosity.

Use of hydrated lime [Ca(OH)2] has a positive influence on the drop resistance and compressivestrength of green and dry pellets. Also there is a good influence of Ca(OH)2 on the final strengthof fired pellets.

The addition of Ca(OH)2 results in a decrease the efficiency of bentonite as a binder by replacing

of the more efficient sodium ion with calcium ion, converting it to the more calcic and lessefficient one, leading to deterioration of the pellet properties. hence in some cases it is not

desirable to add more than one binder at the same time during the pelletization process since itmay results into deterioration of the pellet properties instead of their enhancement. After a

certain amount of Ca(OH)2 is added, some enhancement in pellet properties is achieved whichcan be attributed to the effect of Ca(OH)2 as binder during the pelletization process. The kinetics

of reduction of the pellets containing 0.4 % of bentonite and 4 % Ca(OH)2, having the highest physico-chemical properties, shows that the reduction of these pellets is controlled by interfacial

chemical reaction.

Iron Ore Agglomeration Processes and their Historical Development

There are four types of agglomerating processes which have been developed (Fig 1). They are (i)

briquetting, (ii) nodulizing, (iii) sintering, and (iv) pelletizing.

Fig 1 Agglomeration processes

Briquetting is the simplest and earliest applied process. Fine grained iron ores are pressed in to pillow shaped briquettes with the addition of some water or some other binder under highmechanical compressive pressure. In the nodulizing process, fines or concentrate along with

carbonaceous material are passed through inclined rotary kiln heated by gas or oil. Thetemperature inside the kiln is sufficient to soften but not high enough to fuse the ore. The nodules

vary considerably in composition and are too dense, slaggy, lack required porosity and hence this process could not find great favour. Briquetting and nodulizing are cold binding processes and


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mostly used for the recycling of recovered iron ore wastes in the steel plant. Sintering and pelletizing are the processes of major importance for the iron production.

During 2014, as per World Steel Association, the production of blast furnace iron and direct

reduced iron were 1183 million tons and 73 million tons respectively. Most of this production

has come from iron ore in the form of sinter and pellet. While the preferred feedstock for blastfurnace iron is sinter and/or pellets, that of direct reduced iron is pellets only. Though accurate production data for sinter and pellets are not compiled, but world production of sinter and pellets

together can be safely estimated to be well over 1300 million tons per year to support the iron production of 1256 million tons.

Historically, the feedstock for the world?s blast furnaces was naturally occurring lump ores.

During the mining of iron ores, large amounts were getting generated. These fines since couldnot be used in blast furnace were being dumped. The depletion of deposits of higher quality lump

ores forced the development of sintering of the generated fines in order to use them in the blastfurnace. The depletion of deposits of higher quality lump ores also forced exploration of low-

grade ores that required fine grinding for concentration. These micro fines of high-gradeconcentrates had to be agglomerated for their use in the blast furnace and this has led to the

development of the pelletizing process. These agglomerates, in turn, sharply improved blastfurnace performance and led to a major shift in blast furnace burdening.

History of sintering of iron ore

Middle of nineteenth century, small sintering pot used to be constructed in the copper mining inEngland. The origin of sintering process goes back to 1887 when F. Haberlein and T. Huntington

of England invented the process of agglomeration for sintering of sulphide ores. In this process,the sintering was carried with the sintering bed being blown with air from bottom upwards. The

process was also known as up-draft sintering process. The process was patented on 11th April1905 (Patent no. 786814). The pot sintering methods used is shown in Fig 2.

Fig 2 Pot sintering method


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In 1902, W. Job invented the sintering of pyrite cinder and dusty iron ores with addition of coaland air blowing through the bed from bottom upwards (German patent number 137438). In 1905,

EJ Savelsberg developed the process of iron ore sintering with the sintering mixture containingcoal and coke breeze (German patent number 210742). In 1906, AS Dwight and RL Lloyd both

of USA invented a belt type sinter machine for vacuum sintering. In 1909 Von Schlippenbch

invented a rotary type of sintering machine (German patent number 226033). In 1913, W. Barthdesigned a sintering belt for operation with air blowing from bottom upwards (German patent276424). In 1914, JE Greenawalt was granted a patent (US patent number 1103196) for

rectangular tilting pans for vacuum sintering. The circular type of sintering machine wasinvented in 1930 by VA Sakharnov.

The Huntington and Haberlein process was the best method of sintering prior to Dwight Lloyd

patent number 882517 of 17th March, 1908. This process was capable of producing somesintered material, but the mass of product was unsatisfactory in composition and costly to handle.

It was produced in great pots, in which tons of ore mixed with lime were burned under draftforced upward through the material. Under these conditions a uniform product could not be

produced. The lower portion of the charge, owing to the pressure imposed by the weight of thesuperimposed material, were reduced to a nonporous slag, and while in the upper portions of the

charge large quantities of fines remained unsintered, owing to the agitation of the ore particles,caused by the upward rushing currents of air. The unsintered material required re-treatment and

the large masses of nonporous, thoroughly fused material could not be used in the blast furnaceuntil broken up, at large expense, and even then were unsatisfactory, because of their physical

and chemical structure.

The clumsy pot roasting process of Huntington and Haberlein did not bear comparison with the process disclosed by Dwight Lloyd patent number 882,517.The process described in this patent

eliminated the varying degrees of pressure throughout the mass, and maintained the ore particlesin a state of quiescence during combustion. The means by which this was accomplished were

simple, but effective. Pressures throughout the mass were avoided by treating the ore in a thinlayer. Quiescence of the particles during combustion was attained, either by employing a down

draft with ignition at the top surface, in which case the agitation of the particles was restrained bythe vessel in which they were contained and the pressure of the downward draft, or, if an upward

draft was used, by employing a screen to maintain the quiescence of the particles near the topsurface. In porosity, friability, and chemical structure the sintered product of Dwight Lloyd was

quite ideal for treatment in a blast furnace, and their process was superior to any method of the prior art for preparing fines ores for treatment in a blast furnace.

The first operating machine according to this method was developed by Arthur Smith Dwight(1864 – 1946) and Richard Lewis Lloyd in June 1906 in the copper mine in Cananea built,

Mexico and 1907 the corresponding patent. In 1908 Dwight Lloyd installed sintering apparatusat the plant of the Ohio Colorado Smelting Company at Salida, Colorado. This apparatus was a

continuous type of machine, in which the bed of ore was constantly moved under an igniter andacross section chambers, which maintained a down draft during the process of sintering. The

sintered product was being automatically dumped by the machine after the material has beenignited, moved across the suction chamber, and sintered.


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Prior to 1910, JE Greenawalt, who was a metallurgist of considerable experience, was engagedin the study of processes for the desulphurization of sulphide ores. In the course of his work he

found the use of a porous hearth, upon which the ore was roasted under a down draft of air,resulted in efficient desulphurization, and that the down draft could be utilized in saving volatile

elements of value in the products of combustion, ordinarily carried off through the furnace stack.

He had noted the sintering effect of this process upon the ores under treatment, but it was not his purpose to produce sinter, and in the development of roasting processes his effort was to preventsintering, which impeded complete desulphurization. For this purpose, in his two down draft

patents, Nos. 839,064 and 839,065 (December 18, 1906), he employed rabbles.

Greenawalt developed an intermittent sintering apparatus based on downward draft which wasinstalled at the Modern Smelting Refining Company, at Denver, Colorado around 1909.

Greenawalt process was discovered by Greenawalt a few months ahead of Dwight LloydProcess. The features of the process were very less air leakage and the bottom of the pot was

made of grate or perforated steel plate. Feeding and ignition furnace was movable with sinternormally dropped due to rotation of pot above hopper. This machine consisted of a pan mounted

upon trunnions, in which the material was sintered, downdraft being maintained by a suctionchamber in the pan beneath the bed upon which the ore was sintered. The sintered product was

dumped by turning the pan upon its trunnions, when it was recharged and the process wasrepeated. This process was installed for sintering blast furnace flue dust. It was first commercial

sintering plant of Greenawalt process. Greenawalt patented this process and since 1910, hisapparatus had been used extensively in the treatment of sulphide ores and since 1912 in the

treatment of ferrous ores.

Dwight Lloyd sintering process with down draft became popular and most of the sinter being produced these days is by this process. The two inventors, who founded in 1907 the “Dwight and

Lloyd Metallurgical Company” in New York, not only built themselves a large number of these plants, but licensed the process world, among others at the plant manufacturer Lurgi. The first

machine that sintered Dwight-Lloyd-process iron ore was built in 1910 in the United States. Thefirst Dwight-Lloyd sintering plant in Germany was built in1917.

Though a large numbers of improvements have been made since then in the machine mechanicaldesign and in the process of iron ore sintering, still the basic principle of the process remains the

same.

History of pelletizing

Pelletizing differs from sintering in that a green unbaked pellet or ball is formed and thenhardened by heating. During the development of the sintering process, initial attempts were in

the direction of further improving the process for using micro fines ores. This has led to thedevelopment of a process which was an alternative to sintering. This process was named pelletizing process. In Sweden and Germany, use of major amounts of fines in the sinter mix led

to limited productivity, and thus brought about the first phase of the development in the pelletizing process. The first patent on pelletizing was granted to AG Andersson of Sweden in

1912 (Patent number 35124) and in 1913 to CA Brackelsberg in Germany. A pilot pellet plant


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with 120 tons per day capacity was constructed in 1926 for Krupp at Rheinhausen Steel plant.This plant was dismantled in 1937 for making space to a large sinter plant.

The second phase of development of pelletizing process took place in USA. The principal

nursery of this technique and the source from which the flow of successful modern development

has sprung is the Mines Experiment Station of the University of Minnesota, USA. During the1940s research workers at this station under the direction of Dr. EW Davis and his associatesexamined the problems of utilizing the low grade iron ores of Minnesota. Particular attention was

given to the ferruginous rocks adjacent to the main ore bodies of the Mesabi Range. These low-grade ores (25 % to 30 % Fe) are quite exceptionally hard and abrasive and are known locally as

?taconites?. These ores are the original material from which high grade Mesabi hematites have been evolved by natural leaching and oxidation. The recoverable iron mineral is finely

disseminated magnetite and the ore must be ground to about 80 % of size – 325 mesh forliberation, the concentrate containing about 65 % iron and 8 % silica.

By 1945 research and development at the Station had made considerable progress. Here not only

had a promising concentration technique been evolved but novel ways had been investigated ofusing the very fine concentrate produced. The wet concentrate was balled in a rotating drum and

then hardened by suitable heat treatment in a shaft kiln. The hard pellets (about 15 mm to 25 mmin diameter) were thought to be a suitable blast-furnace feed material and subsequent tests in a

small experimental blast furnace were encouraging. These results attracted the attention of iron producers of the world and inspired some particularly energetic and successful work in Sweden.

In the USA, the steel and ore companies saw in this technique a means of prolonging the rich butdwindling ore resources of Minnesota and Michigan by making available hitherto unusable

material. New companies were formed to explore these possibilities, and research wasintensified. By 1949, it was generally agreed in USA and in Sweden that the best way to prepare

balls from a concentrate was in a rotating drum, but opinion was divided on the heat treatmentmethod.

It was essential that the apparatus used, whatever its form, should (i) give close temperaturecontrol, (ii) require the minimum amount of fuel, i.e. should recuperate sensible heat, (iii) have

reasonably trouble-free and reliable in operation, and (iv) have an adequate unit output.

At first, vertical ‘shaft kilns were used exclusively in pilot-plant research, moist balls ofconcentrate being fed into the top and moving downwards against an ascending flow of hot gases

which first dried them, and then elevated their temperature to the hardening region. Hardened pellets were withdrawn from the bottom of the kiln.

Davies work culminated in 1943 when experimental pellets were fired in a shaft furnace. After

World War II, in 1947, a similar experimental unit was built in Sweden. In 1950s, it becameevident that pelletizing is an economically feasible method of agglomerating fine grainedconcentrate. The first pelletizing plant was commissioned in Sweden where pellets were fired in

shaft furnaces with capacities of 10 to 60 tons per day.

Although the process is intrinsically simple, and therefore attractive, it proved in practice to havesome awkward features. Amongst the difficulties which were most acute in the early 1950s are


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(i) the difficulty of securing even gas distribution, (ii) the difficulty of securing even stockdescent, uniform pellet treatment and trouble free discharge of product, (iii) the difficulty of

securing a high output from one unit, and (iv) uncertainty about ‘scaling-up’ and the mostappropriate kiln shape.

These problems caused the Reserve Mining Company in the USA to seek another way forward.The new line of advance had its genesis in the ‘Lepol kiln process’, used in the cement industry.This process, which was developed in Europe, consists of a balling unit feeding a moving grate

on which the balls are dried and partly hardened. The grate discharges into a rotating kiln wherethe burning process is finished the hot gases from the kiln are ducted back to the grate where

they perform the drying and hardening functions just mentioned.

Mitchell has reported that Dr. Lellep of the Allis- Chalmers Company suggested that the ballingdrum and grate might well be used for producing burned pellets from fine magnetite

concentrates. If such an arrangement could be designed to dry and fire the pellets and torecuperate the sensible heat of the product, it might well solve the problems of fine magnetite

agglomeration relatively cheaply.

Research was continued in the Allis-Chalmers laboratories with encouraging results, and in 1954

the Reserve Mining Company commissioned a 1 000 tons per day experimental machine basedon these principles, but designed mechanically on sinter machine lines by the Arthur G. McKee

Company. The pellets were dried by blowing hot air upwards through the bed, and then hardened by drawing hot gases downwards from special furnaces. Arrangements were made to cool these

pellets on the strand and the hot air recovered was used to dry the wet balls at the feed end, thusreducing fuel consumption. Useful operating experience was obtained and after a few months

trial the Reserve management placed orders (April 1954) for the design of six large machines to be installed at Silver Bay, on the western shores of Lake Superior.

Other companies, however, persisted with the development of the vertical kiln process, again for

magnetite concentrates, and their progress also justified the erection of some large installations:The Erie Mining Company plant at Hoyt Lakes, the largest of its kind, was commissioned in

1957 and includes 24 vertical shaft furnaces.

In the 1950s, therefore, both vertical shaft kilns and moving grate machines were beingdeveloped and applied to the pelletizing of fine magnetite concentrates. Magnetite concentrates

are relatively easy to pelletize. The particles are granular (rather than plate-like) in shape, with ahigh surface area, and with a surface uncontaminated by flotation agents. If properly treated they

oxidize during firing, giving a useful heat release. Oxidation is also associated with grain growthand recrystallization which contribute to the development of the requisite final strength.

However, not all ferrous concentrates are magnetites. In the state of Michigan there are largedeposits of jaspilite in which the iron mineral is a finely divided specular hematite. These oreshave been called the Michigan counterpart of the Minnesota taconites, but the iron mineral is

recoverable by flotation and gravity methods, rather than by magnetic concentration.

Such hematite concentrates present special problems and their exploitation has been a majorconcern of the Cleveland Cliffs Iron Company. In 1956 they began operation at Eagle Mills


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(Michigan) with a grate machine, but since then two very successful larger plants have been builtusing the grate-kiln system, this being, in fact, the Lepol process as a whole applied directly to

the iron ore problem. The system has been developed by Allis-Chalmers and the plantsthemselves have been engineered and constructed by the McKee Company. In general it may be

said that the Lepol process (drying and preheating on a grate and hardening in a kiln) has proved

very successful with hematite concentrates (which do not have the advantage of an exothermicheat of oxidation) whilst for magnetites the ‘straight grate’ or vertical shafts are the usual tools.

Pelletizing of iron ore is a method of Swedish origin, patented in 1912 by AG Andersson(Yamaguchi et al., 2010). The process was developed in the USA in the 1940s, and the first

commercial plant started operation in Babbitt, Minnesota in 1952. The first iron ore pellet plantof the grate-kiln type was established at Humboldt Mine, Michigan in 1960. Allis-Chalmers (a

predecessor company to Metso) have since built around 50 such plants. However, very few ofthe older plants built before 1975 are still in use. Another constructor of grate-kiln plants is Kobe

Steel, who built their first plant in 1966 at Kobe Works, Nadahama, and have since thenconstructed more than ten plants, most of which are still in use.

Since 2000, the grate-kiln process developed by the Shougang Group has been rapidly adopted in

China. The establishment of new grate-kiln plants in China has been very prominent in the lastdecade, with the rise of new fabricators such as Jiangsu Hongda and Citic. There has been an

exponential increase since 2000, driven mainly by installations in China.


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Understanding Sinter and Sinter Plant Operations

Sintering is a process of agglomeration of fine mineral particles into a porous and lumpy mass byincipient fusion caused by heat produced by combustion of solid fuel within the mass itself. The

sintering process is a pre-treatment step in the production of iron, where fine particles of iron

ores and also secondary iron oxide wastes (collected dusts, mill scale etc.) along with fluxes(lime, limestone and dolomite) are agglomerated by combustion. Agglomeration of the fines isnecessary to enable the passage of hot gases during the blast furnace operation.

Sintering has been referred to as the art of burning a fuel mixed with ore under controlledconditions. It involves the heating of fine iron ore with flux and coke fines or coal to produce a

semi-molten mass that solidifies into porous pieces of sinter with the size and strengthcharacteristics necessary for feeding into the blast furnace.

Although simple in principle, sintering plant requires that a number of important factors in its

design and operation be observed to attain optimum performance. A simplified schematic flow

diagram of sintering process is at Fig 1.

Fig 1 Simplified flow diagram of a sintering process

There are basically the following three types of sinters.


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Non flux or acid sinters – In these sinters no flux is added to the iron ore in preparing thesinter mix. Non flux sinters are very rarely being produced these days.

Self fluxing or basic sinters – These are the sinters where sufficient flux is added in thesinter mix for producing slags of desired basicity (CaO/SiO2) in blast furnace taking into

account the acidic oxides in the blast furnace burden.

Super flux sinters – These are the sinters where sufficient flux is added in the sinter mixfor producing slags of desired basicity in blast furnace taking also into account the acidicoxides in the coke ash in addition to the other acidic oxides in the blast furnace burden.

Fluxed sinters have superior high temperature properties in the blast furnace as compared to

lump ore and acid sinters. These improvements include higher softening and meltingtemperatures and higher levels of reducibility.

The flexibility of the sintering process permits conversion of a variety of materials, including

natural fine iron ores, ore fines from screening operations, captured dusts, ore concentrates,return fines not suitable for downstream processing, other iron-bearing materials of small particle

size (sludges, mill scale etc.), and wastes and screenings of lime, limestone and dolomite into aclinker like agglomerate that is well suited for use in the blast furnace.

A sintering plant has become a tremendous success for providing a phenomenal increase in the productivity and saving in coke rate in the blast furnace. Fluxed sinter represents an improved

blast furnace material as compared to sized iron ore. Improvements have been obtained byincorporating the blast furnace flux into the sinter rather than charging it separately at the top of

the furnace, as it is needed to be done with the charging of only the sized iron ore. As per thumbrule, the use of fluxed sinter indicate that for each 100 kg of limestone per net ton of hot metal

removed from the blast furnace burden and charged into the sinter plant to make a fluxed sinter,approximately 20-35 kg of metallurgical coke per ton of hot metal is saved and around 3 % to 5

% improvement in the productivity of blast furnace is achieved. The coke savings results primarily from calcining of limestone on the sintering grate rather than in the blast furnace.

Quality of sinters

Two important properties of sinter are basicity, which is controlled by the amount of

limestone/lime, and strength, which is controlled by coke content.

The blast furnace demands sinter with a high cold strength, low reduction degradation index

(RDI) and high reducibility index (RI) , in a very narrow band of chemistry variation, with thelowest possible fines content, and a good average size. The chemical and structural composition

are very important in sinter, and it is good for the sinter to be stable so that both primary andfinal slags possess adequate characteristics in terms of softening and melting temperatures, liquid

temperature and viscosity for the stable operation of the blast furnace.

It is important to have a high iron content, low gangue content, and basicity of the order of 1.6-2.1. Sinter reducibility, and sinter quality in general, improves with a higher level of hematite

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iron ore pellets and pelletizing processes

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