decreasing blast furnace process costs at iscor long products

8/20/2019 Decreasing blast furnace process costs at Iscor Long Products

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DECREASING BLAST FURNACE PROCESSCOSTS AT ISCOR LONG PRODUCTS

P VermeulenW HandW Seegers

Iscor Long ProductsNewcastle, South Africa

SUMMARY

Since 1995, various improvements in blastfurnace practice have been made at Iscor LongProducts. Coke quality was improveddramatically, which allowed many improvementsin fuel rate, charging practice and a reduction inhot metal production cost. These improvementsinclude an increase in hot blast temperature andpitch injection, charging of sinter fines, and

increased charging of small coke.

Further advances to reduce cost include reduceddrill and taphole clay consumption, and theinstallation of second and third liquid fuelsinjection systems to handle incompatible products.

These improvements allowed Iscor Long Productsto remain competitive during a very demandingperiod in the steel industry.

KEY WORDSCost reduction, coke quality, deadman control,

taphole practice.

1. INTRODUCTION

At Iscor Long Products we operate a 10.1m stavecooled blast furnace, with 2017 m3 workingvolume. The traditional approach was to operateat a blast temperature of 950 oC. This, togetherwith a relatively inferior coke quality (CSR=43,

Ash=15%), resulted in a fuel rate of 560 kg/thm.This paper indicates the improvements made inoperating practice and cost reduction since 1995,as well as the philosophies used to achieve the

results.

2. DECREASING ENERGY LOSSES

In 1992 all the factors influencing our high fuelrate were not known, but the coke quality stoodout from the rest. It was realised that the cokequality was not sufficient for a medium-sized blastfurnace, as a high degree of energy loss occurredthrough the top gas as high top gas temperature.By decreasing coke reactivity from 36 to 30, theenergy loss was greatly reduced.

Laboratory work performed by the authorsshowed that coke with a high reactivity usuallyalso starts to react at a lower temperature. One ofthe main functions of coke in the blast furnace is

to maintain enough structural integrity in the lowerpart of the furnace to support the burden. If thecoke starts to react too high in the shaft, theresulting coke degradation leads to a very lowaverage sizing of the coke reaching tuyere level.

At Iscor Long Products the only effective way tocounteract this was to increase the total coke rate,and to decrease the pitch injection rateaccordingly. This was however a very inefficientand expensive iron making route.

The big breakthrough came in 1996, when a trialwas performed on the charging of low reactivityChinese beehive coke(1).

Table 1 shows the qualities of our 1996metallurgical coke, the Chinese coke, as well asthe current coke quality.

Table 1: Different coke qualities as used at Iscor

Long Products. CSR CRI Ash1996 45 36 14.7Chinese 70 20 10.01998 Best practice 56 28 11.9Current 53 29 12.0

Because Chinese coke has such a low reactivity,care must be taken to avoid using too much. Thephilosophy we applied was to obtain an averagereactivity of 28% to 30%. Our charging pattern isCCOO, with the ability to have a maximum of 12steps per sequence. By replacing one of the coke

steps with Chinese coke, in either an 8 or 12 stepsequence, the Chinese coke could be variedbetween 17 and 30%.

We have not explored higher percentages,because it has not been necessary, and of coursecoke needs to have some reactivity for theregenerative reactions in the blast furnace shaft.

The effect of the lower coke ash, and the cokeash composition is not discussed in this paper.When this work was started, the effect of low ashwas not known as it is today.

At the start of the first trial, the arrival of the lowreactivity coke at tuyere level was dramatic. Itresulted in an immediate decrease in blastpressure standard deviation, and a markedimprovement in dead man quality as the trialprogressed.

The working zones in the blast furnace moveddownwards as expected, and the reduced energyloss was accompanied by a substantial decreasein fuel rate. This is shown in Table 2.

With this knowledge, the local coke quality wasaggressively tackled, as is explained in section 5.Table 2: Summarised results from the 1995Chinese coke trial.



Metallur gicalcoke(kg/thm)

Chinesecoke(kg/thm)

Totalcoke(kg/thm)

Pitch(kg/thm)

Totalfuel(kg/thm)

Baseperiod

475 0 500 53 552

18% 376 85 487 49 53427% 338 120 484 49 531

3. INCREASING HOT BLAST TEMPERATUREAND PITCH INJECTION RATE

As mentioned before, the traditional approach wasto operate at 950 oC blast temperature. One of thereasons was the inferior coke quality, and theresultant damage inflicted on the coke at tuyerelevel.

To compensate for the low blast temperature, ahigh oxygen enrichment practice (3% enrichment)

was used.

Since 1991 the pitch injection rate could not beincreased above 45 kg/thm. Each attempt toincrease injection rate would result in unstablefurnace conditions, tuyere losses, and damage tothe stack refractory system.

In hindsight, this was due to lack of knowledgewith respect to burden distribution control andtuyere parameter control, particularly tuyerevelocity.

The breakthrough came in 1996, with theattendance of the last European IronmakingCongress in Belgium. When Mr. Kadoguchi (2)presented his paper, it was mentioned that thecoal injection rate was no longer 201 kg/thm, but202 kg/thm. This was the key to the successfulincrease of blast temperature and pitch injectionat Iscor Long Products.

The problem of increasing blast temperature andpitch injection rate was solved by taking verysmall steps upwards. Since July 1997, blasttemperature was increased by 10 oC per month,and pitch by 1 kg/thm. At the same time the cokerate was adjusted accordingly, thus forcing thefurnace to accept the changes, and not waste theextra energy into the top gas.

During the rest of the month, the hot blast system,burden distribution parameters and deadmancondition were monitored. Adjustments to burdendistribution were made to maintain a permeabledeadman, and control reduction characteristics inthe shaft.

Each step brought new challenges, as none of theoperating staff had any experience at these highblast temperatures and associated low coke rates.The results are shown in Fig.’s 1 and 2. Fig. 1

shows the blast temperature from 1988 in yearaverages, and month averages from January1997.

Mild steel lances were used for pitch injection.With the increased blast temperature, and thehigh-sulpher oil used in the second injectionsystem (explained later), the lances succumbed topitting corrosion, causing oil to run down theinside of the blowpipe. The resulting flametemperature of the burning oil was far above thelimits of the blowpipe refractory system(3).

When this temporary setback was eliminated,blast temperature could be increased with ease,and much less associated stress!

Fig. 2 shows the increase in pitch injection rate.We currently operate three injection systems, thefirst for a coal-based pitch, the second for a high-sulpher refinery based bottoms product, and thethird for light fuels.

The light fuels are available sporadically, asrefinery streams are sometimes unbalanced. Theproduct is supplied at a very low cost, andcontains zero impurities.

The end result is that blast temperature increasedto 1150 oC (month average, setpoint = 1173 oC),and pitch to 65 kg/thm. This resulted in asubstantial decrease in coke rate and associatedenergy costs per ton of hot metal.

800

850

900

950

1000

1050

1100

1150

1200

1 9 8 8

1 9

9 1

1 9 9 4

J a n - 9 7

A p r - 9 7

J u l - 9 7

O c t - 9

7

J a n - 9 8

A p r - 9 8

J u l - 9

8

O c t - 9 8

J a n - 9 9

A p r - 9 9

J u l - 9 9

O c t - 9 9

Timescale

B l a s t T e m p e r a t u r e

( d e g

C )

FIG. 1. Blast temperature

FIG. 2. Liquid fuel injection rate

20

25

30

35

40

45

50

55

60

65

70

1 9 9 5

J a n - 9 7

M a r - 9 7

M a y -

9 7

J u l - 9 7

S e p - 9 7

N o v - 9 7

J a n - 9 8

M a r - 9 8

M a y -

9 8

J u l - 9 8

S e p - 9 8

N o v - 9 8

J a n - 9 9

M a r - 9 9

M a y -

9 9

J u l - 9 9

S e p - 9 9

I n j e c t i o n r a t e

( k g / t h m

)

Pit ch O il A lcohol



4. DEADMAN CONTROL

With the decreasing coke rate, control ofdeadman permeability became ever more critical.

A simple model was developed describing cokeflow below the cohesive zone. Fig. 3 shows ourconcept of coke flow below the cohesive zone. Itshould be noted that the deadman is fed mostlyfrom coke charged in the middle of the furnace.

A piece of coke in the middle of the furnace willdescend to the top of the deadman, and if a spaceis available, it will enter the deadman. If no spaceis available, it will progress to the flame front to beconsumed by the blast.

Sidewall drainage is influenced by more factors,including tuyere velocity, and direct reduction rate.

Small coke radial charging position, as well ascoke rate is manipulated to control directreduction rate. When sinter is reclaimed fromstockpile, the fines loading increases, and cokerate is increased by 2 kg/thm during such periods.

It has been found that increased small cokecharging, or the radial position of small coke is noteffective enough to maintain sufficient indirectreduction during reclaiming of sinter. Duringnormal operation small coke charging is howeverthe preferred route.

Burden distribution and coke rate is used tocontrol the state of the deadman. Care is taken tomaintain a high degree of material segregation tothe middle of the furnace, ensuring a temperaturepeak of 600 oC above the burden. This ensures a

rich gas in the middle, and that the coke in themiddle is relatively free from alkali deposition. Asthe coke in the middle does not work as hard inthe shaft as the rest of the coke, it is in a better

mechanical condition when it reaches thedeadman.

When the underhearth temperatures start todecrease, operating parameters are verified, andsmall increases in coke rate are applied. Althoughthis increases the fuel rate in the short term, thelong term benefits exceed the short term costimplications by far.

Titanium-rich pellets are charged as part of thenormal burden to counter side-wall wear. Thepellets contain 12% Titania, and 56% Fe. Pelletsare charged to a maximum of 40 kg/thm, andapart from effective hearth sidewall protection,also results in a decrease in hot metal costs.

5. INCREASING COKE QUALITY

With the experience gained from using low

reactivity coke, the Newcastle coke quality wasimproved, using selected imported coals toenhance the coking blend. Traditionally Iscor LongProducts relied totally on locally mined coals, andthis was a major shift in company thinking.

A spreadsheet model was developed(4), usingcoal properties to predict pushability, cokeproperties, blast furnace performance and hotmetal reductant costs.

To predict hot metal reductant costs, variousfactors are influenced by the predicted coke

quality. These factors include the amount of pitchthat can be injected, the amount of small coke thatcan be charged, and blast temperature that canbe used. With the knowledge gained, blasttemperature is now fixed in the model at 1170 oC.

The model has been modified in 1998 toincorporate different blast furnace productivities.

As the end result is hot metal reductant costs, itcame in very handy during the economic crisis oflast year. Blast furnace productivity wasdecreased from 2.6 thm/m 3/24h to 1.8thm/m 3/24h, and the coke quality was decreased

accordingly.

This was by far not technical excellence, but itwas a pure financial decision. As a long productsproducer, the Works benefited substantially fromthis decision in that it could remain competitive inthe market.

The increase in local coke quality of course gavethe opportunity to increase blast temperature andpitch, but also resulted in other advantages. Theamount of small coke (10 – 30mm) has beenincreased from 20 kg/thm to 40 kg/thm.

The small coke is charged in two fractions, a nutfraction (25-30mm), and peas of 10-25mm. Peashave been charged since March 1998, as more

FIG. 3. Coke flow pattern below the cohesivezone.



value could be added by charging it than byselling it to the ferro-alloy industry.

Another advantage was the opportunity to chargefine sinter (-5mm) to the blast furnace. Thispractice was followed successfully until productionlevels decreased, as the expected lower gas flowrates in the furnace shaft did not allow this.

6. METALLURGICAL MODELS AND BURDENDISTRIBUTION MODEL

In 1993 a set of CRM metallurgical models werepurchased, together with a burden distributionmodel developed by Sidmar. For this paper, onlythe burden distribution model is discussed.

After gaining operational experience in the use ofthis model, it has proved invaluable to dailyfurnace operation.

With the inferior coke quality, the aim was toachieve maximum gas-solid interaction, but ascoke quality improved and pitch injection rateincreased, the coke inflection point was moved

away from the wall. The actions taken are thesame as for increased coal injection rate.

A good example of this is the techniquedeveloped for centre-charging coke. Fig. 4 showsa schematic output of the model, with the top half

being a normal discharge, and the lower halfincluding a centre-charged coke.

Centre-charging of coke has been found to bevery effective, but also quite an extreme measureto take.

The charging system offers a lot of flexibility, inthat additives can be charged on the main

charging conveyor anywhere in the ore-sintersandwich. Peas and nuts, for example, are notcharged directly to the wall, as it gives too muchpermeability against the wall. Scrap is charged in

the middle of the furnace, to prevent tuyerelosses.

In 1997 the charging of sinter fines wascommenced, again not against the furnacesidewall, this time to ensure proper indirectreduction directly above the tuyeres.

With the amount of scrap materials charged, theburden distribution model was upgraded toVersion 2, which takes material size distributioninto account.

Our furnace operating philosophy is very similar toSidmar’s, thus the model could be applied withfew changes to the rules.

7. DISTRIBUTION CHUTE DESIGN

One of the cost reduction exercises taken was to

change the design of the distribution chute. In1978 the chute length was increased from 3.0m to3.5m, to achieve the desired trajectories. Thechute employed was of smooth liner plate design,and was used with great success.

As the chute wore through its campaign, moreresistance to flow was created, and the exitvelocity of the material stream decreased.

The trajectories for both a new and an old chutewere measured during the 1993 reline, and on ascheduled shutdown the burden profile was used

to select the correct trajectory for the operation atthe time, and thus fine-tune the model toaccurately predict actual charging profiles.

This chute (design) lasted about nine months inthe furnace, before it had to be changed again. In1998 a stonebox chute was fitted, which issupposed to last four years. It was of greatconcern that the exit velocity would decrease, butfrom lessons learned at Iscor Flat Products, andBHP, Australia, the installed design gave atrajectory very close to that of a new chute.

The advantage is that the trajectories do notchange over time. Based on the last inspection inFebruary 1999, the new design should give alifetime of 4 years. After 20 months in the furnace,the chute showed very little wear.

8. TAPHOLE CLAY AND DRILLCONSUMPTION

The installed taphole length is 2.0m, and inoperation it is maintained at 2.2 to 2.3m with a tar-bonded taphole clay. A dry hearth practice isfollowed in order to avoid drainage problems.

Through meticulous management of clayconsumption, it has been reduced from 780 g/thmto 350 g/thm. As an example, the clay that falls

0

50

100

150

200

250

300

350

400

450

500

B u r d e n

H e

i g h t ( c m

)

Coke

Coke

Ore+Sinter

Ore+Sinter

FIG. 4. Schematic output of burden distributionmodel showing coke centre-charging technique.



out when the claygun is primed, is collected andre-used.

Another breakthrough in reducing clayconsumption was to reduce the number ofnotches injected from five to three. Measurementof taphole length was upgraded, to ensure thattaphole integrity did not suffer as a result. Whenthe desired taphole length is not achieved, drillsize is temporarily increased from 40mm to55mm. This has the advantage that more clay canbe injected before sintering starts, and thustaphole length can be restored to the desiredlevel.

Concerning the drills, a hexagonal bar was used,with laser-cut flights welded on as the drill tip. Thescavenging medium was 5bar compressed air.

Drill consumption averaged 0.6 casts per drill.

Various drillbits were tested, without anysignificant improvement in drill consumption. Thefirst step was to upgrade the flow and pressure ofthe scavenging medium to nitrogen at 14barpressure. Although the results were better, thegeneral consensus was that better results wereachievable.

The problem was that although the scavenging airpressure was increased, the amount of air wasstill not enough to displace the generated dust toa satisfactory degree. This was due to the

throttling effect of the relatively small hole throughthe drill rod. Calculations also showed that thenitrogen had virtually no cooling effect on theouter diameter of the bar due to the thick wallthickness.

The design was changed to a thick-walled pipe,with the same flights. The resulting highernitrogen flow to the drill tip cools the tip with muchbetter efficiency. The idea was to develop aninexpensive drill, able to open only one taphole.However, drill consumption decreased to 2.2casts per drill, using the same practice of drilling

through into the hot metal. At the same time, thecost per drill bit was halved.

Drills are also reconditioned, with the damagedpart cut off, and a new tip welded on.Reconditioned drills do not last as long as newdrills, yielding lives of 1.8 casts per drill. New drillsfrequently yield lives above 4 casts per drill.

9. PRODUCTIVITY IMPROVEMENTS

The effort put into fuel rate reductions of coursegave rise to productivity improvements. Fig. 5

shows the furnace productivity since 1995. Thepeak was reached in 1997, and could have beenmaintained was it not for the steel crisis thatensued the next year.

Current productivity is 2.1 thm/m 3/24h, or 4200thm/day.Of particular mention are the advances made inmaintenance availability. Scheduled shutdownsused to occur at eight week intervals. Byanalysing what equipment was determining, andsolving the relevant technical challenges,shutdown intervals are currently 18 weeks.

After the reline in 1993 a project was started toidentify the equipment determining the scheduledshutdown frequency of 8 weeks.

9.1 Top Temperature Probe

The top temperature probe is manufactured fromstainless steel, with good wear resistance,resistance to chloride and sulpher attack, and hightemperature strength. The design includes flat barstiffeners, and the probe is hinged in the middle ofthe furnace.

Two failure mechanisms existed: thermocouplesfailing prematurely, and the hinging mechanismfailing. The probe had to be changed every 8weeks.

Investigation showed that the material of thestiffeners differed from the rest of the probe. Asthe thermal expansion coefficients of the twomaterials differed, the probe bent upwards, untilmechanical failure of the hinge occurred.

After the material specification was rectified, thethermocouples were shielded from direct attack bythe ascending gas stream. The protectivehousings were specifically designed to not affectthe temperature readings too much. A shieldingplate was attached to the hinge, to further protectthe nitrogen-cooled hinge from high temperaturesin the middle of the furnace..

The last probe was changed after 18 months, dueto mechanical wear.

9.2 Steam Heated Seats

The steam heated lower seals on the Paul Wurthtop frequently leaked, due to pinching of material,

FIG. 5. Productivity since 1995

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

1995 1996 1997 1998 1999

P r o

d u c t i v

i t y ( t h m

/ m 3 w o r k i n g v o

l u m e / 2 4 h

)



0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1995 1996 1997 1998 1999

N o r m a

l i s e d

R a w

M a t e r i a l C o s

t s ( U n

i t s / t h m

)

FIG. 6. Normalised raw material costs since 1995.

build-up of wet material on the seat, and the sealelastomer bursting. The seals had to be changedevery 8 weeks.

By modifying the bifurcated chute to allow formore space between the seal flap and thebifurcated chute, the last outflowing material is notpinched anymore. The control of the steamheated seats were upgraded, and temperaturealarms were installed. Various elastomers werealso tested to give extended lifetime.

Currently the seals are changed every 18 weeks,mostly out of a precautionary basis.

9.3 Radar Burden Level Probes

Stockline probes in use since 1993 weremechanical probes, with servo motors. Theseprobes lasted only 12 weeks due to lost weights

and cables. 3 out of the 6 probes were replacedwith locally manufactured radar units, with thecontrol software written in-house.

The installation has the facility to remove thesource in operation, and calibration can also beperformed in operation. The first radar probe wasremoved after 12 months, to clean the pick-upcone.

The remaining three mechanical probes areoperated once a week, and are used when theburden depth is too low for accurate operation of

the radar probes.Currently the constraint exists that we do not haveenough manpower for the scheduled shutdowns.Geographically we are quite far from experiencedlabour, and we are busy developing/training localcontractors for selected jobs on shutdowns.

The goal is to progress to two stops per year,each with a duration of 16 hours.

10. RESULTS ACHIEVED

Fig. 6 shows the raw material costs since 1995.The largest advantage was obtained in 1999, dueto the cheaper coal blend used for cokemanufacture.

Fig. 7 show the total cost per ton of hot metal,also normalised to indicate the reduction achievedsince 1995.

11. CONCLUSION

In the blast furnace, more than 90% of the totalcost/thm is invested in the raw material feedstock.

Again, a substantial amount is the cost of energy. Advances in technology have been around toallow the energy input to be optimised, throughcoal selection, coke quality, fuel injection and highblast temperatures.

Iscor Long Products has managed to apply thesetechnological advances, through soundengineering principles, to significantly improve fuelrate, and thereby markedly decrease processcosts.

The application of advanced burden distributionmodels, plus the dramatic reduction in plantoutage time has further improved furnace stability,resulting in subsequent increased productivity,and refinement of fuel rate.

The results achieved over the last three years inparticular have enabled the Works to remaincompetitive during probably the most demandingperiod experienced in the steel industry.

Reference:(1) P Vermeulen – First Chinese coke trial, 1996,

Internal report, Iscor Long Products.(2) K Kadoguchi et al – Long term operation with

200 kg/thm of Pulverised coal injection rate atKakogawa Works – 3 rd European IronmakingCongress Proceedings, ISBN 3-514-00607-5.

(3) D du Plessis – Lance failure report, 1998,Internal report, Iscor Long Products.

(4) H Erasmus, W Skinner – Coal to cokepredictive model, Internal Iscor Long ProductsModel.

0.9

0.92

0.94

0.96

0.98

1

1.02

1995 1996 1997 1998 1999

FIG. 7. Normalised total costs since 1995.

decreasing blast furnace process costs at iscor long products

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