JENA. S.. and RA VI\SINGADI. ST. CotnmissiuniJl£ :lnd operating :1Il induction furnace at Zimascn (KwcKwe Division) 10 melt high­

carbon rerrochromilllll. INFACON fi, Pmcl.'eding,f of II'I! 6Th IIIII.'/",{/f;ollu/ Fcrmalloys COlIgrc,u, Capt' 1'011'11, Volume I. Johannesburg,

SAIMM. 1992, pp. 113-118.

Commissioning and Operating an Induction Furnaceat Zimasco

(KweKwe Division) to Melt High-carbonFerrochromium

S. JENA and S.T. RAVASTNGADI

Zill/osco, KweKwe, Zimbabwe

A 6,4 t induction furnace was commissioned at Zimi.lsco (Kwekwe Division) inMay 1989 with the object of reclaiming minus I mm high-carbon ferrochromiumfines that had previously been stockpiled.

Over the years. the Division had stockpiled a large accumulation of fines, whichhad previously been considered to be a waste product of no market value.

Current arisings are presently accumulating at a considerable nHe.The projecl marks the first time worldwide that an induction furnace has been

used 10 remelt high-carbon ferrochromiull1 alloy. The results obtained to dateconfirm that inductive power can be used successfully to melt high-carbonferrochromium fines. The furnace is now operating close to its design operatingparameters.

This paper describes the operating experience from commissioning to the timc ofwriting. Major aspects covered include the training of personnel in the refraclory­re-Iining procedure. the sintering method, and the furnace melting operations.

IntroductionZimasco (KweKwe Division) produces 190 kl of high­carbon ferrochromium (HCFeCr) per annum whcn runningat full capacity. Considerable quantities of high-carbonferrochromium smaller than 12,5 mill are generated whenthe initial high-carbon ferrochromium ingots from theelectric-arc furnaces are crushed and screened to meetcustomer's requirements. These fines are subjected to agravity-separation technique using hydraulic jigs toproduce a coarse product (I to 12.5 111111), which can be soldto a limited market, and a fine fraction (-I mm). This fineproduct is stockpiled, and the Di vision has over the yearsbuilt up a large stockpile, which represents a seriolls loss ofrevenue. The fines can be recycled to the electric-arcfurnaces but only at limited rates because of theirsignificant effect on the charge conductivity and furnacetemperatures.

The induction furnace project, which is the subject ofthis paper,was conceived to reclaim the -1 ml11 high-carbonferrochromium fines and convert them to a saleable lumpproduct. A 6,4 t induction furnace was acquired in October1988 and commissioned in May 1989.

Process FIowsheetThe wet feed material for the induction furnace istransferred from the gravity-concentration plant by road

transport. It is then stored and sun-dried 011 a vast concretepad. Feed to an induction furnacc should be as dryaspossible. The moisture levels of the current feed haveaveraged less than 0,2 per cent. From the pad, the driedfeed material is moved either to a strategic storage bunkeror to an inground hopper. The strategic storage bunker.which is covered, was designed to allow for continuousfeeding operations during the wet season. From the in­ground hopper, the feed materi<ll is transferred via aconveyor system to a weigh fl<lsk. which is mounted onload cells, The conveyor belt is interlocked to the weighflask so that the belt will automatically start once tbe setmass of material in the l"lask has been depleted. From theweigh flask, the feed material passes through a screwfeeder and a vertical feed chute, and then into the furnace.

After the melting process, the crucible is tiltedhydraulically, and both the mollen alloy and the slag arepoured into cast steel moulds. The alloy and slag are cooled inthe moulds for a period of 6 hours, The ingot is then sllippedoff the moulds, and placed on the ground for a further 1,5hours of cooling. Following this stage. the ingOl is placed on apallet. It is quenched in water for :.J period not exceeding 4hours, and then handled and processed in exacLly the samemanner as other ingots from the electric-arc furnaces.

A materials flowsheet for the smelter is shown in Figure I,and a diagrammatic flowsheet of the induction-furnaceoperations is shown in Figure 2.

COMMISSIONING AND OPERATING AN INDUCTION FURNACE AT ZIMASCO 113

FIGURE I. Flowsheet of materials to the smelter

IOmingO min20 to 30 % ofcrucible capacity1 650°C

24 h per day.

Pouring temperatureFurnace operating cycle

Visits to other induction-furnace operations, both localand overseas, were arranged for the team. The purpose ofthese visits was to familiarize the team with inductjon­furnace equipment and operations.

Critical tasks for the operation were identified before thestart-up process. Safe job procedures for the tasks were thenformulated. A period of 3 months was devoted to training andeducation of the project team on the written job procedures.

A commissioning engineer from the equipmentmanufacturers and a refractories specialist from a UK­based company provided valuable information duringinformal lecture sessions. During the start-up period, aspecial roster was formulated to afford the four shift­supervisory crews adequate exposure to supervised meltingoperations. The roster was designed to allow melting andpouring operations to be conducted between 0700 hoursand 1900 hours each day. The other 12 hours wereallocated to ingot-quenching and mould-preparationactivities.

After the furnace had been in continuous operation forabout six months, a UK-based induction-furnace operationtechnician spent a fortnight with the operators to optimizethe operational and lining-installation techniques.

Equipment DesignThe induction furnace consists of one 3,5 MW solid-staternductotherm Power-Melt pack and two coreless tiltablefurnace crucibles, with a nominal capacity of 6,4 teachplaced side by side. Auxiliary equipment with suitabledisconnect arrangements to turn the power on or off toeither of the crucibles alternately is also fitted.

Each crucible assembly consists of a separate standing unit,6,4 m long, 4,26 m wide, and 3,35 m high, placed side hy sidewith steel-sheet enclosures and a steel anti-skid furnace­working deck. The melting units are placed inside theseenclosures, complete with water-cooled copper coils,laminated steel shunts, and hydraulic tilting equipment.

Each crucible is fitted with a hydraulically driven swing lidcontaining an inspection and a feed port fitted with their ownmanually operated lids. Each crucible is lined with a rammedand spiked dry refractory lining. Figure 3 shows a sectionthrough the induction rumace.

The rate of melting of the feed material is about 5,5 tlh,with a 1,5 h pouring cycle. One crucible is used at a time.The furnace is run on a continuous 24 h basis. Currently, afull campaign from start to finish takes about 105,6 hbefore the refractory lining fails. At that point, the othercrucible is brought on line.

The design critetia are as follows:Maximum power 3 500 kWFurnace frequency 150 HzOperating voltage 1 600 VFurnace power source II kY, 3 phase, 50 HzNominal capacity of crucible 6400 kgMass of pour 4 500 kgDuration of melt 80 minDuration of pouring plusinspectionsMelt cycleHeel size

Saleablealloy

Organizational Structure of the InductionFurnace

The day-to-day functions of the section are coordinated by aforeman who has a line function over the entire shift crew.Theforeman is a staff employee who works normal hours. Theshift crew, which is headed by a supervisor, works a four-shiftsystem roster on a 24-hour basis. A furnace operator, whoreports to the supervisor but has no Line authority over thecasting-bay crew, is responsible for the panel, melting, andpouring opemtiollS. The casting-bay personnel, which includethe overhead-crane driver, two crane slingers, and cast-steelmould preparers, are primarily responsible for the mouldpreparation and the alloy-ingot cooling and quenchingprocesses. A conveyor attendant mans the feed system.

Saleablealloy

Saleablealloy

!INDUCTION FURNACE ~

Grade 9

TrainingFollowing the setting up of an organizational structure for theproject, a crew of supervisory and operating personnel wasselected from the existing electric-arc furnace labourcomplement. Since the project was a new one, emphasisduring the selection process was placed on experience inpanel-room operations and reliability of character. A qualifiedmetallurgist was attached to the project as teum leader.

114 INFACON6

Strategic concrete. bunker2OOOm3

Gravity·separationplant -

"~~ /Il~n ~nderground hopper~, 45m3

~~ett ~ ~fines

NO',,"0,,0

Furnace

Weigh flask7m3

r Screw feederl Feed rale 6 tIt1

f1r~mOUld ~ In~ot ~

U!XJ" ~!*-i~Quenching pond

FIGURE 2. Diagrammatic flowshccl for me induction furnace

Refractory Lining - Description andInstaUation

19300.0.

1700 °C

Grain size range

Density (rammed)

Operating temperaturelimit

The furnace is lined with an alumina-magnesia monolithicrefractory with the following specifications:

AI,OJ 84,6%

MgO 14,6%

Other 0,8% (CaO, Na,O, K,O,SiO" Fe,03, and Cr,03)

0,1 mm~5 mm

2,85 t/m3

Each lining requires 4 t of refmctory material.

The crucible coil is first covered on the inside with alayer of refractory plaster (coil screed) 5 mm thick. Thecoil screed is then dried using either furnace power or anexternal heal source placed in the crucible. The dried coilscreed is then lined with a 2,2 mm thick flexible l1IamicMicanite paper. A base block cast from magnesite­alumina based refractory materials forms the bottom of the

.'

.'

:~.'.'~

", ------I:.:.:.;~r'," :.:••.:.'." ... '/..- .

.'.'.'"

.'.

.'

Feed pon

Movable lid / Inspection port

~...,....,....:..... "":~' ~. aslable relractory

~~ -~1:""""" ::±======:.b...,---··..'fu.·.I......... ".' '5- ~F.~; ....."'.} . ;:: ,

Aim block......, ~

Disch 98spout

Furnace shell

---~Shunt -. I-Coil I --.

t--Coli screed_~

,r--U"''''Refraclory linin~

t--~

FIGURE 3. Section through the induction rurnaceDelon(zed water

tank..3,Sm3

FIGURE 4. Cooling-water circuits

Furnace

Heat exchangerf--

Municipalwaler supply

'0'emergency

Induction-fumace

extemal Jnll Rolurn pumpswater·coollng pumps 1 capacIty· 600 Vmln

porunll

s._"""", 'L J~tVmin per ~. _

fJr------'ooocooo."'"""'" --'-----,

dosecj.clraritwater pumps

In the event that the internal cooling circuit cannot be runon deionized water, e.g. during a complete pump failure orbreakdown of the standby alternator, municipal water isintroduced into this circuit. Water circulation in the circuitwiIJ then be driven by the pressure of the municipal water.Figure 4 shows the cooling-water circuits.

Cooling-water SystemThe following three cooling-water systems are associatedwiLh the induction furnace:

(i) an open circuit that draws water from an existing arc­furnace evaporative-cooling water system

(ii) a closed circuit ftlled with deionized water

(iii) municipal water.

COMMISSIONING AND OPERATING AN INDUCTION FURNACE AT ZIMASCO 115

furnace. This block is used as a pusher block to remove thespent lining at the end of each campaign.

The installation of the lining starts with alSO mm layerof lining refractory material over the pusher block. Acylindrical steel former is placed centrally on the bottomrefractory layer and secured in that position by woodenblocks jammed between the former and the coil screed. Theannulus between the former and the coil screed is filledwith refractory material in layers, each layer being about150 mm deep. Each layer is compacted with an immersionvibrator. and the top surface is roughened with spikeshefore the next layer is added. The ftlling and compactioncontinue until the annulus is filled.

During the lining process, the prevention of grain-sizesegregation and contamination of the refractory liningmaterial during handling are critical. Size segregation leadsto shorter lining life. To avoid segregation, the refractorymaterial, which is delivered in 25 kg bags, is mixedthoroughly before use. Ten bags at a time are emptied into aclean steel tray and are mixed using shovels. Compaction iscarried out according to the manufacturer's specifications.

To guard against contamination of the refractorymaterial, strict hygiene is maintained on the workingplatform throughout the installation procedure.

Sintering of the LiningThe two main components in the lining material, MgO andA1203, are present as a mixture in the as-suppliedcondition. For the refractory material to form an effectiveinsulation layer against the coil, the dry powder has to besintered at a high temperature to form the spinel(MgAI,04)'

The procedure currently in use at Zimasco is a resull ofnUUlerous modific.ltions to the original programmesupplied by the refractory-materials manufacturers. Thesintering process involves the melting out of the former bymeans of the following procedure.

Mild-steel scrap is placed io the cold former, which isheated using furnace power. The rate of heating iscontrolled so that the temperature of the former rises at arate of 200°C per hour. The former and the mild-steelscrap start melting at the same temperature, around 1100 DC,and a small steel bath is formed. This serves as the start-upmelt. Charging of remelt materials is started once there is amolten-steel bath, and continues till the crucible is full. Thebath is further heated to a temperature of 1700 °C andmaintained at that level for a period of 1,5 hours to ensurecomplete fritting of the refractory material. This marks theend of the sintering cycle, which at present takes 12 hoursto complele.

Melting OperationsThe induction-melting process is an established classicalmethod of heating metals. It involves four basic steps:

(a) current flow in a coil(b) inducement of magnetic fields

(c) flow of current in the charge

(d) heating of the charge due to resistance to currentflow.

This paper makes no attempt at describing in detail theprinciples of induction melting but instead, looks at themelting operation as practised by Zimasco.

116

The melting of the dry fines alwuys starts with a heel.This is necessary because fine material (-1 mm) is notinductive at the frequency and voltage values at which theZimasco equipment is operated. The size of the heel variesbetweeo 20 and 30 per cent of the crucible capacity. Beforethe addition of feed to the crucible begins, the operatorselects a predetermined batch mass on his automatic weigh­feed controller. Although the operator can vary the feedrate, additions are normally steady throughout the heat andaverage 3,6 tIh. Operational experience has shown thatexcessive temperatures of the molten bath below the'bridge' can cause lining erosion and a metal breakthrough,which can result in damage to the coil. Too much feed canresult in bridge formation on the melt surface. Too littlefeed can result in overheating of the already moltenmaterial (superheating). The operalor therefore tries at alltimes to match the feed rate and the power-setting on thecontrol panel. The operator also regularly checks thephysical condition of the melt.

An immersion thermocouple is used to measure thetemperature of the melt towards the end of each heat.Arrangements are being made to inst<JlI an overhead opticalpyrometer that will monitor the surf~lce temperaturecontinuously. The final temperature of the melt is currentlyset at 1650 °C maximum. Before the final melt is pouredinto a cast-steel mould, a sample of the slag is taken forchemical analysis. After pouring, two metal samples aretaken from either side of the full mould for chemicalanalysis. This process is repeated on every heat.

Process Control'The furnace is controLled from a panel in the control roomon the charge floor. The following controls are available.

Power ControlA large-diameter control dial is used to set the desiredpower to the furnace. The circuit design is such that the setpower is maintained even as melting conditions in thecrucible change. A countdown kilowatt-meter indicates theprogressive power-input summation, and can also be usedto run to a set power input. Furnace selector switches arefitted to enable the two furnaces to be used alternately.

Feed ControlFeed to the furnace is controlled automatically by acomputer. The amount to be fed for each heat is preset, andthe system will deliver the desired amount each time. Thefeed rate is controlled from a feed-control console situatednext to the furnace. The operator adjusts the feed rate tosuit the rate of melting in the furnace, which in turndepends on the power setling.

Voltage Frequency MonitorsFurnace voltage and frequency are read off from direct­reading dials on the panel.

Ground/Molten Leak DetectorThe ground-leak system continuously monitors earthleakage across the furnace lining. The system will indicate,and shut off the power when metal penetrates the furnacelining to the induction coil, when excessive moisture ispresent in the furnace lining, or when a low groundresistance exists in the electrical system.

INFACON6

Melt-temperature ControlThe temperature of the meh is taken at regular intervalsduring the mch cycle using immersion themlOcouples. Themeasured tempcralUre is displayed automatically on adigit<.ll panel mounted on the control-room wall.

The carbon monoxide that is formed in reaction [II]distorts the crystal lattice of Cr20J and facilitates theremoval of oxygen atoms from the chromitc ore. Reaction1101 proceeds much more actively than that of Cr203because a 2-electron exchange is easier than one involving3 electrons. It follows therefore lhat reaction [9] acts as acatalyst for reaction III]. which forms the basis 01" thedecarbonization of ferrochromium.

Table I gives a comparison of the feed and the resultantalloy and slag from the induction-furnace process and Ihesubmerged-arc furnaces.

The average number of heats per refractory campaigncurrently stands at 70.

TABLEtCIIEMICAL ANALYSES OFTIIE FEED ANDTJ-IE RESUlTA1''T ALLOY AND

SLAG FROM THE INDUCTION-FURNACE PROCE.'iS AND SUBMERGED­ARC FURNACE PROCESS

The production of alloy in 1990 was very close 10 Ihedesign target. Had it 110t been for teething problems withelectrical earth leakage during the last quarter of 1990, theactual production would have surpassed the target by awide margin. Figure 5 shows the monthly production ofgross alloy from commissioning 10 December 1990. Theenergy requirements for the induction process are muchlower than lhose ror the submerged-arc furnace. Thespecific power consulllption per ton of saleable alloyproduced in the induction furnace is about 27 per celll ofthe value for the submerged-arc process.

Furnace availability has generally been lower than that ofthe submerged-arc furnaces. Dowl1limc has been dueprimarily to earth-Ienkage faults. burst flexible powerhoscs. and auxiliary-powcr failure.

Figure 6 shows the availability of thc induction furnacefor the period May 1989 to December 1990.

ConstituentInduction fumace Submerged-arc

% furnace, %

Remell feedCr 56,00Fe 20,00C 5,40S 0,042 N/AP 0,022Mn 0,22Total silicon 7,00Contained slag 15.00

SlagCr 5,23 6.63SiO, 40,83 28,65CaO 2,71 3,30MgO 26,60 29,79AI,o, 24,81 25,59

AlloyCr 66,29 66,62Fe 25.18 24,12Si 0,92 1,46C 7.05 6,94S 0.044 0,05P 0,013 0,012Mn 0,22 0,22

[91[101

[I I [

3 Cr2+

2 Cr + 3 C.

3 CrO

Cr,O, + 3 C

Metallurgical and Operating PerformanceThe induction-furnace melting operations have severaladvantages compared with Zimasco'!'j other three-phasesubmerged-arc processes.

The raw materials employed in the process. including therefractory lining material, are cheaper. resulting in a lowerproduction cost.

Fewer process variables are encountered. The chromiulllrecovery in the induction furnace is nbout 8 per cenl higherthan in the submerged-arc process (approximately 80 percent). This factor represents another cost advantage.

When the chromium recovery and power requirementsare compared with those of submerged-arc operation. itmust be remembered and emphasized that this is a meltingoperation, not it chromite-~melting operation that involve~

reduction reactions. The feed-to-product ratio is 1,2. andthis depends largely on the slag contained in the feedmaterial. Operational experience has shown that the life ofthe refactory lining is also influenced strongly by the slagin the feed material. with lower values resulting in betterlining performance. The lines fed to the furnace currcJ1Ilycontain between 10 and 20 per cent slag.

The operation 01" the induction furnace results in thoroughmixing of the continuously fed charge and the molten bath.The swirling of the molten alloy further results in thepromotion of oxidation reactions with atmospheric oxygcll.Some of the proposed simplificd reactions arc as follows:

4 Cr (alloy) + 3 0, = 2 Cr,OJ IIICr,O, + 3 C (alloy) = 3 CO (g) + 2 Cr 121

Fe (alloy) + 1/2 0, = (FeO) [3]

(FeO) + C (alloy) = Fe + CO (g) 14]

(FeO) + CO (g) = Fe + CO 2 (g) [5]

Si (alloy) + 0, = (SiO,) [61

SiO, + 2C = Si + 2 CO (g) 1712 (FeO) + (SiO,) = 2 (FeO) SiO, (slag). 18]

It is believed Ihat reaclions [I] to [81 explain the lowersilicon levels in the alloy from the induction furnacecompared with the levels associated with the original alloyfrom submerged-arc furnaces.

Our practice to date is to sample the slag produced afterevery heat and analyse for total chromium and silicacontents. Typical values for the chromium and silicacontents are around 5 to 40 per cent respectively. Thisinformation is mainly used for statistical comparisons.

Work is in progress on an experimental stage. which isaimed at 3djusting the slag regime so as to further reducethe carbon content of the alloy. The experimental procedureinvolves the addition of certain amounts of chromite oreand quartz to the molten mixture wilhin the crucible of theinduction furnace.

The following reactions are thought to influence thedecarbonization process:

CrO, + 2 CrCrJ + + Cr

COMMISSIONING AND OPERATING AN INDUCTION FURNACE AT ZIMASCO 117

TARGET

J JASON DJ FM AMJ JASON DMonth

FIGURE 5. Monthly prodLH.:lilJll of gross alloy since commissioning(Julle J 9H9 10 December 1990)

Lessons since Commissioning(I) A combination of mild steel and pig iron has been used

as the initial sinter material. As the cold-charge sintermethod employed involved the melting out of a mild­steel former, high-carbon rerrochromiull1 could not belIsed because of its higher melting temperature.

(2) Analysis of refractory-lining wear indicated that thegreatest erosion of refractory material occurred in thelower sections of the crucible. This subsequently led tothe introduction of a bOltom-tapered former designed toincrease the thickness of the refractory material in thesusceptible areas.

(3) To minimize the accelerated rates of lining wear causedby excessive stirring action. power-setting limits havebeen introduced for low bath levels.

(4) The early dip-thermocouples had moisture-absorbingrefractory sheaths, which caused spark emissions duringtemperature measurements. These thermocouples will bereplaced by ones without the refractory sheath to preventsplashing.

(5) Engineers have managed to overcome power-supplyproblems by redistributing units connected to the stand­by power equipment. Proper insulation of the inductivecoil is vital if earth-leakage faults are to be avoided.Rapid and innovative quick methods of replacing burstpower-leads hoses have also been developed.

(6) The continuous monitoring of voltage and frequencyreading at a constant bath level has enabled theoperators to forecast when failure of the refractorylining can be expected.

(7) Sinter material will be poured into ingot moulds for re­use on subsequent campaigns. This will result in betrerpacking density during the all-important sinteringprocess.

(8) Visual indicators showing system conditions are fittedon the panel and these indicate faults in the capacitor,

118

-TARGET

-

- r--

l- f-

LL LLMJ J ASONDJ FMAMJJ ASOND

Month

FIGURE 6. Availability of Ihc induction furnace(May 1989 to Deccmbcr 1990)

over-voltage, internal cooling-waler system, externalcooling-water system, and furnace seleclor switches.

ConclusionsInductive power has been applied to the melting of high­carbon ferrochromium fines at Zimasco. Unit operationshave been largely successful, with the output exceeding thetargets. Future work will focus on improving productquality and reducing the cost of production. As regardsproduct quality, new methods of casting the alloyseparately from the slag will be examined. fnvestigationswill also be carried out on the possibility of reducing thecarbon content in the alloy in situ.

Cost analysis indicates that refractory-lining materialscontribute significantly to the total variable costs of eachcampaign. Attention is being paid to increasing the numberof heats obtained from each campaign by extending the lifeof the refractory lining.

The fUlure will also be devoted to nptimizing the meltingoperation by aUlomatic linkage of key process-controlparameters, e.g. actual power input and mass feed rates.

AcknowledgmentsThe authors thank the management of Union CarbideZimbabwe for permission to publish this paper. We alsoacknowledge the technical input provided by leA(Zimbabwe), Inductotherm (USA). and Capital RefractoriesLimited (U K) throughout the commissioning and operationof the Zimusco induction furnace. The contribution of theengineering department, production foreman, and operatorsto the induction-fumace projecl is gratefully acknowledged.

ReferenceI. Induclotherm Corp., Rancocas. NJ 08073, USA. (1984).

Power Trak an.d POlVer Melt systems manualoperating procedures and lI/aillfclIOIlCC instructions.

INFACON 6