DC Arc Furnace Technology Applied to Smelting Applications

Kjell Bergman (kjell.bergman@centromet.se)

Bjom Kjellberg (bjom.kjeliberg@centromet.se)

Norra Ringvagen, 36S-722 15 Vastedis

Sweden

ABSTRACT

DANIEL! CentroMet has developed aunique DC technology especially suitablefor melting ofprereduced iron, a conceptthat also should be suitable for smeltingapplications.

The furnace concept offers a more compactfurnace design as compared to traditionalsubmerged arc furnaces due to the operationat higher impedance and an increasedflexibility with respect to the use of varioustypes of charge materials. The furnaceoperation does not require the same controlof the burden resistivity and gaspermeability as is the case with thetraditional submerged reduction furnace.Ore fines can be used without pelletizingand ferroalloy fines can be remelted. Thefurnace operation can also easily beswitched over from production of oneferroalloy grade to another.

The energy loss from the submerged arcprocess is lower compared to the open arcprocess. The more compact furnace design,the higher power density and the far lowerelectrical loss in the furnace secondarysystem compensate for this increased energyloss. The compact design makes it easier toseal the furnace for improved energyrecovery from the process gases.

The furnace can be designed with one, twoor three electrodes, depending on the actualsmelting process and the required power.

The conductive bottom is designed withconductive refractory material or, as analternative, with water-cooled anodes as forsteel making applicationsThe mechanical design of the furnace can beidentical to the steelmaking furnace, i.e.tiltable shell with swingroof or with non­tiltable shell as for the traditional reductionfurnace. The "steelmaking" design offers thepossibility to use an exchangeable shell forincreased flexibility and reduced downtimefor refractory maintenance.

In principle, except for processes whichinvolve compounds with high vaporpressures, the proposed furnace technologycan be used for the majority of reductionprocesses where submerged arc furnacetechnology is used today.

1. INTRODUCTION

Ferroalloys are used primarily in the steelindustry for reduction, deoxidation andalloying to achieve the required chemicaland physical properties for the final product.Manganese is used in virtually allcommercial steel grades, silicon in mostgrades and chromium in all stainless grades.Molybdenum, nickel, vanadium, titanium,boron and niobdenum, colombium andcalcium are also frequently used. Aluminumis used in most steel grades, but aluminumand ferroaluminum are not considered asferroalloys.

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The ferroalloys are added as either iron­based master alloys, carbides, oxides or thebase metals.

Most iron-based or carbide alloys areproduced by carbothermic reduction of 'oxides in electric smelting furnaces. Somealloys are produced by using aluminum orsilicon as reductants. Such reactions arestrongly exothermic consequently there is noneed for any additional electrical energysupply. However, production of silicon andaluminum require significant electricalenergy, therefore the net energy source iselectricity.

The ferroalloy production is consequentlyconcentrated not only in countries withmineral resources but also to countries withlow cost electrical energy.

2. THE SUBMERGED SMELTINGFURNACE

The majorferroalloys, Si, Mn and Cr are allproduced by carbothermic processes. Allsilicon is produced in the conventionalsubmerged furnace. Ferromanganese withlow Mn content can be produced in a blastfurnace, while the majority of theferromanganese is produced in thesubmerged furnace. Most ferrochromium isprocessed in the submerged furnace but asignificant volume is also produced in thesingle electrode DC furnace at Samancor,RSA, the Plasmarc process.

The submerged furnace is principally acounter-current reactor where the carbonmonoxide from the reduction process passesthrough the burden. The charge is preheatedand to some extent prereduced. The energyconsumption is close to that theoreticallyexpected for the actual smelting process.

The heat generation by the supply ofelectrical energy to a submerged furnace isdistributed between direct arcing, currentpassing between the electrodes and between

the bath and the electrodes. The charge mixmust therefore be controlled to give thecorrect resitivity. This is a severe limitation,mainly for the carbon reductants but also forthe slag chemistry in slag processes.

Too much fines in the charge mix can createproblems with pressure buildup with a riskfor random process behavior, increasedenergy consumption and decreased metalyield. Too much coarse material can createproblem with reduced contact between thecarbon monoxide and the charge.

A submerged furnace should preferably beoperated with lumpy ore or high qualitypellets or briquettes. Most of the mineraldeposits used today require mineralprocessing and consequently anagglomeration process before use in thesubmerged furnace.

Successful use of fine material in submergedprocesses are reported from pilot plant trialswith FeSi smelting in a 2 MVA singleelectrode DC furnace. Up to 20 % of the rawmaterials were fed as sand and millscalethrough a hollow electrode.' Such feedingsystem is except for CaCz smelting not usedfor submerged arc furnaces.

The submerged furnace can be characterizedby:

• low energy consumption• low power density, some 300 kW/mz

hearth area for a FeCr furnace• low impedance, some 2 * 10-3 Ohm for a

FeCrfurnace

3. PLASMA FURNACES

The need to use low-grade mineral depositsand consequently ore processing is thedriving force for development ofplasmaprocesses. During the years a number ofprocesses have been developed but only twoof them are used for commercial large-scaleproduction of ferroalloys. Smaller plasma

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furnaces are used for special smeltingapplications.

SKF Steel in Sweden has developed aPlasma Furnace for various smeltingapplications.2 Two large-scale commercialunits are commisioned, Plasmachrome inMalmo, Sweden and Plasmadust inLandskrona, Sweden. The Plasmachromeoperation is discontinued while thePlasmadust, Figure 1, still is in operation.

Figure 1. Principle flowsheet forPlasmadust

The plasma furnace is a coke filled shaftfurnace where metal fines are injected intothe lower part of the shaft. The injection isintegrated with the plasma torches used forheat generation. The electrical energyconsumption is high compared to submergedarc furnaces. The concept is based on theuse of fines and on recovery of the energy inthe off gas. No major technologicalproblems are reported with the operation.

The Plasmarc process, where ore fines,reductants and slag additives are chargedthrough a hollow electrode in a singleelectrode DC furnace, originates from theELRED trials by ASEA in the earlyseventies Figure 2.3

The Plasmarc furnace is more compactcompared to the submerged furnace. Thepower density is higher, some 600 kW/m2

and the furnace operates with higherimpedance, some 5 * 10-3 Ohm.

Figure 2. Schematic view of the ELREDfurnace

The furnace concept was developed into achromite smelting process by the work atMINTEK, RSA, in the late seventies andearly eighties. The first commercialoperation started with a 16 MVA furnace atMiddleburgh Steel, RSA, in 1984. Thefurnace was upgraded to 40 MVA a fewyears later. Samancor, RSA, is presentlyoperating two Plasmarc furnaces.

Unlike the submerged arc furnace, thePLASMARC furnace is an open bathprocess. Consequently, compared to thesubmerged arc furnace, the energy losses arefar higher; typically higher by some 1000kWh per ton of charge-chromium. Thishigher energy consumption is compensatedfor by the use of fines, higher chromiumyield, the ability to remelt metal fines and anoverall better raw materials flexibility.

The material feeding system adds somelimitation to the process. The slag chemistrymust be carefully controlled to avoidclogging. Excessive slag foaming can alsocreate feeding problem. The change of thegraphite electrode is complicated andreduces the furnace productivity.

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The chromite smelting is a carbon saturatedprocess. The bottom anode can therefore bebased on carbon as the conductive media.Electrically conductive magnesia-carbonbricks with metal sheets between the bricks ­build up the bottom. The bottom lasts forseveral years without repairs. This can becompared to the 500-1000 heats for thesame anode concept used for steelmakingapplications. The reason for the short life forsteelmaking application is the far higheroxygen potential, frequent bottom exposureto slag and air and the temperaturefluctuations caused by the batch operation.

4. THE DANIELI TWIN ELECTRODEDC FURNACE

The DANIEL! CENTROMET Arc Furnaceconcept is based on experience acquiredsince 1970 in the design and manufacturingof equipment for electric steel making. Thefurnace concept includes not only thefurnace itself but also all auxiliaryequipment required for the utilisation ofmodem process technologies.A recent development in the DANIEL! DCEAF technology is the Twin Electrode DCEAF concept

At HYLSA, Flat Product Division,Monterrey, Mexico a DANIEL! 135 tonnesTwin Electrode DC EAF with an installedpower of 208 MYA was started up October30, 1998, Figure 3. The furnace is designedfor melting 100% DRI charge.

The maximum power density is 4300 kW/m2

while the normal power density is 3000kW/m2

. The typical operating impedance is8 * 10-3 Ohm.

Figure 3, the Twin Electrode DC EAF atHYLSA

Conventional DC furnaces for steel meltingapplication have only one electrode. Themain features of the twin electrode conceptare:

• reduced electrode consumption• arc deflection towards the furnace

center.

The consumption of graphite electrodes ismainly a result of side oxidation and tipwear. A recent update of the empiricalrelationships by Bowman indicates onlyminor differences in specific tipconsumption between AC and SingleElectrode DC applications.4 For theprediction of electrode consumptionBowman suggests the following formulae:

AC filrnaces

3 [kOX*1t*kED*DE*Lox+]WEAC =--*

TPH 0.013 * I~AC * tu

DCfurnaces, single electrode concept

1 [kOX *1t *k ED *DE *Lox +]WEDC =--*TPH 0.0124 * I~DC * tu

where

WEX = Electrode consumption, X=AC andDC respectively, kg/t

(1)

(2)

]

1

~1

1

J

For the case of a multiple electrode DCfurnace however, equation (2) can be written

I I I II I I I ,aclIvvac

I I I,I .---- I

.>K I Ivv CLI' acI -,- I I I II I I I I

I I I II I I "cC'" 'cc

I I I

The corresponding arc power and electrodecurrent is shown in Figure 5.

16014060 80 100 120EAF tap weight, tonnes

40

legend: Wac = AC EAFWdc1= Single Electrode DC EAFWdc2= Twin Electrode DC EAF:

Based on the conditions above, the electrodeconsumption has been calculated andcompared with the consumption for modernHigh Impedance EAF's. The result is shownin Figure 4.

Relative Graphite Consumption

1.3

1.21.1

1

0.9

0.80.7

0.6

(3a::0.3

20

Production, t/hconstant depending on oxygenblowing practice.

DE + DTIP

2D ENominal electrode diameter, mElectrode tip diameter forelectrode in "steady state"condition, mElectrode current, X=AC and DCrespectively, , kATime utilisation factor, poweronltap-to-tap

TPHkox

Number of graphite electrodesTotal electrode current, kA

Figure 4. Relative electrode consumption asfunction ofEAF tap weight

A fair comparison of various EAF conceptsshould be based on some commonconditions with respect to tap-to-tap timeand installed power.

Since the productivity, TPH, is dependent onpower, charge weight and energyrequirement, the calculation of anticipatedgraphite consumption could be based on thefollowing:

25

50

75

50

25

75

Figure 5. Arc power and total DC electrodecurrent as function of EAF tap weight

The twin electrode concept has lowerelectrode consumption than both theconventional AC furnace and the singleelectrode DC furnace at high power input.

Arc Power, MW Total DC Electrode current, ~A

150 ,-------,----,,.-----.-----,1--'1-----,-------, 150ectro e current

125 t---l---~f----+--~==::;:?:::::::.----I-___J125

100 1----t-------iI-----:!---=-------i---+------c=!-------1 100

= specific arc power kW/t to beequal (700 kW/t)= AC arc power equal to DCarc power= tap-to-tap time to be equal= time utilisation to be equal= fix relation between DC arcvoltage and AC arc voltage,UDC = 1.5*UAC

= selected DC arc voltage beinga function of DC current= electrode diameter being afunction of electrode current

mtuV DCAC

PSARC

U ADC

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The EAF current conducting system isinfluenced by electromagnetic forces, whichare created by the current in adjacentconductors. For the case in which the currentin adjacent conductors flows in the samedirection, this force will tend to pull the twoconductors towards each other. However, inthe case in which the current flows inopposite directions, the two conductors willtend to be pushed away from each other; seeFigure 6.

Figure 6, arc deflection.

The forces acting on parallel conductorswith the length L and at a distance "a" fromeach other can be calculated:

F = L * 1l!(2*7t*a) * i1 * i2

where

L = length of the conductora = distance between the conductorsIJ. = magnetic permeabilityIj = instantaneous current value

The consequence of this relationship for theconventional three electrode AC furnace is adeflection towards the slag line. This cancreate problems not only by excessive slagline wear but also mechanical stresses on theelectrode including the arms and masts.

In a single electrode DC furnace the arcdeflection is a function of electrode currentand the magnetic flux generated byconductors adjacent to the arcing zone. By aproper routing of the conductor arrangementunderneath the furnace bottom it is possibleto minimize the magnetic flux in the arcingzone and thereby ensure a vertical arc andconsequently no hot spot zone.

In the case of a twin electrode DCconfiguration however, the two arcs will bedeflected towards the furnace center.Continuous feeding of charge material intothe arc zone favors a fast melting and willalso limit the superheating of the melt.

The bottom anode concept for a steelmakingfurnace must be designed differently to asmelting furnace. The discontinuoussteelmaking operation with largetemperature variations, frequent exposure toaggressive oxidized slag and frequent directcontact between the metallic charge and theconductive bottom reduces the bottom lifecompared to a smelting furnace.

The DANIEL! anode design is optimised forsteelmaking. The most importantrequirements on a bottom anode designcould be summarised as follows:

• Long campaigns between maintenance• Easy and swift maintenance• Quick start up procedure in cold

condition• Quick start up procedure in hot

condition after bottom inspection• Capability to withstand intense oxygen

injection into the melt• High current conduction capacity• Reliable, consistent and safe operation• Possibility to supervise anode current

conduction conditions• Possibility to continuously supervise the

anode thermal condition

In a high productivity unit it is ofimportance to limit maintenance of theanode(s) to periods with scheduled shutdowns not related to the anode itself, e.g., incombination with slag line repair.Having the above mentioned design criteriain mind DANIEL! decided to develop awater-cooled bottom anode, figure 7. After aresearch and development stage, whichincluded theoretical calculations,simulations as well as laboratory tests, theselected solution was installed in an

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industrial scale plant at the BalboaSteelworks in Spain.

Figure 7.The DANIEL! water-cooled bottomanode

The bottom anode system comprises fourwater-cooled anodes embedded in therefractory lining. The anode is built with anupper part made of steel and a lower copperpart, welded to the steel part. The lowercopper part is with a tubular section and aninner top surface machined to form finsarranged in a spiral configuration. This'ensures a high water velocity over the entiresurface, which together with the contributionfrom the fins results in a high heat transfercapacity.) .

So far, the performa~ce with this anodedesign is better than expected. The power ontime between the replacement exceeds 3000hours for the HYLSA installation ascompared to 500-1000 hours for anyalternative design.

5. CHROMITE SMELTING IN ATWIN ELECTRODE DC FURNACE

The possibility to use a modified twinelectrode concept for smelting applicationshas been studied. The furnace and processconcept have some similarities to the presentoperation of the PLASMARC process. ThePLASMARC concept is based oncontinuous feeding of material through a

hollow graphite electrode. This favors agood recovery of fines but the system alsoadds some restrictions to avoid clogging inthe electrode.

The experience with feeding 100 % hot DRIdirectly into the arcing zone between thetwo electrodes in the HYLSA furnace ispromising. The observations so far do notindicate more dust losses as fines ascompared to batch charging. The furnaceoperates close to equilibrium and there is nosign of losses due to insufficient reductionof the DRI.

Chromite and DRI is entirely different andthe kinetics for chromite reduction must befurther evaluated. The power density for theHYLSA furnace is far higher compared tothe PLASMARC process. The residence'time for the charged material isconsequently shorter.

Recent studies at the Royal Institute ofTechnology in Stockholm 6.7 on reduction ofchromiumoxide in liquid slag with carbonshows not only that the slag viscosity andthe temperature have the expected influence.More interesting is the conclusion thataddition of metal, preferably carbonsaturated, increases the reaction rate. Thedifficulty of nucleation of solid chromiumresults in an incubation period. Once a liquidmetal phase is formed or introduced theincubation time can be shortened oreliminated. Remelting metal fines shouldconsequently improve the reaction kineticsfor chromite in processes where the mainpart of the reduction takes place in the slagphase. .

The energy balance for a hypotheticalchromite smelting operation in the HYLSAPIanos furnace can illustrate the use of theDANIEL! twin electrode DC furnace forsmelting application.

The calculation is based on followingassumptions:

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• There are no process limitations. Thisassumption is necessary for thecalculations and must be studied infuture pilot and full-scale trials.

Table 2 shows the global materials balance.The dust is calculated as dust collected inthe bagfilter. Mg(g) and SiO(g) leaving thefurnace are oxidized to MgO and Si02respectively. The Cr yield is 91.9 %.

kg % Cr % C % Si % Fe

Metal 10000 56.7 7.2 2.3 33.8

Metal, slag 11 56.7 7.2 2.3 33.8

• The raw materials composition is shownin Table 1.

Cr203 FeO Si02 CaO AI203 MgO C% % % % % %

Chromite 50.0 25.0 5.0 1.0 10.0 9.0Carbon 0.9 6.2 0.9 2.7 0.9 88.5

. Lime 10.0 90.0Slag

Dust

% Cr203 % Si02

569 3.8 12

131 35 33 10

%A1203 %MgO

35 28

5 12

Table 1. Raw materials composition Table 2. Materials balance

• The dust generation is 5 % of the chargeexcept for elements forming specieswith high vapor pressure and for carbon.This is more than observed in HYLSA.

Figure 9 3 shows the energy consumptionfor the smelting reactions.

• The slag is reduced to 3.8 % Cr203. Theslag contains 2 % metal as immerseddroplets.

ENERGY FOR CHROMITE SMELTING

16%

• The furnace is closed.

Figure 3 shows the system boundary.

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o METAL11 SLAGo DUST_GAS

o REACTIONS57%

Figure 9. Energy consumption for thesmelting reactions.

Water DT Qm3/h C MW

Roof 950 8 8.8Wall 1070 4 5.0Anodes 160 0.3 0.1

Figure 9 does not include the furnace ·losses.The measured losses to the water-cooledparts of the HYLSA furnace are shown inTable 3.

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The furnace is operated at a temperaturesome 30 C over the estimated slagliquidus temperature.

•

• The slag from the chromite must befluxed. Lime and Si02can both be usedto flux the actual system. Lime is usedto minimize the Si02content to avoidenergy consumption by reduction ofSi02.

. .-.-_.." ..---"

Figure 8. System boundaryTable 3. Losses to water-cooled parts in theHYLSA furnace

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Table 4. Energy consumption, no preheating

Base 1000 kg chargechromium

These losses are measured for the normalHYLSA operation, continuous feeding ofDRI at flat bath conditioning and 1550 QCsteel temperature. The assumed operationtemperature for chromite smelting is1750 QC. The major part ofthe loss is byradiation from the bath. The losses to theroof and wall can be estimated:

Q = QORI *(2023/1823)4Q = QORI * 1.5

Smelting

PowerElectrical lossThermal lossProductivityElectrical energy

3211

966

21.721.34513

kWh

MWMWMW1000 kg/hkWh

The losses from the refractory shell aresome 1 MW. The total thermal losses forchromite smelting is:

Q =21.7 MW

The losses in the power supply system arecontrolled by the current:

This electrical energy consumption is some500-1000 kWh higher as compared to mostsubmerged operations. Figure 10 shows theannual output for some chromite smelters.8

The regression fits exactly to 3800kWh/1000 kg FeCr assuming typical valuesfor availability and power factor.

FeCr FURNACES

Figure 10. Production capacity for FeCrsmelters.

140

•

100 120

U 10c0TON• - - tOCOTON THEORETlCAl

• DC.120MVA,25C• DC. 120MVA, 1050C

-!Jne;lIrlOClOTON

R~= 0.88

80

.-0

6040

o

o"

INSTAllED TRANSFORMER CAPACITY, MVA

IT} 0

250z:e§ 200

~U<{

150a.<{uzg

100u=>0 0Qera.

50<t o 0=>z ,uz<{ o .r· :!.J

0 20

= sum of the individual resistanceson the primary side

= current on the transformersecondary side

= sum of the individual resistanceson the secondary side= current on the transformer primary

where

Q

The loss for the Hylsa furnace is 6.0 MW for150 kA. The corresponding power is 96MW. The main part of the losses is causedby the resistance in the electrodes.

Total furnace losses:

Qtot = 27.7 MW

Table 4 shows the productivity andconsumption of electrical energy.

The higher electrical energy consumptionfor the described furnace concept is to someextent compensated for by the increased rawmaterials flexibility, the use of raw materialsfines and a high Cr yield. The furnaceconcept also offers good possibilities forcharging preheated and prereduced material.

The huge amount of CO gas leaving thefurnace can be used for prereduction andpreheating. Figure 11 show the equilibriumconditions for prereduction of chromite.The reduction of chromite starts at some 900QC. The electrical energy consumption for acharge preheated to 75 % of the equilibrium

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conditions at 1050 cC can be estimated tosome 3400 kWh, Table 5.

1c 11\ I J I II I I \1 I L!g)I I ~! I 1 I

I I .r\ I II I I /1 \ I

I ICr2FcO~ I I Fe

I I _J~~Cr?~3 !-j~--

kmol

o500 700 900 1100 1300

Temperature

1500 C

• Arcing zone concentrated in furnacecenter providing quick melting ofcontinuously charged materials

• Flexibility in accepting various chargematerials on form of lump or fines

• The ferroalloy composition can easily bechanged to meet changed marketdemand.

• More simple and faster control oftheprocess compared to submerged arcoperation

Base 1000 kg chargechromium

6. CONCLUSIONS

Figure 11. Equilibrium for chromitereduction.

Table 5. Energy consumption, preheating to1050 cC.

References

1. Kjellberg, Orrling. DC Arc Sin~le

Electrode Smelting Furnace, 411

International Ferro Alloys Congress, RioDe Janeiro August 31-September 3 1986

2. Wiklander, Herlitz, Santen·. The FirstYear ofOperation at Swedchrome,Electric Furnace ConferenceProceedings Vo145, Chicago 1987

" Stenquist, Bowman. High-Power,.).

Graphite-Cathode DC Arc Plasma-Properties and Practical Applicationsfor Steelmaking and FerroalloysProcessing. Plasma Technology inMetallurgical Processing. ISS 1987

4. Bowman. An Updated Model forElectrode Consumption. UCAR paper1987

5. Bergman. Twin Electrode DC powered

JEAF Among the latest improvement inDANIELI Electric Arc Furnace

JTechnology, ISS New MeltingTechnologies, Florida 1997 ]

6. Pei. Chromite Smelting Reduction -

]Kinetic and Thermodynamic Studies.Doctoral Thesis Royal Institute ofTechnology Stockholm 1994 ]

7. Gornerup. Studies ofSlag Metallurgy in ]Stainless Steel Making. Doctoral Thesis IRoyal Institute of TechnologyStockholm 1997

8. Ferroalloy Directory and Data Book,Edition 5. Metal Bulletin

9. Thermodynamic data and calculationsare based on HSC version 4.1,Outukumpu Research Oy, Pori Finland.

2431 kWh

96 MW6 MW

21.7 MW28.1 1000 kg/h3421 kWh

Smelting

PowerElectrical lossThermal lossProductivityElectrical energy

• Compact mechanical design requiringmlI1l1num space

• High productivity due to high powerdensity, 4-5 MW/m2

The energy consumption in an "open arc"furnace for smelting applications, similar tosteel melting practice, is higher compared tothe traditional submerged arc process. Thisis a result of heat losses to water-cooled roofand side wall panels typically used infurnaces designed for steel making.The DANIEL! Twin Electrode DC concepthowever, provides some specific featuresthat can compensate for this. Based on theexcellent performance when meltingcontinuously charged DRI the followingfeatures can be identified:

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