Technologies for cost reduction in EAFs - Steelworld

F o c u sTechnologies forcost reduction inEAFs- By B.V.R.Raja, N.Pal, P.L.Talwar, N.P.JayaswalAlloy Steels Plant,Steel Authority of India Ltd.,IntroductionElectric Arc Furnace (EAF) steelmaking accounts for 33.10% of theworld steel capacity and is consideredto be the only route for making highalloy steels. Technologicaldevelopments have been continuouslytaking place in EAF process for thepast 40 years to improve uponcapacity, productivity, efficiency andquality of the steels with hugeemphasis on cost reduction. Withadvantages of lower investment costscoupled with better flexibility towardsthe input raw materials, energy inputsand steel grade mix, the world steelproduction through this route isprojected to touch the magic figure of50% by 2010.Technological developmentsAlternate scrap useThe major contributor for highproduction cost in EAF has been theraw materials wherein, metallic chargewith 100% cold scrap accounts for87% of the total cost of raw materials.Steel scrap has been the main sourceof metallic feed from EAF steelmaking. The consumption is to the tuneof 900 Kg/T of liquid steel from a totalcharge of 1080 Kg/T of liquid steel ifthe metallic charge is mostly scrap.The physical and chemicalcharacteristics of the scrap have adirect influence on the performanceof EAF melt shop and its operatingcost. Hence, the scrap price is crucialto the economies of electric steelmaking.The advent of continuous castinghas reduced the internal scrap arisingwhich is of high quality from a level of20% to 10% and further decreasing.The only growth in availability of scrapis from consumer durables andautomobile industries which is ofvariable quality. There has been agrowing trend in scrap preparationfacilities throughout the world toupgrade the quality and recovernotable non-ferrous metals throughrecycling. All this lead towards use ofhigh quality scrap with the upgradedquality scrap from fragmentizing unitsas metallic feed in EAF for cutting thecost.Scrap preheating systemsIt is a well known fact that 20%of the energy generated in EAF leavesin the form of offtake gases.Preheating systems have evolved invarious plants that use the waste gasesto preheat the scrap about 250°C -350°C and charged in the furnace toresult in energy savings of 40 Kwh/tonne. Fuchs’ Shaft Preheater helpsto integrate scrap preheating with theEAF wherein the first bucket of scrapis charged into the furnace with theroof open and rest through the shaft.Scrap preheating is done in the shaftby the offtake gases and there areabout four oxy-fuel burners of 3 MWeach installed in the side wall of theshell.Consteel process is a methodwhich involves sized scrap conveyedinside a tunnel like shell, hot furnacegases and gases from natural gasburners in the tunnel preheat the scrapand fed continuously to EAF. Davy-Clecim twill shell electric arc furnacein which scrap is preheated to 800°C-1000°C in one vessel by burners, whilethe other vessel is dedicated to electricmelting. These processes werereported to reduce the powerconsumption by 200 Kwh/tonne ofliquid.Emerging low costmetallic feedsThe majority of scrap is consumedby Basic Oxygen Furnace units resultingin shortage of scrap for electricfurnace steel makers. The rocket highprices of imported scrap is a cause ofworry for EAF steel units. This led thesteel makers through electric arcfurnace route to look for viable optionsfor substituting scrap by alternate lowcost metallic feeds. Considering thescrap shortage, EAF steel meltingSteelworld • April 2005 • 1

F o c u sshops are considering usage ofmetallic feeds like Sponge Iron and HotMetal for reducing the production cost.Sponge iron ( DRI/HBI )The evolution of sponge iron asmetallic feed in EAF has been mainlydue to its cost advantage over scrap.It is a high quality metallic productproduced from iron ore/pellets and canbe used as a suitable feed stock aspartial substitution to scrap in EAF. Itoffers benefits like guaranteed uniformand predictable compositioncontaining low amounts of sulphur,phosphorous & tramp elements alongwith environmental friendliness duringusage. The typical composition ofsponge iron is shown in Table 1.Coal based sponge iron hascarbon content of 0.25% andmetallization of 90% while gas basedsponge iron has carbon of 1.6% andmetallization to the tune of93%.Sponge iron has bulk density of2 – 3.3. t/cu.m. and is higher thanthat of most scrap. The actual densityis more than that of slag in thefurnace to facilitate sponge ironmelting at the slag/metal interface.When sponge iron is used, thereis a need to reduce the iron oxide andto neutralize the acid gangue. Thereduction of 1% iron oxide requiresapprox. 2.3 Kg of carbon and 12 Kwh/ton of liquid steel. The neutralizationof acid gangue (Al 2O 3+ SiO 2) requires20 Kg of lime and power to the tuneof 11 Kwh/tonne of liquid steel toachieve required slag basicity.Continuous charging is the widelypracticed method wherein chargingstarts after creation of partial moltenpool in the bath and is continuedthrough the charge bins provided alongthe roof. This charging system enablesbetter distribution of charge withimproved bath heat transfer and slagmetalmixing. It lowers heat losses andhelp in producing stable arc withimproved productivity and up to 80%DRI in the charge can be successfullyfed through this method.The optimum benefits aregenerally believed to be attained whenthe melting of DRI takes place at theslag/metal interface whichnecessitates appropriate feed rate.The feed rate further is dependent onthe chemical composition of DRI andthe bath temperature. Generally, feedrates of 27 to 35 Kg/min./MW ofapplied power are maintained.However, higher the metallizationTOTAL Fe 87% - 94% P 0.007% - 0.05%METALLIC Fe 76% - 89% SiO2 2% - 4%METALLIZATION 86% - 93% Al2O3 0.60% - 2.7%C 0.20% - 2.4% CaO + MgO 0.2% - 3%S 0.01% - 0.03%Table 1 - Typical composition of sponge iron(optimimumrequired levelis 93%),lower isthe powerconsumptionand higher yield which necessitatescarbon injection systems to take careof metallization problems. It has beenobserved that for every 1% decreasein metallization, the increase in powerconsumption is to the tune of 18 Kwh/ton. Productivity about 1 ton per houris reported for every 10% increase inDRI usage.There has been a new trend incontinuous feeding of hot DRI withtemperatures of 500ºC - 600ºC forthe units which has accessibility to DRIfurnace. Introducing hot DRI surge binbetween the DRI furnace and EAF, upto 80% DRI in the charge issuccessfully fed. With hot DRIcharging, the effect of metallization iscompensated by the temperature ofDRI itself thus possessing highpotential in saving energy. Thispractice can reduce powerconsumption by 20 Kwh/T of liquidsteel for each 100ºC increase incomposite charge temperature. Energysavings of 105 Kwh/t for 80% DRIcharge compared to 52 Kwh/t for 40%DRI charging have been reported.Electrode consumption gets reducedby 30% when continuous feeding ofDRI as it produces stable steel bathreducing the possibility of electrodebreakage , compared to theconventional 100% cold scrap chargepractice. This is due to higherproductivity, decreased electrodeoxidation, less frequency of roofopening meaning less thermal shock,less CO in furnace atmosphere andless electrode breakages.Hot metal (Liquid Pig Iron)This is a low cost metallic feedfor EAFs which have access to blastfurnace or cupola. The typicalcomposition of hot metal is shown inTable 2.C Mn Si P S3.75% 0.48% 0.85% 0.18% 0.035%Table 2 - Typical composition of hot metalThe sensible heat of molten pigiron at the melting point is around1084 MJ/mg. Assuming the heatefficiency of EAF at 80%, this heat willbe equivalent to about 1350 MJ/mg(375 Kwh/Ton) of liquid pig iron. It isestimated that 1 Ton Hot Metal + 25Kg Lime = 0.92 Ton of clear scrap +40 Kg of carbon + 330 Kwh. To derivemaximum benefit from the addition ofliquid pig iron, it is necessary to lookat parameters like the composition ofthe charge mix, thermal condition ofthe charge at the time of hot metaladdition and proper melting practiceto take care of high phosphorous andcarbon.Hot metal is added in EAF throughspout ladles after roof opening. It isadded after partial melt formation inEAF. If hot metal addition is carried outto a full melted over oxidized steel bathin EAF, reactions occur leading toviolent boiling. Hot liquid metal ifadded soon after power on, in contactwith cold scrap immediately freezes2 • Steelworld • April 2005

F o c u sdue to chilling effect. Hence, additionof liquid metal on sufficiently heatedscrap seems to be the most efficientcharging practice. This is because, asthe scrap gradually melts, the alreadyliquid pig iron gets refined under theaction of iron ore and lime in thecharge.The amount of hot metal to becharged depends on the grade of steelto be made and the amount of oxygendeemed efficient. Successful hotmetal practices reveal usage to theextent of 60% of the total charge. Themajor problem with hot metal isdephosphorization of the melt in EAF.This is taken care through addition ofsufficient amount of iron ore in thecharge that facilitates high FeO (15%- 20% ) and 75% dephosphorizationbefore oxygen blow at melt down stage.Carbon is removed by iron ore partiallyand the required level of carbonachieved through oxygen injection athigh rates. It is noticed that hot metalposes no problem with regard to qualityrequirements of carbon and low alloysteels. Energy savings of 130 Kwh/T,increase in productivity by 25% andreduced electrode consumption by15% are achieved through use of 60%hot metal in the charge.Hot metal + Sponge ironThe new trend for cost effectiveproduction processes in EAF iscombined use of hot metal and coalbased sponge iron by completelysubstituting scrap. With hot metalusage, initial slag of EAF has low FeOcontent unless sufficient amount ofiron ore is charged which makes theslag viscous leading to poorphosphorous removal and poor arcefficiency caused by non-foaminess ofthe slag. Due to high carbon in hotmetal, the decarburization raterequirement is as high as 0.25% C/min which limits its usage in EAF dueto low depth/volume ratio. Bycontinuous feeding of coal based DRI,dephosphorization becomes fasterand FeO of DRI promotes foamy slagpromoting decarburization thusreducing oxygen blowing time. Even,increase in yield to the tune of 2%achieved compared to 80% DRImethod.The various practices followed areusage of hot metal between 40% to60% and the remaining with DRI.Productivity improvement by 50%,energy savings of 200 Kwh/T andelectrode consumption reduction by40% have been reported through thismethod.Low ow cost energy inputsAs energy accounts for around12% of the production cost, there isan increasing trend in application ofoxygen as chemical energy input forreducing energy consumption. Today,oxygen to the tune of 30 Nm 3 /t is beingused in many EAF units in the world toreduce melting time & powerconsumption coupled with recovery ofenergy through CO post combustion.Moveable oxygen injection systems likesubmersible hand lance, consumablepipe manipulators, water cooled lancesor retractable burners and fixed oxygenintroduction systems like oxy-fuelburners, supersonic jet burners, carboninjection lance with oxygen and virtuallance burners are widely used. Savingsin energy is about 3 Kwh/Nm 3 oxygenper ton of liquid steel when one kg ofcoke is added in EAF. Evolution ofthese technologies helped to meet10% energy requirements andproductivity improvement of around8%.Ultra high powered furnacesIt is a well known fact that higherthe rate of power input, higher thethermal efficiency due to reduction intime dependant heat losses. Thisresulted in development of Ultra HighPowered EAF units with rating of morethan 700 KVA per ton. However, higherrate of power increases arc currentwhich would increase electrode tiperosion. Hence, with higher powerinput, the applied voltage has to bemore which can be achieved throughuse of longer arcs. These longer arcsled to high refractory erosion. To takecare of this problem, water cooledpanels were developed to replacemuch of the furnace side wall and roofrefractory. Though the application ofpanels drastically reduced therefractory consumption, the heatlosses increased which made EAFsteel makers to splash molten slag forimproving thermal efficiency. Otherdevelopments that took place tocounter the long arc operation includefoamy slag practice. In this practice,carbon and oxygen are injected in bathincreasing the slag cover depth from0.1 m – 1.0 m acting as additionalenergy burying the arc and improvingthermal efficiency. The process led topower savings to the tune of 40 Kwh/T along with better slag deoxidation.Bath stirringThis development helped toabsorb the high power inputs presentlyin use coupled with savings in yield tothe tune of 5 Kg/T and energy about14 Kwh/T. Also, bath stirring resultedin producing slag with low FeO & MnOSteelworld • April 2005 • 3

F o c u scontents decreasing aluminumconsumption.Slag free tapping systemsSlag free tapping technologiesevolved for improvement in steelquality and also productivity throughlow temperature tapping and shortertap duration. Eccentric Bottom Tappingprocess is the one where the bottomhalf of the furnace is fitted witheccentrically located tap hole bywidening the tapping area andremoving the conventional spout.Submerged Tap Hole method is amodification to the conventional taphole with the tap hole spout extendingthe hearth profile allowing steel tappingwithout vortexing. Though, this processis cheaper and efficient, the tappingtimes and temperature losses are inexcess compared to other techniques.Slide gate Tapping technique is ahydraulically operated linear slidinggate valve mounted on the outlet taphole of the furnace. Here, the slidegate remains in open mode throughoutthe melting period and the tap hole isfilled with filler material by use ofpusher rod with the filler materialremoved prior to tapping by scoopdevice or by mild oxygen lancing. In thistechnique, the gate valve is closedonce slag just starts coming in. Thebenefits accrued through thesesystems include production of low “P”steels, extra low “S” steels, decreasein treatment time of secondary refiningunits and reduced aluminumconsumption.Automation systemsComputer assisted steel makinghas evolved through development ofsensors and programming of powerinput, charging practice, amount ofoxygen lancing and adjustments ofchemistry during melting & refining.Process control equations throughprogramming are applied for assessingthe overall furnace performance as wellas calculating requisite amounts ofoxygen and power input for achievingthe desired tapping conditions forcarbon and temperature. Least CostMix programmes are developed withprocess cost elements like yield, timeand energy as a means to cut downcosts.ConclusionsContinuous developments hasreshaped EAF steel making process tomeet the steel requirements of theworld. Modifications are still on in EAFto bring in further improvements inproductivity, energy economy,application of cheaper metallic feeds,reduced electrode & refractoryconsumption to make EAF as thelowest cost steel producing route inthe world. !!!4 • Steelworld • April 2005