(PDF) Ironmaking and Steelmaking

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Iron Making and Steel MakingProf. Gour Gopal RoyDepartment of Metallurgical and Materials EngineeringIndian Institute of Technology, KharagpurLecture – 02Various Routes of SteelmakingIn this lecture, Various Routes of Steelmaking has been discussed. The major route ofsteelmaking is by the BF-BOF route. BF stands for Blast Furnace and BOF stands forBasic Oxygen Furnace. Basic oxygen furnace mostly represent the LD furnace. Anotherroute is DRI-EAF route. DRI stands for Direct Reduced Iron, or sponge iron. And EAFstands for Electric Arc Furnace. In this route, DRI along with hot metal is refined inEAF. The third route is smelting reduction (SR) route, where hot metal equivalent toblast furnace is produced using non-coking coal in especial SR reactor. One of thepopular SR unit is the COREX process. Direct steelmaking is a potential route ofproducing crude steel in a single reactor using non-coking coal.Figure 1.1 BF-BOF route of steelmakingNext let us discuss about what is the BF-BOF route. The reactor where liquid iron isproduced is called the blast furnace. Inputs to blast furnace are coke, made in a cokeoven using special grade of coal called the coking coal; sinter, made by agglomeratingundersize iron ore and iron bearing solid waste to some extent. And then you havecalibrated lumpy iron ore, those are run of mine ores. Run of mine ores represents lumpy
iron ore directly from mines, which are calibrated in size like 40 mm in diameter. Sinteris a better iron burden in blast furnace than lumpy ore due to its better strength andreactivity and consequently it has a large share for iron burden charge. Then flux isadded to partition impurities like sulphur and silicon in the slag. Obviously, you requirehot air blast for combustion of carbon to generate the required heat to run the furnace.Carbon burns at the bottom of the furnace generating heat to melt the reduced metalliciron to liquid iron (hot metal), which is an alloy of iron containing 3.5-4% carbon,~1wt% Si, 0.5% Mn, 0.05% S and 0.4%P. The other liquid product from blast furnace isslag, which is mostly used in cement industry, the cleaned BF gas is partially used in BFstove for air preheating and rest in downstream application.Hot metal from BF goes to BOF, or LD furnace for steelmaking, where the liquid iron ischarged along with some coolant like scrap up to 25 percent. And BOF is an autogenousprocess where heat is generated by the impurity oxidation and the heat generated is morethan to sustain the process, which allows charging of some cold charge like scrap ascoolant. The other input to BOF process are pure oxygen and the flux. And, what youget as products are crude liquid steel and liquid slag. But unlike BF slag, LD or BOF slagcontains FeO and P2O5significantly and more importantlyCaO/SiO2 ratio is notappropriate for cement making. So, mainly it remains unutilized. It is partially used insintermaking. LD slag might require some post processing to get that into cementmaking, but economically viable process has not yet been found. Lots of investigationsare going on to find avenues to utilize the LD slag, which is otherwise is piling up near tothe plant site.So far, majority of crude steel production is through BF-BOF route because both thefurnaces used are rugged, well tested and efficient. From the point of view of chemicaland thermal efficiency, blast furnace is fantastic, and it is a very beautiful reactor. Onlyproblem in the blast furnace is that it requires very stringent raw material requirement.For example, coke has to be used in place of coal, sinter and pellets in place of frazilelumpy ore, which requires separate reactors, like coke ovens, sinter and pelletizingplants, which pollutes atmosphere. Strength of the burden material is important becauseas the burden moves down through the blast furnace, those undergoes impact andabrasion and if they do not have sufficient strength they will generate fines and reducethe bed permeability and hampering the counter current gas solid reaction in the dry zone
of the blast furnace. In the wet zone at the lower part of the furnace coke has a great roleto play by providing the path for gas passage. BF is a complicated reactor combining dryzone, wet zone through an interface called the cohesive zone and maintain a nice balanceheat and chemical exchange and quality of the burden is very important to run suchunique and complex process.On the other hand, the major limitation of BOF is that it has limited capacity to acceptthe solid charge. Being an autogenous process, it can accept on 25% solid charge only ascollant. But today, solid charge are increasing in terms of DRI/sponge iron (by utilizingnon-coking coal and reformed natural gas) and therefore, a reactor with more flexibilityin terms of acceptance of solid and liquid charge in any proportion like, EAF is gettingan edge.Now, let us go to the DRI -EAF route, i..e, the direct reduced iron and the electric arcfurnace route. DRI/sponge iron is produced in reactors called the rotary kiln or the shaftfurnace. In rotary kiln is a coal based DRI unit where lumpy iron ore is reduced usingnon-coking coal. Required heat is also generated by burning coal fines inside the reactor.Shaft furnace are the gas based DRI units where reformed natural gas is used to reducethe iron ore pellets/lumpy iron ore. Natural gas is reformed by nickel catalyst to CO andH2, which are subsequently is passed into shaft reactor for reducing iron ore in solid statetosponge.Now-a-day,synthetic gaspreparedbycoalgasificationisalsobeingconsidered for DRI production in shaft reactor.
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