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(PDF) Ironmaking and Steelmaking

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Added on  2021-03-16

(PDF) Ironmaking and Steelmaking

   Added on 2021-03-16

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Iron Making and Steel Making Prof. Gour Gopal Roy Department of Metallurgical and Materials Engineering Indian Institute of Technology, Kharagpur Lecture – 02 Various Routes of Steelmaking In this lecture, Various Routes of Steelmaking has been discussed. The major route of steelmaking is by the BF-BOF route. BF stands for Blast Furnace and BOF stands for Basic Oxygen Furnace. Basic oxygen furnace mostly represent the LD furnace. Another route is DRI-EAF route. DRI stands for Direct Reduced Iron, or sponge iron. And EAF stands for Electric Arc Furnace. In this route, DRI along with hot metal is refined in EAF. The third route is smelting reduction (SR) route, where hot metal equivalent to blast furnace is produced using non-coking coal in especial SR reactor. One of the popular SR unit is the COREX process. Direct steelmaking is a potential route of producing crude steel in a single reactor using non-coking coal. Figure 1.1 BF-BOF route of steelmaking Next let us discuss about what is the BF-BOF route. The reactor where liquid iron is produced is called the blast furnace. Inputs to blast furnace are coke, made in a coke oven using special grade of coal called the coking coal; sinter, made by agglomerating undersize iron ore and iron bearing solid waste to some extent. And then you have calibrated lumpy iron ore, those are run of mine ores. Run of mine ores represents lumpy
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iron ore directly from mines, which are calibrated in size like 40 mm in diameter. Sinter is a better iron burden in blast furnace than lumpy ore due to its better strength and reactivity and consequently it has a large share for iron burden charge. Then flux is added to partition impurities like sulphur and silicon in the slag. Obviously, you require hot 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 metallic iron 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 is slag, which is mostly used in cement industry, the cleaned BF gas is partially used in BF stove for air preheating and rest in downstream application. Hot metal from BF goes to BOF, or LD furnace for steelmaking, where the liquid iron is charged along with some coolant like scrap up to 25 percent. And BOF is an autogenous process where heat is generated by the impurity oxidation and the heat generated is more than to sustain the process, which allows charging of some cold charge like scrap as coolant. The other input to BOF process are pure oxygen and the flux. And, what you get as products are crude liquid steel and liquid slag. But unlike BF slag, LD or BOF slag contains FeO and P2O5significantly and more importantly CaO/SiO2 ratio is not appropriate for cement making. So, mainly it remains unutilized. It is partially used in sintermaking. LD slag might require some post processing to get that into cement making, but economically viable process has not yet been found. Lots of investigations are going on to find avenues to utilize the LD slag, which is otherwise is piling up near to the plant site. So far, majority of crude steel production is through BF-BOF route because both the furnaces used are rugged, well tested and efficient. From the point of view of chemical and thermal efficiency, blast furnace is fantastic, and it is a very beautiful reactor. Only problem 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 frazile lumpy ore, which requires separate reactors, like coke ovens, sinter and pelletizing plants, which pollutes atmosphere. Strength of the burden material is important because as the burden moves down through the blast furnace, those undergoes impact and abrasion and if they do not have sufficient strength they will generate fines and reduce the bed permeability and hampering the counter current gas solid reaction in the dry zone
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of the blast furnace. In the wet zone at the lower part of the furnace coke has a great role to play by providing the path for gas passage. BF is a complicated reactor combining dry zone, wet zone through an interface called the cohesive zone and maintain a nice balance heat and chemical exchange and quality of the burden is very important to run such unique and complex process. On the other hand, the major limitation of BOF is that it has limited capacity to accept the solid charge. Being an autogenous process, it can accept on 25% solid charge only as collant. But today, solid charge are increasing in terms of DRI/sponge iron (by utilizing non-coking coal and reformed natural gas) and therefore, a reactor with more flexibility in terms of acceptance of solid and liquid charge in any proportion like, EAF is getting an edge. Now, let us go to the DRI -EAF route, i..e, the direct reduced iron and the electric arc furnace route. DRI/sponge iron is produced in reactors called the rotary kiln or the shaft furnace. In rotary kiln is a coal based DRI unit where lumpy iron ore is reduced using non-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 reduce the iron ore pellets/lumpy iron ore. Natural gas is reformed by nickel catalyst to CO and H2, which are subsequently is passed into shaft reactor for reducing iron ore in solid state to sponge. Now-a-day, synthetic gas prepared by coal gasification is also being considered for DRI production in shaft reactor.
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