Question 5 of 7: Iron and Steelmaking (20 marks)

Question 5 — Iron and Steelmaking (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Three major feed materials. The burden charged at the top of a blast furnace consists of iron-bearing material — sinter, pellets or lump ore, essentially Fe₂O₃ and Fe₃O₄; metallurgical coke; and flux, normally limestone (CaCO₃) with dolomite where magnesia is wanted in the slag. A fourth feed enters at the bottom: the hot blast of preheated air, together with injected auxiliary fuel such as pulverised coal.

(b) The function of coke. Coke performs four distinct duties. First, it is the fuel: burning at the tuyeres, $\mathrm{2\,C + O_2 \rightarrow 2\,CO}$, it releases the heat that raises the burden to reaction temperature and melts iron and slag, generating raceway temperatures near 2000 °C. Second, it is the source of the reducing gas: that combustion produces CO, which does the bulk of the reduction as it ascends the stack, $\mathrm{Fe_2O_3+3\,CO\rightarrow2\,Fe+3\,CO_2}$. Third, it is the direct reductant at high temperature via the Boudouard reaction, $\mathrm{C+CO_2\rightarrow2\,CO}$, which becomes favourable above about 1000 °C and also carburises the iron, giving hot metal its characteristic 4 % carbon and a melting point some 350 °C below pure iron's. Fourth, coke is the only burden component that stays solid all the way to the hearth, so it provides the permeable skeleton through which gas ascends and liquid iron/slag descend — without it the furnace would choke.

(c) The function of limestone. Limestone is the flux. It calcines in the stack, $\mathrm{CaCO_3\rightarrow CaO+CO_2}$, and the lime combines with the acidic gangue (mainly silica and alumina) to form a fluid, low-melting slag, $\mathrm{CaO+SiO_2\rightarrow CaSiO_3}$. The slag collects gangue and coke ash for tapping, absorbs sulphur from the metal (the furnace's main desulphurising route), and, floating on the hot metal, protects it from re-oxidation.

(d) The products. Three streams leave the furnace: hot metal (pig iron), tapped near 1500 °C carrying roughly 4–4.5 % C plus Si, Mn, P and S; the slag, a calcium aluminosilicate sold as a cement supplement or aggregate; and the top gas, leaving at 100–250 °C with about 20–25 % CO and 20 % CO₂, cleaned and burned in the hot-blast stoves and power plant.

(e) Advantages of oxygen over air in steelmaking. The case rests on removing the nitrogen that makes up 79 % of air. Thermally, every mole of oxygen delivered as air drags 3.76 mol of nitrogen that must be heated and vented; eliminating it leaves enough heat surplus for a basic oxygen furnace to run on the chemical energy of the blow alone and absorb 25–30 % scrap as coolant. Kinetically, oxygen reaches the bath at much higher partial pressure without the diluent, so decarburisation is far faster — a 300 t heat blows in about 20 minutes, against eight to ten hours for the open-hearth process it displaced. On metallurgical quality, dissolved nitrogen drops to 20–40 ppm from 60–120 ppm for air-blown steel, avoiding strain-ageing embrittlement. And on the environmental/off-gas side, gas volume to be cleaned falls roughly fourfold and the off-gas becomes a concentrated, recoverable CO stream.

(f) Deoxidation of steel. At the end of the oxygen blow the bath carries 400–800 ppm dissolved oxygen; left in, it would react with carbon on freezing, $\mathrm{[C]+[O]\rightarrow CO(g)}$, blowing gas and porosity into the ingot. Deoxidation ("killing") adds elements with greater oxygen affinity than iron so a stable oxide forms and floats to the slag, in ascending strength: manganese ($\mathrm{Mn+[O]\rightarrow MnO}$, weakest, cheapest, also fixes sulphur as MnS); silicon ($\mathrm{Si+2\,[O]\rightarrow SiO_2}$, usually with Mn since the resulting silicate is liquid and separates readily); and aluminium ($\mathrm{2\,Al+3\,[O]\rightarrow Al_2O_3}$, strongest, defining a "fully killed" steel, with residual Al also pinning austenite grain size via AlN).