Question 3 of 7: Iron and Steelmaking (20 marks)

Question 3 — 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. To these three solids must be added the fourth feed that enters at the bottom, the hot blast of preheated air at roughly 1000–1200 °C, together with injected auxiliary fuel such as pulverised coal, oil or natural gas.

(b) The function of coke. Coke performs four distinct duties, which is why nothing has fully replaced it in over two centuries of practice.

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 the iron and slag, generating raceway temperatures near 2000 °C.

Second, it is the source of the reducing gas. That same combustion produces carbon monoxide, which does the bulk of the reduction as it ascends through the stack:

Third, it acts as the direct reductant at high temperature by regenerating CO through the Boudouard reaction, $\mathrm{C + CO_2 \rightarrow 2\,CO}$, which becomes favourable above about 1000 °C and sustains the reducing potential deep in the furnace. It also carburises the iron, giving hot metal its characteristic 4 % carbon and a melting point some 350 °C below that of pure iron.

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 and slag descend. Ore and flux soften and fuse in the cohesive zone; if coke did not hold the bed open, the furnace would choke. This structural role is the reason coke strength after reaction is specified as tightly as its chemistry.

(c) The function of limestone. Limestone is the flux. It calcines in the stack, $\mathrm{CaCO_3 \rightarrow CaO + CO_2}$, and the lime produced combines with the acidic gangue of the ore — principally silica and alumina — to form a fluid slag of low melting point:

The slag serves three purposes: it collects the gangue and coke ash so they can be tapped away from the iron; being basic, it absorbs sulphur from the metal, the principal desulphurising mechanism available inside the furnace; and, floating as a separate layer on the hot metal, it protects the iron from re-oxidation. Slag basicity, usually the CaO/SiO₂ ratio, is the operator's main lever on both fluidity and sulphur capacity.

(d) The products. Three streams leave the furnace. The hot metal (pig iron) is tapped at about 1500 °C carrying roughly 4–4.5 % C, 0.5–1.5 % Si, 0.5–1 % Mn, plus phosphorus and sulphur. The slag is tapped separately and, being a calcium aluminosilicate, is sold as a cement supplement or aggregate rather than discarded. The top gas leaves at 100–250 °C containing about 20–25 % CO, 20 % CO₂ and the balance nitrogen; with a calorific value near 3.5 MJ/m³ it is cleaned and burned in the hot-blast stoves and the power plant.

(e) Advantages of oxygen over air in steelmaking. The case for tonnage oxygen rests on removing the nitrogen that makes up 79 % of air.

The thermal advantage is the largest: every mole of oxygen delivered as air drags 3.76 mol of nitrogen with it, and that nitrogen must be heated to bath temperature and vented, carrying its sensible heat away. Eliminating it leaves a heat surplus that lets a basic oxygen furnace run entirely on the chemical energy of the blow with no external fuel and absorb 25–30 % scrap as coolant.

The kinetic advantage follows: with the diluent gone, oxygen reaches the bath at far higher partial pressure, so decarburisation is much faster — a basic oxygen furnace blows a 300 t heat in about 20 minutes, against eight to ten hours for the open-hearth process it displaced.

The metallurgical quality advantage is that dissolved nitrogen drops sharply, typically to 20–40 ppm against 60–120 ppm for air-blown Bessemer steel; nitrogen causes strain-ageing and embrittlement, and low-nitrogen steel is required for deep-drawing and line-pipe grades.

The environmental / off-gas advantage completes the case: gas volume to be cleaned falls roughly fourfold, the off-gas is a concentrated, recoverable CO stream rather than a nitrogen-diluted waste, and refractory life improves at higher flame temperature with lower flame volume.

(f) Deoxidation of steel. At the end of the oxygen blow the bath carries 400–800 ppm dissolved oxygen. Cast as-is, falling oxygen and carbon solubility on freezing would let them react, $\mathrm{[C]+[O]\rightarrow CO(g)}$, blowing gas and leaving a porous ingot; residual dissolved oxygen also embrittles the steel. Deoxidation ("killing") removes it by adding elements with greater oxygen affinity than iron, so that a stable oxide forms and floats to the slag. In ascending order of deoxidising power:

• Manganese, $\mathrm{Mn + [O] \rightarrow MnO}$ — the weakest and cheapest, added as ferromanganese; also fixes sulphur as MnS rather than low-melting FeS.
• Silicon, $\mathrm{Si + 2\,[O] \rightarrow SiO_2}$ — stronger, usually added with manganese since the resulting manganese silicate is liquid at steelmaking temperature and separates readily.
• Aluminium, $\mathrm{2\,Al + 3\,[O] \rightarrow Al_2O_3}$ — the strongest, taking dissolved oxygen to a few ppm and defining a "fully killed" steel; residual Al also forms AlN precipitates that pin the austenite grain size.

Calcium, added last as a finishing treatment, modifies solid alumina inclusions into liquid calcium aluminates that neither clog the casting nozzle nor form stringers that damage toughness.