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. 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, with it, a melting point some 350 °C below that of pure iron.

Fourth — and this is the duty most often omitted in an exam answer — coke is the only component of the burden that remains 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, and the reason coke rate cannot be driven to zero however much auxiliary fuel is injected.

(c) The function of limestone. Limestone is the flux. It calcines in the stack, $\mathrm{CaCO_3 \rightarrow CaO + CO_2}$, and the lime so 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 the coke ash so they can be tapped away from the iron; being basic, it absorbs sulphur from the metal, which is the principal desulphurising mechanism available inside the furnace; and by floating as a separate layer on the hot metal it protects the iron from re-oxidation. Slag basicity, usually expressed as 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, and it carries flue dust that is recycled to the sinter 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, and the consequences run through the whole process.

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 from ambient to bath temperature and then vented, carrying its sensible heat away. Eliminating it leaves a large surplus of heat in the vessel, which is what allows a basic oxygen furnace to run entirely on the chemical energy of the blow with no external fuel, and to absorb 25–30 % scrap as coolant — scrap that would otherwise need melting elsewhere.

The kinetic advantage follows: with the diluent gone, oxygen reaches the bath at far higher partial pressure, so decarburisation proceeds much faster. A basic oxygen furnace blows a 300 t heat in about 20 minutes, against the eight to ten hours an open-hearth furnace needed for the same tonnage. Productivity per unit of capital is transformed.

The metallurgical quality advantage is that dissolved nitrogen in the finished steel 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 a requirement for deep-drawing and line-pipe grades; the inability to make it was the reason the Bessemer converter disappeared.

The environmental and off-gas advantages complete the case. The gas volume to be cleaned falls by roughly a factor of four, so gas-handling plant is smaller; the off-gas is a concentrated CO stream of real calorific value that can be recovered rather than a nitrogen-diluted waste; and the lower flame volume with higher flame temperature improves refractory life per tonne. Against all this stands the cost of the air-separation plant and the need for careful lance and refractory management at the higher local temperatures — a trade the industry settled decisively in favour of oxygen from the 1950s onward.

(f) Deoxidation of steel. At the end of the oxygen blow the bath is deliberately over-oxidised, carrying 400–800 ppm of dissolved oxygen. If that steel were cast as it stands, the falling solubility of oxygen and carbon on freezing would let them react in the mould, $\mathrm{[C]+[O]\rightarrow CO(g)}$, blowing carbon monoxide and leaving a porous, blowhole-ridden ingot; the residual dissolved oxygen would also embrittle the steel and precipitate oxide inclusions. Deoxidation, or “killing”, removes it by adding elements whose affinity for oxygen exceeds that of iron at steelmaking temperature, so that they form a stable oxide which floats out into the slag.

The metals used, in ascending order of deoxidising power, are:

• Manganese, $\mathrm{Mn + [O] \rightarrow MnO}$ — the weakest of the three and the cheapest, added as ferromanganese. It is used first as a bulk scavenger and also fixes sulphur as MnS rather than the low-melting FeS that causes hot shortness.
• Silicon, $\mathrm{Si + 2\,[O] \rightarrow SiO_2}$ — added as ferrosilicon, and much stronger than manganese. Silicon and manganese are usually added together because the resulting manganese silicate is liquid at steelmaking temperature and so separates far more readily than solid silica alone.
• Aluminium, $\mathrm{2\,Al + 3\,[O] \rightarrow Al_2O_3}$ — the strongest and the final addition, capable of taking dissolved oxygen down to a few parts per million. It is what defines a fully killed steel. Aluminium has the further benefit that the residual dissolved aluminium forms fine AlN precipitates which pin the austenite grain boundaries, giving the fine grain size that is specified for structural and pressure-vessel grades.

Calcium, added as a calcium–silicon alloy or by wire injection, is used as a finishing treatment rather than a bulk deoxidiser: it modifies the solid alumina clusters into liquid calcium aluminates that neither clog the casting nozzle nor form the stringers that damage transverse toughness.