The Blast Furnace

A mixture of crushed iron ore, coke and limestone is fed into the top of the blast furnace. It is named after blasts of air, which are heated up to 1000^oC and fed into the bottom of the furnace, the air contains 20% oxygen.

The steps in a blast furnace take place as follows;

1. In the blast furnace , coke is oxidised to carbon dioxde which is an exothermic reaction:
C(s) + O₂(g) CO₂(g) Delta H = -392 Kj mol^-1

2. The carbon dioxde reacts endothermically with coke to form carbon monoxide:
CO₂(g) + C(s) 2CO(g) Delta H = +172Kj mol^-1

3.Iron(III) oxide in the ore is reduced to iron by the carbon monoxide in a reaction which is overall exothermic:
Fe₂O₃(s) + 3CO(g) 2Fe(s) + 3CO₂(g) Delta H = -27Kjmol^-1
The iron falls to the bottom of the blast furnace, where it melts.

4.Alternatively, iron(III) oxide is reduced to iron(II) oxide by carbon monoxide:
Fe₂O₃(s) + CO(g) 2FeO(s) + CO₂(g)

5.Then, in parts of the furnace where the temperature is high enough, coke reacts with iron(II) oxide, reducing it to metallic iron:
FeO(s) + C(s) Fe(s) + CO(g)

All the reactions involving iron are simple reductions:
Fe³⁺ + 3e^- Fe
Fe³⁺ + e^- Fe²⁺
Fe²⁺ + 2e^- Fe
The reductants are either carbon or carbon monoxide, which themselves are oxidised.

Using limestone to remove the impurities;
The limestone , CaCO₃, is added to remove impurities in the iron ore. Most iron ores are between 30% and 95% iron oxides. The rest is mainly made up of the impurities silicon(IV) oxide, SiO₂ (sand or clay) and aluminium oxide, Al₂O₃.

1.At the high temperatures inside the blast furnace, limestone decomposes:
CaCO₃(s)

CaO(s) + CO₂(g)

2.The calcium oxide reacts with the silcon(IV) oxide and aluminium oxide:
CaO(s) + SiO₂(s) CaSiO₃(s) (calcium silcate)
CaO(s) + Al₂O₃(s) CaAl₂O₄(s) (calcium aluminate)

The mixture of calcium silicate and calcium aluminate is a liquid at blast furnace temperatures and is called slag. This slag does not mix with the molten iron, but forms a separate layer that floats on top of the iron (less dense).Slag is tapped off and not wasted as it can be solidified, broken up and used as hard core in the foundations of buildings and roads, and also as an ingredient of cement.

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