Chemistry

Extraction fo ron from Haematite :
1. Dressing of the ore :
The big lumps of the ore are broken into small pieces and then washed with water to remove clay, sand and other adhering impurities. The ore thus becomes ready for treatment in the blast furnace.
2. Smelting in the blast furnace :
The concentrated ore is mixed with coke and limestone. The mixture is charged at the top of a blast furnace. The following reactions occur in the furnace.
(i) As the charge comes down to the 873 K region, the iron oxide is reduced by the ascending carbon monoxide gas produced by the burning of coke.

The iron thus obtained is called sponge iron.
(ii) At the 1273 K region, the silica is converted to slag.

(iii) At the 1573 K region, sponge iron melts and dissolves carbon, phosphorus, silica, etc. The slag also fuses. The molten mass collects at the base of the furnace. The slag floats over it. The molten iron is taken out as required. This iron is called pig iron.
Function of limestone : Limestone is decomposed to give quicklime.
CaCO₃ → CaO + CO₂
Quicklime combines with impurities like sand to form a molten slag (calcium silicate).
CaO + SiO₂ → CaSiO₃
The slag floats on the surface of molten iron. It is taken out through a hole from time to time.
The formation of calcium silicate as slag not only removes unwanted silica but also keeps iron away from being oxidized.