Iron is mainly extracted from haematite, Fe2O3 by reduction process.
Haematite ore contains silica (SiO2), alumina (Al2O3) and phosphates as impurity or gangue.
Coke is used for the reduction of ore.
To remove acidic gangue SiO2, a basic flux CaO is used which is obtained from lime stone CaCO3.
The extraction process involves following steps :
(1) Concentration of an ore : The powdered ore is concentrated by gravity separation process by washing it in a current of water. The lighter impurities (gangue) are carried away leaving behind the ore.
(2) Roasting : The concentrated ore is heated strongly in a limited current of air. During this, moisture is removed and the impurities like S,
As and phosphorus are oxidised to gaseous oxides which escape.
After roasting, the ore is sintered to form small lumps.
(3) Reduction (or smelting) : The roasted or calcined ore is then reduced by heating in a blast furnace.
The blast furnace is a tall cylindrical steel tower about 25 m in height and has a diameter about 5-10m lined with fire bricks inside.
Blast furnace has three parts :
- the hearth,
- the bosh and
- the stack.
At the top, there is a cup and cone arrangement to introduce the ore and at the bottom, tapping hole for withdrawing molten iron and an outlet to remove a slag.
The roasted ore is mixed with coke and limestone in the approximate ratio of 12 : 5 : 3.
A blast of hot air at about 1000 K is blown from downwards to upwards by layers arrangement. The temperature range is from bottom 2000 K to 500 K at the top. The charge of ore from top and the air blast from bottom are sent simultaneously. There are three zones of temperature in which three main chemical reactions take place.
(i) Zone of combustion : The hot air oxidises coke to CO which is an exothermic reaction, due to which the temperature of furnace rises.
C + 1/2O2 → CO ΔH= – 220kJ
Some part of CO dissociates to give finely divided carbon and O2.
2CO → 2C + O2
The hot gases with CO rise up in the furnace and heats the charge coming down. CO acts as a fuel as well as a reducing agent.
(ii) Zone of reduction : At about 900°C, CO reduces Fe2O3 to spongy (or porous) iron.
Fe2O3 + 3CO → 2Fe + 3CO2
Carbon also reduces partially Fe2O3 to Fe.
Fe2O3 + 3C → 2Fe + 3CO
(iii) Zone of slag formation : At 1200 K limestone, CaCO3 in the charge, decomposes and forms a basic flux CaO which further reacts at 1500 K with gangue (SiO2, Al2O3) and forms a slag of CaSiO3 and Ca3AlO3.
CaCO3 + CaO + CO2.
CaO + SiO2 → CaSiO3
12CaO + 2Al2O3 → 4Ca3AlO3 + 3O2
The slag is removed from the bottom of the furnace through an outlet.
(iv) Zone of fusion : The impurities in ore like MnO2 and Ca3(PO4)2 are reduced to Mn and P while SiO2 is reduced in Si. The spongy iron moving down in the furnace melts in the fusion zone and dissolves the impurities like C, Si, Mn, phosphorus and sulphure. The molten iron collects at the bottom of furnace. The lighter slag floats on the molten iron and prevents its oxidation.
The molten iron is removed and cooled in moulds. It is called pig iron or cast iron. It contains about 4% carbon.
