Extraction of iron

 Iron is extracted from its oxide ores. The ores are crushed and concentrated by gravity method. Magnetite is further concentrated by magnetic method. The concentrated ore is calcined to remove volatile impurities and make the ore porous. The calcined ore contains SiO2 as the chief impurity. It is mixed with lime stone (CaCO3) as flux and coke as reducing agent and the mixture is charged into a blast furnace (a very tall steel tower lined inside with fire bricks). The charge is heated by a hot blast of air preheated at 1000 K blown in through narrow pipes (tuyers) at the base of the furnace. Here coke burns producing heat and temperature rise to 2100 K.

C + O2 --------------> CO2 + heat

The temperature falls to 500 K towards the top of the furnace due to the endothermic reaction.

CO2 + C ---------------> 2CO - heat

Carbon monoxide reduces iron oxide to iron in the middle and upper regions of the furnace.

3Fe2O3 + CO -----------------> 2Fe3O4 + CO2

Fe3O4 + 4CO -----------------> 3Fe + 4 CO2

The molten iron formed at the lowest zone dissolve impurities like C, Si, Mn, S, P etc. and gets collected in the hearth of the furnace. Above 1000 K limestone decomposes to CaO, which combine with silica in the hot regions of the furnace to form a slag, which gets collected over molten iron.

CaCO3 --------------> CaO + CO2

CaO + SiO2 --------------> CaSiO3 (slag)

The molten iron and the molten slag are removed through their respective tap holes. The iron from blast furnace contains about 4% carbon and one to two percent other impurities like Mn, Si, S and P. It is called pig iron. Pig iron is remelted and cast into moulds, this is known as cast iron.

 Wrought Iron
This is the purest form of iron. it is obtained by oxidising the impurities of cast iron by heating with haematite. Carbon is oxidised to CO. Si, P and Mn are also oxidised and form a slag which is removed by passing through rollers. Wrought iron is 99.5% pure. It is malleable and ductile and used for making chains, bolts, nails etc.

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