A mixture of roasted ore,limestone and coke is fed into the blast furnace from the top by means of cup and cone arrangement and a hot blast of air is sent through means of tuyers near the base. As a result of the different temperatures attained at different levels in the furnace the iron oxide gets reduced to iron.
The various reactions at different zones are listed below,
In the first stage coke burns to produce CO2 which rises:
C+O2 → CO2 + heat
This reaction is exothermic and hence the temperature is about 1775K.This region is called combustion zone.
In the stage CO2 rises up it reduced to CO with the coke:
CO + C → 2CO - Heat
This reaction is endothermic and in this region the temperature falls to1475-1575K. Fe2O3,
if present gets reduced to iron by hot coke and the spongy iron in the upper region gets melted.
This region is called as fusion zone.
In the middle portion of the furnace where the is about 1075 to 1275K . Limestone decomposes to produce CaO which combines with silica to form slag.
CaCO3 → Cao + CO2
Cao + SiO2 → CaSio2
This region is called as slag formation zone.
Near the top of the furnace where the is about 875K, the oxides of iron are reduced to iron by CO .
Fe2O3 + CO → 2FeO + CO2
FeO + CO → Fe + CO2
This is called as reduction zone and iron formed moves down and melts in the fusion zone. The molten slag is removed first and then the molten metal. The iron thus formed is called is pig iron. The pig iron after remelting in a vertical furnace is known as cast iron because it can be cast into moulds.
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