The smelted ore is introduced in the furnace by lowering the cup and cone arrangement and at the same time furnace is lit and a blast of hot air is sent upwards through the Tuyeres. On the basis of variation in the temperature, there are four zones where different chemical changes occur.
- Combustion zone: this is lowest part of furnace where the temperature is about 1500 - 1600 C.
C + O2 CO2H = -97 kcals
CO2 + C 2CO
- Reduction zone:
2Fe2O3 + CO 2Fe3O4 + CO2
Fe3O4 + CO 3FeO + CO2
Fe + CO Fe + CO2
Iron formed is called sponge iron. In this reduction zone heat is also evolved due to which CO partially decomposes.
2CO CO2 + C
- Slag formation zone: This is the central zone where the temperature varies from 800-1000 C.
CaCO3 CaO + CO2
Impurities in the ore are reduced to Si, P and Mn, respectively.
P4O10 + 10C 4P + 10CO
SiO2 + 2C Si + 2CO
MnO2 + 2C Mn + 2CO
These are partly absorbed by iron and partly by slag.
- Zone of fusion: This zone is above combustion zone. The temperature ranges between 1200-1500oC. The spongy iron melts at 1300 C and collects at the bottom of hearth. The slag being lighter floats over the molten iron and prevents oxidation of molten metal. The slag and molten metal are removed from their respective holes. The molten metals is run into moulds and is allowed to solidify.
Iron obtained from the blast furnace is called pig-iron. It contains 93% iron, 5% carbon and rest silicon. The pig-iron is remelted and cast into moulds. This is known as cast iron.
There are three commercial varities of iron depending on their carbon content.
- Cast iron - most impure form, containing carbon from 2.5 to 5%.
- Wrought iron - purest form of iron, containing carbon from 0.25% to 0.3%.
- Steel - most important commercial variety of iron, containing carbon from 0.25 to 2%