Question

Name different zones in the Blast furnace. Write the reactions taking place in them.

Answer 1

(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.

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.

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.