Question

Reactions taking place in the furance.

Answer 1

Near the bottom of the furnace (zone of combustion, 2170 K), coke first combines with air to form CO₂ which then combines with more coke (zone of heat absorption, 1425 K) to form CO. The CO thus produced acts as the reducing agent and reduces iron oxide to spongy iron near the top of the furnace (zone of reduction, 823 K)

C + O₂ → CO₂ ; ∆H = - 393.3 kJ (Exothermic) 

CO₂ + C → 2CO; ∆H = + 163.2 kJ (Endothermic)

Fe₂ O₃ + CO₂ \(\xrightarrow{823k}\) 2 Fe0 + CO₂

Fe₃ O₄ + CO \(\xrightarrow{823k}\) 3 FeO + CO₂

But the further reduction of FeO to Fe by CO occurs around 1123 K.

FeO + CO \(\xrightarrow{1123k}\) Fe + CO₂

However, direct reduction of iron ores (haematite, magnetite etc.) left unreduced around 823 K, occurs completely to iron by carbon above 1123 K.

Fe₂ O₃ + 3 C \(\xrightarrow{>1123k}\) 2 Fe + 3CO

2 Fe₂ O₃ + 3 C \(\xrightarrow{>1123k}\) 4 Fe + 3 CO₂

Limestone which acts as flux, decomposes at 1123 K (zone of slag formation) to form CaO which then combines with silica to form slag

CaCO₃ \(\xrightarrow{1123k}\) CaO + CO₂ ;

CaO + SiO₂ → CaSiO₃ (slag)

At the lower part of the furnace (zone of fusion, 1423-1673 K) the spongy iron melts and dissolves some carbon, S, P, SiO₂ , Mn, etc.

The molten slag being less dense floats over the surface of the molten iron. The molten iron is tapped off from the furnace and is then solidified to give blocks of iron called Cast iron or Pig iron.