Question
Describe the extraction of iron in a blast furnace. What reactions occur in each temperature zone? Why is limestone added, and what is the role of slag?
(CBSE 12 boards ask the complete zone-wise description; JEE Main tests specific reactions)
Solution — Step by Step
Three materials are fed from the top: iron ore (haematite, Fe₂O₃), coke (C), and limestone (CaCO₃ — the flux). Hot air is blasted from the bottom through tuyeres. The furnace has distinct temperature zones from bottom (hottest) to top (coolest).
Near the tuyeres, coke burns in the hot air blast:
This provides all the heat for the furnace. The CO₂ rises upward.
CO₂ reacts with more coke to form CO:
This CO is the actual reducing agent. It reduces iron ore in stages:
The iron produced is pig iron — it contains about 4% carbon and other impurities.
Limestone decomposes:
The CaO (basic flux) reacts with SiO₂ (acidic gangue impurity):
Slag is lighter than molten iron, so it floats on top and is tapped off separately.
flowchart TD
A["Top: Charge inlet<br/>Fe₂O₃ + Coke + Limestone"] --> B["Zone of reduction<br/>700-1000°C<br/>Fe₂O₃ → Fe₃O₄ → FeO → Fe"]
B --> C["Zone of slag formation<br/>800-1000°C<br/>CaO + SiO₂ → CaSiO₃"]
C --> D["Zone of combustion<br/>~2000°C<br/>C + O₂ → CO₂"]
D --> E["Bottom: Molten iron collected"]
C --> F["Slag tapped off<br/>(lighter, floats on iron)"]
G["Hot air blast<br/>through tuyeres"] --> D
style D fill:#ff6b6b,stroke:#333
style E fill:#ffeb3b,stroke:#333Why This Works
The blast furnace is a continuous counter-current system — the charge moves down while hot gases rise up. This means the hottest zone is at the bottom (where combustion happens) and the coolest zone is at the top (where the fresh charge enters).
CO is a better reducing agent than carbon at these temperatures because the reaction becomes thermodynamically favourable above 700°C (check the Ellingham diagram). The Ellingham diagram shows that above ~700°C, the line for CO formation dips below the line for CO₂ formation — making CO the dominant reducing agent.
Alternative Method
For quick recall, remember the sequence top to bottom: Feed → Reduction → Slag → Combustion → Iron collected. The temperature increases as you go down. The mnemonic: “Fresh Rice Smells Cooked Immediately” (Feed, Reduction, Slag, Combustion, Iron).
Common Mistake
Students write that carbon (coke) directly reduces Fe₂O₃ to Fe. While direct reduction by carbon does occur to a small extent in the lower zones, the primary reducing agent is CO, not C. The coke mainly serves two purposes: (1) generating heat by combustion, and (2) producing CO which does the actual reduction. Writing “C reduces Fe₂O₃” without mentioning CO will lose marks in board exams.