Question
Describe the extraction of iron in a blast furnace. What reactions occur in each temperature zone, and how is slag formed?
Solution — Step by Step
Preheated air blasts through the tuyeres (nozzles) at the bottom. Coke () burns:
The CO produced here is the main reducing agent for iron ore. This zone provides the heat that drives the entire furnace.
As the charge (ore + coke + limestone) descends, CO reduces the iron oxide in stages:
The iron produced is called pig iron (contains ~4% carbon and other impurities). It melts and collects at the bottom.
Limestone () decomposes:
The CaO (flux) combines with SiO2 (gangue) to form slag:
Slag is less dense than molten iron, so it floats on top and is tapped off separately. This is a beautiful example of acid-base chemistry in metallurgy — CaO (basic) reacts with SiO2 (acidic).
graph TD
A[Charge: Ore + Coke + Limestone] -->|Enters from top| B[Zone of reduction: 500-900C]
B --> C[Fe2O3 reduced to Fe by CO]
C --> D[Zone of slag: 800-1000C]
D --> E[CaO + SiO2 = CaSiO3 slag]
E --> F[Zone of combustion: ~2000C]
F --> G[C + O2 = CO2, CO2 + C = 2CO]
G -->|CO rises up| B
F --> H[Molten iron collects at bottom]
E --> I[Slag floats on iron]Why This Works
The blast furnace is a counter-current system — the charge moves down while hot gases (CO) move up. This ensures maximum contact and efficient reduction. CO is preferred over direct carbon reduction because it is a gas and contacts the ore particles more uniformly.
The three-stage reduction () happens because each oxide has a different stability. The Ellingham diagram shows that CO can reduce iron oxides at these temperatures but not oxides like or — which is why aluminium extraction requires electrolysis, not a blast furnace.
Alternative Method
For exam questions about the flux-gangue reaction, remember the rule:
- Acidic gangue (SiO2) needs a basic flux (CaO from limestone)
- Basic gangue (MnO, FeO) needs an acidic flux (SiO2)
The flux always has the opposite acid-base character of the gangue. This principle applies to all metallurgy questions, not just iron.
Common Mistake
Students often write the reduction as a single step: ”.” While this is the overall equation, the actual process occurs in three stages at different temperatures. CBSE board exams specifically ask for the stepwise reactions, and writing only the overall equation loses marks. JEE Main has also tested which intermediate oxide forms at which temperature.