Blast Furnace
Summary: The blast furnace extracts iron from hematite (Fe₂O₃) using coke (carbon), limestone (CaCO₃), and hot air. Iron ore is reduced to molten iron; limestone removes sand impurities as slag. Tags: igcse chemistry metals extraction iron Created: 2026-07-14 Last Updated: 2026-07-23
Raw Materials
| Material | Formula | Function |
|---|---|---|
| Hematite (iron ore) | Fe₂O₃ | Source of iron |
| Coke (carbon) | C | Fuel (burns to produce heat) and reducing agent |
| Limestone | CaCO₃ | Removes sandy impurities (SiO₂) as slag |
| Hot air | O₂ / N₂ | Supports combustion of coke |
How the Blast Furnace Works
The blast furnace operates continuously at high temperatures (~1500°C). Materials are added from the top; hot air is blasted in from the bottom (tuyères).
Step 1: Coke Burns to Produce Heat and CO₂
C(s) + O₂(g) → CO₂(g)
This reaction is exothermic — it provides the heat needed for the process.
Step 2: CO₂ Is Reduced to Carbon Monoxide
CO₂(g) + C(s) → 2CO(g)
Carbon monoxide is the actual reducing agent in the furnace.
Step 3: Iron(III) Oxide Is Reduced by Carbon Monoxide
Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g)
Molten iron drips to the bottom of the furnace and is tapped off.
Step 4: Limestone Removes Impurities
CaCO₃(s) → CaO(s) + CO₂(g) — Thermal decomposition
CaO(s) + SiO₂(s) → CaSiO₃(l) — Formation of slag
The slag (calcium silicate, CaSiO₃) is molten and floats on top of the molten iron, protecting it from re-oxidation. Slag is tapped off separately and used for road construction and cement.
Summary Equations
| Reaction | Equation |
|---|---|
| Coke combustion | C + O₂ → CO₂ |
| CO production | CO₂ + C → 2CO |
| Iron reduction | Fe₂O₃ + 3CO → 2Fe + 3CO₂ |
| Limestone decomposition | CaCO₃ → CaO + CO₂ |
| Slag formation | CaO + SiO₂ → CaSiO₃ |
Products of the Blast Furnace
| Product | Composition | Uses |
|---|---|---|
| Pig iron | ~95% Fe, ~4% C, impurities | Brittle — converted to steel |
| Slag | CaSiO₃ | Road building, cement |
| Flue gases | CO₂, N₂, CO | Heat exchangers to preheat air |
Why Not Use Carbon Directly?
Carbon monoxide (CO) is a better reducing agent than solid carbon because:
- CO is a gas → it can surround and penetrate the iron ore more effectively
- CO reduces Fe₂O₃ at a lower temperature than solid carbon
From Pig Iron to Steel
Pig iron from the blast furnace contains ~4% carbon, making it brittle (it snaps rather than bends). It is converted to steel in a basic oxygen furnace:
- Oxygen is blown through molten pig iron
- Carbon is oxidised to CO/CO₂, reducing carbon content to <2%
- Other elements (Cr, Ni, Mn) are added to make alloy steels
Key Facts
- Hematite (Fe₂O₃) is the main ore of iron
- Coke (C) provides heat and reducing agent
- Limestone (CaCO₃) removes sand as slag
- Hot air supports combustion
- Fe₂O₃ + 3CO → 2Fe + 3CO₂ — the key reduction reaction
- CaO + SiO₂ → CaSiO₃ — slag formation
- Iron extraction uses carbon reduction because iron is BELOW carbon in the reactivity series
- Slag protects molten iron from re-oxidation