Blast Furnace – IGCSE Chemistry Definition
IGCSE Chemistry definition of blast furnace: the industrial furnace for extracting iron from its ore. Covers reactions, raw materials, and exam tips for 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
The blast furnace is the most detailed industrial process in the metals topic. Paper 4 often gives 4–6 marks for describing the reactions inside the furnace. You must know the raw materials, the three key equations, and the role of limestone.
The 0620 definition
A blast furnace is a large industrial furnace used to extract iron from its ore (haematite, Fe₂O₃) by reduction with carbon monoxide.
Raw materials and their roles
| Raw material | Chemical formula | Role |
|---|---|---|
| Iron ore (haematite) | Fe₂O₃ | Source of iron |
| Coke | C | Burns to produce heat and CO |
| Limestone | CaCO₃ | Removes sandy impurities |
| Hot air | O₂ (and N₂) | Reacts with coke; provides the “blast” |
The three key reactions
1. Coke burns in hot air
C + O₂ → CO₂ (exothermic — provides heat, ~2000 °C)
2. Carbon dioxide reacts with more coke
CO₂ + C → 2CO (produces the reducing agent)
3. Carbon monoxide reduces iron ore
Fe₂O₃ + 3CO → 2Fe + 3CO₂
This is the key reduction equation. Carbon monoxide is the reducing agent — it removes oxygen from the iron oxide.
Removing impurities
The ore contains sandy impurities (silicon dioxide, SiO₂). Limestone removes them:
CaCO₃ → CaO + CO₂ (thermal decomposition)
CaO + SiO₂ → CaSiO₃ (slag)
Slag is molten and less dense than iron, so it floats on top and is tapped off. It is used in road-building and cement manufacture.
Products
- Molten iron: collected at the bottom of the furnace (most dense)
- Slag: floats on the iron, tapped off separately
- Waste gases: CO₂ and unreacted N₂ exit at the top
The iron produced is impure (about 96% iron, with carbon impurities). It is brittle and is converted to steel by removing most of the carbon.
Worked exam question
In the blast furnace, iron ore is reduced by carbon monoxide. (a) Write the equation for the reduction of iron(III) oxide. [1] (b) State the role of limestone. [2] (c) Explain why aluminium cannot be extracted in a blast furnace. [1]
Mark scheme
(a) Fe₂O₃ + 3CO → 2Fe + 3CO₂ [1]
(b) Limestone decomposes to calcium oxide [1]; calcium oxide reacts with SiO₂ / sandy impurities to form slag / CaSiO₃ [1]
(c) Aluminium is more reactive than carbon / above carbon in the reactivity series, so carbon cannot reduce aluminium oxide [1]
Common exam mistakes
- Saying carbon directly reduces the iron ore — it is carbon monoxide (CO) that acts as the reducing agent, not solid carbon.
- Forgetting the role of limestone — it removes impurities, not the iron itself.
- Writing the wrong formula for iron ore: it is Fe₂O₃ (iron(III) oxide), not FeO or Fe₃O₄.
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