A blast furnace runs day and night, turning a rock called hematite into molten iron. Four raw materials go in at the top and bottom; a chain of reactions makes the reducing agent, strips the oxygen from the ore and carries the sand away as slag.
The raw materials are hematite (iron ore, Fe2O3), coke (carbon), limestone (CaCO3) and hot air. Coke burns: C + O2 → CO2, giving heat. Carbon dioxide reacts with more coke: CO2 + C → 2CO. Carbon monoxide reduces the ore: Fe2O3 + 3CO → 2Fe + 3CO2. Limestone decomposes, CaCO3 → CaO + CO2, and the calcium oxide removes sand: CaO + SiO2 → CaSiO3 (slag).
Fig. 1: the blast furnace
Section 01
The reactions, bottom to top.
Follow the gases up the furnace. Each reaction feeds the next.
1. Heat: coke burns in the hot air blast: C + O2 → CO2. This is exothermic and heats the furnace to about 1500 °C.
2. Reducing agent: carbon dioxide reacts with more coke: CO2 + C → 2CO.
3. Reduction: carbon monoxide removes oxygen from the ore: Fe2O3 + 3CO → 2Fe + 3CO2. The iron melts and runs to the bottom.
4. Limestone: thermal decomposition, CaCO3 → CaO + CO2.
5. Slag: calcium oxide (basic) reacts with silicon dioxide (acidic sand impurity): CaO + SiO2 → CaSiO3. Molten slag floats on the iron and is tapped off separately.
Examiner trap
The main reducing agent is carbon monoxide, not carbon: it is a gas that reaches every part of the ore. And limestone does not reduce anything; its job is to remove the sandy impurities as slag.
Stage 1 · Learn
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Which substance is the main reducing agent in the blast furnace?
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What is the ore of iron used in the blast furnace?
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Why is limestone added to the blast furnace?
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What is the purpose of the hot air blast?
Section 02
From pig iron to steel.
The iron from the furnace, pig iron, contains about 4% carbon, which makes it brittle.
Converting to steel: oxygen is blown through molten iron to burn off most of the carbon as carbon dioxide; calcium oxide removes other impurities such as silicon and phosphorus as slag.
Adding elements: measured amounts of carbon and other metals are then added to make steels with the required properties.
Uses of the slag: road-building and cement.
Stage 2 · Exam
Exam-style questions
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Which equation shows the formation of carbon monoxide in the blast furnace?
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Slag is calcium silicate. Which equation shows its formation?
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In Fe2O3 + 3CO → 2Fe + 3CO2, which statement is correct?
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Why does slag float on the molten iron?
Skill unlocked
Extracting iron, mastered.
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