Extraction of Metals and the Blast Furnace
Most metals occur as compounds in ores. Extracting them means reducing the metal ion to the metal — and how you do it depends entirely on the metal’s position in the reactivity series.
1. Choosing the method
ABOVE carbon (K, Na, Ca, Mg, Al) → ELECTROLYSIS BELOW carbon (Zn, Fe, Cu) → REDUCTION with CARBON Very unreactive (Ag, Au) → found NATIVE (as the uncombined element)
Why: carbon can only displace a metal less reactive than itself. For metals above carbon, electrolysis is the only option — but it is far more expensive because of the electricity needed.
Extraction is always REDUCTION — the metal ion gains electrons to become the metal. Confusing oxidation with reduction was recorded, as was general uncertainty about identifying which is which in these reactions.
2. Extracting iron — the blast furnace
The three raw materials:
IRON ORE (haematite, Fe₂O₃) — the source of iron COKE (carbon, C) — the reducing agent and fuel LIMESTONE (calcium carbonate, CaCO₃) — removes impurities
Plus hot air blasted in at the bottom.
The gas blown in is HOT AIR (oxygen) — a recorded error guessed “slag” or “carbon”. The air provides the oxygen to burn the coke.
The reactions, in order
1. Coke burns in the hot air (very exothermic — provides the heat): C + O₂ → CO₂
2. Carbon dioxide is reduced by more coke to carbon monoxide: CO₂ + C → 2CO
3. Carbon monoxide REDUCES the iron ore — the key step: Fe₂O₃ + 3CO → 2Fe + 3CO₂
4. Limestone decomposes: CaCO₃ → CaO + CO₂
5. Calcium oxide removes the sandy impurity as slag: CaO + SiO₂ → CaSiO₃ (slag)
Learn the order. Confusion about the order of reactions in the blast furnace was recorded — carbon must burn first to make CO₂, which then becomes CO, which then does the reducing.
The roles — a favourite exam question
| Substance | Role |
|---|---|
| Coke | burns to provide heat, and forms carbon monoxide, the reducing agent |
| Carbon monoxide | reduces the iron oxide to iron |
| Limestone | decomposes to calcium oxide, which removes acidic impurities (silicon dioxide) as slag |
| Hot air | supplies oxygen to burn the coke |
The reducing agent is CARBON MONOXIDE, not coke itself. A recorded error named coke — coke is the source of the CO and the fuel, but it is CO that reduces the ore in the main reaction. (Carbon can also reduce the ore directly at high temperature, but CO is the answer expected.)
Uncertainty about the role of limestone was recorded twice — it removes impurities. It decomposes to CaO, which is basic, and reacts with the acidic SiO₂ impurity to form slag, which floats on the molten iron and is drained off.
Molten iron is denser than slag, so the iron collects at the bottom and the slag floats above it.
Redox in the furnace
Iron oxide is REDUCED (loses oxygen; Fe goes from +3 to 0) Carbon monoxide is OXIDISED (gains oxygen; C goes from +2 to +4)
So CO is the reducing agent and Fe₂O₃ is the oxidising agent.
3. Extracting aluminium — electrolysis
Aluminium is above carbon, so it must be extracted by electrolysis of molten aluminium oxide (from bauxite).
The oxide is dissolved in molten CRYOLITE to LOWER the melting point, saving a great deal of energy.
At the cathode (negative): Al³⁺ + 3e⁻ → Al (reduction) At the anode (positive): 2O²⁻ → O₂ + 4e⁻ (oxidation)
The carbon anodes burn away and must be replaced regularly, because the oxygen produced reacts with them at the high temperature.
Electrolysis is expensive because of the large amount of electricity required — which is why it is only used for metals above carbon.
4. Zinc
Zinc is below carbon, so it is extracted by reduction with carbon after roasting the ore:
ZnO + C → Zn + CO
Zinc is a GAS in the furnace because its boiling point (907 °C) is below the furnace temperature — so it distils off as a vapour and is condensed. Confusion about why zinc is a gas was recorded.
Zinc oxide is not zinc chloride — confusing them was recorded. The oxide is what’s reduced.
5. Mistakes that cost marks
Choosing the wrong method for the metal’s position.
Saying extraction is oxidation — it is reduction.
Naming coke as the reducing agent instead of carbon monoxide.
Getting the blast furnace reactions out of order.
Not knowing limestone’s role.
Saying carbon dioxide supports burning — it does not; it puts fires out.
Forgetting cryolite or its purpose.
Wrong half-equations for aluminium.
Not explaining why electrolysis is expensive.
Frequently asked questions
How is the extraction method chosen? By the metal’s position relative to carbon in the reactivity series.
Which metals are extracted by electrolysis? Those above carbon — including aluminium.
What are the three raw materials in the blast furnace? Iron ore, coke and limestone (plus hot air).
What is the reducing agent? Carbon monoxide.
What is the main reduction equation? Fe₂O₃ + 3CO → 2Fe + 3CO₂.
What does limestone do? It decomposes to calcium oxide, which removes acidic impurities as slag.
Why is cryolite used? To lower the melting point of aluminium oxide, saving energy.
What happens at the cathode in aluminium extraction? Al³⁺ + 3e⁻ → Al.
Why do the anodes need replacing? The oxygen produced burns them away.
Why is zinc a vapour in the furnace? Its boiling point is below the furnace temperature.
Quick revision checklist
- I can choose the extraction method from the reactivity series
- I know extraction is reduction
- I know the three raw materials and the hot air
- I can write the blast furnace reactions in order
- I know carbon monoxide is the reducing agent
- I can explain the role of coke and limestone
- I know what slag is and why it floats
- I can identify what is oxidised and reduced
- I know aluminium needs electrolysis and why
- I know the purpose of cryolite
- I can write both half-equations for aluminium
- I know why the anodes burn away
These notes cover the extraction of metals in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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