9.6 Extraction of metals
- Syllabus
- 0620–2026–2027
- Topic
- 9.6
- Level
- —
Learning objectives
9.6.1Ease in obtaining metals from their• Describe ease in obtaining metals from their ores, related to the position of the metal in the reactivity series9.6.2Iron extraction from hematite in a• Describe iron extraction from hematite in a blast furnace: coke burns for heat/CO2; CO2 is reduced to CO; CO reduces iron(III) oxide; limestone decomposes to calcium oxide; slag forms. Symbol equations not required9.6.3Main ore of aluminium is bauxite and• State: the main ore of aluminium is bauxite and that aluminium is extracted by electrolysis9.6.4Symbol equations for the extraction of• State the symbol equations for the extraction of iron from hematite (a) C + O2 → CO2 (b) C + CO2 → 2CO (c) Fe 2O3 + 3CO → 2Fe + 3CO2 (d) CaCO 3 → CaO + CO2 (e) CaO + SiO2 → CaSiO39.6.5Aluminium extraction from purified• Describe aluminium extraction from purified bauxite/aluminium oxide: role of cryolite, why carbon anodes are replaced, and electrode reactions with ionic half-equations. Bauxite purification not required
Relate extraction difficulty to reactivity
The more reactive a metal is, the more strongly it is combined in its ore and the harder it is to extract.
| Reactivity position | Typical occurrence or extraction route | Reason |
|---|---|---|
| very unreactive | may occur native as the element | little tendency to form compounds |
| below carbon | heat the oxide with carbon or carbon monoxide | carbon removes oxygen from the less reactive metal oxide |
| above carbon | electrolysis of a molten ionic compound | carbon cannot reduce the stable oxide |
Iron, zinc and copper are below carbon and their oxides can be reduced using carbon or carbon monoxide. Aluminium is above carbon, so aluminium oxide is electrolysed instead.
Electrolysis is not reserved only for aluminium, but it is required for metals above carbon. Do not propose carbon reduction for aluminium, magnesium, calcium, sodium or potassium compounds.
Describe iron extraction in the blast furnace
| Stage | Blast-furnace change | Purpose |
|---|---|---|
| 1 | coke burns in hot air to form carbon dioxide | releases heat |
| 2 | carbon dioxide reacts with more coke to form carbon monoxide | makes the reducing agent |
| 3 | carbon monoxide reduces hematite, iron(III) oxide, to iron | produces molten iron |
| 4 | limestone thermally decomposes to calcium oxide and carbon dioxide | makes a basic oxide |
| 5 | calcium oxide reacts with acidic silicon(IV) oxide impurity | forms calcium silicate slag |
The main inputs are hematite, coke, limestone and hot air. Molten iron and slag leave separately; waste gases include nitrogen, carbon dioxide and some carbon monoxide.
Coke has two linked roles: its combustion supplies heat, and it generates carbon monoxide, which removes oxygen from iron(III) oxide. Limestone removes silica impurity through lime and slag formation.
Carbon dioxide does not reduce hematite; carbon monoxide does. Limestone does not directly become slag: it first decomposes to calcium oxide, which then reacts with silicon(IV) oxide.
State how aluminium is obtained from bauxite
Bauxite is the main ore of aluminium and contains mainly aluminium oxide. Aluminium is extracted from aluminium oxide by electrolysis.
| Ore | Main aluminium compound | Extraction process |
|---|---|---|
| bauxite | aluminium oxide, Al₂O₃ | electrolysis of the molten electrolyte |
Aluminium lies above carbon in the reactivity series, so carbon cannot remove oxygen from aluminium oxide; electrical energy is used to reduce aluminium ions.
Hematite is an iron ore, not an aluminium ore. Bauxite is not itself the lowering agent or electrolyte solvent; purified aluminium oxide supplies the aluminium ions.
Write the five blast-furnace equations
| Process | Required equation |
|---|---|
| coke burns | C + O₂ → CO₂ |
| carbon monoxide forms | C + CO₂ → 2CO |
| hematite is reduced | Fe₂O₃ + 3CO → 2Fe + 3CO₂ |
| limestone decomposes | CaCO₃ → CaO + CO₂ |
| slag forms | CaO + SiO₂ → CaSiO₃ |
The hematite equation is redox: iron(III) oxide is reduced and carbon monoxide is oxidised. Limestone undergoes thermal decomposition. Slag formation is an acid–base reaction between basic CaO and acidic SiO₂.
Check atoms across every equation. In the reduction equation, Fe₂O₃ requires 2Fe and three CO molecules produce three CO₂ molecules.
Keep the two carbon oxides in sequence: coke first forms CO₂, then CO₂ reacts with more carbon to form CO. Do not reverse the slag equation or replace CO with CO₂ as the reducing agent.
Explain aluminium electrolysis
Purified aluminium oxide is dissolved in molten cryolite. Cryolite lowers the operating temperature and improves electrical conductivity, reducing the energy needed compared with melting pure aluminium oxide.
| Electrode | Ion change | Half-equation | Product |
|---|---|---|---|
| negative cathode | Al³⁺ gains electrons; reduction | Al³⁺ + 3e⁻ → Al | molten aluminium |
| positive carbon anode | O²⁻ loses electrons; oxidation | 2O²⁻ → O₂ + 4e⁻ | oxygen |
The oxygen produced at the positive anodes reacts with their carbon: C + O₂ → CO₂. The carbon anodes are therefore consumed and must be replaced regularly.
Aluminium forms at the cathode, not the anode. Cryolite is a solvent and lowers operating temperature; it is not a catalyst, aluminium source or protection for the carbon anodes. Details of bauxite purification are not required.