Extracting aluminium by electrolysis
Aluminium is more reactive than carbon, so reduction with carbon won't work (C4.2). Instead, electrolysis of molten aluminium oxide is used. This is energy-intensive — one of the most electricity-hungry industrial processes in the world.
The ore: bauxite → aluminium oxide
Bauxite is the natural ore. After purification, you get aluminium oxide, Al₂O₃. Pure Al₂O₃ has a very high melting point (2072 °C), which would make electrolysis impossibly costly.
Cryolite — the key trick
Aluminium oxide is dissolved in molten cryolite (Na₃AlF₆) at about 950 °C. Cryolite:
- Lowers the operating temperature.
- Provides a conductive ionic liquid in which Al₂O₃ dissolves.
This dramatically reduces electricity costs.
The cell
- Cathode: graphite lining of the steel tank (negative). Molten Al collects at the bottom and is tapped off.
- Anode: graphite blocks dipping into the melt (positive).
- Electrolyte: molten Al₂O₃ dissolved in cryolite.
Half-equations (HT)
- Cathode: Al³⁺ + 3e⁻ → Al (reduction)
- Anode: 2O²⁻ → O₂ + 4e⁻ (oxidation)
Overall: 2Al₂O₃ → 4Al + 3O₂.
Why anodes need replacing
The oxygen produced at the anode reacts with the hot graphite to form carbon dioxide (and CO): C + O₂ → CO₂
The graphite anodes burn away and have to be replaced regularly — a major running cost.
Energy and environmental costs
- Huge electricity demand (often hydroelectric or located near cheap energy).
- CO₂ emissions from burning anodes.
- Bauxite mining damages ecosystems.
But Al is hugely valuable: low density, corrosion-resistant, recyclable (recycling Al uses 5% of the energy of new extraction — see C10).
⚠Common mistakes
- Forgetting cryolite. The exam wants its purpose: to lower the m.p. and hence energy use.
- Saying anode produces only oxygen — at GCSE we usually say O₂ but recognise it reacts with the carbon anode → CO₂.
- Confusing aluminium oxide with bauxite — bauxite is the ore; we use purified Al₂O₃.
- Saying the cathode is positive — cathode is negative (where Al³⁺ is reduced).
Links
Builds on C4.9, C4.10. Sets up C10.4 (alternative methods like phytomining) and C10.6 (recycling Al).
AI-generated · claude-opus-4-7 · v3-deep-chemistry