Extraction of Aluminium from Bauxite
Section: 9. Metals | Syllabus: Cambridge IGCSE Chemistry (0620)
Bauxite and Electrolysis
Aluminium's main ore is bauxite, which is mainly aluminium oxide (Al₂O₃) mixed with impurities. Aluminium is above carbon in the reactivity series, so it cannot be extracted by reduction with carbon - electrolysis is used instead. (The purification of bauxite into pure aluminium oxide is not required in detail - it is simply purified before electrolysis.)
Worked Example
- Question: name the main ore of aluminium; name the process by which aluminium oxide is broken down into its elements; name the substance added to aluminium oxide to reduce the operating temperature of this process. [3]
- Answer: bauxite [1]; electrolysis [1]; (molten) cryolite [1]
The Role of Cryolite
Pure aluminium oxide melts at about 2050°C - far too hot to be economical. Instead, the aluminium oxide is dissolved in molten cryolite (Na₃AlF₆), which lowers the operating temperature to about 950°C and saves a huge amount of energy. Cryolite is a solvent - it is not used up in the process.
| Electrode | Material |
|---|---|
| Cathode (−) | graphite, lining the cell |
| Anode (+) | graphite blocks dipping into the molten mixture |
Molten aluminium collects at the graphite-lined cathode; oxygen gas is released at the graphite anodes
Reactions at the Electrodes
In the molten mixture, aluminium oxide is ionised: Al₂O₃(l) → 2Al³⁺(l) + 3O²⁻(l)
| Electrode | Half-equation | Process |
|---|---|---|
| Cathode (−) | Al³⁺ + 3e⁻ → Al | reduction - molten aluminium forms and is tapped off |
| Anode (+) | 2O²⁻ → O₂ + 4e⁻ | oxidation - oxygen gas is released |
Overall: 2Al₂O₃(l) → 4Al(l) + 3O₂(g)
Why the Carbon Anodes Need Regular Replacing
The oxygen gas produced at the anode reacts with the hot graphite anode itself:
C(s) + O₂(g) → CO₂(g)
This gradually burns the anodes away, so they must be replaced regularly - adding to the cost of extraction.
Common Mistakes
- ❌ Mixing up which electrode does the reducing and which does the oxidising
- ✓ reduction (gain of electrons) happens at the cathode (Al³⁺ → Al); oxidation (loss of electrons) happens at the anode (O²⁻ → O₂)
- ❌ Thinking cryolite is a reactant that gets used up
- ✓ cryolite is a solvent - its role is to dissolve the aluminium oxide and lower the melting point, not to react
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