Aluminium is too reactive to be reduced by carbon, so it is won by electrolysis. The trick that makes it affordable is dissolving the oxide in molten cryolite, and the price is paid in electricity and in carbon anodes that slowly burn away.
The ore is bauxite, purified to aluminium oxide, Al2O3. It is dissolved in molten cryolite, which lowers the operating temperature and improves conductivity, and electrolysed with carbon (graphite) electrodes. Aluminium forms at the cathode and oxygen at the anode. The oxygen reacts with the hot carbon anodes to form carbon dioxide, so the anodes must be replaced regularly. Extended: Al3+ + 3e− → Al and 2O2− → O2 + 4e−.
Fig. 1: the electrolysis cell for aluminium
Section 01
The electrolysis cell.
Aluminium oxide melts above 2000 °C. Dissolving it in molten cryolite lets the cell run at about 950 °C.
Electrolyte: aluminium oxide dissolved in molten cryolite. The ions Al3+ and O2− are free to move.
Cathode (−): the carbon lining of the cell. Aluminium ions gain electrons to form molten aluminium, which collects at the bottom and is siphoned off.
Anodes (+): carbon blocks. Oxide ions lose electrons to form oxygen.
Anodes burn away: at 950 °C the oxygen reacts with the carbon: C + O2 → CO2. The anodes are replaced regularly.
Overall: 2Al2O3 → 4Al + 3O2.
Examiner trap
Cryolite is not a catalyst and not the source of aluminium: it is a solvent that lowers the temperature needed, saving energy. And it is the anodes, not the cathode, that must be replaced, because oxygen forms there and burns the carbon.
Stage 1 · Learn
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What is the ore of aluminium?
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Why is aluminium oxide dissolved in molten cryolite?
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What forms at the cathode?
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Why must the anodes be replaced regularly?
Section 02
Half-equations and costs.
Extended. As in all electrolysis, reduction happens at the cathode and oxidation at the anode.
Cathode: Al3+ + 3e− → Al (reduction, gain of electrons).
Anode: 2O2− → O2 + 4e− (oxidation, loss of electrons).
Costs: huge amounts of electricity; energy to keep the electrolyte molten; replacing the anodes. Aluminium plants are often built near cheap hydroelectric power.
Recycling aluminium uses only about 5% of the energy needed to extract it from bauxite.
Stage 2 · Exam
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(Extended) Which half-equation shows the reaction at the anode?
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Why can aluminium not be extracted by heating its oxide with carbon?
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Which is the overall equation for the electrolysis of aluminium oxide?
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Why is recycling aluminium better than extracting new aluminium?
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