4.13 Extracting aluminium by electrolysis

Why electrolysis?

Aluminium is more reactive than carbon, so it cannot be extracted by heating its ore with carbon. Instead, aluminium oxide (from the ore bauxite) is electrolysed.

Why cryolite?

Aluminium oxide has a very high melting point, over 2000 °C. Melting it would need a huge amount of energy.

Instead, it is dissolved in molten cryolite. This mixture melts at only about 950 °C, which saves energy and reduces costs. The ions are free to move in the molten mixture.

At the electrodes

• Cathode (the carbon lining of the cell, negative): aluminium ions gain electrons and form molten aluminium, which collects at the bottom and is tapped off. \mathrm{Al^{3+}} + 3\mathrm{e^-} → Al
• Anode (carbon blocks, positive): oxide ions lose electrons and form oxygen. 2\mathrm{O^{2-}} → O₂ + 4\mathrm{e^-}

Overall: 2Al₂O₃ → 4Al + 3O₂

Why the anodes must be replaced

The anodes are made of carbon. At this high temperature, the oxygen produced reacts with the carbon to form carbon dioxide:

C + O₂ → CO₂

So the anodes gradually burn away and must be replaced regularly.

Why aluminium is expensive to extract

• Electrolysis uses a huge amount of electricity.
• Energy is needed to keep the electrolyte molten (about 950 °C).
• The carbon anodes have to be replaced.

This is one reason why recycling aluminium is so worthwhile: it uses only about 5% of the energy needed to extract new aluminium.