Extraction of Aluminium

Aluminium does not occur as a metal in rocks. Its main ore is bauxite, which contains aluminium oxide (alumina), Al₂O₃. Because aluminium is very high in the reactivity series, it cannot be extracted by heating with carbon. Instead, it is obtained by electrolysis.

Why electrolysis is needed

Aluminium oxide is very stable. Carbon is not strong enough to remove oxygen from it. Electrolysis uses electricity to pull the compound apart into aluminium metal and oxygen.

From bauxite to a molten electrolyte

The bauxite is purified to aluminium oxide (details not required). Pure Al₂O₃ melts at about 2050°C, which is too high. It is dissolved in molten cryolite (sodium aluminium fluoride) to:

• lower the operating temperature to about 950°C
• increase electrical conductivity

Think of cryolite like salt sprinkled on ice: it helps things melt at a lower temperature.

The electrolytic cell

A steel tank lined with carbon (graphite) is the cathode (negative). Carbon blocks hang down as anodes (positive). The electrolyte is molten Al₂O₃ in cryolite. Direct current is passed through.

Reactions at the electrodes

• Cathode (reduction): aluminium ions gain electrons and become aluminium metal. $$\text{Al}^{3+} + 3e^- \rightarrow \text{Al}(l)$$
• Anode (oxidation): oxide ions lose electrons to form oxygen gas. $$2\,\text{O}^{2-} \rightarrow \text{O}_2(g) + 4e^-$$

Overall (idealised): $$2\,\text{Al}_2\text{O}_3(l) \rightarrow 4\,\text{Al}(l) + 3\,\text{O}_2(g)$$

In the real cell, hot oxygen reacts with the carbon anodes: $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$.

Why the carbon anodes are replaced

The oxygen made at the anode slowly burns the carbon anodes to carbon dioxide. The anodes wear away and must be replaced regularly.

Products and collection

Liquid aluminium collects at the bottom (it is denser than the electrolyte) and is tapped off. Oxygen forms at the anode but mostly reacts to make carbon dioxide.

Common misconceptions

• The electrolyte is molten, not aqueous. Water would be decomposed instead.
• Bauxite is an ore containing aluminium oxide; it is not pure aluminium.
• Carbon does not reduce Al₂O₃ in a furnace; aluminium is too reactive.

Industrial note: The process uses a lot of electricity, so aluminium smelters are built where electricity is cheap.