How is aluminium extracted using electrolysis?

Aluminium is extracted from its ore, bauxite, using electrolysis in a process called the Hall-Héroult process.

The extraction of aluminium involves several key steps. Firstly, the bauxite ore is purified to produce aluminium oxide, a white powder from which aluminium can be extracted. This is done through a process called the Bayer process. The bauxite is mixed with sodium hydroxide and heated under pressure. The aluminium oxide dissolves in the sodium hydroxide to form sodium aluminate, while impurities such as iron oxides and silicates do not dissolve and can be filtered out.

The next step is to convert the sodium aluminate back into aluminium oxide. This is achieved by adding more aluminium oxide to the solution, which causes the sodium aluminate to precipitate out as aluminium hydroxide. The aluminium hydroxide is then heated to a high temperature to remove any remaining water, leaving pure aluminium oxide.

The final step is the electrolysis of the aluminium oxide to produce aluminium. This is the Hall-Héroult process. The aluminium oxide is dissolved in a bath of molten cryolite, which lowers the melting point of the aluminium oxide and increases the conductivity of the solution. The mixture is then subjected to a strong electric current. The aluminium ions are reduced at the cathode to form aluminium metal, while the oxygen ions are oxidised at the anode to form oxygen gas.

The aluminium metal sinks to the bottom of the electrolysis cell, where it can be tapped off and cooled to form solid aluminium. This process is continuous, with more aluminium oxide being added to the electrolysis cell as the aluminium is extracted. The oxygen gas produced at the anode reacts with the carbon anodes to form carbon dioxide, which is released into the atmosphere. This is a major source of carbon emissions in the aluminium industry.