Aluminium ExtractionOxford AQA IGCSE Chemistry: Revision notes

Aluminium is a reactive metal — it sits above carbon in the reactivity series. Carbon can only displace metals that are less reactive than itself (like iron) by reduction. Because aluminium is more reactive than carbon, carbon cannot remove the oxygen from aluminium oxide, so a chemical reduction method will not work.

Instead, aluminium is extracted by electrolysis, which uses electrical energy rather than a more reactive element to split the compound apart.

The ore used is bauxite, which is purified to give aluminium oxide (Al₂O₃). Pure aluminium oxide has a very high melting point (over 2000°C), so melting it directly would use huge amounts of energy and be very expensive.

To solve this, the aluminium oxide is dissolved in molten cryolite (a compound of aluminium, sodium and fluorine). This mixture melts at a much lower temperature (around 900°C) than pure aluminium oxide, which lowers the energy costs of the process considerably.

During electrolysis of the molten Al₂O₃/cryolite mixture:

• At the negative electrode (cathode): Al³⁺ ions gain electrons (reduction) and molten aluminium metal collects and is tapped off
  • Al³⁺ + 3e⁻ → Al
• At the positive electrode (anode): O²⁻ ions lose electrons (oxidation) to form oxygen gas
  • 2O²⁻ → O₂ + 4e⁻

This matches the general rule for electrolysis: positive ions (cations) are attracted to and discharged at the negative electrode, negative ions (anions) are attracted to and discharged at the positive electrode.

The anodes in this process are made of carbon (graphite). Because the process runs at a high temperature, the oxygen produced at the anode reacts with the hot carbon electrode itself:

C + O₂ → CO₂

This gradually burns away the carbon anodes, so they must be regularly replaced — an ongoing cost of the process.