Metal Extraction By Electrolysis (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Extraction by electrolysis

• Electrolysis is used to extract metals that are too reactive to be reduced by carbon, or that react with carbon

• These metals are extracted by electrolysis of their molten compounds

• Electrolysis requires large amounts of energy:

• To melt the compounds so that ions are free to move

• To produce the electrical current needed for the process

• This makes electrolysis considerably more expensive than carbon reduction

Extracting aluminium

• Aluminium is above carbon in the reactivity series

  • This means that it is extracted by electrolysis

• Its main ore is bauxite, which contains aluminium oxide, Al₂O₃

The electrolytic cell for extraction of aluminium

• Aluminium oxide is dissolved in molten cryolite to form the electrolyte

  • Aluminium oxide alone has a melting point of over 2000°C, which means melting it directly would be extremely expensive

  • Dissolving it in cryolite lowers the melting point of the mixture, significantly reducing energy costs

  • Cryolite does not interfere with the reaction

• The mixture is placed in a steel electrolysis cell lined with graphite

  • The graphite lining acts as the cathode (negative electrode)

• Large graphite blocks act as the anodes (positive electrodes)

• Aluminium is produced at the cathode

• Oxygen is produced at the anode

Why do the anodes need to be replaced?

• The oxygen produced at the anode reacts with the carbon in the graphite anodes:

C (s) + O₂ (g) → CO₂ (g)

• This causes the anodes to gradually wear away and they must be replaced regularly