Extraction of Aluminium from Bauxite (Cambridge O Level Chemistry)

• Aluminium is a reactive metal, above carbon in the reactivity series 
• Its main ore, is bauxite, which contains aluminium oxide
• Aluminium is higher in the reactivity series than carbon, so it cannot be extracted by reduction using carbon
• Instead, aluminium is extracted by electrolysis  

Diagram showing the extraction of aluminium by electrolysis

• Bauxite is first purified to produce aluminium oxide, Al₂O₃
• Aluminium oxide is then dissolved in molten cryolite 
  • This is because aluminium oxide has a melting point of over 2000°C which would use a lot of energy and be very expensive
  • The resulting mixture has a lower melting point without interfering with the reaction
• The mixture is placed in an electrolysis cell, made from steel, lined with graphite
• The graphite lining acts as the negative electrode, with several large graphite blocks as the positive electrodes
• At the cathode (negative electrode): 
  • Aluminium ions gain electrons (reduction) 
  • Molten aluminium forms at the bottom of the cell
  • The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell

Al³⁺ +  3e^-   →  Al 

• At the anode (positive electrode):
  • Oxide ions lose electrons (oxidation)
  • Oxygen is produced at the anode:

2O^2- →   O₂ + 4e^-

• The overall equation for the reaction is:

2Al₂O₃ → 4Al  +  3O₂

• The carbon in the graphite anodes reacts with the oxygen produced to produce CO₂

C (s) + O₂ (g)   →   CO₂ (g)

• As a result the anode wears away and has to be replaced regularly
• A lot of electricity is required for this process of extraction, this is a major expense