Extraction of Aluminium from Bauxite

• 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
• The electrolysis of aluminium

  A) graphite anode

  B) graphite cathode

  C) molten aluminium

  D) purified aluminium

  E) dissolved

  F) molten cryolite

  G) steel case

  
• 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
  • 2O2- →   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