Aluminium Extraction

• 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

  • mixture has a 950 C melting point

  A) graphite anode

  B) steel case

  C) purified aluminium

  D) dissolved

  E) molten cryolite

  F) molten aluminium

  G) graphite cathode

  
• If you are a Core student, you do not need to explain the process of 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:
  • 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
• Use OIL RIG to remember whether oxidation or reduction has occurred at the electrodes:

  A) oxidation

  B) loss

  C) reduction

  D) gain

  
• A mixture of aluminium oxide and cryolite is used because cryolight can lower the overall melting point of the mixture so that it reduces the cost in extracting

  • cryolite is the solvent
• The positive electrode are replaced in regular intervals because the reactions between the oxygen and carbon wears down the anode, requiring replacements which makes it expensive
• Aluminium is expensive as it needs a lot of electricity to react, replacing electrodes
• Why is the positive electrode always continually replaced?


  When the molten aluminium oxide and cryolite mixture is electrolysed the product at the positive electrode is oxygen.

  This product reacts with the positive electrode because the positive electrode is made of carbon.

  This results in the production of carbon dioxide.

  This reaction means that the positive electrode must be continually replaced.