Extraction of Aluminium
- 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 electrolytic cell for extraction of aluminium
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:
2O2– → O2 + 4e–
- 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
