Extraction of Aluminium by Electrolysis
Diagram Showing the Extraction of Aluminium by Electrolysis
Aluminium Ore (Bauxite)
The bauxite is first purified to produce aluminium oxide Al2O3. Aluminium Oxide has a very high melting point so it is first dissolved in molten cryolite producing an electrolyte with a lower melting point, as well as a better conductor of electricity than molten aluminium oxide. This reduces expenses considerably making the process more efficient.
The electrolyte is a solution of aluminium oxide in molten cryolite at a temperature of about 1000 °C. The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell. The cell operates at 5-6 volts and with a current of 100,000 amps. The heat generated by the huge current keeps the electrolyte molten. A lot of electricity is required for this process of extraction which is a major expense.
The overall equation is:
2Al2O3(l) ⟶ 4Al(l) + 3O2(l)
Reaction at the Negative Electrode:
The Aluminium melts and collects at the bottom of the cell and is then tapped off:
Al3+ + 3e– ⟶ Al
Reaction at the Positive Electrode:
2O2- – 4e– ⟶ O2
Some of the oxygen produced at the positive electrode then reacts with graphite (Carbon) electrode to produce carbon dioxide gas:
C(s) + O2(g) ⟶ CO2(g)
This causes the carbon anodes to burn away, so they must be replaced regularly.