Electrolysis of Aluminium Oxide

When aluminium oxide (Al2O3) is electrolysed, it reacts with oxygen to form alundum and with carbon to form carbon dioxide. It is the process of this reaction that is used to extract pure aluminium from its ores.

The first step in the extraction of pure aluminium from bauxite is to convert it into pure aluminium oxide, using the Bayer Process. This is done by heating aluminium hydroxide, a compound of aluminium and water, to a temperature of about 1100 degrees Celsius.

This is done to lower the high melting point of aluminium oxide and to reduce the energy needed for the electrolysis. The aluminium oxide is then electrolysed in solution in molten cryolite, another aluminium compound.

The current flowing through the cell is a very powerful electric current that is able to decompose the aluminum oxide into aluminium ions and oxygen ions at both the cathode and anode. At the cathode, the aluminum ions are reacted to form alundum and at the anode the oxygen ions are reacted to form carbon dioxide.

One reason that electrolysis is expensive is because the anode needs to be constantly replaced. This is because the anode contains graphite, a metal that reacts with oxygen to form carbon dioxide.

The anode also contains impurities from the bauxite, including some other metal oxides that don’t react with the sodium tetrahydroxoaluminate solution. These impurities are removed from the solution by filtration. They are then cooled and “seeded” with some previous produced aluminium hydroxide, so that the new aluminium hydroxide can be precipitated from it.