The extraction of aluminium oxide by electrolysis is a process that requires large amounts of electricity. This is because the ions in aluminium oxide need to be free to move so that electricity can pass through them.
To reduce the energy required, molten cryolite is used as the electrolyte because it has a lower melting point than aluminium oxide. This allows the ions in aluminium oxide to move freely at a lower temperature and therefore reduces some of the energy costs involved in extracting aluminium.
During the electrolysis of aluminium oxide, the negative electrode is made of carbon and is called the cathode. The positive electrode is made of graphite and is called the anode.
When the ions in aluminium oxide are electrolysed they lose their electrons and become neutral atoms which sink to the bottom of the tank. They can then be drained off as liquid aluminium.
The ions in aluminium oxide are then dissolved in a solution of molten cryolite. This reduces their melting point to 950degC which means that they are more likely to dissolve in water.
A second reaction takes place at the cathode when the alumina ions are purified by electrolysis. This is a simple chemical reaction that involves oxygen and carbon.
Aluminium is extracted from bauxite, which is a white powder that is produced by mining and processing the ore. It contains silica and ferric oxide as impurities, which are removed using gravity separation or magnetic separation.