BBC Bitesize Electrolysis of Aluminium Oxide

Aluminium is the most abundant metal in the world but it is quite expensive, mainly because of the amount of electricity required to extract it from its ores. This is because the metals are often too reactive to be extracted by reduction with carbon and so must be electrolysed.

The most commonly used aluminium ore is bauxite and it contains a mixture of impure aluminium oxides. It also contains a number of other impurities including iron oxides and silicon dioxide. These need to be removed from the bauxite before it can be turned into pure aluminium. The extraction of the aluminium from the bauxite is done by electrolysis. The bauxite is dissolved in molten cryolite which has a lower melting point than aluminium oxide and so allows the ions in the bauxite to move at a much lower temperature. The use of cryolite reduces the energy costs involved in this process.

During electrolysis a d.c. electrical current passes through a conducting liquid called an electrolyte. This is the molten mixture of bauxite and cryolite that is being broken down by the electric current. The negative electrode (cathode) in the electrolysis cell is made from graphite and attracts positive aluminium ions – Al3+. The positive anode is made from the more reactive aluminium oxide which attracts the negatively charged oxygen ions – O2-.

The ions combine to form aluminium metal and oxygen gas. The balanced half equation for this reaction is 4Al3+ – 6O2- + 12e-. This shows that four positively charged aluminium ions combine with six negatively charged oxide ions to produce the aluminium metal and oxygen gas. The electrolyte is then cooled and the solution is seeded with some previously produced aluminium hydroxide. This ensures that the ions from the anode are not attracted to the cathode and that the solution continues to break down.