Extracting metals using electrolysis

Metals are found in ores combined with other elements. Electrolysis can be used to extract a more reactive metal from the ore.

Bioleaching is used to extract small quantities of metals from mining waste heaps. Electrolysis can be used to extract the metal from the leachate solution produced during the process.

Extraction of aluminium

Aluminium is the most abundant (found in large quantities) metal in the Earth's crust. It is expensive, largely because of the amount of electricity required in the extraction process.

Aluminium ore is called bauxite. The bauxite is purified to produce aluminium oxide, a white powder from which aluminium can be extracted.

The extraction is done by electrolysis. The ions in the aluminium oxide must be free to move so that electricity can pass through it. Aluminium oxide has a very high melting point (over 2000°C) so it would be expensive to melt it. Aluminium oxide does not dissolve in water, but it does dissolve in molten cryolite. This is an aluminium compound with a lower melting point than aluminium oxide. The use of cryolite reduces some of the energy costs involved in extracting aluminium.

A cell for aluminium extraction: a steel case is lined with a graphite cathode and filled with purified aluminium ore dissolved in molten cryolite. Graphite anodes are inserted and molten alumninium is drawn off from the bottom.A cross section through an electrolysis cell for aluminium extraction

The diagram shows an aluminium oxide electrolysis cell. The negative electrodes (cathodes) and the positive electrodes (anodes) are made of graphite, a form of carbon.

During electrolysis:

  • positively charged aluminium ions gain electrons from the cathode, and form molten aluminium
  • oxide ions lose electrons at the anode, and form oxygen molecules

The oxygen reacts with the carbon in the electrodes, forming carbon dioxide which bubbles off. Carbon is therefore lost from the positive electrodes, so they must be replaced frequently. This adds to the cost of the process.