Aluminium is the most abundant metal on Earth, but it is expensive, largely because of the amount of electricity used in the extraction process.
The extraction is done by electrolysis, but first the aluminium oxide must be melted so that electricity can pass through it. However, aluminium oxide has a very high melting point (over 2,000°C) so it would be expensive to melt it.
Instead, it is dissolved in molten cryolite – an aluminium compound with a lower melting point than aluminium oxide. The use of molten cryolite as a solvent reduces some of the energy costs involved in extracting aluminium by allowing the ions in aluminium oxide to move freely at a lower temperature.
The diagram shows an aluminium oxide electrolysis cell. Both the negative electrode (cathode) and positive electrode (anode) are made of graphite, which is a form of carbon.
Al3+ + 3e– → Al (reduction – gain electrons)
The molten aluminium sinks to the bottom of the cell, where it is tapped off.
Oxide ions lose electrons at the positive electrodes and are oxidised to oxygen gas:
2O2– → O2 + 4e– (oxidation – lose electrons)
This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, so they gradually burn away. As a result, the positive electrodes have to be replaced frequently. This adds to the cost of the process.
There are a number of important factors to consider when choosing the site of an aluminium extraction plant. It should be:
Anglesey, in north Wales, was chosen as a suitable site for an aluminium extraction plant. However, this plant was shut down in 2013 after the nearby power station was decommissioned.