Extracting aluminium
Aluminium is the most If there is a lot of something, it is described as being abundant. metal on Earth, but it is expensive, largely because of the amount of electricity used in the extraction process.
Aluminium A rock containing enough quantities of a mineral for extraction to be possible. is called bauxite (Al2O3). The bauxite is A substance that has been separated from other substances is said to be purified. to yield a white powder – aluminium oxide (also known as alumina) – from which aluminium can be extracted.
The extraction is done by The decomposition (breakdown) of a compound using an electric current., 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 When something is broken up in a liquid and no longer exists. in A term used to describe a liquid substance (eg rock, glass or metal) formed by heating a solid. cryolite – an aluminium compound with a lower melting point than aluminium oxide. The use of molten cryolite as a The liquid in which the solute dissolves to form a solution. 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 A conductor used to establish electrical contact with a circuit. The electrode attached to the negative terminal of a battery is called a negative electrode, or cathode. The electrode attached to the positive terminal of a battery is the positive electrode, or anode. (cathode) and positive electrode (anode) are made of graphite, which is a form of carbon.
Aluminium Electrically charged particle, formed when an atom or molecule gains or loses an electron/electrons. receive Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. at the negative electrode and are The loss of oxygen, gain of electrons, or gain of hydrogen by a substance during a chemical reaction. to aluminium All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons.:
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.
Choice of aluminium extraction site
There are a number of important factors to consider when choosing the site of an aluminium extraction plant. It should be:
- in close proximity to a power station, in order to provide the large supply of electricity needed for the electrolysis
- near the coast to allow for the import of raw materials
- near roads and railway lines to allow for the product to be taken to where it is needed
- near a town or city, so that workers have somewhere to live close by
- away from built-up areas, so that the noise and pollution of the site do not affect the local population
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.