Electrolysis Electrolysis and other metals

Aluminium is probably the most important of the more reactive metals which requires electrolysis to be extracted from their ore. Its lightness, strength, resistance to corrosion and good electrical conductivity make it a very important metal in our modern society.

Aluminium forms a protective oxide layer on exposure to air which allows it to be used in high tension power lines, planes, ships, building and cans to name but a few. If it were not for this protective oxide layer, its high reactivity would make it impossible to use safely.

This was shown to be the case in the Falklands War, where many of the British war ships had aluminium superstructures, because it reduced their overall weight. When one of these ships was hit with a Cruise missile, it caught alight and the aluminium started to melt and then burn itself, totally destroying the ship.

Electrolysis
The ore of aluminium is called bauxite, a brown, rusty colored mineral containing aluminium oxide or alumina (Al₂O₃). Bauxite is usually found in areas of high rainfall and is the product of leaching aluminium oxide from surface rock over millions of years. Alumina is first chemically separated from bauxite ore by dissolving the bauxite in sodium hydroxide solution, filtering off the unwanted colored components and then extracting the alumina by precipitation.

Aluminium oxide, or alumina, is a very stable compound and requires lots of energy to extract the aluminium metal. This energy is best supplied by passing an electric current through a molten mixture of alumina and cryolite (sodium aluminium fluoride) which melts and "dissolves" the alumina at a lower temperature than would would be required otherwise. The electric current is passed though the cell, or vessel holding the molten mixture, from one electrode (the cathode) to another electrode (the anode).

Aluminium is produced at the cathode by positively charged aluminium ions accepting free electrons to form aluminium atoms. At the anode, electrons are effectively stripped of negatively charged oxide ions, which then combine with carbon at the anode to produce carbon dioxide gas.

The aluminium is then drained from the cell and the carbon dioxide escapes to the atmosphere. This process consumes so much electricity, that aluminium is sometimes referred to as "solid electricity"!

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