Chemguide: Core Chemistry 14 - 16


This page looks at the extraction of aluminium from purified bauxite, together with some uses of aluminium.


Aluminium is too high in the reactivity series to extract it from its ore using carbon reduction. The temperatures needed are too high to be economic.

Instead, it is extracted by electrolysis. The ore is first converted into pure aluminium oxide, and this is then electrolysed in solution in molten cryolite - another aluminium compound. The aluminium oxide has too high a melting point to electrolyse on its own.

At this level, you are unlikely to need to know how the aluminium oxide is obtained from the bauxite.

Aluminium ore

The usual aluminium ore is bauxite. Bauxite is essentially an impure aluminium oxide. The major impurities include iron oxides, silicon dioxide and titanium dioxide.

Conversion of the aluminium oxide into aluminium by electrolysis

The aluminium oxide is electrolysed in solution in molten cryolite, Na3AlF6. Cryolite is another aluminium ore, but is rare and expensive, and most is now made chemically.

The electrolysis cell

Start by watching a bit of video which gives some idea of the scale of production.

The diagram shows a very simplified version of an electrolysis cell.

Although the carbon lining of the cell is labelled as the cathode, the effective cathode is mainly the molten aluminium that forms on the bottom of the cell.

Molten aluminium is syphoned out of the cell from time to time, and new aluminium oxide added at the top.

The cell operates at a low voltage of about 5 - 6 volts, but at huge currents of 100,000 amps or more. The heating effect of these large currents keeps the cell at a temperature of about 1000°C.

The electrode reactions

These are very complicated. For chemistry purposes at this level, they are always simplified.

This is the simplification:

Aluminium is released at the cathode. Aluminium ions are reduced by gaining 3 electrons.

Al3+ + 3e-    Al

Oxygen is produced initially at the anode.

2O2-    O2 + 4e-

Note:  The equations on the video are scaled up so that 12 electrons are transferred over-all. In an exam, if you are asked for the electrode equations, keep it simple as above.

However, at the temperature of the cell, the carbon anodes burn in this oxygen to give carbon dioxide and carbon monoxide.

Continual replacement of the anodes is a major expense.

Uses of aluminium

Aluminium is usually alloyed with other elements such as silicon, copper or magnesium. Pure aluminium isn't very strong, and alloying it adds to it strength.

Aluminium is especially useful because it

  • has a low density;

  • is strong when alloyed;

  • is a good conductor of electricity;

  • has a good appearance;

  • resists corrosion because of the strong thin layer of aluminium oxide on its surface. This layer can be strengthened further by anodising the aluminium.

    Anodising essentially involves etching the aluminium with sodium hydroxide solution to remove the existing oxide layer, and then making the aluminium article the anode in an electrolysis of dilute sulphuric acid. The oxygen given off at the anode reacts with the aluminium surface, to build up a film of oxide up to about 0.02 mm thick.

    As well as increasing the corrosion resistance of the aluminium, this film is porous at this stage and will also take up dyes. (It is further treated to make it completely non-porous afterwards.) That means that you can make aluminium articles with the colour built into the surface.

Some uses include:

aluminium is used forbecause
aircraftlight, strong, resists corrosion
other transport such as ships' superstructures, container vehicle bodies, tube trains (metro trains)light, strong, resists corrosion
overhead power cables (with a steel core to strengthen them)light, resists corrosion, good conductor of electricity
saucepanslight, resists corrosion, good appearance, good conductor of heat

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© Jim Clark 2020