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Aluminium is extracted from it's ore by electrolysis. The electrolyte has to be molten ore, if a solution was used (as in the extraction of copper) hydrogen gas would be produced instead of the pure metal.
The next problem lies in that aluminium ore, bauxite, has a very high melting point - over 2000 degrees Celsius! Huge amounts of energy would be needed to melt it. However, by adding a little of another less common ore, cryolite, the melting opint reduces to 950 degrees Celsius.
The electrodes are made of carbon, with the positive anodes being inserted into a carbon tank which is the negative cathode. The bauxite and cryoltie mixture is melted and a huge current is passed through it. The positively charged aluminium ions are attracted to the tank, where they make a layer of molten aluminium.
At the anode, oxygen is formed. This reacts with the carbon anodes to make carbon dioxide, and eventually wears away the anodes so they need to be replaced.
The layer of molten aluminium is tapped off into large crucibles.
Aluminium is still very expensive to extract, which is why it is important to recycle aluminium cans etc, to save on the huge amount of energy needed to extract more.