The extraction of aluminium from its ore is expensive and not very easy, but because it’s light and alloys easily, Al is so valuable that it’s worth the effort. Australian Al is mostly mined as bauxite – a composite of various hydrated Al ores.
Bauxite looks like Martian sand – not very interesting, but here’s an image anyway.
Round here, Bahrain has bauxite mines also. Bauxite melts at more than 2300K – impossibly high – which would waste a lot of heat if we had to melt it ‘as-is’.
Cryolite (Na3AlF6, sodium hexafluoroaluminate) is a rare mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, which ran out in 1987. The difficulty of separating aluminium from oxygen in the oxide ores was overcome by the use of cryolite as a ‘flux’ to dissolve the oxide mineral(s), making them a liquid at much lower temperatures, thus saving money which would otherwise be used to heat the ore up to its much higher melting point. Cryolite itself melts below 900°C (1173K), which is quite achievable and it can dissolve the aluminium oxides sufficiently well to allow easy extraction of the aluminium by electrolysis. Considerable energy is still required for both heating the materials and the electrolysis, but it is much more energy-efficient than melting the oxides themselves. Today, natural cryolite is too rare to be used for this purpose, synthetic sodium aluminium fluoride is produced from the common mineral fluorite for this purpose.
The point here is we need the purified aluminium oxide from bauxite in liquid form so the ions can move to be able to electrolyse it to extract aluminium. But, its melting point is too high, hence the cryolite.
How does it work? Here’s a schematic of the apparatus.
Notice the actual casing is the cathode. The carbon graphite anodes are attacked by the oxygen produced at them and need changing quite often.
Here are the equations:
Al+++ + 3e– → Al
electrons gained at cathode by Al – REDUCTION of Al
2O— → O2 + 4e–
electrons lost at anode by oxygen ions – OXIDATION
Please notice that no electrons are lost or gained overall, their movement is a flow of charge around the circuit which we could measure with an ammeter.
For information: the electrical energy needed to produce aluminium is relatively high. To make 1kg of aluminum requires about 15 kilowatt-hours of electrical energy. This amount of aluminum can be used to make about 50 Coke cans or an energy equivalent of a 100 watt light bulb burning for three hours is needed to make one aluminium can.