How Electrolysis Produces Aluminium Oxide

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Aluminium is very useful but it occurs in nature chemically bound to other elements so it's hard to smelt. In the 19th century people learned how to extract aluminium oxide (AlO2) from Bauxite using electrolysis. This is a process which uses d.c. electrical energy to bring about chemical changes by splitting a compound in an electrolyte solution of freely moving ions which carry the charge of the electric current.

Bauxite is the natural alumina ore of aluminium and it has a high melting point. This is why it has to be dissolved in molten cryolite before being electrolysed. This lowers the melting point and saves energy because it takes more heat to melt aluminium oxide than to liquefy alumina.

The dissolved alumina is made the negative electrode (cathode) in the electrolytic cell which also contains graphite. The carbon in the graphite serves as a good conductor of electricity so it is a very effective anode. The aluminium oxide ions move to the cathode, where they lose their electrons and become molten aluminium (MgAlO). The oxygen ions from the anode also lose their electrons to form oxygen gas, which bubbles off the cathode as water. This reaction is known as the Hall-Heroult electrolytic process.

Pure aluminium isn't very strong, so it's usually alloyed with copper and other metals like magnesium, silicon or iron. It resists corrosion because it has a very thick layer of protective aluminium oxide on its surface which is difficult to penetrate. This can be enhanced artificially by treating the aluminium with sodium hydroxide solution and making it the anode in an electrolysis of dilute sulphuric acid. The oxygen given off at the anode reacts with the aluminium to make a thicker layer of protective oxide.

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