Aluminium and its Extraction

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Bauxite is an orange-red igneous rock, which occurs naturally in the lithosphere, from which Aluminium is extracted. It contains 30-54% alumina, Al2O3 and other impurities such as clay, Iron (III) Oxide (Fe2O3), Silica (SiO2), and Titania (TiO2). Australia was the top producer of bauxite in 2007, with almost one-third world share.

The Bauxite has to be purified by a process known as The Bayer Process. The Bayer process is the main industrial method of refining bauxite to produce alumina. The Bauxite rocks are crushed to smaller rock sizes. Then the bauxite is digested by mixing it with a hot solution of sodium hydroxide, NaOH at 175oC. This only dissolves the oxides of aluminium and silicon, but not the other impurities.  The solution is further purified by filtering out the solid impurities. Carbon dioxide gas is then bubbled through the solution, which creates a weak carbonic acid, neutralising the solution and causing the aluminium oxide to precipitate, but leaving the other silicon impurities. The remaining solution is filtered once again and boiled to remove the water. The resultant product is purified aluminium oxide.

After the purified aluminium oxide has been produced, the aluminium can be separated by a process known as The Hall-Heroult method. The alumina is dissolved in molten cryolite (Na3AlF6). Then mixture is then heated to approximately 980oC, (aluminium oxide usually melts at a higher temperature, but due to the added aluminium fluoride, a considerable amount of energy is saved). The mixture is then placed in a carbon lined bath and a large electrical current is passed through it. This forms aluminium at the cathode and oxygen gas at the carbon anode. The oxygen gas reacts with the anode to give off carbon dioxide gas. The transformer generates a current from 220kA to 340kA, with a voltage of 1-2kV from 110kV; this shows how strong the bonds are between the aluminium and oxygen ions, as a large amount of electrical energy is used.

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The solid aluminium that is formed is denser than the molten cryolite (at 1000oC) and therefore sinks to the bottom of the bath, which taken out. The liquid aluminium is removed by a vacuum tube called a siphon; this saves the use of high energy pumps.


The yield of a metal is the amount of product (i.e. a metal) which is derived from a chemical extraction process, which is often expressed as a percentage of the theoretical maximum amount. It is very important ...

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