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Aluminium ore which is called bauxite is purified by the Bayer Process.The ore is mixed with a hot concentrated solution of sodium hydroxide. The sodium hydroxide dissolves the oxides of aluminum and silicon and leaving other impurities including iron oxides as solid.The mixture is then filtrated to remove the insoluble impurities.The filtrate which now contains the oxides of aluminum and silicon is treated by bubbling carbon (iv)oxide gas through the solution.The Carbon (iv)oxide forms weak acidic solution of carbonic acid that then neutralizes the sodium hydroxide from the first treatment. The treatment by neutralization will also selectively precipitate the aluminum oxide and leave the silicate in solution.The mixture is then filtrated to separate it.Thereafter,the purified aluminum oxide is heated to evaporate the water yielding a white powder, aluminium oxide, from which aluminium can be extracted. Aluminium oxide having a very high melting point (over 2,000°C),it is mixed with cryolite, a mixture of sodium fluoride and aluminum fluoride, and heated to about 980 degrees Celsius to melt the solids. The mixture melts at a much lower temperature than aluminum oxide would by itself. The use of cryolite reduces some of the energy costs involved in extracting aluminium as it lowers the melting point.The hot molten mixture is then electrolyzed.Aluminum ions are reduced to Aluminium metal that forms at the negative electrode(cathode) and sinks to the bottom of the tank, where it is tapped off. At the anode, oxygen is produced from the oxide ions.Both the anode and cathode materials are carbon in the form of graphite, which also is oxidized and must be replaced quite frequently.
Mr Masinde answered the question on April 24, 2018 at 15:50
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