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Aluminium Extraction from the Lithosphere

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Pure aluminium is a relatively soft, silvery white metal. When exposed to air, a thin coating of Aluminium oxide gives it a dull lustre. It is three times less dense than water, and has great strength when alloyed, it doesnt rust and has high electrical conductivity, and Aluminium is also ductile, making it a very useful metal. Aluminium readily makes alloys with copper, zinc, magnesium, manganese and silicon. Aluminium foil is 92-99% pure aluminium. Other uses of aluminium alloys include computers (all Apple MacBooks are made from aluminium), cooking utensils, transportation (aircraft, rockets and cars mainly because of their high strength-to-weight ratio), packaging, water treatment, street lighting etc.

Aluminium is the most abundant metallic element in the lithosphere, thought to be 7.5 to 8.1%, it is rare to find it isolated. Due to its high reactivity it forms a high-energy bond with oxygen, thus making it difficult to extract. Therefore it has to be refined form alumina, using the Hall-Heroult process (below). Alumina is produced by the Bayer Process from bauxite and used in the production of aluminium metal. It is also used as a refractory material, which is a material which keeps its strength even at high temperatures. These materials are usually used in linings of furnaces and kilns, incinerators and reactors.

Bauxite is an orange-red igneous rock, which occurs naturally in the lithosphere. 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. In the process the bauxite is digested by mixing it with a hot solution of sodium hydroxide, NaOH at 175oC. This dissolves the oxides of aluminium and silicon, but the other impurities dont. The solution is further purified by filtering the solid impurities. Carbon dioxide gas is then bubbles through the solution, which forms a weak carbonic acid, this neutralises the solution and causes the aluminium oxide to precipitate, but leaving 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, aluminium can be separated from it by a process known as The Hall-Heroult method. The alumina is dissolved in molten cryolite (Na3AlF6). The mixture is then heated to approximately 980oC, this is much lower than the normal temperature that is required to melt aluminium oxide, this is because of the aluminium fluoride which is also added, this saves a considerable amount of energy. 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.

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.

During the Bayer Process the solid residue is known as red mud and is hard to dispose. The red mud has pH values in excess of 13.2. The solution to this, is to neutralise it with seawater. The resulting water has a pH less than 9.0 and is safe enough to discharge back into the sea. Th e electrolysis process produced exhaust, mainly carbon dioxide, this is allowed to escape in the fume hood. Hydrogen fluoride (HF) is formed from the cryolite, which is a highly corrosive gas and is reactive to glass, which means that they are covered in plastic. These gases are treated in a close-by plant which dissolves the HF into water to neutralise it. The pollution and large amounts of energy required was a problem in the past, but the use of improved filter systems and hydroelectric power plants has resolved this issue to some extent.

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