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| Metallurgy of Aluminum |
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| Pure aluminium is a silvery-white metal with many desirable characteristics. It is light, nontoxic (as the metal), nonmagnetic and non sparking. Pure aluminium is soft and lacks strength, but alloys with small amounts of copper, magnesium, silicon, manganese, and other elements have very useful properties. ~Aluminium is the third most abundant element in the Earth's crust after oxygen and silicon |
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| Aluminum is extracted from its oxide ore "Bauxite". The extraction of aluminum involves the following steps. |
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 Purification of Bauxite |
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Electrolysis of Alumina |
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Refining |
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| Bauxite contains two main impurities Fe2O3 and SiO2. These impurities must be removed in order to get good quality aluminum. These impurities make the aluminum brittle and liable to corrosion. Depending upon the nature of impurity present in bauxite, any one of the following methods can be used for the purification of bauxite. |
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| Bayer's process involves concentration and dehydration of bauxite to alumina (Al2O3). |
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| Treatment with NaOH |
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| Bauxite is treated with hot, concentrated NaOH solution. |
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| Aluminium oxide reacts with NaOH forming an aqueous solution of sodium aluminate. The impurities do not react. |
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| On filtration, impurities are separated from the sodium aluminate solution. |
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| Treatment with HCl |
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| The filtrate containing sodium aluminate is acidified with hydrochloric acid to form a precipitate of aluminium hydroxide. |
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| On filtration sodium chloride solution is obtained as a filtrate and is discarded. The pure aluminium hydroxide obtained as a gel is collected. |
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| Action of Heat on Al(OH)3 |
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| The precipitate of Al(OH)3 is heated to a high temperature to obtain pure alumina. |
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| Thus, pure alumina is obtained from bauxite. |
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| In this process, the pure alumina (Al2O3) obtained from Bayer's process is electrolytically reduced to aluminium. |
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| An electrolytic cell used is called the Hall's cell. |
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| The Hall's cell is a steel tank lined inside with a layer of graphite which is made the cathode. The electrolyte consists of a mixture of fused (molten) alumina and cryolite. Cryolite is a flux and helps in lowering the melting point of alumina. A series of graphite rods dipped into the electrolyte serve as the anode. |
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| When electricity is passed through the cell, the molten alumina dissociates forming Al3+ and O2- ions, |
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| The Al3+ ions capture electrons of the cathode and get deposited at the bottom of the tank as molten aluminium. |
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| The O2- ions lose electrons to the anode and release oxygen gas. Some amount of carbon dioxide and carbon monoxide are also released, due to the reaction of oxygen atoms with the graphite anode. |
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| Ionic Equations |
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| The molten aluminium thus collected is removed from an outlet at the bottom of the cell. |
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