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CHIEF
ORES OF ALUMINUM
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Aluminium
does not occur in free state, but it is widely distributed in nature in
the form of compounds. The chief minerals of aluminum
are either silicates or oxides. SILICATES:Kaolin Al2O3.2SiO2.2H2O Potash felspar K2O. Al2O3.6SiO2 Potash mica K2O.3Al2O3.6SiO2.2H2O OXIDES:Bauxite Al2O3.nH2O Gibbisite Al2O3.3H3O Diaspore Al2O3.H2O Corundum Al2O3 FLOURIDES:Cryolite Na3AlF6
SULPHATES:Alunite K2SO4.Al2(SO)3.4Al(OH)3 |
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EXTRACTION
OF ALUMINUM
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Aluminum
is extracted from its oxide ore "Bauxite". The extraction of aluminum
involves the following steps.
Purification of Bauxite.
Electrolysis of Alumina.
Refining. |
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PURIFICATION
OF BAUXITE TO ALUMINA
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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 in bauxite any one of the following methods can be used for the purification of bauxite. | |||
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HALL’S
METHOD
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| Hall’s
method is used for the purification of bauxite if it contains both Fe2O3
and SiO2 in excess.
DETAILS OF THE PROCESS: When finely divided bauxite is fused with sodium carbonate it dissolved to form sodium aluminate, while the impurities of Fe2O3 and SiO2 are left unaffected. Al2O3.nH2O + Na2CO3 ® 2NaAlO2 + CO2 + (n) H2O The fused mass is extracted with water leaving behind both impurities. Sodium aluminate is then heated up to 50oC to 60oC in the presence of CO2. The precipitates of aluminum hydroxide are formed. 2NaAlO2 + 3H2O + CO2 ® 2Al(OH)3 + Na2CO3 The precipitates of Al(OH)3 are washed to remove Na2CO3, dried and ignited at about 1500oC to get pure alumina. 2Al(OH)3 ® Al2O3 + 3H2O |
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