ALUMINIUM
 
CHIEF ORES OF ALUMINUM
 
   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
EXTRACTION OF ALUMINUM
 
    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
 
    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.
HALLíS METHOD
 
    Hallís 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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