Aluminium is highly reactive and does not occur in the free state. However, it is widely distributed and it is the third metal in abundance on earth after Oxygen and Silicon.
Aluminium exists primarily as Alumino-Silicates (i.e. as Felspar, NaAlSi3O8, or KAlSi3O8, or CaAl2Si2O8), in igneous rocks and as Clays, H4Al2Si2O9, in sedimentary rocks.
Extraction of Aluminium is done by the method of Hall–Héroult cell.
This method makes use of the amphoteric (both acidic and basic nature) of Aluminium salts.
Natural occurrence of Aluminium: Bauxite Al2O3.2H20
Clay Al2O3.2SiO2.2H2O
Cryolite NA3AlF6
Extraction method
Bauxite, an impure form of Aluminium oxide, is first treated with NaOH to obtain pure Aluminium oxide, removing impurities such as Iron(III) oxide and sand (SiO2). By adding excess of NaOH to Bauxite forms NaAlO2 (Aluminate) and Na2SiO3 (silicate) in aqueous form. And other basic materials remain insoluble. They are removed by filtration.
The solution is heated with weak acid in order to precipitate out Al3+ ions as Al(OH)3.
Finally we can separate Al(OH)3 from silicate solution by filtering, then Al(OH)3 is heated to obtain pure Al2O3.
The purified Aluminium oxide is then dissolved in molten Cryolite – Na3(AlF6) .Cryolite is used to reduce the working temperature of Hall–Héroult cell from 2070C to 800-900C
During the electrolysis the negativity charge oxide ions are attracted to anode where they oxidizes to form oxygen gas
2O-(l) → O2(g) + 4e
The positive charge Aluminium ions are attracted to the cathode where they reduce to form Aluminium metal
Al3+ + 3e → Al(l)
The molten Aluminium is collected at the bottom of the cell and it is siphoned out at regular intervals. At the working temperature of the cell, the oxygen liberated react with the graphite anodes. The anodes burn out and have to be replaced by a regular basic.
The electrolysis of Aluminium oxide is a continuous process in which a vast amount of electricity is used. Approximately 15kWh of electricity is used to produce 1kg of Aluminium.
The great electricity demand of an Aluminium plant puts serous limits on its location. Cheap and plentiful electricity (e.g. hydroelectricity) is desirable
The high cost of Aluminium makes recycling economically effective.
