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Sunday, September 9, 2012

Hall - Herault Process

Principle Hall Héroult

The Hall – Herault Process is the major industrial process for production of Aluminium. It involves dissolving alumina in molten crytolite, and electrolysing the molten salt bath to obtain pure aluminium metal.
Bauksit is an alloy of aluminium oxide (Al2O3), iron (III) oxide (Fe2O3), and silikat oxide (SiO2) so there are 2 step in obtaining pure aluminium :
1.       Step 1 : Purification reaction with alkali NaOH
Al2O3 in bauksit will be reacted with NaOH :
       Al2O3(s) + 2OH-(aq) + 3 H2O  2[Al(OH)4]-(aq)
SiO2 in bauksit will be reacted with NaOH :
       SiO2(s) + 2OH-(aq)   SiO3-2(aq) + H2O(l)
With gases CO2, Al(OH)4 solution would be reacted and producting Al(OH)3(s) and will be filtrated     
       Al(OH)4(aq) + CO2(g) Al(OH)3(s) + CO3-2(aq) + H+(aq)
Pure Al2O3 would be obtain by heating Al(OH)3 :
       Al(OH)3(s)   Al2O3(s)  + 3H2O(g)             
2.       Step 2 : Al2O3 electrolysis
Electrolysis cell made from stell that coated by grafit. This grafit function as a katode and the anode made from carbon. Al2O3 would be dissolved in kryolite (Na2AlF6). The reaction would be
       Katode :               AlF4-(aq) + 3e-    Al(s) + 4F-(aq)         or
                                    2Al3+ + 6e-    3Al(s)
      Anode :                2AlOF54-(aq) + C   CO2(g) + AlF6-3(aq) + AlF4-(aq) + 4e-     or
                                   3O-2(aq)  + 3/2CO2(g)    3/2CO2(g) + 6e-
The overall reaction would be :
       2Al2O3(S) + 3C(s)       4Al(s) + 3 CO2(g)

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