Sunday, September 9, 2012
Hall - Herault Process
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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