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Wednesday, June 26, 2019

Extraction of Aluminium:

Important ores of Aluminium:
(1) Corundum (Al2O3)
(2) Diaspore (Al2O3.H2O)
(3) Bauxite (Al2O3.2H2O)
(4) Gibbsite (Al2O3.3H2O)
(5) Cryolite (AlF3.3NaF) or Na3AlF6
(4) Alunite (K2SO4.Al2(SO4)3.4Al(OH)3
(5) Spinel (MgOAl2O3)
(6) Felspar (K2O.Al2O3.6SiO3) or KAlSi3O8
(7) China Clay or Kaoline [Al2O3.2SiO2.2H2O]
(B) Purification of Bauxite:
(1) Baeyer’s Process: By Bayer’s process commercially it is being carried out (for red bauxite) In other words this process is applied to the bauxite ore containing ferric oxide (red bauxite Fe2O3) as chief impurity. Ore roasted to convert ferrous oxide to ferric oxide.
(2) Hall’s Process: This process is also applied to the bauxite ore containing ferric oxide (red bauxite Fe2O3) as major impurity. This process ore is fused with Na2CO3.
(3) Serpeck’s Process: This process is applied to the bauxite ore containing Silica (White bauxite SiO2) as major impurity.
 NOTE:Silicone Volatile at this temperature and removed easily.

Electrolytic reduction of Al2O3:
Electrolysis of molten mixture: HALL HEROULT PROCESS:
Cathode           :           Iron-tank lined with bricks Carbon
Anode              :           Graphite rods (Carbon)
Electrolyte       :           Molten [Al2O3 (5%), Na3AlF6 (85 %), CaF2 (5%), AlF3 (5%) ]
Temperature    :          900 degree centigrade 
Reactions at Electrodes:
According to the first theory the following reaction occurs:
As Cryolite has greater electrochemical stability it does not dissociate. It only increases the dissociation of  Al2O3  But the second theory states that, Cryolite undergoes electrolytic dissociation first then Al+3  goes to the cathode, produced F2 at anode then reacts with Al2O3   produces .

1 comment:

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