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ELECTROLYSIS
OF ALUMINA
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The electrolysis of alumina is carried out in a steel tank lined inside with graphite. The graphite lining serves as cathode. Anode is also made of graphite rods hanging in the molten mass. The electrolyte consists of alumina dissolved in fused cryolite(Na3AlF6) and fluospar(CaF2). Cryolite lowers the melting point of alumina to 950C and fluospar increases the fluidity of the mass so that the liberated aluminum metal may sink at the bottom of the cell. When electric current is passed through this mixture, the aluminum is collected at the cathode in molten state and sinks at the bottom and is tapped off. |
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IONIZATION
OF ALUMINA:
2Al2O3 ® 6O-2 + 4Al+3 AT THE CATHODE: 4Al+3 + 12e- ®4Al AT THE ANODE: 6O-2 ® 3O2 + 12e- C + O2 ®CO2 |
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REFINING
OF ALUMINIUM
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| Aluminum metal produced by the electrolysis of alumina is 99% pure but contains impurities of Fe, Si and Al2O3. Aluminum is further refined by hoop’s method. | |||
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HOOPE’S
METHOD
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| This
is an electrolytic process. Electrolytic cell is made of iron, which is
lined with carbon at the bottom. Itcontains three
layers of fused mass. The lower layer consists of an alloy of impure aluminum
with copper. This layer serves as anode. The middle layer consists of a
solution of cryolite (Na3AlF6)
and barium fluoride. The upper layer consists of pure aluminum and serves
as cathode. These three layers are separated from
each other due to difference in specific gravity. During electrolysis Al+3 ions from the middle layer migrate to the upper layer where they are reduced to aluminum by gaining 3 electrons. Equal numbers of Al+3 ions are produced in the lower layer. These ions migrate to the middle layer. Pure aluminum is tapped off from time to time. Hoope’s method gives 99.99% pure aluminum. ELECTRO-CHEMICAL CHANGES: Na3AlF6 ® 3NaF + AlF3 AlF3 ® Al+3 + 3F- At the cathode: Al+3 + 3e- ®Al At the anode: Al ®Al+3 + 3e- Overall reaction: Al+3 + Al ®Al + Al+3 |
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