Question

In the extraction of aluminium from bauxite, the first step is the dressing of the ore by Baeyer’s process. Give balanced equations for the conversion of impure bauxite to pure alumina using a concentrated solution of NaOH.

  • Metallurgy
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Answer

Conversion of impure bauxite to sodium aluminate:

Al2O3.2H2O + 2NaOH {\overset{ 150^o C - 200^oC}{\longrightarrow}} 2NaAlO2 + 3H2O

Conversion of sodium aluminate to aluminium hydroxide:

NaAlO2 + 2H2O {\overset{ 50^o C - 60^oC}{\longrightarrow}} NaOH + Al(OH)3

Conversion of Al(OH)3 – to pure alumina – heat on Al(OH)3:

2Al(OH)3 {\overset{ 1100^oC}{\longrightarrow}} Al2O3 + 3H2O

Precipitated Al(OH)3 is ignited to give pure alumina [Al2O3]

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Give reasons for the following pertaining to Hall Herault’s process.
(i) The fusion temperature of the electrolyte has to be lowered before conducting the electrolytic reduction.
(ii) The constituents of the electrolyte in addition to one part of fused alumina contains three parts of cryolite and one part of fluorspar.
(iii) A layer of powdered coke sprinkled over the electrolytic mixture, protects the carbon electrodes.
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