Question

In the extraction of aluminium :
(i) Which soln. is used to react with bauxite as a first step in obtaining pure aluminium oxide.
(ii) The aluminium oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide. Write the equation for this reaction.
(iii) Name the element which serves as the anode and cathode in the extraction of aluminium.
(iv) Write equation for the reaction at the cathode during the extraction of aluminium by electrolysis.
(v) Give the equation for the reaction at the anode when aluminium is purified by electrolysis.

  • Metallurgy
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Answer

(i) Sodium hydroxide is used to react with bauxite as a first step in obtaining pure aluminium oxide.

(ii) Conversion of Al(OH)3 – to pure alumina by heating Al(OH)3

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

(iii) Element of Anode and Cathode are:

  • Cathode — Carbon lining [gas carbon] of the cell.
  • Anode — Thick carbon rods (graphite) attached to copper clamps dipping into fused electrolyte.

(iv) 2Al3+ + 6e– ⟶ 2Al

(v) Al – 3e– ⟶ Al3+ (Hoope’s process – Electrorefining of Aluminium)

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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.
(iv) It is preferred to use a number of graphite electrodes as anode, instead of a single graphite electrode.

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