Question

State the function of (i) Sodium hydroxide (ii) Cryolite (iii) Fluorspar in the metallurgy of aluminium in Hall Herault’s process.

  • Metallurgy
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Answer

(i) Sodium hydroxide: Used to purify bauxite by dissolving alumina and separating impurities.

(ii) Addition of Cryolite & Fluorspar

  1. Lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
  2. Enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
  3. Addition of Cryolite enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

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Metallurgy

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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