Question

Answer the following questions based on the extraction of aluminium from alumina by Hall- Heroult’s Process.
(i) What is the function of cryolite used along with alumina as the electrolyte.
(ii) Why is the powdered coke sprinkled on top of the electrolyte.
(iii) Name the electrode from which aluminium is collected.

  • Metallurgy
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Answer

(i) Addition of cryolite —

  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. enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

(ii) The layer of powdered coke is sprinkled over the electrolytic mixture as :

  1. it prevents burning of carbon electrodes in air at the emergence point from the bath.
  2. it minimizes or prevents heat loss by radiation.

(iii) Cathode

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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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