Question

Describe the role played in the extraction of aluminium by each of the substances listed.
(1) Cryolite
(2) Sodium hydroxide
(3) Graphite

  • Metallurgy
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Answer

1. Addition of Cryolite :

  1. lowers the fusion point of the mixture i.e., the 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.

2. Sodium hydroxide is added to bauxite ore during the purification of bauxite. Bauxite is reacted with a conc. solution of NaOH under pressure for 2 hrs as a first step in obtaining Al2O3. The impurities present in bauxite mainly Fe2O3 and SiO2 remain unaffected with conc. NaOH as impurities are not amphoteric. Bauxite, being amphoteric reacts with the base forming sodium salt [sodium aluminate] and water. Hence, impurities are separated out.

3. Thick Graphite rods attached to copper clamps dipping into fused electrolyte are used as anode.

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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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