Question

The following questions are relevant to the extraction of Aluminium :
(i) State the reason for addition of caustic alkali to bauxite ore during purification of bauxite.
(ii) Give a balanced chemical equation for the above reaction.
(iii) Along with cryolite and alumina, another substance is added to the electrolyte mixture. Name the substance and give one reason for the addition.

  • Metallurgy
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Answer

(i) Bauxite is reacted with a conc. solution of NaOH (caustic alkali) 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 salt [sodium aluminate] and water. Hence, impurities are separated out. Therefore, caustic alkali is added to bauxite ore during purification of bauxite.

(ii) Balanced chemical equation for the conversion of impure bauxite to sodium aluminate is given below:

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

(iii) Fluorspar is added to the electrolyte mixture as it lowers the fusion point of the mixture i.e., the mixture fuses around 950°C instead of 2050°C.

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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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