Question

What happens in the Bayer’s process?
A.$A{l_2}{O_3}$ goes into solution as soluble $Al(OH)_4^ -$ while other basic oxides as $Ti{O_2}$ and $F{e_2}{O_3}$ remain soluble.
B.$A{l_2}{O_3}$ changes to $AlN$ which in turn decomposes to ${H_2}O$.
C.$A{l_2}{O_3}$ changes to $A{l_2}{(C{O_3})_3}$ which changes to $AlC{l_3}$.
D.None of the above is correct.

Answer 1

Alumina is changed to sodium meta aluminate which is soluble. The impurities which are insoluble are filtered by the filtration process. Then, the solution which contains sodium meta aluminate is mixed with the aluminium hydroxide which precipitate sodium meta aluminate to aluminium hydroxide.

Complete step by step answer:
The Bayer process is used for the refining of bauxite which produces alumina or aluminium oxide. Bauxite is the ore of aluminium which contains silica, iron oxides, titanium dioxide and 30 – 60% of aluminium oxide.
In the Bayer process, the bauxite with sodium hydroxide is heated in a pressure vessel at temperature of 150 – 200 degree Celsius. Then, in the process of extraction, the aluminium is dissolved as sodium aluminate. This extraction process will change aluminium oxide to sodium aluminate. This will also dissolve silica which will form sodium silicate. The remaining components of bauxite will not dissolve. Then, the fine particles are removed and solid impurities are filtered off. The waste which does not dissolve is extracted after the aluminium compounds.
Some aluminium hydroxide is then used for the manufacture of aluminium sulfate, sodium aluminate etc. It can also be used as filler in rubber and in plastics as fire retardant.
Therefore, from the above we can conclude that the correct option for this question is option (D).

Note: The majority of aluminium in the ore gets dissolved in the Bayer process therefore, 1.9 to 3.6 tons of bauxite produces 1 ton aluminium oxide. When the aluminium oxide produced is over 90% then, the Hall – Heroult process is used to produce aluminium.