Question

The process used for purification of $ A{l_2}{O_3}.2{H_2}O $ is:
(A) Magnetic separation
(B) froth flotation
(C) Leaching
(D) liquation

Answer 1

Hint: $ A{l_2}{O_3}.2{H_2}O $ (Bauxite) is the most important ore of aluminium, contains $ 30 - 60\% $ aluminium oxide, and titanium oxide. Bayer $ ' $ s process also known as leaching is the principle industrial means of refining Bauxite to produce alumina. It was developed by Carl Joseph Bayer.

Complete step by step solution:
Alumina is the main component of bauxite ( $ A{l_2}{O_3}.2{H_2}O $ ). Bayer $ ' $ s process also known as leaching is the principle industrial means of refining Bauxite to produce alumina.
These are the steps involved:
Step $ 1 $ : Digestion
In this step, bauxite ore is crushed and then heated with sodium hydroxide at the temperature around $ 150 - 200 $ degree Celsius.
Step $ 2 $ : filtration
In this step, the mixture is clarified to remove impurities. All the components other than alumina and silica present in bauxite do not dissolve. The undissolved solids settle down forming red mud. This red mud is then discarded from the solution by using a rotary sand trap.
The filtration process converts the aluminium oxide to soluble sodium aluminates, $ 2NaAl{O_2} $ , as per the equation:
 $ A{l_2}{O_3} + 2NaOH \to 2NaAl{O_2} + {H_2}O $
Step $ 3 $ : precipitation
By using heat exchangers, the clear remaining mixture is added to precipitators which turns the mixture to cool liquor.
Step $ 4 $ : calcination
 $ 90\% $ of gibbsite manufactured is converted to alumina by washing, drying, and then heating the aluminium hydroxide in fluid flash containers at a temperature of $ 1010 - 1260 $ degree Celsius.
 $ 2Al{(OH)_3} \to A{l_2}{O_3} + 3{H_2}O $
More than $ 90 $ percent of alumina manufactured is used to produce aluminium by the Hall-Heroult process.
Hence, the process used for purification of $ A{l_2}{O_3}.2{H_2}O $ is leaching.
So, the correct option is (C): leaching.

Note:
Leaching involves the treatment of the ore with a suitable reagent as to make it soluble while impurities remain insoluble. The ore is removed from solution by suitable chemical methods.