Question

In the Bayer's process, the leaching of alumina is done by using:
a. $N{{a}_{2}}C{{O}_{3}}$
b. $NaOH$
c. $Si{{O}_{2}}$
d. $CaO$

Answer 1

Hint : Alumina is a common name for Aluminum oxide ($A{{l}_{2}}{{O}_{3}}$) which is composed of Aluminum and Oxygen molecules. It is extracted from the naturally occurring ore of Aluminum called bauxite.

Complete answer :
Extraction of alumina from its chief ore bauxite is a multi-level process including various steps.

Digestion
The bauxite ore is first crushed, washed or milled in order to remove any silica present in it, which will reduce the quality of the resultant product. Then it is heated in a pressure vessel with caustic soda at a temperature of 150-200 ℃.
$A{{l}_{2}}{{O}_{3}}+2NaOH\to 2NaAl{{O}_{2}}+{{H}_{2}}O$

Filtration
The residue formed after the reaction with caustic soda (NaOH) is removed from the solution with the help of the sedimentation process forming red mud. This process is leaching.

Precipitation
The formed clear mixture is added to precipitators by using heat exchangers, which cools down the mixture. The alumina can then be crystallized from the solution via a precipitation process which carries out the following reaction:
$Al{{(OH)}_{4}}^{-}+N{{a}^{+}}\to Al{{(OH)}_{3}}+N{{a}^{+}}+O{{H}^{-}}$

Calcination
Finally, coarse crystals are separated through classification, and 90 percent of the gibbsite manufactured is converted into alumina by washing, drying, and then heating aluminium hydroxide in a calciner or rotary kiln at temperature 1010-1260 ℃ to yield alumina as,
$2Al{{(OH)}_{3}}\to A{{l}_{2}}{{O}_{3}}+3{{H}_{2}}{{O}^{-}}$

So, the correct option is (b).

Note : Pure alumina, is extracted from the mineral bauxite using hot concentrated sodium hydroxide solution in the Bayer Process. Further, the Hall-Héroult process is used to obtain Aluminum from the ore.