Question

The significance of leaching in the extraction of aluminium is:

A. It helps to remove the impurities like ${\rm{Si}}{{\rm{O}}_{\rm{2}}}$, ${\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3}$ etc. from bauxite ore
B. It converts the ore into oxide
C. It reduces melting point of the ore
D. It eliminates water from bauxite

Answer 1

We know that aluminium is extracted from bauxite ore. On purification of bauxite, a white powder, that is, aluminium oxide obtained from which aluminium is extracted. The extraction of materials from a carrier into a liquid is termed as leaching.

Complete step by step solution:
Leaching of alumina from bauxite:

There are many impurities bauxite such as iron oxide, silicon oxide and titanium oxide. The bauxite ore is concentrated by reacting powdered ore with concentrated ${\rm{NaOH}}$ at a temperature range of 473-523 K and 35-36 bar pressure. In this way leaching of aluminium in the form of sodium aluminate takes place and impurities are left behind.

${\rm{A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}\left( s \right) + 2{\rm{NaOH}}\left( {aq} \right) + 3{{\rm{H}}_{\rm{2}}}{\rm{O}}\left( l \right) \to 2{\rm{Na}}\left[ {{\rm{Al}}{{\left( {{\rm{OH}}} \right)}_4}} \right]\left( {aq} \right)$

The neutralization of aluminate solution is done by passing carbon dioxide gas which results precipitation of ${\rm{A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}$. At this stage, the solution is injected with freshly prepared sample of hydrated ${\rm{A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}$ that causes precipitation.

$2{\rm{Na}}\left[ {{\rm{Al}}{{\left( {{\rm{OH}}} \right)}_4}} \right]\left( {aq} \right) + {\rm{C}}{{\rm{O}}_{\rm{2}}}\left( g \right) \to {\rm{A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}.x{{\rm{H}}_{\rm{2}}}{\rm{O}}\left( s \right) + 2{\rm{NaHC}}{{\rm{O}}_{\rm{3}}}\left( {aq} \right)$

The remaining sodium silicate present in the solution. On filtering, drying and heating of hydrated alumina gives ${\rm{A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}$again.
${\rm{A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}.x{{\rm{H}}_{\rm{2}}}{\rm{O}}\left( s \right)\mathop \to \limits^{1470\;{\rm{K}}} {\rm{A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}\left( s \right) + x{{\rm{H}}_{\rm{2}}}{\rm{O}}\left( g \right)$

So, from the above explanation it is clear that leaching in the process of extraction of aluminium is done to remove impurities such as iron oxide, silicon dioxide and titanium oxide.
Hence, the correct option is A.

Note:
Students may be confused about extraction and leaching. Using liquid extraction of something from solid is leaching but extraction of something from either solid or liquid is extraction.