Question

Electrolytic reduction of alumina to aluminium by Hall-Heroult process is carried out:
(A) In the presence of $NaCl$
(B) In the presence of fluorite
(C) In the presence of cryolite which forms a melt with higher melting temperature
(D) in the presence of cryolite which forms a melt with lower melting temperature

Answer 1

Hint: The extraction of aluminium from bauxite (having formula $A{l_2}{O_3}.{H_2}O$ ) has two major steps, one is concentration of aluminium oxide in bauxite or purification of bauxite (Bayer’s process) and second is obtaining aluminium from purified bauxite and alumina. The process of extracting aluminium from alumina is called the Hall-Heroult process.

Complete step by step answer:
In Hall-Heroult process the electrolytic reduction (the separation of ions by the passage of electricity) of aluminium oxide is carried out at a very high temperature (approximately ${980^ \circ }C$ ) to extract Al from its compound $A{l_2}{O_3}$ .
The melting point of alumina is very high ( ${2000^ \circ }C$ ) . It is very difficult to attain such a high temperature. Hence a compound cryolite ( $N{a_3}Al{F_6}$ ) is added to alumina to lower down its temperature. Cryolite also increases the conductivity of molten alumina and helps in the process of electrolytic reduction.
Hence the answer to this question will be option ‘D’ (i.e. in the presence of cryolite which forms a melt with lower melting temperature).
Additional information:
Mostly metals occur in the form of their oxides, sulphides and carbonates in the earth’s crust. They are mined from earth’s crust and purified using different processes. This is called metallurgy. Aluminium occurs as its oxide called bauxite and it is extracted by the process called Hall-Heroult process. Bauxite is the most important ore of aluminium but it contains only 30–60% aluminium oxide and rest are impurities including a mixture of silica, various iron oxides, and titanium dioxide. The process of removing impurities and concentrating ores is called leaching and the process of electrolytic reduction at high temperature is called smelting.

Note:
It should be remembered that cryolite is added during smelting of aluminium due to two reasons, one is to lower down its melting point and second is to increase its conductivity.