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 lower melting temperature
D.In the presence of cryolite which forms a melt with higher melting temperature

Answer 1

Hint: We must have to know that alumina is aluminium oxide and is obtained from bauxite. Also the melting point of Aluminium is very high.

Complete step by step answer:
As we all know, aluminium is an abundant material but has high silica content when it occurs naturally in nature. That is why commercially, it is extracted from bauxite (also known as aluminium ore) using electrolysis.
This extraction occurs in two stages, with the first stage being the purification of bauxite to get pure aluminium oxide (commonly known as alumina).
When bauxite is added to the caustic soda solution and is heated under pressure, we get the soluble aluminate (III) and \[NaAl{\left( {OH} \right)_4}\]
 \[{\text{A}}{{\text{l}}_2}{{\text{O}}_3} + {\text{2NaOH + 3}}{{\text{H}}_2}{\text{O }} \to {\text{ 2NaAl(OH}}{{\text{)}}_4}\]
The impurities do not react with the caustic soda and hence, we can be easily removed in the form of sludge. The remaining filtrate has aluminate. Aluminium hydroxide crystals are then added to the filtrate so that there is a precipitation of aluminium hydroxide. This precipitate is filtered and dried.
 ${\text{NaAl(OH}}{{\text{)}}_4} \to {\text{ Al(OH}}{{\text{)}}_3}{\text{ + NaOH}}$
Now, to get pure aluminium oxide (alumina), we need to heat it strongly. The melting point of aluminium oxide (alumina) is very high so cryolite is added to bring it down to 950℃. This will help save costs by reducing the heating expenses immensely. Cryolite also increases the electrical conductivity reducing the time taken as well.
$2{\text{Al(OH}}{{\text{)}}_3} \to {\text{ A}}{{\text{l}}_2}{{\text{O}}_3}{\text{ + 3}}{{\text{H}}_2}{\text{O}}$

So, the solution to this question is C. At a lower melting point, we will melt with the presence of cryolite.

Therefore, Electrolytic reduction of alumina to aluminium by Hall-Heroult process is carried out in the presence of cryolite which forms a melt with lower melting temperature
Additional Information: we can also use the cryolite in glass and enamel industries and for the manufacturing of insecticide.

Note: We can also extract or refine alumina from aluminium oxide (bauxite) by Bayer process. In this process, we can produce pure alumina by hydrometallurgical extraction and hence, bauxite ore is ground and then digested in highly caustic solutions at elevated temperatures.Commercial industries using this method for the production of aluminium.