Question

In the extraction of aluminium from alumina, liquid cryolite is used. Give reason.

Answer 1

Aluminium is extracted from aluminium oxide or alumina which has a very high melting point. For economical purposes, we have to reduce the melting point of alumina without affecting the fluidity of molten material.

Complete step by step answer:
-First of all we will talk about extraction of aluminium.
Aluminium is a soft silvery white material having a property of corrosion resistance. We already know that aluminium is the most abundant mineral found in the earth’s crust. It makes 8% of the earth’s crust and is the third most abundant element after oxygen and silicon. We mainly extract aluminium from the bauxite ore. Bauxite is an important ore of aluminium with a molecular formula of $A{l_2}{O_3}.x{H_2}O$.
One of the major ways for aluminium extraction are from aluminium cryolite ($N{a_3}Al{F_6}$) and aluminite through the Hall Heroult’s process.

-Let us now discuss the Hall-Heroult process.
This process is widely used in the extraction of aluminium. We should know that, in Hall-Heroult’s process, pure $A{l_2}{O_3}$ is mixed with cryolite ($Ca{F_2}$ or $N{a_3}Al{F_6}$). A steel vessel with the lining of carbon and graphite rods is used. The carbon lining present acts as cathode and graphite acts as anode. After passing electricity aluminium ions are created at the cathode from the aluminium oxide, which then sink down because they are heavier than the cryolite solution. On the other side, at the positive anode, the oxygen from the aluminium oxide forms and responds to carbon dioxide ($C{O_2}$) with the graphite carbon. At the end there is accumulation of pure aluminium at the cathode. This is how aluminium is extracted from alumina and liquid cryolite.

-We will now talk about the use of liquid cryolite in this process.
During the Hall Heroult’s process molten cryolite or liquid cryolite is used as a solvent for aluminium oxide or alumina ($A{l_2}{O_3}$). Cryolite plays an important role in the refining of aluminium. The melting point of aluminium oxide is very high (more than 2000$^ \circ C$) and so melting it would be a very expensive process. Hence we use cryolite to decrease the melting point of aluminium oxide from 2000 - 2500$^ \circ C$ to 900 - 1000$^ \circ C$. Cryolite also increases the conductivity of the solution mixture. By it the process of extraction of aluminium becomes economical by reducing the energy costs involved. We have successfully explained extraction of aluminium and significance of using cryolite solution in the above discussion.

Note: We should know that aluminium is the most abundant (found in large quantities) metal in the Earth's crust and yet it is expensive. It is because of the large amount of electricity required in the extraction process. We don’t use carbon for extraction of aluminium because aluminium is more reactive than carbon so it must be extracted from its compounds using electrolysis.