Question

Describe the following.
i) The role of cryolite in electro-metallurgy of aluminium.
ii) The role of carbon monoxide in the refining of crude nickel.

Answer 1

Hint: Cryolite, and carbon monoxide are used in the metallurgy of aluminium, and refining of crude nickel respectively. From the refining itself we can relate it to the purification of nickel. In the electro-metallurgy there is need to maintain the specific properties, and cryolite is used to maintain that. Identify the properties, and describe the role in detail.

Complete step by step answer:
First, we will discuss the part (i), i.e. the role of cryolite in electro-metallurgy of aluminium.
As we know, aluminium is highly reactive metal according to the electrochemical series, and it is obtained from the bauxite ore by the process of electrolysis.
While performing the electrolysis, there is conversion of ore into aluminium oxide, and then molten cryolite is used. The aluminium oxide formed has a very high melting point.
So, the molten cryolite is used, as it will help to reduce the melting point to about 1250 K, and it increases the electrical conductivity of aluminium oxide.
Now, let us discuss the part (ii), i.e. the role of carbon monoxide in the refining of crude nickel. The refining of crude nickel is done by Mond’s process.
In this process, there is a reaction between nickel, and carbon monoxide. Thus, we can say that nickel tetracarbonyl is obtained as the product.
Further nickel tetracarbonyl is decomposed at higher temperature to obtain a pure nickel.
Thus, we can represent the process as:
Ni + 4CO $\rightarrow$ Ni(CO)$_4$ $\xrightarrow{450-470K}$ Ni (pure) +4CO
So, in the end we can conclude that for part (i) cryolite is used to maintain the melting point of aluminium oxide, and for part (ii) carbon monoxide is used as a reactant in the Mond’s process to obtain pure nickel.

Note: Don’t get confused about how cryolite can increase the electrical conductivity of aluminium oxide. As mentioned it maintains the melting point of the mixture, so it increases the solubility of aluminium oxide in the solution which leads to the increase in electrical conductivity while performing the electrolysis.