Question

What is the role of graphite in the electrometallurgy of aluminium?

Answer 1

The process of extraction of metals by electrolysis of their salts is called electrometallurgy. Aluminium metal is extracted from the electrolytic reduction of alumina ($A{{l}_{2}}{{O}_{3}}$). Graphite electrodes are oxidized by oxygen liberated in the electrolytic cell.

Complete step by step solution:
In the process of extraction of aluminium, purified$A{{l}_{2}}{{O}_{3}}$ is mixed with cryolite ($N{{a}_{3}}Al{{F}_{6}}$) and fluorspar ($Ca{{F}_{2}}$) to form a molten mass which is covered with a layer of powdered coke. The electrolysis of the molten matrix is carried out in an electrolytic cell made of iron maintained around 1173 K. The process of extracting Al metal by electrolysis of a mixture of purified $A{{l}_{2}}{{O}_{3}}$ and $N{{a}_{3}}Al{{F}_{6}}$ is known as Hall and Heroult process.
Graphite is used as anode which is useful in the reduction of alumina oxide to the Al metal. The reactions taking place at anode and cathode during the electrolytic reduction of alumina are given below:
At cathode: $2A{{l}_{2}}{{O}_{3}}\to 4Al+3{{O}_{2}}$ or $A{{l}^{3+}}(melt)+3{{e}^{-}}\to Al$
At anode: $\begin{aligned} & C(s)+{{O}^{2-}}(melt)\to CO(g)+2{{e}^{-}} \\\ & C(s)+2{{O}^{2-}}(melt)\to C{{O}_{2}}(g)+4{{e}^{-}} \\\ \end{aligned}$
From the above reaction it is clear that the graphite electrode reacts with the oxygen released at cathode to form carbon dioxide ($C{{O}_{2}}$) and carbon monoxide (CO).
Therefore, the role of graphite as an anode in the extraction of Al is to check the formation of oxygen (${{O}_{2}}$). In other words, we can say its role is to prevent the oxidation of Al back to $A{{l}_{2}}{{O}_{3}}$by oxygen.

Additional information: For one kg of aluminium to be liberated at the cathode, 0.5 kg of graphite anode is burned or oxidized. That is why the mass at the carbon anode slowly decreases and thus, they are needed to be replaced regularly.

Note: It is to be noted here that if any other metal is used instead of graphite as the anode, the metal will be degraded by the oxygen released at anode. Also, the oxygen will convert the metallic Al obtained at the cathode back to its oxide if it is not utilized.