What is the role of graphite rod in the electrometallurgy of aluminium?
In the electrometallurgy of aluminium, a fused mixture of purified alumina (Al2O3), cryolite (Na3AlF6) and fluorspar (CaF2) is electrolysed. In this electrolysis, graphite is used as the anode and graphite-lined iron is used as the cathode. During the electrolysis, Al is liberated at the cathode, while CO and CO2 are liberated at the anode, according to the following equation.
Cathode : `Al_(" "("melt"))^(3+) + 3e^(-) -> Al_(l)`
Anode : `C_((s)) + O_(" "("melt"))^(2-) -> CO_((g)) + 2e^(-)`
`C_((s)) + 2O_(" "("melt"))^(2-) -> CO_(2(g)) + 4e^(-)`
If a metal is used instead of graphite as the anode, then O2 will be liberated. This will not only oxidise the metal of the electrode, but also convert some of the Al liberated at the cathode back into Al2O3. Hence, graphite is used for preventing the formation of O2 at the anode. Moreover, graphite is cheaper than other metals.
In the electrometallurgy of aluminium, oxygen gas is evolved at anode. O2 reacts with graphite or carbon (graphite electrodes) to form carbon monoxide and carbon dioxide. In case if some other metal electrodes is used as anode, then oxygen will react with aluminium formed during the process to form aluminium oxide(Al2O3) which will pass into the reaction mixture resulting into wastage of Al. Since graphite is cheaper than aluminium, its wastage or can be tolerated.