Q.What is the role of graphite rod in the electrometallurgy of aluminium?

Molten alumina (dissolved in cryolite) is electrolysed using a carbon-lined steel vessel as cathode and thick graphite rods dipped into the melt as anode. $Al^{3+}$ is reduced to molten Al at the cathode; at the anode, $O^{2-}$ is oxidised to $O_2$, which reacts with the hot graphite carbon to form CO and CO$_2$, consuming the anode over time. …

Step 1 – Setup of the Hall–Héroult electrolytic cell

In the electrolytic extraction of aluminium, purified alumina is dissolved in molten cryolite (with some fluorspar) inside a steel vessel lined internally with carbon, which serves as the cathode. A number of thick graphite (carbon) rods are dipped into the molten electrolyte from above — these serve as the anode.

Step 2 – Electrode reactions

On passing a strong direct current:

• At the cathode (carbon lining): $Al^{3+}$ ions are reduced to molten aluminium metal, which, being denser, collects at the bottom of the cell and is periodically tapped off.

• At the anode (graphite rods): oxide ions are oxidised, releasing oxygen gas.

Step 3 – Why the graphite rods get consumed

The oxygen gas generated at the anode, at the high operating temperature of the cell (~1140–1200 K), reacts with the carbon of the graphite anode itself:

This means the graphite anode is continuously burnt away/oxidised during operation, and so it has to be periodically lowered and eventually replaced as it wears down — this is a routine operating requirement of the process, unlike the carbon cathode lining, which is not consumed in the same way.

Step 4 – Summary of role …

Identify graphite's role as the anode, write the anode half-reaction (oxidation of $O^{2-}$ to $O_2$), and note the key follow-on fact: the liberated $O_2$ reacts with the graphite its …

• Saying graphite acts as the cathode — the carbon LINING of the steel vessel is the cathode; the dipped graphite RODS are specifically the anode. …