Question

What is the role of graphite rod in the electrometallurgy of aluminimum ?

Answer 1

Graphite rods act as anode during the electrolytic reduction of alumina (Hall-Heroult processs). At anode, $O_2$ gas is produced which react with the carbon of anode (rods) to produce $C{O}_2$ gas, so these graphite rods are consumed slowly and need to be replaced from time to time.
$A{l}_2{O}_3\rightleftharpoons 2A{l}^{3+}+3O^{2-}$
At Cathode~$A{l}^{3+}+3e^{-}⟶Al$
At Anode~$3O^{2-}⟶3\left[O\right]+6e^{-}$
$3\left[O\right]+3\left[O\right]⟶3O_{2}C\left(s\right)+O_2\left(g\right)⟶C{O}_2\left(g\right)$