Question

The following question is related to the extraction of aluminum metal by the electrolysis process:

(b) During electrolysis, is it necessary to renew the anode from time to time?

Answer 1

Extraction of Aluminum

• For the extraction of aluminum, purified Bauxite$\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ undergoes electrolysis in a solution of molten Cryolite $\left({\mathrm{Na}}_3{\mathrm{AlF}}_6\right)$.
• Carbon lining acts as cathode in this case.
• Carbon rods or slabs act as anode in this case.

Chemical Reactions

• The electrolytic reaction is given below.

${\mathrm{Al}}_2{\mathrm{O}}_3\rightleftharpoons 2{\mathrm{Al}}^{3+}+3{\mathrm{O}}^{2-}$

• The reaction at the cathode is given below.

${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}\to \mathrm{Al}$

• The reaction at the anode is given below.

$$\begin{gathered} {\mathrm{O}}^{2-}-2{\mathrm{e}}^{-}\to \mathrm{O} \\ \mathrm{O}+\mathrm{O}\to {\mathrm{O}}_2\uparrow \end{gathered}$$

Carbon rods are periodically replaced in the electrolytic extraction of Aluminum because Oxygen $\left({\mathrm{O}}_2\right)$ released at the anode interacts with the carbon anode to produce Carbon monoxide $\left(\mathrm{CO}\right)$ and Carbon dioxide $\left({\mathrm{CO}}_2\right)$. As a result, the carbon anode is depleted during the process.