Question

Which among the following statement(s) is(are) true for the extraction of Aluminium from Bauxite?

• A. Hydrated ${\mathrm{Al}}_2{\mathrm{O}}_3$ precipitates, when ${\mathrm{CO}}_2$ is bubbled through a solution of Sodium aluminate.
• B. The addition of ${\mathrm{Na}}_3{\mathrm{AlF}}_6$ lowers the melting point of Alumina.
• C. ${\mathrm{CO}}_2$ evolved at the anode during electrolysis.
• D. The cathode is a steel vessel with a lining of Carbon.

Answer 1

Answer: (A), (B), (C), (D)

The cathode is a steel vessel with a lining of Carbon.

The explanation for the correct option:

a. Hydrated ${\mathrm{Al}}_2{\mathrm{O}}_3$ precipitates, when ${\mathrm{CO}}_2$ is bubbled through a solution of Sodium aluminate.

1. Pure alumina is obtained from bauxite ore by Baeyer's process.
2. When Bauxite ore $({\mathrm{Al}}_2{\mathrm{O}}_3.2{\mathrm{H}}_2\mathrm{O})$ is heated with a concentrated solution of $\mathrm{NaOH}$, alumina dissolves forming Sodium aluminate$\left(\mathrm{Na}\right[\mathrm{Al}{\left(\mathrm{OH}\right)}_4\left]\right)$.
3. When ${\mathrm{CO}}_2$ is bubbled through a solution of Sodium aluminate, hydrated alumina gets precipitated.

$$\begin{gathered} 2\mathrm{Na}\left[\mathrm{Al}{\left(\mathrm{OH}\right)}_4\right]\left(\mathrm{aq}\right)+2{\mathrm{CO}}_2\left(\mathrm{g}\right)\to {\mathrm{Al}}_2{\mathrm{O}}_3.{\mathrm{xH}}_2\mathrm{O}\left(\mathrm{s}\right)\downarrow +2{\mathrm{NaHCO}}_3\left(\mathrm{aq}\right) \\ \mathrm{hydrated}\mathrm{alumina} \\ \left(\mathrm{precipitate}\right) \end{gathered}$$

b. The addition of ${\mathrm{Na}}_3{\mathrm{AlF}}_6$ lowers the melting point of Alumina.

1. Aluminium is extracted from fused alumina by the electrolysis process.
2. Fused alumina is a bad conductor of electricity.
3. So, cryolite $\left({\mathrm{Na}}_3{\mathrm{AlF}}_6\right)$ and fluorspar $\left({\mathrm{CaF}}_2\right)$ are added, reducing its melting point and making it a good conductor of electricity.

c. ${\mathrm{CO}}_2$ evolved at the anode during electrolysis.

1. During electrolysis, at the anode, the following reaction takes place. $$\begin{gathered} \mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}^{2-}\left(\mathrm{melt}\right)\to \mathrm{CO}\left(\mathrm{g}\right)+2{\mathrm{e}}^{-} \\ \mathrm{C}\left(\mathrm{s}\right)+2{\mathrm{O}}^{2-}\left(\mathrm{melt}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+4{\mathrm{e}}^{-} \end{gathered}$$
2. So, ${\mathrm{CO}}_2$ is evolved at the anode.

d, The cathode is a steel vessel with a lining of Carbon.

1. The cathode is a steel vessel with a lining of Carbon for the electrolysis of purified alumina.

From the above statements about the extraction of Aluminium from Bauxite. These statements are true.

Hydrated ${\mathrm{Al}}_2{\mathrm{O}}_3$ precipitates, when ${\mathrm{CO}}_2$ is bubbled through a solution of Sodium aluminate.

The addition of ${\mathrm{Na}}_3{\mathrm{AlF}}_6$ lowers the melting point of Alumina.

${\mathrm{CO}}_2$ evolved at the anode during electrolysis.

The cathode is a steel vessel with a lining of Carbon.

So, options (a),(b),(c) and (d) are the correct options.