Question

How are impurities removed from bauxite?

Answer 1

Impurities from Bauxite can be removed by following steps:

1. The Bauxite ore is first concentrated with the solution of caustic soda $\left(\mathrm{NaOH}\right)$ at $140°\mathrm{C}-150°\mathrm{C}$.
2. Aluminium oxide present in ore gives sodium meta aluminate while the impurities (red mud) are left behind.

$$\begin{gathered} {\mathrm{Al}}_2{\mathrm{O}}_3.2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)+2\mathrm{NaOH}\left(\mathrm{aq}\right)\stackrel{∆}{\to }2{\mathrm{NaAlO}}_2\left(\mathrm{aq}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Bauxite}\mathrm{sodium}\mathrm{hydroxide}\mathrm{sodium}\mathrm{meta}\mathrm{water} \\ \mathrm{aluminate} \end{gathered}$$

3. Ferric oxide and silica impurities are removed by filtration. The sodium meta aluminate is diluted with water so that it precipitates as aluminium hydroxide.

$$\begin{gathered} {\mathrm{NaAlO}}_2\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{Al}{\left(\mathrm{OH}\right)}_3\left(\mathrm{s}\right)+\mathrm{NaOH}\left(\mathrm{aq}\right) \\ \mathrm{aluminium}\mathrm{sodium} \\ \mathrm{hydroxide}\mathrm{hydroxide} \end{gathered}$$

4. The precipitate of Aluminium hydroxide is filtered, washed and ignited at $1000°\mathrm{C}$ to form pure alumina.

$$\begin{gathered} 2\mathrm{Al}{\left(\mathrm{OH}\right)}_3\left(\mathrm{s}\right)\stackrel{∆}{\to }{\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{pure}\mathrm{alumina} \end{gathered}$$