What is meant by leaching?

1. Leaching is a method used in the concentration of ore.
2. It involves dissolving the ore in a suitable solvent in which the impurities remain insoluble.
3. For example - Bauxite which is an ore of Aluminium is treated with concentrated NaOH at a temperature ranging from $473-523K$ forming a compound of Aluminium while other impurities $({\mathrm{Fe}}_2{\mathrm{O}}_3,{\mathrm{TiO}}_2)$ remain insoluble. $$\begin{gathered} {\mathrm{Al}}_2{\mathrm{O}}_3.2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)+2\mathrm{NaOH}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{473-523\mathrm{K}}{\to }2\mathrm{Na}\left[\mathrm{Al}{\left(\mathrm{OH}\right)}_4\right]\left(\mathrm{aq}\right) \\ \mathrm{Bauxite}\mathrm{Sodium}\mathrm{hydroxide}\mathrm{Sodium}\mathrm{aluminate} \end{gathered}$$
4. Silica $({\mathrm{SiO}}_2)$ which is one of the impurities forms Sodium Silicate and dissolves in the Sodium hydroxide solution.$$\begin{gathered} {\mathrm{SiO}}_2\left(\mathrm{s}\right)+2\mathrm{NaOH}\left(\mathrm{aq}\right)\stackrel{473-523\mathrm{K}}{\to }{\mathrm{Na}}_2{\mathrm{SiO}}_3\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Silica}\mathrm{Sodium}\mathrm{hydroxide}\mathrm{Sodium}\mathrm{silicate} \end{gathered}$$
5. Sodium aluminate solution is then treated with ${\mathrm{CO}}_2$ forming hydrated Alumina. $$\begin{gathered} 2\mathrm{Na}\left[\mathrm{Al}{\left(\mathrm{OH}\right)}_4\right]\left(\mathrm{aq}\right)+2{\mathrm{CO}}_2\left(\mathrm{g}\right)\to {\mathrm{Al}}_2{\mathrm{O}}_3.{\mathrm{xH}}_2\mathrm{O}\downarrow \left(\mathrm{s}\right)+2{\mathrm{NaHCO}}_3\left(\mathrm{aq}\right) \\ \mathrm{Sodium}\mathrm{aluminate}\mathrm{Hydrated}\mathrm{alumina}\mathrm{Sodium}\mathrm{bicarbonate} \end{gathered}$$
6. Hydrated Alumina is washed, purified and undergoes heating to form pure Alumina.
7. This process is used to extract pure Alumina from Bauxite ore. Hence, Leaching is a concentration process.

$$\begin{gathered} {\mathrm{Al}}_2{\mathrm{O}}_3.{\mathrm{xH}}_2\mathrm{O}\left(\mathrm{s}\right)\stackrel{∆}{\to }{\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{xH}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Hydrated}\mathrm{Alumina}\mathrm{Alumina} \end{gathered}$$