Solution 2: Aluminium contains silica $\left(\mathrm{SiO}_{2}\right)$, iron oxide $\left(\mathrm{Fe}_{2} \mathrm{O}_{3}\right)$ and titanium oxide $\left(\mathrm{TiO}_{4}\right)$ as impurities. These impurities can be removed by the process of leaching.
During leaching, the powdered bauxite ore is heated with a concentrated ( $45 \%$ ) solution of $\mathrm{NaOH}$ at $473-523 \mathrm{~K}$, where alumina dissolves as sodium meta aluminate and silica as sodium silicate leaving $\mathrm{Fe}_{2} \mathrm{O}_{3}, \mathrm{TiO}_{2}$ and other impurities behind:
$\mathrm{Al}_{2} \mathrm{O}_{3}(s)+2 \mathrm{NaOH}(a q)+3 \mathrm{H}_{2} \mathrm{O}(l) \stackrel{473–23 k}{\rightarrow} 2 \mathrm{Na}\left[\mathrm{Al}(\mathrm{OH})_{4}\right](a q)$ Sodium meta – aluminate
$$
\mathrm{SiO}_{2}(l)+2 \mathrm{NaOH}(a q) \stackrel{473-523 \mathrm{~K}}{\longrightarrow} \mathrm{Na}_{2} \mathrm{SiO}_{3}(a q)+\mathrm{H}_{2} \mathrm{O}(l)
$$
Sodium silicate
The impurities are filtered off and solution of sodium meta-aluminate is neutralised by passing $\mathrm{CO}_{2}$ when hydrated alumina separates out while sodium silicate remains in solution. The hydrated alumina thus obtained is filtered, dried and heated to give back pure alumina. $\mathrm{Al}_{2} \mathrm{O}_{3} \times \mathrm{H}_{2} \mathrm{O}(s) \stackrel{1473 \mathrm{~K}}{\longrightarrow} \mathrm{Al}_{2} \mathrm{O}_{3}(s)+x \mathrm{H}_{2} \mathrm{O}(g)$