Leaching is a process in which powdered ore is treated with certain reagents, which dissolves the ore leaving behind impurities in undissolved form. Aluminium ore is concentrated by this method, bauxite $\left(\mathrm{Al}_2 \mathrm{O}_3 \cdot 2 \mathrm{H}_2 \mathrm{O}\right)$ contain ferric oxide, titanium oxide and silica as impurities.
Bauxite is treated with aqueous solution of caustic soda. Alumina get dissolved forming soluble sodium aluminate leaving behind impurities.
$
\mathrm{Al}_2 \mathrm{O}_3+6 \mathrm{NaOH} \longrightarrow 2 \mathrm{Na}_3 \mathrm{AlO}_3+3 \mathrm{H}_2 \mathrm{O}
$