In the extraction of aluminium, the significance of leaching is to concentrate pure alumina (Al₂O₃) from bauxite ore. Bauxite usually contains silica, iron oxide, and titanium oxide as impurities. In the process of leaching, alumina is concentrated by digesting the powdered ore with a concentrated solution of NaOH at 473-523 K and 35-36 bar. Under these conditions, alumina (Al₂O₃) dissolves as sodium meta-aluminate and silica (SiO₂) dissolves as sodium silicate leaving the impurities behind.

Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) \(\overset{475-523K}{\underset{35-36\ bar}\longrightarrow}\) 2Na[Al(OH)4](aq)

Alumina

SiO₂(l) + 2NaOH(aq) \(\overset{475-523K}{\underset{35-36\ bar}\longrightarrow}\) NaSiO₃(aq) + H₂O(l)

Silica                            Sodium silicate

The impurities are then filtered and the solution is neutralized by passing CO₂ gas. In this process, hydrated Al₂O₃ gets precipitated and sodium silicate remains in the solution. Precipitation is induced by seeding the solution with freshly prepared samples of hydrated Al₂O₃.

2Na[Al(OH)₄](aq) + CO₂(g) → Al₂O₃.xH₂O(s) + 2NaHCO₃(aq)

Hydrated alumina thus obtained is filtered, dried, and heated to give back pure alumina (Al₂O₃).

Al₂O₃.xH₂O(s) \(\overset{1470K}\longrightarrow\) Al₂O₃ + xH₂O(g)