The following steps are used to separate alumina from silica in bauxite ore that contains silica:
To begin, the powdered ore is digested in a concentrated NaOH solution at 473 523 K and 35 36 bar pressure. The contaminants are left behind when alumina ( Al2O3 ) and silica ( SiO2 ) leach as sodium aluminate and sodium silicate, respectively.

Al₂O₃ (s) + 2NaOH(aq) + 3H₂O (f) ⇒ 2Na [ Al ( OH)₄ ](aq)
Alumina sodium aluminate

SiO₂ + 2NaOH(aq) ⇒ Na₂SiO(aq) + H₂O(l)
Silica Sodium silicate

Carbon dioxide gas is now circulated through the solution we obtained to neutralise the aluminate present in it. The sodium meta – aluminate precipitates as hydrated alumina as a result of this.

2Na [ Al ( OH)₄ ](aq) + CO₂ ⇒ Al₂O₃.xH₂O(s) + 2NaHCO₃(aq)
hydrated alumina

Because sodium silicate does not precipitate, it is filtered out. To obtain pure alumina, the hydrated alumina is dried, heated, and filtered.

Al₂O₃.xH₂O(s) ⇒ Al₂O₃(s) + xH₂O( g) [1470 k]