How can you separate alumina from silica in a bauxite ore associated with silica? Give equations, if any.

Explanation

Bauxite, a principal ore of aluminium contains silica, iron oxide and titanium oxide as impurities. The ore is digested with a concentrated solution of NaOH at 473-523 K and 35-36 bar pressure. `Al_2 O_3` is leached out as sodium aluminate and silica as sodium silicate while impurities are left behind.
`Al_2 O_3 (s) + 2NaOH (aq) + 3H_2 O to 2Na[Al(OH)_4]` (aq) The aluminate in the solution is neutralised by passing `CO_2` gas and hydrated `Al_2 O_3` is precipitated.
`2Na[Al(OH)_4] (aq) + CO_2 (g) to Al_2 O_3 . xH_2 O + 2NaHCO_3 (aq)`
The sodium silicate remains in the solution while hydrate alumina is filtered and dried.
The hydrated alumina thus precipitated is filtered , dried and heated to give back pure alumina.
`Al_2 O_3 . x H_2 O overset(1473 K)to underset("Alumina")(Al_2 O_3 (s)) + x H_2 O`