Question

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

Answer 1

To separate alumina from silica in bauxite ore associated with silica, first the powdered ore is digested with a concentrated NaOH solution at 473 − 523 K and 35 − 36 bar pressure. This results in the leaching out of alumina (Al₂O₃) as sodium aluminate and silica (SiO₂) as sodium silicate leaving the impurities behind.

`Al_2O_(3(s)) + 2NaOH_((aq))+3H_2O_(l) -> 2Na[Al(OH)_4]_(aq)`

Alumina                                                        Sodium aluminate

 

`SiO_2 + 2NaOH_((aq)) -> Na_2SiO_(3(aq)) + H_2O_((l))`

Silica                                 Sodium silicate

Then, CO₂ gas is passed through the resulting solution to neutralize the aluminate in the solution, which results in the precipitation of hydrated alumina. To induce precipitation, the solution is seeded with freshly prepared samples of hydrated alumina

`2Na[Al(OH)_4]_((aq)) + CO_(2(g)) ->Al_2O_3xH_2O_(s) + 2NaHCO_(3(aq))`

Sodium aluminate                           Hydrated alumina  Sodium hydrogen carbonate

During this process, sodium silicate remains in the solution. The obtained hydrated alumina is filtered, dried, and heated to get back pure alumina.