NCERT Solutions
Class 12 Chemistry
General Principles and Processes of Isolation of Elements

Q.13
How can you separate alumina from silica in a bauxite ore associated with silica? Give equations, if any.
To separate alumina from silica in bauxite ore associated with silica, firstly, the
powdered ore is digested with a concentrated NaOH solution at a temperature
of 473 – 523K and 35 – 36 bar pressure. This results in the leaching out of
alumina (Al 2O3) as sodium aluminate and silica (SiO2) as sodium silicate leaving the impurities behind.
Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2 Na [Al (OH) 4] (aq)
Alumina
SiO2 + 2NaOH (aq) → Na 2SiO3 (aq) + H2O (l)
Silica
After this, CO2 gas is passed through the resultant solution to neutralize the
alumina present in the solution, this results in the precipitation of hydrated
samples of alumina. To include precipitation, the solution is seeded with freshly
prepared samples of hydrated alumina.
2Na [Al (OH) 4] (aq) + CO2 (g) à Al2O3x2H2O (s) + 2NaHCO3 (aq)
Sodium aluminate Hydrated Alumina Sodiumhydrogencarbonate
During this process, sodium silicate remains in the solution. The hydrated
alumina obtained is then filtered, dried, and heated to get back pure alumina.
Al2O3x2H2O (s) à Al2O3(s) + xH2O (g)
Hydrated Alumina Alumina