Bauxite usually contains silica (SiO2), iron oxides and titanium oxide (TiO2) as impurities. The powdered ore is digested with a concentrated solution of NaOH at 473 – 523 K and 35 – 36 bar pressure. Al2O3 is leached out as sodium aluminate leaving the impurities behind. (SiO2 is also leached as sodium silicate).
Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2Na[Al(OH)4](aq)
The aluminate in solution is neutralised by passing CO2 gas and hydrated Al2O3 is precipitated. At this stage, the solution is seeded with freshly prepared samples of hydrated Al2O3 to induce the precipitation.
2Na[Al(OH)4](aq) + CO2(g) → Al2O3 xH2O + 2NaHCO3(aq)
The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure Al2O3.
AI2O3⋅xH2O \(\overset{1470K}{\longrightarrow}\) Al2O3( s)+xH2O(g)
