In this method, the ore is treated with aqueous alkali to form a soluble complex. For example, bauxite, an important ore of aluminium is heated with a solution of sodium hydroxide or sodium carbonate in the temperature range 470 – 520 K at 35 atm to form soluble sodium meta- aluminate leaving behind the impurities, iron oxide and titanium oxide
Al2O3 (S) + 2NaOH (aq) + 3H2O (l) → 2Na[Al(OH)4] (aq)
The hot solution is decanted, cooled, and diluted. This solution is neutralised by passing CO gas, to the form hydrated Al2O3 precipitate.
2Na[Al(OH)4](aq) + CO2 (g) → Al2O3. xH2O(s) + 2NaHCO3 (aq)
The precipitate is filtered off and heated around 1670 K to get pure alumina Al2O3.
