Question

Describe the leaching of aluminium ore.

Answer 1

Bauxite is the most significant resource from which aluminium is typically derived. SiO₂, iron oxides, and titanium oxide (TiO₂) are common impurities found in bauxite. When Al₂O₃ leaches out as sodium aluminate (and SiO₂ as sodium silicate), it is concentrated by digesting the powdered ore with a concentrated solution of NaOH at 473-523 Kand 35-36 bar pressure. They leave the pollutants behind.

\[\ce{Al2O3_{(s)} + 2NaOH_{(aq)} + 3H2O_{(l)} -> 2Na2[Al(OH)4]_{(aq)}}\]

After the aluminate in the solution is neutralised and hydrated Al₂O₃ precipitates, CO₂ gas is now carried through the solution. To speed up precipitation, freshly made samples of hydrated Al₂O₃ are now added to the solution as seeds.

\[\ce{2Na[Al(OH)4]_{(aq)} + CO2_{(g)} -> Al2O3 * xH2O_{(s)|} + 2NaHCO3_{(aq)}}\]

The sodium silicate remains in the solution. Hydrated alumina is filtered, dried and heated to obtain pure Al₂O₃.

\[\ce{Al2O3 * xH2O_{(s)} ->[1470K] Al2O3_{(s)} + xH2O_{(g)}}\]