How is pure alumina obtained from bauxite by leaching process.

Explanation

Bauxite is the principle ore of aluminium. It is associated with iron oxides, titanium oxides, and `SiO_(2)`. Powderd ore is digested with 45% aqueous solution of caustic soda at 473K-523K and 35-36 bar pressure. `Al_(2)O_(3)`, goes in to solution forming soluble sodium aluminate silica and dissolves as sodium silicate.
`underset("Bauxite ore")(Al_(2)O_(3).2H_(2)O(s))+2NaOH(aq)+H_(2)O to underset("Sodium metaluminate")(2Na[Al(OH)_(4)](aq))`
`SiO_(2)(s) +2NaOH (aq) to Na_(2)SiO_(3) (aq) +H_(2)O(l)`
Impurities are separated by filtration. The filtrate contains sodium aluminate, sodium silicate and unreacted NaOH. It is neutralized by passing `CO_(2)` through it. At this stage freshly prepared hydrated alumina is added as a seeding agent, to induce further precipitation of hydrated alumina from the solution.
`2Na[Al(OH)_(4)](aq) +CO_(2)(g) to Al_(2) O_(3) .x H_(2)O (g) darr +2NaHCO_(3)(aq)`.
Sodium silicate remains in the solution. Hydrated alumina is filtered, dried and heated strongly to get back pure `Al_(2)O_(3)`.
`Al_(2)O_(3).x H_(2)O (s) overset(1470K) to underset("Alumina")(Al_(2)O_(3(s)))+xH_(2)O_((g))`.