Explain the process of leaching of alumina from bauxite.

Explanation

The principle are of aluminium , bauxite usually `SiO_(2)` iron oxides and titanium oxide `(TiO_(2))` as impurities . Concentration is carried out by digesting the powdered ore with a concentrated solution of NaOH at `473-523K` and `35-36` bar pressure. The way `Al_(2)O_(3)` is leached out as sodium aluminate (and `SiO_(2)` too as sodium silicate) leaving the other impurities behind :
`Al_(2)O_(3)(s)+2NaOH(aq)+3H_(2)O(l)rarr2Na[Al(OH)_(4)](aq)`
The aluminate is alkaline in nature and is neutralised by passing `CO_(2)` gas and hydrated `Al_(2)O_(3)` is precipitated .At this stage , the solution is seeded with freshly prepared samples of hydrated `Al_(2)O_(3)` which induces the precipitation of `Al_(2)O_(3)xH_(2)O` ,
`Al_(2)O_(3)xH_(2)O`
`2N[Al(OH)_(4)](aq)+CO_(2)(g)rarrAl_(2)O_(3)xH_(2)O(s)+2NaCH_(3)(aq)`
The sodium silicate remains in the solution and the insoluble hydrated alumina is filtered, dried and heated to give pure `Al_(2)O_(3)` :
`Al_(2)O_(3).xH_(2)O(s)overset(1470K)rarrAl_(2)O_(3)(s)+xH_(2)O(s)`