Describe the three steps involved in the leaching of bauxite to get pure alumina.

Explanation

Step.1: The ore of aluminium is bauxite. Bauxite is contaminated with several impurities such as Silica `(SiO_(2))`, iron oxide and titanium oxide`(TiO_(2))`. Concentration is carried out by digesting the powder ore with corfcentrated NaOH solution at 473523K and pressure of 35-36 bars. `Al_(2)O_(3)` is leached out as sodium aluminate and `SiO_(2)` as sodium silicate leaving behind the impurities.
`Al_(2)O_(3)2H_(2)O_((s))+2NaOH_((aq))rarr2Na[Al(OH)_(4)]_((aq))`
Step.2: The sodium aluminate is neutralised by passing carbon dioxide gas and hydrated `Al_(2)O_(3)` is ppt. The solution is treated with fresh hydrated alumina which induces precipitation
`2Na[Al(OH)_(4)]_((aq))+CO_(2(g))rarr Al_(2)O_(3).xH_(2)O_((s))+2NaHCO_(3(aq))`
Step.3: The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give pure alumina.
`Al_(2)O_(3).xH_(2)O_((s))overset(1470K)rarr Al_(2)O_(3(s))+x.H_(2)O_((s))`