Explain alkali leaching in the extraction of aluminium.

Explanation

(i) In this method, the ore is treated with aqueous alkali to form a soluble complex.
(ii) 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.
`Al_(2)O_(3(s)) + 2NaOH_((aq)) + 3H_(2)O_((l)) rarr 2Na[Al(OH)_(4)]_((aq))`
(iii) The hot solution is decanted, cooled, and diluted. THis solution is neutralised by passing `CO_(2)` gas , to be form hydrated `Al_(2)O_(3)` precipitate.
`2Na[Al(OH)_(4)]_((aq)) + CO_(2(g)) rarr Al_(2)O_(3).xH_(2)O_((s)) + 2NaHCO_(3(aq))`
(iv) The precipitate is filtered off and heated around 1670 K to get pure alumina `Al_(2)O_(3)`.