What is the role of leaching in the extraction of aluminium ? Write all the chemical reactions involved in it.

Explanation

Aluminia usually contains silica `(SiO_(2))` iron oxide and titanium oxide `(TiO_(2))` as impurities. These impurities can be removed by the process of leaching. During leaching, the powdered bauxite ore is heated with a concentrated (45%) solution of NaOH at 473-523 K, when alumina dissolves as sodium meta-aluminate and silica `(SiO_(2))` as sodium silicate leaving the impurites behind,
`underset("Alumina")(Al_(2)O_(3)(s))+2NaOH(aq)+3H_(2)O(l) overset(473-523K)to underset("Sodium mets- aluminate")(2Na[Al(OH)_(4)](aq))`
`underset("Silica")(SiO_(2)(l)+2NaOH(aq) overset(473-523K) to underset("Sodium silicate")(Na_(2)SiO_(3)(aq))`
The impurities are filtered off and the solution of sodium meta-aluminate is neutralized by passing Co, when hydrated alumina separates out while sodium silicate remains in the solution. The hydrated alumina thus obtained is filtered, dried and heated to give back pure alumina.
`Al_(2)O_(3)xH_(2)O(s) overset(1473K)to Al_(2)O_(3)(s)+xH_(2)O(g)`
Thus, the significance of leaching in the extraction of aluminium is to prepare pure alumina from the bauxite. ore.