What is the significance of leaching in the extraction of aluminium?

Explanation

The principal ore of aluminium is bauxite. It usually contains silica `(SiO_2)`. Iron oxide `(Fe_2O_3)` and titanium oxide `(TiO_20` 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 `473-523K`, when alumina dissolves as sodium metaaluminate and silica `(SiO_2)` as sodium silicate leaving `Fe_2O_3,TiO_2` and other impurities behind.
`Al_2O_3+2NaOH+3H_2Ooverset(473-523K)to2Na[Al(OH)_4]`
`SiO_2+2NaOHoverset(473-523K)toNaSiO_3+H_2O_(1)`
The impurities are filtered off and the solution of sodium metaaluminate is neutralised by passing `CO_2` when hydrated alumine separates out while sodium silicate remains in the solution.
`2Na[Al(OH)_4]_(aq)+2CO_2toAl_2O_3.xH_2Odarr2NaHCO_3`
the hydrated alumina thus obtained is filtered, dried and heated to give back pure alumina.
`Al_2O_3+xH_2O`
Thus the significance of leaching in the extraction of aluminium is to prepare pure alumina from the bauxite ore