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

Explanation

Bauxine is the principle ore of aluminium. It usually contains silica `(SiO_2) `, iron oxide ` (Fe_2O_3) ` and titanium oxide `(TiO_2) ` as impurities. These impurities can be removed by the process of leaching. During leaching, the powdered bauxine ore is heated with a concentrated (45%) solution of NaOH at 473 - 523 K and 35 - 36 bar pressure when alumina dissolves as sodium meta - aluminate and silica `(SiO_2) ` as sodium silicate leaving ` Fe_2O_3, TiO_2 ` and other impurities behind.
` underset ("Alumina")(Al_2O_3(s)) + 2 NaOH (aq) + 3 H _2 O (l) overset ( 473 - 523K) underset (35 - 36" bar pressure") to underset ("Sodium meta - aluminate")(2Na[Al(OH)_4](aq) `
` underset ("Silica")(SiO_(2)(l)) + 2 NaOH (aq) underset (473 - 523K) to underset ("Sodium silicate")(Na_2SiO_3(aq)) + H_ 2 O (l) `
The impurities are filtered off and the solution of sodium meta - aluminate is neutralized by passing ` CO_ 2 ` when hydrated alumina separates out while sodium silicate remains in the solution.
` " "2Na [Al(OH)_4](aq) + 2 CO _2 (g) to Al_2 O _3. x H_2O(s) downarrow + 2 NaHCO_3(aq) `
The hydrated alumina thus obtained is filtered, dried and heated to give back pure alumina.
` " " Al _2 O _ 3 . x H _2 O (s) overset ( 1473 K ) to Al_2 O _3(s) + xH_2O (g)`
Thus, the significance of leaching in the extraction of aluminium is to prepare pure alumina fro the bauxite ore.