Question -
Answer -
In the extraction of aluminium,the significance of leaching is to concentrate pure alumina (Al2O3) from bauxite ore.
Bauxiteusually contains silica, iron oxide, and titanium oxide as impurities. In theprocess of leaching, alumina is concentrated by digesting the powdered ore witha concentrated solution of NaOH at 473-523 K and 35-36 bar. Under theseconditions, alumina (Al2O3) dissolves as sodiummeta-aluminate and silica (SiO2)dissolves as sodium silicate leaving the impurities behind.
The impurities are then filteredand the solution is neutralized by passing CO2 gas. In this process, hydrated Al2O3 gets precipitated and sodium silicate remains inthe solution. Precipitation is induced by seeding the solution with freshlyprepared samples of hydrated Al2O3.
Hydrated alumina thus obtained isfiltered, dried, and heated to give back pure alumina (Al2O3).
