Question

Describe the term Leaching.

Answer 1

Used when the ore is soluble in some suitable solvent 

(i) eg. Leaching of Alumina From Bauxite 

• Bauxite usually contain SiO₂, Iron oxides, TiO₂ etc as impurities 

• Powdered ore is digested with conc NaOH at 473 - 523 K and 35-36 bar pressure 

• Al₂O₃ and SiO₂ are leached out as Sodium aluminate and Sodium silicate. 

• The impurities are left behind 

• The aluminate is neutralised by passing CO₂ so that hydrated aluminium oxide is precipitated(seeded with fresh Hydrated Al₂O₃ 

• The sodium silicate remains in the solution 

• Hydrated alumina is Filtered, dried and heated to give pure Al₂O₃ 

(ii) eg. - Leaching of Silver and Gold 

• Leached with a dilute solution of NaCN or KCN in presence of air 

• The metal is obtained later by Displacement