1.2.3 Leaching Method Quiz

Leaching Method

• Leaching is based on the solubility of the ore in a suitable solvent and the reactions in aqueous solution.
• Crushed ore is allowed to dissolve in a suitable solvent, the metal present in the ore is converted to its soluble salt or complex while the gangue remains insoluble.

Cyanide Leaching

• Concentration of gold ore involves leaching crushed ore with aerated dilute solution of sodium cyanide.
• Gold is converted into a soluble cyanide complex: 4Au (s) + 8CN- (aq) + O2 (g) + 2H2 O (l) → 4[Au(CN)2]- (aq) + 4OH- (aq).
• Gold can be recovered by reacting the deoxygenated leached solution with zinc through cementation: Zn (s) + 2[Au(CN)2]- (aq) → [Zn(CN)4]-(aq) + 2Au (s).

Ammonia Leaching

• Ammonia selectively leaches nickel, copper, and cobalt from crushed ore by forming their soluble complexes: [Ni(NH3)6]2+, [Cu(NH3)4]2+, and [Co(NH3)5H2O]3+ respectively.
• Iron(III) oxides/hydroxides and aluminosilicate remain as gangue.

Alkali Leaching

• Bauxite, an important ore of aluminum, is heated with a solution of sodium hydroxide or sodium carbonate to form soluble sodium meta-aluminate: Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2Na[Al(OH)4] (aq).
• The hot solution is decanted, cooled, and diluted, and then neutralized by passing CO2 gas to form hydrated Al2O3 precipitate.
• The precipitate is filtered off and heated around 1670 K to get pure alumina Al2O3.

Acid Leaching

• Leaching of sulphide ores such as ZnS, PbS, etc., can be done by treating them with hot aqueous sulphuric acid.
• The process involves conversion of insoluble sulphide into soluble sulphate and elemental sulphur: 2ZnS (s) + 2H2SO4 (aq) + O2 (g) → 2ZnSO4 (aq) + 2S (s) + 2H2O.