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Questions and Answers
What is the purpose of alkali leaching?
What is the purpose of alkali leaching?
To form a soluble complex and separate impurities from the ore.
How is bauxite treated in alkali leaching to extract aluminum?
How is bauxite treated in alkali leaching to extract aluminum?
Bauxite is heated with a solution of sodium hydroxide or sodium carbonate.
How is the alumina precipitate obtained from the alkali leaching solution?
How is the alumina precipitate obtained from the alkali leaching solution?
By neutralizing the solution with CO2 gas.
What is the final step to obtain pure alumina from the precipitate?
What is the final step to obtain pure alumina from the precipitate?
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How is acid leaching different from alkali leaching?
How is acid leaching different from alkali leaching?
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Study Notes
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.
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Description
Test your knowledge on the leaching method based on the solubility of ores in suitable solvents and reactions in aqueous solutions. Learn about examples like cyanide leaching for gold ore concentration.
