JEE Main 2022 · 29 June · morning · Single correct · +4 / −1
In isolation of which one of the following metals from their ores, the use of cyanide salt is not commonly involved?
- (a) Zinc
- (b) Gold
- (c) Silver
- (d) Copper
Answer and solution
Answer (d) Copper
Solution
Chemistry ma’am · 2 steps
Track where cyanide appears: leaching for gold and silver, flotation for zinc, nowhere for copper.
- Cyanide as leaching agent. Gold and silver ores are treated with dilute NaCN or KCN in air. The metal dissolves as a soluble cyanide complex and is then displaced by zinc. This is the MacArthur–Forrest cyanide process.
4Au + 8CN⁻ + 2H₂O + O₂ → 4[Au(CN)₂]⁻ + 4OH⁻
Metal Where cyanide is used Role of cyanide Gold Leaching of native ore Forms soluble [Au(CN)₂]⁻ Silver Leaching of argentite, Ag₂S Forms soluble [Ag(CN)₂]⁻ Zinc Froth flotation of ZnS mixed with PbS NaCN is the depressant that keeps ZnS out of the froth Copper Flotation with pine oil, roasting, smelting, self-reduction No cyanide step - So. Copper is extracted from copper pyrites by froth flotation, roasting, smelting and self-reduction (2Cu₂O + Cu₂S → 6Cu + SO₂), with no cyanide anywhere. The answer is (d), copper.
The idea to keep. Cyanide plays two roles in metallurgy: leaching agent for gold and silver, and depressant for ZnS in froth flotation. Remember both, and the odd one out is easy to spot.
Answer from the NTA final answer key. Solution by PrepWiser.