JEE Main 2021 · 24 February · morning · Single correct · +4 / −1
Which of the following ore is concentrated using group 1 cyanide salt ?
- (a) Sphalerite
- (b) Malachite
- (c) Siderite
- (d) Calamine
Answer and solution
Answer (a) Sphalerite
Solution
Chemistry ma’am · 3 steps
Cyanide during concentration means froth flotation, and froth flotation means a sulphide ore.
- Role of NaCN. In froth flotation, NaCN acts as a depressant. Sphalerite (ZnS) is often mined together with galena (PbS). NaCN forms a cyanide complex on the ZnS surface, so ZnS stays in the water while PbS rises with the froth.
ZnS + 4NaCN → Na₂[Zn(CN)₄] + Na₂S
- Only sulphide ores. Froth flotation works because sulphide particles are wetted by oil and carried up by the froth. Carbonate ores are not concentrated this way, so they never need a cyanide depressant.
Ore Formula Type Usual concentration method Sphalerite ZnS sulphide froth flotation, with NaCN as depressant Malachite CuCO₃.Cu(OH)₂ basic carbonate gravity separation Siderite FeCO₃ carbonate magnetic separation Calamine ZnCO₃ carbonate gravity separation - So. Sphalerite is the only sulphide ore here, and NaCN is used in its froth flotation. The answer is (a).
The idea to keep. When a cyanide salt shows up in ore concentration, think froth flotation and a depressant. That points to a sulphide ore, classically ZnS separated from PbS.
Answer from the NTA final answer key. Solution by PrepWiser.