The chemical used as depressant in froth flotation process while separating two sulphide ores is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(NaCN\)
In froth flotation the powdered ore is stirred with water, a collector (pine oils, fatty acids, xanthates) that makes the sulphide particles water-repellent, and a froth stabiliser (cresols, aniline); the ore particles rise with the froth. Two sulphide ores can be separated by adding a depressant, which keeps one of them from coming to the froth. For an ore containing ZnS and PbS the depressant is NaCN: it selectively prevents ZnS from coming to the froth (it forms a zinc cyanide complex on the ZnS surface) but allows PbS to float.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Froth Flotation Method – “… Sometimes, it is possible to separate two sulphide ores by adjusting proportion of oil to water or by using ‘depressants’. For example, in case of an ore containing ZnS and PbS, the depressant used is NaCN. It selectively prevents ZnS from coming to the froth but allows PbS to come with the froth.”