In process of froth floatation, which is used for the removing gangue from sulphide ores. Depressant used is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → NaCN
In froth floatation, a depressant is added when two sulphide ores have to be separated. For an ore containing ZnS and PbS, NaCN is the depressant: it selectively prevents ZnS from coming into the froth, while PbS comes up with the froth.
Why the other options are incorrect
Option (1) Xanthates — Incorrect. Xanthates are collectors; they increase the non-wettability of the mineral particles so that they attach to the air bubbles.
Option (2) Cresols — Incorrect. Cresols are froth stabilisers.
Option (4) Aniline — Incorrect. Aniline is also a froth stabiliser, not a depressant.
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 Floatation Method – “This method has been in use for removing gangue from sulphide ores. In this process, a suspension of the powdered ore is made with water. To it, collectors and froth stabilisers are added. Collectors (e.g., pine oils, fatty acids, xanthates, etc.) enhance non-wettability of the mineral particles and froth stabilisers (e.g., cresols, aniline) stabilise the froth.”
Froth Floatation 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.”