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During froth flotation of galena (PbS) contaminated with zinc blende (ZnS), sodium cyanide is added. Its role is to
Adepress ZnS by forming a surface complex
Bfloat both PbS and ZnS together
Cincrease the froth volume
Ddissolve the PbS as a cyanide complex
Answer & Solution
Correct answer: A. depress ZnS by forming a surface complex
1. When two sulphides are present, a **depressant** selectively stops one from floating.
2. NaCN reacts with ZnS to form a layer of $Na_2[Zn(CN)_4]$ on its surface, keeping ZnS wetted by water so it sinks.
3. PbS is unaffected and floats with the froth, achieving separation.
_Source: Samacheer Kalvi Class 12 Chemistry Vol.1, Unit 1 'Metallurgy', §1.2.2_
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