In process of froth floatation, which is used for the removing gangue from sulphide ores. Depressant used is:

\(NaCN\)

The correct answer is option 3. \(NaCN\).

Froth flotation is a widely used method for concentrating sulphide ores. The basic principle involves adding specific reagents that make the desired ore hydrophobic (water-repellent) so that it can attach to air bubbles and rise to the surface, forming a froth. This froth can then be skimmed off, leaving behind the impurities, or gangue, in the water.

Key Reagents in Froth Flotation:

Collectors: These reagents make the ore particles hydrophobic so they can attach to the air bubbles. Xanthates are common collectors for sulphide ores.

Frothers: These stabilize the froth, ensuring that the air bubbles do not break before they can be separated. Cresols are examples of frothers.

Depressants: These are substances added to the mixture to prevent certain types of ores (usually impurities or unwanted sulphide ores) from forming froth. This is crucial when multiple sulphide ores are present and need to be separated selectively.

The Role of Depressants in Froth Flotation:

Depressants are used when more than one sulphide ore is present. For example, in the extraction of lead sulphide (PbS) and zinc sulphide (ZnS) from a mixed ore, we want to float PbS while ZnS remains in the aqueous phase. A depressant is added to selectively inhibit the flotation of one sulphide ore (in this case, ZnS), while allowing the other to float.

Sodium Cyanide (\(NaCN\)) as a Depressant:

Sodium cyanide (\(NaCN\)) is a common depressant used in the froth flotation process for sulphide ores. In ores where zinc sulphide (ZnS) and lead sulphide (PbS) are present, NaCN selectively depresses ZnS, preventing it from forming froth. When \(NaCN\) is added, it reacts with Zn²⁺ ions from zinc sulphide, forming a soluble complex \([Zn(CN)_4]^{2-}\). This keeps the zinc in solution, allowing lead sulphide (PbS) to float and be separated.

Reaction of \(NaCN\) with Zinc Sulphide:

\(ZnS + 4NaCN \longrightarrow Na_2[Zn(CN)_4] + Na_2S \)

This reaction ensures that zinc does not float to the surface, while PbS remains unaffected by the depressant and floats.

Explanation of the Options:

Xanthates: These are collectors, not depressants. They help the sulphide ores attach to the air bubbles by making them hydrophobic. Hence, xanthates increase the flotation of sulphide ores, rather than preventing it.

Cresols: Cresols act as frothers, which stabilize the froth and help form a stable foam for the attachment of ore particles. They do not act as depressants.

Sodium Cyanide (\(NaCN\)): \(NaCN\) acts as a depressant in the froth flotation process. It selectively prevents zinc sulphide from forming froth by converting it into a soluble complex, leaving the more desired lead sulphide to float.

Aniline: Aniline is not commonly used as a depressant in the froth flotation process for sulphide ores. It does not play a significant role in depressing any of the sulphide ores in froth flotation.

Example: In a real-life scenario of extracting lead sulphide (PbS) and zinc sulphide (ZnS) from a mixed ore, NaCN would be added to selectively prevent ZnS from floating, allowing PbS to be recovered first. After this, the zinc sulphide can be floated separately by adjusting the pH or using another collector. This selective depression is crucial in metallurgical processes for purifying ores.

The correct answer is 3. \(NaCN\)

Sodium cyanide is the depressant used to selectively inhibit zinc sulphide from floating, allowing other sulphide ores (like lead sulphide) to be separated through froth flotation.