The chemical used as depressant in froth flotation process while separating two sulphide ores is:

\(NaCN\)

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

Froth flotation is a widely used method for concentrating and separating minerals, particularly for the extraction of metals from their ores. In this process, finely ground ore is mixed with water to form a slurry. Various chemicals are then added to this slurry to selectively separate different minerals based on their surface properties.

Key Components:

Collectors: Chemicals like pine oil, xanthates, etc., that increase the hydrophobicity of the mineral particles, allowing them to attach to air bubbles and rise to the surface.

Frothers: Substances like pine oil and cresols that stabilize the froth formed on the surface.

Depressants: Chemicals that selectively prevent certain types of minerals from forming froth by making them hydrophilic (water-attractive), so they stay in the aqueous phase.

Role of Depressants in Froth Flotation

Depressants play a crucial role when dealing with mixtures of sulphide ores, particularly when two or more types of sulphide minerals are present. They work by selectively making one type of ore hydrophilic, preventing it from attaching to the air bubbles and thus separating it from the hydrophobic ore that forms the froth.

Example: Separation of Lead Sulphide (\(PbS\)) and Zinc Sulphide (\(ZnS\))

Consider a mixture of lead sulphide \((PbS)\) and zinc sulphide \((ZnS)\), a common scenario in mineral processing.

Challenge: Both lead sulphide and zinc sulphide are sulphide minerals and have similar surface properties, meaning they both could potentially attach to the air bubbles and rise to the surface, making separation difficult.

Solution: Sodium cyanide (\(NaCN\))** is added as a depressant. \(NaCN\) selectively affects zinc sulphide without impacting lead sulphide. In the presence of \(NaCN\), the following reaction occurs:

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

The zinc sulphide reacts with sodium cyanide to form a soluble complex \(Na_2[Zn(CN)_4]\), which remains in the solution and does not rise with the froth. The lead sulphide, unaffected by \(NaCN\), remains hydrophobic and is collected in the froth.

Outcome: Lead sulphide \((PbS)\) floats to the top with the froth and is separated, while zinc sulphide \((ZnS)\) stays in the aqueous phase.

Why \(NaCN\) Works:

Selective Complex Formation: Sodium cyanide (\(NaCN\)) selectively forms a complex with zinc ions \([Zn(CN)_4]^{2-}\), making the zinc sulphide particles hydrophilic (water-attractive). This prevents zinc sulphide from attaching to air bubbles, ensuring that only lead sulphide forms the froth.

No Effect on Lead Sulphide: \(NaCN\) does not react with lead sulphide, so the lead sulphide remains hydrophobic (water-repellent), enabling its separation via froth flotation.

Other Options Considered:

\(NaCl\): Sodium chloride is a neutral salt and does not have the specific chemical properties needed to act as a depressant in froth flotation.

\(NaOH\): Sodium hydroxide is a strong base and can adjust the pH of the solution but does not selectively depress any particular sulphide ore in the froth flotation process.

\(NaNO_2\): Sodium nitrite is an oxidizing agent and is not typically used as a depressant in froth flotation processes.

Conclusion:

Sodium cyanide (\(NaCN\)) is the correct depressant used in the froth flotation process when separating lead sulphide (\(PbS\)) from zinc sulphide (\(ZnS\)). It selectively prevents the flotation of zinc sulphide by forming a soluble complex, allowing lead sulphide to be efficiently separated in the froth.

Thus, the correct answer is option 3: \(NaCN\).