In the metallurgy of silver and that of gold, the respective metal is leached with dil. solution of \(NaCN\) from whic silver and gold is later recovered by:

Replacement with \(Zn\)

The correct answer is option 3. Replacement with \(Zn\).

In the metallurgy of silver and gold, the respective metal is typically leached with a dilute solution of sodium cyanide \((NaCN)\) to form a soluble complex, which is later recovered by a process known as "replacement with zinc" or "Merrill-Crowe process."

Here is how the process works:

1. Leaching with Sodium Cyanide (NaCN): In this initial step, the silver or gold-containing ore is crushed and then treated with a dilute solution of sodium cyanide \((NaCN)\). Sodium cyanide is a highly toxic chemical that forms complexes with the silver or gold ions present in the ore. The cyanide ions (\(CN^-\)) complex with the metal ions, forming soluble complexes. For example:

For silver: \(2Ag(s) + 4CN^-(aq) + O_2(aq) + 2H_2O(l) \rightarrow 2[Ag(CN)_2]^- + 2OH^-(aq)\)

For gold: \(Au(s) + 2CN^-(aq) \rightarrow [Au(CN)_2]^- \)

2. Recovery by Replacement with Zinc (Merrill-Crowe Process): In this crucial step, the dissolved silver or gold ions in the cyanide solution are recovered by the addition of metallic zinc. Zinc has a higher affinity for cyanide ions than either silver or gold does. Therefore, when zinc is introduced into the cyanide solution, it displaces the silver or gold from the cyanide complexes, forming insoluble silver or gold-zinc complexes. The reaction can be represented as follows:

For silver: \(Zn(s) + 2[Ag(CN)_2]^- \rightarrow Zn(CN)_4^{2-} + 2Ag(s)\)

For gold: \(Zn(s) + [Au(CN)_2]^- \rightarrow Zn(CN)_4^{2-} + Au(s)\)

3. Precipitation: The insoluble silver or gold-zinc complexes formed in the solution precipitate out as fine powders or sludges. This precipitate, containing silver or gold along with zinc cyanide complexes, is then separated from the cyanide solution through filtration or sedimentation.

4. Further Processing: The silver or gold precipitate obtained from the Merrill-Crowe process is then subjected to further refining processes to obtain pure silver or gold. These processes may include smelting, where the precipitate is heated to high temperatures to separate the metal from impurities, and electrolysis, where electric current is passed through a solution to deposit pure silver or gold onto electrodes.

This method of recovery is efficient and widely used in the mining industry for extracting silver and gold from ores containing relatively low concentrations of these precious metals. It offers a practical and economical means of obtaining high-purity silver and gold for various industrial and commercial applications.