Question

Extraction of gold and silver involves leaching with CN^– ion. Silver is later recovered by

• A. Liquation
• B. Distillation
• C. Zone refining
• D. Displacement with Zn

Answer 1

The Correct Option is D

The recovery of gold and silver from their ores often involves a hydrometallurgical process called leaching. In the context of the question, the leaching of silver involves the use of cyanide ions (CN^–). Here's how the process generally works:

1. The ore containing silver is treated with a dilute solution of sodium cyanide (NaCN), which dissolves silver and gold due to the formation of their respective cyanide complexes:
   4Ag + 8NaCN + O_2 + 2H_2O → 4Na[Ag(CN)_2] + 4NaOH
2. After the leaching process is complete, the solution contains the silver in the form of a complex ion, [Ag(CN)₂]^–.
3. The recovery of silver from this complex solution is then achieved through the process of displacement by adding zinc dust. This method is known as the Merrill-Crowe process:
   2Na[Ag(CN)_2] + Zn → 2Ag + Na_2[Zn(CN)_4]
   In this reaction, zinc, being more reactive and having a higher reduction potential than silver, displaces silver from its complex, leading to the precipitation of silver.

Let's examine why "Displacement with Zn" is the correct answer and not the other options:

• Liquation: This process is typically used for low melting metals like tin and not applicable to silver recovery from cyanide solutions.
• Distillation: This method is used to separate volatile components, not applicable in dissolving and precipitating metals from complex solutions.
• Zone refining: Used for purifying semiconductors, not for extracting or recovering metals like silver from leach solutions.

Therefore, the correct choice for recovering silver from the cyanide solution is "Displacement with Zn".