JEE Challenger
More from General Principles and Processes of Isolation of Elements
ChemMedAdv 2019 • Paper 2

Cyanide Process for Gold Extraction and Intermediate Identification

In the cyanide extraction process of gold, the metal is leached out from its ore using CN−\text{CN}^- in the presence of Q\mathbf{Q} in an aqueous medium to form R\mathbf{R}. Subsequently, R\mathbf{R} reacts with T\mathbf{T} to yield Au\text{Au} and Z\mathbf{Z}. Identify the correct option(s).

Options

A

Q\mathbf{Q} is O2\text{O}_2

Correct
B

T\mathbf{T} is Zn\text{Zn}

Correct
C

Z\mathbf{Z} is [Zn(CN)4]2−[\text{Zn}(\text{CN})_4]^{2-}

Correct
D

R\mathbf{R} is [Au(CN)4]−[\text{Au}(\text{CN})_4]^-

Topics & Concepts

General Principles and Processes of Isolation of ElementsMetallurgyCoordination Compounds

Step-by-Step Solution

To determine the correct options, let us analyze the cyanide process (MacArthur-Forrest process) for the extraction of gold (Au\text{Au}):

  1. Leaching Step: Gold (Au\text{Au}) is extracted from its ore by treating it with a dilute solution of cyanide ions (CN−\text{CN}^-) in the presence of atmospheric oxygen (O2\text{O}_2) and water. Oxygen acts as an oxidizing agent to oxidize gold from oxidation state 00 to +1+1, forming a soluble cyano-complex of gold: 4Au(s)+8CN−(aq)+2H2O(l)+O2(g)→4[Au(CN)2]−(aq)+4OH−(aq)4\text{Au}(s) + 8\text{CN}^-(aq) + 2\text{H}_2\text{O}(l) + \text{O}_2(g) \rightarrow 4[\text{Au}(\text{CN})_2]^-(aq) + 4\text{OH}^-(aq)

    Comparing this with the given statement:

    • The metal is leached in the presence of Q=O2\mathbf{Q} = \text{O}_2.
    • The complex formed is R=[Au(CN)2]−\mathbf{R} = [\text{Au}(\text{CN})_2]^-.

  2. Recovery / Displacement Step: Gold is then precipitated from the solution by adding a more electropositive reducing agent, zinc (Zn\text{Zn}), which displaces gold from the complex: 2[Au(CN)2]−(aq)+Zn(s)→[Zn(CN)4]2−(aq)+2Au(s)2[\text{Au}(\text{CN})_2]^-(aq) + \text{Zn}(s) \rightarrow [\text{Zn}(\text{CN})_4]^{2-}(aq) + 2\text{Au}(s)

    Comparing this with the given statement:

    • The reducing agent added is T=Zn\mathbf{T} = \text{Zn}.
    • The complex formed is Z=[Zn(CN)4]2−\mathbf{Z} = [\text{Zn}(\text{CN})_4]^{2-}.

Evaluating the given options:

  • (A) Q\mathbf{Q} is O2\text{O}_2: Correct
  • (B) T\mathbf{T} is Zn\text{Zn}: Correct
  • (C) Z\mathbf{Z} is [Zn(CN)4]2−[\text{Zn}(\text{CN})_4]^{2-}: Correct
  • (D) R\mathbf{R} is [Au(CN)4]−[\text{Au}(\text{CN})_4]^-: Incorrect, because R\mathbf{R} is [Au(CN)2]−[\text{Au}(\text{CN})_2]^-.

Thus, the correct options are A, B, and C.

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