Identify the process involved

In the cyanide extraction of silver from argentite ore $\text{Ag}_2\text{S},$ silver is first leached with a cyanide solution in the presence of oxygen.

Leaching reaction

The silver sulphide reacts with sodium cyanide and oxygen to form the soluble argentocyanide complex: $\text{Ag}_2\text{S} + 4\text{NaCN} \rightarrow 2\text{Na}[\text{Ag}(\text{CN})_2] + \text{Na}_2\text{S}$ In practice, oxygen helps in the oxidation process during dissolution. Hence, the oxidising agent is $O_2$.

Recovery of silver from the complex

Silver is then displaced from the cyanide complex by adding zinc dust: $2\text{Na}[\text{Ag}(\text{CN})_2] + \text{Zn} \rightarrow \text{Na}_2[\text{Zn}(\text{CN})_4] + 2\text{Ag}$

Here, zinc loses electrons and reduces silver ion/complex to metallic silver. Therefore, zinc acts as the reducing agent.

Evaluate the options

• A: $O_2$ and $CO$ — incorrect, $CO$ is not used here.
• B: $O_2$ and $Zn$ dust — correct.
• C: $HNO_3$ and $Zn$ dust — incorrect, $HNO_3$ is not the oxidising agent in cyanide extraction.
• D: $HNO_3$ and $CO$ — incorrect.

Final answer

The oxidising and reducing agents used are: $O_2 \text{ and Zn dust respectively}.$