Oxidizing and Reducing Agents in Cyanide Extraction of Silver

In the cyanide process (MacArthur-Forrest process) for the extraction of silver from argentite ore ($\text{Ag}_2\text{S}$), the chemical steps involved are:

1. Leaching (Dissolution of Ore): The finely powdered argentite ore is treated with a dilute aqueous solution of sodium cyanide ($\text{NaCN}$) in the presence of air ($\text{O}_2$). The reversible reaction is: $\text{Ag}_2\text{S} + 4\text{NaCN} \rightleftharpoons 2\text{Na}[\text{Ag}(\text{CN})_2] + \text{Na}_2\text{S}$ Oxygen from air oxidizes the $\text{Na}_2\text{S}$ formed into $\text{Na}_2\text{SO}_4$ (or colloidal sulfur), thereby preventing the backward reaction and driving the equilibrium forward: $4\text{Na}_2\text{S} + 5\text{O}_2 + 2\text{H}_2\text{O} \longrightarrow 2\text{Na}_2\text{SO}_4 + 4\text{NaOH} + 2\text{S}$ Thus, $\text{O}_2$ acts as an oxidizing agent.

2. Precipitation (Reduction of Silver): The filtered solution containing the soluble dicyanoargentate(I) complex, $[\text{Ag}(\text{CN})_2]^-$, is treated with scrap zinc or zinc dust. Zinc acts as a more electropositive metal and displaces silver from the complex by reducing $\text{Ag}^+$ to elemental silver ($\text{Ag}^0$): $2[\text{Ag}(\text{CN})_2]^- + \text{Zn} \longrightarrow [\text{Zn}(\text{CN})_4]^{2-} + 2\text{Ag} \downarrow$ Here, $\text{Zn}$ dust acts as a reducing agent ($\text{Zn} \to \text{Zn}^{2+} + 2e^-$).

Therefore, the respective oxidizing and reducing agents employed are $\text{O}_2$ and $\text{Zn}$ dust.

Correct Option: B