Extraction of Metals (MacArthur-Forrest Cyanide Process)

Chemical Leaching & Displacement Equations:

$$\begin{aligned} \text{Leaching Step:} &\quad 4\text{Ag} + 8\text{NaCN} + 2\text{H}_2\text{O} + \text{O}_2 \longrightarrow 4\text{Na}[\text{Ag(CN)}_2] + 4\text{NaOH} \\ \text{Precipitation Step:} &\quad 2\text{Na}[\text{Ag(CN)}_2] + \text{Zn} \longrightarrow \text{Na}_2[\text{Zn(CN)}_4] + 2\text{Ag} \end{aligned}$$

Concept: Utilizes the stability of soluble cyano-complexes to selectively leach native silver or gold ores via an oxidation process, followed by single-displacement precipitation using a more electropositive transition metal reducing agent.

Notation:

• $$\text{Na}[\text{Ag(CN)}_2]$$ = Soluble sodium dicyanoargentate(I) complex;
• $$\text{Na}_2[\text{Zn(CN)}_4]$$ = Soluble sodium tetracyanozincate(II) complex;
• $$\text{Zn}$$ = Highly electropositive metal acting as the reducing agent.