Which of the following is not added during the extraction of silver by cyanide process?

In the extraction of silver from cyanide process (also Mac-Arthur Forrest process), the siliver compound dissolve in $\mathrm{NaCN}$ solution forming a soluble complex salt, in presence of air, then silver is precipitated from this complex salt by the addition of $\mathrm{Zn}$, because $\mathrm{Zn}$ being more reactive displaces the Ag. $\begin{aligned} \mathrm{Ag}_2 \mathrm{~S}+4 \mathrm{NaCN} & \rightleftharpoons 2 \mathrm{Na}\left[\mathrm{Ag}(\mathrm{CN})_2\right]+\mathrm{Na}_2 \mathrm{~S} \downarrow \\ \mathrm{Na}_2 \mathrm{~S}+2 \mathrm{O}_2 & \longrightarrow \mathrm{Na}_2 \mathrm{SO}_4 \\ 2 \mathrm{Na}\left[\mathrm{Ag}(\mathrm{CN})_2\right]+\mathrm{Zn} & \longrightarrow \mathrm{Na}_2\left[\mathrm{Zn}(\mathrm{CN})_4\right]+2 \mathrm{Ag} \downarrow\end{aligned}$