Question

What is the process of leaching?

Answer 1

Leaching

• “Leaching is often used if the ore is soluble in some suitable solvent.” eg, acids, bases, and suitable chemical reagents.

Procedure for leaching

• In silver and gold metallurgy, the metal is leached with a dilute solution of Sodium cyanide $\left(\mathrm{NaCN}\right)$ or Potassium cyanide $\left(\mathrm{KCN}\right)$ in the presence of oxygen.
• Later, a replacement reaction is used to obtain the metal.
• The reaction is as follows:-
• $4\mathrm{M}\left(\mathrm{s}\right)+8{\mathrm{CN}}^{–}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 4{[\mathrm{M}{\left(\mathrm{CN}\right)}_2]}^{-}\left(\mathrm{aq}\right)+4{\mathrm{OH}}^{–}\left(\mathrm{aq}\right)$
• In the above reaction $\mathrm{M}$ is $\mathrm{Ag}$ or $\mathrm{Au}$.
• $2{\left[\mathrm{M}{\left(\mathrm{CN}\right)}_2\right]}^{-}\mathrm{aq}+\mathrm{Zn}\left(\mathrm{s}\right)\to {\left[\mathrm{Zn}{\left(\mathrm{CN}\right)}_4\right]}^{2-}\left(\mathrm{aq}\right)+2\mathrm{M}\left(\mathrm{s}\right)$

Example

• The sliver is obtained by the leaching process is as follows:-
• $4\mathrm{Ag}\left(\mathrm{s}\right)+8{\mathrm{CN}}^{–}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 4{[\mathrm{Ag}{\left(\mathrm{CN}\right)}_2]}^{-}\left(\mathrm{aq}\right)+4{\mathrm{OH}}^{–}\left(\mathrm{aq}\right)$
• $$\begin{gathered} 2{\left[\mathrm{Ag}{\left(\mathrm{CN}\right)}_2\right]}^{-}\mathrm{aq}+\mathrm{Zn}\left(\mathrm{s}\right)\to {\left[\mathrm{Zn}{\left(\mathrm{CN}\right)}_4\right]}^{2-}\left(\mathrm{aq}\right)+2\mathrm{Ag}\left(\mathrm{s}\right) \\ \mathrm{Silver}\mathrm{cyanide}\mathrm{Zinc}\mathrm{cyanide} \end{gathered}$$