Question

The MacArthur-Forrest process is used for the extraction of:
A) \[\text{ Cu }\]
B) \[\text{ Ag }\]
C) \[\text{ Fe }\]
D) \[\text{ Zn }\]

Answer 1

Hint: The Mcarthur Forest process is also known as the cyanide process. The process involves the treatment of native metal with the dilute solution of $\text{ NaCN }$ or $\text{ KCN }$. The metals get oxidized which then combine with the cyanide $\text{ C}{{\text{N}}^{-}}\text{ }$ions forming a soluble complex cyanides while the impurities remain unaffected and are then filtered off. The metal dissolves in the solution as a complete general reaction for the leaching of native metal is given as follows:
\[\begin{matrix}
  \text{4M(s)} & \text{+} & \text{8C}{{\text{N}}^{-}}(aq) & \text{+} & \text{2}{{\text{H}}_{\text{2}}}\text{O(}l\text{)} & \text{+} & {{\text{O}}_{\text{2}}}(g) & \to & 4{{\left[ \text{M}{{\left( \text{CN} \right)}_{\text{2}}} \right]}^{-}}(aq) & + & 4\text{O}{{\text{H}}^{-}}(aq) \\
   {} & {} & {} & {} & {} & {} & \text{Air} & {} & \text{(soluble complex)} & {} & {} \\
\end{matrix}\]

Complete step by step answer:
The ores are usually found mixed up with large amounts of non-metallic impurities such as sand, mica, limestone, feldspar, earthy and rocky impurities.
The process of removal of unwanted impurities from the ore is called the ore concentration.
Leaching is a chemical method of concentration and is useful in case the ore is soluble in a suitable solvent. In this method, the powdered ore is treated with certain reagents in which the ore is soluble but the impurities are not soluble.
Mac Arthur –Forest Process is also called the cyanide process. It is a leaching process for the extraction of gold or silver.
Leaching is used to concentrate silver and gold ores. The ore containing native metal is treated with the dilute solutions of $\text{ NaCN }$ or $\text{ KCN }$ in the presence of atmospheric oxygen.
The silver is leached as follows:
\[\begin{matrix}
   \text{4Ag} & \text{+} & \text{8NaCN} & \text{+} & \text{2}{{\text{H}}_{\text{2}}}\text{O} & \text{+} & {{\text{O}}_{\text{2}}} & \to & 4\text{Na}\left[ \text{Ag}{{\left( \text{CN} \right)}_{\text{2}}} \right] & + & 4\text{NaOH} \\
   {} & {} & {} & {} & {} & {} & {} & {} & \begin{align}
  & (\text{Sodium dicyanoargentate(I))} \\
 & \text{ (soluble complex)} \\
\end{align} & {} & {} \\
\end{matrix}\]
If instead of native silver, silver glance, or argentite $\text{ (A}{{\text{g}}_{\text{2}}}\text{S) }$is used for leaching of silver, the initial reaction forming the soluble complex is reversible.
$\begin{matrix}
   \text{A}{{\text{g}}_{\text{2}}}\text{S} & \text{+} & \text{4NaCN} & \rightleftharpoons & \text{2Na}\left[ \text{Ag(CN}{{\text{)}}_{\text{2}}} \right] & \text{+} & \text{N}{{\text{a}}_{\text{2}}}\text{S} \\
   {} & {} & {} & {} & \begin{align}
  & (\text{sodium dicyanoargentate (I))} \\
 & \text{ (Soluble complex)} \\
\end{align} & {} & {} \\
\end{matrix}$
Sodium sulphide, thus formed, is oxidized to sodium sulphate by blowing air into the solution. This helps the reaction to occur in the forward direction.
$\text{ }4\text{N}{{\text{a}}_{\text{2}}}\text{S + 5}{{\text{O}}_{\text{2}}}\text{(air) + 2}{{\text{H}}_{\text{2}}}\text{O }\to \text{ 2N}{{\text{a}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\text{ + 4NaOH +2S }$
The above solution after filtration and removing insoluble impurities is heated with zinc to get silver.
$\begin{matrix}
   \text{2Na}\left[ \text{Ag(CN}{{\text{)}}_{\text{2}}} \right] & \text{+} & \text{Zn} & \to & \text{N}{{\text{a}}_{\text{2}}}\left[ \text{Zn(CN}{{\text{)}}_{\text{4}}} \right] & \text{+} & \text{2Ag} \\
   {} & {} & {} & {} & (\text{Complex}) & {} & {} \\
\end{matrix}$
This chemical method of the concentration of silver or leaching of silver is called the MacArthur Forrest Cyanide process.
Hence, (B) is the correct option.
Additional information:
The gold is extracted the same way as the silver. The reactions are as follows:
\[\begin{matrix}
   \text{4Au} & \text{+} & \text{8KCN} & \text{+} & \text{2}{{\text{H}}_{\text{2}}}\text{O} & \text{+} & {{\text{O}}_{\text{2}}} & \to & 4\text{K}\left[ \text{Au}{{\left( \text{CN} \right)}_{\text{2}}} \right] & + & 4\text{KOH} \\
   {} & {} & {} & {} & {} & {} & {} & {} & \begin{align}
  & (\text{Potassium dicyanoaurate(I))} \\
 & \text{ (soluble complex)} \\
\end{align} & {} & {} \\
\end{matrix}\]

Note: The zinc plays a crucial role in removing the silver from the impurities. The zinc is a reducing agent. It undergoes the oxidation from 0 to $+2$ , and it reduces the silver from $+1$ to 0. The zinc assists the formation of pure metal from the complexed silver.