Question

# Name the important ore of silver. Write all steps and reactions involved in the cyanide process for the extraction of silver from its ore.

Hint: Silver is generally found in combined states and its common ore is silver glance or Argentite. This ore contains impurities which have to be removed before metal can be extracted. This impure ore is treated with acid or base so that the ore particles dissolve while impurities do not.

The important ore of silver is ${\text{A}}{{\text{g}}_2}{\text{S}}$ (Argentite) from which silver can be extracted in the following steps-
The finely powdered ore is concentrated by the froth floatation process which is based on wettable properties of ores and the impurities associated with them.
It is then treated with $5$% dilute solution of sodium cyanide $\left( {{\text{NaCN}}} \right)$ and air is passed into the solution. The precipitate of silver cyanide is formed.
${\text{A}}{{\text{g}}_2}{\text{S + NaCN}} \rightleftharpoons {\text{2AgCN + N}}{{\text{a}}_2}{\text{S}}$
The precipitate of silver cyanide initially dissolves in excess of sodium cyanide to form soluble complex-sodium argento cyanide.
${\text{A}}{{\text{g}}_2}{\text{S + NaCN}} \to {\text{Na}}\left[ {{\text{Ag}}{{\left( {{\text{CN}}} \right)}_2}} \right]$
The oxygen present in the air converts sodium sulphide to sodium sulphate and as a result the reaction proceeds in forward direction and backward reaction is prevented.
$4{\text{N}}{{\text{a}}_2}{\text{S + 5}}{{\text{O}}_2}{\text{ + 2}}{{\text{H}}_2}{\text{O}} \to {\text{2N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4}{\text{ + 4NaOH + 2S}}$
The solution is filtered to remove the insoluble impurities. It is treated with a more electropositive metal like zinc which can easily displace silver from the complex and silver is obtained in the form of precipitate.
$6\%$ ${\text{2Na}}\left[ {{\text{Ag}}{{\left( {{\text{CN}}} \right)}_2}} \right] + {\text{Zn}} \to {\text{N}}{{\text{a}}_2}\left[ {{\text{Zn}}{{\left( {{\text{CN}}} \right)}_2}} \right] + 2\mathop {{\text{Ag}}}\limits_{{\text{ppt}}{\text{.}}}$
This ppt. is purified by an electrolytic method in which cathode is pure silver, anode is impure silver. The electrolyte used is ${\text{AgN}}{{\text{o}}_3}$ with $1 \% {\text{HN}}{{\text{O}}_3}$. On passing current, pure silver gets deposited at cathode while impurities get deposited at anode.

Hence pure silver is obtained by this method from argentite.

Argentite can only exist above the temperature of ${173^ \circ }{\text{C,}}{177^ \circ }{\text{C,}}{179^ \circ }{\text{C}}$. When it cools down to ordinary temperature it turns into monoclinic polymorph called acanthite. It reduces metal availability and toxicity in contaminated soils. It is also used as a polish in sterling silver.