Question

Describe the extraction of silver from its sulphide ore by the cyanide process.

Answer 1

The ore argentite is the primary source from which silver is extracted. The MacArthur-Forrest cyanide process is the method used to extract silver from its ores. The basis for this procedure is that silver dissolves in a dilute sodium cyanide solution to form sodium argentocyanide, a soluble compound. Scrap zinc can be added to this compound to extract silver. The following steps are used to carry out the process.

Concentration: The sulphide ore is crushed, powdered, and then concentrated by the froth floatation process.

Treatment with sodium cyanide: A diluted sodium cyanide solution (0.4-0.7%) is applied to the concentrated ore, and for many hours, the combination is constantly stirred by an air current. Silver from the ore enters the solution as the soluble complex sodium argentocyanide.

\[\ce{Ag2S + 4NaCN <=> \underset{(or sodium argentocyanide)}{\underset{Sodium dicyanoargentate (I)}{2Na[Ag(CN)2]}} + Na2S}\]
Na₂S is oxidized into Na₂SO₄ by the air blow. As a result, the reaction’s reversibility is confirmed, and it moves forward.
\[\ce{2Na2S + 2O2 + H2O -> Na2S2O3 + 2NaOH}\]
\[\ce{Na2S2O3 + 2NaOH + 2O2 -> 2Na2SO4 + H2O}\]

Precipitation of silver: After filtering, the solution from the previous stage is treated with leftover zinc. Because zinc is more electropositive, it precipitates silver out of the complex’s solution.
\[\ce{2Na[Ag(CN)2] + Zn -> \underset{zincate (II)}{\underset{Sodium tetracyano}{Na2[Zn(CN)4]}} + 2Ag \downarrow}\]
Silver thus obtained is in the form of a dark amorphous mass.

Fusion: When a bright compact mass is produced, the precipitated silver is filtered, crushed, and dried, and fused with borax.

Purification: Zinc, copper, and gold impurities are typically present in the resulting silver. The electrolytic refining method is used to purify this. In an electrolytic tank, a thin sheet of pure silver serves as the cathode and the impure silver block as the anode. The electrolyte is a silver nitrate solution with 1% nitric acid. Pure silver deposits itself on the cathode when an electric current flows through it. Zinc and copper impurities enter the solution, and any gold that may be present is removed as anode mud.