Q.How is silver extracted from Argentite ore ?

Argentite ($Ag_2S$) is leached with dilute sodium cyanide in the presence of air to form a soluble silver–cyanide complex, and metallic silver is then obtained by displacing it from this complex with zinc dust — the MacArthur–Forrest cyanide process, the hydrometallurgical method for extracting silver.

1. Crushing and concentration: The argentite ore ($Ag_2S$) is crushed and concentrated by the froth flotation process (it is a sulphide ore).

2. Leaching with cyanide (formation of the soluble complex): The concentrated ore is treated with a dilute solution of sodium cyanide ($NaCN$, ~0.4–0.6%) in the presence of atmospheric oxygen (air is blown through). Silver forms a stable, water-soluble dicyanoargentate(I) complex:

$Ag_2S + 4NaCN \rightarrow 2Na[Ag(CN)_2] + Na_2S$

The by-product sodium sulphide is oxidised by the air present, which also helps drive the leaching reaction forward (prevents reversal):

$2Na_2S + 2O_2 + H_2O \rightarrow Na_2S_2O_3 + 2NaOH$

3. Filtration: The clear solution containing the soluble complex $Na[Ag(CN)_2]$ is filtered off from the insoluble gangue.

4. Precipitation of silver (displacement by zinc): Metallic silver is precipitated from this solution by adding zinc dust (zinc, being more electropositive, displaces silver from the complex):

$2Na[Ag(CN)_2] + Zn \rightarrow Na_2[Zn(CN)_4] + 2Ag\downarrow$

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