Question

Describe the principle involved in the cyanide process used for the extraction of silver.

Answer 1

1. The cyanide process is also called the MacArthur-Forrest process.
2. It is based on the fact that silver ore (mainly argentite, Ag₂S) dissolves in dilute sodium cyanide solution to form a soluble complex called sodium argentocyanide.
3. The chemical reaction is:
   \[\ce{Ag2S + 4NaCN <=> 2Na[Ag(CN)2] + Na2S}\]
4. Air is continuously blown in to oxidize Na₂S to Na₂SO4, preventing the reverse reaction and driving dissolution forward.
5. Silver dissolves as the dicyanoargentate complex in the solution.
6. Silver is then recovered by adding zinc dust, which displaces silver from the solution:
   \[\ce{2Na[Ag(CN)2] + Zn -> Na2[Zn(CN)4] + 2Ag\downarrow}\]
7. The silver precipitates out as a dark mass, which is filtered, dried, and purified.
8. This process efficiently extracts silver from low-grade ores.