Silver is extracted from the argentite ore by the Mac-Arthur and Forrest’s cyanide process. The various steps involved in this process are as follows.
Concentration
The crushed ore is concentrated by froth-floatation process.
Treatment of the ore with NaCN
The concentrated ore is treated with 0.4-0.6% solution of sodium cyanide for several hours. The mixture is continuously agitated by a current of air, so that
Ag present in the ore is converted into soluble sodium argento complex.
Ag2S + 4NaCN ↔ 2Na [Ag(CN)2] + Na2S [Sodium argento cyanide (soluble)]
Extraction of silver from silver coins
Silver coins are Ag-Cu alloys. Ag can be obtained from such an allloy by dissolving the alloy in concentrated HNO3, a solution containing mixture of AgNO3 and Cu(NO3)2 is obtained. The solution is boiled to expel excess of nitric acid, then the solution is treated with con.HCl, silver is precipitated as AgCl. AgCl is separated and converted to silver by fusing with excess Na2CO3.
4AgCl + 2Na2CO3 → 4Ag↓ + 4NaCl + 2CO2 + O2
The silver thus obtained, is purified by fusion with borax and followed by electrolytic purification.
