PRECIOUS METALS 



101 



chloride. The sodium thiosulphate method has given way very 

 largely to the calcium thiosulphate method, which is practically 

 identical in apparatus and method of treatment, save with cal- 

 cium in place of sodium as a solvent. The latter process is known 

 as the Kiss process from its inventor. But all these in which 

 calcium thiosulphate enters as a solvent are now replaced by the 

 cyanide process. 



Cyanide Process. This method of treatment is based upon the 

 fact that when silver sulphides, arsenides and antimonides, are 

 treated with a solution of potassium cyanide or sodium cyanide, 

 a double cyanide of silver and potassium, or silver and sodium 

 is formed. The solution is far more concentrated than in the 



FIG. 69. Nevada Hills mill, Fairview, Nevada, for cyaniding silver. 



case of the treatment of gold-bearing ores with potassium cyanide, 

 because the silver minerals are less soluble in a cyanide solution 

 than the gold ores. The solution is filtered to remove all sedi- 

 ment, and allowed to settle to a perfectly transparent liquor. 

 It is then drawn off into precipitating tanks, and the metal 

 reduced to the elemental state by granulated zinc, zinc shavings, 

 zinc dust, as in the treatment of gold. (See Fig. 69.) 



Electrolytic Process. Silver is separated from argentiferous 

 copper ores electrolytically by sulphuric acid and copper sulphate. 

 The copper and iron are dissolved at the anode, while gold, silver 

 and platinum are precipitated at the cathode. In the modifica- 

 tion of this process, known as the Moebius process, large amounts 



