PRECIOUS MET A I < 87 



did not catch all the gold. The silver-plated amalgamated cop- 

 per plate, which has taken its place, saves 15 per cent, more gold 

 than the old copper plates, and the gold caught above the 

 blankets is 16.72 per cent, greater. The old copper plates were in- 

 efficient for three reasons: (1) They tarnish quickly, and the 

 amalgam passes over the tarnished surface. This has to be re- 

 moved by washing the plates with KCN solution. (2) The amal- 

 gam is loosened from the plates by the washing with KCN and 

 mechanically lost. (3) The chemical loss of amalgam by the 

 same agent through solution is great. The total loss of gold 



FIG. 65. Iron Crown quartz mill, Newsome Creek, Idaho. (After W. 

 Lindgren, U. S. Geological Survey.) 



is frequently 10 per cent., but by improved methods in washing 

 and general treatment of the ore 95 to 98 per cent, of the total 

 gold content is recovered. (See Fig. 65.) 



Cyanide Process. The ore is crushed and treated with a solu- 

 tion of KCN, when a double cyanide of gold and potassium is 

 obtained. The double cyanide is often catalogued KAu(CN)j, 

 and from this solution the gold is precipitated by metallic zinc. 

 One-half pound of zinc is required per ton of solution, the total 

 cost then per ton for precipitation is 12 cents; and a profit can 

 be obtained where only 3 grains of gold exist in a ton of solution. 



