234 CHARLES E. MUNROE 



ceeds up and down from compartment to compartment until 

 it reaches the exit pipe of the box. The Mercur mill is equipped 

 with long sheet iron boxes having wooded partitions wedged 

 into place; these can easily be removed for cleaning up, the 

 slimes being all brushed together. At the Cripple Creek 

 mill, the slime discharge pipes of the zinc boxes lead from 

 the side of each compartment and discharge the slime into 

 a trough leading to a tank. 



When the potassium cyanide solution comes into con- 

 tact with the gold in the leaching vat, the gold is dissolved, 

 forming potassium aurocyanide. 



According to Christy, when the solution of potassium 

 aurocyanide, containing, as it usually does, an excess of 

 potassium cyanide, is brought into contact with the zinc, 

 the reaction may be represented by 



2 KAuCy2 + 3 Zn + 4 KCy + 2 H2O = 2 (ZnCy2 . 2 KCy) + K202Zn+ 2 H2 + 2Aui 



but in the absence of free potassium cyanide the successive 

 actions taking place may be summed up in the following 

 equation : 



4 KAuCy2 + 4 Zn + 2 H2O = 2 ZnCy2 + ZnCy2 . 2 KCy + K202Zn + 2 H2 + 2 Au. 



Christy also says : According to the substitution reaction, 

 one atom of zinc replaces two atoms of gold, or 1 ounce of 

 zinc should precipitate 6.2 ounces of gold; whereas, as every- 

 one knows, in practice one ounce of zinc will precipitate only 

 one fifth to one fifteenth of an ounce of gold, or thirty to 

 ninety times less than the amount called for by the reaction 

 by substitution. According to the reactions I have sug- 

 gested, in the absence of free cyanide of potassium and 

 caustic potash 1 ounce of zinc should precipitate 3.1 ounces 

 of gold; in the presence of a moderate excess of cyanide of 

 potassium it should precipitate 2.06 ounces. The apparent 

 discrepancy that seems still to remain between theory and 

 practice is in reality due to the facts, first, that the free alkali 

 (potash in particular) formed in the solution of the gold, or 

 added to neutralize the free acid in the ore, also dissolves the 

 zinc as potassium zincate; second, that an excess of potassium 

 cyanide dissolves the zinc on its own account, both as the 

 double cj^anide and as the zincate of potassium; third, it 

 should also be remembered that water containing dissolved 



