METALS AND CYANIDE PROCESSES 239 



only with solutions containing more than 0.1 per cent of 

 cyanide, the weaker solutions not being acted upon by zinc. 

 An improvement of the method is the addition of lead to the 

 zinc, whereby the combination of the two metals forms a 

 galvanic couple, which also reacts with weaker solutions, such 

 as are employed, for example, in the treatment of slimes. 



"In the Siemens-Halske process the solution flows through 

 compartments very similar to the zinc boxes above described, 

 but the zinc shavings are here replaced with lead strips (0.1 

 pound per square foot) or shavings hung between iron plates 

 placed vertically and longitudinally in the box, about 4 inches 

 apart. The lead strips are connected with the negative, and the 

 iron plates with the positive, pole of a dynamo, and the solu- 

 tion is thus electrolytically decomposed, the gold being placed 

 on the lead cathode. The iron plates are wrapped in canvas 

 to prevent short circuiting. The current employed is from 

 2 to 3 volts, giving a current density of about 0.06 amperes 

 per square foot of cathode. Once a month the lead sheets 

 are removed and replaced, and the gold coated lead is melted 

 and cupelled, yielding a bullion of 0.880 fine m gold and 0.100 

 in silver. The litharge is sold to smelters. The solutions 

 passing through the treatment boxes are collected in tanks, 

 and are made up to a proper strength by adding the necessary 

 KCy. 



'The cost of the Siemens-Halske process is slightly greater 

 than that of zinc precipitation, and the percentage of ex- 

 traction is about the same. But the Siemens-Halske process 

 may be applied to any solution, weak or strong." 



From the description of its mode of operation it is evi- 

 dent that the straight cyanide process is readily applicable 

 to free milling ores which are free from mineral substances 

 that would foul the solution or consume the cj^anogen salt, 

 and the gold in which is finely divided and so exposed that 

 the cyanide solution may come in intimate contact with it. 

 In old tailings which have been exposed for years to the ac- 

 tion of the weather, whereby the sulphurets have been oxi- 

 dized and the acids and salts formed, dissolved and^washed 

 away, there has been found much material ready at hand 

 for immediate treatment in the simplest manner. The prob- 



