THE EXTRACTION OF GOLD, ETC. 313 



with 1 per cent, solutions the metallurgists of the Rand 

 soon cut this down to O'T, or 0*4 per cent., and the limit 

 seems now to have been reached in the treatment of slimes, 

 in which solutions containing o'ooi per cent, are in use, or 

 only one-third of an ounce of cyanide in a ton of water. 



The question of dilution is intimately bound up with 

 that of precipitation. In the early days when the zinc 

 process only was practised, it was found that the gold was 

 precipitated more rapidly and completely when a consider- 

 able excess of free cyanide was present, and it was common 

 to make up the strength of the solution coming from the 

 leaching vats before passing it through the zinc boxes. In 

 particular, pure sheet zinc is quite unable to precipitate gold 

 in the absence of free potassium cyanide, although the fili- 

 form zinc, consisting of thin threads, often as much as a yard 

 long and »Vth inch wide, slowly extracts gold from the same 

 solution. 1 Christy suggests 2 that the cause of this is that 

 hydrogen is set free by the first action of the aurocyanide of 

 potassium on the zinc, thus : — 



2KAuCy 2 + 2Zn + 2H 2 = 2KOH + H 2 + 2ZnCy 2 + 2Au 



(+ circ. 45 calories). 

 This equation is equivalent to stating that the K ions in 

 the aurocyanide are replaced by zinc, as follows : — 

 2(K + AuCy 2 ) + Zn = (Zn + 2AuCy 2 ) + 2K 

 followed by a rearrangement of the molecule of zinc-gold- 

 cyanide, and the replacement of the gold by another atom 

 of zinc. This mechanism of change seems the more prob- 

 able when it is remembered that the K ions in KCy itself 

 are displaced by zinc in the presence of water, and that 

 the whole reaction is more" strongly exothermic than that 

 of simple displacement of gold by zinc. 



Assuming, then, that hydrogen and zinc cyanide are set 

 free as above, imperceptible layers of both would be formed 

 on the surface of the sheet zinc, which would thus be soon 

 protected from further action. Hydrogen would also be set 

 free on the surface of the thread zinc, but in this case the 



1 " The Precipitation of Gold by Zinc-Thread from Dilute and Foul 

 Cyanide Solutions," by A. James, Am. Inst. Mag. Eng., Feb., 1879. 



2 "The Solution and Precipitation of Gold," ibid., 1896. 



