496 SCIENCE PROGRESS. 



caustic soda or lime to neutralise the acids and acid salts 

 in " weathered ' : pyritic ores, and the reduction in the 

 strength of solutions, the favourite "strong" solution now 

 containing from 0*25 to 0*30 per cent, of available KCy, 

 while solutions as weak as o"Oi per cent, are found in many 

 cases to be equally efficacious if a somewhat longer time is 

 allowed. In accordance with the experience in the extrac- 

 tion mills MacLaurin 1 has found that gold and silver in the 

 form of plates are most rapidly dissolved by solutions con- 

 taining from ci to 0*4 per cent, of cyanide, the maximum 

 rate being observed with solutions containing about 0*25 

 per cent, of cyanide of potassium. A fairly rapid rate of 

 dissolution however is still observable when only 0*005 P er 

 cent, of KCy is present. 



Cyanide of potassium acts rapidly when free oxygen is 

 present in large quantities, as for example when gold floats 

 on the solution, with its upper side dry. Under such con- 

 ditions, cyanide is at least as rapid in action as chlorine, but 

 in proportion as the supply of free oxygen falls off, the rate 

 of dissolution of gold in cyanide becomes slower, and when 

 air is excluded as rigidly as possible, hardly any action can 

 be observed. In practice these facts are of importance. 

 Finely divided gold in ordinary ores is dissolved in two or 

 three days. When concentrates containing a large propor- 

 tion of sulphides are being treated, however, free oxygen is 

 absorbed by the pyrites as well as by the solution, and the 

 treatment lasts as much as two or three weeks with a 

 corresponding increase in the destruction of the cyanide by 

 the minerals in the ore. An artificial supply of oxygen 

 or of oxidising agents shortens the time required, but in- 

 creases the waste of cyanide. Moreover, when pyritic ores 

 or concentrates contain much marcasite and, in many cases, 

 when copper sulphides or some other minerals are present, 

 the process is useless, enormous quantities of cyanide being 

 converted into other compounds before any gold is dissolved. 



The process is therefore of limited application to com- 

 plex ores and concentrates generally, although of wide 

 applicability to ores comparatively free from sulphides, 



1 Jour. Chem. Soc, loc. cit. and vol. Ixvii., p. 199, 1895. 



