Preg-robbing phenomena in the cyanidation of sulphide gold ores

Abstract

The ability of pyrite and chalcopyrite to adsorb gold was examined. Experiments were conducted in cyanide deficient solutions with and without activated carbon. Chalcopyrite was shown to be very strongly preg-robbing. It competed with activated carbon to remove the majority of gold from solution. Pyrite was also strongly preg-robbing, and in cyanide deficient solutions adsorbed the majority of the gold in preference to activated carbon. The degree of adsorption onto the ore or the activated carbon was a function of the kinetics of the adsorption process. The role of the cyanide complexes of copper, silver, zinc, nickel and iron was also examined. It was found that these complexes counteract preg-robbing, and serve to stabilise the aurocyanide complex by precipitating prior to gold precipitation. When cyanide or cyanide-species were present in solution, the adsorption onto activated carbon was found in general to take preference to the adsorption onto the minerals. The ability of a sulphide ore to adsorb Au(CN) 2 − from solution was found to be a function of how rapidly the ore consumed cyanide and precipitated metal–cyanide complexes. A mechanism was proposed where the gold is reduced at the chalcopyrite surface through the formation of an adsorbed intermediate. The gold was reduced, along with the oxidation of chalcopyrite to form copper–cyanide complexes in solution. Reduction of gold on the pyrite was observed to occur with the release of zinc as an impurity into solution, and a mechanism for this reaction was proposed.