An investigation of the selective leaching of gold over copper using ammoniacal cyanide

Copper minerals are often associated with gold ores. These create processing problems since cyanide solutions are not selective for gold alone. Copper minerals are also leached by cyanide, and typically each one per cent of 'reactive' copper in the ore consumes about 30 kg t(-1) cyanide. Once in solution, copper cyanide species create further processing problems as they adsorb onto carbon along with the dicyanoaurate ion. These problems make conventional cyanidation of gold ore uneconomic if the ore contains too much copper. As a result, methods of economically processing gold-copper ores are becoming of increasing importance. One of the more promising methods of treating these ores is by selective leaching of the gold by an ammoniacal cyanide solution, a method first described by Hunt in 1901. This method is already in commercial use in Mauritania, treating copper/gold tailings. In general terms, the addition of 1 -2 g L-1 ammonia to a standard cyanide leach solution causes a significant increase in gold extraction and decrease in copper extraction. However, early testwork on this system has concluded that the optimal leach conditions are critically dependent on ore composition, and the mechanism is not well understood, making predictions of best conditions impossible. This paper presents both a kinetic and electrochemical study aimed at better understanding the selective leaching process. The kinetic study involves a comparison of the dissolution rates of both gold and copper in oxygenated solutions containing mixtures of cyanide, ammonia and copper(I). Dissolution rates were measured using a rotating quartz crystal microbalance. The leaching behaviour of copper and gold in these systems was found to be rather similar, and so does not explain why the system is selective towards gold dissolution over copper. It was found that the suppression of copper dissolution in this system is not due to a decrease in rate of copper leaching but rather to the re-precipitation of copper compounds as the reaction proceeds. An electrochemical study was carried out in an attempt to clarify some of the mechanistic aspects of the leaching process.