ULTRA FINE MILLING FOR THE RECOVERY OF REFRACTORY GOLD

Refractory gold ores may be defined as those which yield low recoveries of gold when treated by the conventional grinding and cyanidation process. This refractory behaviour can vary from mild to severe with gold recoveries of 85% down to less than 30% [1]. A common cause of refractoriness is encapsulation of fine gold within the matrix of sulphide minerals, particularly arsenopyrite and pyrite. The size and location of the gold within the sulphide matrix determines to a very large degree the nature of the process required for its liberation and recovery. Submicroscopic gold is liberated by destruction of the sulphide with processes based on the use of thermal, chemical or biological oxidation. If the encapsulated gold is coarser in size, ranging say, from 1 or 2 microns to approximately 20 microns, then liberation can also be achieved by ultra fine milling (UFM). The Western Australian goldfields, like many others, contain numerous orebodies which are refractory in their primary zones. The mineralogy of these ores shows great variation, as does the gold recovery by various processing options. [2] The 'best' process can only be chosen with confidence after a careful metallurgical evaluation programme based on good sampling, process mineralogy and testing various processing options; eg UFM, biological oxidation, pressure oxidation, roasting, etc., coupled with environmental and economic factors. For the past few years, Ammtec has been involved in numerous refractory gold projects. This paper presents some of the results from these process development programmes.