PROCESSING OF REFRACTORY GOLD ORES

During the last ten years, increasing attention has been given by operating companies, research organizations, and engineering and consulting companies to the recovery of precious metals from refractory ores. More ore bodies have been found where the gold is not amenable to simple cyanidation alone, and low recoveries have resulted from metallurgical testwork. In the simplest of terms these are refractory ore bodies. Reasons for ores being refractory will be highlighted. In addition to deposits that are totally refractory, another increasingly common occurence is where the surface ore body is well oxidized and non-refractory and frequently overlays large tonnages of non-oxidized refractory ore. In these cases, the initial metallurgical operation will be conventional, with an 'add-on' refractory treatment process at a later stage. This paper will review some of the existing techniques and operations that are being used for the treatment of refractory gold ores, with particular emphasis on the techniques that are well established, such as Roasting and Pressure Oxidation, together with the developing technology of Biological Oxidation. Part of the paper will discuss costs. It is becoming increasingly apparent that pretreatment can be very expensive and as more plants are built and operated, better cost data is becoming available. Approximate and relative capital and operating costs will be discussed and their major components defined. Other important issues that will be addressed are the effect of preconcentration (such as flotation ahead of the pretreatment stage), on project economics and the importance and cost of neutralization and effluent control. The best choice of a processing route to treat a refractory ore will in most cases be site specific, due mainly to the metallurgical and mineralogical characteristics of the ore. The successful application of these processing techniques can result in mining operations that would otherwise have been impossible. However for an increasing number of low grade refractory ores, the estimated capital and operating costs mean that very careful project evaluation will be necessary, particularly at current gold prices.