These ores, in which the gold is locked into pyrite or arsenopyrite, or present as tellurides, are becoming more important. The Kalgoorlie ores in particular, contain a significant amount of gold tellurides. Thus current studies at Murdoch University on the reactivity of gold tellurides are very pertinent [10]. Gold tellurides prove to be unreactive to CN-, though reactive to CI2, and again show evidence of gold and tellurium oxide passivating films. Their electrochemistry proves to be quite challenging [10] . Normally, however, roasting decomposes the tellurides but there is some evidence which suggests that the volatility of telluriates leads to gold plating in the roaster.

In general, refractory gold ores may be treated in a number of ways and over the past few years, 5 approaches have been taken, all with varying degrees of success depending on the ore.

For some ores liberation is no problem and direct cyanidation works well if the pyrite is finely ground. But traditionally, these ores are simply roasted to oxidise the iron and arsenic and liberate the finely dispersed gold for cyanidation. However, the oxidation roast may not liberate the gold fully, or give unexpectedly low gold recoveries. In Western Australia, investigating this problem which is believed to depend upon the roasting temperature, salinity and degree of sintering or porosity of the calcine. It has been found that the calcine becomes less porous as the roasting temperature increases. This is backed up by recent work which showed that regrinding the calcine led to a greater % Au recovered by cyanidation. This aspect is being followed up on local calcines. Furthermore, recent work indicates that the salinity of the ore or ground water may lead to the formation of volatile gold chlorides in the roaster and gold losses in the fume or on the roaster walls.

gold_ore

The problem with oxidation roasting is that it is exothermic and therefore difficult to control the temperature. Thus another approach currently under study is the thermal decomposition of pyrite to pyrrhotite under controlled

conditions. This produces S and avoids the SO2 problem from the roasters. A well developed porous structure is readily observed when arsenopyrite and pyrite crystals are thermally decomposed to remove sulfur.

Much overseas interest has focussed on pressure leaching and bacterial leaching as a means of oxidising the pyrite under controlled conditions to liberate the free gold. Pressure leaching, developed largely by Sherritt in Canada, is no doubt fast and efficient and leaves an insoluble iron oxide or iron arsenate residue. However, bacterial leaching, whilst slower, is a rapidly developing new technology which is promising for certain ores. Initial bacterial leaching studies has now lead to the establishment of a local company, to test and evaluate the process commercially on Australian concentrates.

A comparison of these processes carried out on the Porgera deposit [26] showed pressure leaching to give the highest gold recovery.

In general, the amount of gold leached by cyanidation after oxidation is directly proportional to the degree of oxidation of the sulfide.

Oxidation roasting often gives low values due to sintering of the calcine, whilst bacterial leaching sometimes gives low values because of the mineralogy of the ore. Pressure leaching proves to be most efficient – but it is usually the most expensive option.

As shown, pressure leaching gives high gold recoveries from a variety of ore types, whilst direct leaching variable poor recoveries.

Optimum conditions: 170–190CC 1500-2000 kPa O2

However as pointed out the process of choice may be determined not only by direct capital and operating costs but other factors such as environmental pollution, technological risk, by-product values, and safety – when dealing with arsenical ores.

Leaching free gold with Cyanide

The introduction of carbon-in-pulp technology to the gold industry has rekindled the dormant research and development interests in gold metallurgy. Over the past decade many new developments have emerged which have capital’ :ed on the advances in science and technology in other disciplines. Five years ago, Australian producers were catching-up with the new C.I.P. technology and learning of its idiosyncracies. They quickly realised that Australian conditions did not match those of South Africa and America and that each plant or ore body needed its own refinement Since 1982 several workshop courses on C.I.P. Technology have been presented in South Africa and advances have been made both overseas and in Australia which broaden the scope of investigations as producers struggle to cope with the decreasing grades and increasing complexity of gold ores [4]. Through the Australian Minerals Industry Research Association (AHIRA) and the Western Australian Mining and Petroleum Research Institute (WAMFRI), the industry has funded various applied research projects into various areas of processing – including carbon fouling and regeneration, and gold elution.

Gold metallurgy covers many facets including leaching, concentration on carbon, elution and recovery. This paper attempts to briefly cover each aspect in turn with particular regard to Australian contributions and future directions and developments.

Although leaching with cyanide is not a recent advance, it illustrates the importance of linking fundamental research with plant practice which is essential to all facets of the process. Leaching with cyanide is one aspect which metallurgists have successfully applied for over 100 years yet it is only in the last 20 years that scientists have been able to explain why leaching with CN- is so slow and varies from ore to ore. It is now established that the gold leaching rate is usually diffusion controlled, but depending on conditions, different factors such as CN~ and Oz diffusion, or Oz reduction on the mineral may limit the rate. It is not possible to increase the rate significantly because gold is passivated by cyanide or oxide films.

gold_metallurgy

The mechanism established by Cathro and Koch at CSIRO over 20 years ago involves separate anodic and cathodic processes where gold dissolves via passivating films at one site, and oxygen is reduced at other sites (Scheme 1). Of particular interest was their observation that certain heavy metal impurities can enhance the rate by disrupting the film. This mechanism has been further refined by Nicol in recent years.

The electrochemists can show this heavy metal effect by looking at the corrosion currents of gold at particular potentials. Only trace concentrations of Pb and Hg {~10-6 M) are required to enhance the rate of gold dissolution when typical operating potentials are applied. (Figure 1).

The metallurgist now knows that the use of more CN~, H2O2, O2 or pressure may not give much improvement in rate due to the passive film on gold and that heavy metal additives may help. However many pulps already contain trace heavy metals or may scavange them from solution; thus each ore has to be tested. At the Kambalda plant, for example, Pb(NO3)2 is added to the leach tanks, but it is not clear how effective it is because the presence of sulphate in the water leads to precipitation of PbSO4.