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.

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