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The now extinct Us Bureau of Mines conducted numerous studies of gold in pyrite, and sulfide ores over the years, until their untimely demise in the 1990's. Much of the referenced studies and case applications discussed in this brief article came from USMB reports, which even after they ceased to exist, still continue to provide some good scientific information to those working in the mining and mineral processing industries. Sulfide ores, and pyrites in particular, have caused and continue to cause difficulty in recovering the gold values from these ores. In most instances, visible gold can not be seen under microscopic examination of pyrite gold ores. However, if the ore is ground to -200 mesh, most times, a few specs of gold can be visibly seen under microscopic examination. Based upon numerous case studies, it appears that gold particles in pyrite are generally fine, ranging from 75 microns down to 2 or 3 microns. The incidence of fine gold's quantity also appears to be related to the gold concentration, as well. If the assay is 2 ounces per ton, there are generally larger gold particles present along with the typically fine micron sized gold. When the grade diminishes to say, 0.2 ounces per ton, there are generally only micron sized particles associated with the pyrites in the ore. This is not absolute, but it does appear to be the trend. Recovery of gold in sulfide ores has fallen into several categories. First is froth flotation, and second is cyanidation of the ores. Using cyanide to recover gold from sulfide ores generally results in 30 to 35% recovery, and the best I have heard of is around 50% recovery. The fine coating of a iron compound definitely appears to be a key component of the inability of cyanide to efficiently leach low grade pyritic ores. Perhaps there is some room here for a pretreatment, to remove this iron coating and then make the fine particles susceptible to cyanide leaching. To my knowledge, this has never been attempted. Generally speaking, cyanide leaching of low grade pyritic ores is economically unsatisfactory in today's market. Low grade ores that do not respond well to cyanide or flotation are generally referred to a "refractory" ore. Which usually means that it needs to be roasted to release the gold. Roasting has to be one of the most expensive methods of recovering gold with current environmental considerations, and is usually cost prohibitive, except in a few circumstances, where grade and volume justify the economics. Gravity concentration tends to recover the pyrite with the gold, and only removes the lighter minerals, such as quartz, from the ore. The most difficult particles to gravity concentrate are the fines, and 75 microns to 2 microns are definitely very fine. So, even if the ore were ground to 2 microns, it may not be recoverable using current gravity technology. That appears to leave froth flotation as still the best method of recovering the gold from pyritic ores. Sulfide ores, such as chalcopyrite, sphaelerite, galena, and pyrrhotite, and mixtures of these ores have historically been found with micron size gold particles included in them. This generally involves frother, several collectors (promoters) and possibly some modifying or depressing agents as well. Geologists and mineralogists have many theories why and how the gold occurs with the pyrite, and I will not enter into any of these areas, since I am concerned with liberating the gold from the ores. I will say that from my understanding, many seem to agree that in sulfide gold ores, the occurrence of gold appears to occur as a replacement of other minerals, possibly iron. One USBM paper summed up five noticeable characteristics of auriferous pyritic ores. The gold occurs as tiny flakes on the crystallographic planes of the pyrite. The gold flakes are very small in size, 5-10 microns. The pyrite in which small amounts of gold occurs is of crystalline variety (primary pyrite). The characteristics of primary pyrite are a absence of porosity, an extreme brittleness, a resistance to oxidation, and the existence of gold possessing a weak susceptibility to magnetism (due to a fine coating of a iron compound). My theory is that if some economical pre-treatment in a heap leach situation, could nullify the effects of the iron coating, chemically, then the pyritic ore could be effectively and economically leached with cyanide in the heap leach. This would make the recovery of gold from pyritic sulfides economical, and negate spending hundreds of millions of dollars on roasting plants. But in the end, it all comes down to cost, which would be the most cost effective. We know roasting is almost prohibitively expensive, so I would think that treating in a heap leach, prior to cyanidation, would be much more cost effective. Charles Kubach, Mining & Mineral Processing Engineer |
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