FIT-FOR-PURPOSE PRECIOUS METALS LEACH SYSTEMS: MATCHING LEACH STRATEGIES TO SOURCE CHARACTERISTICS

摘要

The gold industry has been supporting research into alternative lixiviants over a number of decades. However, it may sometimes appear to the casual observer that the industry is no closer now than it was 20 years ago to implementing a feasible solution at multiple sites. Although one industrial calcium thiosulfate plant has been commissioned, not many new processes have progressed beyond pilot scale. Often the problem lies in the fact that one-size-fits-all lixiviants are sought and it is clear that such a panacea does not exist. Ore mineralogy and chemistry, environmental conditions and water availability, mobilisation of toxic deleterious elements and creation of toxic byproducts remain challenges, over and above the general required attributes such as sufficiently fast kinetics, low reagent consumption, low reagent price, reagent and gold complex stability, and the need to recycle reagent which mostly involve expensive solid-liquid separation equipment. Moreover, as leaching is only one part of the process, and metal recovery from solution, reagent recovery and recycle, waste bleed streams and downstream recovery, refining and waste treatment processes need to be technically and economically feasible while still offering lowered environmental and safety risks. Cyanide based processing has been particularly robust given a combination of the requirement of limited (if any) piloting required, biodegradability in tailing impoundments, and a process that is well understood by engineering companies (yet complexities remains often underestimated due to poor understanding of reagent-gangue mineral interactions).This paper will explore a few of alternative lixiviants and contrast it to cyanide leaching. It will look at the attributes that modern lixiviant systems need to have to make them technically, environmentally and economically attractive from an overall processing perspective. It will be shown that an intelligent and engineered approach is required in modern gold extraction. In many cases cyanide may still suffice, but a fit-for purpose lixiviant evaluation is required to ensure that leaching processes are optimal for the ore, the site, the environment and the process economics. One of the key challenges in modern processing engineers is the lack of understanding of the overall process chemistry, mineralogy and by-product formation and a lack of risk appetite for process chemistries that are not well understood. Often process engineers are more comfortable with changes in mechanical process technology than changes in the chemistry that underpins the process. This risk aversion to changes in process chemistry is even more pronounced amongst mining engineers, geologists and mining company executives making the selection of alternative process chemistries a hard sell up the management chain and subsequently to company shareholders where analysts' views are even more conservative.