INTRODUCTION The widespread use of cyanide in gold ore processing is a fact of life. Up to 1M tonnes are consumed each year in thousands of gold mines around the world. In many countries illegal miners obtain cyanide on the black market to process hand mined ores. The leach effluent is discharged directly to the nearest river. Unfortunately, this practice is not limited to small mines: Some larger operations discharge effluents intentionally, some in a catastrophic unintentional accident. These incidents simply help perpetuate the cyanide myth. Cyanide is hazardous; but it is the best and cheapest known solvent for gold and silver. The present recommended compliance levels for discharge to TSF impoundments are not always guaranteed to protect wildlife, and the cost of detox can be a deterrent to good practice. The simple answer is to solve the problem at the source, i.e. by thoroughly detoxifying in the process plant to prevent downstream problems. Today, there are numerous processes that involve partial recovery or destruction techniques, but they come at a significant cost, and don’t always give satisfactory results. Some processes also add metals such as copper or zinc to the circuit. This paper describes a commercially proven process for solving this problem, a process that can provide significant environmental and operational cost benefits. THE CASE FOR CYANIDE, CARBON AND RECYN Cyanide is an excellent solvent, not only for non-refractory gold and silver, but also for some copper minerals. This has been seen as a negative attribute, especially for high copper ores where cyanide consumption is increased and it complicates the detox process. If the copper can be economically recovered, along with the cyanide, then being non-selective can be turned to an advantage. Gold/silver ores rely on a good solubility for both metals, which is favored by high cyanide levels, particularly for silver. Often cyanide levels are restricted to keep down consumption and detox costs, but this can also reduce metal recovery levels. By eliminating detox costs, cyanide levels can be increased to better suit metal recovery economics. The effort to find a replacement for cyanide has persisted for over 100 years. The hope has been to find a less problematic solvent, which is more publically acceptable. No acceptable alternative has been found, failing due to being less efficient, less economic, not environmentally better, or often a combination of all three deficiencies. We can more easily accept the superior efficiency of cyanide if we solve the environmental concern and turn its non-selectivity to an advantage. An adjunct to the non-selective “problem” of cyanide is the search for a selective adsorbent. Carbon is partially selective and has a fairly complex elution process. For most gold circuits, carbon is currently considered the best choice. It has a proven performance record and robust treatment methods. The most common alternative is a gold specific resin, but this is also not so applicable to gold/silver ores, or if copper is to be simultaneously recovered. Resin is the best alternative if preg-robbing ores are processed or problem levels of mercury are encountered. Having chosen a non-selective solvent and partially selective adsorbent we are left with the challenge of cleaning up what is left solubilized in the tailings. Here the answer is more obvious, a non-selective resin .
ALTA 2017 Gold-PM Proceedings
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