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Recovering Cyanide from Gold Plant Tailings Dr SGS MINERALS SERVICES TECHNICAL PAPER 2003-02 2003 THE ECONOMIC AND ENVIRONMENTAL CASE FOR RECOVERING CYANIDE FROM GOLD PLANT TAILINGS DR. CHRIS A. FLEMING, PH. D. –– SGS ABSTRACT The ability to recover cyanide from gold plant tailings has been known almost as long as the cyanidation process has been practiced, but there has been little incentive to process tailings in this way in the past. With the development of technologies that make it possible to recover and recycle free or complexed cyanide directly from gold plant slurry tailings, which indicates, on paper at least, significant cost savings compared to cyanide destruction, coupled with the introduction of legislation to seriously limit the discharge of cyanide to the environment in most gold-producing regions of the world, many companies are evaluating the cyanide recovery alternative. There is arguably an even more compelling reason for mining companies to consider cyanide recycling. With the growing storm of negative public opinion that the use of cyanide in the mining industry is attracting – following several highly publicized spills over the last couple of years – the time has come for the gold industry to demonstrate environmental diligence and stewardship in the use of this commodity that is so vital for their industry. There is no doubt that the widespread implementation of cyanide recycling will reduce the impact of the cyanidation process on the environment, both by reducing the risk of spills (with less cyanide being transported from manufacturing plants to gold mines), and by reducing the loading of toxic and nontoxic metals and ions in the tailings. This paper briefly discusses the pros and cons of the different processes that are available for the treatment of gold plant tailings to regenerate and recovery cyanide for recycling, and presents a technoeconomic argument for the incorporation of this technology into many gold plant flowsheets today. INTRODUCTION of the general public for the gold form in which it exists in gold plants, mining industry, due to several highly to hydrocyanic gas, HCN. The negative publicized cyanide spills in recent years perception of this highly toxic gas, Interest in the recovery of cyanide (breaches of tailings dams, accidents and concerns about the ability of the from gold and silver plant tailings has involving trucks carrying cyanide to gold mining industry to handle it safely in heightened in recent years. This plants, etc.). a processing plant, has probably been interest has been spurred by several the single largest impediment to the factors: The cost of recovering and recycling implementation of cyanide recovery • Increasingly stringent regulations cyanide from tailings will generally throughout the gold producing world, technology in the gold industry. In this be lower than the cost of purchasing governing discharge limits for free and regard, the mining industry can gain new cyanide. When this cost benefit total cyanide to tailings ponds and the confidence and experience from the is added to the lower cost of tailings environment. chemical industry, which manufactures detoxification, a strong case can • The increasing cost of active chemical sodium and calcium cyanide for the gold frequently be made for cyanide recovery. treatment versus passive (natural industry, and handles large quantities of degradation) treatment of tailing, to HCN gas routinely and safely. In most cases, cyanide is recycled by bring them in line with environmental converting it from the free or complexed regulations. • The worldwide trend to processing of more complex gold ore bodies, which is usually accompanied by higher levels of cyanide consumptions and greater concentrations of cyanide (particularly the copper cyanide complex) in tailings. • The growing negative perception SGS MINERALS SERVICES TECHNICAL BULLETIN 2003-02 2 If the cyanide is present in the tailings scaling problems, such as encountered ion exchange is based on the water- as free cyanide (pKa = 9.4), it is possible at DeLamar Mine in Idaho, USA and balance in the plant. If preconcentration to convert >99% of the cyanide to HCN more recently at the Cerro Vanguardia is unnecessary, or is achieved by ion gas by lowering the pH of the tailings to plant in Argentina (Radcliffe, 2000). exchange, circumventing volatilization about 7. • Operating costs are lower for cyanide results in a simpler, safer and lower-cost recovery plants treating solution than plant. CN- + H+ <-> HCN (1) those treating pulp. The main operating cost is the sulphuric acid consumed Most of the process options for cyanide If the cyanide is present as a metal in lowering the pH to the desired final recovery have been tested at SGS cyanocomplex (Cu,Zn,Ni,Co etc), the pH pH value. When treating solutions, this Lakefield Research over the last few must be reduced to more acidic values consumption is close to stoichiometric years, in the course of several laboratory (which vary with the strength of the (i.e. half a mole of sulphuric acid per and pilot plant investigations. Cyanide complex) to break down the complex mole of cyanide). When treating pulp, tailings solutions from several operating and produce HCN gas. The addition of the acid consumption can be 2 to 10 gold plants and new exploration sulphide ions enhances this reaction by times higher, depending on the final properties have been tested, and the forming metal sulphide precipitates. The pH required and the acid-consuming results of these investigations were chemistry of cyanide recovery from constituents in the ore. published recently (Fleming, 2001). The metal cyanide complexes has been • When the main source of cyanide discussion below is drawn from these reviewed recently by Fleming (2001). consumption is the reaction to form experiences. copper cyanide complexes in solution PRACTICAL CONSIDERATIONS (as is frequently the case), an added PROCESS ALTERNATIVES benefit of treating solution rather An important consideration when than pulp is the opportunity afforded DIRECT RECOVERY WITHOUT evaluating process options for the to easily recover and sell the copper PRECONCENTRATION, BY TAILINGS precipitate. The sale of these by- treatment of the gold or silver plant RECYCLING tailings for cyanide recovery, is whether products can have a significant positive In order to ensure good leaching kinetics to treat the tailings directly as a slurry, impact on cyanide recovery economics. and high overall gold recovery, it is or to first separate the solids and liquids always necessary to add more cyanide and treat only the liquid phase. In making Despite the many positive attributes during leaching than will be consumed in this decision, the following factors must of treating solutions rather than pulps, the leaching process. This excess cyanide be taken into consideration: the cost and efficiencies of solid/ reports to the tailings as uncomplexed • The cost of solid/liquid separation. liquid separation will render cyanide or free cyanide, and it is often possible If the solids and liquids have already recovery from solution unattractive in to recycle a portion of this cyanide at been separated (in Merrill Crowe or some cases. In these situations, resin minimal cost. A number of gold plants heap leaching, for example) this is not a in pulp processes offer the best solution around the world have adopted this factor. If they have not been separated, and examples of this technology are approach. the cost of solid/liquid separation discussed below. must be included as part of the cost The basic requirements to achieve this to recover cyanide, and this can be In fact, ion exchange technology objective are firstly to recycle solution significant, particularly for high-clay, should always be considered, even rather than slurry, and secondly, to slimy or viscous slurries. In most cases, when treating solutions for cyanide satisfy an overall water balance in the the washed solids will still have to be recovery, as the resin processes will plant. These requirements are met quite treated by a detoxification process usually upgrade the strength of the naturally in a heap leach operation, where to remove residual soluble cyanide cyanide solution 20 to 100 fold prior to the residual cyanide in the pregnant species. acidification/precipitation. This in turn will solution emerging from the bottom of • Cyanide recovery by direct acidification lower the overall capital cost and improve a heap is recovered by simply recycling of tailings, followed by volatilization safety in the final cyanide regeneration this solution (after gold and/or silver and reneutralization of the HCN gas steps of the process. In many cases, the recovery) back to the top of the heap. It (known as the AVR process) is very cyanide strength in the eluate from an is not so simple in a milling operation, much faster and more efficient from ion exchange process will be sufficiently but some of the residual free cyanide solution than from pulp, requiring lower high for direct recycling to leach, can be recovered by thickening the mill volumes of air flow per unit volume of circumventing the need for volatilization slurry prior to leaching, and again tailings treated and smaller equipment. of HCN (AVR). thickening the leach plant tailings prior to Capital costs per unit volume of tailings discharge. For example, if the feed and feed are therefore lower for an AVR The volatilization part of the AVR discharge from cyanidation/gold recovery plant treating solution than for one process is a relatively high capital cost can be thickened to 60% solids, and the treating pulp. In addition, attempts to component of the overall process. leach/gold recovery operation is implement the AVR process on tailings The decision on whether or not to conducted at 40% solids, the net result slurry have been dogged by severe preconcentrate by volatilization or is that about half of the free cyanide in SGS MINERALS SERVICES TECHNICAL BULLETIN 2003-02 3 the tailings is recovered and recycled.
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