A kinetic study that compares the leaching of gold in the cyanide, thiosulfate, and chloride systems
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I. INTRODUCTION
GOLD is an integral part of the Australian economy, being one of the country’s highest export earners. For the last 100 years, cyanidation has been the dominant means of recovering gold from its ores. However due to the worldwide environmental and public concerns over the use of cyanide in gold extraction, it has become difficult to obtain approval to construct new leaching plants. Hence, there has been a lot of attention given to the study of alternative lixiviants in recent years. Although thiosulfate leaching appears to be the most promising of the alternatives, there is still interest in the application of the chlorine/chloride system. Not surprisingly, there have been a number of publications on the use of either thiosulfate[1–4] or chlorine/chloride[5–8] to extract gold from its ores. However, there have only been a limited number of fundamental kinetic studies of gold leaching in these alternative systems. In particular, there has been little emphasis on comparing the gold-leaching kinetics with those achievable in the traditional cyanide system. In any event, it has recently been reported that the dissolution of gold in cyanide solutions is much more complex than many people believe, with the leach rate of gold being critically dependent on trace amounts of impurities in either the gold or the leach solutions.[9] Since naturally occurring gold contains various amounts of silver, measuring gold-leaching kinetics with pure gold provides misleading results; more representative results are obtained by utilizing a gold sample containing small amounts of silver.[10] The aim of the present article is twofold: (1) to determine whether the leaching behavior of gold in the thiosulfate and chloride systems is affected by the purity of the gold sample; and
(2) to derive conditions under which the leaching rate in alternative systems is similar to that of the cyanide system. II. EXPERIMENTAL METHODS Mass changes were measured using the rotating electrochemical quartz crystal microbalance (REQCM), which is shown in Figure 1. This technique is the most accurate means of measuring metal leaching rates; the sensitivity is 1 ng, and the reproducibility in measuring gold leaching rates is within 5 pct. The details of the oscillation circuit and frequency measurement are described elsewhere.[11] All experiments were carried out using solutions prepared from analytical-grade reagents and millipore water. All experiments were performed at 20 ⬚C and at a rotation rate of 300 rpm, unless otherwise specified. Prior to each experiment, gold was electroplated onto the platinum-coated quartz electrode at 25 A m⫺2 from a solution containing 0.02 M potassium dicyanoaurate, 0.23 M potassium cyanide, and 0.086 M potassium carbonate. The gold/silver alloy was prepared by the same procedure, with the exception that 0.5 mM silver nitrate was added to the solution, resulting in a deposit containing approximately 5 pct silver by mass (which, for simplicity, will be referred to as gold/silver). For the gold thiosulfate system,
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