P.L. Breuer

Thiosulfate leaching kinetics of gold in the presence of copper and ammonia

Abstract In recent years a significant amount of research has focused on thiosulfate as an alternative lixiviant for gold. However few fundamental studies have been undertaken due to the complexity of the solution chemistry. This study investigates the effect of various parameters on the gold leaching kinetics and the undesirable homogeneous copper(II) reduction by thiosulfate. The leach kinetics are measured at essentially constant solution conditions using a rotating electrochemical quartz crystal microbalance, which measures mass changes in real time and hence leach rates within minutes. The gold leaching kinetics were found to be chemically controlled and the rate was found to decrease with time as the copper(H) concentration decreased. This can be attributed to passivation of the gold surface by the products of the copper(II)-thiosulfate reaction. Optimum leach conditions derived from this study are 0.4M ammonia, 0.1M thiosulfate, pH > 11.4, and temperature at 30°C.

An electrochemical study of gold leaching in thiosulfate solutions containing copper and ammonia

The electrochemistry of gold in thiosulfate solutions containing copper and ammonia was studied using a combination of standard electrochemical techniques and the rotating electrochemical quartz crystal microbalance. It was found that the cathodic half reaction, the reduction of copper(II), occurs readily in the potential region where gold should be oxidised to gold thiosulfate. However, the gold oxidation half reaction in thiosulfate solutions alone is hindered; the presence of ammonia is required for the reaction to occur at an appreciable rate. It was also shown that copper(II) affects the gold oxidation half reaction; this is why copper(II) is more effective than other oxidants at leaching gold in thiosulfate solutions. One complication with using copper(II) as an oxidant is that it also reacts with thiosulfate, which results in a decrease in copper(II) concentration. Such a decrease in [Cu(II)] was shown to cause a decrease in the rate at which the gold oxidation half reaction proceeds. The effect of temperature and thiosulfate concentration on the oxidation of gold was also investigated.

The reduction of copper(II) and the oxidation of thiosulfate and oxysulfur anions in gold leaching solutions

Due to the increasing environmental and public concerns associated with cyanide leaching of gold, much of the current research in gold hydrometallurgy is focussed on alternative systems, such as thiosulfate. One of the major problems with the thiosulfate leaching system is the undesirable reaction between thiosulfate and copper(II). The current paper presents a study on the effect of various anions on the rate of this reaction. It will be shown that the presence of anions such as sulfate, chloride and phosphate significantly reduce the rate at which copper(II) is reduced by thiosulfate. The presence of other oxysulfur species such as sulfite, tetrathionate and trithionate were also studied. These species were found to be readily oxidised by copper(II) and hence greatly increased the rate of copper(II) reduction. The rate of copper(II) reduction by tetrathionate is relatively unaffected by the addition of anions; however, anion addition was found to significantly influence the rate of copper(II) reduction by sulfite and trithionate. Based on these results, inner sphere mechanisms were proposed for the reaction of copper(II) with thiosulfate, sulfite and trithionate. The kinetics of copper(II) reduction is thus very complex and can be represented by a number of parallel-series reaction sequences.

Copper catalysed oxidation of thiosulfate by oxygen in gold leach solutions

The environmental and public concern over the continued use of cyanide in the recovery of gold has grown in recent times due to a number of recently publicised environmental incidents. Of the alternative lixiviants, thiosulfate appears to be the most promising, though the considerable amount of research conducted on thiosulfate leaching of gold over the last three decades has not resulted in its commercial introduction. Perhaps the largest contributing factor to this is the poor understanding of the thiosulfate leach solution chemistry, especially the oxidation of thiosulfate in the presence of copper(II) and oxygen. It has been shown in this research that the oxidation of thiosulfate in the presence of copper(II) and oxygen is very complex with the rates of copper(II) reduction and thiosulfate oxidation being significantly faster in the presence of oxygen. The higher initial rate of copper(II) reduction indicated that oxygen increases the rate of copper(II) reduction to copper(I) by thiosulfate, though the mechanism for this remains unclear. The rates of thiosulfate oxidation and copper(II) reduction were also shown to be affected differently by the presence of anions. This is consistent with thiosulfate oxidation occurring via two mechanisms, with one of these mechanisms involving the oxidation of thiosulfate by copper(II) and the other involving the oxidation of thiosulfate by the intermediate superoxide and hydroxide radicals formed as a result of copper(I) oxidation by oxygen. The effect of various parameters on the rate of thiosulfate oxidation and the copper(II) concentration are also shown.

The cyanide leaching of gold in solutions containing sulfide

Abstract A large proportion of the gold processed in the 21st century will be recovered from sulfide ores. Since the sulfide minerals are to some extent soluble in cyanide solutions, there will always be some sulfur species present in the leach solution. It has been shown that soluble sulfide hinders the rate of gold leaching, forming a protective layer of the type Au/Sx. Electrochemical studies of the constituent half reactions: gold oxidation; and oxygen reduction, were consistent with this view. The effect of sulfide and cyanide concentrations on the leaching reaction were investigated, and it was shown that the sulfur formed is chemically attacked by cyanide, resulting in higher leach rates at higher cyanide concentrations. The effect of lead was also studied and is discussed in detail.

The impact of thiosulfate oxidation products on the oxidation of gold in ammonia thiosulfate solutions

Abstract The continued use of cyanide in the recovery of gold is being questioned due to environmental and public concerns. The present paper describes research into the gold thiosulfate leaching system, which is considered to be the most promising alternative to cyanide. The thiosulfate system is complicated by the homogeneous reaction between two of the reactants, copper(II) and thiosulfate. This reaction produces copper(I) and polythionates. It is generally accepted that oxygen addition is thus required in order to regenerate copper(II) from copper(I). Despite this, no one has previously studied the effect of oxygen addition on the kinetics of gold leaching in thiosulfate solutions. This paper presents a study on gold leaching in thiosulfate solutions containing oxygen. It will be shown that the leaching reaction is severely hindered as the reaction between thiosulfate and copper(II) occurs in the presence of oxygen. It is postulated that an intermediate product of the oxidation of thiosulfate by copper(II) in the presence of oxygen is responsible for the slow leaching kinetics. This effect was not observed when oxygen is excluded from the solution. Disulfite (S 2 O 5 2− ) was identified as a possible cause of this effect, as experiments performed with the addition of disulfite also resulted in very low gold oxidation rates.

The importance of controlling oxygen addition during the thiosulfate leaching of gold ores

Abstract This paper presents a study of the copper–ammonia–thiosulfate system for the leaching of gold. The major problem with the thiosulfate system is that for solutions containing copper(II), ammonia and thiosulfate, the chemistry is complex and continually changes due to the homogeneous reaction between copper(II) and thiosulfate. The present paper highlights the effect the changing solution chemistry has on the gold leaching reaction. It will be shown that the gold leach rate decrease as the reaction between copper(II) and thiosulfate proceeds. Although the copper–thiosulfate reaction can be affected by the addition of species such as phosphate, little improvement to the leach kinetics were observed. Thus, the use of oxygen to reoxidise copper(I) to copper(II) was studied, and it will be shown that the oxygen addition rate is critical to leaching, as too much oxygen results in very slow leaching kinetics. Experiments conducted using gold powder and an oxide gold ore showed a similar trend.