The leaching and adsorption behaviour of gold ores

© 2000 Dr. Kevin Lee ReesThere are many non-idealities in gold processing which are becoming increasingly important as simple free-milling oxide ores are depleted. During cyanidation and adsorption onto activated carbon, these effects include competing metal-cyanide complexes, preg-robbing and the mineralogy of the ore. This thesis considers these areas from an experimental and modelling viewpoint. In this research it is shown that a variety of metal-cyanide species can be used to leach gold from certain refractory ores. A mechanism is proposed which involves the dissociation and precipitation of these metal-cyanide complexes, where the cyanide from the dissociation of the complex is used to leach gold from the ore. The stepwise dissociation of cyanide from copper-cyanide complexes can also be used to leach gold from a variety of oxide and sulphide ores. The copper cyanide's fourth ligand readily dissociates at low cyanide concentrations for use in gold leaching. The extent of preg-robbing onto mineral surfaces was also shown to be related to the metal-cyanide complexes in solution, with these complexes serving to inhibit preg-robbing. However, free cyanide was found to be much more important in preventing preg-robbing. This phenomenon occurs in cyanide-deficient solutions and the extent of preg-robbing is related to the cyanide consuming ability of the ore, rather than simply the ore's mineralogy. Activated carbon was found to compete strongly with the ore for aurocyanide in solution, although when no cyanide was present the kinetics of the two processes governed the distribution of gold between the ore and the carbon. Gold was found to reduce at the chalcopyrite surface and a mechanism for this reduction was proposed. For pyrite, a combination of reduction and physical adsorption was observed. The effect of preg-robbing on the enhanced extraction of gold in the presence of activated carbon was closely examined for a number of different ore types. For the Telfer oxide ore, which was not found to be preg-robbing, enhanced gold extraction was observed at low cyanide concentrations, but not at high concentrations, as the gold was completely extracted by cyanide alone. The flotation feed and copper concentrate showed an enhancement in gold extraction due to the activated carbon adsorbing gold that would otherwise have been preg-robbed. However, this mechanism did not explain the behaviour of the pyrite concentrate. For this ore the enhanced extraction was due to a combination of equilibrium and preg-robbing effects. Diagnostic leaching was used to show that mineralogy was not an important factor in the enhanced gold extraction in the presence of activated carbon for the ores. Kinetic models were developed for the leaching of gold from the ore, adsorption onto activated carbon and preg-robbing at the ore surface. A variable order empirical model of the Mintek form was used to describe leaching, while adsorption was modelled using the film-transfer model. A model of this form was also used for preg-robbing using the Freundlich isotherm to describe the equilibrium between the solution and the surface of the ore. These models successfully accounted for the individual sub-processes and when combined into an overall batch model were used to show the effect of preg-robbing on gold extraction from ores of different mineralogy. The effect of preg-robbing was found to be closely related to the kinetics of the preg-robbing and adsorption processes. This batch model was extended to a continuous model that simulated three leaching and seven carbon-in-Ieach tanks. The effect of processing the oxide and sulphide ores studied was examined and the model shown to satisfactorily describe the extraction of gold from the Telfer oxide ore. A sensitivity analysis of the effect of process changes on this ore was then undertaken, where the correct operating response to preg-robbing ores and process changes were determined