Browsing by Author "Van der Merwe, Pieter F.(Pieter Francois)"
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- ItemFundamentals of the elution of gold cyanide from activated carbon(Stellenbosch : Stellenbosch University, 1991) Van der Merwe, Pieter F.(Pieter Francois); Stellenbosch University. Faculty of Engineering. Department of Process Engineering.ENGLISH ABSTRACT: Most research work on activated carbon systems has been concerned only with the adsorption process. Nevertheless, controversy still exists on the mechanism by which metal cyanide complexes such as Au(CN)2 are adsorbed onto activated carbon, and whether these complexes are decomposed to irreversibly adsorbed species, such as AuCN and Au( 0l. As these species determine directly the need for cyanide during the elution process, the first part of this thesis was devoted to a study of the reversibility of adsorption of metal cyanides on activated carbon and the ·role that oxygen plays in the adsorption reactions. It was found that the formation of AuCN was mainly a function of the type of carbon used and that oxygen did not promote the decomposition of Au(CN)2 to AuCN. Although the dissolved oxygen level affected the equilibrium loading of anionic metal cyanides, it did not participate directly in the adsorption reaction. Au(Dl was formed only under drastic conditions of low pH's and high temperatures. It was now possible to select a carbon on which no irreversibly adsorbed species were formed in order to study the mechanism of the desorption of- gold cyanide and to determine the need for cyanide in the elution process. Although the loaded carbon contained only Au(CN)2, the presence of cyanide was found to drastically enhance the elution of the gold. It was shown that the cyanide present in the elution step of an AARL elution was less important than the cyanide in the pretreatment solution. It was concluded that the cyanide passivates the carbon for the adsorption of anionic metal cyanides by reacting with the functional groups on the carbon surface. However, the presence of high concentrations of cations promotes the formation of neutral ion-pairs with the anionic metal cyanide on the carbon and thereby hinders the desorption of the metal cyanide in the pretreatment solution. Once the bulk of the cations are removed in the elution stage, the desorption of the gold increases rapidly. As the concentrations of cyanide and the spectator cations, as we 11 as their reactions, were found to affect the gold cyanide equilibrium, these factors had to be accounted for in modelling the gold elution process. The system was simplified by using only one cation, namely potassium. Models were then developed to simulate the changes in· cyanide and potassium concentrations during the elution of the gold cyanide. This enabled the calculation of a shifting gold cyanide equilibrium isotherm as a function of time and position in the column. The elution of the gold cyanide was found to be diffusion controlled only under conditions of weak desorption. Under these conditions, the desorption of the gold cyanide was simulated with a model accounting for mass transfer between the micropores and the macropores of the carbon, surface diffusion in the macropores, and film diffusion through the liquid layer surrounding the carbon particles. Under strong elution conditions (i.e. high temperatures and efficient pretreatments) the resistanc• to mass transfer becam~ negligible and the desorption of the gold could be modelled with an equilibrium mode 1. The equilibrium mode 1 was shown to be applicable to both the Zadra .and the AARL elution processes. Simul at i ans of exp·erimental data were presented as evidence of the validity of the models. Furthermore~ the models were used to perform a parametric sensitivity analysis and to predict the elution profiles during a continuous elution process.