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Thesis (MTech (Chemical Engineering))--Cape Technikon, Cape Town, 2000 / Due to equilibrium constraints and the relatively slow kinetics of the cyanidation of gold ores, calcium or potassium cyanide is added to the leaching stage in excess to that required theoretically. This, in many situations, result in large concentrations of free cyanide present in the effluent streams from gold plants. In view of the toxicity of cyanide and the fact that cyanide is fatal in small dosages, authorities have been forced to tigl1ten up plant discharge regulations. Therefore, it is vital to remove cyanide from industrial effluent, not only to meet standard requirements, but also to recover the cyanide as a means of reducing chemical costs. The aim of this study is to recover, rather than destroy, free cyanide from effluent streams via a metal impregnated carbon-in-column configuration. The first part of the study focused on the mechanism of free cyanide recovery by metal impregnated carbon and the factors influencing the kinetics of the process in a batch reactor. The second part concentrates on the optimisation of such a process m a column configuration, and subsequently to recover the cyanide from the carbon. In the batch experiments it was found that impregnated metal carbon outperformed virgin carbon for free cyanide removal both from a kinetic and equilibrium point of view. Furthermore: the presence of other metal cyanides in solution with free cyanide has a negligible effect on the performance of the metal (silver) impregnated activated carbon to remove free cyanide. Moreover, scanning electron micrographs revealed distinct differences in appearance of metal impregnated carbons, which ultimately responds differently to the removal of free cyanide. Although the kinetics of adsorption in the column experiments was found to be slower when compared to that experienced in a batch reactor, preliminary results show that a column configuration could be suitable for a free cyanide recovery on a large scale. Furthemore, a sensitivity analysis using the kinetics of adsorption and equilibrium cyanide loading as criteria, has been conducted on the column configuration. In these studies the effects of different bed volumes, competitive adsorption with other species present, different flow rates, different column diameters and initial cyanide concentrations on the process have been evaluated. These results were plotted as break-through curves, and the mass transfer zone (MTZ) was determined. It was found that impregnation in an air atmosphere yields a product with a higher capacity than in a nitrogen atmosphere, compromising carbon through combustion. Under a nitrogen atmosphere a more robust product is formed. As can be expected, lower linear velocities and/or larger bed volumes as well as lower initial free cyanide concentrations improve the fraction of cyanide removed in a column configuration.

Several series of ligands, designed to chelate silver(I) specifically in the presence of base metals, have been synthesised. The ligands include: - dithiodiamide compounds, prepared by the condensation of acetanilide derivatives with 1,2-dibromoethane; propanenitrile and propanoic ester derivatives prepared from pyridine-2-carbaldehyde via the Morita-Baylis-Hillman reaction; and novel malonamide ligands from the reaction of diethyl malonate with a range of primary amines. The malonamide derivatives were prepared under both conventional thermal and microwave-assisted conditions, the latter proving to be highly efficient. The ligands were all characterised using a combination of spectroscopic and, where appropriate, elemental analysis; in one case, the structural assignment was confirmed by single-crystal X-ray analysis. The fragmentation patterns in the electron-impact mass spectra of the malonamide derivatives have been explored using high-resolution and meta-stable peak scanning techniques. Complexes of the malonamide ligands with copper(II) and silver(I) have been synthesised, and examination of these complexes has revealed distinct differences in their co-ordination preferences towards silver(I) and copper(II). Tentative, computer-modelled structures for the complexes have been proposed using the available spectroscopic and elemental analysis data. Computer modelling, at the Molecular Mechanics level, has also been used to assess the capacity of the ligand systems to adopt conformations suitable for the chelation of tetrahedral silver(I). Solvent extraction studies have been undertaken using aqueous metal ion solutions and various organic solvents. The dithiodiamide derivatives typically presented solubility problems, but one of the ligands, N,N´-bis(3-chlorophenyl)-3,6-dithiaoctanediamide, exhibited significant but slow extraction of silver(I) into toluene. The malonamide derivatives, however, proved to be readily soluble in ethyl acetate and, in some cases, exhibited good to excellent selectivity for silver(I) in the presence of the base metals copper and lead. Atomic absorption analysis revealed rapid equilibration times (<15 min) and high extraction efficiencies over a wide pH range (2.78 - 9.0). Metal selectivity has been determined by ICP-MS analysis of the residual silver, copper and lead present in the aqueous phase after 15 min, and one of the ligands, N,N´-bis(2-benzylsulfanylethyl)malonamide, exhibits excellent (≥ 96 %) silver(I) specificity.

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No description available.