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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:cput/oai:localhost:20.500.11838/883 |
| Date | January 2000 |
| Creators | Dippenaar, Francois |
| Publisher | Cape Technikon |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | http://creativecommons.org/licenses/by-nc-sa/3.0/za/ |
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