Metal sulphide precipitation : effect of operational parameters on particle characteristics and process efficiency

Doctoral Thesis


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University of Cape Town

Acid mine drainage (AMD) is one of the major, long-term environmental challenges facing the minerals processing industry. Uncontrolled discharges have polluted thousands of kilometres of rivers, as well as surface and groundwater bodies with acidic effluents high in dissolved metals and sulphate. Conventional treatment technologies rely on oxidation, neutralisation and precipitation. While these may be effective they are expensive and not sustainable in the long term. Several biological treatment technologies, based on the activity of sulphate reducing bacteria (SRB), have been successfully developed and applied at both laboratory and industrial scale. The SRB uses sulphate as the terminal electron acceptor and with a suitable electron donor produce bicarbonate alkalinity and sulphide. These can be used to neutralise acidic effluents and effect precipitation of dissolved metal ions as metal sulphides. However, a number of challenges exist around the precipitation step, particularly where the recovery of valuable metals is desired. Metal sulphide precipitation reactions are inherently driven by extremely high levels of supersaturation. As a result, metal sulphide precipitation reactions are difficult to control and a large number of small particles are formed during the process. This leads to significant technical challenges with respect to solid-liquid separation and subsequent recovery of the precipitate. Despite the theoretically high metal removal, related to the low solubility of metal sulphides, practical efficiency is often significantly lower. Previous studies have shown that metal precipitation reactions require controlled physico-chemical conditions and the control of high levels of supersaturation to achieve optimum efficiency. The objective of this study was to extend this approach to metal suphide systems.Initial data indicated that conventional techniques to manage supersaturation were not effective and subsequent work focused on characterising the effect of reaction conditions on particle properties and investigating downstream processing options. The intention of this research was to bridge the gap between highly fundamental studies and practical application of sulphide precipitation technologies.

Includes bibliographical references (p. 127-141).