Browsing by Author "Mokone, T P"

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    Effect of post-precipitation conditions on surface properties of colloidal metal sulphide precipitates
    (Elsevier, 2012) Mokone, T P; Lewis, A E; van Hille, R P
    Metal sulphide precipitation is important in several hydrometallurgical processes. However, challenges exist in solid–liquid separation and recovery of the colloidal precipitates produced in some systems. This study presents the effect of downstream processing options on the surface properties of colloidal particles produced during copper and zinc sulphide precipitation. XRD and EDAX characterisation indicated the copper precipitate was a mixture of covellite (63%) and copper hydroxysulphate (37%), while the zinc sulphide was more pure, but less crystalline. The effect on surface charge and aggregation tendency of different concentrations of background electrolyte (1–100 mM KCl), suspension pH, aqueous sulphide, a divalent (Ca2+) and trivalent (Al3+) cation were studied. The magnitude of the negative surface charge increased with increasing suspension pH (pH 6 to pH 11) for both copper and zinc precipitates. The addition of aqueous sulphide (0.84 mM) to the zinc precipitate resulted in a significant decrease in the zeta potential and suppressed aggregation, due to adsorption of the negatively charged sulphide ions. This effect was reduced at high ionic strength. A higher sulphide concentration was required to replicate the phenomenon with the copper precipitate due to sulphidisation of the copper hydroxysulphate initially. Addition of a divalent cation (Ca2+) to the suspension had little effect on the surface charge of the particles and did not promote aggregation. However, addition of a small amount (0.5 mM) of Al3+ ions resulted in a significant change in surface charge (−30 to −10 mV for copper and −23 mV to −5 mV for zinc) and subsequent aggregation. The results of this study show that downstream processing of colloidal metal sulphide precipitates, produced where supersaturation cannot be managed, can lead to effective solid–liquid separation, by changing the surface properties of the precipitate.
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    Effect of post-precipitation conditions on surface properties of colloidal metal sulphide precipitates.
    (Elsevier, 2012) Mokone, T P; Lewis, A E; van Hille, R P
    Metal sulphide precipitation is important in several hydrometallurgical processes. However, challenges exist in solid–liquid separation and recovery of the colloidal precipitates produced in some systems. This study presents the effect of downstream processing options on the surface properties of colloidal particles produced during copper and zinc sulphide precipitation. XRD and EDAX characterisation indicated the copper precipitate was a mixture of covellite (63%) and copper hydroxysulphate (37%), while the zinc sulphide was more pure, but less crystalline. The effect on surface charge and aggregation tendency of different concentrations of background electrolyte (1–100 mM KCl), suspension pH, aqueous sulphide, a divalent (Ca2+) and trivalent (Al3+) cation were studied. The magnitude of the negative surface charge increased with increasing suspension pH (pH 6 to pH 11) for both copper and zinc precipitates. The addition of aqueous sulphide (0.84 mM) to the zinc precipitate resulted in a significant decrease in the zeta potential and suppressed aggregation, due to adsorption of the negatively charged sulphide ions. This effect was reduced at high ionic strength. A higher sulphide concentration was required to replicate the phenomenon with the copper precipitate due to sulphidisation of the copper hydroxysulphate initially. Addition of a divalent cation (Ca2+) to the suspension had little effect on the surface charge of the particles and did not promote aggregation. However, addition of a small amount (0.5 mM) of Al3+ ions resulted in a significant change in surface charge (−30 to −10 mV for copper and −23 mV to −5 mV for zinc) and subsequent aggregation. The results of this study show that downstream processing of colloidal metal sulphide precipitates, produced where supersaturation cannot be managed, can lead to effective solid–liquid separation, by changing the surface properties of the precipitate.
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    Metal sulphides from wastewater: Assessing the impact of supersaturation control strategies
    (IWA Publishing, 2012) Mokone, T P; van Hille, R P; Lewis, A E
    Metal sulphide precipitation forms an important component of acid mine drainage remediation systems based on bacterial sulphate reduction. However, the precipitation reaction is inherently driven by very high levels of supersaturation with the generation of small particles with poor solid–liquid separation characteristics. In this study, the effect of strategies used to manage supersaturation was investigated during copper and zinc sulphide precipitation reactions. Initial batch studies showed the origin of sulphide (biological or chemical) had no significant effect on the efficiency of zinc sulphide precipitation. For copper, low metal removal efficiency was obtained at metal to sulphide molar ratios below 1.6 in the synthetic sulphide system. This was improved in the biogenic sulphide system, due to the presence of residual volatile fatty acids, but the presence or absence of particulate organic matter had no effect on recovery. Subsequent studies, conducted using synthetic sulphide solutions in a seeded fluidised bed reactor with multiple reagent feed points (2FP and 6FP) and different recirculation flow rates (300 and 120 mL min−1) showed efficient zinc sulphide precipitation, but limited (<10%) deposition on the seeds. Increasing the number of sulphide feed points (2–6) reduced precipitate loss as fines by approximately 10%. Zinc sulphide fines could be effectively recovered from suspension by settling under quiescent conditions. In the copper system, metal recovery was low (ca 40%) due to the formation of very small copper sulphide particles (mean particle size of ca 0.01 μm). Increasing the number of reagent feed points did not affect supersaturation to the extent of altering particle characteristics. The copper sulphide fines could not be recovered by settling, remaining in a stable colloidal suspension due to their highly charged surfaces (zeta potential −50 mV). The change in recirculation flow rate had a limited effect (ca 5% improvement) on process efficiency. The results show that the extremely high supersaturation prevalent during metal sulphide precipitation is difficult to control using conventional approaches and suggest that the seeded fluidised bed reactor is not suitable for this application.