Browsing by Author "Lewis, A E"
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- ItemOpen AccessEffect of dissolved precipitating ions on the settling characteristics of copper sulphide(2013) Nduna, M; Rodriguez-Pascual, M; Lewis, A ESurface properties of metal sulphides have a great significance in various areas of engineering and science, such as acid mine drainage, contaminant sorption, and metal separation. In various attempts at producing metal sulphide particles from synthetic solutions, prodigious quantities of nuclei that grow only to colloidal dimensions have been frequently reported. This copious nucleation is promoted by the high levels of supersaturation that characterize most precipitation reactions. Colloidal particle formation in precipitation-based separation processes results in sub-optimal solid-liquid separation, which is alleviated by the production of more highly crystalline particles or agglomerates. The current work approaches this challenge from an electrochemistry perspective, by measuring surface charge potential of precipitant particles during metal sulphide precipitation with respect to the concentration of metal sulphide lattice ions in solution. Electrophoresis was used to measure the metal sulphide particle zeta potential and the settling properties were obtained by performing settleability measurements using an Imhoff settling cone. A suspension of copper sulphide particles was precipitated from synthetic solutions of copper and sulphide ions at equimolar concentrations. Immediately after precipitation the copper sulphide particles had a zeta potential of -50 mV and a settleability of about 7 ml.l-1. With the addition of copper ions the settleability increased by a factor of nearly three times and the zeta potential also increased to a maximum of -40 mV. A decrease in zeta potential to a minimum of -60 mV was observed after the addition of sulphide ions and this was associated with a settleability of 0 ml.l-1.
- ItemRestrictedEffect of post-precipitation conditions on surface properties of colloidal metal sulphide precipitates(Elsevier, 2012) Mokone, T P; Lewis, A E; van Hille, R PMetal 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.
- ItemRestrictedEffect of post-precipitation conditions on surface properties of colloidal metal sulphide precipitates.(Elsevier, 2012) Mokone, T P; Lewis, A E; van Hille, R PMetal 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.
- ItemOpen AccessInvestigating the control of manganese sulphide precipitation(2007) Chiang, Y L; Nathoo, J; Lewis, A EThe generation and control of manganese sulphide particle size distribution using various mixing configurations were investigated. Specifically, this paper discusses how varying mixing intensities on a macro and micro scale affect the resultant manganese sulphide particle size distribution (PSD) at a constant concentration. The mixing variations were achieved using various Y- and T-mixers as premixing devices into a jacketed, agitated vessel. The reagents used to generate the seeds were aqueous manganese sulphate and sodium sulphide solutions. The results showed that the absence of micromixing resulted in very poor control of the particle size distribution, the particle size and the number of particles produced. This was due to the fact that the macromixing time is much greater than the corresponding precipitation time, resulting in different mixing regions existing within the reactor. From the micromixing experiments, the T–mixer was found to be the most effective mixing device at lower concentrations due to the T-mixer providing a faster mixing time than the other mixing configurations, thereby effectively localizing the supersaturation and hence controlling the precipitation. The results obtained from the direct addition without the extension pipe (i.e. short mixing time) indicated a general increase in mean particle size and decrease in total particle number at reagent concentration of 0.007 mol.dm-3 as compared to the corresponding micromixing experiments. The phenomenon may be due to incomplete crystallization at the immediate exit of the mixing device. Further experiments need to be conducted before any conclusions can be drawn about the precipitation mechanisms.
- ItemRestrictedMetal sulphides from wastewater: Assessing the impact of supersaturation control strategies(IWA Publishing, 2012) Mokone, T P; van Hille, R P; Lewis, A EMetal 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.
- ItemRestrictedRecovery of Na2SO4·10H2O from a reverse osmosis retentate by eutectic freeze crystallisation technology.(Elsevier, 2010) Reddy, S T; Lewis, A E; Witkamp, G J; Kramer, H J M; van Spronsen, JThe increasing amount of waste water and effluent from South Africa’s mining industry forms a growing problem, which processing requires sustainable solutions in which both the water and the dissolved component can be reused. Eutectic freeze crystallisation (EFC) has been identified as a key technology that is not only energy efficient, but also produces ice and salt products of high quality. Unlike reverse osmosis membrane systems, EFC can treat both dilute and concentrated systems minimising waste water volumes. In this paper it is shown that freeze and eutectic freeze crystallisation can be used for the processing of a reverse osmosis retentate stream containing 4% NaSO4 and a number of impurities (F, Cl, K, Li, Mg, Ca, NO3 and NH4), producing both pure water and NaSO4•10H2O crystals. The influence of the impurities on the eutectic point and on the crystal structure of mirabilite was investigated using EFC technology investigated for a pure binary system, for a synthetic reverse osmosis retentate as well as for a concentrated NaCl system. In addition, investigations into the recovery and purity of mirabilite for these streams were conducted.
- ItemOpen AccessReducing water consumption at Skorpion Zinc(2011) Bhikha, H; Lewis, A E; Deglon, D ASynopsis: The minerals industry is committed to the principles of sustainability. Reducing water consumption is a priority area, especially for regions of water scarcity. This paper presents a systemic optimization of the water balance of the Skorpion Zinc refinery with the aim of reducing water consumption. An Aspen Plus simulation of the process is used. The validity of the simulation is tested by measuring key output variables and comparing results to plant data. A number of water minimization scenarios are investigated, including unit operation and circuit configuration changes. The scenarios leading to the largest reduction in water consumption are through the full recycle of treated effluent water, which results in water savings of up to 19%. Reducing process water and/or recycling of untreated water is prohibited by the build-up of trace elements, which affect product purity. The Skorpion process already features a highly optimized water balance, with unit operational changes merely resulting in a shift in the water balance. Consequently, the largest area for improvement is through the reuse of effluent water.
- ItemRestrictedThree phase mixing studies for nickel precipitation(Elsevier, 2003) Roberts, M; Lewis, A EHydrodynamics, temperature, pH and various other physico-chemical factors influence the morphology of nickel produced via hydrogen reduction. The focus of the current work is the effect on hydrodynamics of changing the impeller and reactor configurations in a 75 l stirred vessel with draft tube and baffles. The aim was to determine which configuration resulted in maximum particle suspension and local gas hold-up while using the minimum impeller speed and power consumption. A response surface methodology of experimental design was employed. This ensured that the number of candidate variables to be tested was reduced to a minimum and that interactive effects between variables were taken into account. The impeller configurations tested were a single Rushton turbine, a single axial flow impeller, and a double impeller system consisting of a combination of the two. The reactor configurations, tested at different gas flow rates, were varied to test the effects with and without baffles. It was found that optimum mixing could be achieved using a baffled vessel with an upper axial flow impeller and a lower Rushton turbine, and by keeping a minimum impeller clearance from the vessel bottom. This is in agreement with [Mineral Processing, 1–2 August 2002].
- ItemRestrictedUsing fractal structure to describe nickel crystal morphology(Springer Verlag, 2003) Lewis, A E; Roberts, MThe morphology of nickel crystals can be quantified using measurements of fractal structure. In the study described in this article, fractal dimensions of cross sections of 16 nickel samples were determined, and the numerical analysis indicates that the more rugged surfaces are characterized by higher fractal dimensions although the relationship between morphology and fractal dimension is relatively weak. However, the data also show that fractal dimension is a better predictor of morphology than apparent density, which is the measurement currently used. The nickel samples were also subjected to an avalanching disc technique that has the potential to separate out the differences in flow in the different types of nickel powder. The differences in flow between powders were ascribed to the similar gross morphologies of the particles, and the presence of shattered individual particles in the mix.