Browsing by Department "Crystallisation and Precipitation Research"

Now showing 1 - 11 of 11
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    Effect of dissolved precipitating ions on the settling characteristics of copper sulphide
    (2013) Nduna, M; Rodriguez-Pascual, M; Lewis, A E
    Surface 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.
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    Effect of Operating Conditions on Ice Characteristics in Continuous Eutectic Freeze Crystallization
    (Wiley-VCH Verlag, 2014) Chivavava, Jemitias; Rodriguez Pascual, Marcos; Lewis, Alison Emslie
    The efficacy of eutectic freeze crystallization (EFC) in crystallizers and in subsequent downstream washing or filtration steps is a direct function of size and morphology of the crystalline products. These product characteristics are influenced by supersaturation and residence time. To investigate the effect of residence time and supercooling on ice product characteristics in a continuous EFC process, crystallization experiments were performed in an EFC crystallizer with Na2SO4 aqueous solution. Crystal size distribution and morphology during the crystallization process were analyzed by digital image processing. Longer residence times enhanced the mean crystal size and disk-shaped ice crystals were formed. Increasing supercooling resulted in a larger mean ice equivalent diameter.
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    Gypsum seeding to prevent scaling
    (2022-03-02) Chagwedera, Taona Malvin; Chivavava, Jemitias; Lewis, Alison Emslie
    Eutectic freeze crystallization (EFC) is a novel separation technique that can be applied to treat brine solutions such as reverse osmosis retentates. These are often a mixture of different inorganic solutes. The treatment of calcium sulphate-rich brines using EFC often results in gypsum crystallization before any other species. This results in gypsum scaling on the cooled surfaces of the crystallizer, which is undesirable as it retards heat transfer rates and hence reduces the yield of other products. The aim of this study was to investigate and understand gypsum crystallization and gypsum scaling in the presence of gypsum seeds. Synthetic brine solutions were used in this research because they allowed an in-depth understanding of the gypsum bulk crystallization process and scaling tendency without the complexity of industrial brines. A cooled, U-shaped stainless-steel tube suspended in the saturated solution was employed as the scaling surface. This was because a tube-shaped surface enabled the introduction of a constant temperature cold surface in the saturated solution and most industrial EFC crystallizers are constructed from stainless steel. Gypsum seeding was effective in decreasing the mass of scale formed on the heat transfer surface. The most effective seed loading was 0.25 g/L, which reduced scale growth rate by 43%. Importantly, this seed loading is six times the theoretical critical seed loading. The seeding strategy also increased the gypsum crystallization kinetics in the bulk solution, which resulted in an increase in the mass of gypsum product. These findings are relevant for the operability and control of EFC processes, which suffer from scaling problems. By using an appropriate seeding strategy, two problems can be alleviated. Firstly, scaling on the heat transfer surface is minimised and, secondly, seeding increases the crystallization kinetics in the bulk solution, which is advantageous for product yield and recovery. It was also recommended that the use of silica as a seed material to prevent gypsum scaling should be investigated in future studies.
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    Heat and mass transfer effects of ice growth mechanisms in pure water and aqueous solutions
    (American Chemical Society, 2014) Kapembwa, Michael; Rodriguez-Pascual, Marcos; Lewis, Alison Emslie
    Interactions between heat and mass diffusion determine growth mechanisms during ice crystallization. The effects of heat and mass transfer on ice growth in pure water and magnesium sulfate solution were investigated by studying the evolution of the gradient of the refractive index using color Schlieren deflectometry. For pure water, the gradient of the refractive index of water was used to calculate the temperature and therefore the local supersaturation. Its effect on the ice crystal growth rate and morphology was studied. It was found that, for local supersaturations greater than 2.8, the morphology was dendritic ice, with a growth rate 2 orders of magnitude higher than that for layered growth. During dendritic growth, 3−16% of the heat of crystallization diffused to the liquid side, which is counter to current understanding. At the transition (between the time of partial melting of the dendritic ice and the beginning of the layered ice growth), a higher supersaturation than that responsible for layered growth was observed. For ice growth from an aqueous salt solution, a mass and thermal diffusion boundary layer in front of the growing ice was created by diffusion of the solutes from the ice and by the release of heat of crystallization.
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    In Situ Growth Measurements of Sodium Sulfate during Cooling Crystallization
    (Wiley-VCH Verlag, 2014) Egan, Timothy; Rodriguez Pascual, Marcos; Lewis, Alison Emslie
    Combined rainbow Schlieren deflectometry (RSD) and liquid crystal thermography (LCT) served as in situ non-intrusive techniques to determine local concentration and temperature fields based on refractive index gradients in solution during crystallization of sodium sulfate from aqueous solution. The combination of RSD with LCT allowed the decoupling of concentration and temperature effects. Convective mass transport happened during periods of fast, dendritic growth, but even higher levels occurred during the densification of the dendritic crystals, indicating significant mass deposition in this phase. The convection developed mainly as a result of concentration gradients as opposed to temperature gradients. A supersaturation field map was generated and growth kinetics of the sodium sulfate salt was determined. The kinetics followed a power law relationship, with constants in line with those in the literature.
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    Industrial Crystallization: Fundamentals and Applications
    (Cambridge University Press, 2015-07-01) Lewis, Alison Emslie; Seckler, Marcelo; Kramer, Herman; Rosmalen, Gerda
    Bridging the gap between theory and practice, this text provides the reader with a comprehensive overview of industrial crystallization. Newcomers will learn all of the most important topics in industrial crystallization, from key concepts and basic theory to industrial practices. Topics covered include the characterization of a crystalline product and the basic process design for crystallization, as well as batch crystallization, measurement techniques, and details on precipitation, melt crystallization and polymorphism. Each chapter begins with an introduction explaining the importance of the topic, and is supported by homework problems and worked examples. Real world case studies are also provided, as well as new industry-relevant information, making this is an ideal resource for industry practitioners, students, and researchers in the fields of industrial crystallization, separation processes, particle synthesis, and particle technology.
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    A model for the zeta potential of copper sulphide
    (Elsevier, 2014) Nduna, Moses; Lewis, Alison Emslie; Nortier, P
    A model is proposed for the zeta-potential versus pH curve of Covellite (CuS). The classical Gouy–Chapman–Stern treatment of the adsorption of protons is applied to a model surface derived from the crystallographic data. The surface charges are obtained by an original adaptation of the concepts from the MUSIC model to a sulphide, i.e. the analysis of the chemical bonding in Pauling’s electrostatic valence framework to derive partial surface charges. The authors develop a software for the fitting of p –pHcurves using IPHREEQC for the calculation of the zeta potential at each pH point and Matlab to drive the calculation of the whole pH set and the optimisation of the parameters: capacitances of the double layer,site densities, charges and acidity constants. The model successfully reproduces the experimental data,considering only two sulphided surface sites: introduction of an elemental sulphur layer or consideration of hydroxyl surface sites is not required to explain the charging behaviour of Covellite when carefully precipitated under nitrogen.
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    Modelling techniques for biological reaction systems: 1 Mathematical description and model representation
    (Water Research Commission, 1988) Billing, Alison Emslie; Dold, Peter Lorimer
    This paper is the first in a series of three which deals with modelling and numeric techniques for biological reaction systems. A matrix for­ ma rov1des a usef\11. method for model presentation. The matrix ensures clarity as to the compounds, processes, reaction terms and s01chio.metry compnsmg the model. It allows ready comparison of different models and facilitates incorporating the model in a computer slffiulauon program.
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    Selenium impurity in sodium sulphate decahydrate formed by Eutectic Freeze Crystallization of industrial waste brine
    (The Southern African Institute of Mining and Metallurgy, 2013) Apsey, Grant; Lewis, Alison Emslie
    Eutectic freeze crystallization (EFC) is a novel technique for the recovery of pure salt and pure water from hypersaline waste brines. It is therefore a promising technology for the treatment of industrial waste waters. The impurities caused by crystallizing salt out of multi-component brines by EFC have not yet been investigated, however. To these ends, the selenium impurity found in sodium sulphate, produced from the waste brine of a platinum operation, was investigated. It was believed that the similarity between sulphate and selenate ions allowed isomorphous substitution of selenate ions into the sodium sulphate crystals, which was the likely cause of impurity uptake. It was found that the presence of sodium chloride in the industrial brine promotes the uptake of selenium, while ionic strength of the brine and mass deposition rate of sodium sulphate did not have a significant effect on the selenium uptake. Isomorphous substitution is predicted to be the most significant mechanism by which all impurities will be taken up when applying EFC to other industrial waste brines.
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    Treatment of a multicomponent mining effluent using calcium hydroxide in a fluidized bed crystallizer
    (2016) Maharaj, Chiara; Lewis, Alison; Chivavava, Jemitias
    Wastewater which primarily emanates from mining operations and manufacturing industries, has the potential for re-use if treated effectively. These wastewaters, which are typically characterized by high concentrations of dissolved inorganic salts are often disposed in evaporation ponds, which promotes the risk of ground water pollution and land wastage. Moreover, this forfeits the potential benefits of valuable salts recovered. The aim of this project was to investigate the treatment of multicomponent saline wastewater rich in sodium and magnesium sulphates, since these salts are prevalent in most wastewater streams. The intention was to treat the wastewater with a calcium hydroxide (Ca(OH)2) suspension in a laboratory scale seeded fluidised bed crystallizer, thereby precipitating gypsum and magnesium hydroxide. The objectives of this study were to investigate how the chosen reactor configuration, feed stream and reagent characteristics affect the conversion and recovery of gypsum and magnesium hydroxide over a range of wastewater concentrations. Particular focus was on reducing the formation of fines through the use of seeds and to get an insight into the possible precipitation mechanisms. It was important that the resulting precipitate product quality favoured effective separation from the treated water stream for re-use. Preliminary experiments were conducted over a feed concentration ranging from 1.5 g/L - 120 g/L (total sulphate salts) which was contacted with a stoichiometric amount of calcium hydroxide with respect to the sulphates in the stream, that is a Ca:SO4 ratio of 1:1 in the fluidised bed crystallizer. These experiments identified a feasible feed concentration range for operation (8 000 -35 000 mg/L). High inlet concentrations (≥ 50 000 mg/L) were not feasible due to rapid formation of a large mass of precipitates which disrupted fluidisation and caused the reactor contents to be elutriated. These high concentrations resulted in high rates of accumulation which necessitated the need for frequent intermittent product removal.
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    Treatment of textile wastewaters using Eutectic Freeze Crystallization
    (IWA publishing, 2014) Randall, Dyllon; Zinn, C; Lewis, Alison Emslie
    A water treatment process needs to recover both water and other useful products if the process is to be viewed as being financially and environmentally sustainable. Eutectic Freeze Crystallization (EFC) is one such sustainable water treatment process that is able to produce both pure ice (water) and pure salt(s) by operating at a specific temperature. The use of EFC for the treatment of water is particularly useful in the textile industry because ice crystallization excludes all impurities from the recovered water, including dyes. Also, EFC can produce various salts with the intention of reusing these salts in the process. This study investigated the feasibility of EFC as a treatment method for textile industry wastewaters. The results showed that EFC can be used to convert 95% of the wastewater stream to pure ice (98% purity) and sodium sulfate.