Browsing by Author "Chivavava, Jemitias"
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- ItemOpen AccessDeveloping methods to access sensitive industrial wastewater information in South Africa (with treatment in mind)(2018) Harding, Genevieve E; Lewis, Alison; Chivavava, JemitiasSouth Africa is a water stressed country, therefore it is important to understand water use and wastewater generation. Previous research and workshops have identified gaps in the characterisation and remediation of wastewaters in South Africa. Wastewater management can take advantage of wastewater as a valuable resource. However, treatment is required to recover this value, while characterisation is required to develop treatments. Yet wastewater characterisation information is often poorly reported. The nature of industrial wastewaters (in terms of volume, location and composition), and the norms of wastewater characterisation reporting (in terms of quality and accessibility) formed the basis for two research questions. A major component of this research was developing methods to access sensitive wastewater information. Relational approaches were based on building relationships through phone calls, emails, meetings and site visits. Formal, legal requests for were made with application in terms if the Promotion of Access to Information Act (PAIA). Even though wastewater information is not confidential, it is not readily accessible. 87 people from 42 companies or institutions were contacted; 14% of interactions lead to shared data or a meeting, and 12% shared resources. Key industries of interest were: pulp and paper, fish processing, power generation, mining and petroleum. Previous estimates of South African industrial wastewater volumes ranged from 70 – 350 Mm3 /annum. The pulp and paper industry contributed between 28 and 43% of this volume; petroleum contributed 9 to 26%. Both industries were located inland and in coastal regions of South Africa. These industries were most concerned with COD. Mining and power generation contributed 10 – 15% and 7 – 14% respectively. These industries were located inland, and were concerned with total dissolved solids, and specifically sulphate, sodium and chlorides. The fish processing industry contributed between 0 and 23% of volumes, depending whether wastewaters released to a marine environment were included. Seven parameters were reported for over half of the streams considered (65 in total). These parameters were: pH, volume, electrical conductivity, nitrogen, sulphate, sodium and COD. Sulphate and sodium were dominant ions. Calcium was not measured, even though discharge limits were listed in environmental licenses. Characterisation information was reported for compliance and not for treatability. The parameters measured should be expanded to include important parameters for treatability. Industry, research institution and governmental bodies can work together to identify such parameters and develop locally relevant treatments. It is recommended that possible synergies between these groupings be enhanced to improve wastewater management. But an atmosphere of trust and transparency is required to facilitate synergistic relationships. The legal framework in South Africa can be used to motivate for transparency with respect to wastewaters.
- ItemMetadata onlyEffect of Operating Conditions on Ice Characteristics in Continuous Eutectic Freeze Crystallization(Wiley-VCH Verlag, 2014) Chivavava, Jemitias; Rodriguez Pascual, Marcos; Lewis, Alison EmslieThe 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.
- ItemOpen AccessEffect of operating conditions on product quality in continuous eutectic freeze crystallization(2013) Chivavava, Jemitias; Lewis, Alison EmslieThe efficiency of gravitational separation in a staged continuous Eutectic Freeze Crystallization process is largely influenced by the quality of solid products exiting the crystallizer. The particle size distribution and shape of the crystalline products affect the efficiency of the washing and filtration steps hence the ultimate purity of the product water and or salts. These characteristics are the net effect of the crystallizer operating conditions such as the level of supersaturation, residence time and magma density. In order to control the mentioned product characteristics, the mechanistic interaction of crystallizer operating conditions and product morphology and size needs to be understood. This work was aimed at investigating the effect of residence time and supersaturation on the product quality in continuous Eutectic Freeze Crystallization and a 2L glass crystallizer equipped with scrapers was employed for the investigations.
- ItemOpen AccessGypsum seeding to prevent scale formation and improve separation efficiency(2021) Chagwedera, Taona Malvin; Chivavava, Jemitias; Lewis, AlisonEutectic 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 reducing the yield of other products. Gypsum crystallizes in small quantities compared to ice and mostly as fines because it is at lower concentrations. This also results in the entrapment of gypsum by the ice during gravity separation of the crystallization products. The aim of this study was to firstly investigate and understand gypsum crystallization and gypsum scaling in the presence of gypsum seeds. Secondly, it was to investigate the effect of gypsum seeding on gypsum separation efficiency. 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. Cooling crystallization experiments were conducted to investigate the effect of gypsum seeding on gypsum scaling, and EFC experiments were conducted to investigate the effect of gypsum seeding on gypsum separability from the ice. Gypsum seeding was effective in decreasing the mass of scale that formed on the heat transfer surface. Gypsum seeds improved gypsum crystallization kinetics in the bulk solution which resulted in an increase in the mass of gypsum product. Gypsum seeding marginally increased melted ice purity, although the proportion of gypsum which crystallized as fines in the suspension decreased significantly. This was because gypsum deported to the ice fraction mainly through mother liquor entrainment which was contrary to the expectation at the start of the research. The hypothesis was that gypsum reported to the ice fraction mainly through entrapment. It was recommended that the use of silica as a seed material to prevent gypsum scaling should be investigated in future studies. In addition, the treatment of calcium sulphate-rich brines using EFC should be done in stages with the first stage focusing on maximum gypsum removal using a residence time of 5 hours or more.
- ItemOpen AccessGypsum seeding to prevent scaling(2022-03-02) Chagwedera, Taona Malvin; Chivavava, Jemitias; Lewis, Alison EmslieEutectic 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.
- ItemOpen AccessIce scaling in continuous eutectic freeze crystallization(2016) Jooste, Debora; Lewis, Alison Emslie; Chivavava, JemitiasEutectic Freeze Crystallization (EFC) is a novel and potentially cost effective technique to treat industrial brines by the simultaneous crystallization of ice and salt under sub-eutectic conditions. Previous research has demonstrated that the formation of an ice scale layer on the cooling surfaces of indirectly cooled crystallizers severely decreases heat transfer. This increases the mechanical energy requirements and overall operational cost of the process. The energy efficiency of EFC as a wastewater treatment and resource recovery technology is, therefore, dependent on effective control of ice scaling. This research focused on determining the induction time, defined as the time between initial nucleation and scale layer formation, where shorter induction times are associated with more severe scaling tendencies. The experimental work was conducted in a hybrid crystallizer-separator with a 2 litre crystallization zone fitted with a mechanical scraping device. The effect of the driving force for heat transfer, scraper speed and the solute type and concentration of inorganic electrolyte impurities in a binary eutectic Naâ‚‚SOâ‚„-Hâ‚‚O system was investigated. Induction time decreased with an increase in the driving force for heat transfer, due to a lower wall temperature and an increased driving force for crystallization as a result of the higher heat flux. An increase in scraper speed resulted in an increase in induction time, due to the more frequent removal of the thermal boundary layer and better distribution of supersaturation and magma throughout the crystallizer. The induction time was found to be specific to dissolved ionic species as a result of unique electrostatic interactions between the cooled wall and ice layer surface. Induction time showed an increase with an increase in concentration of electrolyte impurities, due to the increased mass transfer limitation of solute molecules away from the growing ice front.
- ItemOpen AccessInvestigating the Effect of Surface Properties on Ice Scaling in Eutectic Freeze Crystallization(2021) Motsepe, Lerato; Chivavava, Jemitias; Lewis, AlisonEutectic Freeze Crystallization (EFC) is an innovative technology that can be applied to treat reverse osmosis (RO) waste streams (brines), to produce pure salt and water. Scaling of the heat exchanger (HX) surface by both ice and salt is currently one of the major drawbacks in the industrial implementation of EFC. At present scaling is controlled by the use of mechanical scraping, which is susceptible to mechanical breakdown, thus reducing the overall process efficiency. Previous studies have shown that lower surface energy materials delay the onset of freezing, and that smooth surfaces reduce nucleation and adhesion sites, thereby reducing the probability of scale formation. Therefore, this study aimed to investigate how the HX surface properties affect ice scaling in EFC, without the influence of mechanical scraping. Copper, Aluminium, Stainless Steel 316 and Brass were the selected HX materials. Ice scaling on the HX materials was investigated using a near eutectic 4 wt.% Na2SO4 aqueous solution, in a crystallization test cell uniquely designed to mimic the region near the HX wall of a crystallizer. The Differential Interference Contrast (DIC) technique was used to study the formation of the initial ice scale layer on the HX material used in the test cell. This method of observation was effective, asfor the first time in a continuous system, the crystallization of the initial ice scale layer was observable in-situ and in real-time. Therefore, with this method, it was possible to investigate the evolution of the predominantscaling modes(nucleation and growth), which differed for the different HX surfaces. The difference was proposed to be due to their distinct surface free energies and surface topographies. The effect of surface free energy and topography on the scaling induction time was investigated while operating at similar heat fluxes (similar cooling rates) for all the metals. The scaling induction time decreased with an increase in the surface free energy, with the Aluminium as an outlier. The recorded scaling induction times for Brass, primary-SS316 and Copper were 92.54, 70.95 and 54.06 min, respectively. Aluminium recorded the longestscaling induction time of 134.74 min. Both the polytetrafluoroethylene (PTFE) coated-SS316 and the primary-SS316 HX surface were used to investigate further the effect of surface free energy on the scaling induction time. The PTFE-coated-SS316 was found to increase the scaling induction times 2.79-fold at a coolant temperature of -15°C, compared to that of the primary-SS316. However, at -20°C and -25°C, the scaling induction times on both surfaces were comparable, which indicated that the benefit of using a low surface free energy material was limited by the cooling rate of the system. It was also found that the scaling induction times were shorter when using a rough-SS316 HX plate, compared to the primary-SS316, because of the larger surface area available for heat transfer. The end of the scaling induction time was characterised by the heterogeneous nucleation and subsequent growth of the ice on the HX surfaces. There was no direct correlation between the HX surface free energy and the nucleation and growth rates. This was because the Brass, Aluminium, SS316 and Copper plates all consist of different surface topographies which also influenced the nucleation and growth rates. However, the nucleation rates consistently increased when the scaling induction times were longer, regardless of the HX material used. The presence of deep sharp crevices on the primary-SS316 also enhanced nucleation rates. These deep sharp crevices created regions of high local supersaturation, where heterogenous nucleation predominated. It was, therefore, reasonable to conclude that the ice scaling induction time was increased by using smooth materials and those of lower surface free energy. The scaling mode was dependent on the surface topography and length of the ice scaling induction time, as longer ice scaling induction times resulted in heterogenous nucleation dominated scaling mode and vice versa. Materials that had a low surface free energy and were smooth minimised the nucleation rate, resulting in a reduced overall scaling rate.
- ItemOpen AccessInvestigation on the Effect of Mesomixing on Crystal Quality during Antisolvent Crystallization of Nd2(SO4)3·8H2O(Multidisciplinary Digital Publishing Institute, 2023-07-31) Baloyi, Tinjombo Octavious; Chivavava, Jemitias; Lewis, Alison EmslieRare earth elements (REEs) are essential for permanent magnets that are vital for wind turbines and electric vehicles motors (EV), and are also used in a range of high-tech devices such as smartphones, digital cameras, and electronic displays. Nickel metal hydride (NiMH) batteries have been identified as a potential source due to their short lifespans and an anticipated boom in the production of EV. The aim of this study was to investigate the effect of mesomixing on crystal quality in a non-confined impinging jet mixer (NCIJM) during antisolvent crystallization of 3.2 from a synthetic leach solution of NiMH battery using ethanol at an O/A ratio of 1.1. The jet streams were supplied at a Reynolds number (Re) between 7500 and 15,000. The product slurry was allowed to further crystallize in a stirred batch crystallizer at a Re of 13,000 for 45 s. An average yield of 90% was achieved. Laser diffraction and scanning electron microscopy (SEM) were used for size analysis. The initial results were inconclusive due to the secondary mixing effect in the stirred batch crystallizer. Therefore, the experiments were repeated, and samples were collected immediately after mixing in the NCIJM onto a porous grid placed on a high absorbance filter paper to abruptly halt crystallization. The samples were analysed using a transmission electron microscope (TEM), and the acquired images were processed using ImageJ to obtain crystal size distributions (CSDs). It was found that the enhanced mesomixing conditions resulted in smaller crystal sizes and narrower CSDs. This was because the nucleation rate was found to be mass-transfer-limited, such that higher mesomixing intensities promoted the nucleation rate from 6 × 10 12 to 5 times; and, therefore, favoured the formation of smaller crystals. In parallel, intensified mesomixing resulted in uniform distribution of the supersaturation and, hence, narrowed the CSDs.
- ItemOpen AccessSelective recovery of salts from a ternary eutectic system in EFC using seeding(2019) Aspeling, Benita Jean; Chivavava, Jemitias; Lewis, Alison EmslieIndustrial and mining saline streams are often multi-component in nature. Much research within Eutectic Freeze Crystallization (EFC) has focused on the crystallization of ice and single salts from aqueous solutions. However, as a single salt and ice are crystallized, the concentration of the non-crystallizing salt species increase until the system is saturated with more than two species. In such a situation, the sequence and rate of crystallization of each species depends on both the kinetics of crystallization of each salt and the interaction between the different species. Seeding could be employed to control kinetics and thereby achieve selective recovery from multi-supersaturated systems. The aim of this study was therefore to determine the effect of seeding on the yield and purity of the salt product in a system supersaturated with two salts and ice. A eutectic MgSO4-Na2SO4-H2O system was chosen for this study as these salts are prevalent in saline waste streams in South Africa. A continuous 2 ℓ jacketed, scraped and stirred glass crystallizer was seeded with Na2SO4∙10H2O, MgSO4∙11H2O and ice. The initial salt seed loading and initial supersaturation were varied. The operating conditions used were 30 minutes residence time, a coolant temperature of -11°C, and operating temperature of approximately - 5.0 to -5.1°C. An increase in seeding mass was found to increase the yield and proportion of the seeded salt in the salt product due to an increase in salt growth rate. However, in all experiments it was found that MgSO4∙11H2O crystallized out at fractions higher than the eutectic thermodynamic ratio, indicating a higher selectivity towards this salt. Furthermore, the introduction of 30 g of MgSO4∙11H2O seeds produced a pure salt product (above 99.4 wt.% purity) and the highest salt yield. A similar mass of either seeding material resulted in a similar total mass of salt product. This was attributed to MgSO4∙11H2O crystallizing as the majority salt, and therefore its kinetics played a major role in the total salt yield. Initial supersaturation was found to have no significant effect on steady state salt purity and yield. This study showed that multiple steady states exist within this system at the same operating conditions but different initial seeding conditions. Seeding was found to have the potential to engineer the salt purity of the overflow and underflow split fractions by changing the individual salt average particle sizes. Therefore, this study showed that selectivity recovery of one salt is possible in a multi supersaturated system through seed engineering.
- ItemOpen AccessTreatment of a multicomponent mining effluent using calcium hydroxide in a fluidized bed crystallizer(2016) Maharaj, Chiara; Lewis, Alison; Chivavava, JemitiasWastewater 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.