Browsing by Author "Nathoo, Jeeten"

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    Antisolvent gibbsite crystallisation from synthetic Bayer liquor
    (2009) Chiang, Yu-Lun; Lewis, Alison Emslie; Nathoo, Jeeten
    The current Bayer precipitation of gibbsite is notoriously slow and is considered to be the rate limiting step of the Bayer process. The present industrial reactor configuration involves precipitating gibbsite inside a series of large stainless steel agitated vessels due to the slow precipitation kinetics of gibbsite from pregnant Bayer liquor. The scope of this study is to investigate the effectiveness of using water as an antisolvent to increase the overall gibbsite yield and the associated particle behaviours under isothermal conditions. The experiments were performed in a series of batch experiments which were divided into two subsets. Firstly, the effect of different caustic concentrations present in the anti solvent on gibbsite yield was tested. Secondly, the effect of different seed loading (50 and 100 grams seeds per liter of synthetic liquor) on aggregation and fines formation were investigated. All batch experiments were conducted isothermally at 80°C under a closed system using a baffled stainless steel 316 agitated vessel. The experimental results indicated water to be a feasible medium for the isothermal antisolvent crystallisation of gibbsite from synthetic pregnant Bayer liquor. It was found that the extent of overall gibbsite yield exhibits a power law correlation to the change in the antisolvent caustic concentration with R2 ～1 (R2 = 0.99974). This finding is important as the regressed curve will be used to correlate gibbsite yield as a function of anti solvent purity. From the seed loading experiments, an Increase In seed loading under constant temperature and initial liquor supersaturation resulted in an overall decrease in product particle size due to a rise in inter-particulate attrition. However, crystal growth was not dominant during any period of the experiments with aggregation being more prominent for the system with the lower seed load. Gibbsite morphology determination from SEM revealed hexagonal solids which, in the presence of a higher seed loading indicated signs of fracture due to attrition. Quantitative evidence of this mechanism is shown USIng the discretised population balance method by Bramley et al. (1996) with the aid of the computational technique developed by Hounslow (2005) in Mathematica. Results generated using this method show decreasing aggregation with liquor de supersaturation and increased seed loading. The overall particle behaviours from the current investigation are validated by results obtained from previous gibbsite studies under very similar conditions. Although the gibbsite yield can be increased drastically in the presence of an anti solvent, the average particle size generated from this study is still too small to be commercially feasible due to the increase in initial supersaturation. Thus it is recommended that further particle size optimisation may be conducted inside a fluidised bed reactor where particles generally experience lower shear than inside a conventional agitated vessel.
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    Industrial application of eutectic freeze crystallization
    (2011) Jivanji, Rineshkumar Bharat; Lewis, Alison Emslie; Nathoo, Jeeten
    South African industries are currently generating increasing quantities of hypersaline brine streams. These wastewater streams need to be processed in an ecologically friendly and economically favourable method to produce both purified water and salts.
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    An investigation into the removal of aluminosilicates scaling species by activated alumina
    (2010) Mbedzi, Ndishavhelafhi; Emslie, Alison; Nathoo, Jeeten
    Gas condensates from the coal conversion plants contains trace amounts of inorganic species such as Si, Ca and Al ions, which cause scaling in downstream processes. Silica has been identified as the main constituent of the scale materials in geothermal plants. In order to prevent scaling, silica ions need to be removed or reduced. Alumina has been shown to successfully remove both silica and calcium from waste water streams. However, it also causes an increase in the aluminum concentration through dissolution. The mechanism of the silica and calcium uptake by alumina is not fully understood. In this study, the mechanism of silica uptake by alumina was investigated through an extensive literature review and experimental work on the alumina and silica chemistry when in solution. The chemistry of the alumina in suspension can be used to explain its reactions with other species (both inorganic and organic) in solutions. Activated alumina chemistry in suspension under alkaline conditions was investigated. The results showed that small amounts of alumina particles can undergo transformation into its hydrated phases and consequently aluminate (Al(OH)4 -) species are leached out from the pellets and dissolve in solution with subsequent precipitation when in solution. The inorganic species uptake can be attributed to the species interacting with the Al in solution and the hydrated phases of alumina. The results on the inorganic species uptake by alumina showed that a break through point is never attained. This indicates that the inorganic species removal by alumina cannot be attributed exclusively to an adsorption process. Hence, the mechanism of species removal was suggested to be a combination of adsorption and surface precipitation/reaction. Since alumina is costly, its application in wastewater treatment is dependent on its ability to be regenerated. As a result, the second objective of this study was to investigate the regeneration of the alumina by unloading the silica from the loaded alumina using various reagents and subsequently testing the effectiveness of the alumina with a second loading. The reagents used to unload the loaded alumina were sulphuric acid, sodium hydroxide and sodium gluconate at varying concentrations. The three reagents showed an increase in Si unloading with an increase in reagent concentrations. Sulphuric acid showed an unloading capacity of up to about 50% and 70% for batch and continuous unloading respectively. On the other hand, sodium hydroxide showed Si unloading of up to about 50% and 40% for the batch and continuous unloading of loaded alumina respectively under the investigated concentrations. The unloading of Si from saturated alumina using sodium gluconate was only conducted batch-wise as it only achieved a 6% unloading for the concentrations investigated. However, even though the scaling species were eluted from the alumina bed this did not improve/restore the loading capacity of alumina but rather kept the performance of the alumina at the same level that it was before the regeneration process. Also during the unloading of silica from alumina, excessive alumina dissolution was observed when using 0.25M and 0.65M NaOH.
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    Optimisation of electrolyte composition and operating parameters for the electropolishing of 304 stainless steel
    (2003) Nathoo, Jeeten; Lewis, Alison Emslie
    Includes bibliography.
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    Phosphate precipitation as struvite from municipality wastewater
    (2009) Ramaru, Rendani; Lewis, Alison Emslie; Nathoo, Jeeten
    Tshwane Municipality produces approximately 4Ml/day of dewatering liquors arising from municipal sludge from a wastewater treatment plant. The sludge being dewatered is a combination of an anaerobically digested primary sludge and an undigested waste activated sludge. High phosphate and nitrogen are released into water during the anaerobic treatment process. High concentrations of these nutrients in the wastewater lead to eutrophication, which is a major environmental problem (Doyle et al., 2000). In view of the above problem, the aim of this project was to investigate the feasibility of precipitating phosphate in the form of struvite (MgNH4PO4.6H2O) from the Tshwane Municipality dewatering liquors and consequently recovering it as a valuable and marketable product. The project was investigated in three sequential stages consisting of: 1. An aqueous thermodynamic modelling study using OLI Systems Inc. Stream Analyser Version 2.0.57 to determine the precipitation conditions required for the removal of phosphates in the dewatering liquor. 2. Bench'scale laboratory experiments to investigate the feasibility of phosphate precipitation as struvite under different pH conditions, Mg: P molar ratios and Ca:P molar ratios. 3. Fluidised bed experiments to establish the characteristics of phosphate removal at varying levels of supersaturation. The results suggest that the pH of the system has a significant effect on phosphate removal due to its influence on the availability of PO4 3, NH4 + and Mg2+. According to the thermodynamic modelling results, phosphates are removed as Ca3(PO4)2 and Mg3(PO4)2.8H2O. Removal of phosphate as Mg3(PO4)2.8H2O starts at a pH above 7 with the maximum removal achieved at pH 10. Thereafter, phosphate removal as Mg3(PO4)3.8H2O drops to zero due to Mg(OH)2 precipitation being favoured at the higher pH values. Thermodynamic modelling shows that a high NH4 +:P molar ratio is required to induce struvite precipitation. For sample C1, which had a PO4 P and an NH4 + concentrations of 145mg/L and 65mg/L respectively, the bench scale experiments showed that approximately 30%, 80% and 92% of the phosphate was removed at pH 8, pH 9 and pH 10, respectively. The NH4 +:P molar ratio in ii the precipitate decreased to zero at pH 10 because NH3 formation is favoured at this pH. The XRD analysis showed that the precipitate was Mg3(PO)2.22H2O. On the other hand, it was shown that phosphate was removed as struvite, for sample C2, which had PO4 P and NH4 + concentrations of 93mg/L and 57 mg/L, respectively. The NH4 +: P molar ratio of sample C1 was 0.99 while the NH4 +: P molar ratio of sample C2 was 1.35. However, it was shown that phosphate was removed as Mg3(PO)2.22H2O under pH 10 conditions. Thus, high NH4 +: P molar ratios (i.e. 1.35) in the waste water favours struvite precipitation with the optimum pH level for phosphate removal as struvite being pH 9. Bench scale experiments showed that, amorphous calcium phosphate precipitation is favoured over struvite precipitation at high Ca:P molar ratios. At a Ca:P molar ratio of 0.85:1, it was shown that no struvite was formed. The bench scale experiments showed that the phosphate conversion increased at high Mg:P molar ratios. However, the effect of increasing Mg:P molar ratio on phosphate conversion was significant at pH 9. While at pH 10 there was insignificant increase of phosphate conversion as Mg:P molar ratio was increased probably due to formation of MgOH+ ions at high pH levels. It was shown that high Mg:P molar ratios slightly limits calcium conversion. The calcium conversion was reduced by approximately 10% when Mg: P was increased from 1:1 to 1.4:1 for systems operated at pH 9 and pH 10 conditions. The supersaturation ratio of amorphous calcium phosphate is 104 greater than the magnesium compounds (i.e. struvite and Mg3(PO4)2.22H2O) supersaturation ratio as a result the effect of increasing Mg:P on calcium conversion was relatively very small. The fluidized bed reactor experiments showed that high Mg:P molar ratios increased the phosphate removal as a result of the increased free Mg2+ concentration. However, the high Mg:P molar ratios resulted in an increased supersaturation which led to the formation of many fine particles. There was an increase in overall conversion from 68% to 83 % when the Mg:P molar ratio was increased from 1:1 to 1.2:1. On the other hand, the removal decreased from 45% to 38% when the Mg:P molar ratio was increased from 1:1 to 1.2:1. Moreover, struvite with high purity was produced at a high Mg:P molar ratios (i.e. Mg:P =1.2). Fluidized bed reactor experiments showed that as the recycle ratio was increased the PO4 P conversion also increased. Thus, large amount of struvite particles were recycled to the fluidized bed reactor. As a result, struvite particles provide favourable nucleation sites for struvite precipitation hence high conversion were achieved at higher recycle ratio.