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A dissertation submitted to the faculty of Engineering and the Built Environment University of Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering January, 2019 / The prevalence and treatment of low grade, finely disseminated iron ore has resulted in the production of primary and secondary slimes that constitute potential resources. Slimes processing is hindered by the particle size limits of current process equipment and this dissertation explores the potential of coupling selective flocculation with magnetic and gravity separation to improve separation efficiencies. Base case tests (without selective flocculation) were conducted on the SLon-100 (laboratory scale pulsating Wet High Intensity Magnetic Separator) and the laboratory scale Reflux Classifier (RC). The base case tests were conducted to determine the optimal intensity for the SLon-100 and a semi-batch test was done on the RC to determine the effect of increasing water fluidisation rates on the response variables namely Fe concentrate grade, Fe concentrate recovery and separation efficiency. Thereafter selective flocculation conditions were optimised by coupling the process with magnetic separation in order to determine the effect of the operating variables on the response variables mentioned above. A Box-Behnken design was utilised and the ANOVA models developed for the significant response variables were used to optimise the selective flocculation process by simultaneously maximising the response variables whilst minimising the three factors (sodium oleate, paraffin dosage and conditioning time). The optimised selective flocculation conditions were then coupled with the RC in order to compare magnetic and gravity separation with and without selective flocculation. The optimised selective flocculation conditions (1 kg/ton sodium silicate; pH 10; 500g/t sodium oleate; 1431.1g/t paraffin and 4.6 min conditioning time) coupled with magnetic separation showed improved metallurgical performance when compared to the base case test. Selective flocculation coupled with magnetic separation improved the magnetic product Fe grade from 52.28±0.38% to 59.21±0.42% Fe whilst simultaneously improving the separation efficiency from 40±1.46% to 56.8±2.0% and maintaining the Fe concentrate recovery within the 95% confidence limits (69.9% to 72.1%). These results were achieved under laboratory and ideal conditions and may differ from industrial scale results. Inconclusive results were achieved with selective flocculation coupled with the RC and additional testwork is recommended / MT 2019

Flocculation has been widely adopted as one of the most effective methods to remove colloidal particles in water or wastewater treatment. Synthetic flocculants are conventionally used because of their high flocculating efficiency and cost-effectiveness. However, they have been reported to have hazardous properties and implicated in some serious health problems including senile dementia and neuro-toxicity, as well as being recalcitrant in the environment. Consequently, efforts are being geared away from the use of synthetic flocculants in water and wastewater treatment. Hence, the need for safe and eco-friendly flocculants has become imperative. Compared with synthetic flocculants, bioflocculants have special advantages such as safety, biodegradability and harmlessness to the environment and humans; attributes which make them potential alternatives in water treatment, downstream as well as fermentation processes. In the current study, the potentials of bacterial isolates recovered from Algoa Bay in the Eastern Cape Province of South Africa for bioflocculant production were investigated. The bacterial isolates were identified by polymerase chain reaction (PCR) as belonging to the Bacillus genus. The analysis of 16S ribosomal deoxyribonucleic acid (rDNA) nucleotide sequence of isolate M72 showed 99 percent similarity to Bacillus toyonensis strain BCT-7112 and was deposited in the GenBank as Bacillus toyonensis strain AEMREG6 with accession number KP406731. Likewise, the 16S rDNA nucleotide sequences of isolates M69 and M67 showed 98 percent sequence similarity to Bacillus licheniformis strain W7 and Bacillus algicola strain QD43 respectively; and M67 isolate was subsequently deposited in the GenBank as Bacillus sp. AEMREG7 with accession number KF933697.1. The results of the nutritional requirements and fermentation conditions revealed that optimum inoculum size for REG-6 production was 4 percent (v/v), while 5 percent (v/v) and 3 percent (v/v) were most favourable for MBF-W7 and MBF-UFH production respectively. Glucose was the best carbon source for the production of bioflocculants (REG-6 and MBF-UFH) by Bacillus toyonensis AEMREG6 and Bacillus sp. AEMREG7 respectively, while maltose supported optimum bioflocculant (MBF-W7) production by Bacillus specie. Inorganic nitrogen (NH4NO3) was the favoured nitrogen source for both REG-6 and MBF-W7 production, while mixed nitrogen sources [yeast extract + urea + (NH4)2SO4] supported the maximum production of MBF-UFH. The initial medium pH for REG-6 was 5, while MBF-W7 and MBF-UFH were both maximally produced at the initial pH of 6. After a 96 h cultivation period under optimal culture conditions, 3.2 g of purified REG-6 with a maximum flocculating activity of 77 percent was recovered from 1 L fermented broth of Bacillus toyonensis AEMREG6. Yields of 3.8 g and 1.6 g pure bioflocculants with the respective highest flocculating activities of 94.9 percent and 83.2 percent were also obtained from 1 L, 72 h-fermented broths of Bacillus licheniformis and Bacillus sp. AEMREG7 respectively. Furthermore, all the three bioflocculants (REG-6, MBF-W7 and MBF-UFH), displayed thermal stability within the temperature range of 50 to 100 oC, with strong flocculating activities of over 80 percent against kaolin suspension over a wide range of pH range (3–11) and relatively low dosage requirements of 0.1-03 mg/ml in the presence of divalent cations in the treatment of kaolin clay suspension and Thyme River waters. Chemical composition analyses of the bioflocculants showed them to be glycoproteins with a predominantly polysaccharide backbones as shown by the following carbohydrate/protein (w/w) ratios: 77.8 percent:11.5 percent (REG-6); 73.7 percent:6.2 percent (MBF-W7) and 76 percent:14 percent (MBF-UFH).

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The aim of this study was to determine the mechanism of Bayer residue flocculation. Hematite was chosen as the test substrate as it is a common Bayer residue mineral. Batch settling tests were used to gain an understanding of the aggregation mechanism and to compare the effect of different parameters on flocculation performance. Flocculant adsorption isotherm measurements were related to changes in flocculation performance. Infrared spectroscopy was used to ascertain the configuration of adsorbed flocculant on the hematite surface.Batch settling tests showed that under strong caustic conditions hematite is naturally coagulated and that flocculation occurs via a bridging mechanism. This was confirmed by results which showed that factors which affect the bridging efficiency of the flocculant had an impact on aggregation. In particular, temperature and caustic concentration were found to greatly influence flocculation performance. This is due mainly to changes in the viscosity of the liquor, but may also be linked to the kinetics of particle-particle and particle-flocculant collisions resulting in a less efficient aggregation process. Ionic strength did not impact on performance as the flocculant was at a limiting size for synthetic liquors containing TC >/= 50 and TA >/= 10.Increasing ionic strength did not increase the adsorption density of the flocculant on hematite nor did altering the salt cation species from Na(subscript)2+ to Ca(subscript)2+. It can be concluded, therefore, that the flocculant is chemisorbed through surface complexation, since if it were electrostatically bound an increase in flocculant adsorption should have been observed with increasing ionic strength or cation charge. The surface complexation mechanism was supported by infrared results which showed that the flocculant vibrational bands were shifted on adsorption. The magnitude and direction of ++ / the shift suggests a bridging bidentate structure at pHs >/+ 11, while a monodentate structure exists at pH 7. In the presence of calcium there is also some electrostatically adsorbed flocculant at pH 7, with the calcium being in a bidentate chelating structure, but this is not observed at much higher pHs.The flocculant had an adsorption isotherm best described by a Langmuir-Freundlich expression with a monolayer coverage of ~ 164 mu g m(subscript)-2 of hematite. The adsorption density was lowered by the presence of carbonate and silicate and the action of both is thought to be due to their adsorption on active sites blocking polymer adsorption. Carbonate has an impact on flocculant adsorption at concentrations > 10 mg g(subscript)-1 while in the case of silicate ~0.2 mg g(subscript)-1 is required for the adsorption density of the flocculant to be affected. While it has been confirmed that silicate does adsorb on hematite, it was not possible to determine whether this was adsorption of a silicate species or an aluminosilicate species.X-ray photoelectron spectroscopy (XPS) showed conclusively that sodium is not involved in the adsorption of the carboxylate to the hematite surface. The lower peak shift between the backbone carbon and the carboxylate carbon suggests that the carboxylate is bonded directly to iron which has a low effective charge.Flocculant adsorption was atomistically modelled using decanoate and decandioate molecules. Modelling supported the results from XPS and infrared analysis and showed the carboxylate oxygen atoms in both organic molecules bonded directly to the surface iron atoms. Adsorption was preferred on near unhydrated surfaces with the most stable adsorption configuration being a non-symmetrical bridging bidentate structure as inferred from the infrared results.

Polyacrylamide is widely used as a flocculant but the influence of the molecular mass distribution upon flocculant activity is poorly understood. This thesis outlines the successful characterisation of ultrahigh molecular mass polyacrylamide solutions in terms of discrete solvated polymer coils and coil agglomerates. These features were correlated with the observed flocculation, demonstrating a number of solution state features required to improve flocculation activity.Aqueous solutions of polyacrylamide exhibit time-dependant behaviour affecting viscosity and polymer agglomeration. Improving the solvation of the polymer suppressed the agglomerates, as did manipulation of hydrogen bonding through the presence of salts. Limiting agglomeration through improved solvation apparently lowers a barrier to polymer interaction, such that the coils disperse but become more susceptible to reagglomeration under mild shear. These solvent modifications did not fully suppress the agglomerates.To fractionate polyacrylamide into a molecular mass distribution, flow field-flow fractionation (flow FFF) was chosen, coupled to a multi-angle laser light scattering (MALLS) photometer and a differential refractometer for molecular mass and concentration sensitive detection, respectively. For the first time, the analysis of high molecular mass polyacrylamides in water using the flow FFF-MALLS technique has been reported. However, a number of difficulties needed to be overcome, including shear artefacts, sample agglomeration and polymer-membrane interactions.Both polyacrylamide standards and commercial flocculants were amenable to the technique. Commercial flocculants were found to vary not only with regard to viscosity and mean molecular mass, but also with sub- and supramicron size agglomerates. The flow FFF technique is size-sensitive, which for some low molecular mass samples displayed an ++ / unusually wide elution profile, in apparent conflict with the molecular mass sensitive MALLS detector. It was concluded that polyacrylamide in solution exists simultaneously in three states: discrete polymer coils, entanglements of a several coils, and agglomerates with supramicron diameters.This thesis concluded with a comparison between the characterised polyacrylamides and observed flocculation activity on a standard kaolin substrate. Results show polymer with supramicron agglomerates produce the largest and most shear-resistant kaolin aggregates with a definite optimum agitation intensity. Higher molecular mass flocculants with less agglomeration bind fine particles under more gentle conditions but are deficient under increasing stress. Flocculants exhibiting coil entanglements showed poor activity and formed only small aggregates. A modified flocculation mechanism was proposed, in which supramicron polymer agglomerates play a critical role.

Graduation date: 1994