Marek Pawlik

An investigation of model quartz floc structure and composition was made using X-ray computed tomography to understand how floc breakage could contribute to consolidation in the dewatering process.It was concluded that floc macro-structure, defined by the connectivity and arrangement of sub-units in medium- and large-sized flocs plays an important role in the formation and breakage of flocs. Sub-units in flocs were found to be connected by lower solids concentration regions and the shared surface area between sub-units was found to decrease as a function of distance from the floc centroid. These observations indicated a tendency for flocs to break in a large-scale deformation mode (i.e., rupture along sub-unit boundaries), rather than smaller-scale fragmentation or attrition.From compression tests evaluated with CT scans and correlated with the gravimetric method, it was found that negligible changes in the solids concentration of flocs occurred at low applied pressures (
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Colloidal gels of cellulose nanocrystals (CNCs) were prepared by increasing ionic strength. The ultimate objective was to investigate stabilization of zirconia suspensions by using a small amount of CNC by changing electrolyte concentration or pH. First, the rheological behavior of CNC hydrogels in the presence of a monovalent electrolyte (NaCl) as a function of CNC and salt concentration was explored using a variety of linear and nonlinear rheological tests. Two step-yielding behavior was observed for CNCs in the presence of high electrolyte concentration from amplitude sweep experiment. The first yielding corresponds to the maximum in the loss modulus at the crossover point between storage and loss modulus due to disconnection of link between CNC clusters. The second yield stress is due to the deformation of clusters to smaller flocs and individual nanorods (corresponding to the change in the slope of loss modulus at higher shear strains). Small angle light scattering measurements, confocal laser scanning microscopy and polarized microscopy images confirmed the gradual breakup of clusters to smaller ones and eventually to nearly individual fibers with an increase in the applied shear strain and rate.CNCs were used as a dispersant to stabilize water-based zirconia suspensions. The zirconia suspensions as functions of solid content, pH and CNC concentration were studied to produce stable highly concentrated suspensions. The achieved stability was found to be due to the adsorption of CNC nanofibers around the zirconia particles revealed by Scanning Electron Microscopy (SEM) images. Suspensions with the lowest viscosity and highest stability over 24 h achieved at pH 4 i.e., 30 wt.% zirconia particles stabilized with the addition of 1 wt.% CNC.Finally, the effect of CNC and NaCl concentration was studied on the stability, adsorption, zeta potential, size, and rheology of slurries. The results confirm that the adsorption capacity of CNC on the surface of zirconia particles increases as salt concentration increases, associated with rise in the viscoelastic properties and denser structure on the surface of adsorbent. It has been concluded that zirconium oxide suspensions of high concentration (>30 wt.%) can be effectively stabilized by using a small amount of CNC (
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In relation to separation between apatite and hematite by froth flotation, surface-chemical interactions between apatite and hematite were studied as a function of pH and constituent ion concentrations. The surface charge characteristics of the minerals and their mixtures were determined using zeta potential measurements. Aggregation and dispersion phenomena were followed using laser light scattering techniques. Measurements of the amount of fine hematite attached to a large apatite crystal were conducted to assess the extent of slime coatings in the system. The results were supplemented by apatite solubility studies, and by measurements of calcium and phosphate adsorption on hematite under selected conditions.Calcium and phosphate released by apatite into supernatant strongly affected the zeta potential and aggregation-dispersion of hematite. The various phosphate species were attracted towards the positively charged hematite surface below the iso-electric point of the mineral (pH 6.8) while calcium cation showed high affinity towards the hematite surfaces above the iep value.Extensive coating of apatite by fine hematite occurred in the pH range from 7 to 9, and no coatings formed at pH 10-11 in the absence of calcium and phosphate ions. As the constituent ion concentrations increased in background solution, the amount of fine hematite on the crystal surface became independent of pH. These results were explained by a dispersing effect of phosphate at lower pH and by a coagulating effect of calcium at high pH.Co-adsorption of phosphate and calcium ions on the hematite surface was also observed, and it was proposed that calcium cations at pH 10-11 adsorbed in the inner Helmholtz plane while phosphate co-adsorption proceeded into the outer Helmholtz plane. Calcium adsorption caused hematite aggregation, while co-adsorption of phosphate led to partial dispersion. In apatite-hematite mixtures, addition of apatite resulted in hematite dispersion, while removal of apatite caused hematite aggregation. It was recognized that the tested minerals systems were under non-equilibrium conditions in terms of apatite dissolution. The aggregation-dispersion phenomena were interpreted in terms of variations in calcium and phosphate ion concentrations in solution in the presence and absence of apatite, and the resulting changes in the adsorption of those ions on hematite.

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Geopolymers (GPs) are a class of inorganic materials which can be used as construction and refractory cements and as functional materials for environmental applications. GPs are low CO₂ emissions binders with high durability that can replace traditional cementitious materials. However the effects and interactions of processing parameters on the different stages of GP setting (“geopolymerization”) are still under scrutiny and the molecular mechanisms and rate limiting steps controlling the setting kinetics are unknown. The crystallization in GPs, which ultimately controls their performance in advanced applications such as water purification and toxic waste encapsulation, is a poorly investigated topic.This dissertation provides new experimental evidences on the role of chemical composition and curing process on metakaolin-based GPs. Steady state and dynamic rheological studies, contact angle tests, microstructural (SEM), structural (XRD and FTIR) and mechanical analyses lead to better understanding of the fundamental transformations occurring during geopolymerization. GPs were seeded with different oxides and zeolites to determine the rate limiting step, increase the reaction rate and control the crystallization. This work contributes to clarification the complex effects of soluble silica on the geopolymerization process. It is shown that soluble and colloidal silicates (such as Na₄SiO₄ and Na₂SiO₃) can act as seeding agents, changing the geopolymerization rate limiting step at temperatures T≥35°C. However, they also slow down the reaction rate, possibly by forming passivation layers on the metakaolin particles, thus producing a more chemically stable and mechanically stronger amorphous gel. Silicates also decrease the water requirement in GPs and thus the porosity. Under certain conditions silicates can increase the percentage of crystalline Faujasite in GPs, but the crystallization process requires higher curing temperatures and times (T>40°C and t>4 days, depending on the amount of silicates). The alkali metals have also a structure-directing role in crystallization of GPs in the form of zeolite, favoring faujasite structure. Water has a templating effect in GPs, favoring the structure of zeolite LTA-type over hydrosodalite. This work also illustrates the compromises that need to be made when selecting appropriate processing parameters to tailor the rheology, structure and properties of geopolymers for specific applications.

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Aggregation and dispersion phenomena in the quartz-hematite system in the presence of polymers were analyzed through the measurement of adsorption density of polymers, turbidity of mineral suspensions, and zeta potential distributions. All the tests were performed on single minerals as well as on their 1:1 mixtures (by volume). A procedure was developed to determine the composition of the tested supernatants using scanning electron microscopy in order to enhance interpretation of zeta potential distributions obtained for mineral mixtures. Two carboxymethyl celluloses and a sodium lignosulfonate were chosen as polymeric additives.It was found that carboxymethyl celluloses selectively adsorbed on hematite in mixtures with quartz and that the selectivity of adsorption was a result of a high affinity of the polysaccharides towards the hematite surface. Lignosulfonate exhibited low affinity type of adsorption on both minerals although preferential adsorption on hematite at pH 5.5 was also observed. At pH 5.5, carboxymethyl celluloses acted as flocculants towards hematite, while lignosulfonate functioned as a coagulant. The polymers strongly dispersed hematite at high pH. The polymers also showed dispersing capabilities towards quartz regardless of pH. Selective flocculation of residual hematite from mixtures with quartz was observed at low pH despite extensive heterocoagulation between the minerals. Lignosulfonate behaved as a selective dispersant of hematite at high pH despite similar adsorption densities of the polymer on the tested minerals.It was generally determined that conditions (pH, polymer concentration) leading to strong dispersion were associated with very narrow zeta potential distributions (10-20 mV), indicative of uniform surface charging characteristics of the tested particle population. In contrast, polymer-induced aggregation of mineral particles (flocculation or coagulation) was accompanied by broad zeta potential distributions (up to 70 mV) resulting from non-uniform adsorption of the polymers on mineral particles and surface charge heterogeneities. Such broad zeta potential distributions were also characteristic of heterocoagulated mixtures in the absence of polymers.Several recommendations were made regarding measurements of zeta potential distributions in mineral mixtures. In order to characterize the entire population of a mineral mixture, such measurements should be performed on very dilute samples of constant turbidity to limit the differential settling of aggregated components.

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The gas dispersion and foaming properties of aqueous solutions of fatty acids of different hydrocarbon chain lengths were assessed through measurements of bubble size distributions, gas hold-up, foam volume and growth rate. The adsorption behavior of the tested fatty acids at the gas-solution interface was assessed and the results were supplemented by measurements of the partition of each surfactant between the bulk solution and foam phase as a result of continuous aeration.Two mechanisms of gas dispersion were identified depending on the pH and speciation of the tested solutions. Solutions of long chain fatty acids containing colloidal precipitates at low pH exhibited low surface tensions, and only a small decrease in bubble sizes was observed for such solutions compared to bubble sizes measured in water. This relatively small change in bubble sizes could theoretically be predicted based only on the corresponding change in the surface tension of the solutions. In contrast, true solutions of long chain fatty acids affected bubble sizes to a much greater extent even though their surface tension values were higher and in some cases comparable to the surface tension of water. A combination of the surface tension and surface tension gradient effects was found to be operative in this case. Experimental results strongly suggested that the associated acid species were more surface-active and more capable of reducing bubble sizes than the dissociated carboxylate anion. The ability of the surfactants to quickly generate a large foam volume was found to be a strong function of the chain length. Although bubble size measurements in bulk solution pointed towards similar gas dispersing abilities of fatty acids of different chain lengths, their foamabilities under the same conditions were remarkably enhanced by increasing chain length. Creation of large volumes of persistent foam was correlated with strong tendency to partition into the foam phase. Overall, the gas dispersing properties of fatty acids were comparable to those of a weak frother such as methyl isobutyl carbinol (MIBC), while only the foaming capabilities of hexanoate were similar to those of MIBC. Longer chain fatty acids were much stronger foaming agents than MIBC.

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Eight oil sands ores were tested in order to quantify the levels of humic acids in these samples through the alkali extraction test originally developed to determine the oxidation of bituminous metallurgical coals. The test gives a concentration of humic acids released from ores, which in combination with the measurement of the total organic carbon content in the alkali extracts provides a measure of ore/bitumen weathering. It was found that poor ores exhibited the highest tendency to leach large amounts of humic acids per gram of bitumen in the samples which was quantified using the absorbance at 520 nm obtained from the UV/visible spectra.The results of contact angle measurements of water on bitumen showed that bitumen became more hydrophilic as pH increased, and that the hydrophobicity of bitumen drastically decreased when the sample was artificially oxidized. Additionally, the results suggested that humic acids make bitumen hydrophilic only if they are part of the internal/surface bitumen structure. Slurries of good ores displayed higher yield stresses than slurries of poor ores. This result is explained by the higher bitumen concentration existing in slurries of good ores which leads to more aggregation. Additionally, it was shown that bitumen oxidation/hydrophobicity also affected the rheology of oil sands slurries which also explains that slurries of poor ores displayed lower cohesion/aggregation than slurries of good ores. Yield stress data agreed with data obtained from power draw measurements that showed that good processing ores required more power for mixing. Extraction data obtained from flotation experiments indicated that the role of humic acids naturally present in the ores was basically that of a depressant of bitumen since poor ores contained the highest proportion of humic acids per gram of bitumen. Overall, it is possible to assess the processability of oil sand ores by quantifying the occurrence of humic acids in the ores, and to correlate ore processability with the rheology of oil sands slurries. Although poor ores are characterized by lower viscosities and lower power requirements during mixing, the presence of humic acids in these ores and their depressing action also contribute to lower bitumen recoveries.

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Six carboxymethyl celluloses (CMC) of different degrees of substitution, molecular weights, and molecular weight distributions (MWDs) were researched as dispersants of oil sand slurries. The molecular weights and MWDs were determined by analytical ultracentrifugation. Viscometric studies on dilute solutions indicated semi-flexible, random coiling behavior of the polymers assuming an extended conformation in distilled water and coiled conformation in a dilute sodium chloride solution. It was found that the ionic strength, rather than pH or temperature, had the strongest effect on the intrinsic viscosity and conformation of CMC. Calculations of the persistence length and expansion factors of the polymers showed that the lowest molecular weight CMC was most flexible among the tested samples. As rheological and sedimentation tests showed, addition of CMC stabilized oil sand slurries towards aggregation and settling. All the polymers dispersed oil sand slurries by adsorbing on the solid particles and preventing mineral–mineral interactions. The molecular weight of the polymers was a more important factor than the degree of substitution in dispersing the slurries. The role of CMC was also to enhance the liberation of bitumen from solids. This role was analyzed through contact angle measurements in which it was demonstrated that all the CMC samples accelerated bitumen displacement and detachment from the illite surface. The CMC sample of the lowest molecular weight was found to be most effective in promoting bitumen displacement from the illite surface, and this action was attributed to the small effective size and high flexibility of the chain, which allowed the polymer to very closely approach the three-phase point of contact between the mineral, water, and bitumen. Wettability studies also revealed that CMC interacted very weakly with bitumen and did not permanently change the natural hydrophobicity of bitumen. Since a good rheological dispersant should not render bitumen hydrophilic and prevent bitumen extraction, the weak wetting action of CMC at the bitumen-solution interface is actually highly desirable. Overall, this dissertation demonstrated that CMC could play a significant role in improving the processability of low-grade oil sand ores, particularly since the polymer was also shown to be effective at neutral pH and low temperature.

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