Stokes’ law that is traditionally used for modeling the sedimentation of flocs, incorrectly assumes that the floc is solid and spherical. Consequently the settling rates of flocs cannot be estimated using the Stokes law. The application of fractal dimensions to study the internal structure and settling of flocs formed in lime softening process was investigated. An optical microscope with motorized stage was used to measure the fractal dimensions of lime softening flocs directly on their images in 2 and 3D space. The fractal dimensions of the lime softening flocs were 1.15-1.27 for floc boundary, 1.49-1.90 for cross-sectional area and 2.55-2.99 for floc volume. Free settling tests were used for indirect determination of 3D fractal dimension. The measured settling velocity of flocs ranged from 0.1 to 7.1 mm/s (average: 2.37 mm/s) for the flocs with equivalent diameters from 10µm to 260µm (average: 124 µm). Floc settling model incorporating variable floc fractal dimensions as well as variable primary particle size was found to describe the settling velocity of large (>60 µm) lime softening flocs better than Stokes’ law. Settling velocities of smaller flocs (<60 µm) could still be quite well predicted by the Stokes’ law. The variation of fractal dimensions with lime floc size in this study indicated that two mechanisms are involved in the formation of these flocs: cluster-cluster aggregation for small flocs (>60 µm) and diffusion-limited aggregation for large flocs (<60 µm). Therefore, the relationship between the floc fractal dimension and floc size appears to be determined by floc aggregation mechanisms. The settling velocity of lime softening flocs was also modeled by a general model that assumes multiple normally distributed fractal dimensions for each floc size. The settling velocities were in the range of 0-10mm/s and in good agreement with measured settling velocities (0.1-7.1mm/s). The Stokes’ law overestimates the settling velocity of lime flocs. It seems that the settling velocity of flocs is mainly controlled by aggregation mechanisms and forming large floc does not guarantee improved sedimentation. The multifractal analysis of lime softening flocs showed that these aggregates are multifractal and a spectrum of fractal dimensions is required to describe the structure of an individual floc.

Stokes’ law that is traditionally used for modeling the sedimentation of flocs, incorrectly assumes that the floc is solid and spherical. Consequently the settling rates of flocs cannot be estimated using the Stokes law. The application of fractal dimensions to study the internal structure and settling of flocs formed in lime softening process was investigated. An optical microscope with motorized stage was used to measure the fractal dimensions of lime softening flocs directly on their images in 2 and 3D space. The fractal dimensions of the lime softening flocs were 1.15-1.27 for floc boundary, 1.49-1.90 for cross-sectional area and 2.55-2.99 for floc volume. Free settling tests were used for indirect determination of 3D fractal dimension. The measured settling velocity of flocs ranged from 0.1 to 7.1 mm/s (average: 2.37 mm/s) for the flocs with equivalent diameters from 10µm to 260µm (average: 124 µm). Floc settling model incorporating variable floc fractal dimensions as well as variable primary particle size was found to describe the settling velocity of large (>60 µm) lime softening flocs better than Stokes’ law. Settling velocities of smaller flocs (<60 µm) could still be quite well predicted by the Stokes’ law. The variation of fractal dimensions with lime floc size in this study indicated that two mechanisms are involved in the formation of these flocs: cluster-cluster aggregation for small flocs (>60 µm) and diffusion-limited aggregation for large flocs (<60 µm). Therefore, the relationship between the floc fractal dimension and floc size appears to be determined by floc aggregation mechanisms. The settling velocity of lime softening flocs was also modeled by a general model that assumes multiple normally distributed fractal dimensions for each floc size. The settling velocities were in the range of 0-10mm/s and in good agreement with measured settling velocities (0.1-7.1mm/s). The Stokes’ law overestimates the settling velocity of lime flocs. It seems that the settling velocity of flocs is mainly controlled by aggregation mechanisms and forming large floc does not guarantee improved sedimentation. The multifractal analysis of lime softening flocs showed that these aggregates are multifractal and a spectrum of fractal dimensions is required to describe the structure of an individual floc.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1965. / Typescript. Vita. Description based on print version record. Includes bibliographical references.

Hydrocyclone density-driven particle separation may offer up improved settling performance for wastewater treatment facilities experiencing poor settleability. Hydrocyclones are fed mixed liquor through the feed inlet and experience a centrifugal motion that separates solids based on density. The variation in hydrocyclone nozzle sizes will report different calculated hydraulic and mass split percentages for the overflow and underflow. Previous research conducted with hydrocyclones have at multiple full-scale facilities used a 10 m3/hr hydrocyclone to promote better settleability as well as aid the formation of aerobic granular sludge (AGS). There has been a multitude of settling improvement experiments and initiatives for full scale wastewater treatment. However, little research has been produced utilizing larger hydrocyclones (20 m3/hr) at a full-scale wastewater treatment facility during continuous operation. Two Hampton Roads Sanitation District (HRSD) plants served as sites for this research: James River (JR) Wastewater Treatment Plant located in Newport News, VA and Urbanna (UB) Wastewater Treatment Plant located in Urbanna, VA. Both treatment facilities have utilized the hydrocyclone for more than two years, to fulfill wasting requirements. The JR plant operates the hydrocyclone continuously for wasting purposes, while UB only uses the hydrocyclone for approximately 30-45 minutes per day. In order to evaluate the effectiveness of the hydrocyclone and its overall impact on settleability at the JR plant, eight hydrocyclones were installed. JR samples were taken from the underflow sample port (representing a mixture of underflow samples representing the number of hydrocyclones operational at the sample time) and overflow samples were taken from the outfall point of a single hydrocyclone. The UB plant only operated one 5 m3/hr hydrocyclone on Treatment Train 1 during wasting operations, while Treatment Train 2 served as the control train for the duration of this research. Hydrocyclone performance at JR was assessed through direct measurement of hydraulic and mass split of the underflow and overflow components, initial settling velocity (ISV), sludge volume index (SVI), and SVI5/SVI30 ratio. UB hydrocyclone and settling performance was measured by ISV, SVI5, SVI30, and SVI5/SVI30 ratios during different comparison experiments: hydrocyclone vs. no hydrocyclone, hydrocyclone vs. polymer addition, and hydrocyclone with polymer addition to Train 1 vs. polymer-only addition to Train 2. Nutrient concentrations from both treatment trains were collected and analyzed to determine any significant changes based on hydrocyclone use. T-test statistical analysis, and a dose response analysis included direct measurements of the ISV, SVI5, SVI30, mass split percentages, along with the effect of polymer with and without the use of a mechanical selector. Hydrocyclone settleability measurements at JR over time revealed a statistically significant positive correlation with the ISV, SVI5, and SVI30 measurements of the aeration effluent. Therefore, the hydrocyclone statistically had a strong impact on three settling parameters that are instrumental in determining overall settling efficiency. Statistically, no strong correlation was determined between the hydrocyclone operation and the total phosphorus (TP) concentration in the secondary effluent, or the ferric addition to the secondary clarifiers. The dose response based on the underflow ISV rate provided understanding of the nozzle comparison and the effect it provided to the underflow sample. Hydrocyclone performance at UB was hindered by the re-seed of Train 1 (inDENSE™) due to over wasting, and most of the data were not representative. Before the re-seed, hydrocyclone performance was improving the overall settleability of the mixed liquor in comparison to Train 2 (Control). All settling parameters measured were in favor of the hydrocyclone operation. After the re-seed the plant mixed liquor changed microbial populations for a brief time and was not representative of the overall treatment efficacy. The hydrocyclone did provide a quicker settling velocity than the polymer addition when the polymer addition was steady, and through both polymeric spikes. Polymeric addition to both trains, while inDENSE™ train still employing the hydrocyclone did not provide any conclusive data as to whether polymer addition with the use of a hydrocyclone was more effective than polymer-only addition. Nutrient profiles from UB did not provide any change in NH4-N, NO3-N, NO2-N, or PO4-P, with the hydrocyclone being operational or not on the secondary clarifier effluent. / Master of Science / Wastewater treatment facilities rely on settling tests to be indicators for plant settling performance. A way to improve plant settling is to separate the sludge on a density basis and retain the dense sludge in the system for better performance downstream, while the less dense sludge is taken out of the system. By implementing a mechanical device that can ensure the separation of dense material and be retained in the system can aide in improved plant settling performance by improving settling parameter measurements. With the ability of using a mechanical device (a hydrocyclone) to physically separate sludge on a density-basis, it will improve settling measurements of the plants taken by operators on a daily basis.

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The main objective of the present work is the removal of inclusions from silicon scrap and metallurgical grade silicon. To reach this goal, two various routes are investigated. First, settling of SiC particles from molten silicon followed by directional solidification is reported in this thesis. Then, removal of SiC and Si3N4 inclusions in silicon scrap by filtration with foam filters and wettabilities of silicon on graphite materials are studied. To supply the increasing needs of the photovoltaic industry it is necessary to produce a low cost silicon feedstock. One of the many routes established from the industry is the Solsilc project. This project aims to produce solar-grade silicon by carbothermal reduction of silicon, based on the use of very pure raw materials. The high carbon content of about 700 mass ppm of the silicon in the form of SiC particles, needs to be removed before the Solsilc silicon could be used as a feedstock to PV industry. Settling of SiC particles in molten silicon was investigated. This part of the work was in cooperation with SINTEF Materials &amp; Chemistry. Two experiments were conducted and the cast silicon ingots were analyzed by light microscopy and LECO carbon analyzer. The results showed that the number of inclusions in the middle of the ingots was less than in the bottom and top. The removal efficiency was above 96% in the middle part of an ingot and the total carbon content measured by LECO was &lt; 25 mass ppm. The difference in density between the particles and the melt gives the SiC particles a relatively high settling velocity leading to a high removal efficiency. Pushing and engulfment of SiC particles by solidification front was also studied. Directional solidification of silicon that followed settling pushes the particles to the top of the ingot. The presence of SiC particles in the middle of the ingot is explained by engulfment. Top-cut silicon scrap represents a considerable loss of the PV silicon. Removal of inclusions from the silicon scrap would make it possible to recycle it to feedstock in the PV cell production. This was carried out by filtration with ceramic foam filters. Carbon and SiC foam filters with various pore sizes were employed in the filtration experiments. They were provided by Eger-Sørensen, a Norwegian company and Foseco AB in Sweden. The top-cut silicon scrap came from REC-Scan Wafer. Characterization of inclusions in silicon scrap before and after filtration experiments took place. Two techniques were developed and used in this work. First, extraction of inclusions by acid dissolution of the silicon was carried out. The SiC and Si3N4 particles collected afterwards were analyzed and counted by automated light microscopy. In the second technique, silicon samples were ground and polished with diamond paste. Microscopic analysis consisted of measuring the surface area of the inclusions found in the silicon samples. Results show that inclusions in top-cut solar cell silicon scrap are needle-like Si3N4 particles and round SiC inclusions. The removal efficiency for a 30 ppi SiC filter is more than 99%. The inclusions remaining after filtration are mainly SiC particles smaller than 10 µm. The experiments show that the filtration efficiency increases with decreasing filter pore size. Some filter cakes that mainly consist of large Si3N4 inclusions are found on the top surface of the filter. Deep bed filtration is the mechanism responsible for the removal of small particles. After taking into consideration various models for the foam filters the main conclusion is that interception seems to be the main removal mechanism of inclusions in silicon. Settling appears to play a minor role for our system. A new model named branch model explains better the experimental results. Due to the low wetting angle between molten metal and the filter material, capillary forces drive the melt through the filter. Therefore, the melt velocity through the filter is high. This justifies the usage of potential flow in the branch model.   It is shown that molten silicon may be contaminated in contact with the refractories. Since purity for solar cell silicon is crucial, contamination must be minimized. Graphite crucibles may be a source of relatively high levels of Al, Fe, and P. In the filtration process, wettability of the molten silicon with the filter material is very important. Thus, spreading and infiltration of molten silicon into the graphite substrates were also investigated in this thesis. Five different graphites were provided by Svenska Tanso AB. They are in use as refractories in the PV industry and vary from each other in porosity, density, and average pore size. The sessile drop technique is employed to study the wetting behavior of molten silicon on the graphite materials. The measured contact angles show that molten silicon does not initially wet carbon materials. However, due to the chemical reaction between Si and C, a SiC layer is formed in the interface between molten silicon and the graphite. Formation of this layer lowers the contact angles finally reaching equilibrium wetting angles of molten silicon with SiC materials. Spreading of molten silicon is affected not only by the reaction formed SiC layer, but also by the surface finish. The final contact angles, also called equilibrium contact angles, decrease with increasing surface roughness of the graphites. Infiltration of silicon into graphites is mainly related to the average pore size of graphite materials. Materials with large pores are penetrated deeper by the liquid silicon. Zero contact angles of the silicon with graphites are found in materials with both high surface roughness and large average pore size. These results indicate that graphites for use in the PV industry should have a small average pore size. The surface of the graphite in direct contact with silicon should be smooth (low roughness).

This study has analyzed Pb-210 activity in cores taken from northern coastal South China Sea (SCS) and the area west of the Luzon Strait (LS) in order to estimate the sedimentation rates and the mass accumulate rates which allow a direct comparison with measured settling particulate fluxes. Also, the particulate and dissolved Po-210 and Pb-210 distributions in a water column located to the west of the LS were measured and compared with earlier data. Core samples were collected at Stations G and H (box cores) during the ORI-688 cruise (July, 2003) and at M1, F and 1 (box cores) during the ORI-722 cruise (June and July, 2004). The water column samples were collected at S7 during the ORI-763 curise (August, 2005). Stations H, G and 1 are located in the continental shelf of the northern SCS; Stations, M1, F and S7 are located in the deep water area west of the LS. The water content of the cores at F and M1 west of the LS is mostly around 40%~55%, while that of the cores at H, G and 1 in the coastal area of the northern SCS ranges about 25%~37%. The total organic matter as estimated from the total loss on ignition at F and M1 is, respectively, about 6% and 12% on the average. The coastal cores contain about 3%~8% total organic matter. The Pb-210 activity generally decreases with the core depth but reaches a constant at a certain depth, below which the excess Pb-210 vanishes in the core. Based on the excess Pb-210 distributions in the cores, the estimated sedimentation rates vary between 16 and 52cm/100yr. At M1 station, the mean Pb-210 flux and the mean mass flux are, respectively, 129 dpm/m2/d and 0.55 g/m2/d as determined from the deepest sediment trap(2848m) (Chung et al., 2004). These values are much smaller than what were estimated from the cores: the Pb-210 flux at 201 dpm/m2/d and the mass flux at 5.3 g/m2/d. The Pb-210 flux obtained from the trap is 65% of that measured from the core, and the mass flux from the trap is only 10% of that from the core. These large differences may arise from an over estimation of the sedimentation rate in the core (due to bioturbation) and near-bottom lateral transport of sediments from elsewhere. Compared to the basin west of the LS, the mass flux at the coastal area is much higher but the Pb-210 flux is much lower, probably due to the fact that shallow water has little amount of Pb-210 to be scavenged and no boundary scavenging effect has been observed. The Pb-210 specific activity at S7 station ranges from 155 to 900 dpm/g; the Po-210 activity at this station ranges from 78 to 507dpm/g. The particulate Po/Pb ratio is about unity at 1800m depth, but the ratio at all other depths is less than unity, indicating that the Po-210 is deficient relative to Pb-210 in particles at this station. The total (dissolved + particulate) Po/Pb ratio in the water column is generally less than unity, showing Po-210 deficiency relative to Pb-210 probably due to absorption and/or adsorption of Po-210 by plankton and other organisms. The Po/Pb ratio for the water column averages about 0.6, corresponding to a mean residence time of 0.83yr for Po-210 removal.

In kraft pulp mills, lime is used to convert sodium carbonate to sodium hydroxide (Ca(OH)2). The causticizing reaction precipitates lime mud which is washed, dewatered, and calcined in a lime kiln to generate lime for reuse. Clean, dry, and more stable lime mud helps reducing the energy usage of the kiln, improving burner flame stability, minimizing ring formation, and alleviating emissions of reduced sulphur gases from the kiln stack. The dewatering efficiency of lime mud is greatly affected by the mud and liquor properties, and the equipment design and operation. The properties of the mud vary continuously due to changes in the liquor strength, lime quality and dosage, which is known as the “liming ratio”. Many studies have been carried out to relate lime mud properties to dewatering and filtration behaviours, the mechanisms by which lime mud becomes difficult to settle and filter are not well understood. A systematic study was therefore conducted to examine the effect of the liming ratio on the settling rate and filterability of lime mud. The results show that the mud settling rate and filterability decreased with an increase in liming ratio. The effect was more noticeable as the liming ratio exceeded a critical level leading to an overliming condition. The results also show that the particle size of the resulting lime mud did not appreciably change with liming ratio. Therefore, the decrease in settling rate and filterability cannot be attributed to the smaller particle size of Ca(OH)2 compared to that of lime mud as commonly believed. Rather, it was caused by a change in zeta potential of Ca(OH)2-containing mud particles.This study also shows that the zeta potential of the mud slurry increases proportionally to the free lime content in the lime mud. This suggests that the zeta potential can be used to indicate the extent of overliming in the causticizing plant. The correlation between zeta potential and free lime content can be used to develop an on-line overliming monitoring system to help regulate theamount of lime addition to the system to achieve optimum operating conditions for the mud settling and filtering equipment.

In kraft pulp mills, lime is used to convert sodium carbonate to sodium hydroxide (Ca(OH)2). The causticizing reaction precipitates lime mud which is washed, dewatered, and calcined in a lime kiln to generate lime for reuse. Clean, dry, and more stable lime mud helps reducing the energy usage of the kiln, improving burner flame stability, minimizing ring formation, and alleviating emissions of reduced sulphur gases from the kiln stack. The dewatering efficiency of lime mud is greatly affected by the mud and liquor properties, and the equipment design and operation. The properties of the mud vary continuously due to changes in the liquor strength, lime quality and dosage, which is known as the “liming ratio”. Many studies have been carried out to relate lime mud properties to dewatering and filtration behaviours, the mechanisms by which lime mud becomes difficult to settle and filter are not well understood. A systematic study was therefore conducted to examine the effect of the liming ratio on the settling rate and filterability of lime mud. The results show that the mud settling rate and filterability decreased with an increase in liming ratio. The effect was more noticeable as the liming ratio exceeded a critical level leading to an overliming condition. The results also show that the particle size of the resulting lime mud did not appreciably change with liming ratio. Therefore, the decrease in settling rate and filterability cannot be attributed to the smaller particle size of Ca(OH)2 compared to that of lime mud as commonly believed. Rather, it was caused by a change in zeta potential of Ca(OH)2-containing mud particles.This study also shows that the zeta potential of the mud slurry increases proportionally to the free lime content in the lime mud. This suggests that the zeta potential can be used to indicate the extent of overliming in the causticizing plant. The correlation between zeta potential and free lime content can be used to develop an on-line overliming monitoring system to help regulate theamount of lime addition to the system to achieve optimum operating conditions for the mud settling and filtering equipment.

<p> A bench scale continuous settling unit was constructed and its operation compared to results of batch settling tests. The particulate solids used were polystyrene spheres with a mean diameter of 285 microns. The concentration profile within the slurry in both the batch and the continuous studies was observed using a light extinction technique.</p> <p> The results show that for the material used in this study, the solids flux limitation of the continuous settler could not be exceeded: the limiting condition in the operation was always the clarification capacity or upward velocity. The batch flux plot shows that this will be the case for any material which exhibits a flux plot that is essentially a single concave curve downwards.</p> / Thesis / Master of Engineering (MEngr)