The Pennsylvania State University. The Graduate School. College of Engineering ELECTROSTATIC & ELECTROKINETIC DEWATERING OF MATURE FINE TAILINGS

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1 The Pennsylvania State University The Graduate School College of Engineering ELECTROSTATIC & ELECTROKINETIC DEWATERING OF MATURE FINE TAILINGS A Thesis in Chemical Engineering by Anwesha Basu Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science August 2013

2 The thesis of Anwesha Basu was reviewed and approved* by the following: Darrell Velegol Distinguished Professor of Chemical Engineering Thesis Advisor Themis Matsoukas Professor of Chemical Engineering Kyle Bishop Assistant Professor of Chemical Engineering Andrew Zydney Walter L. Robb Chair and Professor of Chemical Engineering Head of the Department of Chemical Engineering *Signatures are on file in the Graduate School

3 iii ABSTRACT Mature Fine tailings (MFTs) are the byproducts from the mineral processing operations of bitumen mining. They are alkaline suspensions containing high fines (particles < 44 µm) content. Dewatering and consolidation of MFTs are important for both recycle of water to the oil extraction units and reclamation of clay. However, those are difficult to achieve by sedimentation alone due to low hydraulic conductivity of the tailings. In this dissertation, we have looked at how different surface phenomena like colloidal destabilization and electrokinetic transport mechanisms can bring about water separation in MFTs. Suppression of charges on a colloidal particle electrostatically to allow aggregation has always been a popular means of dewatering the tailings. In this study, we used several versions of a commercial coagulant with non-ionic and anionic polymeric flocculants to treat three different extraction tailings obtained from the respective resources. The polymers were tested for their ability to flocculate the tailings by zeta potential and floc size measurements. Diffusiophoresis is a well-understood flow mechanism that can produce micro-flows even in the inaccessible micro and nano-channels present in a porous medium, for e.g. the concentrated tailings deposits. In our system of MFTs, we observed that flows induced by ionic gradients due to dissolution of inorganic salts like calcium carbonate aid dewatering by a phenomenon called diffusioosmosis. The role of such flows in enhanced dewatering of MFTs is explored in this study in great detail. Thus, the above methods, though governed by different scientific mechanisms, are primarily dependent on the surface charge of the tailings. Although, the results presented here may not give a direct comparison between the dewatering efficiencies for each of these studied iii

4 iv methods, but can definitely help us appreciate the fact that both of them can simultaneously be utilized towards achieving better water separation from the tailings and therefore improved consolidation of clay sediments. Keywords: mature fine tailings, dewatering, destabilization, diffusioosmosis, layering and consolidation. iv

5 v TABLE OF CONTENTS List of Figures... v List of Tables... vi Acknowledgements... vii Chapter 1 Introduction Motivation Research Objectives Overview of Thesis Original Contribution... 5 Chapter 2 Background Tailings and Water Treatment Current Techniques Coagulation and Flocculation Studies for the Oil Sands Tailings Electrokinetic Dewatering Step Forward Chapter 3 Effect of Chemical Treatment (addition of coagulants and flocculants) on field MFTs Surface Phenomena and Properties of Clays Electrical Double Layer DLVO Theory Materials and Methods Materials Zeta Potential Measuring Experiments Floc Size Experiments Optical Microscopy Results and Discussion Zeta potential and size distribution of the starting MFT samples Starting percent solids of the tailings have negligible effect on the zeta potential measurements Impact of addition of inorganic and organic coagulants to MFTs Impact of addition of NP flocculants to the tailings Impact of addition of CP with NP Zeta potential and size distribution analyses with the model clay systems: Kaolinite and Bentonite Zeta potential and size distribution of model clays measured in 1mM KCl solution Impact of addition of cationic coagulant CP v

6 vi Effect of recycled water chemistry on the flocculation properties of the clays Impact of medium molecular weight CP2 on the size distribution of the model clay kaolinite Impact of low molecular weight CP1 on the model clays Impact of anionic polymer (AP) on the model clays Impact of CP and anionic polymer AP on the model kaolinite clay Impact of non-ionic polymer NP on the clays Impact of the combination of high molecular weight CP3 and NP on the model kaolinite clay Chapter 4 Salt Dissolution-Induced Fluid Pumping in MFTs Introduction & Background What causes diffusiophoretic flows to occur? Calcium carbonate (CaCO 3 ) particles acting as micro battery Calcium Carbonate in Dewatering of MFTs Materials and Methods Materials and Characterization Settling Experiments Zeta Potential Measurement Diffusioosmosis Experiments Water chemistry analysis Characterization of sediment layers Results and Discussion Water separation by settling Electrostatic Destabilization Diffusioosmostic Flow Studies Improved Dewatering by Diffusioosmosis Consolidation of tailings Quality of water Chapter 5 Conclusions and Future Work Conclusions Future Work Appendix A Preparation of the tailings sample for study Appendix B Characterization of the layers Appendix C Model for the Concentration Profile References vi

7 vii LIST OF FIGURES Figure 1-1. (a) A typical tailings pond in Canada (b) Clay sediment needs further remediation Figure 1-2. (a) Image showing water separation at the top of a drop of MFT that was placed on a piece of PDMS block coated with calcium carbonate powder. Image showing dewatering (b) when the tailings sample is allowed to settle on its own (c) when calcium carbonate is added to the MFT Figure 2-1. CHWE process used for bitumen extraction Figure 2-2. Schematic Flow Chart of CT Process (Syncrude Canada) Figure 2-3. Electroosmosis in a capillary Figure 3-1. Schematic of an Electrical Double Layer Figure 3-2. The DLVO Plot Figure 3-3. (a) Malvern Zetasizer Nano ZS (b) Folded capillary cell Figure 3-4. Beckman Coulter Particle Size Analyzer Figure 3-5. Zeta potential of MFT 1, MFT 2 and MFT 3 tailings measured in different background electrolytes: DI water, recycled water and 1mM KCl. MFT 2 tailings showed the lowest value of zeta potential among the three tailing samples. All the tailings have a lower (closer to zero) zeta potential when each was diluted in their respective recycled water Figure 3-6. Size distribution of MFT 1, MFT 2 and MFT 3 tailings as measured in DI water: MFT 2 tailings are seen to have a higher fraction of larger particles compared to MFT 1 and MFT 3 tailings Figure 3-7. Zeta potential of MFT 1, MFT 2 and MFT 3 tailings versus ferric sulfate dosage measured in 1mM KCl solution. It was seen that the MFT 2 tailings were much easier to neutralize with aqueous ferric sulfate than the other tailings. However, all the tailings required a very high dosage of the coagulant to effect complete charge neutralization Figure 3-8. Zeta potential of MFT 1, MFT 2 and MFT 3 tailings versus polymer dosage CP3 when measured in 1mM KCl solution at a constant ph 6.5. MFT 2 and MFT 3 tailings require quite a lower polymer dosage than MFT 1 to reach the isoelectric point. Overall, it can be seen that the tailings require a very high dosage of the coagulant to effect complete charge neutralization vii

8 viii Figure 3-9. (a) Particle size distribution of MFT 1 tailings with increasing CP3 dosage. (b) Particle size distribution of MFT 2 tailings with increasing CP3 dosage. (c) Particle size distribution of MFT 3 tailings with increasing CP3 dosage Figure Particle size distribution of MFT 3 tailings with increasing amounts of low molecular weight CP Figure Measured zeta potentials of (a) MFT 3, (b) MFT 2 and (c) MFT 1 tailings over time with the addition of the optimum * dosage of NP. The starting solids state of the tailings was % and a polymer dose of approximately 3500 ppm, 2000 ppm and 3000 ppm were added to the MFT 3, MFT 2 and MFT 1 tailings, respectively Figure Measured zeta potential of tailings with increasing levels of NP. Only a slight change in zeta potential is observed with increasing polymer dosage for the tailings Figure Size distribution of flocs formed by addition of NP to the MFT 1 tailings. The average size distribution did not change much from the initial condition Figure Size distribution of flocs formed by addition of NP to the MFT 1 tailings pre-treated with a constant 2000 ppm of CP Figure Size distribution of flocs formed by addition of NP to the MFT 2 tailings pre-treated with a constant 1000 ppm of CP3. Here, it can be seen that the average size distribution of the particles did not change much with increasing non-ionic polymer concentration Figure Particle size distribution of kaolinite and bentonite clays diluted in MFT 3 recycled water (RW3) Figure Zeta potential of the clay particles diluted in (a) MFT 3 recycled water (RW3) and (b) MFT 2 recycled water (RW2) when treated with CP3. All measurements were taken in a 1mM KCl solution. It can be seen that kaolinite reaches its isoelectric point at a lower polymer dosage when diluted in MFT 2 recycled water. Bentonite, on the other hand, shows very insignificant surface charge reduction within the studied range. This behavior is similar to the tailings Figure Floc size distribution of kaolinite clay diluted in MFT 2 recycled water with increasing concentrations of CP Figure Floc size distribution of bentonite clay diluted in MFT 3 recycled water with increasing concentration of CP viii

9 ix Figure Floc size distribution of kaolinite clay with increasing concentration of medium molecular weight CP2. Increasing the amount of polymer did not have an impact on the floc size Figure Zeta potential of the clay particles diluted in MFT 2 recycled water when treated with low molecular weight CP1. All measurements were taken in a 1mM KCl solution. It can be seen that kaolinite reached its isoelectric point at around 600 ppm of polymer concentration. Bentonite, on the other hand, showed very insignificant surface charge reduction within the studied range. This behavior is similar to the tailings Figure Floc size distribution of kaolinite clay with increasing concentration of low molecular weight CP1. An increase in the amount of polymer increases the average the floc size, but over dosage beyond the isoelectric point shows a reduction in the average size Figure (a) Floc size distribution of kaolinite clay diluted in MFT 3 recycled water with increasing concentration of CP3 added while keeping the concentration of AP at 1000 ppm. An increase in cationic polymer dosage showed an increase in average particle size. (b) Floc size distribution of kaolinite clay diluted in MFT 3 recycled water with increasing concentration of AP added. Increase in polymer dosage showed an increase in average size Figure 3-24.Optical microscope images for kaolinite clay samples diluted in MFT 3 recycled water treated with (a) 1000 ppm of CP3 and 1000 ppm of NP, (b) 2000 ppm of CP3 and 1000 ppm of NP and (c) 3000 ppm of CP3 and 1000 ppm of NP. The floc size distribution of kaolinite clay became better with increasing concentration of the cationic polymer Figure 4-1. Schematic showing the directions of the particle and fluid flow constituting diffusiophoresis for a system where both the particle and the substrate are negatively charged, and the diffusivity of the anion of is larger than the diffusion constant of the cation. The flows around the particle due to electrophoresis and chemiphoresis are shown by the corresponding black and red border on the arrow. If the diffusivities of the ions or the surface charge on the particle or substrate were reversed, the electrophoretic and electroosmotic flows shown in this schematic would reverse their directions, whereas the chemiosmotic and chemiphoretic components would remain unchanged Figure 4-2. (a) CaCO 3 micro particle pumping of 1.4 µm sulfate-functionalized polystyrene latex tracer particles (spsl). (b) Two interacting CaCO 3 micro particle pumps Figure 4-3. (a) Scanning Electron Microscopic image clearly shows poly-dispersity both in terms of size and structure. (b) Particle Size Distribution of the original tailings sample ix

10 x Figure 4-4. (a) Centrifuge tube with MFT and salt (b) Centrifuge used for sedimentation experiments Figure 4-5. (a) The diffusioosmotic study setup using two square capillaries of different sizes (b) Optical Microscope: vertical set up for the experiments Figure 4-6. (a) Schematic showing the set up for settling experiments indicating the heights of the sediment and the supernatant. Settling curves for (b) 1g and (c) 2000 g Figure 4-7. (a) Plot of zeta potential of the diluted tailings treated with salts over time. Calcite addition reduces the zeta potential of the fines from -39 mv to -26 mv after about 3 hours. Calcium sulfate reduces zeta much more, closer to the isoelectric point. (b) Time lapse images of the fines in the tailings before and after calcite addition. The fines are seen to aggregate over time thereby giving rise to bigger aggregates as seen Figure 4-8. (a) Schematic of Diffusioosmosis experimental set up. A smaller glass capillary (0.3 mm I.D.) placed inside a larger glass capillary (0.9mm I.D.) with the ends sealed with wax. The fine particles suspended in DI water are introduced in the outer capillary while different salts like calcium carbonate, calcium sulfate and sodium sulfate or DI water (as control) are introduced in the inner capillary. (b) Microscopic image of the capillary system at time = 2 minutes for 0.5 mm calcium carbonate and (c) 5 mm calcium sulfate solution. The direction of the electric field and the direction in which the fines move are shown by the pointers Figure 4-9. (a) Microscopic image of the capillary system at time ~ 2 minutes for 0.5 mm calcium carbonate in the outer capillary and the MFT fines inside the inner capillary (b) DI water. The direction of the electric field and the direction in which the fines move are shown by the pointers. DI water system, however, has no electric field generated as there is no ion gradient Figure Image showing colored solution of calcium carbonate settles through DI water Figure Velocity profile for tailings fines in calcium carbonate concentration gradient of 0.5 mm and calcium sulfate concentration gradient of 0.5 mm at a distance of about 150 mm from the mouth of the capillary. The velocities of the fines inside the inner capillary are almost an order higher in a CaCO 3 gradient compared to the same CaSO 4 gradient Figure (a) Image of the five visually distinct layers formed on CaCO 3 treatment. (b) EDS for the topmost layer showing maximum Carbon in the sample x

11 xi Figure (a) EDS for the supernatant layer showing no detectable calcium peak (b) Particle size distribution of the supernatant layers for the untreated and the treated MFT samples xi

12 xii LIST OF TABLES Table 3-1. Zeta potential of tailings in different solutions with ph Table 3-2. Zeta Potential of MFT 3 tailings with different dosages of a coagulant at different initial solids content Table 3-3. Zeta potential of fines in the tailings at a lower CP3 dosage as measured in 1mM KCl solution: The yellow color highlights the minimum dosage at which flocs were seen forming visually for different tailings Table 3-4. Zeta potential of fines in the tailings with low molecular weight CP1 dosages: The yellow color highlights the minimum dosage at which flocs are seen forming visually for different tailings Table 3-5. Zeta potential of fines in the tailings with high molecular weight CP3 followed by increasing dosages of NP measured in 1 mm of KCl solution Table 3-6. Zeta potential of model clays in different solutions with ph Table 3-8. Zeta potential of model clays treated with AP Table 3-9. Zeta Potential of model clays with NP dosage Table 4-1. Concentration of different species in the treated MFTs xii

13 xiii ACKNOWLEDGEMENTS I would humbly and wholeheartedly like to thank my thesis adviser and guide Professor Darrell Velegol for his unparalleled support and contribution both technically and otherwise towards my research. I must admit that he has been more of a friend and family, which is special when you are some thousands of miles away from your own. I wish to thank Dr. Harpreet Singh, Dr. Michael Poindexter and Dr. Wu Chen for their excellent guidance and encouragement throughout the course of my project during my internship at Dow. Special thanks to all dear Velegol lab mates, Saul Garcia, Jason Tubbs, Julie Anderson, Nicole Wonderling and Karol Confer for their great help in various lab activities and valuable discussion throughout the project. My friends in State College have been most amazing in lifting my spirits up whenever I have been low. Thanks to them too for every sip of coffee and interesting conversations shared and all other good fun. I want to thank the funding sources (NSF IDR Grant ) for providing financial support. The financial support from Dow Oil & Gas is also gratefully appreciated. I am forever grateful to my family for their love and support. No acknowledgement would be good enough to express my gratitude and love towards all four of my parents. And most importantly, I would take the privilege of thanking my most loving husband, Somnath, for being there always in all good and bad. His guidance and support in every aspect of life is something I m blessed with. xiii

14 Chapter 1 Introduction 1.1. Motivation Athabasca oil sands deposit in Alberta (north Canada) alone produces billion barrels of oil, which is believed to be enough to meet the oil requirements of Canada for the next 400 years. Like other mining operations (in-situ, surface etc.) water-based extraction of oil sands produces large amounts of waste composed of quartz, clays, residual bitumen and water, commonly known as tailings 1. This slurry is disposed into a tailings pond where the coarse particles (mainly the sand and silt) settle rapidly leaving a stable suspension of fine (<44 µm) particles in the middle and a clear supernatant layer of water on the top. This water is recycled to the bitumen extraction units 2. However, the intermediate stable suspensions have a very poor water releasing property. After about 3 5 years of settling, they form mature fine tailings. The MFTs are predicted to take centuries to consolidate without any treatment 3. Thus, the increasing storage of these sediments in the tailings ponds imposes huge pressures not only on the landscape but also on the operating cost required for extracting the natural resources. It has been reported that there are currently more than 170 square kilometers of tailings ponds in Alberta alone and are predicted to be 250 square kilometers by

15 (a) (b) 2 Figure 1-1. (a) A typical tailings pond in Canada (b) Clay sediment needs further remediation. During Spring 2012, I was trying to devise some dewatering experiments with the MFT sample that were obtained from the University of Alberta. During one such event, I added a drop of the original tailings to a piece of poly dimethyl siloxane (PDMS) block coated with CaCO 3 (powder). After a while, it was surprising to observe water separation on top. As a control, I placed a drop of the original tailings on a PDMS block without CaCO 3 and didn t see any such separation taking place. I went on to conduct a few more experiments to see if calcium carbonate aided in water separation by settling and found that it did, even at a very low concentration of the salt. This interesting piece of observation inspired us to study this problem further. (a) (b) (c) Figure 1-2. (a) Image showing water separation at the top of a drop of MFT that was placed on a piece of PDMS block coated with calcium carbonate powder. Image showing dewatering (b) when the tailings sample is allowed to settle on its own (c) when calcium carbonate is added to the MFT. 2

16 3 Besides colloidal destabilization, different electrokinetic transport mechanisms have found successful applications in several industrial processes like waste water treatments, enhanced oil recovery, drug delivery etc. One of Velegol laboratory group s expertise lies in engineering and studying electrokinetic flows in the inaccessible narrow (mostly dead end) channels of any porous medium. So, we were motivated to understand and learn if inducing such flows in the porous tailings beds could be utilized for improved dewatering Research Objectives After observing an improved water separation with the calcium carbonate addition to MFTs, as shown in Figure 1.2 (b)-(c), we decided to examine the physics behind it thoroughly. Additionally, there was an interesting, visually distinguishable, stable sediment layering in the tailings that was observed due to divalent calcium salt addition. Thus, we also went on to characterize and analyze each of those layers to understand the reason behind their formation. During an internship opportunity at the Dow Chemical (Oil & Gas) in Fall 2012, I got to work on a few different kinds of extraction tailings collected from various sources to study the effect of chemical treatment on the surface charge of the particles and vice versa. The size of the aggregates and their shear stability were also studied carefully to predict the best performance of the additives for consolidation of tailings. Thus, the main objectives of this dissertation are to find the answers to the following research questions: 3

17 Aggregation: 4 How do the surface charge and size of the tailings vary with the addition of several versions of a cationic coagulant alone or in combination with an uncharged or a charged flocculent? How does the aggregation behavior of model clays (kaolinite and bentonite) compare with the extraction tailings when treated with the same chemical additives? Does the presence of CaCO 3 in low concentration affect the aggregation and dewatering rate of MFTs? Electrokinetics: Does electrokinetic pumping occur due to the CaCO 3 treatment of MFTs? Overall, this research is aimed towards finding out an efficient and economical way of recycling water from the MFTs produced by oil sands extraction processes by (1) surface charge reduction (using different polymeric flocculants aided by coagulants) and (2) diffusioosmosis (using inorganic salt, not necessarily a coagulant), the results for dewatering and consolidation, thermodynamic and surface chemistry mechanisms of action Overview of Thesis The thesis contains five chapters. Chapter 1 is an introduction of the thesis, including the motivations, research questions or objectives, thesis outline and original contributions. Chapter 2 is a review of literature related to the properties and current treatment technologies of oil sand 4

18 5 tailings, principles and theory of electrostatics and electrokinetics and their applications in dewatering. Chapter 3 presents the detailed study on the chemical treatment of tailings with different coagulants and flocculants. This is a part of the report on the work done in Dow Oil & Gas for the Tailings Remediation Team, Freeport, TX. To comply with the disclosure policies of the company, the names of the sources of the extraction tailings and the different additives used are not revealed by their chemical name or composition. Chapter 4 presents the operation of diffusioosmotic dewatering model tests, procedure and results, which were carried out to investigate the performance and effects of addition of an inorganic salt to the tailings. The results are discussed, including the dewatering rates, the final water content and the velocities of fluid flow. The characterizations of the consolidated layers of the tailings are also discussed in detail. Chapter 5 includes a summary of the thesis, conclusions and a recommendation for future research Original Contribution Here, we are proposing a novel micro battery system comprising of an inorganic salt like calcium carbonate that can pump both water and the fine particles of the tailings in a desired direction by imposing an ionic gradient. The concentrated porous beds of the tailings have numerous dead end channels or pores in them which are relatively inaccessible. This lowers the hydraulic conductivity of the MFTs and thus deteriorates the dewatering rate. Based on the effect that the added salt has on the zeta potential of the fines, either the electrostatic destabilization or the diffusioosmotic pumping would take precedence in dewatering the MFTs. Calcium carbonate being sparingly soluble in water, has a concentration of less than 1mM at saturation. Thus, it 5

19 6 does not effectively compress the Debye layer of the fine particles to promote aggregation. Due to an overall low concentration of ions in the system, the calcium carbonate treatment of MFTs separates water which is relatively free of Ca 2+. This is an advantage considering the fact that even an amount of 30 ppm of Ca 2+ in the separated water makes it inappropriate for recycling to the bitumen extraction units 5. Also the fact that introducing an electric force field in the system of MFTs settling on account of its own weight, seems to stabilize the system further as we see visually distinct stable layers of materials form by some kind of spontaneous sorting based on the size and density of the particles. This can be potentially responsible for the recovery of residual bitumen along with the other lightweight organic materials (bitumen, naphtha etc.) present in the original tailings by sorting due to effective density. This is supported by the fact that the topmost fraction of the layers constitutes of most of the carbon present in the original sample whereas the lower layers are roughly sorted based on their densities. 6

20 7 Chapter 2 Background The commercial extraction of bitumen from oil sand is performed by Clark s Hot Water Extraction (CHWE) process, which requires large amounts of warm water and generates high volumes of oil sands tailings as waste. After a few years of settling, they form mature fine tailings. MFTs are alkaline suspensions (between ph 8 and ph 9) containing un-recovered bitumen, sands, clays, salts, soluble organic compounds and water. Figure 2-1. CHWE process used for bitumen extraction. They are hydraulically transported and stored in massive tailings ponds with the hope that the solids will settle, consolidate and dewater by gravity. Since about two to four barrels of fresh water are needed for a barrel of oil recovery by surface mining, there is a pressing need for 7

21 recycling of water from the tailings 6. 8 Unfortunately, the inventory of mature fine tailings contained in the tailings ponds continues to grow at an alarming rate for the foreseeable future. Thus, there are growing concerns regarding their accumulation and impact on resource conservation and reclamation risks. Also, according to the Alberta Energy and Resources Conservation Board (ERCB) released Directive 074, the reclamation of tailings is critical. The present oil sands projects of different companies are aimed towards reducing the volume of the MFTs and creating trafficable surfaces, meaning that the deposits must have a certain minimum shear strength. The current goal is to achieve the strength and structure necessary to form a trafficable surface with minimum undrained shear strength of 10 kpa within five years Tailings and Water Treatment After the tailings are discharged into the tailings ponds, the coarse solids segregate fairly easily, while most fine and the ultrafine solids stay suspended in water. Fine solids or fines, are defined as being < 44 µm in size and the ultra fine solids are defined as being < 0.3 µm in size. After a few years, a dense layer of mature fine tailings (MFT) develops in the bottom portion of the ponds. MFTs consolidate very slowly and are theoretically predicted to settle completely after a century 8,9. As a result, it is very important to treat the MFTs so as to improve their settling rate for efficient water recycling and also to reduce the volume of the tailings ponds. The most popular physical treatments to deal with tailings are centrifugation 10, filtration 11, electrophoresis 12 and electro-coagulation 13, though these methods are still not in fullscale commercial use. Addition of chemicals like sodium silicate, inorganic and organic 8

22 9 coagulants, organic flocculants and most recently carbon dioxide to treat the MFTs is becoming popular 14. One of the main requirements of the recycle water is to maintain the buffering capacity as bitumen recovery is sensitive to the chemistry of the process water. Thus, to keep the salt concentration low, additional steps of water treatment and softening are required. As mentioned earlier, calcium and magnesium levels of even ppm in the recycled water are considered to be harmful to bitumen recovery Current Techniques Gypsum (CaSO 4 2H 2 O) is widely used by bitumen producers like Syncrude, Suncor and CNRL to consolidate tailings by the composite tailings (CT) process 15. In this process, the MFTs are mixed with coarse sand and gypsum at 4:1 sand to fines ratio which can give a high solid content of approximately 60%. The added calcium ions adsorb on the surface of the fines (which have negatively charged basal plane surfaces), thereby destabilizing the tailings suspension and leading to quicker settling of the solids and easier separation of recycle water 16. In CT process, coarse solids are added to gypsum-treated MFT to produce non-segregating tailings (NST). However, the major shortcomings of this method are: (1) the recycle water contained a significant amount of calcium ions which is harmful to the bitumen extraction process (2) CT is yet to produce effective trafficable tailings deposits as per the requirements set forth by the Alberta government and (3) emissions of toxic gas like hydrogen sulfide (H 2 S) by the anaerobic reduction of sulfate (SO4-2 ) with the residual bitumen present 17. 9

23 10 Figure 2-2. Schematic Flow Chart of CT Process (Syncrude Canada) 18 As a solution to this, a commercial flocculent named Percol 727, which is a high molecular weight, anionic co-polymer of acrylamide and acrylates has been used to treat the tailings 19. Though it provides better settling, it results in a turbid supernatant and thus degrades the quality of the recycle water. The overall performance and effectiveness of flocculants also becomes critical considering the influence of flocculent residue on bitumen extraction. The current research on dewatering of MFTs is primarily focused on using different kinds of polymeric flocculants that will further improve the consolidation of MFTs. Related to the search for improvements, one recent report showed that an organic-inorganic hybrid polymer performed better than the corresponding organic flocculants 20 indicating that combinations of materials are worth investigating. 10

24 2.3. Coagulation and Flocculation Studies for the Oil Sands Tailings 11 Often, the terms coagulation and flocculation are used interchangeably. In this study, the definitions of these are modified from the ones used by Crittenden, et al. and Hunter 21,22. More specifically, coagulation is defined as the addition of an organic/inorganic chemical coagulant to the colloidal suspension to condition the suspended particles. Coagulation destabilizes the particles by reducing surface charges and/or compressing the electrical double layers, forming higher order aggregates upon collision by Brownian motion. Flocculation refers to the formation of macroscopic, loosely packed flocs by aggregation of suspended particles due to the addition of a polymeric flocculent. The large flocs settle rapidly due to their localized higher density versus that of the surrounding medium with the result being a clear supernatant 23. Thus, polymeric flocculants destabilize the colloidal dispersions by binding or bridging the particles together via flocculation. The responsible mechanisms for flocculation are polymer bridging, charge neutralization, polymer-particle surface complex formation and depletion flocculation or combinations of these 24. In a work by Rao, tailings were flocculated by anionic polyacrylamide (PAM) after pretreatment with an optimum dosage of magnesium (Mg 2+ ) and/or calcium (Ca 2+ ) ions. The resulting flocs were reported to carry less than 40 weight % moisture 25. Xu, et al., performed flocculent screening tests on Syncrude extraction tailings 26. It was shown that only anionic polymers with a narrow range of charge densities can flocculate the fine oil sands tailings at a process ph 8.5. It was also shown that higher molecular weight flocculants can form bigger flocs and effect increased settling rates. For most polymers tested, the settling rate was seen to increase with increasing polymer dosage. However, they were unable to obtain a clear 11

25 12 supernatant even at high polymer dosages. The best water clarity was achieved with 0.6 weight % solids content. When 20 ppm of Al-PAM was used in laboratory extraction tests, the tailings showed fast settling rates and smaller final sediment volumes 27. More recent research studies were aimed towards trying different polymers as extraction aids for poor processing ores. It was reported that when 400 ppm of a temperature-sensitive polymer was added to the extraction process, the initial settling rates (ISR) of tailings at 40 0 C increased from 0.2 to 2.5 m/h with a clear supernatant formation 28. Adding low molecular weight and highly charged cationic polymer Percol 368 to the supernatant improved water clarity 29. Sworska, et al. used Percol 727 to flocculate Syncrude fine tailings and found that at alkaline ph (ph > 8), the initial settling rates (ISRs) were low with relatively high solids content (2 weight %) in the supernatant. They observed that the addition of Ca 2+ and Mg 2+ dramatically improved the clarity of the supernatant (<0.3 weight %). For Percol 727, adequate mixing did not show any improvement in the supernatant clarity. However, good mixing conditions improved the distribution of the polymer in the suspension resulting in a lower polymer dosage requirement 30. Polyethylene oxide (PEO) (MW: g/mol) was also used as a flocculent on Syncrude fine tailings. Under suitable mixing conditions, the ISRs were fast. The resulting flocs were stable and could not be easily broken apart 31. To summarize the work on flocculation so far, we see that high molecular weight anionic polymers with a narrow range of charge can effectively flocculate tailings with high settling rates but the supernatant has relatively high solid content. Cationic polymer addition can improve the supernatant clarity, and the flocculation performance is strongly influenced by the mixing conditions. 12

26 2.4. Electrokinetic Dewatering 13 Electrokinetics primarily consists of two phenomena: electrophoresis and electroosmosis. Electrophoresis refers to the movement of a colloidal particle with charged surfaces moving in a stationary fluid under an electric field. The opposite fluid movement in porous media or capillary tubes with electrically charged surfaces refers to as electroosmosis 32. Figure 2-2 shows the electroosmosis and electrophoresis phenomena in a single capillary. Figure 2-3. Electroosmosis in a capillary 33. The main contribution of electrokinetics to dewatering till date has been by electroosmosis. In principle, when ions migrate under the influence of a current, they carry hydrated water molecules along and exert a viscous force on the water around them 34. When electrodes are installed in a porous medium, the pore water is driven from the anode to cathode. By setting up a closed anode and open cathode apparatus, water is drained at the cathode, thereby dewatering and increasing the solid content of the porous tailings sediment between the 13

27 14 electrodes. The Double layer theory is a crucial concept for the understanding of the principle of electrokinetics. The zeta potential is probably the most important factor influencing electroosmotic flow. According to the Helmholtz-Smoluchowski theory, the electroosmotic flow is proportional to the zeta potential 35. For tailings slurry, higher the zeta potential, larger is the water removal rate 36. The addition of various ionic solutions can modify the magnitude of the zeta potential thus modifying the electroosmotic flow Step Forward After studying the literature on dewatering of MFTs, we hypothesized about our observations in Figure 1.2 (a)-(c): (1) Salt addition compresses the thickness of the electrical double layer of the particles, thereby destabilizing the system. That could be solely responsible for improved dewatering performance on calcium carbonate addition like in the case of other chemical treatments mentioned above. (2) Introducing calcium carbonate salt induces an ionic gradient in the system, which can drive particle movement by diffusiophoresis and fluid flow by diffusioosmosis. These directional movements can be a controlling factor for dewatering. These hypotheses will be tested experimentally for their feasibility in the following sections of this dissertation. Also, looking at two different approaches for the same goal can help us broaden our understanding of the system. 14

28 Chapter 3 15 Effect of Chemical Treatment (addition of coagulants and flocculants) on field MFTs This part of the research is aimed towards testing and finding out the effect of several polymeric coagulant and flocculent dosages on the surface charge of the tailings and to assess their aggregation performance. Several versions of a commercial coagulant, cationic polymer CP, (having either low molecular weight CP1, medium molecular weight CP2 or high molecular weight CP3) were used alone or in combination with non-ionic (NP) and anionic polymeric flocculants (AP) to treat three different field derived extraction tailings, often called mature fine tailings (MFT1, MFT2 and MFT3). The polymers were tested for their ability to flocculate the tailings on the basis of the zeta potential and floc size measurements. To complement the tailings studies, model studies on constituent clays of the tailings were also conducted with the same chemical treatments to compare their aggregation behavior. Keeping in mind the three primary goals of this project which are to: (1) study the surface charge and size variation of the tailings with the addition of a low, medium and high molecular weight cationic coagulant alone or in combination with a high molecular weight non-ionic or anionic polymer as a flocculent (2) study the behavior of model clays (kaolinite and bentonite) to compare their results with the tailings, we go on to discuss about a few aspects on the properties of the materials handled and the fundamental scientific terms related to this method of water separation. 15

29 3.1. Surface Phenomena and Properties of Clays 16 As mentioned earlier, complete settling of fines in the tailings without any treatment would take a century or more. The fine (<44 microns) and the ultra-fine (<0.3 micron) particles present in the tailings form a stable colloidal suspension and hinder settling. The fines are mainly composed of clay minerals with different structural and chemical compositions. Thus, information on the clay mineralogy and colloidal dispersion is critical in the understanding of the system. A colloidal dispersion refers to a system having very fine particles with dimensions of a few hundred nanometers to a few microns that are well dispersed in a fluid medium. Such systems can be monodisperse or polydisperse in nature 37. They can be classified as being lyophilic (solvent-loving) and lyophobic (solvent-hating) or reversible and irreversible systems. The dispersion of fine clay particles in water is an example of lyophilic or reversible colloidal dispersion Electrical Double Layer Different kinds of interactions between particles are determined by the surface properties of particles in a colloidal suspension. Such properties affect the stability of the colloidal system. Interaction and stabilization of charged species in an electrolyte is generally explained in terms of the formation of an electrical double layer. In such systems, the charged surface forms an electrical double layer around it by attracting oppositely charged ions from the solution but maintaining the electro-neutrality in the bulk. Figure 3-1 shows the profile of electrical potential 16

30 17 distribution near the charged surface. The surface potential decreases linearly from the surface of the particle until the Stern plane due to the presence of counter ions which are affixed to the surface. Beyond the Stern plane (ψ s ), the potential decreases exponentially with distance. The shear plane refers to the plane where the ions and the solution move around the charged particle and the bound counter ions. The slipping condition applies at the Shear plane, and the potential at the shear plane is known as the zeta potential (ζ). The potential within the electrical double layer region, including the zeta potential, depends on the type of electrolytes and their concentrations 39. Figure 3-1. Schematic of an Electrical Double Layer 17

31 3.3. DLVO Theory 18 The DLVO theory discovered by four research scientists Derjaguin, Landau,Verwey and Overbeek, defines the net force of interaction between two particles in a colloidal suspension to be a summation of van der Waals and electrostatic double layer forces. Other components from the colloidal zoo of forces like the hydration repulsion, depletion forces, hydrodynamic forces etc. are ignored 40. Therefore, the stability of the system is dependent on the net force resulting from the attraction and repulsion between the particles. If the net interaction between the particles is repulsive, the system is said to be stable and well dispersed. On the other hand, if the net interaction between the particles is attractive, the system is unstable, and the particles are prone to aggregation. With all its limitations and approximations, the DLVO theory still provides a good understanding of a complicated colloidal dispersion and the DLVO plot can be used effectively to predict the critical coagulation concentration (C.C.C) for an additive. The C.C.C is defined as the point at which the potential energy barrier opposing coagulation disappears. The ζ- potential decreases as coagulation/aggregation of the particles take place and a quite sharply defined coagulation concentration can be obtained 41. The CCC can be estimated by using an approximate expression for the difference in the potential energy of attraction and the potential energy of repulsion. Assuming that ζ-potential measures the potential of the diffuse layer in the EDL, it is expected that it should be able to provide a good description of the coagulation. The scope of this present study is to examine the effect of coagulation/flocculation in polymer-treated MFT solutions while examining the correlation between ζ-potential and particle size. 18

32 19 Figure 3-2. The DLVO Plot Materials and Methods In this section, we will discuss in detail about the methods of sample preparation, handling and the experimental procedures. Since this was a work done in a scientific research laboratory of a commercial company, the names of the samples and the chemicals used are subject to respective disclosure terms and conditions. However, the codes used for each of those are properly described in the sections. 19

33 Materials 20 For this study, three different sources of mature fines tailings, labeled as MFT1, MFT2 and MFT3 and their respective recycle water samples (RW1, RW2, and RW3), were used. Kaolinite (Al 2 O 7 Si 2.2H 2 O, Lot # BCBF3412V) and bentonite (H 2 Al 2 O 6 Si, Lot # MKBG7538V) clays were studied as model systems for the tailings. They were obtained from Sigma Aldrich, USA. All ph adjustments in this work were done using 1mM NaOH solution. The dispersing medium for zeta potential measurements consisted of a 1mM KCl solution, deionized water or the recycled water (RW) obtained from the respective tailings sources. The three cationic polymer (CP) samples used in this study are low molecular weight CP1, medium molecular weight CP2 and high molecular weight CP3. The anionic polymer (AP) and non-ionic polymer (NP) were used as flocculants. The concentration of all the polymer solutions used was 0.5% w/v, unless otherwise stated. The polymer concentrations are measured in terms of percent solids unless otherwise stated Zeta Potential Measuring Experiments The zeta potential of the fines in the tailings was measured using a Malvern Zetasizer (Nano series ZS), Malvern Instruments Ltd., UK. It is a small-angle static laser light scattering device that measures the velocity of a particle moving in a fluid by micro-electrophoresis. The velocity is then correlated to the zeta potential by different suggested models like 20

34 21 Smoluchowski 42, Huckel 43, etc. The model used in our measurements is the Smoluchowski equation which is given by: v = εeζ η where υ = velocity of the particle, ε = permittivity of the suspending medium, E = the applied electric field, ζ= zeta potential of the particle and η = viscosity of the suspending medium. The Zetasizer, being an optical instrument, has a minimum and a maximum sample concentration requirement which needs to be fulfilled. The sample concentration depends on the optical properties, size and polydispersity of the particles studied. Also, for meaningful measurements of the zeta potential, reference to the dilution medium is critically important. The aim of the sample preparation is to maintain equilibrium between the particle surface and the surrounding medium during dilution/dispersion. The zeta potential measurements should be carried out in special disposable cells called the folded capillary cells designed for microelectrophoresis. They should be checked for reliability by measuring the zeta potential of the standard solution weekly. In our study, the oil sand samples and the model clays were all diluted to a weight % solids loading. The details of the sample preparation for the Zetasizer are given in Appendix A. The standard solution used to check the re-usability of the capillaries was a polystyrene latex solution with a known zeta potential of -42±4 mv. The capillaries that measured a zeta potential deviating from the above mentioned value were discarded from further measurement runs. The diluted tailings and clay samples studied were introduced to the folded capillary cells by a syringe or a disposable pipette, ensuring no air bubbles inside. Then, the cell was inserted in the instrument for zeta potential measurements. The zeta potential was measured at a fixed 21

35 22 temperature of 25 0 C and the unit of measurement is in millivolts (mv). Also, each measured value for a run is an average of three consecutive readings with a standard deviation of less than 5. The refractive index of the tailings was set to be 1.48 (the refractive index for clays is approximately around 1.5), and the external voltage applied for micro-electrophoresis was around 148 V. a) b) Figure 3-3. (a) Malvern Zetasizer Nano ZS (b) Folded capillary cell Floc Size Experiments The particle size distribution of the solids in the tailings slurries and the floc sizes were measured by a Beckman Coulter Laser Diffraction Particle Size Analyzer (LS MW, Beckman Coulter, USA) using the Universal Liquid Module (ULM). It uses single wavelength laser diffraction to measure the particle size distribution. The model chosen to analyze all our size data was the Fraunhoffer model. Dilution of a flocculated sample was needed to keep the obscuration value within a suitable range. This was done by carefully drawing and then releasing a small amount of the sample into the instrument. If the obscuration specifications weren t met, a small amount of the sample was released to a magnetically stirred cell which contained filtered water of the same ph. 22

36 23 For measurements, the bare (i.e. starting or chemically untreated) and the flocculated tailings or clay samples were introduced into the instrument by means of a disposable pipette drop by drop and the resulting particle size distribution was recorded in terms of volume percent. Figure 3-4. Beckman Coulter Particle Size Analyzer Optical Microscopy microscope. The optical microscopy images were obtained by a Nikon Eclipse TE2000-U optical 3.5. Results and Discussion Zeta potential and size distribution of the starting MFT samples. The original tailings (MFT 1, MFT 2 and MFT 3 sources) were first diluted to a 28 weight % solids loading with their respective recycled water. Then, they were further diluted to a 23

37 weight % solids loading by (i) DI water, (ii) respective recycled water (RW) or (iii) 1mM KCl solution for zeta potential measurements. These experiments showed the dependence of the zeta potential measurement on the solution properties like ph and ionic strength (Table 3-1). The measured zeta potential of the fine particles in the tailings was found to be the lowest (closer to zero charge) when diluted in their respective recycled water as there is more collapsing of the electrical double layer of the particles due to higher salt concentration. It was also noted that the zeta potential of the tailings from MFT 2 source is much lower than both MFT 1 and MFT 3 and is thus relatively easier to treat as will be discussed later. The tailings from MFT 1 and MFT 3 were found to have very similar zeta potentials ζ (mv) MFT3 MFT2 MFT1 0 DI RW KCl Figure 3-5. Zeta potential of MFT 1, MFT 2 and MFT 3 tailings measured in different background electrolytes: DI water, recycled water and 1mM KCl. MFT 2 tailings showed the lowest value of zeta potential among the three tailing samples. All the tailings have a lower (closer to zero) zeta potential when each was diluted in their respective recycled water. 24

38 Table 3-1. Zeta potential of tailings in different solutions with ph. 25 MFT 1 MFT 2 MFT 3 Suspending medium ph ζ(mv) ph ζ(mv) ph ζ(mv) Deionized water Recycled water mM KCl MFT3 MFT2 MFT1 vol % log size (µm) Figure 3-6. Size distribution of MFT 1, MFT 2 and MFT 3 tailings as measured in DI water: MFT 2 tailings are seen to have a higher fraction of larger particles compared to MFT 1 and MFT 3 tailings. From Figures 3-5 and 3-6, it is evident that the bare or chemically untreated MFT 2 tailings have a lower average surface charge and a larger average particle size, thus it can be expected to show better flocculation performance than the MFT 1 and the MFT 3 tailings. The latter two MFT samples seem to have a high percentage of fines and higher particle surface charge in them. MFT 1 and MFT 3 have quite comparable size distributions as well. 25

39 Starting percent solids of the tailings have negligible effect on the zeta potential 26 measurements. In order to study the effect of sample concentration on the measurement of zeta potential, starting solids content of the tailings were varied before they were treated with the polymers. In one case, the starting solids concentration was weight % and in the other it was 28 weight %. The polymer dosage was kept consistent in both and was added based on the solids percent of the tailings. Table 3-2 shows the measured zeta potential data for both cases. It should be kept in mind that the experiment starting with 28 weight % solids, following treatment with flocculent was then diluted to weight % to make the zeta potential measurements. It was seen that the starting percent solids doesn t affect the zeta potential readings to a considerable extent. Table 3-2. Zeta Potential of MFT 3 tailings with different dosages of a coagulant at different initial solids content. Suspending medium 1mM KCl ζ (mv) CP3 dosage (ppm) Starting from % solids Starting from 28% solids

40 Impact of addition of inorganic and organic coagulants to MFTs Addition of ferric sulfate solution (inorganic coagulant) to the tailings A 10 volume % solution of ferric sulfate (Fe 2 (SO 4 ) 3 ) was added to the diluted tailings ( weight %) to study charge neutralization. The zeta potentials of the particles were measured by mixing the tailings and the added coagulant solution well with a magnetic stir bar. It was seen that the dosage required to fully neutralize the negative potential of the tailings was around x 10 4 ppm for the MFT 2 tailings and around 6250 x 10 4 ppm for both MFT 1 and MFT 3 extraction tailings MFT1 MFT2 MFT3 ζ(mv) ,100 4,100 6, Fe 2 (SO 4 ) 3 (x1000 ppm) Figure 3-7. Zeta potential of MFT 1, MFT 2 and MFT 3 tailings versus ferric sulfate dosage measured in 1mM KCl solution. It was seen that the MFT 2 tailings were much easier to neutralize with aqueous ferric sulfate than the other tailings. However, all the tailings required a very high dosage of the coagulant to effect complete charge neutralization. It was also noticed that mixing the tailings after coagulant addition did not affect the zeta potential measurements greatly. However, over time, the zeta potential of the tailings continued to increase to more negative values when in contact with ferric sulfate. It is speculated that the 27

41 28 high valent ferric ions of ferric sulfate could gradually interact with certain anionic species held within or on the clay particles and get consumed. This makes less of the coagulant available for charge neutralization and thus causes the zeta potential to become more negative over time Addition of high molecular weight cationic polymer (CP3) solution to the tailings A 0.01 weight % solution of high molecular weight cationic polymer (CP3) was added to the diluted tailings ( weight %) to study charge neutralization. The zeta potentials of the tailings were measured after thorough mixing the tailings sample with the polymer with a magnetic stir bar. The ph of the system was maintained at ph 6.5 for all recordings. It was seen again that the coagulant dosage required to neutralize the negative potential of the tailings was very high. MFT 2 tailings required about 19 x 10 4 ppm, MFT 3 tailings about 25 x 10 4 ppm and MFT 1 tailings about 120 x 10 4 ppm of the polymer to neutralize their surface charges completely. The plot below (Figure 3-8) shows that MFT 1, even though being similar to MFT 3 in both initial charge and size properties, seems to be quite difficult to treat using an organic coagulant like CP3. It was also noted from the experiments that the zeta potential of the tailings wasn t affected by mixing of the suspension after polymer addition. 28

42 29 Figure 3-8. Zeta potential of MFT 1, MFT 2 and MFT 3 tailings versus polymer dosage CP3 when measured in 1mM KCl solution at a constant ph 6.5. MFT 2 and MFT 3 tailings require quite a lower polymer dosage than MFT 1 to reach the isoelectric point. Overall, it can be seen that the tailings require a very high dosage of the coagulant to effect complete charge neutralization Low dosages of high molecular weight cationic polymer (CP3) showed floc formation Lower dosages of the polymer were added to the tailings for zeta potential measurements. As we can see from the data in Table 3-3, the zeta potentials do not show a significant reduction even for a 3000 ppm of polymer addition. However, the flocs were seen to form at a 1000 ppm for the MFT 2 tailings and at 2000 ppm for the MFT 1 and the MFT 3 tailings. This brings us to ask a question as to whether we need to approach the isoelectric point of the tailings to decide on the optimum polymer concentration for effective flocculation. Other surface interactions that do not essentially require surface charge reduction to destabilize a colloidal system could be dominant in this particular case. 29

43 30 Table 3-3. Zeta potential of fines in the tailings at a lower CP3 dosage as measured in 1mM KCl solution: The yellow color highlights the minimum dosage at which flocs were seen forming visually for different tailings. MFT 1 MFT 2 MFT 3 CP3 dosages (ppm) ζ(mv) Particle size distribution of tailings when treated with high molecular weight cationic polymer (CP3) addition The tailings samples treated with CP3 were used for size measurements. The size distribution in the figures 3-9 (a), (b) and (c) show that the average size of the particles increased with increasing dosages of CP3 for all the tailings. The MFT 3 tailings showed a somewhat different behavior beyond the 2000 ppm polymer dosage as the average size of the particles appeared to decrease slightly. This could also be an experimental error as this run was not repeated. The bimodality obtained in the size distribution curves for MFT 1 tailings can be suggestive of the existence of a combination of charge neutralization and polymer bridging effect for the action of this high molecular weight coagulant. 30

44 31 (a) vol % MFT ppm CP3 MFT ppm CP3 MFT MFT ppm CP log size (µm) (b) MFT ppm CP3 vol % MFT2 MFT ppm CP log size (µm) (c) MFT3 vol % MFT ppm CP3 MFT ppm CP log size (µm) MFT ppm CP3 Figure 3-9. (a) Particle size distribution of MFT 1 tailings with increasing CP3 dosage. (b) Particle size distribution of MFT 2 tailings with increasing CP3 dosage. (c) Particle size distribution of MFT 3 tailings with increasing CP3 dosage. 31

45 Effect of low molecular weight cationic polymer CP1 on the tailings 32 A 0.5 weight % solution of low molecular weight CP1 was added to the concentrated (28 weight % solids) tailings samples. The zeta potentials (Table 3-4) and the size distributions (Figure 3-10) were recorded from the analyses. Table 3-4. Zeta potential of fines in the tailings with low molecular weight CP1 dosages: The yellow color highlights the minimum dosage at which flocs are seen forming visually for different tailings. MFT 1 MFT 2 MFT 3 CP1 dosages (ppm) ζ(mv) vol % MFT3 MFT ppm CP1 MFT ppm CP1 MFT ppm CP log size (µm) Figure Particle size distribution of MFT 3 tailings with increasing amounts of low molecular weight CP1. 32

46 33 The low molecular weight coagulant also does not show considerable surface charge neutralization for the dosages shown in Table 3-4. Still, as noted in the table, flocculation is seen to occur visually. It can also be seen from Figure 3-10 that the average particle size distribution increases with increasing polymer concentration for the 3000 ppm experiment, while little to no change was observed when increasing the dosage from 1000 to 2000 ppm. These results seem to indicate that lower dosages of low molecular weight CP1 do not cause noticeable charge neutralization. When comparing Tables 3-3 and 3-4 (i.e. high and low molecular weight CP, respectively), we note that the change in surface potential is quite similar. Also, for the one direct comparison involving particle size distribution (i.e. MFT 3, see Figures 3-9(c) and 3-10), the peak volume percent resides close to 35 µm. Taken together, these results indicate that the molecular weight of the CP does not highly influence charge neutralization of the fines or floc size. For these comparisons, it should be further noted that equal polymer dosages by weight will deliver the same amount of cationic charge to the system, and this seems to dictate the results Impact of addition of NP flocculants to the tailings A non-ionic polymer solution of 0.01 weight % was added to the dilute ( weight %) tailings samples and the zeta potentials were measured. The non-ionic polymer did not alter the surface charge of the system much, as expected, though a slight decrease was seen for each of the tailings due to the displacement of the shear plane caused by the initial adsorption of the polymer on the charged particle surface (Figure 3-11). This displacement of the shear plane is 33

47 34 dependent on the thickness of the adsorbed polymer layer 44. It was also shown that a decrease in zeta potential of the negatively charged fines with increasing NP adsorption is due to a shift of the shear plane and not due to surface charge reduction. The distance of displacement of the shear plane can be assumed to be the same as the thickness of the adsorbed layer. According to an equation 45, the relationship between the normal distance to the surface (x) and the potential ψ(x) in the diffuse phase is given as: tanh zeψ(x) 4kT = tanh zeψ! 4kT exp κ(x δ where z = valence of the counter ions present, k = Boltzmann constant, T = temperature in Kelvin, ψ! = potential of the particles without polymer addition, κ = inverse of the thickness of the electrical double layer and δ = thickness of the adsorbed polymer layer. 34

48 (a) 35 (b) (c) Figure Measured zeta potentials of (a) MFT 3, (b) MFT 2 and (c) MFT 1 tailings over time with the addition of the optimum * dosage of NP. The starting solids state of the tailings was % and a polymer dose of approximately 3500 ppm, 2000 ppm and 3000 ppm were added to the MFT 3, MFT 2 and MFT 1 tailings, respectively. * Obtained from the results of the capillary suction time (CST) tests. Figure Measured zeta potential of tailings with increasing levels of NP. Only a slight change in zeta potential is observed with increasing polymer dosage for the tailings. 35

49 36 Additionally, increasing the dosage of non-ionic polymer reduced the zeta potential to a minor extent (Figure 3-12). For each of the tailings, polymer adsorption appears to be gradually displacing the surrounding cations. Further studies using MFT 1 tailings, starting with 28 weight % solids (Figure 3-13), were conducted to investigate the effect of non-ionic polymer concentration on the particle size distribution. As seen, NP had very little effect. However, visual observations clearly show that NP flocculates the tailings. There are two possible explanations for this. First, it is known from prior studies at Dow that NP can form aggregates that tend to adhere to the walls of the Beckman Coulter particle size analyzer. In such an event, the particle size distribution will be skewed and can partly flaw the results. Second, the analyzer is known to impart enough shear to breakdown weakly formed flocs. If this were to hold for NP, then it would explain why the three results in Figure 3-13 are nearly identical to each other. 4 vol % MFT1+2000ppm NP MFT1+3000ppm NP MFT log size (µm) Figure Size distribution of flocs formed by addition of NP to the MFT 1 tailings. The average size distribution did not change much from the initial condition. 36

50 Impact of addition of CP with NP 37 Using the MFT 1 tailings, the effect of adding two different polymers in combination was studied to determine their influence on charge and size variations. For the data shown in Table 3-5, a high molecular weight cationic coagulant (CP3) was first added to the tailings, and then after about 5 minutes, the non-ionic flocculent was added. For the most part, the combination of CP and NP did not seem to have a significant influence on the measured zeta potentials. Earlier, it was shown that both CP3 and NP, when used alone (see Table 3-3 and Figure 3-12, respectively) resulted in similar zeta potential changes. Thus, there is no additive effect when using this combined treatment. Similar results were obtained for the other two tailings. Table 3-5. Zeta potential of fines in the tailings with high molecular weight CP3 followed by increasing dosages of NP measured in 1 mm of KCl solution. Tailings: MFT 1 alone (-60.2 mv) CP3 dosages (ppm) NP dosages (ppm) ζ(mv)

51 38 A size distribution analysis for the 1000 ppm CP3 series is presented in Figure As more NP is added, the more the resulting sample looks like the results of Figure 3-13 (i.e. a study using only the non-ionic polymer) and not like Figure 3-9(a) where CP3 was used. Thus, whatever system is created by the initial addition of CP, the subsequent addition of NP dominates the behavior. It remains to be seen whether further additions of NP will result in larger floc sizes as noted by the changing small shoulders/peaks in the region of about µm. Further work is also needed to better understand why the size distribution centered around 50 µm decreased when the second polymer was added. The conversion of large flocs to small flocs with increasing levels of non-ionic polymer seems to indicate that floc formation, as experienced by cationic CP, is reversible with the addition of this flocculent. vol % MFT ppm CP ppm NP MFT ppm CP ppm NP MFT MFT ppm CP ppm NP log size (µm) MFT ppm CP3 Figure Size distribution of flocs formed by addition of NP to the MFT 1 tailings pre-treated with a constant 2000 ppm of CP3. The bimodality of Figure 3-14 can be further examined regarding the ratio of peaks at 6 and 50 µm. A higher ratio of CP gives rise to larger flocs, while higher NP gives rise to smaller flocs. The peak for the larger size range (i.e. small volume percent) may be attributed to a 38

52 39 minute contribution of polymer bridging by flocculation with these polymers. It is also worth speculating whether the formation of the two distinct peaks is a result of some combination of surface charge neutralization and polymer bridging by the two polymer species or is due to the independent action of each one of them. Size distribution measurements done by reversing the order of polymer addition could help to answer the above question. This could be a potential future study. The same trends were observed with increasing NP dosage on MFT 1 tailings pre-treated with 2000 ppm and 3000 ppm CP3. A related study using a combination of polymers was next conducted using MFT 2. Back in Figure 3-6, it was noted that MFT 2 s particle size distribution is much different than MFT 1 s (as well as that of MFT 3 s). For MFT 2, the maximum volume percent is centered around µm as seen in Figure 3-6. Addition of CP3 at 1000 ppm or subsequent addition of NP did not change MFT 2 s characteristic particle size distribution. For the four experiments shown in Figure 3-15, larger flocs did not result when analyzed with the particle size analyzer. Still, it is important to keep in mind that flocs were seen visually for each case. 39

53 log size (µm) vol % MFT ppm CP3 MFT2 MFT ppm CP ppm UF MFT ppm CP ppm UF MFT ppm CP ppm UF 40 Figure Size distribution of flocs formed by addition of NP to the MFT 2 tailings pre-treated with a constant 1000 ppm of CP3. Here, it can be seen that the average size distribution of the particles did not change much with increasing non-ionic polymer concentration Zeta potential and size distribution analyses with the model clay systems: Kaolinite and Bentonite Zeta potential and size distribution of model clays measured in 1mM KCl solution The two model clay systems were diluted to 14% solids by recycled water using two sources: MFT 3 and MFT 2. The zeta potential measurements were recorded after a further dilution (to weight % solids) of the samples in 1mM KCl solution (Table 3-6). From Table 3-6 and Figure 3-16, it is seen that kaolinite has a higher particle surface charge and lower fines content than bentonite. 40

54 Table 3-6. Zeta potential of model clays in different solutions with ph. 41 Kaolinite Bentonite Suspending medium ph ζ (mv) Avg. Size (µm) ph ζ (mv) Avg. Size (µm) 1mM KCl vol % log size (µm) Kaolinite Bentonite Figure Particle size distribution of kaolinite and bentonite clays diluted in MFT 3 recycled water (RW3) Impact of addition of cationic coagulant CP Effect of recycled water chemistry on the flocculation properties of the clays The kaolinite and bentonite clays were first diluted to a 14 weight % solids content using MFT 3 and MFT 2 recycled water followed by addition of CP3. Clearly, the dispersing medium had an influence on the measured zeta potential when using kaolinite (Figure 3-16). Such was 41

55 42 not the case for bentonite. The ph of the system was kept constant at ph 7.5 in MFT 3 recycled water and ph 6.9 in MFT 2 recycled water. To cross the isoelectric point, kaolinite needs less CP3 (~1700 ppm less) when diluted in MFT 2 recycled water. 42

56 43 (a) (b) Figure Zeta potential of the clay particles diluted in (a) MFT 3 recycled water (RW3) and (b) MFT 2 recycled water (RW2) when treated with CP3. All measurements were taken in a 1mM KCl solution. It can be seen that kaolinite reaches its isoelectric point at a lower polymer dosage when diluted in MFT 2 recycled water. Bentonite, on the other hand, shows very insignificant surface charge reduction within the studied range. This behavior is similar to the tailings. Size distribution measurements were done with the model clays. In Figure 3-18, kaolinite (diluted in MFT 2 recycled water) was flocculated with CP3. These results were found to be similar to those illustrated in Figure 3-9 where the average particle size distribution increased noticeably at the higher polymer dosage, while little to no change was observed when increasing the dosage from 1000 to 2000 ppm. More research is needed to determine whether or not the model clay kaolinite can suitably represent MFT 3. 43

57 Kaolinite vol % Kaolinite ppm CP3 Kaolinite ppm CP3 2 Kaolinite ppm CP log size (µm) Figure Floc size distribution of kaolinite clay diluted in MFT 2 recycled water with increasing concentrations of CP3. In Figure 3-19, bentonite (diluted in MFT 3 recycled water) was flocculated with CP3. The bimodal size distribution mimics the one shown in Figure 3-8(a) for MFT 1 which was also treated with CP3. In both cases, increasing the dosage of polymer resulted in an increase in the larger particle size fraction. More research is also needed to determine whether or not the model clay bentonite can suitably represent MFT 1. vol % Bentonite ppm CP3 Bentonite ppm CP log size (µm) Figure Floc size distribution of bentonite clay diluted in MFT 3 recycled water with increasing concentration of CP3. 44

58 45 Thus, we see that the size distribution of both kaolinite and bentonite clays increased with increasing coagulant dosage (CP3), which is suggestive of floc formation. The surface charge on the bentonite clays was not neutralized at lower polymer dosages as shown in Figure 3-17 (a) and (b). So, it can be surmised that floc formation in some clays (e.g. bentonite) can be less dependent on surface charge neutralization. This also resembles the behavior of all three MFTs (as seen earlier in Figure 3-7) Impact of medium molecular weight CP2 on the size distribution of the model clay kaolinite A 0.5 weight % solution of polymer CP2 was added to the kaolinite clay (28 weight % solids) diluted in MFT 3 recycled water. The zeta potential measurements showed poor charge neutralization with increasing polymer concentration. A size distribution analysis was done as a function of polymer dosage, and it was seen that the average size did not change significantly. Much unlike the effect produced by the high molecular weight CP3 (as seen in Figure 3-18), this lower molecular weight version showed no significant change in particle size distribution with increasing dosage (Figure 3-20). 45

59 vol % Kaolinite Kaolinite ppm CP2 Kaolinite ppm CP2 Kaolinite ppm CP log size (µm) 46 Figure Floc size distribution of kaolinite clay with increasing concentration of medium molecular weight CP2. Increasing the amount of polymer did not have an impact on the floc size Impact of low molecular weight CP1 on the model clays A 0.5 weight % solution of low molecular weight CP1 was added to the 14 weight % clay solution diluted with MFT 2 recycled water. Kaolinite clay reached the isoelectric point at a fairly low polymer dosage of 600 ppm, whereas bentonite never reached the isoelectric point for the dosages studied (see Figure 3-21). 46

60 47 Figure Zeta potential of the clay particles diluted in MFT 2 recycled water when treated with low molecular weight CP1. All measurements were taken in a 1mM KCl solution. It can be seen that kaolinite reached its isoelectric point at around 600 ppm of polymer concentration. Bentonite, on the other hand, showed very insignificant surface charge reduction within the studied range. This behavior is similar to the tailings. The size distribution studies using low molecular weight CP1 (Figure 3-22) show that the floc sizes increased slightly with increasing polymer loading until the isoelectric point was reached. Higher levels of the low molecular weight version resulted in a slight broadening of the overall particle size distribution. vol % Kaolinite Kaolinite ppm CP1 Kaolinite ppm CP1 Kaolinite ppm CP1 Kaolinite ppm CP1 Kaolinite ppm CP log size (µm) Figure Floc size distribution of kaolinite clay with increasing concentration of low molecular weight CP1. An increase in the amount of polymer increases the average the floc size, but over dosage beyond the isoelectric point shows a reduction in the average size. 47

61 48 From the three studies presented above, it appears that CP, irrespective of molecular weight, is clearly acting as a coagulant (i.e. charge neutralizing additive) and less as a flocculent Impact of anionic polymer (AP) on the model clays Table 3-8 shows the data for the measured zeta potentials for kaolinite and bentonite clays treated with the anionic polymer. These results are in direct opposition with those previously seen using CP and NP. Here, increasing dosages of AP resulted in more negative zeta potentials. Table 3-6. Zeta potential of model clays treated with AP. AP dosages (ppm) ζ (mv) Kaolinite Bentonite Impact of CP and anionic polymer AP on the model kaolinite clay A 0.5 weight % solution of anionic polymer (AP) was added to a kaolinite solution (14 weight %) and allowed to sit for 5 minutes. Then, a 0.5 weight % solution of CP3 was added. 48

62 49 Particle size distribution measurements were conducted for an increasing dosage of CP3 while keeping the AP dosage constant, Figure 3-23 (a), and vice versa, Figure 3-23 (b). From the first study, it can be seen that more cationic polymer results in a larger floc size distribution though the change is not overly dramatic. For the second study, increasing levels of anionic polymer results in the most significant change in particle size when compared to all the previous results shown. 49

63 (a) 6 5 Kaolinite 50 vol % log size (µm) Kaolinite ppm CP ppm AP Kaolinite ppm CP ppm AP (b) vol % log size (µm) Kaolinite Kaolinite ppm CP ppm AP Kaolinite ppm CP ppm AP Kaolinite ppm CP ppm AP Figure (a) Floc size distribution of kaolinite clay diluted in MFT 3 recycled water with increasing concentration of CP3 added while keeping the concentration of AP at 1000 ppm. An increase in cationic polymer dosage showed an increase in average particle size. (b) Floc size distribution of kaolinite clay diluted in MFT 3 recycled water with increasing concentration of AP added. Increase in polymer dosage showed an increase in average size. For a constant cationic polymer dosage of 1000 ppm, the continued addition of the anionic polymer generated an entirely new larger sized fraction of flocculated particles centered around 200 µm. 50

64 Impact of non-ionic polymer NP on the clays 51 A 0.5 weight % solution of NP was added to 14 weight % solution of kaolinite and bentonite clays. The samples were further diluted by addition of 1 mm KCl solution for zeta potential measurements. Table 3-9 shows that the measured zeta potentials for both the model clays show a significant drop with added NP. The yellow highlights in the table represent the minimum dosage of the polymer at which flocs are seen forming visually. When comparing these results with those of Figure 3-11, one can infer that model clays form a system similar but not completely identical to the MFTs. The broader particle size polydispersity, multi-component clays, residual bitumen and presence of salts in MFTs are not fully represented by single component model clay systems. Table 3-7. Zeta Potential of model clays with NP dosage. Starting with 28% Kaolinite Bentonite NP dosage (ppm) ζ(mv) ζ(mv)

65 Impact of the combination of high molecular weight CP3 and NP on the model kaolinite clay To further study the effect of CP3 on the floc size distribution, optical microscope images were taken at a constant loading of 1000 ppm of NP. Figures 3-24(a)-(c) show that increasing CP3 concentration, while keeping the concentration of NP constant, increases the overall floc size distribution of kaolinite clay as indicated by a greater number of large flocs (specifically, dark contrast images).! (a) (b) (c) Figure 3-24.Optical microscope images for kaolinite clay samples diluted in MFT 3 recycled water treated with (a) 1000 ppm of CP3 and 1000 ppm of NP, (b) 2000 ppm of CP3 and 1000 ppm of NP and (c) 3000 ppm of CP3 and 1000 ppm of NP. The floc size distribution of kaolinite clay became better with increasing concentration of the cationic polymer. 52

66 Chapter 4 53 Salt Dissolution-Induced Fluid Pumping in MFTs By far the most popular method of dewatering mature fine tailings has been colloidal destabilization and improved settling of fines with the aid of several commercial coagulants and flocculants as has been discussed in detail in Chapter 3. However, the science of electrokinetic flows in porous beds is particularly attractive for dewatering tailings suspensions of fine particles because of minimum hydraulic flow them. The electrokinetic flows are essentially surface phenomena and are thus insensitive to both the pore size and the pore size distribution. The present study shows that flows induced by ionic gradients due to dissolution of inorganic salts like calcium carbonate can aid dewatering of mature fine tailings by a phenomenon called diffusioosmosis Introduction & Background Diffusioosmosis is a simple physics that can drive fluid flow through the micro channels within the tailings deposits in a desired direction by imposing salt gradients in the system. The dissociated ions of the salt act as micro-batteries pumping water out, depending on the electric field generated. Unlike the pressure-driven convective fluid flows, diffusiophoretic flows can easily access the micro/nano channels of a porous medium. These flows persist as long as the gradient of concentration persists. 53

67 What causes diffusiophoretic flows to occur? 54 Theoretically, diffusiophoresis is a combination of four different types of electrokinetic processes: electrophoresis and chemiphoresis of charged particles, and electroosmosis and chemiosmosis along the charged walls 46. Unlike conventional electrophoresis, no externally applied electric field is required to drive diffusiophoresis. When a salt gradient is introduced in a system, the dissociated ions diffuse from a region of their high concentration to low concentration. However, these ions, owing to their difference in hydration radii, have different diffusivities such that at any given time, the concentration of the fastest ion would be higher than its counterparts from the junction of salt dissolution. Overall, such a system lacks electroneutrality. Thus, the coulombic forces in nature spontaneously set up an electric field to equilibrate the system. Although the resulting electric field for an individual ion-pair is small, the summation of the contributions of all the ions gives rise to a net electric field (~1-10 V cm -1 ), pointing up or down the gradient depending on which of the ions has the higher diffusion constant. It acts on the ions and all charged entities in the system causing them to move by electrophoresis. It also drives fluid flow in response, often referred to as electroosmosis. The direction of the motion of the particle or the fluid in a given salt gradient, however, depends on the sign of the charge on the surface of the particle or the wall and the direction of the electric field. Chemiphoresis and chemiosmosis are similar phenomena occurring in the electrical double layer of counter ions except that the directions of these movements are fixed for a system. 54

68 55 Figure 4-1. Schematic showing the directions of the particle and fluid flow constituting diffusiophoresis for a system where both the particle and the substrate are negatively charged, and the diffusivity of the anion of is larger than the diffusion constant of the cation. The flows around the particle due to electrophoresis and chemiphoresis are shown by the corresponding black and red border on the arrow. If the diffusivities of the ions or the surface charge on the particle or substrate were reversed, the electrophoretic and electroosmotic flows shown in this schematic would reverse their directions, whereas the chemiosmotic and chemiphoretic components would remain unchanged. (Image taken from page 2, Micro/Nanobots for Advanced Oil Recovery (Annual Progress Report and Future Work), Ayusman Sen and Darrell Velegol) Calcium carbonate (CaCO 3 ) particles acting as micro battery For diffusiophoresis in an ionic gradient to take place, the ions generated on dissolution must have significantly different aqueous diffusivity values. Thus, identification of the most prevalent ions in the system is crucial for accurately analyzing the diffusiophoretic and the diffusioosmotic flows. It is because of the fact that CaCO 3 is one of the most abundantly available mineral and that it generates a finite electric field in a system on dissolution, we chose it to be our model salt. CaCO 3 is considered stable against dissolution since they saturate water readily (K sp = 4.96 x 10-9 M 2 at 20 C 47 ). However, when a concentrated solution of CaCO 3 is diluted with 55

69 unsaturated water, dissolution starts at the particle surface into the fluid resulting in the generation of Ca 2+, HCO3 -, and OH - as the primary ions in the solution: At low volume fractions of CaCO 3 particles in the solution, it can be assumed to be dissolving into an infinite bath and eventually disappearing. The solution never becomes fully saturated and thus there is always a persisting concentration gradient theoretically. The diffusion coefficients at 20 0 C of the three ions resulting from each molecule of dissolved CaCO 3 are: D OH - = 5.27 x 10-9 m 2 /s, D HCO3 - = 1.19 x 10-9 m 2 /s and D Ca +2 = x 10-9 m 2 /s. From the discussion above, it is evident that the concentration of OH - ions at a given distance from the region of high salt concentration would be slightly higher than the concentrations of the other two ions CaCO + H O = Ca + HCO + OH Earlier in our lab, Joe McDermott had studied a simple test case of calcium carbonate micro particles driving localized micro flows both in open fluid and close channels and observed pumping of tracer particles in the vicinity of a 5µm CaCO 3 particle. He had also studied the flow fields for two interacting pumps. The figure below shows such pumping phenomena. Figure 4-2. (a) CaCO 3 micro particle pumping of 1.4 µm sulfate-functionalized polystyrene latex tracer particles (spsl). (b) Two interacting CaCO 3 micro particle pumps. 56

70 4.2. Calcium Carbonate in Dewatering of MFTs 57 Calcium carbonate, being sparingly soluble in water (the total ionic strength of dissolved calcium carbonate is less than 1 mm at saturation), does not suppress the surface charge of the fines present in the MFTs significantly and is verified experimentally. In the settling experiments conducted on dewatering, it was observed that CaCO 3 addition in very low concentrations (<1mM) to mature fine tailings (MFT) combined with settling at higher accelerations due to gravity (~2000g) can separate about 74% recyclable water over a period of 10 hours. We find that the electrostatic forces in our systems are not significantly altered by salt addition, and this led us to investigate the physics responsible for such an observation. By conducting experiments in dead-end square capillaries (inner dimension 0.3 or 0.9 mm), we are trying to represent the pores in the tailings deposits. We sought to examine the rate of dewatering due to CaCO 3 compared with other additives such as CaSO 4. Then we sought to identify the mechanism for how CaCO 3 dewatered the MFTs, since this rate was much faster than we might have expected. We find that we can control the directional movement of the fine particles and surrounding fluid by choosing a direction of an imposed ionic gradient in the system. An advantage of using CaCO 3 is that so little of it dissolves in the water (solubility product constant of 4.96 x 10-9 ), relative to for instance CaSO 4 (solubility product constant of 4.93 x 10-5 ). The solution equilibrium of CaSO 4 looks like : CaSO = Ca + SO 4 4 Furthermore, we find that settling experiments with calcium salts give five visually distinct sediment layers that are also characterized in detail in this study. The characterization of the layers has raised the possibility that the remaining bitumen in the MFT samples could be 57

71 readily recovered for productive use. Also, the absence of Ca in the separated water in significant amounts by this method further strengthens its importance Materials and Methods Materials and Characterization Our studies are focused on Mature Fine Oil Sand Tailings manufactured by Syncrude Canada Ltd. The MFTs consist of a thickened suspension of fine clay particles and mineral solids (silt) in water, formed by settling of process tailings during bitumen separation from oil sand. The viscous suspension has a specific gravity of at 20 0 C and is stable at a ph of 8-9. The original tailing sample contains % clay by weight, 1-2 % bitumen, <0.1 % naphtha, and the rest water. The mineral composition of the solids was estimated by X-ray diffraction technique. Quartz is found to be the dominant mineral present. Kaolinite and Muscovite are the primary clay types in the sample (Appendix B). The scanning electron microscope image of the original tailings sample in figure 4-3 (a) shows a coherent orientation of both the phyllosilicate lattices of kaolinite and muscovite in the form of pseudo hexagonal plates and poly-crystals respectively shown by the structures marked by 1 and 2 respectively. The measurement of the particle size of the original suspension by Mastersizer 2000 particle size analyzer (Malvern Instruments, U.K.) in figure 4-3 (b) suggests that about 70 % of the sample comprises of fines (particle diameter < 44 micron). 58

72 59 a) 1 2 b) vol % size (µm) Figure 4-3. (a) Scanning Electron Microscopic image clearly shows poly-dispersity both in terms of size and structure. (b) Particle Size Distribution of the original tailings sample. We obtained calcium carbonate, sodium sulfate and calcium sulfate dihydrate from Fischer Scientific, USA. De-ionized (DI) water was obtained from Millipore Corp. Milli-Q system. 59

73 Settling Experiments 60 The rate of dewatering was monitored during sedimentation experiments. The sedimentation of the tailings samples was observed through bench settling test. A weighed amount of both the untreated and treated samples were taken in 50-mL graduated centrifuge tubes and allowed to settle either at normal or a high (~2000g) gravitational acceleration using a Sorvall Primo Biofuge (Kendro, Germany). The settling at 1 g was allowed for several days until there was no further water separation. The VM-3000 mini vortexer, VWR International was used for mixing. The salts CaCO 3 and CaSO 4 were added in low concentrations ~ 0.5 mm (below their solubility concentrations at 298K) at the bottom of the centrifuge tube and then the MFT was carefully added on top. (a) (b) Figure 4-4. (a) Centrifuge tube with MFT and salt (b) Centrifuge used for sedimentation experiments. 60

74 Zeta Potential Measurement 61 In order to examine how the CaCO 3 affected surface charge, and therefore colloidal stability, the zeta potentials were measured using a Malvern Zetasizer (Malvern Instruments, U.K). Original tailings suspensions and the ones treated with salts were diluted to about 0.01 volume percent in deionized water. The sample was then injected in a folded capillary cell for zeta potential measurements. The ph of the samples for all measurements was kept constant at ph 8 for uniform solution conditions. All ph adjustments were done using 1mM NaOH or 2N HCl. To avoid any coarse particles in the sample, the supernatant was collected from the top of the suspension. The dilution of the tailings is necessary to prepare samples of desired solid concentration suitable for zeta potential measurement by electrophoresis. Ideally for an accurate zeta potential measurement, the slurry sample should be diluted in its own process water for uniformity in the solution matrix. But, in this study de-ionized water was used Diffusioosmosis Experiments The concentrated tailings deposits can be considered to be a porous granular skeleton of clay mass with numerous micro and nano scale channels, most of which have dead ends. Such channels are best represented in our experiments by small sealed square capillaries less than 1 mm wide and 5 mm long. A salt gradient is introduced in the system by injecting a salt solution of fixed molarity in a smaller capillary, with the fines suspended in DI water being in a slightly larger capillary or vice versa. This set up was used to help us study directional particle and fluid flows by diffusioosmosis. To observe and record the pumping behavior, the fine particles with 61

75 62 the salt solution were imaged using a Nikon Eclipse TE2000-U inverted optical microscope, typically at 10x/20x magnification. (a) (b) MFT particles 0.9mm ID Capillary outside 0.2mm x 0.1mm Capillary inside with salt Figure 4-5. (a) The diffusioosmotic study setup using two square capillaries of different sizes (b) Optical Microscope: vertical set up for the experiments Water chemistry analysis To evaluate the quality of water separated in terms of the dissolved metal ion concentration and suspended solids, a few simple analyses were performed. After 10 hours of settling at 2000 g, the supernatant layer was pipetted out and collected. Atomic adsorption spectroscopy (AAS) was used to measure the concentrations of divalent cations in the untreated and treated samples. This technique was used to detect the metal elements in soluble form in the solution. The particle size distribution analysis was conducted on the supernatant by a Mastersizer to study the percentage of fines present in the treated sample compared to the untreated one. 62

76 Characterization of sediment layers 63 The sediment layers were characterized for their structure, size and elemental composition by FESEM, EDS and particle size measurement methods Results and Discussion Water separation by settling Original tailings segregate when allowed to settle under gravitational field. The coarse particles settle more rapidly than the fines as the latter stays well dispersed in aqueous phase. Then, with time, a sharp interface develops between the suspension and a clear fluid layer, which is commonly referred to as the mud line. The reduction in the height of the interface of the sediment and the supernatant was recorded as a function of time for both 1g and 2000g settling conditions. Here, H and h represent the initial height of the suspension and the instantaneous height of the interface at a given settling time, respectively. Figure 4-6 shows the normalized height (h/h) plotted as a function of time at (b) 1g and (c) 2000g with and without salt addition. The plots clearly show that both calcium carbonate and calcium sulfate improve dewatering of the MFTs over settling alone. Calcium sulfate, at a low concentration (below its complete solubility in water), dewaters a given volume of original tailings more than calcium carbonate. This holds true for all settling conditions. It is also seen that a higher volume ratio of salt to sample ensures better water separation for both the salts tested. MFT alone, on the other hand, shows lowest dewatering rate compared to the treated samples. 63

77 64 (a) (b) Sediment height (h/h) :1 CaSO4 1:2 CaSO4 1:1 CaCO3 1:2 CaCO3 MFT only Time (day) (c) 1 sediment height (h/h) :1 CaSO4 1:2 CaSO4 1:1 CaCO3 1:2 CaCO3 MFT only time (h) Figure 4-6. (a) Schematic showing the set up for settling experiments indicating the heights of the sediment and the supernatant. Settling curves for (b) 1g and (c) 2000 g. Settling is done at a higher acceleration due to gravity (g) for faster rates of dewatering. However, separation of water occurs at g as well, showing the same trends although it is a slow process and takes days. 64

78 65 The fact that calcium sulfate performs better than calcium carbonate in settling is further investigated by conducting tests to find the lowering of the surface potential of the tailings by salt addition as discussed in the next section Electrostatic Destabilization It is a known fact by now that adsorption of positively charged calcium ions onto the surface of the negatively charged fine particles reduces the electrostatic force of repulsion between them, thereby enhancing consolidation. Also, there is inherent compression of debye layers of the colloidal particles with salt addition. Therefore, gypsum (hydrated calcium sulfate) is industrially used as an effective inorganic coagulant in the tailings slurries. The dosage used is around 200 ppm of the salt. The zeta potential measurements of the diluted tailings samples with the addition of calcium carbonate (low concentration, ~ 0.5 mm) and calcium sulfate (low concentration ~ 0.5 mm) solutions to the tailings are shown in Figure 4-7 (a). It can be seen that both the salts do bring about compression of the electrical double layers to some extent. However, calcium carbonate addition to the tailings in such low concentration does not significantly destabilize the system when compared to calcium sulfate. Clearly, the zeta potential of solids is dependent on the concentrations of divalent cations in the bulk solution. The comparable dewatering rates observed earlier for both these salt treatments led us to investigate the alternative dewatering mechanism for low ionic strengths. The optical microscope images recorded for the fines after calcium carbonate addition are in support of colloidal aggregation in some form. Thus, the electrostatic destabilization of the 65

79 66 system, despite the small changes in observed zeta potential values, cannot be ruled out completely. The non-uniformity in the particle size, shape and hence surface charge distribution are what make these mixed-clay systems particularly unpredictable in terms of their coagulation behavior. a) ζ (mv) CaCO3 (5ppm) CaSO4 (5ppm) CaSO4 (200ppm) Time (min) b) t=0 t=60 t=120 Figure 4-7. (a) Plot of zeta potential of the diluted tailings treated with salts over time. Calcite addition reduces the zeta potential of the fines from -39 mv to -26 mv after about 3 hours. Calcium sulfate reduces zeta much more, closer to the isoelectric point. (b) Time lapse images of the fines in the tailings before and after calcite addition. The fines are seen to aggregate over time thereby giving rise to bigger aggregates as seen. 66

80 Diffusioosmostic Flow Studies 67 As discussed in the previous sections, introduced gradients of salt concentration due to the differences in ionic diffusivities give rise to a spontaneous electric field in a system lacking electro neutrality. This self-generated electric field drives a diffusioosmotic flow of fluid at charged surfaces, and a complementary diffusiophoretic motion of charged particles. This electric field is similar to an externally applied one in conventional capillary electrophoresis. Here, we study diffusioosmosis-generated flows of fines in sealed channels representing the dead end pores in the tailings deposits. In our system, calcium carbonate dissolves in water producing Ca 2+, HCO3 -, and OH - ions. The fastest moving ion, OH-, diffuses faster than all others and therefore generates a gradient of its concentration in the system. This gradient sets up an electric field in the direction of the overall ionic concentration gradient. The presence of this electric field will cause a series of diffusiophoretic slip velocities at all charged surfaces in the system. In this case, the particles being negatively charged move in a direction towards high calcium carbonate solution concentration. They reverse their direction in an oppositely directed electric field situation for e.g. in sodium sulfate solution (The diffusion coefficients of Na + is 1.5 x 10-9 m 2 /s and SO 2-4 is 1.01 x 10-9 m 2 /s). For calcium sulfate solution, the direction of the electric field, according to the principle of diffusiophoresis is the same as that of calcium carbonate. The diffusion coefficients of Ca 2+ and SO 4 2- are x 10-9 m 2 /s and 1.01 x 10-9 m 2 /s respectively. So, here, although the diffusivity differences are not significantly high, there will still be diffusiophoretic flows. 67

81 68 (a) (b) (c) Figure 4-8. (a) Schematic of Diffusioosmosis experimental set up. A smaller glass capillary (0.3 mm I.D.) placed inside a larger glass capillary (0.9mm I.D.) with the ends sealed with wax. The fine particles suspended in DI water are introduced in the outer capillary while different salts like calcium carbonate, calcium sulfate and sodium sulfate or DI water (as control) are introduced in the inner capillary. (b) Microscopic image of the capillary system at time = 2 minutes for 0.5 mm calcium carbonate and (c) 5 mm calcium sulfate solution. The direction of the electric field and the direction in which the fines move are shown by the pointers. It was seen that both in calcium carbonate and calcium sulfate gradient systems, the fines move towards the high salt concentration i.e. inside the smaller capillary, whereas for sodium sulfate gradient, the particles move in an opposite direction i.e. the particles stay in the bigger capillary and seem to be repelled from the high salt concentration, creating a region of exclusion. Similarly, for the set up where CaCO 3 solution is in the outer capillary with the particles being 68

82 69 suspended in DI water in the inner capillary, the electric field set up is directed such that the particles move from the inner to the outer capillary as shown in Figure 4-9 (a). The set up with DI water in the inner capillary is used as a control in which the particles show no noticeable movement except the quantum mechanical dance about their mean position. Supporting videos are submitted for reference. To avoid the interference of convection or density in our experiments, the set up was arranged in vertical coordinate system for analysis of the particle movements and velocity recording. (a) (b) Fines (-) E Fines (-) DI Figure 4-9. (a) Microscopic image of the capillary system at time ~ 2 minutes for 0.5 mm calcium carbonate in the outer capillary and the MFT fines inside the inner capillary (b) DI water. The direction of the electric field and the direction in which the fines move are shown by the pointers. DI water system, however, has no electric field generated as there is no ion gradient. In the experimental set up shown above, diffusion begins at the junction of the two capillaries due to imposed ionic gradient and thus at t=0, the initial concentration of the salt in the inner capillary is the initial concentration of the salt. Considering very thin double layers for low ionic strengths, we can use one dimensional diffusion equation to solve for the concentration profile along the length of the capillary to achieve the theoretical diffusiophoretic and diffusioosmotic velocities. Such diffusioosmotic flows are seen to drive the particles towards the sealed end of the capillary for a distance of less than a millimeter or so. For the time scales 69

83 70 considered for our analyses, we can assume to have a persistent concentration gradient of salt in the system. The velocities of the fine particles inside the inner capillary were obtained by tracking them through the inbuilt software NIS Elements on the camera of the microscope and were recorded as a function of the radial distance. The diffusiophoretic (U dp ) and diffusioosmotic (V do ) velocities of a charged particle in an electrolyte and the fluid along a charged surface respectively, are given by the following equations 29. U dp ε D D zeζ = η kt + 2k T 2 p n ζ p ln 1 tanh 2 2 ze D+ D z e 4kT n v do ε D D zeζ = η 2 2 kt + 2k T 2 w n ζ w ln 1 tanh 2 2 ze D+ + D z e 4kT n γ! = tanh zeζ! 4kT!!!!!!!!!!!!!!!!!!!!!!γ! = tanh zeζ! 4kT!!!!!!!!!!!!!!!!!!!!!!β = D!"#$%& D!"#$" D!"#$%& + D!"#$"!! (where ε is the permittivity and η is the viscosity of the fluid, k is the Boltzmann constant = 1.38 x J/C, T is the temperature of operation, z is the valency, e is the charge on an electron = 1.6 x C, n(x,t) is the concentration of the salt, D +, D - are the diffusion coefficients of the cation and the anion respectively, ζ p, ζ w are the zeta potentials of the particle and the wall respectively). The flow profiles obtained for both calcium carbonate and calcium sulfate are parabolic in nature as shown in Figure 4-10 (a)-(b). Both the theoretical and experimental diffusiophoretic velocities for fines moving in calcium sulfate gradient are lower than that in calcium carbonate. 70

84 71 This is also supported by the fact that at a given time, fewer fines enter the inner capillary as shown in Figure 4-8 (c). This can be possibly due to the fact that since calcium sulfate suppresses charges on the fines much more than calcium carbonate (as seen in 4.4.2), the difference in zeta potential is lower in the former, which according to the equations above, lowers the diffusiophoretic speeds of the particles. Also, the value of β is lower for calcium sulfate due to a lower difference between the diffusion coefficients of the constituent ions. This is another factor responsible for a lower diffusiophoretic velocity. Here, we infer that the observed flows in the capillaries are neither pressure-driven nor densitydriven as the direction of the particle movement depends only on the direction of the electric field and can be reversed by altering it. Additionally, it was observed that in high salt concentrations (> 10 mm), the flows disappear Improved Dewatering by Diffusioosmosis On introducing a concentration gradient of the salt externally, we are deliberately choosing the direction of the spontaneous electric field in the system. The fine particles, thus, are now acted upon by an additional driving force to make them move towards high or low salt concentration direction depending on the direction of the spontaneous electric field. In our system, we have chosen a positive concentration gradient of calcium carbonate in the vertically upwards direction (against gravity) as we place the salt solution at the bottom of the centrifuge tube. It is due to this that we have an electric field directed against gravity at all lengths of time. The fine particles, being negatively charged, now would be pulled downward not just by gravity but also by diffusiophoresis. The water, on the other hand, both by 71

85 72 diffusioosmosis and chemiosmosis would try to move in the opposite direction. So, the system is essentially pumping water out. Thus, we can expect better dewatering or improved dewatering by a combination of settling, electrostatic destabilization and diffusioosmosis. Even for a case where the salt solution is introduced to the tailings in no particular direction, it would eventually seep through the channels by gravity and by diffusion faster than water such that there will always be a higher salt concentration towards the bottom. This was further verified by a simple test conducted in the laboratory. The calcium carbonate salt solution was mixed with a very small amount of a colored indicator (Fluo-3) in a 50 ml centrifuge tube (Figure 4-10). A few drops of it were added to DI and it was seen that the drops eventually settle eventually building up a high concentration of a colored layer at the bottom of the tube. Thus, for a given pore, the fines would always try to move towards high salt region, pushing the water out. Figure Image showing colored solution of calcium carbonate settles through DI water. These flows, as mentioned before, can generate flows in the pores of the tailings. Thus, the unrecovered fraction of water due to entrapment in the narrow channels and pores can be efficiently extracted by this method. 72

86 CaCO CaSO4 t=220 s t=230 s t=210 s Velocity (µm/s) distance from centre of capillary (µm) Figure Velocity profile for tailings fines in calcium carbonate concentration gradient of 0.5 mm and calcium sulfate concentration gradient of 0.5 mm at a distance of about 150 mm from the mouth of the capillary. The velocities of the fines inside the inner capillary are almost an order higher in a CaCO 3 gradient compared to the same CaSO 4 gradient Consolidation of tailings The MFTs, when treated with calcium carbonate and calcium sulfate salts, separates into five visually distinct layers on settling under any conditions as shown in Figure 4-12 (a). This layering is not observed with MFT settling alone. To study the composition and size distribution, each of these layers were extracted and characterized. Energy-dispersive X-ray spectroscopy (EDS) was conducted on each of the layers which gave some interesting results. The EDS mapping for the topmost layer gave a high peak of Carbon unlike in any of the other layers as shown in figure 4-12(b). The carbon content of the MFTs is attributed to the presence of bitumen and certain other organics like naphtha. The other subsequent layers are formed based on the effective density or size. The EDS maps predicted that the bottom most layer comprises of the heaviest mineral, Muscovite, present 73

87 74 in the sample. This spontaneous sorting or layering of different materials is essentially due to internal reordering or rearrangement of particles. The fact that introduction of an additional force field in the system helps in the transition from a meta-stable to a more stable organization of the materials can be appreciated in this case. The system seems to reach its equilibrium. Apparently, the diffusioosmotic pumping also helps in this internal rearrangement. a) (b) Figure (a) Image of the five visually distinct layers formed on CaCO 3 treatment. (b) EDS for the topmost layer showing maximum Carbon in the sample Quality of water The separated supernatant samples from the settling experiments were collected for analysis. The EDS for the supernatant layer clearly showed no Calcium peak and smaller peaks for other mineral constituents as can be seen in figure 4-13(a). This strengthens the effectiveness of this water separation technique.the AAS runs on the same sample support it further (Table 4-1). 74

88 75 The reduced fines content in the supernatant is further supported by the size distribution plot showing before and after results. Clearly, the volume percent of fines in the supernatant is lower than the untreated one (Figure 4-13 (b)). (a) (b) No Treatment After Treatment 3.0 vol % size (µm) Figure (a) EDS for the supernatant layer showing no detectable calcium peak (b) Particle size distribution of the supernatant layers for the untreated and the treated MFT samples. Table 4-1. Concentration of different species in the treated MFTs. MFT treated with Fe Ca Al SiO 2 (mgfe/l) (mgca/l) (mgal/l) (mgsio 2 /L) CaCO CaSO

89 Chapter 5 76 Conclusions and Future Work 5.1. Conclusions The separation of water from the colloidal suspension of a complex material called MFT has been the central topic of discussion in this dissertation. The fact that two different approaches have been looked at to achieve dewatering is what makes this study interesting. The MFTs obtained from mineral processing are generally difficult to dewater and reclaim. This not only may cost a loss of potentially valuable resources but also can be a huge environmental threat. Slurries containing clays are quite common in the minerals industry. These clay particles are very fine, remain suspended in water for years and also form gel-like network structures with high water content. These structures usually have very small pores and cause dewatering problems even though they are not the major components of the tailings. To summarize the studies on chemical treatment on MFTs based on the experiments conducted, it can be stated that the single component clays (e.g. kaolinite and bentonite), are substantially helpful in the flocculation behavior studies but are not highly representative of the mature fine tailings. Tailings are essentially comprised of a mixture of different types of minerals (e.g. clays, quartz, etc.) with a percentage of bitumen and other organics in it as well. So, in such a complex mineral system, the zeta potential of the particles and their aggregation dynamics would highly depend on the composition. Also, due to the interactions within such systems, the zeta potential can also be predicted to be changing over lengths of time. 76

90 77 It is observed from the model clay system studies that kaolinite clays are relatively easier to treat than bentonite clays. Thus, in terms of the dosage requirements, we can say that bentonite behaves more closely to the MFTs when treated with a cationic coagulant. From the size distribution analysis, clear distinctions were observed between the three MFT samples where MFT 1 showed a clear bimodality when treated with a cationic coagulant or a combination of a cationic coagulant followed by a non-ionic flocculent. A similar bimodal plot was also obtained when bentonite clay was treated with the cationic coagulant. This strengthens the assumption that bentonite clay is similar to the MFT1 in terms of its aggregation behavior. Speaking from a broader perspective, the coagulant CP, regardless of its molecular weight seemed to act as a coagulant and not so much as a flocculent. This can be said keeping in mind the floc sizes that were obtained by the tailings and clay treatments with CP. The combination of the cationic polymer CP followed by an anionic polymer AP resulted in a significant increase in floc size. The floc sizes using this combination were higher than any other treatment studied. This is indicative of floc formation strictly by charge neutralization phenomenon. However, the high molecular weight cationic polymer used, resulted in a better distribution of flocs formed when combined with the non-ionic polymer. Using optical microscopy, it was seen that the density of flocs increased with increasing dosage of the cationic polymer for a fixed non-ionic polymer concentration. The zeta potential and the size distribution analyses have thrown some light on how well the tailings can be aggregated using the studied polymers. These methods allow comparisons between the model clays with those of the mixed mineral systems like mature fine tailings. The positive effect of electroosmotic dewatering of sludge has been known for several years. However, the technology has not yet been successfully applied in industry. It is because 77

91 78 there are several technological barriers to the commercial exploitation of the technology that are yet to be resolved. One of these barriers is a scientifically robust design methodology. Floc strength is an important underlying factor for establishing compliance with ERCB Directive 74, since MFT dewatering can be adversely affected by aggregates being sheared apart during processing. One method for engineering shear-resistant flocs is to make the forces aggregating the particles as strong as possible. As long as shear forces are less than the interparticle forces, the flocs will remain whole but when the flocs break, floc size suffers greatly. Aggregates formed by treating solids with high molecular weight linear polymeric flocculants tend to follow this behavior. The other method for engineering shear-resistant flocs is to make flocs that have weaker interparticle forces, but quickly reform after being sheared apart. Flocs formed from coagulants or branched polymeric flocculants tend to follow this behavior. However, different types of additives can be combined to create flocs that fall somewhere in-between sacrificing a little floc strength for increased recovery, or sacrificing a little recovery for better floc strength. And it is conceivable that the proper combination of additives could even result in performance that exceeded that of the individual components. It was found that for model kaolinite clay, the non-ionic polymer displayed an excellent ability to improve the flocculation performance. In general, CP was found to be sensitive to overdosing. Bentonite clay was rather difficult to treat and was determined not to be very sensitive to the polymer addition in terms of surface charge. The tailings more closely resemble the bentonite clay in their response to polymer treatment. However, improved coagulation using different versions of the cationic polymer CP was supported by the size distribution observations. Thus, these polymers and their combinations can potentially provide an alternative approach for the treatment and disposal of the oil sands tailings. 78

92 79 Phoretic movements like the ones discussed in Chapter 4 can be efficient or maybe indispensable in micro and nano-sized pores and channels of a porous bed of viscous materials like MFTs. In such systems, the pressure-driven flow mechanisms do not work according to the Hagen-Poiseuille equation. The diffusioosmotic flows are generated when a charged particle is placed in the vicinity of a finite concentration gradient of a salt. As already discussed, an electric field in the order of 1-10 V/cm essentially drives particle and fluid flow in the system. An otherwise complex mechanism that is behind the occurrence of such flows, it is the difference in the zeta potential between the particle and the wall that governs the dynamics. Thus, from the experimental results obtained in our system, the hypothesis that diffusioosmosis can be responsible for improved dewatering in MFTs with calcium carbonate seems to be justified. In this study, the role of calcium carbonate on the sedimentation of mature fine tailings, and the underlying mechanism is studied. The experiments carried out on industrial tailings show that the phenomenon of diffusioosmosis can be a potential alternative means of economic and spontaneous dewatering. Also the fact that simply by changing the direction of the concentration gradient of the salt, the movement of the fines can be manipulated makes the process immensely controllable. The concentrated tailings deposits can be visualized as a porous mass of clay with numerous dead end channels. Diffusioosmosis can be a powerful means to generate fluid flow through those channels. Colloidal destabilization through metal ion adsorption is a well-studied physics backed by the classical DLVO Theory 31. But in complex mineral systems like ours, finding the surface charge and thereby modeling the system can be quite challenging. The fact that very low amounts of Ca 2+ are found in the separated water further strengthens the effectiveness of this method. Low concentration of salt added to a system of 79

93 80 tailings can still dewater it by diffusioosmosis is the key point of this study. However, from the results obtained, colloidal destabilization also plays a part in this method. So, the diffusion coefficients of the salt and the suppression of the charges on the surface of the fines (zeta potentials) determine the efficiency of dewatering by diffusioosmosis. This can be particularly useful in a system with particles of different sizes as the diffusiophoretic velocities haves no size dependency Future Work As we saw from the analysis of the results obtained for polymer treatment of the MFTs, the floc strength and maximum recovery do not quite go hand in hand always. Thus, the growth and structure of flocs play an important role in the studied problem. The measurements of aggregate size distribution over time can give rough estimate of the rate of growth but cannot predict their configuration. A parameter called the fractal dimension 49 is known which reflects the internal structure of the aggregates formed and depends on the mechanism of colloidal destabilization. So, it is important to determine the fractal dimension for the above system to estimate the size and nature of the flocs formed. Electrokinetic flows are not regarded economical for industrial sorting and separation operations, particularly for tailings or wastes. The techniques already employed industrially are not economical. The requirements of high voltage and cheap but efficient electrodes make the conventional electrokinetic dewatering techniques rather difficult to implement. For diffusiophoresis, the flows generated last for shorter length scales. Also, the flows generated are not very fast. Overall, it s a slow process as it s based on diffusion of salt in a low 80

94 81 Peclet Number system. However, for a persistent concentration gradient of salt solution in a system, these flows would sustain for larger length scales. The spontaneous sorting of the material based on size or effective density can be a way not only to ensure better dewatering but also to form sediments that can be reclaimed faster than that predicted in literature. Thus this phenomenon needs a deeper analysis to address its limitations. 81

95 Appendix A 82 Preparation of the tailings sample for study The extraction tailings from MFT1, MFT2 and MFT3 sources were 28.5 weight % solids, 38.9 weight % solids and 38.5 weight % solids respectively in their original state. Each of them was diluted first to a 28 weight % solids loading by means of the respective recycled water samples (RW1, RW2, RW3) obtained from the same sources. Two kinds of tailings systems were studied for the zeta potential measurements: In the first system (concentrated), the coagulants and the flocculants were added to the 28 weight % solids state of the tailings. It was then mixed well with a Kar Dynamic Mixer (KDM). A small amount of the flocculated sample was taken and diluted to a weight % solids by various electrolytes (as mentioned) to study the zeta potentials. This was done to meet the concentration requirement of the Zetasizer used. In the second system (dilute), the 28% solids tailings were first diluted to a weight % solids loading by various electrolytes (as mentioned) before they were treated with the studied polymers. The samples were mixed well by means of a magnetic stir bar and then directly used for zeta potential measurements. 82

96 Appendix B 83 Characterization of the layers The mineral composition of the solids is shown by the X-ray diffraction (XRD) pattern in Figure 1. Silica or the sand forms the most abundant mineral in the tailings slurry with kaolinite and Muscovite (a form of illite) being the other predominant constituents. Figure B.1. XRD of the original tailings sample. All the five layers are characterized in terms of their size, structure and elemental composition. The SEM image of the fourth consolidated layer from top resembles the pseudo- 83

97 84 crystalline structure of Muscovite closely. The elemental composition of this layer also supports the presence of Muscovite as the dominant mineral. Figure B.2. SEM of the final sediment layer of the tailings sample after calcium carbonate treatment. 84