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Effect of Floc Size Distribution and Floc Structure on Coal Dewatering

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Effect of Floc Size Distribution and Floc Structure on Coal Dewatering THE UNIVERSITY OF NEW SOUTH WALES SCHOOL OF CHEMICAL ENGINEERING AND INDUSTRIAL CHEMISTRY Effect of Floc Size Distribution and Floc Structure on Coal Dewatering A thesis submitted to the University of New South Wales as fulfllment of the requirements for the degree of MASTER OF ENGINEERING BY TSUI KA FAI, ISAAC May 2000 ACKNOWLEDGMENTS I would like to express my sincere thanks to my supervisors Dr Rose AMAL, Dr Chris VEAL and David CONDINE for their continual guidance and encouragement throughout this thesis. I would also like to thank John Starling and Dr Deyan Guang whose valuable technical assistance I gratefully acknowledge. Finally, special thanks to my family and friends for their love, patience and moral support. ii ABSTRACT In this study the effects of floc micro-properties such as floc structure, size and size distribution on the dewatering of coal by vacuum filtration are investigated. The coal particles were flocculated using different dosages of anionic polymer at different mixing rate prior to the vacuum filtration process. The structure and size distribution of the floes formed at the different flocculation condition were measured using the small angle light scattering technique. Fractal scattering exponent is used to describe the floc structure, and the volume-average size (D[4, 3]) is used to represent the floc average size. The floc size distribution at the different flocculation conditions was studied by comparing the fraction of sub-25.5 micron and the fraction of sub-9 .5 micron in the suspension. The floc micro­ properties are then correlated with the cake macro-properties such as cake porosity, permeability, and ultimately the cake moisture content and the filtration performance. It has been found that the volume average size (D[4,3]) of the floes increases as the polymer dose increases up to 100 ppm and no further increase is evident beyond this point. Almost all the sub-9.5 micron and the sub-25.5 micron of coal fines had been flocculated at the polymer dosage of 20 ppm and 60 ppm, respectively. In terms of floc structure measured by the fractal scattering exponent [FSE], a more compact structure is resulted as the polymer dosage increases up to 20 ppm. Further increase in the polymer dosage beyond 20 ppm decreases the fractal scattering exponent, indicating a looser structure at the high polymer dosage. A strong correlation between the floc micro-properties and the cake properties is observed. The lowest moisture content (22%) is achieved at the condition giving the highest FSE (2.61 ± 0.05, ie. the most compact floes). The highest cake permeability and the shortest cake formation time are obtained at the condition where the fraction of sub-25.5 micron particles in the suspension is negligible. The rapid determination of the floc micro-properties using the small angle light scattering technique and the strong relationship of the measured micro-properties with the cake moisture content and permeability suggest that this technique may be particularly useful as an on-line process control tool to optimise flocculation and dewatering performance in coal industry. Ill Table of Contents Pages Acknowledgments 11 Abstract lll Table of Contents IV List of Figures Vll List of Tables X CHAPTER 1: INTRODUCTION 1 1.1 Coal Slurries 2 1.2 Filtration 2 1.3 Flocculation 3 1.4 Objectives and Overall Approach 4 CHAPTER 2: LITERATURE REVIEW AND BACKGROUND THEORY 5 2.1 Coal 5 2.1.1 Coal Preparation 5 2.2 Dewatering of Coal 6 2.2.1 Different Methods of Coal Dewatering 7 2.2.2 Vacuum Filters 8 2.2.2.1 Drum Filters 8 2.2.2.2 Disc Filters 10 2.2.2.3 Horizontal Belt Filters 11 2.3 Vacuum Filtration 13 2.3.1 Cakes Filtration I Dewatering Theory 14 2.3.2 Classical Filtration I Dewatering Theory 16 2.3.2.1 Cakes Filtration I Dewatering Equation 17 2.3.2.2 Cake Porosity and Permeability Equation 18 2.4 Flocculation 19 2.4.1 The Bridging Theory ofFlocculation 20 2.4.2 Flocculating Agents (Polymenc Flocculants) 22 2.4.3 Flocculation Kinetics 24 2.4.3.1 The Rate ofFlocculation 25 2.4.3.2 The Shear Velocity Gradient 27 2.4.3.3 Floc (Aggregates) Strength and Break-up 29 2.4.3.4 Floc Morphology 30 2.4.4 Flocculation as an aid to Vacuum Filtration 32 iv CHAPTER 3: SAMPLE PREPARATIONS AND EXPERIMENTAL PROCEDURES 35 3.1 Sample Preparation 35 3.1.1 Measurement of Solids Content 35 3.1.2 Flocculant Preparation 36 3.2 Measurement of Floc Size Distribution and Floc Structure 37 3.2.1 Description of the Malvern Master Sizer 39 3.2.2 Flocculation (Conditioning) Procedures 42 3.2.3 Dilution and Particles Size Measurement Procedures 43 3.2.4 Determination ofFractal Scattering Exponent 45 3.3 Determination of Cake Moisture Content 45 3.3.1 Description of the Vacuum Filiation Unit 47 3.3.2 Dewatering Procedures (Vacuum Filtration) 47 CHAPTER 4: RESULTS AND DISCUSSION 49 4.1 Experimental Design and Testing 49 4.1.1 Testing Pumping System 49 4.1.2 Testing Dilution System 51 4.2 Flocculation Results 52 4.2.1 Effect of Flocculation Condition on Floc Size Distribution 54 4.2.1.1 Effect of Polymer Dose on Floc Size Distribution 54 4.2.1.2 Effect of Agitation Speed and Conditioning Time on Floc Size Distribution 58 4.2.2 Effect ofFlocculation Condition on Fractal Scattering Exponent 65 4.2.2.1 Effect of Polymer Dose on Fractal Scattering Exponent 65 4.2.2.2 Effect of Mixing Intensity on Fractal Scattering Exponent 66 4.3 Vacuum Filtration Results 67 4.3.1 Effect of Floc Properties on Cake Moisture Content 67 4.3.1.1 Effect ofFloc Size on Cake Moisture Content 67 4.3.1.2 Effect of Floc Size Distribution on Cake Moisture Content 71 4.3.1.3 Relationship between Fractal Scattering Exponent and Cake Moisture Content 72 4.3.2 Effect ofFlocculation on Cake Structure (Porosity and Permeability) 74 4.3.2.1 Effect of Average Floc Size and Fractal Scattering Exponent on Filter Cake Porosity 75 4.3.2.2 Effect of Average Floc Size Distribution on Filter Cake Permeability 76 80 4.3.3 Effect ofFlocculation Condition on Cake Formation Time V CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS 83 CHAPTER 6: BIBLIOGRAPHY 86 APPENDICES 96 Appendix 1: Calculation of Velocity Gradient (Average Shear Rate) at Different Agitation Condition 96 Appendix 2: Raw Data of Coal Floes Size at Different Flocculation Condition 98 Appendix 3: Fractal Scattering Exponent (FSE) at Different Flocculation Condition 106 Appendix 4: Cake Moisture Content at Different Flocculation Condition 108 Appendix 5: Calculation of Cake Porosity 112 Appendix 6: Calculation of Cake Permeability 116 Appendix 7: Percentage of Floes Size Less Than 9.5 micron at Different Flocculation Condition 120 Appendix 8: Percentage of Floes Size Less Than 25.5 micron at Different Flocculation Condition 122 Appendix 9: Cake Formation Time (CFT) at Different Flocculation Condition 124 Appendix 10: Nomenclature 126 vi List of Figures Pages Figure 2.1 Mechanistic steps involved in filtration/ dewatering of fine suspensions (Parekh and Groppo, 1994) 16 Figure 2.2 Mechanism of bridging flocculation (Akers, 1975) 22 Figure 3.1 A schematic diagram of the Malvern Master Sizer (Malvern Master Sizer instruction manual, 1993) 39 Figure 3.2 A photo offlocculation system 43 Figure 3.3 A diagram of the system used to dilute and measure the size of the coal particle size 44 Figure 3.4 A schematic diagram of the filtration rig 47 Figure 4.1 Particle size distribution of coal sample (flotation concentrates) recovered from Jameson flotation cell in Stratford coal preparation plant 53 Figure 4.2 The variation of particle size for D[4, 3], D[3, 2], D[v, 0.1], D[v, 0.5] and D[v, 0.9] with different flocculant dosage at 500 rpm agitation speed and 30 seconds conditioning time 56 Figure 4.3 The Particles size distribution of coal sample at 200 ppm flocculant dosage, 500rpm agitation speed and 30 seconds conditioning time 57 Figure 4.4 The variation of percentage of floc size less than 9.5 micron with different flocculant dosage at 500 rpm agitation speed and 30 seconds conditioning time 57 Figure 4.5 The variation of percentage of floc size less than 25.5 micron with different flocculant dosage at 500 rpm agitation speed and 30 seconds conditioning time 58 Figure 4.6 The variation of volume mean diameter (D[4, 3]) with different flocculant dosage and different agitation speed at 5 seconds conditioning time 61 Figure 4.7 The variation of volume mean diameter (D[4, 3]) with different flocculant dosage and different agitation speed at 15 seconds conditioning time 61 vii Figure 4.8 The variation of Sauter mean diameter (D[3, 2]) with different flocculant dosage and different agitation speed at 5 seconds conditioning time 62 Figure 4.9 The variation ofSauter mean diameter (D[3, 2]) with different flocculant dosage and different agitation speed at 15 seconds conditioning time 62 Figure 4.10 The variation of diameter at 10 percentage point (D[v, 0.1]) with different flocculant dosage and different agitation speed at 5 seconds conditioning time 63 Figure 4.11 The variation of diameter at 10 percentage point (D[v, 0.1]) with different flocculant dosage and different agitation speed at 15 seconds conditioning time 63 Figure 4.12 The effect of mixing intensity on the mean volume diameter (D[4, 3]) for different polymer dosage 64 Figure 4.13 The effect of mixing intensity on the 10 percentile undersized for different polymer dosage 64 Figure 4.14 The fractal scattering exponent with different flocculant dosage at 500 rpm agitation speed and 30 seconds conditioning time 66 Figure 4.15 The variation of moisture content of filter cakes and mean volume diameter,
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