MASTER'S THESIS Placement of Thickened Tailings Adoption of a Rheology-Oriented Model for Slope Predictions Deniz Dagli Master of Science (120 credits) Civil Engineering Luleå University of Technology Department of Civil, Environmental and Natural Resource Engineering Placement of Thickened Tailings Adoption of a Rheology-Oriented Model for Slope Predictions Deniz Dagli Lule˚aUniversity of Technology Dept. of Civil, Environmental and Natural Resources Engineering Div. of Mining and Geotechnical Engineering 2nd January 2013 ABSTRACT Thickened tailings or “paste” disposal with deposition slopes varying from 2 to 4% results in steeper slopes compared to those of conventional placement (less than 0.5%). As a consequence, thickened tailings disposal ends up with filling more volume per storage area and thus avoids costly & frequent dam raises. Reported observations describe that the discharged slurry typically flows down the slope or “beach” in a confined, self-formed channel, in a macroscopic equilibrium between erosion and sedimentation, defining an overall slope and then spreads out & deposits on a broader area (Simms, et al., 2011). The objective of this study is to describe the slope forming elements and adopt the model for beach slope predictions by Fitton (2007) together with a discussion of simulated results and the effects of rheological & hydraulic parameters. Typical thickened tailings have average particle sizes of 25 to 75 µm with maximum particles of about 0.5 mm. To obtain a conceptually even slope with practically no segregation of particles occurring and no drainage of water taking place, the concentration by volume often needs to exceed 40% (even 45% for some cases). It is reported that these slurries show non-Newtonian behaviour and often found in a supercritical open channel flow state located in the transitional or turbulent zone. When the slurry spreads out, the flow attains a sheet-like, laminar state where particles settle out. The process repeat itself in a cyclic manner and the overall beach profiles are created as a result. Fitton’s model, based on open channel hydraulics (Darcy-Weisbach friction loss con- cept) and sediment transport approach, considers the self-confined open channel flow to be turbulent, based on field and large scale flume observations. The model relates the deposition velocity to maintain the channelized flow, to a Bingham rheological model parameter. The model is utilized to simulate the depositional behavior of thickened tailings slurries having a volumetric solids concentration of 46% (71% mass). This value is used to represent homogeneous (non-segregating) conditions for tailings with an average particle size of 50 µm and a solids density of 2900 kg/m3. Simulation results for slurry flow rates 3 from 25 to 400 m /h correspond to slopes of about 6 and 2%, respectively, expressing a flow rate dependence at yield stresses and Bingham viscosities of up to 30 Pa and 0.1 Pa.s, respectively. Reynolds numbers were from about 500 to 94500 for which the effect iii of channel roughness on predicted slopes was practically negligible within values from 0.3 mm to smooth conditions at 0.05 mm. Calculated slopes were nearly insensitive to the channel shape as long as the width/depth ratio remained constant about 5.5. A rectangular cross-section was used for the calcula- tions but a parabolic channel shape reflects field observations better. Equilibrium yield stress requirement for the flow to come to rest and the required shear strength (cohesion) to be developed upon drying for stability of slopes formed by stacked layers of various thicknesses are demonstrated in schematic examples. iv PREFACE This thesis report is submitted as a part of the graduation work for the fulfillment of a MSc. degree at Lule˚aUniversity of Technology, Dept. of Civil, Environmental and Natural Resources Engineering. After working on the subject of thickened tailings disposal for a considerable amount of time, I would like to use this opportunity to express my gratitudes to the people who have provided invaluable help and support. First, I wish to thank my thesis supervisor, Professor Emeritus Anders Sellgren, for his patience and guidance. Without the time he has put into this work and the effort to carefully examine the work over and over for constant improvement, I would never be able to overcome this challenge. For that, I will always be grateful. I wish to express my thanks to Mr. Thord Wennberg at LKAB for the detailed guided tour of the company’s mineral processing plant and the tailings storage facilities. I also want to thank my examiner, Professor Sven Knutsson, for his support and the encouragement to work on tailings in general. My special thanks go to Mr. Aziz Kubilay Ovacikli for the help he has provided with the coding of the MATLAB algorithm used throughout this work and also for introducing me to report writing with LATEX. Above all, I would like to express my deepest gratitudes to my family for the uncondi- tional love and for the moral, motivational & financial support they have provided during my whole life and my stay in Sweden. Without them I could have never achieved any of this and therefore, I dedicate my share of contribution in this work to them. Deniz Da˘glı Lule˚a, December 2012 v CONTENTS Chapter 1 – Introduction 1 1.1 Mineral Processing & Tailings . 1 1.2 TailingsDisposalMethods .......................... 2 1.2.1 Conventional Tailings Disposal (Dams/Impoundments) . 2 1.2.2 Thickened Tailings Disposal . 5 1.2.3 PasteDisposalinUndergroundMines. 9 1.3 ProblemDescription ............................. 9 1.4 Objective&Scope .............................. 10 Chapter 2 – Open Channel Flow 11 2.1 Flow Resistance & Friction Losses . 11 2.2 FlowRegime&ReynoldsNumber. 13 2.3 FrictionFactor(f)............................... 13 2.4 StateofFlow ................................. 15 2.5 ConceptualExample1 ............................ 17 2.6 Non-Newtonian Fluid & Suspension Flow . 19 2.6.1 Models................................. 19 2.6.2 Non-NewtonianFrictionLosses . 23 2.6.3 ConceptualExample2 ........................ 24 Chapter 3 – Transportation of Tailings Slurries 27 3.1 Solid-WaterMixtureParameters. 27 3.1.1 Solid Content & Density . 27 3.1.2 Particle Size Distribution . 28 3.2 Classification ................................. 28 3.2.1 Settling & Non-Settling Slurries . 28 3.2.2 Deposition Velocity . 31 Chapter 4 – Placement of Tailings Slurries 33 4.1 SlurryDeposition............................... 34 4.1.1 Geotechnical (Slope Stability/Equilibrium) Approach . 34 4.1.2 ConceptualExample3 ........................ 37 Chapter 5 – Adoption of the Rheology-Oriented Model by Fitton 39 5.1 Fitton’sWork................................. 39 5.1.1 ModelDevelopment.......................... 39 5.2 Fitton’s Semi-Empirical Beach Slope Model . 41 5.2.1 Segregating vs Non-Segregating Slurries . 42 5.2.2 Minimum Transport Velocity Equations . 43 5.2.3 FlowResistanceEquation . .. 45 5.2.4 Determination of the Channel Shape for the Open Channel Flow . 46 5.2.5 RunningtheModel.......................... 47 5.3 Validation with the Field Data . 48 Chapter 6 – Results & Discussion 51 6.1 ModelValidation ............................... 51 6.2 Properties of the Tailings Material & Assumptions for the Simulations.. 52 6.3 Results&Discussion ............................. 54 6.3.1 Results................................. 54 6.3.2 StateofFlow ............................. 57 6.3.3 Solid Concentration and Particle Size Distribution . 58 6.3.4 Sediment Transport Approach & Shield’s Diagram . 58 Chapter 7 – Conclusions 61 REFERENCES 62 Appendix A – Parameter Study 67 A.1 EffectoftheYieldStress........................... 67 A.2 EffectofViscosity............................... 68 Appendix B – Flow Simulations 71 viii List of Figures 1.1 Tailings from the mineral processing of iron ore - Schematic flow chart.. 1 1.2 Conventional tailings disposal system. Slopes for deposited tailings are normallylessthan0.5%(Wennberg,2010) . 2 1.3 Discharging with spigots (Fell, et al., 1992) . 3 1.4 UpstreamMethod(Fell,etal.,1992) . 5 1.5 Downstream (a) and Centerline (b) construction methods (Fell, et al., 1992) 6 1.6 Schematic representation of the thickening process (Metso Minerals, 2002). Theterm“pulp”issynonymoustoslurry. 6 1.7 Schematic representation of the thickened tailings discharge method (Fell, etal.,2005) .................................. 7 1.8 Thickened discharge method on an inclined ground (Wennberg, et al., 2008) 8 1.9 Tailings disposal system with a thickener located near the deposition zone (Wennberg,2010)............................... 8 1.10 Thickener location alternatives for LKAB Svappavaara mine (Wennberg, 2010)...................................... 9 2.1 A uniform open channel flow. The slope is normally less than 5◦ which means that φ = sin φ = tan φ and the depth perpendicular to the bottom istakenastheverticaldepth. 12 2.2 Moodychartforpipeflows(AfterMoody,1944) . .. 14 2.3 Transition from sub-critical to supercritical flow . ... 16 2.4 Transition from supercritical to sub-critical flow . ... 16 2.5 Undular hydraulic jump taking place at Fr < 1.7 ............. 17 3 2.6 Open channel equivalent of a pipe flow with 0.15 m diameter and 80 m /h discharge.................................... 18 2.7 Non-Newtonian fluid models, whereγ ˙ is the shear rate (Fitton, 2007) . 19 2.8 Transition point VT , where flow regime changes from laminar to turbulent in a tailings slurry with Bingham like properties. The arrows indicate various approaches for turbulent friction