Grinding Ball Impact Analysis of a Semi-Autogenous (SAG) Mill Shell Liner Using Rocky DEM and ANSYS Mechanical Alexander Herr & Amit Saxena September 17, 2019 The Elecmetal Group 2 The Elecmetal Group Winery Metallurgy Glass Bottle Business 3 Communications Renewable Energy ME Elecmetal at a Glance • ME Elecmetal is part of The ELECMETAL Group with over US$1 billion p.a. in revenues • ME Elecmetal is a leading wear consumable supplier for mining, construction, and energy, with net revenue over USD 600 million p.a. • Range of products: Mill liners, crusher liners, grinding media and GET • Range of services: Service Centers, Engineering and Design, Early Alert, ME FIT(1) System, ME UpTime+ • 1,000 employees • Customers in +35 countries around the world • Manufacturing capacity of +134.500 tons in 5 foundries facilities • Manufacturing capacity of +400.000 tons in grinding media 4 We are Global 5 Analysis of SAG Mill Liner Impacts 6 Study Objective • Develop an impact model with FEA analysis • Develop a 3D DEM model • Show set-up and results from a coupled DEM/FEA simulation and analysis. • Study the influence of operational parameters like speed on impact forces and resulting stresses • Study the influence of liner design on impact forces 7 SAG Mill Operation • Semi-Autogenous Grinding (SAG) Mill • ‘Semi-autogenous’ mills include ore pieces, steel balls (grinding medial), and water. • The mill shell is lined with ‘shell liners’ designed to improve ore comminution and survive long enough to minimize downtime. • Ore particle comminution takes place in a SAG mill by: oreore, ore-ball, and ore-liner impacts. • Steel balls are introduced into mill at a size and rate determined by mill operator. • Balls available in sizes up to 6¼” (160 mm) diameter and weighing 35 lbs. • Water flows through the mill, aiding in the discharge of small 8 particles. SAG Mill With Grinding Media Basic Parts of a SAG Mill • A: Mill Shell – externally visible structure of the mill • B: Shell Liners – protect mill shell during comminution • Consists of high (rose) and low (maroon) lifts in this design • C: Grates – allow for the discharge of small/fine ore particles from grinding chamber • D: Pulp Lifters – (not visible here) behind grates, allow for discharge of ore as the mill spins C B A Internal View of SAG Mill Assembly Operational SAG Mill 9 Study Objective • After a certain period of operation, liners get worn down or broken • Relining — replace the spent liners with new liners • How can we design new liners that are better than the current design? More failure resistant & longer lasting? How to reduce liner failure risk? • Develop better alloys with higher hardness and toughness • Reduce impact force on the liners while This study focuses on measuring impact forces and resulting stresses 10 Toe Nailing at the Grate Ribs Examples of Heavy Peening Peening on the Shell Liner Lifter 11 Metal Flow inside the Grate Slots Methods for Analyzing Impacts Inside a SAG Mill: 1) 2) Analytical Method DEM-Based Methods a) De-coupled DEM/ANSYS b) Coupled Rocky/ANSYS 12 Analytical Model An analytical model for a grinding ball flight inside a SAG mill Absolute Velocity Relative (Impact) Velocity 13 Ball Speed vs Critical Speed • The absolute speed of the grinding ball reaches maximum at 80% CS • The relative speed between the ball and the liner reaches the peak value at 65 to 70% CS 14 Eq. Free Fall Height vs Critical Speed • The impact force shows similar variation pattern with mill speed for all size mill • Maximum impact force occurs at 65% to 70% CS • The larger the mill diameter, the stronger impact force it generates Equivalent free fall height for the ball-liner impact in SAG mills of diameter from 26 to 40 feet, rotation speed from 50% to 85% critical speed 15 Summary of DEM Simulations Discrete Element Method • Discrete Element Method (DEM) – A stochastic numerical method for computing the motion and effects of a large number of particles. • The inside of a mill is a very difficult environment to know what is happening: dark, wet, hot, full of water and dust particles. • Given reasonable assumptions about materials and particles, DEM provides reasonable simulations of the events inside a mill. 17 Features of Rocky DEM • Capable of simulating particles of various shape and stiffness • Increased solving speed with multi-CPU, GPU, and multi-GPU solvers • Full integration into ANSYS Workbench including sharing parameters • Particle breakage models 18 DEM Analysis SAG Mill • SAG mill size: 22’ x 9’ • Variable speed mill • Analyze particle (balls & ore) trajectory • Power draw • Maximum impact force magnitude • Large collision frequency Full view of high lift (upper/left) and low lift (right/lower) shell liner 19 DEM Setup Steps • Select simulation type: • Mill data including: • Simplify geometry • Input data above • Symmetry constraints (if applicable) • Mill shell section • Discharge end • Wear (if applicable) • • • • • Rotational velocity Ore density Particle fill level Particle size distribution Mill power draw Model Setup Within RockyDEM 20 Parameterization in Rocky • Advantages of solving DEM simulations in ANSYS: • Ability to link to other physics: • Any quantity can be parameterized: • Transient stress responses • Include water/slurry with Fluent Setting up an Expression within Rocky DEM • Geometry • Rotational velocity (shown at right and below) • Fill rate, material distribution • Material properties • Etc 21 Coupled Simulation in ANSYS • Rocky transfers impact-induced pressures to ANSYS as surface pressure loads. • Rocky output allows loads to be exported for selected geometries. • Pressure loads are mapped onto applicable surfaces • Mapping is maintained throughout the motion of the part(s). Light-colored high and low lift have a fine mesh in Rocky and loads exported to ANSYS. 22 Simulation Steps 2) Begin rotation (3 – 9 seconds) 12 rpm in this instance Chaotic initial rotation Particles are colored based on translational velocity 1) Fill mill slice section (0 – 2.5 sec) 18% in this instance Note: Symmetry is turned on. Particles that leave one side return on the other. 23 Simulation Steps 3) Steady state rotation (16 – 20 seconds) 12 rpm 24 Comparison of Charge Motion Particle motion at 3 rotational velocities, t = 20 seconds. Particle motion at rotational velocity of 10 RPM. Particle motion at rotational velocity of 12 RPM. 25 Particle motion at rotational velocity of 14 RPM. Power Draw Level Average Power Draw at 3 Rotational Velocities 1200 1000 kW 800 600 400 200 0 10 RPM 12 RPM 14 RPM 26 Parameters from Rocky/ANSYS Design Points in ANSYS Workbench • Design points, shown above, include inputs from SpaceClaim/geometry and Rocky simulation module. • Output stresses come from ANSYS Transient solution. 27 Comparison of Charge Motion Particle motion of 3 geometric modifications. Rotational velocity = 12 RPM, t = 20 seconds. ‘Low’ lifts height increased 30 mm. ‘Low’ lifts raised 30 mm. ‘High’ lifts raised 12.7 mm. 28 ‘Low’ lifts raised 60 mm (equal height to high lift). ANSYS Setup Mesh size is similar (20 mm) to the discretization in Rocky for pressure transfer. Mapped pressure applied to the highlighted surfaces. Lower mill liner surfaces are fixed. 29 ANSYS Transient Stress Results Screen capture of the maximum equivalent stress over the 10 seconds of simulation time (18 sec of Rocky). Video of stresses induced by particle loads - transferred from Rocky within ANSYS Workbench, 12 RPM design point. Note: lower blue stress bands have been modified to show low stress variation over time. 30 Rocky/ANSYS Synced Videos Synchronized video of shell liners of interest in Rocky and ANSYS over 10 seconds of simulation time (@ 12 RPM). 31 Parameters from Rocky/ANSYS • Shown at right, generally stress in the shell liners increases as the rotational velocity increases. Due to randomness, even as particles gain energy – at higher rotation speeds – they may not contact liners directly. Rotational Velocity vs. Max Equivalent Stress (psi) Maximum Equivalent Stress (psi) • Varying Rotational Velocity in ANSYS Workbench 7000 6000 DEM Run 1 5000 DEM Run 2 4000 3000 2000 1000 0 9 10 11 12 Rotational Velocity (RPM) 32 13 14 15 Comments on Fatigue • Although the stresses in this example are relatively low, for larger mills, shell liner impact energies increase exponentially with increases in diameter. • Liner life typically ranges from 3 – 9 months. At 12 RPM an impact every revolution leads to ~500,000 cycles/month. • ANSYS fatigue functions, including the SMART Crack feature, allow modeling of semi-elliptical or arbitrary crack front and visualization of crack propagation. Minor Axis Previous work with semi-elliptical crack analysis has been done to look at the potential for fatigue failure. 33 Major Axis Uncoupled Explicit Dynamic Impact Simulation 34 Evaluate Impact Stresses Example Mill 40’ (12.2 m): Ball Size: 6 1/4 “ Falling Velocity: 52.5 ft/s (16 m/s) Mill Rotational Velocity: 22 ft/s (6.7 m/s) Ball Energy 0.5 kCal (2.1 kJ) To find the worst case impact, particle size, velocity, trajectory, and mass are extracted from the DEM and applied to a grinding ball for an explicit impact analysis. Impact location(s) are chosen at suspected weak points or at locations where breakage is observed. 35 Equivalent Stresses -Surface Impact on Liner Leading Face Radius 36 Equivalent Stress– Cross Section • Compressive Yield Strength = 871.9 Mpa • Tensile Yield Strength = 799 Mpa • Ultimate Tensile Strength = 1235 MPa 37 Reaction Force – Ball/Shell Liner 38 • Max Impact Force = 1.7 MN • Max Impact Force from DEM Calculation = 45kN Kinetic Energy of Impact 39 Facts About DEM • DEM provides a more realistic numerical analysis for liner-particle interactions inside the mill • DEM can capture the effect of mill liner structural factors like lifter height • DEM can also capture the effect of operational parameters like mill speed • DEM provides a direct measure of impact forces on the liner, which is one of great contributors to liner failure in service • DEM has gained common acceptance for understanding the behavior of particles inside the mill • DEM requires validation with experimental calibration 40 Rocky DEM in ANSYS Workbench • • Provides a streamlined method for running mill optimization studies. Frequently these studies vary one or more of these parameters to optimize grinding efficiency: • • • • • Shell liner design (angle and/or height) Total charge level Ball size Ball:Ore ratio Rotational velocity • DEM coupled with impact force results and frequency of collisions helps with better liner design selection through relative comparisons of these parameters • Provides predictive results for new designs to mitigate the risk of future part breakage • The FEA model can be further developed to conduct high-cycle fatigue and predict liner life 41 Acknowledgements: Thank you to Rahul and the Rocky Support Team for their help: Ahmad Saurabh Guilherme 42