Slurry flow in a tower mill

Tower mills are a commonly used device for fine grinding in the mineral processing industry and can be used for dry or wet-grinding applications. In wet grinding, the nature of the slurry flow plays an important role in transporting feed rock and ground fines inside the mill and also assists discharge from the mill. Operating conditions and impeller design can affect the slurry distribution within the mill with some regions of the charge potentially being drier and others saturated. To help understand the slurry distribution and transport we use a two stage modelling process. The Discrete Element Method (DEM) is used to characterise the motion and distribution of the grinding media in the tower mill. The averaged voidage distribution and steady velocity field from the DEM model is then used to define a dynamic porous media in the fluid model. The Smoothed Particle Hydrodynamics (SPH) method is used for modelling the fluid flow because of the free surface and the moving impeller. The one way coupled DEM/SPH model is then used to assess how the fluid distribution and flow pattern of the slurry in a tower mill are to variations in the slurry viscosity.     

Using SPH one-way coupled to DEM to model wet industrial banana screens

Large banana screens with multiple decks are used extensively in the process separation of many valuable export commodities. They are high capacity vibrating screens with a curved profile. Discrete Element Method (DEM) modelling using non-spherical particles has previously provided significant insight into the operation of these dry industrial screens. Here we introduce the use of Smoothed Particle Hydrodynamics (SPH) to model the flow of slurry (water and fine material) through a double deck banana screen. This paper firstly reports on the underlying DEM model of the coarse particulate flow on a full-scale banana screen. We then use Smoothed Particle Hydrodynamics (SPH) to model the transport of fine particle slurry over and through the double deck banana screen. Finally, we combine the DEM with SPH models using a one-way coupling to simulate the effects of adding a slurry flow to coarse particulates on the banana screen. The key outcomes from this study are that; SPH is ideally suited for the high speeds and the high fragmented and filamentary nature of the fluid flow through the screen deck openings; the fluid only (SPH) model of slurry behaves similarly to the DEM approach in that more fluid is screened as the velocity slows, except near the earlier panels on the top deck; and, use of a porous media derived from DEM in one-way coupled approach with SPH produces clear and reasonable changes in fluid structure, separation and wetting of the screens consistent with slurry behaviour. Specifically, the fluid layer was much thicker in the coupled case, with slurry being trapped inside a coarse particle bed and which is sensitive to the fluid viscosity.     

Performance characterisation of AG/SAG mill pulp lifters using CFD techniques

The pulp lifter is an integral component of autogenous (AG) and semi-autogenous (SAG) grinding mills as it controls the throughput, performance and efficiency of mills. The slurry transport from the AG/SAG mill through grate holes into the discharge trunnion is the main function of the pulp lifter. This process develops complex flow behaviour in the region of the grate and pulp lifter. Efficient and effective removal of pulp/slurry from the mill is the key objective of the pulp lifter design.This work aims to understand slurry flow behaviour in pulp lifter sections and its contribution to mill performance using computational fluid dynamics (CFD) modelling and Laser Doppler Anemometry (LDA) measurements applied to a laboratory scale mill. The CFD model is validated against the LDA measurements, and then used to build a cohesive computational framework for modelling industrial pulp lifters, to investigate unique problems associated with their design and performance.     

Slurry flow in mills: grate-pulp lifter discharge systems (Part 2)

Pulp lifters, also known, as pan lifters are an integral part of the majority of autogenous (AG), semi-autogenous (SAG) and grate discharge ball mills. The performance of the pulp lifters in conjunction with grate design determines the ultimate flow capacity of these mills. Although the function of the pulp lifters is simply to transport the slurry passed through the discharge grate into the discharge trunnion, their performance depends on their design as well as that of the grate and operating conditions such as mill speed and charge level. However, little or no work has been reported on the performance of grate-pulp lifter assemblies and in particular the influence of pulp lifter design on slurry transport.Ideally, the discharge rate through a grate-pulp lifter assembly should be equal to the discharge rate through at a given mill hold-up. However, the results obtained have shown that conventional pulp lifter designs cause considerable restrictions to flow resulting in reduced flow capacity.In this second of a two-part series of papers the performance of conventional pulp lifters (radial and spiral designs) is described and is based on extensive test work carried out in a 1 m diameter pilot SAG mill.     

Further validation of DEM modeling of milling: effects of liner profile and mill speed

Grinding mills are usually lined with lifters to improve their efficiency. During the course of operation, the lifters are worn away. This will affect the energy efficiency and capacity of mills and the behavior of the load in the mill and finally leads to a relining to replace the worn lifters. However, the effects of the profiles of lifters are not taken into account in all the previous power models for rotary mills. Discrete element method (DEM) is capable of demonstrating the effects of lifter profiles on mill power and load behavior. In this paper, two types of lifter profiles, square and trapezoidal, are investigated in terms of mill power and load behavior with a 2D mill and a DEM simulator, Millsoft, over a wide range of rotational speed. DEM satisfactorily predicted the load behavior and power draw for different lifter profiles at sub-critical speeds comparing with the experimental results. It is found that the trapezoidal lifters draw more power than the square lifters. An attempt has been made to explain the difference between measured and simulated power using photographs of experimental load behavior.     

Estimating energy in grinding using DEM modelling

The latest state of the art on Discrete Element Method (DEM) and the increased computational power are capable of incorporating and resolving complex physics in comminution devices such as tumbling mills. A full 3D simulation providing a comprehensive prediction of bulk particle dynamics in a grinding mill is now possible using the latest DEM software tools.This paper explores the breakage environment in mills using DEM techniques, and how these techniques may be expanded to provide more useful data for mill and comminution device modelling. A campaign of DEM simulations were performed by varying the mill size and charge particle size distribution to explore and understand the breakage environment in mills using DEM techniques. Analysis of each mill was conducted through consideration of the total energy dissipation and the nature of the collision environment that leads to comminution.The DEM simulations show that the mill charge particle size distribution has a strong influence on the mill input power and on the way the energy is distributed across the charge. The smaller particles experience higher energies while the larger experience less, but this variation is strongly dependent on the mill size. The results also showed that the average particle collision energy increases with increasing mill size, whereas its distribution over particle size is strongly influenced by the mill content particle size distribution. The simulations also captured the energy distribution within different regions of the tumbling charge, with the toe impact region having higher impact energies and the bulk shear region having higher tangential energies. Regardless of the mill size most of the energy is consumed by the particles in the mid-size range, which has the highest percentage mass of the total charge distribution.     

Validation of a model for physical interactions between pulp,charge and mill structure in tumbling mills

Modelling the pulp fluid and its simultaneous interactions with both the charge and the mill structure is an interesting challenge. The interactions have previously been modelled for dry grinding with a combination of discrete element method (DEM), smoothed particle hydrodynamics (SPH) and the finite element method (FEM), where the DEM particles or SPH particles represent the grinding balls and FEM is used to model the mill structure. In this work, the previous model is extended to include fluids with SPH. Wet milling with water and a magnetite pulp, for graded and mono-size charges are numerically modelled and validated. The internal working of the charge and the physical interaction between the charge and the mill structure is studied. The combined SPH–DEM–FEM model presented here can predict the classical DEM results, but can also predict responses from the mill structure, as well as the pulp liquid flow and pressure. Validation is conducted by comparing numerical results with experimental measurements from grinding in an instrumented small-scale batch ball mill equipped with an accurate torque metre. The simulated charge movement is also compared with high speed video of the charge movement for a number of cases. Numerical results are in good agreement with experimental measurements.     

Prediction of coupled particle and fluid flows using DEM and SPH

A coupled DEM (discrete element method) and SPH (smoothed particle hydrodynamics) method is used to predict the motion of the solid particles interacting with slurry flow. DEM simulates the motion of the coarser particulates while SPH simulates the slurry (water and finer particulates). This three dimensional method is demonstrated for multiphase mixing flow on a chute, in a central slice of a 36′ SAG mill and in a Hardinge pilot mill. It is shown to be robust and stable and able to predict complex coupled free surface particle and slurry flows including splashing with saturation levels varying from dry to fully saturated. It is suitable for prediction of multiphase flow in moving equipment including small scale structures such as classification grates.     

Slurry flow in mills: grate-only discharge mechanism (Part-1)

Discharge grates play an important role in determining the performance of autogenous, semi-autogenous and grate discharge ball mills. The flow capacity (grinding capacity) of these mills is strongly influenced by the discharge grate design––open area and position of apertures, as well as the performance of the pulp lifters. As mill sizes have progressively increased and closed-circuiting has become more popular the importance of grate and pulp lifter design has grown.Unfortunately very few studies have concentrated on this aspect of mill performance. To remedy this a series of laboratory and pilot-scale tests were undertaken to study both the performance of grates on their own and in conjunction with pulp lifters. In this first paper of a two-part series the results from the grate-only experiments are presented and discussed, whilst the performance of the grate-pulp-lifter system is covered in the second paper.The results from the grate-only experiments have shown that the build-up of slurry (hold-up) inside the mill starts from the shoulder of the charge, while the toe position of the slurry progressively moves towards the toe of the charge with increasing flowrate. Besides grate design (open area and position of apertures), charge volume and mill speed were also found to have a strong influence on mill hold-up and interact with grate design variables.     

Comparing three DEM charge motion models

In comminution research, recent trends have been made to describe internal dynamics of mills using the discrete element method (DEM). In this work, three modelling approaches to DEM implementation to charge motion modelling are compared these being single ball trajectories, system of individual balls describing the mill charge and charge balls grouped together using an arbitrary discretization scheme. After presenting some charge motion fundamentals as well as a presentation of the three approaches to DEM implementation, charge profile results are presented for a 12 m mill. Further power predictions for a number of ball mills are presented along with a brief internal charge dynamics comparison. A discussion focuses on the needed future research to improving these approaches.     

Characterising porosity of multi-component mixtures in rotary mills

The measurement of porosity presents a significant challenge in modelling of slurry transport in rotary mills. This is due to the aggressive environment within rotary mills. In this paper, a method of measuring the porosity of mill charge, using the positron emission particle tracking (PEPT) technique, is presented. In this work, multiple particles are tracked, in turn. The packing density of each size component is proportional to the residence time distribution of its representative tracer particle, based on the ergodicity of the system. The charge porosity is a linear combination of the packing densities of individual components. The porosity is modelled as a function of mill geometric and operating parameters—mill speed and filling fraction. The results show correlations between porosity distribution and operating parameters.     

Modelling comminution patterns within a pilot scale AG/SAG mill

The patterns of rock comminution within tumbling mills, as well as the nature of forces, are of significant practical importance. Discrete element modelling (DEM) has been used to analyse the pattern of specific energy applied to rock, in terms of spatial distribution within a pilot AG/SAG mill. We also analysed in some detail the nature of the forces, which may result in rock comminution.In order to examine the distribution of energy applied within the mill, the DEM models were compared with measured particle mass losses, in small scale AG and SAG mill experiments. The intensity of contact stresses was estimated using the Hertz theory of elastic contacts. The results indicate that in the case of the AG mill, the highest intensity stresses and strains are likely to occur deep within the charge, and close to the base. This effect is probably more pronounced for large AG mills. In the SAG mill case, the impacts of the steel balls on the surface of the charge are likely to be the most potent. In both cases, the spatial pattern of medium-to-high energy collisions is affected by the rotational speed of the mill.Based on an assumed damage threshold for rock, in terms of specific energy introduced per single collision, the spatial pattern of productive collisions within each charge was estimated and compared with rates of mass loss. We also investigated the nature of the comminution process within AG vs. SAG mill, in order to explain the observed differences in energy utilisation efficiency, between two types of milling. All experiments were performed using a laboratory scale mill of 1.19 m diameter and 0.31 m length, equipped with 14 square section lifters of height 40 mm.     

Towards a mechanistic model for slurry transport in tumbling mills

A new modelling approach to slurry transport in dynamic beds based upon combining space and time-averaged Navier–Stokes equations with a new type of cell model is described. The resulting Ergun-like equation is used to correlate pressure drop with time-averaged distributions of the porosity, superficial fluid velocity and solids velocity for data derived from positron-emission-particle-tracking (PEPT) experiments in a scaled industrial tumbling mill fitted with lifter bars, pulp lifters and a discharge grate and run with particles and re-circulating slurry.     

What is required from DEM simulations to model breakage in mills?

It is quite common to encounter discrete element method (DEM) simulations of mills that present images of the motion of grinding media, summaries of tangential and normal forces, and mill power. The usefulness of this data is questioned, with respect to modelling breakage. This work presents hypotheses of how the DEM simulations can be used as input to comminution modelling, and this guides the data logging and analysis requirements. Techniques are proposed for collecting and using this data in a manner useful for predicting breakage in a comminution device. Individual particle impact histories of contact angle, force, and impulse are required to realistically model breakage. It is argued that the majority of breakage results from cumulative damage, thus it is essential to track individual particle histories to realistically predict the breakage product from a mill.     

Is media shape important for grinding performance in stirred mills?

Models for understanding the basic concepts of fine grinding and how they apply to the design of stirred media mills have not yet matured. While spherical media in tower mills has previously been studied, real grinding media shape in stirred mills can range from spherical (steel/ceramic balls) to highly non-spherical (sand or slag) resulting in very different media and grinding dynamics. Handling the contact mechanics of non-spherical particles is a challenge for numerical models, and very few studies dealing with non-spherical particle shape exist in the literature. Discrete Element Method (DEM) simulations of dry media flow in a pilot-scale tower mill are performed for four cases with different shaped grinding media, in order to understand how flow and energy utilisation within a stirred mill depend on media shape. Differences in media transport, stress distribution, energy dissipation, and liner wear were observed in the tower mill for the spherical and non-spherical cases. A significant departure from sphericity of the media leads to strong dilation of the bed, reduced bulk density, and a reduction in active volume and collisional power levels leading to a reduction in power draw for the mill. In addition, highly non-spherical media tend to pack tightly near the mill walls forming a near solid layer around the inside of the mill shell which results in poorer transport and mixing, as well as increased wear rates on the screw impeller. Grinding performance in stirred mills appears to deteriorate strongly when using highly non-spherical media.     

Understanding fine ore breakage in a laboratory scale ball mill using DEM

DEM models of fine grinding in ball and stirred mills have to date almost entirely focused on the motion of the media and their interaction with the mill configuration. For SAG mills, a large fraction of the feed material can now be accurately represented in DEM models. However, for other mill types with much finer feed materials, such as the second chamber of a cement ball mill, the vast numbers of feed particles makes their explicit inclusion in the models prohibitive. However, it is now feasible to model a periodic section of a laboratory scale ball mill and include the coarser end of the ore size distribution directly in the DEM model. This provides the opportunity to better understand the effect of media on the interstitial bed of powder and of the effect of the powder on the media. The effect of the powder fill level, which is varied between 0% and 150% of the pore space in the media charge, is explored. The distribution of the powder, its effect on power draw and the way in which it contributes to the pattern of energy utilisation is assessed. The simulation results are compared with experimental results from a test at similar ball loading and rotation rate and for several size fractions of ore at a range of powder fill fractions. Tracking the collision histories of specific ore particles within the charge allows estimates of the probability (per unit time) of collision between media and ore particles (the “Selection” function) and of the intensity of each collision which can be used to estimate the severity of breakage using the JKMRC breakage model (the “Breakage” function). The energy spectra indicate that for a typical ore, only very few collisions are large enough to cause damage to the body of each particle. This provides an estimate of the energy efficiency which is less than 10% at even the best operating conditions.     

Experimental observations of lifter parameters and mill operation on power draw and liner impact loading

In mineral processing the mill power plays a major role in the economics of the process and is a critical design criterion. The mill power is influenced by a range of parameters such as: charge and slurry filling, number and geometry of lifters, and mill speed. Deriving the optimum conditions of these parameters should lead to efficient mill operation. Additionally, the optimum utilization of the impact loads that are affected by charge and slurry filling, number of lifters, geometry of lifter and mill speed should result in increased milling efficiency. In this work the influence of these operating parameters were investigated using a laboratory experimental mill. It is found that the power, are affected by number of lifters, lifter height, mill charge and mill speed. Overall the results showed that increasing the mill velocity, number of lifters, and height of lifter and significantly decreasing the mill charge filling results in a higher impact value and impact frequency that may also increase overall efficiency. A simple linear regression relationship has been demonstrated for mill power as a function of lifter spacing (S/H) and mill speed. These parameters give an indication of the possible optimum mill operating conditions in an idealised condition.     

Using DEM to understand scale-up for a HICOM® mill

Scale-up is a process of developing a larger version of a processing machine based on the performance of a smaller machine. For mills, scale-up is made difficult by the different rates at which different physical processes occurring within the machine change with the increasing size. Discrete Element Method (DEM) modelling can now be performed at a range of scales and can be used to help understand scale-up issues for mills. This paper explores the ways in which DEM can be used to assist in the scale-up process for a mill using the HICOM mill as a case study. By choosing speeds at each mill size that have the same charge distribution and structure a scale-up relationship is developed which allows prediction of the power draw with increasing mill size. Similar scale-up relationships are developed for the specific power intensity and the most common collision energy occurring within the charge. The peak loads on the liner and at the nutation point of the mill are key inputs to mechanical design. Their variation with increasing physical size is also explored. Finally, the change in wear behaviour with increasing scale is also determined. This scale-up process can be applied to any form of mill.