Discrete element method (DEM) modelling of evolving mill liner profiles due to wear. Part II. Industrial case study

The wear of tumbling mill liners influences the load behaviour and consequently the performance and efficiency of tumbling mills. The ability to accurately predict the evolving mill liner profile due to wear is therefore of great economic significance since it can be used to determine the optimal initial lifter design for a particular operation. In this paper, we use the discrete element method (DEM) to simulate the load behaviour and predict the wear of two different liners used in industrial mills for the grinding of coal in dry conditions. Our simulated results are compared to industrial measurements in both cases. A good agreement is found between those measurements. Moreover a quick empirical equation is derived for a particular lifter in order to predict the wear without using the DEM.     

Modeling of energy loss to the environment from a grinding mill. Part I: Motivation,literature survey and pilot plant measurements

Grinding mills are generally very inefficient, difficult to control and costly, in terms of both steel and power consumption. Improved understanding of temperature behaviour in milling circuits can be used in the model-based control of milling circuits. The emphasis in this work is to develop a model for the overall heat transfer coefficient as a function of the load volume, mill speed and design of mill shell/liner using the energy balances in order to model energy loss from the mill. The energy loss via convection through the mill shell is accounted for by quantifying the overall heat transfer coefficient of the shell. The ambient temperature allows the model to account for convective loss. Firstly the models of temperature behaviour are developed followed by the development of the model for the overall heat transfer coefficient for the pilot dry batch mill. The practical aspects of the work involved the measurement of the temperatures of the mill load, air above the load, the liners, mill shell and the environmental temperature. Other measurements were: mill power and sound energy from the mill. Energy balances are performed around the entire mill. The model for the heat transfer process is parameterized in Part II.     

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.     

Effects of slurry filling and mill speed on the net power draw of a tumbling ball mill

The pool of slurry is known to lower the power drawn to the mill. An attempt to ascertain this observation by relating load orientation to mill power for a range of speeds and slurry fillings was undertaken.To this end, a Platinum ore (−850 μm) was used to prepare a slurry at 65% solids concentration by mass. The Wits pilot mill (552 × 400 mm), initially loaded with 10 mm balls at 20% volumetric filling, was run at 5 different speeds between 65% and 85% of critical. The net power draw and media charge position were measured. After this, the slurried ore was gradually added to the media charge for slurry filling U between 0 and 3. A proximity probe and a conductivity sensor mounted on the mill shell provided a means of measuring both the position of the media charge and that of slurry. The data collected for the load behaviour and net power draw was later analysed.It was found that Morrell’s model could not fully explain the effect of slurry volume on net power draw especially for an under-filled media charge (i.e., for U < 1). The size of lifters and grinding balls used could be the reason for this. That is why a piece-wise function was curve-fitted to the power data to help make sense of the inconsistencies observed.     

Effects of grinding media shapes on load behaviour and mill power in a dry ball mill

Little attention has been given to the study of grinding media shapes and how they affect load behaviour and power draw in grinding mills. To a large extent the focus has been on how grinding media shapes affect milling kinetics in terms of specific rate of breakage and breakage distribution parameterisation.This study investigated the effects of three media shapes (cylpebs, spherical and worn balls) on load behaviour and mill power draw at various mill speeds and load filling. An inductive proximity probe was used to determine the load orientation of the grinding media charge while a load beam enabled measurement of power draw.The variations in toe and shoulder positions among media shapes were observed. This was also reflected in the power draw. The power increases to a maximum with increasing mill speed for all media shapes. The maximum power draw was reached at different mill speeds for the three studied media shapes.     

Prediction of slurry transport in SAG mills using SPH fluid flow in a dynamic DEM based porous media

DEM modelling of the motion of coarse fractions of the charge inside SAG mills has now been well established for more than a decade. In these models the effect of slurry has broadly been ignored due to its complexity. Smoothed particle hydrodynamics (SPH) provides a particle based method for modelling complex free surface fluid flows and is well suited to modelling fluid flow in mills. Previous modelling has demonstrated the powerful ability of SPH to capture dynamic fluid flow effects such as lifters crashing into slurry pools, fluid draining from lifters, flow through grates and pulp lifter discharge. However, all these examples were limited by the ability to model only the slurry in the mill without the charge.In this paper, we represent the charge as a dynamic porous media through which the SPH fluid is then able to flow. The porous media properties (specifically the spatial distribution of porosity and velocity) are predicted by time averaging the mill charge predicted using a large scale DEM model. This allows prediction of transient and steady state slurry distributions in the mill and allows its variation with operating parameters, slurry viscosity and slurry volume, to be explored.     

Modeling ball impact on the wet mill liners and its application in predicting mill magnetic liner performance

Numerous studies have been conducted in the past and models have been developed to simulate ball motions in horizontal mills. Equations and computer programs have been published to calculate falling velocity of the grinding media upon impacting the mill shell (liner). However, these equations and programs are only suitable for dry mill applications.To accurately simulate the impact of falling balls on the liners in wet mills, the authors have developed models to determine the ball impact location and velocity as the ball contacts the mill liner. Drag and buoyancy forces are considered when the ball submerges in the pulp inside the mill. Models can be used to enhance mill liner design and optimize the operation of horizontal wet mills such as horizontal ball mills.The models are applied to calculate the falling ball impact on the metal magnetic mill liners. Various scenarios are simulated and results are used to predict the performance of magnetic mill liners in various ball mill applications.     

Discrete element method (DEM) modelling of evolving mill liner profiles due to wear. Part I: DEM validation

Since the early 1990s the discrete element method (DEM) gained considerable success in its ability to predict the power draw and the load behaviour in mills as affected by operating conditions. The DEM can also be used to design milling equipment and predict the breakage of particles. A detailed validation of this method is required in order to produce accurate results. In this paper, we assess the ability of the DEM to predict forces exerted by the mill charge on liners. Data obtained on an experimental two-dimensional mill designed in order to record the normal and tangential forces exerted on an instrumented lifter bar was available. The measured results are compared to the DEM simulated results. Good agreement has been found in terms of amplitude of forces and positions of shoulder and toe at low speed.     

Modelling the influence on power draw of the slurry phase in Autogenous (AG), Semi-autogenous (SAG) and ball mills

The slurry level in AG/SAG and ball mills has an influence on their power draw, which in some instances can be profound. This is particularly the case with so-called slurry pooling in AG/SAG mills which can result in significant lowering of power draw. From a predictive viewpoint there are currently no published models that explicitly describe the influence of slurry level on power draw and the relationship between slurry level, slurry flow and pulp lifter design. Citic SMCC Process Technology Pty Ltd has developed such a model and in this paper its structure is described. Examples are given of both AG/SAG and ball mills of how the model responds to changes in slurry level. These examples are used to explain the observed phenomena of slurry pooling and the decrease in power draw observed in ball mills after start-up when feed is first introduced into the mill.     

An experimental investigation of the effects of operating parameters on the wear of lifters in tumbling mills

Lifters are usually used with mill liners to extend their life and to enhance the grinding and crushing efficiency. Although the lifters are durable wear parts but they will gradually worn and consequently their dimensions change during the course of operation. These changes in dimensions have a significant influence on the overall economic performance of the mills. Therefore, it is useful to know the relationship between the mill operation and the lifter profile, and the influence of lifter wear on the change in lifter profile. In this work, a laboratory mill which is capable of producing the required impact and abrasion grinding was operated both wet and dry. Many factors were varied such as: velocity, charge, ore size and different material for the lifters. Also the wear rate on the top and face of lifters are compared in the different conditions. It is found that the mill charge and the mill speed significantly affect the wear rate. Also, the results showed how size distribution affects the wear rate. The results can be interpreted in terms of the wear process in industrial scale mills over different operating conditions. The experimental results provide the possibility of including the lifter wear in optimising mill performance.     

非磁性矿磁性衬板的研制和应用

HM型大型磨机磁性衬板在国内包钢、鞍钢大型铁矿选矿厂都得到了成功运用,磁性衬板用于磁铁矿磨矿使用寿命长、提高磨机作业率、节球、节电。但针对非磁性矿物的应用有一定难度,为此对磁性衬板的结构和磁系进行优化,设计出针对非磁性矿的专用磁性衬板,在不含磁性矿物成分的某铜矿选矿厂推广应用,取得了良好的应用效果。     

Modeling the RTD of an industrial overflow ball mill as a function of load volume and slurry concentration

The role of residence time distribution (RTD) in the accuracy of milling simulation is well appreciated in literature. Accordingly, the development of models that can accurately predict the RTD at various mill operating conditions would be of considerable benefit to the milling industry. In this paper, a 3-parameter RTD model has been derived using the concept of serial stirred mixers with a dead zone. The model parameters were optimized by minimizing the error between experimental tracer response data and model predictions using a MATLAB algorithm. Based on the optimum values of the model parameters, the mean residence time of slurry was evaluated and the effects of ball load volume and slurry concentration examined. The results revealed that the mean residence time of slurry inside the mill is affected to a larger extent by slurry concentration than the ball load volume. An empirical correlation was developed to predict the mean residence time as a function of slurry concentration, ball load volume and slurry feed rate. Over the range of conditions investigated, the mean residence time was observed to vary linearly with slurry concentration and ball load volume but inversely with feed flow rate. Lastly, a test case of the simulated mill product size distribution using the predicted RTD is presented displaying a close match with experimental data.     

Modeling of energy loss to the environment from a grinding mill. Part II: Modeling the overall heat transfer coefficient

The objective of the paper is to develop a model for the overall heat transfer coefficient as a function of the load volume, mill speed, and the design of the mill liners and shell, using the energy balances in order to model energy loss form the grinding mill. The energy loss via convection through the mill shell is accounted for by quantifying the overall heat transfer coefficient of the shell.     

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.     

间断级配高浓度水煤浆制备技术应用研究

陕西延长石油榆林凯越煤化有限公司煤浆制备系统的3台棒磨机一直处于全开状态，无备机，当棒磨机出现故障停机时，后续气化和合成须降低负荷，从而影响甲醇的产量。为降本增效、扩大产能，采用煤科总院研发的第三代间断级配高水煤浆制备技术，对原有制浆系统进行改造。备用一条粗粉生产线，当一台棒磨机检修或故障时启用，以便从根本上解决棒磨机无备机导致生产负荷波动的难题。间断级配高浓度制浆成套技术将选择性破磨机和细磨机有机组合，综合磨矿效率大幅提高，煤浆浓度与单磨机制浆技术相比可提高4个百分点以上。     

Relining efficiency and liner design for improved plant performance

Liners in grinding mills not only protect the mill shell from the aggressive environment inside the mill, but they also play a significant role in the efficiency of grinding. The design of mill liners dictates the charge trajectory and hence the grinding efficiency. The common approach in designing liners is designing for a longer life. However, this approach does not necessarily consider optimum performance over the liner life. It has been observed in many operations that mills under-perform over a significant portion of the liner life (10–25% of the liner life at the beginning and often 5–10% at the end). This paper extends the method proposed by Toor (2013) and Toor et al. (2013) to design liners for performance through investigating the effect of relining efficiency using such an approach in an industrial case.As indicated in the Toor et al. (2013) study, relining efficiency affects the benefits that can be realised by designing liners for efficiency. Russell Mineral Equipment’s Mill Reline Director (MRD) analyses the relining process and provides an accurate estimation of relining time for a given scenario. In this study, five different relining scenarios were simulated and compared against the reference reline (i.e. current liner design) to accurately estimate the time required for relining. This is the first study to demonstrate that incorporating relining constraints in the liner design can be used to inform liner design characteristics of a proper design that meets the requirements of an efficient relining practice.JKSimMet simulation for the industrial case predicts a liner which has same lifter face angle as the current liner design with reduced lifter height from 300 mm to 210 mm, could increase the plant throughput by 8% on average while producing a product with same P80 as the current liner. Considering relining time predictions by MRD for the proposed liner, this study predicts a 3.7% increase on average in throughput per annum. Although the proposed strategy will increase the cost of liners plus relining by 31.5% (i.e. A$ 548,000), the increase in plant throughput is estimated to yield A$ 20.1 M of additional revenue based on data for 12 months to 30 June 2014 from the plant quarterly report.     

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.     

Use of temperature for diagnosing mixing in the IsaMill

This paper describes a methodology to monitor axial mixing in an IsaMill as a function of mill feed flow rate, mill tip speed and media load. Steady state temperature measurements (profiles) along the mill length were used to quantify the degree of mixing during the comminution process. Combined mass and energy balances across the mill length were done to develop a predictive model for slurry mixing in the mill. The temperature measurements can be used for IsaMill control and optimisation.     

Analysis of specific discharge rate functions in industrial scale cement grinding multi-compartment ball mills to assist ball mill modeling

Specific discharge rates in industrial scale multi-compartment cement grinding ball mills were determined using industrial scale data to assist modeling of multi-compartment ball mills. Multi-compartment ball mill model structures were proposed to estimate normalized specific discharge rate functions. Multi-compartment mills were divided into perfectly mixed mill segments to describe material transport mechanism in the mill. Specific discharge rate functions were determined by using the estimated mill hold-up for perfectly mixed mill segments at the steady state condition of the cement grinding circuits. Estimated normalized discharge rate functions were found to be different than the basic pattern observed in semi-autogenous mills due to the differences in design and operational conditions of the mills. Coarse particle accumulation was found to have a significant effect on discharge rate function pattern. Estimated normalized discharge rate functions could be used in modeling of multi-compartment ball mills.