Lifter Bars Research Articles

A positron emission particle tracking investigation of the flow regimes in tumbling mills

Using positron emission particle tracking (PEPT) data we recover key granular rheology ingredients (velocity, shear rate, volume concentration, bed depth) for developing, testing and calibrating granular flow models. In this regard, 5 mm glass beads were rotated within a 476 mm diameter mill fitted with angled lifter bars along the inner azimuthal walls and operated in batch mode across a range of drum rotation speeds that span cascading and cataracting Froude regimes. After averaging the PEPT outputs into representative volume elements, subsequent continuum analysis of the flowing layer revealed a rich coexistence of flow regimes: a quasi-static layer dominated by frictional interactions, a dense, liquid-like layer that is stressed by frictional and collisional interactions, and an inertial layer that interacts mainly through collisions. Combining the inertial number with an empirically formulated dilatancy law and the measured granular rheological ingredients then facilitated the recovery of the total depth-dependent pressure of the free surface layer.

A positron emission particle tracking investigation of the scaling law governing free surface flows in tumbling mills

Positron Emission Particle Tracking (PEPT) measurements are used to track the flow of glass beads within a rotating drum fitted with (and without) lifter bars and operated in the cascading and cataracting Froude regimes. After converting the Lagrangian trajectories of a representative radio‐labeled glass bead (the tracer) into Eulerian fields under the ergodic assumption, the bed shape and kinematics are extracted for steady, fully developed flow conditions. Notwithstanding the azimuthal wall effects introduced by the lifter bars, we show a linear scaling of the local flowing layer thickness (h) with local depth‐averaged velocity and a constant average shear for direct measurements spanning the entire flowing layer (not just the central region), and high Froude regimes (cascading and cataracting) not previously investigated by scaling analysis in the literature. © 2016 American Institute of Chemical Engineers AIChE J, 63: 903–913, 2017

Measurement of shear rates in a laboratory tumbling mill

A methodology for characterising shear rate profiles in tumbling mills based on in situ trajectory fields from Positron Emission Particle Tracking (PEPT) experiments is presented. Experiments were carried out in a laboratory scale mill fitted with lifter bars, pulp lifters and a discharge grate, and run with radioactively labelled particles within re-circulating slurry environments. Using time-averaged velocity distributions of small PEPT tracers, a model for tangential velocity profiles along radial scan-lines is proposed from which the shear rates are derived.

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.

Prediction of mill structure behaviour in a tumbling mill

Computational demands and the lack of detailed experimental verification have limited the value of distinct element method (DEM) modelling approaches in mill simulation studies. This paper presents the results of a study in which the deflection of a lifter bar in a pilot ball mill is measured by an embedded strain gauge sensor and compared to deflections predicted from finite element (FE) simulations. The flexible rubber lifter and the lining in a tumbling mill are modelled with the finite element method (FEM) and the grinding medium is modelled with DEM. The deflection profile obtained from DEM–FE simulation shows a reasonably good correspondence to pilot mill measurements. To study the charge impact on the mill structure two different charges are used in the simulations. The approach is a contribution to the validation of DEM–FE simulations and an introduction to the description of a bendable rubber lifter implemented in a DEM–FEM mill model. It opens up the possibility to predict contact forces for varying mill dimensions and liner combinations. FEM is especially valuable in this case, since there are readily available libraries with material models.

A new predictive model of lifter bar wear in mills

The cost of mill relining and the influence of the liner profiles on mill performance provide a strong motivation to improve liner selection. However, the capability to predict liner life and progressive profile is poorly developed in the industry. A new model has been developed to predict the rate of liner wear based on the main parameters affecting the wear process such as; ore type, relative velocity between the ore and the liners, liner hardness and friction conditions. The resultant model is capable of determining the wear as a function of mill operating and discharge conditions based on the variation of specific gravity of mill contents, geometrical characteristics of the mill and mill velocity. The results from this model are in good agreement with the measured data from the SAG mill at Sarcheshmeh Copper Complex, Iran.

Assessment of mill lifter bar deflection measurements using wavelets and discrete element methods

This paper shows how Partial Least Square Regression (PLS) methods can be used to model sensor data of spectral character. The modelling approach has been applied on a tumbling mill where a strain gauge sensor measures the deflection of a lifter bar when it hits the charge. The deflection of the lifter bar during every mill revolution gives rise to a characteristic signal profile that is shown to contain information on both the charge position and grinding performance. As a signal pre-processing method the discrete wavelet transform is used. It distinctly shows a capability of signal feature extraction where both time and frequency are of interest. Its well-known ability to achieve good data compression without loss of information is also demonstrated, a data reduction ratio of 20:1 is obtained here. Modelling results demonstrate that different operating conditions are well distinguishable from each other and by that the finding of proper operating regimes are highly feasible. Grinding parameters that are normally measured in the laboratory are now readily modelled from the on-line signal. A further objective of this paper is to link the experimentally obtained strain gauge sensor data with computational data from a discrete element mill model (DEM). This enables to visualise the charge motion and helps to interpret the complex phenomena that take place inside a grinding mill measured by the strain gauge sensor. The approach taken is to simulate the behaviour of a rubber lifter when it is exposed to forces from the grinding charge in a two-dimensional DEM mill model using a particle flow code. The deflection profile obtained from the DEM simulation shows a reasonably good correspondence to pilot mill measurements.

Impact wear in mineral crushing

Mining and mineral processing demand wear-resistant machines and components, because the energies and masses of interacting bodies are significant. This paper is concerned with the wear mechanisms, which occur in grinding mills. Conditions of severe-impact wear and impactabrasion, and wear behaviour of the lifter bars are studied. A hammer-mill type device, which can produce high-energy impacts, was used. The experiments were also carried out in conditions with abrasive particles present. The wear behaviour of several PM materials and a few reference materials were compared in certain wear conditions. Different material properties that influence the wear resistance were also studied. The microstructure and homogeneity of the materials were studied mainly with a scanning electron microscope. On the basis of the experimental results the studied materials were classified.

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.

On-line lifter deflection measurements showing flow resistance effects in grinding mills

The deflection that a lifter bar is subjected to when passing through a grinding charge was measured using an embedded strain gauge sensor. The obtained signal profile is analysed and interpreted in relation to charge properties such as toe and charge angle for the grinding charge. The current work has focused on the charge and slurry flow behaviour when solids content is changed and how well the sensor reflects this. Bench scale measurements with a vane type viscometer, roughly evaluated in terms of apparent viscosity is used as a character for the resistance to motion of the slurry. The slurry flow resistance is strongly influenced by solids content and obviously by the addition of a dispersant. The strain gauge sensor reflected this change well, showing that toe and shoulder region of the charge varied in a systematic way. Results obtained also shows that change of slurry flow resistance exert an influence on grinding performance. A multivariate statistical method, partial least squares regression, is applied to the sensor data producing a model that can predict the change in slurry flow resistance. The output from the model also shows good properties to be used as a process-monitoring tool. The predictive capability of the model is believed to be of such quality that it can be used for process control.

Feature Extraction of a Strain-Gauge Signal Using Wavelets for Monitoring of a Tumbling Mill

Techniques that measure the force acting on a lifter bar, when it hits the charge inside a tumbling mill, have got an increased interest because of its direct physical relation to the behavior of the grinding charge. The possibility to combine it with discrete element modeling (DEM), which gives an opportunity to visualize the charge motion, opens new possibilities to understand the complex phenomena that takes place inside a grinding mill. In this work, a method for the measurement of the apparent filling level in a pilot ball mill, the Metso Continuous Charge Measurement system (CCM) has been used. The technique uses a strain gauge sensor, mounted on a flat steel spring, which is fitted into a recess underneath a lifter bar. Deflection of the lifter bar during every mill revolution will then give rise to a characteristic signal pattern depending on different operating conditions.In this work, which should be treated as an introduction, we show how the discrete wavelet transform can be used in multivariate calibration. It will be shown that by using the fast wavelet transform on individual signals as a pre-processing method in regression modeling on CCM measurements, good compression is achieved with almost no loss of information. The predictive ability and diagnostics of the data compressed regression model is almost the same as for the uncompressed.

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The study for producing the flake powders by milling of aluminum foil and gas atomized powders was carried out. The effects of lifter bars on the ball motions and milling of aluminum foils were also investigated. The aluminum foils were laminated each other, elongated, fragmented into small foils and finally formed into the flake powders during the dry ball-milling. The spherical atomized-powders were milled to coarse flake powders with high aspect ratio and then changed to fine flake powders with lower aspect ratio. Even though long times were required for making flake powders by milling of foils, the water covering areas of them were higher than those of powders milled using gas-atomized powders, suggesting aluminum foils were more plastically deformed by micro-forging. On the other hand, as the number of lifter bars increased, the necessary rotation speeds of milling jar for cascading mode and cataracting mode decreased drastically. It was possible to achieve same quality of milled flake powder by using the lifter bars under the lower milling speeds. The painting test showed that the appearance of painted surface was good and optimum content range of aluminum paste in car paint to maximize the degree of gloss was 3-5%.

Effects of lifter bars on the ball motion and aluminum foil milling in tumbler ball mill

Effects of lifter bars on the ball motion during ball milling and characteristics of aluminum flake powders prepared by the ball milling of aluminum foil scraps were studied. When rotation speed of milling jar without lifter bar is below 75% of critical revolutions per minute (rpm), balls slide in the jar. As rotation speed of milling jar increases up to 93% and 140%, the mode of balls motion changes to cascading and cataracting type, respectively. By setting even one lifter bar in the jar, balls move as cascading mode and cataracting mode at 37% and 56% of critical rpm, respectively. As number of lifter bars increases, the necessary rotation speeds of milling jar for cascading mode and cataracting mode decrease drastically. The aluminum foils scrap was milled in the milling jar with four lifter bars and without lifter bar under the rotation speeds of jar corresponding to cascading and cataracting modes. That result shows that it is possible to achieve same quality of milled fine powder by using lifter bars even under the significantly lower rotation speeds of jar.

Optimizing the mill lining and the grinding media in tumbling mills

The grinding mill is probably the most common unit operation in mineral processing. There is, however, much that can still be done in the way of optimization. This paper discusses the results of a well-controlled pilot-scale test in which the performance of a tumbling mill equipped with a conventional rubber lining and lifter bars was compared to a tumbling mill equipped with a smooth rubber lining of the magnetic type. The results indicate that the smooth lining is about 14% more effective with respect to energy consumption. By optimizing the size of the grinding media, the mill capacity can be increased in many grinding operations. In this specific case, grinding with a −20-mm ball charge required 26% less grinding energy than grinding with a −30-mm ball charge. The energy savings can, in most cases, be converted to an increase in capacity. The wear of the grinding media is difficult to assess over a short test period, but the results suggest that the wear is somewhat less when a smooth lining is used. A determination of the wear of the linings was not included in this study, but it is generally accepted that the use of magnetic linings reduces the lining costs dramatically. Computer simulation using the Millsoft code developed by Mishra et al. confirms and explains the increased effectiveness of comminution when a smooth lining and smaller grinding balls are used.

A study of charge motion in rotary mills part 3—Analysis of results

Results of experiments investigating the motion of balls deep within the charge of a rotary grinding mill are presented. The true centre of mass of the entire charge was determined. The angle of repose of the charge was found to be uniquely defined by the centre of circulation of the charge. The angle of the cascading region is determined by the angle of repose of the charge. The inner part of the cascading region and the toe region are responsible for the majority of the grinding work. The face angle of the lifter bars was found to influence both the maximum impact energy and the amount of work done by the charge. Radial segregation of the balls takes place throughout the charge according to both the mass and size of the balls, even in a mill with smooth end faces.

Numerical simulation of charge motion in ball mills — Lifter bar effect

This paper focuses on the use of a numerical tool known as the discrete element method (DEM) to study the motion of ball charge in ball mills. DEM is employed to simulate the motion of individual balls. While doing so, the effect of liner design on the grinding performance of the mill is studied. Two important factors, such as the configuration of the liner and the geometry of the lifting surfaces, are correlated with the overall mill performance. An attempt is made to show the effectiveness of the lifter bars in transmitting power from the mill, shell to the interior of the mill with particular emphasis on large diameter mills.

Measurement of the radial and tangential forces exerted by the load on a liner in a ball mill, as a function of load volume and mill speed

A laboratory grinding mill of diameter 0.36 m was modified to allow the measurement of the radial and tangential forces exerted by the load in the mill on lifter bars of varying profiles. Four load beams were used to achieve these measurements. Electrical conductivity measurements were used to determine when the load was “locked in” to the liners. The mill was instrumented to allow accurate measurement of the torque drawn by the mill. The above measurements were obtained for mill speeds varying between 10% and 135% of critical speed, load volume varying between 0% and 37% of mill volume, for seven different liner profiles. Steel balls of diameter 3, 4 and 5 mm were used as media (equal proportions of each); no ore was used. The measurements provide detailed and quantitative information about the forces exerted on the liner and about the behaviour of the load in the mill. Analysis of these data should provide insights relevent to the optimisation of mill performance with reference to liner and media wear and power consumption, i.e. the major factors determining operating costs in rotary mills.

The effect of liner design on the motion of the outer grinding elements in a rotary mill

A theoretical analysis was made of the motion of an isolated rod or ball, and the way in which it is influenced by a lifter bar of any face angle and of any height. The charge motion of rods in a glass-ended mill was filmed with a high-speed camera. The mill was fitted with a variety of lifter bars with different face angles and heights, and was run at a wide range of speeds. The trajectories of the rods on film were then tracked. The coefficients of friction between the rods and the lifter bar materials were measured under the vibrating conditions that are found in the mill. It was found that there was a good correlation between the theoretical predictions and experimental results over a wide range of conditions. The impact point at which the grinding element strikes the shell of the mill was considered to be of primary importance in the analysis. It was found that as the height of the lifter bar increases from zero to slightly greater than one charge radius, the height has a strong influence on charge trajectories. Thereafter, the lift increases until a critical lifter-bar height is reached. At heights beyond this, the grinding element is projected off the lifter bar prior to reaching the tip, with the height of the impact point increasing slightly and then decreasing to a constant height. The change in the height of the impact point is very small, however, so, in practical terms, an increase in lifter-bar height — once it is higher than the radius of the grinding element — has only a slight effect upon the charge trajectories. An increase in the angle of the lifter bars was found to have a strong influence upon the height of the charge trajectories. A linear relationship was discovered between the speed of the mill and the angle of impact, and changes in the mill speed strongly influence the charge trajectories. Some surprising effects of lifter-bar geometry upon the charge trajectories have been discovered, and are of practical importance. The theoretical model is an improvement on all previous models, and agrees well with experimental results over a wide range of conditions.

Estimation, by use of a Conductivity Bolt, of the Charge Volume and Other Milling Parameters in an Operating Mill

Physical information on the conditions inside an operating mill can be obtained continuously if the bolts that are used to clamp the liners and lifter bars to the shell of the mill are equipped with suitable sensors. The information can be transmitted telemetrically to a conveniently located receiving station. The signals from a bolt that is equipped with an insulated electrode can be used to obtain a measure of the volume of the charge in a mill, and can also be related to the pulp density. Some of the problems associated with the use of this type of bolt are discussed.

Effects of lifter bars on the motion of en-masse grinding media in milling

Cinematographic films were analysed in the quantitative assessment of the effects of rectangular lifter bars on the motions of en-masse grinding elements in an experimental mill. It was found that, while lifter bars eliminate the slip of the grinding charge against an otherwise smooth liner, they increase the dynamic pressure and the intensity of abrasive interactions in the bulk of the en-masse grinding charge. They also increase the intensity of the impactive interactions in a mill as a result of their lifting action, which promotes cataracting of the grinding charge, and their keying-in action, which increases the kinetic energies and the rates of transport of grinding elements through the en-masse regime.