THE EFFECT OF LINER WEAR ON GYRATORY CRUSHING A DEM CASE STUDY

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1 THE EFFECT OF LINER WEAR ON GYRATORY CRUSHING A DEM CASE STUDY Johannes Quist a *, Jochen Franke b, Magnus Evertsson a a Department of Product and Production Development, Chalmers University of Technology, SE Göteborg, Sweden b Scanalyse Pty Ltd, Australia PO Box 1201, Bentley DC 6983, Western Australia Abstract Gyratory crushers are frequently used for first stage sizing in the minerals processing industry and are typically critical path fixed plant equipment. Hence any associated downtime or inefficient operation can have serious consequences for downstream processing and therefore the overall plant productivity. Despite the pressing requirement to tightly control gyratory crusher operation, no accurate, reliable, cost efficient, or practical condition monitoring tools or holistic performance assessment methods have been utilized by practitioners to date. This paper introduces a new method combining laser scanning based CrusherMapper TM software 3D liner shape information and Discrete Element Method (DEM) modelling to assess the effect of measured concave and mantle liner wear on gyratory crusher production. In order to illustrate this effect, two DEM models of an iron ore case study site are presented, one based on the 3D shape of newly installed liners, and the second on that of very worn liner geometry as at the time of replacement. The Bonded Particle Model (BPM) approach is used for modelling the rock material during the crushing sequence inside the crusher and other relevant DEM crushing parameters closely resemble actual production settings. A comparison between resulting modelling output parameters for the two liner geometries provides the basis for aligning actual with desired operational crusher performance. Keywords: Gyratory Crusher, Crushing, Wear, Discrete element modelling, Simulation, Comminution 1 Introduction Liner wear has been a subject of research for a long time in the mining and comminution industry and academia. While most of the efforts have been related to understanding wear rate and mechanisms, few have been targeting the actual effect it has on process operation and machine performance. The total cost of the wear behaviour can be compartmentalized in cost related to the liners and cost related to process performance reduction. In this paper the liner wear on a primary gyratory crusher is investigated in relation to product capacity, particle flow and breakage rate. This is done by simulating a crushing sequence with both new and worn liners using discrete element modelling. The liner geometry is obtained by 3D laser scanning of an Allis-Chalmers Superior gyratory crusher operating at an iron ore site in Western Australia. The crusher is fed with a relatively fine feed distribution since the blasting target is set on approximately 200 mm. However, in order to evaluate the crusher performance in a feasible way the simulated feed is coarser. This work does not directly aim for emulating the exact behaviour in the chosen crusher but rather the effect of wear in a generalized manner. In the mining industry gyratory crushers are the most frequently used type of equipment for the primary crushing stage. The principal design and layout of these machines were developed by Philetus Gates in the late 19 th century. Industrial grade large gyratory crushers built on this principle during the early 20 th century were manufactured by Allis-Chalmers and others. Only minor changes and adaptations have been made since then (Lynch, 2005). In this paper the influence of liner wear on 1

2 crusher performance is investigated for an Allis-Chalmers Superior crusher. Primary gyratory crushers are commonly fed via haul trucks from one or two radial positions. This means that the wear rate of the concave liner varies on every radial position. The detailed CrusherMapper TM 3D geometry for mantle and concave has been captured in both, a very worn and completely new state. The dump pocket has also been scanned in order to capture the feeding conditions correctly. The machine layout and kinematics for cone and gyratory crushers are by definition similar. Hence it is of interest to take into consideration work that has been targeting cone crusher wear and performance also when discussing gyratory crushers. Lindqvist (2006) presented a wear model for cone crushers with the purpose of predicting the change of geometry due to wear as well as how this change affects crushing operation. It was shown that the wear of crusher liners have a strong influence on performance in terms of particle size distribution, capacity and power-draw. In order to keep product quality stable in terms of throughput, particle shape and size consistency, the process needs to be dynamically controlled. Algorithms for control and optimization of cone crushers in real time in order to compensate for process variation and liner wear has been presented by Hulthén (2010). The objective of this research is : to evaluate how liner wear influences crushing performance in terms of capacity and breakage to develop the capability of simulating crushers in DEM using 3D-scanned geometry from real crushing applications. 2 Method The research approach in this work can be regarded as both exploratory as well as constructive in the sense that even though a specific case is investigated, novel knowledge coupled to both fundamental crushing operation as well as DEM modelling has been harvested in the process. The work in this paper covers the following sections : The capture of 3D data of a real operating primary gyratory crusher in order to create a 3D model for evaluation in DEM. Development of a new Bonded Particle Model (BPM) incorporating fraction particle size distribution and meta-particle shape given by 3D scanned rocks. Simulation of three cases of crushing operation in a gyratory crusher: 1. New concave 2. Worn concave with compensation to maintain target CSS 3. Worn concave without compensation 2.1 Discrete Element Modelling The discrete element method (DEM) is a numerical simulation method developed with the purpose of modelling and simulating discrete matter. DEM modelling is most commonly utilized for capturing the bulk flow behaviour in various types of particle based applications. However, during the past years researchers have also used DEM as a method for capturing and understanding breakage in various comminution devices. The modelling of breakage sets higher demand on the modelling effort compared to modelling bulk flow behaviour since more advanced contact models and/or post processing of collision data is needed. Cleary (2001) has made substantial contributions to the field of DEM modelling in comminution as well as in other research fields. Lichter et al (2009) developed an approach for modelling crushing devices based on a population balance model. Impact breakage and crushers have previously been simulated by e.g. (Djordjevic et al., 2003; Khanal, 2005; Schubert, 2005b) utilizing the Bonded Particle Model (BPM) approach. Refahi et al. (2010) modelled and simulated a primary jaw crusher using a BPM model approach. 2

3 1.1 Liner Shape Tracking The most common traditional concave and mantle liner condition monitoring methods are Visual inspection Ultrasonic Thickness Gauging (UTG) These methods cannot provide reliable data for the intended purpose due to a number of limitations (Franke, 2010). Hence crusher operators have been relying on personal experience and a physical trial and error approach to maintenance or operational considerations until CrusherMapper TM became available. CrusherMapper TM for the first time provides an accurate and repeatable condition monitoring method that can be used on mantles in situ and without interruption to production. It also eliminates all safety risks to condition monitoring personnel because it does not require entry into the confined space of the crusher cavity or the dump pocket (Franke, 2010). The three-dimensional data collected captures all liner shape changes in high detail (see Figure 4) and is therefore highly suited as critical input into DEM modelling of gyratory crusher behaviour. This allows for the first time to take into account the highly significant influence of liner shape change over a concave and mantle life cycle when assessing crusher behaviour. No crusher DEM model can hope to be representative of actual installed crusher behaviour if the complete 3D liner shape as well as the change of it over its life cycle is not considered for modelling. 3 Background 3.1 The Gyratory Crusher Gyratory crushers are usually used in the primary crushing stage for high-tonnage operations. The main shaft or spindle is commonly hydraulically mounted hence the vertical position of the spindle can be adjusted. The top bearing dictates the position of the fulcrum position (or pivot point) and an eccentric bushing mounted to the spindle gives the eccentric throw (Wills and Napier-Munn, 2005). In Figure 1 an illustration of a typical gyratory crusher can be seen in form of a cross-section showing the liner geometry. The fulcrum position, chamber design and eccentric throw are mechanical design variables and hence dependent on the selection of crusher (Bearman and Briggs, 1998). The close side setting (CSS) and feed rate are operating variables. The eccentric speed has historically been regarded as a mechanical design variable and is not possible to change during operation for gyratory crushers. However, Hulthén (2011) has recently introduced the eccentric speed as an operating variable for cone crusher control and optimisation with good results. For more information regarding gyratory crusher design and operation, see (Gupta and Yan, 2006). 3

4 Fulcrum point position Mantle Concave OSS Eccentric Angle CSS Figure 1 - Schematic illustration of a typical gyratory crusher showing liners, fulcrum point, open side setting, closed side setting and eccentric angle. The capacity in a gyratory is commonly limited by the open side setting rather than the close side setting due to the relatively low operating speed. Particularly when feeding the crusher with intermittent truck loads hence not being choke-fed. As previously described by Evertsson (2000) crusher capacity is highly governed by the choke feeding conditions and levels in the crusher chamber. When the mantle and concave are subjected to wear, the choke level conditions will change and hence also the capacity (Lindqvist, 2006). 4

5 Z-position [mm] -400 Cross-sectional area between concave and mantle Crossectional area [m 2 ] New Worn-Compensated Worn-Uncompensated Figure 2 - The graph shows the cross-sectional area between the concave and mantle for the three different cases.the data has been extracted by fitting circles to horizontal sections of the 3D-scanned mantle and concave at 10 different vertical positions.the terms Compensated and Uncompensated relates to if the mantles vertical position has been adjusted in order to keep the CSS setting. In Figure 2 and Figure 3 the cross-sectional area for the three different cases is plotted. It can be seen that the choke level position is at the discharge point. In this position the worn-uncompensated case has a larger cross-sectional area then the other two cases implying that a higher capacity is to be expected from the worn-uncompensated case simulation. Several authors have studied the wear behaviour in e.g. milling operations by using discrete element modelling for capturing media-machine interaction and incorporating a wear model in order to predict the wear on mill lifter bars (Powell et al., 2011; Rezaeizadeh et al., 2010). The common strategy is to capture the collision data from every single interaction between media and geometry and apply this information to an external wear model. These investigations usually have the primary purpose of understanding and predicting where material will be worn off the liners and approximately in what rate it does so. In this paper however, the focus is on investigating not the wear behaviour, but what effects it has on the crushing operation. The wear behaviour itself is not predicted but measured through the condition monitoring capabilities of CrusherMapper TM. This in turn forms the reliable basis for representative DEM modelling. The crushing chamber profile and gap have a direct influence on crushing operation hence variations due to liner wear should be considered as a vital parameter for crusher control. 5

6 Z-position [mm] Cross-sectional area between concave and mantle - Discharge detail view Crossectional area [m 2 ] New Worn-Compensated Worn-Uncompensated Figure 3 - The graph shows the cross-sectional area between the concave and mantle at the lower section of the chamber for the three different cases. When feeding any type of gyrating crusher it is evident that the rock mass is distributed evenly around the spider cap in a non-segregated manner. This is fairly easy to achieve for cone crushers and there are multiple solutions available for randomizing and distributing feed material. However for large gyratory crushers the scale and feasibility issues induced by intermittent haul truck feeding leads to difficulties accomplishing ideal feeding conditions. This can result in highly asymmetric wear behaviour on the concave liners as illustrated in Figure 4 and Figure 5. Figure 4 - Unwrapped view of the completely worn concave liners as measured with CrusherMapperTM and used for DEM modelling showing excessive localized wear in orange caused by asymmetric ore dumping layout. 6

7 Figure 5 - Unwrapped view of the completely worn concave liner bottom row as measured with CrusherMapperTM and used for DEM modelling showing highly asymmetric wear causing the differences in production behaviour when compared to completely new concave liners. 3.2 Bonded Particle Model Potyondy and Cundall (2004) presented the bonded particle model giving the possibility to model rock masses and rock breakage by gluing together small particles forming large clusters, here denoted as meta-particles. At a given time-step bonds are formed between particles in contact. Bonds are broken when the loads set upon them exceeds the strength conditions. In Figure 6 a schematic illustration of the bonding beam between two particles is shown. In this work the Hertz-Mindlin with Bonding contact model provided as a default physics module in EDEM has been used. Figure 6 - Schematic illustration of the bonded particle model In each time-step the normal and tangential forces and torques subjected to each bond are calculated, see Eq The stresses are calculated and evaluated according to Eq When the calculated stresses exceed the strength criteria, the bond is regarded as broken. The bond is defined as a circular beam and is the radius of the bonding disc gluing the particles together. and are the normal and shear stiffness s and is the timestep, further, are the normal and tangential velocities and are the normal and tangential angular velocities. (3.1) (3.2) (3.3) (3.4) Where, 7

8 (3.5) (3.6) (EDEM, 2010; Potyondy and Cundall, 2004) When trying to model rock material, several aspects and rock characteristics need to be taken into consideration. Rock material is a very complex type of material with highly differentiated characteristics depending on the geological features of the specific rock type. In this work the scope is not to mimic the full span of rock material features in the BPM model but rather to create a compound behaving similar to rock material when subjected to breakage. To some extent the developed BPM model possesses features such as discontinuity, heterogeneity and anisotropy. The term discontinuity of a rock material is related to the joints, cracks, faults, voids and inclusions inherent by the materials history involving mechanical, chemical and thermal actions subjected to it during millions of years. This type of material weaknesses are commonly referred to as discontinuities (Whittaker, 1992). Rock material is heterogeneous on both a large and small scale and is a composition of more than one mineral type. The mineral grains is often distinguishable to the naked eye and as amorphous particles bound by cementitious materials (Whittaker, 1992). In the case of the BPM model the fraction particles of various sizes corresponds to the mineral grains and the bonds act as the cementious materials. Anisotropy relates to the level of directional properties of the rock material. An isotropic material behaves in the same way if stresses are applied from different directions while an anisotropic material behaves differently. Due to the randomness of the position and size of the fraction particles as well as the bond density inherent by the former, the meta-particles behave in an anisotropic manner. The BPM approach has been used in previous crushing related work as a method for simulating rock breakage in various comminution devices (Khanal, 2005; Schubert, 2005a). In most cases a single to a few bonded meta-particles are created in the following sequence of actions using a static factory approach; 1. A 3D geometry emulating the meta-particle shape is imported, e.g. a cube, sphere or a 3Dscanned geometry. 2. The geometry is defined as a particle factory 3. Particles are created inside the particle factory geometry 4. The fraction particles are bonded at a pre-defined time step This method is suitable for modelling and investigating single particle breakage like in a rock strength test or in an impact crusher. However when the task is to create multiple meta-particles in a crushing sequence simulating the breakage of many particles over time it is a very time-demanding approach and not suitable. An alternative approach for solving the practical problem with creating many metaparticles is to use a particle replacement factory. The idea is to create and use dummy particles as multiple factories and replace them at a given time-step with a set of fraction particles at positions given by an external txt-file containing the coordinates for all the fraction particles. This approach has been utilised in this work and is provided by the EDEM support team at DEM Solutions Ltd.. The ability to control the external coordinate file enables full control over the meta-particle characteristics. In this work it was important to simulate rock shaped meta-particles, hence a new method for creating cluster coordinate files based on rock shape had to be developed. The particle replacement and the creation of the cluster coordinate file based on rock shape are done in the following sequence of actions; 1. A number of dummy particles are introduced to the system using a regular factory 2. At a given time step each dummy particle is replaced by a cluster of fraction particles which position, size and amount is controlled by the cluster coordinate file. The rock shaped cluster coordinate file is created by the following separate sequence of actions made in advance; 8

9 a. A bed of particles are created and packed in a volume larger than the size of the meta-particle to be created. b. A 3D geometry of a rock is imported as a selection bin inside the bed of particles which enables earmarking of all particles placed inside the selection bin space. c. The X, Y, Z position and particle radius of all particles inside the selection bin are exported to a.txt file d. The data is imported to e.g. Excel and reorganized to the correct format 3. The cluster of fraction particles are bonded at one time step post the replacement timestep to form meta-particles Figure 7 - Snapshot from simulation setup phase showing imported meta-particles as well as 3D scanned machine geometry. Fraction particles are coloured by mass. When simulating rock breakage it is of great importance to conduct calibration tests in order to calibrate the breakage towards sample rock. In this case however, due to time and resource limitation, BPM-parameters and strength criteria where chosen from the paper by (Potyondy and Cundall, 2004) instead. The critical experiment setup was to keep all crusher operational parameters as well as all modelling techniques constant for the comparison of the new and completely worn liner cases. This allows for assessment of the effect of liner shape on crusher operation in isolation. 4 Results and discussion In Figure 8 and Figure 9 the breakage of meta-particles can be seen. The meta-particles have a shape that corresponds to real rock. Hence the particle flow is highly stochastic trough the chamber. During crushing particles are damaged through both, surface damage, and crack propagation, splitting the particles into larger fragments. During the compressive gyrating motion of the mantle, the metaparticles experience a relatively high level of lifting. This may imply too low a friction between 9

10 geometry and particle. Potentially the meta-particles are too strong as the original calibrated strength criteria set by Potyondy and Cundall applied to a different rock size. Rock is normally weaker as the size gets larger due to e.g. a higher discontinuity density. Figure 8 - Snapshot from simulation of the worn compensated case showing the bonds forming particles and coloured by the level of normal force. Beams with black colour are representing recently broken bonds. Figure 9 - Snapshot from simulation of the worn compensated case showing a detail view of a particle in the process of breakage. 10

11 Total Mass Within Domain [kg] 4.1 Crusher Performance Figure 10 shows the total mass within the simulated crusher cavity over time, and therefore the ability of the crusher to empty the cavity. In essence, the derivative of the linear regression line plot illustrates throughput capacity. As can be seen the capacity is higher when the concave is worn when compared to new. As the gap is compensated in the worn case to meet the target CSS the capacity drops. However, due to the change in chamber profile and cross-sectional area the capacity is still higher in the worn compensated case compared to the new. This implies that the gap is not the only limiting parameter, but the chamber shape also has a detrimental effect. The capacity is higher in the worn case because particles can fall deeper down the chamber between every gyration and are hence falling through the crusher easier. This also implies that the findings may be dependent on the particle size distribution chosen in the DEM model setup. Throughput Capacity y = x R² = y = x R² = y = x R² = Simulation time [s] New Concave Worn Compensated Liner Worn Uncompensated Liner Figure 10 - The graph shows the total mass within the domain over time for the three different simulated cases In Figure 11 the total amount of intact bonds in the simulation are plotted. This derivative of the linear regression provides an indication of breakage rates. The results show that the breakage rate is highest in the worn-uncompensated case and lowest in the new concave case. The shape of the graph is similar to the capacity graph implying a relationship between the particle flow characteristics and the breakage behaviour. The breakage of a meta-particle splitting it in two large fragments can be seen in Figure

12 Number of Intact Bonds Breakage y = x R² = y = x R² = y = -9974x R² = Simulation time [s] New liner Worn Compensated Liner Worn Uncompensated Liner Figure 11 - The graph shows the total number of intact bonds over time for the three simulated cases. Figure 12 - Snapshot from DEM simulation of the worn- uncompensated case showing how a meta-particle under compressive load in broken into several fragments. Recent broken bonds are shown in black colour 12

13 5 Summary and Conclusions The following general conclusions can be drawn from the findings and results; A new framework for modelling and simulation of gyratory type crushers using DEM in order to evaluate crusher performance has been presented 3D-scanned geometry from real crushing applications provides the quality input data needed for DEM modelling in order to evaluate the influence on wear on the crushing operation. A new BPM model incorporating fraction particle size distribution with a replacement factory has been developed and applied A new BPM model incorporating meta-particle shape corresponding to 3D-scanned geometry of real rock has been developed and applied The following case specific conclusions can be drawn from the findings and results; The production performance behaviour of a gyratory crusher significantly varies for new and worn concaves even though the vertical mantle position is used to compensate for liner wear in order to maintain the target CSS The case study illustrated a more productive crusher behaviour in terms of throughput and breakage when operating with worn concaves rather than new concaves; this observation can be exploited by industry for cases where continuous crusher feed applies in order to achieve higher production by correlating an optimum window of production to respective liner shapes for liner redesign The results indicate that capacity is limited not just by the CSS but also the cross-sectional area The breakage rate is dependent on the chamber profile and the CSS 6 Acknowledgements This work has been carried out within the Sustainable Production Initiative and the Production Area of Advance at Chalmers. The support is gratefully acknowledged. Many thanks to the EDEM support team at DEM Solutions Ltd. 7 References Bearman, R.A., Briggs, C.A., The active use of crushers to control product requirements. Minerals Engineering, 1998, 11(9), Cleary, P., Modelling comminution devices using DEM. Int. J. Numer. Anal. Meth. Geomech., 2001, 25, Djordjevic, N., Shi, F.N., Morrison, R.D., Applying discrete element modelling to vertical and horizontal shaft impact crushers. Minerals Engineering, 2003, 16(10), EDEM, EDEM User Manual. DEM Solutions Ltd., Edinburgh. Evertsson, C.M., Cone Crusher Performance, In Dep. of Machine and Vehicle Design. Chalmers University of Technology, Göteborg. Franke, J., CrusherMapper TM - A New Tool for Crusher Condition Monitoring, In XXV International Mineral Processing Congress (IMPC), Brisbane, QLD, Australia, pp Gupta, A., Yan, D.S., Gyratory and Cone Crusher, In Mineral Processing Design and Operation. Elsevier Science, Amsterdam, pp Hulthén, E., Real-Time Optimization of Cone Crushers, In Dep. Product and Production Development. Chalmers University of Technology, Göteborg. Hulthén, E., Evertsson, C.M., Real-time algorithm for cone crusher control with two variables. Minerals Engineering, 2011, In Press, Corrected Proof. 13

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