Transfer chutes may be a small or low-cost part of a system, but they can easily become a costly bottleneck and maintenance nightmare. In this case study, experts from TUNRA Bulk Solids model a stacker chute in a stacker reclaimer using Discrete Element Modelling (DEM) and scale modelling.
The performance of bulk handling plant depends on all equipment that comprise the system operating efficiently without interruption, and poorly designed transfer chutes often become a costly bottleneck and maintenance nightmare even though the chute itself may appear to be only a small or low-cost part of the plant
Transfer chutes are mostly employed to direct the flow of bulk material from one conveyor belt/feeder to another, often via a three-dimensional path.
Commonly used transfer chute analysis methods include continuum mechanics, Discrete Element Modelling (DEM), and scale modelling. In some cases, more than one approach is used to ensure trouble-free operations.
The continuum method has been developed based on granular dynamics and uses measured properties such as bulk density, internal friction and wall friction. A general assessment of transfer functionality could be obtained within hours.
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However, the analysis of dispersed material stream and build-up in the chute is difficult using continuum approach and relies on many assumptions. With the advancement of simulation techniques, DEM has become increasingly beneficial in solving industrial bulk solids handling problems.
DEM is a numerical method that can be used to simulate the flow of granular bulk solids. The basic principle is to model each individual particle as a separate entity that can undergo a range of forces as observed in reality. These forces typically include gravity and contact forces with other particles and walls, as well as cohesive and adhesive forces if the bulk solid is cohesive in nature.
Calculations for the forces, and resulting displacements, are made for every particle at very small-time steps throughout the simulation. As a result, DEM simulations are often computationally intensive.
Scale modelling is also used for analysing material flow in chutes during operation. The chute walls are typically made of Perspex to allow flow visualisation. As the frictional factor for the Perspex in the model and the liner on site might be different, there may be some discrepancy between the extent of build-up observed in the model and that experienced on site, but the build-up in the model is indicative of the areas where build up would be expected to occur in the full scale.
The objective of this study is to analyse material flow in a stacker chute in a stacker/reclaimer for the Guinea Bauxite Export Project. DEM and scale modelling have been used to ensure an efficient and reliable transfer of the bulk material without spillage or blockage. In addition, analysis of qualitative chute wear has also been conducted.
Chute configuration and operating parameters
Presented in Figure 1 is the concept model of the stacker chute for transferring material from an incoming tripper conveyor to an outgoing boom conveyor, which includes a curved deflector, shuttle hopper chute, and lower chute.
The curved deflector captures the entire stream as it discharges from the head pulley, and then converges the stream to allow compact delivery of the material to the shuttle hopper/chute. The chute incorporated chamfers in corners as a means of reducing the likelihood of fines build-up occurring in the area.
Modelling was conducted using the speeds and the combinations of luffing and slewing angles of the stacker chute. The parameters are outlined in Table 1, where all the dimensions, speeds, and material mass flow rates refer to actual.
DEM modelling
In this study, the DEM software Rocky was used to model the flow in the chute. This software utilises a hysteresis linear spring model for the normal force interactions and an elastic-frictional force model in the tangential direction. Rolling resistance is implemented while adhesive forces between particles and walls and cohesive forces between particles and particles are included through a simple constant force model.
The correct calibration and selection of DEM parameters is one of the most important steps in this simulation method and a great deal of research has been conducted in the field. This has led to the development of laboratory characterisation tests which build upon the well-established test procedures for bulk solids handling applications.
While the validation of the simulation results can sometimes be achieved by employing bench scale model tests, in some cases the opportunity is presented to use full scale validation of the DEM model. This is typically achieved by comparing the DEM model to data taken from site in the form of photos, videos and throughput analysis.
An important aspect of chute design is the knowledge of the relevant bulk solid flow properties. In this study, the modelling parameters within the DEM software are chosen based on the interpretation of the measured flow properties of the bulk material, such as the bulk density, wall friction, and internal friction.
Standard flow property testing was performed for this material (bauxite) using a direct shear tester. The internal strength of the bauxite sample indicated an effective angle of internal friction of approximately 55 to 60 degrees, while the bulk density was approximately 1400 kilograms per cubic metre. The wall friction angle was 35 to 40 degrees, and angle of repose testing revealed an approximate angle of repose of 35 to 40 degrees. In the DEM simulation of chute, the complete particle size distribution of the material could not be used as the computational costs associated with the simulation would be very high. Therefore, the bulk material was modelled with a PSD ranging from 38 millimetres (spherical) to 100 millimetres (shaped) particles.
Scale modelling
Scale modelling of the chute allows verification of the overall performance in operation and determines any potential problems with the design. The model dimensions, velocity and throughout parameters used for the modelling should be correctly scaled to ensure the validity of the modelling results. As the governing forces for the material flow in the chute are inertial and gravitational, the Froude dimensionless number was selected to scale the system from the full scale and model system. As such, the similarity between the full-scale (fs) and model (m) systems was achieved:
Where V is the material velocity (m/s), g is the gravitational constant (m/s2), and D is a size dimension (m). The gravitational constant was the same for the full-scale (fs) and model (m) systems.
The scaling factor of 0.125 was selected for modelling. Then, the calculated velocity scaling factor was 0.353. The modelled tonnages can also be determined based on the scaled size and velocity.
The chutes were made of Perspex to allow flow visualisation. The scaling parameters used in transfer modelling ensure that the modelled trajectories and flow patterns equate to those in full-scale. Therefore, the modelling allows assessment of bauxite trajectories and flow, but it is not designed to model wear and build-up. As previously mentioned, the frictional factor for the Perspex in the model and the wall liner on site are likely different. Therefore, there might be some discrepancy between the extent of build-up observed in the model and that experienced on site, and the build-up in the model is indicative of the areas where build up would be expected to occur in the full-scale.
The scale model of the stacker chute was built in-house in TUNRA Bulk Solids’ facilities (University of Newcastle), and was constructed and set up to replicate the system configuration as shown in Figure 2, which involved the correct angles, speeds and scaled distances.
In the scale modelling exercise, the bauxite top size was selected to represent an approximate scale of 1:8 . The bauxite sample was prepared at a predetermined moisture level at which the flow behaviour of the material corresponds to that of full size. During physical modelling of the transfer chute, a belt cut sample of the bauxite was taken and weighed to validate tonnage. Then, the sample was dried in an oven to determine the moisture content of the bauxite.
DEM simulation results
3D DEM simulation of the flow through the chute was undertaken to assess the flow mode and likely flow velocities through the transfer. Figure 3 shows the bauxite flow through the stacker chute at a throughput of 7500 tonnes per hour when the luffing angle was -13 degrees and the slewing angle was 90 degrees.
The DEM simulation image snapshots present an approximate general flow behaviour and provide insights into the visualisation of a three-dimensional flow through the stacker chute.
After the material impacted the deflector, a small amount of slow-moving material was observed due to the relatively high impact angle and small spacing between the deflector and head pulley. Then, the material stream impacted the centre rear wall of the shuttle hopper/chute, which would help achieve central loading on the boom belt.
DEM results showed that, for the given throughput of 7500 tonnes per hour and material properties, the stacker chute operated without any material choking or blockage.
In the steady state time period considered in the simulation, the material cross-sectional profile on the boom belt shows that the material was loaded centrally and evenly so as to avoid mistracking issues.
From the DEM simulations, it is also possible to extract contours of wall surfaces indicating qualitative impact and shear wear comparisons. The wear contours of the chute are presented in Figure 5, which were averaged over all particle contacts on the chute surfaces analysed over the duration of the simulation. The colour range for the wear contours shows the relative magnitude of wear. For example, red represents the highest magnitude for the wear contours.
The impact and shear wear metrics were calculated in the following manner:
• Shear intensity – the product of the relative tangential velocity and the tangential force transferred between each particle and the boundary at the point of contact. This is a qualitative indication of shear damage of the surface.
• Impact intensity – the product of the relative normal velocity and the normal force transferred between each particle and the boundary at the point of contact. This is a qualitative indication of impact damage of the surface and loading of support structures and idlers.
In practice, wear is known to be dependent on both shear and impact interactions. However, the way in which the two mechanisms interact and develop total wear is complex and, with current research, unable to be accurately quantified. As such, the analysis of wear performance in DEM is currently limited to relative comparisons between different designs or conditions.
Scale modelling results
For the scale modelling analysis, the bauxite sample was loaded into the loop system and all the conveyors were started. During modelling of the stacker chute, it was expected that the moisture of the circulating bauxite would slightly fluctuate due to evaporation. The recorded variation of ±0.5 per cent moisture content was considered to be reasonable. The material flow through the transfer chute was observed for a luffing angle of 10.5 degrees and slewing angle of 90 degrees.
The scale modelling results indicated that the stacker chute would be able to handle the throughput of 7500 tonnes per hour (as referred to actual) without any blockage. No material hang-up was observed. The material profile on the receiving belt showed the material was loaded in a central manner during the steady state time period. Thus, the scale modelling results of flow modes and burden profile on boom belt compared well with DEM simulation.
Conclusions
In this study, analysis of a stacker transfer chute for transferring material from an incoming tripper conveyor to an outgoing boom conveyor has been performed by way of a case study incorporating both DEM and scale modelling. The analysis indicated that, for the given throughput and material properties, the chute would operate without any material choking or blockage. Investigations into the material flow onto the receiving belt indicated that it would be expected to be loaded in a central manner. The DEM simulation results of flow modes and burden profile on boom belt compared well with those based on scale modelling.
This article was originally published in the 13th International Conference on Bulk Materials Storage, Handling and Transportation ICBMH 2019 Proceedings. Permission has been given to ABHR to republish.