Bulk Engineering, Technical articles

Industrial applications of numerical modelling in bulk materials handling

A collaboration of Newcastle businesses, including TUNRA Bulk Solids, HIC Services, and Lindsay Dynan, worked together to improve the throughput of a transfer chute at a prominent Hunter Valley coal mine by using 3D modelling technology.

Dr Bin Chen, Engineering Manager of TUNRA Bulk Solids, discusses the use of numerical simulations to solve industrial problems.

Bulk materials handling operations perform a crucial function in the mining industry and port facilities. It is important that the storage and handling systems be designed and operated to achieve maximum efficiency and reliability. With the advancement of simulation techniques and computer power, Discrete Element Method (DEM) and Smoothed Particle Hydrodynamics (SPH) have been widely used across a range of applications to simulate increasingly complex processes and geometries. The utilisation of extensive numerical modelling has gained growing significance in addressing industrial challenges, particularly in enhancing the design and operational efficiency of bulk materials handling equipment A series of case studies of industrial applications of numerical simulations are presented, including analysis of transfer chute material hang-up and rhino horning, belt wear, hopper wall loads and scrubber chute flow. 

Discrete Element Method (DEM) Modelling

DEM is a numerical method that can be used to simulate the flow of granular bulk solids, with the basic principle being 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 of 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. Although DEM has seen a significant increase in usage over the past few decades, undoubtedly, the accurate calibration and selection of DEM parameters represent crucial steps in the simulation procedure. The DEM modelling parameters can be selected based on an interpretation of the measured flow properties of the bulk material, calibration testing results, and/or site observation/feedback. These properties, which are commonly measured at TUNRA Bulk Solids’ laboratory, include loose poured bulk density, wall friction/adhesion, internal shear angle, particle size distribution, angle of repose, dynamic build-up among others.

DEM case study 1: Increasing the reliability of iron ore chutes

Two chutes, namely CVR123/023 and CVR223/023, experienced problems such as ore accumulation leading to unscheduled downtime and failure of impact idlers on the receiving conveyor due to unfavourable loading conditions. This project unfolded in two phases: Initially, TUNRA was engaged to review the new design already under implementation. Subsequently, Phase 2 involved proposed modifications to the inserted component installed in Phase 1.


Challenges

The new design aimed to transition from a rock-box chute to a sliding chute to accommodate changes in material properties. TUNRA employed DEM simulations utilising parameters derived from flow properties data, field experience, and site feedback. Calibration, a crucial consideration, was undertaken in collaboration with field experts to accurately replicate material flow. The review showed that the proposed lower insert design exhibited reduced build-up, but the inclination angle of the lower wall was inadequate for the specific material. This would likely lead to build-up, resulting in additional maintenance and affecting loading onto the receiving conveyor for sticky materials. The concurrent implementation of the review and modifications due to project timeline constraints meant that the findings only served as risk mitigation in Phase 1, paving the way for Phase 2’s focus on further chute improvement.

Solution

Phase 2 yielded enhanced reliability through key modifications:

  • Optimal selection of the rear wall slope angle to facilitate flow and minimise build-up.
  • Enhanced convergence of insert side walls and higher loading on the insert to better control ore discharge.
  • Extending the lower insert to improve soft loading and loading symmetry, as the gap between the insert’s bottom and the receiving conveyor’s top was deemed excessive.

To illustrate these alterations, Figure 1 depicts the three chutes: the original design, the initial redesign proposal, and the improvements proposed in Phase 2.

Figure 1: Section view showing the flow of sticky ore through the chute (a) initial Design (b) Phase 1 improvement (3rd party) (c) Phase 2 improvement (TUNRA).

Outcomes

The improved performance was subsequently verified on site, as shown in the graph below:

Figure 2: Number of events causing delays (specifically chutes CVR123/023 and CVR223/023).

DEM case study 2: Reducing wear of a coal reclaimer

Belt feeder

Belts undergo various forms of damage during operation, with wear being a prominent cause of belt failure. Extensive research has demonstrated that wear resistance is contingent upon material properties and external wear conditions, including normal pressure. This case study focuses on analysing the wear of feeder belts within a coal reclaimer. 

DEM simulations yielded diverse post-simulation results. Selected results closely aligned with the wear model, and decreasing these values was expected to mitigate feeder belt wear. Visual comparison through surface contours was also possible. Shear and impact intensity surface contours were compared between the original design and the redesign, highlighting wear reduction due to flow insert inclusion. Asymmetrical wear in the existing design was attributed to non-central loading, which was mitigated by the flow inserts.

Figure 4: Isometric view of the flow through the reclaimer using DEM modelling.
Figure 4: Isometric view of the flow through the reclaimer using DEM modelling.

Recommended flow inserts were integrated into the reclaimer’s trouser legs, with wear data collected. Flow inserts perpendicular to the flow were not extended across the entire opening of the trouser leg due to preassembly requirements. Photos of preassembled and installed inserts are shown in Figure 4. On-site data for the original design indicated 5.3 mm top cover wear over 3.2 million tonnes of handled coal. Following flow insert installation, new data demonstrated 1.0 mm top cover wear over 3.0 million tonnes of coal. Feeder belt details remained constant. Wear rate comparison revealed 1.65 x 10-6 mm per million tonnes for the original design and 0.33 x 10-6 mm per million tonnes after insert installation, signifying an approximate 80% reduction in wear. 

Figure 4: Isometric view of the flow through the reclaimer using DEM modelling (left) and flow insert installed into the trouser legs of the transfer chute (right).
Figure 4: Flow insert installed into the trouser legs of the transfer chute (right).

DEM case study 3: Hopper wall loads caused by eccentric discharge

In the context of a case study, Discrete Element Method (DEM) simulations were employed to forecast pressure distributions along the circumferential direction of a hopper during material discharge from a three-legged hopper configuration. In comparison to cylindrical sections where eccentric loads on vertical walls can be estimated via established standards, such as Australia Standard AS3774 (1996) and Eurocode EN 1991-4 (2006), assessing eccentric loads in hoppers poses increased complexity due to non-vertical walls. 

One leg chute in operation

Wall loads were investigated during material discharge from only one leg chute, with DEM simulations conducted. The discharge occurred through an offset flow channel, forming stationary material on the opposite side of the hopper walls, away from the operational chute. Near the hopper bottom, higher normal forces were observed on the far side compared to the near side. This phenomenon was reminiscent of findings in cylindrical sections and aligned with the asymmetric material flow patterns induced by eccentric discharge, affecting the structural integrity of the hopper. The upper section’s normal wall loads were relatively uniform, indicating diminished influence from eccentric discharge at the hopper’s upper periphery.  

Figure 6: Numerical Results for Normal Pressure during Eccentric Discharge (Left) and Pressure Distribution around Hopper Periphery at Hopper Bottom (Right) - One Leg Chute Discharging
Figure 6: Numerical results for normal pressure during eccentric discharge (Left) and pressure distribution around hopper periphery at hopper bottom (Right) – one leg chute discharging

Two leg chutes in operation

Material discharge involved two leg chutes in the hopper, featuring a larger flow channel area than the single-leg chute scenario. Similar trends to the single-leg chute case were observed, with higher loads on the far side near the hopper bottom. The eccentric discharge effect was again evident, impacting normal pressure distributions around the hopper’s periphery. Notably, the extended flow channel in this configuration reduced pressure over a larger circumferential range, owing to the doubled flow channel area resulting from dual-leg discharge.

These DEM-based findings highlight the non-uniformity and eccentricity in wall load distributions during eccentric discharge scenarios in hoppers, particularly near the hopper bottom. These variations emphasise the necessity of accounting for eccentric discharge-induced stress non-uniformities when designing hoppers. These insights contribute to an enhanced understanding of eccentric load behaviour in hoppers.

Figure 7: Numerical results for normal pressure during eccentric discharge (Left) and pressure distribution around hopper periphery at hopper bottom (Right) - two leg chutes discharging.
Figure 7: Numerical results for normal pressure during eccentric discharge (Left) and pressure distribution around hopper periphery at hopper bottom (Right) – two leg chutes discharging.

Smoothed Particle Hydrodynamics (SPH) modelling

Smoothed Particle Hydrodynamics (SPH) is commonly utilised to simulate continuum media mechanics, employing the Navier-Stokes equations for viscous fluid motion. A two-phase model, encompassing water and sediments based on the Herschel-Buckley-Papanastasiou model, was employed for flow behaviour modelling. The water phase utilised established macro parameters (density and viscosity), while the solids phase necessitated parameter calibration, including yield stress, power law index, density, and viscosity.

SPH case study: Optimisation of wet screening operation

A wet screening process required refinement due to observed loading bias of slurry onto one side of screens, potentially hindering the screening process and product moisture levels. Focus was on material flow from scrubber to screen deck, involving a wet scrubber followed by a chute discharging onto a wet screen.

Simulation illustrated biased solid flow towards the northern screen side, aligning with site observations. Comparisons between physical and simulated cases indicated analogous flow characteristics over the weir, particularly in the turbulent region. Notably, discrepancies existed in the model’s representation of reverse flow and droplets compared to site images, attributed to computational limitations.

Given promising preliminary observations, the SPH model underwent additional analysis as a foundation for subsequent designs. Throughput control volumes were implemented for loading analysis, unveiling northern loading biases of 65 per cent through the weir, peaking at 85 per cent at the centre control position, and decreasing to 60 per cent at the chute exit.

Subsequent design improvements were pursued based on SPH model findings. A revised chute geometry showcased a significant reduction in biased loading throughout the chute and onto the screen. This optimised design is poised for installation on-site, following the assessment of SPH baseline model validity and the ensuing modifications.

Figure 8: Baseline SPH scrubber model.
Figure 8: Baseline SPH scrubber model.
Figure 9: Location of throughput analysis boxes.
Figure 9: Location of throughput analysis boxes.

Conclusion

The realm of bulk materials handling operations holds a pivotal role within the mining industry and port facilities. The design and operation of storage and handling systems bear profound implications for achieving optimal efficiency and reliability. As simulation techniques and computational capacities have advanced, DEM and SPH have emerged as instrumental tools, finding diverse applications to unravel complexities in processes and geometries. The escalating significance of extensive numerical modelling is evident in its capacity to tackle industrial challenges, thereby augmenting the design and operational efficiency of bulk materials handling equipment.

The three DEM case studies underline the method’s application in elevating the reliability of iron ore chutes, diminishing wear on coal reclaimer belt feeders, and eccentric discharge-induced hopper wall loads. The SPH case study spotlights its role in optimising wet screening operations. In synthesis, the comprehensive exploration of numerical simulations within the context of industrial materials handling reinforces their transformative potential.  

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