TUNRA Bulk Solids consulting engineer Daniel Grasser examines how experimental testing combined with advanced simulations could optimise bulk materials handling operations, and how targeted modifications can reduce particle degradation.
In bulk material handling, particle degradation commonly occurs during operation.
Here, the severity of degradation depends on the type and size of the material, and the material flow regime.
For example, for bulk materials such as limestone, excessive degradation (including particle fracture) can have significant impacts on the product quality, such as changing the size of the particles.
Understanding where and why degradation occurs is essential for applying countermeasures to ensure a consistent product quality.
A combined approach using experimental fracture testing and discrete element method (DEM) modelling provides an effective way to understand particle degradation.
By linking experimental breakage tests with DEM simulations of material flow, it becomes possible to quantify degradation and to identify design improvements.
Material degradation in the operation
Although bulk materials often pass through multiple transfer points in a typical operation, not all contribute equally to degradation.
DEM simulations can help to analyse and compare the critical locations of the operation. For example, the unloading of material from a tipper truck into a hopper can cause severe particle degradation. In this case, degradation happens largely due to the nature of the material flow. Material enters the operation with high kinetic energy, often falling freely into the hopper before being redirected towards downstream equipment.
These conditions promote high-impact collisions, particularly against steel surfaces, which can significantly increase the likelihood of particle fracture.
In contrast, downstream transfer points tend to operate under more controlled flow conditions at reduced velocities, where material streams are already partially consolidated and impact energies are lower. As a result, degradation in these areas is comparatively lower.
Linking DEM simulations with fracture tests
A key element of assessing particle degradation is the integration of experimental breakage tests with DEM modelling. Here, the material fracture behaviour can be calibrated using laboratory data, ensuring that the predicted degradation mechanisms reflect reality.
In the experiments, rock samples are dropped from different heights. This can be done for two contact regimes: onto a metal plate and onto a bed of accumulated rocks. For each regime, the resulting mass loss due to impact is used as a measure of degradation. These tests can reveal transitions between mild and severe breakage regimes.
Interestingly, these transitions often can be abrupt. At lower drop heights, particles can experience only minor chipping and surface wear. Beyond a certain threshold, however, the extent of fragmentation suddenly increases.
Experimental fracture tests can support quantifying the influence of contact conditions. For example, impacts onto steel surfaces usually lead to substantially higher degradation than impacts onto other rock particles. This highlights that particle degradation is not governed by the theoretical impact energy alone, but also by the nature of the contact interface, and how the energy is dissipated.
These threshold values can be extracted from the experiments and linked to values obtained from DEM analysis.
Evaluating the material degradation
By combining experimental thresholds with DEM-derived impact energies, it is possible to quantify the severity of degradation throughout the operation. As an example, a hopper can be found to contain a significant proportion of impacts within a severe particle breakage regime, while downstream equipment contributes relatively little to overall degradation.
In practical terms, this means that a relatively small fraction of high-energy impacts can be responsible for a disproportionate amount of total particle damage within the operation. Targeting optimisation efforts towards a particular piece of equipment can lead to a significant reduction of particle degradation of the whole operation.
Understanding material flow in a hopper
After a DEM model has been calibrated in terms of bulk material flow and particle fracture, the material flow within the operation can be analysed. The DEM simulations provide insights into particle velocities, impact energies and impact locations, throughout the operation.
For example, a tipper truck discharging material into a hopper. Two critical regions were identified. The first occurs during the initial discharge of material from the truck into the hopper. Here, particles fall from significant heights and can impact directly onto steel surfaces or sparsely covered regions of the hopper. These conditions lead to high-energy collisions and increased breakage.
The second critical region is located at the transition between an apron feeder and a conveyor belt. In this area, the material stream accelerates and impacts the chute, again often involving direct contact with metal surfaces. The combination of high velocity and unfavourable contact conditions can make this zone particularly prone to degradation.
Image: TUNRA Bulk Solids
Strategies for reduction of material degradation
As examples, several strategies for the reduction of material degradation are outlined for a hopper. These strategies include changes to operational parameters as well as structural design elements. Adjusting the position of the truck during unloading can be found to influence the peak impact energies of the particles and, in addition, the cumulated impact energy of the particles.
For example, positioning the truck further from or closer to the edge of the hopper can reduce the intensity of particle impacts. However, while favourable truck positions exist, the benefits should be assessed case-by-case for each individual operation.
Usually, truck positioning can be considered a useful measure but often is not the primary solution. Moreover, the addition of horizontal bars in the upper section of the hopper can be a measure to reduce the velocity of the material stream.
While this concept has the potential to reduce impact energies, specific configurations need to be assessed for each individual hopper. It is important to test the effect of the bars on particle velocities and the reduction of impact energies, for example, using a calibrated DEM model. Moreover, unwanted accumulation of material on the bars should be assessed and the suitability of the design confirmed.
Modern DEM simulations enable the comparison of the specific impact energy of different equipment designs.
Summary
Several important insights can be drawn from this example, which are broadly applicable to bulk material handling systems beyond this specific case. A calibrated DEM model can be used to assess the material flow and resulting particle degradation. In the context of particle degradation, calibration includes both experimental fracture tests and material flow calibration.
From this calibrated model, critical values of impact velocities and resulting impact energies leading to degradation can be extracted. Importantly, the impact velocity of particles should remain below a critical threshold value. This threshold depends on the material type.
Particle degradation can happen gradually, however, critical threshold values leading to sudden fracture can occur. Moreover, contact conditions play a crucial role. To reduce particle degradation, often rock-on-rock interactions are favourable, rather than rock-on-metal impacts.
At the same impact energy, a rock-on-rock contact may not lead to particle fracture, while a rock impacting on a rigid metal surface may fracture. When combined with experimental calibration, DEM certainly is a useful tool to compare different operational designs in terms of material degradation. It allows engineers to visualise the material flow velocities, identify problematic areas, and evaluate design changes.
Reducing particle degradation in bulk handling systems, such as hoppers, requires an understanding of the material flow velocities, contact regimes and critical impact energies.
Modern DEM simulations enable the comparison of different equipment designs to identify strategies for reducing particle degradation. Therefore, the combination of experimental testing with advanced simulations offers opportunities for the optimisation of bulk materials operations.
