Particle breakage, also known as particle degradation or fragmentation, is a process that involves splitting a larger particle into several smaller fragments. This can be desirable or undesirable depending on the application. For example, there are many bulk materials handling equipment whose primary purpose is to crush or break larger particles into smaller ones to reduce particle size enough to prepare the material for further processing downstream in the plant. On the other hand, particle breakage must be avoided or minimized when wanting to guarantee a certain quality of material being conveyed but also to ensure that dust generation (arising from fines) is kept to a minimum as this can have environmental and/or health implications.
Figure 1: Rocky DEM simulation of a cone crusher (left) and SAG mill (right)
Example Application: Using Breakage to Evaluate Transfer Chute Designs
Being able to predict material degradation in transfer chutes is very important to reducing the environmental impact of dust created by fine particles, and to maintaining the overall quality of the material. Until now, these sorts of problems have received very little attention in DEM because of their complexity. But with the Rocky DEM breakage model, these problems can now be accurately simulated.
Figure 2: Comparison of two chute geometries simulated in Rocky DEM: a rock box (left) and hood and spoon (right). Particles are coloured by velocity. Graph shows relative breakage rate.
The Rocky breakage model was used to simulate the flow of coal through two different transfer chutes, as shown above in Figure 2. Apart from chute geometry, all simulation parameters were kept the same. The simulation consisted of over one million particles, half of which were non-round and breakable and the other half were round and non-breakable. The breakage properties for coal were determined via drop weight tests at different heights on an impact plate. Despite the higher particle velocities, the hood and spoon configuration endured much less breakage compared to the rock box chute as shown by the plot of breakage rate.
Example Application: Validating Breakage Accuracy by Simulating an HPGR
Another application where the accuracy of Rocky’s breakage model was validated is presented below. High Pressure Grinding Rolls (HPGRs) are devices that are typically used in the mining and minerals industry. In this application, the particle stiffness was calibrated with respect to a specific force. For the purpose of these simulations, the roll gap was fixed and particle stiffness was varied until the predicted and measured values of the force were in close agreement, as the results presented below illustrate.
Figure 3: Rocky DEM simulation of a HPGR (top left). Results show particle PSD after breakage (top right), specific force (bottom left), and specific power (bottom right).
As can be seen in the plot of specific force (Figure 3, bottom left), a value of 200 MPa for the particle stiffness correlates very well to experimentally measured values. Similarly, the specific power (Figure 3, bottom right) for the same particle stiffness is also in close agreement to experimental results. This also confirms that unlike other applications with much lower consolidation forces (for example, transfer chutes and conveyors), the correct stiffness value plays an important role in obtaining correct quantitative results for force and power draw. This means that the default value of 100 MPa in Rocky is normally conservative for cases that do not involve breakage or high-contact forces.
The PSD obtained after breakage further confirms the accuracy of the breakage model. In the interests of computational time, the PSD was limited to four discrete groups; however, this was still in good agreement with the experimentally measured values. Finally, it is important to note that whilst breakage modelling can increase the accuracy of your predictions, the computational power to process these simulations can be substantially higher, although this can be addressed by taking advantage of GPU processing to reduce the solve time or time-to-solution.
Want to know more about Breakage Modelling in Rocky DEM? Download the white paper below.