FEM and DEM modelling of briquetting of fine powders: multi-scale analysis of briquette structural and mechanical properties

Abstract

Briquetting is a method of particles compaction where fine particles are densified to produce briquette with different sizes and shapes. In briquetting, loose particles are compressed two counter-rotating rolls. The structural and mechanical properties of briquettes are crucial to the downstream operations such as transportation, handling, and coating etc. Therefore, a better insight into those fundamentals governing of briquetting is of great importance to process control and optimization. This work aimed to develop numerical models at different scales to simulate the briquetting behaviour of fine particles. Two numerical techniques, finite element method (FEM) and discrete element method (DEM), were employed in the research. FEM considers compacts as continuum porous materials and can provide full scale simulations. On the other hand, DEM is a particle-based technique and can provide detailed information at the particle scale. The two complementary methods allow to analyse briquettes at different scales. A 2D FEM model was firstly developed to simulate the briquetting process. The Drucker-Prager Cap (DPC) model was adopted to characterize the mechanical response of powders. The model parameters were determined by conducting different experimental tests of die compaction of iron ore fines. The model was validated by comparing the simulation results with experimental data. The relative density and stresses of the briquettes showed inhomogeneous distributions in the flow direction. The parametric studies showed that the feed pressure affected the briquetting process considerably as both relative density and power draw increased almost linearly with increasing feed pressure. However, a non-linear trend was observed with increasing particle-wall friction. The 2D FEM model was later extended to 3D to consider the shape of roll pockets. The 3D simulation results showed that the Von Mises stress and hydrostatic pressure were more inhomogeneous compared to the 2D results. In the roll depth direction, both relative density and Von Mises decreased from the pocket periphery to the centre region. The effects of feed pressure, roll gap and roll speed were analysed. The roll force and relative density increased almost linearly with increasing feed pressure. A non-linear decline in roll force and relative density were observed as roll gap increased. However, roll speed showed limited impact on roll force and briquette mechanical properties. A DEM model with an elasto-plastic adhesion model was developed to simulate briquetting. The DEM model was firstly calibrated by comparing the die compaction results with experiments. In the DEM simulations of briquetting, the force-angular displacement curves showed the oscillatory patterns similar to those observed in the FEM simulations. The relative density at the final stage decreased in comparison with compression stage, and the contact force network was sparser and thinner. The effects of feed pressure and roll speed were studied. The force on the roll increased significantly, and the relative density and strength increased nonlinearly with increasing feed pressure. In contrast, the roll force, relative density, and strength declined as roll speed increased. The formed briquettes were compressed along two different directions to examine the strength and cracking phenomena. When compressed along the roll width, the crack started at the shoulder region and propagated to the centre part of the briquette. With increasing loading, the briquette broke into two parts along the vertical centre line. When compressed on the curve surface, the break force was much lower compared to the roll width direction and the crack initiated at the middle part of briquette and squeezed out vertically. Feed pressure of briquetting showed strong effect on the failure pattern and break force of briquettes. For both cases, the break force increased almost linearly with increasing feed pressure. Multiple cracks were observed for the loading direction along the roll width. However, a more distinct and clearer vertically squeezed crack at the middle periphery region was noticed along the curve surface. The effect of particle shape was investigated by simulating briquetting of tetrahedral particles. The roll force was much lower for the non-spherical particles and a lower relative density was observed for the tetrahedron particles. The force on the roll and the relative density was found to decrease with increasing the non-sphericity. On the other hand, both roll force and relative density increased with increasing feed pressure.