Discrete Element Modelling of Compaction of Wet Granular Materials

Abstract

Powder compaction, as a unit operation in powder/particle technology, is of fundamental importance to many industrial applications. During die compaction, loose powder assembly subjected to an external loading experiences densification and consolidation by different mechanisms depending on particle characteristics and operational conditions. The mechanical strength of formed compact is critical to maintaining geometrical and mechanical integrity of the compact in the downstream operations. Therefore, better understanding of the compaction behaviour requires a link between the die compaction behaviour and the mechanical response of the formed compact.

This study is concerned with a systematic study of the compaction of wet granular materials using particle scale simulation. A parallel program was developed to accelerate the computationally intensive Discrete Element Method (DEM) simulation. The effects of interparticle cohesion due to the presence of water and interparticle bonding were explicitly integrated into the force model. A parametric study focusing on the effect of material properties and particle shape was conducted aiming to provide a scientific insight on the strength response of formed compact.

In the present study, a DEM implementation using Graphic Processing Unit (GPU) computing techniques has been developed. An implementation of multi-grid neighbour searching method for system of large size ratio was proposed. Both the validation and performance analysis were carried out for the packing of spherical particles, showing that the proposed searching method outperforms the classical linked-cell method significantly. Based on the GPU platform, a large scale packing has been simulated at a reasonable cost.

The developed DEM model was calibrated and validated by using experimental data of compaction of wet iron ore fines. With the introduction of the interparticle bonding between contacting particles, the predicted bulk response showed good agreements with experiments in terms of pressure-density relationship during die compaction and stress-strain response during unconfined compression. Three main characteristics of the stress-strain response, namely the stress increasing part, peak value and stress softening state, were captured in the simulations. A shear-dominated bond failure mode was obtained, forming localized shear banding concentrates within the bottom part of the compact. Consolidation pressure, bonding area and bond strength increased the compressive strength of formed compact but had no influence on the dominating bond failure mode. The effects of frictions and sample size on the compaction behaviour were investigated. Particle-wall friction during die compaction was shown to affect the axial inhomogeneity, leading to decreased compressive strength and localized failure zone within the bottom part of the compact. In contrast, interparticle friction strengthens the compact and counteracts the effect of particle-wall friction. Additionally, selection of sample size was shown to be important for consistent and reliable simulation results. When consolidation history is considered, an aspect ratio (>1.5) can provide smaller variation in strength value and a more consistent failure pattern during unconfined compression, which is consistent with experimental observation.

The diametrical compression was performed to determine the tensile strength of formed compact of wet particles. Compared to that of unconfined compression, a tensile-dominated bond failure mode was obtained, resulting into a macroscopic catastrophic tensile crack traveling through the center area of the compact. The bulk tensile strength increases with bond strength and consolidation pressure in different manners. The punch curvature was found to affect the overall compressibility, facilitates vertical force transmission and localizes large forces towards the center of the compact. As a result, the macroscopic failure mode changes from brittle to ductile as the curvature increases. However, the effect of punch curvature can be suppressed by increasing particle-wall friction.

Particle shape plays a key role in the mechanical response of formed compact of wet particles. A non-spherical DEM model for powder compaction was developed and applied to the die compaction and unconfined compression of the ellipsoidal particles. A good agreement with literature data was obtained for the packing of ellipsoidal particles, confirming the validity of the developed non-spherical model. With increasing aspect ratio of prolate ellipsoids, consolidation pressure has a minimum at the aspect ratio of 1.6 while compressive strength saw consistent increase.

A comparative study on the compaction of wet spherical and tetrahedral particles was carried out. The compact of tetrahedral particles exhibits a lower level of compressibility but a larger compressive strength than that of spherical particles. Macroscopic failure was found to be dominated by shear-induced bond breakage. The compact of tetrahedral particles exhibited a wider shear banding, a more dispersed bond breakage and a slower failure process. The compressive strength of formed compact of tetrahedral particles was found to increase with consolidation pressure and particle shape factor. Consolidation pressure has an effect of suppressing the initial imperfections while the increase in shape factor leaded to a wider and more dispersed failure pattern.

DEM simulations were then conducted to investigate the die compaction and unconfined compression of wet non-spherical (tetrahedral) particles, focusing on the effects of material properties (interparticle friction and yield pressure) and operation conditions (friction of the punch). The interparticle friction coefficient was found to affect not merely the degree of particle rearrangement but also the anisotropy of force structure, resulting in an increase in loading stiffness and compressive strength and leading to a transition of the mechanical response from brittle to ductile. On the other hand, yield pressure was found to result in a stiffer response at die loading, an increase in the degree of elastic recovery at unloading and a decreasing compressive strength. During unconfined compression, as the punch friction coefficient increases, the compressive strength gradually increases before reaching a saturation level.

In summary, a GPU-based non-spherical DEM model has been developed and applied to model the compaction of wet granular materials. This work has successfully demonstrated that the DEM model with a proper interparticle bond model is able to study the mechanical strength of compacts.