Mechanistic analysis and computer simulation of impact breakage of agglomerates: Effect of surface energy

Agglomerates are ubiquitous as intermediate or manufactured products in chemical, pharmaceutical and food industries. During handling and processing they may suffer breakage if they are weak. On the other hand, if they are too strong, their dispersion and disintegration could be difficult. The control of their mechanical strength is therefore highly desirable. However, the analysis of agglomerate strength is complex due to the large number of parameters that influence agglomerate behaviour, such as the primary particle size, density and elastic modulus, and the interparticle bond strength. A simple mechanistic model is presented here which relates the number of broken contacts in agglomerate due to impact velocity, interparticle adhesion energy and the particle properties of the particles forming the agglomerate. The model is based on the hypothesis that the energy used to break contacts during impact is proportional to the incident kinetic energy of the agglomerate. The damage ratio defined as the ratio of broken contacts to the initial number of bonds is shown to depend on the dimensionless group, (right triangle), in the form ((pi)V2D5/3E2/3)/ )/(gamma)^5/3, where V is the impact velocity, E the elastic modulus, D the particle diameter, (gamma) the particle density and (gamma)the interface energy. This dimensionless group, (right triangle), incorporates the Weber number, ((pi)DV^2/(gamma)), which was previously shown to be influential in agglomerate breakage, and may be presented in the form, (right triangle)=WeIe^2/3, where Ie=ED/(gamma). The predicted dependency of the damage ratio on the surface energy has been tested using distinct element method (DEM). Four different agglomerates have been formed and impacted against a target for three different values of the surface energy of the primary particles. The simulation results show that the effect of surface energy is better described by the above mechanistic model than by the Weber number alone, as previously used to characterise the impact strength of agglomerates.