Assessing the influence of viscosity and milling bead size on the stressing conditions in a stirred media mill by single particle probes

Abstract

Milling processes in stirred media mills can be essentially described by the collision bead frequency distribution (stress number) and the transferred stress energy distribution. Although general dependencies on the mills’ operating parameters have been extensively studied, the underlying distributions are widely unknown. Recently, we established an experimental protocol to determine the aforesaid distributions by using well-characterized spherical metal particle probes. To this end, the plastic deformation of the particle probes which is caused by stressing in the apparatus is accessed by electron microscopy and is related to single particle compression testing described via a finite element model. Within this account we focus on the influences of grinding media size and fluid viscosity on the stressing conditions in a small-scale horizontal stirred media mill which is operated in open-circuit mode. We show that larger milling beads lead to higher stress energies, whereas an increase of viscosity leads to lower transferred stressing energies. Surprisingly, however, the overall amount of stressed particle probes decreases with the milling bead size. At the same time the fraction of the probes which has been stressed shows many contacts. We relate this effect to the slurry flow through the agitated packed milling bead bed. The effect of viscosity is attributed to squeezing flows which form as a result of approaching milling beads.