Motion and stress intensity of grinding beads in a stirred media mill .1. Energy density distribution and motion of single grinding beads

Numerical calculations concerning the how field, the distribution of the specific energy and the motion of single grinding beads in the grinding chamber of a stirred media mill have been carried out. The calculations are based on steady-state laminar stirring of a Newtonian fluid without grinding media. The flow field of the stirred fluid generates a characteristic distribution of the specific energy. Two zones characterized by a high energy density exist. In these zones the local specific energy is larger than the mean specific energy which is obtained by dividing the total amount of energy dissipated in the grinding chamber by the net volume of the grinding chamber. One zone extends around the stirrer disc whereas the other is located at the grinding chamber wall. The volume of these two zones is only about 10% of the net grinding chamber volume. Approximately 90% Of the entire energy input is dissipated there. Single grinding beads that are exposed to a previously determined flow pattern tend to follow an almost stationary individual trajectory in the grinding chamber. The position of the trajectory depends on the ratio of bead-to-fluid density, the ratio of bead-to-disc radius and the Reynolds number which describes the operating conditions of the stirred media mill. The influence of these parameters can be described by the so-called motion index. Up to a critical value of the motion index, the single bead follows basically the fluid flow and passes through the two zones of high energy density.