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 t he specific energy and the motion of single grinding beads in the grin ding chamber of a stirred media mill have been carried out. The calcul ations 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 chara cterized by a high energy density exist. In these zones the local spec ific energy is larger than the mean specific energy which is obtained by dividing the total amount of energy dissipated in the grinding cham ber 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 grin ding chamber volume. Approximately 90% Of the entire energy input is d issipated there. Single grinding beads that are exposed to a previousl y determined flow pattern tend to follow an almost stationary individu al trajectory in the grinding chamber. The position of the trajectory depends on the ratio of bead-to-fluid density, the ratio of bead-to-di sc radius and the Reynolds number which describes the operating condit ions 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.