One ball mill is not like the other
What makes one ball mill better suited for a specific purpose than another? To understand the factors that differentiate ball mill types, we will first look at their common characteristics. Basically, the working principle of each ball mill is the same: it is based on the concept that the sample material is moved potentially together with grinding balls inside a closed jar. This movement results in strong mixing and crushing effects of the material. The apparent difference, which can be seen immediately, lies in the different way the jars move. The classification of ball mills according to their movement is typically reflected in their names. In a planetary ball mill, for example, a jar rotates on a circular path similar like a planet rotates around the sun, in a mixer mill a jar performs an oscillating shaking movement in a horizontal position and in a drum mill the jar simply rotates around its central axis, see Figure 1.
Ball mills are furthermore characterized by significant differences in the sizes of available grinding jars. Retsch offers mills with jar capacities from 1.5 ml up to 150 l and balls are available from 0.1 mm to 40 mm, see Figure 2.
A third and very important characteristic of a ball mill, which also has a great influence on the result of a milling process, is the power of a mill. Depending on the application, jars should be moved either slowly for gentle material processing or, most commonly, at high speed for effective grinding effects. Here, the maximum speed, which is given as maximum frequency or maximum revolutions per minute (rpm), is often used as a synonym for performance. A more meaningful physical quantity than speed is the acceleration force "g", which is induced by the kinetic energy of a ball mill. In the High-Energy Ball Mill Emax for example, an unrivalled acceleration of 76 g can be obtained, if running at its maximum speed of 2000 rpm.
