ABSTRACT
Ball mills, stirred-ball mills, jet mills and vibration mills are commonly used in preparing ultrafine powders in the chemical industries. The comminution behavior of these mills can only be understood by studying the microproces of particle breakage. For many years researchers studied single- particle breakage, but in actual mills beds of particles break between impacting surfaces. Therefore, in this work the break- age of particle layers between a moving ball and a stationary anvil is studied.
In grinding mills ball-to-ball collisions trap and fracture particles. Curved surfaces between balls trap the intervening particles and balls falling on the particles cause breakage. Similarly, particles are also trapped and broken between “curved” and “flat” surfaces (such as liners) within the mill.
Whenever the stress developed within an individual particle exceeds the compressive strength, the particle breaks. The stress applied to the particle depends on the assemblage of particles present in the zone of breakage. Therefore, a systematic study is needed to learn about fracture behavior of assemblage of particles under specific loads and specific geometry of the impacting bodies. Such a study has been carried out at the University of Utah using a device known as the ultrafast load cell.
These studies were conducted in dry systems only. It is found that, regardless of the number of layers of particles present between impacting balls, only the last two or three layers are broken, as long as the mill allows free movement of particles. The distribution of broken fragments gets finer as the energy of impact increases, but the amount of material broken in an assemblage is roughly the same for all energy inputs. The impacting balls consume some energy upon rebounding. For instance, balls falling through larger heights can carry with them 15% of the input energy during rebound. Another important aspect learned in this project, for which direct evidence is not shown in this series of experiments conducted for WPRI, is that the conversion of input energy into particle breakage is higher at low input energy. This means that balls falling through small heights, in the range of 5-20 cm, are very efficient in breaking particles. In the mill, particles break under compressive stress. Shearing stresses only help the particles to escape from the zone of breakage and may somewhat reduce their apparent strength. Therefore, a combination of compressive and shear stresses is ideal for breakage of particle assemblies.
There are two key features to be considered for any comminution device. First, what is the force or stress, produced by impacting surfaces, and second, what is the size distribution of fragments produced by the applied stress? This report details clearly the distribution of fragments produced upon application of a certain energy. What is needed is a study of forces produced by mass of colliding bodies in the mill. In the case of ball mills, the “lifter” lifts the ball, and as a result the falling ball exerts a force on the bed of particles. In the case of attrition mills, the impeller stirs the mass of ball, which in turn generates a distribution of forces within the ball mass, A study of the forces produced within the mills would complement the work reported here.