Although jet milling is a very energy consuming grinding process it is increasingly used in industry because very fine grinding product with a narrow size distribution is attained without contamination as the milling occurs by inter particle collisions. At Delft University of Technology a project has been started to achieve a considerable energy reduction in jet milling processes. The grinding plant consists of a spiral jet mill in closed loop with an external classifier. Main feature of the system will be an operation control based on in-line particle size measurements using laser diffraction. In industrial practice the operating conditions are often determined by trial and error. To avoid off-spec material the mill is often tuned to lower risk. This results in a relatively large amount of overground material. With respect to the control strategy, the first step was real time particle size monitoring to explore the operating ranges of the jet mill. The controlability is studied in relation to several process inputs and process configurations. A dynamic model of the closed loop grinding plant is developed. Particle transport and size reduction inside the mill show a stochastic behaviour and are described by size and state dependent probability functions. Separate experiments are carried out to derive equations for the breakage kinetics of different materials. Numerical flow simulations are carried out to provide statistic data about the frequency and intensity of collisions between particles in relation to state conditions in the mill. A glass bottom plate will be placed on the mill to observe flow patterns. The influence of several process input variables on the dynamics of the grinding plant and the final product are simulated. Pilot plant experiments are carried out to verify and optimize the dynamic model by direct measurement of the PSD under actual system conditions. The ultimate objective of the dynamic model will be the implementation in a control system. The required setpoints of the millrclassifier system are predicted to obtain the desired product quality at minimum energy use. Furthermore the model can be used for scale up and plant design. q

This paper gives a theoretical basis for calculating Fraunhofer diffraction patterns of arbitrary polyhedron particles. It is shown that this solution can be used for calculating a scatter matrix adapted to the particle shape in a straightforward manner. Some simulations were made to show the difference between the size distribution by volume obtained with a scatter matrix for spheres and with that for the appropriate shape. Finally, some experimentally measured signatures from platelets and rods are evaluated in order to show that the spherical equivalent diameter could be accurately retrieved.

Forward light scattering is a well established technique for measuring particle size distributions. The light intensity fluctuations which can be observed in the diffraction plane of the instrument can be used to stabilize the inversion process [1]. Particle shape information is also present in these fluctuations. It is shown that an azimuthal‐type of detector can be used to extract this information from the statistical correlations of the detector signals.

Although jetmilling is a very energy consuming grinding process it is increasingly used in industry because very fine grinding product with a narrow size distribution is attained without contamination as the milling occurs by inter particle collisions. At Delft University a project has been started to achieve a considerable energy reduction of the milling process. Main feature of the improved grinding system will be an operation control based on in-line particle size measurement using the laser diffraction technique. The pilot plant is equipped with an external classifier for the removal and recirculation of larger particles leaving the jet mill.Phenomena occurring in the mill are not yet well modeled and thus no real optimization of the grinding process can be achieved. Operation conditions of jet mills are still determined by trial and error.This paper is to form the basis of a model which allows a dynamic modeling of the behavior of the jet mill/classifier system. The closed loop grinding system has been split in 3 subsystems (zones), two internal to the mill and one external through which the particles pass. Exchange of particles is not described by complicated residence time distribution functions but by "particle size" dependent probability functions.

No abstract