Aerodynamic classification in a spiral jet mill

• We build the theoretical–numerical model of the classification process in a jet mill.
• The classification condition was found to be size and location dependent.
• The model was validated by experimental results.

In a classical jet-mill, the particles that are forced by air into a circular motion are colliding with other particles as well as with the walls. Such collisions might break the particles thereby reducing the particle size. The air enters the mill at the periphery and exits at the center. The centrifugal and the drag forces are acting simultaneously on each particle in the grinding chamber. These forces define classification phenomenon between finer and coarser particles. For fine particles, the drag force towards the mill exit is larger than the centrifugal force and particles will move out the mill chamber with the air stream. In this research, experiments to define the cut size of particles were conducted. We also tested a number of parameters, such as air flow rate, nozzle angle and nozzle diameter. The classification equation requires knowledge of the radial air velocity and the tangential particle velocity. An extensive numerical investigation led to usable equations that take into account not only all of the above parameters but also the chamber mill diameter, the grinding chamber height, the classifier height, the outlet diameter and the number of nozzles. Inserting the numerical correlations into the force balance resulted in a practical and accurate classification equation. A comparison of the experimental and calculated results showed good agreement for dilute flows.

This research describes an extensive numerical analysis in order to predict the product size of classification process in the jet mill.The figure presents the accumulated particle size distribution measurements for various mill loadings in comparison with the calculated cut size that is represented by a vertical bold line.Figure optionsDownload as PowerPoint slide