Effects of Diffusional Particle Motion on Size Classification in a Spiral Jet Mill

Classification in spiral jet mills, which are widelyused in industryfor fine grinding of particles, is a result of particles smaller thanthe cut size experiencing a larger drag force relative to the centrifugalforce and exiting the mill in the product stream. High jet velocities,gas turbulence, and interparticle collisions may result in particleslarger than the cut size exiting with the product stream due to diffusionalmotion. In this work, we experimentally study such a diffusion fluxby considering particles significantly larger than the cut size andoperating under conditions where particle breakage is negligible.The effect of the classifier tube height and the sizes of the particlesfor pure components and binary and ternary mixtures on the mass fluxand composition of the product stream is reported. The specific massfluxes of the particles exiting the mill decrease with increasingparticle size for the pure components, and the fluxes of the smallerparticles in the mixtures are higher than those of the pure componentsdue to the presence of larger particles. A model for the flux is usedto estimate the effective diffusivity from the measured fluxes.