The sequential elutriation behavior of wide particle size distributions

Abstract

Batch elutriation of a metallurgical-grade silicon powder with a wide particle size distribution in a laboratory scale fluidized bed was studied, highlighting the influence of carryover polydispersity. The smallest elutriable fines, namely superfines (<10μm), whose terminal velocity Ut is far lower than the superficial gas velocity Ug are entrained first, while the largest elutriable particles (Ut≈Ug) begin to be entrained with a delay that is as long as the time required for the superfines to leave the bed, thus inducing sequential elutriation. When no superfines were present, the entrainment was not delayed. This peculiar phenomenon was observed at all of the tested gas velocities (0.05–0.2ms−1) and for different wide particle size distributions belonging overall to the Geldart group A. The superfines thus seem to strongly limit the elutriation of the larger elutriable particles. In addition, the elutriation rate constants were found to increase with increasing superficial gas velocity and with decreasing particle size. When superfines were present, the elutriation rate constant leveled off under a critical size. Increasing the superfine particle content appears to reduce the elutriation rate constant of all of the elutriable particles. These phenomena are related to interparticle interactions within the bed and/or the freeboard and confirm the importance of polydispersity in the elutriation behavior.