Minority charge separation in falling particles with bipolar charge

Fluidization of polymer powders is used extensively in the powder coating industry. During the fluidization process the paint particles become electrically charged due to particle-particle and particle-wall interactions. In the course of this tribocharging, or friction charging process, it has been observed that a bipolarity of charged powder exists. The smaller, or finer, particles tribocharge oppositely to the larger size particles; thus, there exist both positively and negatively charged particles within the fluidized bed. A simple technique for charge measurement and separation has been developed whereby a small sample of bipolar charged powder taken from the fluidized bed is poured from a height and a separation occurs between the majority and minority particles of opposite charge. The amount of separation is a function of charge on the individual particles and the height at which the sample is poured. A computational model, based on techniques of molecular dynamics, has been developed to simulate the trajectories of individual charged particles when a sample of bipolar charged powder is poured. The model takes into account electrical, drag and gravitational forces acting on each particle and confirms the statistically analyzed experimental results. (C) 1998 Elsevier Science B.V. All rights reserved.