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Roll-Type Versus Free-Fall Electrostatic Separation of Tribocharged Plastic Particles

Tribocharged mixed granular materials are commonly separated in the electrostatic field generated between two vertical plate electrodes. However, the existing technologies are not appropriate in sorting coarse granules (size > 2 mm) in which the effect of gravity is stronger than the Coulomb forces. The aim of the experiments reported in this paper is to show that particle sorting is more efficient when done in a roll-type electrostatic separator. The granular materials to be separated, which are a mixture of 50% low-density polyethylene (LDPE) and 50% high-density polyethylene (HDPE), were charged by particle-to-particle and particle-to-wall collisions taking place inside the rotating tube of a custom-designed tribocharging device. The study was performed with charging tubes made of the following four different materials: steel, aluminum, polyvinyl chloride, and polyethylene. Under optimal operating conditions, more than 97% of the LDPE (purity is 97.83%) and 92% of the HDPE (purity is 92.45%) were recovered in the collector of the roll-type electrostatic separator, as compared to only about 60% and 75%, respectively, in the case of a standard free-fall separator.