Shredding as simultaneous size-reduction and tribo-charging operation for improved performances of an electrostatic separation process for granular plastic wastes

Triboelectrostatic separation is a technology which allows the selective sorting of polymer materials based on their surface charging characteristics. Before they are subjected to electrostatic separation, the materials to be sorted are reduced to a proper size (typically less than 5 mm). The present paper aims to evaluate the possibility of using shredding as simultaneous size-reduction and tribo-charging operation and examine the effect of shredding and size reduction on triboelectric charging and electrostatic separation efficiency. The study was performed on typical pieces of PC-ABS and HIPS originating from waste electric and electronic equipment (WEEE). Experiments start by feeding equal quantities of plastic pieces into the shredder. The rotary cutters of the shredder stir the plastic pieces, a charge transfer occurs between the plastic particles by repeated collisions and rubbing against each other. After a certain time, a granular mixture was recovered from the shredder and immediately introduced in a roll type triboelectrostatic separator. The charge of the selectively sorted products was measured using a Faraday cage connected to an electrometer. Four screen mesh diameters were compared (2 mm, 3 mm, 5 mm, and 7 mm). The experimental results confirm that granules can get enough charge during shredding for being successfully sorted by the roll-type electrostatic separator. Particle size is definitively an important factor influencing the outcome of the triboelectrostatic separation. Fine shredding is not favorable, as it requires shredding for extended periods and favors the formation of particle agglomeration. High separation efficiency was obtained for relatively coarse particles.