Tribo-electrostatic separation of plastic flakes originating from packaging waste

Abstract

Free-fall tribo-electrostatic separators are commonly used in the recycling industry for sorting granular waste plastics but are much less effective in the case of flake-like materials of irregular shapes. This work was aimed at demonstrating that by modifying the design of a commercial rotating-cylinder-type tribocharger it is possible to expand the usage of such a separator to processing flakes originating from the shredding of packaging waste. The study was conducted on a mixture of flakes consisting of roughly 60 % PEHD (Polyethylene) and 40 % PP (Polypropylene), with an average size ranging from 8 mm to 10 mm. The plastic flakes were first introduced into the modified tribocharger, then transferred to a vibratory-plate conveyor that fed them as a mono-layer curtain at the upper end of an industrial free-fall electrostatic separator, equipped with two vertical plate electrodes (width: 150 cm; length: 200 cm), energized from two high-potential generators of opposing polarities (+60 kV, −100 kV). Under the action of the electric field, the respectively negatively- and positively-charged PEHD et PP flakes were sorted into distinct compartments. The modified design of the commercial tribocharger enabled the increase of the number and the frequency of the flake/wall and flake/flake collisions that ensure the electric charging. To investigate the impact of residence time, the experiment involved repeated passages of the material through the tribocharger. The effect of the high potential applied to the electrodes on the separation efficiency was also studied, and the results demonstrate the possibility to attain purity levels exceeding 98 %. Finally, the experiments conducted at three different humidity levels of the ambient air demonstrate that this factor has a detrimental effect on the outcome of the tribo-electrostatic separation.