Influence of moisture content and triboelectric charging conditions on the tribo-electrostatic separation of actual granular mixtures of waste plastics

Tribo-electrostatic separation is a simple, cost-effective, and environmentally friendly method to sort the different plastics contained in the ever-increasing quantities of waste electrical and electronic equipment (WEEE) accumulated all over the world and needing recycling. Among the various plastics found in WEEE, Acrylonitrile Butadiene Styrene (ABS) and Polystyrene (PS) are the most prevalent due to their excellent insulating and mechanical properties, making them essential in the electrical and electronic equipment (EEE) industry. However, their similar surface properties and close mass density values significantly limit wet separation methods, such as froth flotation and sink-float methods. Tribo-electrostatic separation was chosen over wet methods because it is a dry process that requires no chemical pretreatment, no expensive wetting reagents, while also eliminating wastewater treatment issues. This method exploits the differences in triboelectric properties of ABS and PS, enabling their separation without altering their chemical structure, which is critical for preserving recyclability. The aim of the present study was to evaluate the effects of several factors (moisture content, particle size, triboelectric charging conditions) that affect the efficiency of a two-stage tribo-electrostatic process employed for processing a mixture consisting of roughly 55 % Acrylonitrile Butadiene Styrene (ABS), 38 % Polystyrene (PS), and 7 % of other granular waste plastics. Heating the mixture significantly reduced the moisture content of the particles, which not only enhances the triboelectric charging efficiency but also considerably improved the outcome of the free-fall electrostatic separation process. The duration of the triboelectric charging process in a rotating-cylinder type device and the configuration of the electrode system that generates the electric field of the electrostatic separator are two other factors that influence the efficiency of the process. After the first stage of electrostatic separation, 74 % of the ABS particles were recovered in a 94 % pure product. Using a two-stage separation scheme, it was possible to recover 82 % of the PS, with a purity of 88 %. A fluidized bed triboelectric charger was found to be more efficient than the rotating-cylinder-type device. Under optimal operating conditions, the purity and recovery of ABS product increased to 97 % and 95 %, respectively.