Set-Point Identification and Robustness Testing of a Free-Fall Triboelectrostatic Separation Process

Identification of the optimal operating conditions and evaluation of their sensibility to changes in certain critical factors are critical issues for the industrial application of electrostatic separation techniques. The authors have already elaborated a procedure for set-point identification and robustness testing of a roll-type corona-electrostatic separation process. The aim of this paper is to validate a similar procedure for optimizing the selective sorting of nonconductive constituents of granular industrial plastic wastes using a free-fall triboelectrostatic separator. Samples of chopped plastic wastes, containing 50% polyethylene (PE) and 50% polyvinyl chloride (PVC) were processed in a custom-designed tribocharging device: a 10 cm diameter, 120 cm long PVC cylinder, rotating at a variable speed. The charged granules were then separated in the electrical field generated by two vertical plate electrodes, and collected in three compartments: PE, Middling and PVC. The controlled process variables were the speed of the PW cylinder, the duration of the tribocharging process and the mass of the sample. Several one- factor-at-a-time experiments, followed by two factorial designs (one composite, the other fractional), were performed based on the following three-step strategy: 1) identifying the domain of variation of the controlled variables; 2) finding the best choice of the set point; 3) assessing the robustness of the process, i.e., testing whether the performance of the system remains satisfactory even when the factors vary slightly around that point.