Set point identification and robustness testing of electrostatic separation processes

Identification of the optimal operating conditions and evaluation of their robustness are critical issues for the industrial application of the electrostatic separation techniques. In spite of extensive investigations performed during recent years, no standard procedure is available for guiding the research of the set point and for minimizing the process sensibility to changes in certain critical factors. The aim of the present work is to formulate a set of recommendations regarding the choice of high-voltage, roll-speed, and feed-rate values for an important class of electrostatic separation applications: the selective sorting of conductive and nonconductive constituents of granular industrial wastes. The experiments were carried out on a laboratory separator, built by one of the authors, with various samples of chopped wire wastes furnished by ENICAB, Biskra, Algeria. Several one-factor-at-a-time followed by two factorial designs (one composite, the other fractional) were performed, based on the following three-step strategy: (1) identify the domain of variation of the controlled variables; (2) found the best choice of the set point; (3) asses the robustness, i.e; test whether the performance of the system remain satisfactory even when the factors vary slightly around that point. The results presented in this paper are strictly valid only for a well-defined category of processed materials, but a similar approach could be adopted for a wider range of electrostatic separation applications.