Set point identification and robustness testing of a triboelectrostatic separation process for mixed granular plastics

The vibratory feeders had already been proven to be efficient devices for tribocharging granular plastics in view of their separation in high-intensity electric fields. In the present work, the vibratory feeder was used as a tribocharger operating with a free fall electrostatic separator. The aim of the study was double: (1) formulate recommendations regarding the optimum values of three control variables of the separator, (2) assess the robustness of the triboelectrostatic separation process. The 50% ABS - 50% HIPS mixture of 4 mm size granules employed in the experiments is typical for the recycling of plastics from waste electrical and electronic equipment. The investigation was conducted according to a composite factorial experimental design and revealed that the separation efficiency significantly depends on the high-voltage applied to the electrode system, and on the position of the splitters that define the geometry of the product collector, but is less affected by the material transport velocity on the tray of the vibratory feeder. Using Taguchi's methodology, it was possible to predict the most robust process design, with respect to the uncontrolled variation of granule size and ABS content. The former of these “noise” factors has a significant effect on the outcome of the process, while the latter has little influence on separation efficiency.