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Mixed granular insulating materials can be electrostatically separated by tribocharging them in a fluidized bed affected by an electric field orthogonally oriented to the direction of the fluidization air. The aim of this paper is to optimize the operating conditions of a novel pilot-scale tribo-aero-electrostatic separator, i.e., maximize the total mass of the granules collected at the two electrodes that generate the electric field. The mixture to be separated was composed of acrylonitrile butadiene styrene (ABS) and high impact polystyrene (HIPS) originating from the recycling of waste electric and electronic equipment. A set of experiments carried out for two durations (30 and 60 s), at two values of ambient relative humidity ( and ), pointed out that the separation becomes efficient beyond a certain value of the initial mass of the mixtures. The experimental design methodology and commercial software MODDE 5.0 were then employed for investigating the effects of two control variables of the industrial process: the speed of the metallic conveyors that serve as high-voltage electrodes and the humidity of ambient air. The effectiveness of separation was found to depend significantly on the latter factor. Surface-potential-decay measurements performed on charged layers of the two sorts of particles facilitate the interpretation of separation results: At high , in contact with an electrode, the ABS granules lose their charge faster than the less hydrophilic HIPS granules. As a consequence, fewer ABS granules are collected at the exit of the tribo-aero-electrostatic separator.