Effect of ambient humidity on the outcome of electrostatic separation process for mixed granular polymers

Recycling of plastics from industrial wastes requires appropriate sorting technologies, such as the free-fall electrostatic separation of tribocharged granular materials. Taguchi´s methodology has already been aplplied to the robust design of an electrostatic separation processes the outcome of which was affected by two hard-to-control “noise factor”: granule size and composition of the materials to be selectively sorted.The aim of this paper was to introduce an additional “noise factor”: the relative humidity of ambient air for the separation of a mixture composed of Acrylonitrile Butadiene Styrene (ABS) and High Impact Polystyrene (HIPS), originating from shredded obsolete computer cases. The objective was to reduce variation in the process outcome by finding operating conditions (pre-charging duration, speed of the fluidization air, high voltage), under which uncontrollable variation in “noise factor” (ambient humidity, the masses of ABS, HIPSand middling collected after separation) has minimal impact on the quantity (mass of middling fraction) and the quality (purity of The HIPS fraction) of the recovered products. The experiments were carried out on a tribo-aero-electrostatic separator. The samples of processed material, contained various proportions of ABS and HIPS. The granule size for both materials is between 1 mm and 3 mm. The ambient relative humidity varied between 40 and 70%, at a temperature of 18 ± 2C°. The design and noise factors were first analyzed using a classical Taguchi approach, then combined into one single experimental design, so that a regression model of the process be fitted. Several additional experiments at lower (30%) or higher (70%) relative humidity have been performed, to assess the feasibility of electrostatic separation in less standard environmental conditions. The conditions of industry application of Taguchi´s methodology are discussed.