Effect of ambient humidity on the outcome of electrostatic separation processes

Taguchi\´s methodology has already been applied to the robust design of an electrostatic separation processes the outcome of which was affected by two hard-to-control "noise factors": granule size and composition of the materials to be selectively sorted. The aim of the present work was to expand that investigation to an additional "noise factor": the relative humidity of ambient air. The objective was to reduce variation in the process outcome by finding operating conditions (high-voltage level, roll speed, splitter position), under which uncontrollable variation in "noise factors" (ambient humidity, copper content in the feed) has minimal impact on the quantity (mass of middling fraction) and the quality (purity of the copper fraction) of the recovered products. The experiments were carried out on a laboratory roll-type electrostatic separator. The samples of processed material were prepared from genuine chopped electric wire wastes (granule size > 1mm and < 2 mm) containing various proportions of copper (5%, 25%, and 45 %) and PVC. The ambient relative humidity varied between 40 and 60%, at a temperature of 20 ± 1°C. 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 asses the feasibility of electrostatic separation in less standard environmental conditions. The conditions of industry application of Taguch\´s methodology are discussed.