Amar Tilmatine

Experimental Study of the Optimum Operating Conditions of a Pilot-Scale Tribo-Aero-Electrostatic Separator for Mixed Granular Solids

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.

A linear-interaction model for electrostatic separation processes

Previous studies based on Box-Wilson and response surface methods have proven that the design of experiments is a powerful tool in improving electrostatic separation performances by controlling the two main parameters of the process: the high-voltage level and the roll speed. The aim of the present paper is to analyze the possibility of deriving a mathematical model capable to reflect the effects of a larger number of factors, as well as their main interactions. At first, the main variables of the process were listed and classified in accordance with the ease of controlling them. Then two experimental designs were chosen, in accordance with Taguchis methodology. The objective was to minimize the middling fraction. The experiments were carried out on a laboratory roll-type electrostatic separator, provided with a corona electrode and a tubular electrode, both connected to a DC high-voltage supply. The samples of processed material were prepared from genuine chopped electric wire wastes (granule size >1 mm and <2 mm) containing 25% copper and 75% PVC. The first experiment consisted of 16 tests, which enabled the derivation of a linear-interaction model comprising 7 variables and 8 interactions. The second experiment consisted of only 8 tests, as the corresponding model took into consideration 4 variables and 3 interactions. A good agreement was obtained between the two models.

Capability evaluation and statistic control of electrostatic separation processes

The variability of the outcome is often deplored by the users of electrostatic separation equipment. This paper is written with the aim of showing how statistical process control methods could contribute to the improvement of users overall satisfaction. In order to validate a procedure for evaluating the short and long term capability of the electrostatic separation process, the authors carried out a series of experiments on subgroups of chopped electric cable wastes, similar to those currently processed in the recycling industry. The first set of experimental data enabled the computation of the upper and lower limits of two types of control charts. The second set of experiments was designed to evaluate the ability of these charts to detect typical situations of abnormal operation of the electrostatic separation equipment. Several recommendations have been formulated based on authors experience as consultants for the recycling industry.

Electrostatic separation processes

Optimization of electrostatic separation processes demands the control of a multitude of factors, including the characteristics of the granular mixtures to be sorted, the feed rate, the configuration of the electrode system, the applied high-voltage and the environmental conditions. The Taguchis experimental designs presented in this article clearly prove that the linear-interaction models of the electrostatic separation processes can reflect the effects of the main factors in a manner that is satisfactory to most case of the practical interest. The Taguchis experimental designs are based on special matrices called orthogonal arrays.

Optimization of a Cost-Effective “Wire-Plate”-Type ESP for Installation in a Medical Waste Incinerator

Although research on electrostatic precipitators (ESPs) has studied and analyzed all the technical aspects of this device, their cost is, nowadays, the major constraint for manufacturers and users. Therefore, medical waste incinerators of hospitals in Algeria, as well as municipal waste incinerators, operate, in most cases, without flue gas filtration system due to the high costs that requires the installation of a cleaning system. This constraint is reflected in the Maghreb and African countries. The objective of this paper is to show that it is possible to build up a cost-efficient ESP, including a high-voltage power supply made up of a static converter and a step-up ferrite transformer. A home-made “wire-plate”-type half-scale prototype ESP was designed and served to optimize the filtration efficiency according to the interelectrode distance and to determine the optimum voltage to be applied without risk of breakdown.

Numerical simulation of tribo-aero-electrostatic separation of mixed granular solids

Numerical simulation can be a powerful tool in the research and development of any new electrostatic process. The aim of the present work is to introduce a simple mathematical model for simulating the outcome of a recently patented tribo-aero-electrostatic separation process for binary mixtures of granular materials. The process is characterized by the fact that the charging of the granules is produced in a fluidized bed device, in the presence of an electric field. The mathematical model assumed that the probability of a granule to be separated can be expressed as a function of the number of impacts with granules belonging to the other class of materials. When this probability was given by a normal distribution law, the results of the simulations differed significantly from those of an experiment conducted with a granular mixture of polyamide and polycarbonate particles, in a laboratory tribo-aero-electrostatic separator. In order to improve the predictive capability of the simulations, a polynomial function derived from the regression of the experimental data was employed for expressing the same probability. Thus, it was possible to calculate the evolution in time of the mass of granules separated at the electrodes for various compositions of the granular mixture. The computed results were in good agreement with the experiments.

Experimental study of the effect of ambient air humidity on the efficiency of tribo-aero-electrostatic separation of mixed granular solids

Mixed granular insulating materials can be electro-statically 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 the present paper is to evaluate the effect of ambient air humidity on the outcome of this tribo-aero-electrostatic separation process, i.e. the 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 s and 60 s), at two values of ambient relative humidity (RH = 20% and RH = 40%), 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 process variables: 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 higher RH, the hydrophilic ABS granules loose faster their charge when in contact with a grounded plate. As a consequence, they are in less number than the HIPS granules to be collected at the exit of the tribo-aero-electrostatic separation chamber.

Robust Design and Capability Evaluation of a Tribo-Aerodynamic Charging Process for Fine Particles

Robustness and capability are critical issues for the industrial application of any novel electrostatic process. The aim of the present work is double: - show how experimental design methodology can contribute to assessing the robustness of a given tribo-aerodynamic charging process; - validate a procedure for evaluating the capability of two tribo-aerodynamic chargers, with respect to the specific requirements of electrostatic separation applications. The results of three fractional factorial experimental designs performed on starch and flour powders, in two devices similar to the tribo-guns employed for electrostatic powder coating, demonstrate that the tribo-aerodynamic charging is robust with respect to the three main control variables of the process: the injection and vortex pressures of the air in the pneumatic circuit, and the material feed-rate. The capability indexes computed for both the straight-type and spiral-type tribo charges were satisfactory.