Karim Medles

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.

Tribo-aero-electrostatic separator for coarse granular insulating materials

The tribo-aero-electrostatic separator achieves the selective sorting of mixed granular insulating materials 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 optimize the design and the operation of such a separator, i.e. maximize the total mass of the granules collected at the two electrodes that generate the electric field. The experiments were performed for four different electrode configurations. The granular mixtures to be separated were composed of Acrylonitrile Butadiene Styrene (ABS) and High Impact Polystyrene (HIPS) originating from the recycling of waste electric and electronic equipment (WEEE). The typical granule size was in the range 2 to 4.5 mm. The highest separation efficiency was obtained when the electric field in the active zone was intensified by the use of an additional electrode connected to the ground and when the collecting electrodes were covered by a thin insulating layer, preventing charge leakage from the particles in contact with them.

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.

Factors that affect tribocharging of PE granules after exposure to a dielectric barrier discharge (DBD)

Triboelectric charging is used in many applications including electricity nanogeneration, safety or electrostatic separation of insulating materials. Insulating granules charge must be the highest in electrostatic separators so that the electric field forces allow their separation. Dielectric Barrier Discharge (DBD), used for ozone production, depollution or surface treatment, is able to change the triboelectric properties of insulating materials. The aim of this study is to improve the triboelectric charging of Polyethylene (PE) particles by exposing them to an atmospheric Dielectric Barrier Discharge. Signal shape, exposure duration, voltage amplitude and frequency, which rule the DBD, are studied. After exposing the particles to DBD, they are charged in a vibratory device during 5 min. Then, particles are put in a Faraday pail connected to an electrostatic voltmeter to measure the acquired charge. Results show that the square waveform has the best effect and that the duration of the DBD treatment should not exceed 10 s, for the amplitude of the applied voltage of 21 kV.

Continuous operation of a fluidized-bed disk-type electrostatic separator for micronized plastic waste

The objective of this work is to highlight the peculiarities of the continuous operating regime of a new tribo-aero-electrostatic separator for the recycling of plastics contained in micronized waste electrical and electronic equipment. The experiments were carried out with an ABS/PS mixture. The particles are charged by triboelectric effect in a fluidized bed in the presence of an electric field created between two disk electrodes connected to high voltage supplies of opposite polarities. The installation described in this paper has several control variables: the high voltage, the mass of the mixture initially introduced into the fluidized bed, the flow of fluidizing air and the rotation speed of the disk electrodes. The influence of each control variable of the machine on the efficiency of the separation was studied by simultaneously and continuously measuring the mass and the electrical charge of the collected products. The quality of the products obtained and the high hourly rates recommend this type of installation for industrial use.

Tribo-electrostatic separation of a quaternary granular mixture of plastics

ABSTRACTMany tribo-electrostatic separation studies of binary mixtures of millimeter-sized plastic particles have been performed. The objective of this work is an experimental investigation for sep...

Premises for industrial application of a two-rotating-disks-type tribo-aero-electrostatic separator for micronized WEEE

Waste electric and electronic equipment (WEEE) recycling technologies often include electrostatic separation of mm-size granular plastics mixtures as a major operation. The aim of this work was to pave the way to industrial application of a new two-rotating-disks-type tribo-aero-electrostatic separator for finely-grinded matter (i.e. granule diameter < 1 mm). The experiments were carried out on binary granular mixtures composed of various proportions of Polypropylene (PP) and Polycarbonate (PC) of size 125 to 1000 microns. The charges and masses of the separated products were continuously and simultaneously recorded using a custom-designed “on-line” measurement system. The purity of the products was evaluated “off-line”, using a colorimetric system. It was thus possible to quantify the performances of the separator under various conditions that simulated industrial operation situations.

Experimental investigation of a roll-type tribo-electrostatic separator for granular waste plastics

The aim of the present study is to evaluate several possibilities of preconditioning a mm-size granular mixture of waste plastics, in view of its electrostatic separation. The experimental set-up was composed of an inclined-rotating-cylinder-type tribo-charging device associated with a roll-type electrostatic separator. The experiments were performed a mixture of mm-size PP and HIPS granules provided by a waste plastics recycle, provided by a waste plastics recycler. In distinct series of experiments, the genuine material was subjected to additional grinding, followed or not by charge neutralization. It was found that feeding the plastic mixture into the tribo-electrostatic separation unit right after grinding gives the best results: the purity and the recovery rates of the products exceed 96%.

Statistical process control of the tribocharging of polymer slabs in frictional sliding contact

Besides the nature and condition of the surfaces in contact, several factors influence the triboelectric charging of polymer slabs in frictional sliding contact: pressure load and relative velocity between the two bodies, number of friction cycles, ambient temperature and humidity. This paper is aimed at pointing out the peculiarities of the statistic control of such a process. Thirty experiments were performed for the optimal combination of factor values (i.e., normal force: 10 N; sliding speed: 55 mm/s; number of sliding cycles: 10) that maximize the absolute value of the average potential at the surface of the tribocharged materials: 1600 kV. The capability index calculated from the experimental data was satisfactory and two Shewart charts were established for the statistical control of the process. Two out-of-control situations were simulated, in order to test the efficiency of the charts.

Triboelectrostatic separation of granular plastics mixtures from waste electric and electronic equipment

ABSTRACT The efficiency of the electrostatic separation of insulating granular mixtures depends on the electric charge carried by the particles. The first objective of this work was to characterize the tribocharging properties of various plastic materials in the composition of waste electric and electronic equipment: polyvinyl chloride (PVC), “high-impact” polystyrene (HIPS), acrylonitrile butadiene styrene (ABS), “high-density” polyethylene (HDPE), polycarbonate (PC), polyamide (PA). Thus, in a first series of experiments, 15 samples composed of binary mixtures of above plastics were initially charged in a fluidized bed device, then separated using a free-fall electrostatic separator. The six plastics could be ordered in a specific triboelectric series: PVC, HIPS, ABS, PEHD, PC, PA. The second objective was to evaluate the efficiency of three tribocharging devices: a static charger, a fluidized bed, and a fan-type device. The triboelectrostatic separation experiments performed with four different binary mixtures (PVC/PA, PC/PA, HIPS/PA, and HIPS/ABS) showed that the fluidized bed is the most effective.