Free-fall Electrostatic Separator Research Articles

Modified electrode configuration of the free-fall tribo-electrostatic separator for the recovery of polyethylene and polypropylene flakes from shredded packaging waste

Free-fall tribo-electrostatic separators have long been used for sorting granular plastic wastes. The aim of this paper is to propose a modified configuration of the electrode system of such a separator and validate its effectiveness for processing a mixture of polyethylene (PE) and polypropylene (PP) flakes originating from the processing of packaging waste at GRANUPLAST Company, Jassans-Riottier, France. The mixture to be treated is not homogeneous and consisted of roughly 2/3 PE and 1/3 PP. The granular mixture samples were charged by triboelectric effect in a fluidized bed device, and then introduced into a laboratory-scale free-fall electrostatic separator. Preliminary tests were conducted to ascertain the mass of the mixture to be introduced in the fluidized bed and the duration of the charging process. Better results were obtained with the modified electrode configuration of the free-fall electrostatic separator. Thus, from 100 g of mixed flakes, 98.2% of PE and 96.6% of PP were recovered with purities of 99.1% and 98%, respectively. In addition, numerical modeling of the PP and PE particles trajectory was performed to simulate the behavior of charged particles in the standard and modified electrode configurations. Work is in progress for the industrial implementation of this tribo- electrostatic separation technology.

Electrostatic separation of HIPS/ABS and HIPS/ABS-PC plastic mixtures from IT equipment using fluidized bed tribocharging

The paper presents an experimental study of the triboelectrostatic separation feasibility of two plastic mixtures, high-impact polystyrene (HIPS)/acrylonitrile butadiene styrene (ABS) and high-impact polystyrene (HIPS)/acrylonitrile butadiene styrene-polycarbonate (ABS-PC). To mirror real-world material recovery situations, the mixtures of 1-2 mm granular samples were sourced from Information Technology (IT) waste. In addition to balanced composition mixtures of 50% component A and 50% component B, heavily unbalanced compositions such as 10% A/90% B and 90% A/10% B were used in this study. The laboratory set-up consists in a free fall electrostatic separator equipped with a fluidized bed tribocharging device, characterized by interchangeable tribocharging chambers. Preliminary tests established the optimal material for the tribocharging chamber and the duration of the charging process. Samples of granular mixture were tribocharged, then introduced into the free-fall electrostatic separator. The analysis of the seven collecting boxes contents shows a recovery rate of approximately 90% and a purity of about 99% for both components. These results demonstrate the feasibility and the high efficiency of the triboelectrostatic separation of balanced and as well as heavily unbalanced composition mixtures of HIPS/ABS and HIPS/ABS-PC, using the fluidized bed for the tribocharging device.

Numerical simulation of the behavior of insulating particles in a free fall tribo-electrostatic separator with four vertical cylindrical electrodes

This paper presents an analysis of the trajectories of tribocharged ABS and PVC particles in a free fall electrostatic separator. The installation that is the subject of this study is equipped with four vertical cylindrical electrodes. The behavior of the particles in this installation is compared with the one in a conventional configuration represented by two plate electrodes. The trajectory of the particles in both configurations is obtained by the numerical resolution of Newton's equation, which takes into account the magnitude and the direction of the electrostatic, gravitational and aerodynamic forces applied to the particles in the electrostatic separation zone. The calculation of the electrostatic force applied to the particles in this zone is performed using a numerical model of the electrostatic field obtained by finite element method implemented by means of the PDE-tool library of MATLAB software. The results of this study show that the use of a four-electrode configuration improves the quality of recovered products as it reduces the nuisible effect of the impacts between particles and electrodes observed in conventional plate-electrode installation. The use of the new configuration requires the development of a collector the shape of which should be adapted to the spatial distribution of the particles recovered at the outlet of the installation.

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

ABSTRACTThe 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.

Modified tribo-charging device for the electrostatic separation of plastics from granular industrial wastes

ABSTRACTTribo-electrostatic separation is the solution of choice for the selective sorting of various plastics contained in a granular mixture. This work is aimed at validating the efficiency of a modified compressed-air-operated tribo-charging device for the electrostatic separation of mixed granular plastics originating from industrial waste. The experiments are carried out with a mixture of mm-size orange and beige PVC granules, using a free-fall electrostatic separator. At optimum operating conditions, 93% of the orange PVC and 96% of the beige PVC were recovered at purities higher than, respectively, 99% and 95%. The numerical simulation of charged particles trajectories facilitates the interpretation of the experimental results and the analysis of the factors that affect the efficiency of the separation process: high-voltage level, configuration of the electrodes system, and position of the feeding unit.

Factors That Influence the Efficiency of a Propeller-Type Tribocharging Device for Granular Plastics

This work is focused on the experimental study of a new propeller-type tribocharging device specifically designed to operate in conjunction with an electrostatic separator for mixtures of granular insulating materials. The device consists of two coaxial aluminum propellers rotating in a polyvinyl chloride (PVC) pipe; it has two control variables: the material feed-rate and the rotation speed of the propellers. Virtual instrumentation using LABVIEW software is used for measuring the mass and the electric charge of the tribocharged samples of four types of polymers: polypropylene (PP); high impact polystyrene (HIPS); high-density polyethylene (HDPE); and PVC. In accordance with the sign and the magnitude of the charge/mass ratio, the four polymers are arranged in a triboelectric series, starting from the negative polarity: PVC—HDPE—PP—HIPS. The effectiveness of the tribocharging process is validated by using a free fall electrostatic separator. The best results (purities and recoveries higher than 90%) are obtained in the case of PVC/HIPS granular mixtures.

Study of charged particles trajectories in free-fall electrostatic separators

The objective of this paper is to develop a mathematical model for computing the millimeter-sized particles trajectories in the free-fall electrostatic separator. The simulation involves the numerical solution of motion equations of the particles subjected to electric and gravitational forces. The air-drag force and the impact of the particles with the electrodes were also considered. The resolution algorithm is implemented as a MATLAB program that uses the results of electric field computation performed with COMSOL software. The model obtained is used to study the factors influencing the quality of the products recovered at the outlet of the separator.

Triboelectrification of granular insulating materials as affected by dielectric barrier discharge (DBD) treatment

The aim of this paper is to point out the influence of dielectric barrier discharge treatment on tribocharging of granular insulating materials. Particles of Polyvinyl Chloride (PVC) and Polypropylene (PP) were subjected to an AC dielectric barrier discharge (DBD) plasma treatment in ambient airprior to tribocharging in a vibratory device. The charge to mass ratio was measured for treated and untreated materials. Electrostatic separation of a mixture of granular materials (PVC and PP) to measure the effectiveness of DBD treatment was evaluated by processing treated and untreated PVC/PP granular mixtures in a free-fall electrostatic separator. The obtained results clearly indicate that DBD has the capability to influence surface charging proprieties of polymer granular materials. In case of short treatment time, typically less than 3 s, a marked increase in the charge to mass ratios was observed for both PVC (about 35%) and PP (roughly 45%). In the same way, the quantity of DBD-treated materials, recovered after electrostatic separation, was increased by about 104% and 30% for PVC and PP, respectively, as compared to untreated case. The DBD treatment time is a key factor to increase the tribo electric effect.

Set-Point Identification and Robustness Testing of a Triboelectrostatic Separation Process for Mixed Granular Plastics

The vibratory feeders had already been proven to be efficient devices for tribocharging granular plastics in view of their separation in high-intensity electric fields. In this paper, the vibratory feeder was used as a tribocharger operating with a free-fall electrostatic separator. There are two aims at embarking on the study: (1) formulate recommendations regarding the optimum values of three control variables of the separator; and (2) assess the robustness of the triboelectrostatic separation. The 50%-ABS-50%-HIPS mixture of 4-mm-size granules employed in the experiments is typical for the recycling of plastics from waste electrical and electronic equipment. The investigation was conducted according to a composite factorial experimental design, and it revealed that the separation efficiency significantly depends on the high-voltage applied to the electrode system and on the position of the splitters that define the geometry of the product collector, but is less affected by the material transport velocity on the tray of the vibratory feeder. Using Taguchi's methodology, it was possible to predict the most robust process design, with respect to the uncontrolled variation of granule size and ABS content. The former of these “noise” factors has a significant effect on the outcome of the process, whereas the latter has little influence on separation efficiency.

Experimental Analysis of a Vibrating Tribocharging Device of Plastic Particles for a Free-Fall Electrostatic Separator

Experimental Analysis of a Vibrating Tribocharging Device of Plastic Particles for a Free-Fall Electrostatic Separator

Experimental and Numerical Modeling of a New Tribo-Electrostatic Separation Process for Granular Plastics Mixtures

The paper aims at optimizing the operation of a free-fall electrostatic separator equipped with a novel tribo-charging device for the processing of granular plastic mixtures. Experiments were performed with a mixture of polycarbonate and polyamide granules (cylinders of 1.5 mm in diameter and 3 mm in length). The samples of granules to be charged were introduced in the gap between the two co-axial cylinders of the tribo-charging device, and were put into turbulent motion for 30 s by two oppositely oriented air jets (pressure: 2 bar). Then, the charged particles were left to fall freely between the vertical plate electrodes of an electrostatic separator. A composite experimental design enabled the modeling and optimization of the tribo-electrostatic separation process as a function of the following control factors: applied high voltage, inter-electrode distance, and the inclination angle of the feeding device. Under optimal conditions, the purity obtained is close to 99%. A numerical model of particle trajectories confirms the conclusions of the experimental study. The simulations point out the effect of each control variable on the outcome of the separation process.

Triboelectrification of Granular Plastic Wastes in Vibrated Zigzag-Shaped Square Pipes in View of Electrostatic Separation

This paper presents an original device for the electrification of granular mixtures in vibrated zigzag-shaped pipes. Spatial movement of the granules introduced in the pipes is controlled by varying the oscillation amplitude and the frequency of a slider-crank mechanism. In the first set of experiments, two sorts of granular plastics (ABS and HIPS) were separately processed through the vibratory tribocharging device. Both ABS and HIPS charged negatively in contact with the aluminum pipes. The absolute value of charge/mass ratio increased with the amplitude and frequency of the vibratory movements to attain a maximum of 26 nC/g for the HIPS particles. In the second set of experiments, 100-g samples of 50% ABS + 50% HIPS were tribocharged, than introduced in a freefall electrostatic separator. A composite experimental design was performed for modeling the process. The output variable was the extraction of ABS while the speed and length of the crank were with the applied voltage the three control variables under investigation. ABS extractions higher than 85% were obtained for optimally chosen values of the control variables.

Séparateurs électrostatiques des particules. Application à la récupération des déchets industriels

Electrostatic separation is a generic term given to a significant class of technologies of industrial waste processing, widely used for the sorting of granular mixtures due to electric forces acting on particles whose average size is approximately 5 mm. The paper describes three electrostatic processes of separation depending on the granular mixture: 1) role-type electrostatic separator: used to sort mixtures of metal/plastic particles (copper/PVC for example); 2) plate-type electrostatic separator: used to sort mixtures of metal/metal particles (copperllead for example); 3) free fall electrostatic separator: used to sort mixtures of plastic/plastic particles (PVC/PE for example).

Electrostatic separators of particles: Application to plastic/metal, metal/metal and plastic/plastic mixtures

Electrostatic separation is a generic term given to a significant class of technologies of industrial waste processing, widely used for the sorting of granular mixtures due to electric forces acting on particles whose average size is approximately 5mm. The focus of this paper is on three electrostatic processes of separation used for processing of different types of mixtures: (i) role-type electrostatic separator, used to sort mixtures containing metal/plastic particles (copper/PVC for example); (ii) plate-type electrostatic separator, used to sort mixtures containing metal/metal particles (copper/lead for example) and (iii) free-fall electrostatic separator, used to sort mixtures of plastic/plastic particles (PVC/PE for example). Experiments carried out on industrial samples using laboratory electrostatic separators confirm the efficiency of these processes and show that the processes can improve the recovery and purity of products resulting from industrial wastes.

Portable Free-Fall Electrostatic Separator for Beneficiation of Charged Particulate Materials

A portable free-fall electrostatic separator capable of analyzing gram quantities of charged powders is presented. Unlike a Faraday pail, in which only the net average charge-to-mass (Q/M) ratio of the particles sampled by the instrument is measured, an electrostatic separator is capable of separately measuring the charge-to-mass ratios of positively and negatively charged sampled powders. Thus, with an electrostatic separator it is possible to measure the mass fractions of powders that are charged with different polarities and the respective charge-to-mass ratios, along with the mass fraction of particles that are uncharged or charged below a threshold level. We describe a method of measuring the total charge of the collected particles in real time by incorporating an electrometer to integrate the current flowing through the collecting electrode to the high voltage power supply. In this manner, both the total charge and total mass of powder deposited on the two electrodes are measured in near real time, providing information on charge-to-mass ratio of the aerosol cloud sampled. Such real time measurements are often needed to analyze the electrostatic charging properties of small quantities of dispersed powder, particularly in such applications where the charge characteristics are of high importance.

Controlling Particle Trajectory in Free-Fall Electrostatic Separators

The purity and recovery of concentrates obtained in industrial free-fall electrostatic separators can be increased by preventing the impacts between the particles and the electrodes. The aim of this paper is to analyze the possibility to control particle trajectories in such separators by modifying the conditions of particle admission in the interelectrode space. The parametric equations of a charged particle trajectory in the electric field between the electrodes of a free-fall separator served for writing a numerical modeling program in MATLAB. The ten control factors of the free-fall electrostatic separation process were employed as input variables of this program: particle charge and dimension, trajectory start point coordinates, feed input angle and initial velocity, electrode length and inclination, interelectrode spacing, and applied high voltage. A custom-designed laboratory free-fall electrostatic separator (length of the electrodes: 1000 mm; standard interelectrode spacing: 300 mm; and nominal high voltage: 90 kV), provided with a fluidized-bed tribocharger, was employed for validating the conclusions of the numerical modeling. The geometrical data of this separator were taken into account for computing the initial velocity nu0 of the particles entering the electric field zone. Another important parameter of the numerical model, which is the granule charge q, was attributed the value measured at the exit of the fluidized-bed tribocharger. The numerical simulations were performed for the two values of the feed input angle: alpha = 4deg and alpha = 8deg, considering polyethylene therephtalate (PET) and polyvinyl chloride (PVC) particles of different sizes. A good agreement was found between the theoretical predictions and the results of the experiments carried out with two binary mixtures: 50% PET/50% PVC and 10% PET/90% PVC. Both the numerical modeling and the experimental study demonstrated that the feed input angle a influences the outcome of the electrostatic separation to a great extent.

Fluidized Bed Device for Plastic Granules Triboelectrification

The study of the triboelectrification process is the key factor to the successful industrial application of electrostatic separation in the field of plastics recycling. Therefore, the aim of the present work is to evaluate an original device for the laboratory study of this process. The original design of the fluidized bed type triboelectrification device has the following distinctive features: (i) transparent walls to observe the fluidization process and the particles moving in the triboelectrification module; (ii) air distributors with different mesh apertures correlated with granule size; (iii) quick replacement of the triboelectrification Plexiglas chamber with other metallic (aluminium, copper) or nonmetallic chambers (polyvinyl chloride—PVC, polyethylene—PE, and polyethylene terephthalate—PET); (iv) possibility of independent operation, for granule charge measurement or integrated operation, for the electrostatic separation experiments using a free-fall electrostatic separator. The results of charge measurement experiments pointed out the different triboelectric behaviour of two types of granules (polystyrene and low-density polyethylene) in contact with the walls of the device. The electrostatic separation experiments carried out on a binary mixture of mm-size polystyrene/polyethylene granules confirmed the effectiveness of the triboelectrification device.

Changing potential for the electrical beneficiation of minerals by chemical pretreatment

The responses of calcite, feldspar and quartz minerals in a laboratory free-fall electrostatic separator after tribocharging with stainless steel, PVC and copper plates were investigated. The effects of H3BO3 and BaCl2 chemical pretreatment on the electrical properties of the above minerals were examined. The changes in the electrical properties are discussed along with the determined charge, dielectric constant and electrical conductivity of the mineral samples. The electrical properties of the minerals and their responses to the electrical field during separation after tribocharging and chemical pretreatment are explored relative to the electrical behavior of the minerals.