Corona-electrostatic Separators Research Articles

Belt-Type Corona-Electrostatic Separator for the Recovery of Conductive and Nonconductive Products From Micronized Wastes

The commercial roll-type corona-electrostatic separators, which are currently employed for the recovery of metals and plastics from millimeter-size granular mixtures, are inappropriate for the processing of finely grinded wastes. The aim of this paper is to demonstrate that a belt-type corona-electrostatic separator could be an appropriate solution for the selective sorting of conductive and nonconductive products contained in micronized wastes. The experiments are carried out on a laboratory-scale multifunctional electrostatic separator designed by the authors. The corona discharge is generated between a wire-type dual electrode and the surface of the metal belt conveyor. The distances between the wire and the belt and the applied voltage are adjusted to values that permit particles charging without having an electric wind that puts them into motion on the surface of the belt. The separation is performed in the electric field generated between a high-voltage roll-type electrode (diameter 30 mm) and the grounded belt electrode. The study is conducted according to the experimental design methodology, to enable the evaluation of the effects of the various factors that affect the efficiency of the separation: position of the roll-type electrode and applied high voltage. The conclusions of this study will serve at the optimum design of an industrial belt-type corona-electrostatic separator for the recycling of metals and plastics from the waste electric and electronic equipment.

Experimental study of a modified dual-type high-voltage electrode for electrostatic separation applications

Several electrode arrangements have been proposed to enhance the efficiency of insulating materials charging by corona discharge. Recent studies pointed out that the presence of metal strips in the proximity of a dual-type high voltage electrode increases the total current measured at the surface of the collecting electrode, decreases the corona onset voltage value and enlarges the reparation of current density as well. The aim of this paper is to evaluate the benefits of using such an electrode arrangement for corona charging of non-conductive particulate materials in belt-type corona-electrostatic separators. The experimental study was carried out with samples of Aluminum and Polystyrene particles in the size class 125–250 μm. The presence of grounded strips reduces the electric wind, which is associated to corona discharge but not tolerated in such processes that involve micronized materials. At the same time, it improves the corona charging conditions of non-conductive materials and as consequence the overall efficiency of the corona-electrostatic separation process. The use of the new electrode configuration is characterized by both high recovery rates and better purities of the separated products.

Numerical and Experimental Study of Insulating Particles Behavior in Roll-Type Corona-Electrostatic Separators

The efficiency of the electrostatic separation process depends on the accurate control of particle trajectories. The aim of this article is to point out the role of the following factors that might affect the behavior of the insulating particles in roll-type corona-electrostatic separators: i) electrode system configuration, ii) applied high voltage, iii) roll speed, iv) particle size, and v) relative humidity of ambient air. The numerical simulation of particle trajectories, based on the balance equation of the electrical and mechanical forces that act on charged insulating particles, took into account both the charging and discharging phenomena. Pauthenier's equation for spheres in a uniform electric field served as the model for particle charging. The equation of particle discharging was established after the experimental study of the potential decay at the surface of a layer of millimeter size PVC granules in contact with an electrode. Samples of the same granular material were then processed in a laboratory roll-type corona-electrostatic separator. The analysis of the products collected after passage through the electric field of the separator validated the accuracy of the numerical simulations.

Effect of spark discharges on the trajectories of insulating particles in roll-type corona-electrostatic separators. Experimental and numerical study

Corona-electrostatic separation is a multi-variable process that has been thoroughly studied in connection with its various applications in the recycling industry. The aim of the present paper is to point out one parasitic phenomenon that adversely affects the efficiency of the separation: the sparks generated at the passage of conductive particles through the electric field zone. The experiments were carried out on a laboratory roll-type corona electrostatic separators, and the sparks were generated by introducing 16 calibrated copper pins in 40-g samples of granular insulating material (PVC; typical granule size: 1.5 mm) that were fed at a constant rate onto the surface of the grounded rotating roll electrode. The distribution of the PVC granules in the 14 boxes of the collector was altered by the occurrence of the spark discharges, as they were accompanied by the annealing of the electric field between the electrodes. The numerical simulation of insulating granules charging and movement under the action of the electric field enabled a better understanding of the interactions between the spark discharges and the other factors that influence their trajectories and affect the efficiency of the separation: roll-speed, particle size and ambient humidity. The particle dynamics equations were solved using an iterative scheme by using the electric field calculated in any point with the commercial software TRICOMP. The good agreement between the predictions made by these simulations and the experimental findings confirms the ability of the mathematical model to reflect the complexity of the physical phenomena. ► Sparks occur between the electrodes at the passage of conductive particles. ► Sparks aneal the electric field and limit the maxim charge of insulating particles. ► Sparks have a stronger effect on the trajectories of coarse particles. ► Purity of the separated products is poorer in the presence of sparks. ► Numerical modeling can provide an accurate description of the phenomenon.

Numerical Modeling of Insulating Particles Trajectories in Roll-type Corona-Electrostatic Separators

The trajectories of the insulating particles in roll-type corona-electrostatic separators depend on the configuration of the electrode system, the applied high voltage, the roll speed, the size of the particles and the relative humidity of the ambient air. The aim of this work was to point out how numerical modeling can be of use in the study of the effects of these factors. Particle charging is modeled using the Pauthenier's equation for spheres in uniform electric field. The equation of particle discharging was obtained after an experimental study of the surface potential decay of a granular layer of insulating material in contact with an electrode. The trajectories are computed based on the balance equation of the electrical and mechanical forces that act on such charged particles. The numerical results are in good agreement with the experimental findings.

A model for computing the trajectories of the conducting particles from waste printed circuit boards in corona electrostatic separators

A model for computing the trajectory of conducting particle from waste printed circuit board (PCB) scraps in corona electrostatic separator is established. Using analytical expression for computing non-uniformity of the electric field in the active zone of the separator and the differential method were used for computing the trajectories of conducting particles in the air, after detachment. The result shows that the trajectory of conducting particle can be computed under various initial parameters ( R, r, L, α, U, n; ρ, r 0) by the computing model and the computing results have a good agreement with the actual separating process. This model offers a possible for designing the new corona electrostatic separator.

Optimization of Corona Electrode Position in Roll-Type Electrostatic Separators

The position of the corona electrode(s) is known to be a key factor of electrostatic separation efficiency, as it influences both the charging conditions of the granular materials on the surface of the roll electrode connected to the ground, and the magnitude of the electric forces exerted on the particles. Response surface methodology was employed for the design of the experiments performed on a laboratory roll-type corona-electrostatic separators, with samples of chopped electric wire wastes typically processed by such techniques. The results of the electrostatic separation tests are discussed in relation to the data obtained from two other experiments, in which corona current and particle charge measurements were performed for various positions of the standard wire-type electrode. The conclusions of this study enabled the formulation of several recommendations for the improvement of the outcome of industrial separation processes (i.e., maximize the weight percentage as well as the purity of the recovered materials).

Unipolar charging of insulating spheres in rectified AC electric fields

Nonfiltered rectifiers and pulsed power supplies are often used for the energization of the high-voltage electrodes in various electrostatic installations. The aim of this paper is to investigate the ionic charging of insulating particles in the pulsatory electric fields specific to such applications. In a first set of numerical simulations, the space charge was considered constant in time, which means that the ion generation is not related to the voltage drop between the electrodes. A second set of simulations was carried out using the assumption that the space charge is generated by the pulsed corona from one of the electrodes. The computed results, which were found in good agreement with the experimental data, show that the amount of charge acquired by a particle depends on the following factors: (1) particle transit time through the electric field zone; (2) space charge density; and (3) ratio between the corona onset voltage and the amplitude of the variable voltage applied to the electrodes. These factors are discussed in correlation with the operating parameters of roll-type corona-electrostatic separators, but the conclusions are valid for a wider group of industry applications.

Factors which affect the corona charging of insulating spheres on plate and roll electrodes

Corona charging of particulates is employed in many electrostatic applications. Pauthenier's equation for the ionic charging of spheres in uniform electric fields cannot be used for particles in the vicinity or in contact with an electrode. Therefore, this paper studies the charging of insulating spheres on plate and roll electrodes affected by monopolar space charge. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (radius: 1.5 mm), when subjected to the corona discharge generated between a high voltage electrode and either a plate or a roll electrode connected to the ground. The spheres were found to be less charged when the grounded electrode was covered by an insulating layer. When the insulating spheres were in contact with the rotating roll, their charge - at a given electric field intensity - was found to reduce with the increase of roll velocity. A similar correlation was found with the initial speed of the particle at the moment of deposing it on the surface of the rotating roll electrode. However, the saturation charge was the same and the increase of the electric field beyond a certain limit determined a diminution of the charge carried by the particles (auto-discharge effect). From a practical point of view, these observations have already served to optimise the operating parameters of roll-type corona electrostatic separators for mixed granular solids.

Dielectric behaviour of particle-contaminated air-gaps in the presence of corona

Abstract The operating conditions of corona-electrostatic separators, of various electroprecipitators, and of other similar high-voltage devices are influenced by the size and shape of the particles which pass through the inter-electrode air-gap. The presence of corona discharge affects the movements and the charge of the particles, and hence modifies the breakdown threshold. The numerical analysis of the electric field, using a charge simulation program, enabled the evaluation of the effects. Two other computer programs were written in order to analyse the conditions in which micro-discharges can occur between charged particles and electrodes of opposite polarities, and to estimate the consequences of such an event on the global dielectric strength of an ionized air-gap. Nevertheless, a systematical experimental study was necessary to fully-understand the phenomena. The basic electrode arrangement consisted in a matrix of corona-emitting points situated above a plate electrode. In some experiments, a grounded metallic grid was interposed between the two electrodes connected to high-voltage supplies of opposite polarities, so that the ionic current density and the intensity of the electric field be independently controlled. Both the computational results and the experimental data demonstrated that the presence of particles in air-gaps affected by corona discharges has a more important influence on the dielectric strength than in the case of a charge-free electric field.

Charges and forces on conductive particles in roll-type corona-electrostatic separators

Mathematical modelling is a powerful tool in the optimization of electroseparation technologies and equipment. This paper proposes the use of a numerical method of field analysis to evaluate more accurately the charges and forces on millimeter-size conductive particles of virtually any shape in contact with an electrode. A program based on the boundary-element method was tested for this purpose, and provided the input data to an algorithm for the computation of particle trajectories. The results were in good agreement with the observations made on a laboratory electroseparator. The effect of space charge on the behavior of conductive particles was studied with a three-electrode system (a grounded grid, between a corona electrode and a metallic plate, energized by two high-voltage supplies of opposite polarities). The accuracy of numerical simulations of electroseparation processes is expected to increase when a mathematical model of particle motion in DC corona fields will be incorporated in the existing computer programs. >

Corona–Electrostatic Separation: An Efficient Technique for the Recovery of Metals and Plastics From Industrial Wastes

Better management of secondary resources, such as copper, aluminium and plastics from electric wire and cable waste requires improved methods of processing of reusable materials. This paper presents three typical examples of recycling technologies, each of them using the capability of electric fields to sort metallic and insulating materials from granular mixture. The efficiency of corona–electrostatic separation as a basic operation of a recycling flowsheet has been demonstrated for feedrates not exceeding 200 kg/hour. At the same time, it has been proved that electroseparation can represent a complement to conventional air–gravity separation of metals and insulators from industrial wastes. The in–plant tests provided useful information for a customs–design of corona–electrostatic separators.

Modelling of high-intensity electric fields - a method to improve the design of corona-electrostatic separators

The equations of the corona-electrostatic field in electroseparators, presented in this paper, can not be solved analytical. Therefore, testing different models of either corona or electrostatic electrodes has proven to be the most reliable way to decide the basic structure of new electroseparators. Conductive sheet and electrolytic tank analogues facilitate optimization of electroseparation equipments.