Alexandru Iuga

Charging of particulates in the corona field of roll-type electroseparators

The physics of corona charging phenomena is of paramount importance for the development of such electrotechnologies as precipitation of dust, spraying of powders and selective sorting of particulate mixtures. This study focused on the use of corona discharge in electroseparation processes. Crude theoretical evaluations pointed out that the required corona current density and high-voltage level in this application depend on various factors, such as material characteristics, feed rate and configuration of the electrode system. Experiments carried out on a laboratory roll-type electroseparator demonstrated the capability of a new corona electrode design to generate an extended and quasi-uniform corona field. Stable corona discharges were obtained when energizing that electrode at both positive and negative polarities from a reversible non-filtered full-bridge high-voltage rectifier. The voltage-current characteristics of the corona discharge from the new electrode were determined under various operating conditions: (i) without material in the inter-electrode gap; (ii) with a monolayer of a 50% aluminium-50% polyvinyl chloride particulate mixture on the surface of the grounded electrode; (iii) with a similar monolayer of metallic particles (99% aluminium); and (iv) with a layer of insulating particles (99% polyvinyl chloride). In each case, attention was paid to the upper acceptable limit of operating voltage, as well as to some side effects of corona discharges, such as audible noise and ozone generation. The efficacy of corona charging was assessed by electroseparation tests involving two groups of particulate materials: (i) barley grains, impurified with weeds and soil particulates; and (ii) granular mixtures of metals and plastics, yielded by chopping electric wire and cable scrap. Several practical considerations were formulated, based on a critical evaluation of the experimental results.

Corona and electrostatic electrodes for high-tension separators

Abstract A systematic reseach has been performed, aiming at the optimization of the corona and electrostatic electrodes of roll-type high-tension separators for mixed granular solids. The theoretical analysis of corona-charging and charge-neutralization processes has led to a crude evaluation of the electrical parameters (corona current per unit of length, power of the corona discharge) which should characterize the electrodes for a certain application. Computer modelling of the electric field has enabled the formulation of several design principles, which have been fully-verified by the experiments. Laboratory tests have been carried out in order to compare the various technical solutions and to state some criteria for electrode evaluation. The preliminary reports on the industrial application of the suggested electrode designs are extremely encouraging.

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. >

ELECTROSTATIC SEPARATION OF INSULATING AND CONDUCTIVE PARTICLES FROM GRANULAR MIXES

Abstract Electrostatic separation is extensively used for the selective sorting of granular mixtures, by means of the electric forces which act on charged or polarized bodies. The roll-type separator with combined corona-electrostatic field has been proved to be the most advantageous solution when the purpose is to isolate conductive particles from non-conductive ones. The paper presents the contributions of the High Intensity Electric Fields Laboratory of the Technical University of Cluj-Napoca to the development of this solution for various industrial applications. The first chapter is devoted to the theoretical analysis of the phenomena and points out the multitude of factors which influence the efficiency of the separation process. A special attention is paid to the study of the corona charging conditions of insulating particles. In the second and third cahpters, the authors show how the results of numerical modeling guided the design of new equipment and the optimization of their operating conditions...

Tribocharging of Granular Plastic Mixtures in View of Electrostatic Separation

The output of any electrostatic separation process is strongly dependent on the effectiveness of particle charging. A novel laboratory-scale fluidized-bed tribo-charging device has been described in a previous paper. The aim of the present work is to asses the effectiveness of two materials, aluminium and polypropylene, that could be employed for the manufacturing of the tribo-charging chamber of that device. The electrostatic separation experiments designed in this purpose were carried out on three samples of a granular polyethylene/polyvinyl chloride mixture, characterized by different compositions. Aluminium and polypropylene were found to be equally effective for the tribo-electrostatic separation of the 50% PVC, 50% PE sample. For the other two samples (10% PVC, 90% PE; 90% PVC110% PE), the best results were obtained with the polypropylene tribo-charging chamber.

Tribocharging and electrostatic separation of mixed granular solids in fluidized bed devices

Granular materials, when fluidized by air or other gaseous medium, acquire electrostatic charge by inter-particle and particle-wall collisions. The effectiveness of particle tribocharging achieved with such fluidization process is crucial for establishing the feasibility of electrostatic separation of mixed granular solid wastes in the recycling industry. The present paper aims at a crude evaluation of the effects of the main factors that influence the efficiency of such a tribocharger, in the presence of an electric field orthogonally oriented to the direction of the fluidization air. The experiments were carried out on binary mixtures taking combinations of two of the three polymeric materials in different tests: (a) virgin polyamide, (b) polycarbonate and (c) polyacetal granules. The variables investigated were the duration of the tribocharging process, the level of the high-voltage applied to the electrode and the composition of the mixture. The analysis of the experimental data revealed the effects of each variable on the mass and the charge of the granules collected at the electrodes.

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.

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.

Modelling of conducting particle behaviour in plate-type electrostatic separators

The charging of conductive particles in contact with an electrode affected by a non-uniform electric field is a physical mechanism of paramount importance for various electrostatic technologies, including the separation of particulate mixtures. The aim of this paper is to model the behaviour of such charged particles in plate-type electrostatic separators. A computer program based on the boundary element method enabled the evaluation of the electric field strength at the surface of the plate electrode of these installations. The results of field computation were then employed for estimating the charge of perfectly-conducting spherical particles, as well as their position on the plate electrode, at the moment of detachment under the action of the electric force acting on them. The data reported in the paper point out the following factors which affect particle detachment and hence the efficiency of the electrostatic separation: the size and specific mass of the particles; the slope of the plate electrode; the dimensions and relative position of the high-voltage electrode; and the level of the high voltage applied to the electrode system. Experiments carried out with two kinds of particles (silver-plated polyamide spheres of radius 1.5 mm and mass 0.0192 g; and steel balls of radius 0.2-0.25 mm and average mass 0.000 45 g) confirmed the conclusions of numerical simulations. The modelling of particle behaviour in contact with plate electrodes facilitates the feasibility studies and can guide the experimental tests, which are needed for the development of any new application of the electrostatic separation method.

Optimal high-voltage energization of corona-electrostatic separators

The selection of the high-voltage supply can play an important role in the optimization of electrostatic separation processes. This paper aims to evaluate the influence of the main high-voltage parameters (waveform, polarity, level) on the efficiency of electrostatic separation, in the case of insulation-metal granular mixtures. A roll-type laboratory high-tension separator was employed for the experimental study, and the tests were carried out with samples of granular materials taken from the technological flowsheet of a recycling plant for electric wire scraps. The oscillograms of the voltage and of the current across the separator proved to be of great use for studying the transition from corona to spark discharges. The experiments, performed under various operating conditions (roll speed, roll radius, high-voltage level, interelectrode distance), show the existence of a strong interdependence between these parameters, the frequency of spark discharges, and the efficiency of the separation process. The reported results suggest that monitoring the frequency of the spark discharges could be of use for controlling the optimum operating voltage for a given electrostatic separation application. Although the full-wave rectifier allows for lower operating voltages than the half-wave rectifier, its general effectiveness in electrostatic separation processes is superior. Good insulation-metal electrostatic separation can be achieved at either positive or negative polarity of the high-voltage supply, but negative electrode energization is recommended for most industrial applications.