Vasile Neamtu

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.

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.

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.

High-voltage supplies for corona-electrostatic separators

The selection of the high-voltage supply can play an important role In the optimisation of electrostatic separation processes. The present work aimed to evaluate the influence of the main high-voltage parameters (waveform, polarity, level) on the efficiency of electroseparation, in the case of insulation-metal granular mixtures. A roll-type laboratory electroseparator was employed for the experimental study and the tests were carried out with granular materials prelevated from the technological flowsheet of a recycling plant for electric wire scraps. The experiments shown the existence of a strong interdependence between the level of the operating voltage and the other electrical parameters, Although the full-wave rectifier allows for lower operating voltages than the half-wave rectifier, its general effectiveness in electroseparation processes is superior. The optimum operating voltage of an electroseparator seems to be slightly lower than the level at which the frequency of the spark discharges tends to exceed 60 min/sup -1/. 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. Good insulation-metal electroseparation can be achieved at either positive or negative polarity of the high-voltage supply, but negative electrode energization is recommended for most industry applications.

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.

Removal of Metallic Particles From Acrylonitrile Butadiene Styrene Wastes Using Electrostatic Separation Methods

Electrostatic separation techniques have been widely used in the recycling industry. The aim of this paper is to assess the possibility of employing them for the beneficiation of the newest and fastest growing category of wastes: the “end-of-life” information technology equipment. The study was performed on samples extracted from a granular acrylonitrile butadiene styrene (ABS) product obtained after the disassembling and shredding of information technology wastes. The samples that typically contained less than 0.1% metals in weight were processed in a laboratory electrostatic separator that enabled the reproduction of the two most important electrode configurations employed in the industry. The design-of-experiments methodology was employed for evaluating the effects of the various electrical and mechanical control variables on the outcome of the separation process. The reported data demonstrate that, by the appropriate adjustment of the input variables of either a plate-type electrostatic separator or a roll-type corona-electrostatic installation, it is possible to remove all the metallic particles from the ABS product, while keeping at a minimum the quantity of plastics lost in the rejected conductive fraction.

Charge-Neutralization Electrodes for High-Tension Separators

Safe and efficient operation of industrial high-tension separators requires a strict control of residual electric charge of the particles emerging the process. Principles of charge neutralization in a roll-type electroseparator are briefly discussed, in order to establish the general features of the corona electrodes that might accomplish this task. A new electrode design is suggested. Its spacific features: small cross-section (neglectable perturbation of the material stream), low corona on-set voltage, good stability of corona discharge during long-time operation, short maintenance time, have been tested on a laboratory model, prior to industrial application. Several design considerations have been formulated, based on a critical evaluation of the experimental results.

High-voltage monitoring in electrostatic separators

High voltage is known to be one of the main control variables in any electrostatic-separation process. From this perspective, the aim of this paper is twofold: to develop an effective high-voltage monitoring system and to demonstrate that it can be a useful tool for controlling the overall operating conditions of an electrostatic-separator system. A custom-designed virtual instrument was employed for processing the experimental data provided by a high-voltage probe, the output of which was connected to an electrometer. In several experiments, the output of the high-voltage probe was also connected to a digital oscilloscope, in order to obtain a better understanding of the variation of the electrode potential after a spark discharge. The laps of time without corona discharge and/or with low electric field intensities could thus be accurately determined, and the impact of the spark discharges on the outcome of the separation process evaluated. The dispersion of high-voltage measured values was found to increase in the presence of the material. The statistical analysis of the data revealed a significant correlation between the standard deviation of the high-voltage and the concentration of metal in the processed material. The results of this paper could be helpful for those seeking the optimization of the operating conditions for the electrostatic separation applications, in which the metal content in the feed materials exhibits substantial fluctuation with time

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.