Unipolar charging of cylindrical insulating particles nearelectrode surfaces

Numerous papers have discussed the ionic charging of insulating spheres in uniform electric fields. However, in certain electrostatic technologies, such as separation and flocking, the particles are often cylindrical in shape, and they get charged on the surface of an electrode or in its proximity, so that existing formulas cannot be used. This paper addresses this problem from both a computational and an experimental point of view. The charge acquired by cylindrical particles of various dielectric constants was evaluated with an original computer program, based on the boundary-element method of field analysis. The computed results show that the position of the particle with respect to the electrodes changes the value of the saturation charge. The experimental setup simulated the charging conditions in a roll-type electrostatic separator. The unipolar space charge was generated by a needle-type electrode. An electrometer was used to measure the charge acquired by millimeter-size calibrated cylinders of polyethylene and polyvinyl chloride on a rotating roll electrode. The experimental results, which were in good agreement with the theoretical predictions, put forward a particle self-discharge effect, at field intensities beyond a well-defined threshold. This kind of information may guide the design of the electrostatic technologies based on the corona charging of granular matter     

Unipolar charging of insulating spheres in rectified AC electricfields

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     

Electrostatic separation of muscovite mica from feldspathic pegmatites

This paper aims at evaluating several electrostatic separation techniques that could be effective for the recovery of mica flakes from pegmatite ores characterized by grain sizes <0.5 mm: 1) triboadhesive separation; 2) dc electric field separation; 3) ac electric field separation; 4) corona field separation using a grounded metallic roll electrode and thermal conditioning; and 5) corona field separation using a grounded metallic roll electrode covered with an insulating layer. The experiments have been carried out on samples containing about 50% mica, 25% feldspar, 15% quartz, 10% iron oxides and hydroxides, as well as iron silicates, with grain sizes ranging between 0.16-0.4 mm. The experimental data confirmed the theoretical predictions: all four separation techniques using high-intensity electric fields improved the grade of muscovite mica concentrate to levels that fulfill the requirements of several industry applications such as welding wire manufacturing. The best results [46.11% SiO/sub 2/, 33.64% Al/sub 2/O/sub 3/, and 11.1% (K/sub 2/O+Na/sub 2/O)] were obtained when performing a three-stage separation using method 5).     

Charging and discharging of insulating particles on the surface of a grounded electrode

The aim of the present paper is to analyze the corona charging of millimeter-size insulating disks, as well as their discharging when they are no longer exposed to the action of an external electric field. The experiments were carried out on a roll-type electrostatic laboratory separator, equipped with a wire-type corona electrode, simulating the actual charging/discharging conditions in an industrial unit. Disks of various sizes were charged on the surface of the roll electrode, then the high voltage supplied to the corona electrode was turned off and the particles were collected in a Faraday pail, connected to an electrometer. The charge measurements were performed at various time intervals from high-voltage turn-off. In this way, the charge decay could be recorded and the discharge process fully characterized. The measured data show that the discharge process depends on the nature, size, and shape of the particles, as well as on the contact conditions between the particles and the grounded roll electrode. These data could guide the design of the electrostatic separation experiments that precede any new industrial application of this technology.     

High-voltage electrode position: a key factor of electrostatic separation efficiency

Industry application of electrostatic separation technologies still faces a major difficulty: good results can be obtained only by adequate control of a multitude of operating parameters. The aim of the present paper is to analyze a key factor of electrostatic separation efficiency: the position of the high-voltage electrodes. Experiments were performed with two types of granular materials: chopped electric wire wastes and foundry sands. The electrostatic separator employed for the tests was provided with a wire-type corona electrode, associated - in some experiments - with a tubular-type electrostatic electrode, at various angular and radial positions, with respect to a rotating roll electrode connected to the ground. The experimental data are discussed in relation to the results of the numerical analysis of the electric field, carried out with a charge simulation program. They show that the outcome of the separation process (i.e., the weight percentage of the conductor and nonconductor fractions, as well as the purity of the recovered materials) depends on the configuration of the electrode system. The position of the electrodes affects both the particles charging conditions on the surface of the roll electrode, and the trajectories of the charged particles in the high-intensity electric field of the separator. Some recommendations could be formulated for the industrial application of the electrostatic separation technology.     

Influence of material superficial moisture on insulation-metalelectroseparation

The efficiency of electroseparating the constituents of a polyvinyl chloride (PVC)-metal (aluminium or copper) granular mixture depends on the moisture content of the materials. Laboratory research has been accomplished in order to determine the acceptable superficial humidity of the granules to be separated. The experimental study has been carried out on a roll-type laboratory separator with corona-electrostatic field. Ten samples of 50% PVC and 50% aluminum or copper, with a moisture content in the range 0相似文献   

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.     

Influence of Material Moisture on the Tribocharging Process of Plastic Granules

The control of the triboelectrification factors is the key for a successful application of electrostatic separation to the recycling of mixed plastics waste. This article focuses on the influence of material moisture on the tribocharging process of three granular materials: polyamide (PA), polyvinyl chloride (PVC), and polyethylene terephthalate (PET).

Different values of moisture content were obtained by immersing the material in water or drying it in a laboratory oven. Afterward, the granular material was tribocharged on the tray of a vibratory feeder and the accumulated charge was measured by an electrometer.

The maximum charge/mass ratio was obtained for the three plastic materials at different values of moisture content: 0.27% for PA, 0.12% for PET 0.1% for PVC. Series of 5 consecutive tribocharging experiments performed on the same sample, show that the first contributes with the greater amount of granules charge while the other four only slightly increment this value. The graphical representation of the evolution of the charge/mass ratio versus number of tribocharging experiment conducts to the conclusion that: i) the charge of the granules tends to saturate; ii) it is useless to excessively increase the duration of the tribocharging process.     

Electrostatic separation processes

Optimization of electrostatic separation processes demands the control of a multitude of factors, including the characteristics of the granular mixtures to be sorted, the feed rate, the configuration of the electrode system, the applied high-voltage and the environmental conditions. The Taguchi's experimental designs presented in this article clearly prove that the linear-interaction models of the electrostatic separation processes can reflect the effects of the main factors in a manner that is satisfactory to most case of the practical interest. The Taguchi's experimental designs are based on special matrices called orthogonal arrays.     

Premises for Statistic Control of a Tribocharging Process for Granular Materials

This article aims to identify the appropriate sampling duration for a tribocharging process on a vibratory feeder device in order to compute the capability indexes and set up a statistical control procedure. The outcome of the process is evaluated as the ratio between the charge and the mass of the granules that exit the tribocharging device during a given laps of time. A virtual instrument developed in LabWiew was used in conjunction with a Faraday cage connected to an electrometer and with an electronic scale, to simultaneously measure the charge and the mass of tribocharged granular plastics, for fixed sampling durations.