EFFICIENCY OF DUAL WIRE-CYLINDER ELECTRODES USED IN ELECTROSTATIC SEPARATORS

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1 EFFICIENCY OF DUAL WIRE-CYLINDER ELECTRODES USED IN ELECTROSTATIC SEPARATORS LAURENŢIU MARIUS DUMITRAN 1, LAURENŢIU VIOREL BADICU 1, MARIUS CRISTIAN PLOPEANU 1,2, LUCIAN DĂSCĂLESCU 2 Key words: Electrostatic separation, Corona discharge, Dual ionizing electrode. The most efficient ionizing electrode used in roll-type electrostatic separators consists of a small diameter ionizing wire attached to a metallic cylinder. Previous studies concerning the numerical computation of the electric field and ionic charge and also the simulation of the metallic and dielectric particle charge processes in the zone located between the ionizing electrode and the grounded roll showed that the geometrical characteristics of the wire-cylinder ionizing electrodes play a very important role for the separator s operation. This paper presents an experimental study concerning the influence of the wire-cylinder and wire-grounded roll distances on the ionic current level and their distribution on the grounded electrode. Finally, the obtained results are discussed and it is shown that, for a given application, the separation efficiency can be increased through the optimization of the ionizing electrode geometry. 1. INTRODUCTION Corona discharge is often used for charging mm-size insulating particles in view of their electrostatic separation from granular mixtures [1 3]. Several models of corona electrodes have been developed for various industry applications. In some cases they consist of a series of needles or a blade fixed on a metallic support. However, the most widely used electrode for roll type electrostatic separators, which is sometimes called dual wire-type electrode, uses a thin metallic wire as ionizing element, attached to a metallic cylinder (Fig. 1). In the electrostatic separators, the dual electrode is located facing a metallic grounded roll that turns with a controlled speed. The granular mixture is deposited to the surface 1 Electrotechnical Materials Laboratory, Faculty of Electrical Engineering, University POLITEHNICA of Bucharest, 313 Splaiul Independenţei St., 642 Bucharest, Romania, dumitran@elmat.pub.ro. 2 University of Poitiers, Laboratoire d Etude Aérodynamique, CNRS UMR 669, Boulevard Marie et Pierre Curie, BP Futuroscope Chasseneuil cedex, France, lucian.dascalescu@univpoitiers.fr. Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 55, 2, p , Bucarest, 21

172 Laurenţiu Marius Dumitran et al. 2 of the roll and is conveyed through the electric field and ionic space charge zone generated by the dual electrode, connected to a DC high-voltage supply (Fig.

2 172 Laurenţiu Marius Dumitran et al. 2 of the roll and is conveyed through the electric field and ionic space charge zone generated by the dual electrode, connected to a DC high-voltage supply (Fig. 1). Fig. 1 Schematized view of a roll type electrostatic separator. One of the main applications of electrostatic separation concerns the recycling of electrical and electronic equipment waste, by sorting the conductive metallic particles from polymeric fragments under the action of the electric field forces. An efficient operation of a roll-type electrostatic separator involves very different trajectories of charged dielectric and metallic particles submitted to the separation forces which depend on the local electric field strength E and particle charge Q p. By consequence, the applied high voltage level, the speed of rotating roll, as well as the position of the ionizing electrode and the characteristics of the corona discharge are the critical parameters that influence the separation efficiency given by the ratio between the resulting unseparated particle mass and the total mass of the processed granular mixture [4, 5]. For many classes of granular mixtures, users practical experience of the electrostatic separation guides the choice of these parameters in such a way to obtain high separation efficiency. The operation conditions must ensure an efficient particle charging process and a maximum field strength in the area located between the corona electrode and the grounded roll. The electric field strength is limited by the gap of the electrodes and the size and properties of particles that transit the corona zone and, in particular conditions, can produce the complete electrical discharges between the electrodes.

3 3 Dual wire-cylinder electrodes used in electrostatic separators 173 Numerical and experimental studies presented in previous papers [6 8] showed that the geometry of dual ionizing electrode has a strong influence on the spatial repartition of electric field and ionic space charge and also on the particle charging process. It is know that for a spherical dielectric particle of diameter d p and permittivity ε r located in the active zone of the separator, the electrical charge Q p depends on the ionic space charge density ρ and the local electric field strength E [9]: 2 dqp ρk Q i s p = Q p 1 s, (1) dt 4ε Q p where K i represents the ions mobility in air and s Q p is the saturation charge. s 2 ε r Q p = 3πε d p E. (2) ε + 2 Moreover, for a spherical conducting particle in contact with the metallic roll, the electric charge Q cp squired by electrostatic induction strongly depends of the local electric field E in accord to the following formula [9]: r Q cp = π ε d p E. (3) 6 By examining the equations (1 3) it results that an efficient charging process of the particles requires high electric field strength and a strong charge injection (high ionic current intensity). In the case of the roll-type electrostatic separators, the particles are deposited on the conductive surface of the grounded electrode and their acquired charge can be easily lost. For this point of view, an important factor concerns the grounded electrode area which receives the ionic current and gives the direct dependence between the ionic current distribution and separation electrostatic forces. From these physical considerations, it results that the basic condition to obtain a high efficiency separation of granular mixture is the existence of a strong field-charge zone where the particles acquire a maximum charge. The experimental results presented in this paper represent an extension of the previous studies [6 8]. Using experimental means, the paper aims at quantifying the influence of the geometry and position of the dual ionizing electrode of the corona current level and their repartition at the surface of the grounded roll. The basic idea is to determine the corona current and its repartition at the surface of the

174 Laurenţiu Marius Dumitran et al. 4 grounded electrode for several distances: wire-dual cylinder electrode h and wiregrounded electrode d (see Fig. 1).

4 174 Laurenţiu Marius Dumitran et al. 4 grounded electrode for several distances: wire-dual cylinder electrode h and wiregrounded electrode d (see Fig. 1). For this purpose, an experimental installation containing several dual ionizing electrodes with variable geometry has been set-up. The obtained results are discussed in view to indicate the influence of the geometrical characteristics position of the dual electrode leading to the most efficient charging process and separation. 2. MEASUREMENTS The experimental set-up has been designed to enable the measurement of current-voltage characteristics, as well as the distribution of the corona current at the surface of the grounded electrode. Taking into account the very important differences between the sizes of grounded roll and corona dual electrode, the experimental set-up was simplified and the grounded roll was replaced by a metallic plate fixed to a special support. The position of the ionizing electrode can be adjusted using a precision mechanical device (Fig. 2). Fig. 2 Experimental set-up: 1 spellman high voltage power supply, 2 Keithley electrometer, 3 mechanical system, 4 corona dual electrode, 5 grounded plate. The corona current was measured using an electrometer Keithley 6487 (Fig. 2). In order to obtain the repartition of the current at the surface of the grounded plate, a thin copper electrode (2.5 mm 2 ) located in the middle of the plate and electrically insulated from the ground served as current probe (Fig. 3). By moving the ionizing electrode relative to the plate it is possible to measure the current received by the copper electrode for various positions.

5 Dual wire-cylinder electrodes used in electrostatic separators 175 The dual ionizing electrode is designed in such a way to rapidly change the wirecylinder distance h.

5 5 Dual wire-cylinder electrodes used in electrostatic separators 175 The dual ionizing electrode is designed in such a way to rapidly change the wirecylinder distance h. Moreover, the dual metallic cylinder can be easily replaced by another insulator cylinder made from epoxy resin. The ionizing electrode used in this study is a wolfram wire having the diameter d wire =.7 mm which is fixed between two metallic cylindrical supports fixed on the dual cylinder (Fig. 3). Fig. 3 Electrodes system: 1 dual metallic cylinder electrode, 2 ionizing wire, 3 cooper electrode, 4 grounded plate. 3. RESULTS AND DISCUSSION Using the experimental set-up presented above, two sets of measurements were carried out: the current-voltage characteristics and the repartition of the ionic current at the surface of the grounded plate. During the experimental measurements, the environmental conditions were controlled in view to reduce the influence of the temperature and humidity variations. To obtain the current-voltage characteristics and the corona current distributions the applied voltage range was from a few kilovolts (corona start voltage) to maximum 4 kv. The influence of the geometrical parameters of the ionizing electrode on the total corona current is showed in Figs. 4 and 5. As expected, the discharge current is strongly influenced by the wire-plate distance d. For an applied voltage of 2 kv, the total discharge current increases with about 5% when d changes from 4 mm to 3 mm (Fig. 4). Though the gap d is important, it must be kept in mind that in

6 176 Laurenţiu Marius Dumitran et al. 6 practical operation the position of the ionizing electrode is limited, in view to avoid complete discharges d = 2 mm d = 3 mm d = 4 mm 2 I [µa] U [kv] Fig. 4 Current-voltage characteristics for h = 2 mm and several distances ionizing wire-grounded plate d. I [µa] h=1 mm h=2 mm h=4 mm h=6 mm U [kv] Fig. 5 Current-voltage characteristics for d = 4 mm and several distances between ionizing wire and dual metallic cylinder h.

7 7 Dual wire-cylinder electrodes used in electrostatic separators 177 J [na/mm 2 ] U = 18 kv U = 2 kv U = 22 kv U = 28 kv U = 3 kv x [mm] Fig. 6 Variation of the current density on the surface of the grounded plate for several values of the applied voltage for d = 5 mm and h = 1 mm d = 4 mm d = 5 mm d = 6 mm J [na/mm 2 ] x [mm] Fig. 7 Variation of the current density on the surface of the grounded plate for U = 22 kv, h = 1 mm and several distances d.

8 178 Laurenţiu Marius Dumitran et al. 8 7 J [na/mm 2 ] d = 3 mm d = 4 mm d = 5 mm d = 6 mm x [mm] Fig. 8 Variation of the current density on the surface of the grounded plate for U = 22 kv, h = 3 mm and several distances d. 25 J [na/mm 2 ] h = 6 mm h = 4 mm h = 3 mm h =1 mm x [mm] Fig. 9 Variation of the current density at the surface of the grounded plate for U = 22 kv, d = 4 mm and several distances h. Another parameter that influences the corona discharge and the current level is h (Fig. 5). When the wire-dual cylinder distance increases, the current increases too (for U = 25 kv, the current increases with about 5 % when h passes from

9 9 Dual wire-cylinder electrodes used in electrostatic separators mm to 6 mm). In fact, the dual metallic cylinder which is supplied to the same high voltage reduces the electric field strength at the surface of the ionizing wire and the charge injection decreases when the wire is close to the dual cylinder. This proves that it is possible to obtain higher current without changing the gap d and the applied voltage. Figure 6 presents the variation of the ionic current density on the surface of the grounded plate for different applied voltage. Though the growth of the current with the applied voltage is obvious, it is important to remark that the extension of the ionic current zone depends on the voltage too. Figures 7 to 9 show the influence of d and h on the current distribution. It can be seen that by changing the gap d, the magnitude of the current density has an important variation. This variation is most important for the small values of the distance h (Figs. 7 and 8). However, the current density variation strongly depends on the distance h. It is clear that when the ionizing wire is remote from the dual metallic cylinder, not only the magnitude of the current increases (Fig. 5), but also the current distribution changes and the covered region become larger. Examining Fig. 9 it results that when h changes from 1 mm to 6 mm, the grounded plate ratio which receive the ionic current is about tree times higher in x direction. When the wire is far away from the cylinder (h > 7 mm), the results show no more influence on the corona current. Based to these observations it appears possible to increase the ionic current by using large wire-dual cylinder distances in real electrostatic separator. According to the granular mixture properties, the parameter h must be chosen in such a way to obtain the maximum particle charging and higher separation efficiency. 4. CONCLUSIONS The experimental results show that the corona discharge and the corresponding ionic current depend on the applied voltage and also on the position and geometry of the ionizing electrode. For practical applications, these parameters must be chosen in such a way to obtain maximum ionic current density and high field strength in the active region of the separator. In the same time, during the separator operation it is necessary to avoid the complete electrical discharge. From this point of view, the choice of d gap and applied voltage value is conditioned by the characteristics of treated granular mixture. The presence of the particles (especially the metallic particles) at the surface of the roll favors the complete inter-electrodes space breakdown, which leads to the separator non-operation periods.

10 18 Laurenţiu Marius Dumitran et al. 1 The obtained results show that the most important parameter that allows the increase of the corona current and the change in the ionic charge distribution at the surface of the grounded roll is the wire-dual metallic cylinder distance h. This parameter can be adjusted for each mixture in such a way to obtain convenient ionic current distribution and the maximum particle charging. Using large wiredual metallic cylinder distance, it results the most favorable magnitude and distribution of the ionic current which lead to an efficient charging process and finally to the increase of the separator s performance. An experimental study concerning the influence of the parameter h on the efficiency of a laboratory electrostatic separator will be presented in a subsequent paper. Received on 3 February, 29 REFERENCES 1. A. D. Moore (Ed.), Electrostatics and Its Applications, Wiley, New York, J. S Chang, A. J. Kelly and J. M. Crowley (Eds.), Handbook of Electrostatic Processes, Dekker, New York, L. Dascalescu, A. Iuga, R. Morar, V. Neamtu, I. Suarasan, A. Samuila, and D. Rafiroiu, Corona and electrostatic electrodes for high-tension separators, J. Electrostatics, 29, pp , R. Morar, A. Iuga, L. Dascalescu, and A. Samuila, Factors which influence the insulation-metal electroseparation, J. Electrostatics, 3, pp , A. Iuga, R. Morar, A. Samuila, and L. Dascalescu, Electrostatic separation of metals and plastics from granular industrial wastes, IEE Proc.-Sci. Meas. Technol., 148, pp , L.M. Dumitram, P. Atten, P.V. Notingher and L. Dascalescu, 2-D corona field computation in configurations with ionising and non-ionising electrodes, J. Electrostatics, 64, pp , L.M. Dumitran, L. Dascalescu, P.V. Notingher, P. Atten, Modeling of corona discharge in cylinder-wire-plate electrode configuration, Journal of Electrostatics, 65, 12, pp , L.M. Dumitran, L. Dascalescu, P. Atten, P.V. Noţingher, Computational and Experimental Study of Ionic Space Charge Generated by Combined Corona-Electrostatic Electrode System, IEEE Trans. Ind. Appl., 42, 2, pp , M. Pauthenier and M. Moreau-Hanot, La charge des particules sphériques dans un champ ionisé, J. Phis. Radium, 3, pp , 1932.

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