Characterization of wire corona electrodes at various discharge gaps in electrostatic separation processes

The development of new applications might impose the use of corona electrodes located at either smaller (s < 25 mm) or larger (s > 75 mm) distances from grounded electrodes than in "classical" electrostatic separators. The aim of the present paper is to characterize such less standard electrode systems. A first group of experiments carried out on a laboratory roll-type electrostatic separator (roll radius: 125 mm) enabled the measurement of current-voltage characteristics of a wire-type corona electrode. A full-wave rectifier, 0-75 kV, 0-6 mA, energized the electrode located at different distances from the grounded roll electrode. In a second group of experiments, the authors evaluated the "pinning" effect exerted by the electric image force on charged insulating particles processed in the same roll-type separator. This was found to depend on both the high-voltage level and the relative position of the electrodes, as they influence the local electric field strength and the intensity of the electro-aero-dynamic effects (corona wind). The set-up employed in the third group of experiments permitted the measurement the distribution of current density at the surface of a large plate electrode, located at different distances from a corona electrode. The experimental observations are discussed in relation with the results of numerical electric field simulations. They suggest the possibility of decreasing or increasing the distance between the electrodes, in response to the needs of various electrostatic separation applications (fine particles, large flakes...).