Characterisation of dual corona electrodes for electrostatic processes applications

The dual corona electrode consists in one or several ionizing elements (wires, needles, blades) attached to a metallic support. The present paper aimed at analyzing the characteristic features of a particular type of corona electrodes, and formulate some recommendations on their design and utilisation. The experiments focused on several models of dual corona electrodes, the ionizing element of which consisted in the edge of a thin metallic blade. The tested devices had similar cylindrical metallic supports, but blades of different sizes. Thus it was possible to investigate the influence of electrode geometry on corona onset voltage and spark-over threshold, in configurations specific to industrial electrostatic processes. Most experiments were performed on roll-type electrostatic separators. A special experimental set-up was employed for the study of the distribution of corona current density at the surface of a plate collecting electrode. For the model the blade edge of which was closer to the metallic support, the corona onset voltage was higher, and the corona discharge affected a smaller area at the surface of the collector. These experiments were discussed in relation with the results of the numerical analysis of the electric field generated by the different electrode configurations. At a given voltage and distance between the ionizing element and the collector, the presence of the cylindrical support diminishes the electric field at the edge of the blade and distorts the field lines. These effects were exploited in the design of the electrode system of an electrostatic separator for the recycling industry.