FACTORS WHICH AFFECT THE CORONA CHARGING OF INSULATING SPHERES ON PLATE AND ROLL ELECTRODES

Corona charging of particulates is employed in many electrostatic appl ications. Pauthenier's equation for the ionic charging of spheres in u niform electric fields cannot be used for particles in the vicinity or in contact with an electrode. Therefore, this paper studies the charg ing of insulating spheres on plate and roll electrodes affected by mon opolar space charge. An electrometer was used to measure the charge ac quired by calibrated spheres of polyamide (radius: 1.5 mm), when subje cted to the corona discharge generated between a high voltage electrod e and either a plate or a roil electrode connected to the ground. The spheres were found to be less charged when the grounded electrode was covered by an insulating layer. When the insulating spheres were in co ntact with the rotating roll, their charge - at a given electric field intensity - was found to reduce with the increase of roll velocity. A similar correlation was found with the initial speed of the particle at the moment of deposing it on the surface of the rotating roll elect rode. However, the saturation charge was the same and the increase of the electric field beyond a certain limit determined a diminution of t he charge carried by the particles (auto-discharge effect). From a pra ctical point of view, these observations have already served to optimi se the operating parameters of roll-type corona electrostatic separato rs for mixed granular solids.