The dispersion of powders by intense electrostatic fields. I. Experimental studies

Abstract

Powder dispersion by electrostatic fields above 0.20 kV/mm has been studied at ambient temperature in a vacuum, air, sulphur dioxide, and insulating liquids, with both conducting and nonconducting powders. All the conducting powders examined were dispersible, but only some of the insulating materials (e.g., alumina) reacted to the field. Alumina was studied in some detail using different media and various electrode geometries. It was concluded that dispersion of insulating powders occurs as a result of a conducting surface film, indicating that charging of the particles is mostly by conduction electrons from the bed. Effects of the dispersing medium were also detected, especially through the use of current-voltage curves. When liquids were used instead of air, for example, pronounced hysteresis occurred, an effect ascribed to the increased settling rate of the particles. Differences in behavior were also demonstrated from one gaseous medium to another, e.g., particulate clouds, which normally show pronounced turbulence in high-voltage air dispersion, became much more uniform in pure sulphur dioxide. In addition, some powders such as lycopodium, which is unreactive to the field in air, dispersed well in sulphur dioxide. This was traced back to the corona-suppressing properties of this gas, demonstrating that production of gaseous ions at field concentrations (such as those produced by sharp electrodes or angular particles) is detrimental to electrodispersion, since ions tend to act as charge carriers in preference to the particles. Further evidence of this came from gamma ray attenuation studies used for measuring cloud densities as a function of the applied field.