Chapter 4
High Voltage Measurement in Corona Electrostatic Separators
High voltage is known to be one of the main control variables of any electrostatic separation process. From this perspective, the aim of the work presented in this chapter is double: develop an effective high-voltage monitoring system, and demonstrate that it can be a useful tool for supervising the overall operating conditions of electrostatic separators. A custom-designed virtual instrument was employed for processing the experimental data provided by a high-voltage probe the output of which was connected to an electrometer. In several experiments, the output of the high-voltage probe was also connected to a digital oscilloscope, in order to obtain a better characterisation of the variation of the electrode potential after a spark discharge. The laps of time without corona discharge and/or with low electric field intensities could thus be rather accurately determined, and the impact of spark discharges on the outcome of the separation process evaluated. The dispersion of high-voltage measured values was found to increase in the presence of the material. The statisticalal analysis of the data revealed a significant correlation between the standard deviation of the high-voltage and the concentration of metal in the processed material. These findings could be helpful for the optimisation of the operating conditions of the electrostatic separation applications where the metal content in the feed materials is characterized by important fluctuations with time.
4.1 High-Voltage Energizing of Corona Electrodes
Electrostatic separation is based on the electrical forces acting on charged or polarized particles in an electric field [45, 48, 54, 101], generated by an electrode system connected to a high-voltage supply. Applied high voltage (HV) should be adjusted close to the threshold beyond which spark discharges could occur between the electrodes, in the presence of the processed material [58]. In this way, one ensures the highest possible intensity of the electric field, which implies best particle charging conditions, and most effective electric separation forces [21, 22, 59].
Indeed, electrostatic induction, which is the prevalent charging mechanism of conductive particles in roll-type separators (Figure 4.1), depends directly of the electric field strength at the surface of the electrode carrier [73]. Corona charging, which affects both conductors and insulators, is also proportional with the electric field strength, and hence with the voltage applied to the electrode system [19].