Chapter 3
Surface Potential Measurement of Insulating Granular Layers
Characterization of the charging/discharging behaviour of granular materials is of paramount importance to the optimum operation of roll-type corona-electrostatic separators [111]. The basic idea behind the work presented in this chapter consists in measuring the decay with time of the potential V in a point at the surface of a monolayer of such a particulate material, by means of an electrostatic voltmeter [94], using a sensing probe close to that surface, but without contacting it [8]. The capacity C of the probe – layer system being constant, the potential V is proportional to the local surface charge density q of insulating granular material, and its decay reflects that of q. The experimental set-up designed for this application is described in sub-chapter 3.1.
The charge decay functions derived from the analysis of the experimental data depend on both the nature of the tested insulator and the environmental conditions, as shown in sub-chapter 3.2. These observations lead to the development of the novel technique for evaluating the separation feasibility of a given granular mixture of insulators, described in sub-chapter 3.3.
3.1 Surface Potential of Insulating Granular Layers
Laboratory studies and in-field observations shown that the charge decay characteristics of the granular materials at the surface of the grounded roll electrode significantly influence the outcome of the electrostatic separation process. An indirect method of charge decay characterization consists in the measurement of the electrical potential at the surface of single- or multi-layer of such materials.