Static electrification of particles takes place in various kinds of particle handling processes. For example, particles are charged so heavily during gas- solids pipe flow that electric sparks are observed periodically through a transpearent part of the conveyor line.
Movements of charged small particles are affected by the charge itself, and the electrification causes various undesirable effects such as adhesion between particles and the wall of process equipments. The equipments are also electrified by impaction between particles and the wall. It may be difficult to reduce the potential of the equipments to zero level because of the electrical resistance and the capacitance between the equipments and the ground. The potential can be lowered by suitable grounding, even the grounding of the equipments can not prevent the electrification of particles.
On the contrary, the grounding may accelerates the electrification of particles. Besides such undesirable effects as adhesion and dust explosion, electrification of particles is utilized positively in electrostatic precipitation, electrostatic separation, electrophotography, and so on.
In the next section, we will briefly discuss the general idea of contact electrification, then the electrification of a particle by impaction on a metal plate, and electrification process of a particle in gas-solids pipe flow.