Charge distribution and state of agglomeration after tribocharging fine particulate materials

Abstract

Particle mixtures of different nonconductive species need to be treated in a tribocharging unit so that they can be separated in an electrostatic field. Such a unit produces an aerosol with particles carrying different positive and negative charges. An efficient separation demands high charges; however, high charges support agglomeration and agglomerates worsen the separation process. This problem has been studied using quartz/calcite mixtures in the size range 20–200 μm using a vibration chamber and a brush aerosol generator as the tribocharging unit. The particle charges vary over a wide range up to almost the theoretical maximum value. The charge distribution depends on the particle size, mixture ratio and atmospheric humidity such that the median value decreases as the particles become smaller, the humidity rises and the fraction in the mixture increases. According to Coehn's rule, quartz carries mainly negative and calcite mainly positive charges. Electrostatic agglomerates containing particles above 10 μm can be broken down by the Coulombic force in the field, but agglomerates with finer particles have to be destroyed before entering the separator. The most effective method is to use a high air velocity inside a slit nozzle. Even a feed with highly charged microfine particles could be deglomerated completely.