Electrostatic separation of powder mixtures based on the work functions of its constituents

Abstract

The objective of this study was to investigate the feasibility of a dry electrostatic process to separate a powder mixture into its components based on their work functions. We studied the dry electrostatic beneficiation of high-sulfur, high-ash Illinois coals and relatively low carbon-containing oil shales, and separation of synthetic mixtures consisting of charcoal and silica to demonstrate the feasibility of such a separation. For nearly complete liberation of mineral inclusions from the organic matrix, both coal and shale need to be ground to a very fine particle size (below 5 ?m). This is typically true for most of the mineral ores. The driving force in the electrostatic beneficiation of coal and shale is the observation that carbonaceous and non-carbonaceous matter can be imparted positive and negative surface charges, respectively, with a copper tribocharger. The polarity of surface charge is found to depend on the work function values of the particles and the tribocharger. Separation tests in a batch laboratory electrostatic separator showed that the efficacy of the electrostatic separation is strongly dependent on the hydrodynamic conditions such as gas velocity, electric field strength, and particle concentration in carrier gas. A dimensionless group called an ‘electrodiffusion number’ was identified which qualitatively described the separation process. Furthermore, the extent of separation was found to be limited by a strong cohesive force acting between the oppositely charged particles which resulted in the formation of agglomerates.