ELECTROSTATIC SEPARATION OF POWDER MIXTURES BASED ON THE WORK-FUNCTIONS OF ITS CONSTITUENTS

The objective of this study was to investigate the feasibility of a dr y electrostatic process to separate a powder mixture into its componen ts based on their work functions. We studied the dry electrostatic ben eficiation of high-sulfur, high-ash Illinois coals and relatively low carbon-containing oil shales, and separation of synthetic mixtures con sisting of charcoal and silica to demonstrate the feasibility of such a separation. For nearly complete liberation of mineral inclusions fro m the organic matrix, both coal and shale need to be ground to a very fine particle size (below 5 mum). This is typically true for most of t he mineral ores. The driving force in the electrostatic beneficiation of coal and shale is the observation that carbonaceous and non-carbona ceous matter can be imparted positive and negative surface charges, re spectively, with a copper tribocharger. The polarity of surface charge is found to depend on the work function values of the particles and t he 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 identifi ed 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 i n the formation of agglomerates.