R. Gupta et al., ELECTROSTATIC SEPARATION OF POWDER MIXTURES BASED ON THE WORK-FUNCTIONS OF ITS CONSTITUENTS, Powder technology, 75(1), 1993, pp. 79-87
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.