Mathematical modeling of separation behaviour of a spiral for washing high ash coal

Abstract

Sustained Coal R&D activity has been crucial to the many successful clean coal technology developments that have been achieved globally. The aim has been to enhance the competitiveness of coal by providing technical and environmental improvements whilst at the same time reducing capital and operating costs. In coal preparation, simpler lower cost alternatives to froth flotation, such as the coal dense medium separator and spiral separators, have been adapted for cleaning of fine coal. A mathematical model has been developed to characterize the separation behavior of a typical high ash coal in a spiral. The modeling framework consists of parametric representation of geometry of the spiral and its trough, particulate flow along the helical path and principal forces acting on a particle during its motion. The elements have been combined seamlessly by assuming that the particles eventually attain dynamic equilibrium in the forward longitudinal direction and static equilibrium in the transverse direction. The resulting force function provides a spectrum of the particles' radial location on the trough according to their size and relative specific gravity. The model predicts relative specific gravity distribution and particle size as a function of equilibrium radial position. Sensitivities of radial equilibrium distribution of particle size and relative specific gravity with respect to mean flow depth have also been investigated. Simulation results are encouraging and validated with the published data. The model provides an analytical tool for better understanding of the separation behavior of particles in a coal-washing spiral.