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

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.