Role of gravitational force on mechanical entrainment of nonmagnetic particles in high gradient magnetic separation

Mechanical entrainment is an important mechanism responsible for the low selectivity in high gradient magnetic separation (HGMS). Previous studies on mechanical entrainment mainly focused on particle motion behaviors or interparticle forces, a very important factor attributing entrainment was neglected, namely the effect of gravi-tational force. In this paper, the role of gravitational force on mechanical entrainment was systematically investigated with varying feeding direction, fluid velocity, size and density of nonmagnetic particles. Results indicated that gravity played a vital role in mechanical entrainment. Nonmagnetic particles were preferentially entrained in deposits of upper surface (relative to gravity direction) rather than upstream surface (relative to flow direction) due to gravity. Mechanical entrainment became more serious with increasing gangue mineral density. Mechanical entrainment of low-density gangue could easily be eliminated with moderate fluid velocity or pulsating frequency, but that caused by large density gangue in upper deposit could hardly be eliminated, even with high fluid velocity or high strength pulsating. Weakening the effect of gravity with nonmagnetic heavy medium (or heavy suspension) should be promising method to enhance selectivity in HGMS, which will be conducted theoretically and experimentally in near future.