A study of vortex magnetic separation (VMS) with a high-speed video system

Abstract

This paper describes the work with a high-speed video system and an image processing computer system on the mechanisms involved in Vortex magnetic separation (VMS). The high-speed video system can clearly display individual particle trajectories at Reynolds Numbers of 6 – 40 which made it possible to measure and calculate particle position and velocity distribution with reference to a wire by means of an image processing computer system. Based on the analysis of variations of particle trajectories and velocity distribution around a single wire at different testing conditions in the process of vortex magnetic capture, it is revealed that the repulsive regions on the magnetized wire make magnetic particles separate from the wire surface at φ < 90°, where φ is the angle measured from the upstream stagnation point on the wire. Although some particles do follow the boundary layer flow to enter the vortex region behind the wire and be captured there, the probability for retention on the downstream side of the wire is reduced. When the repulsive regions on the wire is very small (e.g. H 0 < H s ), the trajectory is different, magnetic particles roll along the wire surface, enter the vortex region and are captured on the downstream of the wire. The nature of the boundary layer flow is another important factor in VMS process. Only those magnetic particles which can stay in the boundary layer can be involved in the downstream capture. The trajectory analysis also shows the inertia force acting on non-magnetic particles is the main factor which allows their escape from the boundary layer around the wire and this results in VMS process producing the secondary magnetic concentration of high grade.