Modelling the size separation of NdFeB magnetic microparticles by magnetophoresis and gravity settling

Abstract

Properties of sintered NdFeB magnets strongly depend on granulation and size distribution of constituent particles, which requires an efficient method for their separation into relatively narrow size fractions. We investigated two methods of magnetic particle separation from a mixture with different sizes using simulation by finite element method: magnetophoresis and gravity settling. In the case of magnetophoresis magnetic particles ranging in diameter from 1 to 10 m were deflected from the direction of continuous laminar flow by a perpendicular magnetic field. Larger particles were deflected from the direction of laminar flow more than smaller particles. The applied flow rate and strength and gradient of the applied magnetic field were the key parameters in controlling the deflection. The gravity settling model simulated spherical particles falling in heptane. Particles of various sizes were divided according to the time they needed to reach the bottom. The model used an axially symmetric fluid-flow simulation in a moving coordinate system connected with the particle, coupled with an ordinary differential equation for the force balance of the particle (gravity and drag force). The grain accelerated from standstill and rapidly reached its terminal velocity. This velocity was approximately proportional to the square of the particle radius, which led to clear separation of 5-10 um particles from those with a diameter of 1 um.