Closed-loop magnetic separation of nanoparticles on a packed bed of spheres

In this work, we consider magnetic separation of iron oxide nanoparticles when a nanoparticle suspension (diluted ferrofluid) passes through a closed-loop filter composed of a packed bed of micro-beads magnetized by an externally applied magnetic field. We show that the capture of nanoparticles of a size as small as 60 nm is easily achieved at low-to-moderate magnetic fields (16–32 kA/m) thanks to relatively strong magnetic interactions between them. The key parameter governing the capture process is the Mason number—the ratio of hydrodynamic-to-magnetic forces exerted to nanoparticles. The filter efficiency, Λ, defined through the ratio of the inlet-to-outlet concentration shows a power-law dependency on Mason number, Λ∝Ma−0.83, in the range of 102