Magnetic Sedimentation in Aqueous Ferrofluids: for Magnetic Density Separation

Abstract

Magnetic Density Separation (MDS) is a recycling technology that can separate several different materials in a single processing step. This requires a dilute ferrofluid that ideally should remain homogeneous in external magnetic fi eld. In practice, magnetic sedimentation and the resulting loss in homogeneity of dilute ferrofluids are an issue. Nevertheless, the practical impact of sedimentation can be limited by minimizing the sedimentation rate, which depends on colloidal stability and size of the particles. In this thesis, the main objectives are: (1) to characterize the rate of magnetic sedimentation in dilute aqueous ferrofluids of potential interest to MDS and (2) to understand the rate at which sedimentation occurs. Several methods for measurement and modelling of sedimentation of magnetic nanoparticles in magnetic elds are presented.

In the experiments, three types of dilute aqueous ferrofluid are analyzed, two ferrofluids received from Urban Mining Corporation, a company developing the MDS technique, and one ferrofluid synthesized in the Van 't Hoff Laboratory in Utrecht via a well-known method to prepare citrate-stabilized ferrofluids. Sedimentation rates of these ferrofluids show signi cant differences, with the ferrofluid synthesized in our laboratory showing sedimentation rates as expected for nanoparticles dispersed as single particles. The other two ferrofluids show signi cantly faster sedimentation rates.

Low-field sedimentation experiments were performed in magnetic fields up to 0.6 T using permanent magnets. Sedimentation was monitored using magnetic measurements and x-ray transmission. High-field sedimentation experiments were performed in a Bitter electromagnet at the HFML in Nijmegen. Sedimentation was monitored by measuring optical transmission profiles.

A theoretical model was developed, with which time-dependent concentration profiles for dilute ferrofluids can be predicted in arbitrary magnetic field and magnetic field gradients. In this model, three main forces are taken into account: (1) the magnetic force on a particle, (2) the force arising from a gradient in osmotic pressure, and (3) the friction force due to movement of the particle relative to the solvent.