Colloidal Stability of Aqueous Ferrofluids at 10 T

Abstract

Magnetic density separation is an emerging recycling technology by which several different waste materials—from plastic products, electronics, or other—can be sorted in a single continuous processing step. Larger-scale installations will require ferrofluids that remain stable at several teslas, high magnetic fields at which colloidal stability was not investigated before. Here we optically monitor the concentration profile of iron oxide nanoparticles in aqueous ferrofluids at a field of 10 T and a gradient of 100 T/m. The sedimentation velocities and equilibrium concentration profiles inform on maintenance or breakdown of colloidal stability, which depends on the concentration and magnetic coupling energy of the nanoparticles. Comparison with results obtained with a small neodymium magnet indicate that stability at moderate fields is predictive of stability at much higher fields, which facilitates the development of new ferrofluids dedicated to magnetic density separation.

Magnetic Sedimentation Velocities and Equilibria in Dilute Aqueous Ferrofluids

“…In summary of the information on hydrodynamic size, obtained from magnetic sedimentation, DLS, and AUC, the three techniques do not agree with each other quantitatively. It is puzzling that the citrate-stabilized particles, which do not form dipolar structures in external field ( Figure 5 a, SE) because of insufficient magnetic coupling energy, 20 sediment more slowly in magnetic fields than expected from their physical size. If sedimentation had been more rapid than expected, this could have been ascribed to cooperative magnetophoresis, with the formation of field-induced structures that are more strongly accelerated than single particles in the magnetic gradient.…”

“… Volume-weighted particle size distributions as found by TEM analysis (blue histograms), VSM analysis (dashed red lines), and the sedimentation equilibrium fit (solid black lines) for (a) citrate ferrofluid, (b) PPEG ferrofluid, (c) FT ferrofluid, and (d) UMC ferrofluid. Parts of this figure were previously published in a different form in ref ( 20 ). …”

“…Profiles are shown at the same time points as in panel (c). Parts of this figure were previously published in a different form in ref ( 20 ).…”

“… 19 On this basis, we recently demonstrated that colloidal stability at fields of up to 10 T can be predicted from the magnetic coupling energy and colloidal concentration. 20 …”

Superconducting magnets for magnetic density separation : a NbTi based demonstrator

“…In the MDS process, illustrated in Figure 1.1, shredded feed particles are immersed in a superparamagnetic fluid (ferrofluid) that flows over a magnet. The fluid is a colloid of superparamagnetic nanoparticles, usually water-based [25], that is magnetically saturated using a magnet that generates a vertical magnetic field gradient [2]. The competition between gravity-acting on the feed particlesand magnetic attraction-acting on the ferrofluid-leads to a net force on the feed material that pushes it up to an equilibrium height z eq , which depends on its mass density.…”