New recycling technology using nanoscience one step closer
Unsustainable demands on the world’s energy and material resources raise the call for practical solutions, and nanosciences have their part to play. For instance, separation of solids according to density are enabled by colloidal dispersions of superparamagnetic nanoparticles. This is of great interest in the recycling of mixed waste. Still large-scale use remains a challenge, as you would need very strong magnets ánd magnetic ferrofluids that remain stable. Researchers from HFML and Utrecht University have found ferrofluids that remain stable at high fields, and proved that stability at moderate fields is predictive of stability at much higher fields. This facilitates the development of new ferrofluids dedicated to magnetic density separation.
Magnetic density separation (MDS) 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. How it works is that a ferrofluid is attracted towards a magnet, and consequently, nonmagnetic objects dispersed inside the fluid are effectively repelled by the magnet.
This approach has long been applied in the diamond industry and in the recovery of nonmagnetic metals, although in both cases, separation yields only two fractions, one that sinks and one that floats. In a recent improvement on this approach, different materials float at separate heights inside the fluid and exit in several density fractions, in a continuous single-step process.
Implementation on industrial scale is complicated by the large distance across which the effective density gradient must be realized. It implies that very strong magnets have to be developed and that the magnetic fluid must remain stable at very high fields. Researchers now show optimal stability is indeed achievable with dilute aqueous magnetic nanofluids at fields up to 10 T and gradients up to 100 T/m. Comparison with results obtained with a small neodymium magnet indicate that stability at moderate fields is predictive of stability at much higher fields.
These insights are useful for the development of new ferrofluids suitable for magnetic density separation. Implementation on industrial scale, however, also depends on many other considerations, not least of which are the economic aspects of building a high-field setup and feeding it with large quantities of high-quality ferrofluid.
Colloidal Stability of Aqueous Ferrofluids at 10 Tesla, Alex M. van Silfhout, Hans Engelkamp and Ben H. Erné, J. Phys. Chem. Lett. 11, 5908-5912 (2020)