Continuous Size Fractionation of Nanoparticles Using Magnetic Field Controlled SMB

Technical relevance of diverse magnetic nanoparticles is rising steadily in numerous and distinct fields. Their application potential is increasing strongly in the field of tumor treatment, for instance, where iron oxide nanoparticles are used to provide magnetic hyperthermia procedures or targeted drug delivery therapies. Advanced quality criteria such as high purities or narrow particle size distributions can only be achieved to a certain extent by the synthesis process. The supply of rare earth particles such as holmium or dysprosium is as well essential for many 21st century technologies such as luminescent material, electronic, catalyst, battery and alloy technologies. Efficient recycling and waste management of such technology-critical materials is crucial as the demand is expected to further grow rapidly within the next few years. Thus, in order to remove undesirable contaminants and/or achieve an improvement regarding particle size distribution, it is often vital to perform a size fractionation after the synthesis process or as part of a recycling process.

For the particle size range in the nanometric scale, however, fractionation processes encounter technical difficulties with only limited solutions discovered, yet. ... mehrWell-known, batch-wise methods such as ultracentrifugation do not provide an efficient, continuous fractionation on an industrial scale for particles of around 50 - 1000 nm in size. However, on a molecular scale, size exclusion is very effective for the fractionation of protein aggregates or ultra small particles of a few nanometers. Here, the mass transport between the liquid bulk phase and the inner surface of the stationary phase is dominated by convection and diffusion, with convection having a significant effect only in the bulk. In the case of nanoparticles larger than 50 nm, however, mass transport by diffusion becomes increasingly ineffective. Hence, an efficient and scalable size separation of particles in a range of 50 nm – 1000 nm is still a major challenge because a complex superposition of forces occurs in this size range. Concludingly, high-resolution size fractionation of magnetic nanoparticles remains an intensively studied scientific field.


In this work, a novel method for continuous fractionation of ultra-fine magnetic particles is being investigated and transferred to a Simulated Moving Bed (SMB) chromatography system, providing a robust process which shows great potential for industrial application. The technique is based on the competition between hydrodynamic and magnetic forces within a chromatography column (see Figure 1). Magnetic forces are induced using a magnetizable matrix, while the hydrodynamic forces are changed by regulating the flow rate through the column. When transferring this technique to a continuous SMB system (see Figure 2), excellent particle recovery rates of up to 99.9 % (w/w) and space-time yields of up to 49 mg/(L*min) combined with high separation sharpnesses as well as feasible scalability are shown. Consequently, the method constitutes a very promising approach for selective size fractionation of nanoscale magnetic particle systems. High method application flexibility is being demonstrated by successful size and susceptibility fractionations of different particle classes, for example ferrimagnetic magnetite particles, paramagnetic dysprosium oxide particles as well as diamagnetic silica particles. In direct comparison to other separation methods such as ultracentrifugation or membrane filtration, the used SMB system offers a continuous operation mode. Furthermore, easy scalability, high space-time yields and a relatively low energy consumption offers capability for large-scale refinement of magnetic nanoparticles.