Conduction-Cooled Nb-Ti Magnet System for Magnetic Density Separation
Gonçalo Tomás is a PhD student in the Department Energy, Materials and Systems. Co(Promotors) are prof.dr.ir. H.J.M. ter Brake, prof.dr.ir. H.H.J. ten Kate and dr. M.M.J. Dhalle from the Faculty of Science & Technology.
This thesis delves into the design, construction, and commissioning of a conduction cooled Nb-Ti superconducting magnet system operating at 4.5 K achieved by a cryocooler, tailored for magnetic density separation (MDS). The magnet was constructed and commissioned at the University of Twente, followed by in-field sorting tests at the facilities of the MDS technology user, the company Umincorp.
MDS stands out as an innovative sorting technology capable of segregating non-magnetic shredded waste based on mass density. This groundbreaking approach combines a magnetic fluid (ferrofluid) with a magnet, generating a vertical magnetic field gradient that imparts an upward force on waste particles within the ferrofluid. The resulting upward force diminishes with distance from the magnet and consequentially, particles with different mass densities remain at distinct distances (equilibrium heights) from the magnet. This facilitates the continuous and parallel sorting of waste with multiple densities, potentially offering a cost-effective and high-throughput technique.
State-of-the-art MDS systems rely on permanent magnet technology. However, these systems encounter limitations in both magnetic field strength and resolution. The restricted magnetic field necessitates highly concentrated ferrofluid, leading to substantial operational expenditure (OPEX) due to fluid loss during operation. Moreover, the limited resolution hampers the sorting of waste with similar mass densities, resulting in diminished quality of the sorted product, particularly for waste streams like household plastics. The constraints posed by permanent magnet technology can be effectively overcome by adopting superconducting magnets, exemplified by the Nb-Ti demonstrator system presented in this thesis.
The designed and constructed magnet, successfully underwent cool-down to its operating temperature in approximately 12 days, as anticipated, and reached nominal current of 300 A without requiring training. Following the successful magnet commissioning, it was relocated to Umincorp’s facilities for preliminary ferrofluid stability and sorting tests, conducted in collaboration with Umincorp personnel and Delft University. The test results were primarily evaluated visually, and overall, all results were deemed satisfactory, particularly considering the instability of the ferrofluid. The system was then moved to the university of Delft for integration into a large sorting system, where further sorting tests will be performed.
In anticipation of the next generation of superconducting magnets for MDS, a study of economics of MDS has been undertaken for three different household waste streams: E-waste; plastics; and low-density plastics. The study reveals that use of a superconducting magnet results in significantly lower total costs compared to a permanent magnet, especially in E-waste sorting. From this study, design guidelines for an optimal superconducting system for each waste stream were derived, providing valuable insights for the development of cost-effective MDS systems.
