Recycling of Additively Printed Rare-earth Bonded Magnets
In this work, we describe an efficient and environmentally benign method of recycling of additive printed Nd−Fe−B polymer bonded magnets. Rapid pulverization of bonded magnets into composite powder containing Nd-Fe-B particles and polymer binder was achieved by milling at cryogenic temperatures. The recycled bonded magnets fabricated by warm compaction of ground cryomilled coarse composite powders and nylon particles showed improved magnetic properties and density. Remanent magnetization and saturation magnetization increased by 4% and 6.5% respectively, due to enhanced density while coercivity and energy product were retained from the original additive printed bonded magnets. This study presents a facile method that enables the direct reuse of end-of-life bonded magnets for remaking new bonded magnets. In addition to magnetic properties, mechanical properties comparable to commercial products have been achieved. This research advances efforts to ensure sustainability in critical materials by forming close loop supply chain.
Near-net shaped polymer bonded rare‐earth transition‐metal (RE-TM) gap magnets with intermediate performance between ferrites and sintered Nd-Fe-B magnets could have many important technological applications in sensors and electric drive technologies due to many unique advantages. Bonded magnets are comprised of two components; hard magnetic powder and nonmagnetic polymer binder. Hard magnetic powders can be permanent magnet materials such as NdFeB, Sm-Co, Sm-Fe-N and Ferrite (ceramic magnets). Typical binders used for the fabrication of bonded magnets include nylon, polyphenylene sulfide (PPS), and thermoset epoxies. Thermoplastic binders can be used to form complex shapes by injection molding or extrusion processes. In a typical compression molding process, isotropic Nd-Fe-B powders are coated with a thermoset epoxy binder and compacted into a simple shape using an uniaxial hot press. An exciting processing technology that is revolutionizing the manufacturing sector, and is beginning to advance the production of bonded magnets is additive manufacturing.
Additive manufacturing (AM) (commonly recognized as 3D printing) processes enable the joining of materials layer by layer to make objects using computer-aided design (CAD) models. It offers a cost-effective and time-efficient approach to manufacture products with complex geometries, advanced material properties and multifunctionality.10 According to Wohlers report 2018, 21% growth rate was reported for the AM industry with market share exceeding $7.3 billion for the year 2017. Compared with conventional manufacturing processes, AM technologies should potentially produce minimum or no wastes, and can be more energy efficient. Nevertheless, some amount of wastes are generated during purging, and in some cases the amount is inadvertently much higher than expected due to human, design and machine errors. For example, laser sintering (LS) – additive manufacturing generates up to 44% waste, and fused deposition modeling (FDM) printers generates nearly 34% of the plastic waste. Considering the exponential growth and high demand for 3D printing manufacturing technologies in both near and long terms, residues generated during AM process is bound to significantly increase, and therefore efficient recycling is inevitable from environmental and waste management standpoints. Most importantly, recycling will help to ensure additional economic benefits for the sustainability of AM processes without limiting the other benefits.
Recently, recycling of magnets containing expensive rare earth elements (REE) have drawn considerable attention due to the concern over potential disruptions in the supply of REEs. To address this challenge, various recycling processes are developed to recover the REEs from preconsumer (manufacturing) scrap and from complex post-consumer end-of-life Nd-Fe-B permanent magnet scrap. Recycling processes of Nd-Fe-B type sintered magnet into bonded magnets have also been reported. However, research and development to recycle polymer bonded magnets have shown little progress because the recovery of rare earth magnetic powder from polymer composites is challenging. For instance, recovery of magnet powders from rare earth bonded magnets by a dissolution process at high temperature (230°C) have been reported in which chemicals for dissolution use at least one solvent from a group comprising tetralin, naphthalene, 1,4-hydroxynaphthalene, naphthol, biphenyl, etc.23 Another recycling method in which magnet powders are separated by heating bonded magnets at temperatures as high as 1200°C to decompose resins, have also been reported. These methods for magnet powder recovery are typically complex and often involve the use of harsh chemicals and/or high temperatures. The complexity of these processes suggests that they may be cost-prohibitive to implement, especially when REEs prices are low. As a result, they cannot be deployed as routine recycling processes. Thus there is a need for developing cost-effective and process-efficient method for reusing and recycling REEs.
Developing deployable magnet recyclying technology requires processes that are energy efficient, potentially profitable when deployed commercially and have less detrimental impact on the environment. For bonded magnets, these conditions are easier to meet if minimal reprocessing is applied to enable reuse, rather than elemental recovery of the REE contents; which requires additional investments in energy and other resources. In fact, elemental recovery is unlikely to result in products that will be used in applications other than permanent magnets because REEs are often recovered as mixed oxides of constituent elements. In general, there needs to be a decision to reuse with minimal reprocessing or elemental recovery of the REEs with less number of processing steps. In our recent study, Sm-Co powder collected from industrial swarfs (wastes generated from post-manufacturing processing of magnets such as grinding, polishing, etc.) was studies for use in magnetic filaments for 3D printing of bonded magnets.
In this work, additively printed Nd–Fe–B bonded magnets were used to develop a recycling process for bonded magnets, enabled by cryogenic pulverization (cryomilling) and subsequent remanufacturing of the bonded magnets. We are unaware of any previous study in which cryomilling enabled the recycling of bonded magnets, particularly those derived from additive manufacturing. Thermal, magnetic and mechanical properties have been studied to evaluate the performance of the recycled magnets. The newly developed method is exemplified here by recycling of nylon bonded Nd–Fe–B magnets.