Researchers use ionic liquids for recycling bonded NdFeB permanent magnets
16 April 2020
Researchers in Europe have demonstrated the use of ionic liquids for recycling bonded NdFeB permanent magnets. An open-access paper on their work is published in the RSC journal Green Chemistry.
Permanent NdFeB magnets are of two types: sintered and (polymer or resin) bonded magnets. Both types are (partly) built up by magnet alloy powders that can exhibit flow characteristics when very fine.
For sintered magnets, the magnet powder is pressed into a shape and punched out with the help of lubricants. The obtained product remains very brittle but, by heating in a vacuum furnace, its density and mechanical strength are greatly improved.
Bonded NdFeB magnets are typically obtained by mixing NdFeB magnet powder with a polymeric binder at an appropriate mass ratio in a mixer or extruder and subjecting the pellet-shaped extrudate to injection or compression molding.
During the last decade, a lot of attention has been devoted to the recycling of sintered NdFeB magnets and these efforts have been reviewed elsewhere. In general, these routes include pyrometallurgical, hydrometallurgical and solvometallurgical approaches, or a combination of them. In contrast, very limited attention has been given to the recycling of bonded NdFeB magnets. This lack of attention can be explained by the much lower rare-earth content of the bonded magnets compared to that of sintered magnets, as well as by the fact that the presence of the polymeric binders or resins makes direct recycling routes (such as hydrogen decrepitation) difficult.
… Ionic liquids (ILs) are known to be excellent green solvents for many types of synthetic polymers and biopolymers. It has also been reported that Lewis-acidic chloroaluminate ionic liquids can be used to dissolve epoxy resins of tantalum capacitors. For these reasons, we decided to explore the use of ionic liquids for the recycling of bonded NdFeB magnets for the first time. This study is also the first one to collect different types of bonded magnets from the global magnet market to simulate a real life-like situation. The dissolution behavior of different types of polymeric binders in ionic liquids was then tested on a selection of bonded magnets. The most promising solvent system was tested on larger scale to prepare a batch of magnet powder that was used to produce new anisotropic epoxy bonded magnets. The magnetic properties of these new magnets were measured and compared to those of commercial counterparts to assess the overall efficiency of the recycling process.—Önal et al.
The researchers, from KU Leuven in Belgium, the University of Birmingham in the UK, and Kolektor Magnet Technology GmbH in Germany, found three main types of polymers in commercial bonded NdFeB magnets: polyamides (PA6 and PA12), poly-p-phenylene sulfide (PPS) and epoxy.
Both types of polyamide resins were easily dissolved by ionic liquids with coordinating anions (chloride, acetate or dialkylphosphate). Removal of the PPS resin was not possible by ionic liquid solvents, but only by using 1-chloronaphthalene and 1,3,5-triphenylbenzene at high temperatures.
Although epoxy could be removed by several ionic liquids, reaction between the NdFeB powder and the ionic liquids was observed. A batch of PA6-bonded magnets was treated with an ionic liquid tributylethylphosphonium diethylphosphate, [P4442][Et2PO4], to selectively remove the polymeric portion. The resulting PA-free magnet powder was found to retain >90% of its original magnetic properties.
The researchers produced two new epoxy-bonded magnets produced with this recycled magnet powder; the magnets showed magnetic properties that were close to those of commercial counterparts, thereby showing the versatility of the process and that the materials loop could be successfully closed.
This work shows that ionic liquid processing of end-of-life bonded NdFeB magnets is a promising method for direct recycling of these magnets. In addition to being environmentally friendly, the process is also a versatile route that allows production of secondary bonded magnets with new design and polymeric material.—Önal et al.
Önal, Mehmet Ali Recai and Dewilde, Sven and Degri, Malik and Pickering, Lydia and Saje, Boris and Riaño, Sofía and Walton, Allan and Binnemans, Koen (2020) “Recycling of bonded NdFeB permanent magnets using ionic liquids”Green Chem. doi: 10.1039/D0GC00647E