The extraction of NdFeB magnets from end of life hybrid and electric vehicle scrap and conversion into master alloy and sintered magnets

Awais, Muhammad ORCID: 0000-0001-8113-9693 (2022). The extraction of NdFeB magnets from end of life hybrid and electric vehicle scrap and conversion into master alloy and sintered magnets. University of Birmingham. Ph.D.

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Abstract

Magnets containing rare earth elements (REE) play a major role in green technologies such as hybrid and electric vehicles and wind turbines. In recent years, the increasing popularity of hybrid and fully electric cars and wind turbines is causing an increase in the demand for these elements. If magnets from these applications are recycled efficiently, it could create a parallel supply chain to the primary route for rare earths. This work is mainly focused on developing the recycling routes for the NdFeB magnets from these end-of-life automotive applications.
In this work, a Toyota hybrid electric and a fully electric Nissan Leaf was dismantled to identify components that could provide a possible stream of NdFeB material for recycling. It was shown that NdFeB magnets were contained in drive motors (1.9 kg), generators (≈ 0.1 kg), power steering motor (0.1 kg), regenerative braking system (0.2 kg) and loudspeaker (≈ 0.03 kg). An attempt was made to extract the NdFeB magnets from these components using hydrogen and the challenges are outlined in this work.
The hydrogenated NdFeB powder produced was then directly recycled into new magnets with varying amounts of coating residue with the addition of neodymium hydride. It was shown that by adding just 2 at.% NdH2.7, more than 90% of remanence and coercivity of the starting magnets can be achieved which are acceptable magnetic properties at an unusually large particle size distribution. Better magnetic properties were achieved in re-sintered magnets when a jet-milled powder was produced from an external provider. However, the porosity in the microstructure of the magnets was evident in both materials, which is thought to be linked to the size of the neodymium hydride used in this work (< 45 μm).
The recycled magnets produced from N42SH magnets were found to have a lower remanence at room temperatures and the loss in magnetic performance widened with the increase in operating temperature. However, the properties of the recycled magnets were comparable to a reference grade (N38SH) up to 100 oC. These recycled magnets were tested in a claw pole prototype along with commercially sourced virgin magnets using the open circuit back EMF test. The performance of virgin magnets was close to the simulated results, however, the test with the recycled magnets showed a 12% drop in performance compared to simulated results. This lower performance of recycled magnets was linked to misalignment caused during the cutting and grinding of the sintered magnets.
In the final part of the project, the extracted magnets were converted into book mould alloys by induction vacuum melting. The ICP analysis of the book mould cast alloy showed that the total rare earths and the oxygen content are slightly lower in most cases than the starting material. The slag was found to contain a slightly higher concentration of the rare earths and oxygen up to 9150 ppm, which indicated that the oxidised rare earths have separated in the slag phase, which is one of the main reasons for looking at this process route.
The book mould cast alloy was subsequently converted to a strip cast alloy and the ICP analysis showed that the amount of total rare earths had further decreased in the strip cast alloy which is likely due to sublimation. However, the SEM micrographs showed a columnar microstructure with little free iron, which should be ideal for sintered magnet production.
The work outlined in this thesis clearly shows that it is technically possible to extract and recycle NdFeB magnets from the end of life automotive waste and produce sintered magnets with comparable magnetic performance to the primary magnets, although of a slightly lower grade. It has also been shown that the economics could be challenging for certain applications due to the position of magnet containing components in the vehicle, due to the design of the components, the fact that they are glued and the choice of coating.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):