Study of the Thermal Demagnetization Process of NdFeB Magnets
Authors: Torta Gianluca , Ciacci Luca , Vassura Ivano , Passarini Fabrizio 
Published: November 2024

Currently, less than 1% of rare earth elements (REEs) are recycled due to the limitations of conventional recycling methods. Waste streams containing NdFeB magnets, such as household appliances, industrial motors, electric vehicle (EV) motors, and consumer electronics, are typically processed using traditional recycling techniques. These methods often involve shredding the material and applying separation techniques like magnetic and eddy current separation, which are designed to recover materials such as copper, iron, aluminum, plastic, and precious metals. However, they do not effectively recover REEs. Studies have shown that the ferrous fraction from typical recycling facilities for ferrous waste contains a REEs concentration which is too low to be economically viable for REE recovery. This creates a pressing need of designing recycling processes able to valorize the REEs fraction. A critical step in these processes is the demagnetization of NdFeB magnets, without which their efficient separation from the waste stream would not be feasible. Demagnetization is necessary when processing waste through mechanical shredding and separation to prevent the strong magnetism of NdFeB magnets from causing operational issues. Issues such as the formation of “meatballs” (clusters of magnetic material) and magnets sticking to ferromagnetic parts of machinery are common when magnets remain magnetized. Moreover, demagnetization facilitates the separation of magnets from non-ferromagnetic fractions in the waste stream. Demagnetization can be achieved using different techniques. One approach is hydrogen decrepitation, where hydrogen is absorbed by the NdFeB magnet, leading to brittle hydride formation that breaks down the magnet into a fine powder, which simultaneously induces demagnetization. Alternatively, thermal treatment can be used to raise the temperature of the magnets above their Curie temperature, at which point they lose their magnetic properties. This study focused on the thermal demagnetization of NdFeB magnets contained within electric motors. Using an experimental design approach, we developed demagnetization curves that correlate the degree of demagnetization with key parameters such as temperature and treatment duration. The study also examined potential differences in demagnetization behavior when magnets were treated as part of an assembled motor versus individually. NdFeB magnets were subjected to varying temperatures and time intervals to generate these demagnetization curves, providing insights into the behavior of both individual magnets and those integrated within motor assemblies. Notably, it was observed that the average degree of demagnetization achieved varied depending on whether magnets were treated within the rotor or individually. This suggests that the assembly within the rotor affects the demagnetization response, resulting in a slower demagnetization process. The experimental design (DOE) method enabled the development of a predictive model that estimates the degree of demagnetization based on time and temperature parameters. Results indicated that the coefficient for temperature (T) in the model consistently exerted a stronger influence on demagnetization than the coefficient for time (t), suggesting that temperature has a greater impact on demagnetization process.