Hydrometallurgical Treatment of Neodymium Magnet Waste

Doctoral thesis, 2018

Recent decades have seen a considerable increase in the usage of rare-earth elements (REEs) in modern technologies and green energy sources. Recycling of REEs out of end-of-life products and E-scrap has become an alternative to mining them out of primary ores due to their supply risk in some countries and the development towards circular economies. Neodymium (NdFeB) magnets are of special interest since they are present in various technological waste streams. They contain considerable amounts of REEs such as Nd, Dy, Pr and some others, for example Gd and Tb, depending on the specific application, making them very attractive for REE-recycling. Apart from REEs, neodymium magnets are made up of around 60% iron, which can pose a challenge in their recycling.

Hydrometallurgical methods such as leaching and solvent extraction are attractive and efficient methods for the recovery of REEs out of NdFeB magnets, albeit with certain drawbacks such as large aqueous and organic waste generation during the process and utilization of some hazardous chemicals. The REEs are normally leached out of the NdFeB magnet waste using strong mineral acids such as HCl, HNO3 and H2SO4 but, despite their excellent leaching properties for REEs out of NdFeB magnets, they pose some risk to the environment because there are still issues with poisonous gas evolution during leaching, regeneration of the used acids, and handling of highly concentrated acids can be a challenge. Furthermore, the extracting agents currently used in the industry for REE-extraction are mostly phosphorus-based and do not follow the CHON principle, meaning it is not possible to incinerate them without either the production of ash or acidic gases.

In this work a comparison of leaching efficiency between the traditionally used mineral acids and organic lixiviants was performed. Magnet powder was successfully leached using fully combustible organic lixiviants (including acetic, citric, maleic, glycolic and ascorbic acid), and new green leaching alternatives were developed. Parameters including acid concentration, leaching time, S/L ratio and temperature were investigated and mitigated. Subsequently, the REEs were selectively extracted from these leachates. For this separation step several phosphorus-based extractants (TBP, D2EHPA, Cyanex 272 and 923) were investigated, alongside TODGA, which follows the CHON principle. The influence of various diluents on the extraction was also studied. It was concluded that REEs can be separated into relatively pure aqueous streams using organic acids instead of mineral acids under certain conditions, while TODGA was efficient at separating REEs from large amounts of Fe in these particular waste streams. A process for the extraction of REEs from organic acids leachates was developed, with promising results.