Recycling of Sintered NdFeB Magnet Waste via Oxidative Roasting and Selective Leaching

Authors: W. Kritsarikan, T. Patcharawit, T. Yingnakorn, S. Khumkoa

Abstract:

Neodymium-iron-boron (NdFeB) magnets classified as high-power magnets are widely used in various applications such as automotive, electrical and medical devices. Because significant amounts of rare earth metals will be subjected to shortages in the future, therefore domestic NdFeB magnet waste recycling should therefore be developed in order to reduce social and environmental impacts towards a circular economy. Each type of wastes has different characteristics and compositions. As a result, these directly affect recycling efficiency as well as types and purity of the recyclable products. This research, therefore, focused on the recycling of manufacturing NdFeB magnet waste obtained from the sintering stage of magnet production and the waste contained 23.6% Nd, 60.3% Fe and 0.261% B in order to recover high purity neodymium oxide (Nd2O3) using hybrid metallurgical process via oxidative roasting and selective leaching techniques. The sintered NdFeB waste was first ground to under 70 mesh prior to oxidative roasting at 550–800 oC to enable selective leaching of neodymium in the subsequent leaching step using H2SO4 at 2.5 M over 24 h. The leachate was then subjected to drying and roasting at 700–800 oC prior to precipitation by oxalic acid and calcination to obtain Nd2O3 as the recycling product. According to XRD analyses, it was found that increasing oxidative roasting temperature led to an increasing amount of hematite (Fe2O3) as the main composition with a smaller amount of magnetite (Fe3O4) found. Peaks of Nd2O3 were also observed in a lesser amount. Furthermore, neodymium iron oxide (NdFeO3) was present and its XRD peaks were pronounced at higher oxidative roasting temperatures. When proceeded to acid leaching and drying, iron sulfate and neodymium sulfate were mainly obtained. After the roasting step prior to water leaching, iron sulfate was converted to form Fe2O3 as the main compound, while neodymium sulfate remained in the ingredient. However, a small amount of Fe3O4 was still detected by XRD. The higher roasting temperature at 800 oC resulted in a greater Fe2O3 to Nd2(SO4)3 ratio, indicating a more effective roasting temperature. Iron oxides were subsequently water leached and filtered out while the solution contained mainly neodymium sulfate. Therefore, low oxidative roasting temperature not exceeding 600 oC followed by acid leaching and roasting at 800 oC gave the optimum condition for further steps of precipitation and calcination to finally achieve Nd2O3.

Keywords: NdFeB magnet waste, oxidative roasting, recycling, selective leaching

References:

[1] J. H. Rademaker, et al., “Recycling as a Strategy against Rare Earth Element Criticality: A Systemic Evaluation of the Potential Yield of NdFeB Magnet Recycling,” Environ. Sci. Technol. 2013, 47, 10129−10136.
[2] W. Benecki, “2016 Global Permanent Magnet Production in Magnetics 2017 the international forum on magnetic applications, technologies & materials January 18-19 Orlando, FL, USA.”
[3] A. Schreiber, et al., “Comparative Life Cycle Assessment of Neodymium Oxide Electrolysis in Molten Salt,” Advanced Engineering Materials, 2020(6).
[4] M.A. Önal, et al., “Recycling of NdFeB Magnets Using Sulfation, Selective Roasting, and Water Leaching 2015,” Springer Science + Business Media: Germany. p. 199.
[5] J.P. Rabatho, et al., “Recovery of Nd and Dy from rare earth magnetic waste sludge by hydrometallurgical process.” The Journal of Material Cycles and Waste Management, 2013. 15(2): p. 171-178.
[6] L. D. Almeida, et al., “Insights into the Thermal Decomposition of Lanthanide (III) and Actinide (III) Oxalates – from Neodymium and Cerium to Plutonium,” Eur. J. Inorg. Chem. 2012, 4986–4999.
[7] T.V. Hoogerstraete, et al., “From NdFeB magnets towards the rare-earth oxides: a recycling process consuming only oxalic acid. 2014,” RSC Publishing: Great Britain. p. 64099.
[8] F. Liu, et al., “Comparison of Different Leaching Media and Their Effect on REEs Recovery from Spent Nd-Fe-B Magnets,” JOM, 2020. 72(2): p. 806-815.
[9] Xin, W., et al., “Phase evolution and oxidation characteristics of the Nd–Fe–B and Ce–Fe–B magnet scrap powder during the roasting process,” High Temperature Materials and Processes, 2020. 39(1): p. 477-488