Presentation Information
[O5-4]Direct reduction of Rare Earth Oxides to Magnets
Rambabu Kuchi1, Jordan Schlage1, Ihor Z. Hlova1, Yaroslav Mudryk1, Robert Kinner2, Shaun O’Donnell3, Rebecca Smaha3, Sage Bauers3, *Matthew J. Kramer1 (1. Ames National Laboratory (United States of America), 2. Powdermet Inc. (United States of America), 3. National Renewable Energy Laboratory (United States of America))
Keywords:
RE2-14-1,Sm2Fe17N3,processing
Traditional production of rare earth (RE) permanent magnets involves melting and casting of the constituents. Refining the metals from the oxides is a costly and typically energy intensive process [1]. On the other hand, the light RE (LRE) metals are compatible with a co-reduction of the RE-oxide with Ca in the presence of Fe [2] since there is no solubility between Fe and Ca even in the liquid state [3]. The reduction of the LRE2O3 proceeds rapidly once the Ca is melted (842°C) in an inert environment and the Ca is mutually immiscible in all the RE metals except Eu and Yb [3]. This provides a direct route to form LRE-Fe intermetallic compounds from LRE2O3 [4, 5]. The challenge with this synthesis route is forming monolithic magnets, which require: 1) formation of phase pure LRE-Fe compounds, 2) separation of the intermetallic compounds from the CaO and other impurities, 3) formation of single grain, single phase powder with sizes of a few microns suitable for forming high coercivity magnets, 4) formation of grain aligned magnets.
In this presentation, we will discuss the synthesis steps to form magnets of Sm2Fe17N3 and Nd2Fe14B directly from their oxides. The advantage of this processing route for the Sm2Fe17N3 is we can produce well-controlled particles of ~ 3 µm with a coercivity (HC) of 10.7 kOe and a maximum energy product (BH)max of 17.3 MGOe for the grain aligned powders [6]. By densifying the Sm2Fe17N3 powders using high-pressure spark plasma sintering, a bulk magnet with a (BH)max of 21.1 MGOe with 88% of theoretical density was produced [7]. The high-temperature (at 1050 °C) processing of the Nd2O3+Ca+Fe+FeB+dispersant mixture results, according to both the literature [5] and our current work, in highly pure Nd2Fe14B powder after Ca-rich by-products are washed out by slightly acidic reagents. We demonstrated that for the Fe particles sized between 10 and 75 µm the optimized calcio-thermic reduction diffusion process can produce XRD-pure hard magnetic material with magnetization (M ~ 164 emu/g) approaching theoretical limit of 169 emu/g. However, the coercivity of the powder sample is rather small (Hc ~ 2 KOe), as expected, due to lack of a RE-rich grain boundary (GB) phase. The next steps in the optimization are to develop the appropriate GB phase to blend in with stoichiometric compound and reduce the overall oxygen content in our material. The goal is to establish the one-step direct metallization route from Nd2O3 to Nd2Fe14B as a viable alternative to the currently dominant strip-casting technology. The techno-economic analysis shows that the process can recuperate the investment cost and become profitable after one year of operation, assuming 80% conversion of Nd from oxide into the Nd2Fe14B compound is achieved.
Acknowledgements. This work was performed at the Ames National Laboratory and supported by the Critical Materials Innovation Hub funded by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Materials and Manufacturing Technologies Office. Ames National Laboratory is operated for the U.S. DOE by Iowa State University under contract DE-AC02-07CH11358.
References
1. Y. Ghorbani, I. M. S. K. Ilankoon, N. Dushyantha and G. T. Nwaila, "Rare earth permanent magnets for the green energy transition: Bottlenecks, current developments and cleaner production solutions," Resources, Conservation and Recycling 212, 107966 (2025).
2. M. Wagner and A. Allanore, "Chemical Thermodynamic Insights on Rare-Earth Magnet Sludge Recycling," ISIJ International 60 (11), 2339-2349 (2020).
3. T.B. Massalski, H. Okamoto, P.R. Subramanian, L. Kacprzak, Binary Alloy Phase Diagrams, 2nd Edition, ASM International, ISBN: 978-0-87170-403-0, 3,589p, 1990
4. S. Okada, K. Suzuki, E. Node, K. Takagi, K. Ozaki, Y. Enokido, Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reduction-diffusion process, J. Alloys Compd. 695 (2017) 1617–1623.
5. S. Kim, V. Galkin, J.R. Jeong, R. Kuchi, D. Kim, Enhanced magnetic and structural properties of chemically prepared Nd-Fe-B particles by reduction-diffusion method through optimization of heat treatments, J. Alloys Compd. 869 (2021), 159337.
6. R. Kuchi, T. A. Seymour-Cozzini, J. V. Zaikina, I. Z. Hlova, M. J. Kramer, Minimizing Particle Aggregation in Sm2Fe17N3 Powders: a CaO-Assisted Reduction-Diffusion Approach, (under review).
7. R. Kuchi, D. Schlagel, T. A. Seymour-Cozzini, J. V. Zaikina, I. Z. Hlova, Exploiting mechanochemical activation and molten-salt-assisted reduction-diffusion approach in bottom-up synthesis of Sm2Fe17N3, J. Alloys Compd. 980 (2024) 173532.
In this presentation, we will discuss the synthesis steps to form magnets of Sm2Fe17N3 and Nd2Fe14B directly from their oxides. The advantage of this processing route for the Sm2Fe17N3 is we can produce well-controlled particles of ~ 3 µm with a coercivity (HC) of 10.7 kOe and a maximum energy product (BH)max of 17.3 MGOe for the grain aligned powders [6]. By densifying the Sm2Fe17N3 powders using high-pressure spark plasma sintering, a bulk magnet with a (BH)max of 21.1 MGOe with 88% of theoretical density was produced [7]. The high-temperature (at 1050 °C) processing of the Nd2O3+Ca+Fe+FeB+dispersant mixture results, according to both the literature [5] and our current work, in highly pure Nd2Fe14B powder after Ca-rich by-products are washed out by slightly acidic reagents. We demonstrated that for the Fe particles sized between 10 and 75 µm the optimized calcio-thermic reduction diffusion process can produce XRD-pure hard magnetic material with magnetization (M ~ 164 emu/g) approaching theoretical limit of 169 emu/g. However, the coercivity of the powder sample is rather small (Hc ~ 2 KOe), as expected, due to lack of a RE-rich grain boundary (GB) phase. The next steps in the optimization are to develop the appropriate GB phase to blend in with stoichiometric compound and reduce the overall oxygen content in our material. The goal is to establish the one-step direct metallization route from Nd2O3 to Nd2Fe14B as a viable alternative to the currently dominant strip-casting technology. The techno-economic analysis shows that the process can recuperate the investment cost and become profitable after one year of operation, assuming 80% conversion of Nd from oxide into the Nd2Fe14B compound is achieved.
Acknowledgements. This work was performed at the Ames National Laboratory and supported by the Critical Materials Innovation Hub funded by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Materials and Manufacturing Technologies Office. Ames National Laboratory is operated for the U.S. DOE by Iowa State University under contract DE-AC02-07CH11358.
References
1. Y. Ghorbani, I. M. S. K. Ilankoon, N. Dushyantha and G. T. Nwaila, "Rare earth permanent magnets for the green energy transition: Bottlenecks, current developments and cleaner production solutions," Resources, Conservation and Recycling 212, 107966 (2025).
2. M. Wagner and A. Allanore, "Chemical Thermodynamic Insights on Rare-Earth Magnet Sludge Recycling," ISIJ International 60 (11), 2339-2349 (2020).
3. T.B. Massalski, H. Okamoto, P.R. Subramanian, L. Kacprzak, Binary Alloy Phase Diagrams, 2nd Edition, ASM International, ISBN: 978-0-87170-403-0, 3,589p, 1990
4. S. Okada, K. Suzuki, E. Node, K. Takagi, K. Ozaki, Y. Enokido, Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reduction-diffusion process, J. Alloys Compd. 695 (2017) 1617–1623.
5. S. Kim, V. Galkin, J.R. Jeong, R. Kuchi, D. Kim, Enhanced magnetic and structural properties of chemically prepared Nd-Fe-B particles by reduction-diffusion method through optimization of heat treatments, J. Alloys Compd. 869 (2021), 159337.
6. R. Kuchi, T. A. Seymour-Cozzini, J. V. Zaikina, I. Z. Hlova, M. J. Kramer, Minimizing Particle Aggregation in Sm2Fe17N3 Powders: a CaO-Assisted Reduction-Diffusion Approach, (under review).
7. R. Kuchi, D. Schlagel, T. A. Seymour-Cozzini, J. V. Zaikina, I. Z. Hlova, Exploiting mechanochemical activation and molten-salt-assisted reduction-diffusion approach in bottom-up synthesis of Sm2Fe17N3, J. Alloys Compd. 980 (2024) 173532.
