Seversk Technological Institute – Branch of State Autonomous Educational Institution of Higher Education “National Research Nuclear University “MEPhi”

Е. К. Grachev, Research Engineer, e-mail: е.k.grachev@gmail.com
А. S. Bujnovskij, Professor, Doctor of Technical Sciences, e-mail: asbujnovskij@mail.ru
А. V. Muslimova, Associate Professor, Candidate of Chemical Sciences, e-mail: klameri7@gmail.com

The effect of the heat treatment process of the R2Fe14B system magnetic alloys (where R is Nd, Pr, Tb, Dy) from spent hard drives on the parameters of the hydrogen decrepitation process is studied and presented. The choice of magnetic alloys of the R2Fe14B system extracted from used hard drives for personal computers as the object of research is due to their low cost, ease of magnet extraction and compact size. The extracted magnets were demagnetized at a temperature of 623 K and an average vacuum (10–2 kPa) for 4 hours. Surface cleaning in order to remove the galvanic coating and the surface oxidized layer was carried out by sandblasting with slag shot and chemical etching with 0.5% sulfuric acid solution with washing in acetone. Then the heat treatment of the purified samples was carried out in an autoclave. Three-stage tempering in an average vacuum with a heating rate of 50 K/min, exposure at a certain temperature and duration at each stage, followed by cooling by quenching the autoclave in a container with water cooled to 278 K, with pre-injection of high purity argon into the autoclave to an overpressure of 200 kPa, which contributes to the growth of microstructure intergranular defects and provides a sharp decrease in temperature, turned out to be optimal. The analysis of the surface microstructure after each stage of heat treatment on a scanning electron microscope revealed a change in the microstructure with the appearance of intergranular cracking. Conducting a preliminary three-stage tempering made it possible to achieve a hydrogen content of 0.46% (wt.) in the alloy during hydrogen decrepitation.

The authors express their gratitude to V. A. Eshchev for the comments and consultations provided during the preparation of the manuscript, as well as M. S. Syrtanov and M. A. Kruglyakov for their assistance in carrying out analytical work to determine the hydrogen content in hydrides.

1. Muammer Kaya. An overview of NdFeB magnets recycling technologies. Current Opinion in Green and Sustainable Chemistry. 2024. Vol. 46. 100884. DOI: 10.1016/j.cogsc.2024.100884
2. Kryukov V. A., Yatsenko V. A., Kryukov Ya. V. Rare earth industry – realizing the opportunities. Gornaya promyshlennost. 2020. No. 5. pp. 68–84. DOI: 10.30686/1609-9192-2020-5-68-84
3. Xiao F., Hu W., Zhao J., Zhu H. Technologies of recycling REEs and iron from NdFeB scrap. Metals. 2023. Vol. 13, Iss. 4. 779. DOI: 10.3390/met13040779
4. Peihong Zhu, Min Liu, Yaxuan Yang, Ke Gao et al. Recycling bonded Nd – Fe – B magnet wastes: recycle, repreparation, and life cycle assessment. ACS Sustainable Chemistry & Engineering. 2024. Vol. 12, Iss. 34. pp. 12858–12868.
5. Schulze R., Buchert M. Estimates of global REE recycling potentials from NdFeB magnet material. Resources, Conservation and Recycling. 2016. Vol. 113. pp. 12–27. DOI: 10.1016/j.resconrec.2016.05.004
6. Rademaker J. H., Kleijn R., Yang Y. Recycling as a strategy against rare earth element criticality: a systemic evaluation of the potential yield of NdFeB magnet recycling. Environmental Science & Technology. 2013. Vol. 47, Iss. 18. pp. 10129–10136. DOI: 10.1021/es305007w
7. Habib K., Wenzel H. Exploring rare earths supply constraints for the emerging clean energy technologies and the role of recycling. Journal of Cleaner Production. 2014. Vol. 84. pp. 348–359. DOI: 10.1016/j.jclepro.2014.04.035
8. Binnemans K., Jones P. T., Blanpain B., Van Gerven T. et al. Recycling of rare earths a critical review. Journal of Cleaner Production. 2013. Vol. 51. pp. 1–22.
9. Prokofev P. A., Kolchugina N. B., Skotnicova K., Burkhanov G. S. et al. Blending powder process for recycling sintered Nd – Fe – B magnets. Materials. 2020. Vol. 13, Iss. 14. 3049.
10. Prokofev P. A., Kolchugina N. B., Dormidontov N. A., Bakulina A. S. et al. A method for manufacturing sintered rare earth magnets from recycled materials. Patent RF, No. 2767131, C1. Applied:18.03.2021. Published: 16.03.2022.
11. Zakotnik M., Tudor C. O. Commercial-scale recycling of NdFeB-type magnets with grain boundary modification yields products with de-signer properties that exceed those of starting materials. Waste Management. 2015. Vol. 44. pp. 48–54.
12. Habibzadeh A., Kucuker M. A., Çakir Ö. et al. Microstructural investigation of discarded NdFeB magnets after low-temperature hydrogenation. Journal Sustainable Metallurgy. 2024. Vol. 10. P. 1141–1155. DOI: 10.1007/s40831-024-00873-8
13. Checa Fernández B. L., Martín J. M., Sarriegui G., Burgos N. Effect of temperature on particle shape, size, and polycrystallinity of Nd – Fe – B powders obtained by hydrogen decrepitation. Journal of Materials Research and Technology. 2023. Vol. 24. pp. 1454–1467. DOI: 10.1016/j.jmrt.2023.03.076
14. Li X. T., Yue M., Zhou S. X., Kuang C. J. et al. Effect of hydrogen pressure on hydrogen absorption of waste Nd – Fe – B sintered magnets. Journal of Magnetism and Magnetic Materials. 2019. Vol. 473. pp. 144–147. DOI:
10.1016/j.jmmm.2018.10.071
15. Michalski B., Szymanski M., Gola K., Zygmuntowicz J., Leonowicz M. Experimental evidence for the suitability of the hydrogen decomposition process for the recycling of Nd – Fe – B sintered magnets. Journal of Magnetism and Magnetic Materials. 2022. Vol. 548. 168979. DOI: 10.1016/j.jmmm.2021.168979
16. Grachev E. K., Buinovsky A. S., Muslimova A. V., Ilekis V. M. et al. Hydrogen decrepitation of NdFeB end-of-life magnets with the preliminary three-step surface cleaning. Russian Journal Applied Chemistry. 2023. Vol. 96. pp. 1086–1093. DOI: 10.1134/S107042722312008X
17. Grachev E. K., Buinovsky A. S., Muslimova A. V., Kartashov E. Yu. et al. Investigation of the process of hydrogen dispersion of the (Nd, Pr, Dy)(Fe, Co)2.6 alloy in specified temperature and pressure ranges. Fundamentalnye problemy sovremennogo materialovedeniya. 2022. Vol. 19, No. 2. pp. 233–242. DOI: 10.25712/ASTU.1811-1416.2022.02.012