JSC Rusredmet Group of Companies (Saint Petersburg, Russia)

К. А. Povetkin, Deputy Head of the Department for Technology Development Implementation, kpovetkin@rusredmet.ru
А. S. Sibilev, Head of the Department for Technology Development Implementation
А. V. Nechaev, General Director, Candidate of Technical Sciences
Е. G. Polyakov, Science Advisor, Doctor of Chemical Sciences, ev-polyakov@mail.ru

Power ful high-coercivity FeNdB-type magnets are widely used in wind power generation, electric transport, municipal and household appliances, and largely determine the current state of these industries. Recently, robotic systems in industry, transportation, and military applications, including a wide variety of unmanned equipment, have become major consumers of such magnets. In order to minimize or completely eliminate dependence on foreign supplies, a number of enterprises in the country are establishing production chains, one of the links of which is the production of neodymium by electrolysis of molten salts. The current state of research on the cathodic reduction of neodymium in fluoride and oxyfluoride melts, which are widely used for its electrolytic production, is considered. Existing views on the mechanism of neodymium reduction at an inert cathode in fluoride and oxyfluoride molten electrolytes are presented. The causes of numerous contradictions in the available literature are identified, including the lack of control over the condition of the starting materials, which are sensitive to moisture in atmospheric air, as well as errors in the design and performance of electrochemical experiments. As a result, the interpretation of the cathodic process as phasic involving the formation of Nd²⁺ ions in an intermediate stage is shown to be untenable. The possibility of using a number of salt mixtures as electrolytes for neodymium production at temperatures above 1000C is considered. The use of sodium and potassium fluorides for this purpose is shown to be impossible due to their insufficiently wide electrochemical window. It is demonstrated that the actual one-step reduction process, Nd³⁺ → Nd, is accompanied by a side reaction involving partial displacement of lithium. Together with the development of electronic conductivity in the melt, this reduces the current efficiency of neodymium. Other factors affecting the performance of the electrolysis process are considered, and approaches to improving its production efficiency are outlined.

1. Zhu H. Rare earth metal production by molten salt electrolysis. Encyclopedia of Applied Electrochemistry. Ed. Kreysa G., Ota K., Savinell R. New York : Springer, 2014. pp. 1765–1772.
2. Siming P., Shihong Y., Zongan L. et al. Development on molten salt electrolytic methods and technology for preparing rare earth metals and alloys in China. Chin. J. Rare Metals. 2011. Vol. 35, Iss. 3. pp. 440–450.
3. Polyakov Е. G., Nechaev А. V., Smirnov А. V. Metallurgy of rare earth metals. 2nd edition. Moscow : Yurait, 2021. 501 p.
4. Кaplan G. Е., Silina G. F., Ostroushko Yu. I. Electrolysis in rare earth metals metallurgy. Moscow : Metallurgizdat, 1963. 360 p.
5. Novoselova A., Smolenski V. Electrochemical behavior of neodymium compounds in molten chlorides. Electrochim. Acta. 2013. Vol. 87. pp. 657–662.
6. Vandarkuzhali S., Chandra M., Ghosh S. et al. Investigation on the electrochemical behavior of neodymium chloride at W, Al and Cd electrodes in molten LiCl-KCl eutectic. Electrochim. Acta. 2014. Vol. 145. pp. 86–98.
7. Yamana H., Park B., Shirai O. et al. Electrochemically produced divalent neodymium in chloride melt. J. Alloys and Comps. 2006. Vol. 408–412. pp. 66–70.
8. Diaz L., Serp J., Chamelot P. et al. Electrochemical behavior of neodymium in molten chloride salts. TMS2017 – Annual Meeting and Exhibition. Feb 2017, San Diego, USA. cea-02437084.
9. Smirnov М. V. Electrode potentials in molten chlorides. Moscow : Nauka, 1973. 248 p.
10. Nikitin D. I., Shuklin D. S., Polovov I. B., Rebrin O. I. Kinetics of electrode processes involving neodymium ions in fluoride-chloride systems. Physics. Technology. Innovation : Proceedings of the III International Youth Scientific Conference. (Yekaterinburg, 16–20 May 2016). Yekaterinburg : UrFU, 2016. pp. 494–495.
11. Li B., Liu S., Wang H., Zhao Z. Electrochemistry for Nd electrowinning from fluoride-oxide molten salts. Rare Metal Technology. 2014. pp. 95–98.
12. Huang С., Liu X. L., Gao Y., Liu S. Z., Li B. Cathodic processes of neodymium(III) in LiF–NdF3–Nd2O3 melts. Faraday Discuss. 2016. Vol. 190. pp. 339–349.
13. Cvetkovic V., Feldhaus D., Vukicevic N. et al. Investigation on the electrochemical behaviour and deposition mechanism of neodymium in NdF3–LiF–Nd2O3 melt on Mo electrode. Metals. 2020. Vol. 10. pp. 576.
14. Cvetkovic V., Vukicevic N., Feldhaus D. et al. Study of Nd deposition onto W and Mo cathodes from molten oxide-fluoride electrolyte. Int. J. Electrochem. Sci. 2020. Vol. 15. pp. 7039–7052.
15. Kwon S., Ryu H.-Y., Cho S-H., Lee J.-H. Effect of the electrolyte composition on the electrochemical behavior of Nd fluoride complex in a LiFNdF3-Nd2O3 molten salt. Journal of Electroanalytical Chemistry. 2020. Vol. 879. 114751.
16. Liu X., Huang C., Li B. The effects of NdF2 on current efficiency of Nd extraction from NdF3-LiF-Nd2O3 melts. Materials Transactions. 2017. Vol. 58, No. 3. pp. 395–399.
17. Holcombe B., Sinclair N., Baker A. et al. Electrolysis in chloride molten salts for sustainable critical metals production and recovery. The Electrochem. Soc. Interface. 2024. Vol. 33, Iss. 1. pp. 49–54.
18. Galyus Z. Theoretical foundations of electrochemical analysis. Moscow : Mir, 1974. 552 p.
19. Bard A. J., Faulkner L. R. Electrochemical methods: fundamentals and applications. 2nd edn. New York : Wiley, 2001. 833 p.
20. Mirceski V., Skrzypek S., Stojanov L. Square-wave voltammetry. Chem. Texts. 2018. Vol. 4. pp. 17–31.
21. Polyakova L. P., Кononova Z. А., Еlizarova I. R., Polyakov Е. G. Voltammetric determination of oxide ion in halide melts. Zhurnal analiticheskoy khimii. 1994. Vol. 49, No. 11. pp. 1228–1232.
22. Polyakova L. P., Polyakov Е. G., Bierrum N. Ya. Voltammetric determination of oxygen in fluoride melts. Elektrokhimiia. 1997. Vol. 33, No. 12. pp. 1443–1446.
23. Sastry R. L. N., Yoganarasimhant S. R., Mehrotra P. N. et al. Рreparation, characterization and thermal decomposition of praseodymium, terbium and neodymium carbonates. J. Inorg. Nucl. Chem. 1966. Vol. 28. pp. 1165–1177.
24. Bernal S., Botana F. J., Garcia R., Rodriquez-Izquierdo J. M. Behavior of rare earth sesquioxides exposed to atmospheric carbon dioxide and water. React. Solids. 1987. Vol. 4, No. 1-2. pp. 23–40.
25. Mlynáriková J., Boča M., Gurišová V. et al. Thermal analysis and volume properties of the systems (LiF– CaF2)eut.–LnF3 (Ln = Sm, Gd, and Nd) up to 1273 K. J. Therm. Anal. Calorim. 2016. Vol. 124. pp. 973–987.
26. Takeda O., Nakano K., Kobayashi F. et al. Solubilities of RE2O3 in REF3-LiF (RE = Nd, Dy) at 1473 K. Journal of Sustainable Metallurgy. 2022. Vol. 8. pp. 1498–1508.
27. Stefanidaki E., Hasiotis C., Kontoyannis C. Electrodeposition of neodymium from LiF–NdF3–Nd2O3 melts. Electrochim. Acta. 2001. Vol. 46. pp. 2665–2670.
28. Stefanidaki E., Photiadis G., Kontoyannis C. et al. Oxide solubility and Raman spectra of NdF3–LiF–KF–MgF2–Nd2O3 melts. J. Chem. Soc., Dalton Trans. 2002. Vol. 11. pp. 2302–2307.
29. Wu X.-Y., Liu F., Li J.-H. et al. Modeling and simulation of neodymium oxide dissolution in molten fluorides. Trans. Nonferrous Metals Soc. of China. 2022. Vol. 32, No. 7. pp. 2414–2427.
30. Hu X., Wang Z., Gao B. et al. Density and ionic structure of NdF3–LiF melts. J. Rare Earths. 2010. Vol. 28. pp. 587–590.
31. Delimarsky Yu. К. Electrochemistry of ion melts. Moscow : Metallurgiya, 1978. 248 p.
32. Abbasalizadeh A., Seetharaman S., Venkatesan P., Sietsma J., Yang Y. Use of iron reactive anode in electrowinning of neodymium from neodymium oxide. Electrochimica Acta. 2019. Vol. 310. pp. 146–152.
33. Kaneko A., Yamamoto Y., Okada C. Electrochemistry of rare earth fluoride molten salts. Journal of Alloys and Compounds. 1993. Vol. 193. pp. 44–46.
34. Thudum R., Srivastava A., Nandi S. et al. Molten salt electrolysis of neodymium: electrolyte selection and deposition mechanism. Mineral Processing and Extractive Metallurgy (Trans. Inst. Min Metall. C). 2010. Vol. 119, No. 2. pp. 88–92.
35. Gibilaro M., Massot L., Chamelot P., Taxil P. Study of neodymium extraction in molten fluorides by electrochemical co-reduction with aluminium. J. Nucl. Mater. 2008. Vol. 382, Iss. 1. pp. 39–45.
36. Hamel C., Chamelot P., Taxil P. Neodymium(III) cathodic processes in molten fluorides. Electrochimica Acta. 2004. Vol. 49. pp. 4467–4476.
37. Chen Z., She C., Zheng H. et al. Electrochemical deposition of neodymium in LiF-CaF2 from Nd2O3 assisted by AlF3. Electrochimica Acta. 2018. Vol. 261. pp. 289–295.
38. Zhanikaeva Z. A. Electrochemical reduction of praseodymium and neodymium ions and the synthesis of compounds based on them : abstract of thesis. … of Candidate of Chemical Sciences. Yekaterinburg, 2007. 24 p.
39. Glagolevskaya A. L., Kuznetsov S. A., Polyakov E. G., Stangrit P. T. Electrochemical reduction of lanthanum trichloride in a molten mixture of equimolar sodium and potassium chlorides. Zhurn. prikl. Khimii. 1987. No. 4. pp. 770–774.
40. Kononov A., Polyakov E. High-temperature electrochemical synthesis and properties of intermetallic compounds of the Ni-Sc system. Part 1. Electrochemical behaviour of Sc(III) in chloride-fluoride melts. Journal of Alloys and Compounds. 1996. Vol. 239. pp. 103–106.
41. Vasin B. D., Ivanov V. A., Naritsyn A. V., Raspopin S. P., Shchetinsky A. V. Corrosion of rare-earth metals of the cerium group in an equimolar mixture of sodium and potassium chlorides. Rasplavy. 1998. Iss. 6. pp. 76–80.
42. Feldhaus D., Tschauner M-T., Friedrich B. Stabilisation of Nd and Pr molten salt electrolysis by feeding LiF to avoid unwanted side reactions. Virtual conference: ERES - European Rare Earth Resources. 2020. DOI: 10.13140/RG.2.2.31892.42881
43. Bredig M. A. Mixtures of metals with molten salts : ORNL report 3391. 1963. 65 p.
44. Haarberg G. M., Thonstad J. Electrochemical properties of metal-molten salt mixtures. J. Appl. Electrochem. 1989. Vol. 19. pp. 789–801.
45. Konstantinov V. I., Polyakov E. G., Stangrit P. T. Cathodic electrolysis of chloride-fluoride melts of tantalum. Electrochim. Acta. 1978. Vol. 23. pp. 713–716.
46. Konstantinov V. I., Polyakov E. G., Stangrit P. T. Cathodic processes at electrolysis of chloride-fluoride and oxyfluoride melts of niobium. Electrochim. Acta. 1981. Vol. 26. pp. 445–448.
47. Chamelot P., Massot L., Taxil P. Lanthanides Extraction Processes in Molten Fluoride Media. In: “Molten Salts Chemistry. From Lab to Applications”. Eds. F. Lantelme, H. Groult. Elsevier, 2013. pp. 521–540.
48. Zhou Q., Liao J., Liao C., Zhao B. Phase equilibrium study of rare earth oxide–fluoride salt system: a review. Metals. 2024. Vol. 14. pp. 314–326.
49. Zhuk S. I., Vlasov M. I. Solubility of rare-earth element oxides in chloride, chloride-fluoride, and fluoride melts of alkali and alkaline-earth metals. Rasplavy. 2024. No. 6. pp. 676–694.
50. Tamamura H., Shimooka T., Utsunomiya M. Proc. Seventh Int. Symp. Molten Salts. 1990. Vol. 90, Iss. 17. pp. 611.
51. Vogel H., Flerus B., Stoffner F., Friedrich B. Reducing greenhouse gas emission from the neodymium oxide electrolysis: Part I: analysis of anodic gas formation. J. Sust. Metallurgy. 2017. Vol. 3. pp. 99–107.
52. Lebedev V. A. Selectivity of liquid-metal electrodes in molten halides. Chelyabinsk : Metallurgiya, 1993. 232 p.