Introduction

Molten salts provide a unique working environment. They offer an ionic, nonaqueous habitat wherein oxide and other ores may be dissolved. Molten salts also provide a relatively wide electrochemical window where the potentials between which one can operate without decomposing the solvent may even be several volts.

Molten salt systems are however widely used not only for industrial electrolysis and treatment of metals (like production of aluminium, magnesium, alkaline earth metals, baths for various alloy heat treatments, carburizing, nitrocarburizing, and silicifying of steel), but they can also be found in such industrial applications as special types of nuclear molten salts reactors, concentrated solar heat exchangers and energy storage media, and high-temperature batteries and fuel cells.1,2,3,4,5,6

The rare earth elements (REEs) constitute the biggest coherent group of chemically similar elements in the periodic table. Though generally unfamiliar, REEs are vital for many industrial applications. The specificity, flexibility, and modality of REEs have given them a high level of technological, economic, and environmental significance. The varied nuclear, chemical, physical, metallurgical, and catalytic properties of the REEs have led to a dramatic increase in the scale and variety of advanced industrial applications of REE over the past few decades. This has given REEs a geopolitical status of strategically very important materials, especially when considering complex and interlinked production chains (e.g., due to the presence of the radioactive minerals in many REE mine deposits) and fragile global supply chains due to unstable geopolitics.7,8,9,10

Neodymium–iron–boron (NdFeB) permanent magnet production is today a key industrial Nd application, considered the main driver of the growth of the REE market in the last 20 years.10 The excellent strength-to-weight ratio of Nd-based magnets has been overriding other competitor materials in terms of density of the magnetic flux. The niche of the industrial use of the Nd-based magnets ranges from miniature hard drivers in laptops, headphones, and other communication systems to green electric technologies like electric cars and wind turbines.

Today, neodymium is industrially produced by high-temperature electrolysis of Nd2O3 dissolved in molten LiF-NdF3 electrolyte (the molar LiF/NdF3 ratio of the initial electrolyte is approximately 0.9). The concentration of the oxide raw material in the molten fluoride electrolyte is around 2 wt.% (0.96 mol.%), and the operational temperature is between 1050°C and 1100°C.1,7

One of the obvious disadvantages of the current industrial electrolysis of neodymium is the very low solubility of Nd2O3 in the molten electrolyte. The low solubility of the oxide in the molten electrolyte influences not only the performance of the cell and oxide feeding operation but is also responsible for the occurrence of the anode effects with the correspondent emission of the very powerful greenhouse perfluorocarbons gases. The current efficiency of the process is also relatively low, 50–80%. Because of the high market demand for primary neodymium, an enhancement of the process is rather required, which would imply higher oxide row material solubility, higher current and energy efficiency, fewer losses of electrolyte, more efficient process control, and the reduction of the emission of perfluorocarbon gases.11

The solubility of the neodymium oxide raw material in molten fluorides and a thorough comprehension of the related phase equilibria and dissolution dynamics are both of theoretical and practical significance for the design of the proper electrolyte compositions and optimal process control and operation. In contrast to comprehensive works done in the field of the cryolite–alumina electrolyte systems,3,12 few have been done in the field of rare earth oxide–fluoride electrolytes. Moreover, there is a discrepancy in the results from various studies in this field. Therefore, the availability of reliable data is rather problematic.

To decide the industrial relevance of a certain melt, in-depth knowledge of the melt’s physico-chemical properties is necessary. One of the most indispensable parameters of molten salt mixtures is evidently the temperature of the primary crystallization (temperature of solid–liquid equilibrium). This temperature determines the region of existence of the liquid phase and is also important information for the construction of a phase diagram of the particular system. Indeed, the phase diagram depicts, in a geometrical way, the thermal stability of the system. The measurement of the temperatures of the primary crystallization (and construction of phase diagram) can also however help in the identification of the solubility of the raw material (solute) in the electrolyte (solvent), which in the case of electrometallurgy is another very important industrial parameter.

The present article deals with phase equilibria of the molten Na3AlF6-NdF3-Nd2O3 system. The phase diagrams of the Na3AlF6-NdF3 and (Na3AlF6-NdF3)eut-Nd2O3 systems were experimentally determined by thermal analysis, and the solidified samples after that analysis were then analysed by x-ray diffraction.

Berul and Voskresenskaya13 were the first to investigate the solubility of Nd2O3 in molten cryolite (Na3AlF6). They found that Nd2O3 in molten cryolite forms NdF3 during the dissolution process. Abbasalizadeh et al.14 studied the fluorination effect of the addition of Na3AlF6 into the molten LiF-Nd2O3 bath. Using XRD analysis of solidified samples, contrary to the findings from the previous work, the authors found that neodymium from neodymium oxide reacts with cryolite and gets substituted in place of aluminium in cryolite to form Na1.5Nd1.5F6 compound. Both above mentioned works are the only ones in the open literature related to the behaviour of Nd2O3 in molten cryolite systems. To our knowledge, there are so far no literature data related to the investigation of phase diagrams of the systems studied in the present work (Na3AlF6-NdF3 and (Na3AlF6-NdF3)eut-Nd2O3).

A comprehensive review of the works related to the dissolution of different REE oxides in different molten fluoride systems done after the work of Berul and Voskresenskaya can be found in the work of Guo et al.15 The important fact related to the solubility of different REE oxides in molten fluoride melts is confirmed by several authors16,17,18—the increased solubility of REE oxides (e.g., Y2O3, CeO2, and Nd2O3) with increasing concentration of REE fluoride (e.g., YF3, CeF3 and NdF3) in both binary and ternary fluoride systems.

The only phase diagram between Na3AlF6 and any REE oxide was reported by Ambrová et al.19 These authors used thermal analysis and reported the phase diagram of the Na3AlF6-La2O3 system. The present system is to be a simple eutectic system with the following coordinates of the eutectic point: 11.5 mol.% La2O3 and 934°C.

Experimental

The phase equilibria of the investigated system were determined by a thermal analysis method. The thermal analysis device used in this work is original uncommercial equipment developed at the Institute of Inorganic Chemistry SAS, Slovakia, for the phase equilibria measurement of the molten fluoride systems. All samples were prepared in a glove box under an inert atmosphere (Ar, 99.999%, SIAD, Slovakia). The powdered samples of 12 g were homogenized in inert atmosphere and then transferred in a platinum crucible into the preheated (ca. 90°C) electric resistance furnace (Fig. 1) under a dried argon atmosphere (99.996%, SIAD, Slovakia). One platinum crucible holds the sample, the other one, the reference material (high-purity powders of Al2O3). The temperature of the furnace was controlled by a Pt10Rh/Pt thermocouple, inserted in the reference material. For the preparation of the samples, the following chemicals were used: NaF (99.9%, Merck, Germany), AlF3 (sublimated, dried at 300°C, min, 99.0%, Slovalco, Slovakia), NdF3 (99.9% Chempur, Germany), and Nd2O3 (99.9% Chempur, Germany). The furnace was first heated at the heating rate of 7°C min−1 to a temperature approximately 50°C above the melting point of the melt. The samples were then kept at this temperature for approximately 50 min. The temperature of primary crystallization and the other heat effects were recorded at the cooling rate of 1.5°C min−1. A computerized measuring device, developed at the Institute of Inorganic Chemistry SAS, Slovakia, was used for the temperature control of the furnace and data acquisition. The temperature of the sample was controlled by a Pt10Rh/Pt thermocouple calibrated using the melting points of the following pure chemicals: NaCl, NaF, and Li2CO3. The accuracy of the temperature measurement of the measured thermocouples was found ± 2°C.

Fig. 1
figure 1

Cross section of the furnace used for the thermal analysis: (1) gas outlet, (2) control thermocouple outlet, (3) upper furnace flange with cooling, (4) water cooling, (5) insulation of the furnace, (6) kanthal heating elements, (7) steel furnace shell, (8) Alsint tube of the furnace, (9) lower furnace flange with cooling, (10) gas inlet, (11) platinum crucible for the reference material (Al2O3), (12) platinum crucible for the sample, (13) Pt10Rh/Pt thermocouples, (14) Alsint radiation shields, (15) measuring thermocouple outlet.

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After the thermal analysis measurements, all the samples were taken for the x-ray diffraction (XRD) analysis. XRD patterns were measured using an Empyrean PANalytical diffractometer with Cu Kα1,2 radiation in Bragg-Brentano geometry and a b filter (Ni). A solid-state PIXcel detector was used to record XRD patterns. The measurements were carried out at room temperature, with 2θ varying from 10° to 90°. Phase analysis was performed with Oxford Cryosystem Crystallographica Search-Match 2.1 software with PDF2 2011 database.

Results and Discussion

Na3AlF6-NdF3

The results of the thermal analysis of the system Na3AlF6-NdF3 are presented in Table I, and the phase diagram of this system is shown in Fig. 2. The investigated system was found to be a simple eutectic one. The NdF3-rich side of the phase diagram was investigated only up to 60 mol.% of NdF3. The coordinates of the eutectic point were in the frame of this range at approximately 49 mol.% NdF3 and 905°C.

Table I Temperatures of primary (tp) and solidus (ts) crystallizations in the molten system Na3AlF6-NdF3
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Fig. 2
figure 2

Phase diagram of the system Na3AlF6-NdF3 experimentally determined by thermal analysis. Squares = temperatures of the primary crystallizations. Circles = temperatures of the solidus crystallization. Dashed lines = formal curves indicating the liquidus and eutectic lines.

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Based on the freezing point depression theory, the number of new species originating from a solute (in our case NdF3) added to a molten solvent (Na3AlF6) can be determined for low concentrations of solute from the following simplified equation.20

$$ \Delta T = \frac{{RT_{{\text{f}}}^{2} }}{{\Delta H_{{\text{f}}} }}k_{{{\text{St}}}} x_{{{\text{NdF}}_{3} }} $$
(1)

Tf (K) is the melting point of the pure solvent (Na3AlF6), Hf (J mol−1) is the enthalpy of fusion of solvent (Na3AlF6), and kst is the so-called Stortenbeker factor, which in fact represents a number of new particles formed/introduced when NdF3 is added and dissolved in molten Na3AlF6. ΔT (K) is the difference between the actual temperature of primary crystallization of the mixture and the melting point of pure solvent (Tf), xNdF3 is the NdF3 molar fraction in the system, and R is the gas constant.

Since the enthalpy related to the melting of pure Na3AlF6 is known (Hf(Na3AlF6) = 106.7 kJ mol−121), theoretical curves representing the introduction of 1, 2, and 3 new species based on Eq. 1 can be calculated. Figure 3 presents the results of that analysis showing how many new species were formed when NdF3 was added to molten Na3AlF6. We can conclude, based on the freezing point depression analysis, that the addition of NdF3 into the molten Na3AlF6 introduced one new species. We can only speculate about the form and structure of that new species formed in this molten system, but the XRD analysis of the solidified samples (done ex-post after the thermal analysis) (Fig. 4) indicates the formation of [NdF4]1− complex anion.

Fig. 3
figure 3

Freezing point depression in molten Na3AlF6 by addition of NdF3 (squares). Calculated curves according to Eq. 1 for 1 (kSt = 1), 2 (kSt = 2), and 3 (kSt = 3) units of foreign particles introduces into the molten Na3AlF6.

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Fig. 4
figure 4

XRD patterns of the solidified samples of the system Na3AlF6-NdF3 as a function of the concentration of NdF3 (mol.%).

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Figure 4 shows the complete XRD analysis of the solidified samples of the system Na3AlF6-NdF3 as a function of NdF3 concentration (mol.%). A1 pattern in this figure represents the XRD pattern of the sample with pure Na3AlF6, while the patterns A2–A9 represent the samples with increasing concentration of NdF3. Besides the signals of the original compounds Na3AlF6 and NdF3, the XRD analysis shows the formation of two new compounds, NaAlF4 and NdOF. NaAlF4 is a product of the interaction between Na3AlF6 and NdF3, while the NdOF is likely the product of the high temperature hydrolysis between the moisture in the atmosphere and NdF3.22 Since there is no NdOF signal at the low and zero concentrations of NdF3, the appearance of oxygen content compound in the XRD analysis must be interrelated with the presence of NdF3 in the investigated samples.

The maximum intensity of the NaNdF4 signals is in the concentration range of 2.5–10 mol.% NdF3. In the concentration range between 10 mol.% and 50 mol.%, the intensity of these peaks is decreasing and completely disappears at the maximal concentration of NdF3 at 60 mol.%.

The first XRD signals of NdOF appear at the concentration of NdF3 at 5 mol.%, and the maximum is reached at the concentration of NdF3 around between 10 mol.% and 20 mol.% and then continually decreases.

To elucidate the possible chemical reaction to form NaNdF4, we must consider the well-known thermal dissociation of molten cryolite systems with the formation of NaAlF4 vapours according to the following reaction scheme.1,3

$$ {\text{Na}}_{{3}} {\text{AlF}}_{{6}} = {\text{ 2NaF }} + {\text{ NaAlF}}_{{4}} $$
(2)

This means that in any cryolitic system in a molten state, the reaction system loses NaAlF4 in the form of vapours and the molten system becomes less acidic (an increase of the cryolite NaF/AlF3 ratio, CR). The final NaF concentration in the system due to reaction (2) increases, and free NaF may react with NdF3 according to the following reaction scheme.

$$ {\text{NaF }} + {\text{ NdF}}_{{3}} = {\text{ NaNdF}}_{{4}} $$
(3)

(Na3AlF6-NdF3)eut-Nd2O3

The results of the thermal analysis of the system (Na3AlF6-NdF3)eut-Nd2O3 are presented in Table II, and the phase diagram of this system is shown in Fig. 5. The composition of the left side of the phase diagram (binary eutectic point of Na3AlF6-NdF3) was set, as a result of the previous part of this work, as 49 mol.% NdF3. The (Na3AlF6-NdF3)eut-rich side of the phase diagram was investigated only up to 45 mol.% Nd2O3. The coordinates of the eutectic point were found to be in the frame of this range at approximately 45 mol.% Nd2O3 and 733°C. The phase diagram of this system seems to be more complex than the simple eutectic system of Na3AlF6-NdF3. The (Na3AlF6-NdF3)eut-rich side of the phase diagram contains, besides the eutectic horizontal line, also two other horizontal lines. One line is located at the temperature range between 860°C and 869°C, and in the concentration range between 0 mol.% and 3 mol.% Nd2O3, another horizontal line is located at the temperature range between 817°C and 825°C and in the concentration range between 3 mol.% and 30 mol.% Nd2O3. These two experimentally determined lines then probably constitute four different fields between the liquidus and eutectic borders in this (Na3AlF6-NdF3)eut-rich side of the phase diagram.

Table II Temperatures of the primary crystallization (tp) and other heat effects (t2, t3) on the cooling curves in the molten system (Na3AlF6-NdF3)eut-Nd2O3
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Fig. 5
figure 5

Phase diagram of the system (Na3AlF6-NdF3)eut-Nd2O3 experimentally determined by thermal analysis. Squares = temperatures of the primary crystallizations. Circles = temperatures of other heat effects on the cooling curves. Dashed lines = formal curves indicating the liquidus and eutectic lines.

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Figure 6 shows an XRD analysis of the solidified samples of the system (Na3AlF6-NdF3)eut-Nd2O3 as a function of Nd2O3 concentration (mol.%). The B1 pattern in this figure represents the XRD pattern of the sample without Nd2O3, while the patterns B2–B8 represent the samples with increasing concentration of Nd2O3. The sample without the addition of Nd2O3 contains, as in the case of the previous Na3AlF6-NdF3 system, only the signals of the following compounds: Na3AlF6, NdF3, NaNdF4, and NdOF. NaNdF3 is a product of the reaction between NaF and NdF3 according to the reaction scheme (3). The formation of NdOF is, like in the previous system, a product of the pyro-hydrolysis between NdF3 and the moisture in the atmosphere.22 Besides the above mentioned compounds, the samples with the higher concentrations of Nd2O3 (B2–B8) also contain the signals of NdAlO3 and NaF.

Fig. 6
figure 6

XRD patterns of the solidified samples of the system (Na3AlF6-NdF3)eut-Nd2O3 as a function of the concentration of Nd2O3 (mol.%).

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Comparing the evolution of the signals of NaNdF4 in both systems shows that in the case of the Na3AlF6-NdF3 system the intensity of the NaNdF4 signals slowly increases upon the addition of NdF3 with the maximum intensity in the concentration range somewhere around 5 mol.% NdF3 and then disappears at higher concentration of NdF3. This is contrary to the system with Nd2O3 where the relatively intense and constant signals of NaNdF4 appear in all samples. These findings indicate that NaNdF4 is in this system formed also by other reaction(s) than by only reaction (3). The occurrence of the relatively constant and intense peaks of NaNdF4 can be explained by the reaction between Na3AlF6 and Nd2O3 according to the following reaction scheme.

$$ {\text{Na}}_{{3}} {\text{AlF}}_{{6}} + {\text{ Nd}}_{{2}} {\text{O}}_{{3}} = {\text{ NaNdF}}_{{4}} + {\text{ NdAlO}}_{{3}} + {\text{ 2NaF}} $$
(4)

This reaction can also explain the presence and evolution of the intensity of the XRD peaks of NdAlO3 in the system with Nd2O3, where the intensity of the signals of the NdAlO3 patterns linearly increases with the concentration Nd2O3 (Fig. 6). Reaction (5) can, on the other hand, explain the complete disappearance of the NdF3 peaks upon the addition of Nd2O3 as well as a “non-linear” evolution of the intensity of XRD peaks of NdOF in the (Na3AlF6-NdF3)eut-Nd2O3 system.

$$ {\text{Nd}}_{{2}} {\text{O}}_{{3}} + {\text{ NdF}}_{{3}} = {\text{ 3NdOF}} $$
(5)

The “non-linear” evolution here means that the intensity of the signals of the NdOF patterns in Fig. 6 first (in the concentration range between 0 mol.% and 5 mol.% Nd2O3) increases, then (in the concentration range 5–30 mol.% Nd2O3) decreases, and then again continually increases in the highest concentrations of Nd2O3 (30–40 mol.%). A similar (“non-linear”) evolution of the intensity of peaks can be seen also in the case of NaF. This phenomenon can be explained by the existence of parallel competitive reactions in the formation of NaF and NdOF.

$$ {\text{NaF }} + {\text{ NdF}}_{{3}} = {\text{ NaNdF}}_{{4}} $$
(6)

In the case of the formation/consumption of NaF, it is the thermal dissociation of Na3AlF6 (2)1,3 and the reactions (4) and (6); in the case of the formation of NdOF, it may be the reaction (5) and the pyro-hydrolysis of NdF3.22

Conclusion

The phase equilibria of Na3AlF6-NdF3 and (Na3AlF6-NdF3)eut-Nd2O3 systems have been analysed using thermal analysis and, for the first time to our knowledge, presented. Both investigated mixtures seem to form a simple eutectic system.

  • The coordinates of the eutectic point in the case of the Na3AlF6-NdF3 system were found to be at approximately 49 mol.% NdF3 and 905°C.

  • The coordinates of the eutectic point in (Na3AlF6-NdF3)eut-Nd2O3 system are at approximately 46 mol.% Nd2O3 and 733°C.

  • It can be concluded that the solubility of Nd2O3 in the molten eutectic system of Na3AlF6-NdF3 is relatively high compared to the solubility of Nd2O3 in the current molten industrial electrolyte (2 wt.%, 0.96 mol.%) for the electrowinning of neodymium (LiF-NdF3).

The solubility of Nd2O3 in the molten eutectic mixture (Na3AlF6-NdF3)eut is surprisingly even higher than the solubility of aluminium oxide in industrial molten cryolite electrolytes (10 wt.%), considering the industrial aluminium electrolysis as a benchmark for any other high temperature electrometallurgical process. The relatively high oxide solubility and relatively low eutectic temperature thus make this molten system very promising for industrial utilization. This fact is even more relevant when the relatively low operating temperature allows using inert anodes (instead of consumable carbon anodes) to produce “green”, CO2-less neodymium with the evolution of oxygen (instead of CO2).

The authors plan to continue in the research on that interesting molten system. The high-temperature in situ techniques like high-temperature XRD and high-temperature NMR (or preparation of deeply quenched samples for conventional room temperature structural and microscopic analysis) can bring a more direct view related to the structure and nature of this molten system. The electrochemical and physico-chemical analysis (electrical conductivity, density, viscosity, surface tension, etc.) of this molten system will be the next important step regarding specifically high-temperature electrometallurgy.