- Email: [email protected]
Use of iron reactive anode in electrowinning of neodymium from neodymium oxide
Electrochimica Acta 310 (2019) 146e152
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Use of iron reactive anode in electrowinning of neodymium from neodymium oxide Aida Abbasalizadeh a, *, Seshadri Seetharaman b, Prakash Venkatesan a, Jilt Sietsma a, Yongxiang Yang a a b
Department of Materials Science and Engineering, Delft University of Technology (TU Delft), Mekelweg 2, 2628CD, Delft, the Netherlands Royal Institute of Technology (KTH), Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2018 Received in revised form 22 March 2019 Accepted 22 March 2019 Available online 23 March 2019
Electrolytic production of metallic neodymium is carried out in fused neodymium fluoride salts containing neodymium oxide. Two major challenges pertaining to neodymium production in fluoride salts are a) low solubility of neodymium oxide in fluoride melt, b) possibility of anodic gas evolution (CO, CO2, CF4, C2F6). In this study, iron is used as a reactive anode in the electrolysis process, promoting electrochemical dissolution of iron into the melt, preventing PFC (perfluorocarbon) gas evolution at the anode. Further, the rare earth oxide is converted to rare earth fluoride by the use of iron fluoride formed as the result of iron dissolution. Thus, the fluoridizing agent is constantly regenerated in-situ which enables the continuous conversion of neodymium oxide feed. The cathodic product is NdeFe alloy which can be used as a master alloy for the production of NdFeB magnets. © 2019 Published by Elsevier Ltd.
Keywords: Rare earths Electrochemical extraction Reactive anode PFC gas Molten salts
1. Introduction Rare earh (RE) metals and their alloys have important applications in different fields of modern technology, in particular in green technologies for clean energy such as wind turbines and electric vehicles [1]. Deoxidation of rare earth metals is very difficult due to the strong affinity of these metals to oxygen. Traditionally, RE metals have been produced by the calciothermic reduction of REF3. However, molten salt electrolysis is becoming the dominant industrial method for the extraction of RE metals from their oxides in the molten salt process [2e4]. Because of negative deposition potential, RE metals cannot be electrochemically extracted in aqueous media. Solubility of the rare earth oxide (REO) in the fluoride electrolyte is an important factor, since it is the dissolved oxide that is subjected to electrolysis. However, the solubility of rare earth oxide in molten alkali fluoride does not exceed 4 wt percent [5e7]. Moreover, neodymium oxide is likely to form neodymium oxyfluoride in molten fluoride salts. Oxyfluoride formation has been reported to affect the neodymium electrodeposition adversely [8]. However,
* Corresponding author. E-mail address: [email protected] (A. Abbasalizadeh). https://doi.org/10.1016/j.electacta.2019.03.161 0013-4686/© 2019 Published by Elsevier Ltd.
the role of neodymium oxyfluoride in the anodic and cathodic reactions is not very clear and needs to be clarified further in detail [8e12]. Another challenge in neodymium production by fluoride electrolysis is the evolution of fluorine associated gasses (PFCs) on the carbon anode when the electrolysis rate exceeds the rare earth oxide feeding and dissolution rate [10]. In the framework of this research for electrochemical extraction of rare earths from their oxides in molten fluoride media, the present research strategy was adopted in order to address the solubility issue of neodymium oxides in fluoride melts. It was shown earlier by the present authors that iron [III] fluoride could be used successfully as a fluoridizing agent for neodymium oxide in molten salts [13]. The conversion of Nd2O3 to NdF3 in molten salts solves the problem of low solubility of the neodymium oxide in molten fluorides to a large extent. Once neodymium fluoride is formed, it can be electrolyzed and subsequently Nd can be deposited on the cathode, along with CO, CO2, CF4/C2F6 (PFCs) evolution on the anode side. However, PFC gas generation on the graphite anode in the rare earth metal/alloy production industry is a serious drawback of this process, in common with aluminum production industry [14,15]. The present study is aimed to address this problem. A reactive anode is employed to overcome the problem of halogen evolution at the anode. Iron is anodically dissolved, generating the fluorinating agent FeF2/FeF3 in-situ in the
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
electrochemical reactor. The NdF3 thus formed can subsequently be processed through the electrolysis route in the same reactor to extract Nd metal as the cathodic deposit. In this concept, the REO dissolution in the molten fluorides would become redundant due to RE oxide conversion into the fluoride as REF3.
147
Moreover, both FeF2 and FeF3 can undergo the electrolysis process and decompose along with Nd on the cathode since NdF3 has higher decomposition voltage ( ENdF3 ¼ 4:7 V, EFeF2 ¼ 2:8 V and EFeF3 ¼ 2:7 V). Hence the cathodic reactions would be: Fe3þ þe /Fe2þ / Fe2þ þ 2e /Fe
(5)
2. Thermodynamic evaluations of the process
Nd3þ þ 3e/ Nd
(6)
In the present study, use of iron anode is suggested for the electrolysis process in which instead of CO/CO2 or CF4/C2F6 gas formation, the anodic reaction would be iron dissolution. FeF3 is added to the system in order to initiate the decomposition process. The dissolved iron at anode side would react with the fluorine ions which are formed as the result of decomposition of added directly FeF3 to LiFeNd2O3 mixture and subsequently FeF2 and FeF3 are formed in-situ in the system:
From Table 1 it can also be concluded that the dissolved neodymium oxide and iron oxide can also participate in the electrochemical process resulting to Nd and Fe extraction on the cathode, respectively. Hence the solubility of these oxides plays a significant role on their participation in the electrochemical reaction process. In order to prevent the accumulation of the oxide in the LiFeNd2O3 salt bath, the undissolved iron oxide should be removed from the system.
Fe2þþ 2F / FeF2 þ 2e
(1)
Fe3þ þ 3F / FeF3 þ3e
(2)
Iron (II) fluoride is more stable compare to iron (III) fluoride. However, depending on the electrolysis conditions FeF3 is formed in the electrolyte. The generated iron fluoride will act as fluorinating agent and convert Nd2O3 to NdF3. From the calculated standard Gibbs energy (DGo) of the reaction (3e4) at 1223 K (950 C) using FactPS database of FactSage: 3FeF2(salt) þ Nd2O3(s) % 2NdF3(salt) þ 3FeO (s) DGo ¼ 215 kJ/ mol (3) 2FeF3(salt) þ Nd2O3(s) % 2NdF3(salt) þ Fe2O3(s) DGo ¼ 221 kJ/ mol (4) It is evident that both FeF2 and FeF3 can react with Nd2O3 to form NdF3, and subsequently Nd3þ ions can further be reduced on the cathode. As the result of the fluorinating reaction, FeO and Fe2O3 are formed in the system. Prior experiments by the present authors confirm the feasibility of the conversion of neodymium oxide to neodymium fluoride using FeF3 as fluorinating agent [13]. In Table 1 the Standard Gibbs free energies of formation of the reacted fluorides and oxides in the system at 1223 K (950 C), calculated using FactSage are given. It is important to note that these reactions refer to the case of the reacting species at their standard state as pure substances. In the real systems, the activity of the components would be lower than unity. From Table 1 it is evident that iron fluoride has lower decomposition voltage than neodymium fluoride. Hence, from the thermodynamic perspective, two scenarios are possible for the formed iron fluoride generated in the system as the result of anodic dissolution. In one scenario iron fluoride acts as the fluorinating agent and convert Nd2O3 to NdF3.
3. Experimental Pure lithium fluoride (98.5%-Alfa Aesar) was used as the electrolyte bath and was mixed with neodymium oxide (Solvay) or neodymium fluoride or iron (III) fluoride (97%-Alfa Aesar) in a glove box. LiF was chosen as the solvent since the deposition potential of the Li in the solvent is more cathodic than the deposition potential of Nd3þ. LiF is also the main solvent used in industry for the extraction of neodymium in molten fluoride process [2]. A schematic of the experimental apparatus used for the electrochemical extraction of metals in molten salt [16] is shown in Fig. 1. A glassy carbon crucible served as the container of the mixtures. The crucible was placed in an electric furnace under the inert argon atmosphere. A water cooling system was used for both flanges at the top and bottom. All experiments were carried out at 950 C with in the limit of ±3 at hot zone. A K-type thermocouple was used for the measurement of the temperature at the bottom of the graphite crucible. 3.1. Experiments related to electrochemical behavior of LiFeNdF3/ FeF3/Nd2O3 systems In order to examine chemical dissolution of iron anode in LiFe Nd2O3 bath an experiment was performed in which an iron bar was inserted in the LiFe Nd2O3 bath at 950 C for 3 h.
Table 1 Standard Gibbs energies and the decomposition potential of different fluorides salts and REOs and REF. Reaction
DGo (kJ/mol) at Decomposition DGo (kJ/mol) at 1223 K (950 C) potential (V) at 298 K (25 C) 1223 K (950 C)
LiF % Li þ 1/2 F2(g) NdF3 % Nd(s) þ 3/2F2(g) FeF3 % Fe(s) þ 3/2F2(g) FeF2 % Fe(s) þ F2(g) Nd2O3 % 2Ndþ 3/2O2(g) Fe2O3 % 2Fe þ 3/2O2(g) FeO % Fe þ 1/2O2(g)
497 1370 780 538 1464 508 184
5.2 4.7 2.7 2.8 2.5 0.88 0.95
584 1590 972 663 1718 744 245
Fig. 1. Schematic diagram of the experimental set-up for electrochemical extraction.
148
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
Cyclic voltammetry, chronopotentiometry and square wave voltammetry experiments were conducted using Parastat 4000 potentiostat (Ametek) and data was recorded using Versastudio software. A molybdenum rod (2 mm diameter) was used as the working electrode and the immersion depth of the Mo rod was measured after each experiment. The auxiliary electrode was an iron rod (silver steel 98%iron - Salomon metals, 3 mm diameter) or graphite. The potentials were referred to a 1-mm diameter platinum wire immersed in the molten electrolyte, acting as a quasireference electrode Pt/PtOx/O2 [17]. 3.2. Experiments related to electrochemical extraction of Nd Electrochemical extraction of Nd was carried out in LiFeNdF3 and LiFeNd2O3 systems. The metallic deposit on the molybdenum electrode after the experiments were analyzed by X-Ray diffractometer (XRD). Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were also performed to analyze the microstructure of the samples. Some samples were also analyzed with Wavelength Dispersive Spectroscopy (WDS) mapping.
4. Results and discussion 4.1. Electrochemical behavior studies of different systems 4.1.1. Electrochemical dissolution of iron anode Experiment related to chemical dissolution of iron anode showed that the length and the mass of iron was not changed after 3 h at 950 C, indicating that iron is not chemically dissolved in the LiF molten salt. Moreover, the positive Gibbs energies of the reactions of Fe with Nd2O3 and LiF in their standard states and as pure substances, proves that chemical dissolution of Fe in LiFeNd2O3 is not feasible: 3Fe þ Nd2O3(s) % 3FeO(s) þ2Nd DGo [T ¼ 950 C] ¼ þ911 kJ/mol (7) 2Fe þ Nd2O3(s) % Fe2O3(s) þ2Nd DGo [T ¼ 950 C] ¼ þ960 kJ/ mol (8) Fe þ 2LiF % FeF2þ 2Li DGo [T ¼ 950 C] ¼ þ462 kJ/mol
(9)
Fe þ 3LiF % FeF3þ 3Li DGo [T ¼ 950 C] ¼ þ763 kJ/mol
(10)
On the contrary, when a current of 2 A was passed between the iron anode inserted 1 cm in the bath and Mo as the cathode in the molten salt electrolyte, after 840 s the cell potential suddenly decreased from 2.5 V and the electrolysis was terminated, showing that all the iron immersed in the salt is consumed. Considering the density of the iron bar, weight of the immersed iron in the salt bath would be 0.55 g and based on Faraday's law, the iron mass dissolution in 840 s for n ¼ 2 would be 0.48 g.
m ¼ MIt=ðnFÞ
(11)
where m is the mass in grams, M is the molar mass in g.mol1, n is the number of exchanged electrons, F is the Faraday constant in C.mol1, I is the current in A and t is the time in s.
Fig. 2. Cyclic voltammograms of LiF-0.3 mol% NdF3 on a Mo electrode (ɸ 2 40 mm) at 950 C, counter electrode: glassy carbon, reference electrode: Pt wire, scan rate: 100 mVs1.
Fig. 3. Cyclic voltammograms of LiF-0.3 mol%NdF3 on different scan rates a Mo electrode (ɸ 2 40 mm) at 950 C, counter electrode: glassy carbon, reference electrode: Pt wire.
4.1.2. LiFeNdF3 system Cyclic voltammetry was carried out on a molybdenum electrode in LiFe0.3 mol%NdF3 melt at 950 C. Reduction of Liþ is the cathodic limit and oxidation of molybdenum is the anodic limit in the potential window of this system. The cyclic voltammogram of
Fig. 4. Linear relationship of Nd3þ reduction peak current density vs. the square root of the scanning potential rate of LiFeNdF3 on. Mo electrode (ɸ 2 40 mm); counter electrode: glassy carbon; reference electrode: Pt, T ¼ 950 C.
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
LiFeNdF3 in Fig. 2, recorded two peaks in the cathodic run at z1.6 V and z2.3 V and two anodic peaks in the reverse run at z2.2 V and z1.5 V versus the quasi-electrode Pt/PtOx/O2. The cathodic peak at 1.6 V and the anodic peak at 1.5 V are associated with reduction and dissolution of neodymium, respectively. The stripping shape of the peaks is typical of deposition and dissolution of a metal. Based on this voltammogram, it is reasonable to assume that Nd3þ is reduced to Nd metal in a single step. It is suggested that Nd3þ ions are deposited from three electron reduction of [NdF6]3- in LiF salt [8]. The present observations are in agreement with the results from other researchers [18,19] which show that the neodymium ion reduction is a single step reaction of Nd3þ to Nd metal. At further negative potentials to 2.3 V, an exponential increase of deposition current is observed. This can be attributed to the decomposition of LiF. The corresponding anodic
peak of lithium dissolution can be observed at ~ -2.2 V. Fig. 3 shows the cyclic voltammograms on the LiFeNdF3 system at different potential scan rates (200 mV s1). The intensity of the peaks is increased with increase in scan rates. The analysis of the curves in Fig. 3 and the relationship between the peak currents and the square root of the scan rates indicates the reversibility of the system. A relationship between maximum intensity of the peak and the square root of the potential scan rate is suggested for the reversible systems [20]:
Ip ¼ 0:61nFSC o ðnF=ðRTÞÞ1=2 D1=2 v1=2
(12)
where F is the Faraday constant (96500 C mol1), S the electrode area in cm2, Co the solute concentration in mol.cm3, n the number of exchanged electrons, D the diffusion coefficient in cm2 s1 and v the potential scanning rate in V s1. The reduction peak current density versus square root of the potential scan rate is plotted in Fig. 4. Based on the relationship between these two parameters and the cyclic voltammogram on Fig. 3, we can assume that LiFeNdF3 system acts as not fully reversible system but a quasi-reversible process. Based on equation (12) the diffusion coefficient of Nd3þ ions can be calculated from the slope of the line in Fig. 4 (being 3.1 A s ½ V1/2 cm2). Assuming n ¼ 3 for Nd3þ/Nd reduction in one step, Co ¼ 5 104 mol cm3 and T ¼ 1223 K, the calculated diffusion coefficient will be 4 105 cm2 s1 which is in good agreement with the result from Hamel et al. [18]. Using the square wave voltammetry technique, the width of the half peak was determined using OriginLab software and is shown in Fig. 5. The number of exchanged electrons for a quasi-reversible system can be calculated based [21]:
W1=2 ¼ 3:52RT=ðnFÞ
Fig. 5. Square wave voltammogram of the LiFe0.3 mol %NdF3 mixture at 10 Hz. Determination of the width (W1/2) of the half peak on Mo electrode (surface area ¼ 0.25 cm2); counter electrode: glassy carbon; reference electrode: Pt; T ¼ 950 C.
149
(13)
where W1/2 is the width of the half peak in volts. R is the universal gas constant (8.314 J mol1 K1), T is the temperature in K, n is the number of exchanged electrons. The number of exchanged electrons for neodymium was calculated to be 2.84 which confirms the
Fig. 6. Cyclic voltammograms of LiFeNd2O3 on a Mo electrode (ɸ 2 40 mm) at 950 C, counter electrode: glassy carbon, reference electrode: Pt wire, scan rate: 100 mVs1..
150
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
Fig. 7. Changes of current with time in the 1.7 V constant cell potential mode of Nd electrolysis in LiFeNdF3 using Fe as anode.
exchange of 3 neodymium electrons in one step reduction. 4.1.3. LiFeNd2O3 system Cyclic voltammetry graph of LiFeNd2O3 system at 950 C on molybdenum working electrode using graphite as the counter electrode and Pt/PtOx/O2 as quasi-reference electrode is shown in Fig. 6. The observed cathodic limit at z -1.5 V is assumed to be Liþ ions reduction and a sharp oxidation peak (z-1.38 V) on the anodic run are associated with the reverse reaction on molybdenum electrode. No other peak related to neodymium reduction or dissolution is present in the LiFeNd2O3 voltammogram. Stefanidaki et al. [8] have claimed that the formation of neodymium oxyfluoride in the LiFeNd2O3 system, prohibit the neodymium reduction [22], hence Nd2O3 cannot be used for Nd extraction in the absence of NdF3 in the electrolyte. 4.2. Nd extraction in LiFeNdF3 using iron anode Nd electrochemical extraction in LiF-10 mol% NdF3 was performed at 950 C using Mo as cathode and Fe as anode. Based on the cyclic voltammetry results on LiFeNdF3 system, electrodeposition was performed at a constant potential mode of 1.7 V. The changes of current with time was recorded and is shown in Fig. 7. After about 3 h (10 ks), the current reaches zero which shows that the iron bar inserted in the salt bath was dissolved in the bulk. The inserted graph in this figure is the cyclic voltammetry of the system, which was performed at the beginning of the experiment to obtain the decomposition voltage of the Nd extraction. A metallic layer was seen after the experiment in the vicinity of anode (shown as analyzed part in Fig. 8). This layer was analyzed by XRD and the results are shown in Fig. 8. It can be seen that there is an accumulation of Fe in the form of FeC0.053 and Fe1.86C0.14 in vicinity of the Fe anode as the result of anodic dissolution. These phases are marked as FeeC on the XRD graph. The peaks related to LiF and NdF3 are marked as salt phase on the XRD graph. The reason that carbide is formed might be due to the carbon presence in iron (silver steel that is used as iron anode contains about 0.95% carbon). The use of graphite crucible can also explain the presence of carbon
Fig. 8. XRD result of the part of the salt under the Fe anode in LiFeNdF3 salt bath.
in the salt bath. The cathodic product on the Mo cathode was analyzed by SEM/ EDS after the experiment. The EDS analysis of the sample shows the deposition of Nd and Fe on the Mo cathode, as is shown in Fig. 9. It should be noted that the concentration of Mo in the phases close to the cathode is high (21 at%). However, during up-scaling of the present process, the presence of Mo in the first cathodic deposited layer may not pose a serious problem. In order to investigate the distribution of the Nd and Fe on the cathode, an EDS mapping was performed which is shown in Fig. 10. It can be concluded from the EDS quantifications and the mappings that an Fe rich NdeFeeMo phase is formed on the cathode. From these results, the formation of Fe15Nd and Fe17Nd2 and Fe16.2Nd2 phases is evident. In the FeeNd binary phase diagram, which is shown in Fig. 11, Fe17Nd2 and Fe17Nd5 are intermetallic compounds [23]. The existence of the Fe2Nd has also been reported in literature [24].
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
151
Fig. 9. SEM image and EDS quantification (in at%) of the deposit on the cathode showing the formation of FeeNd alloy.
Fig. 10. Elemental EDS mapping of the deposit on the cathode showing the distribution of NdeFeeF on the Mo cathode.
4.3. Nd extraction in LiFeNd2O3 using iron anode The extraction of Nd from Nd2O3 using Fe anode was performed at 950 C under argon gas atmosphere. The electrodeposition was performed at a constant potential mode of 1.7 V for 3 h. Mo bar was used as the cathode and 0.1 mol% FeF3 was used in the LiFeNd2O3 salt to initiate the fluoride ion formation in the electrolyte. The deposited material on the Mo electrode and the composition of the bulk were analyzed by XRD and WDS and are shown in Fig. 12 and Fig. 13, respectively. Analyzing the possible mechanisms in the electrolytic process, once the electrolysis is started, FeF3 would decompose and Fe would reduce on the Mo cathode due to cathodic reactions (reaction 5). Anodic dissolution of iron is initiated in the anodic part, leading to the formation of FeF2/FeF3 based on reactions (1e2). The formed iron fluoride can either be decomposed through the electrochemical reaction or react with Nd2O3 to form NdF3 through the chemical reaction based on reactions (3e4). The feasibility of the Nd2O3 to NdF3 conversion by iron fluoride has been experimentally investigated as a part of this project in our lab [13,31,32]. The XRD results in Fig. 12 show the presence of Fe in the form of FeO (wustite) and NdFeO3 phase in the samples from molten salt. Presence of these phases as well as NdF3 proves that fluorination reaction has taken place. The WDS mapping in Fig. 13 clearly shows the formation of Fe layer as the first deposited layer on the Mo cathode. In this figure Nd and Fe co-deposition can be seen on the Fe layer. Decomposition of NdF3 might occur at more positive potentials since NdeFe alloy is reduced in a potential range between the deposition of pure Nd and Fe. This phenomenon has been studied on the TieB, ZreB, TaeB,
Fig. 11. The FeeNd binary phase diagram [25].
SmeCo, TieNb and NdeAl alloy formation by different research groups [26e30]. In all above-mentioned systems, the electrodeposition voltammogram peak of these compounds is identified in a potential between their corresponding pure metals. While the WDS results identify the electrochemical reactions involved in the deposition of Nd as well as FeeNd alloys, the parallel chemical reactions (reactions 3e4) are not ruled out. The presence of FeO and NdF3 shown in the XRD results proves that the conversion has taken place. Further experiments are conducted to investigate the impact of the chemical reactions on the overall process. Moreover, design of a set-up in which the accumulated iron oxide is removed from the salt bath is of great importance for the continuity of the process.
152
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
Fig. 12. XRD result of the sample of LiFeNd2O3eFeF3 electrolyte after electrolysis with iron anode and Mo cathode.
Fig. 13. WDS mapping of the deposit on the cathode in of LiFeNd2O3-0.1 mol%FeF3 using Fe as anode.
5. Conclusion During the extraction of neodymium by electrolytic deposition, problems associated with the choice of the suitable halide, generation of CO/CO2 and PFC gases and the low solubility of the neodymium oxide in molten fluorides are solved by the use of reactive anode and fluoridizing the oxide to fluoride. In this study, a novel method is adopted in which iron is used as anode, promoting electrochemical dissolution of iron into the melt, thus preventing PFC evolution at the anode. Thus, the fluoridizing agent is constantly regenerated in-situ which enables the continuous conversion of neodymium oxide feed to the corresponding fluoride. Once neodymium fluoride is formed, it is subjected to electrolysis and Nd is deposited on the cathode. The cathodic product is NdeFe alloy which can be used as a master alloy for the production of NdFeB magnets. Acknowledgment This research has received funding from the European Community's Seventh Framework Programme ([FP7/2007e2013]) under grant agreement n 607411 (MC-ITN EREAN: European Rare Earth Magnet Recycling Network, Project website: www.erean.eu). This publication reflects only the authors' view, exempting the Community from any liability. The authors acknowledge Ruud Hendrix, Richard Huizenga and Kees Kwakernaak of TU Delft for their contribution in XRD and SEM/EDS analysis. References [1] Y. Yang, A. Walton, R. Sheridan, K. Güth, R. Gauß, O. Gutfleisch, M. Buchert, B.M. Steenari, T. Van Gerven, P.T. Jones, K. Binnemans, REE recovery from endof-life NdFeB permanent magnet scrap: a critical review, J. Sustain. Metal. 3 (1) (2017) 122e149. [2] P. Siming, Y. Shihong, L. Zongan, C. Dehon, X. Liha, Z. Bin, Development on molten salt electrolytic methods and technology for preparing rare earth
metals and alloys in China, Chin. J. Rare Metals 35 (3) (2011) 440e450. [3] S. Seetharaman and O. Grinder US Patent Application Nr. 12/991128, ref. no.: 12057 (2010). [4] A. Abbasalizadeh, S. Seetharaman, L. Teng, S. Sridhar, O. Grinder, Y. Izumi, M. Barati, Highlights of the salt extraction process, J. Miner. Met. Mater. Soc. 65 (11) (2013) 1552e1558. [5] E. Morrice, T.A. Henrie, Electrowinning High-Purity Neodymium, Praseodymium, and Didymium Metals from Their Oxides, U.S. Dept. of the Interior, Bureau of Mines, Washington D.C., 1967. [6] J.E. Murphy, D.K. Dysinger, M.F. Chambers, Electrowinning neodymium metal from chloride and oxide-fluoride electrolytes, in: Light Metals: Proceedings of Sessions, TMS Annual Meeting, Warrendale, Pennsylvania, 1995. [7] B. Porter, E.A. Brown, Determination of Oxide Solubility in Molten Fluorides, U.S. Dept. of the Interior, Bureau of Mines, Washington, DC, 1961, p. 3139. [8] E. Stefanidaki, C. Hasiotis, C. Kontoyannis, Electrodeposition of neodymium from LiF-NdF3-Nd2O3 melts, Electrochim. Acta 46 (17) (2001) 2665e2670. [9] R. Thudum, A. Srivastava, S. Nandi, A. Nagaraj, R. Shekhar, Molten salt electrolysis of neodymium: electrolyte selection and deposition mechanism, Miner. Process. Extr. Metall. (IMM Trans. Sect. C) 119 (2) (2010) 88e92. [10] A. Kaneko, Y. Yamamoto, C. Okada, Electrochemistry of rare earth fluoride molten salts, J. Alloy. Comp. 193 (1993) 44e46. [11] P. Taxil, L. Massot, C. Nourry, M. Gibilaro, P. Chamelot, L. Cassayre, Lanthanides extraction processes in molten fluoride media: application to nuclear spent fuel reprocessing, J. Fluorine Chem. 130 (1) (2009) 94e101. [12] H. Zhu, Rare earth metal production by molten salt electrolysis, in: G. Kreysa, K. Ota, R. Savinell (Eds.), Encyclopedia of Applied Electrochemistry, Springer, New York, 2014, pp. 1765e1772. [13] A. Abbasalizadeh, A. Malfliet, S. Seetharaman, J. Sietsma, Y. Yang, Electrochemical extraction of rare earth metals in molten fluorides: conversion of rare earth oxides into rare earth fluorides using fluoride additives, J. Sustain. Metal. 3 (3) (2017) 627e637. [14] T. Haarberg, A. Solheim, S.T. Johansen, P.A. Solli, Effect of anodic gas release on roult cells, Light Met. (1998) 475e482. current efficiency in Hall-He [15] W.E. Haupin, Electrochemistry of the Hall-Heroult process for aluminum smelting, J. Chem. Educ. 60 (4) (1983) 279. [16] L. Xu, Electrochemical behavior of zirconium in molten LiF-KF-ZrF4 at 600 C, RSC Adv. 6 (87) (2016) 84472. [17] Y. Berghoute, A. Salmi, F. Lantelme, Internal reference systems for fused electrolytes, J. Electroanal. Chem. 365 (1) (1994) 171e177. [18] C. Hamel, P. Chamelot, P. Taxil, Neodymium(III) cathodic processes in molten fluorides, Electrochim. Acta 49 (25) (2004) 4467e4476. [19] T. Nohira, S. Kobayashi, K. Kobayashi, R. Hagiwara, T. Oishi, H. Konishi, Electrochemical formation of Nd-Ni alloys in molten LiF-CaF2-NdF3, ECS Transact. 33 (7) (2010) 205e212. [20] T. Store (1999) PhD Thesis, Norwegian University of Science and Technology: Department of Electrochemistry. p. 22. [21] L.E. Ramalay, M.S. Kraus, Analytical application of square wave voltammetry, Anal. Chem. 41 (11) (1969) 1365e1369. [22] E. Stefanidaki, G.M. Photiadis, C.G. Kontoyannis, A.F. Vik, T. Østvold, Oxide solubility and Raman spectra of NdF3-LiF-KF-MgF2-Nd2O3 melts, J. Chem. Soc., Dalton Trans. 11 (2002) 2302e2307. [23] T.L. Chen, J. Wang, M.H. Rong, G.H. Rao, H.Y. Zhou, Experimental investigation and thermodynamic assessment of the Fe-Pr and Fe-Nd binary systems, Calphad 55 (2016) 270e280. [24] I.A. Santos, S. Gama, Evidence for the stable existence of the Fe2Nd phase in the Fe-Nd system, J. Appl. Phys. 86 (4) (1999) 2334e2336. [25] F.J.G. Landgraf, G.S. Schneider, V. Villas-Boas, F.P. Missell, Solidification and solid state transformations in Fe-Nd: a revised phase diagram, J. Less Common Met. 163 (1) (1990) 209e218. [26] H. Wendt, K. Reuhl, V. Schwarz, Cathodic deposition of refractory intermetallic compounds from flinak-meltsdI. Voltammetric investigation of Ti, Zr, B, TiB2 and ZrB2, Electrochim. Acta 37 (2) (1992) 237e244. [27] O. Makarova, L. Polyakova, E. Polyakov, A. Shevyryov, A. Arakcheeva, Phase composition of cathodic deposits synthesized in FLINAK-K2TaF7-KBF4 melt, J. Min. Metal. B: Metallurgy 39 (1e2) (2003) 261e267. [28] T. Iida, T. Nohira, Y. Ito, Electrochemical formation of SmeCo alloys by codeposition of Sm and Co in a molten LiCleKCleSmCl3eCoCl2 system, Electrochim. Acta 48 (17) (2003) 2517e2521. [29] L.P. Polyakova, P. Taxil, E.G. Polyakov, Electrochemical behaviour and codeposition of titanium and niobium in chlorideefluoride melts, J. Alloy. Comp. 359 (1) (2003) 244e255. [30] M. Gibilaro, L. Massot, P. Chamelot, P. Taxil, Study of neodymium extraction in molten fluorides by electrochemical co-reduction with aluminium, J. Nucl. Mater. 382 (1) (2008) 39e45. [31] A. Abbasalizadeh, A. Malfliet, S. Seetharaman, J. Sietsma, Y. Yang, Electrochemical recovery of rare earth elements from magnets: conversion of rare earth based metals into rare earth fluorides in molten salts, Mater. Trans. 58 (3) (2017) 400e405. [32] A. Abbasalizadeh, L. Teng, S. Seetharaman, J. Sietsma, Y. Yang, Rare earth extraction from NdFeB magnets and rare earth oxides using aluminum chloride/fluoride molten salts, in: D. Lima, I. Borges, W. Leal Filho (Eds.), Rare Earths Industry: Technological, Economic, and Environmental Implications, Elsevier., 2015, pp. 357e373.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Use of iron reactive anode in electrowinning of neodymium from neodymium oxide Aida Abbasalizadeh a, *, Seshadri Seetharaman b, Prakash Venkatesan a, Jilt Sietsma a, Yongxiang Yang a a b
Department of Materials Science and Engineering, Delft University of Technology (TU Delft), Mekelweg 2, 2628CD, Delft, the Netherlands Royal Institute of Technology (KTH), Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2018 Received in revised form 22 March 2019 Accepted 22 March 2019 Available online 23 March 2019
Electrolytic production of metallic neodymium is carried out in fused neodymium fluoride salts containing neodymium oxide. Two major challenges pertaining to neodymium production in fluoride salts are a) low solubility of neodymium oxide in fluoride melt, b) possibility of anodic gas evolution (CO, CO2, CF4, C2F6). In this study, iron is used as a reactive anode in the electrolysis process, promoting electrochemical dissolution of iron into the melt, preventing PFC (perfluorocarbon) gas evolution at the anode. Further, the rare earth oxide is converted to rare earth fluoride by the use of iron fluoride formed as the result of iron dissolution. Thus, the fluoridizing agent is constantly regenerated in-situ which enables the continuous conversion of neodymium oxide feed. The cathodic product is NdeFe alloy which can be used as a master alloy for the production of NdFeB magnets. © 2019 Published by Elsevier Ltd.
Keywords: Rare earths Electrochemical extraction Reactive anode PFC gas Molten salts
1. Introduction Rare earh (RE) metals and their alloys have important applications in different fields of modern technology, in particular in green technologies for clean energy such as wind turbines and electric vehicles [1]. Deoxidation of rare earth metals is very difficult due to the strong affinity of these metals to oxygen. Traditionally, RE metals have been produced by the calciothermic reduction of REF3. However, molten salt electrolysis is becoming the dominant industrial method for the extraction of RE metals from their oxides in the molten salt process [2e4]. Because of negative deposition potential, RE metals cannot be electrochemically extracted in aqueous media. Solubility of the rare earth oxide (REO) in the fluoride electrolyte is an important factor, since it is the dissolved oxide that is subjected to electrolysis. However, the solubility of rare earth oxide in molten alkali fluoride does not exceed 4 wt percent [5e7]. Moreover, neodymium oxide is likely to form neodymium oxyfluoride in molten fluoride salts. Oxyfluoride formation has been reported to affect the neodymium electrodeposition adversely [8]. However,
* Corresponding author. E-mail address: [email protected] (A. Abbasalizadeh). https://doi.org/10.1016/j.electacta.2019.03.161 0013-4686/© 2019 Published by Elsevier Ltd.
the role of neodymium oxyfluoride in the anodic and cathodic reactions is not very clear and needs to be clarified further in detail [8e12]. Another challenge in neodymium production by fluoride electrolysis is the evolution of fluorine associated gasses (PFCs) on the carbon anode when the electrolysis rate exceeds the rare earth oxide feeding and dissolution rate [10]. In the framework of this research for electrochemical extraction of rare earths from their oxides in molten fluoride media, the present research strategy was adopted in order to address the solubility issue of neodymium oxides in fluoride melts. It was shown earlier by the present authors that iron [III] fluoride could be used successfully as a fluoridizing agent for neodymium oxide in molten salts [13]. The conversion of Nd2O3 to NdF3 in molten salts solves the problem of low solubility of the neodymium oxide in molten fluorides to a large extent. Once neodymium fluoride is formed, it can be electrolyzed and subsequently Nd can be deposited on the cathode, along with CO, CO2, CF4/C2F6 (PFCs) evolution on the anode side. However, PFC gas generation on the graphite anode in the rare earth metal/alloy production industry is a serious drawback of this process, in common with aluminum production industry [14,15]. The present study is aimed to address this problem. A reactive anode is employed to overcome the problem of halogen evolution at the anode. Iron is anodically dissolved, generating the fluorinating agent FeF2/FeF3 in-situ in the
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
electrochemical reactor. The NdF3 thus formed can subsequently be processed through the electrolysis route in the same reactor to extract Nd metal as the cathodic deposit. In this concept, the REO dissolution in the molten fluorides would become redundant due to RE oxide conversion into the fluoride as REF3.
147
Moreover, both FeF2 and FeF3 can undergo the electrolysis process and decompose along with Nd on the cathode since NdF3 has higher decomposition voltage ( ENdF3 ¼ 4:7 V, EFeF2 ¼ 2:8 V and EFeF3 ¼ 2:7 V). Hence the cathodic reactions would be: Fe3þ þe /Fe2þ / Fe2þ þ 2e /Fe
(5)
2. Thermodynamic evaluations of the process
Nd3þ þ 3e/ Nd
(6)
In the present study, use of iron anode is suggested for the electrolysis process in which instead of CO/CO2 or CF4/C2F6 gas formation, the anodic reaction would be iron dissolution. FeF3 is added to the system in order to initiate the decomposition process. The dissolved iron at anode side would react with the fluorine ions which are formed as the result of decomposition of added directly FeF3 to LiFeNd2O3 mixture and subsequently FeF2 and FeF3 are formed in-situ in the system:
From Table 1 it can also be concluded that the dissolved neodymium oxide and iron oxide can also participate in the electrochemical process resulting to Nd and Fe extraction on the cathode, respectively. Hence the solubility of these oxides plays a significant role on their participation in the electrochemical reaction process. In order to prevent the accumulation of the oxide in the LiFeNd2O3 salt bath, the undissolved iron oxide should be removed from the system.
Fe2þþ 2F / FeF2 þ 2e
(1)
Fe3þ þ 3F / FeF3 þ3e
(2)
Iron (II) fluoride is more stable compare to iron (III) fluoride. However, depending on the electrolysis conditions FeF3 is formed in the electrolyte. The generated iron fluoride will act as fluorinating agent and convert Nd2O3 to NdF3. From the calculated standard Gibbs energy (DGo) of the reaction (3e4) at 1223 K (950 C) using FactPS database of FactSage: 3FeF2(salt) þ Nd2O3(s) % 2NdF3(salt) þ 3FeO (s) DGo ¼ 215 kJ/ mol (3) 2FeF3(salt) þ Nd2O3(s) % 2NdF3(salt) þ Fe2O3(s) DGo ¼ 221 kJ/ mol (4) It is evident that both FeF2 and FeF3 can react with Nd2O3 to form NdF3, and subsequently Nd3þ ions can further be reduced on the cathode. As the result of the fluorinating reaction, FeO and Fe2O3 are formed in the system. Prior experiments by the present authors confirm the feasibility of the conversion of neodymium oxide to neodymium fluoride using FeF3 as fluorinating agent [13]. In Table 1 the Standard Gibbs free energies of formation of the reacted fluorides and oxides in the system at 1223 K (950 C), calculated using FactSage are given. It is important to note that these reactions refer to the case of the reacting species at their standard state as pure substances. In the real systems, the activity of the components would be lower than unity. From Table 1 it is evident that iron fluoride has lower decomposition voltage than neodymium fluoride. Hence, from the thermodynamic perspective, two scenarios are possible for the formed iron fluoride generated in the system as the result of anodic dissolution. In one scenario iron fluoride acts as the fluorinating agent and convert Nd2O3 to NdF3.
3. Experimental Pure lithium fluoride (98.5%-Alfa Aesar) was used as the electrolyte bath and was mixed with neodymium oxide (Solvay) or neodymium fluoride or iron (III) fluoride (97%-Alfa Aesar) in a glove box. LiF was chosen as the solvent since the deposition potential of the Li in the solvent is more cathodic than the deposition potential of Nd3þ. LiF is also the main solvent used in industry for the extraction of neodymium in molten fluoride process [2]. A schematic of the experimental apparatus used for the electrochemical extraction of metals in molten salt [16] is shown in Fig. 1. A glassy carbon crucible served as the container of the mixtures. The crucible was placed in an electric furnace under the inert argon atmosphere. A water cooling system was used for both flanges at the top and bottom. All experiments were carried out at 950 C with in the limit of ±3 at hot zone. A K-type thermocouple was used for the measurement of the temperature at the bottom of the graphite crucible. 3.1. Experiments related to electrochemical behavior of LiFeNdF3/ FeF3/Nd2O3 systems In order to examine chemical dissolution of iron anode in LiFe Nd2O3 bath an experiment was performed in which an iron bar was inserted in the LiFe Nd2O3 bath at 950 C for 3 h.
Table 1 Standard Gibbs energies and the decomposition potential of different fluorides salts and REOs and REF. Reaction
DGo (kJ/mol) at Decomposition DGo (kJ/mol) at 1223 K (950 C) potential (V) at 298 K (25 C) 1223 K (950 C)
LiF % Li þ 1/2 F2(g) NdF3 % Nd(s) þ 3/2F2(g) FeF3 % Fe(s) þ 3/2F2(g) FeF2 % Fe(s) þ F2(g) Nd2O3 % 2Ndþ 3/2O2(g) Fe2O3 % 2Fe þ 3/2O2(g) FeO % Fe þ 1/2O2(g)
497 1370 780 538 1464 508 184
5.2 4.7 2.7 2.8 2.5 0.88 0.95
584 1590 972 663 1718 744 245
Fig. 1. Schematic diagram of the experimental set-up for electrochemical extraction.
148
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
Cyclic voltammetry, chronopotentiometry and square wave voltammetry experiments were conducted using Parastat 4000 potentiostat (Ametek) and data was recorded using Versastudio software. A molybdenum rod (2 mm diameter) was used as the working electrode and the immersion depth of the Mo rod was measured after each experiment. The auxiliary electrode was an iron rod (silver steel 98%iron - Salomon metals, 3 mm diameter) or graphite. The potentials were referred to a 1-mm diameter platinum wire immersed in the molten electrolyte, acting as a quasireference electrode Pt/PtOx/O2 [17]. 3.2. Experiments related to electrochemical extraction of Nd Electrochemical extraction of Nd was carried out in LiFeNdF3 and LiFeNd2O3 systems. The metallic deposit on the molybdenum electrode after the experiments were analyzed by X-Ray diffractometer (XRD). Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were also performed to analyze the microstructure of the samples. Some samples were also analyzed with Wavelength Dispersive Spectroscopy (WDS) mapping.
4. Results and discussion 4.1. Electrochemical behavior studies of different systems 4.1.1. Electrochemical dissolution of iron anode Experiment related to chemical dissolution of iron anode showed that the length and the mass of iron was not changed after 3 h at 950 C, indicating that iron is not chemically dissolved in the LiF molten salt. Moreover, the positive Gibbs energies of the reactions of Fe with Nd2O3 and LiF in their standard states and as pure substances, proves that chemical dissolution of Fe in LiFeNd2O3 is not feasible: 3Fe þ Nd2O3(s) % 3FeO(s) þ2Nd DGo [T ¼ 950 C] ¼ þ911 kJ/mol (7) 2Fe þ Nd2O3(s) % Fe2O3(s) þ2Nd DGo [T ¼ 950 C] ¼ þ960 kJ/ mol (8) Fe þ 2LiF % FeF2þ 2Li DGo [T ¼ 950 C] ¼ þ462 kJ/mol
(9)
Fe þ 3LiF % FeF3þ 3Li DGo [T ¼ 950 C] ¼ þ763 kJ/mol
(10)
On the contrary, when a current of 2 A was passed between the iron anode inserted 1 cm in the bath and Mo as the cathode in the molten salt electrolyte, after 840 s the cell potential suddenly decreased from 2.5 V and the electrolysis was terminated, showing that all the iron immersed in the salt is consumed. Considering the density of the iron bar, weight of the immersed iron in the salt bath would be 0.55 g and based on Faraday's law, the iron mass dissolution in 840 s for n ¼ 2 would be 0.48 g.
m ¼ MIt=ðnFÞ
(11)
where m is the mass in grams, M is the molar mass in g.mol1, n is the number of exchanged electrons, F is the Faraday constant in C.mol1, I is the current in A and t is the time in s.
Fig. 2. Cyclic voltammograms of LiF-0.3 mol% NdF3 on a Mo electrode (ɸ 2 40 mm) at 950 C, counter electrode: glassy carbon, reference electrode: Pt wire, scan rate: 100 mVs1.
Fig. 3. Cyclic voltammograms of LiF-0.3 mol%NdF3 on different scan rates a Mo electrode (ɸ 2 40 mm) at 950 C, counter electrode: glassy carbon, reference electrode: Pt wire.
4.1.2. LiFeNdF3 system Cyclic voltammetry was carried out on a molybdenum electrode in LiFe0.3 mol%NdF3 melt at 950 C. Reduction of Liþ is the cathodic limit and oxidation of molybdenum is the anodic limit in the potential window of this system. The cyclic voltammogram of
Fig. 4. Linear relationship of Nd3þ reduction peak current density vs. the square root of the scanning potential rate of LiFeNdF3 on. Mo electrode (ɸ 2 40 mm); counter electrode: glassy carbon; reference electrode: Pt, T ¼ 950 C.
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
LiFeNdF3 in Fig. 2, recorded two peaks in the cathodic run at z1.6 V and z2.3 V and two anodic peaks in the reverse run at z2.2 V and z1.5 V versus the quasi-electrode Pt/PtOx/O2. The cathodic peak at 1.6 V and the anodic peak at 1.5 V are associated with reduction and dissolution of neodymium, respectively. The stripping shape of the peaks is typical of deposition and dissolution of a metal. Based on this voltammogram, it is reasonable to assume that Nd3þ is reduced to Nd metal in a single step. It is suggested that Nd3þ ions are deposited from three electron reduction of [NdF6]3- in LiF salt [8]. The present observations are in agreement with the results from other researchers [18,19] which show that the neodymium ion reduction is a single step reaction of Nd3þ to Nd metal. At further negative potentials to 2.3 V, an exponential increase of deposition current is observed. This can be attributed to the decomposition of LiF. The corresponding anodic
peak of lithium dissolution can be observed at ~ -2.2 V. Fig. 3 shows the cyclic voltammograms on the LiFeNdF3 system at different potential scan rates (200 mV s1). The intensity of the peaks is increased with increase in scan rates. The analysis of the curves in Fig. 3 and the relationship between the peak currents and the square root of the scan rates indicates the reversibility of the system. A relationship between maximum intensity of the peak and the square root of the potential scan rate is suggested for the reversible systems [20]:
Ip ¼ 0:61nFSC o ðnF=ðRTÞÞ1=2 D1=2 v1=2
(12)
where F is the Faraday constant (96500 C mol1), S the electrode area in cm2, Co the solute concentration in mol.cm3, n the number of exchanged electrons, D the diffusion coefficient in cm2 s1 and v the potential scanning rate in V s1. The reduction peak current density versus square root of the potential scan rate is plotted in Fig. 4. Based on the relationship between these two parameters and the cyclic voltammogram on Fig. 3, we can assume that LiFeNdF3 system acts as not fully reversible system but a quasi-reversible process. Based on equation (12) the diffusion coefficient of Nd3þ ions can be calculated from the slope of the line in Fig. 4 (being 3.1 A s ½ V1/2 cm2). Assuming n ¼ 3 for Nd3þ/Nd reduction in one step, Co ¼ 5 104 mol cm3 and T ¼ 1223 K, the calculated diffusion coefficient will be 4 105 cm2 s1 which is in good agreement with the result from Hamel et al. [18]. Using the square wave voltammetry technique, the width of the half peak was determined using OriginLab software and is shown in Fig. 5. The number of exchanged electrons for a quasi-reversible system can be calculated based [21]:
W1=2 ¼ 3:52RT=ðnFÞ
Fig. 5. Square wave voltammogram of the LiFe0.3 mol %NdF3 mixture at 10 Hz. Determination of the width (W1/2) of the half peak on Mo electrode (surface area ¼ 0.25 cm2); counter electrode: glassy carbon; reference electrode: Pt; T ¼ 950 C.
149
(13)
where W1/2 is the width of the half peak in volts. R is the universal gas constant (8.314 J mol1 K1), T is the temperature in K, n is the number of exchanged electrons. The number of exchanged electrons for neodymium was calculated to be 2.84 which confirms the
Fig. 6. Cyclic voltammograms of LiFeNd2O3 on a Mo electrode (ɸ 2 40 mm) at 950 C, counter electrode: glassy carbon, reference electrode: Pt wire, scan rate: 100 mVs1..
150
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
Fig. 7. Changes of current with time in the 1.7 V constant cell potential mode of Nd electrolysis in LiFeNdF3 using Fe as anode.
exchange of 3 neodymium electrons in one step reduction. 4.1.3. LiFeNd2O3 system Cyclic voltammetry graph of LiFeNd2O3 system at 950 C on molybdenum working electrode using graphite as the counter electrode and Pt/PtOx/O2 as quasi-reference electrode is shown in Fig. 6. The observed cathodic limit at z -1.5 V is assumed to be Liþ ions reduction and a sharp oxidation peak (z-1.38 V) on the anodic run are associated with the reverse reaction on molybdenum electrode. No other peak related to neodymium reduction or dissolution is present in the LiFeNd2O3 voltammogram. Stefanidaki et al. [8] have claimed that the formation of neodymium oxyfluoride in the LiFeNd2O3 system, prohibit the neodymium reduction [22], hence Nd2O3 cannot be used for Nd extraction in the absence of NdF3 in the electrolyte. 4.2. Nd extraction in LiFeNdF3 using iron anode Nd electrochemical extraction in LiF-10 mol% NdF3 was performed at 950 C using Mo as cathode and Fe as anode. Based on the cyclic voltammetry results on LiFeNdF3 system, electrodeposition was performed at a constant potential mode of 1.7 V. The changes of current with time was recorded and is shown in Fig. 7. After about 3 h (10 ks), the current reaches zero which shows that the iron bar inserted in the salt bath was dissolved in the bulk. The inserted graph in this figure is the cyclic voltammetry of the system, which was performed at the beginning of the experiment to obtain the decomposition voltage of the Nd extraction. A metallic layer was seen after the experiment in the vicinity of anode (shown as analyzed part in Fig. 8). This layer was analyzed by XRD and the results are shown in Fig. 8. It can be seen that there is an accumulation of Fe in the form of FeC0.053 and Fe1.86C0.14 in vicinity of the Fe anode as the result of anodic dissolution. These phases are marked as FeeC on the XRD graph. The peaks related to LiF and NdF3 are marked as salt phase on the XRD graph. The reason that carbide is formed might be due to the carbon presence in iron (silver steel that is used as iron anode contains about 0.95% carbon). The use of graphite crucible can also explain the presence of carbon
Fig. 8. XRD result of the part of the salt under the Fe anode in LiFeNdF3 salt bath.
in the salt bath. The cathodic product on the Mo cathode was analyzed by SEM/ EDS after the experiment. The EDS analysis of the sample shows the deposition of Nd and Fe on the Mo cathode, as is shown in Fig. 9. It should be noted that the concentration of Mo in the phases close to the cathode is high (21 at%). However, during up-scaling of the present process, the presence of Mo in the first cathodic deposited layer may not pose a serious problem. In order to investigate the distribution of the Nd and Fe on the cathode, an EDS mapping was performed which is shown in Fig. 10. It can be concluded from the EDS quantifications and the mappings that an Fe rich NdeFeeMo phase is formed on the cathode. From these results, the formation of Fe15Nd and Fe17Nd2 and Fe16.2Nd2 phases is evident. In the FeeNd binary phase diagram, which is shown in Fig. 11, Fe17Nd2 and Fe17Nd5 are intermetallic compounds [23]. The existence of the Fe2Nd has also been reported in literature [24].
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
151
Fig. 9. SEM image and EDS quantification (in at%) of the deposit on the cathode showing the formation of FeeNd alloy.
Fig. 10. Elemental EDS mapping of the deposit on the cathode showing the distribution of NdeFeeF on the Mo cathode.
4.3. Nd extraction in LiFeNd2O3 using iron anode The extraction of Nd from Nd2O3 using Fe anode was performed at 950 C under argon gas atmosphere. The electrodeposition was performed at a constant potential mode of 1.7 V for 3 h. Mo bar was used as the cathode and 0.1 mol% FeF3 was used in the LiFeNd2O3 salt to initiate the fluoride ion formation in the electrolyte. The deposited material on the Mo electrode and the composition of the bulk were analyzed by XRD and WDS and are shown in Fig. 12 and Fig. 13, respectively. Analyzing the possible mechanisms in the electrolytic process, once the electrolysis is started, FeF3 would decompose and Fe would reduce on the Mo cathode due to cathodic reactions (reaction 5). Anodic dissolution of iron is initiated in the anodic part, leading to the formation of FeF2/FeF3 based on reactions (1e2). The formed iron fluoride can either be decomposed through the electrochemical reaction or react with Nd2O3 to form NdF3 through the chemical reaction based on reactions (3e4). The feasibility of the Nd2O3 to NdF3 conversion by iron fluoride has been experimentally investigated as a part of this project in our lab [13,31,32]. The XRD results in Fig. 12 show the presence of Fe in the form of FeO (wustite) and NdFeO3 phase in the samples from molten salt. Presence of these phases as well as NdF3 proves that fluorination reaction has taken place. The WDS mapping in Fig. 13 clearly shows the formation of Fe layer as the first deposited layer on the Mo cathode. In this figure Nd and Fe co-deposition can be seen on the Fe layer. Decomposition of NdF3 might occur at more positive potentials since NdeFe alloy is reduced in a potential range between the deposition of pure Nd and Fe. This phenomenon has been studied on the TieB, ZreB, TaeB,
Fig. 11. The FeeNd binary phase diagram [25].
SmeCo, TieNb and NdeAl alloy formation by different research groups [26e30]. In all above-mentioned systems, the electrodeposition voltammogram peak of these compounds is identified in a potential between their corresponding pure metals. While the WDS results identify the electrochemical reactions involved in the deposition of Nd as well as FeeNd alloys, the parallel chemical reactions (reactions 3e4) are not ruled out. The presence of FeO and NdF3 shown in the XRD results proves that the conversion has taken place. Further experiments are conducted to investigate the impact of the chemical reactions on the overall process. Moreover, design of a set-up in which the accumulated iron oxide is removed from the salt bath is of great importance for the continuity of the process.
152
A. Abbasalizadeh et al. / Electrochimica Acta 310 (2019) 146e152
Fig. 12. XRD result of the sample of LiFeNd2O3eFeF3 electrolyte after electrolysis with iron anode and Mo cathode.
Fig. 13. WDS mapping of the deposit on the cathode in of LiFeNd2O3-0.1 mol%FeF3 using Fe as anode.
5. Conclusion During the extraction of neodymium by electrolytic deposition, problems associated with the choice of the suitable halide, generation of CO/CO2 and PFC gases and the low solubility of the neodymium oxide in molten fluorides are solved by the use of reactive anode and fluoridizing the oxide to fluoride. In this study, a novel method is adopted in which iron is used as anode, promoting electrochemical dissolution of iron into the melt, thus preventing PFC evolution at the anode. Thus, the fluoridizing agent is constantly regenerated in-situ which enables the continuous conversion of neodymium oxide feed to the corresponding fluoride. Once neodymium fluoride is formed, it is subjected to electrolysis and Nd is deposited on the cathode. The cathodic product is NdeFe alloy which can be used as a master alloy for the production of NdFeB magnets. Acknowledgment This research has received funding from the European Community's Seventh Framework Programme ([FP7/2007e2013]) under grant agreement n 607411 (MC-ITN EREAN: European Rare Earth Magnet Recycling Network, Project website: www.erean.eu). This publication reflects only the authors' view, exempting the Community from any liability. The authors acknowledge Ruud Hendrix, Richard Huizenga and Kees Kwakernaak of TU Delft for their contribution in XRD and SEM/EDS analysis. References [1] Y. Yang, A. Walton, R. Sheridan, K. Güth, R. Gauß, O. Gutfleisch, M. Buchert, B.M. Steenari, T. Van Gerven, P.T. Jones, K. Binnemans, REE recovery from endof-life NdFeB permanent magnet scrap: a critical review, J. Sustain. Metal. 3 (1) (2017) 122e149. [2] P. Siming, Y. Shihong, L. Zongan, C. Dehon, X. Liha, Z. Bin, Development on molten salt electrolytic methods and technology for preparing rare earth
metals and alloys in China, Chin. J. Rare Metals 35 (3) (2011) 440e450. [3] S. Seetharaman and O. Grinder US Patent Application Nr. 12/991128, ref. no.: 12057 (2010). [4] A. Abbasalizadeh, S. Seetharaman, L. Teng, S. Sridhar, O. Grinder, Y. Izumi, M. Barati, Highlights of the salt extraction process, J. Miner. Met. Mater. Soc. 65 (11) (2013) 1552e1558. [5] E. Morrice, T.A. Henrie, Electrowinning High-Purity Neodymium, Praseodymium, and Didymium Metals from Their Oxides, U.S. Dept. of the Interior, Bureau of Mines, Washington D.C., 1967. [6] J.E. Murphy, D.K. Dysinger, M.F. Chambers, Electrowinning neodymium metal from chloride and oxide-fluoride electrolytes, in: Light Metals: Proceedings of Sessions, TMS Annual Meeting, Warrendale, Pennsylvania, 1995. [7] B. Porter, E.A. Brown, Determination of Oxide Solubility in Molten Fluorides, U.S. Dept. of the Interior, Bureau of Mines, Washington, DC, 1961, p. 3139. [8] E. Stefanidaki, C. Hasiotis, C. Kontoyannis, Electrodeposition of neodymium from LiF-NdF3-Nd2O3 melts, Electrochim. Acta 46 (17) (2001) 2665e2670. [9] R. Thudum, A. Srivastava, S. Nandi, A. Nagaraj, R. Shekhar, Molten salt electrolysis of neodymium: electrolyte selection and deposition mechanism, Miner. Process. Extr. Metall. (IMM Trans. Sect. C) 119 (2) (2010) 88e92. [10] A. Kaneko, Y. Yamamoto, C. Okada, Electrochemistry of rare earth fluoride molten salts, J. Alloy. Comp. 193 (1993) 44e46. [11] P. Taxil, L. Massot, C. Nourry, M. Gibilaro, P. Chamelot, L. Cassayre, Lanthanides extraction processes in molten fluoride media: application to nuclear spent fuel reprocessing, J. Fluorine Chem. 130 (1) (2009) 94e101. [12] H. Zhu, Rare earth metal production by molten salt electrolysis, in: G. Kreysa, K. Ota, R. Savinell (Eds.), Encyclopedia of Applied Electrochemistry, Springer, New York, 2014, pp. 1765e1772. [13] A. Abbasalizadeh, A. Malfliet, S. Seetharaman, J. Sietsma, Y. Yang, Electrochemical extraction of rare earth metals in molten fluorides: conversion of rare earth oxides into rare earth fluorides using fluoride additives, J. Sustain. Metal. 3 (3) (2017) 627e637. [14] T. Haarberg, A. Solheim, S.T. Johansen, P.A. Solli, Effect of anodic gas release on roult cells, Light Met. (1998) 475e482. current efficiency in Hall-He [15] W.E. Haupin, Electrochemistry of the Hall-Heroult process for aluminum smelting, J. Chem. Educ. 60 (4) (1983) 279. [16] L. Xu, Electrochemical behavior of zirconium in molten LiF-KF-ZrF4 at 600 C, RSC Adv. 6 (87) (2016) 84472. [17] Y. Berghoute, A. Salmi, F. Lantelme, Internal reference systems for fused electrolytes, J. Electroanal. Chem. 365 (1) (1994) 171e177. [18] C. Hamel, P. Chamelot, P. Taxil, Neodymium(III) cathodic processes in molten fluorides, Electrochim. Acta 49 (25) (2004) 4467e4476. [19] T. Nohira, S. Kobayashi, K. Kobayashi, R. Hagiwara, T. Oishi, H. Konishi, Electrochemical formation of Nd-Ni alloys in molten LiF-CaF2-NdF3, ECS Transact. 33 (7) (2010) 205e212. [20] T. Store (1999) PhD Thesis, Norwegian University of Science and Technology: Department of Electrochemistry. p. 22. [21] L.E. Ramalay, M.S. Kraus, Analytical application of square wave voltammetry, Anal. Chem. 41 (11) (1969) 1365e1369. [22] E. Stefanidaki, G.M. Photiadis, C.G. Kontoyannis, A.F. Vik, T. Østvold, Oxide solubility and Raman spectra of NdF3-LiF-KF-MgF2-Nd2O3 melts, J. Chem. Soc., Dalton Trans. 11 (2002) 2302e2307. [23] T.L. Chen, J. Wang, M.H. Rong, G.H. Rao, H.Y. Zhou, Experimental investigation and thermodynamic assessment of the Fe-Pr and Fe-Nd binary systems, Calphad 55 (2016) 270e280. [24] I.A. Santos, S. Gama, Evidence for the stable existence of the Fe2Nd phase in the Fe-Nd system, J. Appl. Phys. 86 (4) (1999) 2334e2336. [25] F.J.G. Landgraf, G.S. Schneider, V. Villas-Boas, F.P. Missell, Solidification and solid state transformations in Fe-Nd: a revised phase diagram, J. Less Common Met. 163 (1) (1990) 209e218. [26] H. Wendt, K. Reuhl, V. Schwarz, Cathodic deposition of refractory intermetallic compounds from flinak-meltsdI. Voltammetric investigation of Ti, Zr, B, TiB2 and ZrB2, Electrochim. Acta 37 (2) (1992) 237e244. [27] O. Makarova, L. Polyakova, E. Polyakov, A. Shevyryov, A. Arakcheeva, Phase composition of cathodic deposits synthesized in FLINAK-K2TaF7-KBF4 melt, J. Min. Metal. B: Metallurgy 39 (1e2) (2003) 261e267. [28] T. Iida, T. Nohira, Y. Ito, Electrochemical formation of SmeCo alloys by codeposition of Sm and Co in a molten LiCleKCleSmCl3eCoCl2 system, Electrochim. Acta 48 (17) (2003) 2517e2521. [29] L.P. Polyakova, P. Taxil, E.G. Polyakov, Electrochemical behaviour and codeposition of titanium and niobium in chlorideefluoride melts, J. Alloy. Comp. 359 (1) (2003) 244e255. [30] M. Gibilaro, L. Massot, P. Chamelot, P. Taxil, Study of neodymium extraction in molten fluorides by electrochemical co-reduction with aluminium, J. Nucl. Mater. 382 (1) (2008) 39e45. [31] A. Abbasalizadeh, A. Malfliet, S. Seetharaman, J. Sietsma, Y. Yang, Electrochemical recovery of rare earth elements from magnets: conversion of rare earth based metals into rare earth fluorides in molten salts, Mater. Trans. 58 (3) (2017) 400e405. [32] A. Abbasalizadeh, L. Teng, S. Seetharaman, J. Sietsma, Y. Yang, Rare earth extraction from NdFeB magnets and rare earth oxides using aluminum chloride/fluoride molten salts, in: D. Lima, I. Borges, W. Leal Filho (Eds.), Rare Earths Industry: Technological, Economic, and Environmental Implications, Elsevier., 2015, pp. 357e373.