- Email: [email protected]
Anode processes for Nd electrowinning from LiF-NdF3-Nd2O3 melt
Electrochimica Acta 147 (2014) 82–86
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Anode processes for Nd electrowinning from LiF-NdF3-Nd2O3 melt Shizhe Liu a , Lingyun Chen a , Bing Li a, *, Liangliang Wang b , Bo Yan b , Mugen Liu b a b
East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, PR China XuZhou JinShiPengYuan Rare-Earth Material Factory, No.7 Road Energy Development Zone, DaTun PeiCounty, JiangSu, 221611, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 July 2014 Received in revised form 31 August 2014 Accepted 4 September 2014 Available online 6 September 2014
Cyclic voltammetry was applied to characterize oxidation processes of oxygen ions and fluoride ions on a graphite electrode in LiF-NdF3-Nd2O3 melt. The effects of the concentration of Nd2O3, temperature, electrode area on the oxidation of oxygen ions and critical potential have been discussed. Oxidation processes of oxygen ions include adsorption and gas evolution of oxidation products, which is proved by the the smooth and fluctuant cyclic voltammograms. During adsorption process, both the peak current density and the peak potential of oxidation of oxygen ions increase with the increase of Nd2O3 concentration. The peak current density of oxidation of oxygen ions displays a linearity within 2.5wt% Nd2O3 in the melt. The kinetics of the oxidation of oxygen ions is controlled by both electrochemical reaction step and mass transfer. During gas evolution, the oxidation process of oxygen ions is controlled by electrochemical reaction step. Oxidation of flouride ions starts from the critical potential. The critical potential by reverse scan is independent of the electrode area, but depends on electrode area by positive scan. Increasing temperature causes significantly increased the adsoprtion current density for oxidation of oxygen ions. At 950 C, after the critical potential relatively big current peaks involving formaiton of perflouride compounds occurs after the cirtical potential. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: anode processes Nd electrowinning oxygen ions flouride ions cirtical potential
1. Introduction Nowadays, rare-earth metal (REM) is mainly produced by molten salts electrolysis [1], in which fluoride-oxide molten salts are widely used because it is a more environment friendly and high current efficient process than chloride molten salts. However, similar to aluminum electrolysis, fluoride-oxide molten salts are subject to anode effects and emission of perflouride compound (PFC) on a graphite anode. Anode effects will increase the cell voltage and the energy consumption for REM electrowinning, at the same time PFC is the typical greenhouse gasses that cause a more serious effect on the environment than carbon dioxide. So it is necessary to completely understand the anode process of the fluoride-oxide molten salts for the REM electrowinning to effectively control the anode effects and PFC emission. The anode process of aluminum electrolysis has been studied by some researchers. S.S. Nissen and D.R. Sadoway found that during aluminum electrolysis PFCs are generated only when the cell goes on anode effect [2]. H. Zhu has proved that the kinetics of the electrode process is controlled by an interfacial step rather than by mass transfer. H. Zhu suggested that PFC generation can be
* Corresponding author. http://dx.doi.org/10.1016/j.electacta.2014.09.005 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
completely avoided by simply stepping down cell current in small increments [3–5]. Currently, the Environmental Protection Agency and the primary aluminum producers in USA have established the Voluntary Aluminum Industrial Partnership with the goal of substantially reducing PFC emissions [6]. Compared with aluminum electrolysis, REM electrowinning by fluoride-oxide molten salts electrolysis is carried out in a higher temperature and rare-earth fluoride has more aggressive, so it is more difficult to study the electrode processes of rare-earth fluoride-oxide system. Therefore, only a fewer papers of rare earth electrolysis in fluoride-oxide molten salt have been reported until now. G. Wang and H. Zhu have presented the electrode reactions of rare earth electrolysis and analyzed the anode gas composition during Nd electrolysis in LiF-NdF3-Nd2O3 system [7,8]. In the previous work of our laboratory, the anode process in a fluoride melts with low concentration of Nd2O3 has been discussed [9]. A systematic study about the anode processes on a graphite electrode in LiF-NdF3-Nd2O3 molten salts in high temperature for the REM electrowinning is needed. In this paper, cyclic voltammetry has been used to characterize the oxidization processes of oxygen ions and fluoride ions. The influence factors on the processes including the concentration of neodymium oxide, temperature and the electrode area in LiF-NdF3-Nd2O3 system have been discussed.
S. Liu et al. / Electrochimica Acta 147 (2014) 82–86
2. Experiments
3. Results and discussion
The chemicals used in the experiments include LiF (99.9%), NdF3 and Nd2O3 (99.9%), W wire and Mo wire (99.9%), graphite rod (spectrographic purity). In the experiments, LiF-NdF3 eutectic melt (30wt%-70wt%) is weighed and placed in a graphite crucible located in an airtight stainless steel reactor, then heated to 400 C under vacuum and remained for 8 h to remove traces of moisture. Then the melt is further heated to 800 C and maintained for 2 h under argon gas. After pre-treatment, all the electrolytes are stored in an oven at 100 C under vacuum. The initial Nd2O3 concentration in LiF-NdF3 eutectic melt (30wt%-70wt%) is 0.28wt% analyzed by LECO. During experiments, Nd2O3 is added to the above pre-melted electrolyte and heated to required temperatures under argon gas. In the electrochemical measurements, three-electrode system is used, in which graphite rod (6 4 mm) is used as the working electrode and Mo wire (F0.8 mm) as the counter electrode. W wire (F1.0 mm) is used as quasi-reference electrode and then transferred to Li+/Li reference electrode by electrochemical method [9]. All the electrodes and graphite crucible are carefully ground and cleaned before experiments. The electrodes are placed into the melt from the top lid of the stainless steel reactor. All the electrochemical measurements are performed using a PARSTAT2273 (PAR-Ametek Co, Ltd.) with a PowerSuite software package. All the potentials are with respect to Li+/Li reference electrode.
3.1. Background LiF-NdF3 cyclic voltammograms
83
The typical cyclic voltammograms recorded in a graphite electrode at 100 mV/s in LiF-NdF3 (30wt%-70wt%) melt are shown in Fig. 1. During the positive scan in Fig. 1(a), a small oxidation current A starting at 2.1 V is attributed to oxidation of the impurity oxygen ions and the peak current density of A is very small (about 0.080 A/cm2) because of the low level of oxygen ions concentration in the melt. The smooth cyclic voltammogram for peak A is explained by the adsorption of oxidation products of oxygen ions on the graphite electorde without turbulence by the gas evolution. Then the oxidation current density starts to fluctuate at the potential of 2.75 V and the oxidation current density remains the peak current density value until the potential of 3.25 V, then the current density almost linearly increases to 0.38 A/cm2 as the potential further scan from 3.25 V to 4.60 V. The oxidation current B starting at about 3.25 is also attributed to oxidation of oxygen ions under gas evolution conditions. The anode gas products are composed of CO and CO2 which have already been confirmed by H. Zhu [8]. Then the current suddenly falls to a very small value of 0.030 A/cm2 as the potential reaches 4.6 V and slightly increases as the potential increases from 4.6 V to 5.6 V, indicaiting the phenomenon of anode effect similar to aluminum electrolysis [3]. The potential at which the current suddenly falls to a tiny value
Fig. 1. The cyclic voltammograms recorded on a graphite electrode in LiF-NdF3 melt with different addition of Nd2O3 with scan rate of 100 mV/s at 1050 C. (a) Positive scan with 0wt% and 0.5wt% Nd2O3 addition, (b) reverse scan with 0wt% and 0.5wt% Nd2O3 addition, (c) Positive scan with 1.5wt% and 2.5wt% Nd2O3 addition, (d) reverse scan with 1.5wt% and 2.5wt% Nd2O3 addition.
84
S. Liu et al. / Electrochimica Acta 147 (2014) 82–86
is usually named as the critical potential, such as 4.6 V in Fig. 1(a). The reverse scan has shown nearly the same profile as the positive cyclic voltammogram. But the oxidation current for reverse scan has reduced and the critical potential has shifted to about 4.2 V. 3.2. Oxidation of oxygen ions The cyclic voltammograms of graphite electrode at 100 mV/s in LiF-NdF3 (30wt%-70wt%) melt with various Nd2O3 concentration at 1050 C are given in Fig. 1. The oxidation current density for peak A in Fig. 1(a) increases from 0.080 A/cm2 to 0.5 A/cm2 as 0.5wt% Nd2O3 is added to the LiF-NdF3 (30wt%-70wt%) melt. Obviously, as 1.5wt% Nd2O3 is added to LiF-NdF3 (30wt%-70wt%) melt, the oxidation current density for peak A further increases to 0.9 A/cm2, and the peak potential shifts to 3.1 V, as shown in Fig. 1(c). The peak current density for peak A increases to 1.45 A/cm2 and the peak potential shifts to 3.75 V as 2.5wt% Nd2O3 is added to LiF-NdF3 (30wt%-70wt%) melt in Fig. 1(c). The results further prove that the oxidation peak A is caused by oxidation of oxygen ions. The cyclic voltammograms either by positive scan and reverse scan have shown nearly the same profile. The oxidation of oxygen ions on a graphite electrode will produce CO and CO2 according to formula (1) and (2) which has been confirmed by H. Zhu [8].
displays a linearity with Nd2O3 concentration in the range of 2.5wt % Nd2O3 concentration in the melt. By further extending the line in Fig. 2(a) until intersecting with the X-axis, the primary oxide concentration can be obtained by deducing from the intercept with X-axis and it is 0.24wt%, slightly lower than the value analyzed by LECO. Meantime, the linearity between the oxide concentration and the peak current can be used to determine an unknown oxide concentration of the melt. The peak potential for peak A shifts to more postitive one as Nd2O3 concentration increases, and they are almost a linerity too, as shown in Fig. 2(b). The results imply that during the adsorption process, the peak current and peak potential on the graphite anode depend on Nd2O3 concentration in the melt.
3.3. Oxidation of oxygen ions control step
The plot of peak current for peak A derived from Fig. 1(a) and Fig. 1(c) vs Nd2O3 concentration is given in Fig. 2(a). The the peak current density increases with Nd2O3 concentration increase, and
Fig. 3(a) is the cyclic voltammograms recorded on a graphite electrode in LiF-NdF3-0.3wt% Nd2O3 melt with different sweep rates at 1050 C. It can be observed that the peak current density of oxidation of oxygen ions increases with scan rate. At the same time, the peak potential shifts to more positive potential as the scan rate increases from 100 mV/s to 1000 mV/s. The relation of the peak current density of oxidation of oxygen ions vs the square root of scan rate derived from Fig. 3(a) gives a straight line without passing through the origin in Fig. 3(b). The results indicate the kinetics of the adsorption of oxidation products of oxygen ions in LiF-NdF-Nd2O3 is controlled by both electrochemical reaction step and mass transfer, and the electrode process is highly irreversible. Fig. 4 has shown the cyclic voltammograms in the potential range of 3.0 V to 4.9 V on a graphite anode in LiF-NdF3-0.3wt%Nd2O3 melt with different scan rate. Scan rate has almost no effect on the
Fig. 2. The plot of (a) peak current density and (b) peak potential of oxidation of oxygen ions vs. concentration of Nd2O3 in Nd2O3-LiF-NdF3 melt at 1050 C.
Fig. 3. (a) The cyclic voltammograms recorded on a graphite electrode in 0.3wt% Nd2O3-LiF-NdF3 melt with different scan rate at 1050 C. (b) Peak current density of oxidation of oxygen ions vs the square root of scan rate derived from (a).
C+O2 fi CO (g) + 2e
(1)
C+2O2 fi CO2 (g) + 4e
(2)
S. Liu et al. / Electrochimica Acta 147 (2014) 82–86
85
when tempurture increases from 950 C to 1050 C. Obviously, temperature has significantly effects on the oxidation current of oxygen ions. 3.4. Oxidation of flouride ions – perflourides evolution The possible oxidation processes for oxidation of flouride ions on a graphite electrode are given according equations of (3) to (5)
Fig. 4. The cyclic voltammograms recorded on a graphite electrode in 0.3wt% Nd2O3-LiF-NdF3 melt with different scan rate at 1050 C.
current for oxidation of oxygen ions, indicating the kinetics of the oxidation of oxygen ions process in this potential range is controlled by electrochemical reaction step rather than by mass transfer. Because the intense agitation caused by the gas evolution from the graphite electrode has effectively mixed the electrolyte and made a homogeneous distribution of the oxide concentration in the melt. Fig. 5 has shown the effects of temperature on the oxidation of oxygen ions in LiF-NdF3-Nd2O3 melt. The current density of oxidation of oxygen ions increases form 0.5 A/cm2 to 1.5 A/cm2
Fig. 5. The cyclic voltammograms recorded on a graphite electrode in 2.5wt% Nd2O3-LiF-NdF3 melt with scan rate of 100 mV/s, (a) at different temperatures, (b) at 950 C.
2F fi F2 (g) + 2e (DG >0, T 1200 C)
(3)
F +C fi CF4(g) + 4e
(4)
6F + 2C fi C2F6(g) + 6e
(5)
C + 2 F2(g) fi CF4(g) (DG <0,T 1200 C)
(6)
2C + 3F2(g) fi C2F6(g) (DG <0, T 1200 C)
(7)
Obviously, CF4 and C2F6 are more stable than F2 by comparison of the Gibbs free energy in formula (6) to (7). The anode gas by potentiostatic electrolysis on a graphite electrode in LiF-NdF3(30wt%-70wt%) melts at potentials slightly higher than critical potential is mainly composed of CO, CO2, CF4, C2F6 without F2 gas according to H.Zhu [8], which indicates that only CF4 and C2F6 evolve on a graphite electrode during oxidation of flouride ions. Therefore, the anode process in LiF-NdF3(30wt%-70wt%) melt in the potential starting from the critical potential to more positive potential is carried out according to formula (4) and (5). In the cyclic voltammograms in Fig. 1 the starting potentials for oxidation of flouride ions for positive scan and reverse scan are 4.6 V and 4.2 V in the background LiF-NdF3(30wt%-70wt%) melt, and the corresponding oxidation current densities are 0.025 and 0.06 A/cm2 respectively. As 0.5wt% to 2.5wt% Nd2O3 is added to the background LiF-NdF3(30wt%-70wt%) melt, the starting oxidation potential for flouride ions for the positive scan are 4.5V-4.7 V and about 4.3–4.4 V for reverse scan. The corresponding oxidation currents for flouride ions are about 0.05 A/cm2 almost without change with Nd2O3 concentration within the potential of 5.5 V. According to H.Zhu [8], in the positive scan the perflourides including CF4 and C2F6 adsorb on the graphite anode and form a layer of insulating film,which extremely reduces the anode oxidation current to a very tiny level. While during the reverse scan, the perflourides rapidly desorb from the graphite anode and the desorption potential is more negative than the adsorption potential. From the above analysis, the background LiF-NdF3(30wt%-70wt %) melt has shown a more negative starting oxidation potential for fluoride ions. While addition of 0.5wt% to 2.5wt% Nd2O3to the background LiF-NdF3(30wt%-70wt%) melt has resulted in a slightly positive starting oxidation potential for fluoride ions. The electrode area affects the critical potential, that is, during the positive scan the critical potential shifts from 4.3 V to 5.2 V as the electrode area increases from 0.2 cm2 to 1.2 cm2, as shown in Fig. 6. The results indicate that a smaller electrode area is relatively easier to be completely adsorbed and covered by CF4 and C2F6. However, the critical potential for the reverse scan is independent of the electrode area and it is 4.3 V, which is the same value obtained from the positive scan as the electrode area is 0.2 cm2. The results show that the desorption potential of the perflouride gas from graphite electrode is invariable. However, the critical potential for the anode effects obtained by positive scan depends
86
S. Liu et al. / Electrochimica Acta 147 (2014) 82–86
oxidation products, which is proved by the the smooth and fluctuant cyclic voltammograms. During the adsorption process, both the peak current density and the peak potential of oxidation of oxygen ions increase with the increase of Nd2O3 concentration. The peak current density of oxygen ions oxidation displays a linearity up to 2.5wt% Nd2O3 in LiF-NdF3(30wt%-70wt%) melt, which can be used to determine an unknown oxide concentration of the melt. (1) During the adsorption process, the kinetics of the oxidation of oxygen ions in LiF-NdF3-Nd2O3 is controlled by both electrochemical reaction step and mass transfer, and the electrode process is highly irreversible. While during gas evolution, the oxidation of oxygen ions process is controlled by electrochemical reaction step rather than by mass transfer due to intense agitation by the gas evolution from the graphite electrode. The temperature between 950 C to 1050 C has significantly increase the adsoprtion current density for oxidation of oxygen ions. Oxidation of flouride ions starts from the critical potential for the anode effects. The critical potential obtained by reverse scan is independent of the electrode area, whereas depends on electrode area by positive scan. At 950 C in 2.5wt% Nd2O3-LiF-NdF3 melt, the cirtical potential shifts to more negative potential and after the critical potential the anode current density gradually increases and new oxidation peaks occur, which are associated with formation and evolution of PFCs on the graphite anode. Acknowledgements
Fig. 6. The cyclic voltammograms recorded on graphite electrodes with various area in 1.0wt% Nd2O3-LiF-NdF3 melt with scan rate of 100 mV/s at 1050 C. (a) Positive scan, (b) reverse scan.
This work was financially supported by National Natural Science Foundation of China (No.51274102).
References on the electrode area. Maybe the complete adsorption of the perflouride gas is related to electrode roughness. Temperature in the range of 1000 C to 1050 C has almost no effects on the critical potential of anode effects in 2.5wt% Nd2O3-LiF-NdF3 melt as shown in Fig. 5(a). But the critical potential at 950 C is relatively lower in the same melt. At 950 C, the anode current density gradually increases at the potential beyond 5.0 V and about three oxidation peaks occur whitin in the potential of 8.5 V as shown in Fig. 5(b), which are associated with formation and evolution of PFCs on the graphite anode. These peaks correspond to relatively bigger current densities which may result in lots of PFCs emission during Nd electrowinning. 4. Conclusions Oxidation processes of oxygen ions in Nd2O3-LiF-NdF3 melt on a graphite electrode include adsorption and gas evolution of
[1] S. Yan, S. Pang, Development on molten salt electrolytic methods and technology for preparing rare earth metals and alloys in China, Chinese journal of rare earth (2011) 442. [2] S.S. Nissen, D.R. Sadoway, Perfluorocarbon (PFC) generation in laboratory-scale aluminum reduction cells, Light Metals (1997) 159. [3] H. Zhu, D.R. Sadoway, An electroanalytical study of electrode reactions on carbon anodes during electrolytic production of aluminum, light metals (2000) 257. [4] H. Zhu, D.R. Sadoway, The electrode kinetics of perfluorocarbon (PFC) generation, Light Metals-Warrendale (1999) 241. [5] H. Zhu, D.R. Sadoway, Towards elimination of anode effect and PFC emissions via current shunting, Light Metals (2001) 303. [6] E.J. Dolin, PFC emissions reductions: The domestic and international perspective, Light Metal (1999) 56–67. [7] H. Zhu, Rare earth metal production by molten salt electrolysis, encyclopedia of applied electrochemistry (2014) 1765. [8] G. Wang, X. Wang, H. Zhu, Electroanalytical study of electrode processes on carbon anode in lithium fluoride and neodymium fluoride melt, Rare Metal Technology (2007) 533. [9] B. Li, S. Liu, Electrochemistry for Nd electrowinning from fluoride-oxide molten salts, Rare Metal Technology (2014) 95.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Anode processes for Nd electrowinning from LiF-NdF3-Nd2O3 melt Shizhe Liu a , Lingyun Chen a , Bing Li a, *, Liangliang Wang b , Bo Yan b , Mugen Liu b a b
East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, PR China XuZhou JinShiPengYuan Rare-Earth Material Factory, No.7 Road Energy Development Zone, DaTun PeiCounty, JiangSu, 221611, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 July 2014 Received in revised form 31 August 2014 Accepted 4 September 2014 Available online 6 September 2014
Cyclic voltammetry was applied to characterize oxidation processes of oxygen ions and fluoride ions on a graphite electrode in LiF-NdF3-Nd2O3 melt. The effects of the concentration of Nd2O3, temperature, electrode area on the oxidation of oxygen ions and critical potential have been discussed. Oxidation processes of oxygen ions include adsorption and gas evolution of oxidation products, which is proved by the the smooth and fluctuant cyclic voltammograms. During adsorption process, both the peak current density and the peak potential of oxidation of oxygen ions increase with the increase of Nd2O3 concentration. The peak current density of oxidation of oxygen ions displays a linearity within 2.5wt% Nd2O3 in the melt. The kinetics of the oxidation of oxygen ions is controlled by both electrochemical reaction step and mass transfer. During gas evolution, the oxidation process of oxygen ions is controlled by electrochemical reaction step. Oxidation of flouride ions starts from the critical potential. The critical potential by reverse scan is independent of the electrode area, but depends on electrode area by positive scan. Increasing temperature causes significantly increased the adsoprtion current density for oxidation of oxygen ions. At 950 C, after the critical potential relatively big current peaks involving formaiton of perflouride compounds occurs after the cirtical potential. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: anode processes Nd electrowinning oxygen ions flouride ions cirtical potential
1. Introduction Nowadays, rare-earth metal (REM) is mainly produced by molten salts electrolysis [1], in which fluoride-oxide molten salts are widely used because it is a more environment friendly and high current efficient process than chloride molten salts. However, similar to aluminum electrolysis, fluoride-oxide molten salts are subject to anode effects and emission of perflouride compound (PFC) on a graphite anode. Anode effects will increase the cell voltage and the energy consumption for REM electrowinning, at the same time PFC is the typical greenhouse gasses that cause a more serious effect on the environment than carbon dioxide. So it is necessary to completely understand the anode process of the fluoride-oxide molten salts for the REM electrowinning to effectively control the anode effects and PFC emission. The anode process of aluminum electrolysis has been studied by some researchers. S.S. Nissen and D.R. Sadoway found that during aluminum electrolysis PFCs are generated only when the cell goes on anode effect [2]. H. Zhu has proved that the kinetics of the electrode process is controlled by an interfacial step rather than by mass transfer. H. Zhu suggested that PFC generation can be
* Corresponding author. http://dx.doi.org/10.1016/j.electacta.2014.09.005 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
completely avoided by simply stepping down cell current in small increments [3–5]. Currently, the Environmental Protection Agency and the primary aluminum producers in USA have established the Voluntary Aluminum Industrial Partnership with the goal of substantially reducing PFC emissions [6]. Compared with aluminum electrolysis, REM electrowinning by fluoride-oxide molten salts electrolysis is carried out in a higher temperature and rare-earth fluoride has more aggressive, so it is more difficult to study the electrode processes of rare-earth fluoride-oxide system. Therefore, only a fewer papers of rare earth electrolysis in fluoride-oxide molten salt have been reported until now. G. Wang and H. Zhu have presented the electrode reactions of rare earth electrolysis and analyzed the anode gas composition during Nd electrolysis in LiF-NdF3-Nd2O3 system [7,8]. In the previous work of our laboratory, the anode process in a fluoride melts with low concentration of Nd2O3 has been discussed [9]. A systematic study about the anode processes on a graphite electrode in LiF-NdF3-Nd2O3 molten salts in high temperature for the REM electrowinning is needed. In this paper, cyclic voltammetry has been used to characterize the oxidization processes of oxygen ions and fluoride ions. The influence factors on the processes including the concentration of neodymium oxide, temperature and the electrode area in LiF-NdF3-Nd2O3 system have been discussed.
S. Liu et al. / Electrochimica Acta 147 (2014) 82–86
2. Experiments
3. Results and discussion
The chemicals used in the experiments include LiF (99.9%), NdF3 and Nd2O3 (99.9%), W wire and Mo wire (99.9%), graphite rod (spectrographic purity). In the experiments, LiF-NdF3 eutectic melt (30wt%-70wt%) is weighed and placed in a graphite crucible located in an airtight stainless steel reactor, then heated to 400 C under vacuum and remained for 8 h to remove traces of moisture. Then the melt is further heated to 800 C and maintained for 2 h under argon gas. After pre-treatment, all the electrolytes are stored in an oven at 100 C under vacuum. The initial Nd2O3 concentration in LiF-NdF3 eutectic melt (30wt%-70wt%) is 0.28wt% analyzed by LECO. During experiments, Nd2O3 is added to the above pre-melted electrolyte and heated to required temperatures under argon gas. In the electrochemical measurements, three-electrode system is used, in which graphite rod (6 4 mm) is used as the working electrode and Mo wire (F0.8 mm) as the counter electrode. W wire (F1.0 mm) is used as quasi-reference electrode and then transferred to Li+/Li reference electrode by electrochemical method [9]. All the electrodes and graphite crucible are carefully ground and cleaned before experiments. The electrodes are placed into the melt from the top lid of the stainless steel reactor. All the electrochemical measurements are performed using a PARSTAT2273 (PAR-Ametek Co, Ltd.) with a PowerSuite software package. All the potentials are with respect to Li+/Li reference electrode.
3.1. Background LiF-NdF3 cyclic voltammograms
83
The typical cyclic voltammograms recorded in a graphite electrode at 100 mV/s in LiF-NdF3 (30wt%-70wt%) melt are shown in Fig. 1. During the positive scan in Fig. 1(a), a small oxidation current A starting at 2.1 V is attributed to oxidation of the impurity oxygen ions and the peak current density of A is very small (about 0.080 A/cm2) because of the low level of oxygen ions concentration in the melt. The smooth cyclic voltammogram for peak A is explained by the adsorption of oxidation products of oxygen ions on the graphite electorde without turbulence by the gas evolution. Then the oxidation current density starts to fluctuate at the potential of 2.75 V and the oxidation current density remains the peak current density value until the potential of 3.25 V, then the current density almost linearly increases to 0.38 A/cm2 as the potential further scan from 3.25 V to 4.60 V. The oxidation current B starting at about 3.25 is also attributed to oxidation of oxygen ions under gas evolution conditions. The anode gas products are composed of CO and CO2 which have already been confirmed by H. Zhu [8]. Then the current suddenly falls to a very small value of 0.030 A/cm2 as the potential reaches 4.6 V and slightly increases as the potential increases from 4.6 V to 5.6 V, indicaiting the phenomenon of anode effect similar to aluminum electrolysis [3]. The potential at which the current suddenly falls to a tiny value
Fig. 1. The cyclic voltammograms recorded on a graphite electrode in LiF-NdF3 melt with different addition of Nd2O3 with scan rate of 100 mV/s at 1050 C. (a) Positive scan with 0wt% and 0.5wt% Nd2O3 addition, (b) reverse scan with 0wt% and 0.5wt% Nd2O3 addition, (c) Positive scan with 1.5wt% and 2.5wt% Nd2O3 addition, (d) reverse scan with 1.5wt% and 2.5wt% Nd2O3 addition.
84
S. Liu et al. / Electrochimica Acta 147 (2014) 82–86
is usually named as the critical potential, such as 4.6 V in Fig. 1(a). The reverse scan has shown nearly the same profile as the positive cyclic voltammogram. But the oxidation current for reverse scan has reduced and the critical potential has shifted to about 4.2 V. 3.2. Oxidation of oxygen ions The cyclic voltammograms of graphite electrode at 100 mV/s in LiF-NdF3 (30wt%-70wt%) melt with various Nd2O3 concentration at 1050 C are given in Fig. 1. The oxidation current density for peak A in Fig. 1(a) increases from 0.080 A/cm2 to 0.5 A/cm2 as 0.5wt% Nd2O3 is added to the LiF-NdF3 (30wt%-70wt%) melt. Obviously, as 1.5wt% Nd2O3 is added to LiF-NdF3 (30wt%-70wt%) melt, the oxidation current density for peak A further increases to 0.9 A/cm2, and the peak potential shifts to 3.1 V, as shown in Fig. 1(c). The peak current density for peak A increases to 1.45 A/cm2 and the peak potential shifts to 3.75 V as 2.5wt% Nd2O3 is added to LiF-NdF3 (30wt%-70wt%) melt in Fig. 1(c). The results further prove that the oxidation peak A is caused by oxidation of oxygen ions. The cyclic voltammograms either by positive scan and reverse scan have shown nearly the same profile. The oxidation of oxygen ions on a graphite electrode will produce CO and CO2 according to formula (1) and (2) which has been confirmed by H. Zhu [8].
displays a linearity with Nd2O3 concentration in the range of 2.5wt % Nd2O3 concentration in the melt. By further extending the line in Fig. 2(a) until intersecting with the X-axis, the primary oxide concentration can be obtained by deducing from the intercept with X-axis and it is 0.24wt%, slightly lower than the value analyzed by LECO. Meantime, the linearity between the oxide concentration and the peak current can be used to determine an unknown oxide concentration of the melt. The peak potential for peak A shifts to more postitive one as Nd2O3 concentration increases, and they are almost a linerity too, as shown in Fig. 2(b). The results imply that during the adsorption process, the peak current and peak potential on the graphite anode depend on Nd2O3 concentration in the melt.
3.3. Oxidation of oxygen ions control step
The plot of peak current for peak A derived from Fig. 1(a) and Fig. 1(c) vs Nd2O3 concentration is given in Fig. 2(a). The the peak current density increases with Nd2O3 concentration increase, and
Fig. 3(a) is the cyclic voltammograms recorded on a graphite electrode in LiF-NdF3-0.3wt% Nd2O3 melt with different sweep rates at 1050 C. It can be observed that the peak current density of oxidation of oxygen ions increases with scan rate. At the same time, the peak potential shifts to more positive potential as the scan rate increases from 100 mV/s to 1000 mV/s. The relation of the peak current density of oxidation of oxygen ions vs the square root of scan rate derived from Fig. 3(a) gives a straight line without passing through the origin in Fig. 3(b). The results indicate the kinetics of the adsorption of oxidation products of oxygen ions in LiF-NdF-Nd2O3 is controlled by both electrochemical reaction step and mass transfer, and the electrode process is highly irreversible. Fig. 4 has shown the cyclic voltammograms in the potential range of 3.0 V to 4.9 V on a graphite anode in LiF-NdF3-0.3wt%Nd2O3 melt with different scan rate. Scan rate has almost no effect on the
Fig. 2. The plot of (a) peak current density and (b) peak potential of oxidation of oxygen ions vs. concentration of Nd2O3 in Nd2O3-LiF-NdF3 melt at 1050 C.
Fig. 3. (a) The cyclic voltammograms recorded on a graphite electrode in 0.3wt% Nd2O3-LiF-NdF3 melt with different scan rate at 1050 C. (b) Peak current density of oxidation of oxygen ions vs the square root of scan rate derived from (a).
C+O2 fi CO (g) + 2e
(1)
C+2O2 fi CO2 (g) + 4e
(2)
S. Liu et al. / Electrochimica Acta 147 (2014) 82–86
85
when tempurture increases from 950 C to 1050 C. Obviously, temperature has significantly effects on the oxidation current of oxygen ions. 3.4. Oxidation of flouride ions – perflourides evolution The possible oxidation processes for oxidation of flouride ions on a graphite electrode are given according equations of (3) to (5)
Fig. 4. The cyclic voltammograms recorded on a graphite electrode in 0.3wt% Nd2O3-LiF-NdF3 melt with different scan rate at 1050 C.
current for oxidation of oxygen ions, indicating the kinetics of the oxidation of oxygen ions process in this potential range is controlled by electrochemical reaction step rather than by mass transfer. Because the intense agitation caused by the gas evolution from the graphite electrode has effectively mixed the electrolyte and made a homogeneous distribution of the oxide concentration in the melt. Fig. 5 has shown the effects of temperature on the oxidation of oxygen ions in LiF-NdF3-Nd2O3 melt. The current density of oxidation of oxygen ions increases form 0.5 A/cm2 to 1.5 A/cm2
Fig. 5. The cyclic voltammograms recorded on a graphite electrode in 2.5wt% Nd2O3-LiF-NdF3 melt with scan rate of 100 mV/s, (a) at different temperatures, (b) at 950 C.
2F fi F2 (g) + 2e (DG >0, T 1200 C)
(3)
F +C fi CF4(g) + 4e
(4)
6F + 2C fi C2F6(g) + 6e
(5)
C + 2 F2(g) fi CF4(g) (DG <0,T 1200 C)
(6)
2C + 3F2(g) fi C2F6(g) (DG <0, T 1200 C)
(7)
Obviously, CF4 and C2F6 are more stable than F2 by comparison of the Gibbs free energy in formula (6) to (7). The anode gas by potentiostatic electrolysis on a graphite electrode in LiF-NdF3(30wt%-70wt%) melts at potentials slightly higher than critical potential is mainly composed of CO, CO2, CF4, C2F6 without F2 gas according to H.Zhu [8], which indicates that only CF4 and C2F6 evolve on a graphite electrode during oxidation of flouride ions. Therefore, the anode process in LiF-NdF3(30wt%-70wt%) melt in the potential starting from the critical potential to more positive potential is carried out according to formula (4) and (5). In the cyclic voltammograms in Fig. 1 the starting potentials for oxidation of flouride ions for positive scan and reverse scan are 4.6 V and 4.2 V in the background LiF-NdF3(30wt%-70wt%) melt, and the corresponding oxidation current densities are 0.025 and 0.06 A/cm2 respectively. As 0.5wt% to 2.5wt% Nd2O3 is added to the background LiF-NdF3(30wt%-70wt%) melt, the starting oxidation potential for flouride ions for the positive scan are 4.5V-4.7 V and about 4.3–4.4 V for reverse scan. The corresponding oxidation currents for flouride ions are about 0.05 A/cm2 almost without change with Nd2O3 concentration within the potential of 5.5 V. According to H.Zhu [8], in the positive scan the perflourides including CF4 and C2F6 adsorb on the graphite anode and form a layer of insulating film,which extremely reduces the anode oxidation current to a very tiny level. While during the reverse scan, the perflourides rapidly desorb from the graphite anode and the desorption potential is more negative than the adsorption potential. From the above analysis, the background LiF-NdF3(30wt%-70wt %) melt has shown a more negative starting oxidation potential for fluoride ions. While addition of 0.5wt% to 2.5wt% Nd2O3to the background LiF-NdF3(30wt%-70wt%) melt has resulted in a slightly positive starting oxidation potential for fluoride ions. The electrode area affects the critical potential, that is, during the positive scan the critical potential shifts from 4.3 V to 5.2 V as the electrode area increases from 0.2 cm2 to 1.2 cm2, as shown in Fig. 6. The results indicate that a smaller electrode area is relatively easier to be completely adsorbed and covered by CF4 and C2F6. However, the critical potential for the reverse scan is independent of the electrode area and it is 4.3 V, which is the same value obtained from the positive scan as the electrode area is 0.2 cm2. The results show that the desorption potential of the perflouride gas from graphite electrode is invariable. However, the critical potential for the anode effects obtained by positive scan depends
86
S. Liu et al. / Electrochimica Acta 147 (2014) 82–86
oxidation products, which is proved by the the smooth and fluctuant cyclic voltammograms. During the adsorption process, both the peak current density and the peak potential of oxidation of oxygen ions increase with the increase of Nd2O3 concentration. The peak current density of oxygen ions oxidation displays a linearity up to 2.5wt% Nd2O3 in LiF-NdF3(30wt%-70wt%) melt, which can be used to determine an unknown oxide concentration of the melt. (1) During the adsorption process, the kinetics of the oxidation of oxygen ions in LiF-NdF3-Nd2O3 is controlled by both electrochemical reaction step and mass transfer, and the electrode process is highly irreversible. While during gas evolution, the oxidation of oxygen ions process is controlled by electrochemical reaction step rather than by mass transfer due to intense agitation by the gas evolution from the graphite electrode. The temperature between 950 C to 1050 C has significantly increase the adsoprtion current density for oxidation of oxygen ions. Oxidation of flouride ions starts from the critical potential for the anode effects. The critical potential obtained by reverse scan is independent of the electrode area, whereas depends on electrode area by positive scan. At 950 C in 2.5wt% Nd2O3-LiF-NdF3 melt, the cirtical potential shifts to more negative potential and after the critical potential the anode current density gradually increases and new oxidation peaks occur, which are associated with formation and evolution of PFCs on the graphite anode. Acknowledgements
Fig. 6. The cyclic voltammograms recorded on graphite electrodes with various area in 1.0wt% Nd2O3-LiF-NdF3 melt with scan rate of 100 mV/s at 1050 C. (a) Positive scan, (b) reverse scan.
This work was financially supported by National Natural Science Foundation of China (No.51274102).
References on the electrode area. Maybe the complete adsorption of the perflouride gas is related to electrode roughness. Temperature in the range of 1000 C to 1050 C has almost no effects on the critical potential of anode effects in 2.5wt% Nd2O3-LiF-NdF3 melt as shown in Fig. 5(a). But the critical potential at 950 C is relatively lower in the same melt. At 950 C, the anode current density gradually increases at the potential beyond 5.0 V and about three oxidation peaks occur whitin in the potential of 8.5 V as shown in Fig. 5(b), which are associated with formation and evolution of PFCs on the graphite anode. These peaks correspond to relatively bigger current densities which may result in lots of PFCs emission during Nd electrowinning. 4. Conclusions Oxidation processes of oxygen ions in Nd2O3-LiF-NdF3 melt on a graphite electrode include adsorption and gas evolution of
[1] S. Yan, S. Pang, Development on molten salt electrolytic methods and technology for preparing rare earth metals and alloys in China, Chinese journal of rare earth (2011) 442. [2] S.S. Nissen, D.R. Sadoway, Perfluorocarbon (PFC) generation in laboratory-scale aluminum reduction cells, Light Metals (1997) 159. [3] H. Zhu, D.R. Sadoway, An electroanalytical study of electrode reactions on carbon anodes during electrolytic production of aluminum, light metals (2000) 257. [4] H. Zhu, D.R. Sadoway, The electrode kinetics of perfluorocarbon (PFC) generation, Light Metals-Warrendale (1999) 241. [5] H. Zhu, D.R. Sadoway, Towards elimination of anode effect and PFC emissions via current shunting, Light Metals (2001) 303. [6] E.J. Dolin, PFC emissions reductions: The domestic and international perspective, Light Metal (1999) 56–67. [7] H. Zhu, Rare earth metal production by molten salt electrolysis, encyclopedia of applied electrochemistry (2014) 1765. [8] G. Wang, X. Wang, H. Zhu, Electroanalytical study of electrode processes on carbon anode in lithium fluoride and neodymium fluoride melt, Rare Metal Technology (2007) 533. [9] B. Li, S. Liu, Electrochemistry for Nd electrowinning from fluoride-oxide molten salts, Rare Metal Technology (2014) 95.