Electrochemical extraction of neodymium by co-reduction with aluminum in LiCl–KCl molten salt

Journal of Nuclear Materials 433 (2013) 152–159

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Electrochemical extraction of neodymium by co-reduction with aluminum in LiCl–KCl molten salt Yong-De Yan a,b,⇑, Yan-Lu Xu b, Mi-Lin Zhang b,⇑, Yun Xue a,b, Wei Han b, Ying Huang b, Qiong Chen b, Zhi-Jian Zhang a a

Key Discipline Laboratory of Nuclear Safety and Simulation Technology, Harbin Engineering University, Harbin 150001, China Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

b

a r t i c l e

i n f o

Article history: Received 7 July 2012 Accepted 5 September 2012 Available online 15 September 2012

a b s t r a c t The electrochemical behavior of Nd(III) ions in LiCl–KCl and LiCl–KCl–AlCl3 melts on a Mo electrode at 723 K was studied by various electrochemical techniques. The results showed that Nd(III) ions are reduced to Nd(0) through two consecutive steps, and the underpotential deposition of neodymium on pre-deposited Al electrode formed two kinds of Al–Nd intermetallic compounds in LiCl–KCl–AlCl3 solutions. The electrochemical extraction of neodymium was carried out in LiCl–KCl–AlCl3 melts on a Mo electrode at 873 K by potentiostatic and galvanostatic electrolysis. The extraction efficiency was 99.25% after potentiostatic electrolysis for 30 h. Al–Li–Nd bulk alloy was obtained by galvanostatic electrolysis. X-ray diffraction (XRD) suggested that Al2Nd and Al3Nd phases were formed in Al–Li–Nd alloy. The microstructure and micro-zone chemical analysis of Al–Li–Nd alloy were characterized by scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS), respectively. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, the traditional hydrometallurgical process is no more available for lanthanides (Lns) and actinides (Ans) extraction because of their low solubilities in aqueous medium [1]. The pyrochemical separation processes using molten salts have been proposed to be a promising approach for extracting lanthanides and actinides from the fission products. It is because of the high thermal resistance, high radiation and high solubility of molten salts [2]. Researchers have carried out a preliminary study on lanthanides and actinides extraction. Gibilaro et al. [1], Conocar et al. [3] and Serp et al. [4] have demonstrated that the most effective metallic solvent was aluminum for lanthanides and actinides extraction. Since An and Al can interact strongly and yield alloys, it is a promising scheme to extract actinides from the fission products meanwhile remain the lanthanides in the molten salts. The mass content of lanthanides is ten times higher than actinides [5]. When the lanthanides concentration exceeds about 10 wt.% in the melt, the Ans/Lns separation efficiency will be reduced [6]. In addition, after the selective extraction of the Ans, the rest Lns should be

⇑ Corresponding authors. Address: Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, China. Tel.: +86 451 82569890; fax: +86 451 82533026. E-mail addresses: [email protected] (Y.-D. Yan), [email protected] (M.-L. Zhang). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.09.008

extracted from the melts for recycling the molten solvent [2] based on the following reasons: (1) The lanthanides represent 25% of the fission products. Therefore lanthanides extraction is an important tache of the extraction of fission products for storage [7]. (2) The similar chemical properties between lanthanides and actinides make lanthanides extraction more difficult [8–11]. (3) The lanthanides can absorb the neutrons and prevent the transmutable actinides to capture neutron, and hence reduce the efficiency of transmutation [12,13]. (4) Lanthanides can react with cladding metal and form low eutectic melting point alloy [14]. Some researchers have been interested in lanthanides extraction. Gobilaro and co-workers [1,7] have investigated the Nd, Sm and Ce extraction in LiF–CaF2–AlF3 melts and calculated the extraction efficiency (more than 99% of each lanthanide). They have also extracted Gd and Nd on a reactive cathode (Cu, Ni) in LiF–CaF2 melts at 940 °C and 920 °C, respectively. The extraction efficiencies of the two lanthanides were found to be more than 99.8% on both reactive electrodes [15]. Nd is a typical element of the fission products. There is much information about the electrochemical behavior of Nd(III) ions in molten mediums. Fukasawa and co-workers [16,17] have proposed that the reduction process of Nd(III) to Nd(0) involved two steps in LiCl–CaCl2 and LiCl–BaCl2 melts (Nd(III) + e ? Nd(II) and Nd(II) +

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2e?Nd(0)). De Córdoba and co-workers [11] have proposed the reduction of NdCl3 took place in two consecutive steps as well as the oxidation state Nd(II) is stable in the CaCl2–NaCl eutectic mixture. They have also determined the activity coefficient of neodymium in liquid aluminum phase in the temperature range of 973–1073 K. Masset et al. [18] have proposed the reduction of Nd(III) took place in two steps involving Nd(II) and calculated the thermochemical properties of Nd(III) and Nd(II) in the LiCl–KCl melts at 733 K. Hamel et al. [19] have found the Nd(III) reduced to Nd(0) in a one-step process in LiF–CaCl2 melts and the diffusion coefficients of Nd(III) at various temperatures have been calculated. In addition, Kontoyannis et al. [20] in LiF melts at 900 °C, Chamelot et al. [21,22] in LiF–CaF2 at 840 °C have demonstrated the one single step electroreduction of Nd(III) to Nd(0). Since the electrode reaction mechanisms of Nd(III) ions are controversial in different molten salts systems, and few studies have been carried out in LiCl–KCl melts [18], it is necessary to investigate the electrochemical behavior of Nd(III) ions in LiCl–KCl melts. Alloys preparation is another important issue concerning molten salts electrolysis, which has been applied to produce alloys such as Mg–Li based superlight alloys [23–27] and magnetic RE (rare earth) -transition metal alloys [28–30]. Preparation of pure aluminum and rare earth metals by electrolysis is an energy demanding process. Moreover, the traditional method for the production of Al–RE master alloys is mainly by mixing pure aluminum and RE metals directly at the temperature above their melting points. These production processes result in high energy waste. Hence the co-reduction of Al and RE ions in molten salts provides a unique opportunity for the preparation of Al–RE alloys in one step, which can save energy and expenses. It should be pointed out that Li is also likely to be deposited on the Al–Nd alloy under galvanostatic electrolysis with a high current density. The addition of Li to Al–Nd alloy is desirable, because Al–Li alloys have attributes of low density, improved specific strength and high stiffness. At the same time, the existence of rare earth Nd in Al–Li alloy can provide a desirable combination of strength and toughness [31]. This work is dedicated to explore the electrochemical behavior of Nd(III) ions in LiCl–KCl melts on a molybdenum electrode. In addition, the co-reduction mechanism of Al(III) and Nd(III) ions is also studied to verify the feasibility of lanthanides extraction by co-reduction with reductive metal, and to propose an efficacious approach for preparing the Al–Li–Nd bulk alloys.

software package. The working electrodes were molybdenum wires (d = 1 mm, 99.99% purity), which were polished thoroughly using SiC paper, and then cleaned ultrasonically with ethanol prior to use. The counter electrodes were graphite rods (d = 6 mm, 99.99% purity). The active electrode surface was determined by measuring the immersion depth of the electrode in the melts. The reference electrode was a silver wire (d = 1 mm) which dipped into a Pyrex tube containing a solution of AgCl (1 wt.%) in LiCl–KCl (63.7:36.3 mol%) melts. All potentials were referred to this Ag/AgCl couple. 2.3. Auxiliary techniques The Al–Li–Nd alloy was prepared by galvanostatic electrolysis at 873 K. After electrolysis, the alloy sample was extracted from the melts and washed in hexane (99.8% purity) in an ultrasonic bath to remove salts. All these operations were carried out under an argon atmosphere. The treated sample was stored in the glove box for analysis. The deposit was analyzed by XRD (X’Pert Pro; Philips Co., Ltd.) using Cu Ka radiation at 40 kV and 40 mA. The specimen was mounted in thermosetting resins using a metallographic mounting press and then mechanically polished. And then, the microstructure and micro-zone chemical analysis were measured using SEM and EDS (JSM–6480A; JEOL Co., Ltd.). 3. Results and discussions 3.1. Electrochemical behavior of Nd(III) in LiCl–KCl melts on a molybdenum electrode 3.1.1. Cyclic voltammetry and square wave voltammetry Fig. 1 shows the typical cyclic voltammograms obtained from the LiCl–KCl (63.7:36.3 mol%) melts before and after the addition of 1.9  104 mol cm3 NdCl3 on a molybdenum electrode. The dotted curve represents the voltammogram before the addition of NdCl3. Only one cathodic signal C is observed from about 2.36 V which corresponds to the deposition of lithium, and the corresponding anodic signal C0 is associated with the dissolution of lithium. The solid curve shows the voltammogram with the addition of NdCl3. In the solid curve, prior to the cathodic signal C another two cathodic peaks A and B (from about 1.83 and 1.97 V, respectively) are observed, which correspond to the two-step reduction reaction of Nd(III) to Nd(0):

2. Experimental

2.5

2.1. Preparation and purification of the melts

1.5 1.0

B′

0.5

j/A cm-2

The mixture of LiCl–KCl (63.7:36.3 mol%, analytical grade) was dried under vacuum for more than 48 h at 523 K to remove the excess water. And then the mixture was melted in an alumina crucible placed in a quartz cell located in an electric furnace. The melts temperature was measured by a nickel–chromium thermocouple sheathed by an alumina tube. Metal ion impurities of the melts were removed by pre-electrolysis at 2.1 V (vs. Ag/AgCl) for 3 h. Aluminum and neodymium elements were introduced into the bath in the form of dehydrated AlCl3 and NdCl3 powder. Since Ln ions are very sensitive to O2 ions, to avoid the oxidation of Nd(III), HCl was bubbled to the melts to purify the melts and then argon was bubbled to remove the excess HCl [5].

C′

2.0

A′

0.0

B A

-0.5 -1.0 -1.5 -2.0

C

-2.5 -2.5

2.2. Electrochemical apparatus and electrodes All electrochemical techniques were performed using an Im6eX electrochemical workstation (Zahner Co., Ltd.) with a THALES 3.08

-2.0

-1.5

-1.0

-0.5

0.0

E/V vs Ag/AgCl Fig. 1. Cyclic voltammograms of the LiCl–KCl melts before (dotted line) and after (solid line) the adding of 1.9  104 mol cm3 NdCl3 on a molybdenum electrode (S = 0.322 cm2) at 723 K. Scan rate: 0.1 V s1.

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NdðIIIÞ þ e ! NdðIIÞ

ð1Þ

NdðIIÞ þ 2e ! Ndð0Þ

ð2Þ

0.05

0.03

0.02

0.01

0.00 0.0

0.1

0.2

0.3

0.4 1/2

0.5

0.6

0.7

0.8

0.5

0.6

0.7

0.8

-1 1/2

ν /(V s )

(b) 0.07 -

Nd(II) + 2e

Νd (0)

0.06

0.05

-IP/A

0.20

B′

0.03

A′ 0.02

0.10 0.05 -2

Νd (ΙΙ)

0.04

0.15

j/Acm

-

Nd(III) + e

0.04

-IP/A

In the positive-going scan, anodic peaks A0 , B0 attribute to the anodic oxidation processes of the two-step reaction, respectively. The twostep reduction of Nd(III) to Nd(0) has been also observed in LiCl– CaCl2 and LiCl–BaCl2 melts [16]. Fig. 2 shows the CVs obtained in LiCl–KCl–NdCl3 (1.9  104 mol cm3) melts on a molybdenum electrode at different scan rates at 723 K. Concerning the reversibility of the reduction reactions of Nd(III) ions, previous researches have different views. Fukasawa et al. [16,17] determined that both reactions of reduction were reversible. Córdoba et al. [11] thought the both reactions were quasi-reversible. In Fig. 2, the DEP (DEP = |Epa  Epc|) is larger than the value of 2.3 RT/nF or 0.143/0.072 V for a one-electron/ two-electron reaction at 723 K. There are two linear relationships between cathodic peak current (IP) and the square root of scan rate (v1/2) at low scan rates and high scan rates, respectively (see Fig. 3). As a whole, the plot of IP as a function of v1/2 shows a change from reversible, to quasi-reversible and finally irreversible behavior. To further investigate the reversibility of the electrode reactions, the dependences of the cathodic and anodic peak potentials on the logarithm of the sweep rate are plotted in Fig. 4. Up to scan rates of 0.2 V s1, the value of peak potential, EP, is constant and independent of the sweep potential rate. Whereas for higher sweep rates,

(a)

(a)

0.01

0.00

50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s 600 mV/s

-0.05 -0.10 -0.15

A B

-0.20 -0.25

-2.2

-2.0

-1.8

-1.6

-1.4

0.00 0.0

0.1

0.2

0.3

0.4 1/2

-1 1/2

ν /(Vs )

Fig. 3. Linear relationship of (a) Nd(III) and (b) Nd(II) reduction peaks current vs. the square root of the potential scanning rate on a molybdenum electrode (S = 0.322 cm2) in the LiCl–KCl–NdCl3 (1.9  104 mol cm3) melts at 723 K.

-1.2

E/V vs Ag/AgCl

(b) 0.16

A′

-

-1.7

0.12

Nd(II) + 2e Nd(III) + e

Νd(0) Νd(II)

0.08 -1.8

j/A cm

-2

EP/V vs Ag/AgCl

0.04 0.00 -0.04

100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s 600 mV/s

-0.08 -0.12 -0.16

A -2.0

-1.8

-1.6

-1.4

-1.9

-2.0

-2.1

-1.2

E/V vs Ag/AgCl Fig. 2. Cyclic voltammograms for the reductions of (a) Nd(III) to Nd(0) and (b) Nd(III) to Nd(II) in the LiCl–KCl–NdCl3 (1.9  104 mol cm3) melts on a molybdenum electrode (S = 0.322 cm2) at 723 K.

-2.2 -1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

-1

log(ν/V s ) Fig. 4. Variation of the cathodic and anodic peak potentials with the logarithm of the sweep rate at 723 K.

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3.1.2. Open circuit chronopotentiometry Open circuit chronopotentiometry was carried out to investigate the electrodeposition of Nd(III) on a Mo electrode. Firstly, Nd metal was prepared by electrodeposited Nd(III) on a Mo electrode by potentiostatic electrolysis for a short period. Then, the potential was cut off and a transient curve of the open circuit potential was measured during the currentless step. Potential plateau is observed when a two-phase equilibrium at the surface of the electrode occurs [36,37]. Fig. 6 shows the typical open-circuit potential transient curve obtained in LiCl–KCl–NdCl3 (1.9  104 mol cm3) melts on a molybdenum electrode at 723 K. There are three potential plateaus at about: (1) 2.30 V, (2) 1.90 V and (3) 1.71 V. The plateau 1 is interpreted as the Li(I)/Li equilibrium potential. The plateaus 2 and 3 are associated to the equilibrium potential plateaus of Nd(II)/Nd(0) and Nd(III)/Nd(II), respectively. 3.2. Electrochemical behavior of Nd(III) in LiCl–KCl–AlCl3 melts on a molybdenum electrode 3.2.1. Cyclic voltammetry Fig. 7 shows typical CVs before (curve 1) and after (curve 2) the addition of 1.2  104 mol cm3 AlCl3 in LiCl–KCl–NdCl3 (1.9  104 mol cm3) melts on a molybdenum electrode at 723 K. In curve 1, a group of signals C/C0 corresponds to the reduction/dissolution of lithium metal. Signals A/A0 and B/B0 correspond to the Nd(III)/Nd(II) and Nd(II)/Nd(0) electrochemical redox reactions, respectively. After the addition of 1.2  104 mol cm3 AlCl3 in LiCl–KCl– NdCl3 (1.9  104 mol cm3) melts, there are three more couples of cathodic/anodic (D/D0 , E/E0 , F/F0 ) signals are observed (curve 2). Except for the D/D0 system at around 0.95/0.77 V corresponding

0.00

-1.6 3

-1.7

E/V vs Ag/AgCl

-1.8 2

-1.9 -2.0 -2.1 -2.2 1

-2.3 -2.4 50

100

150

200

250

300

t/s Fig. 6. Open-circuit potential transient curve of the LiCl–KCl–NdCl3 (1.9  104 mol cm3) melts on a molybdenum electrode after electrodepositing at 2.32 V vs. Ag/AgCl for 50 s at 723 K.

2.0

1

1.5

C′ D′

1.0

E′

B′

0.5

j/A cm-2

the values of the anodic and cathodic peak potentials shift slightly towards positive and negative ones. These results indicate that the reduction of Nd(III) is quasi-reversible [32]. Since square wave voltammetry is more sensitive and has a higher resolution than cyclic voltammetry [33–35], square wave voltammetry was applied to further investigate the electrochemical behavior of Nd(III). Fig. 5 shows the square wave voltammogram of a solution of NdCl3 at a step potential of 1 mV. There are two peaks (peak A and peak B) at about 1.82 and 1.95 V which correspond to the two-step reduction reactions of Nd(III) to Nd(II) and Nd(II) to Nd(0), respectively. The results of square wave voltammogram are consistent with the ones obtained from the cyclic voltammograms.

2

F′

A′

0.0 -0.5

B

A

F E

D

-1.0 -1.5 -2.0

C -2.5

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

E/V vs Ag/AgCl Fig. 7. Cyclic voltammograms of the LiCl–KCl–NdCl3 (1.9  104 mol cm3) melts before (curve 1) and after (curve 2) adding 1.2  104 mol cm3 AlCl3 on a molybdenum electrode (S = 0.322 cm2) at 723 K. Scan rate: 0.1 V s1.

to the deposition and subsequent reoxidation of aluminum, the cathodic peaks E and F at about 1.32 and 1.43 V should be attributed to the formation of Al–Nd intermetallic compounds. The underpotential deposition of neodymium on pre-deposit aluminum film leads to the formation of Al–Nd alloys.

j/A cm

-2

-0.04

3.2.2. Square wave voltammetry Fig. 8 shows a square wave voltammogram of the LiCl–KCl– AlCl3 (1.2  104 mol cm3)–NdCl3 (1.9  104 mol cm3) melts on a molybdenum electrode at a step potential of 1 mV and frequency of 10 Hz at 723 K. There are five obvious peaks at 0.92, 1.29, 1.42, 1.84, 1.98 V corresponding to the formation of pure Al metal and two different Al–Nd alloys, the reaction of Nd(III)/Nd(II) and Nd(II)/Nd, respectively. The results of square wave voltammogram coincide with the ones obtained from the cyclic voltammograms (Fig. 7).

-0.08

-0.12

A

-0.16 B

-0.20

-2.1

-2.0

-1.9

-1.8

-1.7

-1.6

E/V vs Ag/AgCl 4

3

Fig. 5. Square wave voltammogram of the LiCl–KCl–NdCl3 (1.9  10 mol cm ) melts on a molybdenum electrode at 723 K. Pulse height: 25 mV; potential step: 1 mV; frequency: 10 Hz.

3.2.3. Open circuit chronopotentiometry Fig. 9 shows the typical open-circuit potential transient curve obtained from LiCl–KCl–AlCl3 (1.2  104 mol cm3)–NdCl3 (1.9  104 mol cm3) melts on a Mo electrode at 723 K. There

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0.0

j/A cm-2

-0.1

-0.2

Nd(II)+2e Nd

Formation of Al-Nd alloys

Al(III)+3e Al Nd(III)+e Nd(II)

-0.3

-0.4

-0.5

Li(I)+e Li

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

E/V vs Ag/AgCl Fig. 8. Square wave voltammogram of the LiCl–KCl–AlCl3 (1.2  104 mol cm3)– NdCl3 (1.9  104 mol cm3) system on a molybdenum electrode (S = 0.322 cm2) at 723 K. Pulse height: 25 mV; potential step: 1 mV; frequency: 10 Hz.

-0.8

lanthanides to be enhanced up to 100% [7]. Based on the results of electrochemical studies, electrolytic extraction of neodymium ions by co-reduction with Al(III) ions was carried out via potentiostatic electrolysis in LiCl–KCl–AlCl3 (2.4  104 mol cm3)–NdCl3 (1.9  104 mol cm3) melts on a molybdenum electrode. Due to the low reaction rate and the formation of dendritic deposit at 723 K, the experiment temperature was increased to 873 K to increase the extraction efficiency. According to the results of cyclic voltammograms and square wave voltammogram, the peak potential (1.43 V) for the second Al–Nd intermetallic formation was set as the electrolytic potential. The extraction process can be followed by drawing square wave voltammograms at different time to monitor the electroactive species content of molten salts. In addition, when measuring a new square wave voltammogram, a new molybdenum cathode should be used to ensure the accuracy of the results. Fig. 10 shows a group of square wave voltammograms measured in the extraction process. We can observe that the current density decreases significantly from the square wave voltammograms. The current density decreases more quickly at the initial stage than at the following stage in the electrolysis process. After 30 h electrolysis, the current density was close to zero.

6 5 0.0

4

15h -1.6

-0.4

3

7h 30h

-2

2

j/A cm

E/V vs Ag/AgCl

-1.2

-2.0

1

-0.8

3h -1.2

-2.4

0h 50

100

150

200

250

-1.6

t/s Fig. 9. Open-circuit potential transient curve for a Mo electrode (S = 0.322 cm2) after electrodepositing at 2.35 V vs. Ag/AgCl for 50 s in the LiCl–KCl–AlCl3 (1.2  104 mol cm3)–NdCl3 (1.9  104 mol cm3) melts at 723 K.

are six plateaus at about: (1) 2.31 V, (2) 1.91 V, (3) 1.74 V, (4) 1.40 V, (5) 1.26 V, (6) 0.94 V. The plateau 1 is equilibrium potential plateau of Li(I)/Li. The plateaus 2 and 3 are equilibrium potential plateaus of Nd(II)/Nd and Nd(III)/Nd(II), respectively. The plateau 6 is related to the rest potential of the Al film. Since the Al previously deposited on the molybdenum, the deposited Nd metal reacts with Al and diffuses into the Al electrode. This phenomenon leads to the electrode potential gradually shifts to more positive values. During this process, potential plateau is observed when a composition of the electrode surface is within a range of two-phase coexisting state [38–40]. Therefore, the plateaus 4 and 5 are related to the formation of two different Al–Nd intermetallic compounds.

-2.0

-2.4

-2.0

-1.6

-1.2

-0.8

-0.4

0.0

E/V vs Ag/AgCl Fig. 10. A group of square wave voltammograms of the LiCl–KCl–AlCl3 (2.4  104 mol cm3)–NdCl3 (1.9  104 mol cm3) melts at 873 K after potentiostatic electrolysis at 1.43 V for different electrolysis durations on a molybdenum electrode (S = 0.322 cm2). Pulse height: 25 mV; potential step: 1 mV; frequency: 20 Hz.

3.3. Potentiostatic electrolysis and electrolytic extraction of neodymium ions by co-reduction When both aluminium and neodymium ions are present in the melt, both ions are reduced simultaneously, leading to the formation of an Al–Nd alloy. The formation of Al–Nd alloy leads to the deposition potential of neodymium ions moved to more positive direction by co-reduction with Al(III) ions. This so-called ‘‘depolarization effect’’ allows the theoretical extraction efficiency of these

Fig. 11. The photograph of bulk alloy obtained by galvanostatic electrolysis (1.5 A) for 2 h in LiCl–KCl–AlCl3 (2.0  103 mol cm3)–NdCl3 (1.6  104 mol cm3) melts on a molybdenum electrode (S = 0.322 cm2) at 873 K.

Y.-D. Yan et al. / Journal of Nuclear Materials 433 (2013) 152–159

157

3.4. Galvanostatic electrolysis and characterization of the deposits

Fig. 12. XRD pattern of deposit obtained by galvanostatic electrolysis (1.5 A) for 2 h on a molybdenum electrode (S = 0.322 cm2) in the LiCl–KCl–AlCl3 (2.0  103 mol cm3)–NdCl3 (1.6  104 mol cm3) melts at 873 K.

In order to estimate the extraction efficiency of the extraction process, the total concentrations of Li, K, Al and Nd elements in the melts after potentiostatic electrolysis for 30 h were calculated by ICP. The weight percent of LiCl, KCl, AlCl3 and NdCl3 of the melts after potentiostatic electrolysis were 46.9964%, 52.4930%, 0.4868% and 0.0238%, respectively. We can calculate the extraction efficiency by the following equation [7]:

g¼

Ci  Cf  100% Ci

ð3Þ

where Ci is the initial concentration of Nd(III) in the melts, and Cf is the final concentration of Nd(III) in the melts. The extraction efficiency of the neodymium ion was calculated to be 99.25%.

Since the average electrolytic current was very low under potentiostatic electrolysis, only a small amount of deposit attached on molybdenum electrode was obtained in quite a long time (4– 8 h). It is so difficult for us to extract the Al–Nd alloys from melts and carry out subsequent analyses of solid Al–Nd alloys with such a small mass, using, for example, scanning electron microscopy and inductively coupled plasma analysis. If we want to obtain a larger mass Al–Nd alloy in a relatively short time, we have to perform the electrolysis at a more negative potential or current density. Therefore, galvanostatic electrolysis (1.5 A) was also carried out for 2h in LiCl–KCl–AlCl3 (2.0  103 mol cm3)–NdCl3 4 3 (1.6  10 mol cm ) melts on a molybdenum electrode at 873 K. The photograph of bulk alloy obtained by galvanostatic electrolysis is shown in Fig. 11. The bulk alloy is lump with irregular shape. Fig. 12 shows the X-ray diffraction pattern of alloy sample obtained by galvanostatic electrolysis from the LiCl–KCl–AlCl3 (2.0  103 mol cm3)–NdCl3 (1.6  104 mol cm3) melts. The XRD pattern of the sample shows that Al2Nd, Al3Nd, Al and Nd phases are formed. To examine the distributions of Al and Nd elements in the alloy, SEM and EDS mapping analysis of the alloy sample obtained by galvanostatic electrolysis from the LiCl–KCl–AlCl3 (2.0  103 mol cm3)–NdCl3 (1.6  104 mol cm3) melts were employed (Fig. 13). From the mapping analysis of elements, we can find that Nd element mainly distributes in the white zone in the SEM photograph of the alloy. To further investigate the distribution of Nd, EDS quantitative analysis was carried out (Fig. 14). The EDS results of the points labeled A and B taken from the white zone and black zone indicate that the deposit is composed of the elements of Al, Nd and O. As windows in front of the Si (Li) detector can absorb low-energy X-rays, EDS detectors cannot detect the presence of elements with

Fig. 13. SEM and EDS mapping analysis of the alloy sample obtained from LiCl–KCl–AlCl3 (2.0  103 mol cm3)–NdCl3 (1.6  104 mol cm3) melt at 873 K.

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cated that the underpotential deposition of neodymium on predeposited Al electrode formed two kinds of Al–Nd intermetallic compounds at electrode potentials around 1.32 and 1.43 V, respectively. The extraction efficiency of Nd(III) was estimated to be 99.25% after potentiostatic electrolysis for 30 h. According to the co-reduction process, Al–Li–Nd alloys with Al2Nd, Al3Nd, Al and Nd phases could be obtained directly via co-reduction of Al, Li, Nd on an inert electrode in molten LiCl–KCl–AlCl3–NdCl3 melts. The extraction of Nd(III) ion by co-reduction with Al(III) ion in LiCl–KCl–AlCl3–NdCl3 melts on an inert electrode at 873 K was proved to be feasible. This co-reduction process can also be used for other electrochemical systems for lanthanides extraction and alloys preparation. Acknowledgments

Counts

A

1650 1500 1350 1200 1050 900 750 600 450 300 150 0 0.00

Element Al

Mass%

at.%

Al

44.25

77.48

Nd

54.13

17.13

O

1.62

4.8

Nd Nd

Nd

Nd

O

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00 10.00

References

keV

Counts

B

1650 1500 1350 1200 1050 900 750 600 450 300 150 0 0.00

Element Al

Nd

1.00

Mass%

at.%

Al

98.83

99.78

Nd

1.87

0.22

Nd Nd Nd

2.00

3.00

4.00

5.00

The work was financially supported by High Technology Research and Development Program of China (No. 2011AA03A409 and 2009AA050702), National Program on Key Basic Research Project (No. 2007CB200906), the National Natural Science Foundation of China (21103033, 21101040, and 21173060), the Fundamental Research funds for the Central Universities, the Heilongjiang Postdoctoral Fund (LBH-Z10196 and LBH-Z10207) and China Postdoctoral Science Foundation (20100480974). The authors are particularly grateful to Dr. Tom Mann for kindly discussing various parts of the manuscript.

6.00

7.00

8.00

9.00 10.00

keV Fig. 14. SEM and EDS analysis of the alloy sample obtained from LiCl–KCl–AlCl3 (2.0  103 mol cm3)–NdCl3 (1.6  104 mol cm3) melt at 873 K.

atomic number less than 4, meaning that EDS cannot detect Li [41]. Since EDS is a qualitative and semi-quantitative estimate of elemental concentration, and element Li cannot be detected in the EDS, the sample was analyzed by ICP. The ICP analysis shows that Li content of the alloy is 9.2 wt.%. Therefore, the sample obtained by galvanostatic electrolysis is Al–Li–Nd alloy in such a high current density. At the same time, the results of the EDS demonstrate that the white zone dissolves more Nd (54.13 mass% at point A) than the black zone does (1.87 mass% at point B). 4. Conclusions The electrochemical behavior of Nd(III) ion on a molybdenum electrode in molten LiCl–KCl–NdCl3 at 723 K was investigated by various electrochemical techniques. It was proved that Nd(III) was reduced to Nd metal by two steps which were Nd(III) + e?Nd(II) and Nd(II) + 2e?Nd(0). The electrochemical reductions of Nd(III) to Nd(II) and Nd(II) to Nd(0) were found to be quasi-reversible. The electrode reaction of the LiCl–KCl–AlCl3–NdCl3 solutions indi-

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