The Effects of NdF2 on Current Efficiency of Nd Extraction from NdF3 LiF Nd2O3 Melts

The Effects of NdF

on Current Efficiency of Nd Extraction

from NdF

-LiF-Nd

O

Melts

Xiaolong Liu, Chao Huang and Bing Li

East China University of Science and Technology, 130Meilong Road, Shanghai 200237, P.R. China

In this paper, the cyclic voltammetry was applied to investigate the electrochemical reduction processes of Nd(III) ions in NdF3-LiF melts

with or without excessive metal Nd. Equilibrium experiments were carried out in NdF3-LiF melts with excessive spheric metal Nd to investigate

the relationship between the NdF3:LiF mass ratio and NdF2 concentration. Electrolysis experiments were performed in NdF3-LiF-Nd2O3 melts

with different NdF3:LiF mass ratios to explore the relationship between the NdF2 concentration and the current efficiency.

The results indicated that Nd(III) ions in the melts were reduced in two steps, i.e., Nd(III)→Nd(II) and Nd(II)→Nd(0). NdF2 could be

formed by the comproportionation reaction between Nd(III) and Nd(0) and could stably exist in NdF3-LiF melts containing metal Nd(0). NdF2

mass concentration in the melts decreased from 45.5% to 36.4% with the increase of NdF3-LiF mass ratio from 7:3 to 9:1 in NdF3-LiF melts

containing excessive spheric metal Nd, which resulted in a higher current efficiency during the electrolysis. And the highest current efficiency of about 50% for Nd extraction has been obtained by electrolysis in NdF3-LiF (9:1 mass ratio) melts with Nd2O3 (2%, mass concentration) at

1423 K. [doi:10.2320/matertrans.MK201611]

(Received August 31, 2016; Accepted November 28, 2016; Published February 10, 2017)

Keywords:  NdF2, current efficiency, NdF3-LiF, comproportionation reaction

1.  Introduction

Currently, NdF3-LiF-Nd2O3 melts are mainly used to

ex-tract neodymium (Nd) by molten salts electrolysis due to their higher current efficiency than that of chloride melts, such as NdCl3-KCl melts. However, the current efficiency for

Nd extraction from the NdF3-LiF-Nd2O3 melts is about 70%–

80%, lower than that of aluminium electrolysis (about 92%). According to professor Zhu s research1)_{, the lower current}

ef-ficiency for Nd extraction from NdCl3-KCl melts is partly

caused by Nd consumption and corresponding to Nd(II) for-mation by the comproportionation reaction between the Nd(0) and Nd(III). A. Novoselova et al.2)_{, S.Vandarkuzhali et}

al.3) _{and Hajimu Yamana et al.}4) _{have confirmed that the}

Nd(III) ions in the NdCl3-KCl melts are reduced to Nd(0)

through two consecutive steps, i.e., Nd(III)→Nd(II) and

Nd(II)→Nd(0). Therefore, they have concluded that the

Nd(II) ions can stably exist in molten LiCl-KCl-NdCl3 at

773 K4) _{and LiCl-KCl-CsCl-NdCl}

3 melts after electrolysis at

810–840 K2)_{. But so far the existence of stable Nd(II) ions in}

fluoride melts is ambiguous. The cathodic processes of Nd(III) in fluoride melts revealed by C. Hamel et al.5) _showed

that Nd(III) ions are reduced to Nd(0) in a one-step process in LiF-CaF2 melts. E. Stefanidaki et al.6) have also shown that

Nd(III) ions are reduced to Nd(0) via a three-electron reaction in NdF3-LiF-Nd2O3 melts. But in our previous researches7,8),

we have found that the Nd(III) ions in the NdF3-LiF melts are

reduced through two consecutive steps, similar to the reduc-tion process in the chloride melts. So in this paper, we further investigated the existence of Nd(II) ions in NdF3-LiF melts

with excessive spheric metal Nd. The effect of the NdF3/LiF

mass ratio on the concentration of Nd(II) in NdF3-LiF melts

with excessive spheric metal Nd and on the current efficiency for Nd extraction from the NdF3-LiF-Nd2O3 melts by

elec-trolysis has also been researched.

2.  Experiments

All the experiments were carried out in a graphite crucible or a Mo crucible, which was placed in an airtight stainless steel reactor with cooling water. The reactor was heated in an electric furnace to the designated temperature. A K-type ther-mocouple with an accuracy of ±1 K was used to measure the experiments temperature. Before the experiments, the chemi-cals including LiF (aladdin, 99.9% purity), NdF3 (aladdin,

99.9% purity) and Nd2O3 (aladdin, 99.9% purity) were

pre-treated under argon gas (99.99% purity) according to the fol-lowing steps: First, each chemical was placed in a graphite crucible (spectrographic purity) and then heated to 673 K and remained for 8 h to remove traces of moisture in the airtight stainless steel reactor under the pressure of 0.016 MPa. Then the chemical was further heated to 1073 K and maintained for 2 h at 1073 K under argon gas atmosphere with a flow rate of 1.2 L/h. At last, the chemical was taken out from the reactor after cooling to room temperature (about 298 K) and stored in

a glove box under argon gas with both O2 and H2O levels

below 1 ppm before using.

During the electrochemical measurements, the three-elec-trode system was used, in which the working electhree-elec-trode and reference electrode were W wires (99.9%, dia.1 mm), the auxiliary electrode was graphite rod (spectrographic purity, dia.7 mm). And the electrochemical experiments were per-formed by using a PAR-STAT2273 (PAR-Ametek Co, Ltd.) with a PowerSuite software package. For all the electrochem-ical experiments, the potentials were transferred to Li+_/Li

according to literatures7)_{. In the electrolysis experiments, a}

two-electrode system was used, in which the anode was graphite plate (6 mm ×  25 mm ×  50 mm spectrographic puri-ty) and the cathode was W plate (2 mm ×  8 mm ×  55 mm). Before the experiments, all the electrodes and graphite cruci-bles were polished by SiC emery papers to the mirror finish, washed with distilled water, then dried under argon atmo-sphere.

During the experiments, the salt mixtures (NdF3-LiF,

*

Corresponding author, E-mail: [email protected]

mixtures were further heated to the designated temperature in 5 h under argon gas atmosphere with a flow rate of 1.2 L/h.

3.  Results and Discussion

3.1  Nd(III) ions electrochemical reduction processes

In Fig. 1, the cyclic voltammograms (CVs) on the W elec-trode were measured to investigate the reduction processes of Nd(III) ions in the NdF3-LiF melts. Compared with the

back-ground CV in LiF melts in Fig. 1(a), the CVs in the NdF3-LiF

(50:1 mass ratio) melts in Fig. 1(b) and NdF3-LiF (50:3 mass

ratio) melts with excessive spheric metal Nd(0) in Fig. 1(c) have shown two reduction peaks and two corresponding oxi-dation peaks. As we reported in the previous work7,8)_{, the}

Nd(III) ions in fluoride melts were reduced in two steps, i.e.,

  2 3

spheric metal Nd

Equilibrium experiments were carried out in NdF3-LiF

melts with excessive spheric metal Nd (compared with Nd(III) ions amount in the melts) in Mo crucible at 1323 K to investigate NdF2 concentration in the melts. The Mo crucible

with the above mixture was situated in an airtight stainless steel reactor and heated to 1323 K for 2 h under argon gas atmosphere. Then the mixture was cooled to room tempera-ture (about 298 K) naturally and taken out from the crucible under argon gas atmosphere. The surface morphology of the spheric metal Nd was compared before and after experiments. The mass loss of the spheric metal Nd was calculated. The phase composition of the melts at different positions shown in

Fig. 2 was analyzed by XRD to investigate NdF2

concentra-tion caused by the comproporconcentra-tionaconcentra-tion reacconcentra-tion between Nd(III) and Nd(0).

For the mixture of NdF3-LiF (7:3 mass ratio, total mass of

12.86 g) with excessive spheric metal Nd (5.8 g), the geomet-ric shape of metal Nd has changed from its initial sphere to semiellipse after the equilibrium experiments shown in Fig. 3. And the mass of metal Nd decreased by 35 mass% (2.03 g). The melts phase composition at different positions including on the metal Nd surface and far away from the metallic Nd surface has presented NdF2 phase according to Fig. 4. The

average NdF2 concentration in the melts was about 45.5%

(mass concentration) by calculation according to 2Nd(III) + 

Fig. 1 The cyclic voltammograms recorded on W electrode (a) in LiF melts (b) in NdF3-LiF (50:1 mass ratio) melts (c) in NdF3-LiF (50:3 mass

ra-tio)-Nd (excess) melts with 100 mV/s and an immersion area of 0.53 cm−2

at 1323 K.

Fig. 2 Schematic of equilibrium experiments including the different melts positions for XRD analysis: 1--melts away from the spheric metal Nd; 2--the melts between the spheric metal Nd and Mo crucible; 3--melts on the spheric metal Nd surface; 4--Mo crucible; 5--NdF3-LiF melts; and 6-

spheric metal Nd.

Fig. 3 (a) Initial spheric metal Nd surface morphology before the equilibri-um experiments, (b) metal Nd surface morphology and (c) metal Nd side morphology after the equilibrium experiments in the NdF3-LiF (7:3 mass

Nd(0) =  3Nd(II) reaction.

For the mixture of NdF3-LiF (9:1 mass ratio, total mass of

10 g) with excessive spheric metal Nd (6 g), the geometric shape of metal Nd has changed from its initial sphere to semiellipse after the equilibrium experiments shown in Fig. 5, and the mass of metal Nd decreased by 23 mass% (1.38 g). The melts phase composition at different positions as shown in Fig. 2 has presented NdF2 phase according to Fig. 6. The

average NdF2 concentration in the melts was about 36.4%

(mass concentration) by calculation according to 2Nd(III) +  Nd(0) =  3Nd(II) reaction.

The above results revealed that NdF2 could form and stably

exist because of the comproportionation reaction between

Nd(III) and metal Nd(0). As NdF3-LiF melts composition

changed from 7:3 mass ratio to 9:1 mass ratio, the metal Nd

Fig. 4 XRD patterns of the melts (a) at position 1 of Fig. 2; (b) at position 2 of Fig. 2; (c) at position 3 of Fig. 2 after the equilibrium experiments in the NdF3-LiF (7:3 mass ratio, total mass of 12.86 g) melts with Nd (5.8 g)

at 1323 K.

Fig. 5 (a) Initial spheric metal Nd surface morphology before the equilibri-um experiments, (b) metal Nd surface morphology and (c) metal Nd side morphology after the equilibrium experiments in NdF3-LiF (9:1 mass

ra-tio, total mass of 10 g) melts with Nd (6 g) at 1323 K.

Fig. 6 XRD patterns of the melts (a) at position 1 of Fig. 2; (b) at position 2 of Fig. 2; (c) at position 3 of Fig. 2 after the equilibrium experiments in NdF3-LiF (9:1 mass ratio, total mass of 10 g) melts with Nd (6 g) at

36.4%.

According to the Ref. 9), as NdF3-LiF mass ratio increased

from 7:3 to 9:1, Li+ _{ions concentration in NdF}

3-LiF melts

de-creased and less free Nd(III) ions were released from more stable NdF63− ions. So the comproportionation reaction of

2Nd(III) +  Nd(0) =  3Nd(II) was shifted to the left hand side. Therefore less NdF2 was formed and higher current efficiency

could be obtained.

3.3  Current efficiency for Nd extraction from NdF3-LiF-Nd2O3 melts

In order to explore the relationship between the NdF2

con-centration and the current efficiency for Nd extraction, elec-trolysis experiments were performed in the NdF3-LiF melts

(500 g) with various NdF3:LiF mass ratio of 9:1, 8.5:1.5, 8:2

and 7:3 with Nd2O3 (2% mass concentration) at 1323 K,

1373 K and 1423 K, respectively. The current density was se-lected as Ianode =  3700 A·m−2 and Icathode =  18500 A·m−2, and

the polar distance was 0.056 m during the electrolysis exper-iments. After the electrolysis experiments, the current effi-ciency was calculated in term of eq. (1) and shown in Fig. 7.

η= W

CIt×100% (1)

η: current efficiency; W: the amount of Nd, g; I: the average current, A; t: time, h; C: electrochemical equivalent, g·(A·h)−1_.

According to Fig. 7, the current efficiency increased in dif-ferent NdF3:LiF mass ratio conditions as the temperature

in-creased from 1323 K to 1423 K. This was because the metal Nd obtained in the cathode was subject to aggregate to a big-ger sphere from lots of smaller particles, as shown in Fig. 8. Therefore the smaller specific surface area of the Nd at 1423 K greatly reduced both of the consumption of metal Nd

and NdF2 concentration in the melts by the

comproportion-ation reaction.

As NdF3-LiF mass ratio increased from 7:3 to 9:1, the

cur-rent efficiency for Nd extraction increased from about 3% to 7% at 1323 K, from 5% to 35% at 1373 K and from 5% to 50% at 1423 K, respectively. At the same temperature, the NdF3:LiF mass ratio had a significant effect on the current

efficiency, especially at 1423 K. This was because the con-centration of NdF2 decreased in the melts as the NdF3:LiF

mass ratio changed from 7:3 to 9:1, which was in good agree-ment with our previous results. And the electrolyte composed of NdF3-LiF (9:1 mass ratio) with Nd2O3 (2%, mass

concen-tration) at 1423 K has presented the highest current efficiency of about 50%. Currently, the industrial current efficiency for Nd extraction by electrolysis was about 70% 80%, higher than our experiment value. This was because that the elec-trode distance, current density and temperature in our experi-ments were different from the industrial conditions.

4.  Conclusions

The electrochemical reduction processes of Neodymium

ions in NdF3-LiF melts with or without excessive spheric

metal Nd were measured. The results indicated that neodym-ium ions in the fluoride melts were reduced in two steps, i.e.,

Nd(III)→Nd(II) and Nd(II)→Nd(0). NdF2 could be formed

by the comproportionation reaction between Nd(III) and Nd(0) and stably exist in NdF3-LiF melts containing metal

Nd(0). NdF2 mass concentration in the melts decreased from

45.5% to 36.4% with the increase of NdF3-LiF mass ratio

from 7:3 to 9:1 in NdF3-LiF melts containing excessive

spheric metal Nd, which resulted in a higher current efficien-cy during the electrolysis. And the highest current efficienefficien-cy of about 50% for Nd extraction has been obtained by electrol-ysis in NdF3-LiF (9:1 mass ratio) melts with Nd2O3 (2%,

mass concentration) at 1423 K.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (No. 51274102).

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