I. INTRODUCTION
Molten salts, particularly molten chlorides, are well known as good reaction media for performing selective solubilization or precipitation in chemical reactions, and have already been proposed as a promising route for the treatment of raw materials [1]. Pyrochemical separation processes in molten media have more recently been proposed as a promising option in the nuclear fuel cycle for the future [2, 3]. Since rare earth chloride is extremely sensitive to O2 and H2O, rare earth oxides employed as the raw material of rare earth elements, which greatly simplified the production process and reduced the production cost. But rare earth oxides are usually insoluble in molten chlorides [4]. Thus the direct chlorination of rare earth oxides in the melts was studied in chlorides melts. Sakamura et al. [5] reported that rare earth oxides could be chlorinated by ZrCl4 in LiCl-KCl eutectic melts. Our previous work showed that AlCl3 could chloride the Pr6O11 [6] and Eu2O3 [7] in KCl-LiCl melts.
But AlCl3 and ZrCl4 have a very low melting point and can be sublimed easily [8]. In order to avoid the sublime of chlorination reagent, MgCl2 was chosen to chloride rare earth oxides. Since the melting point and boiling point of magnesium chloride are higher than those of AlCl3 and ZrCl4, to prevent from solvent salt volatility, appropriate solvent was selected by measuring the mass loss of LiCl-NaCl and LiCl-KCl melts by adding MgCl2 at high temperature. And then the chlorination of La2O3 by MgCl2 in the selected LiCl-NaCl melts was explored by a series of techniques. To further confirm the chlorination of La2O3 by MgCl2, galvanostatic electrolysis was carried out to produce Mg-La alloy characterized by XRD and SEM-EDS.
II. EXPERIMENTAL
A. Preparation and purification of melts
The mixture of LiCl-NaCl (60:30 wt.%, analytical grade) and LiCl-KCl(40:50 wt.%, analytical grade) was first dried under vacuum for more than 24 h at 573 K to remove excess water, and then melted in an alumina crucible placed in a quartz cell putted in an electric furnace. The temperature of melts was measured with a nickel chromium-nickel aluminium thermocouple sheathed with an alumina tube. Magnesium and Lanthanum ions were introduced into the bath in the form of dehydrated MgCl2 (99.9%, analytical grade) and La2O3 powder (99.9%, analytical grade). The concentration of La2O3 in the LiCl-NaCl-MgCl2 melts was analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES, Thermo Elemental, IRIS Intrepid II XSP). All experiments were performed in an inert argon atmosphere (99.999%, high pure liquid Argon) to prevent from moisture.
B. Electrochemical apparatus and electrodes
All electrochemical measurements were carried out using an Auto lab electrochemical workstation (Metrohm Co., Ltd.) with the Nova 1.10 software package. A silver wire (d = 1 mm) dipped into a solution of AgCl (0.070 mol/kg) in LiCl-NaCl melts contained in a pyrex tube was used as a reference electrode. All potentials were referred to Ag/AgCl couple. A spectrally pure graphite rod (d = 6 mm) served as the counter electrode. The working electrodes were tungsten (W) wires (d = 0.9 mm, 99.99% purity), which were polished thoroughly using SiC paper, then cleaned ultrasonically with ethanol (99.8% purity) in an ultrasonic bath prior to use.
C. Preparation and characterization of samples
All samples were cleaned in ethanol in an ultrasonic bath to remove salts and stored in a glove box for analysis. These deposits were analyzed by X-ray diffraction (XRD, X’Pert Pro; Philips Co., Ltd.) using Cu Kα radiation at 40 kV and 40 mA. The microstructure and microzone chemical analyses were also carried out using scanning electron microscope with Energy dispersive spectroscopy (SEM-EDS, JSM-6480A; JEOL Co., Ltd.).
III. RESULTS AND DISCUSSION
A. The choice of new chloride system
There are numerous selection criteria for choosing the molten salt system, such as salt stability, salt volatility, solubility of metallic species and so on. It is important to select salt mixtures as solvent medium in order to reduce the salt loss and increase the solubility of RE2O3 in the molten salt. The details are as follow.
The mixture of an anhydrous LiCl-NaCl-MgCl2 (6.0:3.0:1.5 wt.%) and LiCl-KCl-MgCl2 (4.0:5.0:1.5 wt.%) powder in a corundum crucible and heated to a specified temperature in electric furnace under argon atmosphere. The mass loss of the two systems of LiCl-NaCl-MgCl2 and LiCl-KCl-MgCl2 were measured in every 30 min using an electronic balance in the temperature range from 873 K to 1073 K. The comparison of mass loss in these two different systems was shown in Fig. 1. The results showed that the mass loss in the LiCl-KCl-MgCl2 molten salts was much larger than that in the LiCl-NaCl-MgCl2, thus proving that LiCl-NaCl-MgCl2 melts are more stable than LiCl-KCl-MgCl2 melts in the experimental temperature range. According to the experimental results, a new salt system, LiCl-NaCl melts, was selected as a solvent.
B. Chlorination of La2O3 by MgCl2 in the LiCl-NaCl melts
The concentration of La2O3, i.e. the concentration of La(III) ions in LiCl-NaCl melts was determined by ICP. Anhydrous LiCl-NaCl-La2O3 (6.0:3.0: 0.5 wt.%) and LiCl-NaCl-MgCl2-La2O3 (6.0:3.0:1.5:0.5 wt.%) powder in a corundum crucible were heated to 873 K in electric furnace under argon atmosphere and stirred to dissolve fully. Supernatant fluid of molten salts was taken out and analyzed by ICP-AES and XRD. Fig. 2 shows the results of ICP-AES analysis of supernatant fluid with the LiCl-NaCl-La2O3 and LiCl-NaCl-MgCl2-La2O3 melts. The results show that La2O3 can hardly dissolve in the LiCl-NaCl-La2O3 melts, but the concentration of La2O3 in the LiCl-NaCl-MgCl2-La2O3 melts can reach 3.68 wt.%. Evidently, the existence of MgCl2 can accelerate the solubilization of La2O3.
Figure 3(a) shows the XRD pattern of the dissolved supernatant salt of the LiCl-NaCl-MgCl2 melts. It can be seen that the LaCl3 exists in the LiCl-NaCl-MgCl2 melts. Since the rare earth oxides and oxychlorides are insoluble in KCl-LiCl eutectic melts [9], MgO and La2O3 remain solid particles and precipitates in LiCl-NaCl melts. The melts cooled were washed with water in order to remove the soluble salts, and then filter to obtain insoluble substance. The insoluble substance was characterized by XRD shown in Fig. 3(b). The result indicated that the insoluble substance contained LaOCl and MgO. According to Bentouhami [10], LaOCl maybe forms due to the hydrolysis of LaCl3 during the melts being washed with water. From the results mentioned above, we suggest that La2O3 is chlorinated by MgCl2 to form LaCl3.
Figure 4(a) shows the typical cyclic voltammograms (CVs) in LiCl-NaCl melts before and after the addition of 1.0 wt.% MgCl2 on W electrodes at 873 K. In curve 1, the cathodic signal A observed in the absence of MgCl2 in LiCl-NaCl melts is ascribed to the deposition of Li(I) ions, since no alloy or intermetallic compounds exist in the phase diagram of the W-Li binary system [11] at 873 K. In the reverse scanning direction, an anodic peak A’ is corresponding to the dissolution of Li metal. After adding 1.0 wt.% MgCl2, except for the peaks A/A’, peaks C/C’ are associated with the reduction and reoxidation of Mg metal. The results are consistence with our previous work [12]. Fig. 4(b) shows the CVs in LiCl-NaCl-La2O3 (2 wt.%) melts before and after the addition of 1.0 wt.% MgCl2 on W electrodes at 873 K. The shape of curve 3 is the same as the curve 1 in Fig. 3(a), which means that the reduction peak of La (III) ions does not exist, i.e. La2O3 powder is nearly insoluble in the LiCl-NaCl-La2O3 melts. In curve 4, the CVs shows two new redox couples after the addition of 1.0 wt.% MgCl2 in LiCl-NaCl-La2O3 melts. The C/C’ peaks are corresponding to the deposition and subsequent oxidation of Mg metal, and the cathodic peak B observed at about -2.07 V is ascribed to the reduction of La(III) ions. The results are in agreement with the ones reported by Masset et al. and our previous work [13, 14]. The CVs indicate the existence of La(III) ions in LiCl-NaCl-La2O3 melts after the addition of MgCl2.
Figure 5 shows the square wave voltammograms from the LiCl-NaCl-La2O3 melts before (a) and after (b) the addition of MgCl2 at a step potential of 1 mV and frequency of 25 Hz. There is only one peak C at -1.80 V, corresponding to the formation of pure Mg in the LiCl-NaCl-La2O3 melts (Fig. 5(a)). But the two obvious peaks B and C, observed at -1.80 V and -2.07 V, are attributed to the formation of Mg and La metal in the LiCl-NaCl-La2O3 melts after the addition of MgCl2 (Fig. 5(b)). This result indicates that the reduction of La(III)/La(0) is present, in other words, La2O3 can be chlorinated by MgCl2 to form La(III) ions.
C. Preparation and characterization of Mg-La alloy
The existence of La(III) ions in the LiCl-NaCl-MgCl2-La2O3 melts was explored by electrochemical techniques. To further demonstrate the chlorination effect of MgCl2 on rare earth oxide, the galvanostatic electrolysis was conducted in the LiCl-NaCl-MgCl2(10 wt.%)-La2O3(2 wt.%) melts. If we want to obtain a large of mass Mg-La alloy in a relatively short time, we have to perform the electrolysis at a more negative current density. Fig. 6 shows that XRD pattern of Mg-La alloy obtained by galvanostatic electrolysis at 1 A for 3 h. As seen from the XRD pattern, the Mg-La alloy is composed of Mg and La2Mg17.
To examine the distribution of elements of the Mg and La in the Mg-La alloy, a mapping analysis of the elements was employed. Fig. 7 shows a group of SEM and EDS mapping analysis of the Mg-La alloy obtained from LiCl-NaCl-MgCl2 (10 wt.%)-La2O3 (2 wt.%) melts at 873 K. We can find that the La element mainly distributes in the grey zone in the SEM photograph of the alloy. To further investigate the distribution of the element La, a SEM equipped with EDS quantitative analysis was carried out (Figs. 7(d) and 7(e)). The EDS results of the points labeled as A and B taken from grey zone and the dark grey zone indicate that the deposit is composed of elements of Mg and La.
IV. CONCLUSION
According to the determination of the concentration of La(III) in the LiCl-NaCl-MgCl2-La2O3 melts and XRD pattern measurements, La2O3 can be solubilized by MgCl2 chlorination. A series of electrochemical techniques and the formation of Mg-La alloy obtained by galvanostatic electrolysis further confirm the existence of La(III) in the LiCl-NaCl-MgCl2-La2O3 melts. The results indicate that La2O3 can be chlorinated by MgCl2: 3MgCl2 + La2O3 2LaCl3 + 3MgO, which may be used for the treatment of the waste nuclear fuel with pyrometallurgy.
Pulse and ac impedance studies of the electrochemical systems of titanium in LiCl-KCl eutecyic melt at 743K
. T I Min Metall A, 1988, 97: C21-30.An experimental study of molten salt electrorefining of uranium using solid iron cathode and liquid cadmium cathode for development of pyrometallurgical reprocessing
. J Nucl Sci Technol, 1997, 34: 384-393.Cathodic behaviour of europium(III) on glassy carbon, electrochemical formation of Al4Eu, and oxoacidity reactions in the eutectic LiCl-KCl
. J Electroanal Chem, 2007, 545: 141-157. DOI: 10.1016/j.jelechem.2007.01.018Chlorination of UO2, PuO2 and rare earth oxides using ZrCl4 in LiCl-KCl eutectic melt
. J Nucl Mater, 1997, 340: 39-51. DOI: 10.1016/j.jnucmat.2004.11.002AlCl3-aided extraction of praseodymium from Pr6O11 in LiCl-KCl eutectic melts
. Electrochim Acta, 2013, 88: 457-462. DOI: 10.1016/j.electacta.2012.10.045Extraction of europium and electrodeposition of Al-Li-Eu alloy from Eu2O3 assisted by AlCl3 in LiCl-KCl melt
. Electrochim Acta, 2012, 59: 531-537. DOI: 10.1016/j.electacta.2011.11.007Low-temperature synthesis of nanocrystalline ZrP via co-reduction of ZrCl4 and PCl3
. Mater Lett, 2004, 58: 337-3339. DOI: 10.1016/j.matlet.2004.06.033Solubilization of rare earth oxides in the eutectic LiCl-KCl mixture at 450 and in the equimolar CaCl2-NaCl melt at 550
. J Electroanal Chem, 2003, 545: 141-157. DOI: 10.1016/S0022-0728(03)00092-5Physicochemical study of the hydrolysis of rare-earth elements (III) and thorium (IV)
. CR Chim, 2004, 7: 537-545.DOI: 10.1016/j.crci.2004.01.008The Li-W (lithium-tungsten) system
. J Phase Equilib, 1991, 12: 203-203.Electrochemical codeposition of Mg-Li-Gd alloys from LiCl-KCl-MgCl2-Gd2O3 melts
. T Nonferr Metal Soc, 2011, 21: 825-829. DOI: 10.1016/S1003-6326(11)60788-7Electrochemical codeposition of quaternary Mg-Li-Ce-La alloys from molten salt
. Metall Mater Trans B, 2010, 41: 1123-28. DOI: 10.1007/s11663-010-9395-zThermochemical properties of lanthanides (Ln = La, Nd) and actinides (An = U, Np, Pu, Am) in the molten LiCl-KCl eutectic
. J Nucl Mater, 2005, 344: 173-79. DOI: 10.1016/j.jnucmat.2005.04.038