Mutual separation characteristics of neighbouring rare earth elements Nd, Sm, Eu and Gd using stepwise chlorination–chemical vapour transport

Journal of Alloys and Compounds 269 (1998) 88–91

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Mutual separation characteristics of neighbouring rare earth elements Nd, Sm, Eu and Gd using stepwise chlorination–chemical vapour transport a a, b Yan–Hui Sun , Zhi–Chang Wang *, Lei Guo b

a Department of Chemistry, Northeastern University, Shenyang 110006, Liaoning, China Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110015, Liaoning, China

Received 18 October 1997; received in revised form 22 January 1998

Abstract Mutual separation characteristics have been investigated for the neighbouring rare earth elements Nd, Sm, Eu and Gd from binary oxide mixtures Nd 2 O 3 –Sm 2 O 3 , Sm 2 O 3 –Eu 2 O 3 and Eu 2 O 3 –Gd 2 O 3 using a stepwise chlorination–chemical vapour transport reaction mediated by vapour complexes LnAln Cl 3n 13 within 6 h. The separation factor, expressed as atomic ratio for the resulting chlorides, was 6.84 for Nd:Sm, 4.84 for Sm:Eu and 26.5 for Gd:Eu, respectively. As a comparison, the conventional CVT process resulted in the separation factor values of 1.64 for Nd:Sm, 3.19 for Sm:Eu, 1.78 for Gd:Eu and 3.99 for Eu:Gd.  1998 Elsevier Science S.A. Keywords: Rare earth separation; Binary oxide mixture; Chlorination; Chemical vapour transport; Vapour complex

1. Introduction Rare earth element vapour complexes LnAl n Cl 3n13 [1– 10], where Ln5rare earth elements, have recently been used as intermediate materials for the preparation of anhydrous rare earth element chlorides in high purity [3,5,6,8–13]. Mediated by the vapour complexes LnAl n Cl 3n13 and ALnCl 4 [14–18], where A5alkaline metal elements, Adachi and co-workers have successfully developed a chemical vapour transport (CVT) process for rare earth separation [19–27] and rare earth recovery [28,29]. For instance, the reported separation factors, expressed as atomic ratio for the resulting chlorides, within 6 h from the binary chloride mixtures were either 10.8 for Pr:Er [22], 2.3 for Pr:Sm [22] 1.07 for Nd:Pr [20,21] when using AlCl 3 as complex former or 1.04–1.13 for Pr:Nd [20,21] and 1.16–1.33 for Nd:Pr [20,21] when using ACl or an AlCl 3 –ACl mixture as complex former. The reported separation factors became 1.4 within 82 h for Pr:Nd as well as 1.9 within 42 h and 1.7 within 82 h for Nd:Pr from the binary oxide mixture Pr 6 O 11 –Nd 2 O 3 when using K 2 CO 3 s a precursor of KCl [24]. They have also reported the separation results within 12 h from a ternary chloride mixture, PrCl 3 –GdCl 3 –ErCl 3 [22], using AlCl 3 as complex former, and within 82 h from concentrates and crude *Corresponding author. 0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 98 )00162-5

oxides of rare earth minerals [23,25,27], using AlCl 3 , KCl and POCl 3 as complex formers or using K 2 CO 3 as a precursor of KCl. Their results showed many advantages of the dry process over the conventional wet processes. Mutual separation of neighbouring rare earth elements would be one of the most difficult problems in inorganic extraction and separation. In order to search out a suitable CVT reaction condition to increase separation factors between the neighbouring rare earth elements within a relatively short reaction time, we [30,31] recently reported the mutual separation characteristics for the neighbouring rare earth elements La, Ce, Pr and Nd from their binary and quaternary chloride mixtures end oxide mixtures within 6 h under identical CVT reaction conditions. The results showed that the separation factors for the three pairs of neighbouring rare earth elements were much higher from the oxide mixtures than from the chloride mixtures. In all these cases [23–31] both chlorination of the rare earth oxides and CVT of the rare earth chlorides produced were carried out almost at the same time at high temperature for 6–82 h. Very recently, we [32] tried to combine the chlorination at low temperature together with the CVT at high temperature under different atmospheres, resulting in relatively high reaction yield and relatively efficient separation for the elements La, Ce, Pr and Nd from their binary oxide mixtures. That might partly be caused by the selective

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chlorination of the rare oxides at low temperature. The rare earth elements Nd, Sm, Eu and Gd at high purity play an important role in many fields of advanced materials for high technology. Until now, however, there is hardly any CVT information at all for mutual separation of the four rare earth elements. In this paper, we tried to extend the stepwise selective chlorination (SC)–CVT reaction to the mutual separation of the four rare earth elements from their binary oxide mixtures.

2. Experimental details The chemicals used in this study were of 98.0% purity for anhydrous AlCl 3 and $99.9% purity for Nd 2 O 3 , Sm 2 O 3 , Eu 2 O 3 and Gd 2 O 3 . Anhydrous AlCl 3 was further purified by careful sublimation under vacuum. A raw mixture was formed by mixing active carbon with a binary oxide mixture at an atomic ratio of C:Ln:Ln956:1:1. Our preliminary experiments showed that the SC sub-reaction of the raw materials with dry Cl 2 gas at T5800 K for 2 h might result in a yield of 67–92% for the four rare earth elements and a separation factor of 1.20 for Nd:Sm, 1.14 for Sm:Eu, and 1.06 for Gd:Eu. The experimental apparatus and procedure used in this study are the same as those used previously [30,31] except for the details noted below. The stepwise SC–CVT reaction was carried out in a cylindrical alumina reactor tube, 25 mm in inner diameter and 1000 mm in length. Let T denote the highest temperature in the tube reactor, where the raw material was placed. The raw material was chlorinated by dry Cl 2 gas with a flow rate of 20 cm 3 min 21 at T5800 K for 2 h. Within the temperature range of T5800–1300 K, the Cl 2 gas was replaced by a dry CO–HCl mixed gas with the flow rates of 40 and 20 cm 3 min 21 , respectively. The rare earth chlorides produced were then transported with AlCl 3 at T51300 K for 6 h, using a dry CO gas with a flow rate of 40 cm 3 min 21 as carried gas. During the whole reaction, a given pressure gradient was maintained by a subatmospheric pressure of 2.7 kPa at the outlet of the tube reactor. A temperature gradient used in the CVT sub-reaction is shown in Fig. 1. At the end of each run, the amounts of the rare earth chlorides produced were determined from the peak intensity of the characteristic bands [33]: 430.358 nm for Nd 31 , 442.434 nm for Sm 31 , 412.970 nm for Eu 31 , and 342.247 nm for Gd 31 on an inductively coupled plasma atomic emission spectrometry (Perkin Elmer, Optima 3000). As a comparison, a conventional CVT process was also used for the extraction and mutual separation of the same raw materials, where both the chlorination and CVT were carried out at T51300 K for 6 h using AlCl 3 as the complex former under a dry Cl 2 –N 2 mixed gas with flow rates of 15 and 40 cm 3 min 21 , respectively.

Fig. 1. Temperature dependence curve and distribution of NdCl 3 , SmCl 3 , EuCl 3 , and GdCl 3 deposits in an SC–CVT reaction formed from their binary oxide mixtures: (A) Nd 2 O 3 –Sm 2 O 3 , (B) Sm 2 O 3 –Eu 2 O 3 , and (C) Eu 2 O 3 –Gd 2 O 3 .

3. Results and discussion Fig. 1(A–C) show the SC–CVT reaction results of the binary oxide mixtures Nd 2 O 3 –Sm 2 O 3 , Sm 2 O 3 –Eu 2 O 3 and Eu 2 O 3 –Gd 2 O 3 in the form of deposition profiles for the rare earth chlorides vs. fraction numbers (FN) of the receptors. It can be seen that the CVT amount was in the order of Nd.Sm.Eu,Gd, the total separation factor values were 1.53 for Nd:Sm, 2.15 for Sm:Eu and 3.81 for Gd:Eu, and the deposition of all the four elements was the largest at FN51–4. Table 1 lists the largest values of the separation factor and Table 2 shows the values of the separation factors in the case with the highest CVT amounts for the sum of the elements. Here the separation factor values of 2.30 and 6.84 for Nd:Sm, 2.97–4.84 for Sm:Eu and 12.3–26.5 for Gd:Eu, respectively, are much

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Table 1 The largest separation factors between neighbouring rare earth elements Nd, Sm, Eu and Gd from their binary oxide mixtures using SC–CVT and conventional CVT reactions SF a

FN b

(A) SC–CVT reaction Nd 2 O 3 –Sm 2 O 3 6.84 Sm 2 O 3 –Eu 2 O 3 4.84 Eu 2 O 3 –Gd 2 O 3 26.5 (B) conventional CVT reaction Nd 2 O 3 –Sm 2 O 3 1.64 Sm 2 O 3 –Eu 2 O 3 3.19 Eu 2 O 3 –Gd 2 O 3 1.78 3.99 a

11 7 2

(Nd:Sm) (Sm:Eu) (Gd:Eu)

2 10 2 13

(Nd:Sm) (Sm:Eu) (Gd:Eu) (Eu:Gd)

SF5separation factor. FN5fraction number.

b

higher than those for the conventional wet processes, though a combination of electrochemical or photochemical reduction together with solvent extraction might increase the separation factor value to 27.4 [34] or 27.3 [35] for Eu:Sm and 88.3 [34] or 95.5 [35] for Eu:Gd, respectively. As a comparison, Fig. 2(A–C) show the reaction results of the conventional CVT process. It can be seen that the CVT amount was in the order of Nd,Sm.Eu,Gd, the total separation factor values were 1.03 for Sm:Nd, 1.59 for Sm:Eu and 1.07 for Gd:Eu, the deposition of all the four elements was the largest at FN54–6 for Nd 2 O 3 – Sm 2 O 3 at FN54–6 and 12–14 for Sm 2 O 3 –Eu 2 O 3 and at FN53–5 and 10–12 for Eu 2 O 3 –Gd 2 O 3 . Tables 1 and 2 also list the largest values of the separation factor and the values of separation factors in the case with the highest CVT amounts for the sum of the elements, respectively, for the conventional CVT reaction. The separation factor values were 1.64 for Nd:Sm, 3.19 for Sm:Eu, 1.78 for Table 2 Separation factors between neighbouring rare earth elements Nd, Sm, Eu and Gd from their binary oxide mixtures in the case with the highest transport amounts for the sum of the elements using SC–CVT and conventional CVT reactions SF a

FN b

(A) SC–CVT reaction Nd 2 O 3 –Sm 2 O 3 1.27 1 1.07 3 Sm 2 O 3 –Eu 2 O 3 1.38 1 4.26 3 Eu 2 O 3 –Gd 2 O 3 1.51 1 15.1 3 (B) conventional CVT reaction Nd 2 O 3 –Sm 2 O 3 1.22 4 1.06 5 Sm 2 O 3 –Eu 2 O 3 1.19 4 1.70 6 2.53 13 Eu 2 O 3 –Gd 2 O 3 1.74 3 1.73 5 2.25 11 a

SF5separation factor. FN5fraction number.

b

SF a

FN b

1.13 2.30 2.97 3.56 26.5 12.3

2 4 2 4 2 4

1.09 1.37 2.54 2.48 1.72 1.10 3.71

6 5 12 14 4 10 12

(Nd:Sm) (Sm:Eu) (Gd:Eu) (Nd:Sm) (Sm:Nd) (Sm:Eu) (Sm:Eu) (Sm:Eu) (Gd:Eu) (Gd:Eu) (Eu:Gd)

Fig. 2. Distribution of NdCl 3 , SmCl 3 , EuCl 3 , and GdCl 3 deposits in the conventional CVT reaction formed from their binary oxide mixtures: (A) Nd 2 O 3 –Sm 2 O 3 , (B) Sm 2 O 3 –Eu 2 O 3 , and (C) Eu 2 O 3 –Gd 2 O 3 .

Gd:Eu and 3.99 for Eu:Gd, where the former three values were much smaller than those for the SC–CVT process mentioned above. We have determined thermodynamic quantities of the complexation reaction LnCl 3 (s) 1 (3 / 2)Al 2 Cl 6 (g) 5 LnAl 3 Cl 12 (g)

(1)

from Ln5La to Ln5Lu [10]. The reported values of standard enthalpy and entropy were DH 0298 534.662 kJmol 21 and DS 0298 5 23.363 Jmol 21 K 21 for NdAl 3 Cl 12 , DH 0298 527.362 kJmol 21 and DS 0298 5 24.663 0 Jmol 21 K 21 for SmAl 3 Cl 12 , DH 298 523.562 kJmol 21 and 0 21 21 DS 298 5 26.363 Jmol K for EuAl 3 Cl 12 , and DH 0298 5 21 0 21 21 29.062 kJmol and DS 298 52.963 Jmol K for GdAl 3 Cl 12 , respectively, which may result in the pressure ratio of PSmAl 3 Cl 12 / PNdAl 3 Cl 12 51.67, PEuAl 3 Cl 12 / PSmAl 3 Cl 12 5 1.16, and PGdAl 3 Cl 12 / PEuAl 3 Cl 12 51.81 at T51300 K. In other words, the complex pressure at T51300 K was in the order of Nd,Sm,Eu,Gd, which is different from both the SC–CVT reaction order of Nd.Sm.Eu,Gd and the conventional CVT reaction order of Nd,Sm. Eu,Gd mentioned above. This is not surprising since the predominant rare earth vapour complexes should be LnAlCl 6 and LnAl 2 Cl 9 instead of LnAl 3 Cl 12 at such a

Y.-H. Sun et al. / Journal of Alloys and Compounds 269 (1998) 88 – 91

high temperature. However, thermodynamic quantities of the rare earth element vapour complexes LnAlCl 6 and LnAl 2 Cl 9 have been determined only for Ln5Gd [5], not for Ln5Nd, Sm and Eu. On the other hand, the equilibrium constant of the exchange reaction (1 / 2)Ln 2 O 3 (s) 1 Ln9Cl 3 (s) 5 (1 / 2)Ln 29 O 3 (s) 1 LnCl 3 (s) (2) 4

at T5800 K is 61 for Ln / Ln95 Nd / Sm, 2.4310 for Ln / Ln95Sm / Eu, and 2.7310 24 for Ln / Ln95Eu / Gd, respectively, according to the thermochemical data [36,37]. This means the amount of the rare earth chlorides produced in the chlorination sub-reaction should be in the order of Nd.Sm.Eu,Gd, which is the same as the SC–CVT reaction but different from the conventional CVT reaction mentioned above. Moreover, EuCl 2 is more stable and less active to complex with Al 2 Cl 6 to form EuAl 3 Cl 11 than EuCl 3 to form EuAl 3 Cl 12 at around 1300 K [10,22,38,39], which would also greatly benefit the SC–CVT reaction order and conventional CVT order of Sm.Eu,Gd. However, the separation factor values were much smaller than the equilibrium constants of the chlorination and complexation reactions probably due to the kinetic limitations of the SC–CVT reactions.

4. Conclusions This paper presents the first set of experimental data for CVT mutual separation of the neighbouring rare earth elements Nd, Sm, Eu and Gd. The results show that the mutual separation could be within 6 h from their binary oxide mixtures using a SC–CVT process with the separation factors higher than the conventional wet processes.

Acknowledgements This work was supported by the National Natural Science Foundation of China (59574020) and the Ministry of Metallurgical Industry.

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