Stepwise chlorination-chemical vapor transport reaction for rare earth oxide mixtures Gd2O3-Tb4O7, Tb4O7-Dy2O3 and Dy2O3-Ho2O3

Article abstract of DOI:10.1016/S0925-8388(01)01830-8

Stepwise chlorination-chemical vapor transport (SC-CVT) reaction has been investigated for the rare earth oxide mixtures Gd₂O₃-Tb₄O₇, Tb₄O₇-Dy₂O₃ and Dy₂O₃-Ho₂O₃ mediated by vapor complexes LnAl_nCl_3n+3 (where Ln=rare earth elements). The results show selective deposition of lighter elements at higher temperatures. The separation factors are 5.84 for Gd/Tb, 3.72 for Tb/Dy and 3.13 for Ho/Dy when using CO as the CVT carrier gas. A combination of enhanced separation factor with reduced total transport yield is also observed for the Tb₄O₇-Dy₂O₃ system when using argon as the CVT carrier gas.

Full text of DOI:10.1016/S0925-8388(01)01830-8

Journal of Alloys and Compounds 335 (2002) 196–199
L
www.elsevier.com/locate/jallcom
Stepwise chlorination–chemical vapor transport reaction for rare earth
oxide mixtures Gd2O3 –Tb4O7, Tb4O7 –Dy2O3 and Dy2O3 –Ho2O3
b
Yan-Hui Suna, Li-Qing Zhanga, Peng-Xiang Leia, Zhi-Chang Wanga , , Lei Guo
*
aDepartment of Chemistry, Northeastern University, Shenyang 110006, Liaoning, China
bInstitute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110015, Liaoning, China
Received 7 May 2001; received in revised form 5 July 2001; accepted 1 August 2001
Abstract
Stepwise chlorination–chemical vapor transport (SC–CVT) reaction has been investigated for the rare earth oxide mixtures
Gd2O3 –Tb4O7, Tb4O7 –Dy2O3 and Dy2O3 –Ho2O3 mediated by vapor complexes LnAlnCl3n13 (where Ln5rare earth elements). The
results show selective deposition of lighter elements at higher temperatures. The separation factors are 5.84 for Gd/Tb, 3.72 for Tb/Dy
and 3.13 for Ho/Dy when using CO as the CVT carrier gas. A combination of enhanced separation factor with reduced total transport
yield is also observed for the Tb4O7 –Dy2O3 system when using argon as the CVT carrier gas.
reserved.
2002 Elsevier Science B.V. All rights
Keywords: Rare earth compounds; Gas–solid reaction; Thermodynamic properties
1. Introduction
element pairs from Sc–Y to Eu–Gd [1–19]. In this study,
we tried to extend the SC–CVT reaction to the binary
oxide mixtures Gd2O3 –Tb4O7, Tb4O7 –Dy2O3 and
Dy2O3 –Ho2O3, mediated by the vapor complexes
Chemical vapor transport (CVT) reaction has been used
for mutual separation of rare earth elements from their
chloride mixtures, oxide mixtures, concentrates and used
materials via rare earth vapor complexes LnAlnCl3n13 or
ALnCl4 (where Ln5rare earth elements and A5alkaline
metal elements) [1–19]. More efficient separation has been
observed from oxide mixtures than from chloride mixtures
[10,12–14] due to additional carbothermic reduction–
chlorination of the oxides. Moreover, a stepwise chlorina-
tion–chemical vapor transport (SC–CVT) reaction, which
allows the chlorination and CVT to be realized under
different conditions, has resulted in a further improved
separation efficiency between neighboring rare earth ele-
ments [11,15,17–19]. This reaction has also been investi-
gated for the pure rare earth oxides from Sc2O3 to Lu2O3
[20]. Very recently, another vapor separation technique
using rare earth halides has been developed for Pr–Nd,
Nd–Sm and Sm–Dy binaries [21].
LnAlnCl3n13
.
2. Experimental details
The chemicals used in this study were of 98.0% purity
for anhydrous AlCl3 and of $99.9% purity for all the rare
earth oxides. Anhydrous AlCl3 was further purified by
careful sublimation under vacuum. A raw mixture was
formed by mixing active carbon with a binary oxide
mixture of neighboring rare earth elements Ln and Ln9 at
an atomic ratio of C/Ln/Ln956:1:1.
The SC–CVT reaction was carried out in a cylindrical
alumina reactor tube, 25 mm i.d. and 1000 mm length,
with a given temperature and pressure gradient as de-
scribed in previous papers [10–12,15,17–20]. Let T denote
the highest temperature in the tube reactor, where the raw
material is placed. The raw material was chlorinated by dry
Cl2 gas with a flow rate of 20 cm3 min21 at T5800 K for
2 h. The Cl2 gas was replaced by a dry CO–HCl mixed
gas with the flow rates of 40 and 20 cm3 min21, respec-
tively, within the temperature range of T5800–1300 K.
Then, the rare earth chlorides reacted with AlCl3 to form
Until now, the CVT or SC–CVT mutual separation
information is limited only for the neighboring rare earth
*
Corresponding author. Fax: 186-24-2389-0472.
E-mail address: zcwang@mail.neu.edu.cn (Z.-C. Wang).
0925-8388/02/$ – see front matter
PII: S0925-8388(01)01830-8
2002 Elsevier Science B.V. All rights reserved.

Y.-H. Sun et al. / Journal of Alloys and Compounds 335 (2002) 196–199
197
vapor complexes LnAlnCl3n13 which were chemically
transported along the temperature gradient in the tube
reactor at T51300 K for 6 h, using either a dry CO gas or
a dry Ar gas as the carrier with a flow rate of 40 cm3
min21. During the whole reaction, a given pressure
gradient was maintained at a subatmospheric pressure of
2.7 kPa at the outlet of the tube reactor [10–12,15,17–20].
At the end of each run, the amounts of the rare earth
chloride produced were determined from the peak intensity
values of the visible absorption spectra at 342.247 nm for
Gd31, 350.917 nm for Tb31, 353.170 nm for Dy31 and
339.898 nm for Ho31 [22].
3. Results and discussion
Fig. 1(A–C) show the SC–CVT reaction results of the
binary oxide mixtures Gd2O3 –Tb4O7, Tb4O7 –Dy2O3 and
Dy2O3 –Ho2O3, using CO as the carrier gas, in the form of
deposition profiles for the rare earth chlorides produced
versus fraction numbers (FN) of the receptors. Fig. 1(D)
shows the same results for the mixture Tb4O7 –Dy2O3,
using argon as the carrier gas. It can be seen that the
lighter element always concentrates at fractions of higher
temperatures. Table 1 lists the transport yields and sepa-
ration factors. The largest separation factors were 5.84 for
Gd/Tb, 3.72 for Tb/Dy and 3.13 for Ho/Dy when CO was
used as the carrier gas and was 3.91 for Tb/Dy when
argon was used. Table 2 lists the separation factors in the
case with the highest CVT amounts for the sum of the
elements. All the results indicate the potential applicability
of the SC–CVT reaction for rare earth separation from the
three oxide mixtures.
The SC–CVT reaction of these oxide mixtures is more
complicated than that of their pure oxides [20]. It may
mainly be a combination of selective chlorination subreac-
tions of the oxides at T5800 K:
(1/2)Ln2O3(s) 1 (3/2)Cl2(g) 1 (3/2)C(s) 5 LnCl3(s)
1 (3/2)CO(g) (Ln 5 Gd, Dy, Ho)
(1)
(1/4)Tb4O7(s) 1 (3/2)Cl2(g) 1 (7/4)C(s) 5 TbCl3(s)
1 (7/4)CO(g)
(2)
exchange subreactions between the produced chlorides and
the remaining oxides within the temperature range of
T5800–1300 K:
(1/2)Gd2O3(s) 1 TbCl3(s or l) 1 (1/4)CO(g)
5 GdCl3(s or l) 1 (1/4)Tb4O7(s) 1 (1/4)C(s)
(3)
(4)
Fig. 1. Temperature dependence curve, and distribution of LnCl3 deposits
in a SC–CVT reaction formed from their binary oxide mixtures mediated
by the vapor complexes LnAlnCl3n13: (A) Gd2O3 –Tb4O7, (B) Tb4O7 –
Dy2O3 and (C) Dy2O3 –Ho2O3, using CO as the carrier gas, and (D)
Tb4O7 –Dy2O3, using argon as the carrier gas.
(1/4)Tb4O7(s) 1 DyCl3(s or l) 1 (1/4)C(s)
5 TbCl3(s or l) 1 (1/2)Dy2O3(s) 1 (1/4)CO(g)

198
Y.-H. Sun et al. / Journal of Alloys and Compounds 335 (2002) 196–199
Table 1
The total separation factors, total transport yields and largest separation factors of the rare earth chlorides from the binary oxide mixtures Gd2O3 –Tb4O7,
Tb4O7 –Dy2O3 and Dy2O3 –Ho2O3 in a SC–CVT reaction, using AlCl3 as a complex former
Carrier gas
SF
FN
Transport
yield (%)
TSF
Gd2O3 –Tb4O7
Tb4O7 –Dy2O3
Tb4O7 –Dy2O3
Dy2O3 –Ho2O3
CO
5.84 (Gd/Tb)
3.72 (Tb/Dy)
3.91 (Tb/Dy)
3.13 (Ho/Dy)
6
5
Gd: 24.6
Tb: 25.1
Tb: 30.1
Dy: 30.5
Tb: 22.7
Dy: 19.2
Dy: 18.7
Ho: 18.8
1.02 (Tb/Gd)
1.01 (Dy/Tb)
1.18 (Tb/Dy)
1.00 (Ho/Dy)
CO
Argon
CO
1
10
SF, separation factor; FN, fraction number; TSF, total separation factor.
(1/2)Dy2O3(s) 1 HoCl3(s or l) 5 DyCl3(s or l)
(1/2)Tb2O3(s) 1 DyCl3(s or l) 5 TbCl3(s or l)
1 (1/2)Dy2O3(s)
1 (1/2)Ho2O3(s)
(5)
(10)
selective complexation subreactions of the produced chlo-
rides at T51300 K:
In addition, rare earth chlorides evaporate at high
temperatures, though vapor pressures of the chlorides are
always lower than corresponding complex pressures.
The transported amount of LnCl3 in the reactor tube
may mainly be determined by Reactions (1)–(6) and
(8)–(10). The predominant complexes of all lanthanide
elements are LnAl3Cl12 in the range 500–800 K [24] and
LnAlCl6 and LnAl2Cl9 at 1300 K; but thermodynamic
data are lacking for TbAlCl6, DyAlCl6, HoAlCl6 and
TbAl2Cl9 [24–29]. At 1300 K, the SC–CVT reaction of
pure oxides, using CO as the carrier gas, demonstrated
transport yield ratios of 1.52 for TbCl3 /GdCl3, 1.11 for
DyCl3 /TbCl3 and 1.15 for HoCl3 /DyCl3 [20] while the
CVT reaction of pure chlorides showed a slightly weak
transported amount order of Gd,Tb,Dy,Ho [4]. Both
of them suggest a weak complex stability order of Gd,
Tb,Dy,Ho at 1300 K. On the other hand, the solid–
solid or liquid–solid Reactions (3)–(5), (9) and (10) had
much smaller equilibrium constants and ran for a much
shorter time than Reactions (1), (2) and (8). Our prelimin-
ary experiments showed that the selective chlorination of
the raw materials with dry Cl2 gas at T5800 K for 2 h
might result in TbCl3 /GdCl3 50.71, TbCl3 /DyCl3 50.83
and HoCl3 /DyCl3 50.99. Here the small values for the
LnCl3(l) 1 (n/2)Al2Cl6(g) 5 LnAlnCl3n13(g) (Ln
5 Gd, Tb, Dy and Ho)
(6)
and selective CVT subreactions of the vapor complexes
LnAlnCl3n13 (n51–4) along the temperature gradient:
LnAlnCl3n13(g) 5 LnCl3(l or s)
1 (n/2)Al2Cl6(g) (Ln 5 Gd, Tb, Dy and Ho)
(7)
Since nonstoichiometric oxide Tb4O7, which is a mix-
ture of Tb7O12 and Tb11O20, is thermodynamically more
stable than Tb2O3 only below 1000 K in the absence of
reducing atmosphere [23], Reaction (2) may partly be
replaced by
(1/2)Tb2O3(s) 1 (3/2)Cl2(g) 1 (3/2)C(s) 5 TbCl3(s)
1 (3/2)CO(g)
(8)
(9)
and Reactions (3) and (4) may partly be replaced by
(1/2)Gd2O3(s) 1 TbCl3(s or l) 5 GdCl3(s or l)
1 (1/2)Tb2O3(s)
Table 2
Separation factors of the rare earth chlorides obtained from the binary oxide mixtures Gd2O3 –Tb4O7, Tb4O7 –Dy2O3 and Dy2O3 –Ho2O3 in a SC–CVT
reaction, using AlCl3 as a complex former
Carrier gas
CO
SF
FN
SF
FN
Gd2O3 –Tb4O7
Tb4O7 –Dy2O3
Tb4O7 –Dy2O3
Dy2O3 –Ho2O3
1.27 (Gd/Tb)
1.67 (Tb/Gd)
1.11 (Dy/Tb)
1.15 (Tb/Dy)
1.71 (Tb/Dy)
2.52 (Dy/Tb)
1.04 (Dy/Ho)
1.04 (Ho/Dy)
1
2
6
3
3
12
3
5
1.10 (Tb/Gd)
1.12 (Dy/Tb)
3
7
CO
Argon
CO
3.54 (Tb/Dy)
2.18 (Dy/Tb)
1.26 (Dy/Ho)
2
8
2
SF, separation factor; FN, fraction number.

Y.-H. Sun et al. / Journal of Alloys and Compounds 335 (2002) 196–199
199
[2] G. Adachi, K. Murase, K. Shinozaki, K. Machida, Chem. Lett.
(1992) 511.
[3] K. Murase, K. Shinozaki, K. Machida, G. Adachi, Bull. Chem. Soc.
Jpn. 65 (1992) 2724.
[4] K. Murase, K. Shinozaki, Y. Hirashima, K. Machida, G. Adachi, J.
Alloys Comp. 198 (1993) 31.
[5] K. Murase, K. Machida, G. Adachi, Chem. Lett. (1994) 1297.
[6] K. Murase, T. Fukami, K. Machida, G. Adachi, Ind. Eng. Chem.
Res. 34 (1995) 3963.
[7] K. Murase, T. Ozaki, K. Machida, G. Adachi, J. Alloys Comp. 233
(1996) 96.
[8] T. Ozaki, T. Miyazawa, K. Murase, K. Machida, G. Adachi, J.
Alloys Comp. 245 (1996) 10.
[9] T. Ozaki, K. Murase, K. Machida, G. Adachi, Trans. Inst. Min.
Metall. 105 (1996) C141.
[10] Z.-C. Wang, J. Yu, Y.-L. Yu, Bull. Chem. Soc. Jpn. 69 (1996) 2369.
[11] Z.-C. Wang, Y.-H. Sun, Chem. Lett. (1997) 1113.
[12] Z.-C. Wang, J. Yu, Y.-L. Yu, Y.-H. Sun, J. Alloys Comp. 264 (1998)
147.
TbCl3 /GdCl3 and TbCl3 /DyCl3 pairs may indicate the
greater importance of Reactions (8)–(10) than (2)–(4)
even at T5800 K. These may result in the decrease in
transport yield ratios of TbCl3 /GdCl3, DyCl3 /TbCl3 and
HoCl3 /DyCl3 from 1.11 to 1.52 in the case of the pure
oxides [20] to |1.00 in the case of the binary oxide
mixtures as shown in this study. Moreover, the replace-
ment of CO by Ar as carrier gas for the Tb4O7 –Dy2O3
system yielded an enhanced Tb/Dy separation factor at
high temperatures but a reduced total transport yield as
shown in Table 1. This is the same as the Pr6O11 –Nd2O3
system [11]. Here the higher total transport yield under CO
atmosphere may be caused by possible complex formation
between LnCl3 and CO [11]. On the other hand, the
distribution characteristics of LnCl3 in the reactor tube
may mainly be determined by the transport yield of LnCl3
at 1300 K and by the CVT subreaction along the tempera-
ture gradient. This study shows a selective deposition of
lighter elements at higher temperatures for the three binary
oxide systems, which is consistent with the SC–CVT
reaction for the pure oxides [20].
[13] J. Jiang, T. Ozaki, K. Machida, G. Adachi, J. Alloys Comp. 264
(1998) 157.
[14] T. Ozaki, J. Jiang, K. Murase, K. Machida, G. Adachi, J. Alloys
Comp. 265 (1998) 125.
[15] Y.-H. Sun, Z.-C. Wang, L. Guo, J. Alloys Comp. 269 (1998) 88.
[16] T. Ozaki, K. Machida, G. Adachi, Metall. Mater. Trans. B 30B
(1999) 45.
[17] Y.-H. Sun, Z.-C. Wang, L. Guo, J. Alloys Comp. 285 (1999) 73.
[18] Y.-H. Sun, Z.-C. Wang, L. Guo, J. Rare Earths 17 (1999) 178.
[19] Y.-H. Sun, Z.-C. Wang, L. Guo, J. Alloys Comp. 306 (2000) 146.
[20] Z.-C. Wang, Y.-H. Sun, L. Guo, J. Alloys Comp. 287 (1999) 109.
[21] T. Uda, K.T. Jacob, M. Hirasawa, Science 289 (2000) 2326.
[22] R.K. Winge, V.J. Peterson, V.A. Fassel, Appl. Spectrosc. 33 (1979)
206.
[23] O. Knacke, O. Kubaschewski, K. Hesselmann, Thermochemical
Properties of Inorganic Substances, 2nd Edition, Springer, Berlin,
1991.
[24] Z.-C. Wang, L.-S. Wang, Inorg. Chem. 36 (1997) 1536.
[25] S. Boghosian, G.N. Papatheodorou, in: K.A. Gschneidner Jr, L.
Eyring (Eds.), Handbook on the Physics and Chemistry of Rare
Earths, Vol. 23, Elsevier, Amsterdam, 1996, p. 435.
[26] Z.-C. Wang, L.-S. Wang, R.-J. Gao, Y. Su, J. Chem. Soc. Faraday
Trans. 92 (1996) 1887.
4. Conclusions
This paper presents the first set of experimental results
for SC–CVT reaction of the binary oxide mixtures
Gd2O3 –Tb4O7, Tb4O7 –Dy2O3 and Dy2O3 –Ho2O3 me-
diated by the vapor complexes LnAlnCl3n13. The results
show that the mutual separation of the neighboring rare
earth elements from the oxide mixtures could effectively
be realized within 6 h. The separation factors are 5.84 for
Gd/Tb, 3.72 for Tb/Dy and 3.13 for Ho/Dy when using
CO as the carrier gas and 3.91 for Tb/Dy when using
argon as the carrier gas.
[27] L.-S. Wang, R.-J. Gao, Y. Su, Z.-C. Wang, J. Chem. Thermodyn. 28
(1996) 1093.
[28] J. Jiang, T. Ozaki, K. Machida, G. Adachi, J. Alloys Comp. 260
(1997) 222.
Acknowledgements
[29] Z.-C. Wang, L.-S. Wang, J. Alloys Comp. 265 (1998) 153.
This work was supported by the National Natural
Science Foundation of China (59574020 and 59874008).
References
[1] G. Adachi, K. Shinozaki, Y. Hirashima, K. Machida, J. Less-
Common Met. 169 (1991) L1.