Comparative study of the mutual separation characteristics for binary mixed oxides Er2O3-Ln2O3 (Ln = Sc, Y, La, Nd, Sm, Gd and Ho) mediated by vapor complexes KLnCl4

Article abstract of DOI:10.1016/j.jallcom.2005.02.104

Mutual separation characteristics for a series of rare earth elements Sc, Y, La, Nd, Sm, Gd, Ho and Er from their binary oxide mixtures Er ₂O₃-Sc₂O₃, Er₂O ₃-Y₂O₃, Er₂O₃-La ₂O₃, Er₂O₃-Nd₂O ₃, Er₂O₃-Sm₂O₃, Er ₂O₃-Gd₂O₃ and Er₂O ₃-Ho₂O₃ has been investigated using a stepwise chlorination-chemical vapor transport (SC-CVT) reaction mediated by vapor complexes KLnCl₄. The total transported yield of the chlorides produced from the oxide mixtures was in the order of ErCl₃ > ScCl₃, ErCl₃ > YCl₃, ErCl₃ > LaCl₃, ErCl₃ > NdCl₃, ErCl₃ > SmCl₃, ErCl₃ > GdCl₃ and HoCl₃ > ErCl₃, and the total separation factors are 13.0 for Er:Sc, 1.49 for Er:Y, 1.48 for Er:La, 1.15 for Er:Nd, 2.33 for Er:Sm, 2.72 for Er:Gd and 1.10 for Ho:Er. The largest separation factors 1213.8 for Er:Sc, 6.37 for Er:Y, 189.3 for Er:La, 100.6 for Er:Nd, 105.7 for Er:Sm, 27.8 for Er:Gd and 1.14 for Er:Ho in the lower temperature region, while 102.7 for La:Er, only 14.3 for Nd:Er, 16.7 for Sm:Er, 4.0 for Gd:Er and 2.04 for Ho:Er in the higher temperature region were observed, respectively. The results showed the obvious divergence effect of Gd both in the largest separation factors, the total separation factors and total transport efficiency. Furthermore, the results were discussed on the difference of ionic radius of Sc and Y on the one side and the lanthanoid elements of La, Nd, Sm, Gd, Ho and Er on the other hand, and verified that the ionic radius is one of the decisive factors only for lanthanide elements, not for Sc and Y.

Full text of DOI:10.1016/j.jallcom.2005.02.104

Journal of Alloys and Compounds 413 (2006) 175–180
Comparative study of the mutual separation characteristics for binary
mixed oxides Er O –Ln O (Ln = Sc, Y, La, Nd, Sm, Gd and Ho)
2
3
2 3
mediated by vapor complexes KLnCl4
a,∗
b
b
Yan-Hui Sun , Zhen-Fei Chen , Sheng-liang Hu
a
Department of Chemistry, South China Normal University, Guangzhou, 510631 Guangdong, China
b
Department of Metallurgy, Hebei Institute of Technology, Tangshan 063009, Hebei, China
Received 22 September 2004; accepted 28 February 2005
Available online 25 August 2005
Abstract
Mutual separation characteristics for a series of rare earth elements Sc, Y, La, Nd, Sm, Gd, Ho and Er from their binary oxide mixtures
–Sc , Er –Y , Er –La , Er –Nd , Er –Sm , Er –Gd and Er –Ho has been investigated using a
stepwise chlorination-chemical vapor transport (SC-CVT) reaction mediated by vapor complexes KLnCl . The total transported yield of the
chlorides produced from the oxide mixtures was in the order of ErCl > ScCl , ErCl > YCl , ErCl > LaCl , ErCl > NdCl , ErCl > SmCl
ErCl > GdCl and HoCl > ErCl , and the total separation factors are 13.0 for Er:Sc, 1.49 for Er:Y, 1.48 for Er:La, 1.15 for Er:Nd, 2.33
Er
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2 3
O
4
3
3
3
3
3
3
3
3
3
3
,
3
3
3
3
for Er:Sm, 2.72 for Er:Gd and 1.10 for Ho:Er. The largest separation factors 1213.8 for Er:Sc, 6.37 for Er:Y, 189.3 for Er:La, 100.6 for
Er:Nd, 105.7 for Er:Sm, 27.8 for Er:Gd and 1.14 for Er:Ho in the lower temperature region, while 102.7 for La:Er, only 14.3 for Nd:Er,
1
6.7 for Sm:Er, 4.0 for Gd:Er and 2.04 for Ho:Er in the higher temperature region were observed, respectively. The results showed the
obvious divergence effect of Gd both in the largest separation factors, the total separation factors and total transport efficiency. Furthermore,
the results were discussed on the difference of ionic radius of Sc and Y on the one side and the lanthanoid elements of La, Nd, Sm, Gd,
Ho and Er on the other hand, and verified that the ionic radius is one of the decisive factors only for lanthanide elements, not for Sc
and Y.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Rare earth compounds; Liquid–solid reactions; Thermodynamic properties
1
. Introduction
the conventional chemical vapor transport (CVT) reaction
1–10,12–14] or stepwise selective chlorination-chemical
[
Mutual separation characteristics of rare earth elements
vapor transport (SC-CVT) reaction [11,15–22]. More effi-
cient separation has been observed for oxide mixtures than
for chloride mixtures [10,12–14] and using SC-CVT than
using CVT reaction [11,15,17,18], and a further improved
separation efficiency and total transport yields realized medi-
ated by KCl or KCl–AlCl3 as complex former instead of
only by AlCl3 as complex former. Furthermore, the charac-
teristics of CVT or SC-CVT have been discussed from the
nature of the rare earth elements such as the ionic structure,
ionic radius and the electronic configuration by investigat-
ing the mixed oxides which the rare earth elements located
in different period of the periodic table [13,14,18,20–22].
by a dry process so called chemical vapor transport reac-
tion [1–22] have been investigated intensively from sev-
eral aspects, such as different vapor complexes KLnCl4
[
2,3,10,13,14,20–22] or LnAlnCl3n+3 [1,4–19], different raw
materials like as rare earth chloride mixtures, oxide mixtures,
concentrates of ore and the scraps containing of rare earth
elements. Moreover, the temperature gradients [4,22] and
reaction steps for separation has been investigated too, that is
∗
Corresponding author.
E-mail address: sunyanhui0102@163.com (Y.-H. Sun).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.02.104

1
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Y.-H. Sun et al. / Journal of Alloys and Compounds 413 (2006) 175–180
The CVT or SC-CVT mutual separation for the lanthanide
elements showed that the ionic radius difference of two
elements is one of the decisive factors for the separation
efficiency [1–12,15–17,19,21,22], and the separation for Sc
and Y from Sc2O3–La2O3 [13], Y2O3–La2O3 [13,18,20],
Y2O3–Nd2O3 [18,20], Y2O3–Sm2O3 [18,20], Y2O3–Dy2O3
2. Experimental details
The chemicals used in this study were analytic purity for
KCl, active carbon powder and ≥99.9% purity for all rare
earth oxides. A raw mixture was formed by mixing active
carbon and KCl with a binary oxide mixture of rare earth ele-
ꢀ
ꢀ
[14,22], Y2O3–Ho2O3 [14,21], Y2O3–Er2O3 [14,22] and
ments Ln and Ln at an atomic ratio of C:Ln:Ln :K = 6:1:1:1,
which contains 3.0–5.0 mmol of each rare earth element, Ln
Ho2O3–Sc2O3 [21] showed that the ionic structure and the
electronic configuration are the key factors in the CVT reac-
tion.
Recently, Wang’s group has determined the thermody-
namic properties of the gaseous species LnAlnCl3n+3 for
all of rare earth elements including Sc and Y, and found
that LnAl3Cl12 is the predominant gaseous complex species
for the lanthanide series from La to Lu [24–26] while
LnAl2Cl9 for Sc and Y [23] in the temperature range from
ꢀ
and Ln .
The SC-CVT reaction was carried out in a cylindrical
alumina reactor tube, 25 mm inner diameter and 1000 mm
length, with a given temperature gradient as described in
previous paper [10–12,15–22]. Let T denote the highest tem-
perature in the tube reactor, where the raw materials were
placed. The raw material was chlorinated by dry Cl gas with
2
3
−1
a flow rate of 20 cm min at T = 800 K for 2 h. The Cl gas
2
5
00 to 900 K. On the other hand, Adachi and co-workers
2
was replaced by a dry Ar–Cl mixed gas with the flow rates
of 30 and 10 cm min , respectively, within the tempera-
3
−1
[13,14] have reported the CVT characteristics of the binary
oxide mixtures Sc2O3–Y2O3 [13], Sc2O3–La2O3 [13]
and Y2O3–Ln2O3 (Ln = La [13], Dy [14], Ho [14] and Er
ture range of T = 800–1300 K. Then, the rare earth chlorides
reactedwithKCltoformthevaporcomplexesKLnCl , which
4
[14]) mediated by gaseous complexes LnAlnCl3n+3 and
were transported chemically along the temperature gradient
KLnCl4. The experiment results [13,14] indicated that ScCl3
could be easily separated from YCl3 and LaCl3 while the
unexpected results that the lowest CVT efficiency and the
lowest gaseous complexes stability for YCl3 compared with
LnCl3 (Ln = Dy, Ho and Er) were observed. Very recently,
we [18,20–22] reported the mutual separation characteristics
for Sc, Y, Ho and lanthanoid rare earth elements, such as
La, Nd, Sm, Dy and Er from their binary oxide mixtures
Sc2O3–Y2O3 [18,20], Y2O3–Ln2O3 (Ln = La [18,20], Nd
in the tube reactor at T = 1300 K for 6 h with the Ar–Cl car-
rier gas. At the end of each run, the amounts of the rare earth
2
chloride produced were determined from the peak intensity of
3+
the characteristic bands: 337.271 nm for Er , 337.215 nm for
3
+
3+
3+
Sc , 377.433 nm for Y , 333.749 nm for La , 401.225 nm
3+
3+
3+
for Nd , 359.260 nm for Sm , 342.247 nm for Gd and
339.898 nm for Ho , on an inductively coupled plasma
atomic emission spectrometry (Perkin-Elmer, Optima 2000).
3+
[18,20], Sm [18,20], Dy [22], Ho [21,22] and Er [22]),
Sc2O3–Ho2O3 [21] and Ho2O3–Ln2O3 [21] (Ln = Y,
La, Nd and Sm) using SC-CVT reaction mediated by
LnAlnCl3n+3 [18] and KLnCl4 [20–22]. The results mainly
show the largest total transport yields of YCl3 compared
with that of Sc, La, Nd and Sm mediated by LnAlnCl3n+3,
that is YCl3 > NdCl3 > SmCl3 > LaCl3 > ScCl3 [18] while
3. Results and discussion
3.1. Mutual separation for Er2O3–Ln2O3 (Ln = Sc, Y,
La, Nd, Sm, Gd and Ho)
Fig. 1(A–G) show the SC-CVT reaction results of
the binary oxide mixtures Er2O3–Sc2O3, Er2O3–La2O3,
Er2O3–Nd2O3, Er2O3–Sm2O3, Er2O3–Gd2O3 determined
in this study, together with those of Er2O3–Y2O3 [22] and
Er2O3–Ho2O3 [22], in the form of deposition profiles for the
rare earth chlorides transported versus fraction numbers (FN)
of the receptors, taking KCl as complex former and Argon
gas as carried gas in the given temperature gradient for 6 h.
It can be seen that there are few ScCl3 deposited in the
receptor from FN = 1–7, and only a very small amounts of
ScCl3 concentrates in the lower temperature region from
1050 to 900 K (FN = 8–12) indicated that the vapor com-
plexes KScCl4 is stable in the higher temperature range and
decomposed only in the lower temperatures region, while
ErCl3 mainly distributes in the middle temperature range
from 1120 to 980 K (FN = 5–10). For Er–Y system [22],
the main distribution temperature of both ErCl3 and YCl3
concentrated in 1000–750 K (FN = 9–14). For lanthanoid
elements Er–Ln (Ln = La, Nd, Sm, Gd and Ho [22]), the
distribution of chlorides was in accordance with the ionic
NdCl3 > SmCl3 > LaCl3 > YCl3 > ScCl3
mediated
by
KLnCl4 [20], and that of LaCl3 > HoCl3 > NdCl3 > SmCl3 >
YCl3 > ScCl3 [21] and HoCl3 > ErCl3 > DyCl3 > YCl3 [22]
mediated by KLnCl4. Moreover, Sc and La are easily
separated from Y and Ho compared with the other elements.
Generally, the chlorination ability of Er2O3 is as low
as that of Ho2O3 and Y2O3 under normal chlorination
conditions, since the ionic radius of erbium is very similar
to that of holmium and yttrium. So, the chemical vapor
transport characteristics of Er2O3–Ln2O3 should be close
to that of Y2O3–Ln2O3 [18,20] and Ho2O3–Ln2O3 [21]
mediated by vapor complexes LnAlnCl3n+4 or KLnCl4.
Thus, in this study, together with those shown in [22] for
Er2O3–Y2O3 and Er2O3–Ho2O3, we tried to systematic
investigate the mutual separation characteristics for Er, Sc,
Y, La, Nd, Sm, Gd and Ho from their binary oxide mixtures
Er2O3–Sc2O3, Er2O3–Y2O3, Er2O3–La2O3, Er2O3–Nd2O3,
Er2O3–Sm2O3, Er2O3–Gd2O3 and Er2O3–Ho2O3 using
SC-CVT reaction mediated by KLnCl4.

Y.-H. Sun et al. / Journal of Alloys and Compounds 413 (2006) 175–180
177
Fig. 1. Temperature gradients and distribution of ErCl3, ScCl3, YCl3, LaCl3, NdCl3, SmCl3, GdCl3 and HoCl3, deposit in SC-CVT reaction formed from
the binary oxide mixtures: (A) Er2O3–Sc2O3, (B) Er2O3–Y2O3, (C) Er2O3–La2O3, (D) Er2O3–Nd2O3, (E) Er2O3–Sm2O3, (F) Er2O3–Gd2O3 and (G)
Er2O3–Ho2O3.
radius-stability rule, that is ErCl3 with smaller lanthanide
ion radius concentrates in the lower temperature region while
LaCl3, NdCl3 and SmCl3, mainly deposits in the higher tem-
perature region above 1150 K and in the increasing order of
SmCl3 < NdCl3 < LaCl3 with the increase of the lanthanide
ionic radius size. GdCl3 and HoCl3 are all concentrated in the
middle temperature region from 1150 to 980 K (FN = 5–10).
Moreover, the amounts of chlorides ErCl3 deposited in the
higher temperature region was always smaller than that of
LaCl3, NdCl3, SmCl3, GdCl3 and HoCl3 and in the lower
temperature region, the situation was reversed. All of these
are similar to the main deposition region for ScCl3, YCl3,
HoCl3, LaCl3, NdCl3 and SmCl3 in previous paper using
AlCl3 [18] and KCl [20,21] as complex former except for
ScCl3 mediated by ScAlnCl3n+3 [18].
Table 1 lists the total transported yields of LnCl3 produced
from their oxide mixtures in the SC-CVT reaction. The total
transport yields showed that the SC-CVT transport ability is
ErCl3 > ScCl3, ErCl3 > YCl3, ErCl3 > LaCl3, ErCl3 > NdCl3,
ErCl3 > SmCl3, ErCl3 > GdCl3 while ErCl3 < HoCl3. From
La to Er, the transported yield of LnCl3 was increasing
with the decreasing of the ionic radius of rare earth and
the divergence appeared around Sm and Gd, and that of Ho
and Er showed abnormal order too. Table 2 lists the total
separation factors and the largest values of the separation
factor expressed as the atomic ratio for the resulting chlo-
rides. The largest separation factors showed that all the seven
binary oxide mixtures achieved better separation effect in
some receptor by only one circle of CVT reaction except
of Ho2O3–Er2O3. Particularly, it is easy to make mutual

1
78
Y.-H. Sun et al. / Journal of Alloys and Compounds 413 (2006) 175–180
Table 1
The total transported yields of rare earth chlorides LnCl3 produced from their oxide mixtures Er2O3–Ln2O3 (Ln = Sc, Y, La, Nd, Sm, Gd and Ho) through
SC-CVT reaction
Yield (%)
Sc
Y
La
Nd
Sm
Gd
Ho
Er
Er2O3–Sc2O3
Er2O3–Y2O3
Er2O3–La2O3
Er2O3–Nd2O3
Er2O3–Sm2O3
Er2O3–Gd2O3
Er2O3–Ho2O3
1.78
23.12
31.74
36.89
36.75
55.49
40.58
43.54
21.33
24.85
31.96
23.82
14.91
47.72
separation of Sc2O3 from Er2O3 by the SC-CVT reaction
mediated by vapor complex KLnCl4.
3.2. The mechanism of SC-CVT reaction
Fig. 2 gives the total separation factors of Er:Ln, the largest
separation factors of Er:Ln or Ln:Er in the lower or higher
temperature region, resulted from their binary oxide mix-
tures versus the ionic radii, here Ln presented La, Nd, Sm,
Gd, Ho and Y. The largest separation factors Er:Ln in the
lower temperature region is decreasing from 189.3 for Er:La
to 100.0 for Er:Nd, and then increased to 105.7 for Er:Sm
and sharply decreased to 27.8 for Er:Gd and 1.14 for Er:Ho
The SC-CVT reaction mechanism might mainly be a com-
bination of selective carbonthermic reduction-chlorination
sub-reaction of the oxides at T = 800 K:
1
2
3
2
3
2
3
2
Ln O (s) + Cl (g) + C(s) = LnCl (s) + CO(g)
2
3
2
3
(Ln = Er, Sc, Y, La, Nd, Sm, Gd and Ho)
(1)
exchange sub-reactions between the remaining oxides and
the produced chlorides within the temperature range of
T = 800–1300 K:
[22], and the similar phenomena was presented for Ln:Er in
the higher temperature region. Moreover, the total separation
factors decreased from 1.48 for Er:La to 1.10 for Er:Ho [22],
while 2.33 for Er:Sm and 2.72 for Er:Gd which imply that the
CVT separation characteristics also showing the Gd diver-
gence for the lanthanoid elements. In addition, the largest
separation factor 6.37 for Er:Y [22] is much larger than that
1
2
ꢀ
1
ꢀ
2
Ln2O3(s) + Ln Cl3(l or s) = LnCl3(l or s) + Ln O3(s)
2
ꢀ
(Ln
ꢁ
(2)
1
.14 for Er:Ho which showed that the separation characteris-
selective complex formation sub-reactions of the produced
tic for Y by CVT method does not depend on the ionic radius
of Y .
chlorides at T = 1300 K:
3+
LnCl3(l) + KCl(l) = KLnCl4(g)
(Ln = Er, Sc, Y, La, Nd, Sm, Gd and Ho)
(3)
Table 2
The largest separation factors, total separation factors between rare earth
elements Er, Sc, Y, La, Nd, Sm, Gd and Ho from their binary oxide mixtures
using a SC-CVT reaction
SFa
FNb
TSFc
Er2O3–Sc2O3
Er2O3–Y2O3
1213.8 (Er:Sc)
9
13.0 (Er:Sc)
1.49 (Er:Y)
6.37 (Er:Y)
12
14
1
.25 (Y:Er)
189.3 (Er:La)
02.7 (La:Er)
100.0 (Er:Nd)
4.3 (Nd:Er)
105.7 (Er:Sm)
6.7 (Sm:Er)
27.8 (Er:Gd)
.02 (Gd:Er)
2.04 (Ho:Er)
.14 (Er:Ho)
Er2O3–La2O3
Er2O3–Nd2O3
Er2O3–Sm2O3
Er2O3–Gd2O3
9
1
1.48 (Er:La)
1.15 (Er:Nd)
2.33 (Er:Sm)
2.72 (Er:Gd)
1.10 (Ho:Er)
1
9
4
1
10
1
1
10
3
4
Er2O3–Ho2O3
a
8
12
1
SF, separation factor.
FN, fraction number.
TSF, total separation factor.
Fig. 2. Separation factors of the Er–Ln (Ln = Y, La, Nd, Sm, Gd and Ho)
resulted from their binary oxide mixtures vs. the ionic radii. (᭹) Ln:Er; (ꢂ)
Er:Ln; (×) TSF of Er:Ln.
b
c

Y.-H. Sun et al. / Journal of Alloys and Compounds 413 (2006) 175–180
179
and selective CVT sub-reactions of the vapor complexes
KLnCl4 along the temperature gradient:
for ErCl3 while Sc2O3 is only 9.7%, and the chlorides yields
ratio is 5.35 for Er:Sc, 1.23 for Er:Y [22], 2.24 for Er:La, 1.04
for Er:Nd, 1.05 for Er:Sm, 1.33 for Er:Gd and 1.05 for Ho:Er
KLnCl4(g) = LnCl3(l or s) + KCl(s or l)
[22]. This agrees with the results of total transport yields of
ErCl3 > ScCl3, ErCl3 > YCl3, ErCl3 > LaCl3, ErCl3 > NdCl3,
ErCl3 > SmCl3, ErCl3 > GdCl3 and HoCl3 > ErCl3 in this
study. As mentioned above, the SC-CVT results is a com-
bination of selective chlorination in the lower temperature
and the complexation and CVT reaction in the higher tem-
perature.
(
Ln = Er, Sc, Y, La, Nd, Sm, Gd and Ho)
The total transported amount of LnCl3 would mainly
(4)
be determined by the gas–solid reaction (1), solid–solid or
liquid–solid reaction (2) and liquid–liquid reaction (3), while
the distribution characteristics of LnCl3 in the reactor tube
would mainly be determined by the vapor complexes selec-
tive CVT sub-reactions (4) along the temperature gradients.
The largest separation factor of 1213.8 for Er:Sc in this
study is closed to that of 1480 for Ho:Sc [21] and 1100 for
Y:Sc [20] by SC-CVT with KCl as complex former, and much
higher than those for the SC-CVT results of 29.9 for Y:Sc [18]
and CVT results 16.9 for Y:Sc [13] mediated by LnAlnCl3n+3,
and 22.26 for Y:Sc by conventional CVT [13] with KCl as
complex former. This might be explained as follows: vapor
complexes LnAlnCl3n+3 for Sc has higher stability than that
for Y [23], while KLnCl4 for Y, Ho and Er has higher sta-
bility than that of Sc [27]. Thus, the vapor pressure ratio of
LnAlnCl3n+3 is 3.13 for Sc:Y, and that of KLnCl4 is 5.29 for
Y:Sc, 3.65 for Ho:Sc and 1.42 for Er:Sc. On the other hand,
however, the selective chlorination ability of Sc2O3 is much
lower than that of Y2O3, Ho2O3 and Er2O3, so the separa-
tion efficiency for Y:Sc mediated by LnAlnCl3n+3 using a
SC-CVT reaction was weaken.
3
.3. Discussion
The complexation ability of KLnCl4 (Ln = Er, Sc, Y, La,
Nd, Sm, Gd and Ho) is determined by the Gibbs free energy
changes forthe formation (ꢀ G )ofcomplexes KLnCl4. D.S.
Mcphail reviewed the data of ꢀ G at 1000 K for the reac-
◦
f
◦
f
−
1
tion LnCl3(l) + KCl(l) = KLnCl4(g) are −106.8 kJ mol for
−
1
−1
KErCl4, −103.0 kJ mol for KScCl4 [27], −121.0 kJ mol
−
1
for KYCl4 [27], −129.0 kJ mol
for KLaCl4 [27] and
for KNdCl4 [27], −126.0 kJ mol
−
1
−1
−
115.0 kJ mol
for
−
1
KGdCl4 [27], −117.0 kJ mol for KHoCl4 [27] and there
no thermodynamic data for KSmCl4 until now. According
◦
to the ꢀ G data, the ratio of equilibrium vapor pressure of
f
3+
the complexes KLnCl4 at 1300 K can be estimated: 1.42 for
Er:Sc, 0.268 for Er:Y, 0.128 for Er:La, 0.468 for Er:Nd, 0.169
for Er:Gd and 0.389 for Er:Ho, indicating that the formation
ability of vapor complexes is in the order of Er > Sc, Er < Y,
Er < La, Er < Nd, Er < Gd and Er < Ho, which is different from
the SC-CVT results of Er > Y, Er > La, Er > Nd and Er > Gd
in this study.
Moreover, although the ionic radius of Er is similar to
that for Ho3+ and Y , Y, Ho and Er located at different period
3+
3
+
of the periodic table, suggesting result the difference of Y in
ionic structure and electronic configuration to the lanthanide
3+
elements, i.e. the stable structure of [Kr] for Y while [Xe]
10
3+
11
3+
4f for Ho and [Xe] 4f for Er which can result in the
separation efficiency of Er:Sc and Ho:Sc higher than that of
Y:Sc mediated by KLnCl4.
On the other hand, according to thermochemical data
[
28], the ratios of equilibrium constants of the carbonthermic
reduction-chlorinationsub-reaction(1)forEr2O3 andLn2O3,
K1(Er)/K1(Ln)}, which are equivalent to the equilibrium
constant of the exchange sub-reaction (2), were calculated to
In the situation of Er–Y [22], the largest separation factors
6.37 for Er:Y in this study is lower than that of 15.3 for Ho:Y
under the same reaction condition [21], this maybe denoted
by the volatility of complex KHoCl4 is higher than that of
KErCl4 which showed in this study and [14]. But the value
is higher than that of 1.54 for Er:Y, 1.11 for Ho:Y mediated
by KLnCl4 in the CVT reaction [14], this maybe explained
as the selective chlorination in the lower temperature 800 K
in this study while 1300 K in [14].
For Er–Ln (Ln = La, Nd, Sm, Gd and Ho [22]) systems,
the largest separation factors Er:Ln in the lower tempera-
ture region or Ln:Er in the higher temperature region listed
in Table 2 are corresponding to the rule that the larger the
difference between the two ionic radius, the greater the sepa-
ration factor of two lanthanide elements from CVT reaction.
Here the separation factors for Er–Nd and Er–Sm do not
agree with the rule mentioned above. This abnormal phe-
nomena for Nd and Sm observed in the Y–Nd [20], Y–Sm
[20], Ho–Nd [21] and Ho–Sm [21] systems too. This can-
not be explained from the point of view of thermodynamics
since there are no thermodynamic data for KSmCl4 until
now. In additional, the temperature region where the largest
{
4
−2
−11
be, 2.93 × 10 for Er/Sc, 7.0 × 10 for Er/Y, 4.48 × 10
−
9
−7
for Er/La, 7.46 × 10 for Er/Nd, 3.42 × 10 for Er/Sm,
−
8
−4
1
.00 × 10 for Er/Gd and 3.64 × 10 for Er/Ho at 800 K
2
−
−7
and 90.91 for Er/Sc, 4.5 × 10
for Er/Y, 3.62 × 10
−
7
−5
for Er/La, 1.33 × 10 for Er/Nd, 4.72 × 10 for Er/Sm,
−7
−4
1
.43 × 10 for Er/Gd and 9.17 × 10 for Er/Ho at 1300 K.
That is, the amount of chlorides formed is expected as
ErCl3 > ScCl3, ErCl3 < YCl3, ErCl3 < LaCl3, ErCl3 < NdCl3,
ErCl3 < SmCl3, ErCl3 < GdCl3 and ErCl3 < HoCl3, some
of which are different from the SC-CVT results:
ErCl3 > YCl3, ErCl3 > LaCl3, ErCl3 > NdCl3, ErCl3 > SmCl3
and ErCl3 > GdCl3. This may be easy to understand from the
fact that KErCl4 is more volatile and thus more stable than
KLnCl4 (Ln = Y, La, Nd and Gd) deduced from the volatility
of the simple LnCl3 (Ln = Er, Y, La, Nd and Gd) [29].
In addition, our preliminary experiment also showed that
the selective chlorination of the raw materials with dry Cl2
gas at T = 800 K for 2 h might result in an yield of 51.8%

1
80
Y.-H. Sun et al. / Journal of Alloys and Compounds 413 (2006) 175–180
separation efficiency occurred for Er–Ln (Ln = Sc, La, Nd
and Sm) was very near to that of Ho–Ln [21] and Y–Ln
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Acknowledgment
This work was supported by the Natural Science Founda-
tion of Hebei province (501272).
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