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Equilibrium distribution of rare earth elements between molten KCl-LiCl eutectic salt and liquid cadmium
56
Journal of Nuclear Materials 185 (1991) 56-65 North-Holland
Equilibrium distribution of rare earth elements between molten KCl-LiCl eutectic salt and liquid cadmium Masahiro Sakata, Masaki Kurata, Takatoshi Hijikata and Tadashi Inoue Central Research Institute
of Electric Power Industry (CRIEPI),
I1 -1 Iwato Kiia, 2-Chome, Komae-shi,
Tokyo 201, Japan
Received 25 February 1991; accepted 30 May 1990
Distribution experiments for several rare earth elements (La, Ce, Pr, Nd and Y) between molten KCl-LiCl eutectic salt and liquid Cd were carried out at 450, 500 and 6tWC. The material balance of rare earth elements after reaching the equilibrium and their distribution and chemical states in a Cd sample frozen after the experiment were examined. The results suggested the formation of solid intermetallic compounds at the lower concentrations of rare earth metals dissolved in liquid Cd than those solubihties measured in the binary alloy system. The distribution coefficients of rare earth elements between two phases (mole fraction in the Cd phase divided by mole fraction in the salt phase) were determined at each temperature. These distribution coefficients were explained satisfactorily by using the activity coefficients of chlorides and me&is in salt and Cd. Both the activity coefficients of metal and chloride caused a much smaller distribution coefficient of Y relative to those of other elements.
1. Introdnction High level radioactive waste (HLW) from reprocessing of LWR spent fuel contains a small amount of long-lived nuclides, mainly transuranium elements (TRUs) which remain radioactive for more than one million years. If TRUs could be separated from HLW and transformed to short-lived nuclides, not only would waste management be much simpler but also it will be easier to obtain public support for nuclear power generation. Central Research Institute of Electric Power Industry (CRIEPI) has proposed a concept to separate TRUs from HLW by p~ornet~lur~c~ processing and then to transmute them to short-lived nuclides in a metallic fuel FBR [l]. The pyrometallurgical partitioning process is expected to produce a lower amount of secondary radioactive wastes and to favor compact facilities as compared with conventional aqueous processes using solvent extraction, ion exchange, etc., although products with low purity are generally recovered by pyrometallurgical processes [l]. Some processes based on reductive extraction and electrorefining in a molten salt/liquid metal system have been proposed and studied for the reprocessing of nuclear reactor fuels [Z-5] and for the partitioning of radioac-
tive wastes [k-9]. The pyrometallur~c~ partitioning process proposed by CRIEPI consists of (I) microwave heating de&ration to oxides, (2) chlorination to convert oxides to chlorides, (3) reductive extraction to separate TRUs from molten chlorides using liquid Cd-Li and (4) electrorefining to increase the purity of recovered TRUs [I}. Both the denitration and chlorination steps are preprocessing for p~tition~g. In order to evaluate the purity of TRUs recovered by reductive extraction, it is necessary to obtain data concerning separation of TRUs and rare earth elements from each other, because rare earth elements are chemically similar to TRUs and also because the rare earth elements in HLW are about 10 times as large as the TRUs [l]. Prior to separation experiments using TRUs, distribution experiments for several rare earth elements (La, Ce, Pr, Nd and Y) between molten KCl-LiCl eutectic salt and liquid Cd were carried out at 450, 500 and 600°C. The material balance of rare earth elements after reaching the eq~~b~urn and their distribution and chemical states in a Cd sample frozen after the experiment were examined, and data on solubihties of rare earth metals and formation of intermetallic compounds (RECd,) in Cd were obtained. The distribution coefficients of rare earth elements between two phases (mole fraction in the Cd phase divided by mole fraction in the
0022-3115/91/$03.50 0 1991 - Elsevier Science Publishers 3.V. All rights reserved
M. Sakata et al. / Equilibrium distribution of rare earth elements salt phase) were determined at each temperature. The equilibrium distribution of rare earth elements was also analyzed thermodynamically based on the activity coefficients of chlorides and metals in salt and Cd.
/
2. Experimental
Samphng tube (PYREX glass)
2.1. Reagents Polarographic grade KCl-LiCl eutectic salt (41-59 mol%) and unhydrated rare earth trichlorides obtained from Anderson Physics Laboratories were used for the experiment at 450°C. For the experiments at 500 and 6OO”C, mixtures of those reagent grade salts were prepared and purified by bubbling high-purity HCl gas [lo]. Cadmium metal with a purity of 99.999% was melted at 5OO’C along with KCl-LiCl eutectic salt in an argon-atmosphere glove box for removal of oxides, and then the surface of the metal was carefully polished after cooling. Li-Cd alloy (15-85 mol%) used as the reductant was prepared by melting the appropriate amounts of Li and Cd at 500°C along with KCl-LiCl eutectic salt in a glove box. The surface of this alloy was also carefully polished after cooling. Fig. 1. Schematic diagram of apparatus for distribution experiment.
2.2. Apparatus The distribution experiment was carried out by setting the apparatus shown in fig. 1 in the furnace well of an argon-atmosphere glove box. The purity of the glove box atmosphere was maintained by circulating argon gas through copper metal and molecular sieve columns for removal of oxygen and water, respectively. The oxygen content and dew point were maintained less than 0.5 ppm and - 7O”C, respectively.
2.3. Procedure The experimental conditions in this study are summarized in table 1. The temperature of the system was selected from the temperature range in which the liquid phases of salt and Cd exist without severe volatilization loss. The composition of rare earth chlorides in salt was
Table 1 Experimental conditions Run no.
Weight of salt or Cd (g)
Composition of salt (mol%)
Temp. (“C)
1
Salt: 36.52 Cd: 212.63
KCl-LiC1-LaC1,-CeC1,-PrCI,-NdCI,-YCI, 97.45 0.49 0.54 0.49 0.45 0.58
450
2
Salt: 40.07 Cd: 174.20
KCl-LiCl-LaCl,-CeCl,-YCl, 91.86 0.85 0.59 0.70
500
3
Salt: 25.63 Cd: 172.37
KCI-LiCI-LaCl,-CeCl,-YCl, 98.37 0.70 0.42 0.51
600
58
M. Sakata et al. / Equilibrium distribution of rare earth elements
decided, based on the conditions such as the solubilities of rare earth metals in Cd and the concentration dependence of activity coefficients for rare earth chlorides in salt. In the experiments of runs 2 and 3, the distribution of Pr and Nd between salt and Cd was not examined, because those elements were found to have almost the same distribution coefficient as Ce based on the results of run 1. The weighed salt and Cd were loaded in an alumina crucible (SSA-S) which was placed at the bottom of the apparatus as illustrated in fig. 1. The system was then heated to the desired temperature. After melting had occurred, an agitator was lowered into the Cd phase, and the melt was stirred continuously at 75 rpm. The distribution of rare earth elements was controlled by the incremental addition of the Li-Cd alloy to the system. Preliminary experiments indicated that distribution equilibrium is attained in less than 4 h after addition of reductant when the Cd phase is not saturated with rare earth metals. However, when the Cd phase was saturated with rare earth metals, an equilibrium period of at least 8 h was required to ensure the attainment of equilibrium. After the attainment of equilibrium, the samples of salt and Cd were taken by inserting a Pyrex glass tube into the salt or Cd phase and then by withdrawing into the tube with a glass syringe. The salt sample was dissolved in water, while the Cd sample was washed with water for removal of salt contamination, and then was dissolved with nitric acid while heating at about 100°C. Rare earth elements and Cd in the solutions were analysed by ICP atomic emission spectrometry and Li by atomic absorption spectrometry. In the experiment at 450°C (run l), the crucible was taken out of the furnace well immediately after the experiment, and was cooled at room temperature. The frozen sample was treated with water to dissolve the salt. However, as the salt contained a small amount of water-insoluble solid, the solid was separated by filtration through a 0.45 urn filter, and was dissolved with nitric acid while heating at about 1OOT. The elements of both the water-soluble and insoluble fractions of the salt were analyzed by the methods described previously, and the Cl concentration of the water-insoluble fraction was also determined by Mohr’s method. On the other hand, the metal was cut vertically into two sections, and small samples were taken from 13 points (fig. 2) of the cross section of the Cd for analyses. These samples were dissolved and analysed by the methods described previously. X-ray powder diffraction measurements were carried out on both the water-insoluble fraction of salt and Cd to examine the chemical states of rare earth ele-
Fig. 2. Cross section of Cd sample and its sampling points for analyses.
ments of each sample. The cross section of the Cd was examined by scanning electron microscopy with energy dispersive X-ray spectrometry.
3. Results and discussion 3.1. Material balance after distribution equilibrium and formation of intermetallic compounds in Cd In order to clarify the distribution behavior of rare earth elements between molten KCl-LiCl eutectic salt and liquid Cd, the material balances of those elements after distribution equilibrium were examined in the experiments of runs 1 (450°C) and 2 (5OOT). These results are shown in figs. 3 and 4. The total amounts of rare earth elements which were present in the system at each stage were calculated by subtracting the loss caused by sampling of salt and Cd from the initial total amounts in the system. Figs. 3 and 4 indicate that except for the case of the higher amounts of Li consumed, the decrease of rare earth elements in salt and their increase in Cd occur with increasing amount of Li consumed, and the sum of the amounts of rare earth elements in salt and Cd is close to the total amounts present in the system. However, the material balances for the rare earth elements except for Y in run 1 are slightly worse compared with those in run 2, which may be due to some experimental uncertainty. On the other hand, for the higher amounts of Li metal consumed by reduction, the discrepancy between the sum of the amounts of rare earth elements in salt and Cd and their total amounts in the system became much larger, because amounts of rare earth elements equivalent to their decreased amounts in salt didn’t appear in Cd. Figs. 5 and 6 show the relationship between the change (equivalent) of all rare earth elements in salt and Cd and the amount (equivalent) of Li consumed in runs
M. Sakata et al. / Equilibrium distribution of rare earth elements
1 and 2, respectively. The results indicate that the decreased amount of rare earth elements in salt is well balanced by the amount of Li consumed, suggesting that the decrease is due to the reduction of chlorides to metals by Li metal. On the other hand, the increase of rare earth elements in Cd was significantly lower than their decrease in salt or the amount of Li consumed when the higher amounts of Li metal were consumed by reduction. This imbalance may be due to the removal of rare earth metals from the liquid phases of salt and/or Cd with the formation of solids. The salt obtained after the experiment of run 1 (450°C) contained a small amount of water-insoluble solid. Chemical analyses and X-ray diffraction measure-
59
ment of this solid showed that almost all of the rare earth elements in the solid are present in the form of oxychlorides but not in the form of metals or intermetallic compounds. Based on the chemical analyses, the amounts of these oxychlorides produced during the experiment were estimated to be less than 1% for each rare earth element except for Y and about 8% for Y of the total amount of each element in the system. The relatively high percentage for Y is probably due to the contamination of the glove box atmosphere with air during the final stage of the experiment, because only Y was present in the salt phase during the final stage (fig. 3). Thus, the imbalance described earlier can not be attributed to the formation of oxychlorides.
100 Boo A
80
0~0
0
A
-
q 0
AA
60
0 A
-
OBP
La 40
~~~
-
A o
A 0
20
-
0
3
H
O.
A
’
n
IA*
0
60
-
40
-
20
-
rA,A4.;
Y
bAAOo
40
0
t
OA
‘:jo( .
A
60
B
z
0
0
0
0 0
0
A
0
A
0
0 t100
80
q
-
A
q
=u
00~0
40
-
&
D
/::I:,::,::,
oo #.I 0 0
20-
(mol)
EJu
O0
-
Amount of Li consumed
q
A
60
nnn n 0.00.0,
0 Sum of amounts of RE in salt and Cd
A A
0
A
0
A
0
h A . 0
0.01
0.02
Amount of Li consumed
0.03
0.04
(mol)
Fig. 3. Change in material balance of rare earth elements with increasing amount of Li consumed
in run 1 (450°C).
M. Sakata et al. / Equilibrium distribution of rare earth elements
60 n
100
ti”-
-
.
y
100
q
0
qg*n
13
ll Bsi
AA A
80
0
0
-
A
80 A
5s B z
La
-
60
0
40
B
o
A
S
O0
lmn”
80 i? H 5 8
q
IJ
A
.
Ce
-
I
q
0
0
0.
0.02
0.01
Amount of Li consumed
0.03 (mol)
[
0 Amount of RE in Cd
A
0 Sum of amounts of RE in salt and Cd
A
0 OO
A
0
0,
A Amount of RE in salt
0
0
n.
0
0
0
8
40 20
-
q
A
60
40-
0 q
'A
-
y
o-O_
0 100
60-
20
A
0 no
s
o
0 20
73
A
0.02
0.01
0.03
Amount of Li consumed (mol)
Fig. 4. Change in material balance of rare earth elements with increasing amount of Li consumed in run 2 (SOO°C).
Table 2 indicates the distribution of rare earth elements in the cross section of the Cd frozen after the experiment of run 1 (450°C). The rare earth elements 0.04
except for Y were considerably enriched in the lower part, that is, near the bottom of crucible. On the other hand, Y was distributed uniformly in the whole cross 0.03 /’
is
,./’
,.d
0.03
..g.‘.
0.02 ,,/
0.02
.’
0
0
0
..2i’-. .. 0 .
0.01
./....
0.01
6”’
$,q$
+..,
o
.**
,
0
0.01
,
0.02
Amount of Li consumed
I
0 Increased A Decreased
,
0.03
P
0 0.04
(equivalent)
0.01
0.02
0.03
Amount of Li consumed (equivalent)
0 Increased
amount of RE in Cd
A Decreased
amount of RE in salt
Fig. 5. Relationship between amounts of rare earth changed in salt and Cd and amount of Li consumed (45OT).
0
.
elements in run 1
amount of RE in Cd amount of RE in salt
I
Fig. 6. Relationship between amounts of rare earth elements changed in salt and Cd and amount of Li consumed in run 2 (500°C).
A4. Sakata et al. / Equilibrium distribution ofrare earth elements section of Cd relative to other elements. Scanning electron microscope of the cross section of the Cd sample showed that large grains were segregated in the Cd matrix of the lower part. Energy dispersive X-ray analyses showed that these grains contained a greater amount (- 10 wt’%) of rare earth elements than the Cd matrix. Intermetallic compounds of the type RECd,, were identified in the lower part of Cd by X-ray diffraction. This result shows that La, Ce, Pr and Nd are enriched in the form of RECd,, in the lower part, because those elements form intermetallic compounds of this type in Cd-rich solutions [ll]. Although Y forms YCd, in Cd-rich solutions [12], YCd, in the Cd sample was not detected by X-ray diffraction. Thus, the results for the Cd sample indicate that the rare earth elements except for Y formed intermetallic compounds of the type RECd,, which were enriched in the lower part of Cd, during the experiment. Thus, it is reasonable to assume that the imbalance described previously is due to the deposition of rare earth metals as RECd,, in the bottom of the crucible. The imbalance for Y may be also caused by the formation of YCd, or other types of intermetallic compounds, although whether this is the case remains to be confirmed more exactly through experiments. Table 3 indicates the concentrations of rare earth elements in the Cd sample at the time when their material balances became worse in the experiments of runs 1 and 2. For convenience, these concentrations are assumed to be identical to those at the time when the sum of the amounts of rare earth elements in the salt and Cd became less than 80% for run 1 and less than 90% for run 2 relative to the total amounts present in the system, as can be seen by figs. 3 and 4. The solubilities of rare earth metals in Cd which were measured in the binary alloy system [11,12] are also included in this table. The results show that the concentrations of all rare earth elements in Cd at the time when their material balance became worse, that is, at
Table 3 Concentrations their material
of rare earth elements balance became worse Element
Concentration in Cd (wtW) ‘)
Solubility in Cd (wtW) b,
1 (45O’C)
La Ce Pr Nd Y
0.089 0.127 0.122 0.123 0.048
0.15 0.22 0.38 0.65 0.68
2 (500°C)
La Ce Y
0.312 0.262 0.187
0.40 0.75 1.1
a) Assumed to be identical to the concentrations at the time when the sum of the amounts of rare earth elements in salt and Cd became < 80% for run 1 and < 90% for run 2 of their total amounts in the system. b, Published values in the binary alloy system [11,12].
the time when the formation of intermetallic compounds is regarded to have occurred, are significantly lower than the solubilities measured in the binary alloy system [11,12]. This suggests lower solubility of each rare earth metal in Cd by the presence of other metals. Thus, the coexistence of a relatively small amount of rare earth metals appears to have a large effect on the solubility of each metal in liquid Cd. This indicates that a detailed investigation of solubility is necessary for the development of a liquid-liquid extraction system. 3.2. Determination earth elements
in a cross
section
of distribution
coefficients for rare
At a given temperature, the distribution of a trivalent rare earth element (RE) between molten KCl-LiCl eutectic salt and liquid Cd can be expressed by the following general reaction: REC~W,~
K elements
in Cd at the time when
Run no.
+ 3Li (cd) *
The equilibrium Table 2 Distribution of rare earth (run 1: 45O’C) (in wtW)
61
a RE a
RE =
3 LiCl
constant =
aREcl,atj
of Cd
R&Z,,
+
(K,,)
YRE xREY?icl YREcI,XREcI~~~i
3LiCl
(saltj.
can be written
(1)
as
x~ic~ xl!i
in which
Element
Upper part (n = 5)
Central (n=5)
part
Lower part (n=3)
La Ce Pr Nd Y
0.121 *to.048 0.144 f 0.062 0.127kO.055 0.122 f 0.051 0.057 f 0.007
0.071 0.090 0.085 0.087 0.050
0.041 0.049 0.046 0.043 0.014
0.411 0.433 0.362 0.247 0.065
f f f f f
f f f f f
0.052 0.043 0.049 0.080 0.004
a, X and y denote the activity and the mole fraction and the activity coefficient, respectively. If the respective activity coefficients are constant in the system investigated, an apparent equilibrium constant (Kk,) is given by
K’RE
YREcI;/?i =
Key
-YREYLiCl
xRE =
xf?icl
xREc.,x;i
(3)
62
M. Sakata et al. / Equilibrium distribution
-2
-3 -4.2
-4.0
-3.8
-3 6
-3.2
-3.4
-3.0
-2.8
Log DLi O:La
Log DL~ q 10.4 + 3.10 Log Du
n:Ce
Log Dee = 10.5 + 3.01 Log Du
0: Pr
Log DP~ q 10.9 + 3.08 Log Du
l : Nd r:Y
i_Og DNd = 10.5 + 2.98 Log DLI Log DY = 8.3 + 2.94 Log DL,
Fig. 7. Relationship between distribution coefficients of rare earth elements and Li in run 1 (45OOC). Solid lines were obtained by the least-squares method in the range of the experimental data showing a good material balance.
When the distribution
coefficients
X RE Dam=
X
RECI,
tremely low concentrations of Li and rare earth elements. As can be seen by figs. 7 to 9, plots of log D,, versus log DLi at run 1 (45OOC) and run 2 (500°C) give straight lines having a slope of around 3. This indicates that these rare earth elements exist as trivalent species in the salt phase, and that the assumptions made regarding activity coefficients in the deviation of eq. (5) are reasonable in the system investigated. However, the slopes of the regression lines obtained for run 3 (600°C) are in the range of 2.44 to 2.68 (fig. 9), which is 10 to 20% less than the value of 3 expected for trivalent chlorides. This discrepancy may be due to the relatively large uncertainty caused by having only three data points. Table 4 summarizes the apparent equilibrium constants (K&) and the separation factors (Ce basis, i.e. Kk,/K&) which were obtained from the least-squares fits of eq. (5) to the measured distribution coefficients (figs. 7 to 9). The results indicate that the order of KkE values for the rare earth elements is Ki, = K& = KG, > K[, > K;. Thus, the extractabilities of Pr, Nd and Ce are nearly equal, and Y is the least extractable element.
1
, ..’
__.....” 0
G-
_,
-
as
XL, and
eq. (3) can be written log D,,
are defined
oft-are earth elements
= 3 log D,,
(4)
D,;=x,i~l’
in logarithmic + log KiE.
form as (5)
Thus, if the assumptions made regarding the activity coefficients are valid, a plot of log D,, versus log DLi should give a straight line having a slope of 3 for trivalent chlorides. These data plots are shown in figs. 7 to 9. The analytical values of rare earth elements in the Cd sample are assumed to contain some amount of solid phase when the intermetallic compounds were formed, because filtered samples of the salt and metal phases were not taken in this experiment. Therefore, the regression lines between log D,, and log D,, were obtained by least-squares method in the range of the experimental data showing a good material balance. Some data were excluded in this calculation because of the large experimental errors for the Cd samples highly contaminated with salt and for the samples with ex-
-4
1 -4.0
-3.8
-3.6
-3.4
-3 2
Log DLi
0: La
Log DL~ = 10.1 + 3.05 Log Du
n:Ce
Log Dee = 10.8 + 3.11 Log Du
q :Y
Log Dv = 8.5 + 3.02 Log
Du J
Fig. 8. Relationship between distribution coefficients of rare earth elements and Li in run 2 (SOOT). Solid lines were obtained
by least-squares method in the range of the experimental data showing a good material balance.
63
M. Sakata et al. / Equilibrium distribution of rare earth elements
regularly with increasing temperature. Since the separation factor of Y on the basis of La also has such a temperature dependence, the separation of Y and other rare earth elements from each other becomes more difficult with increasing temperature. 3.3. Comparison of distribution coefficients for rare earth elements calculated from activity coefficients with their experimental values
-3.4
-3.6
-2.8
-3.0
-3.2
The standard free energy change general reaction (1) is given by
(AC ” ) for
the
AGo = 3 AGP,icl-
log K,,,
(6)
AGPRECl, = -2.3RT
where AC&, and AC&,, are the standard free energies of formation of Lid1 and rare earth chloride (RECI,), respectively. By combining eqs. (3), (5) and (6), eq. (7) is obtained:
Log Dt_i
3
H”i
log DR,=
310g D,i+
logs
Yi?iCI
2.3RT Fig. 9. Relationship between distribution coefficients of rare earth elements and Li in run 3 (6OO’C).
This can be explained satisfactorily by using the activity coefficients of chlorides and metals in salt and Cd phases, as discussed in the next section. The KiE values for Ce and Y at 500°C are similar to those (KG, = 10.2; KG = 8.1) calculated from the distribution data in a similar LiCl-KCl-NaCl/Cd system which was measured by Lewis and Johnson [13]. Table 4 also shows that the separation factor (Ce basis) for Y increases
Table 4 Apparent equilibrium constants basis) for rare earth elements
and separation
Element
log K;E
Separation factor
450
La Ce Pr Nd Y La Ce Y La Ce Y
10.0 10.5 10.6 10.5 8.5 10.0 10.4 8.4 8.9 9.3 7.7
0.36 1.0 1.3 1.1 9.6x10V3 0.37 1.0 1.1x10-2 0.39 1.0 2.0x10-2
600
(7)
’
in which the second, third and forth terms in the right side are related to the activity coefficients in Cd phase, the activity coefficients in salt phase and the standard free energy change, respectively. When the activity coefficients and the standard free energies of formation are known, the distribution coefficients (D,,) for rare earth elements can be calculated from eq. (7). This calculation was carried out using the appropriate thermodynamic data had been reported, and the results were compared with the experimental values obtained by the present study. The calculation for Nd was not made because of some uncertainty of the activity coefficients of NdCl, in salt which is probably caused by the coexistence of the divalent chloride salt, as noted by McCoy et al. [14].
factors (Ce
Temp. ( ’ C)
500
YREU, +'Og-
Table 5 Standard free energies of formation of chlorides and activity coefficients of chlorides and metals in KCl-LiCl eutectic salt and Cd for calculation of distribution coefficients for rare earth elements at 450°C Element La
Ce Pr Y Li
- AC; (kcal/mol)
209.4 205.0 206.0 196.3 82.7
[14]
y of chloride [14,16]
y of metal
5.0x 10-3 1.4x10-3 3.3x10-3 6.3~10~” 0.63
4x10-‘9 5x10-‘0 2x10-9 3x10-7
U4J51
1x1o-3
64
M. Saknta et al. / Equilibrium
-
distribution
ofrare earth
elements
La
-4.2 -4.0 -3.6 -3.6 -3.4 -3.2 -3.0 -2.6
-4
Log DL,
6
0
Experimental value
-.-.-
Calculated value in case 1 : All
_____
Calculated value in case 2 : Y in Cd from values in table 5 , Y in salt = 1 Calculated value in case 3 : All Y from values in table 5
_
Y = I
-4 6
-4.2
-4.0
-3.8
-3.6
-3.4
-3.2
-3.0
-2.8
Log DL, Fig. 10. Comparison of distribution coefficients for rare earth elements calculated from activity
coefficients
with their experimental
values.
Table 5 summarizes the standard free energies of formation of chlorides and the activity coefficients of chlorides and metals in KCl-LiCl eutectic salt and Cd at 450°C which were used for the calculation of the distribution coefficients for rare earth elements. The free energies and activity coefficients of chlorides are given in the table on the basis of the hypothetical liquid standard state [14]. The activity coefficients of chlorides except for LiCl and metals were measured electrochemically by McCoy et al. [14] and Matsumoto et al. [15], while the activity coefficient of LiCl was calculated from the partial molar excess free energy given by Lumsden [16]. These activity coefficients were assumed
to be constant in this calculation, independent of the concentration of each constituent in the salt or Cd phase, based on the measurements of the concentration dependence of activity coefficients [14,15]. The calculated results are shown in fig. 10, along with the experimental values. First, when all activity coefficients are set equal unity (case l), to the calculated distribution coefficients are 2 to 7 orders of magnitude higher than the experimental values, and shows that Y is the most easily reduced element in spite of the opposite experimental values. Second, when only the activity coefficients of rare earth and Li metals in Cd phase are taken into account (case 2) the calculated
M, Sakata et al. / Equilibrium distribution of rare earth elements
value for Y is much lower than case 1, but there is no significant difference between both the calculated values for other elements. Thirdly, when all activity coefficients are set equal to the values given in table S (case 3), a better agreement is observed between the calculated and experimental values for the rare earth elements. Thus, the distribution coefficients for those elements can be explained satisfactorily by using the activity coefficients of metals and chlorides in Cd and salt, showing that this calculation is useful for the prediction of separation of rare earth elements. It is clear that both the activity coefficients of metal and chloride cause a large downward shift for the distribution coefficient of Y relative to those of other elements, thus Y is the least reduced.
65
rare earth elements. Both the activity coefficients of metal and chloride caused a much smaller distribution coefficient of Y relative to those of other elements.
Acknowledgements We thank H. Miyashiro, T. Matsumoto and Y. Sakamura of CRIEPI and T. Kusakabe of Su~tomo Metal Mining Co., Ltd. for their cooperation in this study. We are indebted to K. Kugimiya of Techno Service Co., Ltd. for his technical assistance. We also thank R. K. Kawaratani for critically reading the manuscript.
References 4. Conclusions The following conclusions can be drawn in the present study on the equilibrium distribution of rare earth elements such as La, Ce, Pr, Nd and Y between molten KCl-LiCl eutectic salt and liquid Cd. (1) It is likely that the solid intermetallic compounds
such as RECd,, were formed at the lower concentrations of rare earth metals dissolved in liquid Cd than those solubilities measured in the binary alloy system. This suggests that the coexistence of a relatively small amount of rare earth metals has a large effect on the solubility of each metal in liquid Cd. (2) For each temperature, the distribution coefficients (mole fraction in the Cd phase divided by mole fraction in the salt phase) for rare earth elements and Li obeyed the relations~p 13,, = Dti K&n, in which Kin is the apparent equilibrium constant. This shows that the rare earth elements exist as trivalent species in the salt phase, and that the respective activity coefficients are almost constant in the system investigated. The temperature dependence for the distribution coefficients shows that the separation of Y and other rare earth elements from each other becomes more difficult with increasing temperature. (3) The extractabilities of Ce, Pr and Nd were nearly equal, and Y was the least extractable element. This could be explained satisfactorily by using the activity coefficients of chlorides and metals in salt and Cd phases, showing that the calculation using the activity coefficients is useful for the prediction of separation of
[l] T. Inoue, M. Sakata, H. Miyashiro, T. Matsumura, A. Sasahara and N. Yoshiki, Nucl. Technol. 69 (1991) 206. [2] L.M. Ferris, J.C. Mailen, J.J. Lawrance, F.J. Smith and E.D. Nogueira, J. Inorg Nucf. Chem. 32 (1970) 2019. [3] J. Oishi, H. Moriyama, K. Moritani, S. Maeda, M. Miyazaki and Y. Asaoka, J. Nucl. Mater. 154 (1988) 163. [4] I. Johnson, J. Nucl. Mater. 154 (1988) 169. [5] DC. Wade and Y.I. Chang, Nucl. Sci. Eng. 100 (1988) 507. [6] M.S. Coops and D.H. S&on, in: Actinide Recovery from Waste and Low-Grade Sources, Eds. J.D. Navratii and W.W. Schulz (Harwood Academic Publishers, Chur, 1982) p. 333. [7] G.A. Jensen, A.M. Platt, G.B. Mellinger and W.J. Bjorklurid, Nucl. Technol. 65 (1984) 305. [8] K. Naito, T. Mats& and Y. Tanaka, J. Nucl. Sci. Technoi. 23 (1986) 540. 191 H.Moriyama, K. Kinoshita, Y. Asaoka, K. Moritani and Y. Ito, J. Nucl. Sci. Technol. 27 (1990) 937. [lo] K. Kim, Y. Sato K. Shimakage and T. Ejima, J. Japan. Inst. Met. 51 (1987) 630 (in Japanese). [ll] R.P. Elliott, Constitution of Binary Alloys, First Supplement (McGraw-Hill, New York, 1985). [12] F.A. Shunk, Constitution of Binary Alloys, Second Supplement (McGraw-Hill, New York, 1985). [13] M.A. Lewis and T.R. Johnson, J. Electroehem. Sot. 137 (1990) 1414. [14] L.R. McCoy et al., to be submitted for publication. [15] T. Matsumoto, H. Miyashiro and Y. Sakamura, CRIEPI Report, Central Research Institute of Electric Power Industry, No. T89913 (May 1990) (in Japanese). [I61 J. Lurnsden, Thermodynamics of Molten Salt Mixtures (Academic Press, London, 1966) p.47.
Journal of Nuclear Materials 185 (1991) 56-65 North-Holland
Equilibrium distribution of rare earth elements between molten KCl-LiCl eutectic salt and liquid cadmium Masahiro Sakata, Masaki Kurata, Takatoshi Hijikata and Tadashi Inoue Central Research Institute
of Electric Power Industry (CRIEPI),
I1 -1 Iwato Kiia, 2-Chome, Komae-shi,
Tokyo 201, Japan
Received 25 February 1991; accepted 30 May 1990
Distribution experiments for several rare earth elements (La, Ce, Pr, Nd and Y) between molten KCl-LiCl eutectic salt and liquid Cd were carried out at 450, 500 and 6tWC. The material balance of rare earth elements after reaching the equilibrium and their distribution and chemical states in a Cd sample frozen after the experiment were examined. The results suggested the formation of solid intermetallic compounds at the lower concentrations of rare earth metals dissolved in liquid Cd than those solubihties measured in the binary alloy system. The distribution coefficients of rare earth elements between two phases (mole fraction in the Cd phase divided by mole fraction in the salt phase) were determined at each temperature. These distribution coefficients were explained satisfactorily by using the activity coefficients of chlorides and me&is in salt and Cd. Both the activity coefficients of metal and chloride caused a much smaller distribution coefficient of Y relative to those of other elements.
1. Introdnction High level radioactive waste (HLW) from reprocessing of LWR spent fuel contains a small amount of long-lived nuclides, mainly transuranium elements (TRUs) which remain radioactive for more than one million years. If TRUs could be separated from HLW and transformed to short-lived nuclides, not only would waste management be much simpler but also it will be easier to obtain public support for nuclear power generation. Central Research Institute of Electric Power Industry (CRIEPI) has proposed a concept to separate TRUs from HLW by p~ornet~lur~c~ processing and then to transmute them to short-lived nuclides in a metallic fuel FBR [l]. The pyrometallurgical partitioning process is expected to produce a lower amount of secondary radioactive wastes and to favor compact facilities as compared with conventional aqueous processes using solvent extraction, ion exchange, etc., although products with low purity are generally recovered by pyrometallurgical processes [l]. Some processes based on reductive extraction and electrorefining in a molten salt/liquid metal system have been proposed and studied for the reprocessing of nuclear reactor fuels [Z-5] and for the partitioning of radioac-
tive wastes [k-9]. The pyrometallur~c~ partitioning process proposed by CRIEPI consists of (I) microwave heating de&ration to oxides, (2) chlorination to convert oxides to chlorides, (3) reductive extraction to separate TRUs from molten chlorides using liquid Cd-Li and (4) electrorefining to increase the purity of recovered TRUs [I}. Both the denitration and chlorination steps are preprocessing for p~tition~g. In order to evaluate the purity of TRUs recovered by reductive extraction, it is necessary to obtain data concerning separation of TRUs and rare earth elements from each other, because rare earth elements are chemically similar to TRUs and also because the rare earth elements in HLW are about 10 times as large as the TRUs [l]. Prior to separation experiments using TRUs, distribution experiments for several rare earth elements (La, Ce, Pr, Nd and Y) between molten KCl-LiCl eutectic salt and liquid Cd were carried out at 450, 500 and 600°C. The material balance of rare earth elements after reaching the eq~~b~urn and their distribution and chemical states in a Cd sample frozen after the experiment were examined, and data on solubihties of rare earth metals and formation of intermetallic compounds (RECd,) in Cd were obtained. The distribution coefficients of rare earth elements between two phases (mole fraction in the Cd phase divided by mole fraction in the
0022-3115/91/$03.50 0 1991 - Elsevier Science Publishers 3.V. All rights reserved
M. Sakata et al. / Equilibrium distribution of rare earth elements salt phase) were determined at each temperature. The equilibrium distribution of rare earth elements was also analyzed thermodynamically based on the activity coefficients of chlorides and metals in salt and Cd.
/
2. Experimental
Samphng tube (PYREX glass)
2.1. Reagents Polarographic grade KCl-LiCl eutectic salt (41-59 mol%) and unhydrated rare earth trichlorides obtained from Anderson Physics Laboratories were used for the experiment at 450°C. For the experiments at 500 and 6OO”C, mixtures of those reagent grade salts were prepared and purified by bubbling high-purity HCl gas [lo]. Cadmium metal with a purity of 99.999% was melted at 5OO’C along with KCl-LiCl eutectic salt in an argon-atmosphere glove box for removal of oxides, and then the surface of the metal was carefully polished after cooling. Li-Cd alloy (15-85 mol%) used as the reductant was prepared by melting the appropriate amounts of Li and Cd at 500°C along with KCl-LiCl eutectic salt in a glove box. The surface of this alloy was also carefully polished after cooling. Fig. 1. Schematic diagram of apparatus for distribution experiment.
2.2. Apparatus The distribution experiment was carried out by setting the apparatus shown in fig. 1 in the furnace well of an argon-atmosphere glove box. The purity of the glove box atmosphere was maintained by circulating argon gas through copper metal and molecular sieve columns for removal of oxygen and water, respectively. The oxygen content and dew point were maintained less than 0.5 ppm and - 7O”C, respectively.
2.3. Procedure The experimental conditions in this study are summarized in table 1. The temperature of the system was selected from the temperature range in which the liquid phases of salt and Cd exist without severe volatilization loss. The composition of rare earth chlorides in salt was
Table 1 Experimental conditions Run no.
Weight of salt or Cd (g)
Composition of salt (mol%)
Temp. (“C)
1
Salt: 36.52 Cd: 212.63
KCl-LiC1-LaC1,-CeC1,-PrCI,-NdCI,-YCI, 97.45 0.49 0.54 0.49 0.45 0.58
450
2
Salt: 40.07 Cd: 174.20
KCl-LiCl-LaCl,-CeCl,-YCl, 91.86 0.85 0.59 0.70
500
3
Salt: 25.63 Cd: 172.37
KCI-LiCI-LaCl,-CeCl,-YCl, 98.37 0.70 0.42 0.51
600
58
M. Sakata et al. / Equilibrium distribution of rare earth elements
decided, based on the conditions such as the solubilities of rare earth metals in Cd and the concentration dependence of activity coefficients for rare earth chlorides in salt. In the experiments of runs 2 and 3, the distribution of Pr and Nd between salt and Cd was not examined, because those elements were found to have almost the same distribution coefficient as Ce based on the results of run 1. The weighed salt and Cd were loaded in an alumina crucible (SSA-S) which was placed at the bottom of the apparatus as illustrated in fig. 1. The system was then heated to the desired temperature. After melting had occurred, an agitator was lowered into the Cd phase, and the melt was stirred continuously at 75 rpm. The distribution of rare earth elements was controlled by the incremental addition of the Li-Cd alloy to the system. Preliminary experiments indicated that distribution equilibrium is attained in less than 4 h after addition of reductant when the Cd phase is not saturated with rare earth metals. However, when the Cd phase was saturated with rare earth metals, an equilibrium period of at least 8 h was required to ensure the attainment of equilibrium. After the attainment of equilibrium, the samples of salt and Cd were taken by inserting a Pyrex glass tube into the salt or Cd phase and then by withdrawing into the tube with a glass syringe. The salt sample was dissolved in water, while the Cd sample was washed with water for removal of salt contamination, and then was dissolved with nitric acid while heating at about 100°C. Rare earth elements and Cd in the solutions were analysed by ICP atomic emission spectrometry and Li by atomic absorption spectrometry. In the experiment at 450°C (run l), the crucible was taken out of the furnace well immediately after the experiment, and was cooled at room temperature. The frozen sample was treated with water to dissolve the salt. However, as the salt contained a small amount of water-insoluble solid, the solid was separated by filtration through a 0.45 urn filter, and was dissolved with nitric acid while heating at about 1OOT. The elements of both the water-soluble and insoluble fractions of the salt were analyzed by the methods described previously, and the Cl concentration of the water-insoluble fraction was also determined by Mohr’s method. On the other hand, the metal was cut vertically into two sections, and small samples were taken from 13 points (fig. 2) of the cross section of the Cd for analyses. These samples were dissolved and analysed by the methods described previously. X-ray powder diffraction measurements were carried out on both the water-insoluble fraction of salt and Cd to examine the chemical states of rare earth ele-
Fig. 2. Cross section of Cd sample and its sampling points for analyses.
ments of each sample. The cross section of the Cd was examined by scanning electron microscopy with energy dispersive X-ray spectrometry.
3. Results and discussion 3.1. Material balance after distribution equilibrium and formation of intermetallic compounds in Cd In order to clarify the distribution behavior of rare earth elements between molten KCl-LiCl eutectic salt and liquid Cd, the material balances of those elements after distribution equilibrium were examined in the experiments of runs 1 (450°C) and 2 (5OOT). These results are shown in figs. 3 and 4. The total amounts of rare earth elements which were present in the system at each stage were calculated by subtracting the loss caused by sampling of salt and Cd from the initial total amounts in the system. Figs. 3 and 4 indicate that except for the case of the higher amounts of Li consumed, the decrease of rare earth elements in salt and their increase in Cd occur with increasing amount of Li consumed, and the sum of the amounts of rare earth elements in salt and Cd is close to the total amounts present in the system. However, the material balances for the rare earth elements except for Y in run 1 are slightly worse compared with those in run 2, which may be due to some experimental uncertainty. On the other hand, for the higher amounts of Li metal consumed by reduction, the discrepancy between the sum of the amounts of rare earth elements in salt and Cd and their total amounts in the system became much larger, because amounts of rare earth elements equivalent to their decreased amounts in salt didn’t appear in Cd. Figs. 5 and 6 show the relationship between the change (equivalent) of all rare earth elements in salt and Cd and the amount (equivalent) of Li consumed in runs
M. Sakata et al. / Equilibrium distribution of rare earth elements
1 and 2, respectively. The results indicate that the decreased amount of rare earth elements in salt is well balanced by the amount of Li consumed, suggesting that the decrease is due to the reduction of chlorides to metals by Li metal. On the other hand, the increase of rare earth elements in Cd was significantly lower than their decrease in salt or the amount of Li consumed when the higher amounts of Li metal were consumed by reduction. This imbalance may be due to the removal of rare earth metals from the liquid phases of salt and/or Cd with the formation of solids. The salt obtained after the experiment of run 1 (450°C) contained a small amount of water-insoluble solid. Chemical analyses and X-ray diffraction measure-
59
ment of this solid showed that almost all of the rare earth elements in the solid are present in the form of oxychlorides but not in the form of metals or intermetallic compounds. Based on the chemical analyses, the amounts of these oxychlorides produced during the experiment were estimated to be less than 1% for each rare earth element except for Y and about 8% for Y of the total amount of each element in the system. The relatively high percentage for Y is probably due to the contamination of the glove box atmosphere with air during the final stage of the experiment, because only Y was present in the salt phase during the final stage (fig. 3). Thus, the imbalance described earlier can not be attributed to the formation of oxychlorides.
100 Boo A
80
0~0
0
A
-
q 0
AA
60
0 A
-
OBP
La 40
~~~
-
A o
A 0
20
-
0
3
H
O.
A
’
n
IA*
0
60
-
40
-
20
-
rA,A4.;
Y
bAAOo
40
0
t
OA
‘:jo( .
A
60
B
z
0
0
0
0 0
0
A
0
A
0
0 t100
80
q
-
A
q
=u
00~0
40
-
&
D
/::I:,::,::,
oo #.I 0 0
20-
(mol)
EJu
O0
-
Amount of Li consumed
q
A
60
nnn n 0.00.0,
0 Sum of amounts of RE in salt and Cd
A A
0
A
0
A
0
h A . 0
0.01
0.02
Amount of Li consumed
0.03
0.04
(mol)
Fig. 3. Change in material balance of rare earth elements with increasing amount of Li consumed
in run 1 (450°C).
M. Sakata et al. / Equilibrium distribution of rare earth elements
60 n
100
ti”-
-
.
y
100
q
0
qg*n
13
ll Bsi
AA A
80
0
0
-
A
80 A
5s B z
La
-
60
0
40
B
o
A
S
O0
lmn”
80 i? H 5 8
q
IJ
A
.
Ce
-
I
q
0
0
0.
0.02
0.01
Amount of Li consumed
0.03 (mol)
[
0 Amount of RE in Cd
A
0 Sum of amounts of RE in salt and Cd
A
0 OO
A
0
0,
A Amount of RE in salt
0
0
n.
0
0
0
8
40 20
-
q
A
60
40-
0 q
'A
-
y
o-O_
0 100
60-
20
A
0 no
s
o
0 20
73
A
0.02
0.01
0.03
Amount of Li consumed (mol)
Fig. 4. Change in material balance of rare earth elements with increasing amount of Li consumed in run 2 (SOO°C).
Table 2 indicates the distribution of rare earth elements in the cross section of the Cd frozen after the experiment of run 1 (450°C). The rare earth elements 0.04
except for Y were considerably enriched in the lower part, that is, near the bottom of crucible. On the other hand, Y was distributed uniformly in the whole cross 0.03 /’
is
,./’
,.d
0.03
..g.‘.
0.02 ,,/
0.02
.’
0
0
0
..2i’-. .. 0 .
0.01
./....
0.01
6”’
$,q$
+..,
o
.**
,
0
0.01
,
0.02
Amount of Li consumed
I
0 Increased A Decreased
,
0.03
P
0 0.04
(equivalent)
0.01
0.02
0.03
Amount of Li consumed (equivalent)
0 Increased
amount of RE in Cd
A Decreased
amount of RE in salt
Fig. 5. Relationship between amounts of rare earth changed in salt and Cd and amount of Li consumed (45OT).
0
.
elements in run 1
amount of RE in Cd amount of RE in salt
I
Fig. 6. Relationship between amounts of rare earth elements changed in salt and Cd and amount of Li consumed in run 2 (500°C).
A4. Sakata et al. / Equilibrium distribution ofrare earth elements section of Cd relative to other elements. Scanning electron microscope of the cross section of the Cd sample showed that large grains were segregated in the Cd matrix of the lower part. Energy dispersive X-ray analyses showed that these grains contained a greater amount (- 10 wt’%) of rare earth elements than the Cd matrix. Intermetallic compounds of the type RECd,, were identified in the lower part of Cd by X-ray diffraction. This result shows that La, Ce, Pr and Nd are enriched in the form of RECd,, in the lower part, because those elements form intermetallic compounds of this type in Cd-rich solutions [ll]. Although Y forms YCd, in Cd-rich solutions [12], YCd, in the Cd sample was not detected by X-ray diffraction. Thus, the results for the Cd sample indicate that the rare earth elements except for Y formed intermetallic compounds of the type RECd,, which were enriched in the lower part of Cd, during the experiment. Thus, it is reasonable to assume that the imbalance described previously is due to the deposition of rare earth metals as RECd,, in the bottom of the crucible. The imbalance for Y may be also caused by the formation of YCd, or other types of intermetallic compounds, although whether this is the case remains to be confirmed more exactly through experiments. Table 3 indicates the concentrations of rare earth elements in the Cd sample at the time when their material balances became worse in the experiments of runs 1 and 2. For convenience, these concentrations are assumed to be identical to those at the time when the sum of the amounts of rare earth elements in the salt and Cd became less than 80% for run 1 and less than 90% for run 2 relative to the total amounts present in the system, as can be seen by figs. 3 and 4. The solubilities of rare earth metals in Cd which were measured in the binary alloy system [11,12] are also included in this table. The results show that the concentrations of all rare earth elements in Cd at the time when their material balance became worse, that is, at
Table 3 Concentrations their material
of rare earth elements balance became worse Element
Concentration in Cd (wtW) ‘)
Solubility in Cd (wtW) b,
1 (45O’C)
La Ce Pr Nd Y
0.089 0.127 0.122 0.123 0.048
0.15 0.22 0.38 0.65 0.68
2 (500°C)
La Ce Y
0.312 0.262 0.187
0.40 0.75 1.1
a) Assumed to be identical to the concentrations at the time when the sum of the amounts of rare earth elements in salt and Cd became < 80% for run 1 and < 90% for run 2 of their total amounts in the system. b, Published values in the binary alloy system [11,12].
the time when the formation of intermetallic compounds is regarded to have occurred, are significantly lower than the solubilities measured in the binary alloy system [11,12]. This suggests lower solubility of each rare earth metal in Cd by the presence of other metals. Thus, the coexistence of a relatively small amount of rare earth metals appears to have a large effect on the solubility of each metal in liquid Cd. This indicates that a detailed investigation of solubility is necessary for the development of a liquid-liquid extraction system. 3.2. Determination earth elements
in a cross
section
of distribution
coefficients for rare
At a given temperature, the distribution of a trivalent rare earth element (RE) between molten KCl-LiCl eutectic salt and liquid Cd can be expressed by the following general reaction: REC~W,~
K elements
in Cd at the time when
Run no.
+ 3Li (cd) *
The equilibrium Table 2 Distribution of rare earth (run 1: 45O’C) (in wtW)
61
a RE a
RE =
3 LiCl
constant =
aREcl,atj
of Cd
R&Z,,
+
(K,,)
YRE xREY?icl YREcI,XREcI~~~i
3LiCl
(saltj.
can be written
(1)
as
x~ic~ xl!i
in which
Element
Upper part (n = 5)
Central (n=5)
part
Lower part (n=3)
La Ce Pr Nd Y
0.121 *to.048 0.144 f 0.062 0.127kO.055 0.122 f 0.051 0.057 f 0.007
0.071 0.090 0.085 0.087 0.050
0.041 0.049 0.046 0.043 0.014
0.411 0.433 0.362 0.247 0.065
f f f f f
f f f f f
0.052 0.043 0.049 0.080 0.004
a, X and y denote the activity and the mole fraction and the activity coefficient, respectively. If the respective activity coefficients are constant in the system investigated, an apparent equilibrium constant (Kk,) is given by
K’RE
YREcI;/?i =
Key
-YREYLiCl
xRE =
xf?icl
xREc.,x;i
(3)
62
M. Sakata et al. / Equilibrium distribution
-2
-3 -4.2
-4.0
-3.8
-3 6
-3.2
-3.4
-3.0
-2.8
Log DLi O:La
Log DL~ q 10.4 + 3.10 Log Du
n:Ce
Log Dee = 10.5 + 3.01 Log Du
0: Pr
Log DP~ q 10.9 + 3.08 Log Du
l : Nd r:Y
i_Og DNd = 10.5 + 2.98 Log DLI Log DY = 8.3 + 2.94 Log DL,
Fig. 7. Relationship between distribution coefficients of rare earth elements and Li in run 1 (45OOC). Solid lines were obtained by the least-squares method in the range of the experimental data showing a good material balance.
When the distribution
coefficients
X RE Dam=
X
RECI,
tremely low concentrations of Li and rare earth elements. As can be seen by figs. 7 to 9, plots of log D,, versus log DLi at run 1 (45OOC) and run 2 (500°C) give straight lines having a slope of around 3. This indicates that these rare earth elements exist as trivalent species in the salt phase, and that the assumptions made regarding activity coefficients in the deviation of eq. (5) are reasonable in the system investigated. However, the slopes of the regression lines obtained for run 3 (600°C) are in the range of 2.44 to 2.68 (fig. 9), which is 10 to 20% less than the value of 3 expected for trivalent chlorides. This discrepancy may be due to the relatively large uncertainty caused by having only three data points. Table 4 summarizes the apparent equilibrium constants (K&) and the separation factors (Ce basis, i.e. Kk,/K&) which were obtained from the least-squares fits of eq. (5) to the measured distribution coefficients (figs. 7 to 9). The results indicate that the order of KkE values for the rare earth elements is Ki, = K& = KG, > K[, > K;. Thus, the extractabilities of Pr, Nd and Ce are nearly equal, and Y is the least extractable element.
1
, ..’
__.....” 0
G-
_,
-
as
XL, and
eq. (3) can be written log D,,
are defined
oft-are earth elements
= 3 log D,,
(4)
D,;=x,i~l’
in logarithmic + log KiE.
form as (5)
Thus, if the assumptions made regarding the activity coefficients are valid, a plot of log D,, versus log DLi should give a straight line having a slope of 3 for trivalent chlorides. These data plots are shown in figs. 7 to 9. The analytical values of rare earth elements in the Cd sample are assumed to contain some amount of solid phase when the intermetallic compounds were formed, because filtered samples of the salt and metal phases were not taken in this experiment. Therefore, the regression lines between log D,, and log D,, were obtained by least-squares method in the range of the experimental data showing a good material balance. Some data were excluded in this calculation because of the large experimental errors for the Cd samples highly contaminated with salt and for the samples with ex-
-4
1 -4.0
-3.8
-3.6
-3.4
-3 2
Log DLi
0: La
Log DL~ = 10.1 + 3.05 Log Du
n:Ce
Log Dee = 10.8 + 3.11 Log Du
q :Y
Log Dv = 8.5 + 3.02 Log
Du J
Fig. 8. Relationship between distribution coefficients of rare earth elements and Li in run 2 (SOOT). Solid lines were obtained
by least-squares method in the range of the experimental data showing a good material balance.
63
M. Sakata et al. / Equilibrium distribution of rare earth elements
regularly with increasing temperature. Since the separation factor of Y on the basis of La also has such a temperature dependence, the separation of Y and other rare earth elements from each other becomes more difficult with increasing temperature. 3.3. Comparison of distribution coefficients for rare earth elements calculated from activity coefficients with their experimental values
-3.4
-3.6
-2.8
-3.0
-3.2
The standard free energy change general reaction (1) is given by
(AC ” ) for
the
AGo = 3 AGP,icl-
log K,,,
(6)
AGPRECl, = -2.3RT
where AC&, and AC&,, are the standard free energies of formation of Lid1 and rare earth chloride (RECI,), respectively. By combining eqs. (3), (5) and (6), eq. (7) is obtained:
Log Dt_i
3
H”i
log DR,=
310g D,i+
logs
Yi?iCI
2.3RT Fig. 9. Relationship between distribution coefficients of rare earth elements and Li in run 3 (6OO’C).
This can be explained satisfactorily by using the activity coefficients of chlorides and metals in salt and Cd phases, as discussed in the next section. The KiE values for Ce and Y at 500°C are similar to those (KG, = 10.2; KG = 8.1) calculated from the distribution data in a similar LiCl-KCl-NaCl/Cd system which was measured by Lewis and Johnson [13]. Table 4 also shows that the separation factor (Ce basis) for Y increases
Table 4 Apparent equilibrium constants basis) for rare earth elements
and separation
Element
log K;E
Separation factor
450
La Ce Pr Nd Y La Ce Y La Ce Y
10.0 10.5 10.6 10.5 8.5 10.0 10.4 8.4 8.9 9.3 7.7
0.36 1.0 1.3 1.1 9.6x10V3 0.37 1.0 1.1x10-2 0.39 1.0 2.0x10-2
600
(7)
’
in which the second, third and forth terms in the right side are related to the activity coefficients in Cd phase, the activity coefficients in salt phase and the standard free energy change, respectively. When the activity coefficients and the standard free energies of formation are known, the distribution coefficients (D,,) for rare earth elements can be calculated from eq. (7). This calculation was carried out using the appropriate thermodynamic data had been reported, and the results were compared with the experimental values obtained by the present study. The calculation for Nd was not made because of some uncertainty of the activity coefficients of NdCl, in salt which is probably caused by the coexistence of the divalent chloride salt, as noted by McCoy et al. [14].
factors (Ce
Temp. ( ’ C)
500
YREU, +'Og-
Table 5 Standard free energies of formation of chlorides and activity coefficients of chlorides and metals in KCl-LiCl eutectic salt and Cd for calculation of distribution coefficients for rare earth elements at 450°C Element La
Ce Pr Y Li
- AC; (kcal/mol)
209.4 205.0 206.0 196.3 82.7
[14]
y of chloride [14,16]
y of metal
5.0x 10-3 1.4x10-3 3.3x10-3 6.3~10~” 0.63
4x10-‘9 5x10-‘0 2x10-9 3x10-7
U4J51
1x1o-3
64
M. Saknta et al. / Equilibrium
-
distribution
ofrare earth
elements
La
-4.2 -4.0 -3.6 -3.6 -3.4 -3.2 -3.0 -2.6
-4
Log DL,
6
0
Experimental value
-.-.-
Calculated value in case 1 : All
_____
Calculated value in case 2 : Y in Cd from values in table 5 , Y in salt = 1 Calculated value in case 3 : All Y from values in table 5
_
Y = I
-4 6
-4.2
-4.0
-3.8
-3.6
-3.4
-3.2
-3.0
-2.8
Log DL, Fig. 10. Comparison of distribution coefficients for rare earth elements calculated from activity
coefficients
with their experimental
values.
Table 5 summarizes the standard free energies of formation of chlorides and the activity coefficients of chlorides and metals in KCl-LiCl eutectic salt and Cd at 450°C which were used for the calculation of the distribution coefficients for rare earth elements. The free energies and activity coefficients of chlorides are given in the table on the basis of the hypothetical liquid standard state [14]. The activity coefficients of chlorides except for LiCl and metals were measured electrochemically by McCoy et al. [14] and Matsumoto et al. [15], while the activity coefficient of LiCl was calculated from the partial molar excess free energy given by Lumsden [16]. These activity coefficients were assumed
to be constant in this calculation, independent of the concentration of each constituent in the salt or Cd phase, based on the measurements of the concentration dependence of activity coefficients [14,15]. The calculated results are shown in fig. 10, along with the experimental values. First, when all activity coefficients are set equal unity (case l), to the calculated distribution coefficients are 2 to 7 orders of magnitude higher than the experimental values, and shows that Y is the most easily reduced element in spite of the opposite experimental values. Second, when only the activity coefficients of rare earth and Li metals in Cd phase are taken into account (case 2) the calculated
M, Sakata et al. / Equilibrium distribution of rare earth elements
value for Y is much lower than case 1, but there is no significant difference between both the calculated values for other elements. Thirdly, when all activity coefficients are set equal to the values given in table S (case 3), a better agreement is observed between the calculated and experimental values for the rare earth elements. Thus, the distribution coefficients for those elements can be explained satisfactorily by using the activity coefficients of metals and chlorides in Cd and salt, showing that this calculation is useful for the prediction of separation of rare earth elements. It is clear that both the activity coefficients of metal and chloride cause a large downward shift for the distribution coefficient of Y relative to those of other elements, thus Y is the least reduced.
65
rare earth elements. Both the activity coefficients of metal and chloride caused a much smaller distribution coefficient of Y relative to those of other elements.
Acknowledgements We thank H. Miyashiro, T. Matsumoto and Y. Sakamura of CRIEPI and T. Kusakabe of Su~tomo Metal Mining Co., Ltd. for their cooperation in this study. We are indebted to K. Kugimiya of Techno Service Co., Ltd. for his technical assistance. We also thank R. K. Kawaratani for critically reading the manuscript.
References 4. Conclusions The following conclusions can be drawn in the present study on the equilibrium distribution of rare earth elements such as La, Ce, Pr, Nd and Y between molten KCl-LiCl eutectic salt and liquid Cd. (1) It is likely that the solid intermetallic compounds
such as RECd,, were formed at the lower concentrations of rare earth metals dissolved in liquid Cd than those solubilities measured in the binary alloy system. This suggests that the coexistence of a relatively small amount of rare earth metals has a large effect on the solubility of each metal in liquid Cd. (2) For each temperature, the distribution coefficients (mole fraction in the Cd phase divided by mole fraction in the salt phase) for rare earth elements and Li obeyed the relations~p 13,, = Dti K&n, in which Kin is the apparent equilibrium constant. This shows that the rare earth elements exist as trivalent species in the salt phase, and that the respective activity coefficients are almost constant in the system investigated. The temperature dependence for the distribution coefficients shows that the separation of Y and other rare earth elements from each other becomes more difficult with increasing temperature. (3) The extractabilities of Ce, Pr and Nd were nearly equal, and Y was the least extractable element. This could be explained satisfactorily by using the activity coefficients of chlorides and metals in salt and Cd phases, showing that the calculation using the activity coefficients is useful for the prediction of separation of
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