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Distribution of lanthanide and actinide elements between molten lithium halide salts and liquid bismuth solutions
J. inorg,nucl.Chem., 1972,Vol. 34, pp. 2921-2933. PergamonPress. Printedin Great Britain
DISTRIBUTION OF LANTHANIDE AND ACTINIDE ELEMENTS BETWEEN MOLTEN LITHIUM HALIDE SALTS AND LIQUID BISMUTH SOLUTIONS* L. M. FERRIS, F. J. SMITH, J. C. M A I L E N and M. J. BELL Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
(Received 4 August 1971) A b s t r a c t - Measurements of the equilibrium distribution of several lanthanide and actinide elements between liquid bismuth solutions and a variety of lithium halide salts were made in the temperature range 600-750°C. The salts included molten LiCI, LiBr and several LiF-BeF2-ThF4 solutions. At each temperature, the distribution coefficients (mole fraction in the bismuth phase divided by mole fraction in the salt phase) for most of the solutes obeyed the relationship Du = ( N"Li) (K*), inwhichn is the oxidation number of solute M n+ in the salt phase and NLi is the mole fraction of lithium in the bismuth phase. Under the experimental conditions employed (NLI ranging between 10-5 and 0-3), thorium and protactinium were in the 4+ oxidation state in the salt phase; La, Nd, U, Np, Pu and Cm were tripositive; and Ba and Eu were dipositive. The values of log K* generally decreased regularly with increasing temperature. The oxidation numbers of Sm, Am and Cf in the salt phase varied between 3+ and 2+, depending on the experimental conditions. Californium existed primarily in the 2+ oxidation state with LiC1 as the salt phase, whereas, in fluoride salts, the californium was tripositive. The variation of the distribution coefficients of Sm, Cf and Am with NL~ at a given temperature could be expressed as Du = N~J (c~+ flNLO, in which a and/3 are constants. INTRODUCTION
A METHOD being considered for the processing of a molten-salt breeder reactor (MSBR) involves the selective reduction of protactinium from the molten fluoride fuel salt into liquid bismuth, followed by the preferential transfer of the rare earths from the fluoride salt through a liquid bismuth phase into an acceptor salt such as LiC1 or LiBr[1-3]. This processing method is based, in part, on data obtained for the equilibrium distribution of several lanthanide and actinide elements between liquid bismuth and lithium halide salts. Some of the data were presented previously [4-6]; the rest are given below. At a given temperature, the distribution of an element, M, between a molten salt containing a lithium halide and a liquid bismuth phase can be expressed as the general reaction MXn¢salt) -4- nLiCni) = Mere) + nLiXcsalt)
( 1)
*Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. 1. L. E. McNeese, In M S R Program Semiann. Progr. Rept. Feb. 28, 1970, p. 277. U S A E C Rep. ORNL-4548 (1970). 2. L. E. McNeese and L. M. Ferris, Trans.ANS 14(1), 84 (1971). 3. Chem. Technol. Div. Ann. Progr. Rept. May 31, 1971, p. 2. U S A E C Rep. ORNL-4682 (1971). 4. L. M. Ferris, J. C. Mailen, J. J. Lawrance, F. J. Smith and E. D. Nogueira, J. inorg, nucl. Chem. 32, 2019 (1970). 5. L. M. Fen'is, J. C. Mailen and F. J. Srnith, J. inorg, nuel. Chem. 33, 1325 (1971). 6. J. C. Mailen and L. M. Ferris, lnorg, nucl. Chem. Letters 7, 431 (1971). 2921
2922
L . M . FERRIS, F. J. SMITH, J. C. M A I L E N and M. J. BELL
in which X is F, C1 or Br; n is the oxidation number of M n+ in the salt phase; and (salt) and (Bi) denote the respective phases. Equation (1) obviously applies only to the distribution of a solute that is in a single oxidation state in the salt phase. An equilibrium constant for reaction (1) can be written as K=
n
?l
NMTMNLIxTLIx NMXnTMxnN~iT~|
(2)
in which N denotes mole fraction and ,/designates an activity coefficient referred to the appropriate pure sofid or liquid. If the various activity coefficients and Nux are practically constant and the distribution coefficient is defined as DM =
(3)
N_..__M_~ NMXn '
Equation (2) can be written as DM = ( N ~ ) ( K * )
(4)
log DM = n log N u + log K~.
(5)
or, in logarithmic form,
A plot of log DM VS. log NLI should give a line of slope n if n is constant and the assumption made regarding the constancy of the activity coefficients is valid. In our previous work with LiF-BeFe-ThF4 and LiF-BeF2 salts[4, 5], we defined n
n
I
DM = (NLi/NLiF) (KM).
(6)
The relationship between K~t and K*, obtained from Equations (4) and (6), is K*=
(K~)/(N~iF).
(7)
In some experiments, M existed in the salt phase in both the 3+ and 2+ oxidation states. The distribution of M between the two phases was considered in terms of the following simultaneous equilibria: M(ai) + 3LiX(salt)
(8)
MX2(salt) -I- 2Li(m) ---- M(ai) q- 2LiX(salt).
(9)
MX3(salt) q- 3Li(Bi) : -
From the expressions for the equilibrium constants for reactions (8) and (9), we obtain N MYM " N L3i X .3 yLiX
NMX3
K
"
8YMXz
N 3 ,3 LiYLi
(10)
Distribution of lanthanide and actinide elements
NMX~=
2 2 N MTMNLix'YLiX
v" ,
Ix-9YMX2
2923
(11)
N 2 ,2 " LiYLi
If the concentration of M is very low in both phases, the distribution coefficient can be expressed as DM = NM(Bi) ----- NM(Bi) NMtsalt) NMX.~+ NMX2"
(12)
Thus, from Equations (10-12), DM =
NM [NMTMNLIxTLiX N M y.M N 2LIX J'LIX[ ,2 q" 3 3 I
I f NLix
8YMX3
LiYLI
9YMX2
(13)
L i y L i _l
and the respective activity coefficients are constant, we can define • N 3
,3
= r i L~rrax KsYmx3Y~i
(14)
• N 2 ,2 ~- )'M LiX TLiX
g
(15)
2 "
9'YMX2'~Li
Substitution of Equations (14) and (15) into Equation (13) yields, in logarithmic form, log DM = 3log NLi -- log (a + flNLi).
(16)
log KMX3 * = -- log a
(17)
log K*x, = -- log ft.
(18)
By definition
It can be shown that the average oxidation number of M in the salt phase is /]M =
(19)
3o~+ 2flNLi o~+,SNLi
Distribution coefficients for two (or more) elements were frequently obtained in the same experiment. Using Dx and Du to represent the distribution coefficients for elements x and y obtained at the same value of NLi, it follows from Equation (5) that a plot of log Dx vs. log D, should give a line having a slope of nx/nu if both nx and n~ are constant. If M~ exists in the salt phase in both the 3+ and 2+ oxidation states and n -- 3 for M u, we can consider the following equilibria: MxXz(sam + Mu(m) MxX2(salt) +
= Mx(m) +
2 / 3 M u ( m ) = Mx(m) +
MuXz(salt)
(20)
2/3MuXa(salt).
(21)
2924
L . M . FERRIS, F. J. SMITH, J. C. M A I L E N and M. J. BELL
From expressions of the equilibrium constants for reactions (20) and (21) and the same assumptions made in the derivation of Equation (16), we obtain log D~ = log D r -- log (~bu+ E~Du 1/3)
(22)
and 3~bu+ 2~uD~ 1/3 =
~bu + ~uD~ 1/3
(23)
in which ~b~and ~ are constants. EXPERIMENTAL
Reagents. The bismuth used in this study was Cominco American Co. 69 grade, and the lithium metal, barium metal, LiC1, LiBr and LiF were reagent grade. The fluoride salts were prepared and purified as noted previously [4]. The lanthanide elements were available either as pure oxides or metals. Uranium was added either as the pure metal (natural U) or as a hydrated oxide (233U). The plutonium metal used contained about 0.1% 241Am. Other actinides (243Am, 244Cm and 2s~Cf) were obtained in 1 N HC1 solutions from the Transuranium Research Laboratory of the Oak Ridge National Laboratory. The protactinium used in experiments with LiC1 and LiBr was 233Pa at the tracer level. The gaseous hydrogen chloride, which was Matheson electronic grade, was used without further treatment. The high-purity gaseous hydrogen bromide was also obtained from the Matheson Co. The hydrogen was Matheson ultrapure grade and was treated further by passage through a catalytic oxidizer and a Molecular Sieve trap. Argon, which was used as an inert cover gas, was purified by passage through traps packed either with uranium turnings [4] or titanium sponge. Either crystal-bar thorium or a lithium-bismuth alloy was used as the reductant. The alloy, which usually contained about 7 at. % Li, was prepared from the pure metals. Apparatus and procedures. The apparatus, general procedure, and sampling technique have been described elsewhere [4-6]. In many of the experiments, distribution coefficients were obtained at only one temperature and the values of n and log K* were determined from the appropriate isotherm. In other experiments, the temperature of the system was varied and values of log K* were calculated from Equation (5), using the value of n obtained from an isotherm and the results of analyses of several sets of samples taken at each temperature. The procedures utilized with fluoride salts were described previously [4]. With LiCi or LiBr as the salt phase, different start-up procedures were used, depending on the elements involved. In most cases, the desired components were loaded, along with bismuth and ovendried LiC1 or LiBr, into a molybdenum crucible. After being heated to about 650°C under argon, the molten two-phase system was treated for at least 24 hr with either an HC1-H2 or an HBr-H2 mixture to dissolve the respective components in the salt phase and to remove oxides from the system. This treatment was followed by sparging of the system with pure argon to remove residual HCI or HBr. After a small amount of thorium had been added to the system to ensure the lowest possible oxide concentration, reductant (usually Li-Bi alloy) was added in increments to effect the systematic reduction and transfer of the solutes from the salt phase to the bismuth phase, as described previously[4, 6]. In some experiments with La, Nd, Eu and Ba, the procedure involved simply heating the salt and bismuth to about 650°C under argon, adding a small amount of thorium to scavenge oxides, and, then, adding the desired metal to the system in an amount such that, at equilibrium, the distribution coefficient was between 0.1 and 1. Finally, reductant was added to the system and the experiment was concluded in the usual manner. The general technique used with the transuranium elements was described previously [6]. Methods for analyzing for the various elements have been discussed elsewhere [4-6]. Data for neptunium presented in this paper were obtained from gamma-spectrometric analysis for the 239Np daughter of 24ZAm. RESULTS AND DISCUSSION
Molten fluoride mixtures as the salt phase. The equilibrium distribution of several solutes (Table 1) between liquid bismuth solutions and various molten
Distribution of lanthanide and actinide elements
2925
Table 1. Values of log K* obtained with LiF-BeF2-ThF4 salt solutions Salt composition (mole %)
LiF
BeFz
68
25
ThF4
Temp. (°C)
7
600 700
69.2
19-4
11.4
550
700 69.6
24-2
6.2
600 700
70.2
23.4
6.4
600 700
72
16
12
73
2
25
600 645 690 600 650 700
73.4
21.4
75
13
5-2 12
600 700
Element
n
log K~
Eu Th Eu Th Eu Th U La Th La Th La Th Eu Th Eu Th Ba Ba Ba Eu Th Eu Th Eu Th Eu Th La
2 4 2 4 2 4 3 3 4 3 4 3 4 2 4 2 4 2 2 2 2 4 2 4 2 4 2 4 3 3 2 4
4.093 10-191 3.534 8.558 4.366 10.745 12.794 6.262 8.614 7.275 9.848 6.330 8-608 4.137 9.929 3.634 8.574 4-049 3-678 3.530 3.840 9.560 3 -566 8.768 3.405 8.264 4.075 9.006 5.809 5.998 3.469 7.738
Nd Eu Th
LiF-BeF2-ThF4 salts was determined at 600-700°C. Plots of log Du vs. log NLi were linear, and the slopes of these plots, within experimental error, gave integral values of n. Values of n and log K* derived from the data are given in Table 1. The uncertainty in each v~lue of log K~ is _+0.1. The values of n obtained for La, Nd, Eu and Th in the present study are the same as those obtained in earlier studies [4, 5] with other fluoride salts. The behavior of barium, the only alkaline earth element studied, was very similar to that of europium. The present data are also in accord with previous data[4, 5] in that the values of log K * obtained with a given salt generally decreased with increasing temperature and, at a given temperature, usually decreased as the "free fluoride" concentration of the salt increased. The "free fluoride" concentration (FF) is defined as
FF = (mole % LiF) -- 2(mole % BeF2) -- 3(mole % ThF4).
2926
L . M . FERRIS, F. J. SMITH, J. C. M A I L E N and M. J. BELL
In an experiment at 600°C, the distribution of lanthanum and thorium between liquid bismuth and L i F - N a F - B e F 2 - T h F 4 (62-10-16-12 mole %) was determined. The values log K*a = 7.38 ± 0.2 and log K* h = 9.57 ± 0.2 are very close to the corresponding values obtained with LiF-BeF2-ThF4 (72-16-12 mole %) as the salt phase [4]. An experiment was conducted at 600°C to determine the distribution of lanthanum and thorium between liquid lead and molten LiF-BeF2-ThF4 (70-1911 mole %). Only a few data points were obtained because the lead phase became saturated with thorium when the thorium concentration reached 190___10 ppm. This concentration is in excellent agreement with the 200 ppm reported by Shaffer et al. [7] for the solubility of thorium in lead at 600°C. The equilibrium lithium concentration in the lead phase was 9 ppm, and the thorium distribution coefficient was 0.0014. These values yield log K* h = 11.47 ± 0.2. The lanthanum distribution coefficient could not be determined accurately, but was found to be less than 0.005. Molten LiCl or LiBr as the salt phase. The equilibrium distribution of barium and several lanthanide and actinide elements between liquid bismuth solutions and molten LiC1 or LiBr was determined in the temperature range 640-750°C. Most of the experiments were conducted at a single temperature, and values of n and log K* were determined from the isotherms represented either by Equation (5) or by Equation (16). In general, the data for the solutes were well represented by Equation (5); i.e. a plot of IogDM VS. IogNLi was linear with a practically integral slope. Typical plots are shown in Figs. 1 and 2; similar data plots have been presented elsewhere[I-3, 6, 8]. The integral value of n was determined from these plots by visual fitting of the data, and the values of log K* (Table 2) were subsequently obtained by fitting the best line of slope n to the data. In a few cases, notably with protactinium, the lithium concentration in the bismuth phase was too low for accurate analysis over the range of NLi where values of DM could be obtained. Values of n and log K* for such elements were determined from plots of the logarithms of the distribution coefficients vs. the logarithms of the distribution coefficients of a solute for which n and log K* were known. A plot of log Dpa vs. log Dxh, using data obtained with LiC1 as the salt phase at 640°C, has been presented elsewhere [9]. Plots of log Dcf vs. log Dcm, using data obtained with LiF-BeF2 (66.7-33.3 mole %) at 600°C, and of log DNp vs. log Dpu, using data obtained with LiC1 at 640°C, are shown in Fig. 3. The fact that most of the plots of log DM vs. log NL~ and of log Dx vs. log D~ were linear supports the assumptions made regarding the constancy of the various activity coefficients in the derivation of Equation (5). The data obtained with the various salt solutions also show that, over the range of conditions where distribution coefficients could be measured, most of the solutes existed in the salt phase in a single oxidation state. In several instances, data were obtained over a range of temperature for 7. J. H. Shatfer, F. A. Doss, W. K. R. Finnell and W. P. Teichert, In Reactor Chem. Div. Ann. Progr. Rept. Dec. 31, 1965, p. 43. U S A E C Rep. ORNL-3913 (1966). 8. L. M. Ferris, F. J. Smith, J. C. Mailen and M. J. Bell, J. inorg, nucl. Chem. 34, 313 (1972). 9. L. M. Ferris and F. J. Smith, In MSR Program Semiann. Progr. Rept. Feb. 28, 1970, p. 291. U S A E C Rep. ORNL-4548 (1970).
2927
Distribution of lanthanide and actinide elements
e u o o u
Q
I0 -~
I0
Lithium
~
in bismuth
IO ~
phase,
IU ~
Iu-'
mole fraction
Fig. 1. Distribution coefficients for thorium, lanthanum and samarium obtained at 640°C with LiC1 as the salt phase.
solutes for which n had been determined at two or more temperatures and had been found to be temperature invariant. Values of log K* were calculated at each temperature, using Equation (5), from the analyses of several pairs of samples. Plots of log K * vs. 1/T(°K) were found to be linear (Fig. 4); thus, the data could be represented as log K * = A + B / T ( ° K ) . Values of,'/and B for several solutes in the temperature range 625-750°C are given in Table 3. The simultaneous distribution of Pu, Cm, Cf and Am between liquid bismuth solutions and molten LiCI was determined both at 640 ° and 700°C. Plots of log Dpu vs. log Dcm were linear with slopes of 1, showing that npu was equal to ncm. The distribution coefficient data for plutonium and curium were also fit quite well by Equation (5) with n = 3. However, plots of log Dcf and log DAm VS. log NLi indicated that californium and americium had not existed in a single oxidation state in the salt phase at 640°C. Consequently, the distribution coefficients for these two elements were fit as a function of the values of NLi, using Equation (16), to determine the constants a and/3. The data were also fit as a function of the distribution coefficients of plutonium and curium, using Equation (22), to determine the constants ~b~ and e~. The nonlinear least-squares fitting of the data was performed using the O R D E A L data analysis program[10]. Typical data plots 10. M. Feliciano, C. W. Nestor, Jr., N. B. Gove and T. D. Calton, Oak Ridge Data Evaluation and Analysis Language. U S A E C Rep. ORNL-4506 (1970).
2928
L. M. FERRIS, F. J. SMITH, J. C. M A I L E N and M. J. BELL =
I0
8tJ o g
"E =a E3 I'C
001 I0 -4
10-3
I0 -2
Mole fractionlithium in bismuthphase Fig. 2. Distribution coefficients for plutonium, curium and californium obtained at 700°C with LiCI as the salt phase.
I00
F
DCf vs. DCm S Salt phase:LIF-BeF2 ~ (66.7-33.3 mole% ./ Temp: 600 "C ~ Slope =l ~ V
/
OI
I
0"01
, ~
e/ Je
1.0
]
I I[Arlll
/ /A J l
DNPV$"Dpu
~ j=
I
I
Salt phaee: LiCl Temp.640 =C
IIIIlII
0.1
I
I
I IIllll
1.0 Distribution
I
I0
coefficient
Fig. 3. Typical plots of log Dx vs. log D u.
(
I IIlll
I00
Distribution of lanthanide and actinide elements
2929
Table 2. Values of log K * obtained with LiCI and with LiBr as the
salt phase
Salt
Temp. (°C)
LiBr LiBr
575 600
LiCI-KCI* LiCI LiCI
600 630 640
LiBr
640
LiC1 LiCI
675 700
LiBr
700
Element Ba Ba La Nd Th Th Eu Ba La Nd Sm Th Pa U Np Pu Am Am Cm Cf Cf La Nd Th Pa U U La Nd Sm Eu Th Pa U Np Pu Am Am Cm Cf Ba Nd
n 2 2 3 3 4 4 2 2 3 3 2 4 4 3 3 3 3 2 3 3 2 3 3 4 4 3 3 3 3 2 2 4 4 3 3 3 3 2 3 2 2 3
log K~ 1.497 ± 0-06 1.443 ± 0.06 9.079 ± 0.2 8.919±0.3 16.16±0.3 15.663 ± 0.1 2.301 ± 0.05 1.702 ± 0.05 7.973 ± 0.2 8.633 ± 0.2 2.886 ± 0.1 15.358 ± 0.2 17.84±0.4 11.278 ± 0.2 10-330±0.2 10.126±0-2 9.955 ± 0.1 7.658 ± 0.3 9.406 ± 0.2 8.966 ± 0. I 5.419±0.1 8.266 ± 0.2 8.834 ± 0.2 14.80 ± 0.7 15-7___0.8 10.6+0.5 10.11 ±0-3 7" t 85 ± 0"2 7.831 ±0"2 2.756±0'1 2.133±0'1 13-772 ± 0.2 15.8±0.4 10.192±0-2 10.369±0-3 10.223 ±0"3 10.377 ± 0"2 7.155±0.1 9.589±0-2 5.37_+0-1 1.316 ± 0.05 8.430 ± 0.2
*LiCI-KC1 eutectic (59-41 mole %).
are shown in Figs. 5 and 6, and all the values of a,/3, ~b~,and eu derived from the data are given in Table 4. Analysis of this information reveals that californium was mostly in the 2+ oxidation state in the LiCI at both 640 and 700°C. The value of log K* = 5.36 +__0.05 for CfC12 calculated from the value o f / 3 at 700°C is
2930
L . M . FERRIS, F. J. SMITH, J. C. MAILEN and M. J. BELL *C
Temperature, 750 I
700 I
650 I
Salt phase: LiCI .J
Sm =r
•
•
: 2i
•
•
•
•
Bo
I
o
I ro.o
95
I to.5
I ~,o
t04/T,*K
Fig. 4. Effect of temperature on some values of log K~ obtained with LiCI as the salt phase. Table 3. Temperature dependence of log K * for several elements, using LiC1 or LiBr as the salt phase: log K * = A + B / T ( ° K ) (temperature range: 625-750°C)
Salt
M
n
A
B
Standard deviation of log K*
LiCI
Ba La Nd Sm Eu Ba Nd
2 3 3 2 2 2 3
- 0.6907 - 2.6585 - 3"3568 0.7518 - 0.1584 --0.0733 4.046
2189 9697 10900 1950 2250 1333 4297
0.02 0.1 0.08 0.05 0.05 0.02 0.1
LiBr
Table 4. Values of a, /3, ~b~, and ~u obtained by fitting americium and californium distribution coefficient data according to Equations (16) and (22)
Temp. Element x (°C) Am Cf
6~ 7~ 6~ 7~
101°a
10rfl
l.ll±0.11 0.42±0.19 10.8±3.2 0±0.62
0.22±0.11 0.70±0.]9 38.1±6.1 44.0±1.0
Element y = Pu
Element y = Cm
~bpu
~bcm
~eu
Ecru
1.41±0"06 0 . 1 ~ ± 0 . 0 2 9 0-24±0"01 0'088±0'01 1.9±0"2 0-092±0-054 0.40±0.06 0.056±0.026 24.6±2"0 15.6±1.5 3.97±0"38 5.79±0.53 4.8±1.7 21.6±0.9 0-2±0"3 8.8±0.4
Distribution of lanthanide and actinide elements
103
2931
3
OM = NLi a +/~ NLi
•
io 2
/
Americium
el = l . l l x l 0 -I°
/~=2.2xl0-e /
"/./
i01
io °
io -I
~ / /
• / /•• /•
o/~,~ Californium_^ .
~
~ =l'OexlO -9
e ~ e e e
e
/ ~ = 3 . 8 1 x l 0 -6
Io -~
io-=.
.
iii
....
i
....
I
......
10-3
Mole fraction
lithium in bismuth phase
Fig. 5. Least-squares plots of IogDAm and logDcf vs. IogNLi, using Equation (16). The data were obtained at 640°C with LiCI as the salt phase.
practically the same as the value obtained by fitting the best line of slope 2 to a plot of log Dcf vs. log NLi (see Fig. 2 and Table 2). The variation of the average oxidation numbers (hM) with distribution coefficients for californium at 640°C and for americium at 640°C and 700°C is shown in Fig. 7. As seen, each set of data confirms that these elements were present in both the 3+ and 2+ oxidation states in the LiC1. In each case, hM decreased with increasing distribution coefficient (i.e. with increasing reductant concentration in the bismuth phase), as expected. The disparity in the values of/~Am at 700°C among the various sets of data appears to be due primarily to a systematic bias in the values obtained for NLi. An indication of this bias is illustrated by the fact that the values of log K * and log K* m increased slightly when the temperature was increased from 640 ° to 700°C rather than decreasing as would be expected from the behavior of the other elements. These results confirm our preliminary evidence [6] that californium is relatively easily reduced to the 2+ state when LiC1 is used as the salt phase. The results of this investigation, along with those from our previous work [4-6, 8], show that thorium and protactinium are invariably in the 4+ oxidation state in the salt phase, regardless of the composition of the salt. Similarly, La, Nd, Pu, and Cm were always found to exist in the 3+ state in the salt phase. The values of n for U, Np and Eu were 3, 3 and 2 under the experimental conditions em-
2932
L.M.
F E R R I S , F. J. S M I T H , J. C. M A I L E N and M. J. B E L L
aM"
u
~
~u +-ol/s
Americium
,*pu.,.41
/
/-
/
I0 °
~- io-~ / L/ E "~ i0 ~
I0
t
~ e /
~ ~,
Californium q~Pu " 2 4 " 6 t p u = 15. 6
I i ilJJll t ~J~,Jl I IIIIII 1 l i l l i lO-Z i0-l I0 ° I0 ~ I0 z I0 z Plutonium distribution coefficient
Fig. 6. Least-squares plots of log DAm and log Dcf vs. log Dpu, using Equation (22). The data were obtained at 640°C with LiC1 as the salt phase. 3
I0 -3
I0 -2 Californium
I0 -3
I0
distribution
I0j
coefficient
3 E
40"C
I¢:::~
I
10-2
'
I
""~. ~-...."
I
iO-J
i0 o
Americium
distribution
1
I0 j
I0 2
I0s
coefficient
5
I0 -i
i0 o
I0 =
Americium
distribution
10 2
I0 3
coefficient
Fig. 7. Variation of t~cf and tZAmwith the respective distribution coefficients. Parameters used to calculate the three types of curves: a, fl, NLI, ; ~be~, ep., Do,, -........ ; ~Cm, ecm, Ocm, . . . . . •
Distribution of lanthanide and actinide elements
2933
ployed, whereas those for Sm, Cf and Am varied both with the composition of the salt phase and with temperature, other conditions being the same. In general, samarium was in the dipositive state; however, it was chiefly in the tripositive state [4] in several LiF-BeF2-ThF4 salts. Californium appeared to exist primarily in the 3+ state in fluoride salts (see, for example, Fig. 3) but, as noted above, was readily reduced to the 2+ state with LiCI as the salt phase. Evidence for the dipositive state of americium has been obtained both with LiCI (this study) and with certain fluorides [5] as the salt phase. Lack of thermodynamic data precludes estimation of (III)-(II) reduction potentials for the various couples. Acknowledgements-The authors thank J. J. Lawrance, J. F. Land and C. T. Thompson for their aid in conducting the experiments. Chemical analyses were provided by the groups of W. R. Laing, W. T. Mullins, and J. H. Cooper of the O R N L Analytical Chemistry Division.
DISTRIBUTION OF LANTHANIDE AND ACTINIDE ELEMENTS BETWEEN MOLTEN LITHIUM HALIDE SALTS AND LIQUID BISMUTH SOLUTIONS* L. M. FERRIS, F. J. SMITH, J. C. M A I L E N and M. J. BELL Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
(Received 4 August 1971) A b s t r a c t - Measurements of the equilibrium distribution of several lanthanide and actinide elements between liquid bismuth solutions and a variety of lithium halide salts were made in the temperature range 600-750°C. The salts included molten LiCI, LiBr and several LiF-BeF2-ThF4 solutions. At each temperature, the distribution coefficients (mole fraction in the bismuth phase divided by mole fraction in the salt phase) for most of the solutes obeyed the relationship Du = ( N"Li) (K*), inwhichn is the oxidation number of solute M n+ in the salt phase and NLi is the mole fraction of lithium in the bismuth phase. Under the experimental conditions employed (NLI ranging between 10-5 and 0-3), thorium and protactinium were in the 4+ oxidation state in the salt phase; La, Nd, U, Np, Pu and Cm were tripositive; and Ba and Eu were dipositive. The values of log K* generally decreased regularly with increasing temperature. The oxidation numbers of Sm, Am and Cf in the salt phase varied between 3+ and 2+, depending on the experimental conditions. Californium existed primarily in the 2+ oxidation state with LiC1 as the salt phase, whereas, in fluoride salts, the californium was tripositive. The variation of the distribution coefficients of Sm, Cf and Am with NL~ at a given temperature could be expressed as Du = N~J (c~+ flNLO, in which a and/3 are constants. INTRODUCTION
A METHOD being considered for the processing of a molten-salt breeder reactor (MSBR) involves the selective reduction of protactinium from the molten fluoride fuel salt into liquid bismuth, followed by the preferential transfer of the rare earths from the fluoride salt through a liquid bismuth phase into an acceptor salt such as LiC1 or LiBr[1-3]. This processing method is based, in part, on data obtained for the equilibrium distribution of several lanthanide and actinide elements between liquid bismuth and lithium halide salts. Some of the data were presented previously [4-6]; the rest are given below. At a given temperature, the distribution of an element, M, between a molten salt containing a lithium halide and a liquid bismuth phase can be expressed as the general reaction MXn¢salt) -4- nLiCni) = Mere) + nLiXcsalt)
( 1)
*Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. 1. L. E. McNeese, In M S R Program Semiann. Progr. Rept. Feb. 28, 1970, p. 277. U S A E C Rep. ORNL-4548 (1970). 2. L. E. McNeese and L. M. Ferris, Trans.ANS 14(1), 84 (1971). 3. Chem. Technol. Div. Ann. Progr. Rept. May 31, 1971, p. 2. U S A E C Rep. ORNL-4682 (1971). 4. L. M. Ferris, J. C. Mailen, J. J. Lawrance, F. J. Smith and E. D. Nogueira, J. inorg, nucl. Chem. 32, 2019 (1970). 5. L. M. Fen'is, J. C. Mailen and F. J. Srnith, J. inorg, nuel. Chem. 33, 1325 (1971). 6. J. C. Mailen and L. M. Ferris, lnorg, nucl. Chem. Letters 7, 431 (1971). 2921
2922
L . M . FERRIS, F. J. SMITH, J. C. M A I L E N and M. J. BELL
in which X is F, C1 or Br; n is the oxidation number of M n+ in the salt phase; and (salt) and (Bi) denote the respective phases. Equation (1) obviously applies only to the distribution of a solute that is in a single oxidation state in the salt phase. An equilibrium constant for reaction (1) can be written as K=
n
?l
NMTMNLIxTLIx NMXnTMxnN~iT~|
(2)
in which N denotes mole fraction and ,/designates an activity coefficient referred to the appropriate pure sofid or liquid. If the various activity coefficients and Nux are practically constant and the distribution coefficient is defined as DM =
(3)
N_..__M_~ NMXn '
Equation (2) can be written as DM = ( N ~ ) ( K * )
(4)
log DM = n log N u + log K~.
(5)
or, in logarithmic form,
A plot of log DM VS. log NLI should give a line of slope n if n is constant and the assumption made regarding the constancy of the activity coefficients is valid. In our previous work with LiF-BeFe-ThF4 and LiF-BeF2 salts[4, 5], we defined n
n
I
DM = (NLi/NLiF) (KM).
(6)
The relationship between K~t and K*, obtained from Equations (4) and (6), is K*=
(K~)/(N~iF).
(7)
In some experiments, M existed in the salt phase in both the 3+ and 2+ oxidation states. The distribution of M between the two phases was considered in terms of the following simultaneous equilibria: M(ai) + 3LiX(salt)
(8)
MX2(salt) -I- 2Li(m) ---- M(ai) q- 2LiX(salt).
(9)
MX3(salt) q- 3Li(Bi) : -
From the expressions for the equilibrium constants for reactions (8) and (9), we obtain N MYM " N L3i X .3 yLiX
NMX3
K
"
8YMXz
N 3 ,3 LiYLi
(10)
Distribution of lanthanide and actinide elements
NMX~=
2 2 N MTMNLix'YLiX
v" ,
Ix-9YMX2
2923
(11)
N 2 ,2 " LiYLi
If the concentration of M is very low in both phases, the distribution coefficient can be expressed as DM = NM(Bi) ----- NM(Bi) NMtsalt) NMX.~+ NMX2"
(12)
Thus, from Equations (10-12), DM =
NM [NMTMNLIxTLiX N M y.M N 2LIX J'LIX[ ,2 q" 3 3 I
I f NLix
8YMX3
LiYLI
9YMX2
(13)
L i y L i _l
and the respective activity coefficients are constant, we can define • N 3
,3
= r i L~rrax KsYmx3Y~i
(14)
• N 2 ,2 ~- )'M LiX TLiX
g
(15)
2 "
9'YMX2'~Li
Substitution of Equations (14) and (15) into Equation (13) yields, in logarithmic form, log DM = 3log NLi -- log (a + flNLi).
(16)
log KMX3 * = -- log a
(17)
log K*x, = -- log ft.
(18)
By definition
It can be shown that the average oxidation number of M in the salt phase is /]M =
(19)
3o~+ 2flNLi o~+,SNLi
Distribution coefficients for two (or more) elements were frequently obtained in the same experiment. Using Dx and Du to represent the distribution coefficients for elements x and y obtained at the same value of NLi, it follows from Equation (5) that a plot of log Dx vs. log D, should give a line having a slope of nx/nu if both nx and n~ are constant. If M~ exists in the salt phase in both the 3+ and 2+ oxidation states and n -- 3 for M u, we can consider the following equilibria: MxXz(sam + Mu(m) MxX2(salt) +
= Mx(m) +
2 / 3 M u ( m ) = Mx(m) +
MuXz(salt)
(20)
2/3MuXa(salt).
(21)
2924
L . M . FERRIS, F. J. SMITH, J. C. M A I L E N and M. J. BELL
From expressions of the equilibrium constants for reactions (20) and (21) and the same assumptions made in the derivation of Equation (16), we obtain log D~ = log D r -- log (~bu+ E~Du 1/3)
(22)
and 3~bu+ 2~uD~ 1/3 =
~bu + ~uD~ 1/3
(23)
in which ~b~and ~ are constants. EXPERIMENTAL
Reagents. The bismuth used in this study was Cominco American Co. 69 grade, and the lithium metal, barium metal, LiC1, LiBr and LiF were reagent grade. The fluoride salts were prepared and purified as noted previously [4]. The lanthanide elements were available either as pure oxides or metals. Uranium was added either as the pure metal (natural U) or as a hydrated oxide (233U). The plutonium metal used contained about 0.1% 241Am. Other actinides (243Am, 244Cm and 2s~Cf) were obtained in 1 N HC1 solutions from the Transuranium Research Laboratory of the Oak Ridge National Laboratory. The protactinium used in experiments with LiC1 and LiBr was 233Pa at the tracer level. The gaseous hydrogen chloride, which was Matheson electronic grade, was used without further treatment. The high-purity gaseous hydrogen bromide was also obtained from the Matheson Co. The hydrogen was Matheson ultrapure grade and was treated further by passage through a catalytic oxidizer and a Molecular Sieve trap. Argon, which was used as an inert cover gas, was purified by passage through traps packed either with uranium turnings [4] or titanium sponge. Either crystal-bar thorium or a lithium-bismuth alloy was used as the reductant. The alloy, which usually contained about 7 at. % Li, was prepared from the pure metals. Apparatus and procedures. The apparatus, general procedure, and sampling technique have been described elsewhere [4-6]. In many of the experiments, distribution coefficients were obtained at only one temperature and the values of n and log K* were determined from the appropriate isotherm. In other experiments, the temperature of the system was varied and values of log K* were calculated from Equation (5), using the value of n obtained from an isotherm and the results of analyses of several sets of samples taken at each temperature. The procedures utilized with fluoride salts were described previously [4]. With LiCi or LiBr as the salt phase, different start-up procedures were used, depending on the elements involved. In most cases, the desired components were loaded, along with bismuth and ovendried LiC1 or LiBr, into a molybdenum crucible. After being heated to about 650°C under argon, the molten two-phase system was treated for at least 24 hr with either an HC1-H2 or an HBr-H2 mixture to dissolve the respective components in the salt phase and to remove oxides from the system. This treatment was followed by sparging of the system with pure argon to remove residual HCI or HBr. After a small amount of thorium had been added to the system to ensure the lowest possible oxide concentration, reductant (usually Li-Bi alloy) was added in increments to effect the systematic reduction and transfer of the solutes from the salt phase to the bismuth phase, as described previously[4, 6]. In some experiments with La, Nd, Eu and Ba, the procedure involved simply heating the salt and bismuth to about 650°C under argon, adding a small amount of thorium to scavenge oxides, and, then, adding the desired metal to the system in an amount such that, at equilibrium, the distribution coefficient was between 0.1 and 1. Finally, reductant was added to the system and the experiment was concluded in the usual manner. The general technique used with the transuranium elements was described previously [6]. Methods for analyzing for the various elements have been discussed elsewhere [4-6]. Data for neptunium presented in this paper were obtained from gamma-spectrometric analysis for the 239Np daughter of 24ZAm. RESULTS AND DISCUSSION
Molten fluoride mixtures as the salt phase. The equilibrium distribution of several solutes (Table 1) between liquid bismuth solutions and various molten
Distribution of lanthanide and actinide elements
2925
Table 1. Values of log K* obtained with LiF-BeF2-ThF4 salt solutions Salt composition (mole %)
LiF
BeFz
68
25
ThF4
Temp. (°C)
7
600 700
69.2
19-4
11.4
550
700 69.6
24-2
6.2
600 700
70.2
23.4
6.4
600 700
72
16
12
73
2
25
600 645 690 600 650 700
73.4
21.4
75
13
5-2 12
600 700
Element
n
log K~
Eu Th Eu Th Eu Th U La Th La Th La Th Eu Th Eu Th Ba Ba Ba Eu Th Eu Th Eu Th Eu Th La
2 4 2 4 2 4 3 3 4 3 4 3 4 2 4 2 4 2 2 2 2 4 2 4 2 4 2 4 3 3 2 4
4.093 10-191 3.534 8.558 4.366 10.745 12.794 6.262 8.614 7.275 9.848 6.330 8-608 4.137 9.929 3.634 8.574 4-049 3-678 3.530 3.840 9.560 3 -566 8.768 3.405 8.264 4.075 9.006 5.809 5.998 3.469 7.738
Nd Eu Th
LiF-BeF2-ThF4 salts was determined at 600-700°C. Plots of log Du vs. log NLi were linear, and the slopes of these plots, within experimental error, gave integral values of n. Values of n and log K* derived from the data are given in Table 1. The uncertainty in each v~lue of log K~ is _+0.1. The values of n obtained for La, Nd, Eu and Th in the present study are the same as those obtained in earlier studies [4, 5] with other fluoride salts. The behavior of barium, the only alkaline earth element studied, was very similar to that of europium. The present data are also in accord with previous data[4, 5] in that the values of log K * obtained with a given salt generally decreased with increasing temperature and, at a given temperature, usually decreased as the "free fluoride" concentration of the salt increased. The "free fluoride" concentration (FF) is defined as
FF = (mole % LiF) -- 2(mole % BeF2) -- 3(mole % ThF4).
2926
L . M . FERRIS, F. J. SMITH, J. C. M A I L E N and M. J. BELL
In an experiment at 600°C, the distribution of lanthanum and thorium between liquid bismuth and L i F - N a F - B e F 2 - T h F 4 (62-10-16-12 mole %) was determined. The values log K*a = 7.38 ± 0.2 and log K* h = 9.57 ± 0.2 are very close to the corresponding values obtained with LiF-BeF2-ThF4 (72-16-12 mole %) as the salt phase [4]. An experiment was conducted at 600°C to determine the distribution of lanthanum and thorium between liquid lead and molten LiF-BeF2-ThF4 (70-1911 mole %). Only a few data points were obtained because the lead phase became saturated with thorium when the thorium concentration reached 190___10 ppm. This concentration is in excellent agreement with the 200 ppm reported by Shaffer et al. [7] for the solubility of thorium in lead at 600°C. The equilibrium lithium concentration in the lead phase was 9 ppm, and the thorium distribution coefficient was 0.0014. These values yield log K* h = 11.47 ± 0.2. The lanthanum distribution coefficient could not be determined accurately, but was found to be less than 0.005. Molten LiCl or LiBr as the salt phase. The equilibrium distribution of barium and several lanthanide and actinide elements between liquid bismuth solutions and molten LiC1 or LiBr was determined in the temperature range 640-750°C. Most of the experiments were conducted at a single temperature, and values of n and log K* were determined from the isotherms represented either by Equation (5) or by Equation (16). In general, the data for the solutes were well represented by Equation (5); i.e. a plot of IogDM VS. IogNLi was linear with a practically integral slope. Typical plots are shown in Figs. 1 and 2; similar data plots have been presented elsewhere[I-3, 6, 8]. The integral value of n was determined from these plots by visual fitting of the data, and the values of log K* (Table 2) were subsequently obtained by fitting the best line of slope n to the data. In a few cases, notably with protactinium, the lithium concentration in the bismuth phase was too low for accurate analysis over the range of NLi where values of DM could be obtained. Values of n and log K* for such elements were determined from plots of the logarithms of the distribution coefficients vs. the logarithms of the distribution coefficients of a solute for which n and log K* were known. A plot of log Dpa vs. log Dxh, using data obtained with LiC1 as the salt phase at 640°C, has been presented elsewhere [9]. Plots of log Dcf vs. log Dcm, using data obtained with LiF-BeF2 (66.7-33.3 mole %) at 600°C, and of log DNp vs. log Dpu, using data obtained with LiC1 at 640°C, are shown in Fig. 3. The fact that most of the plots of log DM vs. log NL~ and of log Dx vs. log D~ were linear supports the assumptions made regarding the constancy of the various activity coefficients in the derivation of Equation (5). The data obtained with the various salt solutions also show that, over the range of conditions where distribution coefficients could be measured, most of the solutes existed in the salt phase in a single oxidation state. In several instances, data were obtained over a range of temperature for 7. J. H. Shatfer, F. A. Doss, W. K. R. Finnell and W. P. Teichert, In Reactor Chem. Div. Ann. Progr. Rept. Dec. 31, 1965, p. 43. U S A E C Rep. ORNL-3913 (1966). 8. L. M. Ferris, F. J. Smith, J. C. Mailen and M. J. Bell, J. inorg, nucl. Chem. 34, 313 (1972). 9. L. M. Ferris and F. J. Smith, In MSR Program Semiann. Progr. Rept. Feb. 28, 1970, p. 291. U S A E C Rep. ORNL-4548 (1970).
2927
Distribution of lanthanide and actinide elements
e u o o u
Q
I0 -~
I0
Lithium
~
in bismuth
IO ~
phase,
IU ~
Iu-'
mole fraction
Fig. 1. Distribution coefficients for thorium, lanthanum and samarium obtained at 640°C with LiC1 as the salt phase.
solutes for which n had been determined at two or more temperatures and had been found to be temperature invariant. Values of log K* were calculated at each temperature, using Equation (5), from the analyses of several pairs of samples. Plots of log K * vs. 1/T(°K) were found to be linear (Fig. 4); thus, the data could be represented as log K * = A + B / T ( ° K ) . Values of,'/and B for several solutes in the temperature range 625-750°C are given in Table 3. The simultaneous distribution of Pu, Cm, Cf and Am between liquid bismuth solutions and molten LiCI was determined both at 640 ° and 700°C. Plots of log Dpu vs. log Dcm were linear with slopes of 1, showing that npu was equal to ncm. The distribution coefficient data for plutonium and curium were also fit quite well by Equation (5) with n = 3. However, plots of log Dcf and log DAm VS. log NLi indicated that californium and americium had not existed in a single oxidation state in the salt phase at 640°C. Consequently, the distribution coefficients for these two elements were fit as a function of the values of NLi, using Equation (16), to determine the constants a and/3. The data were also fit as a function of the distribution coefficients of plutonium and curium, using Equation (22), to determine the constants ~b~ and e~. The nonlinear least-squares fitting of the data was performed using the O R D E A L data analysis program[10]. Typical data plots 10. M. Feliciano, C. W. Nestor, Jr., N. B. Gove and T. D. Calton, Oak Ridge Data Evaluation and Analysis Language. U S A E C Rep. ORNL-4506 (1970).
2928
L. M. FERRIS, F. J. SMITH, J. C. M A I L E N and M. J. BELL =
I0
8tJ o g
"E =a E3 I'C
001 I0 -4
10-3
I0 -2
Mole fractionlithium in bismuthphase Fig. 2. Distribution coefficients for plutonium, curium and californium obtained at 700°C with LiCI as the salt phase.
I00
F
DCf vs. DCm S Salt phase:LIF-BeF2 ~ (66.7-33.3 mole% ./ Temp: 600 "C ~ Slope =l ~ V
/
OI
I
0"01
, ~
e/ Je
1.0
]
I I[Arlll
/ /A J l
DNPV$"Dpu
~ j=
I
I
Salt phaee: LiCl Temp.640 =C
IIIIlII
0.1
I
I
I IIllll
1.0 Distribution
I
I0
coefficient
Fig. 3. Typical plots of log Dx vs. log D u.
(
I IIlll
I00
Distribution of lanthanide and actinide elements
2929
Table 2. Values of log K * obtained with LiCI and with LiBr as the
salt phase
Salt
Temp. (°C)
LiBr LiBr
575 600
LiCI-KCI* LiCI LiCI
600 630 640
LiBr
640
LiC1 LiCI
675 700
LiBr
700
Element Ba Ba La Nd Th Th Eu Ba La Nd Sm Th Pa U Np Pu Am Am Cm Cf Cf La Nd Th Pa U U La Nd Sm Eu Th Pa U Np Pu Am Am Cm Cf Ba Nd
n 2 2 3 3 4 4 2 2 3 3 2 4 4 3 3 3 3 2 3 3 2 3 3 4 4 3 3 3 3 2 2 4 4 3 3 3 3 2 3 2 2 3
log K~ 1.497 ± 0-06 1.443 ± 0.06 9.079 ± 0.2 8.919±0.3 16.16±0.3 15.663 ± 0.1 2.301 ± 0.05 1.702 ± 0.05 7.973 ± 0.2 8.633 ± 0.2 2.886 ± 0.1 15.358 ± 0.2 17.84±0.4 11.278 ± 0.2 10-330±0.2 10.126±0-2 9.955 ± 0.1 7.658 ± 0.3 9.406 ± 0.2 8.966 ± 0. I 5.419±0.1 8.266 ± 0.2 8.834 ± 0.2 14.80 ± 0.7 15-7___0.8 10.6+0.5 10.11 ±0-3 7" t 85 ± 0"2 7.831 ±0"2 2.756±0'1 2.133±0'1 13-772 ± 0.2 15.8±0.4 10.192±0-2 10.369±0-3 10.223 ±0"3 10.377 ± 0"2 7.155±0.1 9.589±0-2 5.37_+0-1 1.316 ± 0.05 8.430 ± 0.2
*LiCI-KC1 eutectic (59-41 mole %).
are shown in Figs. 5 and 6, and all the values of a,/3, ~b~,and eu derived from the data are given in Table 4. Analysis of this information reveals that californium was mostly in the 2+ oxidation state in the LiCI at both 640 and 700°C. The value of log K* = 5.36 +__0.05 for CfC12 calculated from the value o f / 3 at 700°C is
2930
L . M . FERRIS, F. J. SMITH, J. C. MAILEN and M. J. BELL *C
Temperature, 750 I
700 I
650 I
Salt phase: LiCI .J
Sm =r
•
•
: 2i
•
•
•
•
Bo
I
o
I ro.o
95
I to.5
I ~,o
t04/T,*K
Fig. 4. Effect of temperature on some values of log K~ obtained with LiCI as the salt phase. Table 3. Temperature dependence of log K * for several elements, using LiC1 or LiBr as the salt phase: log K * = A + B / T ( ° K ) (temperature range: 625-750°C)
Salt
M
n
A
B
Standard deviation of log K*
LiCI
Ba La Nd Sm Eu Ba Nd
2 3 3 2 2 2 3
- 0.6907 - 2.6585 - 3"3568 0.7518 - 0.1584 --0.0733 4.046
2189 9697 10900 1950 2250 1333 4297
0.02 0.1 0.08 0.05 0.05 0.02 0.1
LiBr
Table 4. Values of a, /3, ~b~, and ~u obtained by fitting americium and californium distribution coefficient data according to Equations (16) and (22)
Temp. Element x (°C) Am Cf
6~ 7~ 6~ 7~
101°a
10rfl
l.ll±0.11 0.42±0.19 10.8±3.2 0±0.62
0.22±0.11 0.70±0.]9 38.1±6.1 44.0±1.0
Element y = Pu
Element y = Cm
~bpu
~bcm
~eu
Ecru
1.41±0"06 0 . 1 ~ ± 0 . 0 2 9 0-24±0"01 0'088±0'01 1.9±0"2 0-092±0-054 0.40±0.06 0.056±0.026 24.6±2"0 15.6±1.5 3.97±0"38 5.79±0.53 4.8±1.7 21.6±0.9 0-2±0"3 8.8±0.4
Distribution of lanthanide and actinide elements
103
2931
3
OM = NLi a +/~ NLi
•
io 2
/
Americium
el = l . l l x l 0 -I°
/~=2.2xl0-e /
"/./
i01
io °
io -I
~ / /
• / /•• /•
o/~,~ Californium_^ .
~
~ =l'OexlO -9
e ~ e e e
e
/ ~ = 3 . 8 1 x l 0 -6
Io -~
io-=.
.
iii
....
i
....
I
......
10-3
Mole fraction
lithium in bismuth phase
Fig. 5. Least-squares plots of IogDAm and logDcf vs. IogNLi, using Equation (16). The data were obtained at 640°C with LiCI as the salt phase.
practically the same as the value obtained by fitting the best line of slope 2 to a plot of log Dcf vs. log NLi (see Fig. 2 and Table 2). The variation of the average oxidation numbers (hM) with distribution coefficients for californium at 640°C and for americium at 640°C and 700°C is shown in Fig. 7. As seen, each set of data confirms that these elements were present in both the 3+ and 2+ oxidation states in the LiC1. In each case, hM decreased with increasing distribution coefficient (i.e. with increasing reductant concentration in the bismuth phase), as expected. The disparity in the values of/~Am at 700°C among the various sets of data appears to be due primarily to a systematic bias in the values obtained for NLi. An indication of this bias is illustrated by the fact that the values of log K * and log K* m increased slightly when the temperature was increased from 640 ° to 700°C rather than decreasing as would be expected from the behavior of the other elements. These results confirm our preliminary evidence [6] that californium is relatively easily reduced to the 2+ state when LiC1 is used as the salt phase. The results of this investigation, along with those from our previous work [4-6, 8], show that thorium and protactinium are invariably in the 4+ oxidation state in the salt phase, regardless of the composition of the salt. Similarly, La, Nd, Pu, and Cm were always found to exist in the 3+ state in the salt phase. The values of n for U, Np and Eu were 3, 3 and 2 under the experimental conditions em-
2932
L.M.
F E R R I S , F. J. S M I T H , J. C. M A I L E N and M. J. B E L L
aM"
u
~
~u +-ol/s
Americium
,*pu.,.41
/
/-
/
I0 °
~- io-~ / L/ E "~ i0 ~
I0
t
~ e /
~ ~,
Californium q~Pu " 2 4 " 6 t p u = 15. 6
I i ilJJll t ~J~,Jl I IIIIII 1 l i l l i lO-Z i0-l I0 ° I0 ~ I0 z I0 z Plutonium distribution coefficient
Fig. 6. Least-squares plots of log DAm and log Dcf vs. log Dpu, using Equation (22). The data were obtained at 640°C with LiC1 as the salt phase. 3
I0 -3
I0 -2 Californium
I0 -3
I0
distribution
I0j
coefficient
3 E
40"C
I¢:::~
I
10-2
'
I
""~. ~-...."
I
iO-J
i0 o
Americium
distribution
1
I0 j
I0 2
I0s
coefficient
5
I0 -i
i0 o
I0 =
Americium
distribution
10 2
I0 3
coefficient
Fig. 7. Variation of t~cf and tZAmwith the respective distribution coefficients. Parameters used to calculate the three types of curves: a, fl, NLI, ; ~be~, ep., Do,, -........ ; ~Cm, ecm, Ocm, . . . . . •
Distribution of lanthanide and actinide elements
2933
ployed, whereas those for Sm, Cf and Am varied both with the composition of the salt phase and with temperature, other conditions being the same. In general, samarium was in the dipositive state; however, it was chiefly in the tripositive state [4] in several LiF-BeF2-ThF4 salts. Californium appeared to exist primarily in the 3+ state in fluoride salts (see, for example, Fig. 3) but, as noted above, was readily reduced to the 2+ state with LiCI as the salt phase. Evidence for the dipositive state of americium has been obtained both with LiCI (this study) and with certain fluorides [5] as the salt phase. Lack of thermodynamic data precludes estimation of (III)-(II) reduction potentials for the various couples. Acknowledgements-The authors thank J. J. Lawrance, J. F. Land and C. T. Thompson for their aid in conducting the experiments. Chemical analyses were provided by the groups of W. R. Laing, W. T. Mullins, and J. H. Cooper of the O R N L Analytical Chemistry Division.