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N
OAK RIDGE NATIONAL LABORATORY
operated by
UNION CARBIDE CORPORATION
for the
U.5. ATOMIC ENERGY COMMISSION
ORNL- TM- 1129
copYNo. - 7C
DATE - May 7, 1965
OXIDE CHEMISTRY AND THERMODYNAMICS OF MOLTEN
LITHIUM FLUORIDE-BERYLLIUM FLUORIDE BY EQUILIBRATION WITH
GASEQUS WATER-HYDROGEN FLUCRIDE MIXTURES
A. L. Mathews*
C. F. Baes, Jr.
*Present address: Western Carolina College, Cullowhee, North Carolina.
A dissertation submitted to the Faculty of The University of Mississippi in
partial fulfillment of the requirements for the Degree of Doctor of Philosophy
in the Department of Chemistry.
LEGAL NOTICE —— - — —— e = —-
This report was prepared os an account of Government sponsored work. Neither the United States,
nor the Commission, nor any person acting on behalf of the Commission:
A. Mokes any warranty or representation, expressed or implied, with respect to the accuracy,
completeness, or usefulness of the information contained in this report, or that the use of
any informetion, apperatus, method, or process disclosed in this report may not infringe
privately owned rights; or
B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of
any information, apparatus, method, or process disclosed in this report,
As used in the cbove, ‘'person acting on behalf of the Commission’* includes any employee or
contractor of the Commission, or employee of such contractor, to the sxtent that such employee
or contractar of the Commission, or employes of such contractor prepares, disseminates, or
provides access to, any information pursuant to his employment or controct with the Commission,
or his employment with such controctor.
iii
ACKNCOWLEDGMENTS
This report is based upon a dissertation submitted to the University
of Mississippi in partial fulfillment of the requirements for the doctoral
degree. The report describes research carried out in the Reactor Chemistry
Division of the Oak Ridge National Laboratory, which is operated by the
Union Carbide Corporaticn for the Atomic Energy Commission. The research
was supported by the Oak Ridge Graduate Fellowship Program of the OCak Ridge
Institute of Nuclear Studies and was directed by a committee appointed by
Dean Lewis Nobles of the University of Mississippi Graduate School which
was composed of Dr. George Vaughan and Dr. Allen Cahill of the Department
of Chemistry at the University of Mississippi and Dr. C. F. Bases, Jr., and
Dr. C. H. Secoy of the Reactor Chemistry Division of the Osk Ridge National
Laborsgtory.
iv
CONTENTS
ACKNOWLEDGMENTS ¢ « ¢ o o o o o « o o o o o s o o o s
LIST OF FIGURES « « « + « o o o o o o s o s s o o o
LIST OF TABIIES - * . . . . . - . . - . - . - - . * .
ABSTRACT . - . . . - . * . . . » - - . . * . . L . .
I.
IT.
I:NITRODUCT ION . - . * e ® * . . » - . . . . . . .
Physical Properties of the LiF-BeF, System . .
Thermodynamic Properties of the Various Possible
SPeCieS . . . . -* . . . . . . . - - * . . »
Thermodynamic Studies of Molten Salt Mixtures
Solubilities of Gases In Melts . . « « « & « & &
Determination of Oxides in Melts « « « « « « o &
Suitable Experimental Approach . « « « « « o« &
Transpiration Method .+ « ¢« ¢ o« ¢ ¢ o & &
Equilibria .« o o ¢« o ¢ o o o o o o &+ o
Saturated Melts .« ¢ ¢ ¢ o ¢« o &+ o & &
Unsaturated Melts . ¢« ¢ ¢« ¢ « o o o &
mE:RMTAL . » * » . . . * » » - . L . - . - »
Chemicals . & ¢ ¢ ¢« ¢ 4 ¢« ¢ ¢ o o o o o s o o
GaBSeS o « o o+ o o s + o & s = o s o o s e .
Melt Components « « o « o s o o s o o o o
Standard Reagents « ¢« o ¢ o o & o o o o o« &
Apparatus . o o o o 4 o s o s s 2 s s 2 s e o
Flow Control Panel . . . & v ¢ o & o o « o
Agueous HF Saturator .« « o« « ¢ ¢ o o o o &
Page
iii
vii
X
G
14
14
16
20
21
23
23
23
23
23
24
24
26
I11.
Anhydrous HF Mixing System
Reaction Vessel . .
Titration Assembly
Gas Volume Measurement
Procedure . « ¢« « ¢« . .
Measurements . . .
Titrations . .
Calculations .
Limitations .
Systematic Errors .
Measured Volume
Influent Pressure
*
L d
Hydrogen Diffusion .
Dead~volume .
Sumexry . . .
Random Exrrors . . .
Melt Composition
Melt Temperature
Titer Precision
Wet-test Meter Temperature
Endpoint Precision . .
Flow~-rate Precision
Statistical Error Analysis
mSULTS - . . - L . - »
Tabulation « « « ¢ o & @
Page
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28
29
29
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30
31
32
32
34
34
36
37
38
38
40
41
41
41
41
42
Saturated Melts .
Unsaturated Melts .
vi
L ] . * .
Determination of Equilibrium Quotients
Saturated Melts .
Unsaturated Melts
Validity of Results
Saturated Melts .
Unsaturated Melts
IVe DISCUSSION ¢ « o » &
Correlation of Q@ . -
Activity of BeF2 and IiF
Thermodynamics of BeFa(l) . . .
Correlation of
Y
Correlation of QO
Summaxry
BIBLIOGRAPHY
APPENDIX A . .
APPENDIX B .+
APPENDIX C . .
- . - - *
Page
45
45
46
54
71
71
72
T4
T4
75
88
92
98
104
106
111
146
177
Fig.
Fig .
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
S
10.
11.
vii
LIST OF FIGURES
Phase Diagram of the LiF-BeF, System (From Thoma,
I'ef- 2) - . . * - - . * - . . . . . . - - . . - . L] . *
Complete Flow-Diagram for Apparatus . . « + « o o o « &
BeO-saturated 0.333 BeFz Showing (a) Calculated and
Observed Partial Pressures, and (b) Linear Correlation
Of Pressures « « o o o o o o
BeO-saturated 0.300 BeFz Showing (a) Calculated and
Observed Partial Pressures, and (b) Linear Correlation
of Pressures in Applicable Reglon « « « « & ¢« + o o &
BeO-saturated 0.300 BeF; during Hz Sparging, Showing
(a) Calculated and Observed Partial Pressures, and
(b) Linear Correlation of Pressures « « « « « « « o &
(a) Dependence of x and y on
r and s with W. Run No. 303
(a) Dependence of x and y on
r and s with W. Run No. 305
(a) Dependence of x and y on
r and s with W. Run No. 306
(a) Dependence of x and y on
r and s with W Run No. 307
(a) Dependence of x and y on
r and s with W Run No. 313
(a) Dependence of x and y on
r and s with W. Run No. 501
W,
W,
and
and
* .
and
(b) Variation of
(b) Vvariation of
(b) Variation of
Page
25
49
50
51
59
60
6l
62
63
64
Fig-
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig-
Fig.
Fig.
Fig.
12.
13.
14.
15.
16.
18.
19.
20.
21.
22.
23.
25.
viii
(a) Dependence of x and y on W, and (b) Variation of
rand s with W Run No. 511 .« « ¢« & ¢ ¢« ¢ ¢ o o « + &
(a) Dependence of x and y on W, and (b) Variation of
rand s with W. Run No. 533 . « « « v v v o o o «
(a) Dependence of x and y on W, and (b) Variation of
rand s with We Run No. 535 . . ¢« . . ¢« ¢ ¢« ¢ ¢ o+ &
(a) Dependence of x and y on W, and (b) Variation of
rand s with Wo Run No. 539 .+ « ¢« ¢ v ¢ ¢ v v o o v &
(a) Dependence of x and y on W, and (b) Variation of
rand s with We Run No. 607 .+ ¢ « o o o ¢ o o o o « &
(a) Dependence of x and y on W, and (b) Variation of
rand s with We Run No. 621 . . « « ¢ « ¢« ¢ &« ¢ o .« &
Correlation of log Q as a Function of Melt Composition
and Temperature « « ¢ ¢« ¢ o ¢ o o o o o o 2 o 2 e o 4 .
Agreement between Observed Q and Value of Q from
Correlation « « + ¢« ¢ ¢ ¢ o o o o o o o o o o s o s o s
Activity Coefficients of LiF and BeFy in Mixtures . . .
Thermodynamic Activities of LiF and BeFz in Mixtures. .
Heat of Fusion of BeFs from Activities at Freezing
TemperatuUreS .+ « o « o o o o o o o o o o « o o s o o o
Correlation of log QA as a Function of Composition
and Temperature « « « « ¢ o o o o o o o o o o » o s
Correlation of log QO as a Function of Temperature
for Various Melt Compositions « « « « ¢ ¢« ¢ ¢« v o & o« &
Solubility of BeO as a Function of Temperature for
Various Melt Compositions « « ¢ ¢ ¢ ¢ ¢ o ¢ o o o o o &
Page
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69
70
85
86
9
102
103
Table
Table
Table
Table
Table
Table
Table
Table
ix
LIST OF TABLES
Equilibrium Constants Predicted from Thermodynamic
Data o ¢ o o o o 4 s o 4 o o o s s o6 e o o v 2 o o o s
Activities of LiF and BeFy from Literature o+ « « o «
Equilibrium Quotients, Q and QA’ Calculated from Data
on Oxide-saturated Melts =+ o s ¢ o+ o o ¢ o ¢ o ¢ ¢ + &
Parameters for Unsaturated Melts from Least Squares
Program « » « o o = o o = o s o o s o o o« s o 4 o o o
Comparison of Calculated and Observed Partial Pressures
for Unsaturated Melts o o ¢ « o o « o o o o s o o o o
Parameters from Correlation of Q as & Function of
Temperature at Specified Compositions ¢ « ¢« + ¢ o o « &
Smoothed Parameters from Correlation of Q as a Function
of Composition and Temperature at the Specified
CompositionNs « o o o o o o o « o o o s o o o o s o o o
Solubility of BeO in Molten LiF-BeFz System « ¢ o - .« &
Page
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101
OXIDE CHEMISTRY AND THERMODYNAMICS OF MOLTEN
LITHIUM FLUORIDE~-BERYIIIUM FLUORIDE BY EQUILIBRATION WITH
GASEQUS WATER-HYDROGEN FLUORIDE MIXTURES
A. L. Mathews C. F. Baes, Jr.
ABSTRACT
The transpiration method was used to equilibrate dilute
gaseous mixtures of HF and H20 in hydrogen carrier gas with
molten LiF-BeF; mixtures varying in composition from 0.25 to
0.80 ReF2, both saturated and unsaturated with crystalline
BeO, in the temperature range 500 to 700°C. The partial pres-
sure data were used to evaluate the equilibrium quotient for
the reaction of HF and Hz0 with solid BeO and dissolved BeFz.
Bquilibrium quotients were also obtained for the formation of
oxide and hydroxide ions in the liquid phase.
These equilibrium gquotients were employed to determine:
(1) thermodynamic activities of LiF and BeFz in the mixtures;
(2) thermodynamics of liquid BeFa; (3) stability of hydroxide
in the melt; and (4) solubility of BeO in the LiF-BeF2 system
as a function of temperature.
I. INTRODUCTION
Molten mixtures of LiF and BeFz have been the subject of numerous
investigations in recent years primarily because of their suitability as
a carrier solvent for UFg in fluid fueled nuclear reactors. In addition,
these solutions are especially worthy of study because the components -
highly ionic IiF and highly associated, more covalent BeFz - represent
extreme types of fluoride salts. Although the molten LiF-BeFz system has
received considerable attention from both a practical and a theoretical
point of view, the study of its chemistry is still far from complete.
According to Everest,l many of the investigations of beryllium fluoride
systems did not take into account the role of moisture and the resulting
hydrolysis products. If this information were available, future investi-
gators could make appropriate experimental adjustments and corrections.
The purpose of the present investigation was to study heterogeneous
reactions of the type
Hao0(g) + fluoride species(soln)’=t HF(g) + oxygen species(soln)
in the molten LiF-Bel, system. From such a study information could in
principle be obtained about: (1) the thermodynamic activities of LiF and
BeF2, (2) the solubilities and stabilities of oxides, and (3) the inter-
action of oxide with the proton and perheps other cations in this molten
fluoride system. Information about the chemical reactivity of the compo-
nents in the molten LiF-BeF; system could be obtained from the thermodynamic
activities of LiF and Belz. The oxide chemistry of this system is of
interest because oxide is a principal impurity to be dealt with in prepara-
tive work and because metal oxides are known to be only sparingly soluble
in LiF-Bel'z mixtures.
The presence of oxide species in the molten IiF-BeF; system, which
is currently being used as the solvent in the Molten-Salt Reactor Experi-
ment at ORNL, constitutes an undesirable impurity since inadvertent pre-
cipitation of sparingly soluble uranium (and other) oxides might result
in unstable reactor operation. One of the steps in the purification of
melts for reactor operation is sparging with a mixture of HF and Hp to
remove oxide. The present study of the equilibria involved would yield
additional guidelines for this treatment.
The reactivity of oxide with beryllium and other cations has been
investigated as a possible means of removal of reactor products, or of
uranium to be reprocessed for later use. In order for proper evaluation
of these methods to be carried out, a thorough understanding of the inter-
actions occurring in melts would be desirable. For example, the use of
H20 as the source of reactive oxygen for oxide precipitating schemes and
the use of HF for removal of oxide require that the stability of the inter-
mediate hydroxide be evaluated in the melts.
In the present study the transpiration method was used to equilibrate
dilute gaseous mixtures of HF and H20 in hydrogen carrier gas with molten
LiF-BeF, mixtures varying in composition from 0.25 to 0.80 BeFz, both sat-
urated and unsaturated with crystalline BeO, in the temperature range 500
to 700°C. The primary data from the measurements were used to evaluate
the equilibrium quotient for the reaction of HF and H20 with solid BeO and
dissolved BeFz. Equilibrium quotients were also obtained for the formation
of oxide and hydroxide ions in the liquid phase. These quantities in turn
could be used to obtain the thermodynamic activities of LiF and BeFy as
well as the solubility of BeO and the stability of hydroxide. Use of the
HF-H20 equilibria as a much needed analytical tool for the determination
of oxide in such melts was also indicated.
Physical Properties of the LiF-BeFz System
Beryllium fluoride is frequently found in the form of a glass rather
than a crystalline solid. The beryllium fluoride glass consists of a ran-
dom network structure in which the beryllium atoms are surrounded tetra-
hedrally by four fluorine atoms and each fluorine atom by two beryllium
atoms.l Iiquid beryliium fluoride retains the polymeric character of the
glass as indicated by its high viscosity. In contrast to the covalent
nature of beryllium fluoride, lithium fluoride is a highly ionic salt.
The addition of LiF to liquid BeFz causes a breaking down of the polymeric
structure, but apparently the tetrahedral BeF42™ groups are retained.
Although it isn't proof of structure in the liquid phase, the fact that
a compound LipBeF,; can be precipitated from the melt may be some indica-
tion of the short range order in the liquid phase.
Phase behavior of the LiF-BeF; system has been studied extensively
by Thomsa., gg_gl.z A copy of their published diagram.(Figure 1) is included
here to illustrate the major characteristics of the system.
The melting point of BeFz is 548°C; the melting point of LiF is 848°cC.
The liquidus temperatures for the BeFz-rich region have been difficult to
obtain because of the high viscosity of these solu.‘bions-3 A brief summary
of the phase studies of various BeF; systems is included in reference l.
Many of these systems parallel those of the much higher melting silicate
glasses.
Studies of the LiF-BeF; system in the temperature range of the pres-
ent work (500 to 700°C) are restricted to the region between the high
ligquidus temperatures at low BeF2 concentrations and high viscosity at
high BeF concentrations.
TEMPERATURE (°C)
650 \ | F
600 | B
\ LIQUID
550 \ . 548
LiF + LIQUID \
500 | 72LiF :Bef, o
450 — ~ / .
\t\ / . BeF, +LIQUID
400 - _— \
LiF + 2LiF -BeF, J 360.3
350 | |
2LiF -BeF, + BeF»
300 | | |
LIF 10 20 30 40 50 60 70 80 90 Bef,
BeF, (mole %)
Fig. 1. Phase Diagram of the LiF-BeF, System (From Thoma, ref. 2).
Thermodynamic Properties of the Various Possible Species
From previous studies of LiF, BeF2, and the other possible species
present, some information can be drawn about the expected behavior of the
system. The thermochemical data for BeF; are summarized in the JANAF
Tables.* There is considerable uncertainty in aH,” of BeFa(1), which vas
derived from the AH_.°
£ 298.15
capacity functions for both the solid and liquid phases. Two sources
for the crystal and the appropriate heat
of error were cited in JANAF. The heat capacity studies were made on
samples of BeF2 which contained BeO and Hz0. Also, the heat of fusion
is uncertain. The value of 2 kecal/mole was used in the tabulation (be-
cause of the similarity of BeFz glass to B203 and Si02) even though a
value of 12.9 kcal/mole was determined from the vapor pressures over solid
and liquid BeFs. Determination of AH%O of Bng(l) would help resolve same
of these difficulties.
The thermochemical properties of BeFa(g) have been based on proper-
ties of the liquid and the heat of vaporization of BeF2(1l) except for the
work of Greenbaum, EE,EQ‘5 who determined equilibrium constants for the
reaction
BeO(s) + 2HF(g) = BeFa(g) + Ha20(g)
over the temperature range 670 to 970°¢. They report that a @lot of log
K vs 1/T yields a least squares slope corresponding to 20.5 * 1.7 keal/mole
for AH} over the temperature range studied, and that a plot of AF} vs T
gives a value of 6.0 * 0.3 cal deg™* mole™* for ASr by a least squares
analysis. If the calculated line for log K vs 1/T using these parameters
were drawn and the reported equilibrium quotients plotted, the line would
fall on the same side of all points. A least squares analysis using all
published data, with equal weighting for all points, gave values of Afii =
20.28 * 0.84 kcal/mole and 85 = 6.67 £ 0.77 cal deg™! mole™*. From their
values Greenbaum, et g_]___.5 reported the AHfo 208 of BeFa(g) as -191.3 +
2.0 kecal/mole and 50298 of BeFa(g) as 52.4 % 0.3 cal deg™! mole™!. Based
cn the recalculation, the values would be -191.5 % 1.1 and 53.1 % 0.8,
respectively.
The vapor pressure of BeFa(i) has been studied extensively.é'lo Sense,
93_25-6’7 studied the vapor pressure from 745 to 1021°C. Two Russian groups8’9
have reported vapor pressure studies. All of these are in general agreement
but have slight differences. The most extensive, and probably the best,
study (550 to 950°C) was that of Greenbaum, g&_g&.lo Since the enthalpy
and entropy of vaporization are reasonably well known from these measure-
ments, a combination of this information with independently determined
entropy and enthalpy of formation of Ber(;) should provide a new means
for evaluating the thermodynamic properties of BeFa(g).
The thermodynamic values of Hz0(g), HF(g), and BeO(s) are well char-
acterized throughout the temperature range of the present measurements
(see reference 4). Thus, if the AHr can be determined for the reaction
H20(g) + BeF2(1) = 2HF(g) + BeO(s) ,
then AH%O of BeFa(1l) can be calculated. The greatest uncertainty in AHfO
for HF(g) is the correction which should be applied for the imperfection
at room temperature. Several recent publications have dealt with the sub-
ject- 1117 Franck and Spalthoff'’ reported that the enthalpy of vaporiza-
tion rises from 89.5 cal/g at 19.4°C to a max of 146 cal/g at 130°C and
decreases at higher temperature. Armitage, g§_§£-12 show that the various
thermodynamic properties can best be explained by assuming that HF exists
in the gas phase principally as monomers and hexamers, but no actual indi=-
cation of the average molecular weight as a function of temperature or
pressure is given. Yabroff, gg.gi-lB have summarized most past work in
their report. They conclude that HF molecules are strongly associated
into polymeric forms and that dissociation is accompanied by large changes
in enthalpy-. Armstrong14 and Feder, g§.§£-l5 have considered the effect
of this association on the heat of formation of HF(g). The average molec-
ular weight of HF(g) at 1 atm and 25°C is 54, at 0.4 atm and 25°¢ is 22,
and at 1 atm and 80°C is 20.16 For pressures as low as a few hundredths
of an atmosphere both HF and Hz0 are reasonably ideal at 25°C and undoubt-
edly are ideal at melt temperatures.
From the thermodynamic gquantities tabulated in JANAF, equilibriuvm
constants were calculated for several conceivable reactions involving HF,
H20, BeFz, BeO, and LiF. These are presented in Table 1 along with the
Table 1. Equilibrium Constants Predicted from Thermodynamic Data
Reactio K at K at
1on 800°K 1000°K keal
BeO(s) + 2LiF(s) == Liz0(s) + BeFa(1) 3 x1071% 2 x1071 52.1
BeO(s) + Hx0(g) == Be(0H)2(s) 5x10°% 1 x107% -12.5
BeF2(1) + 2H20(g) = Be(OH)a(s) + 2HF(g) 1 x10™° 6 x10"° 12.8
2Be0(s) + 2HF(g) == BeFz(1) + Be(OH)2(s) 2 x107% 2 x107™% -37.9
Be(OH)2(s) + 21iF(s) == 2Li0OH(1) + BeFa(l) 1x107% 3x10°% 46.2
BeFa(1l) + H20(g) == BeO(s) + 2HF(g) 2x107% 6 x10°! 25.4
BeFa(g) + Ha0(g) == Be0(s) + 2HF(g) 5x10° 1x10° -21.7
BeO(s) + Hz0(g) == Be(0H)2(g) 2 x1071° 2x10"% 41.6
BeF2(1) == BeFa(g) 1x1078 2x1074 47.1
calculated heats of reaction. Although the equilibrium constants refer to
reactants and products which are pure solids or liquids, they could be ap-
plied to reactions in solution if appropriate activities were used. From
these data the following predictions are made:
(1) Oxygen containing compounds of Li should react to form Be compounds.
If Lip0 were added to an ILiF-BeF2 melt, the Liz0 should react almost
quantitatively to form BeO and LiF. If IiOH were added to an LiF-BeFp
melt, the LiOH should react almost quantitatively to form Be(OH)2 and
IiF.
(2) The formation of Be(OH)2 as a separate solid phase at temperatures as
high as 800%K is very unlikely.
(3) A11 of the stable campounds have low volatilities at the temperatures
of interest.
A more precise determination of the AH. of BeF2(1) and of the activ-
ities of BeFz2 and LiF in the melt would allow more gquantitative predictions
of reactions in the LiF-BeFp system.
Thermodynamic Studies of Molten Salt Mixtures
The determination of thermodynamic activities of melt components in
molten salts has received considerable attention. Blander has swmarized
much of the work through 1962.%7
The relationships between activities and activity coefficients depend
on the choice of concentration units. The most frequent unit for expressing
concentration of mixed solvents is the mole fraction. In molten salts the
ion fraction is frequently used. TFor systems in which all salts contain the
same anion, mole fraction and ion fraction are equal. The term "mole frac-
tion" will be used in the text.
10
According to the Temkin model,l8 in which salts are considered to be
completely ionic, the ideal activity of a component is equal to the product
of ion fractions of its constituents (aij = Xixj)' For a solution which
contains only one anion, j, the ion fraction,Xj, equals one. The ion frac-
tion of each cation is equal to the mole fraction of that component. The
activities of components are usually referred to the pure liguid (super-
cooled if necessary) as the standard state. Occasionally, the activities
are referred to the pure crystalline solid for experimental convenience.
The activities of components in solution have been measured by the fol-
lowing methods: +vapor pressures, freezing point depression, electrode
potentials, and heterogeneous equilibria. The vapor pressure method is
compllicated by the formation of complex species in the vapor phase. Deter-
mination of activities from freezing point depressions requires that the
heat of fusion and the qu for the pure solid and liquid solvent be known.
Electrode potential measurements of activities are often made in cells with
liguid Jjunction. OSuch measurements are limited to dilute solutions, which
are expected to give small liquid junction potentials.
The use of heterogeneous equilibria has thus far been limited. The
19,20 in mixtures with KC1 and NaCl
activity of MgClz has been determined
by use of the equilibrium
MgCla(soln) + 302(g) = Mgo(s) + C1la(g) -
Blood, g§_§£.21 determined the activities of various metal fluorides in
LiF-BeF; mixtures using the eguilibrium
M(s) + xHF(g) == MF_(soln) + (x/2)Ha(g) -
In studies of this type it is important that the solid present be well
characterized and relatively insoluble and, of course, that all phases are
in equilibrium.
11
If the activity of one of the components of a binary mixture is known
as a function of camposition, the other one can be determined by integration
of the Gibbs-Duhem equation. From the activities, such properties as molar
heats of mixing, excess chemical potential, vapor pressure, and phase behav-
ior can be derived.
At present, a general theory of the behavior of melts has not been
developed to the point that activities can be predicted for a system such
as LiF-BeF».
The activities of LiF and BeFz have been reported for a limited number
of cases. Berkowitz and Chupks in 1960 reported the activities in an equi-
molar mixture from relative ion intensities during mass-spectral ana];ysis.22
Recently, Buchler has reported determinations by a more careful mass-spectral
analysis and emf measurements. The emf measurements were conducted at two
temperatures in a concentration cell containing pure BeF2 in one compart-
ment and an LiF-BeFs mixture in the other-23 Blichler used a twin crucible
assembly in his determination of activities with the mass spectrometer to
facilitate comparison of pure compound and mixture-24 The results of these
experiments are compiled in Table 2.
12
Table 2. Activities of IiF and BeF, from Literature
*
ggzga ?ggg aBer* 7BeF2 aLiF ZLiF
0.50% 627 0.443 0.89 0.0246 0.049
0.25° 633 0.027 0.11
0.25° 692 0.039 0.16
0.26° 604 0.016 0.06 1
0.67° 604 0.86 1.3 0.076 0.23
*
Activities referred to BeF2(1l) and LiF(s), respectively.
®From Berkowitz and Chupka, mass spectrometry.
bFrom Bichler, emf.
“From Blichler, mass spectrometry.
Solubilities of Gases in Melts
In addition to the studies of activities in melts, the solubilities
of gases have also been of interest. Watson, g}_§£.25 have studied the
solubility of the inert gases in molten fluorides, including the LiF-BeFa
system. All solubilities obey Henry's law and increase with increasing
temperature and with decreasing atomic weight of gas.
Burkhard and Corbett26 reported the solubility 6f water in molten
LiCl-KCl mixtures. Apparently Henry's law was obeyed for low pressures,
with the deviations above a few millimeters attributed to hydrolysis. How-
ever, no analyses were reported on the gases to determine the amount of
H,0 and HC1l in the gas phase in equilibrium with the melt.
Shaffer, et al. have studied the solubility of HF in molten fluo=-
ride mixtures. In the LiF-BeF2 system28 solubility of HF increases with
13