ORNL-2106 - the Molten Salt Energy Technologies Web Site

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ANP PROJECT PROGRESS REPORT
fixed midway between the two half cells was
formerly used as the cell temperature. In order to
refine the temperature measurements, a calibrated
thermocouple, enclosed in a nickel tube, was sub
stituted for an electrode in a dummy cell. In the
temperature range of interest (550 to 750OC) the
melt temperatures found were 10 f 2OC higher than
the melt temperatures at the center between the two
crucibles. Thus all the temperatures reported in
connection with previous potential measurements
should be revised upward by 10°C. Only the solu-
bility data are sufficiently temperature sensitive
to be appreciably affected.
Application of the temperature correction to the
solubility data previously reported' gave the
following new expressions for the solubility of
MF,.ZrF, in NaF-ZrF, (53-47 mole %):
36.0~ 103
IogN = - + 6.87 , for FeF,*ZrF,
4.576 T
40.1 103
log N = - + 8.36 , for CrF,*ZrF,,
4.576 T
28.8 103
109 N = - + 4.72 , for NiF,.tF,,16
4.576 T
where N is the mole fraction of MF2*ZrF4 and T is
temperature in OK. The corrected heats of solution
and "ideal" melting points are now 36.0, 40.1, and
28.8kcal/mole MF,.ZrF, and 875, 775, and 1060OC
for the Fe, G, and Ni compounds, respectively.
Solubility data obtained by analyses of filtrates
from saturated solutions, as described in the pre-
ceding sections of this chapter ("Equilibrium
Reduction of NiF, by H, in NaF-ZrF," and 'lSolu-
bility and Stability of Structural Metal Fluorides in
Molten NaF-ZrF,"), did not agree with the elec-
trometr ica I I y determined so I ubi I i
t ies, particular I y
in the case of NiF,. Since the filtration measure-
ments were carried out by adding NiF, rather-than
NiF,.tF, as solute (and similarly with FeF, and
Cr F,), the electrometric determinations were re-
peated with NiF, as solute so that the data could
16From this investigation and x-ray and petrographic
examinations of quenches of NaF-ZrF4 melts, NiF, is
now believed to exist in a ternary compound, rather than
as NiF2*ZrF4, in saturated solutions. The equation
for the solubility was revised to include additional
measurements, as well as the temperature correction.
104
be compared. X-ray and petrographic examinations
of the solidified melt from the half cells should
have sufficed to identify either or both of the joins
W
as quasi-binary; in practice, however, both
NiF,*ZrF, and a new phase were found in the
slowly cooled half cells. The new phase is thought
to be a ternary compound, since it has never been
found in the binary systems.
Gradient quenches (510 to 540°C) along the two
joins revealed only the new phase, with no
NiF,*EF,. Such ambiguous results were disappointing,
since on a quasi-binary join the solubility
does not vary with the amount of solute
added, as it does along a random join.
Cells containing MF, in NaF-ZrF, (53-47 mole
%), with no addition of ZrF,, gave the solubilities
of the structural metal salts shown in Table 2.2.10
and Fig. 2.2.4 for NiF, and Table 2.2.11 for FeF,.
The solubilities of the nickel salts along the joins
NiF2eZrF4-7NaF*6ZrF4 and Ni F2-7NaF*6ZrF4, as
well as the values for NiF, obtained by filtration
methods, are plotted in Fig, 2.2.4. The discrepancy
between the results of the various experiments
has not been explained. The electrometrically
determined solubilities appear to be virtually the
some whether the solute is added as NiF, or as
NiF,*tF,, and the same seems to be true for
TABLE 2.2.10. SOLUBILITY OF NiF, IN
NaF-ZrF, (53-47 MOLE %)
&fsoln = 28.8 kcal
Ideal melting paint = 106OoC
Temperature NiF, Solubility
(OC) wt % mole %
553 0.12 0.12
564
0.14 0.15
1
574 0.19 0.20
576 0.20 0.21
650 0.79 0.83
650 0.93 0.97
655 0.88 0.92
669 0.98 1.01
706 1.97 2.0 5
725 2.19 2.29
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