ORNL-2106 - the Molten Salt Energy Technologies Web Site

ANP PROJECT PROGRESS REPORT
TABLE 2.3.1. SUMMARY OF VAPOR PRESSURES FOR THE SYSTEM KF-ZrF, c,
Composition Heat of
Constants
Vaporization
Experimental Temperature (mole X) of ZrF4, &I,,
Range (OC) A €3
ZrF4 KF
(kcal/mole)
in the relation
750-900 65.9 34.1 9.754 8,813 40.4
750-900 59.6 40.4 9.142 8,321 38.1
775-900 55.0 45.0 8.403 7,778 35.6
825-950 49.9 50.1 7.722 7,363 33.7
875-950 47.3 52.4 7.679 7,467 34.2
950- 1000 45.3 54.7 7.649 7,721 35.3
975-1 100 40.4 59.6 7.612 8,465 38.8
1 125-1 300 34.9 65.1 7.387 9,020 41.3
B
log P (mm Hg) = A - -
T '
where T is in OK. These constants are obtained
by treating the data by the method of least
squares. The heat of vaporization, AH,, of ZrF,
is obtained by multiplying the constant B by
2.303 (OR) or 4.576. The values listed in Table
2.3.1 are considered to be more reliable than
those previously published., Apparently there
was some free ZrF, present in some of the earlier
experiments, and for the compositions containing
65.9 to 49.9 mole % ZrF, a reanalysis by least
squares gave the new constants. A plot of the
data from which the equation for each mixture was
obtained is shown in Fig. 2.3.1.
A very odd phenomenon is observed when the
apparent heat of vaporization, AH,, is plotted
against composition (Fig. 2.3.2). A sharp minimum
occurs at 50 mole % ZrF, and the heat of vaporization
increases linearly with composition in both
directions from the minimum. The change below
50 mole % ZrF, is in the expected direction, since
ZrF, becomes more tightly complexed by fluoride
ions. The change above 50 mole % was not
expected; qualitatively it can be explained by
assuming that ZrF, exhibits positive deviations
from Raoult's law when dissolved in KZrF,.
00
(This version of Raoult's law calls for a twoparticle"
depression at the ZrF, end of the
composition range and for zero pressure at the
KZrF, end.) In practice, the vapor pressure of
supercooled liquid ZrF, is not sufficiently well
112
known for a numerically reliable Raoult law to
be established for ZrF, systems at 600 to 800°C.
The System RbF-ZrF,
The vapor pressures of various mixtures in the
system RbF-ZrF, were determined in the same
manner as in the KF-ZrF, system. Reasonably
pure RbF was obtained by selecting clear crystals
from a slowly cooled melt. Analyses showed that
the other alkali metal ions were present in the
following weight percentages: Li, 0.023; Na,
0.005; K, 0.16; Cs, 0.24.
The data for various mixtures are compiled in
Table 2.3.2 and shown graphically in Fig. 2.3.3.
The constants A and B are from the relation
B
log P (mm Hg) = A - -
T '
where T is in OK.
As in the case of the KF-ZrF, system, the heat
of vaporization goes through a minimum at 50 c
mole % ZrF, (Fig. 2.3.4). The linearity with mole
fraction is also similar to that of the KF-ZrF,
system. Thus the effect seems to be a real one
in spite of its unnatural appearance.
A comparison of the effects of the various alkali
fluorides is given in Fig. 2.3.5, in which pressure
is plotted as a function of composition at 912OC,
the melting point of pure ZrF,. Moore8 has
siudied two compositions for the system LiF-ZrF,,
-
and Sense et al.' of Battelle Memorial Institute
and Moore"' have studied the NaF-ZrF, system.
8R. E. Moore, ANP Quat. Ptog. Rep. June 10, 19.55,
ORNL-1896, P 81.
LJ
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