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BeF, in 2LiF 13eF,, this partial pressure was
found to be surprisingly low, that is, 0.03 mm at
700'K to 21 mm at 1000'K. Flirthermore, pre-
liminary direct measurements now in progress con-
firm the low range of these partial pressures. This
suggested that an overpressure of SiF , might pre-
vent, or at least reduce, the attack of the silica
container and prevent the precipitation of BeO.
The compatibility of SiO, with these LiF-BeF ,
melts is primarily associated with the relative
stability of BeF, compared with that of BeO; the
standard free energies of formation at 1000°K are
-206 and -120 kcal/mole respectively. Since
many other metallic fluorides show even greater
stability compared with their corresponding oxides,
it seems that Si0, should not be ruled out as a
container material for molten fluorides without
first estimating the equilibrium position of such
reactions as
X
--SiQ,(s) i MFx(s or I ) + MOx,,(s)
4
X
+ --SiF,(g) , (4)
4
which involve the pure metal fluoride and metal
oxide, and
The latter reaction is the one of most interest,
since it gives the level of oxide contamination
of the fluoride melt. Its equilibrium position may
be judged if activity coefficients or solubilities
can be assigned to MFx and MOxiz in the melt
under consideration. In the present case the activity
of BeF, in 2LiF. BeF, (",0.03) and the
solubility of He0 (-0.01 mole of 0'- per kg of
salt at 600'C) have been measured. From these
and the available thermochemical data the following
equilibrium quotient may be estimated for the
above reaction [Eq. (5)l in 2LiF. BeF, at 600°C:
7C. F. Baes, Jr., "The Chemistry and Thermodynamics
of Molten Salt Reactor Fluoride Solutions," SM-66/60
in Thermodynamics, vol. 1, IAEA, Vienna.
138
From this, it can in turn be estimated that in the
presence of 1 atm of SiF, the oxide ion concentration
at equilibrium with SiO, should be only
6 x lo-, mole/kg.
Another factor to be considered when using silica
is the possible formation of silicates; for
example,
2MF,(s or 2) i 2SiO,(s)
In the present case, for example, Be,SiO, (phen-
acite) should be formed as the stable reaction
product rather than ReQ at low SiF, partial pres--
suies; however, phenacite is not very stable rcl-
ative to 2He0 -I SiO, and should not alter the
above conclusions about the compatibility of sil-
ica with LiF-BeF, melts. In other fluoride sys-
tems, the formation of metallic silicates may be
the controlling factor.
The solubility of SiF, in the molten fluoride
must be considered because possible reactions
such as
could produce high solubilities which not only
would alter the salt phase, but also would cause
the reaction with SiO, to proceed farther than
otherwise expected. Accordingly, the solubility
of SiF , in 2LiF . BeF was estimated by means
of transpiration measurements. The prepurifjed
salt was saturated with SiF, by bubbling a mix-
ture of 0 1 atiii of SiF, in He through the salt
until the effluent and influent compositions were
the same. The dissolved gas was then stripped
with helium and trapped in an aqueous NaOH bub-
bler, the amount of SiF , being equivalent to the
amount of NaOH consumed by its hydrolysis:
SiF,(g) i- 40H- + 4F- + 2H,0 I SiO,(s) . (9)
The solubility was calculated by means of
in which