ORNL-4191 - the Molten Salt Energy Technologies Web Site

THERMOCOUPI-F
FURNACE
I
TO VACUUM
ORNL-DWG 67-1f827
e TO TAYLOR PRESSURE
TRANSMITTER
*GAS SAMPLING
STPT IO ‘\i
PRESSURE
GAUGE k&
Fig. 13.5. Appnratus for Measuring Decomposition
Pressure of Fluoroborate Melts.
is shown schematically in Fig. 13.5. The mclt,
contained in nickel, exerts its vapor pressure on
a mercury manometer and on diaphragm gages. To
remove adsorbed gases, the vessel, containing a
weighed salt sample, is initially evacuated at 100
to 125°C for at least 15 hr. Then to release gases
encapsulated within the salt crystals the sample is
heated about 25OC above its liquidus temperature.
The sample is subsequently cooled down to room
temperature. If the apparent pressure is greater
than 0.5 mm at room temperature, the system is
reevacuated and then reheated and cooled until
the vapor pressure at room temperature is essentially
nil.
After this desorbing procedure, the observed
vapor pressures should be caused only by the
dissociation
NaRF,(2) = BF,(g) + NaF(2) .
Gas samples collected during the vapor pressure
measurements and analyzed mass spectrometrically
confirmed that the vapor was virtually pure BF,.
Usually about 1% SiF, is observed in the vapor,
this probably originates from the reaction of
desorbed KF with the glass parts of the apparatus.
After completing vapor pressure measurements,
the containment vessel is cut open and the contents
160
‘OrOOO r
-e-
FAEASURED
7 - 1-
ORNL DWG 67-41828
65 70 75 80 85 30 95 100
mole % Na BF4
Fig. 13.6. Smoothed Vapor Pressures in the No5F4-
NaF System.
are weighed and chemically analyzed for oxygen
and boron. The oxygen analysis is used as con-
firmation that the system had been leak-tight. ‘The
boron analysis and change in weight are comple-
mentary measurements of how much BE’, was lost
during the course of desorption and vapor pressure
measurement. To obviate coinposition changes in
the melt because of the inevitable loss of some
BF,, a relatively large sample of salt is charged
into the containment vessel.
Measurements have been taken on compositions
between 65 and 100 mole % NaRF4. Some results
are depicted in Fig. 13.6. To a fair approximation,
the data may be represented by the equilibrium
quotient
Q = (pressure of BF,)
P
(molc fraction of NaF)
x - - ~
(mole fraction of NaBF,)