ORNL-1771 - Oak Ridge National Laboratory

AN P QUARTER 1Y PROGRESS R EPOR J
The data presented here were all obtained at
roughly the same gas flow rate (100 ml/min).
However, since the rate of reduction is dependent
on the flow rate and on the geometry of the appe
ratus, the aeswlts must be considered merely as
representative and are subject to some reriation
with conditions; they apply to 3-kg batches in
4-in.-dia pots with hydrogen bubbles rising from
the notched end of a %-in. vertical dip tube im-
mersed to a depth of 5 in.
Reduction of FeF, by H, in NoF-ZrF, System5
C. M. Blood G. M. Wotson
Materials Chemistty Division
Since the reduction of FeF, with hydrogen is the
time-consuming step in purification of NaZrF,
mixtures for reactor use and testing, an extensive
study of the equilibrium constant of this reaction
has been attempted, The reaction
FeF, I H,+Feo + 2HF
is relatively easy to study, since the hydrogen
fluoride concentration in hydrogen tan be accu-
rately determined, and, after filtration of he melt,
accurate determinations of FeF, in the melt can
be obtained. Since at these elevated temperatures
the activities of hydrogen and hydrogen dluoride
can be assumed to beequal to their partiul pressures
in the gas phase, then
where CF~F, is expressed in mole fraction, and
from the re1 ati on ships
and
-,AFo = KT In Kcq
K =-,
Ke 4
the activity coefficient for FeF, at low concentra-
tions in NaZrF, should be easily determined. The
equilibrium constant for the reaction has been
estimated by two different methods: a dynamic
method and an equilibration metliocl.
Dynaraic Method, In preliminary experiments in
which H, was passed through molten NaF-ZrF,
(53-47 mole %) contaminated with know1 quantities
64
Kw
of FeF, or CrF,, the HF concentration of the
effluent H, was studied as o function of the H,
flow rate. The data obtained are plotted in Fig. 5.6,
in which the ratio of HF concentration at a given
flow rate to that found at a standard flow rate (210
ml/iTIiil) is plotted against flaw rate. As antici-
pated, the ratio drops at high flow rates; however,
the ratio shows no sign of leveling off at low flow
rates, as would be expected. Although the curve
shows no justification for the practice, the lowest
flow rate found to be practicable (9 mlhin) was
considered to be equivalent to a zero flow rate
(that is, the flow rate at which the HF concen-
tration could be expected to be at equilibrium),
and experimental values found in subsequent ex-
periments were corrected to the concentration that
they would have shown at such flow rate.
35
30
z
0
2
E 25
W z
z
s 20
LL
I
d
: 15
a
J
E
10
STANDARD FLOW R4TE = 210 ml/mln
a FeF2 + Hp 5 Fe" + 2tiC
0 Cr Fz + H2 C? + 2HF
UNCLASSIFIED
ORhL L7-D:K 2926
0 100 200 300 400
FLOW RATE (rnl/min)
Fig. 5.6. Effect of Gas Flow Rote on Relative
Saturation of Effluent Gas with HF When Passed
Thrcugh Molten NaF-ZrF, (53-47 mole %) Ccn-
tomineted with Known Quantities of FeF, or CrF,.
The experiments were performed by adding known
quantities of FeF, to an NaF-ZrF, (5347 mole 76)
melt in a clean nickel reactor and measuring the HF
concentration of the effluent hydrogen passed at
carefully measured flow rates. The cumulative
total of HF removed was also determined by col-
lection of die HF in caustic solution and titration
of the excess caustic. The FeF, concentration at
any specified time could be calculated from the
FeF, found by analysis on completion of the ex-
periment and the HF yield after the specified time.