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UNCLASSIFIED
ORNL
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Orrind-a81
CONF-571-70
SEP2 1 19
REMOVAL OF URANIUM HEXAFLUORIDE FROM GAS STREAMS
BY SODIUM FLUOKIDE PELLETS*
Leonard E. McNeese
Stanley H. Jury
MASTE.
Oak Ridge National Laboratory
Oak Ridge, Tennessee
To be presented at the American Chemical Society
National Meeting, Chicago, Illinois, August 31,
1964 to September 4, 1964.
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playee or contractor of the Commission, or employee of mech contractor, to the extent that
moet plan or dructor Cowboat, or plesne med meretur meparsan
de maio, o medo non ten un mormado por la plume or antract
with the Cor umjacies, or his employment with such contractor.

*Research sponsored by the U. S. Atomic Energy Commission under contract
with the Union Carbide Corporation.
Olonlik

$
REMOVAL OF URANIUM HEXAFLUORIDE FROM GAS STREAMS
BY SODIUM FLUORIDE PELLETS
Leonard E. McNeese*
Stanley H. Jury*
.
e
.
ABSTRACT
2
1.
.
The removal of uranium hexafluoride from a gas stream containing uranium
hexafluoride and nitrogen by a single layer of sodium fluoride pellets was ·
investigated. Experimental data on the rate and extent of sorption were
obtained in the temperature range 29 to 225°C and uranium hexafluoride concen-
tration range 0.57 to 10.9 mole per cent uranium hexafluoride,
The results of this study indicate that the rate controlling mechanisms
are: transfer of uranium hexafluoride across a stagnant gas filma surrounding
.
the pellet, diffusion of gaseous uragium hexafluoride in the pores of the
t
=
pellet, and diffusion of uranium hexafluoride through a layer of uranium
hexafluoride-sodium fluoride complex covering the unreacted sodium fluoride
in the interior of the pel.let. The crystalline density of the complex,
UF&•2NaF, was determined to be 4.13 grams per cubic centimeter which indicates
that incomplete reaction of the sodium fluoride will occur for pellets in
which the initial volume void fraction is less than 0.807.
A useful model was devised to represent the sorption of uranium hexafluoride
.
S'
.
1
.
by a single pellet of sodium fluoride. The experimental data were correlated
on the basis of the model with a root-mean-square error of 12.0 per cent for
all points.
* Oak Ridge National Laboratory, Oak Ridge, Tennessee.
''
-2-
This paper is concerned with the determination of the rate controlling
mechanisms for the removal of uranium hexafluoride from flowing streams of
uranium hexafluoride in nitrogen by cylindrical pellets (one-eighth-inch right
circular) of sodium fluoride.
As reported in the previous paper, uranium hexafluoride reacts reversibly
with sodium fluoride to form a solid complex UF&•2Naf. The reversible character
of the reaction makes it attractive as a means for separating uranium hexafluoride
from other gases and/or as an alternative to low-temperature cold trapping for
collecting uranium hexafluoride, Differences in the decomposition pressures
of the fluorides that form sodium fluoride complexes are exploited during
alternate sorption and desorption of the uranium hexafluoride in fixed beds
of sodium fluoride to produce a uranium hexafluoride product of high purity.
Information on the effects of various system parameters on the rate of
sorption is needed so that sodium fluoride sorbers may be designed for a wide
In this study, a determination was made of the rate-controlling steps in
the sorption of uranium hexafluoride from a stream of uranium hexafluoride and
-
nitrogen by sodium fluoride in the form of one -eighth-inch right circular
cylindrical pellets at atmospheric pressure. The temperature range covered
was 29 to 225°C, the uranium hexafluoride concentration range was 0.57 to 10.9
mole per cent.
The experimental phase of the study consisted mainly of the determination
of the loading of uranium hexafluoride on single layers of sodiun fluoride
pellets during a prescribed time Interval under a given set of conditions.
Some work was vecessary for preparation of samples of the uranium hexafluoride-
sodium fluoride complex for determination for crystalline density.
JI
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*
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A flow diagram for the equipment used i.n this study 18 shown in Figure 1.
.
.
T
Basically, the equipment provided means for preparing gas mixtures of the
desired composition at a controlled flow rate, means for controlling the
temperature of the gas mixture and the sorption vessel, and means for a second
determination of the flow rate of the two gases used.
The flow rate of uranium hexaf:uoride was set by maintaining a controlled
pressure drop across a calibrated capillary. The nitrogen was metered through
a rotaneter from a constant-pressure nitrogen supply.
The two gases were
LX
introduced into a common line that lej to a gas preheater.
The preheated
gas then flowed through the sorption vessel and through a sodium fluoride trap
'
l
for removal of uranium hexafluoride which passed through the sorption vessel.
Both the gas preheater and the sorption vessel were placed in a constant.
temperature bath in which the temperature was controlled to within 0.5°C of
the desired operating temperature. After removal of the uranium hexafluoride,
the nitrogen was metered by a wet test meter.
A bypass around the sorption vessel and the sodium fluoride trap was
provided so that the gases did not flow through the sorption vessel during
startup and shutdown of the metering system.
The single layer runs were made by using one layer of sodium fluoride
pellets placed between two fcur-and-one-fourth-inch sections of three-millimeter
glass or Monel beads used as entrance and exit sections. As shown in Figure 2,
the bed was constructed from one-and-one-half-inch diameter schedule-forty
nickel pipe. The glass or Monel beads were conditioned before use by exposure
to fluorine at 400°C for one hour.
Prior to a run, the sorption vessel waa loaded and placed in the constant-
temperature bath. At least one hour was allowed for the bed temperature to
reach that of the bath, since it was calculated that after 0.45 hour the difference
between the center-line temperature of she bed and the bath temperature would
UNCLASSIFIED
ORNL-DWG 63-2026

O
ROTAMETER
Rotamere 91
CONSTANT
PRESSURE
N, SUPPLY
CALIBRATED
CAPILLARIES
mm
PRESSURE
GAUGE
SORPTION
VESSEL
PRESSURE
CONTROL VALVE
mon
PREHEATER &
Lam
COIL
UF, SUPPLY
OFF-GAS
WET TEST
METER
Slide 1. Flow Diagram for Equipment Used in the Study of Sorption of Uranium Hexafluoride by Sodium Fluoride.
Let's
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UNCLASSIFIED
ORNL-LR-DWG 66094
1.61"
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RES

res
Inlet -
3/8-inch
1 1/2-inch Schedule 40
Nickel Pipe
-3 mm glass beads
4 1/4" B
9 3/4"
00000
Single layer of 1/8-inch
Sodium Fluoride
1
3 mm glass beads
3
4 1/4"
.
.
3/8-inch
--- Screen
S
Exit
-
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Slide 2. Sorption Vessel Used in Differential-Bed Studies.
time
be less than two per cent of the original temperature difference. Prior to
starting a run, the uranium hexafluoride and nitrogen flow rates were set,
and the stream was allowed to flow through the preheater coil for at least
five minutes in order to establish a constant concentration in the coil.
During this period, the stream was bypassed around the sorption vessel and
the sodium fluoride bed.
A run was started by diverting the stream through
49
the differential-bed. Gas was pissed through the bed for a predetermined
length of time, after which the stream was diverted into the bypass. About
ten seconds were required for effecting the valving changes necessary for
diverting the gas stream. The uranium hexafluoride flow was stopped and the
nitrogen flow was continued through the preheater coi: for two minutes to
free the coil of uranium hexafluoride. The nitrogen flow was then diverted
through the sorption vessel for one minute to free the bed of unreacted uranium
hexafluoride. The bed was then seuled, removed from the constant-temperature
bath, and allowed to cool in a dry box containing dry air. The sodium fluoride
was removed from the reaction vessel in the dry box and placed in a sealed
The sodium fluoride trap
.
container for weighing on an analytical balance.
was treated similarly.
For determination of the crystalline density of the uranium hexafluoride. .
sodium fluoride complex, pellets of sodium fluoride that had been treated with
fluorine for one hour at 400°C were ground with a mortar and pestle in a dry
box. A half-inch layer of the powder in a nickel dish was exposed to a stream
of dilute gaseous uranium hexafluoride (five to ten mole per cent uranium hexa-
fluoride sa nitrogen) at 100°C, after which the resultant material was again
ground with a mortar and pestle in a dry box. The material was repeatedly
exposed to uranium hexafluoride and ground until the desired weight gain had
occurred. Samples of the material and of the original sodium fluoride were then
submitted for crystalline density determination by toluene immersion. The
crystalline density of the complex was determined to be 4.13 g/cms at 26°C.
The partially reacted pellets from a number of runs were sectioned for
determination of the distribution of sorbed uranium hexafluoride.
The contrast
between the pale yellow color of the complex and the white color of sodium
fluoride allowed qualitative determination of the distribution of the uranium
hexafluoride in a pellet by microscopic and photographic means. Partially
reacted pellets, which have been sectioned axially, are shown in Figure 3,
By use of the proper filter, the pale yellow of the complex was made to appear
grey. The pellets in Figure 3 are from a run at 50°C and contain the maximum
quantity of uranium hexafluoride which will be sorbed at that temperature. A
considerable variation in penetration of uranium hexafluoride is observed not
only between individual pellets but also between different areas of the same
pellet. Of particular interest is the slight penetration near the corners of
some pellets since this type of profile is not observed in the case of diffusion
of a substance into a finite cylinder of constant properties. It is felt that
the differences in penetration are due to variation in the density of different
areas of the pellet. The method of manufacture of the pellets, compression of
a powder, would be expected to produce variations in density, with high-density
areas in the unreacted corners of the sectioned pellets. Profiles at other
temperatures are similar to those at 50°C except that less penetration is
observed at higher temperatures and more at lower temperatures.
The over-all process of removal of uranium hexafluoride from a flowing
stream of uranium hexafluoride in nitrogen by a pellet of sodium fluoride is
believed to involve the following processes:
1. The movement of uranium hexafluoride from the gas stream to the
external surface of the pellet, which 18 commonly depicted as the
PHOTO NO M3552
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Hemi..

UNREACTED
AREA
......
Slide 3. Axially-Sectioned Sodium Fluoride Pellets Containing Uranium Hexafluoride
Sorbed at 50°C.
transfer of uraniu hexafluoride across a stagnant nitrogen film
surrour.ding the pellet.
The diffusion of gaseous urunium hexafluoride through nitrogen in the
pores of the pellet.
The diffusion of gaseous uranium hexafluoride through a layer of complex
on the internal sur faces of the pellet.
4. The reaction of uranlum hexafluoride with sodium fluoride below the
complex layer.
Any model proposed to represent the over-all sorption process must take into
account thcse steps whose rates are of the same order of magnitude as the rate
of the over-all process. The model must also account for effects resulting from
deposition of complex in the pores of the pellet.
The most striking characteristic of the system under study is the effect.
of temperature or the rate and on the extent of reaction between uranium hexa-
fluoride and sodium fluoride. For reaction between uranium hexafluoride and
powdered sodium fluoride, an increase in temperature over the range 29 to 100°C
results in an increase in the rate of reaction and the extent of reaction at
..
a specific time. However, in the same temperature range, there is an inverse
.--
.-..
.
effect of temperature on the maximum extent of the reaction of the sodium
.
-
--
fluoride pellets with uranium hexafluoride.
An explanation of this anoma lous effect of temperature on the rate and
extent of sorption for finely divided sodium fluoride and for sodium fluoride
pellets 18 based on the combination of two ideas.
The first idea is that the
temperature dependence of most chemical reactions and of solid-phase diffusion
is of the form e kyk, whereas the temperature dependence of bulk diffusion
of gases is of the form 13/2. At some temperature, a given Increase in tempera- ,,
ture will increase the point reaction rate by a greater amount that the point .
-10-
rate of bulk diffusion. The second idea is that, based on measurement of the
crystalline density of the complex, the pores of a pellet will be closed with
complex before complete reaction of the sodium fluoride can occur. (A maximum
reaction of thirty-three per cent of the sodium fluoride was calculated for
the typical NaF pellets.)
In order to test the hypotheses discussed above it 18 necessary to solve
the equacion for diffusion with chemical reaction allowing for variable
diffusivity and variable reaction rate.
The assumptions shown in Figure 4 were made. Typical experimental data
and calculated results for a typical series of runs are shown in Figures 5, 6, and
7. A total of 108 points were obtained in a set of 12 such series. The root-
mean-square error for all points was 12.0 per cent.
The variation of pellet capacity for UFg with temperature 18 shown in
Figure 8 for several values of initial pellet surface area.
The following conclusions can be drawn from the results of this study:
1.
The apparent mechanisms controlling the rate of sorption of uranium
hexafluoride by sodium fluoride pellets are transfer of uranium hexafluoride
across a stagnant gas film surrounding the pellet, diffusion of gaseous uranium
hexafluoride in the pore space of the pellet, and diffusion of uranium hexa-
fluoride through a layer of uranium hexafluoride-sodium fluoride complex covering
unreacted sodium fluoride.
2. Sorption of uranium hexafluoride at a point in a sodium fluoride pellet
results in a decrease in the volume void fraction, the effective diffusivity
of gaseous uranium hexafluoride, and the reaction rate at the point.
3. The maximum quantity of uranium hexafluoride that can be sorbed at a
point in a pellet of sodium fluoride depends only on the initial void fraction
of the pellet at the point. For void fraction values. less than 0.807,
Incomplete reaction of the sodium fluoride will occur.
.11.
1.
The pellet 18 a sphere having a volume equal to the one-nighth-
inch right circular cylindrical pellets.
2.
The pellet is homogeneous.
W
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Radial symmetry.
The temperature variation in the pollet to negligible.
Radial transfer of Ufo in the pellet is by diffusion of gaseous
UFO through an inert gas in the pores of the pellet.
6. The local rate of reaction between gaseous UF, and unreacted Nar
to of the form:
9 (c.au
dek, e-B/RT
ka + -B7RT
where. C* 18. the decomposition pressure of the complex,
7. The local effective diffusivity for diffusion in the pores of the
pellet 18 of the form:
De Dura-Ne Y€? (1 9/ yle
Slide 4.
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UNCLASSIFIED
ORNL-DWG 63-2609

PELLET LOADING (UF / NoF)
12
EXPERIMENTAL CONDITIONS
T 100°C
Ce 0.57 mole % UFE
G 4.99 103 g/sec.cm2
400
800
1200
1600
2400
2800
3200
3600
4000
2000
TIME (sec)
Slide 5. Comparison of Experimental and Model-Predicted Data Showing Variation of Pellet Loading with
Time at 100°C and 0.57 Mole Per Cent Uranium Hexafluoride.
L
11th
UNCLASSIFIED

PELLET LOADING (9 UFG/0 NoF)
EXPERIMENTAL CONDITIONS
I 100°C
Go 2.45 molo % UF
6 6.12 * 10°3 g/sec.cm2
.
.
400
800
1200
1600
2000
TIME (sec)
2400
2800
3200
3600
4000
Slide 6. Comparison of Experimental and Model-Predicted Data Showing Variation of Pellet Loading with
Time at 50°C and 2.35 Mole Per Cent Uranium Hexafluoride.
"T
UNCLASSIFIED
ORNL-OWO 63-2611
1

-14-
PELLET LOADING (g UFG/NoF)
EXPERIMENTAL CONDITIONS
T 100°C
Ca 8.54 mole % UF
G 1.01 * 10-2 g/sec.cm?
400
2400
2800
4000
800 1200 1600 2000
3200 3600
TIME (sec)
Slide 7. Comparison of Experimental and Model-Predicted Data Showing Variation of Pellet Loading with
Time at 100°C and 8.51 Mole Per Cent Uranium Hexafluoride.
UNCLASSIFIED
ORNL-DWG 63-2745

PELLET CAPACITY (UFg/gNaF)
0.86 m 2/9m
-15-
1.0 m²/gm
1.2 m²/gm
50
80
110
130
TEMPERATURE (°C)
Slide 8. Calculated Values of Effective Pellet Capacity for Pellets having an Initial Void Fraction
of 0.45 Showing Effect of Pellet Surface Area.
.
.
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-16-
4. Cessation of sorption of uranium hexafluoride by pelleted sodium
fluoride occurs when the pores at the external pellet surface have been
filled with complex.
5.
The density of the uranium hexafluoride-sodium fluoride complex
UF8.2NaF is 4.13 grams per cubic centimeter at 26°C.
6. Considerable variation in characteristics exist in individual pellets
as well as between pellets of commercial sodium fluoride.
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DATE FILMED
11/23 /64







VIP
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3
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man
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END
D.