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ORN-P.321
571-24) DTIE-S
jCT 5 1964
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A LABORATORY EVALUATION OF THE CHEMICAL AND PHYSICAL PHENOMENA ASSOCIATED WITH
CYCLIC SORPTION-DESORPTION OF URANIUM HEXAFLUORIDE ON SODIUM FLUORIDE
.
By
Sidney Katz

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LEGAL NOTICE --
TW. report n. poporodu u komt al Covenant sponsors wort. Molther who Uniad
now, no Who Couselom, por may person kung an Douall of the Commission:
A. Mivei my vuruly or repemudom, opred or lupuud, wiu respect to the accu.
racy, completo, or wohl... al the labor mathon contained in wo roport, or what the wo
.way intorution, appunto, pothod, or process dwloud da wo roport may not intrin
prinuly omdraus;
B. Aswas my lattian i na part to the wool, or for domarna rowing from the
one ol way telor anthon, appunto, wohnd, or procou a cloud la we repori.
Au mund ta who sboro, porton acting on to all of the Commiulo" includes may •.
parue or coolractor of the Coun noloa, or a ploy of will contractor, to the one that
oucl oplore or contractor of the couuisutan, or soforo such conuular propanı,
dows alas in, or provider noc w to, way mornalon perinudI W. vaployuct or contract
will do Corinnetou, or No soployoni mwa muca osutor.
*To be presented orally on September 3, 1964 at the American Chemical Society
Meeting, Chicago, Illinois.
Research sponsored by the U. S. Atomic Energy Commission under contract with
Union Carbide Corporation.
The
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CYCLIC SORPTION-DESORPTION OF URANIUM HEXAFLUORIDE ON S ODIUM FLUORIDE
Beds of Naf are used in several processes to selectively sorb and separate
UFO from cther gases. In Slide 1 16 shown a general scheme of such separations.
SLID
The stream containing the UF, passed into the sodium fluoride beds held at 150°C
for the sorption part of the cycle; and in the desorption part of the cycle the
bed temperature is raised to 300° to 400°C.
Historically, Martin, Albers, and Dust in 1951 first reported this formation
and dissociation of double fluorides in the NaF-UF, system. In the decade that
followed that area has been further investigated at QRNL, ANL, ORGDP, PDP, within
the UKAEA and elsewhere with the goal of applying the separatior. to processes for
recovering uranium from reactor fuels. While substantial progress has been made,
several areas remain where improvements and additional fundamental understanding
of the mechanisms involved could be useful. One problem is the failure to sorb
anywhere near to the stoichiometric capacity corresponding to UFG 2NF. A second
problem is the gradual mechanical deterioration of the NaF pellet on successive
SLID
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sorption-desorption cycles. A third problem is lack of information of the sorption-
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desorption characteristics of other gases from which separation may be desired.
At ORNL these three problems have had a 2 pronged attack. First, a static,
laboratory study using undiluted UF, in a gasometric apparatus which I will
describe in this paper. Second, a dynamic, engineering type study of a fixed
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bed with dilute UF, which will be described in the second paper by i. E. McNeese
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this morning. These papers have been arranged so that the introduction to this:
first paper will suffice for the second, the conclusions drawn from the inter-
related data are discussed in the second paper.
I shall first describe the gasometric technique and then the specific experi-
ments. The 3rd Slide presents the gasometric apparatus. For most of the work
ŞLI
discussed here, a 10 millimole sample of NaF or 420 milligrams was placed in a
nickel tray and the tray inserted into this reactor. After sealing and leak
testing the reactor, it was heated to 300°C and exposed to fluorine at 200 mm Hg
pressure for 1/2 br to minimize subsequent wall reactions with UFG. Then the
reactor was maintained at the desired experimental temperature while the sample
was exposed to a measured quantity of a desired gas by opening this valve. The
internal volumes of the various sections of the apparatus were previously call-
brated and therefore the quantities of gas involved in the reaction were known at
all times. There reversible sorption occurred, the steady state pressures could
be measured. Finally at the end of a test, the residual solid in the reactor
could be analyzed chemically.
This apparatus permitted a rapid survey of the effects of variablcs involved!
in sorption-desorption process, the synthesis of compounds by gas-solid reaction,
the measurement of dissociation pressure of chemical systems, and the develop-
ment of conditions most farorable to gas-solid reaction.
Turning now to specific experimental work, the problem of the failure of
sodium fluoride pellets to sorb anywhere near the stoichiometric quantitiy of
UF, is considered first. The 4th slide presents the results of a comparative
SLIDE 4
study of sorption-desorption on pellets and high surface area NaF. You will note
that with the pelleted NaF the first cycle sorption capacity is about 1/3 the
stoichiometric capacity and the 2nd and 3rd cycle capacity are much lower. For
succeeding cycles a continuing gradual increase was noted. These capacities
compare well to those experienced in process applications where the pelleted Ner
is used. With the high surface area NaF the initial cycle capacity is the
18
stoichiometric limit imposed by the composition UF, 2N&F. That complex compound
had been predicted from or exchange studies and was prepared previously by
reaction of gaseous UF with NaF suspended in fluorocarbon oil; that work was
done at ANL. This Naf measured 7 m®/8 surface and was prepared by treating NaF
with a 4-fold molar excess of HF followed by removal of the HF. An alternate
method of preparing high surface NaF 18 by decomposition of the UF 2NaF compound;
surface areas of 11 m/8 have been obtained by the second method. Slide 5 demon-
SLIDE
strates the volume difference between the same weight of pellets, reagent grade
NaF, and high surface NaF; this volume difference suggests that the pellets do
not contain sufficient volume to form the stoichiometric complex.
SLDE
Returning now to the remaining cycles of the high surface area NaF on Slide 4, www
you note a gradual reduction in capacity with cycling. For these cycles the compound
UF2NaF was retained at 320°C for 1 hour to simulate the slower desorption in
packed beds; in another series of cycles where the retention at 320°C was omitted
and the UF was permitted to desorb as the lowesi temperature at which sensible
desorption occurs (280°c) capacity 1066 was less than 1% per cycle.
In another series of sorption cycles with high surface area Naf, the tem-
perature of the NaF was raised between cycles to increasing temperatures until
sintering was detected by a reduction in sorption capacity. The temperature of
490°C for 1 hour resulted in a 15% loss in capacity on the subsequent cycle.
From these tests the following factors listed in Slide 6 are considered
SLIDE 6
important for high sorption capacity of NaF for UFG.
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:
For the second problem, that of deterioration of the NaF pellets, pellets
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SLIDE
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were withdrawn from the reactor for examination and analyses after various stages
in the sorption-desorption cycles. In Slide 7 is shown the penetration by UFO
into the NaF pellet on the first sorption and the absence of penetration on suc-
ceeding cycles; both whole and broken pellets are shown. This complete change
in penetration appears related to the use of undiluted UF, in these tests; in
tests by other investigators using dilute UF, this change is not as evident.
In Slide 8 is demonstrated the roughening of the outer layer of the pellet,
os
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the breaking away of powder, and the sloughing off of a skin layer. These
pellets were withdrawn after the 3rd cycle. The gradual roughening and powder-
ing occurred on all cycles; the sloughing off of a skin was only noted after the
3rd cycle. The skins are very fragile and easily broken.
2
ste
The following factors shown in Slide 9 are evidently important in deterioration
of the NaF pellets.
With regard to the third problem, that of characterizing the sorption-de-
sorption characteristics of other gases, the work with Mofa and WFs had been
completed and compounds analogous to UF2NF have been prepared and the system
dissociation pressures measured; that work has been submitted for publication
to the Journal of Inorganic Chemistry, The dissociation pressures obtained are
SLIDE 1
shown in Slide 10. The stability of the tungsten complex appears to be similar
to that of the uranium complex rather than close to the molybdenum complex as
might be expected.
The compound UF 'NaF and the analogous tungsten and molybdenum compounds
have aloo been prepared by gas solid reactions and the dissociation pressures
measured; they are shown in Slide 11. From this data it is evident that none
of these complexes form at the normal process sorption temperature of 150°C.
These data permit selection of an optimum sorption temperature for best
separation of these gases within reasonable loss limits for UF
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+62-sarapa....
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Slices are ORNL UNCLASSIFIED DRAWINGS numbered volow
Sligo Number
Drawing number
64-6170
611-6174
63-3538
63-3539-1
Photo
64806
E 1
611-6171
64977
Photo
Photo
64807
64-6173
64-1651
64-5528
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UNCLASSIFIED
ORNL DWG. 64-6170
MATRIX GASES
UF, AND CARRIER GAS


NaF BED at
150°C
NaF BED at
300-400°C


UF, IN MATRIX GASES
CARRIER GAS
SORPTION
DESORPTION
SORPTION-DESORPTION OF URANIUM HEXAFLUORIDE
ON SODIUM FLUORIDE
UNCLASSIFIED
ORNL DWG. 64-6174
IMPORTANT REMAINING PROBLEMS
1. Failure to sorb stoichiometrically
to UF 6:2 NaF.
2. Deterioration of Naf pellets.
3. Need for information on sorption of
other gases.
UNCL ASSIFIED
ORNL DWG. 63.3538

SUPPLY 1 GAGE
SUPPLY 2 GAGE
E-HF SUPPLY
SUPPLY 1
PRESSURE INDICATOR
0-331 mm Hg
H
-- UF SUPPLY
SUPPLY 2
WF6 OR M0F6
SUPPLY
SUPPLY 3
PRESSURE RECORDER
0-200 mm Hg
APT
VACUUM AND
GAS-DISPOSAL SYSTEM
BURET
APT
APTT
SUPPLY AND ALSO
SAMPLE POINT
REACTOR
110°C BOX TEMPERATURE
UNCLASSIFIED
ORNL DWG. 63.3539 A
MOLE RATIO
UFO
NOF
UF6 NoF
2.0 — 0.5
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NOF PELLETS
ADSORPTION AT
135-145°C
ACTIVATED NOF
ADSORPTION AT
150-170°C


2.5 —
0.4
3.33 -
0.3
5.0 –
0.2
m
10.0 –
0.1 A
20
HOURS
HOURS
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UNCLASSIFIED
ORNL DWG. 64-6171
FACTORS FAVORABLE TO HIGH SORPTION CAPACITY
1. High surface area sodium fluoride.
2. Avoidance of sintering of the sodium fluoride
by high temperatures.
3. Adequate solid volume.
4. Low desorption temperature.
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仁中
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1
UNCLASSIFIED
ORNL DWG. 64-6173
1.
FACTORS AFFECTING PELLET DETERIORATION
1. Penetration of the UF:
2. Expansion of the crystal lattice and loosening
of surface layers of powder.
3. Exposure of new surfaces after breaking away
of powder.
.
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TEMPERATURE, °C
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DISSOCIATION PRESSURE, mm
DISSOCIATION PRESSURE, mm
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UNCLASSIFIED
ORNL DUO. SASSY
TEMPERATURE, °C
84
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200
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MoFo.NaF
DISSOCIATION PRESSURE, mm
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LEGAL NOTICE

This report was prepared as an account of Government sponsored work. Neither the United
Stales, nor the Commission, rör any person acting on behalf of the Commission:
A. Makes any warranty or representation, expressed or implied, with respect to the accu-
racy, completeness, or usefulness of the information contained in this report, or that the use
of any information, apparatus, method, or process disclosed in this report may not infringe
privately owned rights; or
B. Assumes any liabilities with respect to the use of, or for damages resulting from the
use of any information, apparatus, method, or process disclosed in this report.
As used in the above, "person acting on behalf of the Commission" includes any em-
ployee or contractor of the Commission, or employee of such contractor, to the extent that
such employee or contractor of the Commission, or employee of such contractor prepares,
disseminates, or provides access to, any information pursuant to his employment or contract
with the Commission, or his employment with such contractor.
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