WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
RECOVERY OF HIGH VALUE FLUORINE PRODUCTS
FROM URANIUM HEXAFLUORIDE CONVERSION
John B. Bulko, David S. Schlier
Starmet Corporation
2229 Main Street
Concord, MA 01742
ABSTRACT
Starmet Corporation has developed an integrated process for converting uranium
hexafluoride (UF6) into uranium oxide (as either U3O8 or UO2) while recovering the fluorine
value as useful products free of uranium contamination. The uranium oxide is suitable for
processing into DUAGG and DUCRETE , a depleted uranium oxide aggregate and concrete
form useful in nuclear shielding applications. The fluorine products can be used directly or
processed into other chemical forms depending on market demand.
The conversion process can be divided into two main operations, UF6 conversion to uranium
tetrafluoride (UF4) and subsequent processing of UF4 to uranium oxides with generation of
volatile fluoride gases such as silicon tetrafluoride (SiF4) and boron trifluoride (BF3). The front
end process chemistry, called the ‘6-to-4’ process, involves the vapor phase reaction of UF6 with
excess hydrogen (H2) at about 650°C and at pressures of 101 – 170 KPa in a vertical heated tube
reactor. The reduction products include non-volatile UF4 and gaseous hydrogen fluoride (HF).
Gaseous HF is collected by passing the process effluent through aqueous scrubbers. The solid
UF4 is collected as free flowing powder. Starmet has the only licensed and operational UF6 to
UF4 plant in North America. Located in Barnwell, South Carolina, this facility has the capacity
to convert up to 9 million pounds of UF6 per year to UF4.
Chemistry to further convert the UF4 by-product from the ‘6-to-4’ process has been developed
whereby UF4 is reacted with silicon dioxide (silica, SiO2) at 700°C to produce volatile SiF4 and
coincident uranium oxide. Alternatively, boric oxide (B2O3) has been used in place of SiO2 to
produce BF3. Both fluoride gases possess significantly higher value in comparison to HF, which
is the typical fluoride product recovered from hydrolysis and pyrohydrolysis processing of UF6.
Of greater significance is the generation of products free from uranium contamination which has
historically plagued other fluoride-based products derived from UF6 thereby discouraging
widespread commercial use and diminishing value. Starmet is currently designing a commercial
scale facility to recover these and other high value fluoride products from the immense inventory
of UF6 accumulated through enrichment operations over the last several decades.
INTRODUCTION
Over the past 50 years, the US Department of Energy (DOE) and its predecessors have
stockpiled more than 560,000 metric tons of depleted UF6 at facilities in Oak Ridge, TN,
Paducah, KY and Portsmouth, OH. Depleted UF6 (DUF6) is the non fissionable residue from the
enrichment process used to make nuclear grade enriched uranium for reactors and weapons.
There is currently no use for this material, and DOE is faced with the possibility that the
stockpile will be declared excess. If this action occurs, DOE would be forced to pay for disposal
of their entire DUF6 inventory. Disposal costs have been estimated at $1.4 billion, however,
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
more realistic cost projections based on current technology and capabilities are in the range of
$3-4 billion. To reduce the cost of managing the DUF6 inventory, Starmet Corporation has been
working to develop alternative approaches for production of stable uranium compounds and
recovery of fluorine from UF6.
Starmet Corporation has over 50 years experience in the handling and production of uranium
(U) and uranium chemicals with manufacturing plants in Concord, MA, and Barnwell, SC.
Based on Starmet’s installed capacity to produce more than 9 million pounds/year of UF4 from
UF6, investigations into new processes to economically produce uranium oxide and recover
fluorine from UF4 are underway. The high quality, depleted uranium oxide from these new
processes will be suited to the manufacture of depleted uranium aggregate for DUCRETE .
DUCRETE is a cement based radiation shielding material that uses uranium oxide aggregate in
place of conventional aggregate. By-products of the conversion processes are high value fluorine
compounds and anhydrous HF. These fluorine compounds can be used directly, as fluorinating
agents in the manufacture of organic and inorganic chemicals1,2, or as precursor compounds in
the synthesis of advanced non-oxide based ceramics3-6. The production of high value chemical
by-products provides the potential to realize revenues from uranium processing. By combining
development of new uses for the uranium, such as DUCRETE , with co-production of high value
fluorine chemicals, a technically viable and economically attractive approach for using UF6 is
now available as an alternative to disposal.
PROCESS OVERVIEW
One process being developed at Starmet for conversion of UF6 involves a two step operation.
The first step is the production of UF4 and HF by H2 reduction of gaseous UF6, shown in
equation 1.
UF6(g) + H2(g) → UF4(s) + 2HF(g)
(Eq. 1)
UF4 is a green crystalline solid commonly referred to as ‘green salt’. The second step is the
reaction of UF4 with an oxidizing agent to produce uranium oxide and release of fluorine in the
form of a volatile fluoride gas, MFy, as shown in equation (2). The oxidizing agent is shown here
as MOx, since there are numerous reagents that can be inserted in this reaction.
UF4(s) + MOx(s) → UOx(s) + MFy(g)
(Eq. 2)
The process of reacting UF6 with hydrogen to produce UF4 is well established. Investigations
into the conversion chemistry have attracted both domestic7 and international8-10 interest. Of the
numerous processes to reduce UF6, there are only two which are considered efficient and
economical for converting large inventories of material, namely the “cold wall” method and the
“hot wall” method. In the “cold wall” method, heat is supplied within the reaction zone by
admitting fluorine gas (F2) in addition to H2 and UF6. Heat is released by the reaction between
H2 and F2, raising the reaction temperature to ~1100°C while the reactor walls are maintained
between 150°-200°C. This procedure was developed for treating UF6 highly enriched with U235
isotope where it is essential to eliminate slag buildup in the reactor. Alternatively, the “hot wall”
method features a reaction chamber heated externally whereby the reduction reaction is initiated
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
by heat supplied through the reactor walls. It is this process that Starmet has implemented with
capacity to produce over 9 million pounds UF4 per year. In addition to green salt, the HF byproduct generated in the process can be recovered and sold in either anhydrous or aqueous form.
The market value of 70% aqueous HF is $0.40-0.50/lba while anhydrous hydrogen fluoride
ranges between $0.70-0.90/lbb. Hydrogen fluoride is more easily recovered in aqueous form
while anhydrous recovery is more difficult and expensive to process.
The conversion of green salt to uranium oxide with recovery of fluorine by-products is under
developmentb at Starmet. A pyrometallurgical or fusion approach is used that involves mixing
UF4 with either SiO2 or B2O3 and heating to a temperature sufficient to cause reaction. UF4 is
converted to uranium oxide with production of either SiF4 or BF3, respectively. Both products
are gases that can be easily separated from the solid uranium oxide. Both fluoride gases can be
sold directly in high purity form to markets in the semiconductor industry or they can be used in
the production of high performance ceramics such as boron nitride4 (BN) or silicon nitride11
(Si3N4). These ceramics are used for super abrasives, supertough coatings, refractories and diesel
and turbine engine parts.
The advantage of the new process is that it is inherently lower in operating and capital cost
compared to currently practiced methods. Additionally, the fusion process overcomes the main
objection that has restricted wide scale sale of HF generated by direct steam conversion of UF6 to
oxide, namely uranium carryover and radioactive contamination of the HF by-product. Since the
starting materials for making uranium oxide in the new process are solids, there is no possibility
of carryover into the gas phase fluorine by-product. Testing of the gas phase products has shown
that there is no detectable U in either the SiF4 or BF3 compounds made by this process. In
addition, since all the input materials are of relatively high purity, the products of the process are
also of high purity.
UF6 CONVERSION TO UF4
UF6 is reduced to UF4 by a vapor phase reaction with H2 at approximately 650°C and
pressures in the range of 101 – 170 KPa. A schematic representation of the conversion process is
shown in Figure 1 while a very brief summary of the Starmet operation is given here.
UF6, a white solid compound at ambient temperature, is typically handled and transported in
mild steel cylinders containing approximately 14 tons of material each. To commence the
reduction process, a cylinder containing solid UF6 is loaded into an autoclave and appropriate
connections to the cylinder made for conveying vapor to the reaction zone. Following a
pressurized purge of the system with nitrogen, the autoclave is heated with low pressure saturated
steam to ~100°C, volatilizing the contents and pressurizing the cylinder. Coincident with
pressurization, the cylindrical reactor is heated to ~650°C using wrap around clamshell furnaces.
Upon reaching operating temperature, regulated flows of UF6 and H2 are introduced into the top
section of the reaction tube, with H2 in excess over the stoichiometric requirement. Once the
reaction has been initiated and desired conversion level achieved, excess heat due to the
“exothermicity of reaction” is removed by forced air circulation over the reactor tube.
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
to FLARE
STACK
REACTOR
CHARCOAL
H2
SURGE
BIN
cws
CYCLONES
UF6
Water,KOH
TO VENT
SYSTEM
FILTERS
Nitrogen
purge
Water
HF
ABSORBER
cws
KOH
ABSORBER
AUTOCLAVE
cws
COOLING SCREW
to
ATOMIZER
PRODUCT
HOPPER
HF
SURGE TANK
UF4 TO
PROCESS
Figure 1. UF6 to UF4 Process Flow Diagram
TO HF
RECOVERY
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
UF4 and HF produced by the reduction reaction in the top section of the reactor are cooled as
they pass through the lower zones of the vertical tube and exit into a solids surge bin. The green
salt solids show a tendency to agglomerate on the interior wall of the reactor and must be
removed periodically using externally mounted, air operated vibrators. From the surge bin, the
products pass into an in-line lump breaker for sizing and protection against plugging the product
solids removal system. From the lump breaker, UF4 and by-product gases move into a water
jacketed screw conveyor fitted with a water cooled shaft for final cooling to ~94°C while being
transported to a product hopper. The synthesized green salt is typically stored in standard steel
drums, which are filled inside a ventilated glove box. Material is removed from the product
hopper via a rotary valve that dispenses UF4 into individual product containers.
Meanwhile, reactor off gases including by-product HF and unreacted H2 above the hopper are
passed through a two-stage cyclone system where a majority of the suspended fine particulates
are removed. Final solids removal is accomplished using sintered metal filters. Unreacted UF6
is captured by sorption onto activated charcoal. The UF4-free HF/H2 gas mixture is then passed
to a scrubbing/neutralization system where HF is absorbed into a counter flowing water stream,
leaving hydrogen and traces of HF to react with an in-line potassium hydroxide (KOH) solution
scrubber. The resultant solution of potassium fluoride (KF) is stored for eventual waste
treatment. Although the HF is not currently reclaimed for sale, the anhydrous gas is efficiently
absorbed (99.9%) by the water column, yielding an aqueous solution of approximately 40% HF
by weight. Excess H2 passing through the neutralization process is burned off to remove any
further hazard potential.
The green salt produced in the Starmet process is a very pure, fine powder. A scanning electron
microscope image of UF4 produced at the Barnwell, South Carolina facility is shown in Figure 2.
A predominant fraction of the powder is well below 10 micron diameter with a nearly spherical
particle geometry. Using new Starmet technology, this powder is converted to uranium oxide
with recovery of high value fluorine products.
UF4 CONVERSION TO FLUORIDE PRODUCTS
One fusion method to produce uranium oxide from UF4 uses SiO2 as the oxidizing agent as
shown in equations (3) and (4):
UF4(s) + SiO2(s) → UO2(s) + SiF4(g)
(Eq. 3)
3UF4(s) + 3SiO2(s) + O2(g) → U3O8(s) + 3SiF4(g)
(Eq. 4)
In the absence of oxygen (O2), the favored uranium oxide species produced would be that of
UO2. Thermodynamic calculations12, given in Table 1, for the reaction between UF4 and SiO2
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
Figure 2. Scanning electron microscope image of UF4 produced by H2 reduction of UF6.
Table 1.
Thermodynamic Calculations
For The Reaction Of UF4 With SiO2
T
°C
delta H
kcal
delta S
cal
delta G
kcal _
500.00
600.00
700.00
29.398
28.763
28.441
35.695
34.927
34.578
1.800
-1.733
-5.208
K
3.098E-001
2.716E+000
1.478E+001
indicate this reaction is spontaneous, commencing at temperatures below 600°C where the Gibbs
free energy (∆G) becomes negative. If O2 is added to the system, the free energy of reaction, ∆G,
at 0°C is -4.532 kcal/mol with the preferential formation of U3O8.
An experimental program to verify the chemistry of the process was undertaken at the lab
bench. UF4 derived from defluorination of UF6 was mixed with SiO2 in stoichiometric
proportions and heated in the presence of air to temperatures in the range of 600°-700°C. The
resulting residue was a free-flowing, brown-black powder which was subsequently analyzed by
x-ray powder diffraction. The x-ray diffraction pattern for the reaction residue is shown in
Figure 3 along with a reference pattern for U3O8 (NISTc standard).
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
Relative Intensity
As identified, the experimental residue matches well with the NIST reference standard for U3O8.
The conversion to oxide is essentially complete at greater than 99.9%.
Reaction Residue for
UF4 + SiO2 (fumed silica)
U3O8 std
10
30
50
70
90
110
130
150
2-Theta (degrees)
Figure 3. X-ray diffraction pattern for UF4+SiO2 (fumed silica) reaction residue and U3O8
NIST standard.
The UF4/SiO2 reaction has been performed using two distinct forms of SiO2, namely fumed
silica and diatomaceous earth. The silica material used for the reaction whose results are shown
in Figure 3 was fumed SiO2, possessing high purity (99.8%), high surface area (400m2/gm) and
being essentially amorphous. Another reaction was performed using a mostly crystalline, low
surface area variety composed essentially of common quartz (i.e. sand) in finely ground form.
This reagent is commonly referred to as diatomaceous earth (tradename Celite ). Using similar
reaction conditions, diatomaceous earth was mixed with UF4 and heated to 700°C in the presence
of air. A brown-black residue was produced and analyzed by x-ray diffraction. The diffraction
pattern for the powder product is shown in Figure 4 along with reference data for U3O8. As
shown, diatomaceous earth was effective in converting UF4 to U3O8. No traces of either starting
material or any other phases are present in the uranium oxide residue. A scanning electron
microscope image of the oxide powder is shown in Figure 5.
Relative Intensity
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
Reaction Residue for
®
UF 4 + SiO 2 (Celite )
U 3 O 8 std
10
30
50
70
90
110
130
150
2-Theta (degrees)
Figure 4. X-ray diffraction pattern for UF4+SiO2 (Celite diatomaceous earth) reaction
residue and U3O8 NIST standard.
Figure 5. Scanning electron microscope image of uranium oxide powder produced in the
fusion reaction between UF4 and Celite diatomaceous earth.
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
Verification of the gaseous fluoride by-product was achieved indirectly by analyzing the
material from an in-line adsorbent trap containing KF. The x-ray powder pattern for the
adsorbent is shown in Figure 6 along with the stick pattern13 for potassium hexafluorosilicate
(K2SiF6) superimposed below the experimental data. As shown, the KF adsorbent has been
completely converted to K2SiF6. The reaction occurring in the trap can be given by equation (5):
2KF + SiF4(g) → K2SiF6(s)
(Eq. 5)
The only compound identified is K2SiF6.
Uranium analyses were also performed on the adsorbent material to check for possible U
carryover and contamination of the product stream. Using inductively coupled plasma
spectroscopy (ICP), no uranium could be detected at a detection limit of 1ppm.
To obtain SiF4(g), K2SiF6 is thermally decomposed, yielding the gas at temperatures of 600°630°C. The adsorbent material can then be recycled upstream to continue recovering product gas
from the reaction effluent.
Intensity
Adsorbent Trap From
®
UF4 + SiO2 (Celite )
Fusion Reaction
Stick Pattern for K2SiF6
(#7-217) shown as
reference
10
30
50
70
90
110
130
150
170
2-Theta (degrees)
Figure 6. X-ray diffraction pattern of KF adsorbent material after contact with SiF4
product gas. Theoretical stick pattern for K2SiF6 has been superimposed
below the experimental data.
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
Similar reaction chemistry has been performed using boric oxide as the oxidizing agent to
convert UF4 to uranium oxide. Reactions to produce UO2 and U3O8 are given in equations (6)
and (7):
3UF4(s) + 2B2O3(s) → 3UO2(s) + 4BF3(g)
(Eq. 6)
3UF4(s) + 2B2O3(s) + O2(g) → U3O8(s) + 4BF3(g)
(Eq. 7)
Experimentally, boric oxide was mixed with green salt and reacted at 600°C in the presence of
air. Again, the reactor effluent was passed over an adsorbent material to capture and identify the
fluoride gas evolved in the reaction. The solid reaction residue was a free flowing black powder.
X-ray diffraction analysis of the solid is shown if Figure 7 along with reference patterns for UO2
and U3O8. The black residue contains phases matching well with both UO2 and U3O8.
Relative Intensity
Reaction Residue for
UF4 + B2O3
UO2 ref
U3O8 std
10
30
50
70
90
110
130
150
2-Theta (degrees)
Figure 7. X-ray diffraction pattern for UF4+B2O3 reaction residue and UO2 and U3O8
reference compounds.
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
The x-ray powder pattern of the adsorbent material is shown in Figure 8.
The adsorbent material selected to capture BF3 generated in the fusion reaction was sodium
hydroxide (NaOH). The reactions occurring between BF3 and NaOH in the trap include:
6NaOH(s) + 8BF3(g) → B2O3(s) + 6NaBF4(s) + 3H2O
(Eq. 8)
3NaOH(s) + 2BF3(g) → 3NaF + B2O3(s) + HF(g)
(Eq. 9)
R elative Intensity
As shown in Figure 8, experimental diffraction peaks correlate well with corresponding
theoretical reference peaks9 for NaF and NaBF4, confirming the presence of BF3 in the reaction
effluent. BF3 gas may be recovered from the trap residue through thermal fusion14 of NaBF4
with B2O3 beginning around 400°C.
Adsorbent Trap From
UF4 + B2 O3 Fusion Reaction
Theoretical patterns For
NaBF4 (#11-0671)
▲
10
30
50
70
NaF
90
(#36-1455)
110
2-Theta (degrees)
Figure 8. X-ray diffraction pattern of NaOH adsorbent material after contact with BF3
product gas. Theoretical stick patterns for NaBF4 and NaF have been
superimposed below the experimental data.
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
SUMMARY
From the processes described above, a method of reducing UF6 to UF4 using H2 and subsequent
conversion of UF4 to uranium oxide (U3O8 or UO2) and fluoride by-products has been
demonstrated. Using a “hot wall” reactor at 650°C, UF4 possessing a fine particle size and
spherical geometry is produced, along with anhydrous HF. HF is efficiently absorbed into a
counter flowing water column, producing aqueous HF at concentrations approaching 40 weight
percent. The UF4 reduction product is further reacted with oxidizing agents such as SiO2 and
B2O3, producing stable, free-flowing, uranium oxide powder and volatile fluoride gases such as
SiF4 and BF3. The solid oxide product is suitable for use in manufacturing DUCRETE for
advanced radioactive shielding applications. Carryover of U contamination into the fluoride
product stream has been circumvented, thereby removing restrictions on further use of these
gases in applications ranging from non-oxide ceramics synthesis, the semiconductor industry and
organic chemical manufacture. The fusion process has the distinct feature of flexibility to
produce more than one fluoride by-product by selection of an appropriate oxidizing agent.
Additional fusion processes are being explored to produce a family of fluoride compounds that
will accommodate changing market demand for any one particular material.
FOOTNOTES
a
Source: Chemical Market Reporter, Schnell Publishing Co., New York, 1998.
b
patent applications filed
c
National Institute of Standards and Technology
REFERENCES
1. Chambers, R. D., Fluorine in Organic Chemistry, J. Wiley & Sons, New York, 1973.
2. Grosse, A. V., and Linn, C. B., “The Addition of Hydrogen Fluoride to the Double Bond”, J.
Org. Chem., 3, 26 (1939).
3. Laubengayer, A. W., and Condike, G. F., “Donor-Acceptor Bonding. IV. Ammonia-Boron
Trifluoride”, J. Amer. Chem. Soc., 70, 2274 (1948).
4. Ardaud, P., LeBrun, J. J., and Mignani, G., “Preparation of Boron/Nitrogen Preceramic
Polymers”, U.S. Patent 5,015,607, May 14, 1991.
5. Gebhardt, J. J., Tanzilli, R. A., and Harris, T. A., “Chemical Vapor Deposition of Silicon
Nitride”, J. Electrochem. Soc., 123(10), 1578 (1976).
6. Lee, Y. W., Strife, J. R., and Veltri, R. D., “Low-Pressure Chemical Vapor Deposition of αSi3N4 From SiF4 and NH3: Kinetic Characteristics”, J. Amer. Ceram. Soc., 75(8), 2200
(1992).
7. McLaughlin, D.F. and Nuhfer, K.R., “Pilot Plant UF6 to UF4 Test Operations”, DOE
Contract/Grant DOEAC05-86OR21600, Westinghouse Materials Co. of Ohio, Cincinnatti,
OH, 1991, 489 pages.
WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999
8. Brody, M. and Gates M., “Conversion of Uranium Hexafluoride To Its Tetrafluoride”, UK
Patent 2184106, June 17, 1987.
9. Aquino, A.R, de, Araujo, J.A. de and Rocha, S.M.R. da, “UF6 To UF4 Reduction: Laboratory
Scale”, Proceedings of the General Congress On Nuclear Energy, V.1., Rio de Janeiro,
Brazil, July 5, 1992, p231-233.
10. Jang, I.S., “Preparation of UF4 (Single Step Reduction of UF6 With H2/F2)”, DOE
Contract/Grant DOE AC05-84OT21400, Korea Advanced Energy Research Institute, Seoul,
Korea, 1986, 30 pages.
11. Galasso, F. S., “Pyrolytic Silicon Nitride Prepared From Reactant Gases”, Powder
Metallurgy International, 11, 7 (1979).
12. HSC Chemistry For Windows, Chemical Reaction and Equilibrium software, Version 2.03,
Outokumpu Research Oy, 1994.
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Kirk, D. F. Othmer, editors, Vol. 11, 312. John Wiley and Sons, New York, New York,
1994.
DESIGNATIONS
DUCRETE is a trademark of Lockheed Martin Idaho Technologies Company, Idaho Falls,
Idaho, (1996).
Celite is a registered trademark of the Celite Corporation, Lompoc, California, (1974).