WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999

RECOVERY OF HIGH VALUE FLUORINE PRODUCTSFROM URANIUM HEXAFLUORIDE CONVERSION

John B. Bulko, David S. Schlier

Starmet Corporation2229 Main Street

Concord, MA 01742

ABSTRACT Starmet Corporation has developed an integrated process for converting uraniumhexafluoride (UF6) into uranium oxide (as either U3O8 or UO2) while recovering the fluorinevalue as useful products free of uranium contamination. The uranium oxide is suitable forprocessing into DUAGG and DUCRETE , a depleted uranium oxide aggregate and concreteform useful in nuclear shielding applications. The fluorine products can be used directly orprocessed into other chemical forms depending on market demand. The conversion process can be divided into two main operations, UF6 conversion to uraniumtetrafluoride (UF4) and subsequent processing of UF4 to uranium oxides with generation ofvolatile fluoride gases such as silicon tetrafluoride (SiF4) and boron trifluoride (BF3). The frontend process chemistry, called the ‘6-to-4’ process, involves the vapor phase reaction of UF6 withexcess hydrogen (H2) at about 650°C and at pressures of 101 – 170 KPa in a vertical heated tubereactor. 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 solidUF4 is collected as free flowing powder. Starmet has the only licensed and operational UF6 toUF4 plant in North America. Located in Barnwell, South Carolina, this facility has the capacityto 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 developedwhereby UF4 is reacted with silicon dioxide (silica, SiO2) at 700°C to produce volatile SiF4 andcoincident uranium oxide. Alternatively, boric oxide (B2O3) has been used in place of SiO2 toproduce BF3. Both fluoride gases possess significantly higher value in comparison to HF, whichis 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 hashistorically plagued other fluoride-based products derived from UF6 thereby discouragingwidespread commercial use and diminishing value. Starmet is currently designing a commercialscale facility to recover these and other high value fluoride products from the immense inventoryof 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 havestockpiled 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 theenrichment 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 thestockpile will be declared excess. If this action occurs, DOE would be forced to pay for disposalof 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 beenworking to develop alternative approaches for production of stable uranium compounds andrecovery 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 fromUF6, investigations into new processes to economically produce uranium oxide and recoverfluorine from UF4 are underway. The high quality, depleted uranium oxide from these newprocesses 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 inplace of conventional aggregate. By-products of the conversion processes are high value fluorinecompounds and anhydrous HF. These fluorine compounds can be used directly, as fluorinatingagents in the manufacture of organic and inorganic chemicals1,2, or as precursor compounds inthe synthesis of advanced non-oxide based ceramics3-6. The production of high value chemicalby-products provides the potential to realize revenues from uranium processing. By combiningdevelopment of new uses for the uranium, such as DUCRETE , with co-production of high valuefluorine chemicals, a technically viable and economically attractive approach for using UF6 isnow 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 inequation 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 thereaction of UF4 with an oxidizing agent to produce uranium oxide and release of fluorine in theform of a volatile fluoride gas, MFy, as shown in equation (2). The oxidizing agent is shown hereas 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. Investigationsinto the conversion chemistry have attracted both domestic7 and international8-10 interest. Of thenumerous processes to reduce UF6, there are only two which are considered efficient andeconomical 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 byadmitting fluorine gas (F2) in addition to H2 and UF6. Heat is released by the reaction betweenH2 and F2, raising the reaction temperature to ~1100°C while the reactor walls are maintainedbetween 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 withcapacity to produce over 9 million pounds UF4 per year. In addition to green salt, the HF by-product 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 fluorideranges between $0.70-0.90/lbb. Hydrogen fluoride is more easily recovered in aqueous formwhile anhydrous recovery is more difficult and expensive to process.

The conversion of green salt to uranium oxide with recovery of fluorine by-products is underdevelopmentb at Starmet. A pyrometallurgical or fusion approach is used that involves mixingUF4 with either SiO2 or B2O3 and heating to a temperature sufficient to cause reaction. UF4 isconverted to uranium oxide with production of either SiF4 or BF3, respectively. Both productsare gases that can be easily separated from the solid uranium oxide. Both fluoride gases can besold directly in high purity form to markets in the semiconductor industry or they can be used inthe 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 dieseland turbine engine parts.

The advantage of the new process is that it is inherently lower in operating and capital costcompared to currently practiced methods. Additionally, the fusion process overcomes the mainobjection that has restricted wide scale sale of HF generated by direct steam conversion of UF6 tooxide, namely uranium carryover and radioactive contamination of the HF by-product. Since thestarting materials for making uranium oxide in the new process are solids, there is no possibilityof carryover into the gas phase fluorine by-product. Testing of the gas phase products has shownthat there is no detectable U in either the SiF4 or BF3 compounds made by this process. Inaddition, since all the input materials are of relatively high purity, the products of the process arealso of high purity.

UF6 CONVERSION TO UF4 UF6 is reduced to UF4 by a vapor phase reaction with H2 at approximately 650°C andpressures in the range of 101 – 170 KPa. A schematic representation of the conversion process isshown 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 inmild steel cylinders containing approximately 14 tons of material each. To commence thereduction process, a cylinder containing solid UF6 is loaded into an autoclave and appropriateconnections to the cylinder made for conveying vapor to the reaction zone. Following apressurized purge of the system with nitrogen, the autoclave is heated with low pressure saturatedsteam to ~100°C, volatilizing the contents and pressurizing the cylinder. Coincident withpressurization, 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 topsection of the reaction tube, with H2 in excess over the stoichiometric requirement. Once thereaction 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

Figure 1. UF6 to UF4 Process Flow Diagram

Water Water,KOH

AUTOCLAVE

REA

CTO

R

SURGEBIN

COOLING SCREW

PRODUCTHOPPER

CY

CLO

NES

FILT

ERS

CH

AR

CO

AL

HF ABSORBER

KOHABSORBER

HF SURGE TANK

UF6

H2

TO VENT SYSTEM

to FLARE STACK

to ATOMIZER

TO HF RECOVERY

UF4 TO PROCESS

cws

cws cwsNitrogen

purge

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 asthey pass through the lower zones of the vertical tube and exit into a solids surge bin. The greensalt solids show a tendency to agglomerate on the interior wall of the reactor and must beremoved periodically using externally mounted, air operated vibrators. From the surge bin, theproducts pass into an in-line lump breaker for sizing and protection against plugging the productsolids removal system. From the lump breaker, UF4 and by-product gases move into a waterjacketed screw conveyor fitted with a water cooled shaft for final cooling to ~94°C while beingtransported to a product hopper. The synthesized green salt is typically stored in standard steeldrums, which are filled inside a ventilated glove box. Material is removed from the producthopper 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 arepassed through a two-stage cyclone system where a majority of the suspended fine particulatesare removed. Final solids removal is accomplished using sintered metal filters. Unreacted UF6is captured by sorption onto activated charcoal. The UF4-free HF/H2 gas mixture is then passedto 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) solutionscrubber. The resultant solution of potassium fluoride (KF) is stored for eventual wastetreatment. Although the HF is not currently reclaimed for sale, the anhydrous gas is efficientlyabsorbed (99.9%) by the water column, yielding an aqueous solution of approximately 40% HFby weight. Excess H2 passing through the neutralization process is burned off to remove anyfurther hazard potential.

The green salt produced in the Starmet process is a very pure, fine powder. A scanning electronmicroscope 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 sphericalparticle geometry. Using new Starmet technology, this powder is converted to uranium oxidewith 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 asshown 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 ofUO2. 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 delta H delta S delta G °C kcal cal kcal _ K

500.00 29.398 35.695 1.800 3.098E-001 600.00 28.763 34.927 -1.733 2.716E+000 700.00 28.441 34.578 -5.208 1.478E+001

indicate this reaction is spontaneous, commencing at temperatures below 600°C where the Gibbsfree 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 labbench. UF4 derived from defluorination of UF6 was mixed with SiO2 in stoichiometricproportions and heated in the presence of air to temperatures in the range of 600°-700°C. Theresulting residue was a free-flowing, brown-black powder which was subsequently analyzed byx-ray powder diffraction. The x-ray diffraction pattern for the reaction residue is shown inFigure 3 along with a reference pattern for U3O8 (NISTc standard).

WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999

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)

10 30 50 70 90 110 130 150

2-Theta (degrees)

Rel

ativ

e In

tens

ity

U3O8 std

Figure 3. X-ray diffraction pattern for UF4+SiO2 (fumed silica) reaction residue and U3O8NIST standard.

The UF4/SiO2 reaction has been performed using two distinct forms of SiO2, namely fumedsilica and diatomaceous earth. The silica material used for the reaction whose results are shownin Figure 3 was fumed SiO2, possessing high purity (99.8%), high surface area (400m2/gm) andbeing essentially amorphous. Another reaction was performed using a mostly crystalline, lowsurface 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 similarreaction conditions, diatomaceous earth was mixed with UF4 and heated to 700°C in the presenceof air. A brown-black residue was produced and analyzed by x-ray diffraction. The diffractionpattern for the powder product is shown in Figure 4 along with reference data for U3O8. Asshown, diatomaceous earth was effective in converting UF4 to U3O8. No traces of either startingmaterial or any other phases are present in the uranium oxide residue. A scanning electronmicroscope image of the oxide powder is shown in Figure 5.

WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999

Reaction Residue for UF4 + SiO2 (Celite®)

10 30 50 70 90 110 130 150

2-Theta (degrees)

Rel

ativ

e In

tens

ity

U3O8 std

Figure 4. X-ray diffraction pattern for UF4+SiO2 (Celite diatomaceous earth) reactionresidue and U3O8 NIST standard.

Figure 5. Scanning electron microscope image of uranium oxide powder produced in thefusion 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 thematerial from an in-line adsorbent trap containing KF. The x-ray powder pattern for theadsorbent 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 beencompletely 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 Ucarryover and contamination of the product stream. Using inductively coupled plasmaspectroscopy (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 gasfrom the reaction effluent.

10 30 50 70 90 110 130 150 170

2-Theta (degrees)

Inte

nsity

Stick Pattern for K2SiF6

(#7-217) shown as reference

Adsorbent Trap FromUF4 + SiO2 (Celite®)

Fusion Reaction

Figure 6. X-ray diffraction pattern of KF adsorbent material after contact with SiF4product gas. Theoretical stick pattern for K2SiF6 has been superimposedbelow the experimental data.

WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999

Similar reaction chemistry has been performed using boric oxide as the oxidizing agent toconvert 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 ofair. Again, the reactor effluent was passed over an adsorbent material to capture and identify thefluoride 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 UO2and U3O8. The black residue contains phases matching well with both UO2 and U3O8.

Reaction Residue for UF4 + B2O3

10 30 50 70 90 110 130 150

2-Theta (degrees)

Rel

ativ

e In

tens

ity

U3O8 std

UO2 ref

Figure 7. X-ray diffraction pattern for UF4+B2O3 reaction residue and UO2 and U3O8reference 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 sodiumhydroxide (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)

As shown in Figure 8, experimental diffraction peaks correlate well with correspondingtheoretical reference peaks9 for NaF and NaBF4, confirming the presence of BF3 in the reactioneffluent. BF3 gas may be recovered from the trap residue through thermal fusion14 of NaBF4

with B2O3 beginning around 400°C.

Figure 8. X-ray diffraction pattern of NaOH adsorbent material after contact with BF3product gas. Theoretical stick patterns for NaBF4 and NaF have beensuperimposed below the experimental data.

Adsorbent Trap From UF4 + B2O3 Fusion Reaction

10 30 50 70 90 110

2-Theta (degrees)

Rel

ativ

e In

tens

ity

Theoretical patterns For

����

NaBF4 (#11-0671)

▲▲▲▲ NaF (#36-1455)

WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999

SUMMARYFrom the processes described above, a method of reducing UF6 to UF4 using H2 and subsequentconversion of UF4 to uranium oxide (U3O8 or UO2) and fluoride by-products has beendemonstrated. Using a “hot wall” reactor at 650°C, UF4 possessing a fine particle size andspherical geometry is produced, along with anhydrous HF. HF is efficiently absorbed into acounter flowing water column, producing aqueous HF at concentrations approaching 40 weightpercent. The UF4 reduction product is further reacted with oxidizing agents such as SiO2 andB2O3, producing stable, free-flowing, uranium oxide powder and volatile fluoride gases such asSiF4 and BF3. The solid oxide product is suitable for use in manufacturing DUCRETE foradvanced radioactive shielding applications. Carryover of U contamination into the fluorideproduct stream has been circumvented, thereby removing restrictions on further use of thesegases in applications ranging from non-oxide ceramics synthesis, the semiconductor industry andorganic chemical manufacture. The fusion process has the distinct feature of flexibility toproduce 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 thatwill accommodate changing market demand for any one particular material.

FOOTNOTESa Source: Chemical Market Reporter, Schnell Publishing Co., New York, 1998.b patent applications filedc National Institute of Standards and Technology

REFERENCES1. 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 α-

Si3N4 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.

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8. Brody, M. and Gates M., “Conversion of Uranium Hexafluoride To Its Tetrafluoride”, UKPatent 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. 13. Powder Diffraction File Database, PDF-2 database sets 1-47, Inorganics, JCPDS-

International Center for Diffraction Data, Newton Square, PA, 1997.

14. “Fluorine Compounds, Inorganic”, Encyclopedia of Chemical Technology, 4th Edition, R. E.Kirk, D. F. Othmer, editors, Vol. 11, 312. John Wiley and Sons, New York, New York,1994.

DESIGNATIONSDUCRETE 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).