RECOVERY OF HIGH VALUE FLUORINE PRODUCTS FROM URANIUM HEXAFLUORIDE CONVERSION. John B. Bulko, David S. Schlier

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1 RECOVERY OF HIGH VALUE FLUORINE PRODUCTS FROM URANIUM HEXAFLUORIDE CONVERSION John B. Bulko, David S. Schlier Starmet Corporation 2229 Main Street Concord, MA ABSTRACT Starmet Corporation has developed an integrated process for converting uranium hexafluoride (UF 6 ) into uranium oxide (as either U 3 O 8 or UO 2 ) 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, UF 6 conversion to uranium tetrafluoride (UF 4 ) and subsequent processing of UF 4 to uranium oxides with generation of volatile fluoride gases such as silicon tetrafluoride (SiF 4 ) and boron trifluoride (BF 3 ). The front end process chemistry, called the 6-to-4 process, involves the vapor phase reaction of UF 6 with excess hydrogen (H 2 ) at about 650 C and at pressures of KPa in a vertical heated tube reactor. The reduction products include non-volatile UF 4 and gaseous hydrogen fluoride (HF). Gaseous HF is collected by passing the process effluent through aqueous scrubbers. The solid UF 4 is collected as free flowing powder. Starmet has the only licensed and operational UF 6 to UF 4 plant in North America. Located in Barnwell, South Carolina, this facility has the capacity to convert up to 9 million pounds of UF 6 per year to UF 4. Chemistry to further convert the UF 4 by-product from the 6-to-4 process has been developed whereby UF 4 is reacted with silicon dioxide (silica, SiO 2 ) at 700 C to produce volatile SiF 4 and coincident uranium oxide. Alternatively, boric oxide (B 2 O 3 ) has been used in place of SiO 2 to produce BF 3. Both fluoride gases possess significantly higher value in comparison to HF, which is the typical fluoride product recovered from hydrolysis and pyrohydrolysis processing of UF 6. Of greater significance is the generation of products free from uranium contamination which has historically plagued other fluoride-based products derived from UF 6 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 UF 6 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 UF 6 at facilities in Oak Ridge, TN, Paducah, KY and Portsmouth, OH. Depleted UF 6 (DUF 6 ) 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 DUF 6 inventory. Disposal costs have been estimated at $1.4 billion, however,

2 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 DUF 6 inventory, Starmet Corporation has been working to develop alternative approaches for production of stable uranium compounds and recovery of fluorine from UF 6. 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 UF 4 from UF 6, investigations into new processes to economically produce uranium oxide and recover fluorine from UF 4 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 chemicals 1,2, or as precursor compounds in the synthesis of advanced non-oxide based ceramics 3-6. 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 UF 6 is now available as an alternative to disposal. PROCESS OVERVIEW One process being developed at Starmet for conversion of UF 6 involves a two step operation. The first step is the production of UF 4 and HF by H 2 reduction of gaseous UF 6, shown in equation 1. UF 6 (g) + H 2 (g) UF 4 (s) + 2HF(g) (Eq. 1) UF 4 is a green crystalline solid commonly referred to as green salt. The second step is the reaction of UF 4 with an oxidizing agent to produce uranium oxide and release of fluorine in the form of a volatile fluoride gas, MF y, as shown in equation (2). The oxidizing agent is shown here as MO x, since there are numerous reagents that can be inserted in this reaction. UF 4 (s) + MO x (s) UO x (s) + MF y (g) (Eq. 2) The process of reacting UF 6 with hydrogen to produce UF 4 is well established. Investigations into the conversion chemistry have attracted both domestic 7 and international 8-10 interest. Of the numerous processes to reduce UF 6, 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 (F 2 ) in addition to H 2 and UF 6. Heat is released by the reaction between H 2 and F 2, raising the reaction temperature to ~1100 C while the reactor walls are maintained between C. This procedure was developed for treating UF 6 highly enriched with U 235 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

3 by heat supplied through the reactor walls. It is this process that Starmet has implemented with capacity to produce over 9 million pounds UF 4 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 $ /lb a while anhydrous hydrogen fluoride ranges between $ /lb b. 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 development b at Starmet. A pyrometallurgical or fusion approach is used that involves mixing UF 4 with either SiO 2 or B 2 O 3 and heating to a temperature sufficient to cause reaction. UF 4 is converted to uranium oxide with production of either SiF 4 or BF 3, 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 nitride 4 (BN) or silicon nitride 11 (Si 3 N 4 ). 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 UF 6 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 SiF 4 or BF 3 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. UF 6 CONVERSION TO UF 4 UF 6 is reduced to UF 4 by a vapor phase reaction with H 2 at approximately 650 C and pressures in the range of 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. UF 6, 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 UF 6 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 UF 6 and H 2 are introduced into the top section of the reaction tube, with H 2 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.

4 H2 to FLARE STACK Nitrogen purge UF6 AUTOCLAVE REACTOR SURGE BIN FILTERS CYCLONES COOLING SCREW CHARCOAL TO VENT SYSTEM cws HF ABSORBER Water cws KOH ABSORBER Water,KOH cws PRODUCT HOPPER to ATOMIZER HF SURGE TANK TO HF RECOVERY UF4 TO PROCESS Figure 1. UF 6 to UF 4 Process Flow Diagram

5 UF 4 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, UF 4 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 UF 4 into individual product containers. Meanwhile, reactor off gases including by-product HF and unreacted H 2 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 UF 6 is captured by sorption onto activated charcoal. The UF 4 -free HF/H 2 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 H 2 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 UF 4 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. UF 4 CONVERSION TO FLUORIDE PRODUCTS One fusion method to produce uranium oxide from UF 4 uses SiO 2 as the oxidizing agent as shown in equations (3) and (4): UF 4 (s) + SiO 2 (s) UO 2 (s) + SiF 4 (g) (Eq. 3) 3UF 4 (s) + 3SiO 2 (s) + O 2 (g) U 3 O 8 (s) + 3SiF 4 (g) (Eq. 4) In the absence of oxygen (O 2 ), the favored uranium oxide species produced would be that of UO 2. Thermodynamic calculations 12, given in Table 1, for the reaction between UF 4 and SiO 2

6 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 _ K 3.098E E E+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 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 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).

7 As identified, the experimental residue matches well with the NIST reference standard for U 3 O 8. The conversion to oxide is essentially complete at greater than 99.9%. Relative Intensity Reaction Residue for UF 4 + SiO 2 (fumed silica) U 3 O 8 std Theta (degrees) Figure 3. X-ray diffraction pattern for UF 4 +SiO 2 (fumed silica) reaction residue and U 3 O 8 NIST standard. The UF 4 /SiO 2 reaction has been performed using two distinct forms of SiO 2, namely fumed silica and diatomaceous earth. The silica material used for the reaction whose results are shown in Figure 3 was fumed SiO 2, possessing high purity (99.8%), high surface area (400m 2 /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 UF 4 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 U 3 O 8. As shown, diatomaceous earth was effective in converting UF 4 to U 3 O 8. 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.

8 Relative Intensity Reaction Residue for UF 4 + SiO 2 (Celite ) U 3 O 8 std Theta (degrees) Figure 4. X-ray diffraction pattern for UF 4 +SiO 2 (Celite diatomaceous earth) reaction residue and U 3 O 8 NIST standard. Figure 5. Scanning electron microscope image of uranium oxide powder produced in the fusion reaction between UF 4 and Celite diatomaceous earth.

9 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 pattern 13 for potassium hexafluorosilicate (K 2 SiF 6 ) superimposed below the experimental data. As shown, the KF adsorbent has been completely converted to K 2 SiF 6. The reaction occurring in the trap can be given by equation (5): 2KF + SiF 4 (g) K 2 SiF 6 (s) (Eq. 5) The only compound identified is K 2 SiF 6. 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 SiF 4 (g), K 2 SiF 6 is thermally decomposed, yielding the gas at temperatures of C. The adsorbent material can then be recycled upstream to continue recovering product gas from the reaction effluent. Adsorbent Trap From UF 4 + SiO 2 (Celite ) Fusion Reaction Intensity Stick Pattern for K 2 SiF 6 (#7-217) shown as reference Theta (degrees) Figure 6. X-ray diffraction pattern of KF adsorbent material after contact with SiF 4 product gas. Theoretical stick pattern for K 2 SiF 6 has been superimposed below the experimental data.

10 Similar reaction chemistry has been performed using boric oxide as the oxidizing agent to convert UF 4 to uranium oxide. Reactions to produce UO 2 and U 3 O 8 are given in equations (6) and (7): 3UF 4 (s) + 2B 2 O 3 (s) 3UO 2 (s) + 4BF 3 (g) (Eq. 6) 3UF 4 (s) + 2B 2 O 3 (s) + O 2 (g) U 3 O 8 (s) + 4BF 3 (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 UO 2 and U 3 O 8. The black residue contains phases matching well with both UO 2 and U 3 O 8. Reaction Residue for UF 4 + B 2 O 3 Relative Intensity UO 2 ref U 3 O 8 std Theta (degrees) Figure 7. X-ray diffraction pattern for UF 4 +B 2 O 3 reaction residue and UO 2 and U 3 O 8 reference compounds.

11 The x-ray powder pattern of the adsorbent material is shown in Figure 8. The adsorbent material selected to capture BF 3 generated in the fusion reaction was sodium hydroxide (NaOH). The reactions occurring between BF 3 and NaOH in the trap include: 6NaOH(s) + 8BF 3 (g) B 2 O 3 (s) + 6NaBF 4 (s) + 3H 2 O (Eq. 8) 3NaOH(s) + 2BF 3 (g) 3NaF + B 2 O 3 (s) + HF(g) (Eq. 9) As shown in Figure 8, experimental diffraction peaks correlate well with corresponding theoretical reference peaks 9 for NaF and NaBF 4, confirming the presence of BF 3 in the reaction effluent. BF 3 gas may be recovered from the trap residue through thermal fusion 14 of NaBF 4 with B 2 O 3 beginning around 400 C. Relative Intensity Adsorbent Trap From UF 4 + B 2 O 3 Fusion Reaction Theoretical patterns For 10 NaBF 4 (# ) NaF (# ) Theta (degrees) Figure 8. X-ray diffraction pattern of NaOH adsorbent material after contact with BF 3 product gas. Theoretical stick patterns for NaBF 4 and NaF have been superimposed below the experimental data.

12 SUMMARY From the processes described above, a method of reducing UF 6 to UF 4 using H 2 and subsequent conversion of UF 4 to uranium oxide (U 3 O 8 or UO 2 ) and fluoride by-products has been demonstrated. Using a hot wall reactor at 650 C, UF 4 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 UF 4 reduction product is further reacted with oxidizing agents such as SiO 2 and B 2 O 3, producing stable, free-flowing, uranium oxide powder and volatile fluoride gases such as SiF 4 and BF 3. 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, 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, 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, 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- Si 3 N 4 From SiF 4 and NH 3 : Kinetic Characteristics, J. Amer. Ceram. Soc., 75(8), 2200 (1992). 7. McLaughlin, D.F. and Nuhfer, K.R., Pilot Plant UF 6 to UF 4 Test Operations, DOE Contract/Grant DOEAC05-86OR21600, Westinghouse Materials Co. of Ohio, Cincinnatti, OH, 1991, 489 pages.

13 8. Brody, M. and Gates M., Conversion of Uranium Hexafluoride To Its Tetrafluoride, UK Patent , June 17, Aquino, A.R, de, Araujo, J.A. de and Rocha, S.M.R. da, UF 6 To UF 4 Reduction: Laboratory Scale, Proceedings of the General Congress On Nuclear Energy, V.1., Rio de Janeiro, Brazil, July 5, 1992, p Jang, I.S., Preparation of UF 4 (Single Step Reduction of UF 6 With H 2 /F 2 ), 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, Powder Diffraction File Database, PDF-2 database sets 1-47, Inorganics, JCPDS- International Center for Diffraction Data, Newton Square, PA, Fluorine Compounds, Inorganic, Encyclopedia of Chemical Technology, 4 th Edition, R. E. Kirk, D. F. Othmer, editors, Vol. 11, 312. John Wiley and Sons, New York, New York, 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).