Recovery of High Value Fluorine Products from Uranium Hexafluoride Conversion

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 235 between 150°-200°C. This procedure was developed for treating UF6 highly enriched with U 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 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 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

H2 to FLARE STACK

O Water Water,KOH

R TO VENT SYSTEM

Nitrogen S

purge E cws cws

SURGE F UF6 BIN

O KOH L HF C ABSORBER

Y ABSORBER AUTOCLAVE C cws COOLING SCREW to ATOMIZER PRODUCT HOPPER

HF SURGE TANK TO HF RECOVERY

UF4 TO PROCESS

Figure 1. UF6 to UF4 Process Flow Diagram 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 12 UO2. Thermodynamic calculations , 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 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 c Figure 3 along with a reference pattern for U3O8 (NIST 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) Relative Intensity

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 2 in Figure 3 was fumed SiO2, possessing high purity (99.8%), high surface area (400m /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. WM’99 CONFERENCE, FEBRUARY 28 – MARCH 1, 1999

Reaction Residue for ® UF4 + SiO2 (Celite ) Relative Intensity

U 3O 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.

Adsorbent Trap From ® UF4 + SiO2 (Celite ) Fusion Reaction

Stick Pattern for K SiF Intensity 2 6 (#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.

Reaction Residue for

UF4 + B2O3

UO2 ref Relative Intensity

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)

As shown in Figure 8, experimental diffraction peaks correlate well with corresponding 9 theoretical reference peaks for NaF and NaBF4, confirming the presence of BF3 in the reaction 14 effluent. BF3 gas may be recovered from the trap residue through thermal fusion of NaBF4 with B2O3 beginning around 400°C.

Adsorbent Trap From

UF4 + B2O3 Fusion Reaction Relative Intensity

Theoretical patterns For

NaBF4 (#11-0671) ▲ NaF (#36-1455)

10 30 50 70 90 110 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

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

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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).