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Recovery of High Value Fluorine Products from Uranium Hexafluoride Conversion

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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.
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