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United States Patent 19 (11) 3,806,579 Carles Et Al

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United States Patent 19 (11) 3,806,579 Carles Et Al United States Patent 19 (11) 3,806,579 Carles et al. (45) Apr. 23, 1974 (54) METHOD OF PURIFICATION OF URANUM 3,425,812 2/1969 Cousin et al.......................... 23/352 HEXAFLUORIDE 3,359,078 12/1967 Alter et al............................. 23/326 3,340,019 9/1967 Pierini et al.......................... 23/326 (75) Inventors: Maurice Carles; Jacky Fra, both of Pierrelatte, France Primary Examiner-Carl D. Quarforth 73 Assignee: Commissariat A L'Energie Assistant Examiner-F. M. Gittes Atomique, Paris, France Attorney, Agent, or Firm-Cameron, Kerkam & Sutton 22 Filed: Mar. 22, 1971 21 Appl. No.: 126,682 57 ABSTRACT This invention relates to a method of purification of 30 Foreign Application Priority Data uranium hexafluoride contaminated with impurities such as molybdenum hexafluoride and tungsten hexa Apr. 8, 1970 France.............................. 70. 12685 fluoride. (52) U.S. Cl......................... 423/19, 42318, 423/258 The contaminated uranium hexafluoride is distilled 51 Int. Cl.......................... a a P A 8 is a C01g 43/06 from the liquid phase in the presence of chlorine (58) Field of Search........ 23/326,339, 352; 423/19, trifluoride which forms an azeotrope with the 423/258, 8 impurities to be removed from the uranium hexafluoride, said impurities are withdrawn from the 56) References Cited top of the column and the purified uranium UNITED STATES PATENTS hexafluoride is collected at the bottom of the column. 2,830,873 4/1958 Katz et al.............................. 23/326 1 Claim, 4 Drawing Figures 'ATENTED APR 23 1974 3,806,579 1. 2 METHOD OF PURIFICATION OF URANIUM nium diuranate by bubbling of uranium hexafluoride HEXAFLUORDE through ammonia, dissolving the ammonium diuranate which is formed with nitric acid, extraction of the ura This invention relates to a method of purification of nyl nitrate by tributylphosphate whilst the impurities uranium in the form of uranium hexafluoride which 5 remain in the aqueous phase, stripping of the uranium contains impurities such as molybdenum and tungsten. with a nitric acid solution, action of ammonia on the uranyl nitrate so as to yield ammonium diuranate, cal Uranium hexafluoride is prepared by the action of cination of the ammonium diuranate in order to form fluorine on uranium tetrafluoride, this compound being UO, reduction of UOs to UO, by hydrogen, conversion in turn obtained as a result of the action of anhydrous O of UO, into UF, by anhydrous hydrofluoric acid and hydrofluoric acid on uranium dioxide. The product conversion of UF, into UFs by fluorine. This process thus obtained contains impurities (MoFs, WFs, etc.) involves a large number of stages and calls for substan which have to be eliminated especially for subsequent tial quantities of pure reagents. use in enrichment plants. The method of purification of uranium hexafluoride The methods of purification of uranium hexafluoride 15 in accordance with the invention essentially consists in involve either distillation or solvent extraction. distilling uranium hexafluoride in the presence of chlo The first method of purification consists in direct dis rine trifluoride so as to form an azeotropic mixture with tillation of the gaseous mixture of uranium hexafluo the impurities which are to be removed from the ura ride and molybdenum hexafluoride or of the mixture of nium hexafluoride, said impurities being discharged at uranium hexafluoride and tungsten hexafluoride. the top of the column and the purified uranium hexa Uranium hexafluoride is withdrawn in the pure state fluoride being collected at the bottom of the column. at the bottom of the column. Molybdenum hexafluo ride and tungsten hexafluoride pass out at the top of the The method is applicable to the removal of any impu column in the form of a mixture which is rich in ura rity contained in uranium hexafluoride which forms an nium hexafluoride (approximately 95 percent). 25 azeotropic mixture with chlorine trifluoride provided The second method consists in converting uranium that both miscibility and chemical compatibility exist. hexafluoride into uranyl nitrate and then in extracting the uranium by tributylphosphate. The impurities re The mixture of uranium hexafluoride and chlorine mains in the aqueous phase. trifluoride is wholly miscible over the entire concentra Taking into account the low relative volatility of mo 30 tion range provided that the total pressure remains lybdenum hexafluoride and tungsten hexafluoride with higher than approximately 2,500 mb. Below this value, respect to uranium hexafluoride, a column having a the liquid-vapor equilibrium region encounters a zone large number of plates must be employed for the distil in which two phases are present, one phase being a lation process in order to permit the separation of ura chlorine trifluoride solution which is saturated with nium hexafluoride from its impurities. 35 uranium hexafluoride and the other phase being ura Two types of separation are possible: nium hexafluoride crystals, with the result that the dis either it is sought to obtain 100 percent molybdenum tillation process cannot proceed further. hexafluoride of tungsten hexafluoride at the top of The phase equilibrium diagram of the molybdenum the column and 100 percent uranium hexafluoride hexafluoride-chlorine trifluoride mixture is shown in at the bottom of the column, 40 FIG. 1. The diagram demonstrates the existence of an or it is desired to obtain uranium hexafluoride in the azeotrope which, at a pressure of 3,450 mb, contains pure state at the bottom of the column while limit 12 percent molybdenum hexafluoride (boiling point: ing the quantity obtained at the top of the column 44.2°C). to a few per cent of molybdenum hexafluoride or FIG. 2 shows the phase equilibrium diagram of the tungsten hexafluoride. 45 tungsten hexafluoride-chlorine trifluoride mixture. In the first case, provision must be made for a distilla This diagram demonstrates the existence of an azeo tion column having a large number of plates in order to trope which, at a pressure of 3,450 mb, contains 37.5 carry out the separation process. It is necessary to in percent tungsten hexafluoride (boiling point: 39.2°C). troduce molybdenum hexafluoride or tungsten hexaflu oride in order to set the column at its steady state un 50 FIG.3 shows the curve of temperatures along the dis less the progressive accumulation of impurities at the tillation column in equilibrium with the mixture of UF6 top of the column is first permitted to take place. and ClF. In the second case, the uranium hexafluoride which FIG. 4 is a diagrammatic view of the distillation col leaves the top of the column together with the impuri umn which is employed for carrying out the method. ties must be recovered. Control of the installation is dif 55 The gas to be purified and the chlorine trifluoride are ficult to achieve since the concentration curve in the introduced at an intermediate point G of the distillation column is known only on the basis of analyses, taking column which contains a packing. However, it is possi into account the very small temperature difference be ble to introduce the chlorine trifluoride into the col tween the top and the bottom of the column (the bot umn independently of the gas to be purified. tom of the column is at the boiling temperature of UF6 60 The column is provided at the base with a reboiler R at the pressure under consideration whilst the top of and an overhead condenser C which returns the UFs as the column is at a temperature which is close to the reflux. Purified UF is withdrawn from the bottom of boiling point of UFs, the gases which leave the top of the column whilst the chlorine trifluoride and the impu the column being essentially constituted by UFs). rities are withdrawn from the top of the column. The method of purification of uranium hexafluoride 65 The substantial temperature difference between the by chemical separation takes a long time to carry into top and bottom of the column permits easy separation effect. It involves formation of a precipitate of ammo of impurities from the uranium hexafluoride. As can be 3,806,579 3 4 seen from FIG. 3, said difference is of the order of I. These results show the efficiency of the method for 50C: the temperature at the bottom of the column cor the separation of molybdenum hexafluoride with a responds to the boiling temperature of UFs at the pres small number of plates. sure under consideration (3,450 mb) and the tempera Since the relative volatility of tungsten hexafluoride ture at the top of the column corresponds to the boiling 5 with respect to chlorine trifluoride is greater than in the temperature of the azeotrope. The concentrations case of molybdenum hexafluoride, it was possible to along the column are perfectly defined, thus avoiding carry out tests more rapidly with tungsten hexafluoride, the need for continuous analytical testing of the purity After introduction of 656 g of tungsten hexafluoride, of the bottom product (pure UFs). the concentrations of this product were: The chlorine trifluoride at the top of the column can 10 at the top of the column (point 1): 10 620 vpm (point be regenerated either periodically or on a continuous 2): < 0.465 vpm basis. There is therefore found to be a drop from 10 620 Depending on the quantities of chlorine trifluoride vpm to less than 0.465 vpm with 1.683 m of a packing which are consumed, it is possible to recover this latter: of the “heli-pak' type. 5 either partly, by aeotropic distillation ClF3 - azeo In order to ensure that the tungsten hexafluoride and trope ClFa/MoFa or ClFa azeotrope ClFa/WF, molybdenum hexafluoride did not have any mutual in or totally by trapping molybdenum hexafluoride or fluence, (WF and MoF) were introduced simulta tungsten hexafluoride on sodium fluoride.
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