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Home , Molybdenum hexafluoride, Tungsten hexafluoride, Uranium hexafluoride

United States Patent ti9] [ii] 3,806,579 Carles et al. [45] Apr. 23, 1974

[54] METHOD OF PURIFICATION OF 3,425,812 2/1969 Cousin et al 23/352 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 contaminated with impurities Apr. 8, 1970 France 70.12685 such as hexafluoride and hexa- . [52] U.S. CI 423/19, 423/8, 423/258 The contaminated uranium hexafluoride is distilled [51] Int. CI COlg 43/06 from the liquid phase in the presence of [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forms an azeotrope with the 423/258, 8 impurities to be removed from the uranium hexafluoride, said impurities are withdrawn from the [56] References Cited top of the column and the purified uranium hexafluoride is collected at the bottom of the column. UNITED STATES PATENTS 2,830,873 4/1958 Katz et al ....23/326 1 Claim, 4 Drawing Figures PATENTED APR 2 3 197^ 1806,579

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0 12% 0 37,5%

%MoFe FIG. 1 FIG. 2

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4- 6 5 4 3 2 I FIG. 3

FIG. 4 R 3,806,579 1 2 METHOD OF PURIFICATION OF URANIUM nium diuranate by bubbling of uranium hexafluoride HEXAFLUORIDE through , dissolving the which is formed with , 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 so as to yield ammonium diuranate, cal- Uranium hexafluoride is prepared by the action of cination of the ammonium diuranate in order to form on , this compound being UOs, reduction of U03 to U02 by , conversion in turn obtained as a result of the action of anhydrous 10 of UOz into UF4 by anhydrous and hydrofluoric acid on uranium dioxide. The product conversion of UF4 into UFS by fluorine. This process thus obtained contains impurities (MoF6, WF6, 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 or of the mixture of nium hexafluoride, said impurities being discharged at uranium hexafluoride and . 20 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 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. 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- 5Q oride in order to set the column at its steady state un- 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 C1F3. 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- 55 umn which is employed for carrying out the method, ties must be recovered. Control of the installation is dif- 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- 6Q umn independently of the gas to be purified, torn of the column is at the boiling temperature of UF6 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 UF6 as the column is at a temperature which is close to the reflux. Purified UF6 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 UFe). 65 rities are withdrawn from the top of the column. The method of purification of uranium hexafluoride 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 3,529,233 seen from FIG. 3, said difference is of the order of I. These results show the efficiency of the method for 50°C: the temperature at the bottom of the column cor- the separation of molybdenum hexafluoride with a responds to the boiling temperature of UF6 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 UF6). 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. 15 In order to ensure that the tungsten hexafluoride and either partly, by aeotropic distillation C1F3 — azeo- molybdenum hexafluoride did not have any mutual in- trope ClFs/MoFe or C1F3 azeotrope C1F3/WF6, fluence, (WF and MoF ) were introduced simulta- or totally by trapping molybdenum hexafluoride or 6 e neously. The results were practically identical with tungsten hexafluoride on . The those obtained in the case of each of the products intro- chlorine trifluoride which is inactive on this prod- 20 duced separately. uct is recycled. The method has been carried into effect in a column TABLE I which had the following characteristics: construction entirely of monel, Quantity packing: five sections consisting of two sections of 25 of MoFfl Concentration of MoF„ in vpm 1.683 m and three sections of 0.678 m, Sample springs having contiguous turns wound so as to have test 1 2 3 4 5 6 a rectangular cross-section of 5 mm, 5 154 < 0.88 < 0.88 column diameter; 70 mm, 10 220 3.5 < 0.88 14 321 1.5 <0.88 Values below the heat balance: 2,250 watts. 30 20 431 — < 1.33 limits of 40 802 3.5 < 0.88 analytical Known quantities of MoF6 and WF6 were introduced 100 1898 3.08 <1.32 sensitivity and the concentrations of these products along the col- 220 4317 12.95 < 1.23 umn was then measured. The quantities employed 450 8240 21.13 < 0.88 were: in the case of chlorine trifluoride: 2.4 kg of liquid 35 chlorine trifluoride in the packing; What we claim is: in the case of uranium hexafluoride: 0.8 kg of liquid 1. A method of purification of uranium hexafluoride uranium hexafluoride in the packing. contaminated with the impurities molybdenum hexa- The curve of temperatures along the column is shown fluoride and tungsten hexafluoride in a distillation col- umn comprising the steps of maintaining the column in FIG. 3. Practically quantitative separation of UFS 40 from C1F3 is achieved as is established by analysis (UF6 under a pressure of about 3,450 mb, at a temperature at the top of the column: less than 30 vpm; C1F3 at the of 43°C. at the bottom of the column and at a tempera- bottom of the column: less than 50 vpm). ture of about 95°C. at the top of the column, distilling The quantities of MoF6 were introduced successively contaminated uranium hexafluoride with chlorine tri- and in cumulative values as follows: 45 fluoride which forms an azeotrope with the impurities 5 g, 10 g, 14 g, 20 g, 40 g, 100 g, 220 g, 450 g, to be removed from the uranium hexafluoride, with- After each introduction, the curve of concentrations drawing said impurities from the top of the column and of this product along the column from the top of this collecting the purified uranium hexafluoride at the bot- latter (FIG. 4) was determined analytically. tom of the column. The results obtained were grouped together in Table 50 * * * # . *

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