[1]a 3@4519790 Uj3ited j StateS Pate"t @office Patented une 24, 1969 3,451,790 METHOD OF SEPARATING NEPTUNIUM AND URANIUM VALUES Sidney Katz, Oak Ridge, and George 1. Cathers, Knoxville, Tenn., assignors to the United States of America 5 as represented by the United States Atomic Eniergy Commission No Drawing. Filed May 14, 1968, Ser. No. 728,911 Int. Cl. COlg 57100 U.S. C]. 23-326 9 Claims 10 AIBSTRACT OF THE DISCLOSURE A method of separating neptunium values from a mixture containing neptunium hexafluoride together with 1,5 uranium hexafluoride comprising @contacting said mixture with sodium fliioride at a ternperature of 100' to 250' C., thereby sorbing these hexafluorides on the sodium fluoride, converting the neptunium to a state in which it is more stron.aly sorbed than uranium hexafliioride, and selectively 20 desorbing the uranium hexafluoride by heatin- the sodium fluoride to a temperattire of 25Go to 350' C. Background of the invention 25 The invention described herein was @made in the course of, or under, :a contract with the United States Atomic Energy Commission. Our invention relates tc) processes for separating VOI", 30 tile metal fluorides from each other and more specifically to processes for separatin.- neptunium hexafluoride froni Liranium hexafluoride. In one important method of recovering uranium values fr(>m spent nuclear fuels the fuel is fluorinated to produce 35 uranium hexaffuo-ride, thereby separating uranium values from metals which do not form volatile fluorides. However, some other metals present in the spent fuel, including neptunium, ialso form volatile fluorides, and methods must be provided for separating these volatile ffuorides from 40 uranium hexafluOTide. Separation of uranium hexafluoride from other volatile fluorides -has been achieved by selective sorption of one or more of the volatile fluorides on a nonvolatile metal fluoride. U. S. Patent No. 3,165,376, issued Jan. 12, 1965, to W. R. Golliher teaches a method of sepa- 45 rating neptuniiim and uranium hexafluorides by passing a mixture of these -metal fluc)rides thtough a bed of magnesium fluoride particles. The neptunium hexafluoride is sorbed by the magnesium fluoride and the uranium hexafluoride passes through. VVhile this method is capiable of 50 separating uranium from neptunium values, recovery of the neptunium requires that it be removed from the magnesium fluoride by coiltactin.- the bed with an acidic aqtieous solution, Other volatile metal fluorides sorbed on the M-F2 mass together with neptunium hexafluoride are 55 also removed in this step, and therefore additional separation steps are, required to p-roduce highly pure neptunium values; an additional disadvantage of this method is that the MgF2,inass must ibe thoroughly dried before reuse. 60 Summ-ary of the invention It is accordingly one object of our invention to provide an improved method of separating neptunium values from uranium hexafluoride. 65 It is another object to provide a aas-solid sorption method of separating neptunium values from uraniiim hexafluoride wherein sorbed neptunium values can be removed froin a sorbent bed by contacting the neptuniumloaded bed with a gas. 70 Other objects of our invention will become apparent from the followin.- description and the appended claims. 2 In accordance with our invention we have provided a method of separating neptunium values from a gaseous mixture containing neptunium hexafluoride and uranium hexafluoride comprising: contacting said gaseous rnixture with a sodium fluoride mass maintained at a temperature of 100' to 250' C. whereby uranium and neptunium hexafluorides are sorbed on said mass; selectively reducing the oxidation state of the sorbed neptunium values to a value less than six; reriaoving sorbed uranium hexafluoride from the sodium fluoride mass; and removing substantially uranium-free neptunium values from the resulting sodium fluoride mass. Our method achieves excellent separation of neptunium and uranium values and it is readily incorporated into Iluoride volatiiity processes. Sodium ffuoride is a better sorbent for neptunium hexafluoride than is magnesium fluotide, and neptunium having ahigh degree of purity is readily recovered from the sodium fluoride mass. The desorbed bed may be reused immediately after the neptunium values are removed. Description of the preferred embodiments In the first step of our process a gaseous inixture containing hexafluorides of neptunium and uranium is contacted with a bed of sodium fluoride particles. Methods of contacting gases with solids are well-known and do not form a part of oiir invention. Optimum conditions such as those relating to gas flow rates and sodium fluoride particle size and particle arrangement may readily be determined by workers in the art. The bed of particles is maintained at a ternperature of 100o to 250' C. during the step of contacting the gas mixture with the bed. Witbin this teniperature range neptunium hexafluoride and uranium hexafluoride are sorbed on sodium fluoride. The optimum sorption temperatures for the neptunium and uranium hexafluorides do not overlap. Uranium hexafluo-ride is sorbed -very rapidly at temperatures of 100' C. and greater, but at 175' C. and above the equilibrium pressure is great enough to cause significant losses. The sorption of neptunium hexafluoride is poor at 1 00' C., but it increases with an increase in the temperature and is excellent at 200' C. The pteferred temperature range is from 170' tG 180' C., and, in the preferred method of carrying out the sorption step of our invention, a bed having a temperature variation from the inlet to the outlet is used, the temperature across the bed varying from 150o to 170' C. at one end and from 180' to 2001' C. -at the other end. The gases are sorbed quickly and a bed residence time of 5 to 10 seconds is adequate for a bed containing particles having a surface area of 1 to 3 square meters per gram. The neptuniurn in the neptunium hexafluoride-sodium flUOTide complex is then reduced, thereby producing a highly stable neptunium fluoride-sodium fluoride complex. The valence of neptunium in t-his state corresponds to an average value in the range of 4.5 to 5. The neptunium may readily be reduced merely by cooling the sodium fluoride mass and removing fluorine. In surh a method the sodium ffuoride mass containing sorbed uranium and neptunium fluorides is cooled to a temperature below 100' C., preferably to a temperature in the ringe of 20' to 30' C., and,an inert gas sueh as nitrogen or helium is passed through the cooled bed. Using this method we have found that a period of two hours is long enough to reduce the neptunium. A longer period may be used, but serves no useful purpose. The bed is then heated to a temperature at which uranium hexafluoride is desorbed from the sodium fluoride particles. This temperature may range from 225' to 400' C., preferably 250o to 300' C. An inert gas such as nitrogen or helium is passed through the bed to carry away the desorbed uranium hexafluoride.
[2]3)451,790 3 The sodium fluoride mass, substantially free of uranium values and containing a neptunium fluoride-sodium fluoride complex, is then increased to a temperature of 400' to 500' C., preferably 425' to 475' C., and a fluorinatin.agent such as an interhalogen compound or preferably fluorine is brought into contact with the sodium fluoride. The fluorinatin.- agent oxidizes sorbed neptunium values to neptunium hexafluoride which has a high equilibrium pressure at temperatures in excess of 450' C. The resulting desorbed neptunium hexafluoride -may be collected by passing it into a cold trap maintained at a temperature below -70' C. Having thus described our invention, the following example is given to illustrate it in more detail. EXAMPLE A gaseous mixture containing 640 milligrams of uranium as UF6 and 77 milligrams of neptunium as NpF6 iS passed into a bed of NaF pellets @maintained at a temperature of 175o C. The bed contains 2 -rams of sodium fluoride havin.- a surface area of 1-3 square meters per gram and is disposed in a column 0.75- centimeter in diameter and 5 centimeters long. The mixture is introduced into the column at a rate of 50 milliliters per minute. Ninety-four percent of the neptunium hexafluoride and 99.2 percent of the uranium hexafluoride sorb on the sodium fluoride. The sodium fluoride bed is then cooled to 25' C. and nitrogen is forced throti-h the bed at a rate of 50 milliliters per minute for 120 minutes. The neptunium sorbed on the bed has an average valence of 4.5 to 5. The bed containing the reduced neptunium and the uranium hexafluoride is then heated to a temperature of 275' C. and nitrogen is passed through at a rate of 360 milliliters per minute to, remove desorbed uranium hexafluoride. The desorption time to remove 98.1 percent of the uranium is 4 hours. Over 95 percent of the sorbed neptunium remains on the bed. The sodium fluoride bed containing neptunium values is then heated to a temperature of 450' C. and fluorine gas at a rate of 50 milliliters per minute is passed through the heated bed for 90 minutes. The exit gases are passed into a sodium fluoride trap. The product contains over 80 percent of the startin.- amount of NpF6 and less than 2 percent of the starting amount of UF6- The above example is given to illustrate, not to limit, our invention. Changes in temperatures, concentration of gases, types of inert orfluorinating gases may be made to ac,commodate process requirements and will be apparent to workers in the art. We
