Anion Exchange Separation for the Determination Uranium in Complex Solutions 0. A. Vita, C. R. Walker, C. F. Trivisonno, and R. W. Sparks Goodyear Atomic Corp., Box 628, Piketon, Ohio 45661 Two anion-exchange separations applicable to the determination of uranium in complex nuclear fuel element solutions are described. I n a very selective dual ion-exchange process, uranium is adsorbed on Dowex 1-X8 as the anionic nitrate complex from 1.9M aluminum nitrate-0.lM nitric acid and then as an anionic chloride complex from 8M hydrochloric acid without removal from the column. In a less selective single ion-exchange process that is suitable for many applications, uranium is adsorbed on Dowex 1-X8 from 8M hydrochloric acid. After either adsorption, uranium is eluted and determined by appropriate methods. Uranium has been separated from solutions containing aluminum, beryllium, molybdenum, stainless steel, and zirconium and determined by titration with potassium dichromate. The effects of nitric acid and fluoride ion on adsorption are described. The relative standard deviation is +0.4% per analysis with no significant bias. ION-EXCHANGE SEPARATIONS play a prominent role in the determination of many elements. Uranium is particularly adaptable to the ion-exchange process because it forms several anionic complexes that permit efficient uranium separations from many other elements. Kraus and Nelson ( I , 2 ) have studied in detail the adsorption characteristics of many elements from hydrochloric acid solutions on strongly basic anion-exchange resins and have devised uranium separations based on their findings. Faris and Buchanan (3) reported on similar studies for nitric acid solutions. Marcus (4) studied the anion-exchange characteristics of the actinide elements from nitrate salt solutions. Foreman et a/. (5), found that uranium adsorption on an anionexchange resin is affected by inorganic nitrate salts, and Ockenden and Foreman (6) applied these findings to a separation of uranium from iron in a!uminum nitrate solutions. Vita et a / . (7) extended this uranium separation to many other elements. Thus, anion exchange is an effective tool for separating uranium from strong mineral acids and concentrated nitrate salt solutions. It is particularly adapted, therefore, to the separations required in the analysis for uranium in complex solutions produced by dissolution of nuclear fuel elements. Such solutions vary greatly in composition; in addition to a variety of dissolved metals and dissolution acids, they often contain a high concentration of aluminum nitrate which .is used both as a fluoride complexing and salting agent for the
(1) K. A. Kraus and F. Nelson, Proc. Intern. ConJ Peaceful Uses of Atomic Energy, Geneva, 7, 113 (1956). (2) K. A. Kraus, F. Nelson, and G. E. Moore, J. Amer. Chem. Soc., 77, 3922 (1955). (3) J. Faris and R. F. Buchanan, “Progress in Nuclear Energy, Series IX Analytical Chemistry,” Pergamon Press, London 147-180 (1966). (4) Y. Marcus, M. Givon, and G. R. Choppin, J. Znorg. Nucl. Chem., 25, 1457 (1963). (5) J. K. Foreman, I. R. McGowan, and T. D. Smith, J. Chem. SOC., 1959,738. (6) H. M. Ockenden, and J. K. Foreman, Analyst, 82, 592 (1957). (7) 0. A. Vita, C. F. Trivisonno, and C. W. Phipps, AEC Report GAT-283, Piketon, Ohio (April, 1959).
solvent extraction of uranium. A solution might contain constituents of aluminum alloys, molybdenum alloys, stainless steels, or zircalloys dissolved in hydrofluoric and nitric acids. Uranium contents of such solutions usually vary from 0.1 to 10%. In some cases the solutions are unstable and undergo hydrolytic precipitation which presents additional problems in the uranium separation. Several separation methods for uranium from nuclear fuel element solutions are available (8). These methods include precipitations, solvent extractions, and electrolytic separations. These methods are lengthy and complex, and often a combination of several separation techniques is required to separate the uranium from the different types of nuclear fuel element solutions. Because of these disadvantages, we evaluated and applied anion exchange for the separation of uranium from nuclear fuel element solutions. Anion-exchange separation offers a simpler and more direct approach than the precipitation, solvent extraction, and electrolytic separation methods. The entire separation of uranium is achieved within the ion-exchange column without removal of the uranium except in the final elution. Because most solutions of nuclear fuel elements contain large amounts of aluminum nitrate, they are easily adapted to anion exchange from a salted nitrate-weak acid medium. For many more, anion exchange from strong hydrochloric acid is applicable. Because the anion-exchange behavior of uranium can be varied easily, separation schemes can be designed for different solution types and final uranium measurement methods. In one scheme, for example, uranium is separated by adsorption first from 1.8-2.OM aluminum nitrate solution and then from 8M hydrochloric acid solution without removal from the column. In another scheme, uranium is separated by adsorption from an 8M hydrochloric acid solution. The adsorbed uranium is then eluted and determined. In some cases, separation requirements are simplified because the uranium measurement methods do not require complete separation of uranium. Methods such as the dichromate titration suggested by Davies and Gray (9) can be coupled to either anion-exchange separation to provide an effective method for the determination of uranium in complex solutions. EXPERIMENTAL
Distribution Studies (IO). The Dowex 1-X8 resins (50-100 mesh and 200-400 mesh) were washed with de-ionized water and converted to the nitrate form with nitric acid. The resins were then dried at 110 “C and stored in a desiccator. Distribution coefficients of uranium on Dowex 1 resin from aluminum nitrate solutions were made by batch techniques. One gram of dried resin was equilibrated with 50 ml of acidified aluminum nitrate solution containing 10 mg of uranium. The concentrations of aluminum nitrate were (8) R. J. Jones, AEC Report TID-7029, Washington, D. C. (Oct.,
1963). (9) W. Davies and W. Gray, UKAEA, TRG Report 716(D), January 1964. (IO) E. R. Tompkins, J. Chem. Educ., 26, 32, 92 (1949). ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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Figure 1. Effect of aluminum nitrate concentration on distribution coefficients of uranium on Dowex 1-XS
varied from 0.2 to 2.8M, and concentrations of nitric acid also were varied from 0.1 to 3.28M. After the equilibration, the aqueous phases were analyzed for uranium by the dibenzoylmethane colorimetric method. The distribution coefficients were derived using the relationship : Kd =
Concentration of U per gram of dried resin Concentration of U per ml of solution
Column Studies. Ion-exchange columns were prepared with pre-soaked Dowex 1-X8, 50-100 mesh resin. Resin columns were 28 cm long and 1.7 cm in diameter. The resin was initially conditioned with the adsorption medium. For the salted nitrate exchange, 100 ml of 1.9M A1(N03)30.1N H N 0 3 solution was passed through the column at 2-4 ml per min; for the chloride exchange, 100 ml of 8 M hydrochloric acid was passed through the column at the same rate. Solutions containing up to 100 mg of uranium and other elements, such as molybdenum, vanadium, and zirconium, were passed through the columns at 2 ml/min during the adsorption and elution phases. Twenty-five-milliliter fractions were collected and analyzed for uranium and other elements of interest by atomic absorption or colorimetric techniques. From these data, elution curves were constructed, and separation procedures for uranium from various fuel element solutions were devised. Chloride Exchange Procedure. Chloride ion-exchange separations can be used for fuel element solutions which contain negligible quantities of iron or where complete separation is not a requirement of the measurement method. Such solutions would include those prepared by dissolving uranium-aluminum, uranium-beryllium, uranium carbide, uranium-molybdenum, uranium-niobium, and uraniumzirconium fuel elements. By weight or volume, prepare a sample aliquot which contains up to 150 mg of U. Transfer the aliquot into a Teflon (Du Pont) beaker (if fluoride is present), and add 50 ml of concentrated HC1. Boil the solution to near dryness. Add a second portion of concentrated HC1 and evaporate to near dryness again. Re-dissolve the wet residue with 150 ml of 8 M HC1, and transfer the solution to an ion-exchange column preconditioned with 8 M HC1. Pass the sample solution through the column at the rate of 2 ml/min. Wash the column with three 25-ml portions of 8 M HCl. Wash the 466
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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Figure 2. Effect of nitric acid concentration on distribution coefficients of uranium on Dowex 1-X8 from 2.OM Al(NOa)3solutions
column three more times with 25 ml of 6M HC1-0.02M H F solution. (The fluoride, which is not required in the absence of zirconium, is necessary to elute zirconium completely.) Elute the adsorbed U with 250 ml of 0.1MHC1. Theeluate can then be treated appropriately for the final uranium measurement. Dual Exchange Procedure. The salted nitrate-weak acid ion exchange followed by the chloride ion exchange can be used for solutions containing large amounts of iron such as those prepared from uranium-stainless steel fuel elements. This ion-exchange scheme separates uranium rather completely from most of the elements encountered in such fuel element solutions. In addition, this procedure is easily adapted to salted nitrate dissolver solutions. By weight or volume, prepare a sample aliquot which contains up to 125 mg of U. Transfer the aliquot into a Teflon beaker. If the sample is not a nitrate solution, add 50 ml of concentrated "03. Add 1 ml of H202 (to reduce any C P ) , and evaporate to near dryness (about 10 ml) to Add 90 ml of 2.0MA1(N03)3expel excessive H F and "03. 0.1M HNOI solution to the sample, and transfer the solution to a properly preconditioned ion-exchange column. Wash the beaker with about 50 ml of 1.9M Al(NO&-O.lM "01 solution, Pass the solution through the column at a rate of 2 ml/min. Rinse the column three times with 25 ml of 1.9M ~(N03)3-0.1MHN03solution, Rinse the column three times with 25 ml of 8 M HC1. (Use a 1-ml/min flow for the first two rinses to allow for conversion of the column to the chloride form and re-adsorption of the uranium as its anionic chloride complex.) Rinse the column with 25 ml of 6M HC1-O.02M HF solution. (If gross amounts of zirconium are present, repeat the wash step.) Elute the adsorbed uranium with 250 ml of 0.1 M HC1. RESULTS
Adsorption of Uranium from Nitrate Media. The distribution coefficients for uranium between Dowex 1-X8 and aluminum nitrate solutions were obtained by batch equilibration techniques. They depend on the concentration of aluminum nitrate as shown in Figure 1. In column operation, 1.8-2.OM aluminum nitrate solutions are used for the absorption of
,< 1
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Column E f f l u e n t , ml
Figure 3. Separation of uranium from molybdenum by anion exchange
Figure 4. Separation of uranium from vanadium by anion exchange 100 mg U-400 mg V
100 mg U-540 mg Mo A . 200 ml, 1.9M AI(NO&, 0.1M HN03, 0.05M HF B. 25 ml,1.8M Al(NO& C . 50 ml, 7.8M NaN08 D. 75 ml, 8MHCl E . 250 ml, 0.1M HCl F. 512 mg of Mo was retained on column
uranium because at higher salt concentrations the flows are too slow. Uranium distribution coefficients of about 1000 are obtained in this region of salt concentration, and uranium separations are easily achieved. The concentration of nitric acid also affects the adsorption of uranium on Dowex 1-X8 resin from aluminum nitrate solutions. Adsorption of the uranium was made from 2.OM A1(N0J3 solutions in which the nitric acid was varied from 0.1 to 3.3M. The distribution coefficients are shown in Figure 2 . As the nitric acid concentration is increased, the adsorption of uranium on Dowex 1-X8 is decreased. Thus, in column operation, nitric acid concentration of the sample solution should not exceed 0.1 to 0.2M for the most favorable adsorption characteristics for uranium. Fluoride concentration up to 0.5M has little effect on the adsorption of uranium on Dowex 1-X8 from aluminum nitrate solutions. Adsorption of the uranium on Dowex 1-X8 from 2.OM aluminum nitrate-0.6M nitric acid solution was not affected by fluoride concentration ranging from 0.0 to 0.2M. As fluoride concentration increases further to OSM, the adsorbability of uranium decreases slightly. Many other nitrate salts also enhance the adsorption of uranium on Dowex 1-X8 (7). Again, the adsorption is dependent on salt concentration. Anion exchange from the salted nitrate-weak acid medium separates uranium completely from the Group I, 11, 111, and VI11 elements. Thorium, however, behaves like uranium and is not separated from it, while the rare earths and zirconium partially adsorb from
200 ml, 1.9M Al(N03)3, 0.1M "03, 50 ml, 1.9M Al(NO&, 0.1M HNOB 125 ml, 8M HCI 50 ml, 6 M HCI E. 250 ml, 0.1M HCl
A. B. C. D.
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this medium. Chromate and molybdate also adsorb; however, chromium(II1) does not. For an essentially complete uranium separation, the salted nitrate-weak acid ionexchange process can be coupled with a strong hydrochloric acid exchange. Adsorption of Uranium from Chloride Medium. The adsorption characteristics of uranium(V1) on strongly basic anion-exchange resins from hydrochloric acid are extensively reported in the literature (1). Uranium(V1) adsorption on Dowex 1-X8 from hydrochloric acid varies from Kd
