I
WALLACE W. SCHULZ, ERWIN W. NEUVAR, JIMMY L. CARROLL, and RAYMOND E. BURNS Hanford Atomic Products Operation, General Electric Co., Richland, Wash.
Electrodialytic Conversion of Uranyl Nitrate to Uranic Fluoride Salts This electrodialytic process for alkali metal and ammonium uranium(lV) double fluorides can be operated on a continuous basis at 80 to 85% "current efficiency and involves fewer steps than the conventional method and ammonium uranium(1V) double fluorides of the general formula MUF6 can be used as intermediates for preparing both uranium hexafluoride and uranium metal (4, 5 ) . An electrodialytic process for the preparation of such uranic fluoride salts from aqueous uranyl nitrate-the usual end product in uranium separations processes-was proposed by Carson and others in 1954 (2, 3 ) . Essential features of the process, together with pertinent developmental data and a chemical flowsheet, are briefly discussed in this report.
A L K A L I METAL
product is removed from the catholyte external to the cell. The catholyte is continuously concentrated to remove water introduced during cell operation and is then recycled except for a small fraction sent to waste. Hydrofluoric acid and alkali metal (or ammonium) fluoride are continuously added to the recycled catholyte. The over-all cell reaction is given in Equation 1. Equation 2 represents the anode reaction and Equation 3 the cathode reaction.
Process Description
Membrane Properties. Ion exchange membranes for use in the process should exhibit high electrolytic conductivity, high permselectivity, low hydraulic permeability, and a long useful life. A satisfactory combination of these properties is found in Permutit 3142 cation, Permutit 3148 anion (Permutit Co., New York, N. Y.), and in Nepton AR 111 anion (Ionics Inc., Cambridge, Mass.) membranes. The electrical resistance of these membranes, under typical process conditions, ranges from 5 to 40 ohms per sq. cm. These membranes are also chemically stable in process solutions and appear to retain their electrical properties upon prolonged cell operation up to 60' C. The membrane property of greatest significance to over-all process performance is permselectivity, the property of the membrane to permit passage, under a potential gradient, of anions to the exclusion of cations. or the reverse. Cations leaking from the anolyte compartment across the anion membrane into the feed compartment compete with uranium for transport to the catholyte
+ 2H' + 5F- + NH4++ NH4LFj -+ HzO + (1) H z O 2HC + + 2e- ( 2 ) U02++ + 4H+ + 5F- + + 2e--+ UOz++
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Process Concept
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Two major electrochemical steps are performed simultaneously in the electrodialytic process : Uranyl and nitrate ions of the feed solution are separated from each other, and uranyl ions are reduced a t a mercury cathode to the quadrivalent state. The reduction step depends on the previous separation, as nitrate ions can oxidize uranium(1V) ions. The electrodialytic process is shown in simple schematic form in Figure 1. The basic element of the process is an electrolytic cell which is divided into three compartments by an anion exchange membrane and a cation exchange membrane. lJranyl nitrate is introduced into the center (feed) compartment, which is bounded by the two membranes. When a potential is applied, uranyl ions are transported through the cation membrane into the cathode chamber, which contains the appropriate alkali metal (or ammonium) ion and fluoride ion. Simultaneously, nitrate ions migrate through the anion membrane to form nitric acid in the anode chamber. Water is also transported from the feed compartment with both ions. Over-all current efficiencies in the desired range, 80 to loo%, are not feasible with an acid anolyte, hence base is continuously added to the anode compartment. and the cell is operated with an alkaline anolyte. In the cathode chamber, uranium(V1) is reduced to the quadrivalent state and precipitated as a uranic double fluoride. I n continuous operation, solid
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Although the equations are written for the production of NH4UF6, the sodium and potassium salts are prepared in a similar fashion. The ammonium salt can be readily decomposed to uranium tetrafluoride (7) and ultimately converted to either metal or uranium hexafluoride, whereas the other salts are suitable intermediates for preparation of metal but not for uranium hexafluoride.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 1. Uranyl nitrate i s converted to insoluble uranium(lV) double fluorides in a three-compartment electrodialysis cell
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compartment. The net result of such leakage is a decrease in the current efficiency for uranium transport. The same principles apply to leakage of anions from the catholyte to the feed compartment; experience shows, however, that the leakage rate of fluoride ion through the cation membrane under typical operating conditions is not significant with respect to cell current efficiency. Current efficiency decreases, ranging from 3 to 50%, are the result of leakage of cations through the anion membrane. The rate of such leakage is a function of anolyte composition and concentration, cell temperature, and membrane current density. In general, the leakage rate of a particular cation increases with increasing concentration and membrane current density, and it decreases with increasing cell temperature. The effect of cell temperature is an important consideration in establishing a cell operating temperature of 60' C. The relative leakage rates of ammonium, sodium, and calcium ions from alkaline nitrate anolytes-pH 8 to 9and hydrogen ion from a nitric acid anolyte and the corresponding decreases in uranium transport efficiency are shown in Figure 2. These data were obtained with Permutit 3148 anion membrane; comparable rates were obtained with Nepton AR 111 and Nalfilm 2 (National Aluminate Co., Chicago, Ill.) anion membranes. At comparable concentrations, leakage rates for hydrogen ion are about 10 times higher than those for sodium, calcium, and ammonium ions. Even a t 0.01M nitric acid-a concentration undesirably low from conductivity and anolyte flow considerations-there is about an 11% loss in current efficiency. A feasible cell current efficiency at a practical anolyte
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Figure 3. Current efficiency for uranium transport increases at elevated temperature and decreases with increase in feed acidity
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Generally, leakage rates of anolyte cations through anion membranes increase with increasing concentration and membrane current density and decrease with increasing cell temperature
Membrane, Permutit 3148. square inch
Temperature,
cation concentration (>O.lM) requires operation with an alkaline anolyte. The increased leakage of cations through the anion membrane associated with an increased membrane current density must be considered in establishing an operating current density. Other factors which must be borne in mind are the adverse effects of increasing current density on membranr useful life and the decrease in current efficiency for uranium reduction a t increased cathode current densities. Current densities above 2 amperes per square inch, in some instances, cause excessive membrane deterioration. A uniform membrane current density limited to 1 ampere per square inch is considered a satisfactory compromise to effect maximum utilization of available membrane area without undue damage and the realization of a suitable over-ail cell current efficiency. Cell Operating Conditions. ANOLYTE COMPARTMENT. The anolyte is maintained above p H 7 by continuous addition of base. Choice of the base is dictated by the product desired. Ammonia must be used for the production of pure NHdUF5 and, similarly, sodium hydroxide for the production of NaUF6. This restriction is a consequence of leakage of anolyte cations through the anion membrane. In addition to leakage rates of cations through the anion membrane, electrical conductivity and anolyte efRuent volumes must be considered in selecting a satisfactory anolyte concentration. Anolyte cation concentrations from 0.1 to 0.5M are satisfactory. Platinum is the most satisfactory anode material yet tested. Anodic dissolution rates for platinum average 0.05 mg. per ampere-hour over a wide range of probable operating conditions in the anolyte compartment. Stainless steels 304 L and
60' C.
Current density, 1 ampere per
347 are also satisfactory anode materials in an alkaline medium. FEEDCOMPARTMENT. In the present concept of the process, a feed influent containing only uranyl nitrate and nitric acid as the major constituents and having been decontaminated from fission products has been assumed. Three variables of importance with respect to the current efficiency for transport of uranium are the free nitric aciduranium ratio in the influent feed, the feed compartment operating temperature, and the leakage of anolyte cations through the anion membrane. Current efficiency for uranium transport increases slightly a t 60' C.-another reason for operating the cell a t this temperature (Figure 3)-and decreases with an increase in feed acidity. Figure 3 shows the acidity range expected for probable feeds for the process. Water is transported from the feed by electro-osmotic transfer and as hydration water of the mobile ions. The amount of water transferred with the uranyl ion increases from about 2 to 4 ml. per gram of uranium as the feed compartment uranium concentration decreases from 1.5 to 0.1M. The amount of water transported with the nitrate ion is about 10% that transported through the cation membrane. In terms of feed compartment operation, it is possible to select operating conditions under which all the uranium and water added are transferred. However, such operation requires precise adjustment of influent uranium concentration and flow rates; it appears more practical to operate with a small effluent from the feed compartment. Operation with a feed compartment uranium concentration of 1.5M to decrease cell resistance 2nd with a feed effluent corVOL. 50, NO. 12
*
DECEMBER 1958
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URANYL ION CONCENTRATION, .M Figure 4. Highest current efficiency for uranium reduction occurs a t low cathode current density land high catholyte uranium concentration Temperature, 60' C. Ammonium ion, 0.1 M Fluoride ion, 0.2 to 0.6M
Figure 5. This Lucite cell, with vertical membranes cind a stacked-trench mercury cathode, permits operation with minimum power requirements at a uniform current density of 1 ampere per square inch
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Cell dimensions. Height, 12.5 inches; width, 6.5 inches, and thickness (inside dimensions), 2 inches. It was operated with 2 2 square inches of cation membrane exposed to the cotholyte solution
responding to 1% of the influent uranium is presently envisioned. This feed effluent may be recycled to some previous point in the uranium separations process or processed in a separate facility for recovery of the uranium. CATHOLYTE COMPARTMENT. I n the catholyte compartment, uranium transported from the feed compartment is reduced at a mercury cathode. Mercury is the only suitable cathode material at which high reduction efficiencies can be obtained a t practical cathode current densities. Continuous renewal of the mercury surface is essential to achieve high reduction efficiencies. Current efficiency for uranium reduction is primarily a function of uranium concentration and cathode current density as illustrated in Figure 4. Highest current efficiencies are obtained at high uranium concentrations and low cathode current densities. Cathode current density must be restricted to about 2 amperes per square inch to obtain greater than SOY, efficiency. Operation at 60' C. in the presence of a slight excess of fluoride and metal (or ammonium) ions over stoichiometric requirements also increases reduction efficiencies. PRODUCTRECOVERY. Process steps associated with product recovery and external catholyte treatment have not yet been studied in detail. A tentative procedure is as follows: Product is removed as a catholyte slurry and collected by filtration, settling, or centrifugation. The product is then washed and dried. A small volume of the catholyte
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and wash liquids are sent to waste while the bulk is concentrated to remove water introduced through the cation membrane and in the product washing step. After cooling to the desired temperature, the catholyte is then recirculated to the catholyte compartment. In continuous operation, hydrofluoric acid and the appropriate alkali metal fluoride (or ammonium fluoride) are added to the catholyte to maintain constant composition. In this external catholyte treatment, the small amount of uranium, which can be sent to waste economically, necessitates a relatively low catholyte bleed rate. As a consequence, ionic impurities (fission and stainless steel corrosion products) transferred from the feed compartment will build up in the catholyte to a concentration which could adversely affect the decontamination potential of the process. An alternative procedure would be to remove catholyte equivalent to the volume of water introduced during cell operation. Additional processing steps to recover uranium from the bleed could be compensated for by an increased decontamination potential. CELL DESIGNCONSIDERATIONS. An important feature of cell design is a satisfactory arrangement of membranes and liquid mercury cathode. The choice of geometrical configuration is largely determined by the desirability of operating with minimum power requirements at a uniform membrane current density of 1 ampere per square inch. Attainment of uniform membrane cur-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
rent density requires a parallel arrangement of membranes and electrodes. With vertical membranes, a parallel arrangement can be approximated by constructing the cathode as a series of stacked trenches, parallel to, and equidistant from the cation membrane (Figure 5). In the laboratory cell, each trench was fitted with a paddle stirrer so operated as to continuously break through the mercury surface. Agitation in each cell compartment ensures uniformity of solution composition. The materials of construction of cell components must be resistant to attack by any of the individual or mixed compartment solutions at temperatures at least as high as 60' C. Lucite, used in laboratory cells, is suitable for construction. Literature Cited (1) Rernhardt H. A,, Gustison, R. A., Posey, J. C., AEC Rept. IC-410 (June 1,
19491. (2) Carson, W. N., Jr., AEC Rept. HW32144 (June 15, 1951). (3) Carson, W. N., Jr., Van Meter, W. P., .4EC Rept. HW-39457 (Oct. 11, 1955). (4) Marinsky. J. A , . AEC Rept. ORNL ' 1979 (Aprill7, 19g6). (5) Tolley, W. B., AEC Rept. HW-35814 (Feb. 1, 1955). RECEIVED for review April 7, 1958 ACCEPTED September 29, 1958
Division of Industrial and Engineering Chemistry, Symposium on Preparation and Recvcle of Feed Materials for Kuclear Fuel, 133;d Meeting, ACS, San Francisco, Calif.; April 1958.
