I
CARL W. KUHLMAN, Jr., and BRUCE A. SWINEHART Mallinckrodt Chemical Works, St. Louis, Mo.
Production of Uranium Tetrafluoride Thermal D a m a g e Effect When uranium tetrafluoride is made from uranium trioxide, properties of the reacting solids are altered at high temperatures to hinder completeness of hydrofluorination
IN
THE United States uranium tetrafluoride is produced from uranium trioxide made by thermal decomposition of uranyl nitrate. In both Europe and the United States this trioxide is reduced by hydrogen to uranium dioxide, which is in turn hydrofluorinated to uranium tetrafluoride. In Europe either ammonium diuranate (ADU) or uranium peroxide, prepared by precipitation from uranyl nitrate liquor, is converted to uranium trioxide by heating to a suitable temperature (6. 7 ) . Despite extensive research both in Britain and the United States, the complex physical chemistry of these reactions is not yet fully understood (7, 3-5, 8, 70, 7 7 , 73). Uranium trioxides of identical chemical composition react with hydrogen a t widely different rates (Figure 1). At least five distinct x-ray diffraction patterns have been reported for uranium trioxide (2, 3, 74-77). Structures of these forms have not been completely identified, and they are known empirically as crystal structure Types I, 11, 111, IV, and an amorphous form. Although correlations of the reactivity of the trioxide with crystal type have been reported (75), correlations have
also been found between the rate of reduction of a uranium trioxide and its surface area independent of the particular structure type (Figure 1). Amorphous uranium trioxide has high surface area and great reactivity. Type I11 trioxide, the crystal form produced in commercial scale denitrations, has wide variation of reactivity, depending upon the manner of denitration. Rate and temperature of denitration, presence of impurities, and type of equipment employed have a n important influence on the properties of the product. Rate of hydrofluorination of a uranium dioxide depends upon the temperature at which it had been prepared (Figure 2). This dependence of uranium dioxide reactivity on reduction temperature has been correlated with the surface area, which depends chiefly on the surface area of the uranium trioxide starting material. Above 650' C., however, the surface area also depends inversely on the reduction temperature ( 7 , 72). The magnitude of this sintering effect depends on the microstructure of the trioxide. Uranium dioxide made from a trioxide with small surface area does not sinter above 650' C.
as extensively as those from a starting material of greater area. As yet, no quantitative correlation between this surface area effect and the effect of hydrofluorination rate has been reported. I t has been suggested that the detailed course of the reduction reaction also influences the surface area of the product (8, 72). If the reaction is carried out in such a way as to avoid the formation of UaOs-in other words, if the initial part is carried out below the thermal decomposition temperature of uranium trioxide-less sintering of the product occurs. This has been interpreted in terms of a greater sensitivity of U308 to sintering that uranium dioxide (Figure 3). The reduction rate of all types of dioxide is a n increasing function of the temperature. If uranium trioxide is reduced a t various temperatures for a fixed time, the per cent of unreacted uranium trixode in the product is inversely related to the temperature a t which the reduction is carried out. The hydrofluorination reaction, is considerably more complex. Several contrasting types of behavior occur during the hydrofluorination re-
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4
Figure 1. Uranium trioxides of identical chemical composition react with hydrogen at widely different rates
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Figure 3. Surface area changes in reduction of uranium trioxides ( 9 )
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Figure 4. Contrasting types of behavior occur during hydrofluorination After l ' / ~ hours in anhydrous hydrogen fluoride. trioxide for 2 hours at 600' C.
action (Figure 4). Powders characterized by Type A show relatively little reactivity-large quantities of unreacted oxide in the product; but the amount of this unreacted oxide decreases with increasing temperature throughout the range of the data. Type B powder is much more reactive, but it too results in better conversion a t higher hydrofluorination temperatures. The third type of material shows a maximum conversion a t some intermediate temperature, The trioxide that gave curve A in Figure 4 was made in large scale continuous denitration equipment. Curve B was obtained with oxide made in stirred denitration pots. This same stirred bed denitrated uranium trioxide was micronized to produce the starting material for curve C. The micronization process simply reduces the aggregates to less than 1-micron diameter. Hydrofluorination of a uranium dioxide derived from ammonium diuranate follows a curve similar to Type C, but more exaggerated. The surface area and aggregate size data in Figure 4 categorize the types of uranium dioxide, showing these three types of hydrofluorination behavior.
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HYDROFLUORINATION TEMPERATURE P C J
Each uranium dioxide prepared b y hydrogen reduction of a Type 111 uranium
The effect shown in curve C, in which higher temperatures are unfavorable to complete hydrofluorination, is frequently called "thermal damage." The term suggests that the properties of the reacting solids are altered a t high temperatures to prejudice completeness of the hydrofluorination reaction. Lister and Gillies in Britain have interpreted this thermal damage in terms of a competition between two rate processes-the rate of reaction between hydrogen fluoride and uranium dioxide and the rate of sintering of the tetrafluoride product. As hydrogen fluoride is admitted into contact with a bed of dioxide, the particles react with the gas, forming a layer of uranium trifluoride outside the particle. Subsequent reaction is influenced by the relative rate of diffusion of gas through the layer of the reaction product. If the temperature of the particles or aggregates is sufficiently high, the tetrafluoride layer will begin to sinter, interfering with access of gas to the unreacted uranium dioxide in the interior of the particle. 'This sintering begins at an appreciable rate in the vicinity of 600' C. (Figure 5 ) . The fact that hydrofluorination of
some uranium dioxide samples is more complete a t programmed temperature than under isothermal conditions indicates the presence of some thermal damage as IQW as 430' C. (Figure 6), considerably below the 600' necessary for sintering. This apparent disagreement is believed explainable in terms of the transient particle temperature due to the heat of the reaction. The temperatures plotted are the nominal gross temperatures of the bed, and do not necessarily reflect localized particle temperatures. A consideration of the very rapid rate a t which the initial portions of these reactions occur even at 400' C. leads to the postulate of particle temperatures appreciably higher than either gross bed temperatures or gas phase temperatures measured by normal methods. Thermobalance data have indicated that the uranium dioxide made in these experiments undergoes about 50% of hydrofluorination in less than 5 minutes at 400' C. If it is assumed that not more than 50y0 of this heat is transferred to the apparatus, simple heat balance calculations lead to the result that individual particle or aggregate temperatures a t least 200' higher than the nominal temperatures measured are reasonable. Furthermore, particle temperatures are a direct result of the intrinsic reactivity of the powder,
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TEMPERATURE " C .
Figure 5. 600' C.
Appreciable sintering occurs above about
After heating for 20 minutes a t various temperatures
450
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HYDROFLUORINATION TEMPERATURE
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Reduction 2 hours at 600' C. Isothermal hydrofluorination, 1.5 hr. a t temperature 0 Programmed 0.5 hr., 200' to thermal temperature, 1 hr. at thermal temperature
0
VOL. 50, NO. 12
DECEMBER 1958
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HYDROFLUORINATION TEMPERATURE
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Figure 7. Different types of hydrofluorination behavior are explained on the-basis of this model
or the rate a t which the initial part of the reaction occurs. This effect of the initial rate of the hydrofluorination reaction on its completion is shown in Figure 6. If the initial part of the reaction is carried out a t a low temperature, better conversions are obtained than when the reaction was carried out isothermally at the terminal temperature. Presumably programming of the temperature slows the reaction in its early stages, dissipating a greater fraction of the heat evolved. A similar effect has been reported for the use of a dilute hydrofluoric acid-water mixture to slow down the initial part of the reaction. This principle of programmed reaction temperature is used in large scale technology. The different types of hydrofluorination behavior have been explained on the basis of the model shown in Figure 7. The three distinct powder types of uranium trioxide have been postulated to differ in the degree of aggregation of the intrinsic particles. Type A material is presumed to consist of large particles of low porosity, Type B, of small aggregates of powder of very high surface area and of equivalently very small ultimate particle size, Type C, of relatively large aggregates of fairly large surface area. Powders that have intrinsically high initial reactivity result in high particle temperatures promoting sintering. If the state of aggregation is considerable, formation of a relatively impervious coating of sintered uranium tetrafluoride tends to trap within the sintered shell an appreciable fraction of unreacted uranium dioxide. The percentage of this material will be determined by the rate of sintering, related to the nominal bed temperature of the reacting solids and the rate a t which heat of reaction is added to the sensible heat of the bed. If the aggregate size is small, the frac-
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Figure 8. This product of a hydrofluorination reaction exhibited thermal damag-e. Sample contained 7% unconverted UOz. Magnification 1 O O O X
tion of entrapped dioxide is less noticeable, although conditions for a sintered product are ideal. If a uranium trioxide powder has poor intrinsic reactivity, the rate at which heat of reaction is evolved is sufficiently slow so that it may be dissipated into the gas phase, and aggregate temperatures differ little from gas stream temperatures. Under these conditions sintering will be relatively slight and difficult to detect. The brown centers of uranium dioxide surrounded by shells of uranium tetrafluoride can be distinctly seen in samples showing thermal damage (Figure 8). T o the eye the shells have a glassy appearance under the microscope that does not reproduce in photographs.
Conclusion
The factors that determine the uranium tetrafluoride content of a product made by reduction and hydrofluorination of uranium trioxide under a particular set of conditions are numerous and interrelated in a complex manner. Because the bed temperature of the reacting solid has a n important effect on the rate and completeness of both reactions, the behavior of the uranium trioxide depends on the size of the sample, the apparatus, and the detailed manner in which the two reactions are carried out. Because of the thermal damage effect in reacting solids, the intrinsic reactivity of an oxide is not necessarily proportional to the amount of conversion to uranium tetrafluoride obtained in a particular green process. A partitularly active uranium trioxide may lead to a poor conversion to uranium tetrafluoride in a process that produces a satisfactory product from a less reactive starting material.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
literature Cited
(1) Anderson, J. S., Harper, E. A., Moorbath, s.,Roberts, I. F. J., Atomic Energy Research Establishment, Harwell, Berkshire, England, AERE-C/R-886 (August 1952). ( 2 ) Baenziger, N. C., Ames Laboratory, U. S. Atomic Energy Comm., ISC-99 (AECD-3237) (October 1948). (3) Bridge, J. R., Melton, C. W., Schwartz, C. M., Vaughan, D. A., Battelle Memorial Institute, BMI-1110 (July 1956). (4) Dawson, J. K., Wait, E., Alcock, K., Chilton, D. R., .I. Chem. Soc. 1956, p. 3531. (5) Farrar, R. I>,, Jr., Smith, H. A,, J . Phvs. Chein. 59, 763 (1955). (6) Goldschmidt, B., Vertes, Intern. ConJ. on Peaceful Uses of Atomic EnerEy (United Nations), 8, 152, 156 (195.5). (7) Grainger, L., Zbid., 8, 149 (1955). (8) Lister, B. A. J., Gillies, G. M., “Procrss Chemistry,” pp. 19-35, hlcGraw-Hill, New York. 1956. (9) Lister, B.’A. J., Gillies, G. M., AEREC/R-1650 (March 1955). (10) Lister, B. A. J., Richardson, R. J., AERE-C/R-1874 (October 1954). (11) Moore, R. N., Maness, R. F., Hanford Atomic Products Operation, HW-37321 (July 1953). (12) Kotz, K. J., Jr., “Sintering of Black Oxide,” National Lead Co. of Ohio, NLCO-684 (April 10, 1957). (13) Orrick, N. C., Union Carbide Nuclear Co., K-1081 (December 1953). (14) Perio, P., Bull. chzm. soc., 1953, 776. (15) Vaughan, D. A., Bridge, J. R., Schwartz, C. M., Battelle Memorial Institute, BMI-1205 (July 9, 1957). (16) Wait, E. J., J . Inorg. and Nuclear Chem. 1, 309 (1955). (17) Zachariasin, \V. H., A c t a Cryst. 1, 265 (1948 ), RECEIVED for review May 17, 1958 ACCEPTED October 13, 1958 Division of Industrial and Engineering Chemistry, Symposium on Preparation and Recycle of Feed Materials for Nuclear Fuel, 133rd Meeting, ACS, San Francisco, Calif., April 1958. Work done by Uranium Division, Mallinckrodt Chemical Works, under the auspices of the Atomic Energy Commission, Prime Contract W-15-208Eng.-8.
