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J. L. POWELL,l W. R. FORESHEE, and SEYMOUR BERNSTEIN Union Carbide Nuclear Co., Paducah, Ky.
Recovery of Uranium Hexafluoride Vapors Absorption in a fluidized bed of uranium tetrafluoride when standard methods could not be used was the.answer to recovery of a valuable chemical in the gaseous diffusion process for separating uranium isotopes
THE
uranium hexafluoride used in the gaseous diffusion process for the separation of uranium isotopes is manufactured by high temperature techniques (7, Z), wherein purified uranium tetrafluoride as green salt is combined directly with elemental fluorine. The vent gases consist of appreciable quantities of oxygen and nitrogen, some hydrogen fluoride and fluorine, and small amounts of UFO. Prior to venting, most of the UFO is removed; small but valuable amounts remain, normally lost.
p.p.m. with low quality green salt. These data were obtained at 400' F. with 2 to 3y0 fluorine in the gas. The recovery efficiency for U F O was essentially unaffected by the quality of the green salt. However (Figure l ) , the absorption efficiency for fluorine was a function of green salt quality. The higher the quality, the lower the efF.ciency for fluorine recovery. T h e presence of uranium oxides in the lower quality green salt undoubtedly accounts for the increased absorption eEciency for fluorine.
a batchwise fluid bed until breakthrough of UFO (Table I). Results of analyses of the green salts after exhaustion indicate that as much as 0.38 pound of UFe per pound of green salt or 0.041 pound of fluorine can be recovered at 400' F. if low quality green salt is used. UFa can be efficiently recovered from a gas containing 2 to 3% fluorine u p to a bed loading of 0.085 pound of UFG per pound of green salt with the high quality green salt and 0.077 pound of UFB with the low quality salt.
ExperimentaI
Uranium Hexafluoride Recovery Efficiency us. UP6 Concentration. The UF6 concentration was reduced to less
Chemistry of Absorption and Regeneration
A batch-type fluidized bed reactor (24 inches deep) was constructed from a 2-inch Monel pipe. Gaseous mixtures of air, 0 to 3y0fluorine, and 0 to 10,000 p.p.m. of UF6 were fed to the bed a t various rates. A temperature of 400" F. gave optimum UFO recovery and a gas velocity of 0.4 to 0.5 foot per second fluidized the UF4. Pressure drop was about 1 p.s.i. per foot of fluidized bed. T o investigate the effect of the green salt quality on the capacity of the absorption process, tests were made with green salts of both high and low UF4 content. Particle size distribution was approximately 8OyOthrough 100 mesh, 60y0 through 200 mesh, and 3ooj, through 325 mesh. Additional laboratory tests were made to determine the effect of bed loading and UFO concentration on the efficiency of the fluid bed for absorbing UFe and fluorine. Concentrations of 100 to 4000 p.p.m. UFe and 2 to 3y0 fluorine were studied in particular (Figure 1 and Table I). UFO and Fluorine Recovery Efficiency us. Bed Loading and Green Salt Quality. Prior to exhaustion of the bed, a t least 98% of UFB was recovered with concentrations greater than 100 p.p.m. when high quality green salt was the absorbing medium and greater than 300
than 10 p.p.m. regardless of inlet concentration as long as the bed was not near exhaustion. This suggests that the outlet concentration is only a function of UF6 vapor pressure over the intermediates formed, if the gas has sufficient retention time in the fluidized bed. Bed Loading at Exhaustion. The quantities of material that could be absorbed by the high and low quality green salts were determined by operating
UF6 $. 7UF4 F? 2U4F17 (1 ) 2UFs 3U4F17 7uzF~ (2) UFG UzFs ~2 3UF5 (3) Below about 600" F. UF4 reacts with fluorine to produce the same uranium fluoride intermediates. These reactions also take place a t elevated temperatures, but the intermediates formed are completely oxidized by fluorine to the gaseous sexivalent state. As all these intermediates are solids with low vapor pressures, both UF6
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80
Table 1. Bed Loading at Exhaustion for Absorption of UF6and F2 at 400' F. Bed Loading/Lb. Powder Quality High
DenGreen Salt Analysis, ____ sity, G./Cc. % FreeQuality UF4 UOzFz UOZ flow Packed High Low
98.8
0.2 78.1 10.4
1.0 11.5
3.0 2.8
4.2 3.6
Present address, Union Carbide Nuclear Co., Uravan, Colo. 1
The most stable forms of uranium fluorides exist in the quadrivalent and sexivalent states. The quinquevalent and other intermediate states such as UzFg and UBI7 are known to exist and to have volatilities which are a function of the temperature and the ratio of sexivalent to quadrivalent uranium. Upon heating, all these intermediates decompose a t varying rates to form UF4 and UFO. The following reactions (3, 4 ) occur to various degrees to form solid uranium fluoride intermediates, essentially stable a t room temperature.
Lowa BED LOADING-LB UFe /LB UF,
Figure 1 . At 400' F. the fluorine recovery decreases as bed loading and green salt quality increase
Lowb
Gas UF6, Fz, air UFe, air UFe, F2, air Ups, air Fa, air
Green Salt UF e F2 0.085
0.011
0.15 0.077
0.023
... ...
0.38
...
0.041
Exhaustion taken as 10 p.p.m. UFe leaving bed. a
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AUGUST 1959
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fluorine could be absorbed by U F 4 if the temperature is low enough to prevent the fluorine from completely oxidizing the uranium fluorides to the gaseous sexivalent state. T h e temperature must be high enough to permit completion of the reaction between U F I and UF6 if complete recovery is to be obtained. Regeneration of the bed containing absorbed uranium and fluorine by reduction of the various uranium fluoride intermediates occurred at 400' F. with hydrogen as the reducing agent. Reaction products include UF4 and hydrogen fluoride. The net reactions for the absorption and regeneration processes would, therefore, be UFO Hz -+ UF4 -F 2HF (4) Fz Hs + 2HF (5) If sufficient quantities of UFh are available, as in UFc manufacturing plants, the absorption bed need not be regenerated. If not, the absorption bed can be regenerated with hydrogen or other gaseous reducing agents. Regeneration has a n economic disadvantage, as partial conversion of UFd to UF6 by fluorine in the absorption processes would be lost and would require generation of additional fluorine.
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Fluidized Bed Technique Because green salt suitable for fluidization is available at UFs manufacturing facilities, it is desirable to utilize it directly without additional processing. A:fluidized bed of the powder is ideally suited to an absorption process, because this technique provides excellent contact between solids and gases as well as optimum temperature control. A fluidized bed reactor requires booster pumps to offset pressure losses, but its use is justified by the savings over a tray- or screw-type reactor. T h e reactor, consisting basically of a cylindrical shell with a diffusion plate a t the bottom, can often be fabricated and installed for less capital investment than other types of solid-gas contactors.
Pilot Plant Tests T o provide plant scale design criteria, a pilot plant consisting of a 6-inch-diameter reactor with a 24-inch bed was operated continuously 8 hours a day. Fluorine surges were studied in particular to determine if temperature control might be lost, permitting formation of UFB by direct reaction w:th fluorine. These tests confirmed the laboratory tests and indicated the feasibility of continuous powder feed and withdrawal, and showed that the exhausted bed need not be regenerated with hydrogen if sufficient green salt is available. The capacity of the bed was unaffected by 10.5% hydrogen fluoride in the inlet gas and the bed temperature was easily controlled at 400" f 15' F. when fluorine concentration averaged as high
920
* Green 5 o l t
UFS and fluorine are recovered b y absorption in UF4 which i s recycled back to the UFc manufacturing area
as 17% for 2.5 hours. I n no case, at a bed temperature of 400' F., was the concentration of UF6 leaving the bed greater than entering. Recovery of UFs was better than 99% from a gas mixture containing approximately 16% UF6 and 12% fluorine. The fluid bed technique provided excellent temperature control of the fluidized powder. Steam cooling and electrical heating gave satisfactory performance. A bed temperature of 400" F. gave optimum uranium recovery. Above approximately 500" F., recovery efficiency of uranium decreased appreciably. Below 400" F. absorption eRciency for fluorine decreased. Satisfactory fluidization of the green salt was obtained with superficial gas velocities between 0.15 and 0.75 foot per second. Powder retention times between 1.6 and 4.2 hours and a bed depth of approximately 2 feet gave satisfactory recovery of both UFe and fluorine. No caking occurred in the fluidized bed reactor and particle attrition was not a problem.
Plant Scale Application A plant scale fluidized bed absorber was installed at the UFs manufacturing plant at Paducah, Ky., during July 1958. This reactor operates at 400" f 25" F. between 14 and 16 p.s.i.a., and handles between 25 and 95 c.f.m. of vent gas. Bed temperature is maintained electrically during operation and stcam coils provide cooling. Green salt ranging betlveen 75 and 99% UF4 is conveyed on a vibrating conveyor to a feed hopper, then fed into the bottom of the fluidized bed by a 21/2-inch screw conveyor. Fluidized powder overflows from The reactor 3 feet above the diffusion plate and falls by gravity with the unreacted gases through a n inclined pipe into a cyclone separator. A powder seal at the bottom of the cyclone separator prevents leakage of gas. The green salt containing the absorbed material is removed from this seal leg by a screw conveyor and fed into a product hopper, which feeds a take-off conveyor that routes powder to flame reactors in the UFs manufacturing area. The vent gases are compressed from
INDUSTRIAL AND ENGINEERING CHEMISTRY
11 to 1 6 p.s.i.a. by a Root's-type compressor before entering the reactor The gases enter the bed of green salt through a drilled Monel diffusion plate and maintain the bed in a semifluidized condition. The unreacted gases leave the reactor through a cylone separator. where most of the dust burden is removed, then pass through a primary filter and a back-up filter, both of which utilize porous metal filter elements in a steel shell. These filters are blown back periodically with dry air or nitrogen.
Conclusions Uranium hexafluoride can be economically and efficiently recovered from a gas stream by absorption in a fluidized bed of UF4 at 400" F. Operating costs are nominal, consisting mostly of maintenance charges; no chemical or reprocessing costs are incurred and only minor operating costs are required. Recovery eficiency for LF6 is essentially unaffected by the presence of highly reactive gases such as elemental fluorine and hydrogen fluoride; however, a portion of the fluorine is absorbed and decreases the maximum bed loading that can be obtained. Recovery efficiency is only very slightly affected by the quality of the green salt used, although the portion of fluorine absorbed is markedly reduced by increased green salt quality.
Acknowledgment The authors acknowledge laboratory investigations of G. H Conner, W. R. Golliher, T. J. Mayo, and W. R. Ross massler, and pilot plant work of C. C. Littlefield. D. C. Brater, and J. H. Pashley.
Literature Cited (1) Chem. Eng. News 37, No. 12, 40 (1759). (2) Cronan, C. S., Chem. Eng. 66, 140-3 (March 23, 1753). 13) Katz, J. J., Rabinowitch, E., "Chemistry of Uranium," Part I, 1st ed., pp. 382-92, McGraw-Hill, New York, 1751. (4) Si,ymons, J. H., "Fluorine Chemistry, vol. I, pp. 59-60, Academic Press, hew York, 1750. RECEIVED for review November 20, 1958 ACCEPTED April 20, 1959 Division of Industrial and Engineering Chemistry, 125th Meeting, ACS, Boston, Mass., April 1959.
