I
S. H. SMILEY, D. C. BRATER, C. C. LITTLEFIELD, and J. H. PASHLEY Oak Ridge Gaseous Diffusion Plant, Union Carbide Nuclear Co., Oak Ridge, Tenn.
Quantitative Recovery of Uranium Hexafluoride from a Process Gas Stream Methods have been developed for recovery of uranium hexafluoride from gas streams which differ greatly in size and composition ..--
u
RANIUM HEXAFLUORIDE is the stable compound used in the gaseous diffusion process for separation of uranium isotopes. At room temperature, it is a colorless, volatile solid which forms transparent crystals of high refractive index. The crystals sublime without melting under atmospheric pressure; at higher pressures. they melt to a clear. colorless, mobile liquid of high density. The triple point occurs a t 147.3" F. and 22.0 p.s.i.a.; vapor pressures at 135', 70°, and - 5 j 0 F., are 14.7. 1.6. and 0.002 p.s.i.a., respectively.
Cold Trapping Cranium hexafluoride is recovered from gas streams by solid condensation (cold trapping) in both large- and smallscale operations because of equipment simplicity and the high efficiency attained over a wide range of flows and concentrations. Batch traps? from which uranium hexafluoride can be drained or vaporized, are used. because they are reliable and versatile, and require very little operator attention. Limited attempts to develop a continuous cold trap ( 4 . 3 8 )have met with only moderate success; the economic incentive is insufficient to \varrant thr effort to solve mechanical problems associated with such a unit. The holding capacities of
1
vessels used as traps range from a few grams to thousands of pounds of uranium hexafluoride. Flow and condensation rates are increased by various configurations of baffling and finning inside the vessel. A trap can be unloaded by heating to melt or vaporize the uranium hexafluoride for transfer to a storage cylinder. The trapping surface or core is made of parallel plate fins, brazed or welded to U-tubes which conduct the heat transfer medium (Figure 1). The core is thus fixed a t one end and is relatively free of thermal stresses. If a different fluid is used for heating than for cooling, separate heating tubes must be provided. Another type of trap has the refrigeration tubes attached to the shell. The heat transfer area is composed of egg crate fins held in circumferential rings, bonded to the shell. Copper is most suitable for this type because of its good heat transfer properties and because the assembly can be brazed to give good contact between fin assembly and shell. The trap is heated by electrical elements mounted internally and externally. I n cold traps, the approach temperature-the AT at the cold end of the trap-is about 15' F., and heat transfer coefficients are 0.5 to 1.5 B.t.u./hr.sq. ft.-' F. (29, 36). Traps can ordi[SECTION
narily be filled to 40 to 120 pounds of uranium hexafluoride per cubic foot of free volume. Temperatures var) from about 40' F. for the first in a series of traps to liquid nitrogen temperatures for final stripping. At very low temperatures. some contaminants, principall) hydrogen fluoride, are condensed and must be vented during heating to avoid excessive pressures. Basic design equations applicable to condensation of a solid have been derived by Thompson (36), who assumed that the condensable gas diffused across the boundary film under the influence of a partial pressure gradient and that this film is the same as for conveciike heat transfer. Thus, coefficients of heat and mass transfer can be related directl!( 7 ) . After the heat transfer coefficient has been determined, experimentally or by calculation, area requirements for a given situation may be estimated. Thompson's method does not allow for changes in condensation rate as solid builds up in the trap, although an empirical multiplier (plugging factor) has been applied for similar traps of different sizes. It is assumed that the temperature of the condensed solid on the \call is equal to the wall temperature and the gas film coefficient does not change as free flow area decreases. Thus, condensation at a point is assumed constant dur-
1
I
*,
19 15 IO
eJ GAS OUTLET 4
GAS INLET
r S E A L STRIP
-
-
LSEAL
STRIP
LFIN SUPPORT S - P L A T E LA SECTION
=INSTRUMENT CONNECTION
c-c
INSTRUMENT CONNECTION 8 B A F F L E S ONLY )
[ SHELL
4-k
1 OMITTING S H E L L )
Figure 1. The internally cooled cold trap is made of parallel plate fins brazed or welded to U-tubes VOL. 51, NO. 2
FEBRUARY 1959
191
VENT T O CARBON
RECYCLE TO COMPRESSOR
TRAPS
1 MOLS/HR 1
MOL %
-4O'F
CONDENSER
PRODUCT UF6
ABSORPTION
UFg FEED FROM COMPRESSOR
NE cgF16
00125 I488
0 5 59 5
Figure 2. The uranium hexafluoride absorption-distillation process was tested on a pilot-plant scale Basis, 1 mole o f uranlurn h e x a f l u o r i d e in f e e d gas
ing filling; if this is not true, premature trap plugging or loss in trapping efficiency can result. Therefore, the effect of solid accumulation on the surface should be considered in design, which requires estimation of both thermal resistance of the solid and change in gas film coefficient with velocity. These effects are counteracting; by proper selection of fin spacing, constant rate of condensation as the trap loads can be secured. Density and thermal conductivity values chosen for solid uranium hexafluoride are somewhat arbitrary, because the proportion of voids in the solid layer as a function of operating conditions is not well known. I n tests using uranium hexafluoride solidified from the liquid, the thermal conductivity was determined to be approximately 0.25 B.t.u./hr.-ft.-OF. (39). This value, compared to that reported for the liquid (0.092), is considered reasonable because the conductivity of the solid is usually somewhat higher than that of the liquid. The product of the solid density and the above thermal conductivity is about 75 B.t.u.-lb./sq. ft./' F.-
sq. ft.-hr. I n rate of condensation experiments, however, estimated conductivities as low as 9.0 B.t.u. were obtained (23). Thus, conductivity values for the solid deposits should fall in the range of 9 to 75 B.t.u. These data were obtained at slightly above room temperature; values might be somewhat higher with lower wall temperatures. The density of uranium hexafluoride solid is 303 pounds per cubic foot at 145' F. and 315 pounds at 77' F. (27). As the solid condensed in a cold trap usually contains a high percentage of voids, the apparent density should be much lower than the true density; from the standpoint of design, it is best to be conservative in selection of density and therm31 conductivity values. Based on the work of Colburn and Edison (a),the gas phase will become saturated in uranium hexafluoride at some point in the cold trap, if the heat exchanger area is large enough to accommodate the superheat of the entrance gas. After saturation occurs, part of the condensation will take place in the main body of the gas as a fog or mist. Although it is satisfactory to
3.2 E
2.4
rption Of UFb In Heavy 011
200 OF And /S p r i o L = 4 0 0 0 l b / h r - s q tf
?2 u x
08
0 0
50
IO0
IM
Figure 3.
1 92
2 50
200
E X I T G A S FLOW
300
350
RATE, Ib/hr.-sq. it.
Agreement of data with two solvents is good
INDUSTRIAL AND ENGINEERING CHEMISTRY
400
450
assume for the purpose of rate of condensation calculations that all the material is deposited where formed, a portion will be entrained in the exit gas from the last trapping section. Thus, it is necessary to estimate the amount of such carry-over, to size a fume-removal device. The following empirical relationship ( 7 7) between velocity and 'fractional carry-over has been used. Pounds UFs leaving trapping section in fume/pounds UFs fed = 0.1 - us.* (1) where u = velocity of noncondensable gas through the trap. A metal textile filter will remove up to 99% of the fume with gas stream velocities of 0.25 to 0.50 foot per second; at 1 foot per second, efficiency decreases considerably. Equation 1 shows that. at 1 foot per second, 10% of the uranium hexafluoride fed would be carried out of the trap as fume; under these conditions, rapid filter plugging occurs. The technique of designing cold traps is not advanced sufficiently to permit full confidence in the performance predicted by calculation, particularly with respect to holding capacity. The trap must often perform over a range of conditions which may not be explicit until operating experience has been obtained. Thus, a prototype should be tested or fairly large safety factors incorporated into the design. The satisfactory results from this procedure and the availability of tested designs make further accumulation of knoivledge of basic uranium hexafluoride cold trap design economically unwarranted.
liquid Condensation Condensation of uranium hexafluoride as a liquid is often attractive where moderate or large quantities must be removed from a stream containing small amounts of noncondensables. Gas streams dilute in uranium hexafluoride are more difficult to handle, as higher operating pressures are required and compression problems can become acute; furthermore, the amount of uranium hexafluoride in the vapor to be vented is sufficiently large to require recovery by other means, such as cold trapping. Recovery of uranium hexafluoride by liquid condensation has several advantages over cold trapping. Refrigeration requirements are minimized, condensation equipment is smaller and less costly, and material is removed continuously instead of batchwise. Because uranium hexafluoride does not exist as a liquid under its own vapor pressure below the triple point, the exit gas from a condenser must always be above this temperature and uranium hexafluoride partial pressure. The amount of uranium hexafluoride leaving the system-not removed by condensation-is determined primarily by the ,total flow of noncondensables and system pressure. As noncondensable concentration increases,
NUCLEAR TECHNOLOGY higher system pressures are required for reasonable recovery efficiencies. Liquid condensation has been applied successfully at operating pressures u p to 50 p.s.i.a.; a t the higher pressures needed when noncondensable concentration is larger, compressor performance is of dubious reliability. Calculations for liquid condensation are made by standard techniques; the following uranium hexafluoride properties at 160' F. (a reasonable operating temperature) can be used. Viscosity, 0.71 cp. Density, 224 lb./cu. ft. Thermal conductivity, 0.092 B.t.u./hr.-ft.-
-.
OR
Latent heat of vaporization, 12,400 B.t.u./ lb. mole Surface tension, 17 dynes/cm.
Absorption in Inert liquid Media LTranium hexafluoride can be removed from streams containing high concentrations of noncondensable gases by absorption in an inert solvent and recovered as the pure compound by fractional distillation. Performance of packed columns with perfluorodimethylcyclohexane (CsFls) as the solvent has been studied on a pilot-plant scale ( 7 3 ) . A material balance and flow diagram are shown as Figure 2. Absorption. Absorption of uranium hexafluoride in CaF,G is characterized by solvent temperature rise, decrease in volumetric gas flow, and increase in liquor flow from the top to the bottom of the tower. Ideally, the packing in an absorption column should be gradu-
ated in size to maintain optimum mass transfer conditions through the tower length. Because such a gradation would have been difficult to achieve, the 4-inchdiameter pilot-plant unit was divided into two sections, the bottom 2 feet containing stacked lengths of metal tubes and the top 14l/2 feet packed with dumped Raschig rings. ,4bsorption was studied with uranium hexafluoride feed rates of 600 to 1900 pounds per hour per square foot and concentrations of 29 and 56 mole yo, total inlet gas flows of 1600 to 3800 pounds per hour per square foot, and solvent flows of 4400 to 12,000. No significant change in the temperature of the upper 12'/2 feet of the packed section was observed, showing that essentially all mass transfer took place in the lower 4 feet of the unit; thus, the use of the higher flow capacity packing at the bottom of the tower was justified. The temperature of the solvent increased from about 40' F. a t the inlet to 100' to 150' F. at the outlet. From experimental data, values of the , height of a gas transfer unit, H O ~and the over-all mass transfer coefficient, KGa, were calculated on the basis of the twofilm theory of absorption (33, 34). where
HOG= height of transfer unit based on over-all gas phase resistance, feet HQ = height of transfer unit based on gas film resistance, feet H L = height of transfer unit based on liquid film resistance, feet rn = slope of equilibrium curve
g,
(76,77) at dilute end of tower
GM
LM
= superficial velocity of gas, pound
mole per square foot per second = superficial velocity of liquid, pound mole per square foot per second
The exit gas rates were used to correlate the data, as most of the tower operated isothermally at exit gas conditions. I n general, HOG varied little with varying gas and liquid rates, probably because of entrainment of the entering solvent, which contained small amounts of uranium hexafluoride by the outlet gas. The resulting bias in outlet gas compositions gave higher Hoc values at the higher flow rates, tending to suppress the effect of increasing mass transfer rate by increasing gas or liquid flows. The extent of entrainment is indicated by the good agreement of the data points with a predicted curve based on the Clolburn equation and the assumption that the outlet gas contained 4.75 mole yo solvent. Ideally, Hoc is independent of driving force, and the curve of the true Hoc LIS. inlet solvent concentration should be a horizontal line. Thus, the difference between experimental values and a horizontal line through points at low uranium hexafluoride concentrations should be a measure of entrainment. HOC values were also essentially independent of uranium hexafluoride concentration in the inlet gas. This is to be expected, as nearly all the mass transfer occurred in the first two transfer units, which represent about 20% of the tower length. Any effect of inlet gas density on HOGwould be limited to the bottom of the tower, and the effect on the average value would be slight.
Figure 4. Pressure drop in 4-inch diameter absorption tower packed with dumped 3/8-inch Raschig rings i s shown for various air, uranium hexafluoride, and C8F16 solvent flows
20
I O
h
08
d 06
04
0 2
= 3000 / h / b r -sq I f Of
1
UF6
~
, CA,R,
Ib./hr.-sq.
1
~
ft
VOL. 51, NO. 2
FEBRUARY 1959
193
Absorption rates decreased as the tower approached a flooding condition, but separation was still accomplished at vapor and liquid rates about 85% of the flooding rates predicted from the correlation of Lobo (27). In Figure 3, the mass transfer ccefficients are plotted as a function of gas flow and compared with data from work by Landau (25)with a heavy fluorinated oil. Because HOGis weak function of G.M, and
varies almost directly with G. Reasonable agreement is obtained between the data with the two solvents. To compare the over-all mass transfer coefficients of the heavy oil system at 200" F. and 14.7 p.s.i.a. with those of the C8Fl6 system at 45' F. and 40 p.s.i.a., it was necessary to multiply the former values by 1.38. This factor corrects for effects of total pressure on gas composition and of temperature on diffusivity and solvent properties. Variations in solvent flow rate had little or no effect on the mass transfer coefficient-an apparent contradiction to the findings of Landau. The difference can be explained by Equation 2, in which the over-all gas transfer unit is the gas side transfer unit plus the product of (m G / L ) and the liquid film transfer unit. In Landau's work, ( m G / L ) varied from 0.5 to 4; in this investigation, it was between 0.01 and 0.1 : therefore, the importance of the 1iq;lid film in the uranium hexafluoride-C8FI6 absorption was decreased by a fzctor of about 50 relative to the uranium hexafluoride-heavy oil absorption. Variations of pressure drop across 14.5 feet of dumped 3/s-inch Raschig rings and 2 feet of stacked 4-inch lengths of 0.6-inch thin-walled tubes for various flows are shown in Figure 4. In Figure 4, left, is for air flow through dry packing, 2 for the countercurrent flow of air and 6000 pounds of CaF16 per hour per square foot through the packing, and 3 for combined air and uranium hexafluoride flow countercurrent to a fixed C ~ F flow I ~ of 6000 pounds per hour per square foot. The curves are similar in shape. At low air flows, curve 3 appears to be an extension of 2, indicating that the bottom 2 feet of the tower containing stacked tubular packing almost completely removed uranium hexafluoride, so that the flow condition in the Raschig ring section (which contributes most to over-all tower pressure drop) was similar to the flow of air alone and C I F ~ ~ . The scatter of points on the vertical section of curve 3 is due to variations in the absorption temperatures in the 2-foot section, which allowed varying amounts of uranium hexafluoride gas to enter the Raschig ring section and affected tower pressure drops accordingly. &a
1 94
Figure 4, right, shows the approximate curve obtained with constant air and uranium hexafluoride flows and variable CsF16 flow. As the solvent flow decreased from a mass velocity of 12,000 pounds per hour per square foot, the pressure drop decreased as expected, because of decreasing liquid resistance; however, at 6000 pounds per hour per square foot C8F16 flow, pressure drop was minimum, and a further decrease in flow increased pressure drop. The explanation again involves performance of the tubular packing section. As solvent flow decreased, absorption temperature a t the bottom increased and resulted in a large effective gaseous uranium hexafluoride flow in the tower. Distillation. Fractionation of uranium hexafluoride-csFl6 sohtions is simple (76). Relative volatility varies almost linearly with concentration from 6 a t dilute uranium hexafluoride concentrations to 2 at high concentrations. The instrumentation required to control product purity is not critical because of the large temperature difference between 10 and 90% uranium hexafluoride. The reflux ratio can be controlled to give desired product purity according to the temperature at a selected point with the column operating at a fixed pressure. The distillation tests were made in a 6-inch-diameter column packed with 163/4 feet of I/2-inch Raschig rings. Approximately two thirds of the test program was continuous runs, to produce a pure solvent bottoms for use in the absorption experiments and obtain data on the separation efficiency of the still packing. The remaining runs were made under total reflux to determine HETP values for separation of C8F16 and uranium hexafluoride. The distillation data were analyzed using laboratory vapor-liquid equilibria data (76, 77). The number of theoretical plates was determined by the Ponchon-Savarit method (32) over the distillation range where sensible heat effects are large. At the top of the tower, where the usual simplifying assumptions of negligible sensible heat effects, heat of mixing, heat loss. and approximately equal latent heats are valid, and where the equilibrium curves can be represented by linear equations, the following equation was used:
R
where n = plate number in enricher starting at top X , = mole yoCSFIG in liquid leaving plate
The holdup of uranium hexafluoride per foot of column was also determined. The still column was filled with uranium hexafluoride, so that all thermocouples in the packed section read the
n
XD
INDUSTRIAL AND ENGINEERING CHEMISTRY
=
mole 70 CSFIG in distillate
k
= reflux ratio = slope of equilibrium line in enricher
For total reflux ( R +
a),this
reduces to
The data obtained in the experimental tests are plotted in Figure 5 as H E T P us. boil-up rate. For both total and 5 to 1 reflux ratios, HETP values fall on the same curve for the range of liquid and vapor rates explored. The HETF for total reflux decreases with increasing boil-up to a rate which corresponds to 7070 flooding and then increases ab. ruptly. Pressure drop across the packing exhibited a similar abrupt change at the high boil-up rate. The high percentages of flooding obtained with the column functioning as a separator indicate that the Lobo flooding correlation is valid for the uranium hexaflu~ride-caF,~distillation system. The H E T P value at 5Oy0 flooding is 1.2 feet (Figure 5). This can be compared qualitatively with two literature Sources. Pigford and Colburn (37) tit? 0.6 foot for the standard, hydrocarbon mixture, n-heptane and methylcyclohexane, using '/z-inch Raschig rings. When this is multiplied by the square root of the ratio of the Schmidt number (in liquid phase) of the uranium hexafluoride-CaF,6 system to that of the standard hydrocarbons, an HETP range of 1.3 to 1.5 feet is obtained. Page (30) gives the ratio of the HETP values of a uranium hexafluoridebromine trifluoride system to the standard hydrocarbons as 1.5. This value cannot be corrected as above, because the required physical properties for evaluation of the Schmidt number for uranium hexafluoride-bromine trifluoride are not known; however, it can be interpreted as a lower limit on the H E T P of the uranium hexafluoride-CsF16 system because the mobility of the C S F , ~in the latter case is less than that of the bromine trifluoride on the basis of molecular weight and molecular size. Thus, from the literature, the minimum HETP expected is 0.9 foot, and the highest is 1.5 feet (at Soy0 flooding), the experimental result being 1.2 feet.
NUCLEAR TECHNOLOGY PERCENT
FLOODING
4.0
-
c
3.0
&-
kLI1
8
2.0
1 .O
0
400
200 RATE OF
VAPOR
600 BOIL-UP,
I b . I hr. O F
800 CgFle
Figure 5. Height equivalent to a theoretical plate in distillation is minimum at uranium hexafluoride-C8Fla 7070 of flooding temperature of boiling uranium hexafluoride at column pressure. Weighed increments of uranium hexafluoride were withdrawn, and the column was allowed to regain equilibrium after each withdrawal. From a study of the temperature profiles and knowledge of thermocouple spacing and quantity of uranium hexafluoride removed, column holdup was estimated. The data obtained and the predicted holdup as calculated by the method of Jesser and Elgin (79) are: Boil-Up Rate, Lb./Sq. Ft.-Hr. 1070
Holdup, Lb.,/Cu. Ft. Calcd. Exptl.
2395
5.8 8.9
3310
11.0
5.9
7.1 8.7
.Agreement is excellent between calculated and experimental values a t the low boil-up rate, but experimental values at the high boil-up rates are 207, less than the calculated holdup.
Solid Absorbents Sodium Fluoride. The reaction mechanism for sodium fluoride and uranium hexafluoride involves formation of the double salt UF6.3NaF with the evolution of 118 B.t.u. per pound of uranium hexafluoride reacted (6, 28). Because at 212' F., uranium hexafluoride pressure in equilibrium with the compound is only 2.1 X 10-6 atm., essentially complete recovery is possible. I n experiments with sodium fluoride powder (surface area of 1 sq. meter per gram), the theoretical maximum loading was reached in less than 5 minutes a t 212' F. Uranium hexafluoride may be generated by heating to about 750" F., at which temperature the uranium hexafluoride partial pressure is over 1 atm. Some
decomDosition to a nonvolatile form of uranium occurs, but addition of a small amount of fluorine to the inlet gas stream will suppress this reaction. Certain condensable impurities, which would contaminate the product if recovered by cold trapping alone, may be separated from the uranium hexafluoride, either by passing through while uranium hexafluoride is trapped or by being retained when uranium hexafluoride is liberated after absorption ( 5 ) . A secondary cold trapping or condensation system would be required to condense uranium hexafluoride from the regeneration gases. Hydrogen fluoride in the gas stream to be treated can seriously limit the trapping efficiency of sodium fluoride. Sodium bifluoride is formed, and the equilibrium concentration of uranium hexafluoride in the gas phase rises to satisfy the new system N a F . H F and UFs after the free sodium fluoride component vanishes. Although not confirmed experimentally, the "poisoning" effect of hydrogen fluoride, qualitatively a t least, is well known. I n fact, uranium hexafluoride can be removed from a sodium fluoride trap by passing hydrogen fluoride through it (72). 'The sodium fluoride method would probably be uneconomical where other methods are suitable; however, if hydrogen fluoride concentration is low, sodium fluoride traps may be profitably employed to remove traces of uranium hexafluoride from a vent gas stream. A practical operating system would consist of a series of batch beds, cycled for absorption and regeneration. For large-scale operation, fluid and moving beds might be considered ; however, design, operational, and economic problems involving materials handling, re-
actor performance, and final recovery can be anticipated. Uranium Tetrafluoride. An effective method foi- gas stream stripping has been applied in uranium hexafluoride preparation ( - 3 ) . Fluorination system vent gas streams, which usually contain 0.05 to 0.10% uranium hexafluoride, can be scrubbed completely by action of uranium tetrafluoride in a fluidized bed reactor. The uranium tetrafluoride reacts with uranium hexafluoride to form U 4 F 1 7 , UzFS, or UFs, which increase in volatility with the proportion of uranium hexafluoride to uranium tetrafluoride. The equilibrium pressures of uranium hexafluoride above these compounds are 8.6 X lop6, 3.2 X and 0.13 atm., respectively, a t 400' F. (27). Theoretically therefore, it is possible by formation of intermediates to reduce uranium hexaff uoride concentration in a gas stream to well below 100 p.p.m. Factors affecting the efficiency of a uranium tetrafluoride trapping system have been studied in both 2- and 6inch-diameter experimental fluid-bed reactors (TO, 78, 26). This type of reactor was selected because the high heat and mass transfer coefficients attained allow good temperature control, and the gas-solids contact is excellent. Tests in the larger unit were made with continuous feed and withdrawal of solids; those in the smaller reactor, with batch charges of uranium tetrafluoride. Because some gas streams encountered in uranium hexafluoride manufacturing and use contain fluorine and/or hydrogen fluoride in addition to uranium hexafluoride, the effects of these gases were investigated. In all cases, a finely ground uranium tetrafluoride, which contained less than 5% plus 80-mesh and about 5070 minus 325-mesh particles, was employed. The powder fluidized well, and no difficulty with caking was noted. Uranium hexafluoride recovery was good a t 155" to 400' F., superficial gas velocities of 0.2 to 0.8 foot per second, inlet uranium hexafluoride concentrations of 100 p.p.m. to 11%, and fluorine and hydrogen fluoride concentrations up to 17 and 1070,respectively. When powder temperature was increased to 500' F., efficiency significantly decreased. C)peration ~ . i t l ihigher gas velocities or inlet concentrations was not studied. At 400' F., outlet uranium hexafluoride concentrations of less than 10 p.p.m. were obtained in batch tests with inlet concentrations of 100 to 3500 p.p.m. The capacity of the pohvder bed appeared to be 0.10 to 0.35 pound of uranium hexafluoride per pound of uranium tetrafluoride. The variation in the amount of uranium hexafluoride reacted was probably due to uranium tetrafluoride surface area difVOL. 51, NO. 2
0
FEBRUARY 1959
195
ferences. With a continuous powder feed to a single-stage reactor and a uranium tetrafluoride-uranium hexafluoride ratio of 1 5 to 1, recovery efficiencies were somewhat lower, but still greater than 97%, with inlet uranium hexafluoride concentrations of 0.35 to 11%. Outlet uranium hexafluoride concentrations were directly proportional to inlet concentrations; thus, shortcircuiting of a small amount of the gas probably occurred in the larger reactor. Introduction of fluorine did not decrease the efficiency of the uranium hexafluoride recovery operation, but some intermediate fluorides were formed by reaction of fluorine with uranium tetrafluoride and the capacity of the powder bed was lowered. At 400” F., about one half of the fluorine was consumed. No effect of hydrogen fluoride concentration on capacity was noted. Uranium hexafluoride could be recovered from the spent solids by heating, but experience has shown that the intermediate compounds sinter badly at relatively low temperatures, and low regeneration rates would be expected. As such a trapping system would normally be operated where facilities for preparation of uranium hexafluoride are available, however, the intermediate fluorides can be charged to a fluorination reactor and converted completely to uranium hexafluoride; both uranium and fluorine collected by the uranium tetrafluoride would be recovered in a usable form (uranium hexafluoride). Other Solid Trapping Materials. Use of a bed of solid absorbent to remove relatively small quantities of uranium hexafluoride from a gas stream is often attractive because of low investment and operating costs for the trapping cycle, although costs of recovering uranium from the absorbing material may be high. Trapping materials which remove uranium by chemical reaction are carbon and soda-lime mixtures. Calcium sulfate, calcium fluoride, and alumina (9, 75) function primarily through physical adsorption, although some permanent retention of the uranium may result from reaction with impurities and adsorbed water. The presence of fluorine and/or hydrogen fluoride in the gas stream is of importance in deciding which material to use. Soda-lime will react with hydrogen fluoride and fluorine; carbon and calcium sulfate react with fluorine. Hydrogen fluoride is physically adsorbed by any of the materials, although the effect is probably more significant for alumina, calcium fluoride, and calcium sulfate. Thus, large amounts of fluorine and/or hydrogen fluoride in the gas can reduce considerably the trap capacity for uranium hexafluoride. Commercial activated carbon, which normally contains an appreciable quantity of adsorbed water, traps uranium
1 96
hexafluoride both by hydrolysis to uranyl fluoride and reduction to lower fluorides, such as UF4, U4Fl,, U ~ F Q , and UF6, with formation of fluorocarbons ranging from CF, to a waxlike material (37). The capacity for uranium is high; uranium concentration in the spent reagent is as much as 50y0 by weight. Very little uranium hexafluoride is physically adsorbed, as evidenced by lack of successful removal by heating or pumping at high vacuum. Carbon has two disadvantages: Fluorine can react with it explosively and/or form compounds which explode on impact; and traps must be designed to be dimensionally safe from the criticality standpoint, since carbon is an excellent neutron moderator and very high loading of uranium may be obtained.
Other Recovery Methods Uranium hexafluoride may be scrubbed from a dilute gas by contact with water; a solution of uranyl fluoride and hydrogen fluoride results. An alkaline solution is often used instead of water to neutralize the acid (2, 20, 24). Ethanolamine is also an effective scrubbing agent (24). A metallorganic complex of quadrivalent uranium is formed in a basic, noncorrosive solution, and the uranium can be precipitated quantitatively as the hydroxide by addition of water. Streams either very dilute or moderately concentrated in uranium hexafluoride may be stripped completely by reaction with trichloroethylene in the vapor phase (7, 35). The product is a pure uranium tetrafluoride. Uranyl fluoride can be made by using steam instead of trichloroethylene (74, 22).
literature Cited (1) Baker, J. E., Klaus, H. V., Schmidt, R. A., Smiley, S. H. (Oak Ridge Gaseous Diffusion Plant), Union Carbide Nuclear Co., K-1271 (May 31, 1956). (2) Beck, C. B., Belaga, M. W., Zonis, I. S., Engineering Practice School, Massachusetts Institute of Technology, K-881 (March 3, 1952). (3) Brater, D. C., Smiley, S. H., “Progress in Nuclear Energy,” Ser. 111, “Process Chemistry,” vol. 2, Pergamon Press, in nrcsq r-
(4y-Bresee, J. C., Larson, P. R., IND. ENG. CHEM.49, 1348-54 (1957). (5) Cathers, G. I., Nuclear Sci. and Eng. 7) 2, 768-77 (1957). nett, M. R., Jolley, (6) Cathers, G. I., Bennett, 958). ENG.CHEM.50, 1709 (1958). R. L., IND. llburn, A. P., Zbid., (7) Chilton. T. H., Colburn, 26, 1183-7 (1934). (8) Colburn, A. P., Edison, A. G., Zbid., 33, 457 (1941). (9) Colvin, W. L., Union Carbide Nuclear Co., personal communication. (10) Connor, G. H., Golliher, W. R., Mayo, T. J., Rossmassler, W. R., ,Paducah Gaseous Diffusion Plant, Union Carbide Nuclear Co., KY-230 I .
.,
t\---. o r t In. 19571. --> ---
(11) Cooper, d T . ,Resnick, I., Kellex Corp., K2-245 (Oct. 11, 1945). (12) Dietrich, W. C., Rowan, J. H.,
INDUSTRIAL AND ENGINEERING CHEMISTRY
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