Development of a Pressurized Cation Exchange Chromatographic Process for Separation of Transplutonium Actinides John T. Lowe', William H. Hale, Jr., and Donald F. Hallman Savannah River Laboratorj, E . I . d u Pont de Nemours & Co., A i k e n , S C. 29801
A pressurized ion exchange system has been developed for the separation of large quantities of highly radioactive isotopes. As much as 250 p g of 2'2Cf and 190 grams of "Cm in a single batch have been separated from lanthanide fission products with 25- to 55-micron resin and a high pressure pump to force the eluting agent solution through the columns at a flow rate of 16 ml/cm' min. The advantages of the system are an approximately threefold-higher flow rate than with conventional columns and, for radioactive systems, considerable reduction in radiolytic gassing and radiation damage to the resin.
F o r several years cation exchange chromatography has been used with 50- t o 100-mesh particle resins for the isolation of individual fission product lanthanides, particularly '"Pm (Wheelwright and Myers, 1965; Wheelwright et al., 1966). More recently, 50 grams of 244Cmwas separated from power reactor fuel using conventional ion exchange columns containing 50- to 100-mesh (150- t o 300-micron) resin (Wheelwright et al., 1968). Although flow rates up t o 6 ml/cm2 min were used in these columns, radiolytically produced gassing presented considerable problems. In a recent program at the Savannah River Plant, several kilograms of 244Cmand z43Amhave been produced by irradiation of a mixture of plutonium isotopes (Groh et al., 1965). I n a recent development a t Oak Ridge Xational Laboratory, -400-mesh cation exchange resin in pressurized columns has been used for the rapid separation of small quantities of lanthanides (Campbell and Buxton, 1970) and actinides (Baybarz, 1969). Several milligrams of lanthanides or actinides have been separated by elution development with aqueous solutions of a-hydroxyisobutyric acid and 20- to 40-micron Dowex (Dow Chemical Co.) 50W-X12 resin. This pressurized ion exchange process has advantages over both conventional ion exchange and solvent extraction. Pressurization eliminates the bed disruptions caused by radiolytically produced gases in gravity-fed columns, and a severalfold increase in flow rate minimizes radiolytic resin degradation. The pressurized ion exchange process requires less accurate control of concentrations and flow rates than the solvent extraction process. I n addition, ion exchange requires only stainless steel equipment, whereas tantalum or Zircaloy-2 is required for the chloride solutions in solvent extraction (Eubanks and Burney, 1966). The work described in this paper demonstrates the feasibility of a large-scale production system when a small particle resin, with more rapid kinetics, is used.
' To whom correspondence
Process
Because cation exchange resin shows little selectivity among lanthanides and actinides, a chelating agent must be used to separate cations of the individual elements into separate bands. Two types of chromatographic development are used in actinide separations: displacement development and elution development. Displacement development is superior for large-scale separations; rates of band movement and product concentrations are approximately 10-fold greater than those in elution development. Displacement development involves the following steps: the resin is loaded with a cation (the barrier ion) that has less affinity for the resin than the ions to be separated; a mixture of the ions to be separated is loaded onto the column; a chelating agent having different complex stability with each of the cations t o be separated is pumped through the column. The cations of each element in the actinide-lanthanide mixture form separate bands and are eluted from the column in the order of decreasing magnitude of the complex stability constants. There is a binary zone, or overlap region, between each band of pure component, because the column is not a t equilibrium a t finite flow rates. Although a direct calculation of the kinetics is not possible except for the simplest models, all nonequilibrium conditions can be included in one experimentally determined parameter, the theoretical plate height. Process Performance Calculation
The length of the overlap region between two pure bands determines the quality of a separation achieved in displacement development chromatography. The relation between the length, L , of the overlap region and the theoretical plate height, h , is given (Powell and Spedding, 1959) as
should be addressed. Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 1, 1971
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1800
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1
200
1
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Figure 1 .
IP vs.
0 28
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fractional void space
Both H o and Hi are considerably smaller than H , for commercially available resins suitable for this type of process (Boyd and Soldano, 1954). For 8% cross-linked resin [8% divinylbenzene (DVB) in polystyrene], H, is 1.2 cm a t 25" and 0.15 cm a t 80°C. H , can be decreased when a resin of lower cross-linkage is used. Resin containing less than 4% DVB has other characteristics which limit its usefulness, principally swelling. The development tests described in the following section show that an 8 5 crosslinked resin, Dowex 50W-X8, gives adequate separations. As Equation 4 shows, the particle diffusion term varies as the square of the resin particle radius, r , but only as the first power of the elutriant flow rate, V. Thus it is an advantage to use fine particle resin. The limiting particle size is determined by the pressure drop that can be tolerated for the application considered. The pressure drop across a column packed with spherical particles can be calculated from the Ergun Equation (Bird et al., 1960):
Resin particle diam, 55 microns
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-
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Flow, 16 ml/cm' min
If Re is < l o , the fluid is in the laminar flow region. For all cases considered in this discussion. with flow rates up to 16 ml/cmLmin, Re is 1, indicating laminar flow in the columns. The second term of the Ergun equation contributes negligibly to the pressure drop. and IP varies linearly with solution velocity. Values of LP were calculated for particle diameters of 25 through 55 microns,
1 I
2oo 015
25
35
45
55
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Resin Particle Diameter, microns
0
Figure 2. L P vs. resin particle diameter at various temperatures Void fraction,
t =
f
v, E
1.4
0 26
Flow, 1 b ml/cm' min
where R 1 and R , are the ratios of the concentrations of the first component eluted t o the second component, at the leading and trailing edges of the binary zone, respectively, and N is the separation factor. For chelating agents that form very strong 1-to-1 complexes with the cations t o be separated, the separation factor is the ratio of the two cation-chelate complex stability constants. The theoretical plate height, h , is the sum of three terms (Helfferich, 1962) that account for the finite resin particle size ( H , ) , the rate of diffusion in the aqueous and the rate of film surrounding the resin particle (H,). diffusion in the resin particle (HD).
N o = 1.64 r
( 21 (3)
0.9
1
0.8
1
I 50
I 55
I 60
I 70
I 65
Temperature,
OC
Figure 3. Effect of temperature on theoretical plate height Flow rate, 13.5 ml/cm' min Eluant, 0.05 DTPA, pH 6.0 Resin, Dowex 50W-X8, 30-60 microns
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I 75
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Equimolar solutions of samarium, neodymium. and praseodymium were loaded onto the resin and separated by eluting two band lengths with 0.05M diethylenetriaminepentaacetic acid (DTPA), adjusted to pH 6 with ",OH (Wheelwright et al.. 1966). Zn'. was the barrier ion in all tests. Typical curves which show the effect of temperature and flow rate on the theoretical plate height, h, (calculated from Equation 1) are shown in Figures 3 and 4. A small-scale facility for the separation of up t o 1 gram of 3'4Cm from associated fission product lanthanides was designed after the nonradioactive tests demonstrated the feasibility of rapid ion exchange. Three columns were constructed of 304L stainless steel, each 2'2-ft long, with a G-porosity stainless steel frit to support the resin. Column diameters were 0.87, 0.43, and 0.18 in. (with wall thicknesses of 0.065, 0.035, and 0.035 in., respectively). A more complete description of the small-scale facility has been presented by Hale and Lowe (1969). A similar facility was used to separate 1 gram of '"Pm from a mixture of fission product lanthanides (Lowe. 1969). Following these tests, the pilot-scale system described in the next section was designed and tested. Pilot-Scale System
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I
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I
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14
16
18
Flow Rate, ml/cm2 min Figure 4. Effect of flow rate on theoretical plate height Temperature, 70" C Eluant, 0.05 DTPA, pH 6.0 Resin, Dowex 50W-X8, 30-60 microns
fractional bed void spaces, t , of 0.20 to 0.30, and elutriant temperatures of 25" to 70" C. The variation of IP with the fractional void space, C , at various temperatures is shown in Figure 1. IP increases rapidly with decreasing t . Random packing of unequal spheres can give values of e as low as 0.13 (Wise, 1952), but the minimum value of e in a bed of uniform spheres is 0.26 (Boerdyke, 1952). Therefore. the range of particle size must be reduced as much as possible. A P is also temperature sensitive, approximately twice as large at 30' as a t 70°C, indicating another reason for heating the columns. The variation of AP with particle diameter at c = 0.26 is shown in Figure 2. For the desired pressure drop limit of approximately 1000 psig, 35-micron resin can be used if the elutriant temperature can be maintained a t 2 50" C. I n every test, AP has been higher than that calculated from the Ergun equation because of the relatively wide diameter range of the resin particles used.
The pilot-scale system consists principally of four columns, a feed tank, and a resin transfer tank. The columns are constructed of 304L stainless steel, schedule 80 pipe, and are 4 ft in length with nominal diameters of 4, 3, 2 , and 1 in., respectively. A positive displacement pump feeds solutions to the columns. The system is designed so that the pump can feed the ion exchange columns directly (with nonradioactive solutions), or by water displacement through a pressurized feed tank (for delivery of radioactive feed to the columns). Radioactive solution is never transferred through the pump. Each line leading from the pump passes through a relief valve to prevent overpressurization of the system. Each column is also provided with a pressure relief valve. To prevent backup of radioactive solution, each feed line passes through a spring-loaded check valve, a surge tank. and another check valve. Gamma monitors are placed near the feed lines where these lines enter the shielded cell wall. A warning is sounded in the event of any activity backup. The column system (Figure 5), except for the pumps. is placed in a cell to provide gamma and neutron shielding.
Preliminary Tests
To determine the effects of temperature and flow rate on the theoretical plate height, pressurized displacement development was first tested with nonradioactive solutions. Dowex 50W-X8, -400-mesh resin was used in all tests. The resin was hydraulically graded t o 25 t o 55 microns.
P
P W Manuoily Operated Valves
t% Check Valves
& 6
Sojenaic Valves i s a f e t y ) ?.lief
Valves
Figure 5 . Flow diagram for columns Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 1, 1971
133
and 700 psig. and finally to the 1-in. column at 61 ml min and 300 psig. S i n e hours after the start of the run, u 4 C m breakthrough was detected from the 1-in. column, and products were collected as shown in Figure 6. The analyses for curium. americium, and californium. are shown in Table
I.
E f f l u e n t from I-inch Column, liters
Figure 6. Typical elution diagram
The cell walls consist of 3 ft of either high density concrete or leaded glass. The design of the cells has been described by Coogler et al. (1965). Pilot-Scale Tests
In a typical run with the pilot-scale columns, feed is loaded onto the top 30 to 4OCc of the resin in the 4-in. column. No separation is obtained in this step. T h e eluting agent (0.05MDTPA, adjusted to pH 6.0 with ",OH) is started through the 4-in. column as soon as the loading step is completed. The bands are eluted successively through the 4-, 3-: 2-, and 1-in. diam columns. Successively smaller columns are used because the length of the overlap zone is independent of column diameter (Equation I), but the amount of material in this zone increases as the square of the diameter. The size of the final column is designed to give optimum separation of curium and americium from "'Eu. The elution order for the actinides and major fission product lanthanides is Cf, Cm, Am. Eu, Sm, Pm. S d . Pr: and Ce. For all pairs of adjacent elements except californium and curium. there is an overlap region between bands of pure components. Literature data (Baybarz. 1965) show that curium forms a stronger complex with DTPA than does californium. indicating that curium should elute ahead of californium. A more recent measurement (Wallace and Hinton, 1969) has shown that the stability constant of Cf-DTPA is - 8 times larger than that of Cm-DTPA. As an example of the pilot-scale tests, the feed for Test 4 contained 74.9 grams of ',*Crn: 23.9 grams of "jArn, 143 pg of '"Cf, and 1.6 moles of fission product lanthanides (with -1500 Ci of I4'Ce and -15 Ci of 'j4Eu). The columns were loaded with Dowex 50W-X8, 25- to %-micron resin in the Zn'- form. Column temperature was 70°C. The desired elution rate of 16 ml'cm' min was obtained on each column without exceeding the design pressure of 1000 p i g . Feed was loaded onto the top 30 to 4 0 5 of the 4-in. column, and elution with DTPA was begun at 1.18 liters1 min and 900 psig. After 40 min, breakthrough of ""Cf was detected, effluent was directed to the 3-in. column, and the DTPA flow was reduced t o 660 m1,min at 850 psig. Two hours later, breakthrough of "'Cf was detected from the 3-in. column, and a 30-liter product fraction containing 139 pg of '"Cf was collected. Effluent was then directed to the 2-in. column for 2 hr at 280 ml/min
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Separation of gamma emitting impurities from the '"Cm product is shown in Table 11. The objective of the separations process is to produce '''Cm with the gamma dose rate from fission products less than loc; of the gamma dose rate from the curium. Decontamination factors of approximately 7000 and 2000 are required for "'Ce and "'Eu, respectively. Immediately after collection. Fractions 1, 2, and 3 were acidified to 4M HNO?. I n Fractions 1 and 2, DTPA was destroyed by alpha radiolysis and high acidity in 1 to 2 days (Bibler, 1969). Destruction of DTPA in lowcurium fractions (such as Fraction 3 ) is slow: and such solutions must be radiolyzed with an external 6uCosource. Zn'., Cu'-, and Ni'. have been used as barrier ions. Although CmA and Am'- are efficiently retained, C f ' leaks through the zinc barrier resulting in a large californium product volume (20 to 50 liters). C u ' ~ and Ni' retain Cf3-, but an undesirably large amount of curium elutes with the californium. Also Cu' is reduced to copper metal in the resin phase, presumably by radiolytic degradation products of DTPA. Because collection of a large volume of californium essentially free of curium is preferable to a smaller volume of californium contaminated with curium, Zn'- is used as the barrier ion. In-line Instrumentation
The run is continuously monitored by several different instruments to simplify collection of product fractions. Movement of the bands down each column is followed by a portable neutron probe, which is sensitive to the '"Cf and r'4Cm spontaneous fission neutrons. Most of the analytical control system is installed outside the shielded cell, as shown in Figure 7 . A stainless steel tube runs through the cell wall, past three separate radiation detection systems, and returns to the cell. The radiationdetection systems are: A BF; tube which detects neutrons from '"Cf and ",Cm; a NaI scintillation detector coupled to a single-channel analyzer which detects "'Eu: and a lithium-drifted germanium detector and low-energy Table I. Composition of Product Fractions '''Cm,
Californium Product Fraction 1 2 3 Total 5 of feed
G
-"Am, G
-"Cf, g G
