RAPtD ION EXCHANGE SEPARATIONS Chromatographic Separation of Transplutonium Elements Using High Pressure Ion Exchange DAVID 0 . C A M P B E L L
Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830 The high pressure ion exchange method, utilizing very finely divided resin, a high pressure drop, and large flow rates, was demonstrated by separating the six transplutonium elements, americium, curium, berkelium, californium, einsteinium, and fermium, using chromatographic elution from Dowex 5O-Xl2 resin with a-hydroxyisobutyric acid. All the actinides were separated in good yields and purities in times as short as 1 hour. Product yields were generally better than 99% with substantially less than 1% impurity from other actinides. Purity is limited ultimately by tailing of the elution bands at concentrations about one 1000th of their peak concentrations. Berkelium was not separated from europium. The high pressure system is advantageous for processing highly radioactive materials because the high flow rate permits reduced exposure time, the high pressure controls bubble formation from radiolytic gas, and the improved resolution yields excellent separations.
THEhigh-pressure
ion exchange system, described by Campbell and Buxton (1970), has now been applied to the separation of transplutonium elements. The advantages of this system-decreased time of exposure of the feed to the resin (which minimizes radiation damage), the use of high pressure (which tends to solubilize radiolytic gases, and thus eliminate gassing in the column) and the improved resolution-were expected to be especially useful for such highly radioactive operations. This paper describes ten separations, each involving some or all of the elements americium, curium, berkelium, californium, einsteinium, and fermium, by use of the highpressure ion exchange technique. Experimental
Apparatus. The equipment was similar to that described previously (Campbell and Buxton, 1970), except that all the equipment was installed in glove boxes for alpha containment. The pump and pressure gage were located in one glove box, from which all radioactive materials were normally excluded. Appropriate solutions were pumped through a %-inch stainless steel tube into the second glove box, in which all process equipment was installed. The ?/k-inch transfer line was enclosed in polyethylene tubing that extended between the boxes, so that double containment was provided. The sample line and valve arrangement was similar to that described previously. The radioactive feed solutions were injected into the sample line via syringe for loading on the column. Any one of three sample lines, with capacities of 4, 10, and 50 ml., could be connected between the valves; the feed volume determined the one that was used. The feed solution was displaced to a column from one of the sample lines by pumping either water or ammonium formate solution. I n some cases, parts of this system were bypassed during elution by connecting a %-inch Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
high-pressure Teflon jumper tube across the components. Four resin columns were installed in the glove box (Table I). Three were 26 inches long and made of standard %-inch stainless steel tubing of 32- or 65-mi1wall thickness; these were jacketed for temperature control. The fourth, a small column made from %-inch tubing, was used for loading in one run (run 9). The resin beds were supported by stainless steel frits, 0.5 micron pore size, press-fitted into standard tubing couplings. An alpha monitor, consisting of a small silicon diode supplying a signal to a count rate meter, was used to monitor the alpha activity of the column effluent as the drops formed-i.e., before they fell into the collection tubes. The limit of detection with the detector located 1 cm. from the tip of the effluent line was about 5 x lo5 c.p.m. per ml. for alpha activity, but gamma-and possibly beta-activity interfered to some extent. No maintenance of the pump was required, and all components of the high-pressure system operated satisfactorily. Reagents. All experiments were made with Dowex 50-X12 resin hydraulically graded to either 10 to 20 or 20 to 40 microns (Scott, 1968). Elutions were made with a-hydroxyisobutyric acid (AHIB), partially neutralized with ammonium hydroxide to pH 4.4; the commercial acid was used without purification. Ammonium formate
Table 1. Description of Resin Columns
Column
Bed Length, Cm.
Cross Section, Sq. Cm.
Resin Volume, MI.
Resin Size, Micmns
A B C D
66 66 66 2
0.17 0.17 0.081 0.08
11.2 11.2 5.35 0.16
20-40 10-20 20-40 10-20
95
the same sample plates used for alpha determinations, but at a higher voltage. The beta counts were corrected for alpha activity and the effect of the higher voltage. However, the resulting beta counts include all beta emitters such as europium, zirconium, niobium, and ruthenium (in addition to berkelium). Since some of these beta emitters were probably present in all of the experiments except run 4, the beta activity is a measure of berkelium in this experiment only. I n some experiments, qualitative gamma scans of the product fractions were made with a NaI scintillation spectrometer.
wash solution was made by partially neutralizing formic acid to pH 4.4 with ammonium hydroxide. Run Conditions. The experimental conditions for the ten runs are summarized in Table 11. I n runs 1 through 3, americium and curium were separated, in order to test the equipment and to define operating procedures for the other experiments. Runs 4 through 8 were separations in which actual solutions from the Transuranium Processing Plant (TRU) at ORNL (King et al., 1968) were used as feed. Product fractions from previous tests were used in runs 9 and 10 to study tailing of the elution bands. The process solutions (runs 4 to 8) contained large amounts of chloride and cations such as lithium (King et al., 1968). These were removed by adding ammonium hydroxide to precipitate the actinide hydroxides, which were then dissolved in dilute nitric acid. The dissolver solutions (runs 5 and 6) and Tramex product solutions (runs 7 and 8) contained impurities in concentrations sufficient to precipitate with ammonia, and thus carry the actinides; to the LiCl solution (run 4), 0.5 mg. of lanthanum was added to serve as a carrier. Actinide loadings were on the order of 1 mg. or less, but polyvalent impurities which competed for resin sites may have been very much greater. All elutions were followed by a cleanup step in which a large volume of 0.5M AHIB, a t pH 6, was pumped through the column and its associated system. However, this did not remove all activity from the system, and there was always at least a few hundred counts per minute per milliliter in the column effluent. Analyses. All sample fractions were analyzed by gross alpha counting with a gas proportional counter, and by alpha pulse analysis using silicon surface barrier detectors; this was adequate to define the elution curves of einsteinium, californium, curium, and, to a fair extent, fermium and americium. Berkelium is a weak beta emitter, and various fission products, including especially europium, interfere with its determination. Beta activities of the fractions expected to contain berkelium (between the californium and curium peaks) were determined by counting
Results and Discussion
Americium-Curium Separations. Good products were obtained from the americium-curium separations (runs 1 to 3), and operation of the equipment was trouble-free. Apparent separation factors (the ratio of eluent volumes to the peak concentration for americium and curium, respectively) were between 1.36 and 1.39, but there is some uncertainty in this because the effective displacement volume of the system was not known precisely. At room temperature, these actinides, like lanthanides, were eluted in about one fourth the volume required a t 80°C. I n the range studied, flow rate had no discernible effect. Use of the ammonium formate wash (pH 4.4) caused the bands to elute in less volume in run 2. In all but one of the ten runs, the feed was a dilute nitric acid solution, from which hydrogen ion loads quantitatively. This acid subsequently elutes, decreasing the pH of the eluent and causing a pH gradient, decreasing downward, somewhere in the resin column. If the actinides try to move down the column faster than this band of lower pH, their elution will be delayed and their separation decreased. I n most of the following work, a wash with an ammonium formate solution of the same pH as the eluent was used to circumvent this problem. Peak positions were much more reproducible when such a wash was used. All elution bands had the same shape; the concentration rose rapidly to a maximum, dropped sharply by a factor
Table II. Run Conditions
Run No. 1 2 3 4 5
Column A A
A A C
Temp., C.
Feed
"4
Ident."
Vol., ml.
Wash, Ml.
AmCm AmCm
4 4 4 1.6 2.0
0 15 0 0 10
80 23 23 80 80
AmCm LiClb Disb
6 7
C C
80 80
Tram'
1.o 2.0
10 20
8
C
80
Tramb
2.0
20
9
A
80
Syn 1
21
0
B+D
80
Syn 2
40
4
10
Disb
Eluent MAHIB 0.32 0.32 0.25 0.25 0.26 0.32 0.32 0.25 0.32 0.25 0.32 0.20 0.15 0.25 0.20
Eluent Vol.,MI. 130 60 130 400 75 60 100 62 80 150 50 165 75 60 300
Flow Rate, Ml. Min.-' Cm.? 7.1 1.2-12.5 6-15 6-13 26.5 26.5 24.7 26.5 26.5 24.7 24.7 12.0 12.0 12.0 12.0
"AmCm. Mixtures of 2MCmand 243Am,ca. 3 x 10' c.p.m. each, in dilute "0,. LiC1. Heavy element fraction from LiCl anion exchange purification of Tramex product, plus 0.5 mg. of La carrier, in 0.5M "Os. Dis. 1500 dilution o f TRU Processing Plant dissolver solution from highly irradiated 240Putqtgets, in 0.25M HN03. Tram. Tramex product solution from same source, canuerted to 0.25M "01. Syn 1 . Cf and Es product fractions from run 4, diluted to 0.125M A H I B , pH 4.4. Syn 2. Cf, Es, and Cm product fractions from runs 4 and 9, acidified to p H 1.5. Described by King (1968).
96
Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 1 , January 1970
of several hundred, and then decreased only slowly during the rest of the elution. This tailing effect is clearly shown in all the figures, and is discussed below. Tailing to this extent would be below the limit of detection in the stable rare earth studies reported by Campbell and Buxton (1970). Yields and purities of the americium and curium products were approximately 99%, in spite of the tailing. At some sacrifice in yield, curium could be obtained in higher purity, but americium purity is limited by the curium tailing. Separation of Heavy Elements in Process Solutions. Excellent separation of the first six transplutonium elements was obtained, but certain fission products, if present, appeared in some products. Each elution band tailed over the rest of the run, and this limited the purity of products that eluted later. However, product purity greater than 99% (by alpha count analysis) was usually obtained. The feed for run 4 contained all the californium and heavier elements, a major fraction of the berkelium, and only a small part of the curium and lighter actinides originally present in the targets. This material had been substantially decontaminated from fission products. Both the gamma and fast neutron radioactivities of the feed sample, measured a t contact, were 500 to 700 mr. per hour. Results of the elution are plotted on Figure 1. The small amount of fermium was detected just before the einsteinium band. Einsteinium, californium, and curium were satisfactorily separated, but each, in turn, tailed across the rest of the elution. Curium, remaining on the column from the previous tests, tailed across the entire run. I n general, tailing occurred a t levels less than one 1000th of the peak concentration for each element, and slowly decreased with eluent volume. There was no evidence for forward tailing. This run clearly demonstrated that excellent heavy element products can be obtained by this technique. Yields and purities were: einsteinium, 98% yield, contaminated with about 1% L4uPu(curium daughter) and less than 1'; curium; californium, 99.65 yield with about 0.1% einsteinium impurity and less than 0 . 1 s plutonium; and curium, quantitative recovery, with other actinides being lower than the peak by more than five orders of magnitude. These figures are based on alpha count analyses. The curve for berkelium (Figure 1) is based on beta counting of the same plates that were used for alpha determinations, as described above. Measurements of the beta and gamma spectra of these samples indicated the
presence of no interfering activities, and the original feed was very pure with regard to beta emitters, such as europium, ruthenium, zirconium, or niobium. Accordingly, it is believed that the beta counts in this case provide a good measure of the berkelium concentration, at least for samples in which the beta count is substantially greater than the alpha count. The radiation level of the feed in runs 5 and 6 was about 500 mr. per hour a t contact, but less than 4 mr. per hour a t the face of the glove box with the sample on the resin column. A much larger sample could have been used advantageously. This material contained very large amounts of rare earth fission products and other impurities, compared to the actinides. The alpha determinations yielded elution curves generally similar to those in Figure 2; however, all concentrations were lower by more than two orders of magnitude because of the small feed samples. Well defined bands were obtained for einsteinium, californium, curium, and americium, although the total count in the einsteinium band was so low that contamination could lead to a significant error. Beta counting was attempted on a few samples, but berkelium could not be determined directly because of the high levels of fission product activities. Certain isotopes yielded elution bands along with the actinides. The rare earths lWTband '"Eu contributed gamma bands between einsteinium and californium, and between californium and curium, respectively. A gamma spectrum corresponding to cesium appeared just ahead of the curium product, and 6o Co appeared with americium. Gamma and x-radiation from americium and curium were also apparent, in agreement with the alpha determinations. Cerium-praseodymium-144 was found in the 0.5M, pH 6, AHIB cleanup solution. The effluent from loading and the ammonium formate wash contained the bulk of the ruthenium and zirconium-niobium, These two experiments showed that einsteinium, californium, curium, and americium can be satisfactorily separated from each other and from most of the fission products by the high-pressure ion exchange procedure. Berkelium cannot be determined without additional chemical separation, including especially removal of europium. If there is interest in the lanthanides that arise from heavy element fission, these elements could be recovered as reasonably pure products. The feed for runs 7 and 8 contained all of the actinides heavier than plutonium, but it had been largely decontaminated with respect to fission products. Curium-244
/
J
/
/
/
T /
f
1
\
Y-
Figure 1. Separation of transplutonium elements enriched in heavy actinides 66 cm. X 0.17 sq. cm. column. Flow rate 1 ml. per minute = 6 ml. per sq. cm. per minute. Dowex 50-X12, 20 to 40 microns. Eluent 0.25M, pH 4.4 AHIB. 80" C.
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
97
Figure 2. Rapid separation of six transplutonium elements i n 1 hour 66 cm. X 0.081 sq. cm. column. Flow rote 2 ml. per minute = 24 ml. per sq. cm. per minute. Dowex 50-X12, 20 to 4 0 microns. Eluent 0.26M, pH 4.4 AHIB, 62 ml. 0.32M,, pH 4.4 AHIB, 80 ml. 80" C.
contributes about 99.9% of the alpha activity of this material. The feed contained approximately 5 x 10'' c.p.m. (gross alpha), thus alleviating the detection problems that had resulted from the low activity levels in runs 5 and 6. Radiation levels were low, substantially less than 5 mr. per hour, a t the face of the glove box. Runs 7 and 8 were programmed in advance; the volumes of the product fractions were chosen to give good separation of the individual actinides. Reproducible results were obtained as long as an ammonium ion wash was used to displace acid from the column and the same eluent solution was used. In most cases, volumes were determined by timing the fractions, since this was more accurate than volume measurement. The pressure drop was about 800 p s i . I n run 7 (Figure 2), following the ammonium formate wash, a %-inch high-pressure Teflon tube was connected directly to the column preheater, bypassing the valves and sample line, so that any feed or contaminant remaining in the valves or lines would not be transferred to the column. In run 8 the valve and sample lines system was used; the separation was slightly poorer, and slightly more tailing was observed. Alpha analyses of the fractions showed good separation, with well-defined peaks for fermium, einsteinium, californium, curium, and americium. As usual, 240 Pu, the daughter of 244Cm,appeared throughout, but primarily in the early fractions. Beta counting showed a broad band between californium and curium. Gamma scans clearly showed the presence of '"Eu in this band, and decay measurements indicated substantially more europium than berkelium present initially. The width of the curve is a result of the presence of both berkelium and europium, which were not well separated. I n addition, gamma scans showed the presence of terbium with or just after the einsteinium peak; 239Np(243Amdaughter) in the wash 98
and early fractions, and possibily also following americium; and substantial amounts of lJ4 Ce-lJ4Pr in the cleanup solution following actinide elution. Ruthenium and zirconiumniobium were also present in measurable amounts, and behaved as described above. Band Tailing. Although separations such as those shown in Figures 1 and 2 are adequate for both analytical and process applications, it would clearly be advantageous if the tailing of the bands could be decreased. The tailing seems to continue more or less indefinitely; even after washing the column with many column volumes of cleanup AHIB or ammonium citrate solution, the subsequent run was generally characterized by a continuation of the tailing from the predominant activity in the previous run, usually curium. Band tailing is not a unique characteristic of the highpressure system, or of high flow rates, and results very similar to these are generally observed. In a study of the californium-curium tracer separation with conventional equipment (Aly and Latimer, 1967), tailing was substantially decreased by carefully cleaning the top of the column above the resin bed after the actinides are loaded, but before elution. This suggests that, in their work, tailing may have been caused by a small amount of material attached physically or chemically to the walls of the column or to the top of a quartz wool plug above the resin. Some variations in procedure were used in the runs described above in order to investigate the effects of eluent composition, flow rate, temperature, and the use of the jumper line to bypass the valves and sample line, which might retain radioactive material; of these, only the use of the jumper line possibly decreased the level of the tailing. Two runs were made to test two specific approaches to the problem, and, although tailing was not eliminated, both are of interest. I n run 9, the feed consisted of californium and einsteinium products from a previous elution, which were diluted with water without changing the pH (4.4); this avoided feed preparation steps which cause large changes in the chemical environment. The results (Figure 3) show an excellent separation of californium and einsteinium, and perhaps lower levels of tailing than were observed in the other runs. However, tailing was still significant; extrapolation of the tails back under the peaks yielded concentrations one 3000th to one 5000th that of the peaks. Essentially quantitative yields of both the californium and einsteinium were obtained, and each product contained less than 0.1% of the other by alpha count. Curium and californium, remaining on the column from the previous run, appeared throughout; these constituted the small amount of impurity in the first elution band. Run 9 demonstrates that the extreme emphasis usually placed on sharp loading of the actinides is not essential to obtain quantitative separations. Certainly, the narrower the initial band, the easier the separation. However, an efficient chromatographic system can still separate the bands, even if they occupy a significant part of the column, either because of large loadings or, as in this case, loading with a relatively low distribution coefficient, probably around 100. Run 10 was made to investigate the possibility that a small fraction of the activity might be retained, by some mechanism, on the top of the resin bed, and then Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
ml ELUENl
Figure 3. Separation of Es and Cf 66 cm. X 0.17 sq. cm. column. Flow rate 2 ml. per minute = 12 mi. per sq. cm. per minute. Dowex 50-X12, 20 to 40 microns. Eluent AHIB. 80" C.
might slowly re-enter the eluent stream, causing tailing. Two resin columns, D and B , were used. The feed was loaded at 25 ml. per minute per sq. cm. on column D , which was then washed with 4 ml. of ammonium formate to remove the hydrogen ion. Analysis of the effluent showed that the actinides, some 500 displacement volumes, had loaded quantitatively. Column B was then connected to the outlet of column D. The eluent (0.20M AHIB) was pumped directly to column D ,bypassing the sample line and valves used for feed injection, from which it passed through column B. After approximately 6 to 7 ml. of eluent had passed through column D (about 7 5 displacement volumes, which moved the actinides to column B ) the eluent flow was bypassed around it, directly to column B . Thus, the entire part of the system exposed to feed solution during loading, including the loaded resin, was isolated during the rest of the elution. The pressure drop was about 1700 p.s.i. during the elution. Excellent separation was achieved; yields greater than 99% of both californium and einsteinium were obtained, each product containing much less than 1% of the other. However, tailing was still evident, and, in fact, was more pronounced than in the curves shown in Figure 3. Extrapolation of the tails back under their peaks yielded a concentration about one 1000th of that of the peak.
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
The use of a separate, small loading column was aemonstrated, and this may have application to processing problems. Run 10 demonstrated that loading may be accomplished at a very high flow rate by use of a short column containing very fine resin. This reduces radiation damage by decreasing the exposure time of the resin to the highest actinide concentrations, which occur in the loaded resin band. Also, substantial amounts of contaminants are washed out during such a loading step. After loading, the small column may be coupled to a longer chromatographic column where the separation occurs. Although band tailing has not been eliminated, it has been possible, by reasonably careful operation, to reduce the concentration in the tails to three to five orders of magnitude less than that in the peaks. Scale-up. Larger ion exchange columns were fabricated, up to 4 feet long and 0.5 inch in diameter, and a highpressure system was assembled and installed in a hot cell. This equipment was subsequently used for the final separation and purification of the transcurium elements in the TRU facility a t ORNL (Baybarz, 1968), and all californium, einsteinium, and fermium products (some 6 mg. of californium and 30 mg. of einsteinium) have been processed through high-pressure columns. Excellent product purity has been obtained. Acknowledgment
The author is indebted to J. H . Cooper and his group in the Analytical Chemistry Division, Oak Ridge National Laboratory, for many of the analyses and for preparation of some of the feed samples, and to S. R . Buxton for assistance in carrying out the experimental work. Literature Cited
Aly, H. F., Latimer, R. M., J . Znorg. Nucl. Chem. 29, 2041 (1967). Baybarz, R. D., Oak Ridge National Laboratory, personal communication, 1968. Campbell, D. O., Buxton, S. R., IND.ENG.CHEM.PROCESS DESIGNDEVELOP. 9, 000 (1970). King, L. J., et al., U. S . Atomic Energy Comm., Rept. ORNL-3954 (1968). Scott, C. D., Anal. Biochern. 24, 292 (1968). RECEIVED for review February 7 , 1969 ACCEPTED September 29, 1969 Division of Nuclear Chemistry and Technology, 155th Meeting, ACS, San Francisco, Calif., April 1968. Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Cow.
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