I
T. A. BUTLER and E. E. KETCHEN Oak Ridge National Laboratory, Oak Ridge, Tenn.
Solvent Extraction Separation of Cerium and Yttrium
. . . from
O t h e r Rare Earth Fission Products Fractionat ion of m ult ikilocurie quantities of rare earth fission products produces highly pure Ce144in good yield
A process has been developed for the specific BATCH SOLVENT EXTRACTION
purpose of separating Y91 and Ce144 from the gross rare earth fission products fraction. I t involves the extraction of these two elements into di(2-ethylhexy1)phosphoric acid (D2EHPA) in Amsco. The process has been incorporated in the ORNL Fission Products Pilot Plant chemical flow sheet ( 4 )to provide a pure Gel" fraction and a Pm147 rare earth fraction for subsequent separation and purification. Since Y91 and Cela represent the bulk of the ionizing radiation, Pm147 can be recovered by conventional ion exchange methods on relatively lightly shielded columns without excessive radiation damage to the resins. The rare earth fission product fractions used in the Fission Products Pilot Plant (F3P) experiments had been separated from the gross mixed fission products by carefully controlled hydroxide precipitation, followed by acid oxalate precipitation. The fraction was converted to a HNOs solution relatively free of other radioisotopes such as Sr89--90, Zr95-Nb96, Cs137, Tc99, and, in particular, Ru108, which has a deleterious effect on the CelU extraction. Classical precipitation methods are readily adaptable to the removal of cerium from the trivalent rare earths. The Ce(II1) may be oxidized to Ce(IV), which is then precipitated from the trivalent rare earths as the hydroxide, phosphate, or iodate. On the other hand, the separation of yttrium by known precipitation or fractional crystallization methods is not so readily adaptable to pilot plant production operations. Solvent extraction appeared to be a good method for YSl removal as well as for better removal and recovery of from the rare-earth fraction. From the data of Peppard and others (2, 3) it appeared feasible to extract yttrium from a HCl solution of mixed rareearth fission products into a 0.75M D2EHPA solution in toluene. Preliminary experiments in this laboratory showed :
Yttrium can be satisfactorily extracted from a rare earth fission product soluinto a 0.5M tion which is 1.ON " 0 3 solution of D2EHPA in Amsco, with only a small Pm147 loss. K M n 0 4 is satisfactory for the oxidation of Ce(II1) in a 1N " 0 8 solution, and subsequent extraction into a 0.5M solution of D2EHPA in Amsco is nearly quantitative. 0 Ce(1V) may be recovered from the organic phase by reduction to Ce(II1) with HzOz prior to extraction into aqueous 2N "0s. This article describes, first, the laboratory experiments which were used as a basis for the equipment design and chemical flow sheet used in the F3P and, second, the experimental productiontype runs made in the pilot plant and the chemical flow sheet modifications that were necessary because of the unique conditions characteristic of large-scale operations. This process is useful in separating yttrium and cerium from the rare earth fission product feed on a pilot-plant scale. I t may also be useful in laboratory-scale separations of yttrium and cerium from the rare earth fission products.
Experimental Materials. The D2EHPA (Union Carbide Chemicals Co.) contained 91 .5y0 D2EHPA, 0.6y0 mono(2-ethylhexy1)phosphoric acid (M2EHPA), and very small quantities of the pyroester. It was satisfactory for this investigation, and no further purification was made, Amsco 125-82 (American Mineral Spirits Co.) was used as purchased as a diluent for the D2EHPA. The c a h l a t e d coniposition of the yttrium-rare earth mixture (excluding cerium) produced by fission of U235 is as follows: 6.64% Y, 12.9% La, 13.1% Pr, 49.8% Nd, 6.0% Pm, 11.0% Sm, 0.43% Eu, and 0.13% Gd. This mixture was combined with varying amounts of cerium. A synthetic mixture of this composition, but without promethium and gadolinium, was pre-
pared from pure rare earth compounds. The extraction and precipitation of yttrium and the rare earths were followed with the help of the radioactive isotopes Y91, Ce144, Pm147, Eu152-154, and Pm148. The tracers YQ1,GelM,Pm147, and Eu162--164 from fission are commercially available; Pm14* was produced by the Pm147 (n, y ) Pm148 reaction. Determination of Distribution Coefficient. The distribution coefficient K d is defined as the ratio of the concentration of the element in the less dense organic phase to the concentration of the element in the more dense aqueous phase after the equilibration of the two immiscible liquids. A standard 10minute contacting period by mechanical shaking in a glass separatory funnel was allowed for the attainment ofequilibrium, unless otherwise noted. After this contacting period, the phases were permitted to settle for 10 minutes before separation. The concentration of the element was determined radiometrically. Except where noted, the simulated rare earth feed solution contained 2.5 grams of the yttrium and rare-earth mixture per liter, 1.0 gram of cerium per liter, and the specified amount of "08. Unless otherwise noted, the organicto-aqueous phase ratio was 1 to 3 by volume. Results and Conclusions
Distribution Coefficient. The distribution coefficients were determined for Y, Eu, Pm, Ce(III), and Ce(1V) between a simulated yttrium-rare earth fission product feed solution and D2EHPA in Amsco. The K d values for Ce(1V) were determined after the oxidation of Ce(II1) by use of varying amounts of K M n 0 4 (up to 0.05 mole per liter of aqueous phase). The time needed for practically complete oxidation and extraction of the cerium varied with the molarity of the D2EHPA. Distribution coefficients of 2000 to 10,000 were obtained for this step. Distribution coefficients for Ce(II1) were VOL. 53, NO. 8
AUGUST 1961
651
Table 1. Distribution Coefficients for Rare Earth Elements and Yttrium
"oa in Phase, Cation
AT
Eu( 111)
1 1 1 1
Pm(II1) Y(II1) Ce(II1) Ce(1V) Ce(II1) Ce(II1) Pm(II1) Ce(IV) Y(II1)
1
2 2 2 10 10
-
Variation of Kd with D2EHPA Molarity in Organic Phase
0.5M
0.507 0.127 77.7 0.0614 -104u,b
0.0074d 0.0164 0.015
1 , O - F
0.433 235 0.162 -104aIc 0.073 0.08
855"
0.563
1.33
a &(ITr) lid's resulted from oxidation with KAIn04 in described RS03 medium. 35-min. equilibration period. 15-min. equilibration period. For 0 . 2 5 M DBEHPA. Addition of 0.01 mole of Kh41i0, per liter of l 0 N "01 as holding oxidant.
determined following the reduction of Ce(IV) in the aqueous phase by 1 M HzOz in 2N Hh-03. Results are shown in Table I . Effect of Radiation of the Aqueous Phase. Irradiation of solutions of Nah-03 has been described by Mahlman (7). At 95" C., H N O z (the major product of radiation under process conditions) will be destroyed, thus making it possible to remove the accumulated nitrite before the solvent extraction process is begun. However, the intermediate radiation decomposition products, as they are formed, compete with the Ce(II1) for the KMn04. As a result, it is necessary to use more than equivalent quantities of K M n 0 4 to complete the cerium oxidation. Effect of Radiation on the Organic Phase. Since the organic phase is subjected to ionizing radiation energies up to 10 watt-hr. per liter during the course of the extraction, the effect of gamma radiation on a 1M D2EHPA solution in contact with a 1 N H N 0 3 solution was investigated. Equal volumes of 1M D2EHPA in Amsco and a 1N HXOs solution were contacted for different lengths of time in a Coco gamma radiation field. After the irradiation period, the organic phase was used to determine the K d for Cc(II1) extraction from a 1 N " 0 3 rare earth fission product solution. Results are shown in Table 11. The results show that gamma radiation up to 85 watt-hr. per liter has little effect on the K d for Ce(II1) extraction The M2EHPA is an expected product of radiation and probably has a greater effect on the distribution ratio for the trivalent rare earths than othei radiation products. Peppard and others (2) have shown that the addition of
652
M2EHPA as an impurity to the D2EHPA resulted in a greater change in the Kd for Ce(II1) than in the K d for yttrium or any other rare earth of higher atomic weight than cerium. Therefore, the above experiment indicated that the change in Kd for Ce(II1) arising from radiation was small and that the radiation damage to the organic phase is negligible. While the products of radiation in the organic phase were not expected to change the Kd for Ce(IV), some of the products may compete with the Ce(II1) by reacting with KMn04. For example, %-ethylhexan01may be produced and would be oxidized by KMn04. A batch of 0.5M D2EHPA in contact with lAr " 0 3 was exposed to a Coco gamma radiation field having an energy equivalent to 90 watt-hr. per liter. The K , for cerium in the Ce(II1) oxidation step was determined and was the same as for the nonirradiated 0.5MD2EHPA. However, twice as much K M n 0 4 was needed to maintain an excess of KMnO4 in the irradiated organic phase. Cerium(II1) Oxidation Time. Since longer contacting rimes require more KMnO, to maintain an excess in radioactive solutions, it is essential that the contacting time be kept to a minimum. Kinetic studies showed that K d values of 4000 are attainable in 6 minutes in 1 M D2EHPA, while it takes 30 minutes to reach a K d of 4000 in 0.5M D2EHPA. The oxidation of Ce(II1) to Ce(1V) and the extraction time may be decreased 10 minutes by using a 10-minute Ce(II1) oxidation and extraction period into 1 M D2EHPA in a 1-to-6 organic-toaqueous phase proportion, followed by a 10-minute contacting period after diluting the 1M D2EHPA to 0.5M.A K d of 4000 for Ce(1V) was attained by this method, thus cutting the oxidationextraction time by a third. Reduction of MnOz. When Ce(II1) is oxidized in a KMnOk-HN03 solution in contact with D2EHPA in Amsco, MnOz is formed. The MnOz has two adverse effects on the process. First, MnOz prevents a sharp separation of the organic and aqueous phases. Secondly, the MnOz formed under process conditions tends to sorb the trivalent rare earths.
A wild reducing agent was sought to solubilize the MnOz in a " 0 3 solution without appreciable reduction of Ce(IV) in the organic phase. Citric acid was excellent for this purpose. Since Ce(IV) is almost all in the organic phase and citric acid is soluble only in the aqueous phase, the citric acid has a minimum of contact with the Ce(1V). In practice, after the Ce(1V) extraction was completed, citric acid was added in an equimolar quantity to the KMn04 previously used, and then it was contacted for a suitable time. Since the products of radiation tend to reduce Ce(1V) to Ce(II1) in the organic phase, the interval of time between The completion of the Ce(II1) oxidation and the time when the phases are separated must be kept to a minimum Since the reaction of citric acid with MnOz occurs within the abovementioned time interval, this reaction time must be kept to a minimum. However, if the phases are contacted for too short a time after the citric acid addition, MnOl will still cause trouble. Experiments were performed to determine the optimum contacting tirnc after the citric acid addition. Table I11 shows the effect of the length of contacting time after the citric acid addition on the KCj for promethium and for Ce(1V). The Kd values in Table I11 indicate that a period of 10 to 20 minutes would be the best contacting time after the citric acid addition if the solution were not subjected to higher radiation fields. Since the radiation tends to reduce Ce(IV), a 10-minute contacting period after the citric acid addition was selected for this step. Effect of Organic Phase Loading on Kd. As increasing amounts of rare earths are extracted into the organic phase. the effect of loading the organic phase on K d becomes significant. The effect of ehe cerium loading in the organic phase on the K d for cerium and promethium was investigated. The cerium concentration in the rare earth fission product feed solution was varied from 1 to 5 grams per liter. The variation of K d with initial concentration of cerium is shown in Table IV along with the percentage of cerium removed
Table II. Gamma Radiation Has Little Effect on K d for Ce(ll1) Extraction
Table 111. Effect o f Contact Time with Citric Acid on K d for Pm and Ce(IV)
D2EHPA in Amsco
10 minutes was the optimum contact t i m e
INDUSTRIAL AND ENGINEERING CHEMISTRY
Energy, Watt-Hr./Liter 0 10
20 35 85
Kd 0.162 0.172 0.172 0.170 0.168
0 10 20 30
0.300 0.115 0.0604
0.0592
2000 1790 720 207
SOLVENT EXTRACTION SEPARATION =EED---'\
-.
I
Table IV. Ka and Ce Removal Vary with Change in Initial Ce Concentration Organic to aqueous ratio = 1 to 3
Initial Ce Conc. in Aqueous Phase, Grams/Liter
Ce Removal, %
Kd
2700 1790 1250 600 325 2 50 200
1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0
100
99.9 99.8 99.7 99.5 99.0 98.7 98.3 97.5
by 0.5M D2EHPA in a 1-to-3 organicto-aqueous phase ratio. The rare earth fission product feed solution was 1N in " 0 3 and 1M in NaN03. The effect of cerium loading in the organic phase on the K d for promethium is shown in Table V. In the first experiment the K d for promethium was determined without oxidation of the Ce(II1). This Kd value is related to the promethium loss into the organic phase when yttrium alone is removed. In the second and third experiments, the Ka for promethium was determined after the MnOz dissolution following the Ce(II1) oxidation and extraction step. Since the citric acid reduction of MnOn is not complete in practice, the promethium loss into the organic phase in a cerium removal step would be expected to lie between 1 and 4%. Effect of Ruthenium Contamination. The effect of ruthenium on the yttrium and cerium removal steps was investigated. While ruthenium had little or no effect on the yttrium removal step, the addition of 0.1 7 gram per liter of ruthenium (equivalent to the ratio of Ru to rare earth in fission) to the simulated feed caused the K d for cerium to drop from lo4 to about 1. The Kd for cerium varied widely, depending on the contacting time in the cerium extraction step. The K d for ruthenium into 0.33M D2EHPA in Amsco is 12. Some of the ruthenium plated out on the glass sepa-
Table V. As Ce Loading in Organic Phase Increases, K d for Pm and Pm losses Decrease
Expt.
No. 1 2 3
Ce in Organic Phase, Grams! Liter 0.02 1.0
3.0
P m Loss, Kd
for Pm
0.125 0.084 0.0265
%
3.7 2.7 0.9
STORAGE TANK
ORGANIC WASTE GANlC
5TE
Equipment for solvent extraction of Ygl and Ce144from gross fission productislocated in a hot cell in the Fission Products Pilot Plant at ORNL
C e PRbCESS-
V!iSTE
' U S CELL
PRECIPITA P-4
ratory funnel; the remainder, still dissolved in the organic, could not be back-extracted with HzOz into an acid solution with the cerium.
Fission Products Pilot Plant Equipment and Materials. The separation equipment (above) for the solvent extraction of Y91 and Ce144from the gross rare earth fission product is located in a hot cell in the F3P at ORNL. Samples can be jetted from the various tanks in the solvent extraction cell to a manipulator cell which has a viewing window, manipulator hands, and equipment needed for process-control chemistry. The storage tank S-I, the contactors (C-1 and C-2), and the precipitator (P-1) are equipped with agitators. The contactors, precipitator, and the filter (F-1) are equipped with heating and cooling coils to adjust the temperature of the solution. The solutions are transferred from one vessel to another by vacuum steam jets and air lifts. Pneumatic density type probes are set to cut off the air lift and valves when there is a density difference in the liquid between the two probes. When the air lift is shut off, there is still about a liter of the aqueous phase in the bottom of the contactor and in the lines. This liter of hold-up solution may be washed out of the contactor with a solution of the same acidity as the feed. Rare Earth Fission Product Feed. The rare earth fission product feed material comes into the solvent extraction processing cell as a HNOa solution, resulting from dissolving a rare earth
FILTER F - l
oxalate precipitate in 8 to 12N " 0 3 . The feed, with steam jet dilution and washes, is about 4N in "03 in about 80 liters of solution. The gross rare earth fission product solutions which have been processed may be divided into two types. The first type is short-cooled rare earth fission products (3 to 12 months out of the reactor). The process was originally designed for this type of waste in which the yttrium and rare earth concentration is about 2.5 grams per liter and the cerium concentration is 1 gram per liter. The second type is long-cooled rareearth fission product (over 2 years out of the reactor). This type has a lower Celu and Prnl47 ratio with respect to the total rare earths than the short-cooled type. The YQl is almost completely decayed, thus eliminating the need for a separate yttrium removal step. Procedure. The results of the laboratory investigations were used as a basis for developing a chemical flow sheet for pilot plant scale operations on actual rare earth fission product mixtures. The general flowsheet (p. 654) shows the process for a short-cooled rare earth fission product. In the longcooled rare earth fission product, the yttrium removal step was omitted. The processing of a short-cooled rare earth fission product is described here. The rare earth fission products fraction resulting from previous separations in the plant was jetted into the contactor (C-1) for the yttrium removal step. The acidity was adjusted to 1N HNOs (in 100-liter volume) with NaOH, and the solution was then contacted with 33 liters of 0.5M D2EHPA to extract VOL. 53,
NO. 8
AUGUST 1961
653
r(ljno4 C-RC
43bEOUS
I
ACID,
33-liters AOJEOUS S T R ' " 2 N HNOq
!00-llrers AQUEOUS WAS-;
3z-liters
AQUEOUS ( 9 4 % ~ m ' ~3%"'', ~ , < : T oC e ' 4 4 i
1-
AQUEOUS ( ~ V ~ Y2 ~ % ' , ~ TO lWASTE
