1
R. E. McHENRY and J. C. POSEY O a k Ridge National Laboratory, O a k Ridge, Tenn.
Separation of Strontium-90 from Calcium
. . . by
Solvent Extraction A continuous, multistage liquid-liquid extraction process uses: A buffered aqueous phase An alcohol additive which reduces the effect of organic phase loading on K d and makes possible the use of stronger DZEHPA, with a consequent production increase
T H E
LONG-LIVED
FISSION
PRODUCTS
are separated in the F3P (Fission Products Pilot Plant) at Oak Ridge National Laboratory (3) from solutions resulting from the recovery of uranium and plutonium from irradiated uranium fuel elements. In the fission products recovery process, the alkaline earth .elements are separated and concentrated by precipitation as carbonates. This concentrated fraction contains the fission products strontium and barium, as well as the calcium which has entered the process solution as an impurity in process water and reagents during previous steps in the recovery process. The weight of barium produced in fission is about equal to that of strontium, but the weight of calcium introduced as an impurity may be large relative to that of the fission product strontium. One alkaline earth fraction contained 22,000 curies of Srw and 20 grams of calcium to each gram of strontium. To meet product purity specifications, a process was required to remove at least 99% of the calcium from the SrQo. I t was desirable also that a t least 95% of the Sr90 be recovered. The high purity specifications for the Sr9O product were established from consideration of its intended use in electric power generators for remote areas. Since the F3P contains equipment for batch precipitations, batch extractions, and continuous countercurrent extraction, processes for the purification of strontium which could utilize this equipment were examined on a laboratory scale. The batch precipitation methods, based on the selective precipitation of either calcium or strontium, were found to be impractical in the case of the particular strontium fraction involved because of the excessive number of plant operations required, excessive losses, or inadequate purity of product. Preliminary laboratory work with di(2ethylhexy1)orthophosphoric acid (D2EHPA) had shown that separation fac-
tors between calcium and strontium of 30 to 40 could be obtained under suitable conditions. As a result of these preliminary data, the extractive separation of calcium from strontium with D2EHPA was investigated both in batch and countercurrent processes. The batch extraction method was found to be impractical because of the multiple plant operations required for the 20 to 1 weight ratio of calcium to strontium. However, it does have application to weight ratios of 5 to 1, or less. The multiple stage continuous countercurrent extraction method was found to be capable of operating at greater than the minimum separation at the required production rate in the existing F3P mixer-settler equipment.
Theoretical The use of D2EEIPA for the extraction of lanthanides has been studied by Peppard and others (4-6). They have shown that D2EHPA exists largely as a dimer in the organic phase and that
each triply charged lanthanide ion in the organic phase is associated with three dimers or six formula units of D2EHPA. Coleman and others ( 7 ) have investigated the extraction of uranium by D2EHPA. They reported that a t low uranium concentrations the doubly charged uranyl ion is associated with two dimers or four formula units of D2EHPA but that at higher uranium concentrations larger ratios of uranium to D2EHPA occur. They also reported that the addition of certain organic compounds to the organic phase affected the distribution coefficients. Notably, with respect to the present work, alcohols lower the distribution coefficients. From the above work it is concluded that D2EHPA (dissolved in an inert diluent) reacts, a t least at low organic phase loading, as follows :
nL,.++ + 2(HR)z.,,
M(HRz)z,,,
+
2Ha,.+
(1)
where M + + is the extracted ion and the subscripts refer to the aqueous and or-
Basic features of single mixer-settler stage provide means for approaching equilibrium distribution of extracted ions between two phases VOL. 53, NO. 8
AUGUST 1961
647
ganic phases, respectively. An equation for the distribution coefficient (Kd) of either calcium or strontium ion can be derived from Equation 1 :
Equation 2 shows that the value of
K d for a particular ion varies inversely with the square of the hydrogen ion concentration and directly as the square of the free D2EHPA concentration. Also the ratio of the distribution coefficients of calcium and strontium is equal to the ratio of the equilibrium constants of the two ions from Equation l . Since there is, of necessity, a concentration gradient of the solutes from stage to stage, it is necessary to reduce the effect of organic loading on the variation of the distribution coefficients if high organic loading is to be obtained. Also some means must be provided to control the concentration of hydrogen ions which are liberated by the transport of solute to the organic phase. The production rate of a solvent extraction process with DZEHPA for separation of calcium and strontium is directly proportional to the calcium concentration in the organic phase, if the quantity of calcium exceeds that of strontium, This is due to the fact that the calcium leaves the system in the organic phase. As can be seen from the stoichiometry of Equation 1, the calcium removal capacity of the organic phase will increase with D2EHPA concentration in that phase. However, a hydrogen ion buffer must be found which will control the pH of the aqueous phase a t levels such that the Kd values are suitable for multistage mixer-settler operation. The higher the D2EHPA concentration, the lower the required pH. The buffer system which would control the pH at the lowest level and yet be compatible with the process was an acetic acidacetate system. The lack of compatibility of other buffers was due to problems of corrosion, insoluble salt formation, and complex ion formation. Two methods of reducing the distribution coefficients were investigated. The acetic acid concentration in the aqueous phase was varied, and the effect of an alcohol in the organic phase was studied. Experimental
Equipment. The laboratory size mixer-settler used in these experiments contained 11 stages and measured 20 X 8 X 8 inches. The flow capacities for these mixer-settlers were approximately 2 liters per hour each for the organic and aqueous phases. This unit was an ORNL-designed miniature version of a
648
type developed at the Knolls Atomic Power Laboratory by Coplan and others (2). One important feature of the KAPL design is the use of the pumping action of the impellers in the mixing chambers to maintain automatically the organic-aqueous interface at the desired level in the settlers. Two mixer-settlers were used in the F3P solvent extraction facility: a 20stage separation unit and an 11-stage solvent recovery unit. These units were of the same type as the laboratory unit but were designed for higher organic and aqueous flow rates. Flowsheet Test Procedure, In tesring the flowsheets, the mixer-settlers were operated at the required flow rates with the particular feed, aqueous, and organic solutions required until a steady state in the effluent streams was reached. The flow rates were maintained by suitable metering pumps. Samples of the effluent streams were taken hourly and analyzed to determine when steady state was reached. When the analyses of the effluent samples indicated attainment of steady state, samples were taken of both phases of alternate stages throughout the mixersettler. These stage samples were analyzed for solute concentration in both phases, and the pH of the aqueous phase was measured. Analyses. During the test runs calcium and strontium were added to the feed, and in addition a radioactive tracer of either Ca4j or Sr35 was added to the feed so that counting techniques could be used to determine the concentrations of the solute in question. Another run was then made under identical conditions with the other tracer to determine the behavior of the other solutn. With a known quantity of radioactive tracer added to the feed, the count rate of a sample could be converted to concentration of the element. Samples containing strontium tracer were analyzed by counting the SrSj in a gamma scintillation counter. The Ca45 in the calcium tracer runs was determined by using beta counting techniques.
Table I. Distribution Coefficient of Strontium between 0.2M D2EHPA in Amsco Diluent and Acetic Acid Solution Acetic acid has a limited effect on K d values
Acetic Acid Conc., M 0.1 0.5 1.0 2.0 5.0
Distr. Coeff., Kd
1.16 1.04 0.90 0.94 1.02
2.4 2.3 2.2
2.1 1.8
The effect of 2-ethylhexanol on the distribution coefficients of both calcium and strontium is shown in Figure 1. The alcohol decreases the distribution coefficients of both calcium and strontium t o a greater extent at low loading than at high loading. This effect is believed to be the result of an association compound formed between the alcohol and DZEHPA. This compound acts in a manner analogous to that of weak acid when titrated by a strong base. At zero or low loading, a small concentration of free D2EHPA is present in dynamic equilibrium with the association compound. -4s a result of the low concentration of free DZEHPA in the Amsco diluent, the distribution coefficients of metallic ions are low. As additional extractable ions are added to the system and the free D2EHPA molecules are consumed by reaction with these ions, they are replaced by disassociation of the association compound. Therefore. the concentration of free D2EHPA and, consequently, the distribution coefficients of metallic ions
1000
5m
Results and Discussion
The distribution coefficients of strontium as a function of acetic acid concentration in the aqueous phase are shown in Table I. These data show that when the pH is adjusted with acetic acid the distribution coefficients are affected not only by the hydrogen ion concentration but by the acetic acid concentration as well. I t is apparent that the distribution coefficients can be reduced to only a limited degree with acetic acid. This is in contrast to the general agreement with theory which was obtained when the pH was adjusted with strong acid and the concentration of solutes other than calcium and strontium was negligible.
INDUSTRIAL AND ENGINEERING CHEMISTRY
02
, 0
1 2 3 moles 3 F ALCOHOL PER
4 71018
5 OF DZEHPA
6
Figure 1. Distribution coefficients of calcium and strontium between 0.5M D2EHPA and aqueous acetate solution a t pH 2.7 are depressed b y 2-ethylhexanol Effect is much greater at low organic phase loading
STRONTIUM-90 S E P A R A T I O N 500,
Table
Flowsheet 1 2 3 4
5 6
II.
These Successful Flowsheets W e r e Tested in 1 1-Stage Laboratory Mixer-Settler Unit
Organic Phase (Amsco) 2-EthvlDBEHPA hexaiol conc., conc., M M 0.1 0.2 0.5 0.4 0.4 0.4
Aqueous Phase, Scetic Acid Conc., M 0.1
2.5 0.1 2.5
1.0 1.0 0.1 0.1 0.1
are only moderately sensitive to organic phase loading in a system using 2-ethylhexanol in the organic phase. This ability of 2-ethylhexanol to reduce the effect of loading is very useful. Since the calcium-to-strontium ratio in the feed is 20 on a weight basis or 45 on a molar basis and since the calcium must leave the system in the organic stream, the organic phase loading will be much lower in the stages near the aqueous exit than in the stages from the feed point to the organic exit. When the alcohol is present, a much larger fraction of the ultimate calcium-carrying capacity of the organic phase can be utilized without excessive variation in the distribution coefficients between stages than can be utilized in its absence. The alcohol does lower the stage separation factors (ratio of the distribution coefficients) (Figure 2). However, if a stage separation factor of 15.8 (the lowest value in Figure 2) is used to calculate the multiple stage unit separation factor, values of 4 X lo6 and 1 X 10l2 are obtained for the 11- and 20-stage mixer-settler units, respectively, when ideal operation is assumed. This overall separation factor is more than adequate to achieve the desired separation. Six successful flowsheets were tested in the 11-stage laboratory mixer-settler unit. One of these flowsheets was tested in the pilot plant 20-stage solvent extraction system. These flowsheet conditions are summarized in Table 11. Flowsheets 1, 2, and 3 illustrate the use of acetic acid concentration, D2EHPA concentration, and relative flows to achieve usable values for the extraction factors, &O/A in all parts of the cascade. The organic stream in Flowsheet 1 lacks calcium-carrying capacity. However, this flowsheet would be excellent if used with a feed containing a low ratio of calcium to strontium. Flowsheet 2 allows an increase in the organic phase loading a t the expense of increased acetic acid concentration and aqueous phase volume, both of which are un-
Relative Volume Flows Organic Aqueous Feed
Organic Phase Loading at Exit, Grams/ Liter
1 2 2 1 1 1
0.5 1.0 2.5 2.0 4.0 3.0
1 1 1 1 1 1
0.133 0.26 2.1 0.133 0.26 0.166
desirable with regard to the subsequent processing of the SrW. In Flowsheet 3 the calcium and strontium are fed into the system in a large volume of dilute solution. In the stages from the feed point to the organic exit, the distribution coefficients are suppressed by the organic phase loading. In the stages from the feed point to the aqueous exit, the distribution coefficients are higher, but the extraction factor K,O/A is reduced to a reasonable value by a large aqueous flow. This flowsheet has the same disadvantages as Flowsheet 2 but to an exaggerated degree. It would involve Srgolosses during start-up and shut-down unless calcium were added at the feed stage. I t would also be sensitive to moderate changes in the calcium strontium ratio in the feed. In Flowsheets 4 and 5, 2-ethylhexanol was used to lower the value of the distribution coefficients and reduce the effect of loading. Flowsheets 4 and 5 differ only in that the feed rate and, consequently, the organic phase loading are twice as high in 5 as in 4. Flowsheet 6 was run in the full-size 20-stage pilot plant equipment with the organic and aqueous flow rates of 25 liters per hour. This flowsheet was essentially
I 10
I 0
1
2 3 4 5 moles OF ALCOHOL PER mole OF DZEHPP
Figure 2. Ratio of calcium and strontium distribution coefficients between 0.5M D2EHPA and aqueous acetate phase at pH 2.7 is lowered b y 2-ethylhexanol
the same as 4 and 5,'but with intermediate organic loading. The calcium and strontium contents of the organic and aqueous streams leaving the mixer-settlers are given in Table 111. These values are based on samples taken while running at steady state. Flowsheets 1, 4, and 6 were completely tested with both Ca45 and Srss tracer runs. Only the calcium removal was measured in Flowsheet 5. However, since this flowsheet differed from Flowsheet 4 only in that the organic phase loading was doubled and increased loading depressed the distribution coefficients, it is probable that the strontium recovery exceeded that obtained in Flowsheet 4. Only the strontium recovery was measured while
--I
r--I
20-STAGE MIXER SETTLER UNIT
I
-
Sr SOLUTlONy
I
ORGANIC STREAM
STREAM
-
t
L'Co - - FREE _____
6
! 1-STAGE MIXER
-4-
1
0.f N ACETIC
I
---A
SETTLER UNIT
I
--I
1.0N NITRIC ACID
Co IN NITRIC ACID TO WASTE
Producing flowsheet for pilot plant uses 20-stage primary mixer-settler unit VOL. 53, NO. 8
AUGUST 1961
649
AMSCO DILUENT I
STAGE NUNEER 0
1 2 3 4 5 ORGANIC PHASE LOADING (g.OF
6
7
Co PER
liter)
8
Figure 3. Concentration profile of calcium and strontium was determined using flowsheet 6 (Table 11) in 20-stage production mixer-settler
Figure 4. Distribution coefficient of calcium varies greatly with organic phase loading
running Flowsheets 2 and 3. If normal stage separation factors are assumed, calcium removal was good. The cascade profile of the full-scale test of the 20-stage pilot plant facility is shown in Figure 3. These data are based on samples taken after shutdown on the mixer-settler unit after running Flowsheet 6. Some of the lower values may be slightly high because of cross contamination of samples during analysis and from residual contamination from the hot cell equipment. Similar profile data collected during the testing of Flowsheet 5 showed that in some stages the calcium loading of the organic phase had exceeded the theoretical limit of 0.25 mole of combined calcium and strontium per mole of di(2-ethylhexy1)phosphoric acid predicted by the reaction. Laboratory tests of the distribution coefficient of calcium at high levels of calcium loading were performed, and the results are shown in Figure 4. These data show that in the presence of 2-ethylhexanol at high loading, a molecular pecies containing less than four formula units of
Pilot Plant Process. The process flowsheet of the pilot plant is shown (p. 649). The feed solution, containing 1.5 grams of swontium per liter and 30 grams of calcium per liter as the acetate salt, is fed at the rate of 3.3 liters per hour into the stage-9 mixer. The organic stream contains 0.40 mole of D2EHPA per liter and 2.5 moles of 2ethylhexanol per liter in Amsco diluent. The organic phase enters the stage-1 mixer at the rate of 25 liters per hour. The aqueous stream entering the stage-20 mixer consists of 0.1M acetic acid at a rate of 25 liters per hour. The organic stream is scrubbed prior to re-use by countercurrent extraction throughout 11 mixer-settler stages by 1.OM " 0 3 at a rate of 12.5 liters per hour. The calculated decontamination is almost quantitative.
Table 111.
D2EHPA per calcium ion is present. There is little danger of the failure of Flowsheet 5 because of inability of the organic phase to carry the calcium in cases of moderate overfeeding. Back extraction tests were performed to establish the fact that calcium and Y90 (the radioactive daughter of Sr9o) could be successfully stripped from the D2EHPA-alcohol solution by 1.OM " 0 3 . The distribution coefficients obtained and the calculated decontamination factors for 11 countercurrent stages are given in Table IV.
Table IV. Decontamination of 0.4M D2EHPA-2.5M 2-Ethylhexanol in Amsco by 1.OM HNO8
Exptl. Distr.
Calcd. Decontamination Factor for 11 Countercurrent Stages
Element
Cceff., Kd
Ca
0.0176
4
Y
0.097
1
x 101' x 1010
Flowsheet Performance
Calcium and strontium values a r e based an samples taken a t steady state
Flowsheet
Tracer Element
Steady State, Hr.
No. of
Samples
Split of Tracer Element, To In organic In aqueous
Ca Sr
3
Sr Sr
1 1.25
100 0.3 0.6 0.05 100
4.25
Ca
6
Sr
4
Ca Ca
3.25
Sr
650
Time at
4 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.7 99.94 99.95 1.0
0 99.7 99.4 99.95
0 99.3 0.06
0.05 99.0
Literature Cited
(1) Coleman, C. F., Brown, K. B., Moore, J. G., Crouse, D. J., IND. ENG. CHEM. 50, 1756-62 (1958). (2) Coplan, B. V., Davidson, J. K., Zebroski, E. L., U. S. At. Energy Comm. Rept. AECU-2639, Aug. 8,1953. (3) Lamb, E., Seagren, H. E., Beauchamp, E. E., Proc. LT.N. Intern. Conf. Peaceful Uses At. Energy, 2nd, Geneva, 1958, 20, 38 (1959). (4) Peppard, D. F., Ferraro, F. R., Mason, G. W., J. Inorg. @ Nuclear Chem. 7, 231-4 (1958). (5) Peppard, D. F., Mason? G. I%'., Driscoll, W. J., Sircnen, R. J.! Ibid.,7, 276-85 (1958). (6) Peppard, D. F., Mason, G. I$'., Maier, J. L.. Driscoll, W. J., Ibid., 4, 334-43 (1957). RECEIVED for review September 15, 1960 ACCEPTEDApril 11, 1961 Division of Industrial and Engineering Chemistry, 138th Meeting, ACS, New York. Scptember 1960.
