I
G. B. BARTON, J. L. HEPWORTH, E. D. McCLANAHAN, J.r, R. L. MOORE, and H. H. VAN TUYL Hanford Atomic Products Operation, General Electric Co., Richland, Wash.
Chemical Processing Wastes
Recovering Fission Products INTEREST
has increased sharply in penetrating radiation and its practical uses. Several investigations have been aimed at using beta or gamma radiation or x-rays to improve natural materials or develop entirely new materials (4, 6, 7, 9, 73, 77). Industrial radiography and medical treatment also require large gamma sources ( 7 ) . Generally, gamma radiation would be employed where deep penetration is desired, and beta sources would be used for surface irradiation or for thin materials. Beta radiation also has special uses such as in atomic batteries and luminescent devices (70). A variety of radiation sources have been used experimentally and might well be employed in large-scale application. Each source could have advantages for particular application. Market studies (8, 74) (admitted11 uncertain) suggest that all are competitive and that if any of these applications find general use, immense quantities of ionizing radiation would be required. Fission products may be generally used in either of two ways-i.e., as mixed fission-product sources or as separated individual fission products. The use of cooling fuel elements is an example of a mixed fission product source, whereas cesium-137 is an example of a separated source material. The mixed source, where it can be used, may be the cheapest, but it has several distinct disadvantages such as the changing spectrum and intensity of radiation with consequent variable depth dose characteristics as a function of time. Such a source will also be of relatively low specific activity and may suffer from appreciable internal absorption of energy if the fission products have not been separated from the uranium and other extraneous materials which make u p a fuel element, or from the inert materials often present in waste solutions. There are also troublesome administrative difficulties of an accountability and criticality nature where a source contains fissionable uranium or plutonium. From considerations of fission yield and half life, cesium-137 seems to be the nuclide of choice as a gamma source for industrial use. The long half life and convenient gamma energy are nearly optimum for many applications. Sim-
21 2
ilarly, strontium-90 is the logical candidate if a long-lived energetic beta source, uncontaminated by gamma radiation, is desired, Zirconium and niobium have an inconveniently short half life but account for most of the gamma energy in short-cooled plant waste (1 Year or less) and therefore account for most of the gamma dosage from a mixed fissionproduct source. Where long usage is not required or where the source can be periodically replenished, zirconium-niobium may well be economically competitive with cesium-137 or cobalt-60 ( 5 ) . Ruthenium-I 06 and cerium-144 are probably of lesser interest except for special applications such as in fabricating heat sources for remote use. Therefore, effort in these laboratories has been primarily directed toward developing flowsheets for recovering radio-cesium from waste solutions. For recovering uranium and plutonium from spent reactor-fuel elements. solvent extraction techniques are generally employed-tri-n-butyl phosphate is commonly used as the solvent with nitric acid as salting agent. This is recovered by distillation for re-use. The bulk of the fission products are eliminated in the aqueous waste from the first contactor and will be found in the bottoms from the nitric acid recovery unit Where uranium fuel elements are employed and jackets or cladding material are removed prior to dissolution, this concentrated waste solution will consist principally of nitric acid with small amounts of corrosion products, process additives, and minor quantities of waste uranium and plutonium. Such a solution is nearly ideal as a raw material for recovering specific fission products. Composition of the typical plant waste (designated IWW) employed in this investigation is as follows:
INDUSTRIAL AND ENGINEERING CHEMISTRY
Composition of 1W W Waste Constituent "03
Fe~(S0d3 Uo~(N03)z NaNOn Na~S04 Total fission products a
Molarity 7.6
0.053 0.02 0.38 0.05 -O.Ola
Cesium concn., about 0.0006.1.1.
Besides this work, a substantial effort on fission-product recovery has been carried on a t several other AEC sites, particularly at Arc0 (2) and at Oak Ridge where Rupp and his associates have developed a successful cesium recovery process (72) based on the fractional crystallization of cesium aluminum sulfate (cesium alum). This process, already carried through the pilot plant stage and used for the production of kilocurie cesium sources, has the important advantage of being applicable to a wide variety of feed solutions. The major drawbacks seem to be that the alum precipitate is both bulky and not very selective. Hence, crystallizations must be repeated numerous times to get a pure and concentrated product. In the present work, because only a single fairly simple waste had to be dealt bvith? a more selective precipitant and a large volume reduction seemed desirable. Although Hanford is a rather recent entry into the fission-product recovery field, it has a long history of wastetreatment research. A large amount of work was done and a plant process developed for removing cesium and strontium activities from alkaline plant wastes by scavenging with ferrocyanides of iron or nickel (75). This experience was valuable in developing the present recovery process, because preliminary experiments with a variety of potential precipitants including tetraphenylboron, cobaltinitrite, silicotungstate, and ferriand ferrocyanides indicated that the latter were most promising. Although the two fields are related, the objectives and ground rules in waste treatment and fission-product recovery are distinctly different. I n waste treatment, long-lived or biologically damaging radioactive nuclides must be removed to negligible levels before the waste can be discharged. This may require reducing the concentrations to low levels such as 0.1 microcurie per milliliter or perhaps even to drinking water tolerances. These large decontamination factors, of the order of a thousand to a million or higher, require multiple operations and large excesses of precipitant (3). For recovery, however, a high specific-activity product is desired and a yield of 90 to 95%, corresponding to decontamination factors of 10 to 20, is
satisfactory. Removing this fraction of activity may simplify waste-storage, but this is not a primary objective. The immense quantities of waste now in storage tanks a t Hanford were not seriously considered for fission product recovery. It was felt that removal from the tanks would be an expensive procedure and that extraneous materials present would cause serious process complications. Current production should be ample to satisfy any probable demand for the foreseeable future, Experimental
Scouting studies and experiments to determine the effects of various chemical variables were performed with synthetic solutions traced with either Oak Ridge tracers or a small spike of plant-waste solution. Inactive elements such as cesium, strontium, ruthenium, and zirconium were added to bring the total fission-product concentrations to flowsheet level and to reveal interferences which these might cause. Analyses, performed by beta, gamma, or gamma spectrometer counting, were preceded, when necessary, by standard radiochemical separations. Following the tracer level experiments, the flowsheets were further tested on full level plant-waste solutions to determine if trace impurities present in plant solutions interfered or if the precipitates experienced any serious radiation decomposition. Because of intense radiation, these experiments were done remotely behind shielding in over-thetop type laboratory brick piles. Discussion and Results
In preliminary experiments, the metal ferro- and ferricyanides were the most promising of several reagents evaluated for recovering fission-product cesium. These findings were supported by the fact that nickel ferrocyanide was satisfactory in the large-scale waste treatment of certain first cycle Hanford waste solutions and that no objectionable radiation-induced decomposition had occurred Therefore, these reagents were further tested, the effects of variables determined, and flowsheets for recovery and packaging were evaluated. Comparing Cesium-Scavenging Effectiveness from Acid Solution. A series of coformed ferro- and ferricyanides was compared for completeness of cesium precipitation from a 2.5M nitric acid solution. This would simulate recovery from an unconcentrated firstcycle waste stream where nitric acid has not been recovered or from a concentrated waste after partial neutralization of excess acid (Table I). Solutions of the metal ion and ferro- or ferricyanide were added separately in chemically I
equivalent quantities to a cesiumcontaining solution of the indicated composition, stirred, allowed to digest at room temperature for 1 hour, and then centrifuged and assayed. Nickel ferro- and ferricyanide gave about 95% cesium recovery with only 0.001M nickel. Iron(II1) and copper(11) ferrocyanide are also fairly effective, but inferior to nickel. In each case, the cesium recovery decreases rapidly with decreasing concentration of precipitant. Other ferro- and ferricyanides tested were much less effective in removing cesium from qcid solution. Bulk volume of the centrifuged precipitates differ significantly and nickel ferrocyanide gave the smallest volume-Le., the most favorable volume reduction. This is important because it determines the size of filtration or centrifugation equipment required for plant application. Variables i n Nickel Ferrocyanide Scavenging of Acidic Wastes. The nickel ferrocyanide concentration used in Table 11, 0.005M, is greater than desired for obtaining a high specific activity in the precipitate; consequently, poor recovery under these conditions indicates a serious interference. With nitric acid alone, cesium recovery was quite high and reproducible except in 10M acid where the ferrocyanide was oxidized rather rapidly to ferricyanide. In the other experiments in Table 11, the acid concentration was not critically important in the range 0.1 to 3M. With 0.1M iron added before cesium scavenging, the recovery dropped to only 90%. The uranium concentration expected in the waste will also cause a slight reduction in cesium recovery, but does not interfere nearly as much as iron. Therefore, direct scavenging from unneutralized 1WW waste does not appear attractive. Sodium sulfate and nitrate at the expected concentrations do not reduce cesium recovery appreciably. Ammonium nitrate, which would be introduced into the process by the use of gaseous ammonia or urea hydrolysis io precipitate the iron as ferric hydroxide (to remove this interference), reduces cesium recovery only slightly, even a t high concentrations. Further experiments together with the earlier work of Burns, Clifford, and Brandt in this laboratory indicate that somewhat higher concentrations of scavanger (0.01M nickel and 0.005M ferrocyanide) will remove about 90% of the cesium from 1WW neutralized to 1 M acid, but that poorer cesium recovery is obtained with other metal ferrocyanides, or with 1WW before neutralization. Since the plant waste contains all the other fission products along with the cesium, the radiochemical purity of the recovered cesium will also be poor. The interfering iron can be removed, however, by neutralizing the
waste prior to ferrocyanide scavenging. This has an added advantage-most of the fission products, other than cesium, will be carried down on the ferric hydroxide precipitate, thus improving the purity of the cesium product. Cesium-Scavenging Effectiveness of Ferrocyanides from Neutralized Waste Solutions. Because of the difficulties described, which decreased specific activity and radiochemical purity of the product when cesium precipitation was made from acid solution, the various scavengers were also evaluated a t higher pH. This corresponds to partial or complete neutralization of the waste with ammonia or caustic to precipitate the iron and uranium, followed by recovery of the cesium from the neutralized supernate (1WW-NS) (Tables I11 and IV). Table 111shows the minimum concentration of cation required to precipitate 95% of the cesium under the indicated conditions and employing slight excesses of ferrocyanide. Table IV indicates the acid range over which reasonably good cesium carrying is observed. Nickel ferrocyanide has the widest useful acid range but zinc ferrocyanide scavenges effectively at the lowest concentration and gives a product with the highest cesium content, For this reason, attention centered on zinc ferrocyanide in subsequent flowsheet development. The precipitate with zinc is somewhat slow in forming a t the low concentrations investigated. About an hour is required for maximum cesium recovery. Heating to 80" C. decreases the time for equilibrium to about 15 minutes, but this also decreases cesium recovery somewhat. Composition of Cesium Zinc Ferrocyanide Precipitate. Composition of the cesium nickel and cesium copper ferrocyanide precipitates described corresponded approximately to CszM3[Fe(CN)6]2 and that with iron(II1) to C S F ~ ( C N ) ~ .Those with the other ferrocyanides (cobalt, cadmium, iron(II), and manganese) are similar to nickel and copper, but with an additional excess of MPFe(CN)e. With zinc, however, the precipitate appears to be a definite compound, at least when the zinc, cesium, and ferrocyanide are added in roughly stoichiometric amounts. This analyzes as CszZnFe(CN)e, and appears to have a lower solubility than zinc ferrocyanide. I t gives a sharp detailed x-ray diffraction pattern which indicates that it is isomorphous to potassium zinc ferrocyanide [ K Z n F e (CN)e] (77). The crystal is face centered cubic (Figure 1)' with a cell parameter of 10.2 A. us. 9.98 A. for the potassium compound. The calculated crystallographicdensity is about 3.44 grams per cc. In other experiments, with a thermal balance, cesium zinc ferrocyanide which has been dried in air a t room temperature was found to contain approximately two molecules of water. One molecule VOL, 50, NO. 2
FEBRUARY 1958
213
Table I.
Precipitation of Cesium with Coformed Metal Ferro- and Ferricyanides Metal Ion C'oncn , .If
Cs lie( ,
P p t , Vol
70
%
0.001 0.0005
97 30
0.1 0.1
0 * 002
95 90
1.0 0.5
0.5
0.001
98 55
Zinc
0.001
30
0.2
Uranyl
0.002
20
0.5
0.001 0.0005
95 20
0.5 0.2
Precipitant
Ferrocyanidea Nickel Iron(II1)
0.001
Copper (11)
0.002
Ferricyanide Nickel
0.001
0
0.0
Copper(I1)
0.002
40
1.0
Uranyl
0.002
0
0.5
('eiiuiii aiid 0 131 urea present
is lost at 100' C.; however, it is necessary to heat to about 190' C. to remove the second. The degree of hydration does not have appreciable effect on the diffraction pattern. The freshly precipitated cesium zinc ferrocyanide is white in color but changes to blue on drying. Process Variables in Cesium Zinc Ferrocyanide Precipitation. Effects of the more important process variables which have not been previously discussed as well as possible interferences are shown in Tables Lr and VI. The optimum pH range for cesium recovery with cesium zinc ferrocyanide is from about 2 to 7. Recovery is relatively insensitive to p H in this range but drops off rapidly at higher and lower acidities. The effect of excess zinc or
Table II. Interferences on CesiumCarrying of Nickel Ferrocyanide (10-4M Cs, 0 0 0 5 X nickel ferrocyanide, 1.M nitric acid unless otherwise specified) Constituent
"01
Molal Conm
Recovery,
10 -4 1
99.5 99.5 99.5 95
3 10"
Fe(N03)~
0.01 0.1
99 90
UOz(N03)z
0.01 0.1
99 95
NaA3Od
0.4
99
NaN03
8
99.5
NHaN03
1
99 98
5 a
%
Extensive oxidation.
2 14
ferrocyanide, over that corresponding to the stoichiometry of the precipitate, is shown in Table V for a typical pH of 4 . 5 . The most complete cesium recovery is obtained at the stoichiometry point, and decreases with either excess zinc or ferrocyanide. At a very high pH (8.5), best cesium recovery with zinc is obtained with a large excess (fiftyfold or more) of ferrocyanide. Effects on cesium recovery of other ions which may be present in the 1WWS S were investigated (Table VI). Only oxalate and carbonate affect cesium recovery appreciably, and these will not be present in the IWW-NS under normal conditions. Acidification to remove carbonates after neutralization may be required in some cases. Final pH adjustment \vith carbonate-free caustic should then circumvent carbonate interference. Removal of cesium from synthetic 1WW-NS was also measured at lower cesium concentrations. The solutions at p H 5 were made equimolar in zinc and ferrocyanide, allowed to stand 1 hour at room temperature, and centrifuged for 1 5 minutes. In one of the series shown, a constant concentration of zinc ferrocyanide was employed and the cesium concentration varied. In the other, the ratio of zinc ferrocyanide to cesium was held constant while both concentrations were decreased. As indicated in Table VI, cesium recovery is satisfactory in most cases, but the specific activity is somewhat low at low cesium concentrations. Cesium Recovery Flowsheets. I n the flowsheet of Figure 2, solution is neutralized to precipitate iron, uranium, and most of the fission products. The pH of the supernate is then adjusted to the proper range for cesium-zinc ferrocya-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2" C'
0.2
Iron(II1)
2 531 nitriL acid, lO-'.lf
F.
@
Figure 1.
cn
Cesium zinc ferrocyanide
Molecular weight, 547, cesium, 49 w:ight
70
Table 111. Metal Ferrocyanides Precipitants for Cesium
as
(Neutralized wajte) (Pynthetit 1WK-KS pH 4 4 X 10 '11 ~ebiuiii Metal ion -light exce-s of fetiocyanide)
Conm JI a
Metal 1011
a
*
