I
H. H. VAN TUYL and R. 1. MOORE Hanford Laboratories Operation, General Electric Co., Richland, Wash.
Recovery of Fission Product Cesium from Acidic Wastes Fission product cesium can be recovered from slightly acidic, high salt solutions by carrier precipitation with zinc ferricyanide, zinc cobalticyanide, or nickel ferrocyanide. After further purification the cesium can be used in radiation sources
PUREX-TYPE
processes for the recovery of uranium and plutonium from spent reactor fuel elements yield a fission product solution which consists principally of nitric acid, with lesser amounts of corrosion products, process additives, and unrecovered uranium and plutonium ( 5 ) . Because fission products are present in relatively high concentrations, this solution is an excellent starting material for fission product recovery. In previous investigations, the solution was neutralized and cesium was recovered from the supernatant by precipitation as cesium zinc ferrocyanide (7). The cesium precipitate so obtained is highly pure-almost half cesium on a dry weight basis. However, the neutralization step involves formation and removal of a rather voluminous precipitate consisting largely of hydrous ferric oxide a n d sodium diuranate. One alternative is to recover cesium from unconcentrated wastes, which do not contain the substantial amount of iron introduced by corrosion of the evaporator. Another is to use ferricyanides or cobalticyanides as precipitants; these reagents do not form insoluble compounds with ferric ion, and are acceptable cesium precipitants in the presence of moderate amounts of iron (S), These processes for recovering cesium from acidic waste solutions were therefore investigated.
Experimental
Synthetic solutions containing either cesium-134 tracer or a small spike of plant waste solution were used for scouting studies and for determining the effects of chemical variables. Inactive elements such as cesium, strontium, and ruthenium were added to bring the total fission product concentration to expected levels and to reveal possible interferences. Analyses were performed by beta, gamma, or gamma spectrometer counting, 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 whether there were any interferences from trace impurities present in plant solutions and whether the precipitates experienced any serious radiation decomposition. Because of the intense radiation, these experiments were carried out remotely behind shielding in over-the-top laboratory brick piles. I n experiments with unconcentrated waste solutions, urea was added as a nitrite suppressor to prevent oxidation when ferrocyanides were used. Other reagents were added, and the solutions were stirred, allowed to stand for 15 minutes, and centrifuged for 5 minutes. I n experiments with concentrated waste
solutions, the solutions were neutralized to pH 0 to 1 with sodium hydroxide, reagents were added, and the solutions were stirred, allowed to stand for 1 hour, and centrifuged for 15 minutes. The supernatant liquids from all experiments .were then sampled and analyzed as described above. In experiments with full activity level solutions, the supernatant liquid was resampled and analyzed after several days to determine the extent of radiation decomposition of the precipitate. Discussion and Results
Solution Compositions. The unconcentrated waste (HAW) is a nitric acid solution containing small amounts of uranium and sodium nitrite (Table I). The concentration of fission products is low. The aluminum content will probably not exceed 0.01M, and only minor amounts of corrosion products are expected. I n further processing unconcentrated waste, much of the nitric acid is recovered by distillation for re-use in the plant. Therefore, recovery processes involving the unconcentrated waste stream should not interfere with acid recovery. The acid concentration should not be changed appreciably, nor should constituents be introduced which would decrease the value of recovered acid. VOL. 51, NO. 6
JUNE 1959
741
Table I. Purex-Type Waste Solutions Contain Fission Products in Nitric Acid Constituent HNOa
U Fe Na NOz-
sod--
The
...
0.3
...
0.4
0.01 0.01
Total fission products
cs
Molar Concentration HAW lWW 2.2 6 0.002 0.03
0.002 0.0001
concentrated
waste
1
...
0.02 0.001
solution
(IWW) is of rather variable chemical composition, depending mostly on the extent of concentration achieved in acid recovery operations and the irradiation history of the uranium being processed. The amount of iron in wastes depends largely on the amount of corrosion of process equipment, and probably increases rapidly if additional acid is recovered by distillation. Chromium and nickel are also introduced as corrosion products from stainless steel equipment and are present in variable amounts. Aluminum may be present in amounts up to 0.1M. Concentrations of salts in concentrated waste solution are dependent on the extent of evaporation in the acid recovery portion of the process. Recovery from Unconcentrated Sufficient Wastes. NITRITEREMOVAL. nitrite is present in unconcentrated waste to oxidize over 0.01M ferrocyanide to ferricyanide, and the dark color of ferricyanide is detectable with as little as 0.0001M nitrite. The investigation of metal ferrocyanides as precipitants for cesium requires that nitrite concentration be reduced low enough to prevent oxidation of ferrocyanides. The addition of 0.01M urea as nitrite suppressor delayed, but did not prevent, oxidation of 0.005M ferrocyanide, but 0.05M urea prevented detectable oxidation. Heating the solution at 90° C. for 10 minutes and cooling to room temperature removed nitrite sufficiently to prevent total oxidation of 0.005M ferrocyanide, but some ferricyanide color was still observed. Hot synthetic unconcentrated waste with no nitrite added oxidized ferrocyanide, so cooling to about room temperature after heating is required before ferrocyanide precipitation. Nitrite removal techniques were not investigated further, as either urea or heating appears to be satisfactory. COMPARISON OF PRECIPITANTS. Nickel ferro- or ferricyanide or iron ferrocyanide seem to be suitable for precipitating cesium from unconcentrated waste, but copper and uranyl ferro- or ferricyanides and zinc ferrocyanide are less satisfactory (Table 11). Precipitate vol-
742
umes with nickel ferrocyanide are much trated waste affects nickel ferrocyanide solubility. Solutions of 0.001M nickel smaller than with the other reagents investigated. ferrocyanide in 2M nitric acid did not FULL ACTIVITYLEVELEXPERIMENTS. form precipitates when potassium, soTo obtain cesium recovery of over 90%, dium, magnesium, calcium, strontium, a large excess of reagent is required cobalt, aluminum, chromium, chloride, (Table 111). With ferric ferrocyanide, sulfate, urea, hydroxylamide, or hydraabout a ninefold excess over stoichiozine was added in concentrations of up metric CsFeFe(CN)e is required; with to 0.1M. However, addition of cesium nickel ferrocyanide, about a sixfold exor ammonium ions at concentrations cess over CszNia [Fe(CN),lz is required, from 0.0001 to 0.1M produced a prein good agreement with tracer level excipitate reproducibly. (Cesium is presperiments. The slight decrease with time ent in the waste, and ammonium ion indicates that radiation effects are small. is introduced by urea hydrolysis.) The precipitates from these experiments Recovery of cesium from unconcenall contained about 5% of the fission trated waste by nickel ferrocyanide is substantially increased by adding amproduct zirconium-niobium, 1% of the monium ion, and the minimum nickel cerium, and less than 2% of the ruthenium, in addition to the cesium. ferrocyanide concentration for adequate EFFECTSOF AMMONIUM ION. Both cesium recovery is reduced by a nickel and uranyl ferrocyanides are solufactor of about 5 (Table IV). The mechanism for this improved cesium ble in 2M nitric acid to the extent of recovery is not certain, but may involve about 0.002M. I n contrast, precipitacoprecipitation with ammonium nickel tion of ferric and cupric ferrocyanides ferrocyanide. I n contrast, cesium reand nickel ferricyanide is reproducible, covery on ferric ferrocyanide or nickel even at 0.0001M. Precipitation of ferricyanide precipitates is somewhat 0.001M nickel ferrocyanide in water is reduced by addition of ammonium ion reproducible, but on acidification to 2M with nitric acid, the precipitate dissolves. possibly, by formation of the compounds This is in apparent conflict with other NH4FeFe(CN)6 and NH&iFe(CN)e indata (Tables I1 and 111), and suggests stead of the corresponding cesium comthat some other constituent of unconcenpounds.
Table II.
Nickel Ferro- and Ferricyanides Are Suitable as Precipitants for Cesium from Unconcentrated Wastes (Synthetic HAW, nitrite suppressed with 0.1M urea mThen ferrocyanides were used)
Metal Ion Nickel
Metal
Molar Concentration FerroFerricyanide cyanide
0.001 0.0004 0.00075
0,0005
0.0002
... ...
0.0003
Ferric
0.003
0.0015
0.0015
0.0008
...
0.0015
Cupric
Uranyl Zinc
0.002
0.001
0.001 0.0015
0.0005
0.002
0.001
...
0.0015
...
0.001
0.0005
... ... 0.0005
0 0002 9
Cesium Recovery,
Precipitate Volume,"
%
%
97 30 95 20
0.1 0.1
0.5 0.2
... ... 0.001
95 90 0
0.5 No precipitate
... ...
98
0.5
55
0.2
40
1
20
0
0.5 0.5
30
0.2
0.001
... ...
0.001
1
As % of initial solution volume.
Table ill.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Metal Ion Ferric
Nickel
High Cesium Recovery from Full Level Unconcentrated Waste Requires Large Excess of Reagent Molar Concentration Metal Ion Ferrocyanide 0.005
0.004
0.002 0.001 0.0003
0.0015
0.00075 0.0002
0.005
0.0025
0.002 0.001 0.0004
0.001 0.0005 0.0002
5 min.
Cesium Recovery, % 5 hr. 14 days
99.4 98 86
99.6 98 94
67
70
99.7 98 99
99.2 98 99 70
70
98 92
76
..
99 98 98
..
FISSION P R O D U C T CESIUM R E C O V E R Y Ammonium Ion Affects Recovery of Cesium (Synthetic HAW, 0.1M urea added when ferrocyanides used)
Table IV.
Nickel
Ferric
... ... ... ... ... ...
0.002
0.0004 0.0002
0
0.001 0.01 0.1
e..
,
. e .
0.0003
e..
0 * 0002 0.0002
0
0.0001 0.0001
0 0.001 0.01 0.1
... ... ... ...
0 0.001 0.01 0.1
0.0002 0.0002 0.0002 0.0002
Table V. Zinc Ferri- and Cobalticyanide Are Superior to Ferrocyanides . as Precipitants for Cesium (Synthetic lWW, neutralized with sodium hydroxide to 1M acid. 0.01M anion and stoichiometric amount of metal ion) Cesium Recovery Precipitant
%
Ferrocyanide only Nickel ferrocyanide Zinc ferrocyanide
70 79 67
Ferricyanide only Nickel ferricyanide Zinc ferricyanide
2.5 81 89
Cobalticyanide only Nickel cobalticyanide Zinc cobalticyanide
2.1 82 88
Precipitate Volume, 7% 5.5 5.5 5.0 1.0 23 1.8
.
0.001 0.001 0.001 0.001
0 0.01
0.01
... ... ...
0.0015
... ... ...
Molar Concentration Ammonium Ferrocyanide
1.0 21
2.0
Recovery from Concentrated Wastes. COMPARISON OF PRECIPITANTS. A variety of ferrocyanides, ferricyanides, and cobalticvanides were investigated. Zinc ferricyakde and zinc covbalticyanide were generally superior to the ferrocyanides (Table V). Of the ferrocyanides, nickel ferrocyanide is superior to zinc ferrocyanide, or to ferrocyanide alone, and precipitate volumes are comparable for all of the ferrocyanides investigated. These data agree with previous work on cesium recovery from partially neutralized concentrated wastes (Z), which showed that nickel, copper, or ferric ferrocyanides are satisfactory cesium precipitants, nickel being slightly superior to the others. Although no cesium recovery values approached the 95% which was set as a reasonable goal, nickel appears to be the best cation to use with ferrocyanide for cesium recovery from concentrated waste. No significant differences between
'
Cesium Recovery, Ferricyanide
... ... ... ... ... ... ... ...
0.001 0.001 0.001 0.001
... ... ... ...
% 96 98 99.5 99.9 30 99 15 95 97 96
95 91 51 46 42 31
ferri- and cobalticyanides are apparent. Small precipitates are obtained with ferri- or cobalticyanides either alone or with added zinc, but with the addition of nickel the precipitate volumes increase excessively. Best cesium recovery was obtained with the zinc compounds, but nickel was only slightly poorer in this respect. From both recovery and precipitate volume considerations, however, it appears that zinc is much superior to nickel. The data with cobalticyanide agree rather well with previous work by McKenzie (6), who showed that zinc and cobalt cobalticyanides are superior to nickel and copper as cesium precipitants, but that ferric cobalticyanide does not precipitate cesium appreciably. The presence of nickel in concentrated waste as a result of corrosion of stainless steel equipment could causO large precipitate volumes in cesium recovery operations with ferri- or cobalticyanides as precipitants. However, experiments with full activity level concentrated waste containing the expected amount of nickel, and using zinc ferri- and cobalticyanides as cesium precipitants, resulted in good cesium recovery and small precipitate volumes. This demonstrates that the presence of nickel in waste solutions will not increase the volume of zinc ferri- or cobalticyanide precipitates excessively. Because nickel ferrocyanide is inferior to zinc ferricyanide and zinc cobalticyanide from considerations of cesium recovery, precipitate volume, and expected iron interference, only zinc ferri- and cobalticyanides were selected for further investigation. EFFECTSOF EXCESSREAGENTS. Recovery with ferricyanide is best with high excess zinc, but the precipitate volume is also significantly larger under these
conditions (Table VI). Precipitation with stoichiometric amounts of reagents gives almost as good recovery and only one third as much precipitate. Both recovery and precipitate volume with cobalticyanide are best at stoichiometric conditions, and become slightly poorer with excess zinc ion. With both ferriand cobalticyanide, an excess of anion reduces cesium recovery and increases precipitate volumes markedly. This increased volume probably results from formation of insoluble compounds of iron, uranium, or aluminum with ferrior cobalticyanides. Therefore, the use of stoichiometric amounts of reagents appears to represent the best compromise. EFFECTSOF MAJORWASTE STREAM CONSTITUENTS. Recovery with nickel ferrocyanide is affected only by very high concentrations of acid, or by iron and uranium concentrations in excess of 0.1M (Table VII). At the concentrations of these constituents expected in concentrated waste, only iron will constitute a serious interference. However, because of the rather large amount of iron present in concentrated waste, the use of nickel ferrocyanide does not appear attractive.
Table VI. Stoichiometric Amounts of Reagents Represent the Best Compromise for Cesium Recovery (Synthetic lWW, neutralized to 1M acid with sodium hydroxide) Cesium PreMolar Concentration R ~ - cipitate Ferri- Cobaltz covery, Volume, Zinc cyanide cyanide yo % 0.1 0.05 0.03 0.015 0.015 0.015 0.015
0.01 0.01 0.01 0.01 0.02 0.05 0.1
0.1 0.05 0.03 0.015 0.015 0.015 0.015
... ... ... ... ... ... ...
...
... ... ... ... ... ...
90 89 85 84 82 70 39
4.5 4 3 1.5
0.01 0.01 0.01 0.01 0.02 0.05 0.1
86 88 87 90 86
3 3 2.5 2 2.5 13 20
66
57
4
5a 5a
Very dark solution. Precipitate did not settle or centrifuge well. Difficult to determine volume. LI
With ferri- and cobalticyanides, high concentrations of nitric acid, sodium nitrate, and ferric nitrate reduced cesium carrying. The combination of high sodium nitrate and 0.1M nitric acid reduced cesium recovery even more significantly. Uranium, aluminum, and sulfate do not affect recoveries appreciably, and can therefore probably be tolerated in the expected concentrations. The effect of iron is highly dependent VOL. 51, NO. 6
JUNE 1959
743
on the concentration, with 0.1M iron being innocuous but 0.3M being highly detrimental. Because small amounts of iron have little effect on cesium recovery, diluting the concentrated waste to reduce the iron concentration or modifying the waste concentrators to minimize corrosion may permit more satisfactory cesium recovery. With the concentrations of constituents listed in Tabie I, recoveries are expected to be about 80% for both ferri- and cobalticyanides, which is somewhat below the desired 95% recovery. FULL ACTIVITYLEVELEXPERIMENTS. Full level concentrated waste was neutralized to about p H 0 with sodium hydroxide and divided into two portions. One portion was diluted with an equal volume of water. Samples of full level concentrated waste were made 0.02M in zinc and 0.01M in ferricyanide or cobalticyanide. Samples of diluted concentrated waste were made 0.01M in zinc and 0.005M in ferricyanide or cobalticyanide. Thus the ratio of zinc and anion to cesium was maintained constant in the two sets of experiments. The solutions were stirred, allowed to stand for 1 hour, and centrifuged for 15 minutes. Samples were taken immediately after centrifugation, and after
Table VII.
Reagent
Cesium Recovery Is Affected by Acid and Salt Concentrations Cesium Recovery, % Concentration. Kickel Zinc Zinc M ferrocyanide” ferricyanideb cobalticyanideb
3
>99 >99 >99
10
95
92 45
6 6
>99d
98C 99
9 7c
99d
980 9W
97c 89e
10-4
0.1 1
NaNOs NaN03
standing for 2, 4, and 8 days (Table VIII). I n both sets of experiments, recovery with ferricyanide is better than with cobalticyanide, and dilute concentrated waste is better than full level concentrated waste. From tracer level experiments it appears that the improved recovery with dilution results from both a reduction of ferric ion concentration and a reduction in total ionic strength. The data agree well with tracer level experiments, and no radiation decomposition of the precipitate with time is apparent. The precipitates from high activity level experiments were washed with dilute nitric acid to remove adsorbed fission products. The nitric acid washes removed over 9070 of the cerium, strontium, uranium, plutonium, and curium associated with the precipitates. About half of the zirconium and niobium but only very little ruthenium were removed. About 10% of the precipitated cesium was removed from the precipitates from full level concentrated waste, and 4% was removed from the precipitates from diluted concentrated waste. These data show that retention by the precipitates is strong with cesium and ruthenium, smaller with zirconium and niobium, and not ap-
+ Na2S04
6 6
>99
99 97
98 95 90 32
4- 0.5
+ 0.5
>99d
0.1
95d
971 966
92e 92
0.01 0.1 0.3
99d 90d 79d
964
916
946 816
91’
96e 96O
918 918
0.01
Al(N0a)sf
>99
0.01 0.1
80”
10d4M cesium and 0.005M nickel ferrocyanide. 0 . O O l M cesium and 0.005M zinc ferricyanide or cobalticyanide. pH 4. 1M nitric acid. 8 0.1Mnitric acid. Plus 5M sodium nitrate and 0.4M sodium sulfate.
a
~
~~
Table VIII. Solution Full level Diluted
744
~~~~
~
Recovery of Cesium from Full Level Concentrated Waste Is Improved b y Dilution Anion 1 Hour 2 Days 4 Days 8 Days Ferricyanide Cobalticyanide
89 72
86 75
89 80
Ferricyanide Cobalticyanide
98 94
97 94
96
INDUSTRIAL AND ENGINEERING CHEMISTRY
94
91 83 95 96
preciable with other activities. The washed precipitates contained, in addition to cesium, about 10% of the amounts of zirconium and ruthenium originally present in the solution, 2% of the niobium, and less than 1% of the original amounts of cerium, strontium, uranium, plutonium, and curium. Conclusions
At least of the cesium in concentrated waste can be recovered by precipitation with zinc ferricyanide or cobalticyanide, but somewhat poorer recoveries and much more voluminous precipitates can be expected with nickel ferrocyanide. While any of these three precipitants may be used for cesium recovery, zinc ferricyanide appears to be best. The precipitate volumes in each case are rather large compared to the basic-side cesium zinc ferrocyanide process because a large excess of reagents must be used in the acid-side process. This causes the cesium content of the precipitate to be considerably lower than desired for direct use as radiation sources. Therefore cesium must probably be further concentrated for such use. The major advantage of the acid-side process as compared to the basic-side cesium zinc ferrocyanide process (7) is the elimination of the hydrous oxide precipitation step. The disadvantages include the relatively low specific activity and poor radiochemical purity of the cesium product. If radiation decomposition occurs on prolonged storage (4, requiring further processing to a stable cesium compound ( 3 ) , these objections are less serious. Therefore, the feasibility of cesium recovery from slightly acidic waste solutions has been demonstrated on a laboratory scale. literature Cited (1) Barton,
G. B., Hepworth, J. L., McClanahan, E. D., Jr., Moore, R. L., Van Tuyl, H. H., IND.ENG. CIIEM. 50, 212 (1958). (2) Burns, R. E., Clifford, W. E., Brandt, R. L., “Recovery of Fission Products from Hanford Wastes. Recovery of Cesium from Purex Wastes,” Hanford Lab. Operation, HW-31444, (April 1954). (3) Goodall, C. A., “Recovery of Fission Product Cesium from Chemical Processing Waste Solutions Containing Aluminum Nitrate,” Zbid., HW-49658 (April 1957). (4) Hepworth, J. L., McClanahan, E. D., Jr., Moore, R. L., Office of Technical Services, Dept. Commerce, Washington 25. D. C.. HW-48832 (June 1957).
cedures for Handling Purex and Redox High Activity Waste Streams,” HW47761 (January 1957). RECEIVED for review August 2 5 , 1958 ACCEPTED January 13, 1959 Northwest Regional Meeting, ACS, Portland, Ore., June 1958.
