Liquid-Liquid Extraction of Uranium and Plutonium from Acetate Solution with Triiso-octylamine Separation from Thorium and Fission Products F. L. MOORE Oak Ridge National laboratory, Oak Ridge, Tenn.
,A new and rapid method for the liquid-liquid extraclion of uranium and plutonium from acetate solution is based on the use of triiso-octylamine dissolved in xylene or other organic solvents. Uranium and/or plutoniumare separated from thorium, alkalies, alkaline earths, rare earths, zirconium, niobium, ruthenium, iron, protactinium, americium, and other elements which d o not form anionic species under the conditions described. The technique may b e used for either tracer or macro quantities of uranium. Several practical applications of the method in radiochemical analysis and purifications are proposed.
0
the striking properties of the uranyl ion in aqueous solution is the capacity of forming anionic complexes. Previous work has described the selective liquid-liquid extraction of anionic uranyl species from sulfuric acid (2, S), hydrochloric acid (3, 4), and nitric acid (3, 5) solutions with long-chain amines. This paper describes recent studies on the selective extraction of uranium from acetic acid solution with several of these reagents. The extraction of the uranyl ion from acetic acid solution with longchain tertiary amines was first observed (3) in 1950. The present approach was to employ an anion capable of forming extractable anionic complexes with the uranyl ion but possessing relatively low affinity for forming similar complexes with other elements. From chemical considerations, i t appeared reasonable that there would be even greater selectivity in the acetate system than in the sulfate, chloride, or nitrate systems. The existence of anionic uranyl triacetate is well known ( I ) . Most of the present work involved the use of the long-chain tertiary amine, triiso-octylamine, dissolved in xylene and previously described (4). NE OF
The mechanism of extraction is thought to be of this type: R3S
+ HUOz(CH3COO)a R3NHU02 (CHaCOO)3
where R& = a tertiary amine. The tertiary amine and its salt with uranyl triacetate are essentially insoluble in aqueous solutions but possess high solubility in most organic solvents. EXTRACTIONS FROM ACETIC ACID SOLUTION WITH TRIISO-OCTYLAMINE-XYLENE SOLUTION
Uranium-233 Tracer. The extraction of uranium-233 tracer with 5% (w./v.) triiso-octylamine-xylene a s a function of acetic acid concentration is shown in Figure 1. Aqueous solutions of varying acetic acid concentration containing 2 x lo4 a counts per minute per ml. of uranium-233 tracer were extracted with equal volume portions of 5% triiso-octylamine-xylene for 2 minutes a t room temperature. Extractions were performed in 50-ml. glass centrifuge cones using high-speed motor stirrers with glass paddles. After centrifugation for 1 minute in a clinical centrifuge, each phase was analyzed for uranium-233 tracer. Figure 1 indicates that it readily extracts from dilute acetic acid solution. Two extractions from 0.5111 or 1M acetic acid yielded essentially quantitative
recovery of the uranium-233 tracer. The decreasing extraction with increasing acetic acid concentration probably results from the repression of the ionization of the acetic acid to cause a depletion of acetate ions available for the formation of the extractable uranyl acetate species. I n early experiments, some mechanical difficulty was encountered with foam formation in the amine-acetate system which the substitution of other common solvents for xylene did not alleviate. Further experiments indicated the following to be efficient foam preventers or defoamers but with certain disadvantages : Mineral acids, including hydrochloric, nitric, or sulfuric, interfere with the formation of extractable uranyl acetate species and also tend t o lower the selectivity for uranium because certain metal ions extract from these acids.
Table 1. Effect of Ammonium Acetate Concentration on Extraction of Uranium2 3 3 Tracer from 0.5M Acetic Acid
Aqueous UraniumPhase. 233 -CHi-’ Tracer, COOXHa, Extracted, M % Remarks ... 95 2 Foamy emulsion, slow phase separation 0 05 9 3 . 6 Rapid phase separation, slight amount of foam 0 10 90.4 Excellent mechanical behavior 0.20 84.3 Excellent mechanical behavior
Acetone in small amounts readily broke the foam, but it possessed the disadvantages of high vapor pressure - L L and flammability. 1 _ -~ L-L C 2 4 6 8 4 0 4 2 ’ 4 Various salts. such as ammonium ACETIC ACIC CONCENTRATION, M acetate, are particularly useful. Although ammonium acetate buffers the Figure 1. Extraction of uranium-233 tracer with 5% triiso-octylaminesolution, it decreases the extraction of xylene from acetic acid solution uranium (Table I). Equal-volume porVOL. 32, NO. 9, AUGUST 1960
1075
Table II. Effect of Various Reagents for Stripping Uranium-233 Tracer from 5% Triiso-octylamine-Xylene Solution
Strippant
Uranium-233 Tracer Stripped, % ’ 86.5 95.55 89.2
79.8 91.3
96.2 ~. ~
98.2 98.4 >99.9 >99.0 Second stripping removed >99%.
tions of O.5M acetic acid solutions containing uranium-233 tracer and varying concentrations of ammonium acetate were extracted for 2 minutes with 5% triiso-octylamine-xylene. After centrifugation for 1 minute, the phases were analyzed for uranium-233 tracer.
Table 111. Extraction of Uranium-233 Tracer from 0.5M Acetic Acid Solution as Function of Triiso-octylarnine Concentration
Concentration of Triisooctylamine in Xylene,
% 5
10 20
30
Uranium-233 Tracer Extracted, % 94.3 98.0 99.0 99.1
5 minutes was 93.4, 94.0, 93.5, and 93.9, respectively. Two-minute scrubbing periods of the organic phases with equal-volume portions of distilled water and 0.5M acetic acid solution gave losses from the organic phases of 27 and 2%, respectively. Several aqueous solutions may be used t o strip the uranium-233 tracer from the 5% triiso-octylamine-xylene solution (Table 11). The organic phases containing 5 X lo4 a counts per minute per ml. were stripped for 2 minutes with equal-volume portions of various stripping agents. The data in Table I1 also reflect the fact that the uranyl ion forms organic soluble anionic complexes in dilute sulfuric acid and higher concentrations of nitric acid with longchain amines. Similarly, neither would moderate t o high concentrations of hydrochloric acid be expected to strip the uranium from the organic phase (4). Ammonium bicarbonate is an excellent strippant for uranium in the acetate system and it possesses the added advantage (over other alkali salts) that ammonium salts may be readily volatilized to give essentially solid-free plates requisite for precise
Table IV. Extraction of Macro Quantities of Uranium from Acetic Acid with Triiso-octylamine-Xylene Solution
Triisooctylamine Acetic in Acid, Xylene, M % 0.5 20 0.5
0.5 Uranium-233 tracer could be extracted moderately well from 1M ammonium acetate by increasing the concentration of the triiso-octylamine in xylene. Extractions of lil1 ammonium acetate solutions containing uranium233 tracer for 2 minutes with equalvolume portions of 5, 10, 20, and 30% triiso-octylamine in xylene yielded recoveries of 14, 35, 59, and 70%, respectively. The addition of 3% (v./v.) butyl Cellosolve t o the 5% triiso-octylaminexylene solution produced a solvent which gave good mechanical behavior in the acetate system. Therefore, butyl Cellosolve was used as a solvent modifier in subsequent work, unless otherwise stated. The extraction of uranium-233 tracer from 0.5M acetic acid as a function of time was investigated by performing extractions for varying periods with equal-volume portions of 5% triisooctylamine-xylene. The per cent uranium-233 extracted in 0.5, 1, 2, and 1076
ANALYTlCAL CHEMISTRY
0.5 1 1 1
20
20 20 20 30 20
Uranium Concn., MgJm. U233tracer 3.46
5.86 6.92 U233tracer U233 tracer 2
Uranium Extracted,
%
99.4 97.9 96.2 94.8 99.5 99.6
99.1
Table V. Extraction of Macro Uranium (5 Mg./MI.) as Function of Acetic Acid Concentration
CH3- Uranium COOH, Extracted, Remarks ;I-1 70 0.5 97.5 Good phase behavior 1 2
98.7 98.7
3
97.8
Good phase behavior Slower phase separation; some foam, but phases clear up after centrifugation Slower phase separation; some foam, but phases clear up after centrifugation
alpha counting. Table I11 shows the effect of concentration of triiso-octylamine. Aqueous solutions 0.5iM in acetic acid containing uranium-233 tracer were extracted for 2 minutes with equal-volume portions of xylene containing varying concentrations of triiso-octylamine. EXTRACTION
OF MACRO
QUANTITIES
OF
URANIUM
The extraction of macro quantities of uranium is a function of the uranyl ion, acetate ion, and amine concentration. Experiments in which 0 . 5 M acetic acid solutions containing 1 mg. per ml. of uranium were extracted for 2 minutes 11-ith equal-volume portions of 5% triiso-octylamine-xylene resulted in approximately 10% loli-er Sields than similar experiments n ith uranium-233 tracer. It was necessary to use higher concentrations of amine to accommodate macro amounts of uranium satisfactorily. The same extraction procedure used for the tracer experiments was used except that milligram amounts of uranium were used with the uranium233 tracer. All phases n-ere analyzed for uranium-233 by alpha counting and ionic analyses for uranium. Table IV indicates that excellent extraction of macro quantities of uranium is possible from 0.5M or 1 M acetic acid solution with 2070 triieo-octylamine-xylene. Good mechanical behavior was observed in all cases. Higher concentrations of triiso-octylamine gave excellent extraction of uranium but tended to produce foams. Also, acetic acid concentrations greater than approximately 2ill tended to give mechanical difficulties by the formation of foamy emulsions. The standard extraction procedure selected for further study was as follows : Aqueous phase, 1M acetic acid, 5 mg. per ml. of uranium Organic phase, 20’33 (w./v.) of triisooctylamine-xylene containing 3% (v./v.) of butyl Cellosolve
Table V shows the extraction of uranium under these conditions as a function of acetic acid concentration. A second extraction with an equal-volume portion of the solvent gave quantitative recovery. Saturation of the solvent with acetic acid prior to the extraction of the uranium did not appear to lower the recovery appreciably. Several scrubbing experiments were performed in rrhich the organic phases prepared under the standard conditions given above JTere thoroughly mixed for 1 minute with equal-volume portions of scrubbing agents. The uranium losses from the organic solutions using 0.5M acetic acid, 1M acetic acid, and
distilled water as scrubs were 1.4, 1.3,and 2.5%, respectively. The macro uranium in the 20y0 triiso-octylamine - xylene stripped quantitatively into an equal-volume portion of 1M ammonium bicarbonate in a 2-minute mixing period similar to the tracer work described earlier, SEPARATION OF URANIUM AND PLUTONIUM FROM OTHER ELEMENTS
Decontamination studies n-ere made using three techniques. The data given here refer strictly to the acetic acid or dilute nitric acid-acetic acid system. The presence of other anions may change the extraction characteristics. Direct Extraction of Uranium from Acetic Acid Solution. Tracer solutions of the major fission products were prepared in 1M acetic acid solution and extracted according t o the standard evtraction procedure. Typical results are shown in Table VI. Uranium recoveries averaged 98 to 999;.,. Scrubbing the organic phase n ith various solutions could be used to achieve further decontamination of the uranium. However, no additional work was done in this respect, because one rarely will be dealing with a pure acetic acid solution of uranium or plutonium. Direct Extraction of Uranium from Nitric Acid-Acetic Acid Mixtures. Cranium is ordinarily dissolved in nitric acid solution, and the feasibility of utilizing the uranyl acetate extraction from dilute nitric acid was investigated. Table VI1 shows the recovery of uranium from 1M acetic acid containing dilute nitric acid. Standard extraction conditions were used Rith the exception of the nitric acid addition. Thus, one can effect an excellent recovery of uranium from dilute nitric acid-acetic acid solutions. The nitric acid present prevents the formation of anionic uranyl acetate but, upon mixing, the tertiary amine extracts the nitric acid, thereby allowing the formation and extraction of anionic uranyl acetate. Kitric acid concentrations greater than 0.5M were not investigated because, once the solvent is saturated with nitric acid, the aqueous nitric acid concentration would be high enough to prevent the formation of the extractable anionic uranyl acetate. Also. the selectivity of the acetate system nould be lost, inasmuch as some elements form extractable anionic nitrate species in this system. Table VI11 indicates typical data. Cesium, strontium, and europium do not extract. The extraction of ruthenium, zirconium, and niobium was variable (10 to 50%). These elements may be removed effectively from the uranium by scrubbing the organic phase with 5M hydrochloric acid solution (4). For instance, when the organic extracts were scrubbed
with three equal-volume portions of 5M hydrochloric acid solution for 2 minutes each, additional decontamination is achieved (Table VIII). Loss of uranium from the organic phase under these conditions is negligible. The uranium may be stripped from the organic phase by mixing well with a double-volume portion of 0.1M hydrochloric acid solution. Approximately 85y0recovery of the uranium is possible using this procedure. A second stripping step removes the remaining uranium from the organic phase. Decontamination was found to be slightly better, using macro amounts of uranium in all cases. Hydroxide Precipitation Followed by Extraction from Acetic Acid Solution. Acetic acid solutions of uranium and plutonium are not ordinarily encountered, and i t is usually necessary t o perform a hydroxide precipitation followed by dissolution in acetic acid t o secure 5 solution suitable for extraction. T K O carriers have been studied in this respect. IRON HYDROXIDE. Because iron is a
Table VI. Direct Extraction of Fission Products from 1M Acetic Acid Solution
Tracer Extracted, yo
