THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in U. 8. Patent Office)
4:. .
VOLUME 62
(0Copyright, 1958, by the American Chemical Society)
NUMBER 2
MARCH 3, 1958
THE EXTRACTION OF URANIUM(V1) FROM ACID PERCHLORATE SOLUTIONS BY DI-(2-ETHYLHEXYL)-PHOSPHORIC ACID I N n-HEXANEl BY C. F. BAES,JR.,RALPHA. ZINGARO~ AND C. F. COLEMAN Oak Ridge National Laboratory, operated by Union Carbide Nuclear Company, Oak Ridge, Tenn., and the Agricultural and Mechanical College of Texas, College Station, Tex. Received April 1 , 1067
The distribution of uranium(V1) between acidic aqueous .perchlorate solutions and solutions of di-(2-ethylhexy1)-phosphoric acjd J(RO)zP02H, DPA] in n-hexane has been examned as a function of various extraction variables at constant aqueous ionic strength. The organic solutions involved have been investigated by means of isopiestic molecular weight determinations and viscosity measurements. The distribution behavior a t low uranium levels conforms approximately to the relation Eao = CTJ(~)/CU(~) = KC'DPA/C%in which CU~O) and are, respectively, the organic and the aqueous phase uranium(V1) concentrations CDPAis the organic extractant concentration and CH is the aqueous acidity. Since the isopiestic results indicate that 6 P A is dimeric in hexane solutions over the concentration range used (0.01-1M DPA), the.principal extraction reaction a t low uranium levels appears to be UO*++(a) 2(HX)2(o)p UO2X4H2(o) 2H+(a) in which X denotes the anion (RO)~POI-. As the ratio CU(O)/CDPAincreases above 0.25 the isopiestic results indicate molecular association which increases as saturation is approached (CU(O)/CDPA + 0.5). donsistent with this, above CY(O)/CDPA = 0.45 the viscosity rises sharply. These results, together with the extraction behavior in the vicinity of saturation, are discussed in terms of the formation of chain polymers of the type H X Z U O Z X ~ J O ~ - X ~ U O ~ X ~ H .
+
Introduction It was recognized at Oak Ridge and at Hanford in connection with tributyl phosphate extraction studies in 1949, that the hydrolysis products of this reagent (monobutyl- and dibutylphosphoric acid) are themselves effective extractants for uranium and other metals. Subsequent investigations of the use of alkylphosphoric acids as liquid-Iiquid extraction reagents, primarily for uranium, have been reviewed by Blake, et U Z . , ~ in reports dealing mainly with di-(2-ethylhexyl)-phosphoric acid extraction of uranium(V1) from acidic sulfate solutions. Dialkylphosphoric acids have generally been assumed t o extract uranium(V1) by means of the ionexchange reaction (1) Presanted at the ACS meeting, Miami, Florida, April 7-12, 1957. (2) Summer Reaearoh Participant with the Oak Ridge National Laboratory, 1955. sponsored by the Oak Ridge Institute of Nuclear Studies. Chemistry Department, A. and M. College of Texas, College Station, Texas. (3) (a) C. A. Blake, K. B. Brown and C. F. Coleman, Report ORNL-1903, June (1955): (b) C. A. Blake, C. E'. Coleman, K. B. Brown, D. J. Crouse and A. D. Kelmers, Report ORNL-2172, Deo. 18, 1956.
UOz++(a)
+
+ 2HX(o)
UOoXz(o)
+ 2H+(a)
(1)
in which X represents the anion (RO)zP02-. The results of Stewart and Hicks4 and Blake, el ~ l . , ~ appeared generally consistent with this reaction t o the extent that they showed the expected 2:l reagent to uranium mole ratio in uranium-saturated organic solutions, and close to the expected second power dependence on reagent and aqueous acid concentrations. However, these authors also reported results which are not consistent with this reaction. They suggested that this might, be due to complications such as the solubility of the extractant in the aqueous phase,3 exchange between the acidic hydrogen in the reagent and alkali metal cations in the aqueous phasela the use of additives in the organic diluent, and the presence of complexing anions in the aqueous phase.'S2 Another extraction variable not yet considered, which might be responsible for the inconsistencies noted, lies in the possibility that dialkylphosphoric acid molecules may be associated in the organic solutions. Dimerization of dibutylphosphoric acid in benzene has been reported by Wagner,s and of
129
(4) D. C. Stewart and T. E. Hicks, Report UCRL-861, 1950.
130
C. F. BAES,JR.,RALPHA. ZINGAROAND C. F. COLEMAN
Vol. 62
the analogous dialkylphosphinic acids (R2P02H) by Kosolapoff and Powell. In the present investigation the distribution of uranium(V1) between acidic perchlorate solutions (of ionic strength 2 M ) and n-hexane solutions of di-(2-ethylhexyl)-phosphoric acid (hereinafter referred to as DPA) was measured and the nature of the resulting organic solutions was examined. This dialkylphosphoric acid was chosen because it is an effective extractant of negligible water solubility, readily prepared in good purity,
tion of solutions resulting from prolonged runs revealed no detectable uranium( VI) reduction; however, a strong odor was present which was not associated with freshly pre ared solutions. The mode of this decomposition which evid)ently takes place in the light-tight container used in these measurements is unknown at present. Extraction Measurements.-Measured volumes (usually 40 ml.) of aqueous and organic solutions of known composition were introduced into a separatory funnel and shake: mechanically in a thermostat for one hour a t 25 f 0.02 . Upon separation of the equilibrated phases, which was usually rapid, a portion of one or both solutions was taken for analysis. Usually each measurement was made by a single contact of freshly repared solutions; however, in some cases involving DPif solutions nearly saturated with Experimental uranium, it was found convenient to contact the organic Di-(2-ethylhexy1)-phosphoric Acid.-The DPA reagent phase with a series of uranium-barren aqueous solutions to was prepared by purification of “Tergitol” Wetting Agent obtain a series of equilibrium distributions. Rate measurements which were made during the present P-28,7 a commercially available aqueous solution of sodlum di-(2-ethylhexy1)-phosphate. Tpe final product, as indi- investigation indicate that, in accord with previous obsercated by potentiometric titration, possessed a minimum vations” the extraction of uranium(V1) by DPA is rapid purity of 99% dialkylphosphoric acid and a maximum of and reversible, equilibrium usually being reached in less than 2 minutes of shaking. 0.1% of the monoalkyl acid.8 Throughout the acid range (0.1-2 M ) used in these measn-Hexane.-Fisher laboratory-grade n-hexane, which was used throughout as the organic solvent, was further urements, no appreciable exchange was detected between the purified for most of the measurements by scrubbing with acidic hydrogen in DPA and aqueous sodium ion. I n addiconcentrated sulfuric acid, followed by sodium hydroxide tion, there was no appreciable distribution of sodium perand water washes. While darkening of the sulfuric acid chlorate (used to adjust the ionic strength to 2 M ) to the indicated the removal of unsaturated hydrocarbons from hexane phase. Karl Fischer titrations of various equilithe hexane, there was no detectable difference between the brated 0.1 M DPA solutions indicated that no water of hyextraction results obtained using the treated and the un- dration was associated with either the free DPA reagent or the uranium-DPA complexes f0rmed.l’ treated diluent . Uranium was analyzed by one of four methods depending Reagent Solutions.-Stock solutions of DPA in hexane were analyzed by potentiometric titration us. standard base mainly on the quantity of uranium present in the sample. in 70% ethanol. The agreement between values calcu- For the relatively few samples containing more than ea. lated from the titration curves and those based on weight 0.1 mmole of uranium, volumetric dichromate titration of material used was within 0.5%. The nitrate-free uranyl was used.Ia The accuracy of this method is ca. =t0.2%. perchlorate solution used was prepared as previously de- The majority of samples in the range 10-e0.1 mmole scribed.0 Perchloric acid and sodium perchlorate solutions uranium were analyzed potentiometrically using the procedure and automatic microtitrator described by Allen.13 were prepared from reagent grade chemicals. The usual accuracy of this method was ca. &2%. Other Uranium( VI)-DPA Decomposition in Hexane .-The organic solutions resulting from uranium extraction, ini- samples in this range were analyzed by the colorimetric tially yellow in color, were found to develop a greenish tinge thiocyanate procedure of Crouthamel and Johnson,” giving after several days storage on open shelves. After prolonged results of about the same accuracy. For the few cases in mmole of uranium was present, fluorostanding a blue-green color (similar to that of tetravalent which less than uranium in aqueous phosphoric acid solution) resulted. metric analysis was used.16 Above 5 X 10-8 mmole an Brief spectrophotometric examination of such solutions re- accuracy of *lo% usually was obtained. Below this level vealed absorption curves with a pronounced maximum at the uncertainty increased rapidly. Before analysis, the aqueous perchlorate solutions were ca. 650 mp, which is characteristic of uranium(1V). Uranium(V1)-DPA solutions stored in the dark at room tem- often concentrated by first adjusting the acidity to 0.1 M perature showed no apparent reduction after several and contacting with a relatively small volume of 0.1 M months. The reproducibility of the uranium(V1) extrac- DPA (Eo, lo4). The separated organic phase was dition measurements indicates that no significant decomposi- luted ten-fold with hexane and extracted with several small tion occurred during the period required to equilibrate and volumes of 6 A4 hydrochloric acid (Eoa 0.001). This analyze the solutions involved. Because of an observed acid solution was washed with hexane to remove entrained slow decrease in viscosity of nearly saturated uranium( VI) DPA, evaporated to dryness and the residue taken up with solutions on exposure to light,lO such measurements were sulfuric acid, ready for analysis. Organic solutions were made in a shielded thermostat on freshly prepared solutions. prepared for uranium analysis by dilution to 0.01 M in DPA In the isopiestic measurements some difficulty, presumably with hexane and subsequent hydrochloric acid extraction, caused by solution decomposition, also was encountered. evaporation and dissolution in sulfuric acid. Isopiestic Measurements.-The isopiestic a paratus Runs at low reference solute concentrations (
