Determination of Uranium(lV), Uranium(Vl), and Total Uranium in Phosphoric Acid Process Solutions by Tri-n-Octylamine Extraction and Polarography Isaac Hodara and Isaac Balouka Israel Atomic Energy Commission, Nuclear Research Centre-Negeu,
Uranium(V1) i s separated from U(IV) by extraction with tri-n-octylamine (TOA) in xylene from a mixture of phosphoric and hydrochloric acids. Uranium(V1) is backextracted from the TOA-xylene solution with dilute hydrochloric acid and is determined by polarography in 4N HCI medium. For total uranium determination, U(IV) i s oxidized with ferric chloride prior to extraction. Impurities present in the phosphate rock, including vanadium, molybdenum, and chromium, do not interfere. The method i s simple, rapid, and accurate. Six to eight determinations can be carried out per day by a trained technician. The relative standard deviation is 2% at the 95% confidence level for samples containing 0.05 to 0.2 mg uranium per milliliter.
A PROCESS FOR EXTRACTING uranium as by-product from the Israel Mining Industries (IMI) process for phosphoric acid production was developed in this laboratory. The uranium concentration of the Israeli Negev phosphate rock is 100 to 200 ppm (1). In the IMI process (Z), the rock is decomposed with hydrochloric acid. U(1V) tends to remain in the insoluble residue, whereas U(V1) dissolves in the HCI. When the uranium is recovered from the residue (3), the rock is attacked under reducing conditions (iron powder present) which causes most of the U to remain in the residue. In an alternative procedure (4,the rock is attacked in the absence of reducing agents, and U remains in the dissolution liquor to be recovered later as by-product in one of the subsequent stages of phosphoric acid purification. The phosphate rock also contains a number of minor constituents at the same concentration level as that of uranium. Table I gives the results of complete analysis of a representative rock sample. The concentrations of the major components (Ca, Poda-, CO,, and F-) are consistent with the composition of a carbonate-fluoroapatite of the approximate formula Calo(P04,C03)82-3. Table I1 gives the composition of three solutions taken from various pilot-plant stages. Obviously, direct measurement of the U(1V) and (VI) concentrations is not possible in such complex media. However many spectrophotometric and polarographic methods for total uranium concentration are available (for reviews, see References 5 and 6). All are based on the preliminary (1) I. Alter, E. Foa, Z. Hadari, G. Peri, and J. Trocker, Proc. Int. Conf Peaceful Uses At. Energy 1958, Vol. 3, p 253, P/1607 (Geneva, 1958). (2) A. Baniel and R. Blumberg in “Phosphoric Acid,” A. V. Black, Ed., 1968, Marcel Dekker, Inc., New York, Vol. 1, part 11, p 889. (3) Z. Ketzinel, D. Yakir, J. Rosenberg, J. Shashua, M. Hassid, and Y . Volkman, Proc. Symp. Recovery of Uranium from Its Ores and Other Sources, Sao Paulo, Brazil, August 1970, IAEASM-135/12, International Atomic Energy Agency, Vienna, 1971. (4) Z. Ketzinel, D. Yakir, and Y. Volkman, Israel Patent 33444, submitted November 1969. (5) G. L. Booman and J. E. Rein in “Treatise on Analytical Chemistry,” I. M. Kolthoff and P. J. Elving, Ed., Interscience, New York, N. Y., 1962, Part 11, Vol. 9, Section A, pp94-101, 115-128. ( 6 ) S. I. Sinyakova in “Analytical Chemistry of Uranium,” D. I. Ryabchikov and M. M. Senyavin, Ed., Israel Program for Scientific Translations, Jerusalem, 1963, pp 134-176.
Beer-Sheva, Israel
separation of uranium by solvent extraction, ion exchange, or precipitation except neutron activation analysis (7), where the uranium concentration is sured meadirectly. Existing controlled-potential coulometry methods have been developed either for pure synthetic solutions or for simple mixtures of thorium and uranium oxides. For example, Zittel and Dunlap (8) use a 6% solution of sodium tripolyphosphate (NaSP3OI0)adjusted to pH 7.5-9.5 as supporting electrolyte, which would not be suitable for the phosphate rock solution unless the great amount of Ca ions and various electroactive ions such as V, Mo, and Fe were removed beforehand. Similarly, the same electroactive species would interfere in the method proposed by Boyd and Menis (9) who determine the U(IV)/U(VI) ratio in thorium oxide-uranium oxide mixtures by controlled potential coulometry. Tserkovnitskaya and Bykhovtseva (10) determine U(1V) in mixtures of UO, and u01containing iron by amperometric titration of U(1V) with arsenazo. One method (11) specifically designed for the determination of the U(IV)/U(VI) ratio in the phosphate rock is known from the literature. According to it, the sample is dissolved in cold HCl or H 3 P 0 4 ,and U(1V) is precipitated by cupferron with Ti(1V) as carrier. The uranium concentration is determined by fluorimetry. Although the method is long and tedious, the authors claim that the results are reliable. The method given here is based on the selective extraction of U(V1) by tri-n-octylamine (TOA) in xylene. Although TOA extracts both U(1V) and (VI) from HC1 (12) or from H N 0 3(13)media, it was found in the present study that it will not extract U(1V) that is complexed with phosphate ions. To our knowledge, the prevention of U(1V) extraction by complexation with phosphate has not been used previously for separating quantitatively U(1V) from U(V1) by extraction with TOA. For the measurement of total U concentration, U(1V) is oxidized to U(V1) before extraction. In a separate sample, the H3P04 concentration of the aqueous phase is adjusted to 0.6M in order to prevent the extraction of U(1V) so that only U(V1) is extracted. Uranium is back-extracted from TOA with 0.1N HCl, the aqueous phase adjusted to 4N HCI, and the U determined in this solution by polarography. Combined with dissolution methods under inert atmosphere, the method may be applied, in addition to process streams, to the determination of U(IV)/U(VI) ratio in phosphate rocks. (7) S. Amiel, ANAL.CHEM., 34, 1683 (1962). (8) H. E. Zittel and L. B. Dunlap, ibid., 35, 125 (1963). (9) C. M. Boyd and 0. Menis, ibid., 33, 1016 (1961). (10) A. Tserkovnitskaya and T. T. Bykhovtseva, Russ. J. Anal. Chem., 22,79 (1967). (11) R. S. Clarke and Z . S. Altschuler, Geochim. Cosmochim. Acta, 13, 127 (1968). (12) F. L. Moore, ANAL.CHEM., 30,908 (1958). (13) W. E. Keder, J. C. Sheppard, and A. S. Wilson, J. Znorg. Nucl. Chem., 12, 327 (1960).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
1213
Table I. Representative Composition of Negev Phosphate Rock Major constituents Wt
z
z
Wt 0.3 0.3 2.0 0.1 0.015 0.2
28.0 MgO 37.2 A1203 coz 13.2 Hz0 F 3.2 Fe SiOI 1.5 U 1.8 Organic matter so3 Minor constituentsa PPm As < 10 Sb TI
