The precipitation and dissolution of uranium(VI) from aqueous
solutions containing 10-3 M uranyl nitrate and various concentrations
of orthophosphoric and nitric acid is / surveyed. The concentration
regions where (U02)s(P04h and U02HP04 are precipitated
and/or dissolved are presented by means of typical precipitation
bodies. The influence of the initial concentration ratio of nitric/
orthophosphoric acid in the precipitation system on the morphology
of (U02la(P04)2 particles formed is shown by electron micrograps

M. Branica

M. Wrischer

N. Pavković

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CCA-509 CROATICA CHEMICA ACTA 40 (1968) 127 541.18.041 :546.185.791.6 Original Scientific Paper Precipitation and Solubility of Uranyl Orthophosphates. II.* Heterogeneous Equilibria in Solutions of: U02(NOa)2 - HaP04 - HNOa N. Pavkovic,** M. Wrischer, and M. Branica Department of Physical Chemistry, Institute »Ruder BoskoviC«, and Laboratory of Physical Chemistry, Faculty of Science, University -of Zagreb, -Zagreb, Croatia, Yugoslavia Received April 6, 1968 The precipitation and dissolution of uranium(VI) from aqueous solutions containing 10-3 M uranyl nitrate and various concen-trations of orthophosphoric and nitric acid is/surveyed. The concen-tration regions where (U02)s(P04h and U02HP04 are precipitated and/or dissolved are presented by means of typical precipitation bodies. The influence of the initial concentration ratio of nitric/ orthophosphoric acid in the precipitation system on the morphology of (U02la(P04)2 particles formed is shown by electron micrograps. I NTRODUCTION The precipitation and dissolution of (U02)a(P0 1) 2 and U02HP04 in solutions containing nitric acid has found its application in the uranium production technology1- 2• Up to now only the solubility behaviour of U02HP04 in nitric and per-chloric acid has been reported3•4• However, there is not much information available on the precipitation of_ both (U02) 3(PO,i.) 2 and U02HP04 and on the solubility behaviour of (U02) 3(P04) 2 in solutions containing nitric acid5•6 • In our previous paper7 the influence of changes in concentrations of uranyl nitrate and orthophosphoric acid on the precipitation and dissolution processes of uranium (VI) has been shown. In the present work the concentration of uranyl nitrate was kept constant (1 X 10-3 M) and the influence of changes in the concentrations of orthophosphoric and nitric acid on the mentioned processes was studied. EXPERIMENTAL All chemicals were reagent grade. The systems were obtained by mixing equal volumes of solutions of uranyl nitrate and phosphoric acid, the solutions of pho-sphoric acid contained different amounts of nitric acid. The method of the prepa-ration of the systems, their analyses and the instruments used have been the same as in the previous work7 • * Presented in part at the 2n• Yugoslav Symposium on Reactor Materials, Her-ceg Novi, October 1965. Part I.: N. Pavkovic, M. Branica, and B. Tezak, Croat. Chem. Acta 40 (1968) 117. **Part of the Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Chemistry at the Faculty of Science, University of Zagreb. 128 N. PAVKiOVIC, M. WRISCHER, AND M. BRANICA RESULTS The precipitation of uranium (VI) from solutions containing 10-3 M uranyl nitrate and varying concentrations of phosphoric acid (from 2 X 10-s M to 4 X 10-2 M) and nitric acid (from 10-s to 10-1 M) is surveyed by means of the typical precipitation bodies3•9 which are represented in the lower parts of Figs. 1 and 2. The distribution of (U0 2) 3(P04}e and U02PH0 4 species, as detected by fluorescent microscopy and verified by chemical analyses is presented as a function of the concentrations of phosphoric and nitric acids. The systems were aged in contact with . the mother liquor for 24 hours (Fig. 1) and 30 days (Fig. 2) respectively. On the upper parts of Figs. 1 and 2 two kinds of pH curves are shown. The first one (curves 1) denotes the pH values ~c:r<r"O pH 2 PRECIPITATE ~ _ _q..fl I I f{'l?'2 ~e&-8 I I I CLE AR S 0 LU~ I 0 N -6 - --1 - 1~-'t-1 UO,CNOill 1--A.e--etl- OOOOO- O 000-0-0- 0 00 O-Q):)~ 00 r:,, IJ,,. UEAR SOLUTION O (U021J (PO:,}i () t-llXTURE 'i: e U02HPOL '-' -5 M 0 z z ':' :r:: >--:i ::> ~ u -4 o_ z U> 0 u 0 g -3 -3 -4 -5 LOG. CONCN . HJP04 (M) pH 0 z :r:: -6 z_-4 u z 0 (J 0 9-3 -2 1 1 ~ PR~~~~ _....-.~ L._s;i-23-f' 2 ~~:IHJ!-ll~ CLE AR SO LUTION -2 -3 LOG. CONCN. HJP04 -4 [Ml -5 Fig. 1. Lower part. Typical precipitation body cf the system: 1 X 10-• M U02(NOs)2 - HsPO, (var .) -- HN03 (var.) 24 hours after mixing, at room temperature. Upper part: pH curve 1 corresponds to the system: 1 x 10-s M U02(N03)2 - H 3PO, (var.) and pH curve 2 to the system: 1 X 10-3 M U02(NOa)2 - H 3P04 (var.) - HN03 (var.) Full triangles demonstrate the clea.r solutions which show a more intensive fluorescence than the solutions of uranyl nitrate of the same concentrations. Fig. 2. Typical precipitation body of the 1 X 10-a M U02(NOah - H aPO• (var.) - HNOs (var. ) systems at room temperature. The precipitates were in contact with the mother liquor for 30 days. for the mother liquor of the systems: U02(NOa) 2 (10-3 M) - H 3P04 (var.) without any addition of nitric acid (corresponding to the pH curve in Fig. 1, paper No. F). The second one (curves 2, Figs. 1 and 2) denotes the pH values for the solutions of corresponding composition but with different amounts of nitric acid added to the systems at the precipitation boundary. Comparing Figs. 1 and 2 changes of the solid with time can be seen. Observations on a great Figs. 3a-b. Electron micrographs of the precipitates obtained from t h e systems listed in Table I made 24 hours after mixing. The systems wer e aged at r oom temperature I . · . ~ !.· .. '\.·.· ... .·. ~} - 1 ul Figs, 4a-d. Electron micrographs of the precipitates obtained from the systems listed in Table I made 2'4 hours after mixing_ Th e systems were aged at room temperature. URANYL ORTHOPHOSPHATES. U . 129' number of systems have shown that complete equilibrium between the solid phase and the mother liquor is attained after 10 to 14 days. At the precipitation boundary in the range of excess of poosphoric acid U02HP04 is the stable form and the clear solutions in these concentration regions exhibit very intensive fluorescence when excited by UV light. In the range of excess of uranyl nitrate (U02)g(P04 ) 2 was found to be the stable solid phase (Fig. 2) . The fluorescence of the clear solutions in this part of the precipitation boundary has an comparable intensity to intensity of the fluorescence of a 10-3 M uranyl nitrate solution. The electron micrographs 3a and 3b show the morphology of particles which precipitate in the uranyl nitrate-orthophosphoric acid system without addition of nitric acid. In Fig. 3a a mixture of particles of (U02) 3 (P04 ) 2 (small crystals) and of U02HP04 (much bigger rectangular crystals) is shown while in Fig. 3b only crystals of (U02)g(P04 ) 2 can be optically selected. From the electron-micrographs 4a-d 'the influence of the addition of increasing con-centrations of nitric acid on the morphology of the (U0 2) 3(P04L particles can be seen. In Table I are listed the concentrations of orthophosphoric acid, nitric acid, their ratio and the pH of the mother liquor of the corresponding precipitation systems. It is evident (Figs. 4a-d) that the crystals become larger with increasing concentrations of nitric acid in the systems. At exstreme concentrations of nitric acid large, rod-like crystals were formed (Figs. 4c, d) . The influence of nitric acid on the growth of U02HPQ4 crystals was less evident. TABLE I Initial Initial I pH Composition Fig. concn. (M) concn. (M) I Ratio of of the of the ppt. I the cone. No. of of HNOa/HaP04 mother 24 hours HaP04 HN03 I liquor after mixing 3 a 3 x 10- 3 - - 2.50 I (U02)s(P04h and U02HP04 3 b 3 x 10- 3 - - 2.50 (U02)3(P04h 4 a 3 X 10- 3 1 x 10- 4 0.033 2.50 (U02)a(P04)2 4 b 3 x 10-3 3.0 X 10-3 1.0 2.31 (U02)s(P04)2 4 c 3 x 10-3 6.6 x 10-3 2.2 2.14 (U02)s(P04)2 4 d 2 x 10-4 2.8 X 10-3 1.4 2.55 (U02)s(P04)2 In all sy stems uran yl nitrate concentration was kept const a nt at 1 X io·s M . DISCUSSION Comparing the results described in this and the previous paper7 we may draw some conclusions as to the influence of changes in the initial pH (as achieved by the addition of nitric acid) on the precipitation and dissolution processes in the system UOz(N03 ) 2 (1 X 10-3 M) - H3P04 (var.) - HN03 (var.). Similarly as in the system without nitric acid in the entire concentration range investigated only precipitates of (U02 ) 3 (P04L and U02HP04 were fo rmed. Their distribution was independent of the initial pH, i. e. of the concentration of nitric acid added (Fig. 2) up to approximately equimolar concentrations of nitric and phosphoric acid. The slopes of the precipitation boundaries at nitric acid concentrations higher than 3 X 10-3 M indicate the formation of several different soluble 130 N. PAVNOVIC, M. WRISCHER, ANL M. BRANICA complex uranyl orthophosphates whose hydrogen to phosphate ratios differ from the hydrogen to phosphate ratios of the respective solid phases. It is also obvious from these data, as well as from the different fluorescence behaviour of the mother liquors which were in equilibrium with (U02) 3(P04h and U02HP04 , that different complex species are in equilibrium with these two solids. Of interest is also the significant effect of the initial concentration ratio of nitric/orthophosphoric acid in the systems on the morphology of the pre-cipitated (U02) 3(P04 ) 2 particles. This effect is especially pronounced in the vicinity of the precipitation boundary and thus might be due to highly prolonged precipitation processes. The calculation of more probable soluble species and their equilibrium constants will be done in one of the following paper. Acknowledgments. We wish .to thank Prof. B. Tefak for his interest in this work and Dr. H. Fiiredi for reading and discussing the manuscript. REFERENCES 1. Processing of Low-Grade Uranium Ores, Proceedings of Panel, Vienna, 27 June--1 July 1966, International Atomic Energy Agency, Vienna, 1967. 2. C. D. H a r r i n gt o n and A. E. Rue h 1 e. Uranium Production Technology, D. V. Nostrand Company, Inc., New York, 1959 pp. 15-17, 24-25. 3. G. R. Le ad er, Report CN-2195 (1944). 4. J. M. Schreyer and C. F. B a es Jr., J. Phys. Chem. 59 (1955) 1179. 5. A. D. Ryon and D. W. Kuhn, Report Y-315 (1949). 6. A. D. Ryon and D. W. Kuhn, Report Y-381 (1949). 7. N. Pav k o vi c, M. Br an i ca, and B. Tezak, Croat. Chem. Acta 40 (1968) 117. 8. B. Tezak and collaborators, Disc. Faraday Soc. 42 (1966) 175. 9. H . F ii red i, Complex Precipitation Systems, Chapter 6 of The Formation and Properties of Precipitates by A . G. W a 1 ton, Interscience Publ., New York, 1967. IZVOD Talozenje i topljivost uranil ortofosfata II. Heterogena ravnotefa u otopinama: U02(N03h-H3P04-HN03• N. Pavkovic, M. Wrischer i M . Branica Ispitano je talozenje i otapanje urana(VI) iz vodenih otopina koje su sadrfavale 10-s M uranil nitrat i razne koncentracije ortofosforne i dusicne kiseline. Koncentra-ciona podrucja, gdje se taloze ili otapaju talozi sastava (U02)s(P04)2 i U02HP04 prikazana su opCim precipitacionim dijagramima. Elektronskim mikrofotografijama prikazan je utjecaj pocetnoga koncentracio-nog odnosa dusicna/ortofosforna kiselina na morfologiju nastalih cestica taloga (U02)s(P04)2. ODJEL FIZICKE KEMIJE INSTITUT »RUDER BOSKOVIC« I FIZICKO-KEMIJSKI ZA VOD PRIRODOSLOVNO-MATEMATICKI FAKULTET SVEUCILISTE U ZAGREBU Primlj eno 6. travnja 1968. 

Precipitation and Solubility of Uranyl Orthophosphates. II. Heterogeneous Equilibria in Solutions of: U02(NO3)2 - H3P04 - HNO3

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