"72
JAMES
M. SCIIREYER
VOl. 77
[CONTRIBUTION FROM OAK RIDGENATIONAL LABORATORY, CARBIDE AND CARBON CHEMICALS COMPAXY]
The Solubility of Uranium(1V) Orthophosphates in Phosphoric Acid Solutions BY JAMES M. SCHREYER RECEIVEDSEPTEMBER 16, 1954 Solubility measurements in the U02-PzOs-HzO system have been made a t 25 Z!Z 0.1" in the range from 1.5 t o 15.24 M total dissolved phosphate. The existence of U(HP0&.6H20 and U(HPO&.H3PO4.H20 as the equilibrium solid phases has been demonstrated by use of the wet residue method of Schreinemakers. A method of preparation of U(HP0&?.4H20is presented.
I. Introduction Investigations in the uranium(1V) orthophosphate system reported prior to the present work are incomplete with regard to solubilities as well as the composition of the solid phases in equilibrium with the phosphate solutions. Some early investigators, Rammelsberg, Arendt and Knop, Chataing. Aloy,' and Schaap, Andrews and Gates,2 reported the preparation of U(HP0&.2H20 by various precipitation methods. Aloy' also reported the preparation of U(HP04)z 5H20. As a result of an investigation aimed a t characterizing phosphates of tetravalent uranium, Pannell and Rubino3 reported some solubility measurements of uranium(1V) phosphates in aqueous solutions of nitric, hydrochloric, sulfuric, phosphoric, perchloric and acetic acid in the pH range of 0.5 to 2.5. Unfortunately, the samples were shaken mechanically for 4.5 hours which the investigators recognized as an insufficient time to attain eyuilibrium. Milward4 described the titration of uranium(1V) sulfate solution with 0.1 M Na2HP04, and reported that an inflection point on the titration curve indicated the formation of U3(P04)4 under these conditions. The present investigation was undertaken in order to determine the solubility of uranium(1V) orthophosphates in phosphoric acid solutions. For these measurements, it was expedient to characterize the starting solid, to study the rate of oxidation of uranium(1V) in phosphoric acid solutions and to determine the rate of attainment of equilibrium in the various solutions.
11. Experimental A. Stability of Uranium(1V) Orthophosphate Solutions. -To obtain information on the stability of uranium(1V) toward air oxidation in phosphoric acid solutions, two soluin 8.50 M and tions containing 0.02010 -If 0.003300 M U+4 in 3.18 M Podw3,respectively (prepared by dissolving U(HPOa)24HzO in the appropriate phosphoric acid solution), were vigorously aerated at 25'. Aliquots were removed a t various time intervals for periods of 15 t o 17 hours and analyzed for ~ r a n i u m ( 1 V )through ~ the direct addition to ferric chloride solutions. No significant change in the concentration of uranium(1V) was found during these periods of aeration. The stability tomard air oxidation of uranium(1T") in phosphoric acid solutions is greater than has been reported by Nichols6 for uranium( IV) chloride solu(1) J. W. hlellor, " A Comprehensive Treatise o n Inorganic a n d Theoretical Chemistry." Vol X I I , I.onKmans, Green a n d Co.. London, r\-ew York, Toronto, 1932, p. 128 (2) W. B. S c h a n p , I-. J Aodrrwr and J. W. Gates. Tennessee Eastman Corp., CD-1002 ( 3 ) James 11. Pannell and Ernilia 11. Kubino, Mineral Engineering 1,aboratory. MITG-245. September 15, iY50. ( 4 ) G. I-. X I i l v a r d , CRL-AE-liG, Chemical Research Laboratory, Teddington, A i i g i i s t , 1950. ( 5 ) J. I f . Schreyer and C. F.Raes, Jr., Oak Ridge National Laborat o r y , Y-12, ORNL-1292, M a y i , 1922: A n n l . C k e m . , 2 5 , 6+4 (1953). ( R ) .4.11. Nichols J r . , BC-12, April 2 0 , 1W(i
tions and by Sill and Peterson? for uranium(1V) sulfate solutions, indicating a comparatively stronger complex formation between uranium(1V) and phosphate in acid solutions. As a result, i t was uniiecessary t o remove oxygeii from the solubility mixtures above 3.18 M total phosphate prior t o shaking, and the filtration of the equilibrium mixtures could be performed without protecting the mother liquors from air. Stability studies below 3.18 M total phosphate were not made because below this concentration the uranium(1V) phosphate solubility was found t o be too low for detection of small changes in uranium( IV) by the available analytical methods. Operations in t h e solubility determinations in this region were carried out in a nitrogen atmosphere. B. Analysis of Solids and Wet Residues.-Difficulty was encountered in dissolving the uranium( IV) phosphate solids and wet residues in sulfuric or perchloric acid. Kitric acid was not used because of interference with the determination of uranium when a Jones reductor is used for reduction. I t was found t h a t these uranium(1V) phosphate solids and wet residues could be dissolved in sulfuric acid by the addition of a small amount of hydrogen peroxide. The excess hydrogen peroxide could not be removed from the resulting uranium(V1) solution b y boiling. Analysis of these boiled solutions gave high results, probably because the hydrogen peroxide was not destroyed in the Jones reductor. This is in accord with the results of Sill and Peterson' who reported t h a t hydrogen peroxide is formed in the Jones reductor in the presence of air. The interference of hydrogen peroxide was eliminated in the foIIoxkg manner. The uranium( \'I) soIutions containing hydrogen peroxide were diluted t o a desired volume. Aliquots were removed and titrated with a permanganate solution to a light pink end-point in order t o destroy the hydrogen peroxide. The solution was then heated to boiling to remove the dissolved oxygen. If the pink color disappeared, more permanganate was added until the color returned. This solution, containing a slight excess of permanganate, was cooled and analyzed for uranium by the dichromate m e t h ~ d . I~t was found t h a t permanganate or manganous ions as high as 0.001 -11 did not interfere with t 11e uranium det erm inat ion. Phosphate determinations were made on these oxidized solutious by the modified magnesium ammonium phosphate method 8 C. Preparation and Purification of U ( H P O ~ ) ~ ~ H Z O . Uranium( IV) orthophosphate is a light green hydrogel when freshly precipitated from aqueous solution. Except for the color, it resembles freshly precipitated hydrous aluminum oxide. The detailed procedure for preparing samples of C(HPOa)2.4H10for use in the solubility studies is given below. About 500 ml. of uranium(V1) phosphate solution containing approximately 0.25 hl UOs++, 3 J l HsPOa and 3.5 t o 4.5 'If H2SOawas reduced in a Jones reductor. In a previous report,6 it was shown that under these condition.; uranium(T'1) is reduced quantitatively t o uranium( I\'). Uranium(1V) phosphate was precipitated from this reduced solution by slowly diluting with water until the concentration OF sulfuric acid was approximately I M. Vigorous stirring was required during the dilution and for several hours afterwards to produce a uniform product. The gelatinous precipitate was separated b y filtering with suction through a coarse fritted glass filter. The solid was washed twice with "liter portions uf 1 .I/ i i ) Claude \E' Sill and Heher 13, Peterson, Rbrreau 01 Mines. B h l RI-4882, June, I952 18) J . Xf. Schreyer and C 17 Baes, Jr., Oak Ridge National Labora l o r y Y-11'. O K S I . - l 5 7 8 , June 3 0 , 1953; 'I'HIS I M I H N A L , 7 6 , : 9.8 f 0.1 AI ~ 0 ~ 3 . During this investigation, U ( H P 0 4 ) ~ 4 H z 0was GRAMS OF P p O 5 PER 100 GRAMS OF MIXTURE, prepared and identified by both chemical and XFig. 2.-The system UO*-PpO;-HpO a t 25". ray diffraction analysis. A graph is presented which depicts the solubility Additional information to substantiate the existence of the above-mentioned equilibrium solid phases was obtained of uranium (IV) phosphate over the range of 1.5 to from an X-ray diffraction pattern. All of the patterns ob13.24 M total phosphate. tained with slurries from 1.85 to 10.9 il.I total phosphate Acknowledgments.-The author wishes to exwere identical, indicating the same solid phase over this range, but were different from the reference pattern for press his appreciation to the Oak Ridge National U(HPOa)24H20. S-RaJ- patterns for slurries from 11.76, Laboratory, .lnalytical Division, under the super12.42, 14.97 and 16.24 total phosphate were identical vision of Dr. M. T. Kelley and Mr. C. D. Susano, with each other, but were different from the patterns for for all solid phase analyses as well as the phosphate U(HPO4)2.4H20 or U(HPO4h6HrO. The concentrations of uranium(1V) and phosphate in the analyses of all solutions, to Air. H. W. Dunn, ISOmother liquor a t the transition point between r ( H P 0 4 ) ~ tope Analysis Methods Laboratory, Oak Ridge 6H?O and U(HPO4)2.H3PO4~H20 were determined to be 0.62 National Laboratory, who performed the X-ray f 0.02 .U and 9.8 =t0.1,[I. respectively, from an enlarge- diffraction analyses. The author wishes to acment of Fig. 1. T h e density of the transition mother liquor (1.63 f 0.03 g./ml.) was determined from a plot of densities knowledge the helpful suggestions of Dr. L. R. as a function of the phosphate concentration. Calculations Phillips, Oak Ridge Xational Laboratory, hiaterials ~.. Chemistry Division, during the course of this study. MOTHER LIQUORS CALCULATED 0
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(10) Roozehoom, Z p h r s i k . C h e m . 12, 369 (1893). ( I I ) 2.'. A . H. Schreinrmakrr, i h i i . . 11, 73 (18R3J.
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