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J. Phys. Chem. 1996, 100, 220-223
Solubilities of Uranium(IV) Dioxide in Magnesium Chloride, Calcium Chloride, and Aluminum Chloride Melts: A Comparative Study Sheng Dai,* L. M. Toth,* G. D. Del Cul, and D. H. Metcalf Chemical Technology DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6181 ReceiVed: July 25, 1995; In Final Form: September 26, 1995X
High-temperature near-IR absorption spectroscopy was used to study the dissolution of UO2 in molten MgCl2, CaCl2, and AlCl3 melts. The study reveals that UO2 is most soluble in molten AlCl3, followed by the melt of MgCl2. The solubility of UO2 in molten CaCl2 is too small to be measured with optical spectroscopy. This strong dependence of the solubility on the cations of the melts was rationalized by the use of the FloodForland-Grjotheim thermodynamic cycle.
Introduction There has been a recent resurgence of interest in the development of nuclear reactors using molten salts to carry nuclear fuel.1 Liquid-fuel systems in the form of molten salts can operate at high temperature without chemical decomposition, and they can reach high operating temperatures at much lower vapor pressures as compared with water/steam systems. In addition to the application in the reactor research, molten salts have been widely used in processing nuclear fuels and as pyrochemical reaction media to produce metals electrolytically.2,3 In all these applications, the need for reliable methods to measure the thermodynamic properties of uranium compounds in molten-salt media is apparent. Among others, an important point is the solubility of radionuclide compounds in moltensalt media. In this paper, we present an experimental and theoretical investigation of uranium(IV) dioxide (UO2) solubility in molten MgCl2, CaCl2, and AlCl3. The solubilities were determined by using visible and near-IR absorption spectroscopy. Compared with other solubility determination methods, the accuracy of this measurement is less affected by the presence of impurities in samples. The relation between solute solubilities and molten-salt solvent properties has received some attention.2,4-7 A thermodynamic model called the Flood-Forland-Grjotheim cycle (FFG cycle) has been proposed by Blander to rationalize solute solubilities in molten-salt solvents. This model makes use of a simple thermodynamic cycle, that permits calculation of the solubility product of an insoluble salt in a molten-salt solvent. Specifically, Blander and his co-workers have successully applied this cycle in calculating the solubilities of silver halide in molten potassium nitrate.4 Kucera and Saboungi have used this cycle to explain the solubility difference between MgO and CaO in the CaCl2 melt.2 Some recent applications of the FFG cycle have been summarized in a review by Blander.6 In this paper, measured solubilities of solid UO2 in molten MgCl2, CaCl2, and AlCl3 are analyzed via this FFG cycle. This analysis enables us to understand the thermodynamic driving force for the dissolution of UO2 in the selected molten halide melts. Experimental Section Reagents. Uranium dioxide (UO2) usually contains uranium trioxide as an impurity. Hence, UO2 was further purified by heating in a flowing atmosphere of H2/Ar (4 mole %) at 450 * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0220$12.00/0
°C. The preparation and purification of uranium(IV) tetrachloride and uranium(IV) oxychloride have been reported previously.8 The UCl4 was purified by loading the nominally pure starting material in a silica tube and pumping it down to ∼20 µm Hg at 650 °C to remove any moisture or HCl. The tube was then sealed, and UCl4 was sublimed from one end of the tube to the other at 510 °C. The UOCl2 was prepared by the reaction of stoichiometric amounts of UCl4 and Sb2O3 at 150 °C and then steadily raising the temperature to 300 °C, where the SbCl3 by-product distilled from the UOCl2 product in a temperature gradient. Anhydrous MgCl2 was synthesized by the decomposition of the corresponding carnallite salt, followed by distillation in a double-bulb fused-silica vessel.9 The purification of the other starting materials (CaCl2 and AlCl3) has also been described previously.9 All molten salt solutions were prepared under helium atmosphere in a drybox of MgCl2 > CaCl2 even if the AlCl3 did not react further to form U(AlCl4)xCl4-x. The inclusion of this additional step only enhances the solubility of UO2 in AlCl3. This prediction of the solubility order by the FFG model agrees well with our experimental results. Clearly, the difference in solubilities among the three molten-salt systems can be qualitatively attributed to that of ∆G°0. Since the solubility of UO2 in molten MgCl2 is quite small, we expect that Henry’s law applies and can assume that aUOCl2 and aMO in eq 8 equals xUOCl2 and xMO, respectively. The solubility of UO2 in molten MgCl2 at temperatures greater than 877 °C is 0.127 × 10-1 mole fraction. The amount of MgO formed during the dissolution of UO2 in molten MgCl2 should equal this value, according to eq 4. Boghosian et al.7 have recently measured the solubility of MgO in molten MgCl2. The solubility is less than 4 × 10-3 mole fraction at 850 °C. Therefore, some of the MgO formed during dissolution of UO2 in molten MgCl2 is expected to precipitate as an insoluble solid. Thus, the following equation holds:
µ°MgO ) µ* MgO + RT ln xMgO
(9)
Insertion of this equation in eq 8 gives When the molten MCl2 is in equilibrium with solid UO2, then the sum of the three free energies for eqs 4-6 is zero. Thus,
∆G° + µ*UOCl2 - µUOCl2 + RT ln aUOCl2 + µ* MO - µMO + RT ln aMO ) 0 (7) and
-RT ln Ksp ) -RT ln aUOCl2aMO ) ∆G°1 + ∆Gst2 + ∆Gst3 (8)
-RT ln xUOCl2 ) ∆G°1 + ∆Gst2
(10)
This equation is consistent with our experimental observation that the plot of ln xUOCl2 vs 1/T is linear. Since the values of xUOCl2 were experimentally measured at various temperatures, the free energy of change (∆Gst2 ) for step 2 at the corresponding temperatures can be calculated from eq 10. The calculated ∆Gst2 ’s at various temperatures (Table 1 ) are consistent with
Solubilities of Uranium(IV) Dioxide
J. Phys. Chem., Vol. 100, No. 1, 1996 223
our previous experimental observation that UOCl2 is highly soluble in molten MgCl2.8,11 Estimation can be made concerning the concentration of U4+(solvated) by considering the following FFG cycle:18
complex between UCl4 and AlCl3. This binary compound is formed by the oxygen exchange between uranium(IV) and aluminum(III) during dissolution. The UO2-solubility difference in the three molten salt solvents can by rationalized by invoking the FFG cycle.
UO2 + 2MgCl2 ) UCl4(l) + 2MgO(s)
(11)
UCl4(l) ) UCl4(infinite dilute)
(12)
Acknowledgment. Authors wish to thank a reviewer for his helpful suggestions. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-ACO5-84OR21400 with Martin Marietta Energy Systems.
The free energy of change for the first reaction (eq 11) at 850 °C is about 124 kJ/mol. As compared to the first process, the free energy of change for the second process (eq 12) is normally negligibly small. Hence, the mole fraction of U4+(solvated) is approximately equal to 1.7 × 10-6 on the basis of the equilibrium constant calculated using only the free energy of change for eq 11. As pointed out in the previous paragraph, our molten salt solution contains saturated MgO. Therefore, the concentration of the oxide is the solubility (3.6 × 10-3 at 850 °C) of MgO in molten MgCl2.7 One can then calculate the formation constant for UO2+(solvated) from U4+(solvated) and O2-(solvated) (U4+ + O2- ) UO2+). The formation constant at 850 °C is estimated to be 1.6 × 106 on the basis of our experimentally determined UO2+(solvated) concentration (1.0 × 10-2 mole fraction). Conclusions The dissolution of UO2 in molten MgCl2 gives rise to the same species as that obtained from the solvation of UOCl2 by molten MgCl2. This species is assigned to the uranium(IV) oxychloride complex. Since the near-IR extinction coefficient of the oxychloride complex is temperature-insensitive, this allows us to accurately determine the solubilities of UO2 in molten MgCl2 at various temperatures by using near-IR absorption spectroscopy. No detectable optical absorption spectrum corresponding to the uranium oxychloride complex was observed for CaCl2 melts in contact with solid UO2. This indicates that the solubility of UO2 in molten CaCl2 is much less than that in molten MgCl2. The solubility of UO2 in molten AlCl3 was found to be the greatest. The solvated species is the binary
References and Notes (1) Venneri, F.; Bowman, C.; Jameson, R. Phys. World 1993, August, 40. (2) Tumidajski, P. J.; Blander, M. J. Phys. Chem. 1995, 99, 9992. Kucera, G. H.; Saboungi, M. L. Metall. Trans. 1976, 7B, 213. (3) Chiotti, P.; Jha, M. C.; Tschetter, M. T. J. Less-Common Met. 1975, 42, 141. (4) Blander, M.; Luchsinger, E. B. J. Am. Chem. Soc. 1964, 86, 319. (5) Saboungi, M.-L.; Blander, M. J. Electrochem. Soc. 1975, 122, 1631. (6) Blander, M. In Molten Salt Chemistry; Mamantov, G., Marassi, R., Eds.; D. Reidel Publishing Company: Dordrecht, The Netherlands, 1987; pp 17-62. (7) Boghosian, S.; Godo, A.; Mediaas, H.; Ravlo, W.; Ostvold, T. Acta Chem. Scand. 1991, 45, 145. (8) Toth, L. M.; Felker, L. K.; Hunt, R. D.; Brunson, R. R.; Loghry, S. L. Sep. Sci. Technol. 1993, 28, 781. (9) Dai, S.; Young, J. P.; Begun, G. M.; Coffield, J. E.; Mamantov, G. Microchim. Acta 1992, 108, 261. (10) Dai, S.; Toth, L. M.; Del Cul, G. D.; Metcalf, D. H. J. Chem. Phys. 1994, 101, 4470. (11) Dai, S.; Toth, L. M.; Del Cul, G. D.; Metcalf, D. H. Inorg. Chem. 1995, 34, 412. (12) Malinowski, E. R.; Howery, D. G. Factor Analysis in Chemistry; Wiley: New York, 1980. (13) Irish, D. E.; Ozeki, T. in Analytical Raman Spectroscopy; Grasselli, J. G., Bulkin, B. J., Eds.; John Wiley: New York, 1991. (14) Sun, Y.-P.; Sears, D. F., Jr; Saltiel, J. Anal. Chem. 1987, 59, 2515. (15) Hush, N. S. AdV. Inorg. Chem. 1963, 1, 391. (16) Janz, G.; Dampier, F. W.; Lakshminarayanan, G. R.; Lorenz, K.; Tomkins, R. P. Natl. Stand. Ref. Data Ser.; Natl. Bur. Stand. 15. Molten Salts, 1968, Vol. 1. (17) Gruen, D. M.; and McBeth, R. L. Inorg. Nucl. Chem. Lett. 1968, 4, 299. (18) Since there is some MgO formed with MgCl2 solvent, the sum of the standard free energies for eqs 11 and 12 is directly related to the solubility of UO2 in the form of U4+(solvated).
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