Occlusion and ion exchange in the molten (lithium chloride+potassium chloride+alkaline-earth chloride) salt+zeolite 4A s
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tive salt disposal are being developed at Argonne National Laboratory (ANL). In some cases, the radioactive constituents can be removed from the molten salts by passing them through a zeolite 4A ionexchange column, allowing the salt to be reused and decreasing the volume of the waste produced. In addition to its major component (70 wt pct), the (LiCl ⫹ KCl) eutectic salt (x(LiCl) ⫽ 0.59, x(KCl) ⫽ 0.41), the process salt comprises at least 16 other chloride and iodide salts, including NaCl, CsCl, SrCl2, UCl3, and PuCl3. Hence, a systematic study of the thermodynamics of occlusion and ion exchange in the molten (LiCl ⫹ KCl ⫹ MeCln) salt ⫹ zeolite systems was initiated. The thermodynamic behavior of these systems strongly depends upon the charge of the cation Men⫹. Results obtained with univalent alkali metal cations of NaCl, RbCl, and CsCl have been reported previously.[1] This work is a report on the results of studies involving multivalent alkaline-earth and actinide cations of CaCl2, SrCl2, and UCl3. II.
EXPERIMENTAL
All experimental procedures used were identical to the ones described in detail previously,[1] except that samples of molten salt were analyzed only by inductively coupled plasma–atomic emission spectrometry (ICP-AES). DUSAN LEXA, formerly Assistant Chemist with the Chemical Technology Division, Argonne National Laboratory, Argonne, IL 60439, is Head of the Radioactive Waste Management Department with the Austrian Research Centers GmbH-ARC, A-2444 Seibersdorf, Austria. Contact e-mail: [email protected] Manuscript submitted June 14, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS B
THEORY AND CALCULATIONS
Pertinent research into the ion-exchange and occlusion properties of zeolite 4A (Na12(AlSiO4)12), hereafter called “zeolite,” and the underlying theory have been recounted previously.[1] The analytical composition of the salt and zeolite samples was reported in weight percent. These data were converted to mole fractions as follows: in the salt phase, (s) (s) (s) x(s) Li ⫹ xK ⫹ xNa ⫹ xMe ⫽ 1, and in the zeolite phzase, (z) (z) (z) x(z) Li ⫹ xK ⫹ xNa ⫹ xMe ⫽ 1. Calculations of the sto-
ichiometric coefficients of Li, K, Me, Si, and Cl in the saltloaded zeolite were performed by normalizing the amount of Al in moles per 100 g of sample to a stoichiometric coefficient of 12.0 and applying the normalization factor thus obtained to the amounts of Li, K, Me, Si, and Cl in moles per 100 g. Because of zeolite stoichiometry, the expected value of the Si stoichiometric coefficient was also 12.0. It served as a check of the analytical procedure and the calculations and, with few exceptions, equaled 12.0 ⫾ 0.5. Regarding occlusion, the goal of this study was the determination of the zeolite occlusion capacity,[1] as well as its possible dependence on the salt-loaded zeolite composition. Such composition dependence is expected from simple geometrical arguments, given the limited zeolite cage volume and the different radii of the ions involved. Specifically, the occlusion capacity (y) can be expressed as[2] V
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