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Ind. Eng. Chem. Res. 2003, 42, 4208-4212
SEPARATIONS Two-Site Equilibrium Model for Ion Exchange between Monovalent Cations and Zeolite-A in a Molten Salt Michael F. Simpson* and Mary Lou D. Gougar Argonne National Laboratory-West, P.O. Box 2528, Idaho Falls, Idaho 83403-2528
A two-site model has been derived and tested against experimental data for monovalent species ion exchange in molten chloride salt/zeolite A. This system is of interest for the application of spent nuclear fuel treatment. Fission products and transuranics that accumulate in a molten salt electrorefining process can be preferentially removed and eventually stabilized in a waste form, using zeolite A. An excellent fit of Cs-Rb-Na-Li-K data to the model was found with different selectivity sequences for the two sites. Cesium was found to be the preferred cation in the R-cage sites, while lithium was preferred in the framework sites. Introduction Argonne National Laboratory (ANL) has developed an electrochemical process for separating uranium from plutonium and fission products in sodium-bonded, spent nuclear fuel.1,2 This process, referred to as electrorefining, is carried out in a bath of molten, eutectic LiClKCl at 500 °C. While the uranium is collected on a solid cathode, the fission products, transuranics, and sodium are oxidized and partitioned into the salt phase as chlorides. Fission products in the fuel include Group I and II elements (Cs, Rb, Ba, Sr, etc.) and lanthanides (La, Ce, Nd, Pr, etc.). These species are allowed to accumulate in the salt over several batches of fuel treatment. Eventually, the salt must either be discarded or be treated to remove the contaminants. Depending on the composition of the fuel, regeneration of the salt may be driven by a number of different factors. The melting point of the salt may be raised above the operating temperature of the electrorefiner by the buildup of NaCl. There may be excessive heat generated due to decay of radioactive species. The plutonium concentration may become high enough to cause a criticality safety concern. To prevent any of these problems from occurring during the currently ongoing treatment of Experimental Breeder Reactor-II (EBRII) spent nuclear fuel, the salt is periodically removed from the electrorefiner and incorporated into a ceramic waste form. At the same time, pure LiCl-KCl is fed into the electrorefiner as makeup salt. In the ceramic waste process, the salt is blended with small particles of zeolite-4A to form salt-occluded zeolite. This saltoccluded zeolite is mixed with small particles of glass powder and converted to sodalite at a high temperature.3-5 However, this process is not optimized with respect to waste generation. In addition to the fission products and sodium, LiCl-KCl is thrown away in the ceramic waste. If the fission products and sodium could * To whom correspondence should be addressed. Tel.: (208) 533-7256. Fax: (208) 533-7735. E-mail:
[email protected].
be preferentially removed while returning LiCl and KCl to the electrorefiner, there could be a significant decrease in the amount of waste generated. A zeolite ionexchange column has been proposed for removing contaminants from the LiCl-KCl, allowing the salt to be returned to the electrorefiner.6 Zeolite from such a column would be highly loaded with fission products and plutonium. This saturated zeolite from the column could be blended with glass powder and converted into the sodalite ceramic waste form. Since the contaminants are concentrated in the zeolite using an ion-exchange process, there is expected to be about a 50% decrease in mass of waste using this method rather than simply blending all of the salt with zeolite and glass. Previous Work. Early work on the application of ion exchange with zeolite-A to removal of fission products from LiCl-KCl was led by Ahluwalia et al.7 Columns loaded with zeolite pellets were contacted with molten salt flowing at a constant rate. Samples of the effluent salt were used to quantify the breakthrough of each individual fission product. While it was demonstrated that the zeolite-A had a high selectivity for the fission products that needed to be removed from the salt, slow ion-exchange kinetics made it difficult to run the column process to obtain an equilibrium extent of separation. Non-zero concentrations of critical species such as Cs+ were observed shortly after breakthrough of the salt, even when employing residence times of approximately an hour. In support of this column experimental work, a comprehensive model was derived by Ahluwalia et al.7 The model accounted for ion-exchange equilibria as well as kinetics. It did a reasonable job of predicting breakthrough times for the various species. However, it did not always predict the correct ordering of the species in the effluent. Ahluwalia’s model relied upon an exchange factor approach for modeling equilibrium. And intracrystalline diffusion was assumed to be the ratelimiting step for determining the kinetics of the ionexchange reaction. These appear to be questionable decisions, perhaps explaining the discrepancies between experiment and model.
10.1021/ie020896b CCC: $25.00 © 2003 American Chemical Society Published on Web 07/31/2003
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While the study performed by Pereira et al. involved looking at the effect of several species ion exchanging with the zeolite simultaneously in a flow system, Lexa and Johnson employed a different approach.8 In their work, only three-component systems were studied, and the experimental setup included a small sample of zeolite being contacted with nonflowing salt for typically 1 h. Several different batch experiments were run, and each contained LiCl, KCl, and one other species. NaCl was typically involved as a fourth species, but only in low concentrations. The idea was to simplify the experiment enough to be able to create a clear picture of what each species was doing in the system. Some interesting observations were made in this study. For one, it was found that the number of chloride ions per unit cell in the zeolite had a strong dependence on the composition of the occluded cations. Cesium, for instance, takes up much more space inside of the zeolite than lithium or potassium. Furthermore, it was found that from the Group I elements examined, only cesium is expected to be selectively removed from the salt by ion exchange with the zeolite. Neither sodium nor rubidium exhibit a selectivity for the zeolite higher than lithium and potassium. Gougar et al. have combined the multicomponent approach of Pereira and the batch contacting approach of Lexa to gain further insight into the salt/zeolite-A ion-exchange reactions.9 Some experiments were run using radioactive salt containing real fission products as well as uranium and transuranics. This work is largely still in progress. Current Scope. What appears to be lacking from the work done to date is a predictive model for the salt/ zeolite equilibrium. An attempt was made to this end in the paper by Ahluwalia et al. using exchange factors.7 However, exchange factors were shown to vary unpredictably for this system and were fit using functions that were not based on first principles. Thus, the objective of our work was to derive a better equilibrium model based on first principles. The body of experimental data which has been selected for fitting this model is that of Lexa and Johnson. After successfully fitting the threecomponent data, it should be possible to scale the model to encompass as many components as necessary. Ultimately, this equilibrium model can be used to help predict the performance of columns or batch ionexchange processes.
tretrahedrally coordinated Al. Thus, when modeling the ion-exchange process, it will generally be necessary to use mole equivalents instead of moles for calculating concentrations. And from the formula for zeolite-A, it is known that there are 12 mole equivalent framework sites per mole of pseudo unit cells. It is a more complicated issue to determine the maximum loading in the occluded sites of the zeolite as demonstrated by Lexa and Johnson.8 Loading the R-cages with chloride salts is believed to be a spacefilling process. So the number of equivalents of ions that can be loaded into these sites per unit cell is likely to be a function of the size distribution of the ions being occluded. The charge of the ionic species is also important because a trichloride will likely occupy a different volume than three monovalent chlorides. For the framework sites, the ion exchange proceeds as shown in the equation below: n+ m+ m+ mAn+ (framework) (2) s + nBz1 h mAz1 + nBs
Framework sites are denoted by the subscript “z1”. The salt phase is denoted by the subscript “s”. The following kinetic equation is a result of balancing the forward and reverse reaction rates. n m n k1Cm A,sCB,z1 ) k2CA,zlCB,s
(3)
If all of the cations are monovalent, it is possible to explicitly solve for the concentration of one of the species in the zeolite framework in terms of measurable saltphase concentrations. In the equation below, S1 is the concentration of ion-exchange sites in the framework (12 mole per mole of zeolite pseudo unit cells).
CA,z1 )
kA,1S1CA,s kA,1CA,s + kB,1CB,s
(4)
For extending the system to multiple monovalent components, it can be shown that the following eq 5 is applicable. Note that it is similar to a Langmuir equation with the exception that it requires that all sites be occupied all of the time. The Langmuir equation itself is based on sorption, where it is possible for some sites to be empty.
Equilibrium Model It is known that cations may assume two different kinds of positions within salt-loaded zeolite-A. They can be occluded as chloride molecules in the zeolite’s R-cages, or they can be bound to the framework, balancing the negative charge from the tetrahedrally coordinated aluminum sites. As a matter of convention, we will refer to the pseudo unit cell of zeolite-A, which is shown below in eq 1. n+ M12/n (AlO2)12(SiO2)12
(1)
The framework cations are denoted in this equation as Mn+. In the formula above, it is implied that there is a single type of cation, but this is generally not the case. Virtually an infinite number of combinations of monovalent, divalent, and trivalent cations can be used to balance the framework negative charge introduced by
Ci,z1 )
ki,1S1Ci,s
∑j
(5)
kj,1Cj,s
For ion exchange using fission product-loaded LiClKCl, there may be as many as 20 different major cations in the salt. However, many of these are divalent and trivalent, requiring more complicated formulas than the type given in eq 5. This extension of the model to variable-valent cations will be addressed in a future publication. Since experiments have been performed with monovalent-only cations, the model in its current form may be tested. As previously mentioned, there are believed to be two different kinds of sites in the zeolite for ion exchange. Equation 5 pertains to framework sites. Very similarly,
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eq 6 can be derived for ion exchange in the occluded sites.
Ci,z2 )
ki,2S2Ci,s
∑j kj,2Cj,s
ki,1S1Ci,s
∑j
kj,1Cj,s
+
ki,2S2Ci,s
∑j
moles per mole of unit cells in zeolite
mole fraction in salt phase
(6)
Unlike in the case of framework sites, S2 is not easily determined and is even expected to vary as a function of salt composition. Predictive models for S2 have not yet been derived. Another important difference between the two kinds of sites is that a different set of k values are assigned to each. This distinction is based on the likelihood that there are probably different factors that lead to selectivity in each type of site. It is not possible to measure the concentration of any of the species specific to a particular kind of site. Rather, a measurement is made of the total amount of a given species exchanged into the zeolite. So eqs 5 and 6 must be combined to yield an observable concentration.
Ci,z )
Table 1. Data for Molten Chloride Salt/Zeolite-A Ion Exchange at 550 °C after 1 h of Exposure
(7)
kj,2Cj,s
All of the values for ki,1 and ki,2 can be scaled arbitrarily for the equation as written above. So one of the cations should be nominally selected to be a reference species and assigned k values of unity. Comparison with Experimental Data. To test this model, data were used from the paper by Lexa and Johnson who ran three sets of salt/zeolite equilibrium experiments.8 Each one involved a ternary mixture of monovalent chloride species (LiCl-KCl-CsCl or LiClKCl-RbCl or LiCl-KCl-NaCl). They reported concentration data in terms of weight percent for these ternary salt mixtures in contact with zeolite-4A for varying periods of time. These results indicated that equilibrium was achieved in an hour or less. Thus, all data used in the current analysis were from 1-h contacting experiments. Lexa and Johnson’s concentration values for 1-h contacting experiments have been converted to mole fraction in the salt phase and moles per unit cell in the zeolite phase and are presented in Table 1. Concentrations of each species in the zeolite were determined by scaling them relative to aluminum, which is known to have a concentration of 12 mole equivalents per mole of unit cells. Note that in the paper by Ahluwalia et al., data were presented that indicated equilibration times much longer than 1 h are required for salt in contact with zeolite-A.7 However, those experiments were performed with a salt mixture containing nine different salt compounds, including +2 and +3 cations. It is speculated that competitive ion-exchange effects slowed the equilibration process for those systems. The +3 species, in particular, appear to diffuse much more slowly into the zeolite than +1 species. Equation 7 was used to fit this set of data, optimizing with respect to ki,1 and ki,2. Before computing the leastsquares fit, S2 was calculated for each data point by summing the concentrations of the cations in the zeolite and subtracting the concentration of framework sites. S2 was found to vary from 6.3 to 12.6 mole per mole of unit cells, depending on the distribution of cations in the zeolite. The low values were due to the presence of high concentrations of Cs or Rb. The high values were
Li
K
Na
Cs
0.564 0.565 0.567 0.565 0.564 0.559 0.523 0.465 0.400 0.348 0.301 0.290 0.556 0.565 0.565 0.560 0.560 0.566 0.551 0.509 0.471 0.428 0.395 0.357 0.563 0.506 0.479
0.391 0.391 0.389 0.390 0.391 0.396 0.375 0.338 0.305 0.265 0.224 0.212 0.401 0.393 0.394 0.398 0.398 0.394 0.381 0.363 0.335 0.311 0.279 0.251 0.387 0.353 0.333
0.00811 0.00804 0.00795 0.00803 0.00791 0.00480 0.00561 0.00275 0.00678 0.00442 0.00676 0.00741 0.00622 0.00601 0.00602 0.00610 0.00604 0.00591 0.00105 0.00228 0.00269 0.00351 0.00407 0.00431 0.0496 0.141 0.188
0.0371 0.0365 0.0360 0.0365 0.0374 0.0403 0.0956 0.194 0.288 0.383 0.468 0.490
Rb
0.0363 0.0351 0.0352 0.0359 0.0356 0.0344 0.0669 0.125 0.191 0.257 0.322 0.387
Li
K
Na
Cs
16.1 16.1 16.2 16.0 16.0 15.7 13.5 12.1 11.3 10.8 10.1 10.0 14.3 15.0 14.5 14.9 15.0 15.2 15.9 15.4 14.1 14.1 12.7 11.3 17.5 16.8 16.7
4.56 4.62 4.70 4.47 4.65 4.51 3.48 2.86 2.42 2.02 1.69 1.62 5.17 5.06 5.30 5.01 5.28 5.35 4.89 3.73 3.43 2.61 2.38 2.21 4.85 4.58 4.46
0.169 0.179 0.182 0.179 0.177 0.0770 0.0328 0.0458 0.106 0.179 0.340 0.361 0.129 0.131 0.131 0.129 0.119 0.130 0.0209 0.0626 0.0612 0.0960 0.104 0.157 1.09 2.50 2.88
1.89 1.91 1.84 1.90 1.88 2.17 3.59 4.73 5.64 6.15 6.53 6.41
Rb
0.758 0.788 0.755 0.745 0.721 0.731 1.49 2.56 3.46 4.25 5.13 5.00
Table 2. Optimal Values for ki,1 and ki,2 parameter
species
type of site
value
k1,1 k2,1 k3,1 k4,1 k5,1 k1,2 k2,2 k3,2 k4,2 k5,2
Li K Na Cs Rb Li K Na Cs Rb
framework framework framework framework framework occluded occluded occluded occluded occluded
1 0.18 0.32 0.058 0.15 1 0.83 0.77 5.4 2.0
due to high concentrations of Li and K in the zeolite. Since Cs and Rb have much larger atomic sizes than Li and K, it appears likely that steric factors play a large role in determining S2. It can be envisioned that a computationally demanding, space-filling model might be necessary to accurately predict S2 for different cation distributions. However, effort is currently being expended on determining a simple approach to calculating S2 that does not require demanding computational work. Table 2 gives the values of ki,1 and ki,2 that result in the best fit to the model, using the least-squares method. Lithium has been selected as the reference species, and each of its constants have accordingly been fixed at 1. Interestingly, the two different ion-exchange sites exhibit unique selectivity sequences as seen below. And they are each different than the selectivity sequence for aqueous ion exchange in zeolite-A.10
Li > Na > K >Rb > Cs (framework) Cs > Rb > Li > K > Na (occluded) Na > K > Rb > Li > Cs (aqueous systems) In Figures 1-3, the quality of the fit is demonstrated. For each set of experiments, the zeolite loading in
Ind. Eng. Chem. Res., Vol. 42, No. 18, 2003 4211
Figure 1. Experimental versus calculated zeolite loadings for CsCl-LiCl-KCl tests ([ ) Cs; 9 ) Li; b ) K).
Figure 2. Experimental versus calculated zeolite loadings for RbCl-LiCl-KCl tests ([ ) Rb; 9 ) Li; b ) K).
equivalents per unit cell is plotted versus the mole fraction in the salt. Note that the curves are not wellbehaved because calculated zeolite loading is a function of the concentration of each species in the salt phase. And with each new data point, all of the concentrations are changing. So it is not possible to plot the zeolite loading of species i with respect to the mole fraction of i in the salt with all other species’ salt mole fractions held constant. Still, curves are drawn to make the graphs more easily readable. Otherwise, it would be very difficult to distinguish between the measured loading values and those which were calculated by the model. Conclusions A two-site model has been shown to closely fit experimental data for a monovalent molten chloride salt/zeolite-A ion-exchange system. One site is associated with bonding to the zeolite framework, balancing negative charge from tetrahedrally coordinated alumi-
Figure 3. Experimental versus calculated zeolite loadings for NaCl-LiCl-KCl tests ([ ) Na; 9 ) Li; b ) K).
num. The other site is in the zeolite’s R-cages, bonded to chloride ions. The parameters obtained in this exercise indicate that there is a marked difference in selectivity for the two different kinds of sites. Lithium is the preferred cation in the framework sites, while cesium is preferred in the occluded space. Experimentally, it has been found that the maximum loading in the R-cages decreases significantly with the increasing concentration of large cations such as cesium and rubidium. However, there has yet to be a predictive model developed for quantifying this effect. So this is an area where future work will be focused. Challenges also still exist for expanding this model to include divalent and trivalent cations. Derivation of the equations for describing equilibrium behavior in this system will be noticeably more complex in switching from monovalent to multivalent species. And subsequently, numerical methods may need to be applied to the problem. In support of expanding the model to multivalent species, experimental work has already been performed and will be the subject of forthcoming publications. It is of particular interest to determine whether this model can be successfully scaled to include the numerous monovalent, divalent, and trivalent cations found in the molten electrorefiner salt. If it can be scaled as needed, this model will be a valuable tool for designing a zeolite/salt ion-exchange process to be incorporated into a commercial process for treating spent nuclear fuel. Acknowledgment The authors would like to acknowledge Dr. Dusan Lexa’s experimental work in this area as well as his helpful comments and suggestions for this paper. Support of this research was by the U.S. Department of Energy under Contract W-31-109-ENG-38. Literature Cited (1) Laidler, J. J.; Battles, J. E.; Miller, W. E.; Ackerman, J. P.; Carls, E. L. Prog. Nucl. Energy 1997, 31, 131. (2) Benedict, R. W.; McFarlane, H. F. Radwaste Mag. 1998, July, 23. (3) Goff, K. M.; Benedict, R. W.; Bateman, K. J.; Lewis, M. A.; Pereira, C.; Musik, C. A. Spent Fuel Treatment and Mineral Waste
4212 Ind. Eng. Chem. Res., Vol. 42, No. 18, 2003 Form Production and Testing at ANL-West. Proc. Int. Top. Mtg. Nucl. Hazard Waste Manage. 1996, 3, 2436. (4) Simpson, M. F.; Goff, K. M.; Johnson, S. G.; Bateman, K. J.; Battisti, T. J. Toews, K. L.; Frank, S. M.; Moschetti, T. L.; O’Holleran, T. P. A Description of the Ceramic Waste Form Production Process from the Demonstration Phase of the Electrometallurgical Treatment of EBR-II Spent Fuel. Nucl. Technol. 2001, 134, 263. (5) Hash, M. C.; Thalacker, J. P.; Simon, D. R.; Burns, G. L.; Woodman, R. H. Environmental Issues and Waste Management Technologies in the Ceramic and Nuclear Industries IV; Marra, J. C., Chandler, G. T., Eds.; 1999; p 229. (6) Pereira, C.; Hash, M. C.; Lewis, M. A.; Richmann, M. K.; Basco, J. Mater. Res. Soc. Symp. Proc. 1999, 556, 115. (7) Ahluwalia, R. K.; Geyer, H. K.; Pereira, C.; Ackerman, J. P. Ind. Eng. Chem. Res. 1998, 37, 145.
(8) Lexa, D.; Johnson, I. Occlusion and Ion Exchange in the Molten (Lithium Chloride-Potassium Chloride-Alkali Chloride) Salt + Zeolite 4A System with Alkali Metal Chlorides of Sodium, Rubidium, and Cesium. Metall. Mater. Trans. 2001, 32B, 429. (9) Gougar, M. L. D.; Simpson, M. F.; Battisti, T. J. Ion Exchange of Fission Products Between Zeolite and a Molten Salt. Light Metals 2002; The Minerals, Metals, & Materials Society: Warrendale, PA, 2002; p 37. (10) Breck, D. W.; Eversole, W. G.; Milton, R. M.; Reed, T. B.; Thomas, T. L. J. Am. Chem. Soc. 1956, 78, 5963.
Received for review November 8, 2002 Revised manuscript received May 30, 2003 Accepted June 2, 2003 IE020896B
