(44a) Kinetic Study of Ion Exchange Between Multivalent Cations and Zeolite-4A in a Molten Salt

One of the important steps in pyrochemical spent fuel processing is removal of spent electrolyte from electrorefiners. This electrolyte is a molten chloride salt consisting of LiCl, KCl, NaCl, actinide chlorides, and fission product chlorides. Current technology being used for processing spent fuel from Experimental Breeder Reactor-II (EBR-II) at Idaho National Laboratory (INL) involves absorption of the salt into zeolite-4A followed by thermal consolidation of the material into a ceramic waste form. An alternative salt treatment process involving ion exchange into zeolite-A has been proposed and studied to allow for the recycle of most of the LiCl-KCl back to the electrorefiner and, thus, minimize waste volume generated from the process. Previous experimental work and model development on ion exchange between zeolite-A and molten chloride salts has been focused on the equilibrium condition.^1-4 However, an understanding of the fundamental kinetics of ion exchange for this system is needed for optimal design and operation of this process.

Ion exchange kinetics experiments have recently been performed and are reported in this paper. Testing was performed using a lab-scale furnace operating at 500°C in an argon atmosphere glovebox with the moisture and oxygen contents below 0.1 ppm to prevent contamination of the zeolite or salt. Cesium (Cs), strontium (Sr), and neodymium (Nd) were selected as the surrogate fission product species to capture monovalent, divalent, and trivalent cation behavior for ion exchange between zeolite-4A and molten eutectic LiCl-KCl salt. Concentrations of Cs ternary salt (CsCl-LiCl-KCl), Sr ternary salt (SrCl₂-LiCl-KCl), and Nd ternary salt (NdCl₃-LiCl-KCl) were varied from 1.0 to 5.0 wt%. Zeolite-4A pellets were loaded in a rotating stainless steel mesh basket and lowered into a magnesium oxide crucible containing the molten salt mixture. The ion exchange experiments were run from 2 to 12 hrs. Samples of the zeolite phase were taken at prescribed intervals of time. Those samples were then dissolved with various acids and analyzed using an inductively-coupled mass spectrometer (ICP-MS) to measure ion loading in the zeolite. Exchange of each cation was measured using this technique as a function of time.

Preliminary results of this study reveal that Cs loading in the zeolite reaches its equilibrium level after approximately 30 minutes of contact time, similar to the results reported in literature.^3-4 However, Nd exhibits unexpected behavior. It reaches its maximum loading in the zeolite at approximately 100 minutes of contact time and then decreases over subsequent time intervals. It is expected that there is a possibility of neodymium oxy-chloride formation during this process. Further detailed studies and results, including ion exchange of Sr, will be presented and discussed.

References:

1. S. Phongikaroon and M. F. Simpson, ?Two Site Equilibrium Model for ion exchange between multivalent cations and zeolite-A in a molten salt,? AIChE Journal, Vol. 52, 1736-1743 (2006).

2. M. F. Simpson, and M.L. Dunzik-Gougar, ?Two-site equilibrium model for ion exchange between monovalent cations and zeolite-A in a molten salt, Industrial & Engineering Chemistry Research, 42, 4208-4212 (2003).

3. D. Lexa, and I. Johnson, ?Occlusion and ion exchange in the molten (lithium chloride-potassium chloride-alkali metal chloride) salt + zeolite 4A system with alkali metal chlorides of sodium, rubidium, and cesium,? Metallurgical and Materials Transactions B, Vol. 32B, 429-435 (2001).

4. M. L. Dunzik-Gougar, M. F. Simpson, and B. E. Scheetz, ?Two-site equilibrium model for ion exchange between multi-valent cations and zeolite-A in a molten salt,? Microporous and Mesoporous Materias, Vol 84, 366-372 (2005).