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

Abstract

Interaction between molten salts of the type LiCl-KCl-MeCl (Me ϭ Na, Rb, Cs, x MeCl ϭ 0 to 0.5, x KCl /x LiCl ϭ 0.69) and zeolite 4A have been studied at 823 K. The main interactions between these salts and zeolite are molten salt occlusion to form salt-loaded zeolite and ion exchange between the molten salt and salt-loaded zeolite. No chemical reaction has been observed. The extent of occlusion is a function of the concentration of MeCl in the zeolite and is equal to 11 Ϯ 1 Cl Ϫ per zeolite unit cell, (AlSiO 4 ) 12 , at infinite MeCl dilution. The ion-exchange mole fraction equilibrium constants (separation factors) with respect to Li are decreasing functions of concentration of MeCl in the zeolite. At infinite MeCl dilution, they are equal to 0.84, 0.87, and 2.31 for NaCl, RbCl, and CsCl, respectively, and increase in the order Na Ͻ Rb Ͻ Cs at identical MeCl concentrations. The standard ion-exchange chemical potentials are equal to Ϫ(0.0 Ϯ 0.5) kJиmol Ϫ1 , Ϫ(0.4 Ϯ 0.3) kJиmol Ϫ1 , and Ϫ(6.5 Ϯ 0.5) kJиmol Ϫ1 for Na + , Rb + , and Cs + , respectively.

I-NERI ANNUAL TECHNICAL PROGRESS REPORT: 2006-002-K, Separation of Fission Products from Molten LiCl-KCl Salt Used for Electrorefining of Metal Fuels

“…In this case, diffusion coefficient (D) can be related to the exchanged amount of each species at any given time (Q t ) (Refs. 3 and 4): S f Dt r 6 Q / Q t [4] where Q is the exchanged amount at equilibrium, and r is the radius of the zeolite particle. By analyzing the near time-zero slope of Eq.…”

“…By analyzing the near time-zero slope of Eq. [4], the value of D can be related to a rate constant (K) by plotting Q t /Q f versus t 1/2 ; that is S D r 6 K [5] Moreover, by dimensional inspection, it can be seen that K 2 has the dimension of (time) -1 showing that the kinetics reaction associates with the first order reaction rate under the mass transfer limited. The third approach can also be used to determine the rate constants k. Here, k i is the forwarding rate constant of species i of the reaction type presented by Ref.…”

“…Strontium reaches at its equilibrium stage approximately after 3 hrs (Ref. 4). The samples of salt and zeolite phases were taken every 30 minutes interval up to 6 hrs in the first experiment, to observe the time to reach the equilibrium stage.…”

“…The value of D and K were determined by using Eqs. [4] and [5]. Figure B1 Tables 5 and 6 for cesium and strontium, respectively.…”

“…3) Previous experimental work on ion exchange between zeolite-A and molten chloride salts has been almost entirely focused on the equilibrium condition (Refs. [4][5][6]. As a result, fundamental kinetics information essential for design and operation analysis of this process is lacking.…”