Purifications and subsequent exposures to zeolite were conducted in the same 4-in.-IPS nickel vessel. Numerous 20-mil aliquots of Linde 4A, sodium form, No. 8 mesh zeolite were equilibrated to constant weight (five weeks) over CaClz*6H20 solid and saturated aqueous solution at 24.5"C. Each aliquot was dried in a filter tube for 18 hr at 375째C with argon purge. The filter tube was then transferred under continuous argon purge to the extraction vessel and slowly lowered into the molten chlorides at 650OC. Salt was then pushed alternately into and out from the tube by pressure fluctuations. Inflow was damped by autopressurization of argon in the top of the tube and was terminated upon contact of salt with an electric probe. Contact times were approximately 20 sec per stroke; contacted volumes were 10% of salt inventory per stroke. Although 50 strokes were usually made per aliquot, no significant increase of loading was noted from 20 to 100 strokes. The exposed zeblife was recovered as loose 'spheres, unaltered in appearance except for a film of salt on the surface. It was very radioactive.and could be monitored with a survey meter. Since the specific activity involved an indeterminate amount of salt, however, the amount of cerium removed per aliquot was computed from physical inventory, and the decreasing radioactivity of filtered salt samples removed after each batch exposure. The data obtained are shown in Table 8.1. Within the precision of the data the distribution of cerium between the molten chloride and the solid zeolite phases can be expressed empirically by the equation
established, cumulative errors introduced in material balance calculations prevented further evaluation of the rareearth removal process in this experiment. Chemical analyses for sodium in the salt showed a steady increase and indicated, as shown in Table 8.1, that exchange with lithium was essentially complete. This finding is.contrary to the reported equiliirium quotient DNJDLi = 78.4 for this zeolite in LiCl at 650"C,'4 and suggests that a processing plant for a molten-salt thermal reactor would require that Zeolite 4A be in the lithium form, with 'Li. Analyses for aluminum in the salt were several hundred ppm but showed no rising trend. Zeolite 4A is initially floated by molten lithium chloride, but filling of the void space between crystallites with LCl causes the spheres to sink to the bottom. Complete filling of the void space between crystallites would result in the consumption of approximately 0.16 g of lithium per gram of dry, lithium form, Zeolite 4A. Molecular inclusion of lithium chloride within crystallite cavities would increase this consumption by possibly 50%.' This consideration involving conservation of materials would need to be included in the final evaluation of a zeolite process. Further experiments are intended to measure the removal of other fBsion products, both divalent and trivalent, from molten LiCl by zeolite. Wide variations may be found since the possible mechanisms include ion exchange, reaction with the zeolite structure, and formation of molecular inclusion complexes.
In (ppm Ce in salt) = 12.163 + 1.35 In (g Ce/g zeolite) for concentrations down to 4600 ppm in the salt phase. ' Although an effective experimental procedure was
14. W. A. Platek and J. A. Marinsky,J. Phys. Chem 65,2118 (1961). 15. R. M. Barrer and W. M. Meier,J. Chem Soc. 299 (1958).
Table 8.1. Removal of cerium from molten LCl by Zeolite 4A Number of exposuresto 14 g of No. 8 mesh dry Linde Zeolite 4A
1 2 3 4 5
ppm Ce in salt
Grams of cerium removed per gram of zeolite
Fraction of total sodium exchanged
14,200 11,500 9,000 7,800 6,200 4,600
0.133 0.125 0.115 0.054 0.071 0.067
0.98 0.65 0.73 0.92 0.96 0.72