FLUORIDE REMOVAL FROM MOLTEN-SALT SOLUTIONS BY SICL4 SPARGING

Molten salts containing fluoride ion react destructively with Zeolite 4A through a mechanism that is typical of fluoride reactions with oxyg en-bearing ceramics such as silica. The resulting product is one that no longer contains the molecular sieve framework and is therefore usel ess as an ion-exchange medium. Variations in either the temperature be tween 500 and 740 degrees C or in the fluoride/chloride concentration ratio from 0.05 to 1.0 produced little improvement in the stability of the zeolite framework with respect to fluoride ion attack. Other ion- exchange media such as mica have been considered, but they should be e qually affected by the fluoride ion. It was therefore determined that the fluoride ion must first be removed from molten salt mixtures if al uminosilicate zeolites are to be used as an ion-exchange media in cont act with them. Gaseous SiCl4 has been found to remove fluoride ion in molten salts to at least 0.1 mol % by the reaction: 2CaF(2) + SiCl4 = 2CaCl(2) + SiF4 up arrow. The efficiency of the process is largely det ermined by the effectiveness of the gas\melt contact. Attractive featu res of this reaction include ease of separation of unreacted SiCl4 fro m SiF4 for eventual recycle of both.