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 oxygen-bearing ceramics such as silica. The resulting product is one that no longer contains the molecular sieve framework and is therefore useless as an ion-exchange medium. Variations in either the temperature between 500 and 740[ddot]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 equally affected by the fluoride ion. It was therefore determined that the fluoride ion must first be removed from molten salt mixtures if aluminosilicate zeolites are to be used as an ion-exchange media in contact with them. Gaseous SiCl 4 has been found to remove fluoride ion in molten salts to at least 0.1 mol % by the reaction: The efficiency of the process is largely determined by the effectiveness of the gas∅melt contact. Attractive features of this reaction include ease of separation of unreacted SiCl 4 from SiF 4 for eventual recycle of both.