Investigation of the properties of lithium fluoride based sorbent

Abstract

The results of an investigation of the physicochemical and sorption properties of pelleted lithium fluoride based sorbent obtained using ammonium carbonate as the porophore are presented. It is shown that the properties of granular lithium fluoride using ammonium carbonate are similar to those of a sorbent obtained on the basis of lithium fluoride and ammonium fluoride. The results of sorption separation of the mixture WF 6 -HF on granular sorbents are presented. The sorbent possesses selectivity to hydrogen fluoride and makes it possible to remove efficiently the volatile fluorides of refractory metals while strictly maintaining the quality of the sorbent and the conditions under which sorption and desorption are performed.Selective catching of hydrogen fluoride on lithium fluoride has been well studied and has found application for obtaining and purifying uranium hexafluoride. At the first step in the development of the technology, powdered lithium fluoride with an extended surface, which together with high sorption activity had drawbacks also, was used. For example, hydrogen fluoride extraction on LiF powder requires a horizontal apparatus as adsorbents, which makes it impossible to attain the required contact time between the adsorbate and the sorbent, so that the production capacity is limited. Vertical adsorbents filled with powdered materials are impossible to use because of the high gas-dynamic resistance of the layer. To improve the efficiency of sorption extraction of hydrogen fluoride, a technology was developed for making pelleted lithium fluoride based sorbents. It incorporates charge formation, granulation, and heat-treatment of the sorbents and uses ammonium fluoride as a porophore agent [1].Ammonium fluoride is a corrosive substance. Corrosion activity, accompanied by the formation of hydrogen fluoride and ammonia gases, is highest in the presence of decomposition during drying and calcination. At temperatures above 140°C, ammonium fluoride decomposes via the reaction NH 4 F ↔ NH 3 ↑ + HF↑. This reaction is reversible: on cooling recombination occurs with ammonium fluoride being formed. On an industrial scale, when outgoing gases containing NH 3 and HF come into contact with the cold walls of ventilation pipes and pipelines they recombine forming ammonium fluoride in the form of solid corrosive deposits. As the sorbents undergo calcination, the action of hydrogen fluoride vapors on the process equipment results in strong corrosion of the metal surfaces of the furnace chambers, drawing boats, and baking pans while the products of corrosion, in turn, contaminate the sorbent. The contaminated sorbent cannot be used to purify volatile fluorides, since the products of corrosion form chemical compounds with the latter.Thus, ammonium fluoride is best used as a porophore only on laboratory and research scales. A porophore that does not have these drawbacks must be used for prototype and commercial production.This article presents the results of investigations of the physico-mechanical and sorption propert...