Production of aluminum fluoride by thermal decomposition of aluminum double fluorides

摘要

In the production of aluminium fluoride, solid aluminium hydrate is intimately mixed with such an amount of a solid acid fluoride salt of ammonia that it contains at least six and preferably twelve atoms of fluorine for each molecule of alumina used, and the mixture heated at 100--200 DEG C., until a double aluminium ammonium fluoride is formed, after which the temperature is gradually raised to 500--600 DEG C., to decompose the double salt and yield anhydrous aluminium fluoride. If desired, the second heating step may be omitted and the double fluoride recovered as such. The initial materials may contain free or hygroscopic moisture. Vapours evolved in the second heating step are collected and reformed into ammonium acid fluoride for re-use by combination with hydrofluoric acid; for instance they may be first passed through aluminium hydrate to remove ammonium acid fluoride and then into aqueous hydrofluoric acid. The ammonium acid fluoride may be prepared in situ as by using normal ammonium fluoride in the initial mixture, or by mixing aluminium hydrate with an ammonium compound such as the carbonate, and heating the mixture in the presence of hydrofluoric acid gas. The process may be carried out continuously in an externally heated rotating tube having a temperature gradient of 150--600 DEG C., which contains initially a charge of ammonium acid fluoride. Hydrate of alumina is fed into the cool end of the tube and hydrofluoric acid at the hot end. In the furnace the double salt is first formed and then decomposed at higher temperatures, and the vapours evolved react with the hydrofluoric acid to form ammonium acid fluoride which condenses at the cool end of the tube and reacts with further supplies of aluminium hydrate. The process may be carried out in aluminium vessels, or in the case of the continuous process, also in vessels constructed of iron or steel.