GB735617A - Improvements relating to the manufacture of anhydrous silicon, boron and metal halides - Google Patents

Abstract

Chlorides, bromides and iodides volatile at temperatures below 1200 DEG C. of silicon and boron and of metals other than the alkali metals, alkaline earth metals and rare earth metals, specifically those of Ti, Si, Ge, Zr, Hf, Th, Al, B, Nb, Ta, Fe, Sn, Mo, W, As, Sb, Ga, are prepared by heating to at least 500 DEG C. an anhydrous mixture of an alkali metal double fluoride of the desired metal and the chloride, bromide or iodide of magnesium, calcium or lithium and withdrawing the desired vaporized chloride, bromide or iodide leaving as residue the fluoride of magnesium, calcium or lithium, and that of the alkali metal employed. The starting materials may be individually or collectively rendered anhydrous prior to reaction, suitably in situ, for example by heating to 350 DEG C., until the pressure can be maintained under vacuum pumping, as low as 200 microns. Stoichiometric proportions of pure recrystallized reactants are preferred and the reaction residue may be recycled to produce the double fluoride employed or converted to the bromide, chloride or iodide employed. The reaction mixture is preferably not fused and the reactants are preferably mixed and charged as briquettes. If the reaction mixture is fused, the reaction may be carried out in a diluent bath of alkali metal halides or the alkali metal double fluoride may be charged incrementally to a fused bath of the molten chloride, bromdie or iodide of magnesium, calcium or lithium. The reaction chamber should be under vacuum or swept with an inert gas to entrain the evolved vapours and conduct them to a cooling zone. The reaction residue, for example magnesium and potassium fluoride, may be employed to produce the alkali-metal double fluoride of the desired metal by aqueous phase reaction, for example, by reaction with aluminium sulphate. Lithium and calcium fluoride residues are suited for a fusion process of alkali metal double fluoride production or they may be converted to chlorides and recycled.