Patent GB-871649-A (GB)
Title: Process for the recovery of the halides of niobium and tantalum from their adducts with phosphorus oxychloride
IPC: C01G33/00
Filed: 1957-11-11
Abstract: Niobium or tantalum halides are recovered from adducts of the halides of those metals with phosphorus oxychloride (see Specification 833,569) by reacting the adduct with an alkali metal halide to form a double salt with the metal halide constituent of the adduct, and then, after separating the phosphorus oxychloride liberated, subjecting the double salt to thermal decomposition.  The adducts may be formed by reacting niobium and tantalum pentachlorides in the solid or vapour state with liquid or vaporous phosphorus oxychloride and the solid compounds obtained are normally freed from excess phosphorus oxychloride.  The reaction of the phosphorus oxychloride adducts with the alkali metal halides is normally effected below 500 DEG C. either at atmospheric or superatmospheric pressure with the exclusion of oxygen and moisture.  An inert atmosphere such as nitrogen or carbon dioxide is preferred.  The vapours of the adduct may be diluted with an inert carrier such as nitrogen and reacted at 280 to 500 DEG C. with solid potassium chloride which may itself be diluted with an inert solid such as carbon.  Phosphorus oxychloride liberated is condensed.  Alternatively anhydrous potassium chloride may be heated with the molten adducts or with a solution of the adducts in an inert solvent such as thionyl chloride.  The thermal splitting of the double salts formed is normally effected at 600 to 800 DEG C. whereby a phosphorus-free halide of niobium or tantalum sublimes and phosphorus chloride remains as a solid residue.  The decomposition is preferably effected under reduced pressure in the absence of oxygen.  The formation of the double salts and their decomposition may be effected in separate fluidised beds.  According to Examples (1) a tantalum pentachloride/phosphorus oxychloride adduct was dissolved in thionyl chloride and anhydrous finely pulverised potassium chloride added.  The mixture was heated in an atmosphere of carbon dioxide under reflux, filtered and the filtrate evaporated to dryness.  The residue was heated at 600 DEG C. in nitrogen to obtain tantalum pentachloride and the phosphorus oxychloride was recovered by fractional distillation of thionyl chloride.  Niobium pentachloride may be similarly prepared.  (2)A tantalum pentachloride phosphorus oxychloride adduct was treated with finely divided potassium chloride in an atmosphere of carbon dioxide in a vessel fitted with a reflux condenser and a dephlegmator.  The latter condensed the vapours to liquid which fell dropwise on to the heated potassium chloride.  Phosphorus oxychloride liberated was separately recovered and on completion of the reaction the potassium tantalum chloride double salt was heated in oxygen free nitrogen at 600 DEG C. to obtain tantalum pentachloride.  Niobium pentachloride may be similarly prepared. (3) A vaporized stream of tantalum pentachloride phosphorus oxychloride adduct diluted with a nitrogen was passed through a layer heated to 370 DEG C. of granular potassium chloride.  On completion of the reaction the temperature was raised to 550 DEG C. whereupon the tantalum pentachloride sublimed.  (4) Phosphorus oxychloride/tantalum pentachloride adduct was heated with potassium fluoride in nitrogen and the liberated phosphorus oxychloride removed.  The residual double salt of potassium fluoride and tantalum pentachloride was decomposed at 500 DEG C. to volatilize the tantalum pentachloride.  (5) A phosphorus oxychloride/niobium pentachloride adduct was heated up to 400 DEG C. with potassium fluoride and the resulting double salt was decomposed at 500 DEG C. Niobium pentachloride was volatilized and the blue residue contained a small proportion of tetravalent niobium which could be recovered as pentachloride by heating in chlorine.