Patent GB-876802-A (GB)
Title: Process for the reduction of double salts of niobium or tantalum pentahalides
IPC: C01G33/00
Filed: 1957-12-23
Abstract: Double halides of an alkali metal or alkaline earth metal with niobium and/or tantalum in a valency state less than 5, are prepared by reducing an anhydrous double salt or an alkali or alkaline earth metal and pentavalent niobium and/or tantalum with hydrogen at temperatures above 280 DEG C.  According to a modification, the formation of the double salts to be reduced is carried out simultaneously with their reduction to the lower valent state by contacting tantalum and/or niobium pentahalides with alkali or alkaline earth metal halides in the presence of hydrogen.  Any halide may be employed but particularly preferred is a double salt of niobium or tantalum pentachloride with an alkali metal, in particular sodium, fluoride or an alkali or alkaline earth metal chloride.  The preferred salt is of general formula Kx[MCl8-y] where M is niobium or tantalum and x+y is from 4 to 7.  The initial double salts are preferably prepared by reacting at least one mol of alkali metal halide or at least 0.5 mol of an alkali earth metal halide with the niobium or tantalum pentahalide.  Such reaction may be effected in an inert atmosphere such as nitrogen or carbon dioxide under atmospheric or superatmospheric pressure and temperatures normally from 300-500 DEG C.  Thus niobium or tantalum pentachloride diluted in an inert carrier such as nitrogen may be reacted at 300-500 DEG C. with potassium chloride in a shaft or tubular furnace. The potassium chloride may be mixed with an inert carrier. Alternatively anhydrous potassium chloride may be heated with niobium or tantalum pentahalide in the molten state or in the presence of an inert solvent such as thionyl chloride.  Reduction of the halide is generally effected at 300-400 DEG C. to produced the double salt with niobium or tantalum in the tetravalent state, at about 500 DEG C. to produce trivalent niobium or tantalum and at 600 DEG C. to produce the monovalent state.  At temperatures above 650 DEG C. reduction to the metal is likely to occur.  It is preferred in all cases to reduce the double salt initially at temperatures at 450-500 DEG C. thereby obtaining a reduced salt containing tetravalent niobium or tantalum.  Reduction may be effected at atmospheric or superatmospheric pressure in a shaft or tubular furnace containing the pentavalent double salt which may if desired be mixed with alkali or alkaline earth metal halide. Alternatively reduction may be effected in a liquid phase by treating the pentavalent double salt with hydrogen in a molten salt bath or an inert non-ionised chlorine-containing solvent such as thionyl chloride or sulphuryl chloride.  Reduction may also be effected by passing the pentahalide vapour over or into a melt of an alkali or alkaline earth metal halide.  According to Examples (1) tantalum pentahalide was heated with dry potassium chloride in the presence of thionyl chloride, excess potassium chloride was filtered off and the thionyl chloride evaporated.  A double salt of formula TaCl6 formed was reduced with hydrogen at 400 DEG C. to obtain a double salt of formula K2TaCl6, (2) potassium niobium double halide was reduced at 550 DEG C. with hydrogen to produce a mixture of 8K2NbCl5 and K4NbCl5 which when mixed with its own weight of dried potassium chloride forms a stable melt at 800 DEG C., (3) potassium niobium double halide was mixed with potassium chloride and reduced with hydrogen at 500 DEG C. to obtain a mixture of potassium chloride containing K6NbCl2 and K3NbCl6, (4) niobium pentachloride vapour in a current of hydrogen is passed through a bed of dry potassium chloride granules at 500 DEG C.  The pentachloride is reduced to lower chlorides on the potassium chloride bed to form a lower valent brown-violet double salt, (6) melts of potassium and sodium chloride with or without potassium fluoride are treated with niobium pentachloride and hydrogen whereby the niobium dissolves to form a double salt containing niobium with an apparent valence of 2.8, (7) hydrogen and niobium pentachloride were passed over a potassium chloride melt at 950 DEG C. and niobium metal crystallised out on top of the melt.ALSO:Metallic niobium or tantalum is prepared by reducing with hydrogen a double halide of an alkali or alkaline earth metal and niobium or tantalum at temperatures above 650 DEG C.  In a modification niobium or tantalum pentahalide is passed over the surface of a molten alkali metal halide bath in the presence of hydrogen whereby the niobium or tantalum metal forms on the surface of the melt.  According to an Example (7) a mixture of 10 mols of hydrogen with 1 mol of niobium pentachloride was passed over the surface of a potassium chloride melt at 950 DEG C.  Reduced double salts are formed in the vapour space above the melt which fall back into the melt and are there reduced to the metal.  The latter is formed as crystals and as a cohesive layer on top of the melt.