The manufacture of phosphorus oxychloride addition products and their use for separating niobium and tantalum from mixtures containing them

摘要

An addition product of tantalum or niobium pentachloride and phosphorus oxychloride is prepared by contacting the constituents under anhydrous conditions. Amongst the products, compounds having a ratio of metal pentachloride to phosphorus oxychloride of 1:1 are formed. Tantalum and niobium may be separated by fractional distillation of the addition products. The pentachloride may be reacted with phosphorus oxychloride by dissolution of solid or gaseous pentachloride in the liquid oxychloride at ordinary or elevated temperature, excess oxychloride being then removed by distillation at 100-200 DEG C. at atmospheric pressure or by reacting phosphorus oxychloride vapour (or an inert gas containing phosphorus oxychloride vapour) with the solid pentachlorides, preferably at above the meltingpoint of the addition products, i.e. at 100-180 DEG C., or with the pentachloride in gaseous form, e.g. when leaving a chlorination zone. An initial mixture of halides may be obtained by briquetting slags, concentrates or ores containing niobium and tantalum in oxidized form with carbon and chlorinating with chlorine gas at 400-1000 DEG C.; if the products contain niobium oxychloride they are further chlorinated in the presence of carbon to convert the oxychloride to pentachloride. Chlorides such as silicon, titanium, tin and manganese chlorides may thereafter if desired be at least partly removed by condensing the mixture under suitable conditions. Alternatively, the initial mixture may be prepared by treating niobium and tantalum oxides with phosphorus pentachloride at 200 DEG C. in the absence of air or moisture or with a current of dry carbon tetrachloride or by chlorinating an alloy of niobium and tantalum. If the crude chlorination gases are dissolved in phosphorus oxychloride, addition products with zirconium and titanium chlorides separate in crystalline form and may be removed. The niobium and tantalum addition products may, if desired, be subjected to a preliminary distillation to remove other metals. The separation of niobium and tantalum by fractional distillation may be carried out under reduced pressure or under atmospheric pressure with the exclusion of moisture and in air or an inert gas such as nitrogen or carbon dioxide. The tantalum product has the higher boiling-point. Zirconium, aluminium and iron chloride addition products, if present, remain in the residue. It is desirable to add to the mixture before distillation an inert substance boiling above the tantalum pentachloride addition product, e.g. the addition product with aluminium chloride (or aluminium trichloride itself). If desired the products may be redistilled, and the pentachlorides then recovered by adding the addition products to a solvent at room temperature or an elevated temperature. Suitable solvents are those which dissolve or form addition products with phosphorus oxychloride without decomposing and in which the tantalum and niobium pentachlorides are insoluble, e.g. liquid hydrocarbons and carbon tetrachloride, chloroform and chlorobenzene, and solvents which form new addition products with niobium and tantalum pentachlorides which are insoluble in the solvent and unstable at raised temperatures, e.g. ethyl ether, ethyl acetate and acetone. The phosphorus oxychloride formed may be separated, e.g. by distillation, and re-used in forming further quantities of reaction product and the process made cyclic. Examples are given in some of which in addition to the metals mentioned above, silicon, manganese and tungsten are mentioned as being present in the addition products.