A new process for the separation of hafnium from zirconium
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THE
world's production of zirconium m e t a l is b a s e d almost exclusively on the treatment of two minerals; zircon, zirconium silicate, accounts for the bulk of production although baddeleyite, an impure zirconium oxide, is also treated. These two ore minerals, like all zirconium minerals, contain anywhere from 0.5 t o s e v e r a l percent hafnium which substitutes for zirconium in the m i n e r a l lattice.1 So g r e a t is the chemical similarity of zirconium and hafnium that unless special and costly techniques are followed to r e m o v e the hafnium impurity, all the zirconium m e t a l eventually produced will contain the same amount of hafnium as did the starting concentrate. Zirconium destined for use in nuclear reactors, and this is by far the m a j o r application for zirconium metal, must be substantially hafnium-free. This need a r i s e s bec a u s e the c r o s s section for the capture of thermal neutrons is 0.18 barn/atom for Z r and about 105 barn/atom for Hr. Whereas pure zirconium is r e l a tively transparent to t h e r m a l neutrons, m e t a l containing 2 pct Hf has a capture c r o s s section g r e a t e r than 2 barn/atom. Such materials would seriously interfere with the neutron flux in atomic reactors using t h e r m a l neutrons, and virtually all reactors in service today are of this t y p e . Solvent extraction processes are universally used today t o r e m o v e hafnium from zirconium; such a separation step is shown in Fig. 1, which is a conceptual flowsheet for the treatment of a zircon concent r a t e t o yield nuclear g r a d e zirconium. The f i r s t step in the process is the production of zirconium (hafnium) tetrachloride by high temperature gaseous chlorination of either zircon-carbon composites or zirconium carbide previously prepared by the carbothermic reduction of zircon. The chloride is p u r i fied by distillation, often in the presence of reducing agents, t o yield a high grade, anhydrous Zr(Hf)C14 material. The anhydrous tetrachloride is subsequently dissolved in an aqueous medium and the com-
position of the resulting solution is adjusted with l a r g e amounts of nitric acid, thiocyanic acid, ammonium thiocyanate, and so forth, in preparation for the solvent extraction s t e p . During solvent extraction, the aqueous Zr(Hf)-bearing solution is repeatedly contacted with an organic phase2-~ such as tributyl phosphate in kerosene or methyl isobutyl ketone (hexone) t o effect a separation of the two elements. For either solvent extraction system, the hafniumfree zirconium undergoes various precipitation-dissolution s t e p s until it finally reports as a hydrous zirconium oxide precipitate which is calcined, m i x e d with carbon and subsequently chlorinated t o produce the zirconium tetrachloride feed for the Kroll r e a c tor. Various alternatives t o solvent extraction have been proposed to effect the zirconium-hafnium separation in aqueous solution. Fractional crystallization and fractional precipitation techniques were the f i r s t to be used t o produce s m
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