Kinetics of the vapor phase chlorination of niobium oxychloride by phosgene
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The k i n e t i c s of the homogeneous gas phase r e a c t i o n between n i o b i u m oxychloride and phosgene was studied between 380 ~ and 450~ in a c o n s t a n t volume r e a c t o r . The r e a c t i o n was found to be e s s e n t i a l l y i r r e v e r s i b l e with the only detected r e a c t i o n p r o d u c t s being n i o b i u m p e n t a c h l o ride and c a r b o n dioxide. The data o b t a i n e d were fit by an e l e m e n t a r y s e c o n d - o r d e r r a t e equation. V a l u e s of the A r r h e n i u s f r e q u e n c y factor and a c t i v a t i o n e n e r g y were found to be 1.33 x 106 l i t e r per g mol sec and 21.9 kcal per g mol with s t a n d a r d d e v i a t i o n s of 0.05 • 106 and 1.4, r e s p e c t i v e l y .
O N E p r o b l e m f r e q u e n t l y e n c o u n t e r e d in the c h l o r i n a t i o n p r o c e s s for p r o d u c t i o n of pure n i o b i u m m e t a l is the r e m o v a l of n i o b i u m oxychloride (NbOCI'a) f r o m n i obium p e n t a c h l o r i d e (NbCls) p r i o r to the r e d u c t i o n step. NbOCls is u s u a l l y p r o d u c e d along with NbCls d u r i n g c h l o r i n a t i o n of the o r i g i n a l o r e s and oxides and m a y also be f o r m e d d u r i n g s u b s e q u e n t o p e r a t i o n s by hyd r o l y s i s of the h y g r o s c o p i c NbC15. If p r e s e n t with NbCls d u r i n g the r e d u c t i o n step, the NbOCls will be r e d u c e d to n i o b i u m oxide and s e r i o u s l y d e g r a d e the f i n a l m e t a l product. A p r o m i s i n g approach to the r e m o v a l of NbOCls from NbOCls--NbC15 m i x t u r e s is the c o n v e r s i o n of NbOCls to NbCls by c h l o r i n a t i o n . I n v e s t i g a t i o n s of such c o n v e r s i o n s have been made ~-4 u s i n g v a r i o u s c h l o r i n a t i n g agents (e.g., C12 and c a r b o n , CC14, SOC12, and COC12). F r o m a t h e r m o d y n a m i c standpoint, the r e a c t i o n of NbOCla with these a g e n t s i s highly f a v o r able and, at e q u i l i b r i u m , the c o n v e r s i o n to NbCls is e s s e n t i a l l y complete. T h e s e r e a c t i o n s , h o w e v e r , a r e not i n s t a n t a n e o u s , p a r t i c u l a r l y as the c o n c e n t r a t i o n of the p r i n c i p a l r e a c t a n t , NbOCls, a p p r o a c h e s z e r o . Since n e g l i g i b l y s m a l l NbOCls c o n c e n t r a t i o n s a r e the object of such c h l o r i n a t i o n s , it i s e x p e d i e n t for efficient r e a c t o r design and operation to have s o m e knowledge of the r e action k i n e t i c s involved. The r e a c t i o n of NbOCls with COC12 is of p a r t i c u l a r i n t e r e s t f r o m an i n d u s t r i a l standpoint b e c a u s e of the r e a d y a v a i l a b i l i t y of COClz and its r e l a t i v e economy. F r o m e x p e r i m e n t a l cons i d e r a t i o n s , the r e a c t i o n is h o m o g e n e o u s and p r o c e e d s at a rate which is r e a d i l y a m e n a b l e to c o n v e n t i o n a l inv e s t i g a t i o n . The
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