High temperature chlorination kinetics of a niobium pyrochlore
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T H E k i n e t i c s of c h l o r i n a t i o n r e a c t i o n for a n i o b i u m p y r o c h l o r e has b e e n studied in a high t e m p e r a t u r e (1873 to 2223 K) d i f f e r e n t i a l r e a c t o r . The s t a r t i n g m a t e r i a l was the St-Honor~ (P.Q., Canada) p y r o c h l o r e , of a p p r o x i m a t e f o r m u l a (Nao.Tz, Cao.95, Ao.32) (Nbl.82, Tio.ls, Bo.oa) (08.25, Fo.75), with A r e p r e s e n t i n g r a r e e a r t h m e t a l s and Sr, while B is w r i t t e n for F e and Z r . P r e v i o u s work x has shown that NbO2F is evolded at the e a r l y s t a g e s of c h l o r i n a t i o n (around 1270 K). A p r e l i m i n a r y heat t r e a t m e n t of the p y r o c h l o r e r e s u l t e d in the f o r m a t i o n of a m i x t u r e made up of NaNbOz and CaNb206, which is a well defined s t a r t i n g m a t e r i a l for c h l o r i n a t i o n s t u d i e s . A l a r g e a m o u n t of work is a v a i l a b l e on the s u b j e c t of c h l o r i n a t i o n of n i o b i u m oxides. 2 A m o n g r e c e n t s t u d i e s , c h l o r i n a t i o n of Nb20.~ in the g a s - s o l i d phase, 3 c a l c u l a t i o n of t h e r m o d y n a m i c c o n s t a n t s for this r e a c tion at 298 and 600 K, 4 and the k i n e t i c s of n i o b i u m o x y c h l o r i d e s c h l o r i n a t i o n in a flow r e a c t o r , 5 have b e e n e x a m i n e d . While these r e a c t i o n s o c c u r at low t e m p e r a t u r e and a r e well known, c h l o r i n a t i o n of CaNb206 in the g a s - s o l i d phase is much m o r e difficult and r e a c t i o n s b e c o m e fast only a f t e r fusion (approxim a t e l y 1770 K) has o c c u r r e d . 1 Since v e r y little i n f o r m a t i o n was a v a i l a b l e on this s u b j e c t a c h l o r i n a t i o n k i n e t i c s study of liquid Na20-CaO-Nb20~ was u n d e r t a k e n o v e r the t e m p e r a t u r e r a n g e 1873 to 2223 K.
made. The e x p e r i m e n t a l e q u i p m e n t is r e p r o d u c e d on F i g . 2. A l u m i n a c r u c i b l e s were made of s e m i c y l i n d r i cal s e c t i o n s of a l u m i n a t u b e s 3 m m ID and 5 m m OD, with an o v e r a l l length of 10 m m . When p o s i t i o n e d in the middle of the r e a c t o r , the c r u c i b l e was e n t i r e l y i n c l u d e d in a n e a r l y c o n s t a n t t e m p e r a t u r e s e c t i o n (+25 K). The p r o c e d u r e for t e m p e r a t u r e c a l i b r a tion i n s i d e the r e a c t o r and the t e m p e r a t u r e d i s t r i b u tion obtained have a l r e a d y b e e n d e s c r i b e d . l The e x p e r i m e n t s w e r e c a r r i e d out at a t m o s p h e r i c p r e s s u r e , with c h l o r i n e c o n c e n t r a t i o n s in h e l i u m up to 20 pct, the t e m p e r a t u r e v a r y i n g f r o m 1873 to 2223 K. B l a n k t e s t s w e r e c a r r i e d out to d e t e r m i n e the b e h a v i o r of a l u m i n a in the p r e s e n c e of c h l o r i n e h e l i u m m i x t u r e s . Only above 2173 K and 25 pct c h l o r
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