Kinetics of zinc oxide reduction with carbon monoxide
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NC e x t r a c t i o n by t h e r m a l o r e l e c t r o t h e r m a l m e t h ods involves t h e o v e r a l l reaction: ZnO(s) + C(s) = Zn(g) + CO(g)
[1]
One mechanism for this p r o c e s s postulates a d i r e c t solid-solid reaction between t h e Z n O a n d t h e C,1'2 w h i l e a s e c o n d speculates a chain reaction involving t h e i n t e r m e d i a t e f o r m a t i o n of C O a n d C O z :3-s ZnO(s) + CO(g) = Zn(g) + COz(g)
[2]
CO2(g) + C(s) = 2CO(g)
[3]
T h e f i r s t m e c h a n i s m a p p e a r s to o f f e r m o r e r e s i s t a n c e t o t h e p r o c e s s b e c a u s e it r e s t r i c t s r e a c t i o n t o t h e p o i n t s o f c o n t a c t b e t w e e n the Z n O a n d the C . B e c a u s e o f t h i s , t h e s e c o n d m e c h a n i s m is a c c e p t e d m o r e r e a d i l y , but n e i t h e r o f the s t o i c h i o m e t r i e s i s e n t i r e l y s a t i s f a c t o r y . T h e f i r s t c a n n o t e x p l a i n the f o r m a t i o n o f CO2 i n the f u r n a c e , w h i l e the s e c o n d r e q u i r e s t h e i n i t i a l p r e s e n c e o f C O o r CO2 t o i n i t i a t e r e d u c t i o n . R e a c t i o n s [2] a n d [3] m a y b e i n i t i a t e d e i t h e r by r e s i d u a l o x y g e n p r e s e n t in t h e f u r n a c e d u r i n g s t a r t - u p o r else by o x y g e n g e n e r a t e d by t h e t h e r m a l d e c o m p o s i t i o n o f Z n O :6 Z n O ( s ) = Z n ( g ) + ~-02(g)
[4]
T h e 0 2 f o r m e d in t h i s f a s h i o n c o u l d t h e n r e a c t w i t h t h e C t o p r o d u c e t h e C O r e q u i r e d f o r r e a c t i o n [2]:
c(s) + ~o2(g)
:
co(g)
[5]
All o f t h e s e r e a c t i o n s probably o c c u r to s o m e e x t e n t in a c o m m e r c i a l z i n c s m e l t i n g s h a f t . T h e n e t c o n t r i bution o f each r e m a i n s unknown at t h e p r e s e n t t i m e . T h i s p a p e r i n v e s t i g a t e s r e a c t i o n [2] b o t h t h e o r e t i c a l l y and experimentally, with p a r t i c u l a r e m p h a s i s b e i n g p l a c e d o n isolating t h e c h e m i c a l r a t e p r o c e s s at the s u r f a c e from the physical rate p r o c e s s e s of ext e r n a l m a s s t r a n s f e r and internal diffusion. P r e v i o u s s t u d i e s employed p o r o u s s a m p l e s with u n u s u a l g e o m e t r i e s , w h i c h m a k e it difficult to e s t i m a t e t h e c o n t r i bution of external and internal diffusion. T h e ultimate C. E. G U G E R is Project Scientist, KoppersCo. Inc., Research Departm e n t , Monroeville, Pa. F. S. MANNING is Professor and Head, Department of Chemical Engineering, T h e University of Tulsa, Tulsa, Okla. Manuscript received February 3 , 1971. METALLURGICAL TRANSACTIONS
o b j e c t i v e is to d e v e l o p a p r o c e s s e n g i n e e r i n g m o d e l o r d e s i g n equation that w o u l d e n a b l e prediction and e v a l u a t i o n of m u l t i p a r t i c l e z i n c s m e l t i n g p r o c e s s e s . LITERATURE
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Two types of m e c h a n i s m h
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