On the reaction of solids with mixed gases
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T H E R E a r e m a n y s y s t e m s of p r a c t i c a l i m p o r t a n c e in m e t a l s and m a t e r i a l s p r o c e s s i n g o p e r a t i o n s , w h e r e a solid s p e c i e s is made to r e a c t with a gas m i x t u r e , one o r m o r e c o m p o n e n t s of which p a r t i c i p a t e in the r e a c t i o n . The r e d u c t i o n of i r o n oxides with c a r b o n m o n o x i d e - h y d r o g e n m i x t u r e s in the " d i r e c t r e d u c t i o n p r o c e s s e s " and also in the i r o n b l a s t f u r n a c e , when s t a c k gas i n j e c t i o n is b e i n g e m p l o y e d may be cited as r e p r e s e n t a t i v e e x a m p l e s . O t h e r s y s t e m s of c o n s i d e r able p r a c t i c a l i n t e r e s t include the r e a c t i o n of c a r b o n (coal) with oxygen and s t e a m in the L u r g i or the K o p p e r s T o t z e k coal g a s i f i c a t i o n p r o c e s s e s . In spite of the a p p r e c i a b l e p r a c t i c a l i m p o r t a n c e of these s y s t e m s , r e l a t i v e l y little f u n d a m e n t a l work has b e e n r e p o r t e d on the r e a c t i o n of s o l i d s with gas m i x tures. Notable e x c e p t i o n s to this r u l e a r e r e c e n t p a p e r s by T u r k d o g a n , 1,2 the p r e s e n t a u t h o r s s and two p a p e r s by T s a y , Ray and Szekely, 4,s a l l of which were conc e r n e d with the effect of gas c o m p o s i t i o n on the r a t e at which i r o n oxide p e l l e t s r e a c t with a CO + H2 gas mixture. T u r k d o g a n and V i n t e r s m e a s u r e d the r a t e at which h e m a t i t e p e l l e t s r e a c t with CO + H2 m i x t u r e s at 900~ in a r e g i m e that was c o n t r o l l e d by pore diffusion; they found that the higher was the h y d r o g e n c o n t e n t of the r e a c t a n t gas m i x t u r e , the f a s t e r was the o v e r a l l r a t e of r e a c t i o n . In a d v a n c i n g a s e m i q u a n t i t a t i v e int e r p r e t a t i o n t h e s e a u t h o r s s u g g e s t e d that the o v e r a l l r a t e of r e d u c t i o n was p r o p o r t i o n a l to a p s e u d o b i n a r y diffusion coefficient, the v a l u e of which i n c r e a s e d with a n i n c r e a s i n g p r o p o r t i o n of h y d r o g e n in the gas m i x t u r e . T h i s e x p l a n a t i o n was c o n s i s t e n t with the o v e r a l l t r e n d exhibited by the m e a s u r e m e n t s . In a m o r e r e c e n t c o m m u n i c a t i o n , Szekely and E1T a w i l r e p o r t e d f i n d i n g s on the r e d u c t i o n k i n e t i c s of h e m a t i t e p e l l e t s with CO + H2 m i x t u r e s which w e r e v e r y s i m i l a r to the data p r e s e n t e d e a r l i e r by T u r k d o J. SZEKELY is Professor of Materials Engineering and M. CHOUDHARY is Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Mass. 02139. Y. EL-TAW1L, formerly Graduate Student, Department of Chemical Engineering, State University of New York at Buffalo, is
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