The kinetics of reduction of iron oxides at moderate temperatures
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r-71,55o 9 900] RT
exp L
g.atom O/cm 2 .s .arm.
The e x p e r i m e n t a l data was i n t e r p r e t e d by m e a n s of a n u c l e a t i o n and growth m o d e l .
T H E r e d u c t i o n of i r o n oxides b y g a s e o u s r e d u c i n g agents is a p r i m e e x a m p l e of a c l a s s of h e t e r o g e n e ous r e a c t i o n s whose r a t e s a r e i n f l u e n c e d by c h e m i cal k i n e t i c f a c t o r s on one hand and m a s s (or heat) t r a n s f e r f a c t o r s on the o t h e r . R a t e s which a r e f r e e of t r a n s f e r i n f l u e n c e s , and a r e t h e r e f o r e r e p r e s e n tative of the c h e m i c a l k i n e t i c a s p e c t s of the s u r f a c e r e a c t i o n , a r e t e r m e d the i n t r i n s i c r a t e s of r e a c t i o n . Knowledge of the i n t r i n s i c r a t e s of r e d u c t i o n is of c o n s i d e r a b l e i m p o r t a n c e both f r o m the f u n d a m e n t a l and p r a c t i c a l v i e w p o i n t s . F u n d a m e n t a l l y , such r a t e data is v e r y u s e f u l as it would e n a b l e a c c u r a t e and r e l i a b l e d e t e r m i n a t i o n of the t r u e a c t i v a t i o n e n e r g y (unfalsified by t r a n s f e r effects) of the r e a c t i o n . F r o m a p r a c t i c a l standpoint, t h e s e r e p r e s e n t the m a x i m u m r a t e s of r e d u c t i o n that can be a c h i e v e d with a p a r t i c u l a r r e d u c t a n t at s p e c i f i e d t e m p e r a t u r e and p r e s s u r e . The a c t u a l r a t e s r e a l i z e d i n p r a c t i c e , though, a r e far s h o r t of the i n t r i n s i c r a t e s of r e d u c t i o n b e c a u s e the m a s s (and heat) t r a n s f e r p r o c e s s e s act as i m pediments. The i n t e r p l a y b e t w e e n the c h e m i c a l and m a s s t r a n s f e r p r o c e s s e s in h e t e r o g e n e o u s r e a c t i o n s has r e ceived a g r e a t deal of a t t e n t i o n and a n u m b e r of exc e l l e n t p a p e r s and m o n o g r a p h s have b e e n p u b l i s h e d on this s u b j e c t , x'7 It e m e r g e s f r o m t h e s e e a r l i e r works that the r a t e of a h e t e r o g e n e o u s r e a c t i o n inv o l v i n g a s o l i d and a gas t e n d s to be l e s s i n f l u e n c e d by m a s s (and heat) t r a n s f e r p r o c e s s e s at lower t e m p e r a t u r e s . T h u s , for a given h e t e r o g e n e o u s r e a c t i o n one m a y p o s t u l a t e the e x i s t e n c e of a c h e m i c a l k i n e t i c r e g i m e , at low and m o d e r a t e t e m p e r a t u r e s , within which the m e a s u r e d r a t e s r e p r e s e n t the i n t r i n s i c c h e m i c a l k i n e t i c s of the i n t e r r a c i a l r e a c t i o n ; i l l u m i n a t i n g d i s c u s s i o n s p e r t a i n i n g to the c h e m i c a l k i n e t i c and m a s s t r a n s f e r r e g i m e s in g a s - s o l i d r e a c t i o n s have b e e n p u b l i s h e d in r e c e n t y e a r s . 5-v S. K. EL-RAHAIBYand Y. K. RAO are Graduate Student and Associa
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