Gaseous reduction of iron oxides: Part III. Reduction-oxidation of porous and dense iron oxides and iron
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AND
J. V. VINTERS
The i n t e r n a l r e d u c t i o n of h i g h - g r a d e g r a n u l a r h e m a t i t e o r e in h y d r o g e n and c a r b o n m o n o x i d e , and also the i n t e r n a l oxidation of p o r o u s i r o n g r a n u l e s in CO2-CO m i x t u r e s have been i n v e s t i gated. To a s s i s t the i n t e r p r e t a t i o n of the r a t e d a t a for p o r o u s i r o n and i r o n o x i d e s , r a t e m e a s u r e m e n t s have been m a d e a l s o with d e n s e w u s t i t e , p r e v i o u s l y g r o w n on i r o n by oxidation. The i r o n f o r m e d by r e d u c t i o n of d e n s e w u s t i t e is p o r o u s , s i m i l a r to that o b s e r v e d when p o r o u s h e m atite is r e d u c e d . It is found that the r a t e of d i s s o c i a t i o n o r f o r m a t i o n of w a t e r v a p o r o r c a r b o n d i o x i d e on the i r o n s u r f a c e is about an o r d e r of m a g n i t u d e g r e a t e r than that on the s u r f a c e of wustite. The r e s u l t s of the p r e v i o u s i n v e s t i g a t i o n s u s i n g d e n s e i r o n and w u st i t e a r e in g e n e r a l a c c o r d with the p r e s e n t findings. The r a t e of r e d u c t i o n of h e m a t i t e i n c r e a s e s with i n c r e a s i n g p o r e s u r f a c e a r e a of the r e d u c e d oxide. The r e s u l t s i n d i c a t e that the r a t e of r e d u c t i o n of g r a n u l e s is c o n t r o l l e d p r i m a r i l y by the f o r m a t i o n of H20 o r CO2 on the p o r e w a l l s of wustite. The s p e c i f i c r a t e c o n s t a n t s e v a l u a t e d f r o m i n t e r n a l r e d u c t i o n , u s i n g the total p o r e s u r f a c e a r e a , a r e about ~o to ~ of those f o r d e n s e wustite. T h e s e findings indicate that with p o r o u s w u st i t e o r i r o n , the e f f e c t i v e p o r e s u r f a c e a r e a u t i l i z e d is about 1 to 2 pct of the t o t al p o r e s u r f a c e a r e a . The r a t e of r e d u c t i o n in H2-CO m i x t u r e s is in a c c o r d with that d e r i v e d f r o m the r a t e c o n s t a n t s f o r r e d u c t i o n in H2 and CO.
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e a r l i e r P a r t s of t h i s s e r i e s of p a p e r s , the au t h o r s 1,2 d i s c u s s e d the r a t e p r o c e s s e s i n v o l v e d in the g a s e o u s r e d u c t i o n of n a t u r a l o r s y n t h e t i c h e m a t i t e . S u b s t a n t i a t e d by e x p e r i m e n t a l o b s e r v a t i o n s , it was pointed out that t h e r e a r e 3 b a s i c r a t e p r o c e s s e s which c o n t r o l the g a s e o u s r e d u c t i o n of h e m a t i t e e i t h e r c o l l e c t i v e l y , or individually in c e r t a i n l i m i t i n g c a s e s : I) U n i f o r m i n t e r n a l r e d u c t i o n : The t i m e of r e d u c tion is independent of p a r t i c l e s i z e ; this p r o c e s s p r e v a i l s in s m a l l g r a n u l e s w h e r e gas d if f u s io n in the p o r e s o f f e r s l i t t l e h i n d r a n c e to the r e d u c t i o n p r o c e s s . II) L i m i t i
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