Gaseous reduction of iron oxides: Part II. Pore characteristics of iron reduced from hematite in hydrogen
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demonstrated in the previous paper,1 the reduction in hydrogen of hematite spheroids (>7 mm d i a m ) beyond about 60 pct reduction is controlled principally by gas diffusion in the pores of the iron l a y e r at temperatures from 500° t o 1200°C. The effective H2-H20 diffusivity in the porous iron l a y e r derived from the reduction data was found to d e c r e a s e markedly w i t h decreasing reduction temperature. The d e c r e a s e in the r a t i o (effective diffusivity/molecular diffusivity) with d e c r e a s ing reduction temperature implies that as the r e d u c tion temperature decreases, the pore structure bec o m e s f i n e r . This was substantiated by the appearance of the f r a c t u r e surfaces when viewed by scanning e l e c tron microscopy. The characteristics of porous m e d i a may be des c r i b e d by s e v e r a l m e a n s , e.g. X - r a y and microscopic examinations, measurements of pore volume, pore s u r f a c e a r e a , and effective gas diffusivity. E x a m i n a tions of reduced o x i d e by optical microscopy and particularly by scanning electron microscopy, discussed previously,1 proved t o be most informative. In the present paper, w e report and discuss our work on pore volume, pore surface a r e a , and effective gas diffusivity in porous iron reduced from a high-grade hematite ore. The hematite ore investigated was the s~me as that used in the previous investigation of the rate of reduction in hydrogen.~ The a v e r a g e composition of d r i e d ore was: 96 4- 2 pct Fe203, 1 t o 2 pct FeO, 1 to 3 pct SIO2, and 0.2
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Fig. 1--Connected (---total) pore v o l u m e of iron r e d u c e d from h e m a t i t e ore i n hydrogen at v a r i o u s t e m p e r a t u r e s ; l o w e r c u r v e i s for dehydrated h e m a t i t e ore. VOLUME 2, NOVEMBER 1971-3189
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t h a t t h e s m a l l e s t p o r e s in t h e s e m a t e r i a l s a r e l e s s t h a n 0.01 t ~ m . H o w e v e r , t h e a d d i t i o n a l v o l u m e due t o t h e s e m i c r o p o r e s is o n l y a s m a l l f r a c t i o n o f t h e t o t a l pore volume. A n o t h e r i m p o r t a n t f e a t u r e o f t h e d a t a in Fig. 1 is t h e i n f l e c t i o n in t h e c u r v e s c o r r e s p o n d i n g t o p e a k s in t h e d e r i v a t i v e ~ V / a P . T h e p r e s s u r e r a n g e in w h i c h a c u r v e r i s e s steeply corresponds t o t h e size r a n g e o f the interconnecting pore network. T h e critical r a d i u s , r c , of t h e i n t e r c o n n e c t i n g p o r e s i s d e f i n e d a s t h a t c o r r e s p o n d i n g t o t h e h i g h e s t v a l u e o f O V / O P , i.e. t h e p o i n t of i n f l e c t i o n . T h r e e i m p o r t a n t p a r a m e t e r s , i.e. rc, r m , a n d V, obtained from the p r e s e n t m e a s u r e m e n t s u s
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