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i n v e s t i g a t i o n s on 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 have lent i n c r e a s i n g i m p o r t a n c e to the s t r u c t u r a l changes o c c u r r i n g d u r i n g the f o r m a t i o n of m a g n e t i t e and w u s t i t e . 1 This has b e e n p a r t i c u l a r l y e v i d e n t in the r e d u c t i o n of p o r o u s h e m a t i t e p e l l e t s , w h e r e the r e d u c i n g gas has a c c e s s to the p e l l e t i n t e r i o r . In this event a p a r t i a l p r e s s u r e g r a d i e n t can be set up over the pellet, r e s u l t i n g in the c o n v e r s i o n of l a r g e p o r t i o n s of the p e l l e t to m a g n e t i t e and w u s t i t e p r i o r to the a p p e a r a n c e of i r o n . 2 The p h y s i c a l s t r u c t u r e of t h e s e lower oxides will then i n f l u e n c e the r a t e of final r e duction to i r o n . F o r e x a m p l e , the r e d u c t i o n of h e m a t i t e to m a g n e t i t e is f r e q u e n t l y a c c o m p a n i e d by the f o r m a t i o n of p o r e s which a r e p r e s e r v e d d u r i n g s u b s e q u e n t r e d u c t i o n to w u s t i t e , s'4 The s i z e and d i s t r i b u t i o n of t h e s e p o r e s change with the r e d u c t i o n t e m p e r a t u r e , s so that the c o n d i t i o n s u n d e r which the m a g n e t i t e is f o r m e d m a y have a profound effect on its s u b s e q u e n t r e d u c t i o n to iron. This study into the m e c h a n i s m of r e d u c t i o n of h e m a tite g r a i n s to m a g n e t i t e a i m s to r e l a t e g r a v i m e t r i c m e a s u r e m e n t s on the r e d u c t i o n of h e m a t i t e p e l l e t s and g r a n u l a r h e m a t i t e in CO-CO 2 a t m o s p h e r e s with the a c c o m p a n y i n g s t r u c t u r a l c h a n g e s . EXPERIMENTAL G r a v i m e t r i c r e d u c t i o n t e s t s w e r e c a r r i e d out on c o m m e r c i a l hematite pellets originating from Carol Lake, Canada. In each t e s t a s i n g l e p e l l e t was r e duced at a fixed t e m p e r a t u r e and in a C O - C O 2 gas m i x t u r e of fixed c o m p o s i t i o n . T e m p e r a t u r e s r a n g e d f r o m 500 to l l 0 0 ~ and the CO c o n t e n t of the gas m i x t u r e s v a r i e d f r o m 0.063 to 0.50 atm. However, the m a x i m u m v a l u e of the CO p a r t i a l p r e s s u r e at a p a r A. V. BRADSHAW,formerly Professor of Applied Metallurgy, Imperial College, is Chief of the Divisionof Process Technology, C.S.I.R.O. Minerals Research Laboratories, P.O. Box 136, North Ryde, New South Wales, 2113, Australia. A. G. MATYASis Research Engineer, Chemical Engineering Department, Noranda Research Centre, 240 Hymus Boulevard, Pointe Claire, Quebec H9R 1G5, Canada. Manuscript submitted July 9, 1975. METALLURGICAL TRANSACTIONSB

t i c u l a r t e m p e r a t u r e was l i m i t e d to p r o v i d e m a g n e t i t e as the s t a b l e phase. Reduction t e s t s w e r e also m a d e on g r a n u l a r m a t e r i a l obtained by c r u s h i n g the p