A coupled diffusion model for the reduction of agglomerates of iron oxide granules
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T H E production of sponge i r o n by d i r e c t r e d u c t i o n p r o c e s s e s is gaining a c c e p t a n c e for s m a l l scale steel p r o d u c t i o n as an economic a l t e r n a t i v e to the b l a s t f u r nace. Steel is produced in New Zealand by a r c - m e l t i n g the i r o n product from a SL/RN r o t a r y kiln, using low grade titano m a g n e t i t e beach sands as the raw m a t e r i a l (57 to 60 pct iron). A study has been made of the CO r e d u c t i o n of sand g r a i n s as a background to this Industry. (McAdam 9 1974). In this paper, we c o n s i d e r a model for the r e d u c t i o n of an a g g l o m e r a t e of f e r r o u s oxide p a r t i c l e s by carbon monoxide. Imagine a pellet c o n s i s t i n g of f e r r o u s oxide g r a i n s of v a r i o u s s i z e s and shapes u n i f o r m l y d i s t r i b uted throughout so that each point contains a r e p r e s e n tative s a m p l e of oxide g r a i n s near it. A r e d u c i n g gas of c a r b o n monoxide of fixed c o n c e n t r a t i o n and t e m p e r a t u r e flows past the peI1et sweeping away oxidation products expelled f r o m the p a r t i c l e s . In our model, we i m a g i n e carbon monoxide gas diffusing f i r s t a c r o s s a b o u n d a r y l a y e r c r e a t e d by the gas flow at the pellet s u r f a c e , then through the m a c r o s c o p i c porous m e d i u m c o m p r i s i n g the pellet and finally diffusing into the i n dividual g r a i n s towards a moving b o u n d a r y s e p a r a t i n g u n r e d u c e d oxide from a porous outer shell of reduced iron. At this moving boundary, carbon dioxide is p r o duced and diffuses c o u n t e r - c u r r e n t to the c a r b o n m o n oxide to be swept away f i n a l l y in the gas flowing past the pellet. Two e x t r e m e models of the p r o c e s s of r e d u c t i o n of a g g l o m e r a t e s of iron o r e s by carbon monoxide or hydrogen a r e e x t e n s i v e l y used in the l i t e r a t u r e . The f i r s t (Yagi and Kunii 1) a s s u m e s a homogeneous m e d i u m with a sharp moving i n t e r f a c e between r e a c t e d and u n r e acted solid: a situation holding as a l i m i t i n g case for v e r y s m a l l g r a i n s , or slow gaseous diffusion in the pellet. The second model (Weisz and Goodwin 2) a s s u m e s free m o v e m e n t of r e a c t a n t gas and holds for l a r g e g r a i n s or r a p i d diffusion in the pellet. However, in m a n y p r a c t i c a l i n s t a n c e s the individual g r a i n s a r e
A. McNABB and E. BRADFORD are Scientists, Applied Mathematics Division,Department of Scientific and Industrial Research, P. O. Box 8030, Wellington,New Zealand. G. D. McADAMis Scientist, Chemistry Division,Department of Scientific and Industrial Research, Private Bag, Petone, New Zealand. Manuscript submitted May 7,1974. METALLURGICALTRANSACTIONS B
r e d u c e d v a r y i n g d e g r e e s depending on t h e i r s p a t i a l position in the solid. Recent p a p e r s by Szekely and Evans s and P a p a n a s t a s
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