Causes of the shell and core structure and layered inhomogeneity in iron ore pellets
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within i n d i v i d u a l s h e l l s was w e l l s i n t e r e d . * It is o b v i *The term sinteringis usedherein the acceptedsense of particlesin contact growingtogetherinto a solidbodywhilebelowtheir meltingpoint. Use of this term to denoteprocessesalsoin pelletindurationis to be encouraged. ous, h o w e v e r , that such p e l l e t s would f a i l the q u a l i t y s p e c i f i c a t i o n t e s t s , p a r t i c u l a r l y the t u m b l e t e s t , b e c a u s e the p e l l e t would f r a c t u r e into s m a l l p i e c e s which would b e f u r t h e r a b r a d e d d u r i n g tumbling. The c a s e p o r t r a y e d in Fig. 1 is r a t h e r e x t r e m e and would b e m e t i n f r e q u e n t l y in c u r r e n t o p e r a t i n g p r a c t i c e . It i n d i c a t e s , h o w e v e r , the f e a t u r e s of the d e f e c t p a r t i c u l a r l y c l e a r l y . A r e l a t e d d e f e c t is shown in Fig. 2. This is the d u plex structure or shell-and-core structure consisting of a s i n t e r e d h e m a t i t e s h e l l and a p a r t i c u l a r l y d e n s e l y s i n t e r e d m a g n e t i t e c o r e . The m a t r i x of both is g e n e r a l l y quite u n i f o r m . N e i t h e r the s u r f a c e c r a c k s n o r the c o n c e n t r i c , l a y e r e d i n h o m o g e n e i t y of the s h e l l a r e p r o nounced, but t h e r e is a l a r g e p e r i p h e r a l void. This d e f e c t o c c u r s in the i n d u r a t i o n of m a g n e t i t e p e l l e t s , but can a l s o o c c u r in t h o s e h e m a t i t e p e l l e t s w h e r e c a r b o n a c e o u s fuel has b e e n a d d e d e i t h e r p r i o r to b a l l i n g o r b y r e - r o l l i n g in f i n e l y - g r o u n d coke o r coal. ~7'18 The above d i v i s i o n of the d e f e c t s is p e r h a p s a r t i f i c i a l in that, in s i n t e r i n g m a g n e t i t e p e l l e t s , t h e r e can b e a continuous t r a n s i t i o n b e t w e e n the two. F o r e x a m p l e the p e l l e t i n d i c a t e d b y (A) in Fig. 2 shows s o m e e v i dence of c o n c e n t r i c s h e l l s in the o u t e r m a t r i x while p e l l e t (C) in Fig. 1 c o n t a i n s l e s s d i s t i n c t s h e l l s but a v e r y d i s t i n c t c o r e . N e v e r t h e l e s s the d e f e c t s ought to b e c l a s s e d s e p a r a t e l y s i n c e they f o r m b y d i s t i n c t m e c h a n i s m s and a r e due to d i s t i n c t c a u s e s , a point which will r e c e i v e f u r t h e r e l a b o r a t i o n below.
H. MODEL D E V E L O P M E N T (a) The P h y s i c a l P i c t u r e C o n s i d e r a p e l l e t h e a t e d u n d e r c o n d i t i o n s of a high h e a t t r a n s f e r r a t e to the s u r f a c e . If the v a l u e of the r e c i p r o c a l Biot modulus m =- K~ UR is s i g n i f i c a n t l y s m a l l e r than 1.0, 8 l a r g e t e m p e r a t u r e g r a d i e n t s can r e s u l t within the p e l l e t . The a c t i v a t i o n e n e r g y f o r p e l l e t s h r i n k a g e i s l a r g e . T h e r e f o r
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