The self diffusion of iron in Fe 2 SiO 4 and CaFeSiO 4 melts
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3800 + 500
T
2.74 + 0.29
for Fe~$iO4, and log DFe =
5450 + 620
T
1.93 + 0.37
for CaFeSiO 4. E x c e l l e n t a g r e e m e n t is obtained with the s e l f - d i f f u s i v i t y of i r o n calculated f r o m the m e a s u r e d i n t e r d i f f u s i v i t y of i r o n and oxygen in i r o n oxide m e l t s . C O M P L E T E study of the k i n e t i c s of h e t e r o g e n e o u s r e a c t i o n s at high t e m p e r a t u r e s r e q u i r e s c o n s i d e r a t i o n of the diffusivities of p a r t i c i p a t i n g s p e c i e s in the b o u n d a r y l a y e r s on e i t h e r side of the i n t e r f a c e , as, in a l a r g e n u m b e r of such s l a g - m e t a l t r a n s f e r r e a c t i o n s , diffusion a c r o s s these l a y e r s is the r e a c t i o n r a t e l i m i t i n g step. G e n e r a l l y the diffusivity of the n e u t r a l solute in the m e t a l phase is g r e a t e r than that of the c o r r e s p o n d i n g ionic s p e c i e s in the s l a g phase, and hence, in these r e a c t i o n s , r a t e - c o n t r o l can be sought in the s l a g - s i d e diffusion b o u n d a r y l a y e r . F o r this r e a s o n alone the study of diffusion in s l a g s is of c o n s i d e r a b l e i m p o r tance in e x t r a c t i o n m e t a l l u r g y as, only through knowledge of the s e l f - d i f f u s i v i t i e s of the t r a n s p o r t i n g s p e cies and the effect t h e r e o n of s l a g c o m p o s i t i o n can such complex r a t e - l i m i t i n g m e c h a n i s m s as coupled diffusion be identified. Slag diffusion s t u d i e s a r e also of i m p o r t a n c e f r o m the point of view of t h e i r p o s s i b l e p r o v i s i o n of i n f o r mation on the m i c r o s c o p i c c o n s t i t u t i o n of s l a g s . I n a s much that diffusion is a s t r u c t u r e - s e n s i t i v e p r o p e r t y , it is to be expected that p r o p e r t i e s such as the a c t i v a tion e n e r g y for diffusion should yield i n f o r m a t i o n on t o n - i o n i n t e r a c t i o n s and hence on the n a t u r e of the e n v i r o n m e n t , o r c o - o r d i n a t i o n s p h e r e , of the diffusing entities. The i m p o r t a n c e of slag diffusion is evidenced by the l a r g e n u m b e r of studies which have b e e n conducted. Unfortunately, f r o m the s t r u c t u r e point of view, the c o m p o s i t i o n s of the s l a g s studied in the m a j o r i t y of these i n v e s t i g a t i o n s were c h o s e n p r i m a r i l y on the b a s i s of c h e m i c a l c o m p a t i b i l i t y with r e a d i l y - a v a i l a b l e c o n t a i n e r m a t e r i a l s , e . g . , CaO-A12Os-SiO2 slags in graphite c o n t a i n e r s , or s a t u r a t e d with A1203 or SiO2 in AlzO3 o r SiOz c o n t a i n e r s , o r F e O - c o n t a i n i n g s l a g s in i r o n c o n t a i n e r s . Thus, although q u a n t i t a t i v e i n f o r m a t i o n has b e e n obtained for diffusion in i n d u s t r i a l l y i m p o r t a n t s y s t e m s , the c o m p l e x i t y
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