Kinetics of chromium oxide reduction from a basic steelmaking slag by silicon dissolved in liquid iron
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[1]
and
2 (CrO) + Si : (SiO~) + 2 C__r_r
[2]
The experiments were carried out under an argon atmosphere in a vertical resistanceheated tube furnace. The slag and metal phases were held in zirconia crucibles.The course of the reactions was followed by periodically sampling the slag phase and analyzing for total chromium, divalent chromium, and iron. Results obtained by varying stirring rate, temperature, and composition defined the rate-limiting mechanism for each reaction. The rate of reduction of trivalent chromium (reaction [I] above) increases with moderate increases in stirring of the slag, and increases markedly with increases in temperature. The effects of changes in composition identified the rate-limiting step for Cr ยง reduction as diffusion of Cr +3 from the bulk slag to the slag-metal interface. The rate of reduction of divalent chromium does not vary with changes in stirring of the slag, but increases in temperature markedly increase the reaction rate. Thus, this reaction is limited by the rate of an interfacial chemical reaction. The reduction of divalent chromium is linearly dependent on concentration of divalent chromium, but is independent of silicon content of the metal phase. s behavior of c h r o m i u m and its oxides in s t e e l making s y s t e m s has been the subject of numerous p a s t studies, but most of t h e s e studies w e r e concerned with equilibrium behavior r a t h e r than the m e c h a n i s m s of c h r o m i u m r e c o v e r y . The p u r p o s e of the p r e s e n t i n v e s tigation was to study the reduction of c h r o m i u m oxide by s i l i c o n and to d e t e r m i n e the r a t e - l i m i t i n g steps. The effects of s t i r r i n g r a t e , composition, and t e m p e r a t u r e w e r e investigated to define the c o n t r o l l i n g steps. Because of the difficulty inherent in containing liquid s l a g s and m e t a l s in l a b o r a t o r y c r u c i b l e s at s t e e l m a k i n g t e m p e r a t u r e s only a l i m i t e d number of kinetic studies of s t e e l m a k i n g s y s t e m s has been made. Most of these studies employed c r u c i b l e s of graphite or s i l i c a . G r a p h ite c r u c i b l e s n e c e s s i t a t e a metal phase s a t u r a t e d with carbon while s i l i c a c r u c i b l e s a r e l i m i t e d to use with silica-saturated slags. Three studies have been made of c h r o m i u m t r a n s f e r between slags and molten iron s a t u r a t e d with carbon. McCoy and Philbrook 1 used a rotating c r u c i b l e technique to p r e v e n t contact of the slag with the graphite c r u c i b l e (where c h r o m i u m c a r b i d e s would form). They found the reduction to be r a t e - l i m i t e d by an i n t e r f a c i a l c h e m i c a l r e a c t i o n which was f i r s t o r d e r with r e s p e c t to the c h r o mium content of the slag. JAMES W. ROBISON, JR. is Research Engineer, SpecialMetals Corp., New Hartford, N. Y. ROBERT D. PEHLKE is Professor and Chairman Dept. of Materials& MetallurgicalEngineering,The Unive
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