Catalytic effect of iron on decomposition of carbon monoxide: II. Effect of additions of H 2 , H 2 O, CO 2 , SO 2 and H
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FesC + CO2(g)
[1]
D e c o m p o s i t i o n of c e m e n t i t e : FesC = (Fe3C)* ~ 3Fe + C (gr) [2] C e m e n t i t e is unstable r e l a t i v e to i r o n and graphite at all t e m p e r a t u r e s . The fact that c e m e n t i t e f o r m a t i o n (or H'~tgg c a r b i d e , Fe2C , below about 300~ a l m o s t a l ways a c c o m p a n i e s c a r b o n d e p o s i t i o n s u g g e s t s that the d e c o m p o s i t i o n of the activated complex, which m a y be r e p r e s e n t e d by [Fes(CO)2]* , m a y yield c e m e n t i t e (or H/igg carbide) as an i n t e r m e d i a t e r e a c t i o n product. Through a n o t h e r activated r e a c t i o n , c e m e n t i t e may then decompose to i r o n and graphite. As d i s c u s s e d in P a r t I, the d e c o m p o s i t i o n of c a r b o n monoxide is c a t a lyzed by i r o n and not by c e m e n t i t e . T h e r e f o r e , it follows that if the d e c o m p o s i t i o n of c e m e n t i t e is r e t a r d e d b y the p r e s e n c e of i m p u r i t i e s in the s y s t e m , the c a r b o n deposition will be r e t a r d e d and u l t i m a t e l y stopped when the s u r f a c e of the i r o n is c o v e r e d with c e m e n t i t e . The a c c e l e r a t i o n or r e t a r d a t i o n of r e a c t i o n s [1] and [2] by gaseous additives to c a r b o n monoxide or solid additions to the i r o n c a t a l y s t will a c c e l e r a t e or r e t a r d c a r b o n deposition. R. G. OLSSON and E. T. TURKDOGANare Senior Scientist and Manager, respectively, Chemical Metallurgy, Research Laboratory, U. S. Steel Corporation, Monroeville,Pa. 15146. Manuscript submitted March 28, 1973. METALLURGICALTRANSACTIONS
E v e r s i n c e the f i r s t o b s e r v a t i o n s made by B y r o m ~ and soon after by C a r p e n t e r and Smith, 3 it has b e c o m e g e n e r a l l y r e c o g n i z e d that s u l f u r in the gas inhibits the catalytic action of i r o n in the d e c o m p o s i t i o n of c a r b o n monoxide. In the Stelling p r o c e s s 4 i r o n ore is r e d u c e d to c e m e n t i t e in a CO-COz a t m o s p h e r e without the f o r m a t i o n of f r e e c a r b o n , p r o b a b l y b e c a u s e of the p r e s ence of sulfur in the o r e . It has long b e e n known in the c a s t - i r o n technology that sulfur inhibits g r a p h l t i z a t i o n ; g r a p h i t i z a t l o n of the cast iron is e n s u r e d by the a d d i tion of a s t r o n g d e s u l f u r i z e r such as m a g n e s i u m . Some light on this m a t t e r is p e r h a p s shed by the c o n c l u s i o n of P o d g u r s k i , s b a s e d on his e x p e r i m e n t s and o t h e r s , that the d e c o m p o s i t i o n (graphltizatton) of the c a r b i d e s of i r o n , n i c k e l , and cobalt is m a r k e d l y r e t a r d e d by s m a l l a m o u n t s of sulfur sufficient to produce only a surface monolayer. The n i t r o g e n - b e a r i n g gaseous s p e c i e s such as NH s and (CN)2 a r e also known to r e t a r d the decomposition of c a r b o n monoxide, as d e
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