Desulfurization of iron alloys in vacuo-free energy of formation of SiS (v)
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FRUEHAN AND E. T. TURKDOGAN
The r a t e of d e s u l f u r i z a t i o n of F e - C - S i - S m e l t s i n v a c u o (10 ~m p r e s s u r e ) has b e e n i n v e s t i g a t e d u s i n g 15 kg a l l o y s m e l t e d by induction h e a t i n g . F o r the s u l f u r c o n t e n t s f r o m 0.008 to 0.1 pct i n v e s t i g a t e d , the r a t e is a f i r s t - o r d e r type with r e s p e c t to s u l f u r . O v e r this s u l f u r c o n c e n t r a t i o n r a n g e and in the p r e s e n c e of c a r b o n up to s a t u r a t i o n and s i l i c o n f r o m 0 to 5 pct, the s u l f u r a c t i v i t y is s u f f i c i e n t l y high that the f r a c t i o n a l s u r f a c e c o v e r a g e by a d s o r b e d s u l f u r is w i t h i n 0.75 < 0s < 1. The r a t e equation d e r i v e d to s a t i s f y the e x p e r i m e n t a l findings for the l i m i t i n g c a s e of 0.~ ~ 1 i n d i c a t e s that s u l f u r is evolved p r i m a r i l y via two a c t i v a t e d r e a c t i o n s i n v o l v ing (SIS2)~ and ($2)$ . T h e s e c o m p l e x e s t h e n p r o d u c e SiS and S v a p o r s p e c i e s . At 1600~ the r a t e s of f o r m a t i o n of t h e s e v o l a t i l e s p e c i e s a r e about one fourth of t h o s e for f r e e v a p o r i z a tion of SiS and s u l f u r , r e s p e c t i v e l y . The a p p a r e n t h e a t s of a c t i v a t i o n a r e -47 k c a l for SiS and -37 k c a l for s u l f u r v a p o r . U s i n g a n a p p a r a t u s i n v o l v i n g a K n u d s e n c e l l and the Bendix m a s s s p e c t r o m e t e r , the enthalpy of r e a c t i o n SiS2(s) + Si(s) = 2 SiS(v) has b e e n m e a s u r e d , giving 102.2 k c a l . C o m b i n i n g this with o t h e r t h e r m o d y n a m i c d a t a , the f r e e e n e r g y of f o r m a t i o n of SiS v a p o r is evaluated as 1 Si(s) + ~-S~(v) = SiS(v)
AF ~ = 15,500-- 19.5T
si(z)
f r o m 1000 ~ to 1686~
+ -~S~(v) = SiS(v)
AF ~ = 3 5 0 0 - 12.4T
above 1686~
I T has long b e e n r e c o g n i z e d that the d e s u l f u r i z a t i o n of liquid s t e e l o r hot m e t a l is a c h i e v e d b e s t only u n d e r r e d u c i n g c o n d i t i o n s . D a r k e n 1 showed, for e x a m p l e , that the s u l f u r c o n t e n t of hot m e t a l in e q u i l i b r i u m with l i m e , g r a p h i t e , CaS, and CO at 1 a t m at 1600~ would be a s low as 2 ppm. In the o v e r - a l l r e a c t i o n r e p r e s e n t e d by S(in soln) + CaO(s) + C(in soln) = CaS(s) + CO(g) [i] c a r b o n in s o l u t i o n in i r o n has a dual effect by m a i n t a i n ing r e d u c i n g c o n d i t i o n s in the m e l t and i n c r e a s i n g the a c t i v i t y coefficient of s u l f u r in i r o n . D a r k e n 1 a l s o showed that for effective d e s u l f u r i z a t i o n of s t e e l by lime injection, there should be some silicon in the metal, thus S(in soln) + 2CaO(s) + AS~(insoln) = CaS(s)
+ ~ca2sio4(s)
[2]
The dual effect of s i l i c o n is s i m i l a r to that of c a r b o n . F o r this r e a c t i o n the e q u i
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