The overall kinetics of roasting of chalcopyrite
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Communications The Overall Kinetics of Roasting of Chalcopyrite L . S . LEUNG An i n t e r m e d i a t e step in the r e c o v e r y of copper f r o m c h a l c o p y r i t e is the r o a s t i n g operation in which p a r t i a l r e m o v a l of s u l f u r takes place. This operation is g e n e r ally c a r r i e d out in a m u l t i h e a r t h r o a s t e r or a fluid bed r o a s t e r . The k i n e t i c s of r o a s t i n g of c h a l c o p y r i t e a r e well known. 1"~ F o r r o a s t i n g at the operating t e m p e r a t a r e of about 650~ the following t h r e e s t a g e s a r e b e lieved to take p l a c e 2 Stage 1
2CuFeS2 - - CuzS + FeS + FeS2
Stage 2
FeSz ~ FeS + 89
~. s2
+ o" -
so.
Stage 3 3FeS + 502 - - FesO4 + 3So.
[1] [2] [3]
[4]
Taking m a g n e t i t e (FesO4) as the stable i r o n oxide at r o a s t i n g t e m p e r a t u r e the o v e r a l l r e a c t i o n may be w r i t ten as 2CuFeS2 + 4~Oz - - CuaS + ~ FesO4 + 3So.
[5 ]
F o r a given level o f sulfur removal f r o m chalcopyr i t e two e x t r e m e paths may be envisaged: 1) initially aU " f r e e " s u l f u r in c h a l c o p y r i t e is r e moved, i.e. 25 pct of s u l f u r in c h a l c o p y r i t e is i n i t i a l l y r e m o v e d a c c o r d i n g to Eqs. [1] to [3]. F u r t h e r r e d u c t i o n in s u l f u r takes p l a c e according to Eq. [4]. 2) some c h a l c o p y r i t e r e m a i n s c o m p l e t e l y u n r e a c t e d while the r e s t r e a c t s to c o m p l e t i o n f o r m i n g magnetite, Fig. 1--Minimum air requirements for roasting of chalcopyrite chalcocite and s u l f u r dioxide a c c o r d i n g to Eq. [5]. (basis: 100 lb dry feed containing 24.44 lb sulfur). (1 kg = F o r a given l e v e l of s u l f u r r e m o v a l up to 75 pct of 2.205 lb). s u l f u r in c h a l c o p y r i t e , the c o m p o s i t i o n of c a l c i n e , m i n i m u m a i r r e q u i r e m e n t and heat of r e a c t i o n a r e quite d i f f e r e n t for the two paths. T h e s e a r e c o m p a r e d in T a b l e I and F i g s . 1 and 2. R e a c t i o n P a t h 1 is r e f e r r e d to here as textbook mechanism while P a t h 2 is r e f e r r e d to as proposed m e c h a n i s m . T a b l e I shows that the magnetite c o n t e n t in c a l c i n e is c o n s i d e r a b l y higher for the p r o posed m e c h a n i s m than for the textbook m e c h a n i s m over the p r a c t i c a l r a n g e of s u l f u r r e m o v a l in a r o a s t e r . S i m i l a r l y the heat of r e a c t i o n (Fig. 1) and m i n i m u m a i r r e q u i r e m e n t (Fig. 2) a r e also h i g h e r for the proposed m e c hanis m. In view of these d i f f e r e n c e s a knowledge of what r e a c tion path is followed in p r a c t i c e will be of s o m e i m p o r tance in the d e s i g n and operation of r o a s t e r s , p a r t i c u l a r l y for the design of fluid bed r o a s t e r s . G e n e r a l l y the textbook m e c h a n i s m is accepted. T a b l e I s u g g e s t s that a n a l y s i s of the c a l c i n e p r o d u c
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