Fluidized bed roasting of molybdenite-effect of operating variables

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MoO s + 2 SO2

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is a highly e x o t h e r m i c r e a c t i o n , e . g . heat of r e a c t i o n is about - 2 5 2 K c a l / g m o l of MoS 2 at 600~ The r o a s t ing t e m p e r a t u r e should not exceed about 650~ o t h e r wise MoO s f o r m e d m a y soften o r m e l t due to local M. A. DOHEIM and S. A. RASSOULare Associate Professor and Professor, respectively, Miningand MetallurgicalEngineering Department, Faculty of Engineering, Assiut University, Assiut, Egypt. M. Z. ABDEL-WAHABis Production Manager, Ministry of Industry, Cairo. Manuscript submitted June 5, 1975. METALLURGICALTRANSACTIONS B

o v e r h e a t i n g and its high v o l a t i l i t y . 2's Molybdic oxide b e g i n s to s u b l i m e at about 593~ b e c o m e s quite v o l a t i l e at 704~ and m e l t s at 795~ The s o f t e n i n g or m e l t i n g of MoO s is highly u n d e s i r a b l e e s p e c i a l l y in a f l u i d i z e d bed p r o c e s s due to the a g g l o m e r a t i o n and s i n t e r i n g p r o b l e m s a r i s i n g . The highly e x o t h e r m i c n a t u r e of the r o a s t i n g r e a c t i o n is the m a i n r e a s o n for d e v e l o p i n g hot spots which m a y c a u s e s o f t e n i n g or m e l t i n g of s o l i d s . The n a t u r e of the f l u i d i z e d bed t e c h n i q u e ( i n t e n s e m i x i n g and e x c e l l e n t t h e r m a l p r o p e r t i e s ) r e d u c e s the extent of such effect. Molybdenite r o a s t i n g in m u l t i p l e h e a r t h f u r n a c e s , ~-6 r o t a r y k i l n s , 3'6 and shaft f u r n a c e s 7 h a s b e e n r e p o r t e d . Many i n v e s t i g a t i o n s of MoS 2 r o a s t i n g in fixed beds have also b e e n r e p o r t e d . 4-6,s, ~ The t h e r m o d y n a m i c s of the oxidation of MoS 2 to oxides a r e v e r y f a v o r a b l e and the k i n e t i c s of the r e a c tion a r e v e r y complex. 2'5'7'1~ The i n v e s t i g a t o r s found that the p r o c e s s is c o n t r o l l e d by a f i r s t o r d e r c h e m i cal r e a c t i o n , and it m a y involve d i f f u s i o n c o n t r o l in the e a r l y s t a g e s and s u r f a c e r e a c t i o n c o n t r o l in the l a t t e r s t a g e s . F a c t o r s affecting MoSs r o a s t i n g , e . g . p a r t i c l e s i z e , t e m p e r a t u r e , O~ c o n t e n t of the fluid, w e r e involved in the above s t u d i e s and o t h e r i n v e s t i g a t i o n s 5, a, 11 Some of the above i n v e s t i g a t o r s 2,v' 11 c a l c u l a t e d the a c t i v a t i o n e n e r g y for the r e a c t i o n . F l u i d i z e d bed r o a s t i n g of m o l y b d e n i t e has b e e n r e p o r t e d s'~1-14 in the l i t e r a t u r e . The heat g e n e r a t e d by the r a p i d c o m b u s t i o n of MoS s in a f l u i d i z e d bed m a y be enough to m a i n t a i n the r e q u i r e d t e m p e r a t u r e for the p r o c e s s . On a n i n d u s t r i a l s c a l e , the q u a l i t y and e f f e c t i v e n e s s of c o n t r o l l i n g the t e m p e r a t u r e of the f l u i d i z e d bed r o a