Oxidation kinetics of a lime-copper concentrate pellet
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PROCESSES
for treating sulfide-copper flotation c o n c e n t r a t e s t h a t do not c a u s e a p p r e c i a b l e a i r p o l l u t i o n by SO2 e m i s s i o n a r e n e e d e d . T h e r e c e n t l y d e veloped lime-concentrate-pellet-roast (LCPR) proc e s s 1 i n v o l v e s r o a s t i n g a m i x t u r e of f i n e l y g r o u n d f l o t a t i o n c o n c e n t r a t e s and n e a r l y s t o i c h i o m e t r i c l i m e in the f o r m of a b a l l e d p e l l e t f o l l o w e d by l e a c h i n g in s u l f u r i c a c i d and e v e n t u a l r e c o v e r y of c o p p e r by e l e c t r o w i n n i n g . P e l l e t i z a t i o n i s n e c e s s a r y to r e t a i n n e a r l y a l l of the s u l f u r a s a n h y d r i t e (CaSO4) d u r i n g r o a s t i n g . When a p e l l e t i s r o a s t e d , o x i d a t i o n of the s u l f i d e m i n e r a l g r a i n s o c c u r s at the i n t e r f a c e s e p a r a t i n g t h e unreacted pellet core from a roasted shell, provided t h a t the m i n e r a l p a r t i c l e s a r e not too l a r g e . A l t h o u g h - 2 0 0 m e s h c o n c e n t r a t e s w e r e u s e d in t h e s e e x p e r i m e n t s , c o n c e n t r a t e s a s c o a r s e a s 35 m e s h have b e e n successfully treated. The reaction products are prim a r i l y CuO, Fe203 and CaSO4. If a s u b s t o i c h i o m e t r i c a m o u n t of l i m e i s u s e d at low t e m p e r a t u r e s , s o m e CuSO4 and CuO 9CuSO4 w i l l a l s o be p r o d u c e d . At high roasting temperatures some copper ferrite is formed. S u l f u r d i o x i d e g a s i s p r o d u c e d b y the r o a s t i n g r e a c t i o n and d i f f u s e s out of t h e p e l l e t t h r o u g h p o r e s in t h e r e a c t i o n p r o d u c t s h e l l . The d i f f u s i n g SO2 r e a c t s with l i m e in a d i f f u s e z o n e a d j a c e n t t o t h e c o r e / s h e l l i n t e r f a c e . Although l e s s t h a n 5 p c t of t h e SO2 e s c a p e s t h e p e l l e t u n d e r n o r m a l r o a s t i n g c o n d i t i o n s , t h e SO2 t h a t does escape originates predominantly from near the s u r f a c e of the p e l l e t e a r l y in the r o a s t i n g p e r i o d , when t h e r o a s t e d s h e l l t h r o u g h which d i f f u s i o n m u s t o c c u r i s v e r y thin. T h e p u r p o s e of t h e p r e s e n t s t u d y i s to r e p o r t on t h e single pellet roasting kinetics. The r o a s t i n g r e a c t i o n s and t h e l i m e r e a c t i o n w i t h SO2 a r e e x o t h e r m i e a n d m a y c a u s e o v e r h e a t i n g a n d f o r m a t i o n of c o p p e r f e r r i t e w h i c h i s r e l a t i v e l y i n s o l u b l e in a s u l f u r i c a c i d l e a c h . 2 F o r i n d u s t r i a l s c a l e u p , c o n s i d e r a t i o n s h o u l d be given to a c o n t i n u o u s l y f e d fluid bed r o a s t e r with w a t e r i n j e c t i o n to o b t a i n t e m p e r a t u r e c o n t r o l and t e m p e r a t u r e u n i f o r m i t y t h r o u g h out the b e d .
H. H. HAUNG, form
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