Conversion and extraction efficiencies for ground particles in heterogeneous process reactors
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MANY i m p o r t a n t
metallurgical p r o c e s s e s involve chemical reactions b e t w e e n s o l i d particles and a f l u i d p h a s e . Leaching, roasting, g a s e o u s reduction, and other g a s / s o l i d reactions are examples. Most c o m m o n l y , t h e p a r t i c l e s w e r e p r o d u c e d by p r e v i o u s g r i n d ing a n d particle size classification operations. W h e n t h e valuable m i n e r a l is substantially l i b e r a t e d a n d i s t h e m a j o r c o n s t i t u e n t o f t h e p a r t i c l e , the subs e q u e n t m e t a l l u r g i c a l r e a c t i o n is i ~ e l y to i n i t i a t e at the external s u r f a c e of the particle and involve a s h r i n k i n g u n r e a c t e d p a r t i c l e c o r e , w h i c h m a y be s u r rounded with a s o l i d s h e l l of reaction product. For example, d u r i n g the roasting of m e t a l sulfide flotation c o n c e n t r a t e s , a n o x i d e s h e l l u s u a l l y s u r r o u n d s the u n r e a c t e d s u l f i d e c o r e . In o t h e r c a s e s , s u c h a s o x i d a tive leaching of sulfide concentrates, the c h e m i c a l r e a c t i o n products a r e d i s s o l v e d by t h e f l u i d p h a s e a n d a r e a c t i o n p r o d u c t s h e l l d o e s not f o r m . T h e r e h a v e been n u m e r o u s kinetic s t u d i e s of metallurgical conv e r s i o n reactions w h e r e the u n r e a c t e d core/reaction product s h e l l particle morphology o c c u r s , and e q u a t i o n s d e v e l o p e d to d e s c r i b e t h e k i n e t i c s f o r q u a s i spherical particles u n d e r v a r i o u s rate controlling c o n d i t i o n s a r e r e v i e w e d by L e v e n s p i e l . 1 O f t e n , a relatively s m a l l amount of grinding without l i b e r a t i n g t h e o r e m i n e r a l is s u f f i c i e n t to p e r m i t l i q u i d or g a s e o u s chemical extraction of m e t a l s from r o c k s i n w h i c h the m e t a l l i c m i n e r a l s a r e d i s t r i b u t e d in a g a n g u e m i n e r a l m a t r i x . T h e v a l u a b l e m i n e r a l p h a s e u n d e r g o i n g r e a c t i o n is u s u a l l y a m i n o r c o n s t i t uent of the heterogeneous rock particle. T h e reacting f l u i d m u s t p e n e t r a t e p o r e s of t h e r o c k p a r t i c l e a n d t h e c h e m i c a l r e a c t i o n o c c u r s p r i m a r i l y on i n t e r n a l s u r f a c e s r a t h e r t h a n t h e e x t e r n a l s u r f a c e of t h e p a r ticle. T h e p o r o u s s o l i d m o d e l constitutes the other m a j o r catagory of s o l i d particle reactions. For convenience, the t e r m s "conversion" and "extraction" w i l l be u s e d t o d e s i g n a t e t h e s e t w o c a t e g o r i e s . B o t h m o d e l s a r e i l l u s t r a t e d in Fig. 1. T h e r e h a s b e e n l i t t l e a p p l i c a t i o n of t h e r e s u l t s o f n u m e r o u s laboratory kinetic s t u d i e s of particle r e a c t i o n s in d e s i g
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