Heat transfer in a direct-fired rotary kiln: I. Pilot plant and experimentation
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r o t a r y kiln is a g a s - s o l i d s r e a c t o r that has a long h i s t o r y of u s e i n the m e t a l l u r g i c a l and c h e m i c a l i n d u s t r i e s , and c o n t i n u e s to play a key r o l e today. M e t a l l u r g i c a l p r o c e s s e s e m p l o y i n g the r o t a r y k i l n i n clude the c a l c i n a t i o n of p e t r o l e u m coke 1'2 and a l u m i n a t r i h y d r a t e ; 3 the i n d u r a t i o n 4 and r e d u c t i o n s'6 of i r o n oxide p e l l e t s , the Waelz t r e a t m e n t of z i n c - l e a d o r e s 7 and zinc l e a c h r e s i d u e s ; 8 and the t r e a t m e n t of n i c k e l o r e s and i n t e r m e d i a t e p r o d u c t s .9 On the c h e m i c a l side the r o t a r y kiln is e m p l o y e d to p r o d u c e l a r g e t o n n a g e s of c e m e n t 1~ and c a l c i n e d l i m e s t o n e 11 and d o l o m i t e i n addition to a v a r i e t y of other s o l i d s such as b a r i u m sulfide and t i t a n i u m dioxide .12 A p r i m a r y f u n c t i o n of the r o t a r y k i l n i n e a c h of these p r o c e s s e s is to r a i s e the solids to r e a c t i o n t e m p e r a t u r e and to supply heat f o r e n d o t h e r m i c r e a c t i o n s . T h e h e a t i n g m e d i u m i s n o r m a l l y a gas s t r e a m f l o w i n g through the f r e e b o a r d above the solids which move as a bed t h r o u g h the i n c l i n e d kiln. The m e c h a n i s m of heat t r a n s f e r is v e r y complex b e c a u s e it i n v o l v e s not only the gas and s o l i d s , but also the r o t a t i n g kiln wall. A s shown i n F i g . 1, six heat t r a n s f e r s t e p s i n v o l v i n g t h e s e p h a s e s can r e a d i l y be identified, i) G a s - t o - s o l i d s by r a d i a t i o n and convection; ii) g a s - t o - w a l l by r a d i a t i o n and convection; iii) exposed w a l l - t o - s o l i d s by r a d i a tion; iv) c o v e r e d w a l l - t o - s o l i d s by r a d i a t i o n and conduction; v) conduction t h r o u g h the kiln wall; and vi) s u r f a c e - t o - i n t e r i o r of the bed by bulk m i x i n g . T h e p i c t u r e is f u r t h e r c o m p l i c a t e d by the p r e s e n c e of dust and f l a m e s in the gas and a c y c l i c a l t e m p e r a t u r e v a r i a tion at the i n s i d e wall as it is r e p e a t e d l y c o v e r e d by the bed and exposed. I n t e r n a l d e v i c e s such a s l i f t e r s , t r e f o i l s , d a m s o r h a n g i n g link c h a i n s a l s o c o m p l i c a t e the heat flow i m m e a s u r a b l y . Although heat t r a n s f e r i s a v i t a l p a r t of r o t a r y kiln p r o c e s s i n g and indeed is often the r a t e l i m i t i n g step, it r e m a i n s p o o r l y u n d e r s t o o d . Few e x p e r i m e n t a l s t u d i e s have b e e n u n d e r t a k e n , p r e s u m a b l y b e c a u s e of the difficulty of m a k i n g m e a s u r e m e n t s on a r o t a t i n g v e s s e l , p a r t i c u l a r l y u n d e r i n d u s t r i a l c o n d i t i o n s . Att e m p t s ha
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