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PADILLA

AND

H. Y. SOHN

The r a t e of r e d u c t i o n of s t a n n i c oxide ( c a s s i t e r i t e ) with c a r b o n a c e o u s m a t e r i a l s was inv e s t i g a t e d in the t e m p e r a t u r e r a n g e 1073 to 1273 K, u s i n g t h e r m o g r a v i m e t i c a n a l y s i s . T h e effects of the type, the p a r t i c l e s i z e , and the r e l a t i v e a m o u n t of c a r b o n were studied. The r e s u l t s i n d i c a t e that c a s s i t e r i t e is r e d u c e d d i r e c t l y to Sn p r o c e e d i n g through the g a s e o u s i n t e r m e d i a t e s of CO and CO2. The o v e r a l l r a t e of r e d u c t i o n is c o n t r o l l e d by the oxidation of c a r b o n by C02. A n e n e r g y of a c t i v a t i o n of 220.9 k J / m o l e (52.8 k c a l / m o l e ) was c a l c u l a t e d for the r e d u c tion of Srs with coconut c h a r c o a l within the t e m p e r a t u r e r a n g e 1073 to 1173 K and 323.8 k J / m o l e (77.4 k c a l / m o l e ) with g r a p h i t e within the t e m p e r a t u r e r a n g e 1198 to 1273 K. A d i r e c t c o m p a r i s o n was made b e t w e e n the r a t e of oxidation of coconut c h a r c o a l in COz-CO m i x t u r e s and the r a t e of r e d u c t i o n of SnO2 with coconut c h a r c o a l , which a r e not in a g r e e m e n t . The r e a s o n for this d i s a g r e e m e n t was found to be the c a t a l y t i c a c t i o n of the tin f o r m e d d u r i n g the r e d u c t i o n .

THE

r e d u c t i o n of m e t a l oxides by c a r b o n is an i m p o r t a n t m e a n s of e x t r a c t i n g m e t a l v a l u e s . A g r e a t deal of r e s e a r c h studying the k i n e t i c s and m e c h a n i s m s of such a r e a c t i o n has b e e n r e p o r t e d i n the literature. A l a r g e p o r t i o n of work has b e e n c o n c e r n e d with the r e d u c t i o n of i r o n oxide z-7 C o n t r a r i w i s e , r e l a t i v e l y few s t u d i e s have been r e p o r t e d on the c a r b o t h e r m a l r e d u c t i o n of n o n f e r r o u s m e t a l oxides. 3,8-12 Although t i n is p r o d u c e d by c a r b o t h e r m a l r e d u c t i o n , the m e c h a n i s m of this r e a c t i o n is not w e l l - u n d e r s t o o d . The p r e s e n t study was i n i t i a t e d with the a i m of inv e s t i g a t i n g the k i n e t i c s and m e c h a n i s m of the o v e r a l l r e d u c t i o n p r o c e s s in the c a s s i t e r i t e - c a r b o n powder mixtures. The r e d u c t i o n of tin dioxide by c a r b o n is u s u a l l y r e p r e s e n t e d by the equation: is SnO2(s) + C(s) = S n ( s , l ) + CO2(g).

[1]

However, f r o m the r e s u l t s of e a r l i e r i n v e s t i g a tions z'e,l~ on the r e a c t i o n of m e t a l oxides with c a r bon, the g e n e r a l a g r e e m e n t is that the c o n s e c u t i v e r e d u c t i o n and oxidation r e a c t i o n s o c c u r t h r o u g h the gaseous i n t e r m e d i a t e s CO2 and CO. The d i r e c t r e a c tion b e t w e e n s o l i d oxide and solid c a r b o n is