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T h e t h e r m a l d e c o m p o s i t i o n of Mn203 was s t u d i e d at 1000~ The r a t e of d e c o m p o s i t i o n was i n h e r e n t l y fast and dependent on the m a s s - t r a n s f e r c h a r a c t e r i s t i c s of the g e o m e t r y u s e d . The r a t e of r e d u c t i o n of Mn304 by CO, coconut c h a r c o a l , coke, and p u r e g r a p h i t e was i n v 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 900 to 1200~ It was found that the r a t e of r e d u c t i o n in CO was fast and that the o v e r a l l r a t e of r e d u c t i o n b y c a r b o n was d e t e r m i n e d by the oxidation of the c a r b o n by CO2. In g e n e r a l the r a t e was h i g h e r for higher t e m p e r a t u r e s and for s m a l l e r p a r t i c l e s i z e s of the oxide and the c a r b o n . G a s e s such as He, A r , o r N2 were found to d e c r e a s e the r a t e b e c a u s e of the d i l u t i o n effect due to b a c k diffusion into the i n t e r p a r t i c l e p o r e s of the o x i d e - c a r b o n m i x t u r e . It was f u r t h e r found that the oxid a t i o n of c a r b o n by CO2 was c a t a l y z e d by Mn304. T h i s effect was p a r t i c u l a r l y p r o n o u n c e d when the r e d u c t a n t was e i t h e r graphite or coke.

T HE r e d u c t i o n of m e t a l oxides by c a r b o n has b e e n the s u b j e c t of m a n y s t u d i e s in the p a s t few y e a r s . In the case of the oxides of m a n g a n e s e r e d u c t i o n to the m e t a l is i m p o s s i b l e b e c a u s e the c a r b i d e is f o r m e d f i r s t . 1 It is, however, of i n t e r e s t to study the c a r b o n r e d u c t i o n of the higher oxides of m a n g a n e s e to the lowest oxide, MnO. In the p r o d u c t i o n of m a n g a n e s e m e t a l by the s i l i c o t h e r m i c p r o c e s s it is a d v a n t a g e o u s if the lowest m a n g a n e s e oxide is used as one of the reactants.1 T h e r m a l d e c o m p o s i t i o n of MnO2 to Mn203 can be a c h i e v e d r e a d i l y by h e a t i n g i n a i r to about 450~ w h e r e a s the t h e r m a l d e c o m p o s i t i o n of Mn203 to Mn304 takes p l a c e at about 950~ The f u r t h e r r e d u c t i o n of Mn304 to MnO can be a c h i e v e d by H2, CO or c a r b o n . The p r e s e n t work was u n d e r t a k e n to study the b a s i c f e a t u r e s of i) the t h e r m a l d e c o m p o s i t i o n of Mn203 to Mn304 at 1000~ (that of MnO2 to Mn203 is so r a p i d that a m e a n ingful study was i m p o s s i b l e ) ; ii) the r e d u c t i o n of Mn304 by CO as well as by v a r i ous c a r b o n a c e o u s m a t e r i a l s at t e m p e r a t u r e s b e t w e e n 900 and 1200~

t w e e n the top of the bed and the r i m of the c r u c i b l e was about 6 m m . The a v e r a g e p a r t i c l e s i z e of the Mn2Oa p a r t i c l e s used in t h e s e e x p e r i m e n t s was l e s s than 74 ~ m (