Carbothermic reduction of domestic chromites
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HE U n i t e d States i m p o r t s a l l of its c h r o m i t e to produce f e r r o c h r o m e s u i t a b l e for s t a i n l e s s , heat r e s i s t a n t , and alloy s t e e l p r o d u c t i o n . 1 I n view of the p r e s e n t dependence on f o r e i g n s o u r c e s and the s t e a d i l y i n c r e a s i n g d e m a n d for c h r o m i u m in the s t e e l m a k i n g , r e f r a c t o r i e s , and c h e m i c a l i n d u s t r i e s , the F e d e r a l B u r e a u of M i n e s , as p a r t of its m i s s i o n to develop p r a c t i c a l and e c o n o m i c a l methods for r e c o v e r i n g m i n e r a l s and m e t a l s f r o m lower g r a d e o r e s that m a x i mize e f f i c i e n c y and r e c o v e r y r a t e s , and for c o n v e r t ing r e s o u r c e s to r e s e r v e s , is i n t e r e s t e d in m a x i m i z ing i r o n and c h r o m i u m r e c o v e r y f r o m d o m e s t i c c h r o m i t e s a n d d e c r e a s i n g e n e r g y r e q u i r e m e n t s in p r o c e s s ing t h e s e c h r o m i t e s . P r e r e d u c t i o n , p r i o r to s m e l t i n g , offers s e v e r a l adv a n t a g e s over the d i r e c t s m e l t i n g of c o n c e n t r a t e s or o r e s . 2,a A m o n g t h e s e a d v a n t a g e s a r e the following: 1) P r e r e d u c t i o n can effectively use m o r e r e a d i l y a v a i l able s o l i d c a r b o n a c e o u s r e d u c t a n t s such as coal for p a r t i a l r e d u c t i o n so that e l e c t r i c a l e n e r g y is not r e q u i r e d to r e d u c e the e n t i r e charge and hence should be a t t r a c t i v e where power costs a r e high, 2) l e s s c a r bon is r e q u i r e d d u r i n g s m e l t i n g and t h e r e f o r e the o v e r a l l o p e r a t i o n and s l a g v i s c o s i t y a r e m o r e e a s i l y cont r o l l e d with l e s s a d d i t i o n a l s u l f u r or a s h i m p u r i t i e s i n t r o d u c e d into the s y s t e m , 3) p r o d u c t i v i t y is i n c r e a s e d for a given f u r n a c e s i z e , 4) h i g h e r q u a l i t y s l a g s a r e o b t a i n a b l e s i n c e the FeO content can be m o r e c l o s e l y controlled, 5) m o r e oxygen and v o l a t i l e s a r e r e m o v e d p r i o r to s m e l t i n g , 6) lower e l e c t r o d e c o n s u m p t i o n is p o s s i b l e , 7) p r e h e a t e d m a t e r i a l can be c h a r g e d dir e c t l y into the s m e l t i n g f u r n a c e e f f e c t i v e l y u s i n g the e x c e s s heat g e n e r a t e d in the p r e r e d u c t i o n p r o c e s s , 8) a s m o o t h e r s m e l t i n g o p e r a t i o n m a y be p o s s i b l e , and 9) h i g h e r r e c o v e r i e s a r e r e a l i z e d .
P r e r e d u c t i o n of p e l l e t i z e d c h r o m i t e s with coke b r e e z e or a n t h r a c i t e has b e e n conducted s u c c e s s f u l l y on a c o m m e r c i a l s c a l e in a r o t a r y kiln in J a p a n . The heated p r e r e d u c e d p e l l e t s a r e c o n t i n u o u s l y c h a r g e d into an e l e c t r i c a r
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