On the mechanism of iron oxide reduction by carbon
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IN r e c e n t
y e a r s , a n u m b e r of i n v e s t i g a t i o n s 1-s on the r e d u c t i o n of i r o n oxide using s o l i d fuels have b e e n c a r r i e d out. T h e s e have been m a i n l y c o n c e r n e d with the k i n e t i c s and m e c h a n i s m of reduction 9 It is g e n e r a l l y a g r e e d that, in an i n e r t a t m o s p h e r e , the 2 s t e p m e c h a n i s m involving the c o n s u m p t i o n and r e g e n e r a t i o n of the r e d u c t a n t CO by s o l i d s is the m o s t p r o b a b l e one and u n d e r high v a c u u m c o n d i t i o n s , d i r e c t r e d u c t i o n p r e dominates.5 In this p a p e r , an a t t e m p t h a s b e e n made to d e t e r m i n e the conditions under which the v a r i o u s r e d u c t i o n m e c h a n i s m s a r e o p e r a t i v e . This is b a s e d on the c o n c e p t of m i n i m u m t e m p e r a t u r e of r e d u c t i o n (Train) defined as that t e m p e r a t u r e below which the Boudouard r e a c t i o n c e a s e s to have any e f f e c t on the r e d u c t i o n p r o c e s s .
~'a is the r a t e of r e d u c t i o n of i r o n oxide by c a r b o n p e r unit v o l u m e of the m i x t u r e . The p r e s e n t a n a l y s i s a i m s at d e t e r m i n i n g the l o w e s t t e m p e r a t u r e at which r e a c t i o n [2] can be i n i t i a t e d with r e a c t i o n s [1] and [3] taking p l a c e . This e s s e n t i a l l y m e a n s that at any t e m p e r a t u r e g r e a t e r than Train, the C O / C O , r a t i o a t t a i n e d in the s t e a d y - s t a t e due to r e a c tions [1] and [3] will be g r e a t e r than the e q u i l i b r i u m C O / C O , r a t i o for r e a c t i o n [2], i . e . a t T = Trnin,
(Loo
XCO / mixture \ XCO ] f o r r e a c t i o n [2] w h e r e K e is the e q u i l i b r i u m c o n s t a n t f o r r e a c t i o n [2], and at T < Train,
(
Xco2~
MATHEMATICAL ANALYSIS
> ( X c o 2 ~ eq
XCO / m i x t u r e
\ XCO /
For purposes of analysis, an iron oxide-carbon mixi . e . , {% = O. T h e r e f o r e , ture of volume V and surface area A placed in a stream ~'CO = 2{'1 +{'3 of nitrogen under isothermal conditions is considered. In the most general case, the reduction proceeds through ~co2 = -{'1. the following mechanism:
[10]
[11]
Substituting [10] and [11] in [4] and [5]
C +CO,= 2CO
[1]
FexOy + CO = F e x O y - , + CO~
[2]
FexOy + C = FexOy_l + CO
[3]
In the s t e a d y - s t a t e , m a s s b a l a n c e equations f o r CO, CO~ and N, can be w r i t t e n a s : A . N c o + V '{'CO = 0
[4]
A o N c o 2 + V'{'CO 2 = 0
[5]
i% : o
9
/'CO = 2{'1 + / ' s - {'2
[7]
= ~'2 - {'~
[8]
w h e r e {'1 is the r a t e of c a r b o n g a s i f i c a t i o n r e a c t i o n p e r unit volume of the m i x t u r e , i ' , is the r a t e of r e d u c t i o n of i r o n oxide by CO p e r unit v o l u m e of the m i x t u r e , and
/r
METALLURGICALTRANSACTIONS B
v.
+ {'3)
[12]
v.
[13]
+
Nco2 = 9
-
V ({'s + {'I)-
[14]
~-
M a s s flow between the mixture and surroundi
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