Kinetics of oxidation of copper sulfide
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AND
K. P. A B R A H A M
T h e o x i d a t i o n rate of a c u p r o u s s u l f i d e p e l l e t s u s p e n d e d in a s t r e a m of air w a s f o l l o w e d b y m e a s u r i n g the e v o l u t i o n of S O 2 titrimetrically. T h i n t h e r m o c o u p l e s e m b e d d e d in the c e n t e r of the s a m p l e r e c o r d e d the v a r i a t i o n of t e m p e r a t u r e d u r i n g o x i d a t i o n . T h e r e a c t i o n w a s f o u n d to b e t o p o c h e m i c a l a n d the s a m p l e t e m p e r a t u r e w a s f o u n d to b e h i g h e r than its s u r r o u n d i n g s initially for a b o u t half a n h o u r . A f t e r this initial p e r i o d , the s a m p l e t e m p e r a t u r e d e c r e a s e d to that of the s u r r o u n d i n g s a n d r e m a i n e d c o n s t a n t d u r i n g the rest of the p e r i o d of o v e r 5 hr. T h e a p p a r e n t a c t i v a t i o n e n e r g y f r o m t h e e x p e r i m e n t a l d a t a w a s f o u n d to b e d i f f e r e n t for t h e initial ( n o n i s o t h e r m a l ) a n d s u b s e q u e n t ( i s o t h e r m a l ) p e r i o d s . R a t e c o n t r o l ling m e c h a n i s m s for t h e s e t w o i n t e r v a l s h a v e b e e n p r o p o s e d b a s e d o n i n t e r f a c e c h e m i c a l r e a c t i o n , m a s s t r a n s f e r r e s i s t a n c e , a n d heat t r a n s f e r c o n c e p t s , F a i r a g r e e m e n t is f o u n d b e t w e e n the t h e o r e t i c a l r a t e s b a s e d o n t r a n s p o r t m e c h a n i s m s a n d t h o s e o b t a i n e d e x p e r i mentally.
SEVERAL investigators, I-5 have studied the oxidation of copper sulfides and it has been generally agreed that in the temperature range 750° to 950°C, the oxidation of cuprous sulfide can be represented as 1
Cu2S + i~-O2 = Cu~O + SOz
[i]
AF~50o c = --64,850 cal followed by 1
Cu20 + 7 0 2 = 2CuO
[2]
AF~50o C = --6090 c a l T h e C u - S - O p h a s e s t a b i l i t y d i a g r a m v a l i d in t h e t e m p e r a t u r e r a n g e 477 ° t o 677°C h a s b e e n g i v e n by I n g r a h a m 6 Fig. 1 is a C u - S - O p h a s e s t a b i l i t y d i a g r a m d r a w n , u s i n g s i m i l a r d a t a , at 850°C, t h e temperature o f i n t e r e s t in t h e p r e s e n t w o r k . F r o m t h e d i a g r a m it is q u i t e e v i d e n t t h a t C u 2 0 must initially f o r m a s a n i n t e r m e d i a t e p r o d u c t d u r i n g t h e c o n v e r s i o n of Cu2S t o CuO. A t h i g h e r p a r t i a l p r e s s u r e s o f o x y g e n p r e s e n t in a i r , C u 2 0 i s then c o n v e r t e d into C u O . T h i s p a t t e r n of b e h a v i o r d u r i n g r o a s t i n g h a s a l s o b e e n o b s e r v e d by both Peretti1 and Henderson.2 However, t h e r e s e e m s t o be s o m e u n c e r t a i n t y r e g a r d i n g t h e r a t e - d e t e r m i n i n g s t e p i n v o l v e d in t h e o x i d a t i o n . F o r i n s t a n c e , a c c o r d i n g to P e r e t t i , 1 a diffusive s t e p i n v o l v i n g t h e diffusion o f t h e r e a
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