Oxidation of ferrous sulfate in acid solution by a mixture of sulfur dioxide and oxygen
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J. KOLBE,
AND
H. W. HAYDEN,
JR.
The oxidation of f e r r o u s sulfate with o x y g e n - s u l f u r dioxide m i x t u r e s in aqueous s u l f u r i c acid has b e e n i n v e s t i g a t e d . In the p r e s e n c e of e x c e s s oxygen, s u l f u r dioxide p r o d u c e s fast oxidation of f e r r o u s ion p r o b a b l y b e c a u s e of the f o r m a t i o n of r e a c t i v e r a d i c a l s s u c h a s HSO3, HO2, and OH. The c a t a l y t i c effect of c u p r i c ion and a c t i v a t e d c a r b o n on the r a t e of oxidation has a l s o b e e n studied. By a p p r o p r i a t e l y a d j u s t i n g the SOJO2 in the gas and Fe*3/Fe +2 in the liquid p h a s e , any of the following r e a c t i o n s can be f a v o r a b l y c a r r i e d out: 1) oxidation of f e r r o u s ion, 2) r e d u c t i o n of f e r r i c ion, and 3) g e n e r a t i o n of s u l f u r i c a c i d .
THE
use of s u l f u r dioxide a s a r e d u c i n g agent is well known in h y d r o m e t a l l u r g y . Much i n f o r m a t i o n is not a v a i l a b l e c o n c e r n i n g its a p p l i c a t i o n to oxidation p r o c e s s e s , s u c h as l e a c h i n g sulfide m i n e r a l s or e n h a n c i n g the oxidation r a t e of f e r r o u s ion by oxygen. However, s u l f u r dioxide has b e e n r e c e n t l y u s e d in leaching zinc sulfide I and c h a l c o p y r i t e . 2 In the e a r l y 1920's, a i r - s u l f u r dioxide m i x t u r e was u s e d for oxidizing f e r r o u s s u l f a t e s o l u t i o n s to p r o d u c e f e r r i c sulfate and s u l f u r i c acid at the B u r e a u of M i n e s . 3-6 L e a v e r and T h u r s t o n 3 r e p o r t e d that the u p p e r l i m i t of s u l f u r dioxide in the m i x t u r e v a r i e s f r o m 8 to 10 pct b e f o r e f e r r o u s ion oxidation is s e v e r e l y h a m p e r e d . Oldright, K e y e s , and W a r t m a n 5 p r o d u c e d f e r r i c s u l f a t e and s u l f u r i c acid by t r e a t i n g f e r r o u s sulfate s o l u t i o n with r o a s t e r g a s . In both c a s e s , f e r r o u s s u l fate was m o s t l y oxidized f i r s t followed by the oxidat i o n of SO2 to s u l f u r i c a c i d . K a r r a k e r 7 found a t h i r t y fold i n c r e a s e in the oxidat i o n r a t e of s u l f u r o u s acid by f e r r i c ion in a s o l u t i o n e n r i c h e d with oxygen but did not i n v e s t i g a t e the p o t e n t i a l use of (O2 + SO2) or (air + SO2) m i x t u r e s to oxidize f e r r o u s ion. McNally, Spedden, and Apps s r e p o r t e d that the b i s u l f i t e ion would e i t h e r r e d u c e or oxidize i r o n in solution, depending upon the O2 c o n c e n t r a t i o n in the s o l u t i o n . The I n t e r n a t i o n a l Nickel Company, Canada, p a t e n t e d the p r o c e s s e s for the r e m o v a l of i r o n with (O2 + SO2) or (air + SO2) m i x t u r e s , and i r o n was p r e c i p itated b y a d j u s t i n g the pH of the solution.9'1~ S i m i l a r u s e of SO2 in the oxidation of n o n f e r r o u s heavy m e t a l
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