Electrochemical reduction of arsenic (III) with cadmium metal
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G. GUTIERREZ
M., AND
M. C. FUERSTENAU
The k i n e t i c s of aqueous a r s e n i c (III) r e d u c t i o n to a m o r p h o u s a r s e n i c with c a d m i u m m e t a l a r e p r e s e n t e d . Two r e a c t i o n m e c h a n i s m s a r e o b s e r v e d in this s y s t e m . With a r s e n i c conc e n t r a t i o n s g r e a t e r than a p p r o x i m a t e l y 0.2 gm/1 in solutions containing one m o l a r s u l f u r i c acid, the r e d u c t i o n of a r s e n y l ion on the c a d m i u m s u r f a c e is r a t e controlling. When a r s e n i c c o n c e n t r a t i o n s a r e l e s s than this value, r e a c t i o n r a t e is m a s s - t r a n s p o r t c o n t r o l l e d . The r e d u c t i o n of a r s e n i c (III) with c a d m i u m powder at pH 2 is also p r e s e n t e d . A
z'XQUEOUS p r o c e s s i n g of o r e s and c o n c e n t r a t e s of m e t a l sulfides is d e s t i n e d to b e c o m e one of the m o s t i m p o r t a n t t e c h n i q u e s for r e c o v e r y of many m e t a l s . As a r e s u l t , c o n s i d e r a b l e study of the dissolution, e x t r a c t i o n , and e l e c t r o c h e m i c a l r e a c t i o n s of m e t a l s and t h e i r compounds have been undertaken to e s t a b l i s h m e c h a n i s m s o c c u r r i n g in t h e s e s y s t e m s . T h e s e s t u d i e s have involved p r i n c i p a l l y the c h a r a c t e r i z a t i o n of the b e h a v i o r of the m e t a l l i c constituents of t h e s e m a terials. In many i n s t a n c e s the b e h a v i o r of s e m i m e t a l l i c e l e m e n t s d i c t a t e s the o v e r a l l s u c c e s s of a m e t a l r e c o v e r y p r o c e s s . Well known p r o b l e m s c a u s e d by t h e s e e l e me nts include f o r m a t i o n of toxic g a s e o u s h y d r id e s , f o r m a t i o n of gelatinous p r e c i p i t a t e s , and substantial l o w e r i n g of hydrogen o v e r p o t e n t i a l . One such p a r t i c u l a r l y t r o u b l e s o m e e l e m e n t is a r s e n i c . A r s i n e , which is a r s e n i c hydride, is e x t r e m e l y toxic. F u r t h e r , many a r s e n i t e and a r s e n a t e s a l t s a r e highly insoluble. Atthough a r s e n i c can be e l i m i n a t e d f r o m solution by ap pl i c a t i o n of s e v e r a l t e c h n i q u e s , e l e c t r o c h e m i c a l r e duction with a suitable m e t a l is c e r t a i n l y one of the m o r e a t t r a c t i v e p r o c e s s e s . Not only is application of this technique s t r a i g h t f o r w a r d , but s e l e c t i v i t y is r e g u lated r i g o r o u s l y by the e l e c t r o m o t i v e s e r i e s . The e l e c t r o c h e m i c a l p o t e n ti a l of s e v e r a l r e d u c t a n t a r s e n i o u s acid couples under conditions c o m m o n l y en c ount er ed in m i n e r a l d i s s o l u t i o n , i.e. pH 2.0, 1 g m / 1 m e t a l ion, and 0 . i gm/1 HAsO2, is p r e s e n t e d in Table I. As can be s e e n f r o m this tabulation, s e v e r a l e l e me nt s a r e capable of r e d u c i n g a
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