The adsorption of silver cyanide on activated charcoal
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HA CHO AND
CHARLES
H. P I T T
E x p e r i m e n t s have b e e n c a r r i e d out on the a d s o r p t i o n of s i l v e r cyanide on c h a r c o a l f r o m solution having v a r i o u s c o n c e n t r a t i o n s of s o d i u m , c a l c i u m , free cyanide, and h y d r o g e n ion. It has b e e n found that s o d i u m and c a l c i u m ions e n h a n c e the a d s o r p t i o n of s i l v e r cyanide on c h a r c o a l while f r e e cyanide ions r e d u c e the a d s o r p t i o n . A q u a l i t a t i v e d e s c r i p t i o n of the a d s o r p t i o n b a s e d on the s t r u c t u r e of the e l e c t r i c a l double l a y e r has b e e n p r o p o s e d to explain the effect of s o d i u m , c a l c i u m and free cyanide ions on the a d s o r p t i o n . I n c r e a s ing a c i d i t y of solution e n h a n c e s the a d s o r p t i o n of s i l v e r cyanide on c h a r c o a l except in a c e r t a i n pH r a n g e . The a d s o r p t i o n of s i l v e r cyanide in acid s o l u t i o n s e e m s to be i n f l u e n c e d b y ' t h e zeta potential of the c h a r c o a l p a r t i c l e s r a t h e r than by the a d s o r p t i o n of h y d r o g e n i o n s . The a d s o r b a b i l i t y of d i f f e r e n t s i z e ions on c h a r c o a l has b e e n d e t e r m i n e d . The l a r g e r ions have a g r e a t e r a d s o r p t i o n p o t e n t i a l as shown by the o r d e r of a d s o r p t i o n a m o u n t with gold cyan%de > s i l v e r cyanide > cyanide.
A C T I V A T E D c a r b o n has b e e n used a s an a d s o r b e n t in the gold and s i l v e r i n d u s t r y for m a n y y e a r s . A c t i vated c a r b o n is loaded with gold and s i l v e r c y a n i d e s f r o m cyanide leach solutions and the loaded c h a r c o a l is s m e l t e d or allowed to d e s o r b to r e c o v e r the m e t a l values. Although c o m m e r c i a l u t i l i z a t i o n of the p r o c e s s had been s u c c e s s f u l to s o m e d e g r e e , the b a s i c m e c h a n i s m s of the p r o c e s s have not b e e n c a r e f u l l y i n v e s t i g a t e d . However, s e v e r a l i n v e s t i g a t o r s have a d v a n c e d hypot h e s e s to e x p l a i n the m e c h a n i s m of a d s o r p t i o n . A c cording to F r u m k i n ' s e l e c t r o c h e m i c a l m o d e l , ' oxygen in contact with an aqueous s o l u t i o n of c h a r c o a l is r e duced to hydroxyl groups with l i b e r a t i o n of HzOz: 02 + 2H20 + 2e- = H202 + 2OH-. Since the c h a r c o a l s u p p l i e s the e l e c t r o n s , it a c q u i r e s a positive charge and in o r d e r to m a i n t a i n e l e c t r i c a l n e u t r a l i t y it a d s o r b s a n i o n s p r e f e r e n t i a l l y . A c c o r d i n g to this t h e o r y , e l e c t r o s t a t i c f o r c e s a r e r e s p o n s i b l e for a d s o r p t i o n of a n i o n s on c h a r c o a l s u r f a c e s . S c o t t ' s hypothesis 2 was b a s e d on a c h e m i c a l r e a c t i o n b e t w e e n gold cyanide and c a l c i u m p r e s e n t on the
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