The kinetics of gold cyanide adsorption on activated charcoal
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ACTIVATEDc h a r c o a l
has b e e n used s i n c e the late 1800's to a d s o r b gold and s i l v e r f r o m chloride o r cyanide s o l u t i o n s . A c t i v a t e d c h a r c o a l is p r e p a r e d for this p u r p o s e by s t e a m a c t i v a t i o n which p r o d u c e s a high s u r f a c e a r e a (1200 m2/g) c h a r c o a l composed of a d i s o r d e r e d graphite s t r u c t u r e . The active s i t e s a r e thought to be f u n c t i o n a l groups c o n t a i n i n g oxygen. The c h a r c o a l contains m a c r o p o r e s , t r a n s i t i o n p o r e s and m i c r o p o r e s . 1 The m a c r o p o r e s have an effective r a d i u s of 1000 to 200tt and c o n t r i b u t e only a s m a l l a m o u n t to the total s u r f a c e a r e a . The t r a n s i t i o n a l p o r e s having a n effective r a d i u s of 20 to 1000/~ a c c o u n t for about 5 p c t of the s u r f a c e a r e a while the m i c r o p o r e s (10 to 20A) account for about 95 pct of the total s u r f a c e a r e a . I n i t i a l l y the c h a r c o a l was b u r n e d to r e c o v e r the ads o r b e d gold and s i l v e r but p r e s e n t c o m m e r c i a l p r a c tice calls for d e s o r p t i o n in a b a s i c s o l u t i o n so that the c h a r c o a l may be used s e v e r a l t i m e s . T h i s inv e s t i g a t i o n was c a r r i e d out as p a r t of a p r o j e c t to e l u c i d a t e the n a t u r e and m e c h a n i s m of the a d s o r p t i o n p r o c e s s of gold cyanide on c h a r c o a l . EXPERIMENTAL The a c t i v a t e d c a r b o n used in the e x p e r i m e n t a l work was obtained f r o m Calgon Corp., A c t i v a t e d C a r b o n D i v i s i o n . The c h a r c o a l is made f r o m coconut s h e l l s by high t e m p e r a t u r e s t e a m a c t i v a t i o n . The c h a r c o a l was 10 • 30 m e s h s i z e with a s u r f a c e a r e a of 1150 to 1250 m2/g as d e t e r m i n e d that about 80 pct of the t o t a l a r e a is due to m i c r o p o r e s of 18 to 21A in d i a m . S a m p l e s were p r e p a r e d by o b t a i n i n g u n i f o r m size p a r t i c l e s (20 • 28 mesh) by s i e v i n g the o r i g i n a l c h a r coal supplied f r o m Calgon C o r p o r a t i o n . The m e a n p a r t i c l e d i a m e t e r for this size r a n g e is 0.714 m m . Gold solutions were p r e p a r e d by d i s s o l v i n g 0.5 g of 99.9 pct gold powder in 0.068 M NaCN s o l u t i o n . F o r rapid d i s s o l u t i o n it was n e c e s s a r y to b u b b l e oxygen through the s o l u t i o n while s t i r r i n g . The gold cyanide solution was diluted to 1000 ppm Au and 0.02 M NaCN to make a s t o c k solution which was s u b s e q u e n t l y used to p r e p a r e e x p e r i m e n t a l s o l u t i o n s . The s t o c k solution was a n a l y z e d u s i n g a t o m i c a b s o r b a n c e EUNG HA CHO, formerly with the Department of Metallurgy, University of Utah, is now Assistant Professor, Department of Mineral ProcessingEngineering, West Virginia University, WV. STEVEN N. DIXON is Me
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