Recovery of selenium from copper anode slimes
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S E L E N I U M c a n be r e c o v e r e d f r o m c o p p e r a n o d e s l i m e s by p r e s s u r e l e a c h i n g t h e m t o r e m o v e c o p p e r1 a n d by t h e n s u b j e c t i n g t h e m t o o x i d a t i v e r o a s t i n g a t t e m p e r a t u r e s a b o v e 6 0 0 ° C .2-6 T h e o b j e c t of t h i s s t u d y was t o d e t e r m i n e t h e t h e r m o d y n a m i c a n d k i n e t i c f a c tors w h i c h control the rate of deselenization of s i n g l e s l i m e pellets. P a r t o f the s e l e n i u m c o n t e n t o f t h e s l i m e s i s i n t h e form of free s e l e n i u m which, on heating the pellets, is v o l a t i l i z e d a n d o x i d i z e d in t h e a t m o s p h e r e a r o u n d the pellet: Se + 0 2 - - SeO2
[1]
T h e r e m a i n d e r o f the s e l e n i u m c o n t e n t i n d e c o p p e r i z e d s l i m e s i s m a i n l y i n t h e f o r m of s i l v e r s e l e n i d e w h i c h r e a c t s with oxygen a c c o r d i n g t o the following equations: 2 A g 2 S e + 1.5 0 2 ~ AgzSeO3 (300 ° t o 6 0 0 ° C )
[2]
A g s S e O 3 ~ 2 A g + SeO2 + 0.5 O2 (550 ° t o 6 5 0 ° C )
[3]
EXPERIMENTAL a) M e a s u r e m e n t of Dissociation P r e s s u r e T h e t o t a l p r e s s u r e of s e l e n i u m d i o x i d e a n d o x y g e n a t e q u i l i b r i u m w i t h s i l v e r s e l e n i t e , E q s . [3a] a n d [3b] w a s m e a s u r e d in a V y c o r s p o o n g a g e , Fig. 1 . S i l v e r s e l e n i t e w a s p l a c e d in t h e s a m p l e c h a m b e r a n d t h e w h o l e a p p a r a t u s w a s evacuated. T h e s a m p l e c h a m b e r w a s t h e n s e a l e d a n d t h e gage w a s p l a c e d i n a v e r t i c a l tube f u r n a c e . A s t h e f u r n a c e t e m p e r a t u r e w a s g r a d u a l l y i n c r e a s e d , t h e d i s s o c i a t i o n p r e s s u r e in t h e " s p o o n " i n c r e a s e d c a u s i n g a d e f l e c t i o n of t h e p o i n t e r . Air w a s a d m i t t e d t o the o u t e r envelope through the stopcock until the pointer r e t u r n e d t o its e q u i l i b r i u m p o s i t i o n . T h e a i r p r e s s u r e , m e a s u r a b l e on a n e x t e r n a l m e r c u r y manometer, was then equal to the p r e s s u r e in t h e s a m p l e c h a m b e r . B o t h t e m p e r a t u r e
SEAL T h e c o m b i n a t i o n of E q s , [2] a n d [3a] r e s u l t s i n t h e o x i d a t i v e d e c o m p o s i t i o n of s i l v e r s e l e n i d e t o s i l v e r a n d s e l e n i u m dioxide: AgzSe + O2 ~ 2 A g + SeO2
[4]
E q . [4] h a s not b e e n d e s c r i b e d i n t h e l i t e r a t u r e a s a d i r e c t oxidative decomposition of s i l v e r selenide. Its existence u n d e r favorable experimental conditions w a s e s t a b l i s h e d i n t h i s w o r k a n d w i l l be d i s c u s s e d later. T h e experimental work included thermogravimetric and differential thermal a n a l y s i s s t u d i e s on thin l a y e r s a n d s p h e r i c a l p e l l e t s o f s l i m e s . T h e e f f e c t s of o t h e r c o m
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