Contribution to the study of the kinetics and mechanism of sulfation of zinc oxide
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industrial roasting of sulfides often gives rise
to the f o r m a t i o n of a c e r t a i n a m o u n t of sulfate in a d d i tion to the oxide. In the case of a h y d r o m e t a l l u r g i c a l plant, although p a r t i a l sulfation of the r o a s t e d m a t e r i a l is i n t e r e s t i n g in o r d e r to c o m p e n s a t e for the l o s s e s in s u l f u r i c acid d u r i n g the l e a c h i n g p r o c e s s , the p e r centage of sulfation should be kept u n d e r close cont r o l to avoid a p r o g r e s s i v e build up of s u l f u r i c acid in the solution. In the case of zinc sulfide c o n c e n t r a t e s r o a s t e d in a f l u o - s o l i d r e a c t o r , sulfation o c c u r s on oxide fines taken out of the f u r n a c e to the b o i l e r by the g a s e s . T h e r e is thus an i n d u s t r i a l i n t e r e s t in the study of the sulfation of zinc oxide by g a s e s c o n t a i n i n g SO 2, oxygen, and n i t r o g e n . The sulfation of v a r i o u s oxides by SO3 was studied by v a r i o u s a u t h o r s . 1-6 Dhindaw and S i r c a r 4 studied in the sulfation of Z n O b y a m i x t u r e of SO2 and oxygen. ZnO was in powder f o r m and contained v a r i a b l e a m o u n t s of V2Os, a c a t a l y s t to the f o r m a t i o n of SO3. It was found that without the p r e s e n c e of any V205, t h e r e a c t i o n p r o c e e d e d v e r y slowly. M o r e o v e r , the r a t e of s u l f a tion was found to i n c r e a s e l i n e a r l y with the a m o u n t of V205 p r e s e n t in the s a m p l e . The a u t h o r s concluded that the d e t e r m i n i n g step is the f o r m a t i o n of SOa in the s a m p l e s t h e m s e l v e s . In i n d u s t r i a l p r a c t i c e , the r o a s t e d c o n c e n t r a t e s a l w a y s contain a c e r t a i n amount of Fe2Oa, which can also act a s a c a t a l y s t to the f o r m a t i o n of SO3. In o r d e r to s e p a r a t e the SOs f o r m a t i o n r e a c t i o n and the sulfation r e a c t i o n itself we decided to work with g a s e s a l r e a d y c o n t a i n i n g SO3 o b t a i n e d by p a s s i n g on an e x t e r n a l c a t a l y s t . M o r e o v e r , owing to the fact that the oxide fines coming out of the f l u o - s o l i d r e a c tor a r e p a r t l y s i n t e r e d g r a i n s , we worked with ZnO powder c o m p r e s s e d into p o r o u s p l a t e l e t s , s i m u l a t i n g to some extent the fines c o m i n g out of a f l u o - s o l i d r e a c t o r , except for t h e i r e x t e r n a l g e o m e t r y , which has no i m p o r t a n c e a s far a s e x t e r n a l diffusion is not the r a t e d e t e r m i n i n g step. F r o m a n o t h e r point of view, I n g r a h a m and Kellogg 7 e s t a b l i s h e d the e q u i l i b r i u m conditions between ZnO and ZnSO4. They showed that no t r u e e q u i l i b r i u m may exist between those two compounds b e c a u s e t h e i r r e s p e c t i v e s t a b i l i t y r a n g e s a r e s e p a r a t e d by a b a s i c sulfate ZnO 92ZnSO4. The p u r p o
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