Electric smelting of lead sulfate residues
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M i t s u b i s h i M e t a l C o r p o r a t i o n (MMC) has b e e n u s i n g e l e c t r i c s m e l t i n g for the t r e a t m e n t of lead s u l fate r e s i d u e at its N a o s h i m a S m e l t e r for s e v e r a l y e a r s . The lead sulfate is in a r e s i d u e that r e m a i n s u n d i s solved when copper c o n v e r t e r dusts a r e leached in dilute s u l f u r i c acid to r e c o v e r soluble zinc and cadm i u m . The N a o s h i m a S m e l t e r p r o d u c e s 1000 to 1500 tons of r e s i d u e / y e a r (where 1 ton = 1000 kg). The r e s i d u e is s m e l t e d in an 800 kVA e l e c t r i c f u r n a c e to produce slag, s p e i s s , and l e a d / t i n b u l l i o n which is e l e c t r o l y t i c a l l y r e f i n e d to p r o d u c e l e a d / t i n alloy for s a l e . T y p i c a l l y the r e s i d u e contains 38 pct Pb, 4 pct Cu, 0.8 pct F e , 9 pct Zn, 4 pct As, 3 pct Sn, and 9 pct S. The E l e c t r o l y t i c Z i n c Company of A u s t r a l a s i a L i m i t e d (EZ) has b e e n p r o d u c i n g a lead sulfate r e s i d u e s i n c e the c o m m i s s i o n i n g in 1971 of a new p l a n t at R i s d o n to t r e a t zinc f e r r i t e r e s i d u e by the J a r o s i t e P r o c e s s . 1 The lead sulfate r e s i d u e is a b y - p r o d u c t f r o m this p r o c e s s and is t e r m e d a l e a d / s i l v e r r e s i d u e as it contains s i g n i f i c a n t a m o u n t s of s i l v e r . C u r r e n t l y p r o d u c t i o n is at the r a t e of about 38,000 tons p e r y e a r , a t y p i c a l a n a l y s i s b e i n g 22 pct Pb, 13 pct F e , 10 pct Zn, 10 pct S, and in e x c e s s of 0.7 kg A g / t o n . S i m i l a r lead sulfate r e s i d u e s a r e now b e i n g p r o d u c e d in m a n y e l e c t r o l y t i c zinc p l a n t s following the adoption of i m p r o v e d leaching technologies. Since J u n e 1974, MMC and EZ have b e e n c o o p e r a t i n g in the d e v e l o p m e n t of e l e c t r i c s m e l t i n g of lead sulfate r e s i d u e s for the p r o d u c t i o n of lead b u l l i o n . T e s t i n g of e l e c t r i c s m e l t i n g of l e a d / s i l v e r r e s i d u e f r o m R i s d o n has b e e n conducted in the 800 kVA e l e c t r i c f u r n a c e at N a o s h i m a . The only t r e a t m e n t r e q u i r e d in p r e p a r a t i o n for s m e l t i n g was d r y i n g of the r e s i d u e to 5 pct m o i s t u r e . A s i m p l e m i x t u r e of the r e s i d u e with s i l i c a , l i m e s t o n e , i r o n ore, and coke b r e e z e was then fed d i r e c t l y and c o n t i n u o u s l y into the e l e c t r i c f u r n a c e .
The s m e l t i n g t e s t s w e r e conducted with a t e m p e r a t u r e of 1400 to 1600 K m a i n t a i n e d in the s l a g l a y e r . T h e p r o d u c t s w e r e lead b u l l i o n , slag, and matte, which were tapped f r o m the f u r n a c e at a p p r o p r i a t e i n t e r v a l s . The b u l l i o n g e n e r a l l y contained 90 pct of the lead and s i l v e r that w e r e in t h e s
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