2007512045 ROSENQVIST Phase Equilibria In

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The 1978 Extractive Metallurgy Lecture The Metallurgical Society of AIME Phase Equilibria in the Pyrometallurgy of Sulfide Ores TERKEL ROSENQVIST Although reaction kinetics and process dynamics may greatly affect the engineering of pyrometallurgical processes, the limits of what can be achieved are set by the equili- brium relations. After a discussion of some experimental techniques this lecture reviews the phase equilibria that are of importance for the roasting and smelting of some import- ant sulfide ores. The quaternary systems Zn-Fe-S-O and Cu-Fe-S-O under roasting con- ditions are treated in detail as functions of composition, temperature and oxygen poten- tial. Finally the use of calcium oxide as an absorbent for sulfur as well as a means to modify roasting and reduction processes for copper and iron sulfides is discussed. IN giving this lecture for the 1978 annual meeting of the AIME I have to recall that last summer was exactly thirty years since I first came to the North American continent as a research fellow with the In- stitute for the Study of Metals, us it was called at that time, in Chicago. Knowing that I would be free to work on any subject I liked, provided it had some vague rela- tion to metals I had, before I left Norway, discussed various possibilities with an older colleague. I'wanted to work on oxygen in liquid steel and deoxidafion equilibria, but was advised against it. "Oxygen is so difficult to analyze for. Why don't you work on sulfur instead." As a result of this advice I can today look back on what I will call a thirty years war with fire and brimstone. Although desulfurization of iron and steel is a very important subject, where a great deal of research has been done, and when there still are problems to solve, I will, in this audience of mainly nonferrous metallurgists, talk about problems in connection with the recovery of nonferrous metals from their sulfide ores, and will also give some views on the handling of the sulfur component. As you all know, extractive metallurgy may be divided into pyrometallurgy and hydrometallurgy. An extensive review of the thermodynamics of hydrometal- lurgical treatment of sulfide minerals was recently given by Peters 1 in his 1976 Extractive Metallurgy Lecture. My job will therefore be to discuss the ther- modynamics of pyrometallurgical treatments. I can not do this, however, without pointing out the pioneer work that has been done on this continent during the same thirty years by people like Reinhard Schuhmann, Herbert Kellogg, Tom Ingraham, and others. Their work has been a great inspiration to me, and in this lecture I will also draw on some of their findings. The most important pyrometallurgical processes are roasting and sintering, as applied for example to The Extractive Metallurgy Lecture was authorized in 1959 to provide an out- standing man in the field of nonferrous metallurgy as a lecturer at the annual AIME meeting. TERKEL ROSENQVIST, born in Oslo, Norway, was awarded the Doctorate of Science in 1954 from the Norwegian Institute of Technology. He has been Professor of Extractive Metallurgy there since 1955. He was a Research Fellow in the Institute for the Study of Metals at the University of Chicago in 1947 and METALLURGICAL TRANSACTIONS B more recently a Battelle Visiting Professor at the Ohio State University in 1976 and a visiting Professor at both M.I.T. and the University of Wisconsin-Madison. Dr. Rosenqvist's main research work has been the thermodynamic studies of sys- tems important to pyrometallurgy, in particular, metal-sulfur and metal-sulfur- oxygen systems, and the thermodynamics of copper smelting. He has made ex- tensive contributions to journal publications and is the author of the textbook Principles of Extractive Metallurgy. ISSN 0360-2141/78/0911-0337500.75/0 9 1978 AMERICAN SOCIETY FOR METALS AND VOLUME 9B, SEPTEMBER 1978-337 THE METALLURGICAL SOCIETY OF AIME

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Transcript of 2007512045 ROSENQVIST Phase Equilibria In

The 1978 Extractive Metallurgy Lecture The Metallurgical Society of AIME

Phase Equilibria in the Pyrometallurgy of Sulfide Ores

T E R K E L ROSENQVIST

Although r e a c t i o n k i n e t i c s and p r o c e s s d y n a m i c s m a y g r e a t l y affect the e n g i n e e r i n g of p y r o m e t a l l u r g i c a l p r o c e s s e s , the l i m i t s of what c a n be a c h i e v e d a r e s e t by the e q u i l i b r i u m r e l a t i o n s . A f t e r a d i s c u s s i o n of s o m e e x p e r i m e n t a l t e c h n i q u e s this l e c t u r e r e v i e w s the p h a s e e q u i l i b r i a that a r e of i m p o r t a n c e for the r o a s t i n g and s m e l t i n g of s o m e i m p o r t ant s u l f i d e o r e s . The q u a t e r n a r y s y s t e m s Z n - F e - S - O and C u - F e - S - O u n d e r r o a s t i n g cond i t i o n s a r e t r e a t e d in d e t a i l as f u n c t i o n s of c o m p o s i t i o n , t e m p e r a t u r e and oxygen p o t e n t i a l . F i n a l l y the use of c a l c i u m oxide as a n a b s o r b e n t for s u l f u r as well as a m e a n s to modify r o a s t i n g and r e d u c t i o n p r o c e s s e s for copper and i r o n s u l f i d e s is d i s c u s s e d . giving this l e c t u r e for the 1978 a n n u a l m e e t i n g of the AIME I have to r e c a l l that last s u m m e r was e x a c t l y t h i r t y y e a r s s i n c e I f i r s t c a m e to the N o r t h A m e r i c a n c o n t i n e n t as a r e s e a r c h fellow with the Ins t i t u t e for the Study of M e t a l s , us it was c a l l e d at that t i m e , in Chicago. Knowing that I would be f r e e to work on any s u b j e c t I liked, p r o v i d e d it had s o m e vague r e l a tion to m e t a l s I had, b e f o r e I left Norway, d i s c u s s e d v a r i o u s p o s s i b i l i t i e s with a n older c o l l e a g u e . I ' w a n t e d to work on oxygen in liquid s t e e l and deoxidafion e q u i l i b r i a , but was a d v i s e d a g a i n s t it. " O x y g e n is so difficult to a n a l y z e f o r . Why d o n ' t you w o r k on s u l f u r i n s t e a d . " As a r e s u l t of this advice I can today look back on what I will c a l l a t h i r t y y e a r s w a r with f i r e and b r i m s t o n e . Although d e s u l f u r i z a t i o n of i r o n and s t e e l is a v e r y i m p o r t a n t s u b j e c t , w h e r e a g r e a t deal of r e s e a r c h has b e e n done, and when t h e r e s t i l l a r e p r o b l e m s to solve, I will, in this a u d i e n c e of m a i n l y n o n f e r r o u sThe Extractive Metallurgy Lecture was authorized in 1959 to provide an outstanding man in the field o f nonferrous metallurgy as a lecturer at the annual AIME meeting. TERKEL ROSENQVIST, born in Oslo, Norway, was awarded the Doctorate of Science in 1954 from the Norwegian Institute of Technology. He has been Professor of Extractive Metallurgy there since 1955. He was a Research Fellow in the Institute for the Study of Metals at the University of Chicago in 1947 and METALLURGICAL TRANSACTIONS B

IN

m e t a l l u r g i s t s , t a l k about p r o b l e m s in c o n n e c t i o n with the r e c o v e r y of n o n f e r r o u s m e t a l s f r o m t h e i r sulfide o r e s , and will a l s o give s o m e v i e w s on the h a n d l i n g of the s u l f u r c o m p o n e n t . As you a l l know, e x t r a c t i v e m e t a l l u r g y m a y be divided into p y r o m e t a l l u r g y and h y d r o m e t a l l u r g y . An e x t e n s i v e r e v i e w of the t h e r m o d y n a m i c s of h y d r o m e t a l l u r g i c a l t r e a t m e n t of sulfide m i n e r a l s was r e c e n t l y given by P e t e r s 1 in his 1976 E x t r a c t i v e M e t a l l u r g y L e c t u r e . My job w i l l t h e r e f o r e be to d i s c u s s the t h e r m o d y n a m i c s of pyrometallurgical t r e a t m e n t s . I can not do this, h o w e v e r , without p o i n t i n g out the p i o n e e r work that has b e e n done on this c o n t i n e n t d u r i n g the s a m e t h i r t y y e a r s by people like R e i n h a r d Schuhmann, H e r b e r t Kellogg, T o m I n g r a h a m , and o t h e r s . T h e i r work has b e e n a g r e a t i n s p i r a t i o n to me, and in this l e c t u r e I will a l s o draw on s o m e of t h e i r f i n d i n g s . The m o s t i m p o r t a n t p y r o m e t a l l u r g i c a l p r o c e s s e s a r e r o a s t i n g and s i n t e r i n g , as applied for e x a m p l e tomore recently a Battelle Visiting Professor at the Ohio State University in 1976 and a visiting Professor at both M.I.T. and the University of Wisconsin-Madison. Dr. Rosenqvist's main research work has been the thermodynamic studies of systems important to pyrometallurgy, in particular, metal-sulfur and metal-sulfuroxygen systems, and the thermodynamics of copper smelting. He has made extensive contributions to journal publications and is the author of the textbook Principles o f Extractive Metallurgy. VOLUME 9B, SEPTEMBER 1978-337

ISSN 0360-2141/78/0911-0337500.75/0 9 1978 A M E R I C A N S O C I E T Y F O R M E T A L S A N D THE METALLURGICAL SOCIETY OF AIME

t h e s u l f i d e s of c o p p e r , z i n c and lead, and m a t t e s m e l t ing and c o n v e r t i n g , which a p p l y to c o p p e r , n i c k e l and l e a d s u l f i d e s . C o n t r a r y to the conditions f o r oxide o r e s a d i r e c t r e d u c t i o n to m e t a l with, e.g., c a r b o n o r h y d r o g e n is in m o s t c a s e s not a f e a s i b l e p r o c e s s , but Will be d i s c u s s e d b r i e f l y t o w a r d s the end of this lecture. T H E O R E T I C A L BASIS Although r e a c t i o n k i n e t i c s and p r o c e s s d y n a m i c s m a y g r e a t l y influence and a f f e c t the e n g i n e e r i n g of r o a s t i n g and s m e l t i n g , the t h e o r e t i c a l l i m i t of what can be a c h i e v e d is s e t b y the t h e r m o d y n a m i c s , i.e., the e q u i l i b r i u m r e l a t i o n s . Of i m m e d i a t e i n t e r e s t to r o a s t i n g and s m e l t i n g p r o c e s s e s a r e , t h e r e f o r e , e q u i l i b r i u m r e l a t i o n s in s y s t e m s of the type M e - S - O , the m a i n f e a t u r e s of which a r e shown s c h e m a t i c a l l y in Fig. 1 for a temperature where all condensed phases are solid. In a d d i t i o n to the m e t a l l i c p h a s e Me, we have the sulfide MeS, the oxide MeO and the s u l f a t e MeSO4. F u r t h e r m o r e , we could have had o t h e r s u l f i d e s like MeS2 o r Me2S, and o t h e r o x i d e s like Me203. We could have had t r i v a l e n t s u l f a t e s like Me2(SO4)3 o r b a s i c s u l f a t e s like MeO 9MeSO4. Although t h e s e p h a s e s a r e i n d i c a t e d and t r e a t e d a s if they w e r e p u r e , s m a l l a m o u n t s of s o l i d s o l u b i l i t y can not be e x c l u d e d . T h i s has b e e n d e m o n s t r a t e d by, a m o n g o t h e r s , T u r k d o g a n and c o w o r k e r s 2'3 for c a l c i u m and m a n g a n e s e o x i d e s , which show s m a l l s o l i d s o l u b i l i t i e s of the c o r r e s p o n d ing s u l f i d e s and s u l f a t e s , and by K o r 4 f o r the s o l i d s o l u b i l i t y of s u l f i d e in w u s t i t e and in m e t a l l i c i r o n . C o r r e s p o n d i n g s o l i d s o l u b i l i t i e s in the s u l f i d e and s u l f a t e p h a s e s a r e l e s s w e l l known. A b o v e the e u t e c t i c m e l t i n g t e m p e r a t u r e s , h o w e v e r , e x t e n s i v e liquid m i s c i b i l i t y b e t w e e n m e t a l , s u l f i d e , oxide and s u l f a t e i s the r u l e r a t h e r than the e x c e p t i o n . The gas p h a s e m a y have any c o m p o s i t i o n b e t w e e n p u r e O2 and p u r e $2 and c o n t a i n m o l e c u l e s of SO3 and SO2 a s w e l l a s of the l e s s s t a b l e s p e c i e s SO and $20. S o m e m e t a l s s u c h a s l e a d , tin, a r s e n i c and a n t i m o n y m a y f o r m g a s e o u s m o l e c u l e s with s u l f u r , oxygen o r b o t h . T h i s m a y c a u s e the c o m p o s i t i o n a l a r e a of the gas p h a s e to extend way into the t e r n a r y p h a s e d i a gram. $2

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Fig. 2--Predominance areas for phases in the system Me-S-O at constant temperature, schematic. F o r a s y s t e m with t h r e e c o m p o n e n t s and two o r more phases a complete thermodynamic description a s function of c o m p o s i t i o n and t e m p e r a t u r e would r e q u i r e a t h r e e d i m e n s i o n a l r e p r e s e n t a t i o n . In o r d e r to o b t a i n a two d i m e n s i o n a l f i g u r e one p a r a m e t e r has to be k e p t c o n s t a n t . M o s t w e l l known a r e p e r h a p s the d i a g r a m s f o r c o n s t a n t t e m p e r a t u r e w h e r e the p h a s e e q u i l i b r i a a r e shown a s f u n c t i o n of the p a r t i a l p r e s s u r e or p o t e n t i a l of two c o m p o n e n t s in the gas p h a s e . Such d i a g r a m s w e r e p r e s e n t e d by, e.g., K e l l o g g and B a s u s and a r e shown s c h e m a t i c a l l y in F i g . 2. In this type of d i a g r a m e a c h p h a s e i s s t a b l e within a p r e d o m i n a n c e a r e a , this b e i n g b o r d e r e d by l i n e s which r e p r e s e n t c o e x i s t e n c e with o t h e r p h a s e s . A s p a r a m e t e r s the l o g a r i t h m s of the p a r t i a l p r e s s u r e of 02 and SO2 a r e c h o s e n , t h e s e m o l e c u l e s b e i n g the d o m i n a t i n g ones in r o a s t i n g and s m e l t i n g g a s e s . Ins t e a d of the SO2 p r e s s u r e the p a r t i a l p r e s s u r e of $2 o r e v e n SOs could have b e e n c h o s e n . A l s o , the oxygen p o t e n t i a l could h a v e b e e n given b y the c o r r e s p o n d i n g C O J C O o r H~O/H2 r a t i o s , and the s u l f u r p o t e n t i a l by, e.g., the H2S/H2 r a t i o . In F i g . 3 l i n e s a r e d r a w n c o r r e s p o n d i n g to 1 a i m p r e s s u r e of $2 and SO3 r e s p e c t i v e l y . F u r t h e r m o r e , at the u p p e r left c o r n e r a line is given to show the a p p e a r a n c e of liquid e l e m e n t a l s u l f u r . Any g a s p h a s e b e y o n d t h i s line would be unstable. One c h a r a c t e r i s t i c f e a t u r e of t h i s type of d i a g r a m is that the s l o p e of e a c h c u r v e is given by the s t o i c h i o m e r r y of the c o r r e s p o n d i n g r e a c t i o n . T h u s for a r e a c t i o n of the t y p e : 2MeS+ 30~= 2MeO+ 2SO2 [1]

and taking the a c t i v i t i e s of MeS and MeO e q u a l to unity:T:const.

2 log Pso~ = 3 log p o 2 + log Kz. Thus the s l o p e of 3/2 w i l l a p p l y r e g a r d l e s s of w h e t h e r the m e t a l is lead, c o p p e r o r z i n c . Only the v a l u e for the e q u i l i b r i u m c o n s t a n t K1 will d i f f e r . T h i s type of p r e s e n t a t i o n d o e s not show the effect of t e m p e r a t u r e , h o w e v e r . T h i s c a n be a c h i e v e d b y p l o t t i n g log p o 2 a g a i n s t t e m p e r a t u r e , o r b e t t e r a g a i n s t 1/T, for a c o n s t a n t SOs p r e s s u r e . Such p r e s e n t a t i o n s w e r e , to m y knowledge, f i r s t given by Knacke and c o w o r k e r s . 6 F i g u r e 3 shows a s c h e m a t i c d i a g r a m for a c o n s t a n t SO2 p r e s s u r e of I a i m , ~nd m a y b e c o n sidered a pyrometallurgical Pourbaix diagram. Also in t h i s d i a g r a m c u r v e s f o r 1 a t m of $2 and SOa a r e shown. A s for a given s t o i c h i o m e t r y t h e r e i s a definite r e l a t i o n b e t w e e n the 02 and the SOs p r e s s u r e s the METALLURGICAL TRANSACTIONS B

M

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Fig. 1--Phase equilibria in a s y s t e m Me-S-O under roasting conditions, schematic. 338-VOLUME 9B, SEPTEMBER 1978

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s t a n t for Eq. [5] the e q u i l i b r i u m c o n s t a n t for Eq. [4] as well as the c o r r e s p o n d i n g line in F i g . 3 m a y be calculated. E X P E R I M E N T A L METHODS Although the e q u i l i b r i a b e t w e e n the v a r i o u s p h a s e s m a y be c a l c u l a t e d f r o m t h e r m o d y n a m i c data for the compounds involved, such data a r e often of i n s u f f i c i e n t a c c u r a c y . A b e t t e r way is to m e a s u r e the e q u i l i b r i a d i r e c t l y , in which c a s e any m u t u a l s o l u b i l i t y b e t w e e n the c o m p o u n d s involved, as well as the p r e s e n c e of c o m p l e x p h a s e s which were not f o r e s e e n in the c a l c u l a t i o n s , m a y be detected. T h e choice of method will to a g r e a t extent depend on the e q u i l i b r i u m to be studied. F o r e q u i l i b r i a b e t w e e n s u l f a t e s and oxides t h e s e m a y de d e r i v e d f r o m t o t a l d e c o m p o s i t i o n p r e s s u r e s as given by E q s . [4] and [5]. O n e difficulty is the a g g r e s s i v e n a t u r e of the gas and c o n d e n s a t i o n of SO3 at a r o u n d r o o m t e m p e r a t u r e which p r e v e n t a d i r e c t use of m e r c u r y m a n o m e t e r s . In m e a s u r e m e n t s made a m o n g o t h e r s by W a r n e r and I n g r a h a m v a n i n t e r m e d i a t e bellow was u s e d . In other c a s e s 8 use was made of a flexible s i l i c a m e m b r a n e c o m b i n e d with a c o m p e n s a t i n g a i r or n i t r o g e n p r e s s u r e which could be m e a s u r e d d i r e c t l y , F i g . 4. A f t e r the sulfate had b e e n i n s e r t e d the whole a p p a r a t u s is e v a c u a t e d u n d e r gentle h e a t i n g in o r d e r to r e m o v e a l l a d s o r b e d g a s e s , and the p o s i t i o n of the m e m b r a n e is r e a d on the c a t h e t o m e t e r . The p a r t of the appar.atus that c o n t a i n s the s u l f a t e is then s e a l e d at the f u s e - o f f point. A s the s u l f a t e is heated t o w a r d d e c o m p o s i t i o n a i r or n i t r o g e n is a d m i t t e d to keep the m e m b r a n e in its fixed p o s i t i o n , and the gas p r e s s u r e is r e a d on a m e r c u r y m a n o m e t e r . The a p p a r a t u s thus a c t s as a z e r o method. I n s t e a d of a m e m b r a n e a s i l i c a s p i r a l tube ( B o d e n s t e i n gage) could have b e e n u s e d . One s h o r t c o m i n g of the d e c o m p o s i t i o n p r e s s u r e method

high temp

1//T ---~

Iowtemp

F i g . 3 - - E f f e c t of t e m p e r a t u r e on p h a s e e q u i l i b r i a in the s y s t e m M e - S - O a t c o n s t a n t SOs p r e s s u r e , s c h e m a t i c . PSO2 = 1 a t m , 9 . . . . PSO2 = 0.1 a t m , . . . . . P t o t = 1 a t m .

c u r v e s for any SO2 p r e s s u r e d i f f e r e n t f r o m 1 arm m a y b e obtained by a s y s t e m a t i c shift of the c u r v e s , up or down, as i l l u s t r a t e d by the dotted l i n e s in F i g . 3 for 0.1 arm of SO2. We notice that the oxygen p o t e n t i a l for the e q u i l i b r i a b e t w e e n Me and MeO and b e t w e e n MeS and MeSO4 a r e not affected by the SO2 p r e s s u r e . W h e r e a s for i n t e r m e d i a t e oxygen p o t e n t i a l s , and in the a b s e n c e of i n e r t g a s e s , the SO2 p r e s s u r e is c l o s e l y equal to the t o t a l gas p r e s s u r e in the s y s t e m this is not the c a s e for high or v e r y low 02 p o t e n t i a l s . In addition to SO2 and 02 the gas will at high 02 p o t e n t i a l s contain s o m e SO3 given by the e q u i l i b r i u m : 2 SO2 + O2 = 2 SOs [2]

giving Ptot ~ PS02 + P02 + PS03. T h e r e f o r e , for m e a s u r e m e n t s c a r r i e d out at a t o t a l gas p r e s s u r e of 1 arm the p a r t i a l p r e s s u r e s of O2 and SO3 m u s t be c a l c u l a t e d in o r d e r to o b t a i n the SO2 p r e s s u r e . S i m i l a r l y at v e r y low oxygen p o t e n t i a l s t h e r e m a y be s o m e s u l f u r v a p o r given by the e q u i l i b r i u m : 2 SO~ = S2 + 2 02 [3]

and Ptot ~- PSO2 + P Sf Other gas s p e c i e s s u c h as SO and $20 m a y aiso o c c u r in s m a l l e r a m o u n t s . F i n a l l y , in c a s e s w h e r e e i t h e r the m e t a l or some of its c o m pounds a r e v o l a t i l e t h e i r v a p o r p r e s s u r e s m u s t be i n cluded in the t o t a l gas p r e s s u r e . In F i g . 3 e q u i l i b r i u m l i n e s for a t o t a l gas p r e s s u r e of 1 arm a r e shown as dashed l i n e s for the c a s e s w h e r e t h e s e differ f r o m the l i n e s for 1 a t m of SO2. A t h i r d way of p r e s e n t a t i o n is by the t o t a l d e c o m p o s i t i o n p r e s s u r e , i.e., the p r e s s u r e e x e r t e d when a compound is heated alone or in an i n e r t a t m o s p h e r e . A t y p i c a l e x a m p l e is sulfate d e c o m p o s i t i o n : MeSO4 = MeO + SOsS03 = SO2 + ~02 where:

3athetometerTo m a n o m e t e r11

To gas source

[4]

[5]

e-off point

Ps02 = 2 PO~ and: Ptot = PSO3 + PS02 + PO2 = PSO3 + 3 p o 2. Knowing the t o t a l p r e s s u r e and the e q u i l i b r i u m conMETALLURGICAL TRANSACTIONS B F i g . 4 - - A p p a r a t u s f o r t he s t u d y of d e c o m p o s i t i o n p r e s s u r e s of m e t a l s u l f a t e s . A f t e r K a r w a n et al 8 VOLUME 9B, SEPTEMBER 1978 339

ut Pt-win Teflon-

ZrOz-Cc

02out

Al2Os-t u bes Sample-Pt-gauze----J

Pt/Pt-lO%Rh thermocouple ........ ~j~Fig. 5--Apparatus for the study of equilibria with SO2 + S2 gas mixtures, t2 is that it is l i m i t e d to r e l a t i v e l y n a r r o w t e m p e r a t u r e r a n g e s , u s u a l l y r a n g e s of l e s s than 200~ V a r i o u s types of d y n a m i c or flow m e t h o d s have b e e n u s e d . T h e s e m a y be b a s e d e i t h e r on d e t e r m i n i n g the e q u i l i b r i u m c o m p o s i t i o n of the gas for a given phase c o m b i n a t i o n9 o r on d e t e r m i n i n g the c o m p o s i t i o n of the s o l i d p h a s e s in e q u i l i b r i u m with a given gas flow. %s,l~ The gas m a y be any s u i t a b l e m i x t u r e which d e f i n e s the oxygen and s u l f u r p o t e n t i a l s as for e x a m p l e SO2 + Oe, l~ SO2 + C02 + CO, 2's HeS + H20 + He (Ref. 9) or SO2 + $2.~e'~3 An e x a m p l e of the l a s t type is shown in F i g . 5, which shows the a p p a r a t u s u s e d to study the e q u i l i b r i u m : * *Throughoutthispaperquotationmarksare usedto indicatephasesw~thnoticeablevariationin composition,regardlessof whetherthisis in the metalto metalloidor in the metalto metalratio. 3 " F e S " + 2 SOs = Fe304 + 5/2 Se. H e r e a gas m i x t u r e SO2 + $2 is p r e p a r e d by b u b b l i n g SOs t h r o u g h a bath of m o l t e n s u l f u r kept in a t h e r m o s t a t e d f u r n a c e , w h e r e b y s a t u r a t i o n with s u l f u r v a p o r is a c h i e v e d . T h i s gas m i x t u r e is p a s s e d through a bed of m i x e d s o l i d " F e S " and Fe304 and the effluent gas is s a m p l e d and a n a l y z e d . By c h a n g i n g the t e m p e r a t u r e of the t h e r m o s t a t e d f u r n a c e the c o m p o s i t i o n of the gas could be changed, and was changed u n t i l a c o m p o s i t i o n was found which p a s s e d t h r o u g h the bed of s o l i d p h a s e s without f u r t h e r r e a c t i o n , i . e . , e q u i l i b r i u m had b e e n e s t a b l i s h e d . The s a m e t e c h n i q u e was u s e d x3 to study the e q u i l i b r i u m : CaS + 2 SO2 = CaSO4 + $2. Although the flow method may be u s e d over a s o m e what l a r g e r t e m p e r a t u r e r a n g e than the d e c o m p o s i t i o n p r e s s u r e method it is for a given type of gas m i x t u r e 340-VOLUME 9B, SEPTEMBER 1978

~

02 in

J~ Fig. 6--EMF cell for study of phase equilibria in Me-S-O systems.l i m i t e d to a r a t h e r n a r r o w r a n g e of oxygen o r sulfur potentials. The method which we have found to have the g r e a t e s t v e r s a t i l i t y is the EMF method u s i n g s o l i d Zr02 + CaO e l e c t r o l y t e s , a method which may be used between 600~ and the m e l t i n g t e m p e r a t u r e of the s a m ple. T h i s type of m e a s u r e m e n t s w e r e f i r s t u s e d by L a r s o n and E l l i o t t x4 for s u l f i d e - o x i d e e q u i l i b r i a , and l a t e r by E s p e l u n d and c o w o r k e r s xS-x7 for s u l f a t e - o x i d e as well as for s u l f i d e - o x i d e e q u i l i b r i a . Our work in T r o n d h e i m is b u s e d on the e x p e r i m e n t a l d e s i g n shown in F i g . 6. A s i m i l a r technique is used in Sweden by R o s 6 n and c o w o r k e r s . 18-e~ In this type of a p p a r a t u s a c o n s t a n t gas p r e s s u r e of one a t m o s p h e r e is m a i n t a i n e d over the s a m p l e , which may be e i t h e r a s u l f i d e - o x i d e or a s u l f a t e - o x i d e m i x t u r e . The a t m o s p h e r e c o n s i s t s m a i n l y of SO2, but with s m a l l a m o u n t s of $2, SOs or O2 depending on the type of e q u i l i b r i u m which is b e i n g studied. If the m e a s u r e m e n t s w e r e c a r r i e d out in a flow of SO2 t h e s e m i n o r s p e c i e s would be swept away with the gas flow, u p s e t t i n g the gas c o m p o s i t i o n at the e l e c t r o d e . In our a p p a r a t u s , t h e r e f o r e , a f t e r the i n i t i a l a i r has b e e n swept out, the p a r t of the a p p a r a t u s which c o n t a i n s the s a m p l e is made to f o r m a dead end where the e q u i l i b r i u m gas m i x t u r e is allowed to e s t a b l i s h i t self, and the t o t a l p r e s s u r e of one a t m o s p h e r e is m a i n t a i n e d by m e a n s of a slow flow of SO2 which b y - p a s s e s and c o m m u n i c a t e s with that p a r t of the c e l l . E v e n in this case s o m e l o s s of the m i n o r gas c o m p o n e n t s m a y o c c u r by d i f f u s i o n to the colder p a r t of the a p p a r a t u s w h e r e e l e m e n t a l s u l f u r , a l t e r n a t i v e l y SOs, m a y cond e n s e , o r b y diffusion of oxygen into the SO2 flow. In s o m e c a s e s SO3 may a l s o r e a c t with the l i m e content of the e l e c t r o l y t e to f o r m CaSO4. By c o n s t r i c t i n g the p a s s a g e b e t w e e n the s a m p l e and the c o o l e r p a r t s l o s s e s by diffusion a r e m i n i m i z e d and a r e i n m o s t METALLURGICALTRANSACTIONSB

c a s e s r a p i d l y r e p l e n i s h e d by r e e s t a b l i s h m e n t of e q u i l i b r i u m a r o u n d the s a m p l e . A l s o r e a c t i o n with the s o l i d e l e c t r o l y t e s e e m s to p r e s e n t no p r o b l e m for m o d e r a t e SOs p r e s s u r e s , p r o v i d e d enough s a m p l e is p r e s e n t . S e r i o u s p r o b l e m s a r e e n c o u n t e r e d only for s u l f a t e - o x i d e e q u i l i b r i a at high t e m p e r a t u r e w h e r e the SOs and O2 p r e s s u r e s m a y e x c e e d 0.1 a t m . In m o s t c a s e s an e l e c t r o d e l e a d of p l a t i n u m m a y be u s e d . F o r m e a s u r e m e n t s at v e r y low o x y g e n p o t e n t i a l s , a s f o r the " F e S " - F e 3 0 4 e q u i l i b r i u m , the $2 p r e s s u r e w i l l be s u f f i c i e n t l y high, h o w e v e r , to a t t a c k the p l a t i n u m l e a d . In t h o s e c a s e s a gold l e a d m a y be used, gold b e i n g i n e r t t o w a r d s u l f u r . The o t h e r e l e c t r o d e m a y be any one with a w e l l d e fined o x y g e n - p o t e n t i a l . We u s e p u r e oxygen g a s and a p l a t i n u m l e a d . A l t e r n a t i v e l y a i r o r a N i O - N i couple could have b e e n u s e d . SOME T Y P I C A L SYSTEMS S y s t e m s With One M e t a l A f a i r l y s i m p l e s y s t e m with one m e t a l is that of iron, which i s shown in F i g . 7 c o n s t r u c t e d f r o m own and p u b l i s h e d d a t a of v a r i o u s k i n d s Y ,1%21'22 T h e l o w e r l i m i t of the d i a g r a m is s e t b y liquid s u l f u r , the u p p e r b y 1 a r m of oxygen. F u r t h e r m o r e l i n e s for 1 a t m of s u l f u r v a p o r , a s w e l l a s f o r 1 a t m of SO3 a r e shown. F r o m the v i e w p o i n t of r o a s t i n g the i n t e r e s t i n g f e a t u r e of the d i a g r a m is b e t w e e n about 580 and 1010~ H e r e " F e S " o x i d i z e s to give Fe304, the e q u i l i b r i u m $2 p r e s s u r e b e i n g of the o r d e r of 0.01 a t m at 700~ i n c r e a s i n g s l i g h t l y with i n c r e a s i n g t e m p e r a t u r e . With i n c r e a s i n g oxygen p o t e n t i a l FeaO4 o x i d i z e s to Fe203 which a g a i n m a y be s u l f a t e d to give e i t h e r FeSO4 o r Fe2(SO4)3 d e p e n d i n g on t e m p e r a t u r e . At t e m p e r a t u r e s b e l o w about 580~ " F e S " is no l o n g e r

\\\\

Ps02= lcttm - - -- -- PS02= 0.1atrn

"~,\

CuS04\

~KS. -8o

\

\

-12 Melt

-16"" x

%,

P-208 9

p,0 .,4"

10

11

12

13

1L~

15

Fig. 8--Oxygen potentials in the system Cu-S-O at 1 and 0.1 atm of SO2. s t a b l e in the p r e s e n c e of FesO4 and 1 a t m of SO2, the c o e x i s t i n g s u l f i d e p h a s e b e i n g p y r i t e , FeS2. A t even l o w e r t e m p e r a t u r e s , b e l o w about 450~ FeS2 o x i d i z e s d i r e c t l y to Fe203 and b e l o w about 350~ to FeSO4. F i n a l l y at s o m e t e m p e r a t u r e below a b o u t 150~ FeS2 will, u n d e r e q u i l i b r i u m c o n d i t i o n s and f o r 1 a r m of SO2, oxid i z e to give e l e m e n t a l s u l f u r and FeSO4. T h e r a t e of r e a c t i o n at t h e s e t e m p e r a t u r e s i s m u c h too low, howe v e r , to be m e a s u r e a b l e . In aqueous s y s t e m s , on the o t h e r hand, o x i d a t i o n of " F e S " to give e l e m e n t a l s u l f u r and f e r r i c h y d r o x i d e is w e l l known, 1 e v e n though t h i s r e p r e s e n t s an u n s t a b l e p h a s e c o m b i n a t i o n . A t t e m p e r a t u r e s a b o v e 1010~ and at 1 a r m of SO2 " F e S " and Fe304 f o r m an e u t e c t i c m e l t , li, which on i n c r e a s i n g t e m p e r a t u r e and d e p e n d i n g on the oxygen p o t e n t i a l m a y e x t e n d f r o m c o e x i s t e n c e with liquid s u l f u r 22 to c o e x i s t e n c e with s o l i d m a g n e t i t e 21 A s l i g h t l y m o r e c o m p l i c a t e d s y s t e m is given by that of c o p p e r which i s shown in F i g . 8, a g a i n m a i n l y d e r i v e d f r o m l i t e r a t u r e d a t a . 1~ The interesting feat u r e of t h i s d i a g r a m is that Cu2S m a y be r o a s t e d to give m e t a l l i c c o p p e r ( r o a s t - r e a c t i o n ) above about 700~ a fact which we s h a l l r e t u r n to l a t e r . A n o t h e r f e a t u r e i s the p r o b a b l e e x i s t e n c e of a t e r n a r y e u t e c t i c m e l t b e t w e e n Cu2S, Cu20 and CuSO4 a r o u n d 400~ 24'2s We have m a d e s o m e e x p e r i m e n t s in T r o n d h e i m w h e r e a m i x t u r e of Cu2S and CuSO4 w a s h e a t e d u n d e r one a t m o s p h e r e of SO2. A t 450~ the m i x t u r e was c o m p l e t e l y m o l t e n , w h e r e a s at 550~ it had s o l i d i f i e d due to l o s s of SO2. C o n t r a r y to t h i s o b s e r v a t i o n N a g a m o r i and H a b a s h i 26 have s u g g e s t e d that c u p r o u s s u l f a t e , Cu2SO4, m a y e x i s t u n d e r t h e s e c o n d i t i o n s . Further studies may therefore be needed. T h e t h i r d s y s t e m with one m e t a l to be c o n s i d e r e d is t h a t of l e a d . T h i s is shown in F i g . 9, which i s d e r i v e d m a i n l y f r o m a r e c e n t r e v i e w b y Schuhmann et al, 27 s l i g h t l y m o d i f i e d b y p h a s e d i a g r a m d a t a given VOLUME 9B, SEPTEMBER 1978-341

-5

lo

-10

-15

-20

7

8

9

10

11

12

13

1L~

15

Fig. 7--Oxygen potentials in the system Fe-S-O at 1. atm of 802. Numbers on each line give source references. METALLURGICAL TRANSACTIONS B

-4

o.l Yx ' ~ ~ ..~,-~ ~ -,,: -6 ! ~'-~. ~ ~ ~PbO(t), PbSO-([) ~

",..I, - \

Zn'S04 I~'e203'

(~'

/ 1 -]/

PbSO4(s)

-1

ZnO.2 ZnS04+ Fe203 @

- 2 ZnO'2ZnSO4+ZnFe204~

t -8ol

I

"-

"'~ ~" ,~

~

:L_X ~'~,ebO-~S04\. . . . . .

~9o

~_~Ptot=0"25otto / "'" /Pt/ot

-10

+ZnFe204

-12

~4oo~h

..... ,l~tm 12oo~I ,

"'. . ""~.

~,, :-to ~"./Ot.

~ PbSls) ~

\

~X8oo~I

X

"10~

t Z n,Fe)Fe20~4

~ooo~c" I ,

900%I'. i

6

7

8

9

10 -11 Zn, ZnF

Fig. 9--Oxygen potentials in the system Pb-S-O at 1 atm of SO2. Mainly after Schuhmann et al 27

zoo2z::eil: :::Zn Fe Fig. 10--The system Zn-Fe-S-O for i arm of SO2 and 900~ schematic.by M a r g u l i s e l a l . 2a T h e c h a r a c t e r i s t i c f e a t u r e of this s y s t e m is that at low t e m p e r a t u r e s PbS o x i d i z e s to f o r m lead s u l f a t e s , b a s i c o r n o r m a l . Above 900~ and for 1 ar m of SO2 a m o l t e n m e t a l l i c lead p h a s e m a y be f o r m e d by the r o a s t - r e a c t i o n . Molten lead and m o l t e n PbS a r e c o m p l e t e l y m i s c i b l e , h o w e v e r , and on d e c r e a s i n g o x y g e n - p o t e n t i a l the s u l f u r content of the lead p h a s e w i l l i n c r e a s e . At high oxygen p o t e n t i a l s a m o l t e n m i x t u r e of PbO and PbSO4 w i l l be f o r m e d , the s u l f a t e content of which i n c r e a s e s with i n c r e a s i n g o x y g e n p o t e n t i a l . T o m a k e the s y s t e m e v e n m o r e c o m p l i c a t e d both PbS and PbO a r e r a t h e r v o l a t i l e , m e t a l l i c lab s o m e w h a t l e s s . As a r e s u l t , the c o m b i n e d v a p o r p r e s s u r e PPbS + # P b + PPbO w i l l be a p p r e c i a b l e at al l o x y g e n p o t e n t i a l s , although it w i l l have a m i n i m u m in s o m e i n t e r m e d i a t e r a n g e . In F i g . 9 the t e m p e r a t u r e s a r e given w h e r e the c o m b i n e d v a p o r p r e s s u r e r e a c h e s 0.1 and 0.25 arm. T h i s b e h a v i o r of the s y s t e m m a k e s the r o a s t - r e a c t i o n f o r l e a d a d if f i c u lt p r o c e s s , and c o n s i d e r a b l e s k i l l is n e e d e d to m a n e u v e r b e t w e e n high s u l f u r content in the m e t a l at low oxygen p o t e n t i a l , high lead l o s s e s in the s l a g at high o x y g e n p o t e n t i a l and high v a p o r l o s s e s at high t e m p e r a t u r e . 342-VOLUME 9B, SEPTEMBER 1978

0

-12 .(Zn, Fe)S i (Zn,Fe)S 9 FeSI II

I

,.

,,

I

I

I

i

0

0.2 0.4 0.6 0.8 NFe = rife/(riFe+ nz n)

1.0

Fig. ll--Oxygen potential in system Zn-Fe-S-O as function of molar ratio NFe = nFe./(nFe + ~Zn) for 1 atm of SO2 at 891Oc.16,17 Numbers on each line refer to the three-phase areas shown in Fig. 10.

-2

$o

-l.

,..,,

e*oN

,

"r,oK'00

%,

"~

\

\

T -6c:~ o

\\

N

\\

--8

\ x, \ o 0

mo

\

-12

8

J

9

10

IO;T

9

Fig. 12--Effect of temperature on the oxygen potential for i arm of SO2 for all three-phase combinations in the system Zn-Fe-S-O.16,17 METALLURGICAL TRANSACTIONS B

S y s t e m s With Two M e t a l s An e x a m p l e of a s y s t e m with two m e t a l s , which we have studied e x t e n s i v e l y i n T r o n d h e i m 17 is that of zinc and i r o n . In F i g . 10 the q u a t e r n a r y s y s t e m is shown s c h e m a t i c a l l y a s a t e t r a h e d r o n for a t e m p e r a t u r e of about 900~ We see that t h e r e is an e x t e n s i v e solid s o l u b i l i t y of FeS in ZnS giving the s p h a l e r i t e or m a r m a t i t e p h a s e . T h e r e i s also complete s o l i d m i s c i b i l i t y b e t w e e n Fe304 and ZnFe204, the s p i n e l phase. At a SO2 p r e s s u r e of 1 a t m the s y s t e m is c h a r a c t e r i z e d by five a r e a s w h e r e t h r e e solid p h a s e s coexist: 1) S p h a l e r i t e - p y r r h o t i t e - s p i n e l ("FesO4"), 2) S p h a l e r i t e - Z n O - s p i n e l ( " Z n F e 2 0 4 " ) , 3) ZnO" Z n F e 2 0 4 " - Z n O 9 2 ZnSO4, 4) " Z n F e 2 0 4 " - Z n O 92 ZnSO4Fe203, and 5) ZnO 92 ZnSO4-ZnSO4-Fe2Os. Between the t h r e e - p h a s e a r e a s 1) and 2) s p h a l e r i t e of d e c r e a s i n g i r o n content c o e x i s t s with s p i n e l of i n c r e a s i n g zinc c o n tent. F u r t h e r m o r e , with i n c r e a s i n g oxygen p o t e n t i a l the Fe203 p h a s e will c o e x i s t with s p i n e l of i n c r e a s i n g zinc content. In F i g . 11 the oxygen p o t e n t i a l for c o e x i s t e n c e b e tween the sulfide and oxide p h a s e s at 1 arm of SO2 and 891~ is given as function of the m o l a r r a t i o N F e = nFe/(nFe + n z n ) . 17 S i m i l a r d i a g r a m s w e r e o b t a i n e d for higfier t e m p e r a t u r e s , but show, in addition, the occ u r r e n c e of a n o t h e r (Zn, Fe)S modification, the w u r zite p h a s e . L i n e s for the e q u i l i b r i a b e t w e e n the v a r i ous oxide and sulfate p h a s e s a r e a l s o shown in F i g . 11. T h e s e w e r e d e r i v e d m a i n l y f r o m data for the p u r e zinc s u l f a t e s , ~6 the a s s u m p t i o n b e i n g made that the solid s o l u b i l i t y of i r o n in t h e s e is i n s i g n i f i c a n t , an a s s u m p t i o n which may not n e c e s s a r i l y be t r u e . In d e r i v i n g line 4) use was made of the s t a n d a r d Gibbs e n e r g y of the r e a c t i o n ZnO + Fe203 = ZnFe204, as der i v e d f r o m data given by K u b a s c h e w s k i . =9 F i n a l l y F i g . 12 shows the effect of t e m p e r a t u r e on the oxygen pot e n t i a l for a l l t h r e e - p h a s e c o m b i n a t i o n s in the s y s t e m . E q u i l i b r i u m l i n e s for oxidation of m a g n e t i t e to h e m a tite as for the f o r m a t i o n of i r o n sulfate x6 a r e a l s o i n cluded in this d i a g r a m . It is a n i n d u s t r i a l e x p e r i e n c e that, when r o a s t i n g i r o n r i c h s p h a l e r i t e , the a m o u n t of zinc f e r r i t e f o r m e d is the l e a s t if the r o a s t i n g is c a r r i e d out r a p i d l y and at high t e m p e r a t u r e . A p o s s i b l e e x p l a n a t i o n for this may be that the s p i n e l f o r m e d d u r i n g the i n i t i a l p a r t of the r o a s t i n g p r o c e s s , and which is e s s e n t i a l l y m a g n e t i t e , will oxidize f u r t h e r to h e m a t i t e b e f o r e it has a chance to r e a c t f u r t h e r with ZnO to give ZnFezO4. C o n s i d e r a b l y m o r e c o m p l i c a t e d is the C u - F e - S - O s y s t e m which is shown s c h e m a t i c a l l y in F i g s . 13 and 14 as t e t r a h e d r o n s for t e m p e r a t u r e s a r o u n d 700 and 900~ r e s p e c t i v e l y . In addition to the e x t e n s i v e solid s o l u b i l i t y of FeS in Cu2S, the so c a l l e d b o r n i t e p h a s e , and a m u c h s m a l l e r s o l u b i l i t y of Cu2S in " F e S " , the p y r r h o t i t e p h a s e , we have at a t m o s p h e r i c p r e s s u r e the c h a l c o p y r i t e phase, " C u F e S 2 " . 3~ In addition to the s i m p l e oxides CuzO, CuO, Fe304 and Fe203 we have the c o m p l e x oxides CuFeO2 (delafossite) and CuFe204 (copper f e r r i t e ) . 3~ T h i s l a s t oxide f o r m s l i m i t e d solid s o l u t i o n s .with m a g n e t i t e , the s o l u b i l i t y b e i n g c o m plete only a b o v e about 1000~ s2* F i n a l l y we have the *Theiron-richspinelswillbe denoted"Fe304" the copper-richones "CuFe204." s u l f a t e s C u O . CuSO4, CuSO4, FeSO4 and Fe2(SO4)3 but we know little about t h e i r s o l u b i l i t y in each other or METALLURGICAL TRANSACTIONSB

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