The 1978 Extractive Metallurgy Lecture The Metallurgical Society of AIME

Phase Equilibria in the Pyrometallurgy of Sulfide Ores

TERKEL ROSENQVIST

Although r eac t i on k ine t i cs and p r o c e s s dynamics may grea t ly affect the e ng i ne e r i ng 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 imi t s of what can be achieved a r e se t by the equi l i - b r i u m r e l a t i o n s . Af ter a d i s c u s s i o n of some e x p e r i m e n t a l techniques this l ec ture rev iews the phase equ i l i b r i a that a re of impor t ance for the roa s t i ng and s m e l t i n g of some impor t - ant sulf ide o re 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 under r o a s t i n g con- d i t ions a re t r ea ted in de ta i l as funct ions of composi t ion, t e m p e r a t u r e and oxygen poten- t ia l . F ina l l y the use of ca lc ium oxide as an abso rben t for su l fur as wel l as a means to modify roas t ing and reduc t ion p r o c e s s e s for copper and i ron su l f ides is d i s cus sed .

IN giving this l ec tu re for the 1978 annual mee t ing of the AIME I have to r e c a l l that las t s u m m e r was exact ly th i r ty yea r s s ince I f i r s t came to the Nor th A m e r i c a n cont inent as a r e s e a r c h fellow with the In- s t i tute for the Study of Metals , us it was ca l led at that t ime , in Chicago. Knowing that I would be f ree to work on any subjec t I liked, provided it had some vague r e l a - t ion to me ta l s I had, before I left Norway, d i s c u s s e d var ious pos s ib i l i t i e s with an older col league. I 'wanted to work on oxygen in l iquid s t ee l and deoxidafion equi l ib r ia , but was advised aga ins t it . "Oxygen is so diff icult to ana lyze for . Why don ' t you work on su l fur 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 wil l ca l l a th i r ty y e a r s war with f i re and b r i m s t o n e .

Although desu l fu r i za t ion of i ron and s t ee l is a ve ry impor t an t subjec t , where a g rea t deal of r e s e a r c h has been done, and when the re s t i l l a re p r o b l e m s to solve, I will , in this audience of main ly non fe r rous

m e t a l l u r g i s t s , t a lk about p r o b l e m s in connect ion with the r e c o v e r y of n o n f e r r o u s meta l s f rom the i r sulf ide o res , and wil l a l so give some views on the handl ing of the su l fur component .

As you a l l know, ex t rac t ive me ta l l u rgy may be divided into p y r o m e t a l l u r g y and hydrometa l lu rgy . An ex tens ive r ev iew of the t h e r m o d y n a m i c s of hydrometa l - lu rg ica l t r e a t m e n t of sulf ide 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 Ext rac t ive Meta l lu rgy L e c t u r e . My job wi l l t he re fo re be to d i scus s the t h e r - modynamics of pyrometallurgical t r e a t m e n t s . I can not do this , however , without point ing out the p ioneer work that has been done on this cont inent dur ing the s ame th i r ty y e a r s by people like Re inha rd Schuhmann, H e r b e r t Kellogg, T o m Ingraham, and o thers . The i r work has been a g rea t i n sp i r a t i on to me, and in this l ec tu re I wil l a l so draw on some of the i r f indings .

The mos t i mpor 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 re roa s t i ng and s in t e r ing , as applied for example to

The Extractive Metallurgy Lecture was authorized in 1959 to provide an out- standing 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

M E T A L L U R G I C A L T R A N S A C T I O N S 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 o f Extractive Metallurgy.

ISSN 0360-2141/78/0911-0337500.75/0 �9 1978 A M E R I C A N SOCIETY FOR M E T A L S A N D V O L U M E 9B, S E P T E M B E R 1 9 7 8 - 3 3 7

T H E M E T A L L U R G I C A L SOCIETY OF AIME

t h e su l f ides of copper , z inc and lead, and mat t e s m e l t - ing and conver t ing , which apply to copper , n i cke l and lead su l f ides . C o n t r a r y to the condi t ions for oxide o r e s a d i r e c t r educ t i on to m e t a l with, e.g., ca rbon o r hydrogen is in mos t c a s e s not a f ea s ib 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 th is l e c t u r e .

THEORETICAL 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 may g r e a t l y inf luence and af fec t the e n g i n e e r i n g of r o a s t i n g and sme l t i ng , the t h e o r e t i c a l l i m i t of what can be ach ieved is se t by 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 , equ 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 main 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 F ig . 1 for a t e m p e r a t u r e where a l l condensed p h a s e s a r e so l id .

In add i t ion to the m e t a l l i c phase Me, we have the sul f ide MeS, the oxide MeO and the su l fa te MeSO4. F u r t h e r m o r e , we could have had o the r su l f ides l ike MeS2 or Me2S, and o t h e r ox ides l ike Me203. We could have had t r i v a l e n t su l f a t e s l ike Me2(SO4)3 o r b a s i c su l f a t e s l ike MeO �9 MeSO4. Al though these p h a s e s a r e ind ica ted and t r e a t e d as if they we re p u r e , s m a l l amounts of so l id so lub i l i t y can not be exc luded . Th i s has been d e m o n s t r a t e d by, among o the r s , Tu rkdogan and c o w o r k e r s 2'3 for c a l c i u m and manganese ox ides , which show s m a l l so l id 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 su l f ides and su l f a t e s , and by Kor 4 for the so l id so lub i l i t y of su l f ide in wus t i t e and in m e t a l l i c i ron . C o r r e s p o n d i n g so l id s o l u b i l i t i e s in the su l f ide and su l fa te p h a s e s a r e l e s s wel l known. Above the eu tec t i c mel t ing t e m p e r a t u r e s , however , ex t ens ive l iquid m i s - c ib i l i t y be tween me ta l , su l f ide , oxide and su l fa te i s the ru l e r a t h e r than the excep t ion .

The gas phase may have any compos i t i on be tween pure O2 and pure $2 and conta in m o l e c u l e s of SO3 and SO2 as wel l as of the l e s s s t ab le s p e c i e s SO and $20. Some m e t a l s such a s lead , t in, a r s e n i c and an t imony may fo rm gaseous m o l e c u l e s with su l fur , oxygen o r both . Th i s may cause the c o m p o s i t i o n a l a r e a of the gas phase to extend way into the t e r n a r y phase d i a - g r a m .

$2

T:const .

M ~ SO2

Me MeO 0 2

Fig. 1--Phase equilibria in a s y s t e m Me-S-O under roasting conditions, schematic.

[og po 2 i- 0 0 : : e ~ >1 . . . . . 7 ~ - ~ - ~% ~,~ 0

- . j %x- -- ,~ -- , i I / " / Me MeO ~" l

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 componen t s and two or m o r e p h a s e s a comple t e t h e r m o d y n a m i c d e s c r i p t i o n as function of compos i t i on 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 obta in a two d i m e n s i o n a l f i gu re one p a r a m e t e r has to be kept cons tan t . Mos t we l l known a r e p e r h a p s the d i a g r a m s for cons tan t t e m p e r a t u r e where the phase e q u i l i b r i a a r e shown as funct ion of the p a r t i a l p r e s - s u r e or po t en t i a l of two componen t s in the gas phase . Such d i a g r a m s were p r e s e n t e d by, e.g., Kel logg 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 ig . 2.

In this type of d i a g r a m each phase i s s t ab le within a p r e d o m i n a n c e a r e a , th is be ing b o r d e r e d by l ines which r e p r e s e n t coex i s t ence with o the r p h a s e s . As 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 chosen , t he se m o l e c u l e s be ing the domina t ing ones in r o a s t i n g and s m e l t i n g g a s e s . In- s 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 or even SOs could have been chosen . Also , the oxygen po- t e n t i a l could have been given by the c o r r e s p o n d i n g C O J C O or H~O/H2 r a t i o s , and the su l fur po t en t i a l by, e.g., the H2S/H2 r a t i o . In F ig . 3 l ines a r e d rawn co r - r e s p o n d i n g to 1 a im p r e s s u r e of $2 and SO3 r e s p e c - t i ve ly . F u r t h e r m o r e , a t the upper lef t c o r n e r a l ine is given to show the a p p e a r a n c e of l iquid e l e m e n t a l su l fu r . Any gas phase beyond th i s l ine would be un- s t a b l e .

One c h a r a c t e r i s t i c f e a tu r e of th is type of d i a g r a m is that the s lope of each curve is given by the s to i ch io - 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 . Thus for a r e - ac t ion of the type :

2 M e S + 3 0 ~ = 2 M e O + 2SO2 [1]

and taking the a c t i v i t i e s of MeS and MeO equal to uni ty:

2 log Pso~ = 3 log p o 2 + log Kz.

Thus the s lope of 3/2 wi l l app ly r e g a r d l e s s of whether the m e t a l is lead, copper o r z inc . Only the va lue for the e q u i l i b r i u m cons tan t K1 wil l d i f f e r .

Th i s type of p r e s e n t a t i o n does not show the ef fec t of t e m p e r a t u r e , howeve r . Th i s can be ach ieved by p lo t - t ing 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 aga ins t 1/T, for a cons tan 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 my 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 cons tan t SO2 p r e s s u r e of I a i m , ~nd may be con- s i d e r e d a p y r o m e t a l l u r g i c a l P o u r b a i x d i a g r a m . A l so in th is d i a g r a m c u r v e s for 1 a t m of $2 and SOa a r e shown. As for a given s t o i c h i o m e t r y t h e r e i s a def ini te r e l a t i o n be tween the 02 and the SOs p r e s s u r e s the

338-VOLUME 9B, SEPTEMBER 1978 METALLURGICAL TRANSACTIONS B

0 . , \ f " , ~ , ~ ....

\

high temp 1//T ---~ Iowtemp Fig . 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 .

curves for any SO2 p r e s s u r e d i f ferent f rom 1 arm may b e obtained by a sys t ema t i c shift of the cu rves , up or down, as i l l u s t r a t ed by the dotted l ines in F ig . 3 for 0.1 arm of SO2. We notice that the oxygen potent ia l for the equ i l i b r i a between Me and MeO and be tween MeS and MeSO4 a r e not affected by the SO2 p r e s s u r e .

Whereas for in t e rmed ia t e oxygen potent ia l s , and in the absence of iner t gases , the SO2 p r e s s u r e is c lose ly equal to the tota l gas p r e s s u r e in the s y s t e m this is not the case for high or ve ry low 02 po ten t ia l s . In addit ion to SO2 and 02 the gas wil l at high 02 potent ia ls contain some SO3 given by the equ i l i b r i um:

2 SO2 + O2 = 2 SOs [2]

giving Ptot ~ PS02 + P02 + PS03. The re fo re , for m e a s - u r e m e n t s c a r r i e d out at a to ta l gas p r e s s u r e of 1 arm the pa r t i a l p r e s s u r e s of O2 and SO3 must be ca lcula ted in o rder to obta in the SO2 p r e s s u r e . S i m i l a r l y at ve ry low oxygen potent ia ls there may be some sul fur vapor given by the equ i l ib r ium:

2 SO~ = S2 + 2 02 [3]

and Ptot ~- PSO2 + P Sf Other gas spec ies such as SO and $20 may aiso occur in s m a l l e r amounts . F ina l ly , in cases where e i ther the meta l or some of i ts com- pounds a re volat i le the i r vapor p r e s s u r e s m u s t be in - cluded in the tota l gas p r e s s u r e . In Fig . 3 equ i l i b r i um l ines for a to ta l gas p r e s s u r e of 1 arm a r e shown as dashed l ines for the cases where these differ f rom the l ines for 1 a tm of SO2.

A third way of p r e sen t a t i on is by the tota l decompo- s i t ion p r e s s u r e , i .e. , the p r e s s u r e exer ted when a compound is heated alone or in an ine r t a tmosphe re . A typical example is sulfate decompos i t ion :

MeSO4 = MeO + SOs [4]

S03 = SO2 + ~02

where:

Ps02 = 2 PO~

and:

Ptot = PSO3 + PS02 + PO2 = PSO3 + 3 po 2.

Knowing the to ta l p r e s s u r e and the e q u i l i b r i u m con-

[5]

s tant for Eq. [5] the equ i l i b r ium cons tan t for Eq. [4] as wel l as the co r r e spond ing l ine in Fig . 3 may be ca lcula ted .

EXPERIMENTAL METHODS

Although the equ i l i b r i a between the va r ious phases may be ca lcu la ted f rom t he r modyna mi c data for the compounds involved, such data a re often of insuf f ic ien t accu racy . A be 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 rec t ly , in which case any mutual so lubi l i ty be tween the compounds involved, as well as the p r e s e n c e of complex phases which were not f o r e s e e n in the ca l cu - la t ions , may be detected.

The choice of method will to a grea t extent depend on the e q u i l i b r i u m to be s tudied. F o r equ i l i b r i a be - tween sul fa tes and oxides these may de de r ived f rom tota l decompos i t ion p r e s s u r e s as given by Eqs . [4] and [5]. O ne diff icul ty is the agg re s s ive na tu re of the gas and condensa t ion of SO3 at a round r o o m t e m p e r a t u r e which p r e ve n t a d i r ec 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 among o thers by W a r n e r and Ing raham v an i n t e r m e d i a t e bellow was used. In other cases 8 use was made of a f lexible s i l i c a m e m b r a n e combined with a compensa t ing a i r or n i t rogen p r e s - su re which could be m e a s u r e d d i rec t ly , F ig . 4. Af te r the sulfate had been in se r t ed the whole appara tus is evacuated under gentle hea t ing in o rde r to r e m o v e a l l adsorbed gases , and the pos i t ion of the m e m b r a n e is r ead on the ca the tomete r . The pa r t of the appar.atus that conta ins the sulfate is then sea led at the fuse-of f point . As the sulfate is heated toward decompos i t ion a i r or n i t rogen is admit ted to keep the m e m b r a n e in i ts fixed posi t ion , and the gas p r e s s u r e is r ead on a m e r c u r y m a n o m e t e r . The appara tus thus ac ts as a ze ro method. Ins tead of a m e m b r a n e a s i l i c a s p i r a l tube (Bodenste in gage) could have been used. One sho r t c omi ng of the decompos i t ion p r e s s u r e method

To manomete r 11

To gas source

3athetometer

e-off point

F i g . 4 - - A p p a r a t u s fo r the s tudy 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

METALLURGICAL TRANSACTIONS B VOLUME 9B, SEPTEMBER 1978 339

Fig. 5--Apparatus for the study of equilibria with SO 2 + S 2 gas mixtures, t2

is that it is l imi ted to r e l a t i ve ly na r row t e m p e r a t u r e r anges , usual ly r anges of l e s s than 200~

Var ious types of dynamic or flow methods have been used. These may be based e i ther on d e t e r m i n i n g the e q u i l i b r i u m composi t ion of the gas for a given phase combina t ion 9 or on d e t e r m i n i n g the compos i t ion of the sol id phases in equ i l i b r i um with a given gas flow. %s,l~ The gas may be any su i table mix ture which def ines the oxygen and su l fur po ten t ia l s as for example SO2 + Oe, l~ SO2 + C02 + CO, 2's HeS + H20 + He (Ref. 9) or SO2 + $2. ~e'~3 An example of the las t type is shown in Fig . 5, which shows the appara tus used to study the equ i l ib r ium:*

*Throughout this paper quotation marks are used to indicate phases w~th no- ticeable variation in composition, regardless of whether this is in the metal to metalloid or in the metal to metal ratio.

3 " F e S " + 2 SOs = Fe304 + 5/2 Se.

Here a gas mix ture SO2 + $2 is p r e p a r e d by bubbl ing SOs through a bath of mol ten su l fur kept in a t h e r m o - s ta ted furnace , whereby sa tu ra t ion with su l fur vapor is achieved . This gas mix tu re is pa s sed through a bed of mixed sol id " F e S " and Fe304 and the eff luent gas is sampled and ana lyzed . By changing the t e m - p e r a t u r e of the t he rmos t a t ed furnace the composi t ion of the gas could be changed, and was changed unt i l a compos i t ion was found which passed through the bed of so l id phases without fu r the r reac t ion , i . e . , equ i l i b r ium had been es tabl i shed. The s a m e technique was used x3 to study the equ i l ib r ium:

CaS + 2 SO2 = CaSO4 + $2.

Although the flow method may be used over a some - what l a r g e r t e m p e r a t u r e r ange than the decompos i t ion p r e s s u r e method it is for a given type of gas mix ture

Pt-win

Teflon-

ut

ZrOz-Cc 02out

Al2Os-t u bes

S a m p l e - -

P t - g a u z e - - - - -

J

Pt/Pt-lO%Rh thermocouple . . . . . . . . ~j~ ~ 0 2 in

J~ Fig. 6--EMF cell for study of phase equilibria in Me-S-O systems.

l imi ted to a r a t h e r na r row range of oxygen or sulfur po ten t ia l s .

The method which we have found to have the grea t - es t v e r s a t i l i t y is the EMF method us ing sol id Zr02 + CaO e l ec t ro ly t e s , a method which may be used be- tween 600~ and the mel t ing t e m p e r a t u r e of the s a m - ple . This type of m e a s u r e m e n t s were f i r s t used by L a r s o n and El l io t t x4 for su l f ide-oxide e q u i l i b r i a , and la te r by Espelund and coworkers xS-x7 for sul fa te-oxide as well as for sul f ide-oxide equ i l ib r i a . Our work in T r o n d h e i m is bused on the e x p e r i m e n t a l des ign shown in Fig . 6. A s i m i l a r technique is used in Sweden by Ros6n and coworke r s . 18-e~ In this type of appara tus a cons tant gas p r e s s u r e of one a tmosphe re is main ta ined over the sample , which may be e i the r a su l f ide-oxide or a su l fa te -oxide mix ture . The a tmosphere cons i s t s main ly of SO2, but with sma l l amounts of $2, SOs or O2 depending on the type of equ i l i b r ium which is being s tudied. If the m e a s u r e m e n t s were c a r r i e d out in a flow of SO2 these minor spec ies would be swept away with the gas flow, upse t t ing the gas composi t ion at the e lec t rode . In our appara tus , the re fore , a f ter the in i t ia l a i r has been swept out, the pa r t of the appara tus which conta ins the sample is made to fo rm a dead end where the e qu i l i b r i um gas mix ture is allowed to e s t ab l i sh i t - self , and the to ta l p r e s s u r e of one a tmosphe re is ma in - ta ined by means 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 pa r t of the cel l . Even in this case some loss of the minor gas components may occur by di f fus ion to the colder pa r t of the appara tus where e l e m e n t a l su l fur , a l t e rna t ive ly SOs, may con- dense , or by diffusion of oxygen into the SO2 flow. In some cases SO3 may a lso reac t with the l ime content of the e l ec t ro ly te to fo rm CaSO4. By cons t r i c t ing the passage be tween the sample and the cooler par t s losses by diffusion a re m i n i mi z e d and a re in most

340-VOLUME 9B, SEPTEMBER 1978 METALLURGICAL TRANSACTIONS B

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 equ i - l i b r i u m a round the s a m p l e . A l so r e a c t i o n with the so l id 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 encoun te r ed 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 where the SOs and O2 p r e s s u r e s may exceed 0.1 a tm .

In mos t c a s e s an e l e c t r o d e l ead of p l a t i num m a y be used . F o r m e a s u r e m e n t s a t v e r y low oxygen po ten- t i a l s , as for 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 wi l l be su f f i c ien t ly high, however , to a t t a ck the p l a t i num lead . In those c a s e s a gold l ead m a y be used, gold be ing ine r t t oward su l fu r .

The o the r e l e c t r o d e may be any one with a we l l de - f ined oxygen -po ten t i a l . We use pu re oxygen gas and a p l a t inum lead . A l t e r n a t i v e l y a i r o r a NiO-Ni couple could have been used .

SOME TYPICAL SYSTEMS

S y s t e m s With One Meta 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 i ron, which i s shown in F ig . 7 c o n s t r u c t e d f r o m own and publ i shed da ta of v a r i o u s k i n d s Y ,1%21'22 The lower l i m i t of the d i a g r a m is s e t by l iquid su l fu r , the upper by 1 arm of oxygen. F u r t h e r m o r e l ines for 1 a tm of su l fu r vapor , a s wel l a s for 1 a t m of SO3 a r e shown. F r o m the v iewpoin t of r o a s t i n g the i n t e r e s t i n g f ea tu re of the d i a g r a m is be tween about 580 and 1010~ Here " F e S " ox id i ze 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 be ing of the o r d e r of 0.01 a tm at 700~ i n c r e a s i n g s l igh t ly 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 po ten t i a l FeaO4 o x i d i z e s to Fe203 which aga in may be su l fa ted to give e i t h e r FeSO4 o r Fe2(SO4)3 depending 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 be low about 580~ " F e S " is no longer

-5

l -10

o

-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.

~KS. - 8

o

-12

-16

- 2 0

\ \

\ \

Ps02= lc t tm

- - - - - - PS02 = 0.1atrn

"~, CuS04 \

\ \

\

"" x

P p ,

8 9 10 11 12 13

%,

Melt

0 .,4"

1L~ 15

Fig. 8--Oxygen potentials in the system Cu-S-O at 1 and 0.1 atm of SO 2.

s t ab l e in the p r e s e n c e of FesO4 and 1 a tm of SO2, the c o e x i s t i n g su l f ide phase be ing p y r i t e , FeS2. At even lower t e m p e r a t u r e s , be low about 450~ FeS2 ox id i zes d i r e c t l y to Fe203 and be low 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 wil l , under e q u i l i b r i u m condi t ions and fo r 1 a rm of SO2, oxi- d i ze to give e l e m e n t a l su l fu r and FeSO4. The 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 much too low, how- e 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 the r hand, ox ida t ion of " F e S " to give e l e m e n t a l su l - fu r and f e r r i c hyd rox ide is we l l known, 1 even though th i s r e p r e s e n t s an uns tab le p h a s e combina t ion .

At t e m p e r a t u r e s above 1010~ and at 1 a rm of SO2 " F e S " and Fe304 f o r m an eu tec t i c me l t , l i , which on i n c r e a s i n g t e m p e r a t u r e and depending on the oxygen po ten t i a l may extend f r o m c o e x i s t e n c e with l iquid su l fu r 22 to c o e x i s t e n c e with so l id magne t i t e 21

A s l i gh t ly m o r e c o m p l i c a t e d s y s t e m is g iven by that of copper which i s shown in F i g . 8, aga in ma in ly de - r i v e d f r o m l i t e r a t u r e da ta . 1~ The i n t e r e s t i n g fea - t u r e of th i s d i a g r a m is that Cu2S may be r o a s t e d to give m e t a l l i c coppe r ( r o a s t - r e a c t i o n ) above about 700~ a fact which we sha l l r e t u r n to l a t e r . Ano the 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 eu tec t i c me l t be tween Cu2S, Cu20 and CuSO4 a round 400~ 24'2s We have made 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 where a m i x t u r e of Cu2S and CuSO4 was hea t ed under one a t m o s p h e r e of SO2. At 450~ the m i x t u r e was c o m - p l e t e l y mol ten , w h e r e a s at 550~ it had so l i d i f i ed due to lo s s of SO2. C o n t r a r y to th is o b s e r v a t i o n N a g a m o r i and H a ba sh i 26 have s u g g e s t e d that cup rous su l fa te , Cu2SO4, may e x i s t under t hese cond i t ions . F u r t h e r s t ud i e s m a y t h e r e f o r e be needed .

The 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 tha t of l ead . Th i s is shown in F ig . 9, which i s d e - r i v e d ma in ly f r o m a r e c e n t r ev i ew by Schuhmann et al, 27 s l i gh t ly mod i f i ed by phase d i a g r a m da ta given

METALLURGICAL TRANSACTIONS B VOLUME 9B, SEPTEMBER 1978-341

- 4

-6

o.l Yx ' ",.. ! ~'-~. ~ ~ ~ P b O ( t ) PbSO- ( [ ) I \ PbSO4(s) ~ ~ . .~ , -~ ~ -,,: - , ~ , - -

I "- "'~ ~" ,~ ~ . . . . . . _X ~ _ ~ ~'~,ebO-~S04\

Ptot =0"25otto / " ' . . ~ \ "'" / " " ~ . PbSls) ~ X . . . . . ~,, :-t o P t / o t ,l~tm ~"./Ot. ~X

~4oo~ 12oo~ ~ooo~c" 900% 8oo~

t -8 ol

~ 9

o

-10

-12 h I , I , I ' . i I 6 7 8 9 10

Fig. 9--Oxygen potentials in the system Pb-S-O at 1 atm of SO 2. Mainly after Schuhmann et al 27

0

zoo2z:: ::: eil: Zn Fe

Fig. 10--The system Zn-Fe-S-O for i arm of SO2 and 900~ schematic.

by Margu l i s e l a l . 2a The c h a r a c t e r i s t i c f ea tu re 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 ox id izes to f o r m lead su l fa tes , bas i c or n o r m a l . Above 900~ and for 1 arm of SO2 a mol ten me ta l l i c lead phase may 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 mol ten PbS a r e comple te ly m i s c i b l e , however , and on de- c r e a s i n g oxygen-po ten t i a l the sulfur content of the lead phase wil l i n c r e a s e . At high oxygen potent ia l s a mol ten mix tu re of PbO and PbSO4 wi l l be fo rmed , the sul fa te content of which i n c r e a s e s with i n c r e a s i n g oxygen potent ia l . To make the s y s t e m even m o r e com- p l i ca ted both PbS and PbO a r e r a t h e r vo la t i l e , me ta l l i c lab somewha t l e s s . As a r e su l t , the combined vapor p r e s s u r e PPbS + #Pb + PPbO wil l be app rec i ab l e at a l l oxygen potent ia l s , a l though it wi l l have a m in im um in s o m e i n t e r m e d i a t e r ange . In F ig . 9 the t e m p e r a t u r e s a r e given where the combined vapor p r e s s u r e r e a c h e s 0.1 and 0.25 arm. This behav io r of the s y s t e m makes the r o a s t - r e a c t i o n for lead a diff icul t p r o c e s s , and cons ide r ab l e sk i l l is needed to maneuve r be tween high sul fur content in the m e t a l at low oxygen potent ia l , high lead l o s s e s in the s l ag at high oxygen potent ia l and high vapor l o s s e s at high t e m p e r a t u r e .

Zn'S04 § I~'e203' (~' /

-1 ZnO.2 ZnS04+ Fe203 @ 1

- 2 ZnO'2ZnSO4+ZnFe204~ -] /

:L

+ZnFe204

"10~ t Z 0~4 n,Fe) Fe 2

-11 Zn, ZnF

I ,. ,, -12 .(Zn, Fe)S i (Zn,Fe)S �9 FeS

I I

I I I I i

0 0.2 0.4 0.6 0.8 1.0 NFe = rife/(riFe+ nz n)

Fig. l l--Oxygen potential in system Zn-Fe-S-O as function of molar ratio NFe = nFe./(nFe + ~Zn) for 1 atm of SO 2 at 891Oc.16,17 Numbers on each line refer to the three-phase areas shown in Fig. 10.

-2 $o

"r, oK \ - l . e*o , "~§ \ ,..

, , % N '00

T -6 \ \ ,

c:~ N o \

- - 8 \ \

x, \ o0

mo \

-12 J 8 9 10

IO;T �9

Fig. 12--Effect of temperature on the oxygen potential for i arm of SO 2 for all three-phase combinations in the system Zn-Fe-S-O.16,17

342-VOLUME 9B, SEPTEMBER 1978 METALLURGICAL TRANSACTIONS B

Sys tems With Two Metals

An example of a s y s t e m with two meta l s , which we have studied ex tens ive ly in T r o n d h e i m 17 is that of zinc and i ron . In Fig . 10 the q u a t e r n a r y s y s t e m is shown schema t i ca l l y as a t e t r ahedron for a t e m p e r a - ture of about 900~ We see that there is an ex tens ive solid so lubi l i ty of FeS in ZnS giving the spha le r i t e or m a r m a t i t e phase . The re is a lso complete sol id misc ib i l i ty between Fe304 and ZnFe204, the sp ine l phase. At a SO2 p r e s s u r e of 1 a tm the sys t em is cha r - ac t e r i zed by five a r ea s where th ree sol id phases co- exis t : 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 ( "ZnFe204") , 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 �9 2 ZnSO4- Fe203, and 5) ZnO �9 2 ZnSO4-ZnSO4-Fe2Os. Between the t h r ee -phase a r ea s 1) and 2) spha l e r i t e of d e c r e a s i n g i ron content coexis ts with sp ine l of i n c r e a s i n g zinc con- tent . F u r t h e r m o r e , with i n c r e a s i n g oxygen potent ia l the Fe203 phase wil l coexis t with sp ine l of i n c r e a s i n g zinc content .

In Fig . 11 the oxygen poten t ia l for coexis tence be- tween the sulf ide and oxide phases at 1 arm of SO2 and 891~ is given as funct ion of the mola r r a t io NFe = n F e / ( n F e + n z n ) . 17 S imi l a r d i a g r a m s were obtained for higfier t e m p e r a t u r e s , but show, in addition, the oc- c u r r e n c e of another (Zn, Fe)S modif icat ion, the wur- zi te phase . L ines for the equ i l i b r i a between the v a r i - ous oxide and sulfate phases a re a lso shown in Fig . 11. These were de r ived main ly f rom data for the pure zinc sul fa tes , ~6 the a s sumpt ion be ing made that the sol id so lubi l i ty of i r on in these is ins igni f icant , an a s sumpt ion which may not n e c e s s a r i l y be t rue . In de- r iv ing l ine 4) use was made of the s tandard Gibbs energy of the r eac t i on ZnO + Fe203 = ZnFe204, as de- r ived f rom data given by Kubaschewski . =9 F ina l l y F ig . 12 shows the effect of t e m p e r a t u r e on the oxygen po- ten t ia l for a l l t h r ee -phase combina t ions in the sy s t e m. Equ i l i b r ium l ines for oxidat ion of magnet i te to hema- t i te as for the fo rma t ion of i ron sulfate x6 a re a lso in- cluded in this d i ag ram.

It is an i n d u s t r i a l exper ience that, when r o a s t i n g i ron r i ch spha le r i t e , the amount of z inc f e r r i t e fo rmed is the leas t if the roas t ing is c a r r i e d out rap id ly and at high t e m p e r a t u r e . A poss ib le explanat ion for this may be that the sp ine l formed dur ing the in i t i a l pa r t of the roa s t i ng p r o c e s s , and which is e s s en t i a l l y mag- net i te , wil l oxidize fur ther to hemat i t e before it has a chance to r e ac t fu r the r with ZnO to give ZnFezO4.

Cons ide rab ly more compl ica ted is the C u - F e - S - O sys t em which is shown schema t i ca l ly in F igs . 13 and 14 as t e t r ahed rons for t e m p e r a t u r e s around 700 and 900~ r e spec t ive ly . In addi t ion to the ex tens ive sol id solubi l i ty of FeS in Cu2S, the so cal led born i t e phase , and a much s m a l l e r so lubi l i ty of Cu2S in " F e S " , the pyr rhot i te phase , we have at a tmosphe r i c p r e s s u r e the chalcopyr i te phase, " C u F e S 2 " . 3~ In addi t ion to the s imple oxides CuzO, CuO, Fe304 and Fe203 we have the complex oxides CuFeO2 (delafossi te) and CuFe204 (copper f e r r i t e ) . 3~ Th i s las t oxide fo rms l imi ted sol id solut ions .with magnet i te , the so lubi l i ty be ing com- plete only above about 1000~ s2* F ina l ly we have the

*The iron-rich spinels will be denoted "Fe304" the copper-rich ones "CuFe2 04."

sulfa tes CuO. CuSO4, CuSO4, FeSO4 and Fe2(SO4)3 but we know l i t t le about the i r so lubi l i ty in each other or

0

Fe2(S04)3 / / ~ / Fe2(S04)3 �9 C u S % . H \ / i V CuSm

^ ~ ^ ")~.SO_i /~x / + C u O ' C u S O 4 + H

~ o . o , , H ' "

c o.cosm / / \ M �9 . . o

c u o . c o s o 4 ~ ~ / t J/'//// Ill \ �9 Cu, O.D I/ll \

c,,2o / I ~ ~ ' / , ~ / ~ - <~o II~--§ Fo \

Pl i F e Fig. 13--The system Cu-Fe-S-O for 1 arm of SO 2 and ~- 700~ B n = bornite, Po = " F e S " , Cp = "CuFeS2", D = CuFeO 2, M = "Fe304", H = Fe203.

in the oxide phase although Habashi e t a133 ment ion some sol id so lubi l i ty be tween CuS04 and FeS04.* At

*As a matter of interest Turkdogan 34 has shown recently that MnS04 may take significant amounts of CuS04 and NiS04 into solid solution.

t e m p e r a t u r e above 900~ inc ip ien t mel t ing is ex- pected to occur in pa r t s of the s y s t e m . This appl ies for the t h r e e - p h a s e combina t ions born i t e + " C u F e S z " + " F e 3 0 4 " and " C u F e S 2 " + " F e S " + "FesO4" . The re is a l so indica t ion that some me l t i ng occur s in the su l fa te -oxide combina t ions . F o r s imp l i c i t y the occu r - r e nc e of mol ten phases is d i s r e g a r d e d in Fig . 14. The var ious t h r ee -p lmse equ i l i b r i a shown in F igs . 13 and 14 and which apply for 1 a rm of SO2 a r e de r ived pa r t l y f rom our m e a s u r e m e n t s , 3S par t ly f rom publ ished data, I1'17'19~23'30'36"a8 a n d w i l l be d i s cus sed in some de-

ta i l . F igu re 15 shows the oxygen poten t ia l as funct ion of

the mola r r a t io NFe = n F e / ( n F e + ncu ) at 927~ = 1200 K and for 1 arm of SO2. At very low oxygen po- t en t i a l "CuFeS2" coexis ts with " F e S " and "FesO4" . At a s l ight ly higher oxygen po ten t ia l there is coexis t - ence between born i te , "CuFeS2" and " F e g 3 4 " . With i n c r e a s i n g oxygen poten t ia l bo r n i t e of d e c r e a s i n g i ron content coexis ts with magne t i t e of i n c r e a s i n g copper con ten t , unt i l a point is r eached where me ta l l i c cop- pe r is fo rmed. At a s l ight ly h igher oxygen potent ia l copper and magnet i te r e a c t to give de lafoss i te , CuFeO2. On fu r the r oxidation the phase combina t ions wil l de- pend on whether the mola r r a t io NFe is s m a l l e r or l a r g e r than 0.5. F o r NFe < 0.5 the sequence wil l be: Cu + CuFeO2, Cu20 + CuFeO2, CuO + CuFeO2, CuO + " C u F e 2 0 4 " . F o r NFe > 0.5 the sequence wil l be: CuFeO2 + "Fe~O4", CuFeO2 + Fe203, CuFeO2 + ~'CuFe204 '~, CuO + " C u F e 2 0 4 " . The two las t phase combina t ions apply for 0.5 < NFe < 0.7, whereas for NFe > 0.7 the combina t ion wil l be " C u F e 2 0 4 " + Fe203. F ina l ly , at oxygen p r e s s u r e s that approach 1 arm, cor - r e spond ing to a to ta l p r e s s u r e of about 2 atm, bas i c copper sulfate , CuO .CuSO4, may poss ib ly coexis t with "CuFe204" at 927~ and NFe < 0.67.

METALLURGICAL TRANSACTIONS B VOLUME 9B, SEPTEMBER 1978-343

The copper content of the magnet i te phase was s tudied in sepa ra t e experiments.SS Here mix tu re s of magnet i te e i ther with born i t e and chalcopyr i te or with "CueS" and meta l l i c copper were annea led for s e v e r a l days at va r ious t e m p e r a t u r e s in a s tagnant a tmosphe re of SO2. S imi l a r ly , m i x t u r e s of magnet i te with "CueS" and de la foss i te were annea led in evacuated and sea led s i l i c a tubes . Af ter annea l ing the copper content of the magnet i te phase was d e t e r m i n e d by mic rop robe ana ly- s i s . At a t e m p e r a t u r e of 927~ = 1200 K the copper content of the magne t i t e phase was found to i nc r ea se f rom less than 0.1 wt pct for coexis tence with born i te and chalcopyr i te through 2.7 wt pct for coexis tence with "CueS" and Cu to 3.6 wt pct for coexis tence with Cu and CuFeOe. These va lues join up n ice ly with the 8.4 wt pct found by the Rosdn group 19 for coexis tence with CuFeO2 and Fe2Oa.

The su l fur potential in the s y s t e m is shown on the r ight hand sca le in F ig . 15, and we see that it de- c r e a s e s f rom 1 a tm for PO2 = 10-*e to 10 -20 a tm for P O2 = 10-2. These va lues apply for 1 a tm of SO2 and 927~

At 727~ = 1000 K somewhat s i m i l a r phase com- b ina t ions occur but with the d i f ference that meta l l i c copper, and CuO do not appear . F u r t h e r m o r e , in addi- t ion to CuO. CuSO4, CuSO4 as well as Fee(SO4)s a re fo rmed at high oxygen potent ia ls (see a l so Fig . 16).

The re la t ive s tab i l i ty of CuO �9 CuSO4 and CuFeeO4 may be der ived f rom obse rva t ions made by E r i k s s o n and Rosdn ~ and Wil l i s ~7 accord ing to whom the sol id phase r eac t i on :

CuFe204 + CuSO4 = CuO �9 CuSO4 + FeeO3

is shif ted to the r ight in the t e m p e r a t u r e range 700 to 900~ Thus CuFe204 and CuSO4 may not coexist ,

The effect of t e m p e r a t u r e - o n a l l phase equ i l i b r i a is shown in F ig . 16 for 1 arm of SO2. Here the sol id l ines r e f e r to NFe < 0.5, the dashed l ines to NFe > 0.5, whereas the dotted l ines r e f e r to equ i l i b r i a which a re u n c e r t a i n or e s t ima ted .

Of p a r t i c u l a r i n t e r e s t in the lower oxygen potent ia l

0

~ / D * Sp CuO'CuS04 / : \ / H*Sp

cuo. ,, !so, / , , / ,., �9 / /X /o . , - , . , , , ,

c uo D.M

Cu Fe Fig. 14--The sys t em C u - F e - S - O for 1 arm of SO 2 and ~-900~ Symbols as in Fig. 13. Also, Sp = " C u F e 2 0 4 " .

0

- 2

- 4

o

- 8

Cu O.CuS04 +"Cu Fe2 04"

CuO ",- "Cu Fe204" cuo *CuFeO 2

Cu20+CuFeO 2

�9 I I ycuFe,o,:' t l

| "CuFe2Q~ ,' CuFe02 \l| § , "CuFe20 ~' ~

CuFe02

+ Fe 2 03

CuFeO 2 �9 "Fe30 ~'

-2O

-15

o

t x l~ -10 l

I

-5

Cu+CuFeO 2

cu ."Fe304"

Bornite * "Fe3 04" t t

-I0 ~ Cp',M

. . . . . . _/qnl "CuFeS2" ~Cp+PolPo - 1 2 ~ p l , , . i, �9 ~ 0

0 0.2 0.4 0.6 0.8 1.0 N Fe =

Fig. 15--Oxygen potential in system Cu-Fe-S-O as function of NFe for 1 atm of SO2 and 927~ Symbols as in Fig. 13. . . . . . . . . . . . . estimated.

a r e a is the fou r -phase combina t ion "CueS" + CuFeOe + "Fe304" + FezO3, which for 1 a tm of SO2 is e s t i - mated to occur at about 750~ F u r t h e r m o r e , the equi- l i b r ium l ines for the combina t ions CuFeO2 + "Fe304" + Fe203 and CuFeO2 + "CuFe204" + Fe203 a r e ex- pected to join up in a loop at the t e m p e r a t u r e of com- plete misc ib i l i ty , about 1000~ F o r the t h r ee -phase combinat ions Cu20 + CuO + CuFeOe and CuO + CuFeOe + CuFeeO4 data a r e obtained f rom Floyd and Wil l i s 38 and Schaefer , a8 which a re in good genera l ag reemen t . On the bas i s of our s tudies we get the Gibbs energy change for the react ion:

Cu20 + FeeOs = CuFeO2 AG~ = - 2 + 1 kcal .

This is s igni f icant ly less negat ive than the a b o u t - 12 kcal r epor ted by Barnay et al. 39 The i r va lues can not be reconc i l ed e i ther with our r e s u l t s , or with other publ i shed data .

More unc e r t a i n a r e the phase equ i l ib r i a between CuO, CuO.CuSO4, "CuFe204" and CuFeO2. Although our m e a s u r e m e n t s agree with the genera l behavior de- picted in Fig. 16 the r ep roduc ib i l i t y was poor. In con- s t ruc t ing F ig . 16 use was the re fo re made of the ob- s e r va t i on 1z'37 that CuSO4 and CuFe204 may not coexist , but wil l r eac t to give CuO �9 CuSO4 + Fe203, and fu r the r - more that the fou r -phase combina t ion CuO �9 CuSO4 + CuFeOe + "CuFeeO4" + FeeOa at 1 a tm of SO2 may oc- cur at only one t e m p e r a t u r e , which with the aid of our m e a s u r e m e n t s for the CuFeO + "CuFe204" + FeeO3 equ i l ib r ium is e s t ima ted to be about 750 ~ Thus for 1

344-VOLUME 9B, SEPTEMBER 1978 METALLURGICAL TRANSACTIONS B

8 9 10~_ = 10 11 / I

F i g . l ~ - - E f f e c t of t e m p e r a t u r e on the oxygen p o t e n t i a l f o r 1 a tm of SO 2 fo r 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 C u - F e - S - O , N F e < 0.5 . . . . . . . NFe > 0.5 . . . . . . . e s t i m a t e d .

arm of SO2, "CuFe2Oa" is comple te ly sulfated below that t e m p e r a t u r e .

The t e m p e r a t u r e s given above, both for the "Cu2S" + CuFeO2 + "Fe3Oa" + Fe~O~ and the CuO �9 CuSO4 + CuFeO2 + "CuFe2Oa" + Fe20~ four -phase combina - t ions , were obta ined by ex t rapo la t ion of the cu rves f rom higher t e m p e r a t u r e s , and a r e r a the r u n c e r t a i n . F u r t h e r work is in p r o g r e s s a iming at a more accu ra t e de t e rmina t i on of these t e m p e r a t u r e s .

In Fig . 17 log POz is shown as funct ion of t e m p e r a - tu re for 0.1 arm of SO2, which more c lose ly c o r r e - sponds to i n d u s t r i a l roas t ing condi t ions . Th i s d i a g r a m is cons t ruc ted f r o m Fig. 16 by means of the s to ich io- mer ry of the indiv idual r eac t ions as p rev ious ly de- s c r ibed . By the lower ing of the SO2 p r e s s u r e the p r e - dominance a r e a s for the me ta l l i c copper phase as well as for the va r ious sul fa tes a re shif ted to lower t e m - p e r a t u r e s . This a lso appl ies for the fou r -phase com- b ina t ion CuO .CuSOa + CuFeO2 + "CuFe2Oa" + Fe2Os which now occurs at about 720~ * whereas the four -

*Jacob e t aL n suggest that CuFe204 decomposes to CuO + Fe:O3 below about 680~ but give no experimental evidence to support this statement.

phase combina t ion Cu2S + CuFeO2 "Fe~Oa Fe203 is rep laced by the combina t ion Cu + CuFeO2 + " F e 3 0 4 " + Fe2Os which occu r s at about 700~ T h i s is s o m e - what higher than the value 950 K ~ 680~ r epor t ed by Yund and Kul le rud . 3~

Of p a r t i c u l a r i n t e r e s t to p y r o m e t a l l u r g y is the so cal led su l fa t ing r o a s t of chalcopyr i te concen t ra te for subsequent aqueous leaching. Th i s roas t is usua l ly

c a r r i e d out under condi t ions where both the SO2 and the O2 p r e s s u r e s a re 0.1 arm or l e s s . We see that under these condi t ions fo rmat ion of copper sulfate and hemat i te (Fe203) is poss ib le only be tween about 680 and 750~ in a g r e e m e n t with the t e m p e r a t u r e s found in d i r ec t r oa s t i ng and leaching t e s t s . 4~

MATTE SMELTING

We shall now briefly turn our attention to matte smelting and converting of copper-iron sulfides, a subject which has recently been reviewed by Elliott. 42 If chalcopyrite is subjected to a partial roast above its melting point this will give a molten sulfide (matte) and solid magnetite. On further oxidation liquid metallic copper will be formed, and the copper content of the magnetite phase will increase towards that of copper spinel, CuFe204. Eventually a molten cuprous oxide-iron oxide mixture will be formed. To what extent solid delafossite may exist at, e.g., 1300~ is not clear. According to Yazawa et a143 delafossite melts incongruently at I183~ giving an oxide melt and solid spinel. This temperature is expected to be a function of the oxygen potential, however.

In order to obtain an industrially feasible process a silica flux is usually added to give a liquid iron sili- cate slag, and the predominance area for the slag phase will increase with decreasing activity of FeO in the slag. In the absence of other slag forming constitu- ents the lowest possible FeO activity, about 0.3, is ob- tained at silica saturation. The phase relations for the quinary Cu-Fe-S-O-SiO2 system under these conditions

i . i - I

~ L \ \ c~ uo . "c~ \

I ~ ",~,,~o o,.\ X , ,,--& v o . \

t -z re~o~ -,~/v=..~,,2. "~-\

o~o<.,~ Cu20 \

~o~ . +CuFeO 2 \ ~ ,\Oo

o

�9 ~ 1 7 6 " -10

-12 n " uFeS2" �9 .

10 1 8 9 1

Fig. 17--Effect of temperature on the oxygen potential for 0.1 a t m of SO2 fo r 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 C u - F e - S - O .

METALLURGICAL TRANSACTIONS B VOLUME 9B, SEPTEMBER 1978-345

and 1 a tm of SO~ a r e shown in F ig . 18. ~ At low oxy- gen potent ia l mol ten mat te and mol ten s i l i ca s a tu ra t ed s lag wil l be the s table phases , the i ron content of the mat te d e c r e a s i n g with i n c r e a s i n g oxygen potent ia l . Above a c e r t a i n po ten t ia l the mat te , which at this point is a lmos t pure Cues (white metal) oxidizes by the roa s t r eac t ion to l iquid copper (b l i s te r copper) . On fu r the r oxidat ion sol id sp ine l and a mixed l iquid s lag phase wil l be fo rmed . Fo r the lack of data for these phases only the theo re t i ca l potent ia l for equ i l i b r i um between liquid copper and Cu20 of unit ac t iv i ty is shown. The sp ine l phase , which for coexis tence with white me t a l is a lmos t pure magnet i te , will for 1 a tm of SO2 coexis t with white metal , b l i s t e r copper and s i l i ca s a tu ra t ed s l ag at a t e m p e r a t u r e of about 1300oC 4~,~ .

*In the original paper 44 this temperature was given as 1300~ Later work in our laboratory as well as that of Yazawa and Eguchi 43 indicate a slightly higher temperature.

F i g u r e 18 shows fu r the r that the act ivi ty of Cu20 for coexis tence be tween white meta l and b l i s t e r copper is of the o rde r of 0.1. Th i s co r re sponds accord ing to our s tudies to a copper content of the s lag of 7 to 8 pct. Thus the conc lus ion can be drawn that in o rder to sme l t cha lcopyr i te concen t ra te d i r ec t ly to b l i s t e r copper with a s i l i ceous s lag t e m p e r a t u r e s in excess of 1300~ a re needed, and the s lag wil l conta in a lmos t 10 pct of ~copper.

Whereas in the pas t a l l copper s m e l t i n g has been based on the use of s i l i ca as the f luxing agent for mag- neti te , Yazawa e t al have shown recen t ly that, as an a l t e rna t ive , l ime may be used. 43 L ime r eac t s with i ron oxides to give ca lc ium f e r r i t e s lags , and the low- es t mel t ing t e m p e r a t u r e s , a round 1200~ a re found for s lags with about 20 wt pat CaO. Apar t f rom the fact that this r e s u l t s in a lower s lag volume for a given amount of i r on oxides, s lagging of magnet i te with l ime is p r ac t i c a l l y independent of the oxygen potent ia l . If anything it is more favorable under oxidizing condi- t ions, whereas s lagging of magnet i te with s i l i ca is favored s t rongly by reduc ing condi t ions . As a r e s u l t s , s i l i c a is the be t t e r f luxing agent for low grade mat tes , (Fig. 18), whereas l ime is p a r t i c u l a r l y useful for the b e s s e m e r s tage where i ron -coppe r mat te is blown to b l i s t e r copper.

Another advantage of the ca lc ium f e r r i t e s lags is that they seem to have a s m a l l e r aff ini ty for cuprous oxide than s i l i ca te s l ags . Thus Kuxmann and B uss - mann 4S found for coexis tence between b l i s t e r copper, white meta l and l ime sa tu ra t ed s lag at 1200 to 1300~ and 1 a tm of SO2 a copper content of 3 to 5 pct, or about half the value that apply for s i l i c a sa tu ra ted s lag. The i ndus t r i a l r e su l t s obtained with ca lc ium f e r r i t e s lags in the Mi t sub ish i p roces s appear to be p romis ing , 43 and fu r the r work should ce r t a in ly be done to explore the mel t ing r e l a t ions and the rmody- namics of the C a - F e - C u - O sys tem.

SYSTEM WITH LIME

We shal l now r e t u r n to the solid s ta te and look f i r s t upon the s imple Ca-S-O sys t em. Th i s is shown in Fig . 19, based main ly on data f rom Turkdogan et al 2 and Fykse . 13 In addi t ion to the solid l ines which apply for uni t ac t iv i t i es and 1 a tm of SO2, dashed l ines a re drawn for condit ions where the product aca O "PSO2

-3

o

-7

-9

-5

~ , ,, -'I 0 -i

6 ~.Oc~ ' - -

2 " "

Pso2"l otm I\ \K~%~.., \ -8 aFeO = 0.3 j ~ \

0 P P N

-. ~ . ~ oxro ..i

\ \ . ~ o ,,

s ~ \ \ ,

6.0 6.5 7.0

F i g . 1 8 - - O x y g e n p o t e n t i a l a n d p h a s e e q u i l i b r i a f o r m a t t e smelting and converting at 1 atm of 802 and aFe O = 0.3.

is 10 -1 - 10 -4. The line that gives the equ i l i b r ium be- tween CaS and CaSO4 is independent of this product .

The poss ib le ex is tence of ca lc ium sulf i te , CaSO3, as a s table phase has been much debated. This phase is formed by the act ion of SO2 on CaO at t e m p e r a t u r e s be- tween 350 and 600~ but decomposes exo thermal ly and i r r e v e r s i b l e to CaS + CaSO4 around 700~ 46"48 For the reac t ion :

4 CaSO3 = 3 CaSO4 + CaS.

T a r r a d e l l a s and Bonneta in 47 give AG~ = --30 kcal, and we may safely conclude that CaSOa is the rmody- namica l ly uns tab le at a l l t e m p e r a t u r e s .

The Ca-S-O sys t em has r ece ived a t ten t ion for v a r i - ous r e a s o n s . In the e a r l y 1950's when there was a shor tage of su l fur on the world!s marke t the vas t deposi ts of gypsum and anhydr i te were cons idered a sul fur source , and some SO2 and su l fur ic acid were produced c o m m e r c i a l l y by t h e r m a l decompos i t ion of the sulfate . 49 As it wil l be seen f rom Fig . 19 this decomposi t ion is favored by high t e m p e r a t u r e , mildly reduc ing condi t ions and a low act ivi ty of CaO. The las t condi t ion may be achieved by r eac t ion with some s i l iceous or c layey ma t e r i a l , in which case Por t l and cement may be obtained as an addi t ional product . The p r oc e s s is hampered , however, by i ts r a the r large endo thermic heat of r eac t ion which r e s u l t s in a large fuel consumpt ion .

Today the ma in i n t e r e s t is focused on the abi l i ty of CaO to abso rb su l fur f rom fuel and roas t gases . This can occur in two di f ferent ways: Under r educ ing condi-

3 4 6 - V O L U M E 9B, SEPTEMBER 1978 M E T A L L U R G I C A L TRANSACTIONS B

- 2

-4

- 6

- 8

t - 1 0 o? c ~ o

- -12

-14

- 1 6

6 7 8 9 10

Fig. 19--Oxygen potential in the system Ca-S-O as function of temperature.

2CoS $2 * nS02

I / d "1

2 CoS* 02 (+nS02) I I= I Heot exchcnger I I =2CnO*S2 (+nS02) --I �9

13000C I- 1 _..I

! 2Cc0

02

I Condenser I ~1300C

2S(t)

Fig. 20--Suggested method for recovery of elemental sulfur from CaS.

t ions , as for example in a gas p r o d u c e r or by p a r t i a l combust ion of fuel oi l or na tu ra l gas, su l fur in the fue l wil l be t ied up as CaS. Th is is the b a s i s fo r the use of l ime or burned dolomi te for f ixat ion of su l fur in va r ious sponge i ron p r o c e s s e s , and has a l so been sugges ted as a method for r e m o v a l of su l fur f r o m ca rbonaceous sol id fuels , s~ The CaS f o r m e d may l a t e r be roa s t ed e x o t h e r m a l l y to give CaO, which may be r e t u r n e d to the p r o c e s s , and sul fur dioxide of suff ic ient concen t ra t ion to be conve r t ed into su l fu r i c acid.

An i n t e r e s t i n g f ea tu re in this connect ion is that CaS need not n e c e s s a r i l y be roa s t ed to give only SO2. If the r o a s t i n g is c a r r i e d out with an oxygen de f i c i ency and at high t e m p e r a t u r e a c e r t a i n amount of e l e m e n t a l su l fur wil l a l so be fo rmed . Thus a t 1300~ the equi - l i b r ium constant for the r e a c t i o n :

2 C a S + S O 2 = 2 C a O + ~ S 2

is of the o r d e r of 10 "2. Thus if the r o a s t i n g is c a r r i e d

out with a i r the e q u i l i b r i u m r o a s t gas wi l l contain about 16 pct SO2 and about 1.5 pct $2, c o r r e s p o n d i n g to about 15 pct of the to ta l su l fur be ing p r e s e n t in e l e - men ta l f o r m . If the r o a s t i n g is c a r r i e d out with pu re oxygen or with a SO2 + O2 mix tu re the e q u i l i b r i u m gas wi l l contain about 95 pct SO2 and 5 pct Se. An even h igher p e r c e n t a g e of e l e m e n t a l su l fur may be obtained if the CaO ac t iv i ty i s l owered by the addi t ion of s i l i - ceous or c layey m a t e r i a l . Thus one can imag ine a p r o c e s s whe re CaS is t r e a t e d with SO2 + O~ to give

o

o o o

' '

0 Ca S04(+C u Fe204)

-15

8 9 10 11

Fig. 21--Oxygen potential for univariant equilibria in the sys- tern Ca-Cu-S-O (solid lines) and Cu-Fe-O (dashed lines) as function of temperature.

o o o co

2 U.2C

I ~.~.._Co= 0 �9 ,~

,-P 2 C : ~ c~,,.,

I I I I

8.0 9.0 10.0 11.0

Fig. 22--SO2 potential for univariant equilibria in the system Ca-Cu-S-O (solid lines) and Ca-Cu-Fe-S-O (dashed line) as function of temperature. Calcium ferri tes disregarded.

METALLURGICAL TRANSACTIONS B VOLUME 9B, SEPTEMBER 1978 347

SO2 + $2. Af te r cool ing of the gas and condensat ion of the su l fur the r e m a i n i n g SO2 is mixed with the n e c e s - s a r y amounts of pu re oxygen and is r e t u r n e d to the r o a s t i n g fu rnace . The o v e r a l l r eac t ion , t h e r e f o r e , wi l l be:

2 C a S + O s ( + n S O s ) = 2 C a O + S s (+nSO2)

where n wi l l be of the o r d e r of 20 for pure CaS-CaO. Although this r e a c t i o n is highly e x o t h e r m i c a s y s t e m of heat exchange r s wi l l be needed to heat the c i r c u l a t - ing SOs gas, see F ig . 20.

Burned l ime may a l so be used to r e m o v e sulfur f r o m hot combus t ion ga se s under oxidiz ing condi t ions . In this case ca l c ium sulfa te is f o r m e d . $1-53 The r e - ac t ion is mos t f avo rab l e in the t e m p e r a t u r e range 800 to 1000~ As an example : At a r e s i d u a l oxygen p r e s - s u r e of 10 -s arm the SOs p r e s s u r e in equ i l i b r i um with CaO and CaSO4 at 800~ wi l l be as low as 10 -9 a tm. At l l00~ it wil l be 5 . 1 0 -5 a tm. The r e a c t i o n is h a m p e r e d by a r a t h e r slow r a t e , however , which s e e m s to de- c r e a s e a l so above 1000~ One r e a s o n may be that the r e a c t i o n product , CaSO4, o c c u p i e s a l a r g e r vo lume than CaO, or even l a r g e r than CaCO3 if l imes tone is used as the abso rben t . As a r e s u l t a dense l aye r of CaSO4 is f o r m e d which r e t a r d s diffusion of gases to the i n t e r i o r of the l ime p a r t i c l e s , and l ime e f f i c i enc i e s above 25 pct a r e r a r e l y obtained. Th is l aye r s e e m s to be p a r t i c u l a r l y dense at h igher t e m p e r a t u r e s . 5s If, ins tead of l imes tone , do lomi te is used a be t t e r l ime e f f i c i ency is obtained, the conve r s ion of CaMg(CO3)s into CaSO4 + MgO having a s m a l l nega t ive vo lume change.

L i m e or l imes tone have a l so been sugges ted as add i t ives in the r o a s t i n g of non fe r rous su l f ides . In this way SOs in the r o a s t gas is abso rbed as CaSO4 s imul t aneous with the r o a s t i n g p r o c e s s , a p r o c e d u r e which may poss ib ly be e c o n o m i c a l if t he r e is no m a r k e t for su l fu r ic ac id and if e m i s s i o n of SOs into the a t m o s p h e r e is p roh ib i ted . Such a l i m e - r o a s t of cha lcopyr i t e has been s tudied by B a r t l e t t and Haung 54 and by Hayer and Wong 4s but with r a t h e r di f - f e ren t r e s u l t s . We sha l l f i r s t look upon the t h e r m o d y - namics of the p r o c e s s .

F i g u r e 21 shows the oxygen po ten t ia l as function of t e m p e r a t u r e for the va r i ous un ivar ian t equ i l i b r i a in the s y s t e m s C a - C u - S - O and C u - F e - O . It should be e m p h a s i z e d that, these being un iva r i an t equ i l ib r ia , the SO2 p r e s s u r e is not constant . The SO2 p r e s s u r e s for s o m e of the e q u i l i b r i a a r e shown as function of t e m p e r a t u r e in F ig . 22. We see that if cha lcopyr i t e is dead r o a s t e d in the p r e s e n c e of l ime (curves 5 and 6) the e q u i l i b r i u m produc t at a l l t e m p e r a t u r e s will , in addit ion to CaSO4, be CuO and CuFesO4.* Copper

*If complete equilibrium with solid lime were established, calcium ferrite, Ca2 Fe20s, would have been formed. Since this compound can only be formed through a solid-solid reaction it is disregarded in Fig. 21, but will be considered in Figs. 23 to 25.

sul fa te can not be f o r m e d in the p r e s e n c e of l ime, the equ i l i b r ium:

CuSO4 + CaO = CaSO~ + CuO

being shi f ted to the r igh t at a l l t e m p e r a t u r e s . Th is conclus ion is in a g r e e m e n t with the f indings of Hayer and Wong, 4s who found that in o r d e r to b r ing the cop- pe r va lues into aqueous solut ion a s t r o n g hyd roch lo r i c ac id was needed, in which case a l so s ign i f ican t

348-VOLUME 9B, SEPTEMBER 1978

amounts of i ron w e r e d i s s o l v e d . In con t r a s t Bar t l e t t and Haung 54 found that if the roa s t i ng was c a r r i e d out around 500~ mos t of the copper is conve r t ed to su l - fate, which could ea s i l y be leached, w h e r e a s i ron r e - mained as ac id - in so lub l e oxide. Although the la t te r f indings can not be r e c o n c i l e d with the t h e r m o d y n a m - ics of the s y s t e m they may poss ib ly be expla ined as a r e s u l t of a low r e a c t i v i t y of the l ime used. The c o r - responding SO2 content of the eff luent r o a s t gas wil l in this ca se be h igher , however , than if comple te equi- l i b r i um with CaO had been e s t ab l i shed .

Another poss ib i l i ty would be to t r e a t the sulf ide with l ime under m o r e r educ ing condi t ions . One such pos - s ib i l i ty was sugges ted and s tudied by Ceck and T i e - mann, 5s F o r d and Fah im, $6 and Habashi and Dugdale, 57 who s tudied r e a c t i o n s of the type:

MeS + CaO + Hs = Me + CaS + HsO.

T h e s e r e a c t i o n s a r e analogous to the use of l ime and a reduc ing agent in the r e m o v a l of su l fur f r o m i ron and s t ee l . F o r m o d e r a t e l y noble me ta l s such as cop- per , cobal t and i ron the e q u i l i b r i u m is shi f ted wel l to the r igh t giving e s s e n t i a l l y wa te r vapor as the only gaseous product . Th is is s een in F ig . 21 where curve 1 g ives the oxygen po ten t ia l for reduc t ion of Cu2S in the p r e s e n c e of l ime , which is s een to c o r r e s p o n d to HsOYHs r a t i o s around 100. On the o ther hand, in mos t c a s e s this type of r e a c t i o n is e i the r endo the rmic or only s l ight ly exo the rmic , which means that, in addi- t ion to a r e l a t i v e l y expens ive r educ ing agent , hydro- gen, addi t ional heat e n e r g y mus t be added. Some of the ene rgy thus abso rbed by the p r o c e s s may la ter be r e c o v e r e d as heat if the c a l c ium sulf ide is r o a s t ed to oxide o r to sulfate , but th is may not a lways be con- venient .

Another poss ib i l i t y should be cons ide red . As seen f r o m curve 2 in F ig . 21 a pa r t i a l r o a s t of Cuss ( roas t - reac t ion) in the p r e s e n c e of l ime wil l give me ta l l i c copper and CaSO4 i .e. the p redominance a r e a for the me ta l l i c phase is expanded.* If cha l copyr i t e is roas t ed

*Below about 600~ Cu2 S will be decomposed by lime:

4 Cu2S + 4 CaO = 8 Cu + 3 CaS + CaSO4.

Whereas the analogous reaction is used industrially for decomposition of mercury sulfide, this reaction is considered of less interest for copper.

i ts i ron content wil l go to magnet i te , or , if comple te equ i l i b r i um with l ime is ach ieved , to Ca2Fe2Os. These r e a c t i o n s a r e e x o t h e r m i c and should r e q u i r e no addi- t ional f u e l . As seen f r o m Fig . 22, curve 2, the co r - responding SO2 p r e s s u r e is h igher than for a dead l ime roas t , but s m a l l e r than for a r oa s t in the absence of l ime, p a r t i c u l a r l y at lower t e m p e r a t u r e s .

In o r d e r to tes t whe ther such a r e a c t i o n is poss ib le we made s o m e e x p e r i m e n t s where a m ix tu r e of 1 mole of Cu2S and 2 moles of Cu20 with an e x c e s s of l ime and some magnet i te was heated at 800~ in evacua ted and sea led s i l i c a tubes . The r eac t ion was found to p r o c e e d as indicated, the gases evolved by the roas t r e ac t i on being abso rbed by the l ime . X - r a y and mi - c r o s c o p i c examina t ion of the product showed, in addi- t ion to un reac t ed l ime, m e t a l l i c copper , magnet i te and ca l c ium sulfate as the only phases . M i c r o p r o b e ana lys i s of the magne t i t e phase gave a copper content of 0.7 + 0.2 wt pct .

To what extent the m e t a l l i c copper phase can be s e p a r a t e d f r o m the c a l c i u m s u l f a t e - i r o n oxide mix ture

METALLURGICAL TRANSACTIONS B

is not c l e a r , but should be eva lua t ed . One p o s s i b i l i t y would be by f lo ta t ion , ano the r by l each ing with a m - monia and a i r . M e t a l l i c copper is known to l each m o r e e a s i l y than c h a l c o p y r i t e a s such.

In c lo s ing th is l e c t u r e I wi l l s a y a few words about the c a l c i u m - i r o n - s u l f u r - o x y g e n s y s t e m . Th i s s y s t e m has r e c e i v e d a t t en t ion r e c e n t l y in v a r i o u s connec t ions . Thus B a r k e r 58 r e p o r t s on the r e a c t i o n of p y r i t e with l ime , with o r without c a rbon added, in the t e m p e r a t u r e r a n g e 700 to 1000~ Without c a r b o n a m ix tu r e of CaSO4, CaS, FeaO4 and r e s i d u a l " F e S " was obta ined , w h e r e a s with ca rbon and for t e m p e r a t u r e s above 800~ the p roduc t was a m ix tu r e of e s s e n t i a l l y CaS and m e t a l l i c i ron . T h e s e p r o d u c t s we re s u g g e s t e d as m e a n s to p r e c i p i t a t e coppe r f rom aqueous l each so lu - t i ons . F u r t h e r m o r e , T a n a k a et a159 r e p o r t on s o m e e x p e r i m e n t s where m i x t u r e s of FeS and CaO were " r o a s t e d " with s t e a m to give CaS, FeaO4 and hydrogen , the p u r p o s e be ing to conve r t the e n e r g y of r o a s t i n g into a va luab le r e d u c i n g agent , hydrogen .

W h e r e a s the phase e q u i l i b r i a be tween l ime , i ron and su l fu r at i r o n - and s t e e l m a k i n g condi t ions a r e we l l unde r s tood th is i s not the c a s e be low me l t i ng t e m - p e r a t u r e s and for l e s s r e d u c i n g cond i t ions . One c o m - p l i c a t i n g f ac to r is that l ime and i r o n ox ides f o r m a n u m b e r of double compounds , " c a l c i u m f e r r i t e s " , with 0.5 < NFe < 1.0 and with v a r i o u s f e r r i c / f e r r o u s r a t i o s , e ach of which be ing s t ab le in a c e r t a i n oxygen po t en t i a l r a n g e . 6~ F u r t h e r m o r e , o x y - s u l f i d e p h a s e s of c o m p o s i t i o n FeO �9 CaS and 4 FeO �9 3 CaS have been r e m p o r t e d . 6: In the fo l lowing d i s c u s s i o n for s i m p l i c i t y , only the mos t s t ab le c a l c ium f e r r i t e , Ca2Fe205 wi l l be c o n s i d e r e d , and the oxysu l f ides wi l l , for the t i m e being, be d i s r e g a r d e d .

On the b a s i s of ex i s t i ng t h e r m o d y n a m i c da ta we may c a l c u l a t e the s t ab le phase combina t ions in the r e c i p r o - ca l s y s t e m C a O - " F e O " - C a S - " F e S " as shown in F i g . 23, which r e f e r s to i r on s a t u r a t e d cond i t ions . With i n c r e a s i n g oxygen po ten t i a l the s a m e f igure wi l l , in p r i n c i p l e , apply , but " F e O " , wil l ox id ize to give 1/3 "Fen04" , which l ike " F e O " m a y take some l ime into so l id solu t ion , and even tua l ly to 1/2 Fe203, w h e r e a s CaS wi l l ox id ize to CaSO4.

We s e e that the phase d i a g r a m a t i r on s a t u r a t i o n m a y be d iv ided into t h r e e t h r e e - p h a s e a r e a s A, B and

CoO Ca2Fe2Os(+ Fe) "FeO"

/ /

l / 1~4 FeO.3 CoS / /fl(FeO-CoS

"o / ,, c" I / / t

:oS "FeS" N Fe - ~

Fig. 23--Phase combinations in the system C aO Jc :FeO"-C aS- "FeS" at 600 to 900~ and iron saturation.

C, a n d one t w o - p h k s e a r e a w h e r e CaS c o e x i s t s with " F e O " of v a r y i n g l i m e content . We see a l s o that CaO and FeS may not coex i s t , but wi l l r e a c t to f o r m CaS and " F e O " , a l t e r n a t i v e l y CaS and Ca2Fe205 + F e . If the o x y - s u l f l d e p h a s e s we re c o n s i d e r e d t h e s e would af fec t the phase d i a g r a m as shown by the da shed l ines in F i g . 23, and give the addi t iona l t h r e e - p h a s e a r e a s B ' , B" , C ' and C". The oxysu l f ide p h a s e s a r e ex- pec t ed to o c c u r only at r e l a t i v e l y low oxygen po ten- t i a l s , howeve r . At i n t e r m e d i a t e and h ighe r oxygen po t e n t i a l s a n u m b e r of add i t iona l c a l c i u m f e r r i t e s wil! occu r on the l ine s e c t i o n C a 2 F e 2 0 5 - " F e O " , caus ing f u r t h e r c o m p l i c a t i o n of the phase d i a g r a m . F u r t h e r - m o r e , a t t e m p e r a t u r e s above 880~ and at i r o n s a t u r a - t ion a t e r n a r y eu tec t i c me l t i s f o r m e d in the C - t r i - angle , 6: and at 915~ the mol t en f ie ld r e a c h e s the F e S - " F e O " b i n a r y l ine . 62

On the b a s i s of e x i s t i n g t h e r m o d y n a m i c da t a for the p h a s e s involved, 63 and d i s r e g a r d i n g the o x y - s u l f i d e s a s wel l a s c a l c i u m f e r r i t e s o the r than Ca2Fe2Os, we may now c a l c u l a t e the e q u i l i b r i u m oxygen po ten t i a l for c o e x i s t e n c e be tween any two t h r e e - p h a s e c o m b i n a - t ions in the s y s t e m . The r e s u l t s a r e shown in F ig . 24, which shows a l l s t ab l e phase combina t ions for the " p s e u d o b i n a r y " s e c t i o n C a O - " F e S " a s funct ion of oxygen po ten t i a l and m o l a r r a t i o NFeS = n FeS// (riFe s + n c a O) at 727~ It should be po in ted out that the phase combina t ions which occu r in the r a n g e s m a r k e d A, B and C apply to a l l c o m p o s i t i o n s within t hese t r i a n g l e s (F ig . 23) and not only to c o m p o s i t i o n s a long the " p s e u d o b i n a r y " .

/ |

, A + B * C - -

I caSO

oCo2Fe2~

*Fe20 a

CaSO4*CaO (~) CaSO4+Fea04

*Ca2Fe205 c~ +S02(latm)

J %

-10

" ~ | a 0 o -. COS04 "~

I0

-15 (~) (~) § @ -3.9.~ !

C,I C

Q- CaS+CaO + CaS'g'FeS% FeaO 4 t t31) O') ~ / O +Co2Fe205 ~ ~ O

-

| | ,= -20 o

x CGS+"FeS"+"FeO"~ - -

| -t0.7 | y

CaO+CaS*Fe CaS*"FeS"+ Fe

I I I I

CoO 0.2 0.4 0.6 0.8 "FeS"

NFe S " Fig. 24--Phase combinations as function of oxygen potential in the system CaO-"FeS" at 727~ X = CaS + Ca2Fe205 + "FeO", Y = CaS + Ca2Fe205 + Fe.

METALLURGICAL TRANSACTIONS B VOLUME 9B, SEPTEMBER 1978-349

-10

-15

t O

-20

-25 8 9 10 11

IO?T -

Fig. 25--Oxygen potential as function of temperature for four-phase equilibria in the system Ca-Fe-S-O.

We see that at ve ry low oxygen-poten t ia l s meta l l i c i r on coexis ts with CaS + CaO, a l t e rna t i ve ly CaS + " F e S " , depending on the NFe S va lue . Fo r l i m e - r i c h composi t ions , range A, the phase combina t ions at in- c r e a s i n g oxygen potent ia l will be CaS + CaO + Ca2Fe2Os followed by CaSO4 + Ca(3 + Ca2Fe2Os. Fo r mix tu re s in the C- r ange the sequence will be CaS + " F e S " + " F e O " , CaS + " F e S " + Fe304 and CaSO4 + " F e S " + Fe304. F ina l ly at a log P02 value o f - 1 4 . 2 " F e S " oxi- d izes to give Fe304 + 1 atm of SO2. In range B a s i m i - l a r sequence appl ies , but " F e S " is r ep laced by Ca2Fe205, and the p redominance a r e a s for " F e O " and "Fe304" a re affected sl ightly due to sol id solut ion of l ime . F ina l ly , at high oxygen poten t ia l "Fe304" oxi- d izes to Fe203.

If the oxy-sul f ide phases were cons ide red these would occur in the a r e a s marked X and Y in sec t ion B (Fig. 24). If the other ca lc ium f e r r i t e s were con- s ide red these would occur in sec t ion B at oxygen po- t en t i a l s above that of a r e a X.

In addi t ion to the SO2 potent ia l of 1 a im for oxidation of " F e S " to Fe304 the SO2 poten t ia l s for coexis tence be tween CaS and CaSO4 as well as be tween Fe, " F e S " and " F e O " were ca lcula ted and a re shown to the r ight in F ig . 24. It should be emphas ized that in this d ia - g r a m the SO2 potent ia l is not a s imple funct ion of the oxygen potent ia l .

The effect of t e m p e r a t u r e on the oxygen potent ia l for coexis tence be tween any four sol id phases , co r r e spond- ing to the hor izon ta l l ines in Fig . 24, is shown in Fig . 25. Th i s f igure inc ludes also a l ine for equ i l i b r i um between solid ca rbon and CO + CO2 of 1 aim, as well as m a r k e r s to show the co r r e spond ing HzO/H2 ra t ios .

350 V O L U M E 9B, S E P T E M B E R 1978

We see that in o rder to reduce a mix ture of l ime and i ron sulf ide with ca rbon to give CaS and meta l l i c i ron, t e m p e r a t u r e s in excess of 730~ a re r equ i red . As the r eac t iv i ty of ca rbon is s t i l l low at this t e m p e r a - tu re a somewhat higher t e m p e r a t u r e , e .g . , in excess of 800~ may be needed. 5a

The " r o a s t i n g " of m ix tu re s of FeS and Ca(3 with s t eam 59 p r e s e n t s a more complicated p ic tu re . Since these two phases may not coexist at the t e m p e r a t u r e s in quest ion, they will, even without s t eam, r eac t to give mix tu res of CaS with " F e O " , or with Ca2Fe205 + Fe, depending on the in i t i a l mixing ra t io . When s t e a m is applied to such mix tu res the " F e O " or Fe content wil l be oxidized to Fe~O4 a l t e rna t ive ly Ca2Fe205 giving equ i l ib r ium HeO/H2 ra t io s of about 2, a l t e r n a - t ive ly 1/2, at, e.g., 800~ If the in i t i a l so l id state r eac t ion had been incomple te the c o r r e spond ing H20/H2 ra t io would be undefined, but would be higher than the two equ i l i b r ium va lues . These ca lcula t ions agree with the f indings of Tanaka et al, ~9 who found in i t i a l gas r a t io s of about unity, which as the r eac t ion proceeded i nc r e a se d to between 50 and 100 co r r e spond ing to be- tween 2 and 1 pct hydrogen in the effluent gas. In order to develop a p r o c e s s where i ron sulf ide is used for the product ion of hydrogen it s e e ms n e c e s s a r y , the re - fore, to make the r eac t i on with l ime go to complet ion before s t e a m is in t roduced.

On the other hand, the ove ra l l r eac t ion is only s l ight ly exo thermic , and addi t ional heat wil l be needed to produce s t e a m and to heat the r eac t an t s to the r e q u i r e d t e m p e r a t u r e . A great deal of the energy of r oa s t i ng is used in the reduct ion of Ca(9 to CaS, ene rgy which wil l be r e l e a s e d as heat if the solid product is subsequent ly roas ted to give e i the r CaO or CaSO4. Th i s heat may poss ib ly be used to cover the phys ica l heat r e q u i r e m e n t s of the p r o c e s s .

Even though much work, therefore , r e m a i n s to be done on the " r o a s t i n g " of i ron sulfide with s team, the idea of cons ide r ing sulf ide ores not only as a source of meta l value (and of sul fur value under more favorab le marke t condit ions) , but a lso as a source of energy is an i n t e r e s t i n g one. So far the ene rgy of roas t ing has been u t i l ized in f lash s m e l t i n g as well as in s team b o i l e r s . I ts use to produce hydrogen and s i m i l a r chemica l s is c e r t a in ly worth cons idera t ion .

CONCLUDING REMARKS

If I sha l l t ry to s u m m a r i z e this l ec ture , it would be that the s tudy and mapping of phase equ i l i b r i a in roas t ing and s m e l t i n g complex sulfide o re s is a long and tedious task. In most cases it wil l only conf i rm r e s u l t s obtained in p r a c t i c a l t es t s and in the indus t ry . In a few cases , however , it may point to new poss ib l i - t ies for i n d u s t r i a l appl icat ion, pos s ib i l i t i e s which should be inves t iga ted fu r the r .

ACKNOWLEDGMENTS

The author wants to thank Mr . Arne Espelund and in p a r t i c u l a r Mr. Audun Hofseth for va luable help in p r e - pa r ing this l ec ture . The o r ig ina l r e s e a r c h ment ioned is being suppor ted by the Norwegian R e s e a r c h Council for Science and the Humani t i e s (Norges A lmenv i t en - skapel ige Fo r skn ings r~d ) .

METALLURGICAL TRANSACTIONS B

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M E T A L L U R G I C A L TRANSACTIONS B VOLUME 9B, SEPTEMBER 1 9 7 8 - 3 5 1