Biolix Calco a Distintas Temp

8/18/2019 Biolix Calco a Distintas Temp.

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E L S E V I E R

Hydrometallurgy 43 (1996) 331-344

h y d r o m e t l l u r g y

Electrochemistry of chalcopyri te

C . G 6 m e z

a , ,

M . F i g u e r o a b ,1 , j . M u f i o z a , M . L . B l f iz q u e z

a ,

A . B a l l e s t e r a

a Departamento de Cienc ia de M ater ia les e lngenier la Metal ~rgica , Facul tad d e Cienc ias Qulmicas ,

Univers idad Complutense de Madrid , 28040 Madrid , Spain

b Departamento d e Q ulmica lnorg6nica , F acul tad de Qulmica , Pont i f ic ia Univers idad Cat61ica de Chi le ,

Casil la 306, correo 22, Santiago, Chile

Received 30 November 1995; accepted 21 January 1996

A b s t r a c t

The electrochemical response of a massive chalcopyrite electrode at two different temperatures,

25°C and 68°C, were compared. The electrolyte used in the experiments was an acidic medium

(0.4 g -I -t (NH4)2SO 4, 0.5 g .1 -I MgSO4- 7H20, 0.2 g-1- 1 K2HPO 4 at pH = 2 ) which is

suitable for the growth of the microorganisms involved in the bioleaching process. The chosen

temperatures were optimum for the growth of the mesophilic T h i o b a c i ll u s f e r r o o x i d a n s ) and

thermophilic

S u l f o l o b u s )

microorganisms. The experimental results at both temperatures were

similar and confirmed that, during the anodic dissolution of chalcopyrite, a passive film is formed

on the surface which restricts the oxidation reactions in the medium by diffusional control of the

film. The different responses at the temperatures tested were due to the differing physical structure

of the complex films of the electrochemically formed sulphides, polysulphides and elemental

sulphur.

1 . I n t r o d u c t i o n

C h a l c o p y r i t e (C u F e S 2 ) is t h e m o s t a b u n d a n t o r e o f t h e s u l p h i d e m i n e r a ls o f c o p p e r .

Howeve r , t h i s m i ne r a l i s t he m os t r e ca l c i t r an t t o hyd r om e t a l l u r g i ca l p r oces s e s . Fo r t h i s

r ea s on p y r om e t a l l u r g i ca l p r oces s e s i nvo l v i ng t he s m e l t i ng o f concen t r a t e s [ 1 a r e s t il l the

m o s t e f f e c t iv e w a y o f e x t r a c ti n g t h e c o p p e r f r o m t h i s m i n e r a l. U n f o r t u n a t e l y , th e

e x t r a c t io n p r o c e s s e s p r e s e n t s e r io u s p o l l u ti o n p r o b l e m s , d u e t o S O 2 e m i s s i o n s d u r i n g

* Corresponding author. Fax: + 34 1 394 43 57. E-mail: [email protected]

i Fax: 56 2 552 56 92. E-mail: [email protected]

0304-386X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved.

P H

S0304-3 86X(96)00010- 2


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332 C. Gdmez et al. / Hydrometallurgy 43 1996) 331-344

pyr om e t a l l u r g i ca l r e ac t i ons . The s t r i c t env i r onm en t a l r e s t r i c t i ons i m pos ed on s uch

ope r a t i ons i nvo l ve a g r ea t dea l o f i nves t m en t , wh i ch i nc r ea s e s p r oduc t i ons cos t s . To

t ack l e s uch p r ob l em s , hyd r om e t a l l u r gy , wh i ch i s l e s s con t am i na t i ng [ 2, 3] , o f f e r s chea pe r

ways t o ex t r ac t t he coppe r , de s p i t e t he d i f f i cu l t i e s o f t h i s p r oces s e s even i n s t r ong l y

ox i dan t m ed i a [ 4 ] .

A m o n g t h e h y d r o m e t a l l u r g i c a l p r o c e s s e s , o n e p o s s ib l e e c o n o m i c a l l y p r o f it a b le p r o -

ce s s cou l d be t o d i s s o l ve t he coppe r by b i o l each i ng , t ha t i s t o s ay , t o u s e m i c r oo r gan -

i s m s t ha t c a t a l y s e t he m i ne r a l d i s s o l u t i on . Such m i c r oo r gan i s m s a r e u s ua l l y c l a s s i f i ed

acco r d i ng t o t he t em pe r a t u r e s t hey need t o g r ow, s o t ha t t he s o - ca l l ed m es oph i l i c

m i c r o o r g a n i s m s

Thiobacillus errooxidans, Thiobacillus thiooxidans, Leptospirilum fer-

rooxidans,

e t c . ) d e v e l o p a t a m b i e n t t e m p e r a t u r e s , w h e r e a s t h e r m o p h i l ic m i c r o o r g a n i s m s

Sulfolobus)

n e e d h i g h e r t e m p e r a t u r e s.

The m a i n d i s advan t age o f u s i ng b i o l each i ng t r ea t m en t s wi t h s u l ph i de m i ne r a l s i s t he

s l ow d i s s o l u t i on k i ne t i c s and , i n t he ca s e o f cha l copyr i t e , t he l ow y i e l d s , wh i ch a r e

a t t r i bu t ed t o t he f o r m a t i on o f a s o l i d r eac t i on p r oduc t on t he s u r f ace [ 5 ] . Th i s l aye r

wou l d ac t a s a d i f f u s i ona l ba r r i e r and h i nde r con t ac t be t ween t he l e ach i ng s o l u t i on and

t he cha i copyr i t e , t hus s l owi ng down any s ubs equen t d i s s o l u t i on .

P r e v i o u s b i o l e a c h i n g e x p e r i m e n t s c a r r i e d o u t w i t h t h e r m o p h i l i c m i c r o o r g a n i s m s

( p r i nc i pa l l y Sulfulobus) r epo r t ed i n t he l i t e r a tu r e [ 6 , 7 ] r epo r t h i g he r d i s s o l u t i on r a t e s and

be t t e r y i e l d s t han us i ng m es oph i l i c m i c r oo r gan i s m s ( p r i nc i pa l l y Thioba cillus ferrooxi-

dans)

i n t he l e ach i ng o f t he cha l copyr i t e .

I t ha s been dem ons t r a t ed t ha t cha l copyr i t e and m os t o t he r m e t a l l i c s u l ph i des a r e

d i s s o l ved by m eans o f an e l ec t r ochem i ca l m echan i s m [ 8 ] . Fu r t he r m or e , i n t he l i t e r a t u r e

t he r e a r e a l a r ge num ber o f s t ud i e s wh i ch exp l a i n t he anod i c d i s s o l u t i on o f cha l copyr i t e

i n d if f e r e n t m e d i a [ 9 - 1 4 ] ; h o w e v e r w e h a v e f o u n d n o i n f o r m a t i o n o n th e e x p e r i m e n t a l

c o n d i t i o n s f o r g r o w i n g m i c r o o r g a n i s m s ( t e m p e r a t u r e a n d c u l t u r e m e d i u m ) . F o r t h i s

r ea s on , t he a i m o f t h is wo r k i s to s t udy t he e l ec t r och em i ca l r e s pons e o f a m as s i ve

cha l co pyr i t e e l ec t r ode i n a bac t e r i a l cu lt u r e m ed i um a t tw o t em pe r a t u r e s : 25° C ( a t wh i ch

t e m p e r a t u r e m e s o p h i l i c m i c r o o r g a n i s m s g r o w ) a n d 6 8 ° C ( t h e o p t i m u m g r o w t h t e m p e r a -

t u r e o f Sulfolobus) i n o r de r t o unde r s t and t he d i f f e r ences i n t he d i s s o l u t i on k i ne t i c and

n a t u r e o f t h e p r o d u c t s f o r m e d d u r i n g t h e b i o l e a c h i n g p r o c e s s u s i n g b o t h k i n d s o f

m i c r o o r g a n i s m s .

2 .

Exper imental

M e a s u r e m e n t s w e r e m a d e w i t h a c o n v e n t i o n a l t h r e e - c o m p a r t m e n t g l as s e l e c t ro l y s i s

ce l l to l odge t he w or k i ng e l ec t r ode ( s pec i m en) , t he co un t e r e l ec t r ode ( p l a t inum s p i ra l

w i r e ) a n d th e re f e r e n c e e l e c t ro d e (A g / A g C 1 ) , w h i c h w a s p r o v i d e d w i th a L u g g i n - H a b e r

cap i l l a r y t i p . The ce l l was kep t a t cons t an t t em pe r a t u r e by connec t i ng i t t o a c i r cu l a t i ng ,

t he r m o s t a t i c a l ly con t r o l l ed w a t e r l oop . T he t em p e r a t u r e was f i xed a t 25 ° o r 68 ° + 0 . 1° C.

E a c h w o r k i n g e l e c t r o d e w a s p r e p a r e d f r o m a n a t u r a l s p e c i m e n o f m a s s i v e c h a l c o p y r i t e

f r o m T r a n s v a a l , S o u th A f r i c a (3 5 . 3 % C u , 3 0 . 1 % F e a n d 3 4 . 7 % S ) .

T h e s p e c im e n s w e r e c u b e s h ap e d ( - - 4 - 6 m m e d g e ) a nd w e r e c o n n e c t e d t o a c o p p e r

w i r e b y m e a n s o f a c o n d u c t i v e s il v e r r e si n . F i n a ll y , s a m p l e a n d w i r e w e r e e m b e d d e d i n


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C . G dm e z e t a l . / H y drom e ta l lu rgy 43 1996 ) 331 - 344 333

a n e p o x y r e s i n t o o b t a i n a r e a s o f 0 . 4 - 0 . 5 c m 2 e x p o s e d t o t h e e l e c t r o l y t e . B e f o r e e a c h

t e s t , t h e w o r k i n g e l e c t r o d e s u r f a c e s w e r e p o l i s h e d w i t h 6 0 0 s i l i c o n c a r b i d e p a p e r ,

f o l l o w e d b y u l t r a so n i c c l e a n i n g a n d r i n s in g w i t h d i s ti ll e d w a t e r . T h e e l e c t r o l y t e c o m p o -

s i ti o n , p r e p a r e d f r o m A . R . c h e m i c a l s a n d d i s t il le d w a t e r , w a s a s f o ll o w s : 0 . 4 g . 1-

(NH4)2SO4, 0 . 5 g " 1-1 M g S O 4 • 7 H 2 0 , 0 .2 g " 1 - l K 2 H P O 4 " T h e p H w a s k e p t a t 2 .0

b y t h e a d d i t i o n o f H2 SO 4. B e f o r e e a c h e x p e r i m e n t w a s p e r f o r m e d , 0 . 1 3 1 o f t h is

e l e c t r o l y te w a s p u r g e d i n s id e t h e c e l l w i t h n it r o g e n ( 9 9 . 9 5 % ) f o r 1 5 m i n . A f t e r th a t , a n d

b e f o r e s t a r t i n g e a c h t e s t , e l e c t r o d e a n d e l e c t r o l y t e w e r e p u r g e d a g a i n f o r 2 0 m i n .

T h e e l e c tr o c h e m i c a l e x p e r i m e n t s w e r e c a rr ie d o u t a t c o n st a n t o r p r o g r a m m e d p o t e n -

t i a l s u s i n g a W e n k i n g p o t e n t i o s t a t ( M o d e l L B 8 1 M ) a n d a W e n k i n g v o l t a g e s c a n n e r

( M o d e l M V S 8 7 ). D a t a w e r e a c q u i r e d b y a n a n a l o g u e b o a r d i n s t a ll e d i n a P C . W h e n

n e c e s s a r y , s t i r r i n g w a s a c c o m p l i s h e d w i t h a m a g n e t i c s t i r r e r , t h e w o r k i n g e l e c t r o d e

r e m a i n i n g s t a ti o n a r y . P o t e n t i a l s i n t h e t ex t r e f e r to t h e A g / A g C 1 e l e c t r o d e ( + 0 . 2 0 7 v s

S H E a t 2 5 ° C ) . T w o t y p e s o f e l e c t r o c h e m i c a l e x p e r i m e n t s w e r e p e r f o r m e d , p o t e n t i o d y -

n a m i c p o l a r i z a t io n c u r v e s a n d c y c l i c v o l t a m m e t r y . T h e s e c u r v e s w e r e i n i ti a te d f r o m t h e

r e s t po t en t i a l un l e s s o the rwi se s t a t ed .

3 . R e s u l t s

3 .1 . E / j c u r v e a t 2 5 ° C

P o l a r i z a t io n c u r v e s ( a p p a r e n t c u r r e n t d e n s i ty , j , v e r s u s p o t e n t ia l , E ) w e r e o b t a i n e d b y

a p p l y i n g t o t h e c h a l c o p y r i t e e l e c t r o d e s a s i n g l e t r i a n g u l a r p o t e n t i a l s w e e p ( S T P S )

be tw een t he r e s t po t en t i a l (E R) and t he anod i c (Es , a ) sw i t ch ing po t en t i a l a t t h r ee

d i f f e re n t s w e e p r a t e s ( v ): 2 0 , 5 a n d 2 m V s - ~ . I m m e d i a t e l y a f t e r t h e a n o d i c s c a n a

c a t h o d i c s c a n w a s c a r r i e d o u t f r o m t h e Es, a to the E~.c ( ca thod i c sw i t ch ing p o t en t i a l ).

F i g . l a s h o w s t h e i n i ti a l S T P S a t 2 m V s - i b e t w e e n E R a n d E s . a a t 2 5 °C , w h i c h

p r e s e n t s a n a n o d i c c u r r e n t r e s p o n s e w h i c h c a n b e a r b i t r a r i l y d i v i d e d i n t w o p o t e n t i a l

z o n e s f o r th e d i s c u s s i o n o f th e r e s u l ts . Z o n e I ra n g e s f r o m E R t o 0 . 7 V p o t e n t i a l r a n g e

a n d z o n e I I f r o m 0 . 7 V t o h i g h e r p o t e n ti a l s. T h e c u r r e n t r e s p o n s e i n z o n e I p r e s e n ts a

s m a l l b r o a d p e a k ( A 1 ) , l o c a t e d a t a b o u t 0 . 4 V . I n z o n e I I , a l a r g e i n c r e a s e i n c u r r e n t w a s

r e c o r d e d ( A l l ) a n d a l i m i t i n g c u r r e n t c l o s e t o 0 . 8 5 V w a s r e a c h e d ( A m ) . T h e s w i t c h i n g

p o t e n t i a l s c a n f r o m E s. a p r e s e n t s t h re e c a t h o d i c p e a k s a t 0 .4 V ( C I ) , a t 0 . 2 V ( C l l ) a n d a

l a r g e p e a k s t a r ti n g a t - 0 . 1 5 V ( C l ll ) .

T h i s e l e c t r o c h e m i c a l b e h a v i o u r o f t h e c h a l c o p y r i t e i n t h e n u t r i e n t m e d i u m u s e d a s

e l e c t r o l y t e a t 2 5 ° C w a s s i m i l a r t o t h a t d e s c r i b e d b y W a r r e n e t a l . [ 1 1 ] f o r t h e s a m e

m i n e r a l e l e c t r o d e s i n t h e p r e s e n c e o f H2S O 4. Thus, A l r e p r e s e n t s t he ' p r e w a v e ' , i n

w h i c h c h a l c o p y r i t e i s t r a n s f o r m e d t o C u S , t h r o u g h a n i n t e r m e d i a t e n o n - s t o c h i o m e t r i c

p h a s e ( C u ~ _ x F e l _ y S 2 _ z ), p r o d u c i n g S O a n d C u ( I I ) a n d F e ( I I ) i o n s . I n t h is z o n e o n l y a

s m a l l p o r t i o n o f t h e m i n e r a l i s p r o b a b l y t r a n s f o r m e d . I n r e g i o n I I , a t p o t e n t i a l s m o r e

p o s i t i v e t h a n 0 .7 V ( t r a n s p a s s i v e z o n e ) , t h e o v e r a l l d i s s o l u ti o n o f c h a l c o p y r i t e t a k e s

p l a c e t h r o u g h t h e f o l l o w i n g r e a c t i o n s :

C u F e S 2 ~ C u 2 ÷ + F e 3 ÷ + 2 S ° + 5 e - ( 1 )

C u F e S 2 + 8 H 2 0 ~ C u 2÷ + F e 3÷ + 2 S O ~ - + 1 6 H ÷ + 1 7 e - ( 2 )


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3 3 4 C. Gdmez et al. / Hydrometallurgy 43 1996) 331-344

3.2. Influence o f temperature on E / j prof i le

Si m i l a r t e s ts t o t hos e ca r r i ed ou t on t he cha i copy r i t e a t 25°C we r e pe r f o r m ed a t 68°C.

As i n t he l ow t em pe r a t u r e expe r i m en t , zones I and I I a r e a l s o p r e s en t i n t he po t en t i ody -

n a m i c c u r v e ( F i g . l b ) . H o w e v e r , z o n e I i s m o v e d t o w a r d s m o r e p o s i t i v e p o t e n t i a l s a t

h i ghes t t em pe r a t u r e and peaks C I and CI [ , p r oduc ed i n a s w eep f r o m Es .a = 1.0 to

e l ec t r oneg a t i ve po t en t i a l s and obs e r ved a t 25° C ( F i g . l a ) , d i d no t s eem t o appea r a t

68° C, a l t hough t hey becam e ev i den t when t he s ca l e was s u i t ab l y i nc r ea s ed ( i n s e r t , F i g .

l b ) .

3.3. Influence o f swee p rate v)

Fi g . 2 s hows t he i n f l uence o f s weep r a t e a t 25° C ( F i g . 2a ) and 68° C ( F i g . 2b ) . At t he

l o w e s t t e m p e r a t u r e , c h a l c o p y r i t e o x i d a t i o n d id n o t d e p e n d o n v , b u t a t t h e h i g h e s t

t em pe r a t u r e t he r e was a l i nea r r e l a t i ons h i p be t ween t he l i m i t i ng cu r r en t dens i t y ( J L) and

v l / 2 ( F i g . 3 ) .

F i g . 4 s h o w s t h e d e p e n d e n c i e s o f m a x i m u m p e a k i n te n s i ty ( I v ) a n d i ts c o r r e s p o n d i n g

poten t i a l (Ep ) wi th v 1 /2 and log v 1 /2 a t 25°C for the C l l peak . A s can b e seen , Ip wa s a

l i nea r f unc t i on o f v ] / 2 , wh i ch i nd i ca t ed t ha t the e l ec t r oac t i ve s ubs t ance w as d i s s o l ve d i n

t he e l eca 'o l y t e and r eached t he i n t e r f ace by d i f f u s i on . Mor eove r , t he f ac t t ha t Ep was

dep end en t on t he l og v 1 /2 s how ed t ha t t h is r e ac t i on was no t r ap i d [ 15 ]. T he s m a l l

cu r r en t dens i ti e s de t ec t ed in t he CI peak a t 68°C d i d no t a l low s i m i l a r ana l ys e s .

3.4 . In fluence o f pH on the E / j pro f il e

F i g . 5 s h o w s t h e i n f l u e n c e o f p H o n t h e a n o d i c p o t e n t i o d y n a m i c s o f c h a l c o p y r i t e a t

2 5 ° C a n d 6 8 ° C , r e s p e c ti v e l y . T h e p H h a s n o e f f e c t a t t he l o w e r t e m p e r a t u re , b u t a c l e a r

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p o t e n t la l I V A g /A g C l ) p o t e n t la l I V A g /A g C l )

F i g . 1 . V o l t a m m o g r a m o f c h a l c o p y r i t e a t ( a ) 2 5 ° C a n d ( b ) 68 ° C . v = 2 m V s i . D e t a il o f c u r r e n t p e a k s

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a n d

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C. G6mez et al./Hydrometallurgy 43 1996) 331-344 335

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potential

I V ( A g / A g C i ) p o t e n t i a l I V ( A g / A g C I )

Fig. 2. E ffect of sweep rate (v) on an odic potentiodynamic curves o f chalcopyrite at: (a) 25°C and (b) 68°C.

i n f l u e n c e a t 6 8 ° C . F i g . 6 s h o w s t h e p o t e n t i a l a t w h i c h t h e m a s s i v e d i s s o l u t i o n o f t h e

c h a l c o p y r i t e b e g a n ( E b ) a s a f u n c t i o n o f t h e p H a t t h is t e m p e r a t u r e , t h e o n e b e c o m i n g

m o r e e l e c t r o p o s i t i v e as th e o t h e r d e c r e a s e d , w h i c h m e a n s t h a t a n in c r e a s e i n a c i d i ty

h a m p e r e d m i n e r a l e l e c t r o o x i d a t i o n .

F i g . 7 s h o w s t h e i n f l u e n c e o f p H o n t h e C , 1 r e d u c t i o n p e a k w h e n t h e s c a n w a s

r e v e r s e d f r o m E s, a t o w a r d s m o r e e l e c t r o n e g a t i v e v a l u e s a t 2 5 ° C . A s h i ft o f 0 . 1 5 V i n th e

i n i t ia l p o t e n t i a l o f th e C . ~ r e a c t i o n t o w a r d s t h e m o r e e l e c t r o p o s i t i v e z o n e f o r a d e c r e a s e

o f 1 p H u n i t w a s o b s e r v e d .

1 . 4

~ . 1 . 2

9

E 1.0 /

, ~ 0 . 8

u 0 . 6

o

4 6 8 10 12 14 16

v 1 / 2 1 0 2 1 ( V s ' t ) 1 / 2

Fig. 3. L imiting current density (JL) of zon e I versus v 1/2 at 68°C.


6/14

336

C. Gdmez et a l. / Hydrometallurgy 43 1996) 331-344

. <

E

- 1 . 8 0

0 . 2 5

0 . 2 0

0 . 1 5

0 . 1 0

3

l o g ( v ) 1 / 2

- 1 . 2 0 - 0 . 8 0

I ~ ~ 0 . 1 8 8

L

I

~__ ~-

0 . 1 8 6

~ 0 .182 ~

0 .180 ~

0 . 1 7 8

I ~ I 0 . 1 7 6

6 0 1 2 1 5

v 1 /2 1 0 2 1 ( V s I ) 1 1 2

Fig. 4. Two plots of maximum intensity ( lp) and m aximum Ell peak potential (Er,) versus v ~/2 at 25°C.

3 . 5. I n f l u e n c e o f t he e l e c t r o l y t e s t i r ri n g o n t h e E / j p r o f i l e

A t 2 5 ° C e l e c t r o l y t e s t ir r in g i n f l u e n c e d b o t h t h e a n o d i c p o t e n t i o d y n a m i c c u r v e a n d t h e

i n v e r s e r e d u c t i o n c y c l e o f t h e c h a l c o p y r i t e , F i g . 8 a . I n z o n e I t h e c u r r e n t i n i t i a l l y

i n c r e a s e s l i n e a r l y w h e n t h e p o t e n t i a l i s i n c r e a s e d f r o m 0 . 2 5 t o 0 . 4 0 V , a l t h o u g h t h e s l o p e

1 . 8 5 . 0

p H 1 . 0 r - -

1 .6 2 5 ° C / / p H 1 I 6 8 ° C ~ ,

1 .4 S I / p H 2 .0 4 . 0 P H i / / i

E ° . 5 /

• I

1 . 2 t , ~

/ [ 3 . 0

,~ 1.0

w /

= 0 8 / l

- o 2 . 0

0 . 6

E

/ 1 . 0

0 . 2 a )

0 . 0 l ; I 0 . 0 . . . . . . . .

0 . 0 0 . 2 O A t 0 . 6 0 . 8 1 . 0 1 . 2 0 , 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0

p o t e n t i a l

; V ( A g / A g C I ) p o t e n t i a l I V ( A g / A g C I )

Fig. 5. Effect of pH on the anodic potentiodynamic curves of chalcopyrite at: (a) 25°C and (b) 68°C. v = 5

mVs ~.


7/14

C. G6mez et al . / Hydrometallurgy 43 1996) 331-344 337

2 . 0

1 . 6

1 . 0

I

0 . 4 5 0 . 5 0

\

\

\

©

0 . 5 5 0 . 6 0 0 . 6 5 0 . 7 0

E b V A g / A g C I )

Fig. 6. Relationship between chalcopyrite electrodissolution potential (E b) and the pH at 68°C. v = 5 m V s- 1.

t h e n c h a n g e s a n d t e n d s t o w a r d s a l im i t i n g c u r r e n t u n t i l t h e b e g i n n i n g o f z o n e I I ( E = 0 . 7

V ) . I n t h is s e c o n d z o n e , n o s i g n i f i c a n t i n f l u e n c e o f s t ir r in g o n t h e l i m i t i n g c u r r e n t

r e a c h e d w a s o b s e r v e d . D u r i n g th e r e d u c t i o n c y c le p e a k C . w a s d i s p l a c e d t o w a r d s m o r e

e l e c t r o n e g a t i v e v a l u e s , i n c r e a s i n g th e c u r r e n t d e n s i t y . F ig . 8 b s h o w s t h e s a m e e x p e r i -

m e n t a t 6 8 ° C , w h e n a s l ig h t i n f l u e n c e o f s t i r ri n g o n t h e e l e c t ro c h e m i c a l b e h a v i o u r o f

c h a l c o p y r i t e i s o b s e r v e d .

0 . 0

- 0 . 2

~ ' : : S

0 . 4

p H : 2 . 0 /

E - 0 . 6 t

' ~ '

- 0 . 8

~ C I I I

C

• - 1 . 0 ~

~ p H 1 0

U

- 1 . 4 t

- 1 . 6 ~ ~ - r ~ t ~

- 0 . 6 - 0 . 5 - 0 . 4 - 0 . 3 -0 . 2 - 0 .1 0 . 0

p o t e n t ia l I V A g / A o C I )

Fig. 7. Effect o f pH on the reduction peak CH~ in the cathodic scan from Es,a = 1.0 V at 25°C. v = 5 m V s- J.


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338

C. G6m ez e t a l . / Hydrom etal lurgy 43 1996) 33 1- 34 4

i - - w i t h e t lr r l n g - - - - w i t h

stirring

1 . 0 2 1 ~ .% , A | j i l

3 0 6 8 o C

~ O.g

1 ° 1

0.0 0.0 +

- 2 . 0

- 1 . 0 - 3 . 0

(a ) , ,4 .0

- 0 . 6 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 - 0 .4 - 0 .2 0 . 0 0 .2 0 . 4 0 . 6 0 . 8 1 .0

p o t e n t i a l / V ( A g / A g C I ) p o t e n t i a l / V ( A g / A g C I )

Fig. 8. Effect of electrolyte stirring on voltammogram of the chalcopyrite at (a) 25°C and (b) 68°C. v = 2

mWs- i .

3 .6 . I n f l u e n c e o f C u 2 + a n d F e ~ + a d d i t i o n

I n F i g . 9 a t h e i n f l u e n c e o f t h e a d d i t i o n o f 1 . 0 g • 1 - l o f C u 2 ÷ t o t h e e l e c t r o l y t e a t

2 5 ° C c a n b e o b s e r v e d . T h e a d d i t i o n o f t h is i o n t o t h e e l e c t r o l y t e p r o d u c e d a l a r g e

i n c r e a s e i n th e c u r r e n t d e n s i t y o f t h e w a v e C I i, c o n f i r m i n g t h a t i t d e p e n d s o n t h e c o p p e r

c o n c e n t r a t i o n i n so l u t i o n . F i g . 9 b s h o w s t h e re s u l t o f t h e s a m e s t u d y a t 6 8 ° C a n d , i n t h is

c a s e , a n i n t e n s e c a t h o d i c p e a k C H a p p e a r s , w h i c h i s n o t o b s e r v e d w h e n C u 2 + i s n o t

0 . 0 0 7 , ~ 0 . 0 0 . . . . . . = := : '~ b )

< > I o

m / I I

:

i 1 0 0 ~ _ i'_ : : ° C

- 1 . 2 0 - 3 . 2 0

. . . . . C~

W n h o u t C u 2 +

-1 .4 0 ~ , , 7 l g 'L 'IC u : [ - 4 . 0 0 j - - . , i - - i - 1 g 'L 'IC u 2 +

- 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 - 0 . 3 4 ) .1 0 . 1 0 . 3 0 . 5 0 . 7

p o t e n t i a l I V ( A g / A g C I ) p o t e n t i a l I V ( A g l A g C I )

Fig. 9. Effect of the addition of

Cu 2+

ions on the reduction peak C , in the cathodic scan from Es, a = 1.0 V at

(a) 25°C and (b) 68°C. v = 2 mVs-

1


9/14

C . G Sm e z e t a l . / H y drom e ta l lu rgy 43 1996 ) 331 - 34 4 3 3 9

0 .00

- 0 . 0 5 -

- 0 . t 0 -

~ 41.15 -

C

0

~ .o .2o~

- 0 .25 -

-0 .30 1

-0 .2

-0 ,80

~ / 1 2 0 ~ /

/ 2 5 o c 1 6 0 i / 6 8 o c

- v w ~ o u t F e I I Cl l

. . . . . . W m o u t

Fe + j

2 0 0 . Z ~

- - 1 g-L-1Fe3 + ~ 1 g - L ' l F e 3 +

c . (a ) 1 ( b )

- 2 . 4 0 - ~ - - ~ - - ~ - - - - ~ . . . . . . :r

0 1 0 0 1 2 0 1 4 0 , 6 - 0 . 2 0 . 0 0 . 2 0 1 4 0 . 6

p o t e n t i a l I V A g / A g C I ) p o t e n t l a l I V A g / A g C I )

F i g . 1 0. E f f e c t o f t h e a d d i t io n o f F e 3 + i o n s o n t h e r e d u c t i o n p e a k C ][ ffl_ h e c a t h o d i c s c a n f r o m E s .a = 1.0 V at

( a ) 2 5 ° C a n d ( b ) 6 8 ° C . v = 5 m V s - ] .

pre sen t . F ig . 10 sho ws t he e f f e c t o f add ing 1 .0 g • 1 - ] o f Fe 3+ t o t he so lu t i on a t bo th

2 5 ° C a n d 6 8 ° C . A t t h e l o w e r t e m p e r a t u r e s th e r e w a s a n i n c r e a s e i n c a t h o d i c c o n t r i b u ti o n

o f C ~ a n d C l l . A t 6 8 ° C t h e a d d i t io n o f t h e s a m e a m o u n t o f F e 3 + i n c r e a s e d t h e c u r re n t s

i n v o l v e d a n d s h i f te d th e C . p e a k t o w a r d s m o r e e l e c t r o n e g a t i v e p o t e n t i a l v a l u e s .

3.7. Electrochemical reduction of chalcopyrite

I n t h i s s e r ie s o f e x p e r i m e n t s , t h e c h a l c o p y r i t e e l e c tr o d e u n d e r w e n t p o t e n t ia l s c a n s

f r o m E R up to t he ca thod i c l im i t , Esx , a f t e r wh ich t he d i r ec t i on was changed un t i l t he

0 . 4

0 . 2 ~

~ ' 0 . 0 ~

~ .o.41

= - 0 . 6

- 1 .0 ~

- 1 . 2 ~

- 1 . 4 I

- 0 . 6

A I V A V

2 5 C

( a )

. o =

o l o o . =

p o t e n U a t / V (A g /A gC l )

0 . S

0.0 -1

- 0 . 5

- 1 . 0 -

- t . 5

- 2 . 0

- 2 . 5

- 3 . 0

- 3 . 5

- 4 . 0

-0 .6

6 8 ° C

(b)

- 0 . 4 -O .2 O f 0 . 2

p o t e n t i a l I V (A g /A gC I )

F i g . 1 1 . V o l t a m m o g r a m o f c h a l c o p y r it e a t ( a ) 2 5 °C a n d ( b ) 6 8 °C , v = 5 m V s - ] .


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340 C. Grmez et al. / Hydrometallurgy 43 1996) 331-344

1.8

1 .4

A V I

1.0

A t ~ v / ~ f ~

?

0 , 6

~ 0 . 2

~ - 0 . 2

-o

- 0 . 6

- 1 . 0

- 1 . 4

-1.8

I 1 t I I

- 0 . 6 - 0 . 4 - 0 . 2 0 .0 0 .2 0 .4 0 .6

potenUal I V Ag /AgC I )

Fig. 12. Cyclic voltamm ogram sbetween -0 .5 V and 0.6 V at 25°C. v = 20 mV s- J.

a n o d i c l i m i t (E s, a ) w a s r e a c h e d . T h e e x p e r i m e n t s w e r e c a r r i e d o u t a t 2 5 °C a n d 6 8 ° C .

F i g . 11 s h o w s t h e E / j p r o f i l e s o f c h a l c o p y r i t e a t b o t h t e m p e r a t u r e s f r o m E R t o

Es, c = - 0 . 5 V and t he r eve r se s can u p t o Es , a = i n i ti a l E R . N o te t ha t t he ca thod i c cu r r en t

i n c r e a s e d s h a r p l y a b o v e a p o t e n t i a l o f a r o u n d - 0 . 1 V . T h e a n o d i c c o n t r ib u t i o n s , A j v

a n d A v , a p p e a r in t h e r e v e r s e o x i d a t i o n s c a n o n l y w h e n t h e c a t h o d i c l i m i t is l e s s t h an

- 0 . 4 0 V . T h e p r o f i le E / j a t 6 8 °C ( F i g . 1 l b ) s h o w s a s i m i l a r c a t ho d i c b e h a v i o u r to t h at

o b s e r v e d a t 2 5 ° C ( F i g . l l a ) , a l t h o u g h t h e c u r r e n t d e n s i t i e s w e r e h i g h e r . I n c y c l i c

v o l t a m m e t r y e x p e r i m e n t s a t 2 5 ° C ( F ig . 1 2), w h e n t h e p o t e n t ia l s w e e p s b e t w e e n E s, a ~-

- 0 . 5 V a n d E s . a w e r e c a r r ie d o u t f o r th e l a t te r v a r y i n g b e t w e e n 0 .1 V a n d 0 . 6 V , t h e C I I

p e a k o n l y d e v e l o p e d w h e n E ~ , a w a s g r e a t e r th a n 0 .5 V a n d w h e n t h e a n o d i c p e a k , A l v ,

h a d b e e n p r o d u c e d p r e v i o u s l y ,

4 . D i s c u s s i o n

T h e r e s u l t s c o n f i r m e d t h a t , f o r t h e e x p e r i m e n t a l c o n d i t i o n s m e n t i o n e d a b o v e , t w o

t y p e s o f c h a l c o p y r i t e o x i d a t i o n p r o c e s s e s o c c u r r e d a t 2 5 ° C a n d 6 8 ° C , d e p e n d i n g o n t h e

a n o d i c p o t en t i al : z o n e I ( E < 0 . 7 V ) a n d z o n e I I ( E > 0 .7 V ) . O v e r a l o w r a n g e o f

po t en t i a l s and a t 25° C (F ig . l a ) , t he r e spo nse w as s imi l a r to t ha t o f me t a l s du r ing t he

f o r m a t i o n o f p a s s i v e f i lm s o n t h e i r s u rf a c e ( A I ) , w h i c h i n c r e a s e s w i th t h e p o t e n t ia l . A t

6 8 ° C ( F i g . l b ) , i n t h e s a m e z o n e , a d i f f u s i o n a l l i m i t i n g c u r r e n t a p p e a r e d b e c a u s e t h e

f i l m h i n d e r e d t h e p a s s a g e o f i o n s f r o m t h e c h a l c o p y r i t e s u r f a c e t o t h e s o l u t i o n . T h i s

p h e n o m e n o n m i g h t b e d u e t o a l o w e r d e g r e e o f h y d r at io n o f th e e l e c t ro f o r m e d f i l m a t

t h i s t e m p e r a t u r e . T h e o n l y v a r i a b l e w h i c h h a d a n i n f l u e n c e i n z o n e I a t 2 5 ° C w a s

e l ec t ro ly t e s t i r r i ng (F ig . 8a ) , i n t he absence o f wh ich t he e l ec t rode ox ida t i on was

c o n t r o l l e d o h m i c a l l y b y t h e p h e n o m e n o n o f f i l m g r o w t h . S t i r r i n g r e s u l t e d i n a l i m i t i n g

cu r r en t be ing r eached , wh ich con f i rmed t he d i f fu s iona l con t ro l i n t he f i lm .


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C. G6mez et al. / Hydrornetallurgy 43 1996) 331-344 341

I n a n a t t e m p t t o e x p l a i n t h e f o r m a t i o n o f t h i s f i l m i n t h e prewave or passive zone

( z o n e I ) a t E < 0 . 7 V t h e c h a l c o p y r i t e o x i d a t io n m e c h a n i s m h a s b e e n e x t e n s i v e l y

s t u d i e d b y s e v e r a l i n v e s t i g a t o r s . B i e g l e r a n d H o m e [ 1 6 ] , f o r e x a m p l e , u s e d c y c l i c

v o l t a m m e t r y t o p r o p o s e t h e f o l l o w i n g r e a c t i o n :

Cu Fe S 2 ---* 0 .75 Cu S + 0 .25 Cu 2+ + Fe z+ + 1 .25 S o + 2 .5 e - ( 3 )

w h e r e C u S ( c o v e l l i n e ) a n d S o w o u l d b e i n a r a ti o o f 3 / 5 a n d F e ( I I ) / C u ( I I ) i n th e

s o l u t i o n w a s 4 / 1 .

S t a n k o v i c [ 1 2 ] , u s i n g g a l v a n o s t a t i c p u l s e c h r o n o p o t e n t i o m e t r i c t e c h n i q u e s i n s o l u -

t io n s o f F e U D a n d C u ( I I ) , p r o p o s e d a t w o - s t a g e o x i d a t i o n p r o c e s s . T h e f i r s t s t a g e w o u l d

invo lve t he l i be r a t i on o f Cu 2+ an d ac t a s the l im i t i ng s t ep o f the r eac t i on r a t e , whe rea s

t h e s e c o n d w o u l d i n v o l v e t h e l i b e r a ti o n o f F e 2 + , in a c c o r d a n c e w i t h t h e f o l l o w i n g

s c h e m e :

nC uFeS 2 ~ Cu 2+ + Cu n_ iFenS2n q-- 2 e - ( s l o w ) (4 )

Cu n_ iFenS2n --* Fe 3+ + C u n_ iFen - iS2n q'- 3 e - ( 5 )

A c c o r d i n g t o t h is s c h e m e , t h e d i s s o l u ti o n o f c h a l c o p y r i t e w o u l d l e a d t o t h e f o r m a t i o n o f

a c o p p e r a n d i r o n p o l y s u l p h i d e :

nC uFeS 2 ~ Cu 2+ + Fe 3+ + Cu n_ iFen_ iS2n + 5 e - ( 6 )

H o l l i d a y a n d R i c h m o n d [ 1 7 ] , o n t h e o t h e r h a n d , h a v e s u g g e s t e d t h a t t h e a n o d i c

d i s s o l u ti o n o f c h a l c o p y r i t e in z o n e I o c c u r s s e q u e n t ia l l y a c c o r d i n g t o E q s . ( 7 ) - ( 9 ) , t h e

d e t e r m i n i n g s t e p b e i n g E q . ( 8 ) :

CuF2S 2 ~ C u 2 + -['-F e S 2 + 2 e -

Cu 2+ _ ... ) x / " , . 2+ x~ f '~ . 2+

~ t l ( a d s ) " l- ( 1 - - / ~ U ( a q )

2 +

Cu~aos + FeS 2 ~ C uS + Fe 2+ + S o

(7)

(8 )

(9)

A n y o f t h e a b o v e s o l u t i o n s c o u l d e x p l a i n t h e r e s u lt s w e o b t a i n e d i n t h is s t u d y , i n

wh ich we conc lude t ha t Fe i s d i s so lved i n p r e f e r ence t o Cu [16 ,17 ] ; and t ha t t he

c h a l c o p y r i t e s u r f a c e i s c o a t e d w i t h a l a y e r w h i c h d e c r e a s e s t h e s p e e d o f e l e c t r o n t r a n s f e r

t o ox idan t s , such a s Fe 3÷ , and i on t r anspo r t . The add i t i on o f C u 2÷ and Fe 3÷ t o t he

e l e c t r o l y t e a t 2 5 ° C a n d 6 8 ° C , i n o u r e x p e r i m e n t s , c o n f i r m e d t h e e l e c t r o n c o n d u c t i n g

c h a r a c te r i st i c o f t h e f i l m f o r m e d a t l o w p o t e n t i a l s; s in c e , i n t h e r e s p e c t i v e s c a n s t o w a r d s

m o r e e l e c t r o n e g a t i v e p o t e n t i a l s f r o m E s, a = 1 .0 V , t h e c u r r e n t i n c r e a s e d f o r p e a k s C ,

and C~ (F ig s . 9 and 10 ) , wh ich we re r e l a t ed t o r eac t i ons whe re coppe r and i r on ,

r e s p e c t i v e l y , t o o k p a r t .

T h e i n f lu e n c e o f th e s w e e p r a t e a t h i g h t e m p e r a t u r e s in t h i s p o te n t i a l z o n e a n d t h e

l i nea r r e l a t i onsh ip be tw een t he l im i t i ng cu r r en t ( JL ) and v 1 /2 (F ig . 3 ) i nd i ca t e s t ha t i n

t h i s z o n e a n d a t 6 8 ° C a f i l m w a s f o r m e d w h i c h l i m i t e d t h e o x i d a t i o n r e a c t i o n s w i t h t h e

m e d i u m b y s o l i d s t a t e d i f f u s i o n .

A t 68° C , t he pH p l ay ed a r o l e i n the pa s s ive zon e (F ig . 5b ) and i n t he po t en t i a l (E b )

a t w h i c h m a s s i v e d i s s o l u t io n o f c h a l c o p y r i t e s t a rt e d t o b e o b s e r v e d ( F i g . 6 ) , i n d i c a t in g

t h a t m i n e r a l e l e c t r o o x i d a t i o n w a s h a m p e r e d b y i n c r e a s i n g a c i d i t y .


12/14

342

C. Gdmez et al. / Hydrometallurgy 43 1996) 331-344

I n zone I I , o r t he t r ans pas s i ve ox i da t i on zone , i n wh i ch t he cu r r en t i nc r ea s ed

s ubs t an t i a ll y ( F i g . 1 ), the cha l co pyr i t e beh ave d i n a s i m i l a r way t o o t he r s u l ph i de

m i ne r a l s , s uch a s py r i t e and a r s en opyr i t e [ 18 ], wi t h S o and SO ~- be i ng p r od uced , i n

acco r dance wi t h Eq . ( 1 ) and Eq . ( 2 ) . Th i s l a r ge i nc r ea s e i n cu r r en t wi t h an i nc r ea s e i n

t e m p e r a t u r e c o u l d b e e x p l a i n e d b y t h e i n c r e a s e d i o n i c c o n d u c t i v i t y at t h e c h a l c o p y r i t e -

f i l m - s o l u t i o n i n t e rf a c e , w h i ch w o u l d f a v o u r t he a n o d i c d i s s o l u ti o n o f t h e m i n e ra l . T h e

m a x i m u m v a l u e o f c u r r e n t d e n si ty i n z o n e I I d i d n o t d e p e n d o n e l e c t r o l y te s ti rr in g ( F i g .

8 ) , and t hus we can conc l ude t ha t , du r i ng t he r eac t i on , a l aye r , pos s i b l y e l em en t a l

s u l phur , i s ads o r bed on t he e l ec t r ode l aye r .

I n t he r eve r s e s can , t he i n f l uence o f s t ir r ing , t he s w eep r a t e and t he add i t i on o f i on i c

s pec i e s r e l a t ed t o t he e l ec t r ode ' s na t u r e , m ade pos s i b l e cha r ac t e r i za t i on o f t he r eac t i ons

i nvo l ved i n t he ca t hod i c con t r i bu t i ons o f C I , CII and C m . The add i t i on o f Fe 3÷

f und am en t a l l y m od i f i ed t he zone o f C~ and CI~ (F i g . 10 ), w h i ch w ou l d be r e l a t ed t o the

r educ t i on r eac t i on :

F e 3 + + l e - ~ F e 2÷ E ' = 0 . 4 V ( 1 0 )

T h i s r e a c t i o n c o u l d o n l y b e d e t e c t e d b e c a u s e i t t o o k p l a c e o n a n e l e c t r o c h e m i c a l l y

o x i d i z e d c h a l c o p y r i t e . O n a f r e s h l y p r e p a r e d s u r f a c e o f c h a l c o p y r i t e t h e r e w a s n o

r eac t i on , due t o t he s l ow k i ne t i c s and t he i r r eve r s i b i li t y o f t he Fe 2 + / F e 3÷ co up l e on t h i s

m i ne r a l [10 ]. K ucek i [ 19 ] a l s o reg i s t e r ed t h is r e ac t i on w hen t he cha l cop yr i t e was

a n o d i c a l l y o x i d i z e d u p t o 1 .4 V v s S H E ( - - 1 .2 V v s A g / A g C 1 ) . A s e x p l a i n e d b e f o r e ,

t he C H peak w as r e l a t ed t o C u 2+ i ons , a c co r d i n g t o Eqs . ( 11 ) and ( 12 ) , and n o t t o t he

Fe 3÷ co ncen t r a t i on . How eve r , t he i nc r ea s e i n cu r r en t dens i t y obs e r ved i n t he CII peak

( F i g . 10 ) cou l d be ex p l a i ned by t he f ac t t ha t Fe 3+ was ab l e t o ox i d i ze t he CuS f i lm

f o r m ed on t he cha l co pyr i t e s u r f ace in t he p r ev i o us ano d i c s can and t o p r odu ce Cu 2+

i ons , v i a t he r eac t i on in Eq . ( 3 ) , wh i ch i nc r ea s e s the cu r r en t dens i t y i n th i s C , peak .

A t 2 5 ° C , t h e C H p e a k w a s m o r e p r o n o u n c e d i n t h e c a t h o d i c s c a n , p r o b a b l y d u e t o t h e

s o l i d p r oduc t s f o r m e d on t he ch a l copy r i t e du r i ng t he ano d i c s can t o 1 .0 V ( F i g . l a )

( r eac t i ons i n Eqs . ( 3 ) , ( 6 ) - ( 9 ) ) , a l t hough t h i s was m or e d i f f i cu l t t o obs e r ve a t the h i ghes t

t em pe r a t u r e . M or eo ve r , t he l i nea r dep end ence o f t he he i gh t o f peak Cxl ( I p ) ve r s us v 1 /2

and o f the po t en t i a l ( Ep ) ve r s us l og v t / 2 ( F i g . 4 ) , a s w e l l a s t he depen denc e o f th i s peak

on s t ir r ing o f t he s o l u t i on (F i g . 8a ) , i nd i ca t e the e l ec t r o f o r m a t i on o f t he t h r ee - d i m en -

s i ona l t h i ck Cu2S(s ) l aye r und e r oh m i c con t r o l , a cc o r d i ng to :

S O+ C u 2÷ + 2 e - ~ C u S ( 1 1 )

C u S + C u 2+ + 2 e - ~ C u 2 S + H 2 0 ( 1 2 )

S t i r r ing t he s o l u t i on , coppe r i on add i t i on and pH had an i n f l uenc e i n t he ca t hod i c

zone t h r ough t he r eac t i ons i n Eq . ( 13 ) and Eq . ( 14 ) , t r ans po r t i ng t he e l ec t r oac t i ve

s pec i e s i n t he s o l u t i on ( Cu 2÷ and H ÷ ) and c on t r i bu t i ng t o t he f o r m a t i on o f t he Cu 2S

acco r d i ng t o :

2 C u 2 + H 2 S + 2 e - ~ C u 2 S + 2 H ÷ ( 1 3 )

Ho l l i day [ 17 ] and Tor m a [ 13 ] r eached s i m i l a r conc l us i ons .


13/14

C. Gdmez et al. / Hydrometallurgy 43 1996) 331-344 343

T h e p H a l s o i n f l u e n c e d t h e C

| If peak

a n d t h e r e i s a c l e a r c o r r e l a t io n b e t w e e n a c i d i ty

a n d t h e p o t e n t i a l a t w h i c h t h e r e a c t i o n C ill b e g a n , s o t h a t w h e n p H d e c r e a s e d f r o m 2 t o

1 , t h e s t a r t in g p o t e n t i a l w a s d i s p l a c e d t o w a r d s m o r e e l e c t r o p o s i t i v e v a l u e s ( F i g . 7 ) . P e a k

f i l l , t herefo re ,

m u s t h a v e b e e n p r o d u c e d b y t h e f o l l o w i n g r e a c t i o n s :

2 Cu S + 2H + + 2 e - ~ C u 2S + H 2 S

S o + 2 H + + 2 e - ~ H 2 S

( 1 4 )

( 1 5 )

i n w h i c h t h e E - p H d e p e n d e n c e c a n b e e x p r e s s e d as : E = E ' - 0 . 0 5 9 . p H .

T h e e l e c t r o c h e m i c a l b e h a v i o u r o f t h e c h a l c o p y r i t e d u r i n g a c a t h o d i c s c a n f r o m E R t o

E s. c = - 0 . 5 V ( F i g s . 11 a n d 1 2 ) m i g h t b e a t t ri b u t e d t o t h e p r o g r e s s i v e t r a n s f o r m a t i o n o f

t he ch a l cop y r i t e i n to C u 2 S and Cu 1.9 S [20 ]. Such a t r an s fo rm a t ion wo u ld occ u r t h rough

a s e r i es o f i n te r m e d i a t e s t e p s i n v o l v i n g t h e g e n e r a t i o n o f C u x F e y S z s u lp h i d e s , s u c h a s

C u 9 F e 8 S 4

a n d C u s F e S 4 , F e 2 + , C u 2 + a n d H 2 S . T h e f o r m a t i o n o f m e t a l l ic c o p p e r i s

a n o t h e r p o ss i b il it y [2 1] ( E ° C u 2 + / C u ° = 3 4 0 m V S H E ; + 1 33 m V A g / A g C 1 ) . N o t e

tha t t he a nod i c pea ks A w and A v (F ig . 11 ) and A v l (F ig . 12 ) appea r wh en Es , c

< - 0 . 4 0 V , p r o b a b l y d u e t o t h e f o l lo w i n g r e ac t io n s :

( 1 6 )

C u ° + H 2 S ~ C u 2 S + 2 H ÷ + 2 e -

C u 2 S ~ C u 2 _ x -q- x C u 2 + + 2 x e - ( 1 7 )

(18)

u 2 _ x S ~ C u S + (1 - x ) C u 2 + + 2 ( 1 - x ) e -

W h a t e v e r t h e c a s e , a t b o t h h i g h a n d l o w t e m p e r a t u r e s t h e c h a l c o p y r i t e e l e c t r o d e i s

t r a n s f o r m e d i n t o a C u - r i c h a n d F e - p o o r p h a s e a t E < E R , s o t h a t a t - 0 . 1 V t h e

c h a l c o p y r i t e n o l o n g e r e x i s t s a s a m i n e r a l p h a s e o n t h e e l e c t r o d e s u r f a c e .

5 . C o n c l u s i o n s

T h e r e s u l t s s h o w t h a t t h e c h a l c o p y r i t e i n a n e l e c t r o l y t e o f s i m i l a r c o m p o s i t i o n t o o n e

u s e d i n a c u lt u r e o f m e s o p h i l i c a n d t h e r m o p h i l i c b a c t e r i a b e h a v e s i n a s im i l a r m a n n e r t o

t h e o n e d e s c r i b e d b y o t h e r a u t h o r s i n a s u l p h u r i c a c i d m e d i u m . T h e d i f f e r e n c e s i n t h e

r e s p o n s e a t 2 5 ° C a n d 6 8 ° C a r e o n l y d u e t o t h e d i f f e r e n t p h y s i c a l s t r u c t u r e o f t h e

c o m p l e x f i l m s o f s u l p h i d e s , p o l y s u l p h i d e s a n d e l e m e n t a l s u l p h u r , w h i c h a r e f o r m e d

e l e c t r o c h e m i c a l l y .

A c k n o w l e d g e m e n t s

T h i s s t u d y w a s f u n d e d b y M E C : I n te r m i n is t e ri a l C o m m i s s i o n o f S c i e n c e a n d

T e c n o l o g y a n d P r o g r a m m e f o r S c i e n t i f i c C o o p e r a t i o n w i t h S o u t h A m e r i c a , i n c o n j u n c -

t i on w i th t he Pon t i f i c i a Un ive r s idad Ca t61 i ca de Ch i l e .


14/14

344 C . Gdm ez e t a l . / Hydrom etal lurgy 43 1996) 33 1-3 44

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