ELSEVIER Hydrometallurgy 43 (1996) 331-344

hydrometallurgy

Electrochemistry of chalcopyrite C. G6mez a,*, M. Figueroa b,1, j. Mufioz a, M.L. Blfizquez a,

A. Ballester a a Departamento de Ciencia de Materiales e lngenierla Metal(~rgica, Facultad de Ciencias Qulmicas,

Universidad Complutense de Madrid, 28040 Madrid, Spain b Departamento de Qulmica lnorg6nica, Facultad de Qulmica, Pontificia Universidad Cat61ica de Chile,

Casilla 306, correo 22, Santiago, Chile

Received 30 November 1995; accepted 21 January 1996

Abstract

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 p H = 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 (Thiobacillus ferrooxidans) and thermophilic (Sulfolobus) 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. Introduction

Chalcopyrite (CuFeS 2) is the most abundant ore of the sulphide minerals of copper. However, this mineral is the most recalcitrant to hydrometallurgical processes. For this reason pyrometallurgical processes involving the smelting of concentrates [ 1 ] are still the most effective way of extracting the copper from this mineral. Unfortunately, the extraction processes present serious pollution problems, due to SO 2 emissions during

* Corresponding author. Fax: + 34 1 394 43 57. E-mail: cgcastro@eucmos.sim.ucm.es i Fax: 56 2 552 56 92. E-mail: mfiguero@lascar.puc.cl

0304-386X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0304-3 86X(96)00010- 2

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

pyrometallurgical reactions. The strict environmental restrictions imposed on such operations involve a great deal of investment, which increases productions costs. To tackle such problems, hydrometallurgy, which is less contaminating [2,3], offers cheaper ways to extract the copper, despite the difficulties of this processes even in strongly oxidant media [4].

Among the hydrometallurgical processes, one possible economically profitable pro- cess could be to dissolve the copper by bioleaching, that is to say, to use microorgan- isms that catalyse the mineral dissolution. Such microorganisms are usually classified according to the temperatures they need to grow, so that the so-called mesophilic microorganisms ( Thiobacillus ferrooxidans, Thiobacillus thiooxidans, Leptospirilum fer- rooxidans, etc.) develop at ambient temperatures, whereas thermophilic microorganisms (Sulfolobus) need higher temperatures.

The main disadvantage of using bioleaching treatments with sulphide minerals is the slow dissolution kinetics and, in the case of chalcopyrite, the low yields, which are attributed to the formation of a solid reaction product on the surface [5]. This layer would act as a diffusional barrier and hinder contact between the leaching solution and the chaicopyrite, thus slowing down any subsequent dissolution.

Previous bioleaching experiments carried out with thermophilic microorganisms (principally Sulfulobus) reported in the literature [6,7] report higher dissolution rates and better yields than using mesophilic microorganisms (principally Thiobacillus ferrooxi- dans) in the leaching of the chalcopyrite.

It has been demonstrated that chalcopyrite and most other metallic sulphides are dissolved by means of an electrochemical mechanism [8]. Furthermore, in the literature there are a large number of studies which explain the anodic dissolution of chalcopyrite in different media [9-14]; however we have found no information on the experimental conditions for growing microorganisms (temperature and culture medium). For this reason, the aim of this work is to study the electrochemical response of a massive chalcopyrite electrode in a bacterial culture medium at two temperatures: 25°C (at which temperature mesophilic microorganisms grow) and 68°C (the optimum growth tempera- ture of Sulfolobus) in order to understand the differences in the dissolution kinetic and nature of the products formed during the bioleaching process using both kinds of microorganisms.

2. Experimental

Measurements were made with a conventional three-compartment glass electrolysis cell to lodge the working electrode (specimen), the counter electrode (platinum spiral wire) and the reference electrode (Ag/AgC1), which was provided with a Luggin-Haber capillary tip. The cell was kept at constant temperature by connecting it to a circulating, thermostatically controlled water loop. The temperature was fixed at 25 ° or 68 ° + 0.1°C. Each working electrode was prepared from a natural specimen of massive chalcopyrite from Transvaal, South Africa (35.3% Cu, 30.1% Fe and 34.7% S).

The specimens were cube shaped (-- 4 -6 mm edge) and were connected to a copper wire by means of a conductive silver resin. Finally, sample and wire were embedded in

C. Gdmez et a l . / Hydrometallurgy 43 (1996) 331-344 333

an epoxy resin to obtain areas of 0.4-0.5 cm 2 exposed to the electrolyte. Before each test, the working electrode surfaces were polished with 600 silicon carbide paper, followed by ultrasonic cleaning and rinsing with distilled water. The electrolyte compo- sition, prepared from A.R. chemicals and distilled water, was as follows: 0.4 g . 1- (NH4)2SO4, 0.5 g" 1-1 MgSO 4 • 7 H20, 0.2 g" 1-l K2HPO4" The pH was kept at 2.0 by the addition of H2SO 4. Before each experiment was performed, 0.13 1 of this electrolyte was purged inside the cell with nitrogen (99.95%) for 15 min. After that, and before starting each test, electrode and electrolyte were purged again for 20 min.

The electrochemical experiments were carried out at constant or programmed poten- tials using a Wenking potentiostat (Model LB 81 M) and a Wenking voltage scanner (Model MVS 87). Data were acquired by an analogue board installed in a PC. When necessary, stirring was accomplished with a magnetic stirrer, the working electrode remaining stationary. Potentials in the text refer to the Ag/AgC1 electrode ( + 0.207 vs SHE at 25°C). Two types of electrochemical experiments were performed, potentiody- namic polarization curves and cyclic voltammetry. These curves were initiated from the rest potential unless otherwise stated.

3. Results

3.1. E / j curve at 25°C

Polarization curves (apparent current density, j, versus potential, E) were obtained by applying to the chalcopyrite electrodes a single triangular potential sweep (STPS) between the rest potential (E R) and the anodic (Es, a) switching potential at three different sweep rates (v): 20, 5 and 2 mVs -~. Immediately after the anodic scan a cathodic scan was carried out from the Es, a to the E~. c (cathodic switching potential).

Fig. l a shows the initial STPS at 2 mVs- i between E R and Es. a at 25°C, which presents an anodic current response which can be arbitrarily divided in two potential zones for the discussion of the results. Zone I ranges from E R to 0.7 V potential range and zone II from 0.7 V to higher potentials. The current response in zone I presents a small broad peak (A1), located at about 0.4 V. In zone II, a large increase in current was recorded (All) and a limiting current close to 0.85 V was reached (Am). The switching potential scan from Es. a presents three cathodic peaks at 0.4 V (CI), at 0.2V (Cll) and a large peak starting at - 0 . 1 5 V (Clll).

This electrochemical behaviour of the chalcopyrite in the nutrient medium used as electrolyte at 25°C was similar to that described by Warren et al. [11] for the same mineral electrodes in the presence of H2SO 4. Thus, A l represents the 'prewave' , in which chalcopyrite is transformed to CuS, through an intermediate non-stochiometric phase (Cu~_ x Fel_y S 2_ z), producing S O and Cu(II) and Fe(II) ions. In this zone only a small portion of the mineral is probably transformed. In region II, at potentials more positive than 0.7 V (transpassive zone), the overall dissolution of chalcopyrite takes place through the following reactions:

CuFeS 2 ~ Cu 2÷ + Fe 3÷ + 2S ° + 5e- (1)

CuFeS 2 + 8H20 ~ Cu 2÷ + Fe 3÷ + 2SO~- + 16H÷+ 17e- (2)

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

3.2. Influence of temperature on E / j profile

Similar tests to those carried out on the chaicopyrite at 25°C were performed at 68°C. As in the low temperature experiment, zones I and II are also present in the potentiody- namic curve (Fig. lb). However, zone I is moved towards more positive potentials at highest temperature and peaks C I and CI[, produced in a sweep from Es. a = 1.0 to electronegative potentials and observed at 25°C (Fig. la), did not seem to appear at 68°C, although they became evident when the scale was suitably increased (insert, Fig. lb).

3.3. Influence of sweep rate (v)

Fig. 2 shows the influence of sweep rate at 25°C (Fig. 2a) and 68°C (Fig. 2b). At the lowest temperature, chalcopyrite oxidation did not depend on v, but at the highest temperature there was a linear relationship between the limiting current density (JL) and vl/2 (Fig. 3).

Fig. 4 shows the dependencies of maximum peak intensity (I v) and its corresponding potential (Ep) with v 1/2 and log v 1/2 at 25°C for the Cll peak. As can be seen, Ip was a linear function of v ]/2, which indicated that the electroactive substance was dissolved in the eleca'olyte and reached the interface by diffusion. Moreover, the fact that Ep was dependent on the log v 1/2 showed that this reaction was not rapid [15]. The small current densities detected in the CI! peak at 68°C did not allow similar analyses.

3.4. Influence of pH on the E / j profile

Fig. 5 shows the influence of pH on the anodic potentiodynamics of chalcopyrite at 25°C and 68°C, respectively. The pH has no effect at the lower temperature, but a clear

~,,

E ?

E ,,...

I

0 " 0

,,,i 0

2.0

1.5

1.0

0,5

0.0

-0.5

-1 .0

-1.5

3.0 /

2. I " 0-' 0:2 o:, * . , :

0.0 " . . . .

-1.0 / ~ ~ - - - ~ ~ -2.0 III -3.0

(a) -4 0 v (b) I ' l r I 'T~I ' I '1 '1 '1'1' I '1 ' I I ' l ' ' J ' l ' l '1 ' [ '1 '1 '1 ' t ' i ' I rT ' l ' l r~

-0.6,0.4-0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 potentlal I V (Ag/AgCl) potentlal I V (Ag/AgCl)

Fig. 1. Voltammogram of chalcopyrite at (a) 25°C and (b) 68°C. v = 2 mVs i. Detail of current peaks C l and C . at the highest temperature are given in the insert.

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

2.0

1.8

~. 1.6 E ? 1.4 < E 1.2

-~ 1.0 C 0 -o 0.8

~ 0.6

u 0.4

0.2

0.0

26oc

20 mvs -1 . . . . . . . . 6 mvs "1

. . . . . 2 mvs "1 ///

I : //

5.° I 4.0 68°C

I - - 8 m V s " 1

3.0 ~

t

1.0

/ I ' [

mV. 1 / / ;;/

2 mvs "1 I; i'

(a) 0.0 _L~ , , E (b) b ' ' ' I

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0A 0.6 0.8 1.0 potential I V (Ag/AgCi) potent ial I V (Ag/AgCI)

Fig. 2. Effect of sweep rate (v) on anodic potentiodynamic curves of chalcopyrite at: (a) 25°C and (b) 68°C.

influence at 68°C. Fig. 6 shows the potential at which the massive dissolution of the chalcopyri te began (E b) as a function of the pH at this temperature, the one becoming more electroposit ive as the other decreased, which means that an increase in acidity

hampered mineral electrooxidation. Fig. 7 shows the influence of pH on the C,1 reduction peak when the scan was

reversed from Es, a towards more electronegative values at 25°C. A shift of 0.15 V in the initial potential of the C.~ reaction towards the more electroposit ive zone for a decrease

of 1 pH unit was observed.

1.4

~. 1.2

9

E 1.0 /

,~ 0.8

u 0.6

o

4 6 8 10 12 14 16 v 1/2 102 1 (Vs ' t ) 1/2

Fig. 3. Limiting current density (JL) of zone I versus v 1/2 at 68°C.

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

.< E

-1.80 0.25

0.20

0.15

0.10

3

log (v) 1/2 -1.20 -0.80

I ~ ~ 0.188 L I ~__ ~- 0.186

~ 0.182 ~

0.180 ~

0 .1 7 8

I ~ I 0.176 6 0 12 15 v 1/2 1021 (Vs'I) 112

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

3.5. Influence o f the electrolyte stirring on the E / j profile

At 25°C e lec t ro ly te st irr ing inf luenced both the anodic po ten t iodynamic curve and the inverse reduct ion cycle of the cha lcopyr i t e , Fig. 8a. In zone I the current ini t ia l ly increases l inear ly when the potent ia l is increased f rom 0.25 to 0.40 V, al though the s lope

1.8 5.0 pH 1.0r - -

1.6 25°C / / p H 1 .I 68 °C ~ ,

1.4 SI/pH 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 OAt 0.6 0.8 1.0 1.2 0,0 0.2 0.4 0.6 0.8 1.0 potential ; V (Ag/AgCI) potential I V (Ag/AgCI)

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

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

2.0

1.6

\ \

\

!

1 .0

I

0.45 0.50

\ \ \

©

0.55 0.60 0.65 0.70

E b / V (Ag/AgCI)

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

then changes and tends towards a l imi t ing current unt i l the beg inn ing of zone II (E = 0.7

V). In this second zone, no s ignif icant inf luence of stirring on the l imi t ing current reached was observed. Dur ing the reduct ion cycle peak C . was displaced towards more electronegat ive values, increasing the current density. Fig. 8b shows the same experi-

men t at 68°C, when a slight inf luence of stirring on the electrochemical behaviour of chalcopyri te is observed.

0 . 0

- 0 . 2

~' :: S -0.4

pH:2.0 / E -0.6 t

'~' -0.8 "~ CIII C 0

• o -1.0 ~ ~ pH 1 0

U

-1.4 t

- 1 . 6 ~ ~ - r ~ t ~

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 potential I V (Ag/AoCI)

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

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

i - - with etlrrlng -- -- with stirring

1.0 21~.%, A | j i ! l 3.0 68oC

~ 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 potential / V (Ag/AgCI) potential / V (Ag/AgCI)

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 uence o f Cu 2 + a n d F e ~ + addi t ion

In Fig. 9a the inf luence o f the addi t ion o f 1.0 g • 1- l o f Cu 2÷ to the e lect rolyte at

25°C can be observed. The addi t ion o f this ion to the e lec t ro ly te p roduced a large

increase in the current densi ty o f the w a v e C Ii, conf i rming that it depends on the copper

concentra t ion in solution. Fig. 9b shows the result o f the same study at 68°C and, in this

case, an intense ca thodic peak C H appears, which is not observed w h e n Cu 2+ is not

0.00 7 , ~ 0.00 . . . . . . =:=:'~ (b) <.> I of

m / I I"" ':

i "1"00~ / _i'_ : : " ° C -1.20 -3.20

..... C ~ Wnhout Cu 2+

-1.40~ , , 7 l g'L'ICu: [ -4.00j--. , i--i- 1 g'L'ICu2+ -0.2 0.0 0.2 0.4 0.6 -0.3 4).1 0.1 0.3 0.5 0.7

potential I V (Ag/AgCI) potential I V (AglAgCI)

Fig. 9. Effect of the addition of Cu 2+ ions on the reduction peak C, in the cathodic scan from E s , a = 1.0 V at (a) 25°C and (b) 68°C. v = 2 mVs- 1

C. GSmez et a l . / Hydrometallurgy 43 (1996) 331-344 339

0.00

-0.05 -

-0.t0 -

~ 41.15 - C 0

~ .o.2o ~

-0.25 -

-0.30 1 -0.2

-0,80 1 ~ / 120 ~' /

/ 25oc "1"60 i / 68oc - vw~out Fe 3+ I I Cll . . . . . . Wmout Fe 3+ j

200 . Z ~ - - 1 g-L-1Fe 3+ " " ~ 1 g-L'lFe 3+

c . (a) 1 (b)! -2.40 - ~ - - ~ - - ~ - - - - ~ ...... :r

010 012 014 0,6 -0.2 0.0 0.2 014 0.6 potential I V (Ag/AgCI) potentlal I V (Ag/AgCI)

Fig. 10. Effect of the addition of Fe 3 + ions on the reduction peak C ][ ffl_ the cathodic scan from Es. a = 1.0 V at (a) 25°C and (b) 68°C. v = 5 m V s - ].

present. Fig. 10 shows the effect of adding 1.0 g • 1- ] of Fe 3+ to the solution at both 25°C and 68°C. At the lower temperatures there was an increase in cathodic contribution of C~ and Cll. At 68°C the addition of the same amount of Fe 3+ increased the currents involved and shifted the C . peak towards more electronegative potential values.

3.7. Electrochemical reduction of chalcopyrite

In this series of experiments, the chalcopyrite electrode underwent potential scans from E R up to the cathodic limit, Esx, after which the direction was changed until the

0.4

0.2~

~' 0 .0~

~ .o.41 - = -0.6

-1.0 ~

-1.2 ~

-1.4 I -0.6

AIV AV

250C (a)

.o'= olo o.= potenUat / V (Ag/AgCl)

0.S

0.0 -1

-0.5

-1.0 -

-t .5

-2.0

- 2 . 5

-3 .0

- 3 . 5

-4.0 -0.6

68 ° C

(b)

-0'.4 -O'.2 Of 0 0.2 potential I V (Ag/AgCI)

Fig. 11. Voltammogram of chalcopyrite at (a) 25°C and (b) 68°C, v = 5 m V s - ].

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

1.8

1.4 AVI 1.0 At~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/AgCI)

Fig. 12. Cyclic voltammograms between -0.5 V and 0.6 V at 25°C. v = 20 mVs- J.

anodic limit (Es, a) was reached. The experiments were carried out at 25°C and 68°C. Fig. 11 shows the E / j profiles of chalcopyrite at both temperatures from E R to

Es, c = - 0 . 5 V and the reverse scan up to Es, a = initial E R. Note that the cathodic current increased sharply above a potential of around -0 .1 V. The anodic contributions, Ajv and A v, appear in the reverse oxidation scan only when the cathodic limit is less than - 0 . 4 0 V. The profile E / j at 68°C (Fig. 1 lb) shows a similar cathodic behaviour to that observed at 25°C (Fig. l la) , although the current densities were higher. In cyclic voltammetry experiments at 25°C (Fig. 12), when the potential sweeps between Es, a ~- - 0 . 5 V and Es. a were carried out for the latter varying between 0.1 V and 0.6 V, the CII peak only developed when E~, a was greater than 0.5 V and when the anodic peak, Alv, had been produced previously,

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

The results confirmed that, for the experimental conditions mentioned above, two types of chalcopyrite oxidation processes occurred at 25°C and 68°C, depending on the anodic potential: zone I (E < 0.7 V) and zone II (E > 0.7 V). Over a low range of potentials and at 25°C (Fig. la), the response was similar to that of metals during the formation of passive films on their surface (AI), which increases with the potential. At 68°C (Fig. lb), in the same zone, a diffusional limiting current appeared because the film hindered the passage of ions from the chalcopyrite surface to the solution. This phenomenon might be due to a lower degree of hydration of the electroformed film at this temperature. The only variable which had an influence in zone I at 25°C was electrolyte stirring (Fig. 8a), in the absence of which the electrode oxidation was controlled ohmically by the phenomenon of film growth. Stirring resulted in a limiting current being reached, which confirmed the diffusional control in the film.

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

In an attempt to explain the formation of this film in the prewave or passive zone

(zone I) at E < 0.7 V the chalcopyrite oxidation mechanism has been extensively studied by several investigators. Biegler and Home [16], for example, used cyclic voltammetry to propose the following reaction:

CuFe S 2 ---* 0.75 CuS + 0.25 Cu 2+ + Fe z+ + 1.25 S o + 2.5 e - (3)

where CuS (covelline) and S o would be in a ratio of 3 / 5 and Fe (II) /Cu(II) in the solution was 4 /1 .

Stankovic [12], using galvanostatic pulse chronopotentiometric techniques in solu- tions of FeUD and Cu(II), proposed a two-stage oxidation process. The first stage would involve the liberation of Cu 2+ and act as the limiting step of the reaction rate, whereas the second would involve the liberation of Fe 2+, in accordance with the following scheme:

nCuFeS 2 ~ Cu 2+ + Cu n_ iFenS2n q-- 2e- (slow) (4)

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

According to this scheme, the dissolution of chalcopyrite would lead to the formation of a copper and iron polysulphide:

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

Holliday and Richmond [17], on the other hand, have suggested that the anodic dissolution of chalcopyrite in zone I occurs sequentially according to Eqs. (7)-(9), the determining step being Eq. (8):

CuF2S 2 ~ C u 2+ -['- FeS 2 + 2e-

C u 2 + _...) x/",. 2+ x~f'~. 2+ ~t l (ads ) "l- ( 1 - - / ~U (aq )

2+ Cu~aos ) + FeS 2 ~ CuS + Fe 2+ + S o

(7)

(8)

(9)

Any of the above solutions could explain the results we obtained in this study, in which we conclude that Fe is dissolved in preference to Cu [16,17]; and that the chalcopyrite surface is coated with a layer which decreases the speed of electron transfer to oxidants, such as Fe 3÷, and ion transport. The addition of Cu 2÷ and Fe 3÷ to the electrolyte at 25°C and 68°C, in our experiments, confirmed the electron conducting characteristic of the film formed at low potentials; since, in the respective scans towards more electronegative potentials from Es, a = 1.0 V, the current increased for peaks C , and C~ (Figs. 9 and 10), which were related to reactions where copper and iron, respectively, took part.

The influence of the sweep rate at high temperatures in this potential zone and the linear relationship between the limiting current (JL) and v 1/2 (Fig. 3) indicates that in this zone and at 68°C a film was formed which limited the oxidation reactions with the medium by solid state diffusion.

At 68°C, the pH played a role in the passive zone (Fig. 5b) and in the potential (E b) at which massive dissolution of chalcopyrite started to be observed (Fig. 6), indicating that mineral electrooxidation was hampered by increasing acidity.

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

In zone II, or the transpassive oxidation zone, in which the current increased substantially (Fig. 1), the chalcopyrite behaved in a similar way to other sulphide minerals, such as pyrite and arsenopyrite [18], with S o and SO~- being produced, in accordance with Eq. (1) and Eq. (2). This large increase in current with an increase in temperature could be explained by the increased ionic conductivity at the chalcopyrite- film-solution interface, which would favour the anodic dissolution of the mineral. The maximum value of current density in zone II did not depend on electrolyte stirring (Fig. 8), and thus we can conclude that, during the reaction, a layer, possibly elemental sulphur, is adsorbed on the electrode layer.

In the reverse scan, the influence of stirring, the sweep rate and the addition of ionic species related to the electrode's nature, made possible characterization of the reactions involved in the cathodic contributions of C I, CII and C m. The addition of Fe 3÷ fundamentally modified the zone of C~ and CI~ (Fig. 10), which would be related to the reduction reaction:

Fe 3++ l e - ~ Fe 2÷ E' = 0.4V (10)

This reaction could only be detected because it took place on an electrochemically oxidized chalcopyrite. On a freshly prepared surface of chalcopyrite there was no reaction, due to the slow kinetics and the irreversibility of the Fe2+/Fe 3÷ couple on this mineral [10]. Kuceki [19] also registered this reaction when the chalcopyrite was anodically oxidized up to 1.4 V vs SHE (-- 1.2 V vs Ag/AgC1). As explained before, the C H peak was related to Cu 2+ ions, according to Eqs. (11) and (12), and not to the Fe 3÷ concentration. However, the increase in current density observed in the CII peak (Fig. 10) could be explained by the fact that Fe 3+ was able to oxidize the CuS film formed on the chalcopyrite surface in the previous anodic scan and to produce Cu 2+ ions, via the reaction in Eq. (3), which increases the current density in this C , peak.

At 25°C, the C H peak was more pronounced in the cathodic scan, probably due to the solid products formed on the chalcopyrite during the anodic scan to 1.0 V (Fig. l a) (reactions in Eqs. (3), (6)-(9)), although this was more difficult to observe at the highest temperature. Moreover, the linear dependence of the height of peak Cxl (Ip) versus v 1/2 and of the potential (Ep) versus log v t/2 (Fig. 4), as well as the dependence of this peak on stirring of the solution (Fig. 8a), indicate the electroformation of the three-dimen- sional thick Cu2S(s ) layer under ohmic control, according to:

S O + Cu 2÷ + 2 e - ~ CuS (11)

CuS + Cu 2+ + 2 e - ~ Cu2S + H20 (12)

Stirring the solution, copper ion addition and pH had an influence in the cathodic zone through the reactions in Eq. (13) and Eq. (14), transporting the electroactive species in the solution (Cu 2÷ and H ÷) and contributing to the formation of the Cu2S according to:

2Cu2+H2S + 2 e - ~ Cu2S + 2H ÷ (13)

Holliday [17] and Torma [13] reached similar conclusions.

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

The pH also influenced the C |If peak and there is a clear correlation between acidity and the potential at which the reaction C ill began, so that when pH decreased from 2 to 1, the starting potential was displaced towards more electropositive values (Fig. 7). Peak fi l l , therefore, must have been produced by the following reactions:

2CuS + 2H ++ 2 e - ~ Cu2S + H 2 S

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

(14)

(15)

in which the E - p H dependence can be expressed as: E = E' - 0.059. pH. The electrochemical behaviour of the chalcopyrite during a cathodic scan from E R to

Es. c = -0.5V (Figs. 11 and 12) might be attributed to the progressive transformation of the chalcopyrite into Cu 2 S and Cu 1.9 S [20]. Such a transformation would occur through a series of intermediate steps involving the generation of CuxFeyS z sulphides, such as Cu9Fe8S 4 and CusFeS 4, Fe 2+, Cu 2+ and H2S. The formation of metallic copper is another possibility [21] (E ° Cu2+/Cu ° = 340 mV SHE; + 133 mV Ag/AgC1). Note that the anodic peaks A w and A v (Fig. 11) and Avl (Fig. 12) appear when Es, c < - 0 . 4 0 V, probably due to the following reactions:

(16) 2Cu ° + H2S ~ Cu2S + 2 H ÷ + 2e-

Cu2S ~ Cu2_ x -q- xCu2+ + 2xe - (17)

(18) Cu2_ xS ~ CuS + (1 - x)Cu 2++ 2(1 - x ) e -

Whatever the case, at both high and low temperatures the chalcopyrite electrode is transformed into a Cu-rich and Fe-poor phase at E < E R, so that at - 0 . 1 V the chalcopyrite no longer exists as a mineral phase on the electrode surface.

5. Conclusions

The results show that the chalcopyrite in an electrolyte of similar composition to one used in a culture of mesophilic and thermophilic bacteria behaves in a similar manner to the one described by other authors in a sulphuric acid medium. The differences in the response at 25°C and 68°C are only due to the different physical structure of the complex films of sulphides, polysulphides and elemental sulphur, which are formed electrochemically.

Acknowledgements

This study was funded by MEC: Interministerial Commission of Science and Tecnology and Programme for Scientific Cooperation with South America, in conjunc- tion with the Pontificia Universidad Cat61ica de Chile.

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

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