www.elsevier.com/locate/hydromet

Hydrometallurgy 78

An experimental strategy to determine galvanic interactions

affecting the reactivity of sulfide mineral concentrates

R. Cruza,*, R.M. Luna-Sanchezb, G.T. Lapidusc, I. Gonzalezc, M. Monroya

aInstituto de Metalurgia/Facultad de Ingenierıa, UASLP. Av. Sierra Leona No. 550, Col., Lomas 2a Seccion,

78210 San Luis Potosı, S.L.P., MexicobUniversidad Autonoma Metropolitana Azcapotzalco. Depto. Energıa. Av. San Pablo, No. 180,

Col. Reynosa Tamaulipas 02200, Mexico, D.F., MexicocUniversidad Autonoma Metropolitana Iztapalapa. Depto. Quımica, Apdo. Postal 55-534, 09340 Mexico, D.F., Mexico

Received 7 April 2004; received in revised form 23 November 2004; accepted 10 March 2005

Abstract

The galvanic effect between the distinct associated mineralogical phases in different mineral concentrates was evaluated

using an alternative methodology. Voltammetric studies were carried out in order to analyze the electrochemical reactivity of

pyrite-bearing samples naturally associated with sphalerite, sphalerite-galena and acanthite, respectively. When these phases

were chemically removed from the concentrate, using selective leaching procedures, the voltammetric behavior indicated a

modification of the pyrite reactivity. These modifications were directly associated with the loss of the galvanic interactions. In

this study, it has been shown that even small quantities of the impurities provoke a significant effect on pyrite reactivity. The

strategy proposed here may be employed to predict the reactivity of sulfide minerals in different hydrometallurgical systems.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Acid rock drainage; Carbon paste electrodes; Cyanidation; Galvanic interaction; Galvanic protection; Sulfide minerals

1. Introduction

Galvanic effects, occurring between conducting

and semiconducting minerals in aqueous systems,

play an important role in the aqueous processing of

ores and minerals, such as in flotation and leaching. In

flotation pulps, the galvanic interactions between two

0304-386X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.hydromet.2005.03.006

* Corresponding author. Fax: +52 444 8254326.

E-mail address: rcruz@uaslp.mx (R. Cruz).

or more minerals produce surface coatings, which

affect the floatability of sulfide minerals (Pozzo et

al., 1990). It has also been observed that the type of

coating formed depends, among other factors, on the

mineral/solution ratio (Ekmekci and Demirel, 1997).

In hydrometallurgy, galvanic interactions have been

studied for several leaching and bioleaching systems

(Metha and Murr, 1983; Paramguru and Nayak, 1996;

Madhuchhanda et al., 2000). In these systems, the

galvanic interactions could substantially increase the

leaching of one or both of the minerals that constitute

(2005) 198–208

R. Cruz et al. / Hydrometallurgy 78 (2005) 198–208 199

the galvanic cell, depending on the electrochemical

characteristics of the minerals and on the occurrence

of the distinct sulfides contained in the concentrates

(Nowak et al., 1984; Paramguru and Nayak, 1996;

Abraitis et al., 2003).

For semiconductive minerals, such as sulfides, di-

rect contact of different minerals with dissimilar rest

potentials initiates the galvanic effect. This effect has

been modeled with galvanic cells through the redox

reactions, where the mineral with the higher rest

potential acts as the cathode, which is galvanically

protected, while the mineral with the lower rest po-

tential acts as an anode and its dissolution is favored

through electronic interactions. These interactions

occur between sulfides, involving the flow of elec-

trons from grains with a higher potential to grains with

lower potentials, modifying the Fermi level of both

minerals (Shuey, 1975).

Most of the studies which concern the influence of

various factors on the galvanic interaction between

sulfide minerals have been carried out using galvanic

cells with short-circuited or attached mineral electro-

des, and by comparison of individual electrochemical

behavior of the minerals (Metha and Murr, 1983;

Nowak et al., 1984; Paramguru and Nayak, 1996;

Madhuchhanda et al., 2000). However, the galvanic

interactions with these systems do not reliably repro-

duce the behavior of the complex aqueous systems

used in practice, mainly because of the type of elec-

trical contact between the mineralogical phases. Some

investigators (Ekmekci and Demirel, 1997; Ekmekci

et al., 2003) have studied the galvanic pair effect from

the point of view of flotation, where the analysis of

the interactions between distinct sulfides contained in

the ore is performed on a macroscopic level. The

results obtained do not clearly indicate the effect of

one mineral phase on the other, thus do not allow a

precise determination of the existence of galvanic

protection in certain sulfides. For this reason, it is

important to develop alternative methods for the

study of galvanic interactions between different min-

eral associations under conditions similar to those

found in sulfide mineral concentrates.

In order to evaluate the effect of mineral associa-

tions on the pyrite reactivity in different concentrates,

a different strategy was developed in the present work.

Voltammetric studies were performed in order to an-

alyze the oxidation potential and the oxidation rate of

pyrite samples, naturally associated with sphalerite

(ZnS), sphalerite-galena (PbS) and acanthite (Ag2S),

respectively. The results obtained from theses samples

were compared with the voltammetric behavior of

bimpurity freeQ pyrite samples, which were generated

from the original samples using selective leaching

procedures for sphalerite, galena and acanthite.

The choice of each concentrate was made on the

basis of the electroactivity of its major impurity in a

neutral medium. When the concentrates contained

galena and sphalerite, these impurities presented equi-

librium potential values less positive compared to

pyrite (Madhuchhanda et al., 2000). Lastly, the main

impurity was acanthite, which has an equilibrium

potential more positive than that of pyrite (Luna-San-

chez et al., 2002).

In order to achieve high reproducibility in the

electrochemical results and also to handle particles

more easily with the size distribution of the concen-

trates and residues, carbon paste electrodes were

employed as the working electrodes (Lazaro et al.,

1995). For this kind of electrodes, the presence of

graphite and a non-conductive binding oil does not

affect the electroactivity of mineral particles (Lazaro

et al., 1995).

2. Experimental

2.1. Sulfide mineral concentrates

Four different concentrates were employed. Three

samples contained impurities with lower rest potential

than pyrite (sphalerite, ZnS and galena, PbS), which

were obtained from different mines: Huckleberry and

Brunswick from Canada and Tizapa from Mexico.

The sample containing acanthite as impurity (Ag2S),

with a higher rest potential than that of pyrite, was

obtained from Real del Monte (Mexico). The samples

were carefully prepared to avoid contamination and

were kept in a glass desiccator under vacuum to

prevent the oxidation of pyrite surfaces. The samples

were crushed to the desired particle size range of 105–

150 Am, using an agate mortar. The chemical and

mineralogical characterization of these samples

showed similarity in composition, crystallography

and impurities (Cruz et al., 2001). The mineralogical

composition is shown in Table 1, where the only data

Table 1

Metal sulfide content in pyritic concentrates

Sample Pyrite content

(%)

Sulfide metal

impurities content (%)

Huckleberry 97.0 –

Tizapa 95.5 ZnS 0.9

Brunswick 80.4 PbS 9.2

ZnS 4.3

Real del Monte (RM) 45.0 Ag2S 1.4

Pyrite (PY) 99.0 –

R. Cruz et al. / Hydrometallurgy 78 (2005) 198–208200

presented is for the impurities that strongly affect the

pyrite reactivity, according to data reported by other

authors (Madhuchhanda et al., 2000).

2.2. Impurities leaching procedure

Taking advantage of the galvanic interactions, the

leaching procedure for the samples containing impu-

rities with a lower rest potential than pyrite was a non-

aggressive treatment with CO2 saturated distillated

water. The leaching device and procedure were

designed to promote the natural alteration of the sam-

ples similarly to that obtained by the Humidity Cell

Test Procedure (ASTM, 1996). The devices consisted

of polyethylene Buchner funnels with a filter at the

bottom, in order to retain the concentrate. Each sam-

ple was placed in a cell where it was subjected to 1

day of leaching followed by 3 days of ambient con-

dition exposure for periods of 10 weeks. The solution

of each leaching cycle was collected and analyzed for

pH, SO42�, Fe and Zn concentrations. The leached

mineral samples (residues) were recovered for elec-

trochemical analysis and scanning electronic micros-

copy (SEM) observation.

Acanthite was removed by cyanide leaching since

this mineral phase was not removable from the

sample by the CO2 treatment. Cyanide leaching is

selective for silver and does not significantly attack

the pyrite. For these experiments, pure oxygen was

used as an oxidizing agent. Before preparing the

cyanide solutions, the water was first deionised,

boiled for 20 min and cooled to 20 8C. After this,

nitrogen was sparged for 50 min to ensure that the

dissolved CO2 was liberated. Once the water was

CO2-free, it was used to prepare the cyanide solu-

tion and finally the pH was adjusted to 10.6 with

NaOH.

The cyanide solution was placed in a standard

1-l Pyrex reactor and the flow of the oxidizing agent

was initiated. After 30 min, the reaction was initiated

by introducing the sulfide mineral concentrate. The

kinetic experiments lasted for 8 h. The amount of

concentrate employed was 10 g/l of solution which

had a cyanide concentration of 0.3 M. The system

temperature was maintained at 20 8C. At the end of

each experiment, the residues were filtered, dried and

analyzed for silver and iron to complete the metallur-

gical balance.

2.3. Electrochemical analysis

The electrochemical analyses consisted of cyclic

voltammetry, where an EG & G PARC 273 poten-

tiostat coupled to a PC with the M270 software was

used to impose the electrochemical signal and mea-

sure the voltammetric response. An electrochemical

cell was fitted with three ports: one for the working

electrode (carbon paste electrode, CPE–graphite-

electroactive species), another for the counter elec-

trode (graphite bar, Alfa Aesar, Johnson Matthey

99.9995%) and the last for the reference electrode

(Hg/Hg2SO4/K2SO4 (sat), SSE (E =0.615 V/SHE)).

The working electrode was prepared according to

Lazaro et al. (1995) by mixing 0.5 g natural graphite

powder (Alfa Aesar, 2–15 Am, 99.9995%), 0.5 g of

the electroactive species (concentrates from Huckle-

berry, Tizapa, Brunswick and Real del Monte and

their respective leached residues and Acanthite

from Echo Bay Mine) and silicone oil in an agate

mortar to a homogeneous paste. The quantity of

silicone oil (Sigma, q =0.96 g/mL, m =200 cS)

employed depended on the concentrate or residue.

Once the paste was ready, it was introduced into a 7-

m-long, 0.2-cm interior polyethylene syringe. A plat-

inum wire, welded to a copper wire, was used as the

electrical contact. A three-electrode experimental set

up was employed. Immersion tests were applied to

the concentrates–CPE in the electrolyte to determine

the time independence of the OCP (Open Circuit

Potential). The OCP of the CPE–electroactive spe-

cies was established before conducting the potential

sweep by allowing the electrode system to rest for a

minimum of 5 min or until a stable reading of the

potential was obtained (Table 2). The potential

sweep rate imposed was 20 mV s�1 and was initi-

Table 2

Electrochemical parameters associated with the voltammetric response of the CPE–Mineral for pyrite samples before (fresh concentrate) and

after leaching (residue)

Sample OCPa (V) Eb(I=10 AA) (V/SSE)

c I/Ed Rate (AA/mV) Charge, QR (AC)

Concentrate Concentrate Residue Concentrate Residue Concentrate Residue

Huckleberry �0.27 0.37 0.43 1.86 1.38 37.0 26.1

Tizapa �0.18 0.47 0.42 0.98 1.38 17.8 22.5

Brunswick �0.29 0.44 0.43 0.44 1.33 3.4 30.2

Real del Monte (RM) �0.32 0.31 0.36 2.34 2.18 60.30 21.52

Pyrite (PY) �0.29 0.34 – 2.41 – 31.78 –

The OCP values are also shown. The electrochemical characterization was performed in 0.1 M NaNO3.a OCP—open circuit potential of CPE–fresh mineral concentrate.b E(I=10 AA)—the potential to reach 10 AA for the forward scan in the oxidation pyrite process.c SSE—saturated sulphate electrode.d I/E Rate—the current release as the potential increases.

R. Cruz et al. / Hydrometallurgy 78 (2005) 198–208 201

ated at the OCP. The upper vertex potential was 700

mV. After reaching this value, the potential was

inverted toward to the cathodic zone up to �1000

mV and again, inverted until the OCP was reached.

After each test, a plunger was used to eliminate the

reacted paste; once this was performed, the surface

was homogenized for the following experiment

using a 600-grade silicon carbide emery paper. The

electrolyte solution employed was 0.1 M NaNO3 at a

pH of 6.5. In all cases, purified nitrogen gas was

sparged through the electrolyte solution for 45 min

before the start of the experiments and an inert

atmosphere was maintained within the cell during

the tests.

3. Results and discussion

3.1. Effect of sulfide impurities (galena and sphaler-

ite) on the reactivity of pyrite (concentrates from

Huckleberry, Brunswick and Tizapa)

The pyrite reactivity describes the capacity of this

mineral to react under a given set of conditions. In

ores and concentrates, pyrite reactivity depends,

among other factors, on their mineralogical composi-

tion (Jambor, 1994). The analysis of reactivity can be

carried out either by chemical or electrochemical

methods. In this section the application of cyclic

voltammetry (electrochemical technique) is used to

characterize the reactivity of pyrite in the presence

of impurities (galena and sphalerite) similar to those

contained in natural minerals.

The results obtained from the electrochemical anal-

ysis show that pyrite samples from Huckleberry,

Tizapa and Brunswick (Figs. 1a, 2a and 3a, respec-

tively) exhibit similar voltammetric responses, which

is directly associated with the pyrite oxidation. Fur-

thermore, to evaluate the effect of the carbon paste

alone in the voltammetric response, its analysis, with-

out mineral, in the 0.1 M NaNO3 electrolyte (Fig. 1,

curve CPE, carbon electrode paste) was performed.

The comparison between the CPE and CPE–Mineral

curves shows that the response of the CPE is negligi-

ble and therefore does not interfere with the analysis

of the sulfide mineral concentrate.

The electrochemical oxidation processes observed

in Figs. 1–3 have been studied by Zhu et al. (1993).

They attributed this oxidation process to the oxidation

of pyrite, according to the following reaction:

FeS2 þ 10H2OYFeOOH þ 2SO2�4 þ 19Hþ þ 15e�

ð1Þ

The peak R observed in the inset of Figs. 1–3 is

associated with the reduction of the FeOOH formed

during the pyrite oxidation.

The current–voltage response is usually described

as a function of peak potentials. Since a maximum

current (peak) was not obtained for the pyrite samples

studied here (due to the high concentration of pyrite in

the concentrates), alternative comparison parameters

were established. These parameters were the potential

required to reach 10 AA (EI =10 AA) during the forward

scan in the oxidation of pyrite; the oxidation rate (I/E

rate) of pyrite calculated from the slope of the current–

-20

80

180

280

380

480

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-5

0

5

10

-0.3 0 0.3

Potential (V/SSE)

Cur

rent

(µA

)

a

b

CPE

ab

Peak R CPE

Fig. 1. Evolution of the electrochemical reactivity of Huckleberry pyrite: (a) unleached and (b) residue. The voltammetric responses were

obtained from CPE–Mineral 50% in 0.1 M NaNO3. The scan was started in the positive direction at 20 mV s�1. The CPE curve corresponds to

the Carbon Paste Electrode (CPE) without mineral.

R. Cruz et al. / Hydrometallurgy 78 (2005) 198–208202

potential response during the forward scan (in the

range of 0.55–0.66 V) and the charge associated

(QR) with peak R corresponding to the reduction of

FeOOH formed by pyrite oxidation. These parameters

are summarized in Table 2 and emphasize the differ-

ence between pyrite reactivities.

-20

80

180

280

380

480

-0.1 0 0.1 0.2 0.3

-5

0

5

10

-0.3 0 0.3

Potenti

Cur

rent

(µA

)

Peak R

Fig. 2. Evolution of the reactivity for Tizapa pyrite sample at different

responses were obtained from CPE–Mineral 50% in 0.1 M NaNO3. The

Comparing the values reported in Table 2, it may

be concluded that the oxidation rate (the current re-

lease rates as the potential is increased, I/E ratio)

depends on the nature of the pyritic sample. From

these values, it was also established that the Huckle-

berry pyrite is more reactive (lower EI = 10 AA, higher

0.4 0.5 0.6 0.7 0.8

al (V/SSE)

a

ba

b

leaching stages: (a) unleached and (b) residue. The voltammetric

scan was started in the positive direction at 20 mV s�1.

-20

80

180

280

380

480

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-5

0

5

10

-0.3 0 0.3

Potential (V/SSE)

Cur

rent

(µA

)

a

b

Peak G a

b

Peak R

Fig. 3. Evolution of electrochemical reactivity of Brunswick pyrite sample: (a) unleached and (b) residue. The voltammetric responses were

obtained from CPE–Mineral 50% in 0.1 M NaNO3. The scan was started in the positive direction at 20 mV s�1.

R. Cruz et al. / Hydrometallurgy 78 (2005) 198–208 203

I/E ratio and higher QR) than the other two pyrites

(Tizapa and Brunswick). The chemical data also show

that Huckleberry pyrite presents the highest reactivity,

generating the lowest pH, and the highest SO42� and

iron dissolution (Fig. 4a–c).

The voltammetric analysis of the Huckleberry py-

rite sample indicates that the reactivity of this sample

decreases after leaching (Fig. 1b). On the other hand,

from the chemical analysis of the solution (Fig. 4a–c)

and the SEM images (Fig. 5a and b), it was estab-

lished that this decrease in reactivity was due to the

formation of an amorphous iron oxy-hydroxide layer

on the pyrite surface.

In Fig. 2, it may be observed that the Tizapa

sample shows a different behavior with respect to

that of the Huckleberry concentrate. In this case, the

reactivity of fresh pyrite sample is lower than that of

the Huckleberry (Fig. 2; Table 2). In contrast to the

behavior of Huckleberry concentrate after leaching,

once the Tizapa concentrate has been leached, the

pyrite shows an increase in its reactivity (Fig. 2b;

Table 2) due to a loss of galvanic protection between

the pyrite and the sphalerite. It may be noted in Table

2 that the EI =10 AA value for the residue is lower than

that of the concentrate, which indicates that pyrite is

more easily oxidized once the impurity has been

chemically removed. The I/E ratio and the QR values

(Table 2) corroborated this statement. The chemical

analysis of the leach solutions at different times shows

that a quantitative dissolution of zinc occurs during

the early periods of leaching, which indicated that

sphalerite was rapidly leached out of the sample

(Fig. 4d).

From the data obtained, it was concluded that the

presence of sphalerite in the Tizapa concentrate cre-

ates a galvanic protection of the pyrite, decreasing its

reactivity. This phenomenon indicates that the equi-

librium potential of sphalerite is more negative than

that of pyrite. However, the electrochemical detection

of sphalerite contained in Tizapa is not possible since

it would require a large overpotential due to the low

conductivity or large band gap of this mineral (Shuey,

1975). Hence, the elimination of the sphalerite by

leaching leads to the electrochemical reactivation of

the pyrite (Fig. 2b). Therefore, the leaching of sphal-

erite was promoted by the same galvanic interaction

that protects the pyrite from dissolution.

The voltammetric response of the Brunswick con-

centrate is shown in Fig. 3a. It may be noted that the

amount of energy necessary to oxidize this concen-

trate (Table 2) is higher than in the case of the

Huckleberry pyrite (Fig. 1a). On the other hand, the

presence of an oxidation peak (Fig. 3a, peak G) may

be observed at potential values starting from approxi-

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0

100

200

300

400

500

600

700

800

900

0.0

0.5

1.0

1.5

2.0

2.5

0 20

50

100

150

200

250

300

350

400

Time (weeks) Time (weeks)

Time (weeks)Time (weeks)

Fe

(mg/

l)Z

n (m

g/l)

SO4=

(g/l)

pH

1 3 4 0 21 3 4

0 21 3 4 0 21 3 4

a)

b)

c)

d)

Fig. 4. Evolution of (a) pH, (b) SO42�, (c) Fe and (d) Zn of the solution as a function of the leaching time of pyritic samples: Huckleberry (x),

Tizapa (n) and Brunswick (E).

R. Cruz et al. / Hydrometallurgy 78 (2005) 198–208204

mately 0.2 V. This peak may be attributed to the

oxidation of galena in the Brunswick sample as was

previously demonstrated (Cruz, 2000). The evolution

of pyrite reactivity in this sample shows a similar

Py

Py

a)

Fig. 5. Scanning electron microscopy images of the Huckleberry pyrite s

precipitates and Py indicated with arrows are electrostatic pyrite particles

behavior to that of the Tizapa. Once the concentrate

was leached (Fig. 3b), the I/E rate and the QR (Table

2) increased, suggesting a higher reactivity of the

pyrite. In this case, it may be observed that the oxi-

Py

AP

b)

urface: (a) unleached and (b) leached. Py=Pyrite, AP=Amorphous

.

R. Cruz et al. / Hydrometallurgy 78 (2005) 198–208 205

dation peak of the galena disappears after leaching,

which indicates that it has been removed and that this

mineral phase provides a galvanic protection for the

pyrite in the Brunswick concentrate. For this concen-

trate, the effect of sphalerite on pyrite does not seem

to be noticeable, because the galena has a much larger

influence.

The influence of the impurities contained in the

Brunswick (galena and sphalerite) and the Tizapa

(sphalerite) concentrates shows that, despite their

small quantities, pyrite reactivity is greatly affected.

On the other hand, even though the equilibrium poten-

tials of sphalerite and galena are both more negative

than pyrite, only the galena is electrochemically

detected in the concentrate. In the case of sphalerite,

electrochemical detection is not possible; however the

effect that this phase has on the pyrite is eliminated

once it has been chemically removed. It is important

to note that the combined effect of the galena and

sphalerite provokes the lowest reactivity in the Bruns-

wick pyrite (Fig. 3a and Table 2).

3.2. Reactivity of pyrite in the presence of silver

sulfide (Acanthite) contained in a concentrate (Real

del Monte)

This section illustrates the galvanic effect between

the pyrite and acanthite mineralogical phases

contained in the Real del Monte (RM) concentrate,

which consists of a high percentage of iron sulfide

-50

150

350

550

0 0.2

Potenti

Cur

rent

(µA

)

Fig. 6. Evolution of electrochemical reactivity of the concentrates: (a) R

(PY). The voltammetric responses were obtained from CPE–Mineral 50%

20 mV s�1.

with a small portion of acanthite (see Table 1).

Acanthite is a mineral phase whose equilibrium po-

tential is higher than that of pyrite. This changes the

reactivity (voltammetric response) of the latter, even

when the former is present in minor quantities.

In Fig. 6, the voltammetric results of Real del

Monte, before (Fig. 6a, RM) and after (Fig. 6b,

RML) leaching, as well as that of a pure pyritic

concentrate (Fig. 6c, PY; Table 1) are shown. In the

case of the residue (Fig. 6b, RML; Table 2), the pyrite

reactivity decreases (The I/E ratio diminishes) and the

amount of energy (EI =10 AA) is larger than the other

two concentrates (RM and PY). However, from Fig.

6c it is difficult to observe during the oxidation step

the pyrite reactivity and the contribution of the

acanthite contained in the concentrates, due to the

masking phenomenon between these two mineralogi-

cal phases. The main difference between the concen-

trates and the residue was found in the zone where the

reduction of the products occurs, obtained during the

direct voltammetric oxidation (�200 to 300 mV),

which is shown in Fig. 7 (peak R). The RM (Fig.

7a) shows a higher reactivity due to the fact that the

acanthite contained in this concentrate favors the py-

rite voltammetric oxidation. Once the acanthite was

chemically removed from RM, the residue (Fig. 7b,

RML) presented the same qualitative (form) and

quantitative (current) behavior as the PY (Fig. 7c).

The presence of acanthite as an impurity in the RM

concentrate (Fig. 7a) increases the reactivity of pyrite.

0.4 0.6

al (V/SSE)

a

c

b

eal del Monte unleached (RM), (b) residue (RML) and (c) pyrite

in 0.1 M NaNO3. The scan was started in the positive direction at

Pyrite

Acanthite

Pyrite

AcanthitePyrite

Acanthite

b)

Acanthite

Pyrite

Acanthite

Pyrite

Acanthite

Pyrite

a)

Fig. 8. Scanning electron microscope images of the Real del Monte pyrite surface: (a) unleached and (b) residue.

-7

0

7

-0.2 0 0.2

Potential (V/SSE)

Cur

rent

(µA

)

a

bc

b

aPeak R

c

Fig. 7. Zoom of the electrochemical reactivity of Real del Monte and pyrite: (a) RM before leaching, (b) RML after cyanide leaching, (c) PY

before leaching. The voltammetric responses were obtained from CPE–Mineral 50% in 0.1 M NaNO3. The scan was started in the positive

direction at 20 mV s�1.

R. Cruz et al. / Hydrometallurgy 78 (2005) 198–208206

R. Cruz et al. / Hydrometallurgy 78 (2005) 198–208 207

The voltammetric oxidation of acanthite is unobserv-

able because, as was mentioned earlier, its equilibrium

potential is much greater than that of pyrite. Once the

acanthite is chemically removed from the concentrate,

the exposure of the pyrite contained in RM is similar

to that of the concentrate of nearly pure pyrite (Fig.

7b, RML and Fig. 7c, PY). On other hand, the charge

associated with these reduction processes is directly

related to the quantity of oxidized pyrite. Therefore,

the value of the charge for these processes allows the

comparison of the tendencies mentioned above. The

results are shown in Table 2. In the case of PY, (Fig.

7c) the charge obtained is almost half of that of the

RM (Fig. 7a), which manifests the presence of

acanthite, even though it is present only in small

quantities. Once the RM concentrate has been leached

(Fig. 7b, RML), it may be noted that the value of QR

is similar to those of pyrite from PY and pyrite

contained in Huckleberry concentrate. The chemical

treatment applied (Section 2.2) removed only the

acanthite, and the remaining concentrate is pyrite.

In order to confirm this behavior, the concentrate

(RM) and its residue (RML) were observed using

the Scanning Electron Microscopy (SEM). It may

be noted that the acanthite contained in the con-

centrate (Fig. 8a) was removed after cyanidation

(Fig. 8b), while the pyrite phase was detected in

both cases (Fig. 8a and b).

From the above discussion, it was concluded that

in both cases, the distinct impurities contained in each

concentrate increased or diminished the reactivity of

pyrite, depending on their values at the equilibrium

potential, as well as the electroactivity of the miner-

alogical phases contained in each concentrate.

4. Conclusions

Galvanic interactions depend on the mineralogical

association between the phases present in the concen-

trate. In order to determine the galvanic interactions in

a complex mineral association, an alternative strategy

was proposed in this work to analyze these kinds of

interactions: Initially, the associated minerals were

characterized by cyclic voltammetry to understand

the reactivity of pyrite (main component). Afterward,

a selective leaching procedure was then used to extract

the associated impurities and the residues were also

characterized by cyclic voltammetry. The comparison

of the electrochemical behavior before and after leach-

ing procedure allowed the identification of the galvan-

ic interactions and their effect on the pyrite reactivity.

It has been shown that even small quantities of the

impurities cause a significant modification of pyrite

reactivity. Therefore, the strategy proposed here could

be employed to predict the reactivity of sulfide mine-

rals in different hydrometallurgical systems.

Acknowledgements

R. Cruz and R. M. Luna-Sanchez wish to thank

CONACyT for their postgraduate scholarships. Finan-

cial support for this work came fromCONACyT (Grant

485100-5-25715B). For the first case, Noranda Inc.

(Canada) and Servicios Industriales Penoles (Mexico)

supplied the mineral samples to develop this project.

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