Mechanism of di-isobutyl dithiophosphinate adsorption onto galena and pyrite

Minerals Engineering 19 (2006) 904–911 This article is also available online at: www.elsevier.com/locate/mineng

Mechanism of di-isobutyl dithiophosphinate adsorption onto galena and pyrite E.T. Pecina a

a,*

, A. Uribe b, J.A. Finch c, F. Nava

b

CIMAV, SC (Centro de Investigacion en Materiales), Quimica de Materiales, Miguel de Cervantes 120, Compl. Ind. Chihuahua, C.P. 31109, Chihuahua, Chih., Mexico b CINVESTAV-IPN, Unidad Saltillo, A.P. 663, C.P. 25900, Ramos Arizpe, Coah., Mexico c McGill University, Montreal, Canada H3A2B2 Received 13 May 2005; accepted 7 October 2005 Available online 21 November 2005

Abstract The interaction of sodium-di-isobutyl dithiophosphinate (DTPINa) with galena and pyrite in alkaline media (pH 9) was examined using cyclic voltammetry, open circuit potential and contact angle techniques. Analysis of results suggests that the collector interacts with the minerals by chemical and electrochemical mechanisms according to the following steps: a chemisorption process (without electron transfer) that takes place when the collector interacts with metal species (e.g., of lead) that shows chemical affinity towards the collector; an electrochemical–chemical process developed by the galena that involves two steps: the oxidation of the mineral (electrochemical step) and the formation of a metal–collector precipitate (chemical step); and an electrochemical process (adsorption of collector and dimer formation) that occurs when pyrite is held at a redox potential sufficiently anodic to produce oxidation of the collector.  2005 Elsevier Ltd. All rights reserved. Keywords: Mineral Processing; Flotation collectors; Sulphide ores; Redox reactions

1. Introduction Non-conventional collectors (i.e., collectors different to xanthates) are more frequently used in the Mexican sulphide flotation practice (Uribe-Salas et al., 2000), mainly due to both the presence of silver values and pyrite or pyrrhotite as sulfide gangue minerals. An example of non-conventional collectors is sodium-di-isobutyl dithiophosphinate (DTPINa), known under the trademark of Aerophine 3418A of Cytec Industries (Fig. 1). DTPINa is mainly recommended to float lead, copper and precious metals from minerals containing high levels of pyrite, as is the case of polymetallic ores. Research works on froth flotation of sulfides in the presence of non-conventional collectors (e.g., dithiophosphinates, thionocarbamates, etc.) have demonstrated that the *

Corresponding author. Tel.: +52 614 4394845; fax: +52 614 4391112. E-mail address: [email protected] (E.T. Pecina).

0892-6875/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.10.004

selective activation of the mineral species, is substantially affected by the electrochemical conditions (pulp potential) of the flotation slurry (Gorken et al., 1992; Uribe-Salas et al., 2000). In spite of this, very little fundamental data are available on the electrochemical response of sulfides in presence of DTPINa. It is known that the interaction mechanism of thiol collectors and sulfides is essentially the same. Nevertheless, examinations of non-conventional collectors such as thionocarbamates (Basilio and Yoon, 1992), MBT (Maier and Dobia´s, 1997), etc., have revealed some differences with regard to the xanthates. For example, a chemisorption process (with no electron transfer) that proceeds at lower potentials compared with the potentials at which the electrochemical adsorption of xanthates occurs. According to Basilio and Yoon (1992), this behavior is due to the small pK of the corresponding metal–collector compound. Although it is believed that DTPI interacts with sulfides through mechanisms analogous to those accepted for

E.T. Pecina et al. / Minerals Engineering 19 (2006) 904–911 M=Na+ R=Isobutyl

R

S P

R

SM

Fig. 1. Chemical structure of sodium-di-isobutyl dithiophosphinate (DTPINa).

xanthates, thus far it has not been demonstrated. According to the above, knowledge of the interaction mechanism of DTPINa with common sulfides might contribute to the basic understanding of sulfide flotation chemistry. Pyrite (FeS2) is the most abundant of the sulfide minerals. It is considered as a worthless mineral (i.e., gangue type) in the flotation of the metal-base sulfides (Pb/Cu/ Zn), and is only appreciated when it is associated with precious metals such as gold. The conventional practice of sulfide flotation establishes that successful depression of pyrite may be performed by following a conventional scheme of flotation, where xanthates and traditional flotation reagents are involved. Nevertheless, the increasing complexity of the exploitable mineral deposits, motivates the searching for more selective collectors. This communication addresses the industrial need of rejecting more iron from base metal concentrates and to limit subsequent iron residue production in the lead extraction process. This communication is focused on both the study of the interaction mechanism of DTPINa with galena and pyrite, and the role of redox potential on the hydrophobic properties that the mineral surface develops in the presence of the collector at pH 9; techniques of cyclic voltammetry, open circuit potential and contact angle were used. The results may contribute to the fundamental knowledge of the interaction mechanism that governs the mineral–collector contact, thus contributing to the understanding of the sulfide flotation chemistry. 2. Experimental 2.1. Reagents and minerals Reagents were of analytical grade from Aldrich. The commercial form of the collector DTPINa was used (product name: Aerophine 3418A promoter, AEROPHINE promoters from Cytec Industries Inc.). The collector contains ca. 50% w/w of sodium-di-isobutyl dithiophosphinate and traces of triisobutyl phosphine sulphide impurity (the rest is water). The purity of collector (>95% w/w) is adequate to conduct fundamental work. The experiments were performed with pure crystals of galena and pyrite from a Zacatecas mining district (Mexico). The main impurities of pyrite were 0.006% (by weight) Cu, 0.022% Pb, 0.004% Zn, 0.10% insoluble; and those of galena: 0.012% Cu, 1.30% Zn, 0.29% Fe, 0.10% insoluble.

905

Deionized water with a specific conductivity of 106 X1 cm1 was used in all the tests. All electrolytes were de-aerated with high purity nitrogen for 40 min. Suspensions were prepared with 102 M NaNO3 regulated solutions and the pH was controlled with the addition of diluted solutions of NaOH and HNO3. The experiments were conducted using a conventional three-electrode circuit The cell used in the experiments was a 0.5 l acrylic container. A calomel electrode and a platinum electrode of relatively large surface area were used as reference and counter electrode, respectively. The redox potentials reported here are expressed in terms of the saturated calomel electrode (SCE) (E = 244 mV versus SHE). The electrode potential was controlled with a potentiostat Radiometer-Copenhagen Mod. DEA332 33V/2a. Experiments were performed under nitrogen atmosphere. 2.2. Electrodes A copper wire was firmly bonded to one face of the sulfide crystal with silver epoxy cement (Epotek E4110). The joint and the face of the mineral crystal (galena and pyrite) were sealed with non-conducting epoxy sealant. The uncovered face of the mineral electrode was prepared (and refreshed before each experiment) by consecutively polishing on 600, 800 and 1200 grit silicon carbide paper. The exposed surface area of galena and pyrite electrodes was 0.30 cm2 and 0.17 cm2, respectively. 2.3. Procedure 2.3.1. Contact angle The effect of redox potential on the contact angle between a bubble and the mineral surface, in the absence and presence of DTPINa (104 M), was evaluated at pH 9. As mentioned above, The electrodes were wet polished on silicon carbide paper of successive grades (from 400 to 1000 grit), to provide a surface suitable for contact angle studies. The selected potential was then applied during 5 min. At the end of the conditioning stage, a nitrogen bubble was deposited onto the electrode surface and the angle developed was measured on a photograph taken with a SLR camera equipped with a 105 mm macro lens. The arrangement of the electrodes in the cell allows the surface of mineral electrode to be placed with the working area perfectly flat and facing upward. The galena and pyrite electrodes had a surface area of 1.12 cm2 and 1.3 cm2, respectively. 2.3.2. Open circuit potential (OCP) The open circuit potentials of galena and pyrite are reported after a conditioning period of 10 min has elapsed. The experiments were performed in the presence and absence of DTPINa (103 M), at different conditions of pH and dissolved oxygen.

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The behavior of the experimental results is similar to that reported in the literature (Ahmed, 1978; Abramov and Avdohin, 1997). In this regard, Abramov and Avdohin (1997) claim that the electrode potential develop by most of sulfide minerals in aqueous media is irreversible. This is due to the following reasons: (i) the fact that in the oxidation reaction defining the OCP of the mineral, not only the ions of the particular mineral are involved but also other foreign species such as H+ and dissolved oxygen, and (ii) the fact that the anodic and cathodic reactions do not occur at the same rate. The analysis of the behavior of the OCP of sulfides when di-isobutyl dithiophosphinate is added to the system may provide useful information on the nature of the mineral– collector interaction. Fig. 2 shows that the open circuit potential (rest potential) of galena is affected by the presence of DTPINa, which causes a significant drop. It is also observed that the rest potential of galena in the presence of collector is independent of pH and dissolved oxygen concentration (e.g., oxygen and nitrogen atmospheres). This suggests that the mineral/collector interaction results in a

2.3.3. Cyclic voltammetry The voltammetry tests were performed at pH 9 under nitrogen atmosphere. Voltammograms were obtained at a potential sweep of 20 mV/s for galena and pyrite. The initial and final potential of all voltammograms was that obtained in the open circuit (Ei=0) measurements; the anodic and cathodic edge was set at 800 mV and 1000 mV respectively, unless another value is specified. 3. Results and discussion 3.1. Open circuit potential It is accepted that the open circuit potential (also known as rest potential) of a sulfide mineral electrode immersed in an aqueous solution is a ‘‘mixed potential’’. This is the generic term for a complex system consisting of the cathodic reduction of dissolved oxygen and the anodic oxidation of the mineral surface. The OCP values of galena and pyrite in the absence of collector are presented in Figs. 2(a) and 3(a), respectively.

300 (b) 10-3 M DTPINa

(a) No collector

N2 O2

E, mV vs. SCE

200 100 0 1

-100

3, 4

Galena electrode 2

-200 3

4

5

6

7

8

4

9

5

6

7

8

9

10

pH

Fig. 2. Rest potential of galena as a function of pH and dissolved oxygen, in water and collector solutions. (a) Absence of collector; Curve 1, O2 atmosphere; Curve 2, N2 atmosphere. (b) In presence of 103 M DTPINa; Curve 3, O2 atmosphere; Curve 4, N2 atmosphere. Ionic strength 102 M NaNO3.

300 1

E, mV vs. SCE

200

(b) 10-3 M DTPINa

(a) No collector

2 3

100 4

0 --100 -200

N2 O2 3

4

Pyrite electrode 5

6

7

8

9

pH

4

5

6

7

8

9

10

Fig. 3. Rest potential of pyrite as a function of pH and dissolved oxygen, in water and collector solutions. (a) Absence of collector; Curve 1, O2 atmosphere; Curve 2, N2 atmosphere. (b) In presence of 103 M DTPINa; Curve 3, O2 atmosphere; Curve 4, N2 atmosphere. Ionic strength 102 M NaNO3.

E.T. Pecina et al. / Minerals Engineering 19 (2006) 904–911

3.2. Cyclic voltammetry The electrochemical study was performed using three working electrodes: galena and pyrite. 3.2.1. Galena electrode Galena voltammograms in the presence and absence of DTPINa are presented in Figs. 4 and 5. Since low scan rates permit good reproducibility, galena voltammograms were performed using a sweep rate of 20 mV/s. In the absence of collector, the electrochemical behavior of galena has been fully analyzed and documented. According to the available information (Richardson and Maust, 1976; Gardner and Woods, 1979; Lamache et al., 1981) the assignment of the peaks in Fig. 4 to galena reactions in plain aqueous solutions are the following. The lead oxide (PbO) and the hydrolyzed lead species (Pb(OH)2,

3.2 No collector DTPINa

A2

I, mA·cm-2

2.4 1.6

0.8 A1 0 C2 -0.8 -1000

Galena electrode -500

0

500

1000

E, mV vs. SCE

Fig. 4. Voltammograms of galena in absence and presence of collector. Unbuffered solutions (102 M NaNO3) at pH 9. 103 M DTPINa. Scan rate 20 mV/s.

0.3 No collector DTPINa

A2

0.15 I, mA·cm-2

reaction product that passivates the electrode surface, and that such interaction proceeds via a chemical mechanism. In regard to this behavior, it has been reported in the literature that the adsorption of thiol collectors (e.g., xanthate), passivates galena electrodes. For example, xanthate prevents the adsorption/desorption of hydrogen on platinum electrodes (Woods, 1971). Furthermore, it is worth mentioning that the lead extracted from galena with EDTA, decreases from 8.6 to 4.7 lM/g due to the presence of DTPINa, thus suggesting the passivation of the mineral. In regard to pyrite, the presence of DTPINa has a minor effect on the rest potential of this mineral (Fig. 3). This indifference to the presence of collector is probably due to the lack of affinity of DTPINa for the iron species initially present on the pyrite surface. Owing to the relatively large scattering that exists in the OCP values of pyrite, a definite conclusion may not be drawn, nevertheless, the effect of the collector on the OCP values of the minerals clearly indicates the development of processes of different nature.

907

A1

0

Galena electrode -0.15 -200 0 200 400 E, mV vs. SCE

600

Fig. 5. Voltammograms of galena in absence and presence of collector. Unbuffered solutions (102 M NaNO3) at pH 9. 103 M DTPINa. Eka = 500, Ekc = 160. Scan rate 20 mV/s.

PbOH+, HPbO2) are considered the main species in alkaline media. Accordingly, the current peak (A2) is assigned to the anodic dissolution of galena to produce lead oxide and lead hydroxide: PbS + H2 O () PbO + S + 2Hþ + 2e

ð1Þ

PbS + 2OH () Pb(OH)2 + S + 2e

ð2Þ

At the high anodic limit of the voltammograms presented in Fig. 4, the sulfur is oxidized to a sulfur–oxygen ion, through the following processes: þ  PbS þ 5H2 O () PbO þ SO2 4 þ 10H þ 8e

PbS þ 5H2 O () PbO þ xPbS þ ð2x þ 2yÞOH

S2 O2 3

þ

þ 10H þ 8e



ð3Þ ð4Þ



 () xPbðOHÞ2 þ Sx O2 y þ yH2 O þ ð2y þ 2x þ 2Þe

ð5Þ The hysteresis of peak A2 has been related to the nucleation and growth of elemental sulfur sites (Gardner and Woods, 1979). The cathodic peak (C2), may be assigned to the reverse of processes described in Eqs. (3)–(5), as well as the reduction of lead to the metal (Eq. (6)): 

Pb(OH)2 + 2e () Pb + 2OH

ð6Þ

The anodic electrochemical response of the galena electrode in presence of DTPINa is presented in Fig. 5. The figure shows an increase of current (designed as A1) that initiates at a potential of about 30 mV. This process takes place before the oxidation of the mineral indicating the occurrence of processes such as the formation of a metal–collector compound (e.g., Pb(DTPI)2). In the cathodic scan, the peak C2, associated to the reduction of the oxidation products vanishes, thus supporting the mineral–collector interaction proposed for the anodic range (see Fig. 4). The irreversibility of the metal– collector process is supported by the absence of the reduction peak.

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E.T. Pecina et al. / Minerals Engineering 19 (2006) 904–911 1.2

0.3 No collector

No collector DTPINa

0.9

I, mA·cm-2

0.3

I, mA·cm-2

A2

0.6

0

A1 C2

-0.3

0.15 A1

0 C1 Pyrite electrode

-0.6 -1000

A2

DTPINa

Pyrite electrode -0.15

-500

0

500

1000

-200

0

E, mV vs. SCE

200 E, mV vs. SCE

400

600

Fig. 6. Voltammograms of pyrite in absence and presence of collector. Unbuffered solutions (102 M NaNO3) at pH 9. 103 M DTPINa. Scan rate 20 mV/s.

Fig. 7. Voltammograms of pyrite in absence and presence of collector. Unbuffered solutions (102 M NaNO3) at pH 9. 103 M DTPINa. Eka = 500, Ekc = 160. Scan rate 20 mV/s.

3.2.2. Pyrite electrode Pyrite voltammograms in absence and presence of DTPINa are presented in Fig. 6. According to the available information (Hamilton and Woods, 1981; Chander and Briceno, 1987; Abraitis et al., 2000) the assignment of peaks of pyrite voltammogram are the following. In alkaline solutions, the oxidation of pyrite has been related to the formation of hydrated ferric oxide with the production of sulfur (Eq. (7)) and sulfate (Eq. (8)):

3.3. Contact angle

FeS2 þ 3H2 O () FeðOHÞ3 þ 2S þ 3Hþ þ 3e

ð7Þ

þ  FeS2 þ 11H2 O () FeðOHÞ3 þ 2SO2 4 þ 19H þ 15e

ð8Þ The relative amount of sulfate produced increases as the potential is taken to higher values. According to Hamilton and Woods (1981) sulfate becomes the dominant product above 0.8 V versus SHE. At the high anodic limits imposed on the voltammograms of Fig. 6, sulfate is the dominant sulfur species. The cathodic processes C2 could be associated to the reduction of ferric to ferrous species (Eq. (9)): Fe(OH)3 +e () Fe(OH)2 +OH

The behavior of the contact angle of galena as a function of redox potential in the presence and absence of DTPINa is presented in Fig. 8. The substantial increase of the contact angle in the absence of collector at potentials above 100 mV, may be related to formation of hydrophobic species according to process described by Eq. (1). The results obtained in the presence of collector show that the applied potential does not have a significant effect on the contact angle, thus suggesting that DTPINa chemisorbs onto the mineral, at least for the potentials at which the voltammograms of galena does not show an increase of current (i.e., E < 30 mV). At potentials more anodic than 30 mV, further processes such as the oxidation of collector or the oxidation of the sulfide itself may occur. This behavior correlates well with the OCP results obtained for galena (Fig. 2), which indicate a potential value independent of both pH and concentration of dissolved oxygen. It is important to notice that in the presence of DTPINa, at the potential values where peak A1 is observed, 100

The remaining sulfur could be reduced according to Eq. (10):

80

Fe(OH)3 + S + 3e () FeS + 3OH

ð10Þ

In the presence of DTPINa (see Fig. 7), the voltammogram of pyrite presents the anodic peak A1 (located at above 40 mV), assigned to the electrochemical interaction with the collector. The electrochemical process is related to the oxidation of DTPINa (formation of the dimer). This is supported by the fact that the ferrous species released in the alkaline media undergoes hydrolysis reactions (e.g. formation of hydroxide) instead of the formation of the ferrousDTPI compound (Pecina-Trevin˜o et al., 2003). The peak C1 is related to the cathodic reduction of species produced during the anodic scan.

Contact angle, degrees

ð9Þ

10-4 M DTPINa

60 40 No collector

20 Galena

0 -700 -500 -300 -100 100 300 E, mV vs. SCE

500

700

Fig. 8. Effect of potential on the contact angle of galena in water and 104 M DTPINa solutions. Unbuffered solutions at pH 9. Ionic strength 102 M NaNO3.

E.T. Pecina et al. / Minerals Engineering 19 (2006) 904–911 100 Pyrite Contact angle, degrees

80

10-4M DTPINa

60 Weak adherence 40 20 A1 0 -500

-300

-100 100 300 E, mV vs. SCE

500

700

Fig. 9. Effect of potential on the contact angle of pyrite in water and 104 M DTPINa solutions. Unbuffered solutions at pH 9. Ionic strength 102 M NaNO3.

909

The galena results suggest the DTPINa interacts with the mineral through chemical and electrochemical mechanisms as follows: (i) A chemisorption mechanism (without transfer of electrons), that takes place at cathodic potentials (e.g., below 30 mV). This mechanism is supported by the following observations: galena is hydrophobic at more cathodic potentials than the potential at which current peak A1 is observed (ca. 30 mV). Other studies of thiol type collectors have discussed this process taking into consideration the Ksp of the metal/collector compound (see Basilio and Yoon, 1992). According to the above, the Ksp value of the compound Pb(DTPI)2, may be the basis of the chemisorption process. The well-known reactions of substitution that have been established for thiol type collector are considered to illustrate the chemisorption mechanism:  PB2þ surface þ 2DTPIaqueous () ½PbðDTPIÞ2 surface

i.e., 30 mV, the contact angle of galena is not affected. However, this behavior may be due to the fact that at the collector concentration tested (104 M DTPINa), the contact angle is already high (i.e., full galena floatability (see Pecina-Trevin˜o et al., 2003)). This may reflect the chemical affinity of the collector toward lead species. In tests performed at potentials around 200 mV, the formation of a white precipitate was observed (most probably a metal–collector compound Pb(II)–DTPI). The amount of precipitate increased as the anodic potential gets more anodic. The contact angle of pyrite in presence of DTPINa as a function of the applied potential is presented in Fig. 9 (in the absence of collector the mineral remained hydrophilic in the entire range of potential tested). Pyrite starts to become hydrophobic at a potential of about 0 mV (and in presence of DTPINa), thus suggesting the development of an electrochemical process where the collector is involved. The fact that onset of hydrophobicity of the mineral coincides with the current peak A1 (located at about 40 mV), suggests the oxidation of DTPI is related to the onset of hydrophobicity. At potentials above 0 mV, contact angle starts to rise, and at ca. 100 mV reaches values in the order of 60, due to the increase of concentration of hydrophobic species onto the mineral surface. It is worth mentioning that, in the present case, it is not possible to establish a relationship with the OCP values of pyrite, mainly due to their scattering. As well, it is important to mention that contrary to the phenomenon observed with galena, in these experiments formation of a white precipitate was not observed. 3.4. Proposed mechanisms for the interaction of DTPINa with sulfides to generate a hydrophobic surface The analysis of the results obtained by OCP, cyclic voltammetry and contact angle, leads to the following proposed mechanisms to explain the mineral–collector interaction in alkaline media (pH 9).

ð11Þ

½PbS; OH surface þ DTPI aqueous () ½PbS; DTPI surface þ OH þ

½PbOH surface þ

DTPI aqeous

ð12Þ þ

() ½PbðDTPIÞ surface þ OH ð13Þ

½PbðOHÞ2 surface þ 2DTPI aqueous () ½PbðDTPIÞ2 surface þ 2OH

ð14Þ

A similar mechanism was proposed by Maier and Dobia´s (1997) for the interaction of sulfides with mercaptobenzothiazole (MBT), which is, as DTPINa, a chelating agent. These authors suggest that the mechanism of MBT adsorption onto galena initiates with the chemisorption of the collector through processes that do not involve electron transfer steps. (ii) At potentials above 30 mV, a chemical reaction between the anion DTPI and the metallic sites of the mineral surface may occur to form a metal–collector compound. The mechanism involved may be regarded as a two steps process consisting of an electrochemical reaction (e), such as that represented in Eq. (2), followed by a chemical reaction (c), as the one represented in Eq. (15): 2DTPI + Pb2þ () Pb(DTPI)2

ð15Þ

The overall mechanism being: 2DTPI + PbS () Pb(DTPI)2 + So + 2e

ð16Þ

This mechanism is known as a coupled electrochemical– chemical reaction (ec) (Greef et al., 1985). This proposed ec mechanism correlates well with observations made during the measurements of contact angle on galena, where the precipitation of a metal–collector compound was observed at potentials near (and above) the potential of peak A1 (see description of Fig. 9). Electrokinetic measurements presented elsewhere (Pecina-Trevin˜o et al., 2003) confirm the Pb(DTPI)2 compound as one of the products of the mineral–collector interaction. As stated in standard electrochemical books (Greef et al., 1985), and summarized

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by Basilio and Yoon (1992), in an ec mechanism, the electrochemical potential of the system controls the oxidation of mineral, whereas the Ksp of the metal–collector compound controls the chemical step; that is, the potential defines the availability of metal ions and the Ksp determines if the metal–collector compound may precipitate. The low Ksp value of Pb(DTPI)2 (5 · 1017) indicates that the precipitation of this compound is possible even if the galena is only slightly oxidized (e.g., low concentration of lead ions). (iii) Lastly, it is worth mentioning that in the presence of collector and at potentials more anodic than 650 mV, the oxidation of the mineral (peak A2) is favored over the interaction with the collector. The proposed reaction for very oxidizing potentials is analogous to that established in the presence of xanthates, namely elemental sulfur is oxi2 2 dized to oxy-sulfuric species ðS2 O2 3 ; SO3 ; SO4 Þ. The results obtained with pyrite in alkaline suspensions (pH 9) suggest that the collector interacts with the mineral through electrochemical mechanisms: (i) Electrochemical processes such as adsorption of collector and dimer formation occurring at a potential over 40 mV. The potential of the current increase associated to the oxidation of the collector (adsorption of collector and dimer formation), designated A1, correlates well with the potential of the onset of pyrite hydrophobicity. Analogous to xanthates, in this electrochemical mechanism, the mineral electrode is not directly involved in any oxidation reaction, except for offering a path in the electron transfer, from the site where the collector oxidizes to the site where the oxygen is cathodically reduced. Therefore, the peak A1 of voltammograms of pyrite could be related to the following reactions: Electrochemical adsorption: DTPI () (DTPI)ads + e

ð17Þ

Collector oxidation: 2DTPI () (DTPI)2 + 2e

ð18Þ

4. Conclusions The analysis of the results presented in this paper suggests that sodium di-isobutyl dithiophosphinate interacts with galena in alkaline media (pH 9) by chemical and electrochemical mechanisms through the following processes: chemisorption without electron transfer occurring at cathodic potentials (below 30 mV); the precipitation of a metal– collector compound that takes place at potentials above 30 mV, that involves two steps: the oxidation of the mineral (electrochemical step) and the formation of the metal–collector compound (chemical step). The results for pyrite at pH 9 suggest that sodium-diisobutyl dithiophosphinate interacts with this mineral according to an electrochemical mechanism (adsorption of collector, dimer formation) that occurs at potentials above 40 mV.

The results indicate that selective flotation of galena in the presence of pyrite will be enhanced by using an electrochemical potential below 40 mV. Acknowledgements E.T. Pecina gratefully acknowledges the financial support received from CONACYT Mexico for the support provided to complete Ph.D. studies. The authors acknowledge the helpful comments of Prof. Ignacio Gonzalez (UAM-Iztapalapa). References Abraitis, P., Brandon, N.P., England, K.E.R., Kelsall, G.H., Lenniee, A.R., Patrick, R.A.D., Vaughan, D.J., Yin, Q., 2000. Electrochemical oxidation of pyrite in alkaline electrolytes: an investigation employing cyclic voltammetry, in situ scanning probe microscopy and ex situ X-ray photoelectron spectroscopy. In: Woods, R., Doyle, F. (Eds.), Electrochemistry in mineral and metal processing V (Proc. of the Electrochemical Society Congress). The Electrochemical Society, USA, pp. 206–217. Abramov, A.A., Avdohin, V.M., 1997. Oxidation of sulfide minerals in benefication process. Gordon and Breach Science Publishers, The Netherlands, p. 321. Ahmed, S.M., 1978. Electrochemical studies of sulphide. I. The electrocatalytic activity of galena, pyrite and cobalt sulphide for oxygen reduction in relation to xanthate adsorption and flotation. Int. J. Min. Process. 5, 163–164. Basilio, C., Yoon, R.H, 1992. Mechanism of TCB adsorption on copper and chalcocite. In: Woods, R., Richardson, P.E. (Eds.), Electrochemistry in mineral and metal processing III. The Electrochemical Society, Pennington, N.J., pp. 79–94. Chander, S., Briceno, A., 1987. Kinetics of pyrite oxidation. Miner. Metall. Process. 4 (3), 171–176. Gardner, J.R., Woods, R., 1979. A study of the surface oxidation of galena using cyclic voltammetry. J. Electroanal. Chem. 100, 447– 459. Gorken, A., Nagaraj, D.R., Riccio, P., 1992. The role of pulp redox potential and modifiers in complex sulfide flotation with DTPI. In: Woods, R., Richardson, R. (Eds.), ECS Symposium St Louis, LA (Proc. Int. Symp. Electrochemical Miner. Met. Process., III). Electrochemical Society, Pennington, NJ, PV, pp. 92–117. Greef, R., Peat, R., Peter, L.M., Pletcher, D., Robinson, J., 1985. Instrumental methods in electrochemistry. Ellis Horwood Ltd., West Sussex, England, p. 443. Hamilton, I.C., Woods, R., 1981. An investigation of surface oxidation of pyrite and pyrrhotite by linear potential sweep voltammetry. J. Electroanal. Chem. 118, 327–343. Lamache, M., Bauer, D., Pegouret, J., 1981. Compartement electrochimique de la galene (PbS) dans les conditions de pH proches de celles de la flottation. Electrochim. Acta 26 (12), 1845–1850. Maier, G.S., Dobia´s, B., 1997. 2-Mercaptobenzothiazole and derivatives in the flotation of galena, chalcocite and sphalerite: a study of flotation, adsorption and microcalorimetry. Miner. Eng. 10 (12), 1375– 1393. Pecina-Trevin˜o, T., Uribe-Salas, A., Nava-Alonso, F., 2003. On the sodium-di-isobutyl dithiophosphinate (Aerophine 3418A) interaction with activated and unactivated galena and pyrite. Int. J. Miner. Process. 71, 201–217. Richardson, P.E., Maust, E.E., 1976. Surface stoichiometry of galena in aqueous electrolytes and its effect on xanthate interactions. In: Fuerstenau, M.C. (Ed.), Flotation: A.M. Gaudin Memorial. AIME, USA, pp. 364–392.

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