Copper and xanthate adsorption onto pyrite surfaces: Implications for mineral separation through flotation

International Journal of Mineral Processing 114–117 (2012) 16–26

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Copper and xanthate adsorption onto pyrite surfaces: Implications for mineral separation through flotation Anand P. Chandra a, Ljiljana Puskar b, Darren J. Simpson a, Andrea R. Gerson a,⁎ a b

Minerals and Materials Science & Technology, Mawson Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Infrared Microspectroscopy beamline, Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia

a r t i c l e

i n f o

Article history: Received 4 May 2011 Received in revised form 14 August 2012 Accepted 18 August 2012 Available online 30 August 2012 Keywords: Pyrite Flotation Copper activation Photoemission electron microscopy (PEEM) FTIR

a b s t r a c t The findings of synchrotron based spectromicroscopic investigations of Cu and xanthate adsorption onto pyrite surfaces are reported. Synchrotron based photoemission electron microscopy measurements have revealed that Cu adsorbs only onto specific surface sites on pyrite. The Cu 2p X-ray absorption near edge structure and position (2p3/2 peak at 934.3 eV) confirms the presence of Cu + with transitions from 2p to 4s states being apparent. Moreover, the peak position is typical of Cu + bonded to S, Cu(I)\S. This is confirmed by S 2p X-ray photoelectron spectroscopy with oxidised S 2− species (S −), most likely resulting from electrochemical interaction of solution Cu 2+ and surface S2− species, being observed. Fe 2p X-ray absorption spectroscopy shows that surface Fe does not take part in the Cu adsorption or subsequent restructuring processes. Synchrotron FTIR microscopy analysis of xanthate (potassium ethyl xanthate, KEX) adsorbed onto Cu activated surfaces also shows a heterogeneous distribution on pyrite. The use of a dilute (10−5 M) KEX concentration shows Cu(I)–xanthate as the only surface xanthate containing species. This species may have formed through a chemical process involving previously adsorbed Cu+ and the xanthate from solution. When a greater (10 −3 M) concentration of KEX is used significant concentrations of Cu(I)–xanthate precipitate from solution onto the pyrite surfaces and cannot be easily removed. Under this condition, the predominant surface species remains Cu(I)–xanthate with smaller concentrations of diethyl dixanthogen also being present. The latter species may form in solution although some previous studies also suggest direct surface formation. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction Sphalerite (ZnS) and pyrite (FeS2) frequently occur together in ore deposits along with galena (PbS) and Cu containing minerals such as chalcopyrite (CuFeS2), chalcocite (Cu2S), covellite (CuS) and bornite (Cu5FeS4). The mineral industry's common practice is to first float the Cu containing minerals (if present) followed by galena (Wills, 1997; Woodcock et al., 2007). The tails from the galena flotation are then used as the feed for sphalerite flotation and separation from pyrite, primarily via Cu activation. Effective separation of these two minerals is needed to minimise the Fe component in the final Zn concentrate. Loss of selectivity and unwanted activation of pyrite can occur due to contaminants present in the process water or due to Cu solubilised from Cu bearing minerals (Lascelles and Finch, 2002; Wong et al., 2002). Despite continuous process improvements the problem of pyrite misreporting to sphalerite concentrates still remains (Finch et al., 2007; Natarajan and Nirdosh, 2006). Both activated and unactivated pyrite interacts with thiol collector molecules such as xanthates (Leppinen, 1990; Zhang et al., 1997). ⁎ Corresponding author. Tel.: +61 8 83023044; fax: +61 8 8305545. E-mail address: [email protected] (A.R. Gerson).

However, it is still not clear if the Cu activation and xanthate adsorption mechanisms are an electrochemical or a chemical process (Chandra and Gerson, 2009; Hicyilmaz et al., 2004; Pecina et al., 2006; Zhang et al., 1997). It has been shown by Weisener and Gerson (2000a) that the Cu activation mechanism does not involve a 1:1 exchange of the mineral metal cation with Cu as is the case for sphalerite. Furthermore, it is believed that upon adsorption on to a reactive S site Cu 2 + is reduced to Cu + and that the adsorbed ion does not migrate into the bulk structure (Boulton et al., 2003; Weisener and Gerson, 2000a,b). The structure formed upon Cu adsorption is not fully characterised, however, phases such as CuO, CuS and Cu2S have previously been suggested to occur on Cu activated minerals such as pyrite and sphalerite (Chen and Yoon, 2000; He et al., 2005, 2006; Pattrick et al., 1999). Cu(OH)2 species may also exist on the surface when activation takes place at greater pH (Weisener and Gerson, 2000b). Species such as Cu(I)–xanthate and dixanthogen are also suggested to occur upon xanthate adsorption (Leppinen, 1990; Shen et al., 1998; Zhang et al., 1997). Further details of pyrite copper activation and flotation are provided in Chandra and Gerson (2009) and references therein. Despite decades of research and widespread use in industrial mineral processing circuits, the exact Cu activation of, and collector

0301-7516/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2012.08.003

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2. Methodology 2.1. Pyrite sample and preparation A highly pure pyrite specimen from Ward's Scientific (origin Huanzala, Peru) was used throughout the study. Powder X-ray diffraction (XRD) confirmed pyrite to be the only detectable phase present. ICP-OES (inductively coupled plasma–optical emission spectrometry, Table 1) elemental analysis indicated a S:Fe atomic ratio of 2.00 and the total impurity content to be approximately 0.7 wt.%. The identification of Si suggests the presence of some minor silicate phase that is either present at concentrations below the detection limit for laboratory powder XRD or is amorphous. For PEEM and reflectance FTIR measurements the samples need to be extremely flat (highly polished) and to have parallel horizontal surfaces so that the surface is uniformly illuminated by the beam and remains in focus throughout the measurements. In addition PEEM requires a flat surface with limited surface defects such as pits and depressions to avoid field emission effects, which can distort images and introduce artefacts. For IR measurements a highly polished reflective and flat surface is needed so that the IR beam can reflect off the surface with measurable intensity. Therefore, polished (1 μm mirror finish) pyrite slabs of lateral dimensions of 7–10 mm and thickness of approximately 5 mm were used for both measurements. Polishing of the samples was carried out using silicon carbide grit papers and diamond paste. Polishing of a pyrite surface reduces the natural surface roughness compared to a fracture surface. It should be noted that in Section 3 information on, and subsequent discussion of, surface species were obtained from studies reported in the literature that have primarily used a fractured surface. 2.2. PEEM measurements PEEM measurements were conducted at the Canadian Light Source, soft X-ray microscopy beamline, PEEM end-station. PEEM uses an electron imaging system for magnification and projection of emitted electrons, and provides spectromicroscopic data of high spatial resolution. This technique enables correlation of topographic images with surface chemical information at a sub-micron scale. PEEM is usually capable of X-ray absorption near edge spectroscopy (XANES) imaging by measurement of a fixed electron kinetic energy as a function of incident X-ray energy and if the setup allows, XPS imaging can be conducted by measurement of a range of electron kinetic energies as a function of fixed incident X-ray energy. In general, spatial resolution ranges from b100 nm to b10 nm on new generation PEEM facilities. XANES imaging probes approximately 2 nm to 5 nm into the sample surface while XPS imaging probes approximately 1 nm into the sample surface. Microscopic and spectroscopic information is gathered through parallel measurements where detailed images are taken at each energy point across the absorption edge (or photoelectron peak) of a

selected element. Spectroscopic information is then derived from this series of images (or stacks) by first selecting an area (or areas) of interest and plotting the intensity of the region through the stack. Refer to Chandra and Gerson (2010) and references therein for a detailed explanation of the PEEM technique, different PEEM setups and data processing approaches. A freshly polished pyrite slab was Cu activated for 15 min in 10−5 M copper nitrate solution at pH 5 in N2 purged (for 30 min) and pH adjusted (using sulphuric acid) milli-Q water. N2 was used as the conditioning gas as it has been shown to be beneficial for pyrite Cu activation and flotation (Shen et al., 2001). Solution Eh and pH were monitored throughout the conditioning procedure. The speciation diagram (generated using Visual MINTEQ ver. 2.52 programme) for aqueous Cu species (Fig. 1) shows that at the stabilised solution Eh of 370 mV (SHE) and pH 5, and under the activation conditions used, there would be no precipitation of Cu hydroxides. Therefore the activating species at pH 5 at this solution concentration of Cu can only be aqueous Cu 2+. Cu(OH)2 colloids tend to complicate the activation process by adsorbing non-selectively onto mineral surfaces (Fornasiero and Ralston, 2006; Gerson et al., 1999; Prestidge et al., 1997; Weisener and Gerson, 2000b). Hence, we have chosen a smaller pH (pH 5) for this study than is typical of many industrial plant scenarios. Furthermore, while frequent plant practice is to conduct flotation (especially for separating sphalerite from pyrite) at approximately pH 8–9 (Hayes, 1993; Shen et al., 1998), some plants such as Teck Cominco also carry out flotation at the smaller pH of approximately 5 (Harmer et al., 2008). However, it has been noted that at alkaline pH copper carbonate complexes also become a major species as a result of atmospheric CO2 (O'Dea et al., 2001). After activation the slab was removed from the solution and washed with pH 5 milli-Q water, immediately transferred onto the sample holder and into the UHV chamber. During the transfer process the pyrite surface was air dried. Video mode measurement at the Cu 2p absorption edge was used to survey the surface to identify locations of surface adsorbed Cu. Cu 2p, Fe 2p and O 1s edge XANES spectromicroscopy measurements were conducted at these locations as was S 2p XPS spectromicroscopy. Characterisation of an unactivated sample did not show any surface Cu to be present naturally in the sample. The images (stack) collected at Canadian Light Source (CLS) were analysed using the aXis2000 software available at the website: http:// unicorn.mcmaster.ca/aXis2000.html. The analysis involves alignment of the stack of images prior to extracting site specific spectroscopic data. The extracted data were normalised using the incident photon intensity (I0) to account for the decreasing beam flux during measurements and intensity losses due to the oxide layer on the gold mesh filter that was used to monitor I0. Energy calibration of the XANES data was also conducted from data available at the beamline.

pH 0

2

4

6

8

10

-4.4

-4.8

Log [M]

adsorption mechanism onto, pyrite and the related surface speciation are poorly understood. Synchrotron based photoemission electron microscopy (PEEM) was used to identify the oxidation state and distribution pattern of Cu on pyrite surfaces after activation and to examine the involvement of surface Fe or S (or both) in the Cu adsorption process. Synchrotron based Fourier transform infrared (FTIR) microscopy was also used to identify the mechanism of xanthate adsorption onto activated and unactivated pyrite surfaces in the presence of various concentrations of xanthate.

17

2+

-5.2

-5.6

Cu CuCO3 (aq) CuOH+ Cu(CO3)22-

Table 1 Elemental composition of pyrite sample. S

Fe

Si

Cu

Ca

As

Ag

Pb

Sb

Al

K

Zn

Wt.% 53.1 46.2 0.20 0.003 0.20 0.07 0.002 0.02 0.008 0.01 0.1 0.03

-6.0 Fig. 1. Solution speciation diagram for Cu under the activation conditions used.

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XANES data were further processed using the IFEFFIT software package, Athena (Ravel, 2001; Ravel and Newville, 2005). XANES chemical maps were obtained by normalising images collected at the absorption edge using an image below the absorption edge. This results in chemical maps of species distribution while also removing topographic effects from the images. 2.3. IR microscopy measurements The specular external reflection IR method was used on pyrite samples as they could easily be polished to be reflective, and therefore was the most suitable method for obtaining surface information. The intensity of the reflected signal depends on the angle of incidence, the refractive index, surface roughness and absorption properties of the sample. Infrared measurements were performed at the infrared microspectroscopy beamline at the Australian Synchrotron. The beamline comprises a Bruker Hyperion 2000 IR microscope coupled to a Bruker Vertex 80v FTIR spectrometer. The microscope uses a 100× 100 μm single element narrow-band mercury cadmium telluride (MCT) detector (cut-off 750 cm−1). Spectra were collected in reflectance mode using an Ealing 36× Schwarzschild optic with 0.5 numerical aperture. Optical images of the sample surface were recorded to define the regions of interest prior to IR analysis. Multi-point grid maps were taken by raster scanning the sample using a motorized x-y sample stage. The sample stage was housed in a Perspex enclosure with dry N2 purging to avoid spectral contributions from atmospheric gases such as H2O and CO2. Background measurements were collected off a gold mirror or a fresh pyrite surface and these were subtracted from the sample spectra thus also removing any small contribution from atmospheric gases. Apertures of 5 × 5, 10× 10 or 20× 20 μm were used (depending on the reflected IR intensity) and IR spectra from 3000 to 750 cm−1 were acquired. Depending on the reflected IR intensity 500 to 2000 scans were collected at each sample and background point with a spectral resolution of 4 cm−1. Spectra were collected and processed using Bruker OPUS software version 6.5. Spatial resolution is wavelength dependent (diffraction limited) and can range from 3 to 20 μm for mineral samples. In the 4000 to 750 cm −1 wavelength range IR is approximately 1000 times more penetrating than a conventional X-ray beam and in the 2000– 600 cm −1 range the information depth can range from 25,000 to 83,000 Å (de Donato et al., 1993). However, this technique can provide surface sensitive information for adsorbed species. Reflectance mode IR measurements were performed on Cu activated and unactivated pyrite samples conditioned with potassium ethyl xanthate (KEX), CH3CH2OCS2K, collector molecule. The KEX used was Alfa Aesar 98% purity. A freshly polished pyrite sample was Cu activated as described in Section 2.2, and subsequently conditioned for 2 min in either 10 −5 M (144 g/t) or 10 −3 M (14,400 g/t) KEX solution. Typical industrial usage of xanthate ranges from 5 to 350 g/t (Woodcock et al., 2007). After conditioning the sample was washed (with pH 5 H2SO4 solution) and immediately mounted on the sample stage for reflectance IR analysis. It should be noted that the sample was not washed subsequent to Cu activation. The sample surface was dried under N2 inside the Perspex enclosure which encased the sample stage. Measurements were also performed on unactivated pyrite samples conditioned with the two different concentrations of KEX solution. 3. Results and discussion 3.1. PEEM analysis of Cu activated pyrite PEEM measurements conducted at CLS show that Cu was not evenly adsorbed onto the pyrite surface. Fig. 2a shows a PEEM image taken at the Cu 2p3/2 X-ray absorption spectroscopy (XAS)

energy of 934.3 eV with a 50 μm field of view. This is a raw photoelectron image which includes topographic contributions. To remove the topographic effects this image was normalised with an image recorded at the incident X-ray energy of 928 eV (below the Cu 2p edge). The normalised image is shown in Fig. 2b. The intense region of approximately 6.3 μm diameter represents a Cu rich spot. Spots 1 and 2 in Fig. 2 indicate positions from which Cu 2p, Fe 2p and O 1s XAS, and S 2p XPS were recorded. The total area from which the spectroscopic information was obtained was approximately 5 × 5 μm at each of the marked spots. The O 1s XAS measurements failed to show any discernible O 1s absorption edge at either spot 1 or 2. Cu 2p XAS from spot 1 is shown in Fig. 2c along with Cu metal reference data. XAS data from spot 2 did not show any Cu 2p adsorption edge (as expected from Fig. 2b). The Cu metal XAS peak position was referenced to 932.7 eV. The peak energy of the Cu adsorbed on the pyrite occurred at 934.3 eV which is close to the energy (934.6 eV) observed for chalcocite (Todd et al., 2003). The Cu adsorbed onto the pyrite surface is therefore consistent with Cu+ as expected from previous observations. Cu 2+ has a predominant 3d 9 character in the ground state. According to Pattrick et al. (1997) the 2p edge of Cu in the + 2 oxidation state is characterised by transitions from 2p3/2 and 2p1/2 to 3d states. Cu + has a fully occupied 3d shell (3d 10) and hence there is no 2p to 3d transition, however transitions from 2p states to 4s states can take place. The transition probability of 2p to 4s is however relatively small compared to the 2p to 3d transition (Pattrick et al., 1997; Todd et al., 2003). This is due to the relatively large overlap of 2p and 3d energies compared to 2p and 4s and as such Cu 2+ 2p to 3d transitions tend to have up to 25 times greater intensity than Cu + 2p to 4s transitions (Pattrick et al., 1997). Fig. 3a shows the Fe PEEM image taken at the Fe 2p XAS energy of 708.8 eV which is dominated by topographic effects. A normalised Fe image (using an image recorded at 704.0 eV) is shown in Fig. 3b. The normalised image shows that Fe is reduced in intensity at the position corresponding to high Cu content (spot 1) which is probably due to attenuation of the Fe signal by surface Cu species. The Fe 2p XAS spectra obtained from spots 1 and 2 are shown in Fig. 4. The Fe 2p3/2 energy position and spectral shape is exactly the same as those reported for unactivated pyrite (Garvie and Buseck, 2004; Thole and Van Der Laan, 1988). The Fe 2p3/2 and 2p1/2 peaks of the spectra from spot 1 (concentrated Cu spot) show a notch near the peak apex, however a high resolution spectrum of the peak apex (shown as an inset in Fig. 4) confirmed this to be due to spectral noise. Furthermore, the L3/L2 intensity ratio for both spectra was found to be 2.1 which is close to the value (2.0) for ground state Fe (Doyle et al., 2004; Thole and Van Der Laan, 1988). This confirms that the Fe species at these locations retain the low spin diamagnetic configuration of pyrite Fe 2+. Therefore Cu adsorption does not appear to significantly perturb nearby Fe electronic structure. This outcome is compatible with previously reported observations that suggested that there is no exchange of surface Fe with Cu (Weisener and Gerson, 2000a,b). S 2p XPS (XPS-imaging PEEM) measurements were conducted and the spectra from spots 1 (Cu spot) and 2 (no Cu, as shown in Fig. 2) were extracted. Components of pyrite S 2p XPS spectra have been identified by several authors previously (Demoisson et al., 2007; Nesbitt et al., 1998; Nesbitt and Muir, 1994; Schaufuß et al., 1998a,b). A S 3p to Fe 3d (eg) energy loss feature has also been previously included in S 2p fitting (Acres et al., 2010a,b; Harmer et al., 2004; Klauber, 2003; Nesbitt et al., 2000) based primarily on the work of Fujisawa et al. (1994), and Bronold et al. (1994). It is, however, unclear as to the relative intensity contribution made by such an energy loss peak near 164.9 eV and if it is significant enough to be included in the fits, especially when other S species such as S0 and SO32− are also present in this binding energy region. The asymmetric peak shape of the S 2p spectra has been attributed to this energy loss feature but has also previously been attributed to the high density of states near the Fermi level (Yin

A.P. Chandra et al. / International Journal of Mineral Processing 114–117 (2012) 16–26

19

a

b

Normalised Intensity [arb. units]

c 1.5

1.0

0.5

Cu metal reference Adsorbed Cu spot 1

0.0 930

940

950

960

970

Photon Energy [eV] Fig. 2. (a) Cu 2p images of a pyrite surface after 15 min of Cu activation at pH 5 taken using the incident X-ray energy of 934.3 eV, (b) the normalised image of (a) and (c) Cu 2p XAS spectra from the Cu rich spot 1 and from Cu metal reference.

et al., 1995) with no energy loss feature being included in the fitting (Abraitis et al., 2004; Sasaki et al., 2010). Therefore, to avoid this ambiguity, the maximum spectral measured intensity of the S 2p spectra from the pyrite feed material and the activated surface were first

normalised to 1, and then the spectrum of the feed material was subtracted from the spectra of spot 1 and spot 2, to emphasise changes to S containing species resulting from the activation process (Fig. 5a). This process accounts for any energy loss feature, if present, as the

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A.P. Chandra et al. / International Journal of Mineral Processing 114–117 (2012) 16–26

Fig. 3. Fe 2p image, taken at 708.8 eV (a) and the normalised image (b), of a pyrite surface after 15 min of Cu activation at pH 5.

intensity of this feature would be very similar in all normalised spectra and would be removed on subtraction of the spectra of the feed material. Fig. 5b shows the fitting of components in the difference spectrum of spot 2. Similar components were fitted for spot 1 (not shown). The fitting of the positive and negative regions of the difference spectra was conducted using two separate Shirley background functions.

Normalised Intensity [arb. units]

3.5 3.0 2.5 2.0

708.8 eV 1.5 1.0 0.5 0.0 700

Spot 2 Spot 1 705

710

715

720

725

Photon Energy [eV] Fig. 4. Fe 2p XAS spectra from spots 1 and 2, as shown in Fig. 3. The inset shows a high resolution spectrum of the apex of 2p3/2 peak of spot 1.

The component at 161.0 eV is identified as the S 2p3/2 peak arising from S 2− (Buckley and Woods, 1987; Nesbitt et al., 1998; Nesbitt and Muir, 1994; Schaufuß et al., 1998b) and the region on the activated surface analysed has a relatively smaller surface S 2− component as compared to the feed, causing negative intensity in the difference spectrum. S 2p3/2 has a spin-orbit doublet at + 1.18 eV of 50% intensity. Therefore the S 2− component will also give rise to a 2p1/2 contribution near 162.1 eV. However, the even more reduced intensity of the difference spectra near 162.1 eV indicates that spots 1 and 2 also have a relatively reduced abundance of surface S22 − as compared to the feed, the S 2p3/2 component of which occurs in the binding energy range of 162.0 to 162.7 eV (Demoisson et al., 2007; Descostes et al., 2001; Nesbitt et al., 1998; Nesbitt and Muir, 1994). The smaller S 2− and S22− surface components are most likely the result of partial dissolution during the mineral conditioning and activation processes (Schaufuß et al., 1998a,b). The S22− component gives rise to a S 2p1/2 contribution near 163.2 eV, however; this could not be fitted in the difference spectra due to positive intensity in this region. This increased intensity of the difference spectra suggests the presence of greater amounts of Sn2− species relative to the feed. This species gives rise to a S 2p3/2 component at 163.2 eV with corresponding S 2p1/2 contribution near 164.4 eV (Nesbitt and Muir, 1994; Smart et al., 1999). The intensity of the Sn2− 2p3/2 component also contains an unknown negative contribution from the S22− 2p1/2 intensity. The larger than expected intensity at 164.4 eV, on the basis of the S 2p3/2 Sn2− intensity, suggests the presence of a further additional S component. The species contributing to this intensity may be

A.P. Chandra et al. / International Journal of Mineral Processing 114–117 (2012) 16–26

a 1.2

0.8

Feed Spot 1 - Feed Spot 2 - Feed 163.2

0.6

164.4

Normalised Intensity

1.0

162.3

161.0

0.4

165.6

0.2 0.0 -0.2 168

166

164

162

160

158

Binding Energy [eV]

Normalised Intensity

b

0.3

0.2

Difference Spect S-OH Sn2-

0.1

Fit/Background

S2S22-

0.0

-0.1

-0.2 168

166

164

162

160

158

Binding Energy [eV]

c 0.30

Normalised Intensity

0.25

Difference Spectrum Ox-S2-

0.20

S2Fit/Background

0.15 0.10 0.05 0.00 164

163

162

161

160

Binding Energy [eV] Fig. 5. (a) S 2p spectrum of pyrite feed (surface image not shown) and difference S 2p spectra from spots 1 (Cu spot) and 2 (no Cu). (b) Fitted components of the difference spectrum from spot 2. (c) Fitted S 2p difference spectrum of spots 1 (Cu spot) minus spot 2 (no Cu).

identified as S–OH (2p3/2) with the 2p3/2 contribution near 164.2 eV (Schaufuß et al., 1998a). The relatively greater presence of Sn2− and S–OH and the reduced presence of S 2− and S22−, species are due most likely to oxidation during mineral conditioning and activation. The oxidation may be due to the presence of O2/H2O or the Cu 2+ adsorption processes.

21

The S speciation components resulting from Cu adsorption can be identified from a difference spectrum of spot 1 (Cu spot) and spot 2 (non Cu spot). Fig. 5c shows the fitted difference spectrum obtained by subtracting the normalised spectrum of spot 2 from spot 1. In this instance only, the spectra has been smoothed and also a linear background was used due, in both cases to the relatively high level of noise of the difference spectra. The fitting of the difference spectrum shows the presence of two species with 2p3/2 components at 161.1 and 161.8 eV. The species at 161.1 eV is identified as S 2−. Thus the Cu spot appears to be rich in S 2− species compared to the non-Cu containing spot (spot 2). The 2p3/2 component at 161.8 eV is a new species formed after Cu adsorption and is discussed in the following paragraphs. The surface S 2− species has previously been found to be the most reactive surface S species (Schaufuß et al., 1998a). Such S species are found to be concentrated at surface defect sites, fracture planes and ridges. Surface Fe species have been suggested to be more favourable energetically, than surface S species, to surface redox processes (Rosso et al., 1999). However, surface Fe does not appear to play a role in the Cu activation process of pyrite. It has previously been suggested that under low pH conditions Cu 2+ adsorption onto reactive S sites is a fast single step process followed by reduction of Cu 2+ to Cu + and oxidation of associated S (Weisener and Gerson, 2000b). It therefore appears that the solution Cu 2+ interacts with surface S 2 − species at such surface sites. This may be the fast step as previously suggested. This must then be followed by a redox reaction involving reduction of Cu and oxidation of the S 2− species. This may involve the partial delocalisation of the S electron cloud towards Cu, thus giving it more 3d 10 character typical of Cu +. The adsorption process therefore produces a new oxidised S2− (Ox-S2−) species. According to Schaufuß et al. (1998a), the XPS binding energy of S 2p3/2 shifts 0.8 eV to higher binding energy for each change in oxidation number of surface S species of pyrite. Therefore, the binding energy of S 2− (161.0–161.1 eV) would increase to approximately 161.8 eV after oxidation. Spot 1 (Cu spot) still contains a relatively high concentration of S2− as it may not be spatially/energetically possible for enough Cu to adsorb to oxidise all the S2− species present. Angle resolved S 2p XPS studies of Weisener and Gerson (2000b) found a new component at 163.5 eV binding energy, which they attributed to oxidised S22− (S2 2−ox) species. Laajalehto et al. (1999) also found a new S 2p component at 161.6 eV on pyrite surfaces activated at pH 5, which they attributed to chalcopyrite-type S. Based on this, they suggested that a chalcopyrite like species forms upon Cu activation of pyrite. Recent studies also support the development of a Cu sulphide species upon reduction of Cu 2+ to Cu + which is suggested to occur via an electrochemical process (Peng and Grano, in press). Using Fourier transform infrared spectroscopy employing attenuated total reflection (FTIR-ATR) Leppinen (1990) suggested that the pyrite surface upon activation is fully covered, in contrast to findings from this study, with copper-sulphide like products which more closely resemble Cu2S than CuS. Naveau et al. (2006), using extended X-ray absorption fine structure (EXAFS), suggested that Cu + on pyrite surfaces activated at pH 4 to 6, is in tetrahedral coordination with two S atoms and two O atoms. However, no O edge was discernible at the concentrated Cu region shown in Fig. 3b. An earlier EXAFS study with pyrite activated at smaller pH has indicated that Cu has a distorted trigonal planar position between three sulphur atoms (Weisener and Gerson, 2000a) with an average Cu\S bond length of 2.27 ± 0.02 Å. Cu\O bonds (bond length 2.00 Å) were only shown to occur at alkaline pH resulting from Cu(OH)2 precipitates. Cu activation of other sulphide minerals such as sphalerite is suggested to also result in the formation of polysulphide/oligosulphide like surface environments including S 0 (Buckley et al., 2007; Fornasiero and Ralston, 2006; Kartio et al., 1998; Popov and Vucinic, 1990a,b). No such phases were evident at the concentrated Cu region (spot 1). The Sn2− species present at this spot is most likely present prior to

A.P. Chandra et al. / International Journal of Mineral Processing 114–117 (2012) 16–26

2−

ð1Þ

However, no evidence is provided to suggest that S release into solution is enhanced by Cu activation nor is any evidence provided which suggests that surface S is depleted due to Cu activation. Pyrite Cu activation does not involve migration of Cu into the bulk pyrite structure as demonstrated previously using angle resolved XPS (Weisener and Gerson, 2000b); hence, reaction of Cu with pyrite would not involve bulk oxidation states. Since Cu only reacts with surface states, we suggest the following reaction mechanism that is also consistent with speciation data collected in this study:

0.15

1008

0.05

1158 1172

0.00 1400

1300

1200

1100

1000

900

Wavenumber [cm-1]

1187

CuEX and (EX)2

ð2Þ

1032

0.8

ð3Þ

Eqs. (2) and (3) provide a simplified mechanistic view as it has been demonstrated that pyrite surfaces are highly heterogeneous (Chandra and Gerson, 2010). However, it is not possible to depict a realistic degree of complexity via this form reaction equation. Nevertheless, these equations serve to demonstrate the basic mechanics of the activation reaction vis-à-vis the surface based oxidation of S 2− to S − and the nature of the final adsorbed Cu +. It should also be noted that these equations in no way suggest the final coordination of the Cu +. 3.2. FTIR of KEX (CH3CH2OCS2K) The structure of KEX includes polar and non-polar ends of the molecule. Upon adsorption, it is the S part of the molecule (anion) that binds to the mineral surface so that the non-polar end is oriented towards the solution, rendering the mineral surface hydrophobic. A reflectance FTIR spectrum of KEX (Fig. 6a.) was obtained from KEX deposited on disposable gold mirrors. The spectrum is very similar

0.6

1122

0.4

1107

The stoichiometric Eq. (2) indicates a surface species composed of S 2− attached to three Fe 2 + which are in turn each attached to a further five S22−, making a total of 15. However these S22− species are further shared by S and Fe atoms and hence the denominator of 6. On the converse side of the fracture, to maintain overall charge neutrality, S 0, also attached to 3 Fe is formed. Hence Eq. (2) represents the speciation that may result from the breakage of a single S\S bond. Both S 2− and S0 have been identified as products of pyrite cleavage as has S2 2−(surface) which may form on fracture of Fe\S bonds (Chandra and Gerson, 2011). It has also been proposed that this latter species may interact with surface adsorbed Cu 2+ to give rise to an oxidised surface S dimer (von Oertzen et al., 2007) but we see no evidence for this species in this study. However, this may be due to the relative ratio of solution Cu to surface S species such that at smaller ratios the S 2 − surface species react preferentially. It is also possible to propose a similar stoichiometric equation for the reaction of the S2 2 −(surface) species with Cu 2 +:

1193

6

   15 2− 2þ 15 2− S2 S2 xFeS2ðsÞ þ 2Hþ S2− 3Fe2þ 2 3Fe 6  6  2þ − 2þ þ 15 2− 2þ 15 2− þ S2 S2 xFeS2ðsÞ þ 2HðaqÞ : 3Fe CuðaqÞ → S2 3Fe Cu 6 6

1048

0.10

b

h

6

1138

1346

0.2

1367

1262

1239

1008

1047

ih i 15 15 2Hþ S2− 3Fe2þ S2− S0 3Fe2þ S2− xFeS2ðsÞ þ 2 2 6 6 h ih i 2þ − 2þ þ 15 2− 0 2þ 15 2− S 3Fe S2 S2 xFeS2ðsÞ þ 2Hþ CuðaqÞ → S 3Fe Cu ðaqÞ :

1096

KEX

0.20

1116

2þ

FeS2ðsolidÞ þ 3xCuðaqÞ þ 4xH2 OðaqÞ →Cu3x FeSð2−xÞðsolidÞ þ xSO4ðaqÞ þ þ 8xHðaqÞ :

a

Absorbance [arb. units]

activation as was observed on the pyrite feed (unactivated pyrite sample). While this study also points to the interaction of Cu and surface S species, no development of a separate crystalline phase is seen to occur. von Oertzen et al. (2007) suggested the following reaction mechanism for adsorption of Cu onto pyrite surfaces:

Absorbance [arb. units]

22

1151

1291

1020

0.0 1400

1300

1200

1100

1000

900

Wavenumber [cm-1] Fig. 6. (a) FTIR spectrum of KEX taken with a 5 × 5 μm aperture, average of 500 scans with 4 cm−1 spectral resolution. (b) FTIR spectrum of yellow precipitates from Cu + KEX solution taken with a 10 × 10 μm aperture, average of 500 scans with 4 cm−1 spectral resolution.

to that obtained previously from solid KEX (Leppinen, 1990). During pyrite conditioning a yellow precipitate (most likely CuEX) was observed immediately upon addition of KEX solution into the Cu solution at the end of the activation period (15 min). This was done to replicate the plant situation where the collector is usually added to the activating solution. There were clearly more precipitates when 10 −3 M KEX was used as compared to 10 −5 M KEX. This yellow precipitate was also found to occur on activated pyrite surfaces and could not be removed upon washing (with pH 5 H2SO4 solution) at the end of the conditioning period. The yellow precipitate was then extracted from the solution and deposited onto reflective gold mirror for FTIR measurements (Fig. 6b.) The FTIR spectra of xanthates are normally characterised by three different absorbance features between 1400 and 1000 cm−1 (Leppinen, 1990; Wang and Forssberg, 1996; Zhang et al., 1997). The absorbance peak frequencies correspond to symmetric and asymmetric C\O\C stretching vibrations and C_S vibrations. Table 2 summarises the spectral frequencies of KEX, Cu(I)–xanthate (CuEX) and diethyl dixanthogen (EX)2 obtained from previously published studies. The spectral bands at 1008 and 1048 cm−1 for KEX in Fig. 6a can therefore be attributed to vibrations of C_S. Bands at 1096 and 1116 cm−1 are attributed to symmetric C\O\C vibrations while those at 1138, 1158 and 1172 cm−1 are attributed to asymmetric C\O\C vibrations.

A.P. Chandra et al. / International Journal of Mineral Processing 114–117 (2012) 16–26 Table 2 Spectral frequencies of ethyl xanthate species. Reference values were obtained from Leppinen (1990), Wang (1995) and Zhang et al. (1997). Species

Vibrational modes va (COC)

v (COC)

v (C = S)

KEX

1145–1138, 1171

1052–1049, 1008

CuEX

1197–1193, 1190–1189 1367, 1290, 1263–1261, 1242–1239

1119–1117, 1105–1100 1123–1122, 1049 1151–1150, 1109–1108

1044–1036, 1026–1019

(EX)2

1035–1034, 1009

The yellow precipitate in the Cu + KEX solution was identified as being predominantly Cu(I)–xanthate with a relatively small amount of diethyl dixanthogen. Aqueous Cu 2+ can readily combine with EX − in solution to form an unstable cupric ethyl xanthate species which dissociates into CuEX (cuprous ethyl xanthate) and (EX)2 (Popov and Vucinic, 1990b). The reaction may be represented as: 2þ

−

2CuðaqÞ þ 4EXðaqÞ →2CuðEXÞ2ðaqÞ →2CuEX þ ðEXÞ2 :

ð4Þ

The FTIR peaks that correspond to CuEX have higher absorbance intensity than the peaks that correspond to (EX)2. In Fig. 6b bands observed at 1008 and 1032 cm −1 can be identified as C_S vibrations in CuEX. The 1047 and 1122 cm −1 peaks are attributed to symmetric vibrations of C\O\C in CuEX while the peaks at 1187 and 1193 (shoulder) cm −1 result from asymmetric C\O\C vibrations, both in CuEX. (EX)2 is mainly identified from the set of peaks between 1239 and 1367 cm −1 corresponding to asymmetric C\O\C vibrations. A peak at 1151 cm −1 and a shoulder at 1107 cm −1 are assigned to symmetric C\O\C vibrations of (EX)2. The C_S vibrations of (EX)2 are overwhelmed by the dominant C_S vibrations of CuEX which occur at similar frequencies. However, a definite shoulder can be seen at approximately 1020 cm −1 most likely due to the C_S vibrations of (EX)2.

23

Fe(III) hydroxide with conversion to Fe 2+. According to the mechanism proposed by Valdivieso et al. (2005), as the hydrophobic dixanthogen develops on the pyrite surface there is a subsequent reduction in hydrophilic surface hydroxide. Fig. 7a shows an optical micrograph of a Cu activated pyrite surface after conditioning in 10 −5 M KEX solution for 2 min. Faint yellowish regions could be seen scattered on the pyrite surface. Numerous FTIR measurements were conducted on these yellow features, which displayed noisy peaks in the 1400–900 cm −1 region. Fig. 7b shows a FTIR spectrum obtained from one of these surface areas. Only one spectrum is shown as all the yellowish regions gave rise to the same spectral peaks. Using Table 2 as a guide, Cu(I)–xanthate (CuEX) is identified as being present in the yellowish areas. As it is known that these Cu activation conditions give rise to surface adsorbed Cu+ it is reasonable to assume that a portion of this CuEX arises from interaction of EX with chemisorbed surface Cu +. However, it is not possible using FTIR to distinguish between precipitated, (CuEX)ppt, and the interaction between surface activating Cu and EX, (CuactEX). There was no evidence of (EX)2 on the surface conditioned with 10 − 5 M KEX solution. Surface areas that appeared normal (areas without yellowish coloration) did not show any peaks (not shown) in this frequency range. Patchy precipitate that could not be removed by washing (with pH 5 solution), was found on the pyrite surface after conditioning of the Cu activated surface with 10 −3 M KEX (Fig. 8a). Five spots are marked on Fig. 8a, at which FTIR measurements were conducted. Spot 5 did not show any peaks in the 1400–900 cm −1 region. The spectra from spots 1 and 2 are shown in Fig. 8b. The rest of the spectra (spots 2 to 4) were essentially the same, hence only the spectra from

a

3.3. IR-microscopy of adsorbed KEX

10 µm

b

0.10 Pyrite + Cu + 10-5 M KEX 1031

0.06

1006

0.08

1046

1184

1196

Logarithmic Reflectance

IR-microscopy measurements were performed on 3 sets of samples: unactivated pyrite samples conditioned with 10 −5 M KEX and activated pyrite samples subsequently conditioned in 10−5 or 10−3 M KEX solution. All samples were washed with pH 5 H2SO4 solution after conditioning with KEX, but not subsequent to Cu activation. It has been reported that pyrite can interact directly with collector molecules such as KEX without activation (Leppinen, 1990; Valdivieso et al., 2005). Measurements on pyrite samples conditioned with only KEX were conducted at numerous spots on the pyrite surface. It has previously been found using FTIR measurements that xanthate on an unactivated pyrite surface is present as ferric xanthate, Fe(EX)3, along with (EX)2 (Leppinen, 1990; Wang, 1995). Fe(EX)3 species can be identified by a characteristic intense band between 1260 and 1235 cm −1 (Leppinen, 1990; Wang, 1995). FTIR measurements on pyrite + KEX sample showed some very weak peaks (data not shown) that may correspond to Fe(EX)3 and (EX)2. However, due to the high noise in the FTIR spectrum (not shown), the existence of Fe(EX)3 species cannot be unambiguously established from this study. Studies by Valdivieso et al. (2005) have shown that adsorption of xanthate onto an unactivated pyrite surface increases the concentration of aqueous Fe 2+. The surface oxidation of xanthate to dixanthogen results in a corresponding reduction of the surface

0.04

1121

0.02

0.00 1400

1300

1200

1100

1000

900

Wavenumber [cm-1] Fig. 7. (a) Optical micrograph of a Cu activated pyrite surface after conditioning in 10−5 M KEX solution for 2 min. (b) FTIR spectrum of KEX on Cu activated pyrite surface taken with 10×10 μm aperture, average of 2000 scans with 4 cm−1 spectral resolution.

24

A.P. Chandra et al. / International Journal of Mineral Processing 114–117 (2012) 16–26

a

with monolayer coverage. Shen et al. (2001) also reported cuprous-xanthate as the only xanthate product formed under their reaction conditions. Leppinen (1990) found pyrite xanthate adsorption, and subsequent flotation, to be Cu concentration and pH dependent. He showed that at pH 7 when equal amounts of Cu and xanthate are used, Cu xanthate is the only product formed, however when either relatively greater or smaller concentrations of Cu is used, significant amounts of dixanthogen also form. He further showed that when a smaller concentration of KEX than Cu is used, no EX2 adsorbs to the surface. This is similar to the current findings where no EX2 was seen when the pyrite was conditioned with 10 −5 M KEX. However, significant amounts of (EX)2 may have formed from interaction of KEX with excess Cu 2+ in solution when conditioning with the greater concentration of KEX. Direct surface formation of (EX)2 has previously been suggested for other minerals such as sphalerite (Popov and Vucinic, 1990a,b) and this may also occur on pyrite surfaces under conditions of excess KEX and Cu 2+. In this study the CuEX on the surface (10 −5 M KEX) was found to be located heterogeneously. This is in line with the findings of the PEEM study where the adsorbed Cu was also found to occur at specific sites. Such specific sites may be related to surface defect areas with higher dangling bond densities that may be more reactive than sites of normal bulk type coordination. The CuEX, when 10 −3 M KEX was used, was also found to precipitate heterogeneously, and most likely non-specifically, onto the surface. Therefore during plant-based flotation the concentration of the collector needs to be optimally chosen as solution precipitation would tend to reduce selectivity causing misreporting of pyrite to concentrates.

5

4

3

2

1

0.20

0.10

1148

1234

1118

0.15

1046

1027 1180

1189

Absorbance [arb. units]

Spot 1 Spot 2

1020 1001

b

10 µm

1374 1344

4. Conclusions 0.05

0.00 1400

1300

1200

1100

1000

900

Wavenumber [cm-1] Fig. 8. (a) Optical micrograph of a Cu activated pyrite surface after conditioning in 10−3 M KEX solution for 2 min. (b) FTIR spectra of KEX on a Cu activated pyrite surface taken with 5×5 μm apertures, average of 2000 scans with 4 cm−1 spectral resolution.

position 2 are shown. Using Table 2, CuEX is identified to be present predominantly at spots 1 to 4. A shoulder at 1020 cm −1 with broad bands near 1234, 1344 and 1374 cm −1 indicates that (EX)2 is also present in significant amounts at spot 1. Similar, less intense broad bands from 1234 to 1374 cm −1 were seen in the spectra from spots 2 to 4 suggesting smaller amounts of (EX)2 at these locations. The same yellow precipitates were found heterogeneously located over the entire pyrite surface. FTIR spectra from these areas showed very similar peaks, where CuEX can be identified as the predominant species with lesser amounts of (EX)2. In addition, in some areas which are optically indistinguishable from areas that contained (EX)2 only CuEX could be detected. Hicyilmaz et al. (2004) suggested that the interaction between activated Cu on a pyrite surface and the collector is primarily chemical in nature. Results from Pecina et al. (2006) have also confirmed the chemical interaction of collectors with activated and unactivated pyrite surfaces. The chemical interaction would most probably involve bond formation between xanthatic S and adsorbed Cu (surface Fe under unactivated conditions) onto the pyrite surface (Pattrick et al., 1999). Leppinen (1990), using IR internal reflection surface analysis of activated pyrite surfaces conditioned with ethyl xanthate collector, showed Cu(I)–ethyl xanthate to be the dominant activation product

PEEM measurements revealed that Cu adsorbs only onto specific pyrite surface sites. The Cu 2p edge (2p3/2 peak at 934.3 eV) XANES structure and position confirms the presence of Cu +. The peak position is typical of Cu + bonded to S, Cu(I)\S. This is confirmed by S XPS at the adsorbed Cu spot which showed the presence of a new Ox-S 2− species. This results from the electrochemical interaction of solution Cu 2+ with surface S 2− species. The initial fast step involves Cu 2+ adsorption followed by a redox reaction involving reduction of Cu and oxidation of the related S resulting in the formation of Cu + and an Ox-S 2− (S −) species. No evidence of Cu\O type bonding was found. Furthermore, Fe XAS confirmed that surface Fe does not take part in the Cu adsorption or any subsequent restructuring processes. The IR analyses showed that xanthate also adsorbs heterogeneously onto pyrite surfaces. Adsorption of xanthate onto the pyrite surface that had not been activated may involve ferric xanthate surface species. However, definitive confirmation of the presence of this species could not be obtained. The use of low KEX concentration for conditioning shows Cu(I)–xanthate to be the only surface xanthate containing species. A portion of this Cu(I)–xanthate may form through a chemical process involving adsorbed Cu + and the xanthate from solution. When greater concentration of KEX is used significant amounts of Cu(I)–xanthate precipitates from solution and are deposited onto the pyrite surfaces which cannot be removed through washing. Under this condition, the predominant surface species is found to be Cu(I)–xanthate with smaller amounts of diethyl dixanthogen. The latter species may have formed in solution although some previous studies also suggest direct surface formation. Xanthate adsorption may therefore occur in three ways. The predominant mechanism is the interaction of xanthate from solution with adsorbed Cu. The second mechanism occurs when the concentrations are not optimal (when excess Cu and xanthate exist in solution). In this case direct precipitation of Cu(I)–xanthate on to the mineral surfaces can take place. Such precipitation tends to occur non-selectively and may be detrimental to the flotation process and

A.P. Chandra et al. / International Journal of Mineral Processing 114–117 (2012) 16–26

resulting concentrate grade. The third mechanism is through dixanthogen formation which may occur through physisorption from solution or direct formation on the surface. To a much lesser extent surface Fe may also interact with xanthates in solution to form surface ferric xanthate species. Acknowledgements The support from the University President's Scholarship awarded by the University of South Australia to Dr. Anand Chandra is gratefully acknowledged. This work was funded by the Premiers Science and Research Fund (PSRF) of South Australia, Rio Tinto and BHP-Billiton under the research project New Information for Minerals Processing. Further support was also provided by the University of South Australia under Divisional Small Grant to Dr. Darren Simpson. Supplementary funding for synchrotron measurements was provided by the Access to Major Research Facilities (AMRF) fund (PEEM) and the Australian Synchrotron (FTIR). We are also thankful to beamline scientists, Dr. Uday Lanke of SM beamline, PEEM endstation, Canadian Light Source, and Dr. Mark Tobin of Infrared Microspectroscopy beamline, Australian Synchrotron. Help from Dr. Rong Fan during synchrotron data collection is also gratefully acknowledged. References Abraitis, P.K., Pattrick, R.A.D., Kelsall, G.H., Vaughan, D.J., 2004. Acid leaching and dissolution of major sulphide ore minerals: processes and galvanic effects in complex systems. Mineral. Mag. 68, 343–351. Acres, R.G., Harmer, S.L., Beattie, D.A., 2010a. Synchrotron XPS studies of solution exposed chalcopyrite, bornite, and heterogeneous chalcopyrite with bornite. Int. J. Miner. Process. 94, 43–51. Acres, R.G., Harmer, S.L., Beattie, D.A., 2010b. Synchrotron XPS, NEXAFS, and ToF-SIMS studies of solution exposed chalcopyrite and heterogeneous chalcopyrite with pyrite. Miner. Eng. 23, 928–936. Boulton, A., Fornasiero, D., Ralston, J., 2003. Characterisation of sphalerite and pyrite flotation samples by XPS and ToF-SIMS. Int. J. Miner. Process. 70, 205–219. Bronold, M., Tomm, Y., Jaegermann, W., 1994. Surface states on cubic d-band semiconductor pyrite (FeS2). Surf. Sci. 314, L931–L936. Buckley, A.N., Woods, R., 1987. The surface oxidation of pyrite. Appl. Surf. Sci. 27, 437–452. Buckley, A.N., Skinner, W.M., Harmer, S.L., Pring, A., Lamb, R.N., Fan, L., Yang, Y., 2007. Examination of the proposition that Cu(II) can be required for charge neutrality in a sulfide lattice — Cu in tetrahedrites and sphalerite. Can. J. Chem. 85, 767–781. Chandra, A.P., Gerson, A.R., 2009. A review of the fundamental studies of the copper activation mechanisms for selective flotation of the sulfide minerals, sphalerite and pyrite. Adv. Colloid Interface Sci. 145, 97–110. Chandra, A.P., Gerson, A., 2010. The mechanisms of pyrite oxidation and leaching: a fundamental perspective. Surf. Sci. Rep. 65, 293–315. Chandra, A.P., Gerson, A.R., 2011. Pyrite (FeS2) oxidation: a sub-micron synchrotron investigation of the initial steps. Geochim. Cosmochim. Acta 75, 6239–6254. Chen, Z., Yoon, R.-H., 2000. Electrochemistry of copper activation of sphalerite at pH 9.2. Int. J. Miner. Process. 58, 57–66. de Donato, P., Mustin, C., Benoit, R., Erre, R., 1993. Spatial distribution of iron and sulphur species on the surface of pyrite. Appl. Surf. Sci. 68, 81–93. Demoisson, F., Mullet, M., Humbert, B., 2007. Investigation of pyrite oxidation by hexavalent chromium: solution species and surface chemistry. J. Colloid Interface Sci. 316, 531–540. Descostes, M., Mercier, F., Beaucaire, C., Zuddas, P., Trocellier, P., 2001. Nature and distribution of chemical species on oxidized pyrite surface: complementarity of XPS and nuclear microprobe analysis. Nucl. Instrum. Methods Phys. Res., Sect. B: Beam Interact. Mater. Atoms 181, 603–609. Doyle, C.S., Kendelewicz, T., Bostick, B.C., Brown, G.E., 2004. Soft X-ray spectroscopic studies of the reaction of fractured pyrite surfaces with Cr(VI)-containing aqueous solutions. Geochim. Cosmochim. Acta 68, 4287–4299. Finch, J.A., Rao, S.R., Nesset, J.E., 2007. Iron Control in Mineral Processing, 39th Annual Meeting of the Canadian Mineral Processors 23–25 January. Canadian Institute of Mining, Metallurgy and Petroleum, Ottawa. Fornasiero, D., Ralston, J., 2006. Effect of surface oxide/hydroxide products on the collectorless flotation of copper-activated sphalerite. Int. J. Miner. Process. 78, 231–237. Fujisawa, M., Suga, S., Mizokawa, T., Fujimori, A., Sato, K., 1994. Electronic structures of CuFeS2 and CuAl0.9Fe0.1S studied by electron and optical spectroscopies. Phys. Rev. B 49, 7155–7164. Garvie, L.A.J., Buseck, P.R., 2004. Unoccupied states of pyrite probed by electron energyloss spectroscopy (EELS). Am. Mineral. 89, 485–491. Gerson, A.R., Lange, A.G., Prince, K.E., Smart, R.S.C., 1999. The mechanism of copper activation of sphalerite. Appl. Surf. Sci. 137, 207–223.

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