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Langmuir 1998, 14, 5274-5278
An XPS Study of Sphalerite Activation by Copper I. J. Kartio,† C. I. Basilio,‡ and R.-H. Yoon*,‡ Laboratory of Materials Science, Department of Applied Physics, University of Turku, Turku, Finland, and Center for Coal and Minerals Processing, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0258 Received April 28, 1997. In Final Form: June 17, 1998 X-ray photoelectron spectroscopy (XPS) was used to study the kinetics and mechanisms of sphalerite activation in a 10-4 M CuSO4 solution at pH 9.2. The activation was fast during the first 10 min, after which the rate decreased exponentially. The increase in the amount of copper ion uptake was accompanied by the displacement of zinc ions from the sphalerite surface, indicating that the mechanism of copper uptake is one of displacement rather than adsorption. The XPS data show that the kinetics of activation increased considerably in a deoxygenated CuSO4 solution. Both the conventional XPS and synchrotron radiation XPS (SR-XPS) analyses show that copper activation involves a mechanism in which Cu2+ ions are reduced to the Cu(I) state, while the sulfur in ZnS is oxidized. Activation in the absence of oxygen and at an open circuit results in the formation of a CuS-like product, while in an air-saturated solution copper polysulfides are formed. The latter mechanisms are supported by the appearance of two distinct S(2p) doublets representing S- and S0 species at higher binding energies. When copper-activated sphalerite was conditioned in an air-saturated solution of pH 9.2, the CuS-like activation product is oxidized in preference to unactivated ZnS. The oxidation resulted in a loss of copper from the sphalerite surface, which may be detrimental to flotation.
Introduction Unlike most other sulfide minerals, sphalerite does not respond well to flotation using short-chain thiol collectors. The problem is generally attributed to the high solubility of zinc thiol compounds in water. Also, sphalerite is a poor host for mixed potential electrochemical reactions by which thiol collectors adsorb on other sulfide minerals, because sphalerite is an insulator. Therefore, sphalerite flotation requires activation by heavy-metal ions, usually by Cu2+ ions, to form a conducting layer on the surface, so that thiol collectors can form an insoluble collector coating via the mixed potential reactions. The mechanism of copper activation of sphalerite has been studied extensively. It is generally accepted that Cu2+ ions in solution metathetically substitute the Zn2+ ions on the surface of sphalerite.1,2 However, there are continuing debates on the nature of the activation product. Some researchers2,3 suggested that the activation product is covellite (CuS), while others4-6 suggested it is chalcocite (Cu2S). On the other hand, Buckley and Woods7 analyzed the surface of activated sphalerite using X-ray photoelectron spectroscopy (XPS) and suggested that a bimetallic compound, Zn1-xCuxS, is formed by the metathetical substitution. This conclusion was drawn based * To whom correspondence may be addressed. † University of Turku. ‡ Virginia Polytechnic Institute and State University. (1) Gaudin, A. M. Flotation; McGraw-Hill: New York, 1957. (2) Finkelstein, N. P.; Allison, S. A. In Flotation; A. M. Gaudin Memorial Volume; Fuerstenau, M. C., Ed.; AIME: New York, 1976; Vol. 1, p 414. (3) Cecile, J. L. In Flotation of Sulfide Minerals; Forssberg, K. S. E., Ed.; Elsevier: Amsterdam, The Netherlands, 1985; p 61. (4) Nefedov, V. I.; Salyn, Ya. V.; Solozhenkin, P. M.; Pulatov, G. Yu. Surf. Interface Anal. 1980, 2, 170. (5) Perry, D. L.; Tsao, L.; Taylor, J. A. In Proceedings of the International Symposium on Electrochemistry in Mineral and Metal Processes; Richardson, P. E., Srinivasan, S., Woods, R., Ed.; The Electrochemical Society: Pennington, NJ, 1984; p 169. (6) Prestidge, C. A.; Thiel, A. G.; Ralston, J.; Smart, R. St. C. Colloids Surf. A 1994, 85, 51. (7) Buckley, A. N.; Woods, R.; Wouterlood, H. J. Int. J. Miner. Process. 1989, 26, 29.
on their measurements showing that the S(2p) spectrum does not change significantly during the initial stages of activation. More recently, however, Prestidge et al.6 suggested that elemental sulfur (S0) forms as part of the activation mechanism, in which case the S(2p) spectrum should change during the course of the activation process. In this work, XPS studies were conducted to further investigate the mechanism and kinetics involved in the activation sphalerite by Cu2+ ions under alkaline conditions. The measurements were conducted with and without oxygen in the activating solution to determine if the activation mechanism is affected by the electrochemical conditions of the system. Experimental Section Materials. A natural sphalerite specimen (from Santander, Spain) was cut into a 8 × 8 × 1 mm plate and polished. The specimen contained small amounts of iron (0.035 wt %) and copper (0.012%). The experiments were carried out in a pH 9.2 buffer solution (0.05 M Na2B4O7) containing 10-4 M CuSO4. In some experiments, the buffer solution was deoxygenated by purging with nitrogen gas (99.99% pure) for at least half an hour. In each experiment, the sphalerite sample was wet-polished with silicon carbide paper (600 grit), rinsed with double-distilled deionized water, and then immersed in a solution of interest. After each solution treatment, the sample was rinsed with water, quickly dried in air, and inserted in the evacuation chamber of the spectrometer. XPS Measurements. A Perkin-Elmer ESCA PHI 5400 spectrometer was used to obtain the XPS spectra of sphalerite samples treated under different conditions. Photoelectrons were excited with unmonochromatized Mg KR radiation and analyzed using a hemispherical analyzer with a constant pass energy of 35.75 eV. The vacuum pressure inside the measuring chamber was approximately 10-7 Pa. The diameter of the analyzed spot on sample surfaces was about 1.1 mm, and the takeoff angle of the electrons was 45°. The binding energy (BE) scale of the instrument was calibrated using the Au(4f)7/2 (BE ) 84.0 eV) lines of metallic gold and the Cu(2p)3/2 (BE ) 932.6 eV) lines of metallic copper as references. The spectra were recorded and analyzed using an Apollo Domain 3010 computer with Perkin-Elmer software. After a
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Shirley-type background subtraction, the measured emission spectra were resolved into components with individually optimized mixtures of Gaussian and Lorenzian line shapes. Doublet lines were fitted with two components, keeping their BE differences and intensity ratios fixed. Unmonochromatized Mg KR excitation was used instead of monochromatized radiation to decrease sample charging, which arises when measuring relatively insulating samples such as sphalerite. With unmonochromatized excitation, the sample charging is partly compensated by secondary electrons emitted from the aluminum window of the X-ray tube. Also, the beam size is larger; therefore, the differential charging is minimized. When a shift due to charging was observed in the recorded spectra, the Zn(3p)3/2 component (BE ) 88.4 eV) of the Zn(3p) doublet was used as an internal standard to correct the shift. The commonly used C(1s) line of contaminated hydrocarbons was not used as the internal reference due to the interference from the Zn (KLL) Auger signal. Kartio et al.8 showed that XPS becomes substantially more surface-sensitive, particularly with regard to the S(2p) signal, when a sulfide mineral is excited at about 200 eV rather than at 1256 eV by Mg KR radiation. Therefore, two sphalerite samples were analyzed using synchrotron radiation-excited photoelectron spectroscopy (SR-XPS). The measurements were conducted at Beamline 51, MAX I Synchrotron Source, Lund, Sweden. The excitation energy was selected with a SX-700 monochromator, and the photoelectrons were analyzed with a Scienta 300 hemispherical analyzer at a takeoff angle of 45°. At the excitation energies used in this work, the system used for the present workup has a higher resolution than that of the conventional ESCA using monochromated Al KR excitation.8 Since no flood gun was available with this spectrometer, it was possible to take the spectra of only those sphalerite samples that had been activated long enough to have sufficient conductivity to neutralize the charging caused by the loss of photoelectrons. The binding energies of the SR-XPS spectra were calibrated internally using the values of the bulk components determined with the conventional XPS.
Results and Discussion Kinetics of Activation. The intensity ratio of Cu(2p) to S(2p) emissions, which is designated as I(Cu2p)/I(S2p) in the present work, is used to represent the changes in the copper uptake on sphalerite during activation. Likewise, the intensity ratio of Zn(2p)3/2 to S(2p) emissions (I(Zn2p3/2)/I(S2p)) is used to represent the amount of Zn2+ removed from the surface of sphalerite during the activation process. This approach would be valid only if the sulfur content on the surface remains unchanged during activation. Figure 1 shows these ratios plotted as a function of activation time. As expected, the copper uptake increased with increasing activation time, with a concomitant increase in the amount of zinc lost into solution during activation. These results support, at least qualitatively, the metathetical substitution mechanism
ZnS + Cu2+ ) CuS + Zn2+
(1)
which is widely accepted. Ralston and Healy9 showed that the ratio of the Cu2+ ions abstracted from solution is equal to the amount of Zn2+ ions released into solution. During the first minute, the copper uptake reached more than 25% of the maximum value achieved in 9 h of activation. The rate of copper uptake slowed with time afterward, showing a logarithmic time dependence during the first hour. This behavior is similar to what was observed by Ralston and Healy,9,10 who determined the copper uptake by monitoring Cu2+ ions in solution using an ion-selective electrode in weakly acidic and neutral solutions. (8) Kartio, I.; Laajalehto, K.; Kaurila, T.; Suoninen, E. Appl. Surf. Sci. 1996, 93, 167. (9) Ralston, J.; Healy, T. W. Int. J. Miner. Process. 1980, 7, 175. (10) Ralston, J.; Healy, T. W. Int. J. Miner. Process. 1980, 7, 203.
Figure 1. Changes in copper and zinc ion concentration on the surface of sphalerite activated in a 10-4 M CuSO4 solution at pH 9.2, as measured by the intensity ratios of the Cu(2p)/S(2p) and Zn(2p)3/2/S(2p) spectra. Table 1. Comparison of the XPS Data for Sphalerite with and without Activation in a 10-4 M CuSO4 Solution at pH 9.2 unactivateda
activatedb
BEc
RId
fwhme
emission
(eV)
(%)
(eV)
S(2p)3/2
161.2
100
1.3
1022.1 88.4f
100 100
1.5 2.6
531.7 533.2 284.8 285.8
77 23 88 12
1.8 1.7 1.8 1.5
Zn(2p)3/2 Zn(3p)3/2 Cu(2p)3/2 O(1s) C(1s)
BE (eV)
RI (%)
fwhm (eV)
161.3 162.5 1021.4 88.4f 932.3 933.7 531.6 533.0 284.6 285.9
78 22 100 100 92 8 71 29 81 19
1.2 1.5 1.4 2.6 1.5 1.6 1.7 1.8 1.5 1.4
a Wet-polished. b In 10-4 M CuSO (pH 9.2) for 1 h. c Binding 4 energy. d Relative intensity. e Full width at half-maximum. f Used for calibration; actual measured values were 93.6 eV for the unactivated sample and 88.8 eV for the activated sample.
Also shown in Figure 1 for comparison are the intensity ratios for the sphalerite samples activated in 10-4 M CuSO4 solutions purged with nitrogen gas to expel dissolved oxygen. The copper uptake increased considerably in the deoxygenated solutions, suggesting that molecular oxygen may be detrimental to activation and further that copper activation may also be controlled by an electrochemical mechanism which is not implicated by reaction 1. Identification of Activation Products. Table 1 compares the binding energies (BE) and line widths (fwhm) of the characteristic emissions of the elements of interest before and after copper activation. Unactivated sphalerite was subjected to XPS analysis immediately after wetpolishing, while activated sphalerite was conditioned for 1 h in 0.05 M borate buffer (pH 9.2) containing 10-4 M CuSO4. The unactivated and activated sphalerite samples showed charge shifts of 5.2 and 0.4 eV, respectively, which were taken into account upon calibration of the spectra against the Zn(3p)3/2 emission. The decrease in the charge shift observed with the activated sample indicates that a conducting layer is formed on the sphalerite surface as a result of the activation. There is a considerable scatter in the values of BEs obtained in the present work and those reported in the literature for sphalerite. The BE of S(2p)3/2 emission varies
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from 161.111 to 161.6 eV,7 while that of Zn(2p)3/2 ranges from 1020.912 to 1022.1 eV (this work). The difference in BE between the S(2p) and Zn(2p) lines also varies from work to work. These variations cannot be attributed to the differences in the methods of internal calibration, because the difference in BEs between two lines is independent of calibration. Also, the variation is not likely due to the nonlinearity of the spectrometer.7 It is more likely due to the variations in the conductivities of the sphalerite samples, which may vary with the level of impurities in the mineral. Note that the activated and unactivated samples show different BEs for the Zn2p3/2 lines, which may be attributed to the changes in conductivity upon activation.13 In the present work, the Cu(2p)3/2 spectrum was fitted with two components. The main component at 932.5 eV represents a Cu(I) state, while the minor component at 933.7 eV could be attributed to another Cu(I) state with a slightly different electronic environment or to an asymmetry of the main emission.14 There was no evidence for a Cu(II) state, because the spectra do not show excited final state satellites, typical of the Cu(II) state. Thus, the results obtained in the present work suggest that the copper on activated sphalerite is in the Cu(I) state. Other investigators4,5 also showed that the Cu(2p) spectrum of copper-activated sphalerite is due to Cu(I) species, while Cecile3 suggested that it is due to Cu(II) species. The main Cu(2p)3/2 component was found at 932.5 eV, which is closer to the Cu(2p)3/2 BE for Cu2S5,14 than that for CuS5,15-17 or Cu1.87S.13 On the other hand, covellite (CuS) is known to have two Cu2p states, separated by 1.11.2 eV,18 originating from two different Cu(I) sites in the structure. As shown in Table 1, Cu-activated sphalerite also shows two Cu(2p) states, although the relative intensity of the higher BE component is weak compared to that of covellite. Therefore, it is difficult to make a conclusion as to whether the activation product is CuS, Cu2S, or any other based on the Cu(2p) spectrum alone. As shown in Table 1, the S(2p) spectrum of the unactivated sphalerite shows only one doublet with its S(2p)3/2 BE at 161.2 eV, while the same for activated sphalerite requires at least two doublets to fit the measured spectrum. (In this paper, S(2p) spectra will be represented by their S(2p)3/2 BEs.). Figure 2 shows the S(2p) spectrum of a sphalerite sample activated for 1 h in a 10-4 M CuSO4 solution. The solution was open to air, which allowed the mineral to be activated under an airsaturated condition. The spectrum, obtained with the conventional XPS with Mg KR radiation, was curve-fitted with two doublets with BEs of 161.3 and 162.5 eV. The width of the latter was 0.3 eV larger than that of the former, indicating that this new emission may contain more than one sulfur state. However, because of insufficient surface sensitivity and resolution of the conventional XPS, this cannot be substantiated by the spectrum given in Figure 2. (11) Grzetic, I. J. Eng. Phys. 1984, 26, 16. (12) Clifford, R. K. K.; Purdy, L.; Miller, J. D. Advances in Interfacial Phenomena. AIChE Symp. Ser. 1975, 71, 138. (13) Laajalehto, K.; Kartio, I.; Nowak, P. Appl. Surf. Sci. 1994, 81, 11. (14) Nakai, I.; Sugitani, Y.; Nagashima, K; Niwa, Y. XPS Studies of Copper Minerals. J. Inorg. Nucl. Chem. 1978, 40, 789. (15) Brion, D. Appl. Surf. Sci. 1980, 5, 133. (16) Romand, M.; Roubin, M.; Deloume, J. P. J. Electron Spectrosc. 1978, 13, 229. (17) Gebhardt, J. E.; McCarron, J. J.; Richardson, P. E.; Buckley, A. N. Hydrometallurgy 1986, 17, 27. (18) Laajalehto, K.; Kartio, I.; Kaurila, T.; Laiho, T.; Suoninen, E. Surf. Interface Anal. 1996, in press.
Kartio et al.
Figure 2. Conventional XPS spectra (in the S(2p) region) of the sphalerite activated in a 10-4 M CuSO4 solution at pH 9.2.
The spectrum shown in Figure 2 is similar to that reported by Buckley et al.7 These investigators suggested that the 161.3 eV component is the S(2p) doublet of the S2- species of unactivated ZnS, which does not change during activation, while the 162.5 eV component is due to oxidation of the activation product. Because the XPS data showed no changes in the S2p spectrum during activation, which, in turn, suggested no changes in the sulfur environment, Buckley et al.7 proposed the activation mechanism
ZnS + xCu2+ ) Zn1-xCuxS + xZn2+
(2)
in which Cu2+ ions substitute Zn2+ ions isometrically. The broad S2p component observed at 162.5 eV with the conventional XPS can be further resolved into two components using the more surface-sensitive SR-XPS. Figure 3a shows the SR-XPS spectrum of a sample activated in an air-saturated 10-4 M CuSO4 solution at pH 9.2 for 30 min. The spectrum was obtained at an excitation energy of 209 eV. The curve fitting shows two S2p components at 162.1 and 163.1 eV, which are of the same origin as the single broad doublet at 162.5 eV. The doublet at 163.1 eV has a somewhat lower BE than that of elemental sulfur19 but is characteristic of the sulfurs with formal charge of 0 in polysulfides.20-22 Polysulfides have a chain structure, in which the sulfur atoms at both ends have the formal charge of -1, while those in the middle of the chain have the formal charge of 0. At least part of the intensity of the 162.1 eV doublet shown in Figure 3a may be attributed to the S- species of polysulfides. Thus, the sphalerite sample activated in an airsaturated CuSO4 solution at pH 9.2 shows the presence of polysulfide on the surface. The intensity ratio of the S0 to S- components is approximately 2:1. Assuming that these two doublets originate solely from polysulfides, one can estimate that the polysulfides formed on the sphaerite sample are six-sulfur atoms long, i.e., Cu2S6. Figure 3b shows the S(2p) SR-XPS spectrum of the sphalerite sample activated for 30 min in a 10-4 M CuSO4 solution at pH 9.2. The solution was purged with nitrogen to remove oxygen from the system. As is the case with activation of the mineral in an oxygen-saturated solution, the spectrum also shows the two new components of higher binding energy. However, the intensity of the S(2p) component at 163.1 eV (which may be due to the S0 moiety of polysulfide) was considerably reduced as compared to the case of activation in the air-saturated CuSO4 solution (Figure 3a). On the other hand, the intensity of the component at 162.1 eV (which may be due to the S- moiety of polysulfide), relative to the intensity of the S2component, remained the same as that in the case of activation of the mineral in an oxygen-saturated CuSO4
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Figure 3. SR-XPS spectra of sphalerite activated for 30 min in a 10-4 M CuSO4 solution (a) under air-saturated conditions and (b) under deoxygenated conditions.
solution. The intensity ratio of the S0 to S- species is 1.4:1, from which one can estimate the polysulfide to be Cu2S5. The SR-XPS spectra shown in Figure 3 may be considered to support the activation mechanism proposed by Buckley et al.7 (reaction 2). It is possible that polysulfides found on activated sphalerite are the results of oxidation of the activation product (i.e., Zn1-xCuxS). This might be the case if one looks only at the spectrum of the sphalerite activated in an air-saturated CuSO4 solution. However, polysulfide was also found on the sample activated in a deoxygenated solution. Although it is difficult to completely remove all the oxygen by N2 gas bubbling, the amount of polysulfide found on the surface was too large. It is, therefore, possible that polysulfide formation may be an integral part of the activation mechanism, which is similar to the mechanism proposed by Prestidge et al.6 These investigators proposed an activation mechanism
2ZnS + 2Cu2+ ) 2Zn2+ + Cu2S + S0
(3)
in which elemental sulfur (S0) is formed on the surface along with Cu2S. It is possible that S0 in reaction 3 could actually be part of the polysulfide chain. According to the mechanism of Buckley et al.7 (reaction 2), the S(2p) component observed at 162.1 eV in the SRXPS spectra (or the 162.5 eV component in the conventional XPS spectra) should be attributed to oxidation of the activation product. If this was indeed the case, the intensity of the 162.1 eV component should decrease when sphalerite is activated in a deoxygenated solution. How-
ever, the intensity of the 162.1 eV component, relative to that of the 161.3 eV component, remained practically the same as that in the case of activation in a air-saturated solution. Therefore, the 162.1 eV component might not be due to the oxidation of activation product. Rather, it may actually be due to the activation product itself. It is possible that the activation product may be a CuSlike substance where the copper is in the Cu(I) tate and the formal charge of sulfur is -1. Covellite has its S(2p) doublets at 161.1 and 161.9 eV, with the latter being the stronger of the two.17 One may consider the 162.1 eV component of the activation product (Figure 3a,b) to be the stronger of the two S(2p) components of the CuS-like activation product, with the weaker component having its BE very close to the S(2p) doublet (161.3 eV) of unoxidized ZnS. Note that the 0.8 eV difference between the two S(2p) components of the CuS-like activation product is the same as that observed with covellite. This finding reinforces the likelihood that the activation product is indeed CuS-like. In light of the XPS data obtained in the present work, it may be suggested that sphalerite activated at an open circuit in oxygen-free solutions may form a CuS-like product. Since the copper is in the Cu(I) state as in any sulfide mineral, the activation mechanism may involve coupled solid-state electrochemical reactions, including reduction of Cu2+ ions to the Cu(I) state and oxidation of S2- to S- species. In effect, the solid-state electrochemical reactions result in the formation of Cu-S bonds, as manifested by the two S(2p) components at 161.3 and 162.1 eV. In the presence of oxygen, the CuS-like activation product may be oxidized to form polysulfides. After 30 min of activation in an air-saturated CuSO4 solution at pH 9.2, Cu2S6 is formed as shown in Figure 3a. In the complete absence of oxygen, no polysulfide should be formed, and the 163.1 eV component representing the S0 species of polysulfide should vanish. That the sphalerite sample activated in deoxygenated solutions showed polysulfides (e.g., Cu2S5) at all may be attributed to the difficulty in preventing the activation product from oxidation during the process of activation and transfer of the sample to the UHV system of the XPS spectrometer. The proposed activation mechanism may be useful for explaining the role of oxygen in copper activation of sphalerite. Molecular oxygen may compete with Cu2+ ions for electrons and reduce the copper uptake. Likewise, the proposed mechanism may offer an explanation for the changes in copper uptake with electrochemical potential.19-23 Furthermore, the proposed mechanism explains many of the experimental results reported in the literature. First, it explains the 1:1 exchange between zinc and copper, as shown by Finkelstein and Allison.2 Second, the copper is in the form of Cu(I) rather than Cu(II), as shown in the present work and by others.4,5 Third, the formation of metal polysulfides as sulfur oxidation product explains why no elemental sulfur could be detected on an activated ZnS sample cooled to liquidnitrogen temperature.6 Oxidation of Activated Sphalerite. A sphalerite sample was activated in a deoxygenated pH 9.2 buffer (19) Szargan, R.; Karthe, S.; Suoninen, E. Appl. Surf. Sci. 1992, 55, 227. (20) Luttrell, G. L.; Yoon, R. H. Int. J. Miner. Process. 1984, 13, 271. (21) Termes, S. C.; Richardson, P. Int. J. Miner. Process. 1987, 18, 167. (22) Buckley, A. N.; Woods, R.; Wouterlood, H. J. Aust. J. Chem. 1988, 41, 1003. (23) Yoon, R. H.; Chen, Z.; Richardson, P. E. In Proceedings of the International Symposium on Electrochemistry in Mineral and Metal Processes IV; Richardson, P. E., Doyle, F. M., Woods, R., Eds.; The Electrochemical Society: Pennington, NJ, 1996.
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Kartio et al.
Table 2. Effects of Open-Circuit Oxidation of Activated Sphalerite at pH 9.2 before oxidationa emission S(2p)3/2 Zn(3p)3/2 Cu(3p)3/2
after oxidationb
BEc
RId
fwhme
ACf
(eV)
(%)
(eV)
(%)
161.2 82 162.4 18 g 88.4 100 74.8 100
1.2 1.5 2.5 2.6
25 5 20 10
BE (eV)
RI (%)
161.3 70 162.6 30 g 88.4 100 74.9 100
fwhm AC (eV) (%) 1.3 2.0 2.6 2.5
25 10 22 5
a Activated in deoxygenated 10-4 M CuSO (pH 9.2) for 1 h. b After 4 1 h of activation, kept in a blank borate (pH 9.2) solution for 8 h. c Binding energy. d Relative intensity. e Full width at half-maximum. f Atomic concentration. g Used for calibration; actual measured values were 88.3 eV (before oxidtion) and 90.9 eV (after oxidation).
solution containing 10-4 M CuSO4 for 1 h and, then, replaced in the same buffer solution (containing no CuSO4) for 8 h. The blank buffer solution was allowed to be in equilibrium with air, so that the activated sphalerite could be subjected to oxidation. After the oxidation, the sphalerite sample was taken out of the buffer solution and analyzed by conventional XPS, with the results being given in Table 2. The atomic concentrations (AC) were calculated on the basis of the relative intensities (RI) and experimental sensitivity factors for the S(2p), Zn(3p), and Cu(3p) emissions. These three emission lines were chosen for the quantitative analysis because they have about the same surface sensitivity. The results showed that the activated sphalerite sample became sulfur-rich on the surface after the 8 h of oxidation. The excess sulfur was associated with the S(2p) component at 162.5 eV, which was attributed to polysulfide. Note that the width (fwhm) of this component increased from 1.5 to 2.0 eV upon oxidation, indicating an increase in the chain length of the polysulfide; i.e., the S0/S- ratio increased. The formation of sulfur-rich surface is accompanied by the decrease in the copper-to-sulfur (Cu/S) atomic ratio from 0.33 to 0.14. This is compared with the zinc-to-sulfur (Zn/S) atomic ratio, which remained practically unchanged (0.66 vs 0.63). These results show that the CuS-like activation product is oxidized in preference to ZnS, which is consistent with the ring-disk and stripping voltammetry work reported in another paper.23 Also, the XPS spectrum of an unactivated sphalerite sample showed no sign of significant oxidation after 9 h of oxidation in a blank buffer solution (not shown here). Stewart and Finkelstein24 were the first to show that activated sphalerite can float without collector. These investigators identified neither the electrochemical conditions for the collectorless flotation nor the hydrophobic species responsible for the flotation. Nevertheless, they (24) Stewart, B. V.; Finkelstein, N. P. S. Afri. Nat. Inst. Metall. 1973, 1587.
ruled out elemental sulfur as the hydrophobic species because it was not detected on the activated sphalerite surface. Later investigations by Yoon25 and Craynon26 also showed that Cu-activated sphalerite can be floated without collector but that the flotation is favored under oxidizing conditions. Craynon showed that the collectorless flotation occurs at potentials above approximately 0 mV (SHE). On the basis of his XPS analysis, Craynon suggested that the hydrophobic species responsible for the collectorless flotation is polysulfides (Sn2-). Summary and Conclusions The kinetics of sphalerite activation by copper ions was studied by monitoring the surface species using XPS. The results obtained at pH 9.2 in air-saturated 10-4 M CuSO4 solutions showed that the copper uptake is fast during the first 10 min, after which the rate is reduced. The changes in the rate of copper uptake are the same as those for the displacement of zinc ions from the sphalerite surface, indicating that the mechanism of copper ion uptake is one of displacement rather than adsorption. The kinetics of copper uptake increases in deoxygenated CuSO4 solutions. The mechanism of the sphalerite activation by copper may be summarized as follows: (i) Activation of sphalerite in oxygen-free CuSO4 solutions of pH 9.2 and at an open circuit results in the formation of a CuS-like product, in which the copper is in the Cu(I) state and the formal charge of the sulfur is -1. (ii) Activation of sphalerite in an air-saturated CuSO4 solution of pH 9.2 results in the formation of copper polysulfides, and the copper uptake is less than that in the case of activation of the mineral in a deoxygenated solution. (iii) Oxidation of sphalerite activated in a deoxygenated solution causes preferential oxidation of the CuS-like activation product, resulting in a loss of copper into solution. The finding that copper uptake by sphalerite increases in the deoxygenated solutions may have a significant implication in plant practice. It suggests that the use of an oxygen scavenger or a reducing condition during activation may be beneficial for the copper activation of sphalerite. On the other hand, the presence of oxygen increases the amount of polysulfides and their sulfur chain length. The polysulfide formation may explain the flotation of activated sphalerite without collector under oxidizing conditions. LA970440C (25) Yoon, R. H. Int. J. Miner. Process. 1981, 8, 31. (26) Craynon, J. R. The Collector Flotation of Shpalerite. M.Sc. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA, 1985.
