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XPS Characterization of Chalcopyrite, Tetrahedrite, and Tennantite Surface Products after Different Conditioning. 2. Amyl Xanthate Solution at pH 10 J. A. Mielczarski,*,† J. M. Cases,† M. Alnot,‡ and J. J. Ehrhardt‡ Laboratoire “Environnement et Mine´ ralurgie” UA 235 CRNS, INPL-ENSG, B.P. 40, 54501 Vandoeuvre-le` s-Nancy, France, and Laboratoire de Chimie Physique pour l’Environnement-UMR 9992-CNRS, 54600 Villers-le` s-Nancy, France Received July 17, 1995. In Final Form: December 18, 1995X Characterization of the surface products formed by the interaction of amyl xanthate solution at pH 10 with mineral samples of chalcopyrite (CuFeS2), tetrahedrite (Cu12Sb4S13), and tennantite (Cu12As4S13) was carried out by X-ray photoelectron spectroscopy (XPS). The experimental data collected directly after solution treatment and after successive sputterings have led us to determine the in-depth distribution of the different types of surface products formed by the diffusion of atoms from the bulk to the interface region and their interaction with aerated aqueous xanthate solution. The xanthate adsorption takes place by an electrochemical mechanism which involves the formation of cuprous xanthate (hydrophobic species) and metal hydroxides (hydrophilic species). In general, the following in-depth surface composition was found for the investigated mineral samples in contact with the xanthate solution: (i) the outermost layer contains the cuprous xanthate complex, which causes hydrophobic properties, and a small amount of metal hydroxide species (depending on the mineral sample), and (ii) below this hydrophobic layer are gradual changes leading to a structure with a composition very similar to the one expected for the bulk mineral sample. All the mineral samples show a copper enrichment in the interface region formed as a result of diffusion of copper atoms to the interface provoked by the adsorbed xanthate molecules. Sulfur-enriched structures, which were observed after treatment in solution without xanthate, are not observed in the presence of xanthate solution. The adsorbed amount of xanthate as well as the structure of the outermost layer, which is responsible for the mineral sample’s hydrophobicity, varied dramatically. Tetrahedrite shows the fastest kinetics of xanthate adsorption with the formation of multilayer coverage of surface cuprous xanthate. On chalcopyrite and tennantite the adsorbed amount of xanthate is much lower, close to a monolayer coverage. The differences in the surface composition of these three minerals are governed mainly by a different mobility of copper atoms in their crystalline structures in the presence of xanthate in solution. Other metal atoms present in the mineral samples, i.e. antimony, arsenic, iron, zinc, and silver, do not take a significant part in the formation of the outermost adsorption layer.
Introduction 11
In part X-ray photoelectron spectroscopic (XPS) studies were presented on the alteration of the surface composition of mineral samples of chalcopyrite (CuFeS2), tetrahedrite (Cu12Sb4S13), and tennantite (Cu12As4S13) formed by the interaction with aqueous solution at pH 10. It was concluded that this treatment produces the outermost layer containing hydrophilic species, mainly ferric or cupric (depending on the mineral sample) hydroxides and adsorbed water, as well as a sulfurenriched structure with a gradually changing composition with hydrophobic properties which vary significantly among the minerals. In the case of chalcopyrite the separation between the outermost hydrophilic layer (iron oxides/hydroxides) and the internal hydrophobic sulfurrich layer is more pronounced than for other minerals. In industrial separation processes selective modification of the surface properties of mineral components can be achieved by selective adsorption of organic reagents. In the case of sulfide minerals, xanthate homologues (ROCS2-, R ) an alkyl chain) are the most commonly used reagents. Whereas the adsorption products of xanthate on chalcopyrite were investigated in several works2-6 by the use of infrared spectroscopy, there is very limited work7 which reported results of XPS studies of xanthate interaction with chalcopyrite. The latter pre†
Laboratoire “Environnement et Mine´ralurgie” UA 235 CNRS. Laboratoire de Chimie Physique pour l’EnvironnementsUMR 9992-CNRS. X Abstract published in Advance ACS Abstracts, April 15, 1996. ‡
(1) Mielczarski, J. A.; Cases, J. M.; Alnot, M.; Ehrhardt, J. J. Part 1: Langmuir 1996, 12, 2519.
liminary results show very small changes in the recorded spectra before and after xanthate adsorption, which allows one to conclude that less than a monolayer of xanthate is adsorbed on chalcopyrite. The majority of the XPS studies of the surface structure of chalcopyrite were carried out on samples after treatment in different aqueous solutions containing inorganic ions. Some discussion of those studies and references to those works are reported in part 1.1 The infrared spectroscopic studies carried out by different techniques show mainly two xanthate adsorption products on chalcopyrite;2-4,6 they are dixanthogen (ROCS2)2, which is commonly assumed to be the surface product, and copper xanthate. However, there is no agreement as to whether they are formed simultaneously or in some order and to which one is the initial adsorption product.4-6 In the most recent work5 copper xanthate was reported as the only adsorption product. A very low adsorption of xanthate7 and significant changes in the surface layer of chalcopyrite just after conditioning in water,1 which themselves may provide a higher hydrophobicity, keep the role of xanthate in the hydrophobization of the chalcopyrite surface unclear. The aim of this work is to clarify the role of xanthate in producing (2) Allison, S. A.; Goold, L. A.; Nicol, M. J.; Granville, A. Metall. Trans. 1972, 3, 2613. (3) Ackerman, P. K.; Harris, G. H.; Klimpel, R. R.; Aplan, F. F. Int. J. Miner. Process. 1987, 21, 105. (4) Leppinen, J.; Basilio, C. I.; Yoon, R. H. Int. J. Miner. Process. 1988, 26, 259. (5) Valli, M.; Persson, I. Colloids Surf. 1994, 83, 207. (6) Leppinen, J. Int. J. Miner. Process. 1990, 30, 245. (7) Kartio, I.; Laajalehto, K.; Suoninen, E. Colloids Surf., A 1994, 93, 149.
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Mielczarski et al. Table 1. Mineralogical Composition
sample
composition
chalcopyrite tetrahedrite with silver tetrahedrite without silver tennantite
chalcopyrite >99%, sphalerite (ZnS) tetrahedrite ∼98%, chalcopyrite, pyrite (FeS2), galena (PbS), carbonates tetrahedrite ∼98%, chalcopyrite, pyrite, galena, carbonates tennantite ∼55%, sphalerite ∼42%, pyrite, chalcopyrite, galena
Chart 1
the hydrophobicity of chalcopyrite and collect the maximum information on the interaction of xanthate with tetrahedrite and tennantite. To the best knowledge of the authors, there is no published work on the surface characterization of the surface layer of the latter two minerals after their conditioning in xanthate solution. The long-term goal of these studies is to form a good fundamental basis for a selective separation of these minerals; therefore, it is vitally important to provide detailed information about the type and the amount of the adsorbed xanthate species (hydrophobic products) and the oxidation products (hydrophilic products) formed on the surface of the minerals under different solution conditions. A schematic representation of the complex structure of the investigated system is shown in Chart 1. The interface region represents all the different organic and inorganic species, produced during the contact of the mineral with xanthate aqueous solution. The complex composition of the mineral (two or more cations) causes the amount of possible surface reaction products that can be produced to increase tremendously. A detailed characterization of the interface region of the three minerals, including the depth profiling, will be discussed in this paper. The XPS spectra of the mineral samples were recorded directly after the conditioning in xanthate solution and after applying four different sputtering conditions (layer-by-layer microsectioning), which were initially very soft and then gradually stronger. Since the mineral samples and the experimental strategy used during this work are the same as in the experiments without xanthate,1 the importance of the xanthate influence on the composition and structure of the interface region, which governs the observed changes in the properties of these minerals, can be elucidated. Experimental Section The mineral samples chalcopyrite (CuFeS2), tetrahedrite (Cu12Sb4S13), and tennantite (Cu12As4S13) were procured from the Iberian Pyrite Belt. Whereas the two first mineral samples contain a small amount of other components, the latter sample composition is very complex, with a significant amount of sphalerite. Two different tetrahedrite samples with and without silver content were used in these studies, the majority of the silver substituting copper in the tetrahedrite structure (Table 1). A detailed description of the mineral samples can be found in the previous paper (Part 1).1 The slab sample of minerals
Table 2. Open Circuit Potentials of the Investigated Mineral Samples in Amyl Xanthate Solution (2 × 10-4 M) at pH 10 chalcopyrite
tetrahedrite
tennantite
100 mV, 5 min 105 mV, 30 min
100 mV, 5 min 110 mV, 30 min
120 mV, 5 min 120 mV, 30 min
with dimensions of 10 × 9 mm2 was cut from a massive rock, polished using emery paper and increasingly smaller sizes of alumina powder (to 0.03 µm), and washed with ethanol and water. The amyl xanthate used in these studies as a collector was synthesized from CS2, KOH, and amyl alcohol and then purified by recrystallization from acetone and ether. Distilled water, 18 MΩ, from the Millipore (Milli-Qplus) system, was used throughout the experiments. The other reagents used were all of an analytical grade. The mineral samples were contacted with amyl xanthate solutions at a concentrations of 2.0 × 10-4 M at a pH of 10.0 ( 0.1, adjusted by addition of KOH, immediately after polishing. Hence, they are the samples with “fresh” surfaces exposed to the xanthate solution. The changes in the surface composition after 25 min of immersion in xanthate solution were studied by XPS methods directly after removal of the samples from solution. If necessary the surface of the mineral samples was quickly blotted in order to remove the excess water prior to the spectroscopic analysis. The open circuit potential (OCP) of the mineral samples in xanthate aerated aqueous solution at pH 10 as a function of the immersion time is shown in Table 2. All potentials are reported against the standard hydrogen electrode (SHE). The XPS analysis was performed directly after xanthate adsorption and after consecutive sputtering. This allows us to obtain information on the in-depth distribution of particular surface species produced during contact of minerals with aqueous xanthate solution. The sputtering was used as the preparation technique. A detailed discussion on the choice of the measurement conditions and their description can be found in Part 1 of this work.1 There is also a discussion1 on the uncertainty of the quantitative in-depth analysis using the consecutive sputtering. Habitually four different sputtering conditions were used in these studies, to monitor the surface composition of minerals after adsorption, in the following order: (i) 0.3 mA for 2 min, (ii) 1 mA for 2 min, (iii) 1 mA for 4 min, and (iv) 1 mA for 10 min. After the last sputtering the obtained spectra are very similar to those characteristic for the “fresh” surface of the mineral samples. Empirical sensitivity factors8 were used for the quantitative determination of surface composition. The uncertainty of the quantitative estimation was discussed in detail in our previous work.1 The position of S 2p doublets reported in this paper is the position of the S 2p3/2 splitting component obtained after the fitting procedure.
Results and Discussion Interaction of Chalcopyrite with Amyl Xanthate Solution. The results of the XPS studies of the surface composition of chalcopyrite in contact with xanthate solution in an open to air vessel and after ion bombarding are presented in Figure 1 and Tables 3 and 4. A brief comparison of these results with those obtained after conditioning of chalcopyrite in water at the same pH, without collector,1 reveals that the presence of xanthate in the solution significantly changes the surface composition of the surface layer of chalcopyrite. The highest intensity component of the O 1s line is observed at 532.9 eV (Figure 1a). This position is characteristic of oxygen due to the adsorbed xanthate (8) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymont, R. H.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211.
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Figure 1. XPS lines of chalcopyrite after conditioning in xanthate solution, pH 10, with their fitted components (a), and in depth compositions after consecutive sputtering (b). In part b the absolute areas of the line components are presented.
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Table 3. Binding Energy (eV) of S, C, O, Fe, and Cu Elements and Kinetic Energy (eV) of Cu Auger Line Measured for Chalcopyrite after Conditioning in Xanthate Solution, pH 10, and Consecutive Sputtering core level C 1s
S 2p3/2
O 1s
Fe 2p3/2
treatment
a
b
c
a
b
a
b
c
a
b
Cu 2p3/2 a
Cu Auger
xanthate adsorption and sputtering at (mA min) 0.6 2.6 6.6 16.6
163.2
162.4
161.2
286.6
285.0
532.9
531.4
530.0
710.9
708.4
932.2
918.1
163.1 163.0 162.9 162.9
162.4 162.5
161.2 161.3 161.3 161.3
286.6 286.4 286.4 286.6
285.0 284.9 284.9 285.0
532.9 532.9
531.4 531.4 531.4 531.4
530.0 530.0 530.0 530.0
710.9 709.2 709.0
708.4 708.4 708.4 708.4
932.3 932.2 932.3 932.3
918.0 917.9 917.9 917.8
Table 4. Relative Intensities of Observed Lines in XPS Spectra of Chalcopyrite after Conditioning in Xanthate Solution, pH 10, and Consecutive Sputtering core level S 2p
C 1s
O 1s
Fe 2p3/2
treatment
a
b
c
a
b
a
b
c
a
b
Cu 2p3/2 a
xanthate adsorption and sputtering at (mA min) 0.6 2.6 6.6 16.6
0.044
0.063
0.120
0.080
0.160
0.204
0.140
0.120
0.178
0.121
1.000
0.027 0.017 0.014 0.036
0.032 0.005
0.073 0.045 0.038 0.099
0.028 0.006
0.066 0.023 0.008 0.008
0.066 0.013
0.054 0.024 0.011 0.008
0.058 0.032 0.014 0.016
0.194 0.109 0.037
0.279 0.182 0.171 0.280
1.000 1.000 1.000 1.000
molecules, as was determined in studies of xanthate adsorption on copper and cuprous sulfide.9,10 There are also lower intensity components at 531.4 and 530.0 eV, indicating the formation of hydroxide (component b) and oxide species (component c), respectively. The S 2p line can be fitted by three doublets with the position of the S 2p3/2 splitting peaks at 161.2 and 162.4 eV and about 163.0 eV. The average fwhm (full width at half maximum) values for these three components were (for one of the S 2p split components) about 1.3, 1.4, and 2.1 eV, respectively. The former and latter doublets have positions characteristic of the sample of chalcopyrite,1 while the middle doublet is due to the presence of sulfur in xanthate, which is in good agreement with that found recently for adsorbed xanthate on copper and cuprous sulfide.9,10 The Cu Auger line is somewhat shifted to a position of 918.1 eV (KE, kinetic energy), in comparison to the sample investigated immediately after polishing, indicating some changes in the environment of the copper atoms in the adsorption layer, but the position observed is not similar to that determined for the cuprous xanthate complex at a KE of 916.2 eV.11 There is no evidence for the formation of cupric species (no shake-up satellites); therefore, only cuprous species are expected in the interface region. The C 1s doublet at positions of 285.0 (hydrocarbon) and 286.6 eV (carbon bonded to oxygen) also indicates the adsorption of xanthate on chalcopyrite. It should be noted that at least part of the intensity at 285.0 eV may result from carbon contamination. A significant lowering in the absolute intensities of the Fe 2p, Cu 2p, and Cu Auger lines observed after xanthate adsorption (Figure 1b) indicates that these elements are not present in the outermost layer, which consists mostly of amyl xanthate molecules. The amount of adsorbed xanthate is not high, probably lower than that of a monolayer. This has been concluded on the basis of the following observations: (i) the intensity of the S 2p doublet of the mineral at 161.2 eV is about twice as high as that of adsorbed xanthate at 162.4 eV, indicating a relatively low attenuation signal from the bulk mineral and, hence, also a very thin adsorbed (9) Mielczarski, J.; Werfel, F.; Suoninen, E. Appl. Surf. Sci. 1983, 17, 160-174. (10) Mielczarski, J.; Suoninen, E. Surf. Interface Anal. 1984, 6, 3439. Mielczarski, J. A.; Suoninen, E. Colloids Surf. 1988, 33, 231-234. (11) Mielczarski, J. A.; Suoninen, E.; Johansson, L.-S.; Laajalehto, K. Int. J. Miner. Process. 1989, 26, 181-191.
xanthate layer, (ii) the position of the Cu Auger line is only slightly shifted, which suggests that copper atoms are at least partly bound to the mineral structure, and (iii) soft sputtering essentially diminishes all the features from the recorded spectra which indicate the presence of the adsorbed xanthate molecules. The relatively high intensity ratio between the S 2p doublet of the mineral and the S 2p doublet of the adsorbed xanthate could also be a result of the very high heterogeneity of the xanthate adsorption layer, when a patchlike structure is produced. Inspection of the Fe 2p3/2 line indicates that iron oxides and hydroxides with the high intensity component at 710.9 eV are formed simultaneously with xanthate adsorption. The simultaneous presence of the hydrophobic cuprous xanthate complex and hydrophilic iron hydroxide species indicates that amyl xanthate adsorption takes place according to an electrochemical mechanism:
mineral| + AmX- ) mineral|CuAmX + e (anodic oxidation) 1
/2O2 + H2O + 2e ) 2OH-
(1)
(cathodic reduction) (2)
Iron hydroxide species can also interact with xanthate ions from solution in a nonelectrochemical (replacement) reaction:
Fe(OH)3 + 3AmX- ) Fe(AmX)3 + 3OH-
(3)
The latter process provides a hydrophobic product; hence, the kinetics of the formation and transformation of iron hydroxide to other compounds could have a significant influence on the hydrophobic properties of chalcopyrite in contact with xanthate solution. The XPS spectrum cannot give a definitive answer whether the iron xanthate species are present at the chalcopyrite interface because of overlapping of the Fe 2p3/2 signal for iron xanthate and hydroxide surface species; hence, the component at about 710.9 eV could also represent, at least in part, the iron atoms forming the surface iron xanthate complex. Some help to answer this question is possible by the use of infrared spectroscopy to determine the nature of the surface layer. The infrared results obtained12 for the chalcopyrite sample prepared under similar conditions suggested that cuprous amyl
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Chalcopyrite, Tetrahedrite, and Tennantite
xanthate is the major surface product of xanthate adsorption on the “fresh” surface of the mineral. The amount of iron xanthate species, if any, was below the detection limit. The surface structure after xanthate adsorption is characteristic of a significant excess of copper at the interface region. The calculated atomic ratio is CuFe0.6S1.8, excluding the xanthate sulfur component. This indicates a significant enrichment of the surface layer of chalcopyrite in copper, and the reason for this is the presence of xanthate molecules in aqueous solution. A low-intensity shoulder at the position somewhat lower than 160 eV (Figure 1, S 2p line) may also support the presence of copper enrichment in the interface region. Another interesting feature is that there is not an indication of any enrichment in sulfur, as was observed after holding of chalcopyrite in aqueous solution without xanthate. This will be discussed in detail later. A consecutive sputtering reveals very interesting information about the in-depth surface composition (Figure 1b). The iron oxide/hydroxide species are located just below the outermost hydrophobic layer of xanthate, which is illustrated by an increase of the intensity of this line by more than twice after very soft (0.6 mA min) sputtering (Figure 1b, Table 4). This observation also suggests that the Fe 2p3/2 signal at about 710.9 eV is mainly due to iron oxide/hydroxide species. The absolute copper intensity shows the highest concentration of this element after the third consecutive sputtering (Figure 1b). At the same time the majority of the xanthate molecules are removed from the surface. The observed enrichment in copper surface concentration results from a diffusion of copper atoms to the mineral interface for the formation of a surface curpous xanthate complex. After the last ion bombarding the determined relative intensity ratio of the elements Cu:Fe:S is equal to 1:0.28:0.15, which gives a stoichiometry of CuFe0.6S1.1. This clearly indicates a very strong enrichment in the copper of the interface region of chalcopyrite at a rather significant distance from the interface. It could be seen from Figure 1 that after all of the consecutive sputtering (16.6 mA min) the intensity of the Cu 2p3/2 line decreases and the intensities of the S 2p and Fe 2p3/2 lines increase, implying that the bulk composition was not reached after the longest sputtering used. Close inspection of the S 2p line does not indicate any significant enrichment in the concentration of sulfur close to the interface (MeSx). The relative intensities of the S 2p components of MeSx and the mineral are almost constant after consecutive sputtering. The higher relative intensity of the S 2p line (Table 4) is observed just after xanthate adsorption. This increase is caused mainly by sulfur from the adsorbed xanthate molecules. It should be noticed that the removal by sputtering of all the surface adsorption products, when the almost clean surface of the mineral is studied, also results in a higher intensity of the S 2p line. This is caused by removal of all the adsorption products, which attenuated the S 2p signal from the bulk mineral. The above results show the following in-depth distribution of surface products produced on chalcopyrite treated with xanthate solution: (i) the outermost thin layer contains the adsorbed cuprous amyl xanthate species (surface iron xanthate species are also likely), which form a hydrophobic coverage, (ii) just below this thin layer and probably in the form of patches are iron oxide/hydroxide species, which have hydrophilic properties, (iii) the next layer is strongly enriched in copper atoms whose diffusion to the interface region of chalcopyrite is caused by the (12) Mielczarski, J. A.; Mielczarski, E.; Cases, J. M. Manuscript in preparation.
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presence of xanthate molecules, and (iv) next is the structure with the composition expected for the bulk chalcopyrite. For a schematic representation see also the schematic diagram shown in Figure 7. Another adsorption product which was found on the surface of chalcopyrite by infrared studies is dixanthogen.2-4,6 This compound was not observed in these studies, but it cannot be excluded as a possible adsorption product on the basis of XPS studies because of its very high volatility under UHV conditions. In order to monitor the presence of dixanthogen, a cooling of the sample after adsorption to liquid nitrogen temperature is required prior to introducing the sample to the spectrometer vacuum.13 These measurement conditions were ensured in separate experiments,14 whose results also do not show the presence of dixanthogen under OCP conditions. Dixanthogen was observed on chalcopyrite at higher potentials either in XPS14 or infrared spectroscopic studies.12 Interaction of Tetrahedrite with Amyl Xanthate Solution. The results of the XPS studies of the surface composition of tetrahedrite after conditioning with xanthate solution in an open to air vessel and after ion bombarding are presented in Figures 2 and 3 and Tables 5 and 6. These results clearly reveal that xanthate is adsorbed on the surface of tetrahedrite in a significant amount, since all the signals characteristic of the bulk mineral are strongly attenuated. The Zn 2p3/2 and Ag 3d5/2 lines are not visible, and the S 2p doublet at 161.5 eV characteristic for mineral sulfur and the Sb 3d components show very low intensities. The S 2p line with the major component of S 2p3/2 at 162.4 eV, the O 1s line component at 533.1 eV, the Cu Auger line at 916.3 eV (KE), and the high-intensity components of C 1s at 285.0 and 286.5 eV indicates the presence of the cuprous xanthate complex which forms multilayer coverage. There is no evidence (no shake-up satellites) for the presence of any cupric species. The intensity ratio of carbon C 1s components is close to 1:2, which is the value expected for amyl xanthate adsorbed molecules. The positions of the major components of the S 2p, C 1s, O 1s, and Cu Auger lines agree very well with those reported for adsorbed xanthate molecules on copper-containing substrates.9,10 These lines dominate in the spectrum recorded directly after adsorption; hence, the major adsorption product is a cuprous amyl xanthtate complex. This implies that the tetrahedrite sample will have very strong hydrophobic properties after its contact with xanthate solution. It should be also noted that the O 1s component characteristic of the oxide species (component c, Figure 2a) overlaps the Sb 3d5/2 component. This complication can be overcome easily by the use of the Sb 3d3/2 component at about 539 eV, for the characterization of antimony species in the sample, and its subtraction from the mixed O 1s and Sb 3d5/2 lines positioned at about 531 eV. There is a well established intensity ratio and splitting distance between both Sb 3d components.15 Using this approach, it was possible to determine two antimony components of the Sb 3d5/2 line at 529.4 and 531.0 eV, on the basis of the two Sb 3d3/2 components (Figure 2a), which are characteristic of mineral antimony and antimony oxide/hydroxide species. It is also possible that the higher energy component at least partly is due to an antimony xanthate complex. Additional studies are required to clarify this possibility. The simultaneous presence of a hydrophobic cuprous xanthate complex and hydrophilic antimony (and probably (13) Kartio, I.; Laajalehto, K.; Suoninen, E.; Karthe, S.; Szargan, R. Surf. Interface Anal. 1992, 18, 807. (14) Mielczarski, J. A.; Laajalehto, K. Manuscript in preparation. (15) Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., Muilenberg, G. E., Eds.; Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer Corporation: Eden Prairie, MN, 1979.
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Figure 2. XPS lines of tetrahedrite after conditioning in xanthate solution, pH 10, with their fitted components (a), and in depth compositions after consecutive sputtering (b).
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Table 5. Binding Energy (eV) of S, C, O, Sb, Zn, Ag, and Cu Elements and Kinetic Energy (eV) of Cu Auger Line Measured for Tetrahedrite after Conditioning in Xanthate Solution, pH 10, and Consecutive Sputtering core level C 1s
S 2p3/2 treatment xanthate adsorption and sputtering at (mA min) 0.6 2.6 6.6 16.6
a
b
c
a
O 1s + Sb 3d5/2 b
a
b
c
d
Zn 2p3/2 a
Ag 3d5/2 a
164.1 162.4 161.5 286.5 285.0 533.1 530.8 531.0 529.4 163.6 162.4 161.5 286.5 285.0 163.6 162.4 161.5 286.3 284.8 163.1 162.4 161.5 286.3 284.8 162.8 161.5 286.3 284.8
533.1 533.1 533.1 533.1
530.8 530.9 529.4 530.8 530.7 529.4 530.8 530.6 529.4 530.8 529.4
1022.0 1021.7
368.0 368.0 368.0
Cu 2p3/2 a
Cu Auger
932.3
916.3
932.5 932.5 932.5 932.4
916.6 917.2 917.3 917.4
Table 6. Relative Intensities of Observed Lines in XPS Spectra of Tetrahedrite after Conditioning in Xanthate Solution, pH 10, and Consecutive Sputtering core level S 2p
C 1s
O 1s + Sb 3d5/2
treatment
a
b
c
a
b
a
b
c
d
xanthate adsorption and sputtering at (mA min) 0.6 2.6 6.6 16.6
0.014
0.147
0.011
0.091
0.181
0.128
0.067
0.028
0.024
0.010 0.010 0.016 0.021
0.084 0.046 0.017
0.022 0.053 0.070 0.086
0.049 0.024 0.015 0.005
0.107 0.065 0.034 0.025
0.067 0.027 0.007 0.013
0.055 0.070 0.083 0.049
0.018 0.025 0.032
0.037 0.096 0.183 0.283
Figure 3. Copper Auger L3M45M45 line of tetrahedrite after conditioning in xanthate solution, pH 10, and in depth compositions: (a) simultaneously after adsorption and after consecutive sputtering; (b) 0.3 mA for 2 min; (c) 1 mA for 2 min; (d) 1 mA for 4 min; (e) 1 mA for 10 min.
cuprous) oxide/hydroxide species indicates that amyl xanthate adsorption takes place according to an electrochemical mechanism, as is described in the above section (eqs 1 and 2). Zinc, antimony, and silver are not present in the outermost xanthate adsorbed layer. The two former elements can be detected after a long consecutive sputtering (Figure 2b). Silver is distributed closer to the interface than the other two elements. The intensities of these lines increase gradually, except the Ag 3d line, which appears suddenly after the second sputtering (2.6 mA min), showing a relatively high intensity which barely increases
Zn 2p3/2 a
Ag 3d5/2 a
Cu 2p3/2 a 1.000
0.091 0.125
0.011 0.011 0.014
1.000 1.000 1.000 1.000
after the next ion bombarding. A very small amount of iron was also detected after a long sputtering (results not shown). The consecutive sputtering gradually removes the adsorbed cuprous xanthate molecules from the tetrahedrite surface (Figure 2b). After the third sputtering (6.6 mA min) almost all the adsorbed xanthate species disappeared. The S 2p line shows a major component of S 2p3/2 at 161.5 eV and the Cu Auger at 917.3 eV (KE), which are both characteristic of tetrahedrite1 (Table 5). The changes in the position of the Cu Auger line are shown in Figure 3, which clearly demonstrate a transition from copper bonded to xanthate molecules to copper atoms in the mineral structure. It is interesting to note that the Cu Auger line shows a maximum intensity between the second and third ion bombarding, indicating a strong copper enrichment (Figure 3). This enrichment is caused by diffusion of copper atoms from the bulk mineral to the interface in the presence of xanthate molecules in solution. This phenomenon can be observed only on the basis of the changes of the Cu 2p Auger line, which gives information from a thicker layer (higher kinetic energy of the escape electrons) than the Cu 2p line, which is more surface sensitive (low kinetic energy of the escape electrons). This significant difference in the thickness of the surface layer monitored by the Cu Auger and Cu 2p lines can explain the observed differences in the behavior of these two copper lines and indicates that the copper-rich layer is mainly localized in the region sputtered during the third ion bombarding. The relative intensity ratio of the S 2p broad component at about 163 eV assigned to the sulfur-rich species does not change significantly with consecutive sputtering, which excludes any significant sulfur enrichment in the adsorption layer. After the last sputtering the relative intensity ratio of the elements Cu:Sb:S is about 1:0.28:0.11, which gives a stoichiometry of Cu12Sb3.0S10.2. This result indicates a small increase of the copper concentration in the interface region of tetrahedrite. This is a result of xanthate adsorption, which involves a diffusion of copper atoms to the mineral interface from the bulk, and the consecutive sputtering displays this copper enrichment. Except the latter small surface modification, the surface distribution of other elements is very similar to that expected for the bulk tetrahedrite sample. The above results imply the following in-depth distribution of surface products formed on tetrahedrite in
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contact with xanthate solution: (i) the outermost multilayer contains a cuprous xanthate complex which is responsible for the hydrophobic properties of the sample (a very small amount of antimony oxides/hydroxides is also observed), (ii) below this strong hydrophobic layer gradual changes lead to a structure with a composition very similar to that expected for bulk tetrahedrite. The tetrahedrite sample contains a significant amount of silver and zinc in the bulk sample, and they are not present, as well as antimony, in the outermost hydrophobic layer. For details see also Figure 7. The obtained results indicate that the tetrahedrite sample in contact with xanthate solution at pH 10 will be strongly hydrophobic. Interaction of Xanthate with Tetrahedrite Sample Free of Silver. It is well-known that impurities can play an important role in adsorption of collectors, modifying strongly the composition and structure of the adsorbed layer (see, for examples, refs 16-19). The above described results were obtained for the tetrahedrite sample containing a significant amount of zinc and silver, which are not the elements expected for a pure sample of tetrahedrite (Cu12Sb4S13). The corresponding experiments similar to those described above were carried out on a tetrahedrite sample without silver (with other “impurities” at a similar level) in order to determine the influence of silver content on the adsorption of xanthate. The results obtained are presented in Figure 4. In general the results are similar to those presented above. Xanthate is strongly adsorbed on tetrahedrite, forming a cuprous amyl xanthate complex. Silver does not influence the adsorption of xanthate on tetrahedrite, at least in the amount of present in the investigated samples. In fact, these results were already expected on the basis of the observation (Figure 2) that silver atoms do not participate in the adsorption of xanthate, forming the silver xanthate complex. Nevertheless, there are important reasons to present the experimental results for the sample free of silver and discuss them in detail. Close inspection of the results obtained for the two tetrahedrite samples (Figures 2 and 4) reveals some important differences. First of all the shape of the lines recorded for the silver free tetrahedrite sample shows more fine structure (shoulders, local minima and maxima, saddle and inflection points), which allows us to perform the curve fitting more precisely. The most visible difference is observed for the S 2p line, which can be fitted by two doublets characteristic of the mineral sulfur and adsorbed xanthate sulfur. The third S 2p doublet component, which was assigned to the formation of the sulfurrich layer, was not observed for the silver free tetrahedrite sample. This is a very important observation, which clearly indicates that the third doublet at about 163.0 eV can be misinterpreted as a sulfur-rich layer (known as metal-deficient sulfides20 or polysulfides21), whereas the real reason is the presence of other mineral components, which causes the modification of the environment of the sulfur atoms in the mineral, resulting in the observed broadening of the S 2p line. Another difference between these two tetrahedrite samples is a lack of the second antimony component assigned to oxide/hydroxide species for the sample free of (16) Mielczarski, J. Int. J. Miner. Process. 1986, 16, 179. (17) Blackburn, W. H.; Schwendeman, J. F. Can. Mineral. 1977, 15, 365. (18) Ansell, H. G.; Boorman, R. S. CIM Bull. 1973, 66, 736. (19) Laajalehto, K.; Suoninen, E.; Heimala, S. Int. J. Miner. Process. 1991, 33, 95-102. (20) Buckley, A. N.; Hamilton, I. C.; Woods, R. Investigation of surface oxidation of sulfide minerals by linear sweep voltammetry and X-ray photoelectron spectroscopy. In Flotation of sulfide minerals; Developments in Mineral Processing; Forssberg, K. S. E., Ed.; Elsevier: Amsterdam, 1985; Vol. 6, p 41. (21) Luttrell, G. H.; Yoon, R. H. Colloids Surf. 1984, 12, 239-254.
Mielczarski et al.
silver. There is only one Sb 3d3/2 line component with a position characteristic of the bulk mineral. This indicates that the presence of silver may modify the kinetics of different surface reactions, resulting in some intermediate species being not clearly visible. Summarizing the above observations, although the presence of silver does not change the mechanism and kinetics of xanthate adsorption, it has a significant influence on the shape of the observed XPS line, which can be easily misinterpreted; for example, a broadening of the S 2p line can be interpreted as a sulfur surface enrichment. Interaction of Tennantite with Amyl Xanthate Solution. The results of the XPS studies of the surface composition of tennantite in contact with xanthate solution in an open to air vessel and after sputtering are presented in Figures 5 and 6 and Tables 7 and 8. A brief inspection of these results reveals that xanthate is adsorbed on the surface of tennantite in a small amount because the signal from the bulk mineral (Figure 5a, for example, component c of the S 2p line) is moderately attenuated. Close inspection of the O 1s line indicates also that the formation of oxidized products is very limited although only a small amount of collector was adsorbed. The S 2p doublet at 162.4 eV, the O 1s line at 533.1 eV, and the high-intensity components of C 1s at 284.9 and 286.5 eV (with the relative ratio 1:2) are characteristic, as discussed above, for the cuprous xanthate surface complex. The Cu Auger line (Figure 6) is broad, indicating two components with positions at about 916.5 and 917.2 eV (KE), which suggests that the cuprous xanthate complex is not exactly the same as that observed for multilayer coverage at 916.2 eV (KE). This indicates that copper atoms bonded to xanthate molecules are simultaneously involved in an interaction with the mineral structure. The S 2p line was fitted by three doublets at 161.5, 162.4, and 162.9 eV with fwhm about 1.4, 1.4, and 2.5 eV, respectively. Except the relatively strong O 1s component at 533.1 eV, there are low-intensity components at 531.9 and 530.5 eV, indicating the formation of hydroxide and oxide species, respectively. The As 3d line shows two components at about 42.9 and 45.4 eV which are due to mineral arsenic and arsenic oxides, respectively. These positions are similar to those reported for As2S3 and As2O3 compounds.15 The presence of the hydrophobic cuprous xanthate complex and at the same time hydrophilic hydroxide species indicates that the xanthate adsorption on tennantite also takes place by the electrochemical mechanism described in reactions 1 and 2. Arsenic and zinc are not enriched in the outermost layer; on the contrary, their concentrations are much lower than the bulk concentration. A small amount of iron can be observed in the sample after a strong consecutive sputtering. After the third sputtering (6.6 mA min) all the adsorbed xanthate molecules are removed from the tennantite surface (Figure 4). The S 2p line shows a major doublet at 161.5 eV, and the Cu Auger line is at 917.2 eV (KE), which are characteristic of the bulk tennantite sample. It is interesting to note that the Cu Auger line shows a maximum intensity after the second ion bombarding, indicating a slight copper enrichment in the interface region of tennantite, similar to what was observed for the other two minerals, indicating acceleration of copper atom diffusion from the bulk of the mineral to the interface caused by the presence of xanthate in solution. That the relative intensity ratio of the S 2p broad component at 163 eV temporarily assigned to sulfur surface species and the Cu 2p3/2 line component shows a little increase after the 6.6 mA min sputtering could indicate some enhancement in sulfur content in the adsorption layer. The appearance of the component at about 163 eV could also
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Chalcopyrite, Tetrahedrite, and Tennantite
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Figure 4. XPS lines of tetrahedrite sample without silver after conditioning in xanthate solution, pH 10, with their fitted components (a), and in depth compositions after consecutive sputtering (b).
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Figure 5. XPS lines of tennantite after conditioning in xanthate solution, pH 10, with their fitted components (a), and in depth compositions after consecutive sputtering (b).
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Table 7. Binding Energy (eV) of S, C, O, Zn, As, and Cu Elements and Kinetic Energy (eV) of Cu Auger Line Measured for Tennantite after Conditioning in Xanthate Solution, pH 10, and Consecutive Sputtering core level C 1s
S 2p3/2
O 1s
treatment
a
b
c
a
b
a
b
c
Zn 2p3/2 a
xanthate adsorption and sputtering at (mA min) 0.6 2.6 6.6 16.6
162.9
162.4
161.5
286.5
284.9
533.1
531.9
530.5
1021.6
162.9 162.8 162.7 162.6
162.4 162.4
161.5 161.5 161.5 161.5
286.4 286.2 286.4 286.4
284.9 284.9 284.8 284.8
533.1 533.1
531.9 531.9
530.5 530.5 530.5 530.5
1021.7 1021.7 1021.8 1021.7
As 3d a
b
Cu 2p3/2 a
Cu Auger
45.4
42.9
932.3
916.8
42.7 42.7 42.7 42.6
932.4 932.4 932.4 932.4
917.0 917.1 917.1 917.2
Table 8. Relative Intensities of the Observed Lines in XPS Spectra of Tennantite after Conditioning in Xanthate Solution, pH 10, and Consecutive Sputtering core level S 2p
C 1s
O 1s
As 3d
treatment
a
b
c
a
b
a
b
c
Zn 2p3/2 a
a
b
Cu 2p3/2 a
xanthate adsorption and sputtering at (mA min) 0.6 2.6 6.6 16.6
0.035
0.052
0.167
0.066
0.131
0.076
0.039
0.023
0.628
0.011
0.017
1.000
0.029 0.025 0.044 0.028
0.039 0.017
0.131 0.125 0.145 0.158
0.032 0.010 0.003 0.003
0.090 0.058 0.038 0.015
0.030 0.016
0.019 0.009
0.029 0.023 0.013 0.008
0.694 0.886 1.253 1.533
0.017 0.014 0.017 0.017
1.000 1.000 1.000 1.000
Figure 6. Copper Auger L3M45M45 line of tennantite after conditioning in xanthate solution, pH 10, and in depth compositions: (a) simultaneously after adsorption and after consecutive sputtering; (b) 0.3 mA for 2 min; (c) 1 mA for 2 min; (d) 1 mA for 4 min; (e) 1 mA for 10 min.
be caused by other mineral components in the tennantite sample (Table 1) (see discussion in the above section); therefore, any final conclusion about sulfur enrichment requires additional clarification. After the last sputtering the relative intensity ratio of the elements Cu:As:S:Zn is about 1:0.017:0.19:1.53, which gives a stoichiometry of Cu12As1.6S16.6Zn18.6. The surface shows the composition, which is similar to that expected for the bulk tennantite sample.1 There are two major components, tennantite with copper atoms replaced partly by zinc, and zinc sulfide. The above results suggest the following in-depth distribution of surface products formed on tennantite in
contact at OCP with xanthate solution: (i) the outermost layer contains a small amount of the cuprous xanthate complex and a very small amount of arsenic hydroxide and oxide species and (ii) below this very thin hydrophobic layer is another layer slightly enriched in copper and probably in sulfur, and (iii) then gradual changes lead to the structure with a composition very similar to that expected for bulk tennantite. For details see also the schematic diagram presented in Figure 7. The obtained results indicate that tennantite in contact with xanthate solution at pH 10 will show a limited increase in the hydrophobicity. Comparison of XPS Results of Interaction of Chalcopyrite, Tetrahedrite, and Tennantite with Xanthate Solution. The adsorbed xanthate molecules were observed on all three of the minerals, but the adsorbed amount varies dramatically as well as the structure of the outermost layer which is responsible for the mineral hydrophobicity. The xanthate adsorption takes place by an electrochemical mechanism which involves the formation of cuprous xanthate (hydrophobic species) and metal hydroxides (hydrophilic species). Whereas copper at the interface is mainly involved in the formation of the cuprous xanthate complex, the other elements, iron, antimony, and arsenic, depending on the mineral sample, form mainly hydroxides and oxides. At least a part of the observed oxides are probably produced by dehydration of hydroxides under UHV during the XPS studies. The formation of cuprous oxides/hydroxides in a small amount cannot be excluded because of the overlapping of the Cu 2p and Cu Auger signals from these species with copper signals from the bulk mineral. The cuprous oxides/ hydroxides are present rather as intermediate species, which are transformed to cuprous xanthate species in a replacement reaction similar to that presented in eq 3. All the mineral samples show a copper enrichment in the interface region formed as a result of diffusion of copper atoms to the interface provoked by the adsorbed xanthate molecules. In general the obtained results imply the following indepth distribution of the surface products formed on the minerals in contact with the xanthate solution: (i) the outermost layer contains the cuprous xanthate complex which causes hydrophobic properties (a small amount of metal hydroxide species are also observed) and (ii) below this hydrophobic layer gradual changes lead to the
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Figure 7. Schematic diagram of surface layer composition in the interface region. Areas filled by patterns represent approximate distributions of major surface species. The thicknesses are rough estimations made in the same manner as recently presented.1
structure with a composition very similar to that expected for the bulk mineral sample. A schematic diagram which represents the approximate distributions of different surface species and a rough estimation of the thicknesses of the interface regions found for the three minerals is shown in Figure 7. The adsorption of xanthate on the “fresh” surface of chalcopyrite is very small. The outermost layer shows a mixed composition of the hydrophobic cuprous xanthate complex and the hydrophilic iron (and probably cuprous) oxides/hydroxides. Therefore it can be concluded that only a limited increase in hydrophobicity is expected under these adsorption conditions. A very strong copper enrichment in the surface region of chalcopyrite was found, which at extended adsorption will produce more of the surface cuprous xanthate complex. The tetrahedrite sample shows a very fast xanthate adsorption with multilayer formation of cuprous amyl xanthate, which produces a strong hydrophobic layer on the mineral. The tennantite sample, which contains a lot of other mineral components, mainly sphalerite, shows a very slow adsorption of xanthate with the formation of cuprous xanthate. The formation of oxidation products is also very slow. Consequently a limited increase in hydrophobicity will be observed. The formation of a significant amount of hydrophobic cuprous xanthate requires an extension of the adsorption time in the case of tennantite. Other metal ions present in the mineral samples, i.e. antimony, arsenic, iron, zinc, and silver, do not take an important part in the formation of the outermost adsorption layer. Nevertheless they modify the shape of the XPS line, which could be a reason for a misinterpretation of experimental spectra. For example, special attention is required in the case of assignment of the S 2p doublet
Mielczarski et al.
at about 163 eV to sulfur surface enrichment species because this component may also arise from the presence of other elements in mineral samples (“impurity” mineral components). The presence of silver in tetrahedrite shows clearly this effect. Therefore, the conclusions according the presence of sulfur-rich layers at the interface of tetrahedrite and tennantite are rather temporary. It cannot be excluded that the presence of a significant amount of sphalerite and replacement of copper by zinc in tennantite are the reason for the presence of the S 2p component with a position which can be assigned to sulfurrich species. Comparison of the Surface Composition of the Three Minerals after Their Contact with Aqueous Solution with and without Xanthate. Conditioning of the minerals with xanthate solution at pH 10 is, in fact, also conditioning with water, and the concurrent surface reactions between interaction of minerals with water and with xanthate ions take place. As was described in Part 11 contact of the minerals with water at pH 10 results in significant changes in the interface region. The sulfurenriched structures, which were observed after treatment in solution without xanthate, are not observed in the presence of xanthate solution. In general these three minerals after contact with xanthate solution are covered by the cuprous xanthate layer which increases the hydrophobicity, whereas the treatment in aqueous solution without xanthate produces hydrophilic coverages containing metal oxide/hydroxide species. There are also significant differences in the composition of the mineral surface layers which could have a significant importance in (i) the adsorption performed during an extended conditioning and (ii) the mechanical stability of the outermost adsorbed layers. This will be discussed in detail in this section. The most striking differences are observed for chalcopyrite. Treatment in aqueous solution involves diffusion of iron atoms to the outermost layer with formation of iron oxide/hydroxide species, whereas the presence of xanthate in the solution causes a significant increase of copper concentration in the surface region of chalcopyrite. This increase is much higher than the one required for bonding of xanthate molecules and is observed in a relatively thick (more than 10 nm) surface region of chalcopyrite. Thus, an increase of the conditioning time in xanthate solution should provide good conditions for the formation of the hydrophobic surface cuprous xanthate complex. The second important difference is the formation of a well established sulfur-rich layer under hydrophilic iron oxide/hydroxide coverage, while in the presence of xanthate in aqueous solution no sulfur enrichment was observed. Tetrahedrite shows relatively strong diffusion of copper atoms to the interface with and without the presence of xanthate in solution. As a result, the formation of the outermost thick layer of cuprous and cupric oxides/ hydroxides in aqueous solution at pH 10, and the multilayer of cuprous xanthate in the presence of xanthate in solution are observed. Any enrichment in sulfur at the interface in the presence of xanthate was not observed, especially for the tetrahedrite sample without silver content. The tetrahedrite samples containing silver show the sulfur component at about 163 eV, after treatment in either aqueous solution or xanthate solution, which can be interpreted as a sulfur-enriched species. However, it was found that a similar effect may result from the presence of other mineral components like silver. It is interesting to note that silver and zinc elements present in the mineral samples do not take part in the formation of the outermost layers. It seems that zinc, at least in a
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Chalcopyrite, Tetrahedrite, and Tennantite
small amount, does not produce the additional S 2p components which can be misinterpreted as sulfur enrichment. Tennantite shows relatively slow kinetics of the interaction with aqueous solution without and with xanthate. Therefore, the thickness of the interface region is limited to the thinnest layer consisting of cuprous oxides/ hydroxides or cuprous xanthate, depending on the presence of xanthate in the aqueous solution. Tennantite and tetrahedrite are very similar minerals, and it is rather expected that they will have similar properties. The reason for the major difference in the kinetics of interaction of these two minerals is a fine dispersion of tennantite in
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sphalerite which is present in the tennantite sample in a significant amount, up to 40% of the surface layer. This causes a high resistance of the mineral sample and as a consequence slows all the surface reactions which are electrochemically controlled. Acknowledgment. This work was supported by European Community Project MA2M-CT92-0062. The mineralogical analysis by Dr. P. Marion and the technical assistance of J. Lambert in collection of XPS spectra are gratefully acknowledged. LA950589T
