Langmuir 1996, 12, 6521-6529
6521
Interaction of Amyl Xanthate with Chalcopyrite, Tetrahedrite, and Tennantite at Controlled Potentials. Simulation and Spectroelectrochemical Results for Two-Component Adsorption Layers J. A. Mielczarski,* E. Mielczarski, and J. M. Cases Laboratoire “Environnement et Mine´ ralurgie”, UA 235 CNRS, INPL-ENSG, B.P. 40, 54501 Vandoeuvre-le` s-Nancy, France Received June 10, 1996. In Final Form: September 23, 1996X Spectroelectrochemical studies of the interaction of amyl xanthate solution (C5H11OCS2-) at pH 10 with mineral samples of chalcopyrite (CuFeS2), tetrahedrite (Cu12Sb4S13), and tennantite (Cu12As4S13) at different potentials were carried out by infrared reflection spectroscopy. The experimental data were compared with simulated results in order to evaluate in detail the composition and structure of the adsorbed layers. Two-component adsorption layers were determined on these three minerals. The first adsorption product observed spectroscopically was a cuprous amyl xanthate complex followed by an amyl dixanthogen (dimer, (C5H11OCS2)2). The dixanthogen was always observed together with cuprous xanthate complex. It was produced simultaneously with cuprous xanthate, on chalcopyrite and tetrahedrite, or at potential 100 mV higher than cuprous xanthate formation on tetrahedrite. The produced cuprous xanthate was placed close to the mineral interface, and its amount was limited from a submonolayer to a few monolayers, whereas the dixanthogen forms the outermost layer and a very large thickness could be achieved. There was a strong thermodynamic limitation of the formation of submonolayer coverages at the potentials close to that at which the first xanthate molecules start to adsorb. After monolayer completion the adsorbed amount was time dependent. The dixanthogen was observed at the chalcopyrite surface at submonolayer coverages at potentials very close to that calculated from the thermodynamic data for bulk phase, while for tetrahedrite and tennantite dixanthogen was produced at potentials 100 and 40 mV higher, respectively. The adsorption kinetics of two kinds of surface species were investigated. The relative amount of cuprous xanthate and dixanthogen depends on the availability of copper atoms at the interface, hence, the mobility of the atoms in the interfacial region, which is different for each of the minerals. For slower copper diffusion the produced amount of dixanthogen increases. Selective flotation separation of these three minerals on the basis of differences in optimal adsorption conditions of amyl xanthate (selective changes in mineral hydrophobicity) could be improved.
Introduction In separation processes a selective modification of the surface properties (hydrophobicity) of solid components can be achieved by a selective adsorption of organic reagents. In the case of sulfide minerals the xanthate homologues (ROCS2K, R ) alkyl chain) are most commonly used reagents for separation by flotation. Selective separation of chalcopyrite (CuFeS2), tetrahedrite (Cu12Sb4S13), and tennantite (Cu12As4S13) from their mixtures, important from an environmental point of view (presence of antimony and arsenic), is difficult because all of them contain copper; hence, copper complexing selective reagents will form surface complexes, with all of them modifying similarly their hydrophobic properties. Therefore, other methods than the use of the metal complexing reagent have to be considered in order to modify differently the surface properties of the three minerals. One possible way is to change the potentials of these three minerals in solution to the particular values which ensure the selective formation of the organic hydrophobic layer on each of the minerals. Detailed characterization at a molecular level of amyl xanthate surface products produced on these three minerals under different adsorption conditions (mineral potentials, adsorption time) is the major subject of this work. The infrared spectroscopic studies carried out by different techniques show mainly two xanthate adsorption products on chalcopyrite,1-5 they are dixanthogen X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) Allison, S. A.; Goold, L. A.; Nicol, M. J.; Granville, A. Metall. Trans. 1972, 3, 2613.
S0743-7463(96)00568-9 CCC: $12.00
(ROCS2)2, which is commonly assumed to be the surface product, and copper xanthate (ROCS2Cu). However, there is no agreement on if they are formed simultaneously or in a particular order and which one of them is the initial adsorption product.3-5 In the most recent work4 the copper xanthate was reported as the only adsorption product. A very low adsorption of xanthate6 and significant changes in the surface layer of chalcopyrite just after conditioning in water,7 which themselves may provide a higher hydrophobicity, leave the role of xanthate in the hydrophobization of chalcopyrite surface unclear. It is also reported that chalcopyrite after grinding in dry8 or wet9 conditions showed the formation of iron xanthate as the third adsorption product of xanthate. In spite of this work, important details of the nature and structure of the surface adsorbed layers on chalcopyrite remain unexplained. Concerning tetrahedrite and tennantite, to the best knowledge of the authors, there are only two published works on the surface characterization of the adsorbed xanthate layers by the use of X-ray photoelectron spec(2) Ackerman, P. K.; Harris, G. H.; Klimpel, R. R.; Aplan, F. F. Int. J. Miner. Process. 1987, 21, 105. (3) Leppinen, J.; Basilio, C. I.; Yoon, R. H. Int. J. Miner. Process. 1988, 26, 259. (4) Valli, M.; Persson, I. Colloids Surf. 1994, 83, 207. (5) Leppinen, J. Int. J. Miner. Process. 1990, 30, 245. (6) Kartio, I.; Laajalehto, K.; Suoninen, E. Colloids Surf., A 1994, 96, 149. (7) Mielczarski, J. A.; Cases, J. M.; Alnot, M.; Ehrhardt, J. J. Langmuir 1996, 12, 2519-2530. (8) Mielczarski, J. A.; Cases, J. M.; Barres, O. J. Colloid Interface Sci. 1996, 178, 740. (9) Cases, J. M.; Kongolo, M.; de Donato P.; Mielczarski, J. A.; Barre´s, O.; Bouquet, E.; Franco, A. 1995. In Mineral Processing. Recent Advances and Future Trends; Mehrotra, S. P., Shekhar, R., Eds.; Allied Publishers, Ltd.: New Delhi, pp 13-29.
© 1996 American Chemical Society
6522 Langmuir, Vol. 12, No. 26, 1996
troscopy (XPS)10 and infrared spectroscopy8 for the minerals at open circuit potential (OPC) conditions. Because the XPS method required a dry sample and measurements are carried out in ultrahigh vacuum (UHV), the surface layer composition and structure could undergo some changes in comparison with the original surface layer produced in solution. Infrared reflection spectroscopy is especially well suited for characterization of organic supramolecular structures and does not require the drastic measurement conditions as in the case of XPS method. The aim of this work is to determine, using the infrared external reflection method, the composition and structure of xanthate adsorption products under different adsorption conditions (mineral potentials, adsorption times) in order to find thermodynamic and kinetic limitations of the formation of xanthate inducing hydrophobicity for these three copper minerals. A detailed structure of the adsorption layers has been evaluated on the basis of the comparison of the experimental spectra and the simulated spectra of the hypothetical adsorption layer with assumed composition and structure. Since the mineral samples used during this work are from the same massive rock as those in the recent XPS studies,10 the comparison to those results will also be performed. Experimental Section Materials. The mineral samples chalcopyrite (CuFeS2), tetrahedrite (Cu12Sb4S13), and tennantite (Cu12As4S13) were procured from the Iberian Pyrite Belt. Whereas the first two mineral samples contain a small amount of other components, the latter sample composition is very complex, with a significant amount of sphalerite. Detailed description of the mineral samples can be found in a previous paper (Table 1 in ref 7). The slab sample of minerals with dimensions of 40 × 9 mm2 were cut from a massive rock and polished using emery paper and increasingly smaller sizes of alumina powder (to 0.03 µm) and washed with water. The experimental data presented in this work were collected on the same mineral sample for each mineral in order to avoid sample dependent variation. The potassium amyl xanthate (C5H11OCS2K) used in these studies was synthesized from CS2, KOH, and amyl alcohol, and then purified by recrystallization from acetone and ether. The product obtained was tested spectrophotometrically showing purity of above 99.5%. Distilled water, 18 MΩ, from the Millipore (Milli-Qplus) system, was used throughout the experiments. The other used reagents were all of an analytical grade. Adsorption Study at Controlled Potentials. A standard three-electrode cell and a Tacussel PRT 20-2X potentiostat were used in the electrochemical studies of the three mineral samples. The mineral slab sample was the working electrode, while a platinum wire mesh, separated from solution by frit, served as the counter electrode. Although a Ag/AgCl electrode served as the reference electrode, all potentials are reported against the standard hydrogen electrode (SHE) scale. The electrochemically controlled adsorption was performed from xanthate solutions at a concentration of 2.0 × 10-4 M, containing 0.05 M Na2SO4 at pH 10 ( 0.2 adjusted by adding KOH or H2SO4 solutions. The mineral sample was immersed in this solution immediately after polishing. Hence, it is the sample with “fresh” surface exposed to the xanthate solution. The mineral electrode was linearly swept from open circuit potential (OCP) to the desired potential and held at this potential for 10 min. Next the sample was immersed in water at pH 10 (adjusted by adding KOH) for about 1 s and then immediately introduced to the spectrophotometer for infrared characterization. The sample prepared in the same conditions was characterized also without preceding immersing in water. No noticeable difference was observed after these different pretreatments of the samples. Extended contact of mineral sample with the xanthate solution from 10 to 120 min was also exploited. The electrochemical cell was open to air; therefore the applied potential was restricted to the range (10) Mielczarski, J. A.; Cases, J. M.; Alnot, M.; Ehrhardt, J. J. Langmuir 1996, 12, 2531-2543.
Mielczarski et al. between the OCP and up to 300 mV above this value. The electric charge which passed through the system during the immersion of the mineral in collector solutions was recorded for all samples. This value was used to evaluate the surface coverage by adsorbed xanthate molecules for each investigated sample. The concentrations of amyl xanthate solutions were additionally monitored in UV range by the use of a UV-visible spectrometer (Shimadzu UV 2100). Infrared Analysis. The infrared spectra of all mineral samples after adsorption were recorded on Bruker IFS88 and IFS55 FTIR spectrometers with a DTGS detector and an MCT detector cooled with liquid nitrogen. External reflection spectra were recorded by means of a special reflection attachment with a polarized incident beam at various incident angles. All the optical accessories were from Harrick Scientific Co. The spectrometer was purged with dry air (Balston Filter) to minimize the contribution of a water vapor and carbon dioxide to the recorded spectra. The spectra were taken at 4 cm-1 resolution by coadding up to 500 scans in the 4000-500 cm-1 region. The unit of intensity was defined as -log (R/R0), where R0 and R are the reflectivities of the systems without and with investigated medium (adsorption layer of xanthate), respectively. Because some of the adsorption products could be not very stable at the mineral surface after adsorption and the removal from solution, some of the spectra were recorded as a function of time. This procedure shows that one of the formed products, amyl dixanthogen, is volatile, but relatively stable during the time required for recording the infrared reflection spectra of the adsorbed layer. The lowering in the absorbance intensity does not exceed 5% of the initial intensity.
Results and Discussion Optical Consideration. Detailed description of the infrared external reflection technique applied to the studies of adsorption layers of surfactants on different nonmetallic samples, including minerals, can be found in recent papers.11-22 These successful studies, which provide detailed information about the nature and structure of the adsorbed layer at a molecular level, were possible because of the use of the spectral simulation and theoretical consideration of the systems under investigation. It has been well established that the reflection spectra of ultrathin layers.11,12,15,18-22 on nonmetallic substrates differ significantly from the corresponding transmission spectra of the same free-standing sample even if both of them are isotropic. The reflection spectra are very sensitive to the recording conditions (polarization and angle of the incident beam) because of optical effects. Distinguishing the changes in reflection spectra caused by optical effects is crucial for the interpretation and must be done before relating any difference in band shape, position, and intensity to structural and chemical bonding changes in the adsorbed layer. Therefore, only the combination of the simulated and experimental spectra recorded for different polarizations and incident angles allows us to identify with satisfactory accuracy the composition and (11) Wong, J. S.; Yen, Y. S. Appl. Spectrosc. 1988, 42, 598. Yen, Y. S.; Wong, J. S. J. Phys. Chem. 1989, 93, 7208. (12) Mielczarski, J. A.; Yoon, R. H. J. Phys. Chem. 1989, 93, 20342038. (13) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 94. (14) Mielczarski, J. A. In SPIE 1989, 1145, 489. Mielczarski, J. A.; Yoon, R. H. Langmuir 1991, 7, 101. (15) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927-945. (16) Mielczarski, J. A. J. Phys. Chem. 1993, 97, 2649-2663. (17) Ishino, Y.; Ishida H. Langmuir 1988, 4, 1341. (18) Mielczarski, J. A. Surf. Interface Anal. 1994, 22, 162-166. (19) Mielczarski, J. A.; Mielczarski, E.; Cases, J. M. Colloids Surf. 1994, 93, 97-109. (20) Mielczarski, J. A.; Mielczarski, E. J. Phys. Chem. 1995, 99, 32063217. (21) Hoffman, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304-1312. (22) Mielczarski, J. A.; Mielczarski, E.; Zachwieja, J.; Cases, J. M. Langmuir 1995, 11, 2787-2799.
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Langmuir, Vol. 12, No. 26, 1996 6523
a
b
Figure 2. Optical constants: refractive index n and absorption coefficient k as functions of wavenumber for: (a) liquid form of amyl dixanthogen; (b) cuprous amyl xanthate. Table 1. Optical Constants of Mineral Samples for Two Chosen Wavenumbers 1400 cm-1
Figure 1. Simulated spectra of hypothetical adsorption layers containing one or two components. Spectra are calculated for different incident angles and p- and s- polarizations: (a) chalcopyrite-1 nm cuprous amyl xanthate, 70°, p-polarization; (b) chalcopyrite-1 nm cuprous amyl xanthate, 20°, s-polarization; (c) chalcopyrite-0.1 nm cuprous amyl xanthate-1 nm amyl dixanthogen, 70°, p-polarization; (d) chalcopyrite-0.1 nm cuprous amyl xanthate-1 nm amyl dixanthogen, 20°, spolarization; (e) tetrahedrite-1 nm cuprous amyl xanthate, 20°, s-polarization; (f) tetrahedrite-0.5 nm cuprous amyl xanthate-1 nm amyl dixanthogen, 20°, s-polarization.
the structure of the adsorbed surface layer. A two-step procedure was proposed20 for detailed evaluation of the composition and structure of adsorbed layers on nonmetallic substrates. At first the simulated spectra for isotropic layer were compared with the experimental data. If very similar spectra were obtained, this indicates an isotropic structure of the adsorbed layer and allows an immediately quantitative evaluation of the adsorbed species.22 If there are significant differences in band positions and intensities between the simulated and experimental results, an anisotropic structure and/or chemical changes should be considered.14-16,18,19,21 Detailed knowledge of band assignments allow, in the second step, the performance of a detailed quantitative description of the orientation of the adsorbed molecules on the substrate.15,16,20,21 The optical consideration also permits an optimization of the experimental conditions.12,16,20 It was found from the simulation that the best spectra, with reasonable signal/noise ratio at submonolayer coverages, could be recorded at an incident angle of 70° and p-polarization for chalcopyrite and 20° and s-polarization for tetrahedrite and tennantite. On the basis of these findings spectral simulations were performed for the three minerals covered by one- or two-component hypothetical isotropic layers of cuprous amyl xanthate and amyl dixanthogen with their different thicknesses (Figure 1). These spectra are used extensively in the determination of the composition and structure of the experimentally produced adsorption layer. The spectral simulations were made with the use of the exact equations based on Hansen’s formulas23 for a multilayer system of isotropic and homogeneous phases
1000 cm-1
mineral
n
k
n
k
chalcopyrite tetrahedrite tennantite
4.22 2.65 2.49
0.002 0.02 0.0002
4.07 2.56 2.48
0.005 0.006 0.001
with parallel interface boundaries. Details of the calculation can be found in previous work.12,16 The calculations were performed using a three-phase model: phase 1, air with a refractive index of n1 ) 1.0 and an absorption coefficient of k1 ) 0; phase 2, the adsorption layer; phase 3, the mineral sample. The optical constants of two adsorption products, the cuprous amyl xanthate and amyl dixanthogen, are shown in Figure 2. The optical constants of mineral samples used in these experiments are listed in Table 1. The optical properties of minerals and surface products were determined from their reflection spectra recorded at different incident angles and two polarizations applying the recently reported method.24 Spectroelectrochemical Studies of Amyl Xanthate Interaction with Chalcopyrite. Application of different potentials to the mineral electrode makes it possible to control the surface composition and its structure and, as a consequence, the hydrophobic property of the mineral surface. The compact mineral samples used in these studies allow the characterization of the surface species by the infrared external reflection method and controlling the mineral potential with a higher precision than in the case of a powder type electrode. Results of spectroelectrochemical studies of the surface composition of chalcopyrite contacted with xanthate solution for 10 min at various potentials are presented in Figure 3. These spectra were recorded at the optical conditions, chosen on the basis of optical simulation, which ensure the best sensitivity for the system under investigation. The lowest potential applied in these studies (90 mV) was 20 mV higher than the open circuit potential (OCP); the highest potential value (300 mV) was in the region where the oxidation of chalcopyrite takes place. The spectrum presented in Figure 3a shows no adsorption of amyl xanthate molecules on chalcopyrite at this (23) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380. (24) Mielczarski, J. A.; Milosevic, M.; Berets, S. L. Appl. Spectrosc. 1992, 46, 1040-1044.
6524 Langmuir, Vol. 12, No. 26, 1996
Figure 3. Infrared reflection spectra of chalcopyrite contacted with amyl xanthate solution of 2 × 10-4 M for 10 min, at various potentials. All spectra were recorded for p-polarization and incident angle of 70°.
potential. The lowest detection limit in these spectroscopic studies was estimated, on the basis of the experimental spectra (signal to noise ratio), to be less than half of the statistical monolayer coverage. The absorption bands of xanthate species, at about 1200 and 1034 cm-1, are visible at potential 110 mV (Figure 3b) indicating the formation of cuprous amyl xanthate complex. At a potential only 10 mV higher (Figure 3c) an additional band at 1269 cm-1 is observed showing the presence of dixanthogen. While the amount of cuprous amyl xanthate complex does not increase with the further increase in potential, the amount of dixanthogen increases significantly. Note that only a small increase in potential between 120 and 125 mV (Figure 3c and 3d) results in an about a factor of 3 increase in absorbance of the dixanthogen band at about 1270 cm-1. A future increase of potential to 150 mV involves an additional increase, about 2 times, in the observed intensity of dixanthogen bands. Comparison of the experimental spectra (Figure 3) with the simulated ones (Figure 1) shows several differences. The spectrum recorded at potential 110 mV (Figure 3b) displays very low intensity (submonolayer coverage) and the position of the bands are different from those predicted by simulation (Figure 1a). However, the experimental spectrum is similar to that expected for cuprous amyl xanthate for s-polarization (Figure 1b) with difference only in the intensities of the bands at 1190 and 1044 cm-1. This suggests that the adsorbed amyl xanthate molecules are oriented at the chalcopyrite surface. As has been described,12,15,21 the detailed position of molecules at the interface could be determined if the band assignments are well-known. Because at this time the assignment of particular bands in cuprous amyl xanthate spectrum is not well determined, the detailed quantitative evaluation of molecular orientation was not performed. At 125 mV the major adsorption product is amyl dixanthogen and the experimental spectrum (Figure 3d) is also different
Mielczarski et al.
from the simulated spectrum (Figure 1c) of this product on chalcopyrite surface. The predicted negative bands, very small at 1248 cm-1 and strong at 1016 cm-1 (Figure 1c), are not observed in the experimental spectra. Because the position of positive bands in the simulated and experimental spectra are practically the same, the lack of these negative bands could indicate an organization in the adsorbed dixanthogen layer. The recorded reflection spectra at the incident angle of 20° (not shown) does not show a strong negative band at 1256 cm-1, as predicted by spectral simulation (Figure 1d). The experimental spectra recorded for s- and p-polarizations are almost exactly the same, which indicates that an uniaxial model is a good approximation for orientation of the molecules at the mineral surface. This also suggests that the band at 1256 cm-1 is characteristic, at least to a significant extension, for the molecular group with dipole moment almost vertically oriented to the mineral surface. The above observations were made for the samples prepared between potentials 120 and 200 mV. At 300 mV the negative band at 1254 cm-1 was well established in the experimental spectrum recorded at 20° (not shown) but its intensity was about 3 times lower than that expected from the spectrum recorded at 70° assuming the formation of an isotropic dixanthogen layer. All these data consistently suggest that amyl dixanthogen forms an organized dixanthogen layer on chalcopyrite and this organization declines with the increasing of the adsorption layer thickness above 3 nm. The suggested organization does not mean the formation of a well-oriented rigid solidlike structure. It was demonstrated recently25 that the infrared spectrum of the solid form of ethyl dixanthogen is very different from that of the liquid, in all the band positions, shapes, and intensities. The spectra of the surface layers (Figure 3d-f) show a typical liquidlike spectrum of dixanthogen (Figure 1c), and only the relative intensities of particular bands are changed. It could be considered that the observed organization of amyl dixanthogen is forced by the first layer of oriented cuprous amyl xanthate species formed directly on the chalcopyrite surface. The above discussion indicates that a detailed quantitative evaluation of the amount of adsorbed surface products requires at first the determination of orientation of the molecules in the surface layer. Because, as already mentioned, the assignment of infrared bands is not well determined, it is possible to make only a crude estimation of the surface coverage by the adsorbed two types of xanthate species on the basis of the differences between the experimental and simulated spectra recorded at incident angles of 20° and 70°. The above discussion indicates that bands at about 1200 and 1270 cm-1 are characteristic for the molecular groups which are oriented vertically to the chalcopyrite surface. Most probably they are due to the asymmetric stretching vibration of the COC group in cuprous amyl xanthate and amyl dixanthogen, respectively, as suggested on the basis of the results presented for the ethyl homologue of xanthate.16 This orientation could enhance up to 3 times the intensity of absorbance bands with respect to that predicted for isotropic layers. Therefore the real thickness of the adsorbed layers could be up to 3 times lower than that estimated by simple comparison of the experimental and simulated spectra. This possible discrepancy should be kept in mind during the quantitative discussion of the experimental results. With the assumption of isotropic adsorption layers, the estimated thicknesses of a few (25) Mielczarski, J. A.; Xu, Z.; Cases, J. M. J. Phys. Chem. 1996, 100, 7181.
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Langmuir, Vol. 12, No. 26, 1996 6525
Table 2. Estimation of Thicknesses of Xanthate Surface Adsorption Layers (in nm), Assuming Their Isotropic Structure, Produced at Different Potentials (mV) adsorption potentials
from infrared spectraa CuAmX AmX2
from electrochemical resultsa
Chalcopyrite 110 120 125 150 300
0.6 0.6 0.6 0.6 2.5
130 170 190 340
1.0 2.1 3.0 3.0
115 150 200 400
1.8 2.8 5.0 5.0
2.0 6.0 10.0 22.0
2.0 4.5 11.0 17.0 73.0
Tetrahedrite 3.0 4.5 Tennantiteb 3.5 7.0 11.0 9.5
1.1 2.5 18.5 21.0 8.0 22.5 53.0 60.0
a See text for details. b Extended adsorption time from 10 min to 2 h.
adsorbed two-component (cuprous xanthate, CuAmX, and dixanthogen, AmX2) layers are presented in Table 2. The magnitude of charge associated with the adsorption of amyl xanthate on chalcopyrite surface was also measured. This measurement is very often used to estimate (independently of the spectroscopic results) the amount of the adsorbed products. In this calculation one electron exchange reaction for one xanthate molecule was assumed. In order to calculate the coverage of the mineral electrode by xanthate it was assumed to have a surface roughness factor (ratio between real and geometric surface) of approximately 3. This factor was determined electrochemically for a metallic copper electrode26 prepared in a very similar way as mineral electrodes used in these studies. Taking into account the natural porosity (fractures, cracks and holes) of the mineral sample, in comparison to the copper electrode, the assumed roughness factor could be only underestimated (or overestimated in a statistic thickness of the adsorbed layer). Another source of error in the evaluation of the adsorbed layer thicknesses is the assumption that a current background level is zero. The real background current in the presence of xanthate in the solution is not known and difficult to determine precisely. The coverage calculation is presented in the numbers of amyl xanthate monolayers assuming the occupation area of 28 Å2 per one xanthate molecule.27 This was utilized to determine a statistical monolayer coverage by xanthate-adsorbed ions, independently of the type of the formed surface product, cuprous xanthate complex or dixanthogen. One statistical monolayer is equal to about a 1 nm thick layer. The above consideration clearly indicates that the calculated quantitative results are rather crude estimations. The quantitative estimation on the basis of the electrochemical results shows that the spectrum recorded at 110 mV (Figure 3b) is characteristic of close to 2 statistical monolayers (2.0 nm thick) of coverage by the cuprous xanthate complex (Table 2). The spectrum recorded at 120 mV can be ascribed as a sample with about 4.5 statistical monolayers coverage, while those recorded at 125 and 150 mV are characteristic of nearly 11.0 and 17.0 monolayers. The comparison of the thicknesses of surface (26) Fletcher, S.; Barrades, R. G.; Porter, J. D. J. Electrochem. Soc. 1978, 125, 1960. (27) Gaudin, A. M.; Preller, G. S. Trans. Am. Miner. Metall. 1946, 81, C118.
Figure 4. Infrared reflection spectra of chalcopyrite contacted with amyl xanthate solution of 2 × 10-4 M for 10 min, at potential 300 mV: (a) recorded immediately; (b) recorded after 30 min in spectrophotometer; (c) the sample after washing with ethanol; (d) after second washing with ethanol. All spectra were recorded for p-polarization and incident angle of 70°.
coverage estimated by the two methods (Table 2) shows a large discrepancy with a high overestimation of the amount adsorbed from the electrochemical data. These discrepancies could be even much higher if the real coverage obtained from infrared studies will be available and if the real roughness factor will be lower than that assumed in the above calculation. As discussed above, the coverages estimated on the basis of spectroscopic data presented in Table 2 are the highest possible values. It is interesting to note that at the potential of 300 mV (Figure 3f) significant amounts of cuprous amyl xanthate complex and dixanthogen are observed at the chalcopyrite surface. At this potential chalcopyrite itself begins to oxidize and this oxidation causes the release of the copper ions which form the cuprous amyl xanthate complex at the interface. The charge recorded at this potential was only partly related to the formation of xanthate products; the other significant part is related to the dissolution of chalcopyrite. Comparison of the quantitative data obtained from spectroscopic and electrochemical estimations (Table 2) indicates about 3 times higher value obtained from electrochemical data, which supports the above conclusion. It was observed that the intensities of the dixanthogen absorbance bands in the recorded spectra decrease notably with time (Figure 4a,b) when the chalcopyrite sample was held in the measurement compartment for a very long time. Hence, the dixanthogen is not a very stable adsorption product on the mineral surface. The adsorbed dixanthogen can be removed easily from the chalcopyrite surface by washing of the mineral sample after adsorption with ethanol while the cuprous xanthate complex remains unaffected (Figure 4b,c). It is interesting to note that the intensities of the absorbance bands of the adsorbed cuprous xanthate complex somewhat increase after ethanol washing (Figure 4b,c) and the spectra are almost the same after redundant washing (Figure 4c,d). There is also
6526 Langmuir, Vol. 12, No. 26, 1996
appearance of a negative broad band at about 1110 cm-1 whose presence has to be considered with caution, otherwise it could lead to a misinterpretation of the spectra indicating a significant increase in the band intensities at 1197 and 1034 cm-1 after ethanol washing. The observed band intensities of cuprous xanthate agree well with the simulated data obtained with and without dixanthogen as the coadsorbed product. These observations support the conclusion about a high stability of cuprous xanthate complex on the mineral surface and indicates that the cuprous xanthate complex product is formed close to the surface of chalcopyrite. Thermodynamic and Kinetic Limitation of Xanthate Adsorption. The electrochemical studies of the chalcopyrite-ethyl xanthate system carried out in a nitrogen and air environment were already reported.28 There are no significant differences between the features of the voltammograms presented recently28 and those obtained in the present studies. The difference between the voltammograms recorded in absence and presence of xanthate in solution is very small, especially when other than sodium tetraborate electrolyte was used. The specific influence of the tetraborate electrolyte on the shape of recorded voltammograms was already noticed,28 and the reason is not clear at this moment. The performed XPS studies failed to determine any borate surface complex on chalcopyrite. The major observations which could be made from the electrochemical study of chalcopyrite are (i) the inhibition of chalcopyrite oxidation in the presence of xanthate and (ii) the shift of this reaction to higher potential. There are not well-defined peaks on the voltammograms which could be related to the formation of different xanthate adsorption products as observed, for example, for cuprous sulfide at positive potentials.16,29 Figure 3 shows a very strong increase of the amount of xanthate adsorption products in a very narrow potential region of about 120 mV. For this spectroscopic observation there are no relevant changes in the recorded voltammograms. This indicates that the electrochemical method is not as sensitive to the adsorption of xanthate on chalcopyrite as spectroscopic direct investigation of the mineral surface. There is no clear explanation proposed for the differences in the electrochemical behavior of chalcopyrite and cuprous sulfide. It seems that this problem originates from the fact that voltammetric techniques could be applied to characterize very fast electrochemical processes whereas slower processes or processes with an inert time cannot be monitored by this technique. The formation of dixanthogen is rather a slow process especially at lower potential regions (see also discussion below), and the acceleration of its formation kinetics requires a considerable overpotential. The above results show that amyl xanthate adsorption products appeared on the chalcopyrite at a potential of 110-120 mV after 10 min of adsorption. It was interesting to find more details about a kinetic limitation of xanthate adsorption. Extension of contact time to 1 h significantly changes the surface composition (Figure 5). Whereas at a potential of 80 mV after 10 min of adsorption no xanthate surface product was detected (Figure 5d), a prolongation of the adsorption to 1 h results in the formation of a significant amount (6 nm thick layer) of amyl dixanthogen (bands at 1270, 1047, and 1031 cm-1) and a small amount (about 0.6 nm) of cuprous amyl xanthate (band at 1204 cm-1) (Figure 5e) if isotropic structures of these products are assumed. At a potential 5 mV lower after 1 h of adsorption (Figure 5b), the amount of dixanthogen is (28) Pang, J.; Chander, S. Miner. Metall. Process. 1990, 8, 149-155. (29) Kowal, A.; Pomianowski, A. Electroanal. Chem. Interfacial Electrochem. 1973, 46, 411-420.
Mielczarski et al.
Figure 5. Infrared reflection spectra of chalcopyrite contacted with amyl xanthate solution of 2 × 10-4 M at potentials close to thermodynamic potential of xanthate adsorption for extended adsorption times. All spectra were recorded for p-polarization and incident angle of 70°.
several times lower while the amount of cuprous xanthate shows only a small decrease. A prolongation of the contact time to 2 h (Figure 5c) does not change the surface composition noticeably, indicating an equilibrium or semiequilibrium state. The estimation shows that the intensity of the band at 1204 cm-1 is very similar to that expected for 40% of a monolayer coverage (0.4 nm) of cuprous xanthate complex while dixanthogen forms an about 1.3 nm thick layer. The amount of these two products together suggests the formation of a nearly monomolecular two-component layer at the potential of 75 mV. At a potential of 70 mV both adsorption products, amyl dixanthogen and cuprous amyl xanthate, are at the detection limit, i.e., below 0.2 nm thick layer for cuprous amyl xanthate and 0.4 nm for amyl dixanthogen if they form isotropic structures. Obviously any orientation will decrease this limit by a factor up to 3 for specifically favorable molecular groups orientation (vibrations vertical to interface) whereas absorbance of the other groups could be significantly lower or even show negative bands for unfavorable orientation. The above results clearly show that there is a significant kinetic limitation of the surface product formation with an inert time in the potential region close to the thermodynamic potential. Amyl xanthate produces two adsorption products simultaneously on the chalcopyrite surface with beginning of the adsorption at about 75 mV for an extended adsorption time of 1 h. Therefore, the potential of 75 mV could be assumed as the thermodynamic potential of the initial amyl xanthate surface layer formation on chalcopyrite. There are four different possible mechanisms of amyl xanthate adsorption: (i) the formation of the organized cuprous xanthate complex starting at the potentials of 75 mV; (ii) the formation of dixanthogen which starts also at 75 mV potential that is only somewhat lower than the reversible potential, E ) 86 mV, calculated for an AmX concentration of 2 × 10-4 M on the basis of thermodynamic data30 for the oxidation reaction
AmX- ) 0.5AmX2 + e-
(1)
The adsorbed amount of dixanthogen increases signifi-
Interaction of Xanthate with Minerals
Langmuir, Vol. 12, No. 26, 1996 6527
Figure 6. Infrared reflection spectra of tetrahedrite contacted with amyl xanthate solution of 2 × 10-4 M for 10 min, at various potentials. All spectra were recorded for p-polarization and incident angle of 20°.
cantly with the potential, especially at the beginning of this increase, and with extension of the adsorption time; (iii) the production of the thick layer of the cuprous xanthate complex additionally to the thick layer of dixanthogen at the higher potential region at which chalcopyrite itself begins to oxidize and dissolve significantly; (iv) a possible creation of cuprous xanthate and dixanthogen in the same reaction with copper ion
2AmX- + Cu2+ ) CuAmX + 0.5AmX2
(2)
This process does not play a significant role in the formation of surface products because the amount of dixanthogen experimentally observed is much higher than that of cuprous xanthate, which is contrary to what is expected from the reaction (2). The XPS detailed studies10 of the topmost surface layer of chalcopyrite after amyl xanthate adsorption at OCP indicate the presence of the cuprous xanthate complex as only the adsorption product, in the amount limited to a part of monolayer coverage. This is in good agreement with the results obtained in this work only if the cuprous xanthate adsorption product is considered. However, no presence of amyl dixanthogen was noticed in the XPS studies. This is explained by the low stability (volatile character) of dixanthagen in UHV conditions. Also this comparison shows clearly the advantage of infrared spectroscopy over the XPS method in the determination of composition of organic unstable adsorbed layers. Spectroelectrochemical Studies of Amyl Xanthate Interaction with Tetrahedrite. Results of spectroscopic studies of the surface composition of tetrahedrite contacted with xanthate solutions of 2 × 10-4 M for 10 min at various potentials are presented in Figure 6. The adsorbed xanthate species on tetrahedrite show negative absorbance bands. This is caused by the use of different, compared to chalcopyrite, optimal optical conditions for recording the reflection spectra less disturbed by optical (30) Kakovsky, I. A. In Proceedings, 2nd International Congress on Surface Activity; Schulman, J. H., Ed.; Butterworths: London, 1957; Vol. 4, p 222.
effects (see discussion above and Figure 1). It can be immediately noted that the absorbance bands can be found at any applied potential above OCP (about 100 mV) after a 10 min contact of this mineral sample with the xanthate solution. At all potentials up to 170 mV (Figure 6a-c) the clearly visible bands at about 1200 and 1040 cm-1 are present indicating the formation of the adsorbed cuprous xanthate complex on the mineral surface. The formation of cuprous amyl xanthate layer on chalcopyrite at OCP was also observed in the recent XPS studies.10 At higher than 170 mV potentials an additional adsorption product, amyl dixanthogen, is observed showing the absorbance bands at about 1255, 1039, and 1023 cm-1. The dixanthogen is clearly seen at potential 190 mV which is about 100 mV higher than the reversible potential, E ) 86 mV, calculated for the formation of amyl dixanthogen in reaction 1. It is interesting to note that the observed amount of dixanthogen does not increase significantly with an increase of potential as was found for chalcopyrite. This observation correlated well with the magnitude of charge passed through the system. The quantitative evaluation of the amount of surface products was carried out on the basis of electrochemical data with the same assumptions as for chalcopyrite. The performed calculation indicates that the coverage of the sample prepared at a potential of 130 mV (Figure 6b) is about a statistical monolayer (1.1 nm) (Table 2). At 170 mV (Figure 6c) the coverage is about of 2.5 monolayers and the only adsorbed product clearly visible is the cuprous xanthate surface complex. At an only 20 mV higher potential, i.e., at 190 mV (Figure 6d), a large increase in the passed charge was noted that is equal to about 18.5 statistical monolayers (18.5 nm thick layer) representing both cuprous xanthate and dixanthogen products (Table 2). The increase of the potential to 270 mV does not influence the type and amount of the adsorbed cuprous xanthate and dixanthogen products. However, some increase of the amount of adsorbed dixanthogen is observed at the highest potential of 340 mV (Figure 6f). The very limited increase of the adsorbed xanthate with the significant increase of potential indicates the formation of a very compact adsorption layer which isolates electrically the mineral sample from the collector solution. Additional possible reason for the enormous slowing down of the kinetics of dixanthogen formation is the lower conductivity of the tetrahedrite sample in comparison with chalcopyrite. Comparison of the experimental (Figure 6) and simulated spectra (Figure 1e,f) shows some differences. The spectra recorded at potentials 100-170 mV show absorbance bands at positions of about 1200 and 1040 cm-1 which are very close to the positions of spectral components, i.e., the shoulders in the simulated spectrum at about 1198 and 1044 cm-1 (Figure 1e). The other absorbance components expected at 1190 and 1034 cm-1 (Figure 1e) show very low intensities in the experimental spectra. This suggests that xanthate molecules form an organized layer with some molecular group oriented vertically to interface, therefore, a nonactive for spolarization mode (for details see for example ref 16). The intensities of the bands observed (Figure 6c) indicate the formation of an ∼2.1 nm layer of cuprous xanthate complex if it is assumed that it forms an isotropic layer. Because the spectrum shows an anisotropic structure, the thickness of produced surface layer is in reality lower, and at a perfect match of the electric field vector with the dipole moment of a particular group, the maximum gain in absorbance could reach factor 1.5. The experimental spectra recorded for s- and p-polarizations and the incident angle of 20° were found very similar. This suggest that an uniaxial
6528 Langmuir, Vol. 12, No. 26, 1996
model could be used for description of the organized molecules at the interface.15,20 With this assumption the thickness of the adsorbed layer produced at 170 mV could be estimated as low as 1.4 nm, which is a 1.5 times lower than the value listed in Table 2 for an isotropic structure. As was mentioned above, the assignment of particular bands in cuprous amyl xanthate infrared spectrum is not well determined at this time; therefore the detailed qualitative evaluation of molecular orientation was not performed. At potentials higher than 170 mV the amount of cuprous xanthate complex does not increase significantly, which probably results from the formation of the compact and organized cuprous xanthate surface layer. The major adsorption product formed at higher potentials than 170 mV is amyl dixanthogen which produced 3-4.5 nm thick layers. These coverages and the cuprous xanthate layer of 3 nm are again far below those evaluated from an electrochemical data of 18.5-21.0 nm, even if the isotropic structure is assumed (Table 2). For an anisotropic dixanthogen layer the spectroscopic estimation could be lowered up to 2-3 nm showing a significant overestimation offered by the electrochemical method. The spectrum in Figure 6f is very similar to the simulated spectrum in Figure 1f if the intensities of the bands at 1190 and 1036 cm-1 are significantly lower. This finding indicates that the dixanthogen is adsorbed on top of the oriented surface layer of cuprous xanthate complex (see also the discussion at the beginning of this section). The adsorbed cuprous xanthate complex remains untouched after ethanol washing on the surface of this mineral, which is similar to the observation made for chalcopyrite. This also supports the finding that the cuprous xanthate complex is formed closer to the surface of tetrahedrite than the dixanthogen. The obtained results indicate that amyl xanthate adsorption takes place in two steps: (i) the formation of the organized cuprous xanthate layer at all potentials above OCP in an amount up to a few statistical monolayers; (ii) the formation and coadsorption of dixanthogen layer on top of cuprous xanthate complex that starts at potential 190 mV which is about 100 mV higher than the calculated reverse potential for the dixanthogen formation. Spectroelectrochemical Studies of Amyl Xanthate Interaction with Tennantite. The adsorption kinetics of xanthate on tennantite polarized to different potentials is very different from those described above for chalcopyrite and tetrahedrite. After 10 min of adsorption in a xanthate solution the recorded reflection spectra show very low absorbance bands (low signal to noise ratios) which are difficult to interpret, even for the highest potential applied (400 mV). This observation correlated with the determined magnitude of the charge passed through the system, which also suggests a very low coverage by the adsorbed xanthate molecules. Only an increase of the adsorption time gives a significant increase in the intensities of the observed absorbance bands. For the sample at a potential of 150 mV, the passing charge during 10 min is equivalent to less than a statistical monolayer (the same assumptions were made as in the case of the chalcopyrite sample). After 20 min of adsorption the calculated coverage raises to about 2 monolayers. After 1 and 2 h the coverages reach about 7 and 17 monolayers, respectively. The XPS data obtained after 30 min of xanthate adsorption at OCP show10 the presence of the cuprous xanthate complex as the only surface product in an amount lower than monolayer coverage, which indicates that only a part of the passing charge is related to xanthate adsorption. However, no presence of amyl dixanthogen was noticed by the XPS, which is caused by the instability of this surface product in UHV conditions.
Mielczarski et al.
Figure 7. Infrared reflection spectra of tennantite contacted with amyl xanthate solution of 2 × 10-4 M for 2 h, at various potentials. All spectra were recorded for p-polarization and incident angle of 20°.
Results of spectroscopic studies of the surface composition of tennantite after interaction with xanthate solutions of 2 × 10-4 M for 2 h at various potentials are presented in Figure 7. The lowest potential applied in these studies was, as in the case of other mineral samples, close to OCP (115 mV) of the mineral in the xanthate solution. These spectra show clearly that the adsorption of amyl xanthate on tennantite takes place at all applied potentials, including OCP. The obtained spectra show negative absorbance bands (the optimal optical experimental conditions were the same as for tetrahedrite) at about 1255, 1202, 1041, and 1019 cm-1 indicating the presence of two types of the adsorption layer, cuprous xanthate and dixanthogen, at all potentials. The sample of tennantite contains a significant amount of sphalerite which probably is the reason for the very low conductivity of the mineral sample. The difference in electric properties between the three minerals could explain very slow kinetics of xanthate adsorption on tennantite, which as an electrochemical reaction is sensitive to the changes in conductivity. Electrochemical studies of tetrahedrite and tennantite in the absence and presence of amyl xanthate carried out with potential ohmic drop correction31 show almost identical voltammograms for both samples indicating no noticeable differences in the electrochemical behavior between these two minerals. The observed enormous slowing down of the kinetics of xanthate adsorption could also suggest some surface processes which take place before amyl xanthate starts to be adsorbed in a noticeable amount. It is also possible that part of the xanthate is adsorbed on sphalerite as well. However, this adsorption requires at first an activation of sphalerite by metal ions (Cu or other heavy metal ions) from the solution or by their diffusion to sphalerite surface from bulk sample before xanthate adsorption takes place. The activation process could also be the reason of the observed inert time of the xanthate adsorption. The quantitative evaluation of the surface coverage on the basis of the electric charge which passed through the (31) Nowak, P.; Mielczarski, J. A. Paper in preparation.
Interaction of Xanthate with Minerals
system shows that the spectrum of sample prepared at potential 115 mV (Figure 7a) is characteristic for coverage close to 8 statistical monolayers. This calculation was carried out with the same assumptions as those for chalcopyrite sample. At higher potentials the coverages are at 150 mV (Figure 7b) near 22.5 monolayers, at 200 mV about 53 monolayers, and at potential 400 mV about 60 monolayers (Figures 7c,d) (Table 2). The electrochemical observation of significant increase in the magnitude of charge passed through the system above potential 200 mV and a significant increase in the intensities of the absorbance bands at 1202 and 1022 cm-1 assigned to the cuprous xanthate complex together with the other bands due to dixanthogen suggest that tennantite undergoes oxidation at these potentials. The surface coverages by the two surface components estimated for each sample from spectroscopic data (Figure 7) are shown in Table 2. The very large differences in the surface coverages between the electrochemical and spectroscopical estimations observed at the potentials below and above 200 mV support the conclusion about the oxidation of the mineral under these conditions. The simulated spectra of the expected adsorption products on tennantite are very similar to those obtained for tetrahedrite because of the very similar optical properties of these two minerals (see Table 1). Therefore, the spectra presented in parts e and f of Figure 1 could be also used for the discussion of the experimental results obtained for tennantite. The comparison of the simulated and experimental spectra reveals a similar finding already observed for tetrahedrite; if the simulated spectrum is modified by a significant decreasing of intensity of the absorbance bands at 1190 and 1034 cm-1, a good agreement between both the calculated and experimental spectra is reached. This indicates that the adsorbed cuprous xanthate molecules form an organized layer also on the tennantite surface. As was found in a similar observation for chalcopyrite and tetrahedrite, the adsorbed cuprous xanthate complex remains untouched on the mineral surface after ethanol washing of tennantite. Hence, this is additional evidence that the organized cuprous xanthate layer is formed close to the mineral interface. Comparison of the Results of Xanthate Interaction with Chalcopyrite, Tetrahedrite, and Tennantite. Figure 8 is a graphical presentation of the results obtained which could be used in the determination of the optimal adsorption conditions in order to separate these three minerals from their mixture. It can be easily found that the formation of cuprous xanthate surface complex, one of the possible surface products which plays a significant role in making mineral surfaces hydrophobic, starts to be observed at potentials close to OCP or at OCP for each mineral. The cuprous xanthate forms an oriented layer which is responsible for the good spreading and organization of the second hydrophobic product, i.e., dixanthogen, on the mineral surfaces. The amount of cuprous xanthate produced depends closely on the accessibility of copper ions and is not limited to monolayer coverage. This observation agrees well with the XPS data7,10 on the in-depth copper atoms distribution at the interface of these minerals. Those data showed that the presence of xanthate in solution increases diffusion
Langmuir, Vol. 12, No. 26, 1996 6529
Figure 8. Schematic presentation of thermodynamic and kinetic limitations of the formation of different type of hydrophobic surface products of amyl xanthate on chalcopyrite, tetrahedrite, and tennantite.
kinetics of copper atoms to the mineral interfaces. The most notable difference was observed for chalcopyrite which in the absence of xanthate shows a preferential diffusion of iron atoms to the interface. Cuprous xanthate is a more stable product than dixanthogen, the latter is produced only when the concentration of copper atoms at the mineral interface is insufficient for the interaction with all xanthate ions available at the mineral interface. At the first glance, the lack of large different thermodynamic limitation to produce xanthate hydrophobic products at close to OCP could suggest that the separation of these minerals by flotation is practically impossible. Before this conclusion is made, other experimental observations made in these studies should be carefully considered. For these three mineral samples only tetrahedrite does not show any thermodynamic or kinetic limitations of xanthate adsorption at OCP. After 10 min of adsorption, the formation of xanthate hydrophobic product was found only on tetrahedrite. Amyl xanthate is not adsorbed at OCP on chalcopyrite, and in order to produce relatively quickly a hydrophobic surface layer, a significant increase of chalcopyrite potential, about 50 mV, is required. Two adsorption products are formed on chalcopyrite, cuprous xanthate complex and dixanthogen; they together ensure a high hydrophobicity of the sample. For tennantite a very extended adsorption time is needed for the formation of the amyl xanthate two-component hydrophobic layer. The observed variety in the type of xanthate adsorption products and their amount depending on the mineral potentials (thermodynamic limitation) and the adsorption time (kinetic limitation) can be used to make a more selective separation of these three components from their mixture. Acknowledgment. This work was supported by the European Community (project MA2M-CT92-0062). The authors thank Somincor SA Portugal for providing the mineral samples. LA960568E
