Chemisorption—the thermodynamically favoured process in the interaction of thiol collectors with sulphide minerals

II1TERnlITIOmlL JOORIIm.OF

HIHERm, PROtE|SIUI; ELSEV][ER

Int. J. Miner. Process. 51 (1997) 15-26

Chemisorption the thermodynamically favoured process in the interaction of thiol collectors with sulphide minerals A.N. Buckley a, R. Woods b,,,1 CSIRO, Division of Coal and Energy Technology, P.O. Box 136, North Ryde, NSW 2113, Australia b CSIRO, Division of Minerals, P.O. Box 124, Port Melbourne, Vict. 3207, Australia

Accepted 14 April 1997

Abstracl: Voltammetric investigations show that chemisorption is the thermodynamically favoured process in the interaction of thiol collectors with sulphide minerals. The potential dependence of chemisorption coverage is in accordance with the Frumkin adsorption isotherm. Chemisorbed thiols render sulphide mineral surfaces hydrophobic and induce efficient flotation of mineral particles. UV-Visible, Fourier transform infrared, surface-enhanced Raman scattering, and electron spectroscopic studies provide complementary information on the nature of the adsorbed species. X-ray photoelectron spectroscopic (XPS) studies of xanthate on galena reveal sulphur environments associated with chemisorbed xanthate and lead xanthate but no oxidised sulphur derived from the galena substrate. This finding is explained by electrochemical studies that have shown the surface composition of oxidized galena to relax. Discrepancies between the results of conventional and synchrotron radiation excited XPS studies of the oxidation of galena are discussed. © 1997 Elsevier Science B.V. Keywords." chemisorption;flotation;thiol collectors; sulphide minerals;electrochemistry;XPS

1. Introduction The key chemical step in the froth flotation concentration of sulphide minerals with thiol collectors is the attachment of the collector to the mineral surface. It is now

* Correspondingauthor. Fax: + 61-3-9836-5105;e-mail: [email protected] J Prese:at address: 9 AllambeeAvenue, Camberwell,Victoria 3124, Australia. 0301-7516/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0301-75 16(97)00016-1

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A.N. Buckley, R. Woods/hit. J. Miner, Process. 51 (1997) 15-26

generally accepted that this process is electrochemical in nature. It occurs by separate electron transfer reactions in which an anodic reaction involving the collector is coupled with a cathodic reaction which is typically the reduction of oxygen. The electrochemical mechanism of mineral/collector interaction implies that flotation recovery is dependent on the potential across the mineral/solution interface and such a relationship offers the prospect of monitoring or controlling flotation through the measurement of the potential of electrode probes inserted into the pulp. The development of appropriate measuring techniques and procedures are discussed in other papers in this volume. The electrochemical nature of the mineral/collector interaction also means that the processes that lead to mineral hydrophobicity are amenable to investigation by electrochemical techniques. This is important not only because it allows a detailed knowledge of the reactions that can occur in flotation systems to be established, but also because such knowledge is essential if effective potential monitoring and control methods are to be introduced in practice. Electrochemical investigations have shown (see Woods, 1996 for review) that the product of the anodic oxidation of thiols at sulphide mineral surfaces can be a cbemisorbed thiol, a metal collector compound, or a dithiolate. The important question from the flotation viewpoint is to what extent these species are formed under the conditions pertaining to the plant situation and how they influence the wettability of the mineral. In this communication, we focus attention on chemisorption. As will be demonstrated, this is the thermodynamically favoured process in the interaction of thiol collectors with sulphide minerals and thus occurs at lower potentials than the formation of the corresponding metal thiolate.

2. Underpotential deposition of thiol collectors Voltammetric studies of thiol collectors on a range of sulphide mineral and metal surfaces have revealed the appearance of a prewave on the positive-going scan at a potential below the reversible value for the formation of the metal thiolate. Systems in which such underpotential deposition has been investigated include ethyl xanthate on the metal sulphides galena (Woods, 1971; Gardner and Woods, 1977; Richardson and O'Dell, 1985), chalcocite (Kowal and Pomianowski, 1973; O'Dell et al., 1984; Richardson et al., 1984; Basilio et al., 1985; O'Dell et al., 1986; Woods, 1988; Woods et al., 1990), and silver sulphide (Buckley and Woods, 1995), and ethyl xanthate on the corresponding metals lead (Woods et al., 1997), copper (Szeglowski et al., 1977; Woods, 1988; Woods et al., 1990; Talonen et al., 1991) and silver (Talonen et al., 1991; Woods et al., 1992; Buckley and Woods, 1995). Underpotentiat deposition has also been reported for methyl and butyl xanthates on galena (Gardner and Woods, 1977) and diethyl dithiophosphate on chalcocite (Chander and Fuerstenau, 1974, 1975; Buckley and Woods, 1993). Fig. 1 presents voltammograms recorded at 40 mV s l in 10 3 mol dm -3 ethyl xanthate solution at pH 9.2 for copper and chalcocite (Woods et al., 1990). The arrows mark the reversible potentials for the formation of the bulk metal xanthate. The potential

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for chalcocite has been calculated on the basis of the sulphur product of oxidation of Cu2S to the metal xanthate being CUl.83S (Woods et al., 1990). The voltammograms in Fig. 1 clearly show the underpotential deposition of xanthate on the two substrates considered. The shift in the reversible potential on proceeding from copper to chalcocite reflects the decrease in copper activity on bonding to sulphur. Analogous behaviour is observed with the lead system; oxidation to lead xanthate occurs at a potential 0.48 V more positive on galena (Woods, 1971) than on lead (Woods et al., 1997) and the onset of chernisorption is shifted by a similar value. The similarity between the behaviour of xanthate adsorption on metals and their sulphides, and the observed shift in potential with stoichiometry, indicate that the thiol is bonded to metal atoms in the surface of sulphide minerals, rather than to sulphur. It should not be concluded that chemisorption only occurs with the systems specified above. Chemisorption could occur with many other systems in which it is difficult to separate', chemisorption from other processes that occur in the same potential region. The particular case of xanthate/chalcopyrite will be discussed in a later section of this paper.

3. U n d e r p o t e n t i a l d e p o s i t i o n a n d W a r k ' s m e c h a n i s m o f c o l l e c t o r a t t a c h m e n t

The underpotential deposition of a chemisorbed thiol layer is in agreement with the mechanism proposed more than six decades ago by Wark and Cox (1934). These authors maintained that the theory put forward by Taggart and co-workers (Taggart et al., 1930), that the interaction of thiol collectors with sulphide minerals occurred by "chemical

18

A.N. Buckley, R. Woods/Int. J. Miner. Process. 51 (1997) 15-26

reactions of well-recognized types", was not supported by experimental observations. Sutherland and Wark (1955) pointed out that the solubility product of the products of collector/mineral interaction "differs by factors ranging from a thousand to a thousand million from the solubility product" of the bulk metal-thiol compound. It can be seen from Fig. 1 that the underpotential deposition commences about 180-240 mV below the reversible potential for the formation of a metal xanthate phase. This corresponds to a metal ion concentration in equilibrium with the layer, and hence an equivalent solubility, between 103 and l 0 4 less than that of the bulk phase for a monovalent cation and 10 6 and 108 for a bivalent cation. These values cover a similar range to that specified by Wark.

4. Chemisorption isotherms and Eh-pH diagrams Equilibrium coverages determined in the prewave potential region have been found to fit the Frumkin adsorption isotherm for the chalcocite/ethyl xanthate (Woods et al., 1990), copper/ethyl xanthate (Woods et al., 1990), chalcocite/diethyl dithiophosphate (Buckley and Woods, 1993), silver/ethyl xanthate (Woods et al., 1992) and lead/ethyl xanthate (Woods et al., 1997) systems. The Frumkin isotherm has also been fitted to ethyl xanthate coverages on galena (Nowak, 1993; Buckley and Woods, 1994). The reader is referred to the original papers for details on the determination of the isotherms, the goodness-of-fit, and discussion on the various parameters derived. We wish to focus attention here on the implications of the potential dependence of collector coverage in the prewave region being delineated by the Frumkin isotherm. Firstly, it implies that the attachment of the collector to the sulphide mineral corresponds to that of chemisorption, with collector species being bonded to metal atoms still retained in the surface of the mineral. Secondly, it indicates that the chemisorbed species is not characterized by a single Gibbs free energy, but that the free energy varies with coverage. A further implication of fitting to an isotherm is that thermodynamic information becomes available to characterize chemisorption and hence chemisorbed species can be incorporated in E h - p H diagrams describing the interaction of thiol collectors with mineral surfaces. This allows the stability domains of all thiol species to be included (Woods, 1996). Since the chemisorbed thiol is formed at lower potentials than the metal thiol compound, it will occupy a stability domain in a potential region lower than that for the bulk species.

5. Potential dependence of flotation recovery Since chemisorption is the thermodynamically favoured process in the interaction of thiols with sulphide minerals, chemisorbed xanthate is expected to be the major species formed in the presence of xanthate concentrations relevant to flotation. For each of the metal/thiol and sulphide mineral/thiol systems investigated, measurements of the contact angle as a function of flotation (Woods, 1996 and references therein) have shown that the contact angle reaches the maximum value for the particular hydrocarbon

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Fig. 2. Comparison of flotation recovery and fractional surface coverage of collector for (A) galena with 2.3 × 10-5 mol din-3 ethyl xanthate (Buckley and Woods, 1991); (B) chalcocite with 10-5 mol dm-3 ethyl xanthate (Woods et al., 1990); (C) chalcocite with 10-5 mol dm-3 diethyl dithiophosphate (Woods et al., 1993). "linearrows identify the relevant ordinate scale. chain length (Sutherland and Wark, 1955) before a monolayer is established. The implication of this finding is that chemisorbed xanthate can induce efficient flotation. Such a conclusion has been confirmed for the chalcocite/ethyl xanthate (O'Dell et al., 1984; Woods et al., 1990), chalcocite/diethyl dithiophosphate (Woods et al., 1993) and galena/ethyl xanthate (Buckley and Woods, 1991) systems. Fig. 2 shows a comparison of the potential dependence of flotation recovery and of the coverage of chemisorbed xanthate. It can be seen that, in each case, significant flotation occurs at potentials at which the coverage is low, 50% recovery corresponds to a fractional coverage of < 0.2, and 90% recovery to about half-coverage. Leppinen (1986; see also Leppinen and Rastas, 1986) determined the potential dependence of the flotation of galena with high ethyl xanthate concentrations. Under the selected conditions, a sulphide ion was released to the solution for each two xanthate ions adsorbed. They suggested that the xanthate became chemisorbed on lead atoms in the galena surface and that the dissolution of sulphide ions resulted in corresponding sulphur vacancies. This reaction can be represented as the anodic charge transfer chemisorption of xanthate: X-~Xad~ + e-

(1)

coupled with the cathodic reduction of galena: PbS + y H + + 2 y e - --* PbS 1- y + yHS -

(2)

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A.N. Buckley, R. Woods/Int. J. Miner. Process. 51 (1997) 15-26

When the potential was calculated from the sulphide ion concentration, Leppinen found his results correlated with those of Guy and Trahar (1985) in which the potential was controlled by the addition of redox reagents. Under the conditions employed by Guy and Trahar chemisorption, reaction (1), is coupled with the reduction of oxygen on the sulphide surface: 0 2+2H20+4e

~4OH

(3)

The similarity of the potential dependence reported by Leppinen and by Guy and Trahar would suggest, however, that the surface composition, as well as the potential, is the same under these two different conditions. Buckley and Woods (1996a,b) showed that the same surface composition can be established as a result of relaxation of the surface stoichiometry of galena following any potential perturbation. Thus, the equilibrium composition of a galena surface depends only on the potential across the mineral/solution interface because sulphur or lead vacancies formed at the surface in reaching that potential can be filled by diffusion between the surface and the bulk of the mineral. Therefore, an adsorbate such as xanthate that bonds to a lead atom in a galena surface will give rise to the same coverage/potential relationship irrespective of whether its adsorption occurs by an associated loss of lattice sulphur as dissolved sulphide ion or by coupled electrode processes involving solution species.

6. Complementary spectroscopic investigations 6.1. U V - Visible s p e c t r o s c o p y

Complementing electrochemical investigations with studies using a range of spectroscopies have provided valuable information on the nature of the surface species, including that of the initial chemisorbed layer. Combining electrochemical techniques with UV-Visible spectroscopy of the solution phase circulated through high surface area electrodes has been applied to confirm the identity of the products of various reactions. Using this approach with silver (Woods et al., 1992) and copper (Woods et al., 1994), it has been established that chemisorption involves the attachment of xanthate to the surface without change in the integrity of the collector molecule. The application of UV-Visible spectroscopy is particularly useful in situations where it is difficult to separate thiol oxidation from background currents arising from oxidation of the substrate. Richardson and Walker (1984) and Zachwieja et al. (1987) applied this approach to determine the adsorption of ethyl xanthate on bornite as a function of potential and interpreted their results in terms of a chemisorbed species developing at underpotentials compared to the formation of copper xanthate or dixanthogen. A similar result was obtained for ethyl xanthate on chalcopyrite (Richardson and Walker, 1984). Whereas voltammetry only revealed the formation of dixanthogen, UV-Visible spectroscopy showed that xanthate was abstracted from solution at lower potentials. Indeed, the potential at which the xanthate concentration began to decrease correlated with the potential at which the flotation of a chalcopyrite particle bed was observed to commence. From an estimation of the reversible potential for the formation

A.N. Buckley, R. Woods/Int. J. Miner. Process. 51 (1997) 15-26

21

of copper xanthate on chalcopyrite, and assuming that xanthate chemisorption begins at a simil~x underpotential to that for chalcocite, it was concluded (Woods, 1996) that the onset o1' flotation correlates with the potential at which xanthate chemisorbs. 6.2. FTIR and F T - S E R S spectroscopy

Infrared spectroscopy provides a valuable means of detecting the presence of thiol species at surfaces, identifying the chemical nature of surface compounds, and determining the structure and orientation of the adsorbed entity. The detection of surface species by Fourier Transform Infrared (FTIR) spectroscopy is particularly useful for systems in which it is difficult to discern currents due to adsorption from those arising from other processes. This technique can also be applied to distinguish between two surface species formed simultaneously. For example, it has been shown (Woods et al., 1995) that xanthate chemisorption commences on gold just below the reversible potential of the xanthate/dixanthogen redox couple and reaches a coverage of ~ 0.2 before dixanthogen begins to deposit. SurE,ace enhanced Raman scattering (SERS) also provides spectroscopic information on the nature of species adsorbed on certain surfaces. Roughened silver gives rise to strong surface enhancement and FT-SERS spectroscopy has recently been applied (Buckley et al., 1997) to provide further evidence for the integrity of ethyl xanthate chemisorbed on this metal. The reversible potential for the silver/silver ethyl xanthate couple is - 0 . 0 4 4 V vs SHE for 10 -4 mol dm -3 xanthate, and previous studies have found (Woods et al., 1992) that a surface held in such a solution at a potential just negative of this value is covered with a monolayer of chemisorbed xanthate but the bulk phase cannot form. A silver electrode, roughened appropriately for SERS studies (Roth et al., 1993), was held at - 0 . 0 5 V for 5 min in 10 - 4 mol dm -3 xanthate at pH 9.2. The presence of chemisorbed xanthate, and the absence of multilayer silver xanthate, was verifiec[ from the characteristics of S 2p photoelectron and Ag(MNN) Auger electron spectra (Buckley and Woods, 1995). Comparison of SERS spectra, determined ex situ both before and after the electron spectra, with SERS spectra for multilayer silver xanthate on the roughened silver and for potassium ethyl xanthate, confirmed the integrity of the chemisorbed xanthate and eliminated the possibility of a significant concen~Lration of a xanthate decomposition product adsorbed on the surface. 6.3. Electron spectroscopy

Electron spectroscopy is well established for determining the composition of species present on solid surfaces, and it has been applied widely to flotation systems. Both Mg or AI ]K~ excited X-ray photoelectron spectroscopy (XPS) and synchrotron radiation (SR) excited XPS have proved to be particularly powerful for this purpose. In ~t study of the galena/butyl xanthate system (Shchukarev et al., 1994), for example, the K,~-excited S 2p photoelectron spectrum from a surface after being held at a pote~tial in the chemisorption region was characterised by a single additional doublet shifted by 1.4 eV from that of galena, a binding energy which is 0.3-0.4 eV less than that of lead xanthate. The corresponding Pb 4f spectrum was indistinguishable from that

22

A.N. Buckley, R. Woods/Int. J. Miner. Process. 51 (1997) 15-26

of galena itself, behaviour that is also indicative of chemisorption. It has recently been concluded (Buckley and Woods, 1995) that substrate core electron energy shifts following chemisorption are not always large enough to be detectable by conventional XPS. By contrast, core electron energy shifts for multilayer, metal-compound formation are clearly discernible. Shchukarev et al. (1994) found that the S 2p photoelectron spectrum after holding at a potential above the reversible value for lead xanthate formation could be fitted by two doublets in addition to that for the sulphur in the mineral. The binding energies of these shifted doublets supported their assignment to the sulphur atoms in chemisorbed xantbate and in lead xanthate. The formation of the thiolate involves the removal of lead atoms out of the galena lattice and hence should be accompanied by the formation of a metal-deficient sulphide: PbS + 2 xX---* xPbX 2 + Pb I _x S + 2 x e -

(4)

or elemental sulphur. There was no evidence for such a metal-deficient species or elemental sulphur in the S 2p spectrum. This can be explained by relaxation of the surface stoichiometry, behaviour that has been established (Buckley and Woods, 1996a,b) for galena. The relaxation process results in the surface stoichiometry becoming close to that of the bulk mineral before the electron spectrum of the collector-covered surface can be recorded. The evidence for relaxation of the surface stoichiometry presented by Buckley and Woods (1996a,b) was largely electrochemical. It was supported, however, by XPS investigations that showed P b / O species are formed on a galena surface exposed to air before any detectable new sulphur environments are apparent. This behaviour was explained by lead atoms diffusing to the surface to occupy metal vacancies arising from the development of an overlayer of lead oxide. The K~-excited XPS evidence for incongruent oxidation of galena has been reported previously (e.g., Buckley and Woods, 1984a,b; Laajalehto et al., 1993; Buckley et al., 1994). Recently, Kartio et al. (1996) have challenged the XPS evidence for incongruent oxidation. They carried out SR-XPS on galena fracture surfaces exposed to air at 24°C and ~ 60% relative humidity. The Pb 4f spectra excited with 215 eV radiation revealed the presence of P b / O species after exposure of the mineral for 10 min. The corresponding S 2p spectra indicated that a S / O species, together with smaller amounts of sulphur-rich environments attributed to a lead polysulphide, were formed concomitantly with the Pb/O. After exposure to air for 60 min, the Pb 4f component at a binding energy 0.6 eV higher than for galena accounted for 50% of the total Pb 4f intensity, whereas all the higher binding energy S 2p components accounted for about 40% of the S 2p intensity. Kartio et al. (1996) considered that the observed intensities indicated essentially congruent oxidation, notwithstanding the fact that the surface sensitivity of the S 2p electrons (kinetic energy ~ 50 eV) would have been marginally greater than of the Pb 4f ( ~ 75 eV). The SR-XPS results of Kartio et al. (1996) do not provide an explanation for the electrochemical findings of Buckley and Woods (1996a,b). While acknowledging the superior resolution, the superior signal/noise ratio, the higher S 2p photoelectric cross-section, and the less complicated background of the SR-XPS spectra, their surface

A.N. Buckley, R. Woods/Int. J. Miner. Process. 51 (1997) 15-26

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sensitivity is not markedly greater than that of the Pb(NOO)/S(LMM) X-ray excited Auger spectra reported previously for galena (Buckley and Riley, 1991; Buckley and Woods, 1996a). The Pb(NOO) and S(LMM) electron kinetic energies are ~ 90 and 150 eV, respectively, so their escape depths should be less than a factor of 1.5 greater than those c f the SR-excited photoelectrons. The Auger peaks are of course much broader than even the Ks-excited photoelectron peaks, but the shifts are also greater. In particular, the Pb(NOO) energies for unhydrated PbO are more than 2 eV lower than those fi3r PbS, whereas the Pb 4f photoelectron energies for these materials are similar (Pederson, 1982). The Pb(NOO) energies for other possible galena oxidation products are up to 4.5 eV lower. T h e Pb(N6,7OO) and S(L2,3MM) spectra for galena surfaces exposed to air for 1 min and 60 rain are shown in Fig. 3. The specimen was not cooled in the determination of these spectra in order to enable direct comparison with the data reported by Kartio et al. (1996). In agreement with previous findings (Buckley and Woods, 1996a), the Auger spectra after air exposure for 60 min indicated the formation

24

A.N. Buckley. R. Woods/Int. J. Miner. Process. 51 (1997) 15 26

of a significant surface concentration of P b / O species, but no new sulphur species. The spectra also show an appreciable diminution of the S(LMM) intensity relative to the Pb(NOO). Each S(LMM) spectrum can be fitted adequately by a Gaussian-shape peak when the rising background is subtracted. The Auger spectra are consistent with the formation of a P b / O species covering a largely unaltered sulphide lattice as was proposed previously (Buckley and Woods, 1984a, 1996a). Recent joint investigations by the University of Turku and CSIRO groups (Kartio et al., in prep.) have suggested a basis for resolving the apparent discrepancy in the reported oxidation behaviour. The SR-XPS studies had been carried out on p-type galena, whereas the CSIRO work had been carried out on n-type galena. In a comparison of the anodic oxidation at pH 4.6 of these mineral specimens, significantly less elemental sulphur was detected in the case of the n-type semiconductor. Comparable variation in the rate of appearance of products under other oxidation conditions could arise from compositional differences. For further consideration of this issue, the reader is referred to the paper by Vaughan et al. (1997) in this volume.

References Basilio, C., Pritzker, M.D., Yoon, R.-H., 1985. Thermodynamics, electrochemistry and flotation of the chalcocite-potassium ethyl xanthate system. 114th AIME Annual Meeting, New York, N.Y., Preprint No. 85-86. Buckley, A.N., Riley, K.W., 1991. Self-induced floatability of sulphide minerals: examination of recent evidence for elemental sulphur as the hydrophobic entity. Surf. Interface Anal. 17, 655-659. Buckley, A.N., Woods, R., 1984a. An X-ray photoelectron spectroscopic study of the oxidation of galena. Appl. Surf. Sci. 17, 401-414. Buckley, A.N., Woods, R., 1984b. An X-ray photoelectron spectroscopic investigation of the surface oxidation of sulfide minerals. In: Richardson, P.E., Srinivasan, S., Woods, R. (Eds.), Proc. Int. Syrup. Electrochemistry in Mineral and Metal Processing. The Electrochem. Soc., Pennington, N.J., PV 84-10, pp. 286-302. Buckley, A.N., Woods, R., 1991. Adsorption of ethyl xanthate on freshly exposed galena surfaces. Colloids Surf. 53, 33-45. Buckley, A.N., Woods, R., 1993. Underpotential deposition of dithiophosphate on chalcocite. J. Electroanal. Chem. 357, 387-405. Buckley, A.N., Woods, R., 1994. Xanthate chemisorption on lead sulfide. Colloids Surf. 89, 71-76. Buckley, A.N., Woods, R., 1995. Identifying chemisorption in the interaction of thiol collectors with sulfide minerals by XPS: adsorption of xanthate on silver and silver sulfide. Colloids Surf. 104, 295-305. Buckley, A.N., Woods, R., 1996a. Relaxation of the lead-deficient sulfide surface layer on oxidized galena. J. Appl. Electrochem. 26, 899-907. Buckley, A.N., Woods, R., 1996b. The galena surface revisited. In: Woods, R., Doyle, F.M., Richardson, P.E. (Eds.), Proc. 4th Int. Symp. Electrochemistry in Mineral and Metal Processing. The Electrochem. Soc., Pennington, N.J., pp. 1-12. Buckley, A.N., Kravets, I.M., Shchukarev, A.V., Woods, R., 1994. Interaction of galena with hydrosulfide ions under controlled potentials. J. Appl. Electrochem. 24, 513-520. Buckley, A.N., Parks, T.J,, Vassallo, A.M., Woods, R., 1997. Verification by surface-enhanced Raman spectroscopy of the integrity of xanthate chemisorbed on silver. Int, J. Miner. Process. 51, 303-313. Chander, S., Fuerstenau, D,W., 1974. The effect of potassium diethyldithiophosphate on the electrochemical properties of platinum, copper and copper sulfide in aqueous solution. J. Electroanal. Chem. 56, 217-247. Chander, S., Fuerstenau, D.W., 1975. Electrochemical reaction control of contact angles on copper and synthetic chalcocite in aqueous potassium diethyldithiophosphate solutions. Int. J. Miner. Process. 2, 333-352.

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Gardner, J.R., Woods, R., 1977. An electrochemical investigation of contact angle and of flotation in the presertce of alkyl xanthates, 2. Galena and pyrite surfaces. Aust. J. Chem. 30, 981-991. Guy, P.J., Trahar, W.J., 1985. The effects of oxidation and mineral interaction on sulphide flotation. In: Forssberg, K.S.E. (Ed.), Flotation of Sulphide Minerals. Developments in Mineral Processing 6, Elsevier, Amsterdam, pp. 91-110. Kartio, I.. Laajalehto, K., Suoninen, E., 1996. Recent applications of SR-XPS to surface studies of sulfide minerals. In: Woods, R., Doyle, FM., Richardson, P.E. (Eds.), Proc. 4th Int. Symp. Electrochemistry in Mineral and Metal Processing. The Electrochem. Soc., Pennington, N.J., pp. 13-24. Kowal, A., Pomianowski, A., 1973. Cyclic voltammetry of ethyl xanthate on a natural copper sulphide electrode. J. Electroanal. Chem. 46, 411-420. Laajalehto, K., Nowak, P., Suoninen, E., 1993. On the XPS and IR identification of the products of xanthate sorption at the surface of galena. Int. J. Miner. Process. 37, 123-147. Leppinen, J.O., 1986. On the Interaction between Thiol Collector Ions and Lead Sulfide Surface. Ph.D. Thesis, University of Turku. Leppinen, J.O., Rastas, J.K., 1986. The interaction between ethyl xanthate ion and lead sulfide surface. Colloids Surf. 20, 221-237. Nowak, P., 1993. Xanthate chemisorption at PbS surfaces: molecular model and thermodynamic description. ColloLdS Surf. 76, 65-72. O'Dell, C.S., Dooley, R.K., Walker, G.W., Richardson, P.E., 1984. Chemical and electrochemical reactions in the chalcocite-xanthate system. In: Richardson, P.E., Srinivasan, S., Woods, R. (Eds.), Proc. Int. Symp. Electrochemistry in Mineral and Metal Processing. The Electrochem. Soc., Pennington, N.J., PV-10, pp. 81-95. O'Dell, C.S., Walker, G.W., Richardson, P.E., 1986. Electrochemistry of the chalcocite-xanthate system. J. Appl. Electrochem. 16, 544-554. Pederson. L.R., 1982. Two-dimensional chemical-state plot for lead using XPS. J. Electron Spectrosc. 28, 203-209. Richardson, P.E., O'Dell, C.S., 1985. Semiconducting characteristics of galena electrodes. Relationship to mineral flotation. J. Electrochem. Soc. 132, 1350-1356. Richardson, P.E., Walker, G.W., 1984. The flotation of chalcocite, bornite, chalcopyrite and pyrite in an electrochemical cell. Proc. XVth Int. Miner. Process. Congr., Cannes, Vol. II, pp. 198-210. Richardson, P.E., Stout, J.V. III, Proctor, C.L., Walker, G.W., 1984. Electrochemical flotation of sulfides: chalcocite-ethylxanthate interactions. Int. J. Miner. Process. 12, 73-93. Roth, E., Hope, G.A., Schweinsberg, D.P., Kiefer, W., Fredericks, P.M., 1993. Simple technique for measuring surface-enhanced Fourier Transform Raman spectra of organic compounds. Appl. Spectrosc. 47, 1794-- 1800. Shchukatev, A.V., Kravets, I.M., Buckley, A.N., Woods, R., 1994. Submonolayer adsorption of alkyl xanthates on galena. Int. J. Miner. Process. 41, 99-114. Sutherla~d, K.L., Wark, I.W., 1955. Principles of Flotation. Aust. I.M.M., Melbourne, 489 pp. Szeglowski, Z., Czarnecki, J., Kowal, A., Pomianowski, A., 1977. Adsorption of potassium ethyl xanthate on a copper electrode surface. Trans. IMM 86, C115-118. Taggart, A.F., Taylor, T.C., Knoll, A.F., 1930. Chemical reactions in flotation. AIME Tech. Publ. 312, 3-33. Talonen, P., Rastas, J., Leppinen, J.O., 1991. In situ FTIR study of ethyl xanthate on gold, silver and copper electrodes under controlled potential. Surf. Interface Anal. 17, 669-674. Vaughan, D.J., Becket, U., Wright, K., 1997. Sulphide mineral surfaces: theory and experiment. Int. J. Miner. Process. 51, 1-14. Wark, I.W., Cox, A.B., 1934. Principles of flotation, I. An experimental study of the effect of xanthates on contact angles at mineral surfaces. Trans AIME 112, 189-232. Woods, R., 1971. The oxidation of ethyl xanthate on platinum, gold, copper, and galena electrodes: relation to the mechanism of mineral flotation. J. Phys. Chem. 75, 354-362. Woods, R., 1988. Flotation of sulfide minerals: electrochemical perspectives. In: Mullar, E.A., Gonzalez, G., Barahona, C. (Eds.), Copper 87, Vol. 2, Mineral Processing and Process Control. University of Chile, Santiago, pp. 121-135. Woods, R., 1996. Chemisorption of thiols on metal and metal sulfides. In: Bockris, J.O.M., Conway, B.E., White, R.E. (Eds.), Modern Aspects of Electrochemistry, Vol. 29. Plenum Press, New York, pp. 401-453.

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Woods, R., Young, C.A., Yoon, R.-H., 1990. Ethyl xanthate chemisorption isotherms and Eh-pH diagrams for the copper/water/xanthate and chalcocite/water/xanthate systems. Int. J. Miner. Process. 30, 17-33. Woods, R., Basilio, C.I., Kim, D.S., Yoon, R.-H., 1992. Ethyl xanthate chemisorption isotherms and Eh-pH diagrams for the silver+ water+ ethyl xanthate system. J. Electroanal. Chem. 328, 179-194. Woods, R., Kim, D.S., Yoon, R.-H., 1993. The potential dependence of flotation of chalcocite with diethyl dithiophosphate. Int. J. Miner. Process. 39, 101-106. Woods, R., Basilio, C.I., Kim, D.S., Yoon, R.-H., 1994. Chemisorption of ethyl xanthate on copper electrodes. Int. J. Miner. Process. 42, 215-223. Woods, R., Kim, D.S., Basilio, C.I., Yoon, R.-H., 1995. A spectroelectrochemical study of chemisorption of ethyl xanthate on gold. Colloids Surf. 92, 67-74. Woods, R., Chen, Z., Yoon, R.-H., 1997. Isotherms for the chemisorption of ethyl xanthate on lead. Int. J. Miner. Process., in press. Zachwieja, J.B., Walker, G.W., Richardson, P.E., 1987. Electrochemical flotation of sulfides: the bornite-ethylxanthate system. Min. Metall. Process. 4, 146-151.