Ilkka Kartio

Reactivity of surface chemical states on fractured pyrite

Abstract Synchrotron-radiation-excited photoelectron spectroscopy was used to monitor sulfur chemical states on fractured pyrite surfaces reacted with atmospheric gases. The results demonstrate that there are at least three distinct states at the pyrite surface and each is oxidised at a very different rate in air. The two surface chemical states are more reactive than bulk sulfur, the most reactive surface sulfur component being S 2− . The second chemical state is identified as the surface atom of the first disulfide layer (S 2− 2 ), and the least reactive species are sulfur atoms of disulfide groups beneath the surface layer (i.e., all sulfur atoms having bulk coordination). A model combining the interpretation of sulfur surface species after Nesbitt et al. (Am. Mineral. in press) and the proposed oxidation mechanism of Eggleston et al. (Am. Mineral. 81 (1996) 1036) was developed to explain the initial oxidation processes on pyrite surfaces, where air oxidation of pyrite commences with the oxidation of S 2− sites at the surface.

XPS and SR-XPS techniques applied to sulphide mineral surfaces

Abstract X-ray photoelectron spectroscopy (XPS) is one of the major techniques in basic research of flotation-related surface studies of sulphide minerals. The advantages of the method, in general, are good surface sensitivity, rather straightforward elemental and chemical state analysis and reliable quantification of the data. The most serious disadvantage is the ex-situ nature of the technique. Surface sensitivity of the XPS technique can be considerably improved if an optimal excitation energy is chosen for a characteristic emission of each different element. This can be done by using a synchrotron radiation (SR) source, providing a continuous energy distribution over large energy region, instead of a fixed excitation energy source (either AlK α or MgK α radiation from sealed off X-ray tube) used in conventional XPS. In the case of identification of sulphur species, which often is the main task in surface analysis of sulphide surfaces, this leads to approxinnately an order of magnitude improvement in surface sensitivity and, hence, essentially better possibility to detect surface species of submonolayer coverage. Air oxidation of PbS and FeS 2 has been studied with SR excitation. On PbS cleavage surface, oxidized sulphur (sulphate and/or polysulphide type) and lead species were detected after 10 min exposure time in air at room temperature, indicating congruent oxidation of PbS surface. The oxidation reactions proceeded relatively quickly during the first 4 h of contact with air. SR-XPS experiments with cleaved FeS 2 also show the formation of sulphate as a primary oxidation product in air. However, the initial layer, formed immediately after cleavage, seemed to passivate the surface against further oxidation.

A flotation related X-ray photoelectron spectroscopy study of the oxidation of galena surface

Abstract The initial steps of the oxidation of galena (PbS) surface with special attention to the oxidation in an aqueous environment were investigated by the X-ray photoelectron spectroscopy method (XPS). Galena monocrystals from two different sources were cleaved and exposed to either air or an aqueous environment. The results show that the presence of organic contamination at the galena surface may exert a dramatic effect on the course of its surface oxidation. In the case of aerial oxidation, the presence of impurities changes the reaction path, suppressing the oxidation of sulfide sulfur to sulfate like species selectively. In the case of aqueous oxidation, the presence of organic contamination at the surface (this contaminants are supposed to be mainly the carboxylic acids and their salts) inhibits the oxidation. Very little oxidation products were found on the galena surface contacted with aqueous solutions at the spontaneously established potential, in accordance with the congruent dissolution-oxidation mechanism.

XPS and FTIR study of the influence of electrode potential on activation of pyrite by copper or lead

Abstract Activation of pyrite by either copper or lead ions and subsequent xanthate adsorption on activated surfaces were studied in aqueous solutions of pH 5, 6.5 and 9 under different electrochemical conditions using X-ray Photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). At pH 5, the copper concentration on the surface of pyrite increased strongly when the electrode potential was changed towards cathodic direction. At low activation potentials (Eh≤+100 mV), a change in the chemical state of sulfur on the surface was observed by XPS proposing formation of a copper-containing surface sulfide phase. In electrochemically-controlled xanthate adsorption, the behavior of copper-activated pyrite resembled that of chalcopyrite (CuFeS2). The results indicate that activation of pyrite by copper at pH 5 can be controlled by the potential of the mineral. At pH 9, the copper content on the surface was independent of the potential of pyrite and no changes were observed in chemical state of sulfur if compared to unactivated pyrite in the potential region −100 to +400 mV (SHE). In the case of pyrite activation by lead, no changes were observed in sulfur spectra at either pH 5 or 9 at different electrode potentials. The concentration of lead on the surface increased at pH 5 when potential was changed towards cathodic direction. All lead present at the surface of pyrite was concluded to be in the form of lead(II)-oxygen species and no evidence of exchange between lead and iron was found. Both adsorbed xanthate and dixanthogen were observed on the surface of Cu-activated pyrite after activation at cathodic potentials (−100 to +50 mV) and subsequent treatment in either ethyl or amyl xanthate solution of pH 5 or 6.5 at potential region +350 to 550 mV (standard hydrogen electrode, SHE). In a similar experiments with Pb-activated pyrite, only a faint indication of the adsorbed collector species was found proposing that lead rather depresses than activates pyrite.

A study of galena (PbS) surfaces under controlled potential in pH 4.6 solution by synchrotron radiation excited photoelectron spectroscopy

Abstract The sulfur and lead species formed on galena (PbS) surface at different electrochemical potentials in acetate buffer solution (pH 4.6) were investigated using surface sensitive synchrotron radiation excited photoelectron spectroscopy (SR-XPS). At +50 to +150 mV versus SHE a new, very weak, S 2p component was observed at binding energy of about 2.8 eV higher than that from PbS, indicating that restructuring of the surface, due to oxidation, starts in this potential region. At +250 mV metal polysulfides were observed on the surface. In addition to polysulfides, enrichment of copper, present as a minor element in bulk of the natural galena sample, was detected. Since the quantity of polysulfides on the surface was not sufficient enough to explain the observed increase in S : Pb ratio with potential, it is proposed that copper diffuses from the bulk of the sample filling the vacant sites formed in anodic dissolution of lead. Formation of a metal deficient surface, where the sulfur state remains unchanged, is also possible. Indication of metallic lead, formed in reduction of PbS at −550 mV, was found. The better surface sensitivity gained by using synchrotron excitation instead of a conventional X-ray tube with Mg or Al amode turned out to be essential when studying submonolayer coverages of sulfur species on a sulfide mineral.

Application of electron spectroscopy to characterization of mineral surfaces in flotation studies

Abstract Use of synchrotron radiation (SR) for exciting X-ray photoelectron spectra offers a new possibility for highly surface-sensitive characterization of minerals in flotation studies. The problems common to all X-ray photoelectron spectroscopy (XPS) techniques are possible changes in the sample caused by the need to extract the sample from the process and the evacuation necessary for the XPS measurement. When SR excitation is used, also damage of the sample surface during the measurement may easily occur, probably, by sample heating. SR excited core level X-ray photoelectron spectra of galena (100) surfaces cleaved in vacuum show no evidence of surface electronic state, in contrast to the previous results obtaine for similarly prepared pyrite (100) surfaces. The surface sensitivity of the measurement is essentially better than the sensitivity obtained using conventional excitation (Al Kα or Mg Kα). Preliminary results of the application of the SR excitation method to studies of the surface adsorption layer of ethyl xanthate on copper substrate were ambiguous because of the possibility of sample damage. Measurements of charge transfer between different minerals simultaneously immersed in xanthate solution, combined with XPS measurements of the sample surfaces, allowed conclusions to be drawn for the mechanism of separation of pyrite and chalcopyrite in flotation carried out in alkaline solution (pH 9.2).

Characterization of the ethyl xanthate adsorption layer on galena (PbS) by synchrotron radiation excited photoelectron spectroscopy

Abstract Utilization of synchrotron radiation excitation in photoelectron spectroscopy (SR-XPS) of solid surfaces provides a significant improvement in surface sensitivity of the method. It has recently been shown that almost an order of magnitude increase in relative intensity of the sulfur 2p signal from the very first atomic layer of a sulfide can be achieved by using SR instead of conventional AlKα or MgKα X-rays for photoelectron excitation. In this work, SR-XPS was applied to characterize the xanthate layer adsorbed on galena in 10−4 M potassium ethyl xanthate solution (pH 9.2) in equilibrium with air. Chemisorption of xanthate radicals rather than formation of bulk lead ethyl xanthate was observed. Prolonged irradiation of the sample by the SR beam was observed to decompose the adsorption layer. Means to avoid radiation damage during the SR-XPS measurement is discussed.

The initial products of the anodic oxidation of galena in acidic solution and the influence of mineral stoichiometry

Abstract The oxidation products retained at the surface of galena samples from Broken Hill (lead-rich) and Missouri (sulphur-rich) following mild anodic treatment in acetate solution of pH 4.6 have been determined by X-ray photoelectron spectroscopy (XPS) using excitation both by Al Kα X-rays and by synchrotron radiation (SR). Elemental sulphur was found to be the major oxidation product on both galenas. For short anodization times, S0 could only be detected by the more surface sensitive SR-XPS. Electrochemical studies showed that the quantity of sulphur on a Missouri galena surface did not decrease significantly when the galena was held for 40 min at open circuit. This contrasts with previous studies on Broken Hill galena in which the sulphur coverage was found to decrease to a low value in this time period, a behaviour that was assigned to diffusion of lead atoms from the bulk. The different behaviour of the two galenas can be explained by the different stoichiometry of the Broken Hill and Missouri minerals. According to SR-XPS measurements, the intensity of the S0 component in S 2p spectra for galena surfaces held for 40 min at open circuit following anodization at 0.3 V for 20 s was less than for the equivalent spectra carried out within 7 min of anodization. The S0 intensity was negligible for the Broken Hill mineral in agreement with electrochemical studies. A small but significant decrease in the S 2p intensity was also observed for Missouri galena and this was assigned to accumulation of the sulphur product into clusters. S 2p components at the binding energies expected for lead polysulphide were also observed in the spectra for the Missouri mineral. Conventional XPS revealed a significant variability in the S0 2p intensity following short anodization times and this could also arise from sulphur clustering.

Temperature Controled Photoelectron Spectroscopic Investigation of Volatile Species on PbS(100) Surface

A special sample precooling technique has been utilised to preserve thin layers of volatile chemical species on the sample surface in ultra high vacuum conditions for investigation with conventional and synchrotron excited photoelectron spectroscopy. The precooling system has been constructed for the Scienta 300 electron spectrometer at the beam line 51 at MAX I synchrotron laboratory in Lund (Sweden) and it will be transferred to beam line 411 at the new MAX II storage ring. The spectrometer also includes a possibility for continuous temperature control of the sample during the measurement at temperature range 70 K – 450 K. In this work, temperature controlled measuring technique has been applied in a study of electrochemically treated PbS (100) surface. The measured sulfur 2p core level spectra clearly reveal a layer structure of elemental sulfur and metal polysulfides formed during anodic oxidation of PbS in acidic aqueous solution. Similar measuring technique can also be applied, e.g., in studies of adsorption layers on various substrates.