E. Suoninen

Oxidation of pyrite surfaces: a photoelectron spectroscopic study

Surfaces of pyrite (FeS2) differently prepared in situ and ex situ have been studied before and after contact to air and air-saturated aqueous solutions of 4≤pH≤10 by means of photoelectron spectroscopy. Pyrite surfaces fractured or scraped in situ revealed FeS-like species concentrated in the surface region. Preparation (polishing, grinding, powdering) and prolonged oxidation in air mainly resulted in basic iron sulphate and iron oxide/hydroxide. A promoting effect of an increased surface roughness due to the preparation was observed for the formation of iron oxide/hydroxide compared with sulphate in contrast to the natural oxidation process. Oxidation in air also led to sulphur-rich species identified as iron-deficient regions below monolayer coverage. Similar regions were present at ground surfaces exposed to air-saturated solution of pH4 and pH5. In near-neutral to alkaline solution mainly iron hydroxy-oxide is formed the layer thickness of which was estimated in the range of 0.5 nm (pH5) to 1.7 nm (pH10).

STM and XPS investigation of reaction of galena in air

Scanning tunneling microscopy (STM) was used to study the oxidation of stepped (100) PbS surfaces in air over periods up to 270 min. The systematic growth of reaction products as surface features with lateral dimensions <0.6 nm initially to overlapped regions (#62; 9 nm diameter) was observed. No clear preference for growth at step edges was found but defect sites in the (100) surface appear to provide initiation points for reaction. X-ray photoelectron spectra (XPS) from PbS after successive reaction times suggest that limited adsorption of peroxide (O-) and hydroxide (OH-) species occurs initially but that the predominant products after 60 min are carbonate species from CO2 reaction with O- or OH- sites. After 120 min, hydroxycarbonate species are formed but the S 2p spectra show only sulfide at all times, i.e. no sulfur-oxygen species are formed. The combined results provide both structural and compositional evidence for the mechanism of reaction of galena surfaces to PbCO3 · xPb(OH)2 surface products in air.

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.

XPS studies of xanthate adsorption on pyrite

Abstract X-ray photoelectron spectroscopy (XPS) studies have been carried out on the reactions between natural pyrite with Cu(II) and xanthate ions in air-saturated solutions. The Cu 2p binding energies point out that Cu(II) ions are inserted into the pyrite surface during the activation changing their oxidation state to Cu(I). Chemical shifts and intensity variations of the C 1s, O 1s and Fe 2p spectra provide evidence that a xanthate surface species on pyrite is formed in potassium ethyl xanthate solutions with 6 ≤ pH

On the XPS and IR identification of the products of xanthate sorption at the surface of galena

Abstract XPS investigations of the sorption of methyl, ethyl and amyl xanthates at the surface of lead sulfide and galena, with special attention for the submonolayer region, are described. Lead xanthate in the molecular form as the adsorbing entity is proposed on the basis of the analysis of XPS spectra of the surface after sorption. The uneven distribution of xanthate at the surface, even at low surface coverage, is suggested.

Xanthate adsorption at PbS/aqueous interfaces: Comparison of XPS, infrared and electrochemical results

Abstract In order to elucidate the reasons for the difference in results of measurements of adsorption of ethylxanthate (EtX − ) ion at the surface of lead sulphide and galena, performed using different methods, measurements by X-ray photoelectron spectroscopy and infrared internal reflection spectroscopy were performed on both natural galena and synthetic PbS. The measurements were performed both at spontaneously established potential and under controlled potential conditions. The results obtained using both methods were very similar, showing the formation of submonolayer perpendicularly oriented adsorbed lead ethylxanthate.

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.

An XPS study of adsorption of methyl and amyl xanthates on copper

Abstract Adsorption of methyl and amyl xanthate ions from aqueous solution onto clean copper substrates has been followed by electron-spectroscopic measurements (XPS and AES techniques). Measurements of methyl xanthate adsorption layers confirm the mechanism of xanthate adsorption previously presented on the basis of ethyl xanthate layer observations. Replacement of preadsorbed hydroxyl groups with xanthate ions with simultaneous removal of the carbon-containing impurities is found to be the predominant adsorption mechanism. The velocity of the adsorption is in the case of amyl xanthate much slower than for methyl xanthate, and the resulting amyl xanthate layer is more heterogeneous. Some oxidation of the substrate surface during the first stage of the contact with xanthate solution is observed for amyl xanthate. No signs of dixanthogen can be observed in the adsorbed layers. This indicates adsorption of individual xanthate ions, without further oxidation to dixanthogen, during the formation of the first monolayer, and formation of copper xanthate molecules in the subsequently deposited part of the adsorption layer.

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.