Andrea R. Gerson

XPS of sulphide mineral surfaces: metal-deficient, polysulphides, defects and elemental sulphur

This paper reviews evidence for the assignments of components of the S 2p XPS spectra from sulphide mineral surfaces under different conditions of preparation, oxidation and reaction. Evidence from other techniques confirming assignment of high-binding-energy S 2p components to metal-deficient sulphide surfaces, polysulphides, elemental sulphur and electronic defect structures is considered for specific cases. Reliable assignment of S 2p 3/2 components at 163.6-164.0 eV to elemental sulphur S n 0 can be confirmed by evaporative loss at 295 K and/or observation of S-S bonding by x-ray absorption fine structure (XAFS), x-ray diffraction or vibrational spectroscopy. Assignment to polysulphides S n 2- at 162.0-163.6 eV requires confirmation of S-S bonding by XAFS or vibrational spectroscopy. Metal-deficient lattices can be represented as electronic defects (e.g. vacancies) or restructured surface phases confirmed by diffraction or XAFS evidence. High-binding-energy S 2p 3/2 components can also result from Cu(I) substitution into ZnS with associated oxidation of sulphur as electronic defect sites without S-S bonding, metal deficiency or restructuring. This assignment is confirmed by XAFS evidence.

A review of the structure, and fundamental mechanisms and kinetics of the leaching of chalcopyrite

Most investigators regard CuFeS2 as having the formal oxidation states of Cu(+)Fe(3+)(S(2-))2. However, the spectroscopic characterisation of chalcopyrite is clearly influenced by the considerable degree of covalency between S and both Fe and Cu. The poor cleavage of CuFeS2 results in conchoidal surfaces. Reconstruction of the fractured surfaces to form, from what was previously bulk S(2-), a mixture of surface S(2-), S2(2) and S(n)(2-) (or metal deficient sulfide) takes place. Oxidation of chalcopyrite in air (i.e. 0.2 atm of O2 equilibrated with atmospheric water vapour) results in a Fe(III)-O-OH surface layer on top of a Cu rich sulfide layer overlying the bulk chalcopyrite with the formation of Cu(II) and Fe(III) sulfate, and Cu(I)-O on prolonged oxidation. Cu2O and Cu2S-like species have also been proposed to form on exposure of chalcopyrite to air. S2(2-), S(n)(2-) and S(0) form on the chalcopyrite surface upon aqueous leaching. The latter two of these species along with a jarosite-like species are frequently proposed to result in surface leaching passivation. However, some investigators have reported the formation of S(0) sufficiently porous to allow ion transportation to and from the chalcopyrite surface. Moreover, under some conditions both S(n)(2-) and S(0) were observed to increase in surface concentration for the duration of the leach with no resulting passivation. The effect of a number of oxidants, e.g. O2, H2O2, Cu(2+), Cr(6+) and Fe(3+), has been examined. However, this is often accompanied by poor control of leach parameters, principally pH and E(h). Nevertheless, there is general agreement in the literature that chalcopyrite leaching is significantly affected by solution redox potential with an optimum E(h) range suggesting the participation of leach steps that involve both oxidation and reduction. Three kinetic models have generally been suggested by researchers to be applicable: diffusion, chemical reaction and a mixed model containing diffusion and chemical components which occur at different stages of leaching. Passivation effects, due to surface diffusion rate control, may be affected by leach conditions such as pH or E(h). However, only initial conditions are generally described and these parameters are not controlled in most studies. However, at fixed pH, E(h) and temperature, it appears most likely that leaching in sulfuric acid media in the presence of added Fe(3+) is surface reaction rate controlled with some initial period, depending on leach conditions, where the leach rate is surface layer diffusion controlled. Although bioleaching of some copper ores has been adopted by industry, bioleaching has yet to be applied to predominantly chalcopyrite ores due to the slow resulting leach rates. Mixed microbial strains usually yield higher leach rates, as compared to single strains, as different bacterial strains are able to adapt to the changing leach conditions throughout the leach process. As for chemical leaching, passivation is also observed on bioleaching with jarosite being likely to be the main contributor. In summary, whilst much has been observed at the macro-scale regarding the chalcopyrite leach process it is clear that interpretation of these phenomena is hampered by lack of understanding at the molecular or atomic scale. Three primary questions that require elucidation, before the overall mechanism can be understood are: 1. How does the surface of chalcopyrite interact with solution or air borne oxidants? 2. How does the nature of these oxidants affect the surface products formed? 3. What determines whether the surface formed will be passivating or not? These can only realistically be tackled by the application of near atomic-scale analytical approaches, which may include quantum chemical modelling, PEEM/SPEM, TEM, AFM etc.

The mechanism of the sodalite-to-cancrinite phase transformation in synthetic spent Bayer liquor

Abstract The precipitation of zeolite, sodalite and cancrinite and subsequent phase transformations were investigated under a variety of conditions in sodium aluminate liquor. At sufficiently high SiO 2 relative supersaturation, amorphous sodium aluminosilicate and zeolite precipitated at temperatures as high as 160°C. They subsequently transformed to sodalite and finally to cancrinite. Thus the sequence of the transformation of phases is: [Aluminosilicate species]→Amorphous phase→Zeolite (Linde A)→Sodalite→Cancrinite. It was found that sodalite did not transform to cancrinite in the absence of a liquid medium. The transformation of sodalite to cancrinite was demonstrated to involve a solution-mediated mechanism with sodalite dissolution and subsequent cancrinite precipitation. Neither the amorphous phase nor the zeolite phase precipitates at typical spent Bayer liquor SiO 2 supersaturation.

Kinetics and mechanisms of the leaching of low Fe sphalerite

Abstract The surface speciation and leaching kinetics of 38- to 75-μm sphalerite (0.45 wt.% Fe) particles reacted in O2 purged perchloric acid (at pH 1.0) at 25, 40, 60, and 85 °C over a leach period of 144 h were investigated. In all cases, an initial rapid leach rate is observed followed by a slower leach rate. These two leach regimes can each be adequately modeled using straight-line interpolation, and thus two activation energies (Ea) have been derived. Ea for the fast and slow Zn dissolution rates were 33 ± 4 kJ mol−1 and 34 ± 4 kJ mol−1 respectively, suggesting the same rate-determining step.

Bayer process plant scale : transformation of sodalite to cancrinite

An investigation of the deposition and in situ transformation of scale found in a Bayer process plant has been carried out using X-ray powder diffraction and FTIR studies. Scale samples were analysed as a function of their position in the Bayer process circuit. Scale precipitated during bauxite digestion at approximately 255°C was found to be mostly cafetite but also contained haematite. At 120°C boehmite has been identified as the main scale phase formed from “spent” liquor (i.e. liquors from which Al(OH)3 crystallisation has previously occurred). Three sodium aluminosilicate phases were found to form between 150 and 255°C, sodalite1, sodalite2 and cancrinite although thermonatrite (Na2CO3 · H2O) and calcite (CaCO3) were also observed periodically. The ratios of cancrinite to sodalite1 and sodalite2 to sodalite1 were observed to increase with the temperature of formation, The scale phases found in a cross section of plant scale formed at 150°C show a similar trend on increasing the in situ age of the scale. Comparison with precipitation from synthetic solutions has indicated that the aging mechanism of the sodium aluminosilicate deposits is the same in both cases: sodalite1 (cubic, a ≈ 8.98 A)) → sodalite2 (cubic, a ≈ 8.89 A) → cancrinite (hexagonal, a ≈ 12.70 A, c ≈ 5.18 A). The transformation from sodalite2 to cancrinite has been shown to be the rate determining step in cancrinite formation.

The mechanism of copper activation of sphalerite

Abstract On the basis of recent SIMS and XAFS measurements in conjunction with already published XPS results, a mechanism for the adsorption/absorption of Cu onto sphalerite is proposed. Under conditions of high pH and high nominal surface coverage of the sphalerite by the Cu, Cu(OH)2 colloidal particles are observed on the sphalerite surfaces using SIMS. Under other conditions, SIMS measurements have indicated that adsorption of the Cu is essentially uniform over the sphalerite surface and is not related to low coordination sites on the surface of the sphalerite. Depth profiling of sphalerite surfaces with Cu adsorbed under conditions that do not result in Cu(OH)2 colloidal particles show that the Cu adsorbed/absorbed on the sphalerite surface is largely in the first few atomic layers. XAFS analysis of Cu activated sphalerite has indicated that the Cu occupies a distorted trigonal planar geometry, coordinated to three S atoms, in both surface and bulk sites. In addition Cu(1s), absorption edges in XAFS show that both bulk and surface adsorbed copper have an oxidation state less than +1 with the surface Cu being slightly more oxidised than the bulk absorbed Cu. On the basis of the combined XPS, SIMS, XAFS and solution studies, a model is proposed that, on surface adsorption of Cu, the surface Zn(II) atoms are replaced by Cu(II) atoms which are then reduced in situ to Cu(I). This reduction is accompanied by the oxidation of the three neighbouring S atoms to an oxidation state of approximately −1.5. On bulk absorption of Cu atoms into the sphalerite lattice a distorted trigonal planar configuration is achieved through the breakage of a formerly tetrahedral Zn–S bond. The breakage of this bond results in a 3-fold coordinated Cu plus one S 3-fold coordinated to Zn atoms. The breakage of this bond leads to a greater reduction of the Cu than on surface absorption and also oxidation of the 3-fold coordinated S atom to an approximately −0.5 oxidation state. This model does not invoke any polysulfite or S–S bonded species to explain the higher binding energy components of the S(2p) XPS spectra.

The influence of sodium carbonate on sodium aluminosilicate crystallisation and solubility in sodium aluminate solutions

Abstract Isothermal batch precipitation experiments have been carried out in synthetic Bayer liquors to investigate the effects of sodium carbonate concentration on both silica solubility and the crystallisation of sodium aluminosilicates. At both 90 and 160°C cancrinite (generically defined as a sodium aluminosilicate of space group P6 3 ) is the stable solid phase. Sodalite (generically defined as a sodium aluminosilicate with space group P43n seed transforms to cancrinite at both these temperatures. A high concentration of sodium carbonate in the synthetic liquor causes a decrease in the rate of conversion of sodalite to cancrinite. The solubility of both cancrinite and sodalite decreases as the concentration of sodium carbonate in the synthetic liquor is increased. For instance at 90°C and with 40.0 g dm −3 sodium carbonate in the synthetic liquor after 13 days the sodium aluminosilicate concentration is 0.52 g dm −3 compared to 0.85 g dm −3 with 4.6 g dm −3 of sodium carbonate in solution. At 160°C the sodium aluminosilicate concentration is 0.47 g dm −3 with 40.0 g dm −3 sodium carbonate in solution after 13 days and 0.79 g dm −3 with 4.6 g dm −3 sodium carbonate in solution. Throughout all these experiments a progressive loss of carbonate from the sodium aluminosilicate crystallisation products was observed as a function of time.

The solubility of sodalite and cancrinite in synthetic spent Bayer liquor

The equilibrium SiO2 solubility of sodalite and cancrinite crystals in sodium aluminate solutions containing low concentrations of aluminium (synthetic spent Bayer liquor) has been determined over a range of temperatures (90–220°C) and as a function of solution NaOH and Al(OH)3 concentration for cancrinite. The same SiO2 solubilities, within experimental error, were measured when equilibrium was approached from both above and below SiO2 solubility for spent Bayer liquors in contact with either sodalite or cancrinite crystals. The solubility of sodalite and cancrinite (expressed in terms of SiO2 concentration) increased linearly with increasing temperature. The equilibrium SiO2 solubility of sodalite was higher than that of cancrinite at all temperatures. The solubility of cancrinite increased with increasing concentration of NaOH (3.87–5.42 M) and Al(OH)3 (1.39–2.23 M) in solution. The SiO2 solubilities of four types of dimorphic sodalite/cancrinite mixed phase seed crystals synthesised from Bayer plant spent liquor and synthetic Na2CO3, Na2SO4 or NaNO3 rich NaOH solutions containing dissolved kaolinite were investigated. The solubility of the four types of mixed phase seed crystals was found to be substantially the same as the solubility for pure cancrinite.

Cu(II) adsorption mechanism on pyrite: an XAFS and XPS study

The surface of a <38 μm ground fraction of pyrite was analysed using x-ray photoelectron spectroscopy (xps), x-ray absorption fine structure spectroscopy (XAFS) and inductively coupled plasma atomic emission spectroscopy (ICPAES) prior to and after Cu(II) adsorption under different pH and Cu(II) concentration conditions. Under all conditions copper is found to be coordinated to sulphur on the surface of pyrite with an average bond length of 2.27 ± 0.02 A. A Cu-O bond with a length of 2.00 A due to Cu(OH) 2 precipitation was also observed for a sample with copper adsorbed at pH 8.5 from a solution initially containing 2.84 x 10 -4 mol m -2 Cu (all concentrations given are normalized against the surface area of pyrite in solution). The Cu 2p XPS signals for the samples with adsorbed copper show the presence of Cu(I) ions on the pyrite surfaces in all cases. For the 2.84 x 10 -4 mol m -2 Cu solution at pH 8.5 the pyrite was observed to have both a Cu(I) and a Cu(II) XPS component. The ground pyrite surfaces showed increases in the oxidized S 2 2- (E b = 163.5 eV) signal on copper adsorption with minor increases in Fe(III) oxy- or hydroxy-species observed on pyrite surfaces exposed to long periods of adsorption. It is proposed that the reduction of Cu(II) to Cu(I) on the surface of the pyrite is accompanied by the oxidation of the S 2 2- species present in the pyrite surface. Solution analysis in conjunction with surface analysis has indicated that a 1:1 exchange of Fe(II) ions for Cu(II) ions did not occur during copper adsorption and thus an ion-exchange mechanism is ruled out.

A methodology for quantifying sodalite and cancrinite phase mixtures and the kinetics of the sodalite to cancrinite phase transformation

A simple methodology for determining the cancrinite proportion of a sodalite and cancrinite phase mixture was developed using powder X-ray diffraction. Using this methodology to quantify phase mixtures, the kinetics of the sodalite to cancrinite phase transformation were determined. The transformation reaction leading to cancrinite formation was found to be first order with respect to the relative concentration of sodalite. Over the temperature range 160–240°C, an activation energy for this process was estimated to be 133 kJ mol−1. The techniques of 29Si MAS NMR and FTIR were both found to be unsuitable for quantifying dimorphic phase mixtures due to the ambiguity of the results they produced.