Roger St.C. Smart

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.

The role of surface sulfur species in the inhibition of pyrrhotite dissolution in acid conditions

Abstract Pyrrhotite, in anoxic acidic conditions, exhibits an induction period before rapid dissolution occurs. The length of the induction period is controlled by the amount of surface oxidation products on the mineral surface, acid strength, and temperature. During the induction period there is slow release of iron but little or no production of H 2 S. The induction period is best described as a period of inhibited dissolution, before the onset of H 2 S production and increased rate of iron release of at least 2 orders of magnitude. X-ray photoelectron spectroscopic (XPS) analysis of the acid-reacted surface shows the progress of the dissolution. Four stages of dissolution have been identified. (1) The immediate dissolution of an outermost layer of oxidised iron hydroxide/oxyhydroxide species and oxy-sulfur species. (2) Inhibited, diffusion limited dissolution during an induction period due to iron diffusion through the metal-deficient layer and oxidative dissolution of the polysulfide species. (3) Rapid, acid-consuming reaction of mono-sulfide species under nonoxidative or reductive conditions with production of H 2 S. (4) Inhibited dissolution due to reoxidation of the sulfide surface by oxidising solution species (i.e., Fe 3+ , residual oxygen) to produce polysulfide, elemental sulfur, and oxy-sulfur species. Dissolving synthetic pyrrhotite in similar, but aerated, acidic conditions, results in inhibited dissolution characterised by a lower rate of Fe release, minimal release of SO 4 2− and no release of H 2 S . The XPS sulfur (S2p) spectrum shows sulfate and a form of elemental sulfur on the reacted surface. Only the first two stages of dissolution occur. The second stage differs in this case in that there is a plentiful supply of oxidising species (O 2 ). Two reaction mechanisms are proposed for the dissolution of the iron sulfide lattice of pyrrhotite in acidic conditions. The mechanisms are oxidative and nonoxidative dissolution. Two distinct activation energies are associated with the two regimes. A lower activation energy corresponds to inhibited dissolution with no production of H 2 S. A t 1/2 rate law describes dissolution in air saturated solutions and supports diffusion controlled dissolution under these conditions. A higher activation energy corresponds to rapid dissolution with H 2 S production. The mechanism of dissolution is determined by the state of the surface, particularly the sulfur species.

Nanomorphology of kaolinites; comparative SEM and AFM studies

Nanomorphological structure of well-crystallized Georgia and poorly crystallized North Queensland kaolinite particles have been compared using field emission scanning electron microscopy (SEM) and atomic force microscopy (AFM). In general, there is good agreement in information from the 2 very different imaging techniques. AFM gives more detailed information on step and ledge dimensions, microvalleys and crystallographic orientation of irregularities on basal planes and edges of the crystallites. There are major differences in nanomorphology and surface structure between the 2 kaolin samples with the Georgia kaolin showing 200–500-nm, relatively flat basal planes with some cascade-like step growth 50–100 nm wide. The edges, apparently flat and right-angled in SEM images, appear beveled in AFM images due to artifacts from the aspect ratio of the AFM tip. The North Queensland kaolinite has much more complex surface structure with anhedral crystallites attached to larger particles, high density of steps and nm-scale irregularities (often crystallographically directed). The additional step edge site contribution from the attached crystallites is estimated as a minimum of 6%, giving a total edge contribution above 30% of the kaolinite total surface area. These structures will generate a substantial pH-dependent charge across the surfaces of the North Queensland kaolinite platelets. An idealized, uniform, pH-independent, negatively charged basal plane cannot be assumed from these structures. There is also some evidence, from both SEM and AFM images, of curvature in the thinner, poorly ordered structures of the North Queensland kaolinite particles.

Surface chemistry and rheological behaviour of titania pigment suspensions

Abstract The influence of pH and surface chemical state on rheological behaviour was investigated for variations of silica- and alumina-coated, aluminium-doped titania pigment suspensions. The variation in rheological properties correlates with the change in the pigment surface properties, determined from electrophoresis measurements, atomic concentrations, chemical states and modified Auger parameters derived from X-ray photoelectron spectroscopy (XPS). Pigment suspensions exhibited a maximum yield stress and viscosity at or near the isoelectric point (iep). At a pH where the magnitude of the zeta potential of the titania pigment is high, a low-viscosity, dispersed suspension was obtained. The pH of the maximum yield value of the pigment suspension increases with increasing aluminium hydroxyl group density and decreases with increasing silicon hydroxyl group density. Low-viscosity pigment dispersions were obtained with increasing aluminium surface concentration and further reduced with an increase in the silicon surface concentration. Pigment particle attractions are chiefly dictated by van der Waals forces and heteroaggregation. The pigment aggregate strength therefore depends upon the Hamaker constant of the heterogeneous pigment based on the proportion of the respective surface groups.

THE EFFECT OF SURFACE MODIFICATION BY AN ORGANOSILANE ON THE ELECTROCHEMICAL PROPERTIES OF KAOLINITE

The electrochemical properties of kaolinite before and after modification with chlorodimethyl-octadecylsilane have been studied by electrophoretic mobility, surface charge titration, and extrapolated yield stress measurements as a function of pH and ionic strength. A heteropolar model of kaolinite, which views the particles as having a pH-independent permanent negative charge on the basal planes and a pH-dependent charge on the edges, has been used to model the data. The zeta potential and surface charge titration experimental data have been used simultaneously to calculate acid and ion complexation equilibrium constants using a surface complex model of the oxide-solution interface. The experimental data were modeled following subtraction of the basal plane constant negative charge, describing only the edge electrical double layer properties. Extrapolated yield stress measurements along with the electrochemical data were used to determine the edge isoelectric points for both the unmodified and modified kaolinite and were found to occur at pH values of 5.25 and 6.75, respectively. Acidity and ion complexation constants were calculated for both sets of data before and after surface modification. The acidity constants, pKa1 = 5.0 and pKa2 = 6.0, calculated for unmodified kaolinite, correlate closely with acidity constants determined by oxide studies for acidic sites on alumina and silica, respectively, and were, therefore, assigned to pH-dependent specific chemical surface hydroxyl groups on the edges of kaolinite. The parameters calculated for the modified kaolinite indicate that the silane has reacted with these pH-dependent hydroxyl groups causing both a change in their acidity and a concomitant decrease in their ionization capacity. Infrared data show that the long chain hydrocarbon silane is held by strong bonding to the kaolinite surface as it remains attached after washing with cyclohexane, heating, and dispersion in an aqueous environment.

Kaolinite flocculation structure

Effective flocculation and dewatering of mineral processing streams containing colloidal clays has become increasingly urgent. Release of water from slurries in tailings streams and dam beds for recycle water consumption, is usually slow and incomplete. To achieve fast settling and minimization of retained water, individual particles need to be bound, in the initial stages of thickening, into large, high-density aggregates, which may sediment more rapidly with lower intra-aggregate water content. Quantitative cryo-SEM image analysis shows that the structure of aggregates formed before flocculant addition has a determinative effect on these outcomes. Without flocculant addition, 3 stages occur in the mechanism of primary dewatering of kaolinite at pH 8: initially, the dispersed structures already show edge-edge (EE) and edge-face (EF) inter-particle associations but these are open, loose and easily disrupted; in the hindered settling region, aggregates are in adherent, chain-like structures of EE and stairstep face-face (FF) associations; this network structure slowly partially rearranges from EE chains to more compact face-face (FF) contacts densifying the aggregates with increased settling rates. During settling, the sponge-like network structure with EE and FF string-like aggregates, limits dewatering because the steric effects in the resulting partially-gelled aggregate structures are dominant. With flocculant addition, the internal structure and networking of the pre-aggregates is largely preserved but they are rapidly and effectively bound together by the aggregate-bridging action of the flocculant. The effects of initial pH and Ca ion addition on these structures are also analyzed. Statistical analysis from cryo-SEM imaging shows that there is an inverse correlation of intra-aggregate porosity with Darcian inter-aggregate permeability whereas there is a strong positive correlation of Darcian permeability with settling and primary dewatering rate as a function of pH in suspension. Graphs of partial void contributions also suggest that it is not total porosity that dominates permeability in these systems but the abundance of larger intra-aggregate voids.

A comparison of the dissolution behavior of troilite with other iron(II) sulfides; implications of structure

Abstract Further knowledge as to the nature of the structure of a terrestrial sample of troilite, FeS [stoichiometric iron(II) sulfide] is revealed by a combination of XPS studies and dissolution studies in acid. The XPS analysis of a pristine troilite surface (the sample being cleaved under high vacuum) is compared to that of a surface polished in an inert atmosphere and a surface after reaction in deoxygenated acid. Further comparison is made with polished and acid-reacted surfaces of pyrrhotite (Fe1-xS) and pyrite (FeS2). The pristine troilite S2p spectrum comprises mainly monosulfide 161.1 eV, within the reported range of monosulfide, together with evidence of an unsatisfied monosulfide surface state arising from S–Fe bond rupture. Small, higher oxidation state sulfur contributions, including a disulfide-like state are also present, which suggest the presence of defects due to some nonstoichiometry. The dissolution studies showed that the troilite, in addition to dissolving in acid as an ionic solid to produce H2S, also exhibits some oxidation of sulfur in the surface layers. In addition, a study of the dissolution behavior of troilite under the influence of cathodic applied potential supported the existence of a proportion of the sulfur within troilite needing reduction before dissolution forming HS− or H2S can occur. A significant increase in the dissolution rate was observed with application of −105 mV (SHE), but further stepped decreases in potential to −405 mV and −705 mV resulted in a decreased rate of dissolution, a response typical of an ionic solid. The results of the studies emphasise the viewing of iron(II) sulfides as a continuum. Pyrrhotite has been reported previously to dissolve in acid both oxidatively (like pyrite) and nonoxidatively (like troilite) on the same surface. Dissolution studies using troilite, in Ar-purged acid, indicate that dissolution of this material may not be uniformly nonoxidative. XPS evidence of restructuring of the surface of troilite to pyrrhotite and the surface of pyrrhotite towards a FeS2 type structure, after exposure to Ar-purged acid, is presented.

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.

Copper(II) activation and cyanide deactivation of zinc sulphide under mildly alkaline conditions

The adsorption of copper(II) ions onto zinc sulphide particles at pH 9 was determined by inductively coupled plasma atomic emission spectrophotometry and the resulting surfaces were characterised by X-ray photoelectron spectroscopy. The form of the copper(II) activated zinc sulphide surface is controlled by the copper(II) ion concentration and the activation time. Copper(II) activation, at an equivalent surface coverage of one monolayer, results in a copper-substituted zinc sulphide surface, whereas at surface coverages of ten or more equivalent copper(II) monolayers a copper(II) hydroxide surface phase covers the underlying copper-substituted zinc sulphide phases. Mechanisms are proposed which describe copper(II) activation in terms of precipitation, ion-exchange, redox and diffusion processes. Cyanide treatment removes copper hydroxide and copper-substituted species from the surfaces of copper(II) activated zinc sulphide. Ligand exchange mechanisms for cyanide deactivation of copper(II) activated zinc sulphide are proposed.

A mechanism to explain sudden changes in rates and products for pyrrhotite dissolution in acid solution

A reductive mechanism is proposed to explain the sudden changes from oxidative (acid-produc- ing) to nonoxidative (acid-consuming) dissolution that can occur with pyrrhotite. Typically, in acidic conditions, an initial period of slow dissolution involving no release of H 2S can suddenly change to nonoxidative dissolution, with release of H2S and greatly increased rates of release of both iron and sulfur species. Observations of the change from oxidative to nonoxidative dissolution of pyrrhotite in deoxygenated acid show that the process is temperature sensitive, with solution temperatures of at least 40°C required. The mechanism is correlated with the observation from XPS analysis that pyrrhotite surfaces exhibit metastable chemical states that have trapped electrons. The same negative charge shift is measured for all C, Fe, and S chemical states implying a crystal-wide space-charge surface region. The accumulation of this surface charge during dissolution appears to result in the reduction of oxidised disulfide and polysulfide species back to sulfide, thus inducing nonoxidative dissolution. Reduction is favoured on natural pyrrhotite surfaces polished in an oxygen-free atmosphere. Reduction also occurs with synthetic pyrrhotite that, before dissolution in acid, has undergone only limited oxidation. The mechanism is minimal or nonexistent if, before dissolution in acid, the pyrrhotite (natural or synthetic) is ground either in air or in a N 2 atmosphere. No evidence for this mechanism is found with either polished or ground pyrite dissolving in acid under the same conditions. Reduction of pyrite only occurs with the application of a sufficiently cathodic potential. Copyright