A.N. Buckley

The surface oxidation of pyrite

Abstract The surface oxidation in air and air-saturated aqueous solutions of the iron sulfide mineral, pyrite, has been studied by X-ray photoelectron spectroscopy. Iron sulfate was produced on fracture surfaces within the first few minutes of exposure to air under ambient conditions. Iron oxide was also included in the oxidation products after prolonged exposure which implies that a sulfur product in addition to sulfate must be formed. It is suggested that this product is an iron-deficient sulfide. Elemental sulfur was not evident at surfaces exposed to air. Iron oxide rather than sulfate was present at abraded surfaces exposed to air for a few minutes. Oxidation of pyrite in air-saturated acid solutions resulted in the formation of a surface sulfur layer the extent and nature of which depended on solution composition and exposure time. Sulfate was the only sulfur oxidation product identified in alkaline solutions not containing soluble sulfide, and iron oxide remained at the surface after such treatment. Thin layers of elemental sulfur were observed at fracture surfaces immersed in aerated, dilute sodium sulfide solutions.

An x-ray photoelectron spectroscopic study of the oxidation of galena

Abstract The oxidation of cleaved surfaces of galena (natural lead sulfide) in air at ambient temperature and humidity has been studied by X-ray photoelectron spectroscopy. Analyses were carried out with the sample held below 150 K, a temperature at which elemental sulfur was shown to be retained at a galena surface, and subsequently at ambient temperature. During the initial stages of air-oxidation, the surfaces became enriched with lead due to the formation of lead hydroxide, oxide and carbonate. No oxidised sulfur species were observed in this period and it was concluded that a metal-deficient sulfide was formed. Exposure of galena to air for extended periods resulted in the formation of sulfate. After oxidising galena in air-saturated acetic acid solution, the presence of polysulfide phases was revealed. Minor elements present in the galena were found to migrate to fill some of the vacancies in the metal-deficient sulfide and polysulfide phases.

An XPS investigation of the surface of natural sphalerites under flotation-related conditions

Abstract The surface composition of two natural sphalerites of different iron and lead content has been determined under flotation-related conditions by X-ray photoelectron spectroscopy (XPS). Oxidation of the mineral samples in air-saturated alkaline solutions has been found to be very slow. In the presence of copper (II) species in solution, copper exchanges with zinc, and metal-deficient environments are formed in the sulfide lattice at the surface. Cyanide removes the copper and some sulfur from the surface, but a metal-deficient sulfide layer remains. Treatment of sphalerite with cyanide can also result in lead impurity in the mineral migrating to the surface to form lead cyanide and hydrated lead oxide. Zinc-oxygen species were detected at the surface of sphalerite that had been immersed in alkaline zinc solutions and washed with dilute sodium hydroxide before examination. Copper was found to diffuse to the surface when sulfur was deposited on sphalerite by air oxidation of hydrosulfide ions. The resulting surface was a metal-deficient sulfide rather than an overlayer of elemental sulfur.

An investigation of the sulphur(−II)/sulphur(0) system on bold electrodes

Abstract Sulphur deposited on gold by the anodic oxidation of sulphur(−II) species in solution has been studied by X-ray photoelectron spectroscopy. The initial layer behaved as gold sulphide. Multilayers of sulphur had a lower volatility and a smaller electron binding energy than bulk elemental sulphur, indicating that there is interaction with the underlying gold or gold sulphide. The anodic oxidation of sulphur(−II) to sulphur, and the reverse process, has been investigated on gold using the rotating ring disc electrode technique. Polysulphide ions were formed as intermediates in both processes. Polysulphides were also produced by chemical reaction of deposited sulphur with sulphur(−II) species in solution. The polysulphide intermediates were identified as S 2− 5 at pH 13, a mixture of species with average stoichiometry S 2− 3.3 at pH 9.2 and S 2− 2 , possibly HS − 2 , at pH 6.8.

The surface composition of arsenopyrite exposed to oxidizing environments

Abstract The oxidation of fracture surfaces of arsenopyrite has been investigated by X-ray photoelectron spectroscopy and compared with the oxidation of cobaltite. In air, or in air-saturated alkaline solution, iron and arsenic oxides are formed initially at the surface of arsenopyrite, with the arsenic oxidizing faster than the iron. Sulfur is not directly involved in the initial reaction. In alkaline solution, the oxidized iron and some of the oxidized arsenic are retained at the surface and most of this arsenic has an oxidation state of III rather than V. After immersion in an air-saturated acid solution, oxidized iron and arsenic are removed from the surface which becomes increasingly enriched in sulfur. This sulfur is not present as elemental sulfur in its stable S8 configuration, but is in a form similar to that found in a severely metal-deficient sulfide lattice.

X-ray photoelectron spectroscopy of oxidized pyrrhotite surfaces: I. Exposure to aire

Abstract The oxidation of fracture surfaces of a pyrrhotite mineral of composition Fe 0.89 S at ambient conditions in air has been studied by X-ray photoelectron spectroscopy (XPS). Fe(2p) and Fe(3p) spectra indicated that iron had diffused from the outermost layers of the mineral lattice to form a hydrated iron(III) oxide or hydro-oxide. The corresponding S(2p) spectrum exhibited a shifted component at a binding energy increasing with time of exposure. It is considered that this component arises from the formation of iron-deficient sulfides with the iron content decreasing with increasing oxidation time.

Nitrogen functionality in coals and coal-tar pitch determined by X-ray photoelectron spectroscopy

Abstract The forms of nitrogen in several Australian coals, a coal-tar pitch, mesophase prepared from the coal-tar pitch, and the isotropic phase in equilibrium with the mesophase, have been determined by X-ray photoelectron spectroscopy. In agreement with previous studies, nitrogen in the coals was found to be present predominantly in pyrrolic and pyridinic forms, with the proportion of pyrrolic being greater than pyridinic in each case. A N(1s) component near 401.5 eV was assigned to a protonated or oxide form of pyridinic nitrogen. On this basis, the total proportion of pyridinic nitrogen is comparable with the reported value determined by X-ray absorption near-edge structure spectroscopy for a US bituminous coal. Pyrrolic nitrogen was also found to be the predominant form of nitrogen in coal-tar pitch. The nitrogen content, and the ratio of pyrrolic to pyridinic functional forms, were essentially the same in coal-tar pitch mesophase as in the precursor pitch from which it was prepared. It was concluded that mesogens do not discriminate against the inclusion of nitrogen in 5- or 6-membered rings, but that other nitrogen functionalities such as amines appear to be excluded.

Electrochemical and XPS studies of the surface oxidation of synthetic heazlewoodite (Ni3S2)

X-ray photoelectron spectroscopic (XPS) and electrochemical techniques have been applied to the investigation of the surface oxidation of synthetic heazlewoodite (Ni3S2). The XPS data showed that exposure of the sulphide to air resulted in nickel atoms migrating to the surface to form an overlayer of a hydrated nickel oxide and leave a sulphur-rich heazlewoodite. A hydrated nickel oxide was also produced on immersion of heazlewoodite in acetic acid solution in equilibrium with air, despite nickel being soluble under these conditions. After the acetic acid treatment, the S(2p) spectrum had a component at the binding energy of NiS and a small contribution due to sulphur-oxygen species. Voltammetry with bulk heazlewoodite electrodes, and the ground sulphide in a carbon paste electrode, indicated that, at pH 4.6, the initial anodic product was a sulphur-rich heazlewoodite and that oxidation was inhibited when NiS was formed on the surface. Further oxidation to higher nickel sulphides and elemental sulphur occurred at high potentials. In basic solutions, oxidation was restricted due to the formation of nickel oxide.

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

Abstract 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.

Functional forms of nitrogen in coals and the release of coal nitrogen as NOx precursors (HCN and NH3)

The influence of coal nitrogen on the pyrolytic release of NOx precursors, (HCN and NH3), has been investigated for a range of Australian coals. Functional forms of nitrogen in the coals were determined by X-ray photoelectron spectroscopy. Pyrrolic-type nitrogen predominated (50–60% coal nitrogen), but pyridinic and quaternary forms were also detected. The proportion of pyridinic increased and that of quaternary decreased with increasing carbon contents of the coals. Four coals, ranging in rank from brown to bituminous and in nitrogen content from 0.6–2.0%, dry ash free basis, were pyrolyzed in a fluidized bed reactor at temperatures ranging from 500 to 1100°C, gas residence times of 0.3–0.5 s and particle heating rates of 104 K s−1. Formation of HCN and NH3 occurred at higher temperatures than that required for maximum tar yields; cracking reactions of the tars are a probable source of HCN and NH3. However, the results show that release of nitrogen from structures which are not volatilised as tar also occurs. The major nitrogen-containing components of the tars were identified and quantified by gas chromatography. Nitrogen contained in pyridinic groups was more stable than that in pyrrolic groups; thus, a higher pyrolysis temperature was necessary to release nitrogen as HCN and NH3 from the pyridinic groups. For the range of coals and pyrolysis conditions studied here, however, nitrogen release (as a proportion of total coal nitrogen), in the form of NOx precursors, did not depend on coal type or coal nitrogen content.