Thèse de doctorat : Géosciences : INPL : 2007. / Titre provenant de l'écran-titre.

Sulfide minerals have been suggested to play an important role in regulating dissolved metal concentrations in anoxic environments. Pyrite is the most common sulfide mineral and it has shown an affinity for arsenic, but little is known about the arsenic retention mechanisms of pyrite. In this study, interactions of arsenic with pyrite were investigated in an anoxic environment to understand geochemical cycling of arsenic better and to predict arsenic fate and transport in the environment better. A procedure using microwaves was studied to develop a fast and reliable method for synthesizing pyrite. Arsenic-pyrite interactions were investigated using macroscopic (solution phase experiments) and microscopic (X-ray photoelectron spectroscopic investigation) approaches. Pyrite was successfully synthesized within a few minutes via reaction of ferric iron and hydrogen sulfide under the influence of irradiation by a conventional microwave oven. The SEM-EDX study revealed that the nucleation and growth of pyrite occurred on the surface of elemental sulfur, where polysulfides are available. Compared to conventional heating, microwave energy results in rapid (< 1 minute) formation of smaller particulates of pyrite. Higher levels of microwave power can form pyrite even faster, but faster reaction can lead to the formation of pyrite with defects. Arsenic removal by pyrite was strongly dependent on pH and arsenic species. Both arsenite (As(III)) and arsenate (As(V)) had a strong affinity for the pyrite surface under acidic conditions, but As(III) was removed more effectively than As(V). Under acidic conditions, arsenic removal continued to occur almost linearly with time until complete removal was achieved. However, under neutral to alkaline conditions, fast removal was followed by slow removal and complete removal was not achieved in our experimental conditions. A BET isotherm equation provided the best fit to arsenic removal data, suggesting that surface precipitation occurred at high arsenic/pyrite ratio. The addition of competing ions did not substantially affect the ultimate distribution of arsenic between the pyrite surface and the solution, but changing pH affected arsenic stability on pyrite. X-ray photoelectron spectroscopy revealed that under acidic conditions, arsenic was removed and formed solid phases similar to As2S3 and As4S4 by reaction with pyrite. However, under neutral to alkaline conditions, arsenic was removed and formed As(III)-O and As(V)-O surface complexes, as well as As2S3/As4S4-like precipitates. As pH increases, the amount of arsenic that formed As2S3/As4S4-like precipitates decreased, while the amount that formed As(III)-O and As(V)-O surface complexes increased. Under alkaline conditions, a FeAsS-like phase was also detected.

Th.--Sci. nat.--Nancy 1, 1978. / Rés. angl., esp. et fr.

No description available.

Alkaline pressure oxidation, particularly in the presence of trona as additive, can be used to oxidize high carbonate refractory gold ores as it prevents the formation of CO2 in the autoclave. However, the presence of silica in the ore can lead to the encapsulation of pyrite due to the formation of a passive layer. This phenomenon occurs due to the high solubility of silica in alkaline solutions and its subsequent re-precipitation on the reacting pyrite surface. The present study investigated the chemical composition and thickness of the passive layer on a rotating pyrite surface in an aqueous slurry containing silica sand, sodium carbonate and calcium carbonate at 230°C and under 7 bar of oxygen overpressure. Results obtained from XPS and SEM show that a concentration of 2.5 g/L sodium carbonate gave the maximum thickness of passivation on pyrite and that the passive layer consisted primarily of silicates and aluminosilicates.

Alkaline pressure oxidation, particularly in the presence of trona as additive, can be used to oxidize high carbonate refractory gold ores as it prevents the formation of CO2 in the autoclave. However, the presence of silica in the ore can lead to the encapsulation of pyrite due to the formation of a passive layer. This phenomenon occurs due to the high solubility of silica in alkaline solutions and its subsequent re-precipitation on the reacting pyrite surface. The present study investigated the chemical composition and thickness of the passive layer on a rotating pyrite surface in an aqueous slurry containing silica sand, sodium carbonate and calcium carbonate at 230°C and under 7 bar of oxygen overpressure. Results obtained from XPS and SEM show that a concentration of 2.5 g/L sodium carbonate gave the maximum thickness of passivation on pyrite and that the passive layer consisted primarily of silicates and aluminosilicates.

Pyrite is a ubiquitous mineral in shales. It results from the bacterial reduction of seawater sulphate to sulphide and the reaction of that sulphide with iron to form iron sulphides and subsequently pyrite. Five Jurassic shales were examined. 1 The Upper Oxford Clay (Oxfordian) Warboys Cambridgeshire. 2 The Dunans Clay (Callovian-oxfordian) Isle of Skye. 3 The Lower Oxford Clay (Callovian) East midlands. 4 The Posidonia Shales (Lias ε) Southern Germany. 5 The Dunans Shale (Callovian) Isle of Skye. The five shale units are divisible into three major biofacies "Normal" (Units 1& 2), "Restricted" (3) and "Bituminous" (4 & 5), which show varying pyrite contents and isotopic compositions, that reflect the changing influence of the three limiting factors of pyrite formation (sulphate, organic carbon, and iron). Samples of shale were analysed for fine grained pyrite content and isotopic composition, organic and carbonate carbon, and hydrochloric acid soluble iron. The Normal shales showed low pyrite sulphur (<1.3%) and organic carbon (<2%), degree of pyritization (D. O. P. ) (<0.5) and negative (<-28%o) δ34S values. These are consistent with the formation of pyrite near the sediment surface, limited by organic carbon. In the Restricted shales higher pyrite sulphur (av. 1.4%) and organic carbon (av. 5%), degree of pyritization (0.5 to 0.7) and less negative (>-28%o, <-9%o) δ34S values indicate the continuation of pyrite formation deeper into the sediment, where it is limited by sulphate diffusion. High D. O. Ps in the Posidonia Shales (>0.8) suggest that pyrite formation is limited by the availability of iron in Bituminous shales. Heavier δ34S values (>-28%o) in the Posidonia Shale suggest limitation of sulphate reduction by sulphate diffusion, and lighter values in the Dunans Shale (<-28%o) suggest limitation by organic matter. Petrographic and isotopic studies of shell replacement and associated concretionary pyrite from the Lower Oxford Clay show types of replacement related to the differences between the porewaters within the shell and those of the shale, particularly as regards iron supply.

The form and location of gold in the structure of arsenopyrite and pyrite minerals, and the mechanisms for the mobility agglomeration of gold in arsenopyrite during thermal treatment, have been studied using a combination of Rietveld X-ray diffraction refinement, Convergent Beam Electron Diffraction (CBED) and Atomic Location by Channelling Enhanced Microanalysis. The basic structure of all the arsenopyrite compositions studies, has been shown to be monoclinic P2(subscript)1/c, regardless of the variation in stoichiometry. An increase in the arsenic to sulfur ratio in the natural arsenopyrites was found to be associated with an increase in unit cell dimensions accompanied by expansions within the iron-centred octahedra along the [101] direction of the monoclinic cell and concommitant contractions of the octahedra in the (101) plane. There was no obvious relationship between variation in stoichiometry and structure of arsenopyrite which could provide information as to possible substitution of gold in its structure. However, atomic displacements caused by twinning or disorder, may help to incorporate gold.The synthesis of auriferous arsenopyrites showed that gold has to be in an ionic form to be taken up in the structure. The form of the gold species affects the distribution of gold in the structure, being chemically zoned when derived from a dichloro complex and more evenly distributed when derived from a hydrosulfido complex. It is suggested that rapid crystallisation, with resultant displacement faults along the b-axis, may contribute to higher concentrations of gold in the natural arsenopyrite structure. Electron probe microanalysis showed a possible slight iron-deficiency in some of the auriferous arsenopyrite grains analysed. However, the errors in the analyses were too high to provide conclusive evidence of gold substitution in the iron sites, as has been ++ / proposed in the literature.Analyses of natural and synthetic pyrites showed no deviations in structural parameters which could indicate possible substitution of gold or other impurities within the structure.Electron channelling experiments showed that gold was located on the sulfur sites in pyrite. In arsenopyrite, there was some evidence for gold located on the iron sites, however, most gold was interstitial, probably situated between the octahedra. This location is probably facilitated by the presence of the displacement faults as observed by CBED in the synthetic auriferous arsenopyrite.Breakdown of arsenopyrite under thermal treatment was topotactic along its b-axis, which converts to the a-axis in the pyrrhotite structure, following a reconstruction mechanism based on the preferential removal of arsenic over sulfur. Gold was visually recorded exsolving from the arsenopyrite structure and agglomerating as liquid metal globules as the arsenopyrite was chemically altered during thermal treatment under the Transmission Electron Microscopy electron beam. Gold became mobile on the decomposition of arsenopyrite, but this was not observed until a temperature of approximately 470 degrees celsius was reached. Above the temperature both solid solution and particulate gold became mobile. The interaction of arsenic vapour and gold reduced the melting point of gold.The observations on the effects of arsenic residence time, and the relative mobility of solid solution and particulate gold during the thermal decomposition of auriferous arsenopyrite and pyrite, have significant implications for improved industrial extraction of gold from these minerals.

Thèses (M.Sc.A.)--Université de Sherbrooke (Canada), 2001. / Titre de l'écran-titre (visionné le 20 juin 2006). Publié aussi en version papier.

Thesis ... by George William Plummer.