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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1981. / Microfiche copy available in Archives and Science. / Bibliography: leaves 95-99. / by Simon Muir Peacock. / M.S.

Pyrite (FeS₂) is the major source of sulfur in various coals, and its efficient removal has proven to be a more difficult task than expected. Flotation is generally considered to be the most practicable process for the preparation of coal fines. However, even this technique is usually unable to remove more than 50% of pyrite from a 65-mesh coal sample, which is the typical feed to flotation. There are three major reasons for the low separation efficiency of liberated pyrite from coal by flotation. They include self-induced hydrophobicity of pyrite caused by superficial oxidation, nonselective hydraulic entrainment of pyrite particles into froth product, and incomplete liberation of pyrite from coal that results in composite coal-pyrite particles, i.e., middlings. The present study was undertaken to address problems associated with these recovery mechanisms of pyrite and develop techniques to enhance pyrite rejection in coal flotation. To better understand self-induced hydrophobicity of pyrite, chronoamperometry and voltammetry on freshly fractured electrodes were used to explore incipient oxidation and reduction of the mineral. Voltammetry on rotating ring-disc electrodes (RRDE) was carried out to provide information on soluble species and kinetics of oxidation and reduction processes. X-ray photoelectron spectroscopy (XPS) was used for chemical identification of oxidation products. Galvanic coupling with sacrificial anodes was investigated as a practical method to cathodically protect pyrite and prevent its oxidation. Microflotation tests were conducted under controlled potentials at different solution pH's, and the results were correlated with electrochemical studies. The feasibility of improving pyrite rejection by controlling its surface chemistry was tested in flotation experiments conducted with a 2"-diameter microbubble flotation column and a conventional 5-liter Denver flotation cell. Effects of froth stability on the microbubble flotation of coal were studied with an objective of minimizing hydraulic entrainment of pyrite. The operating parameters were systematically varied to study their effects on water recovery which was used as a measure of froth stability. It has been demonstrated that the upgrading of coal in a flotation column can be significantly improved when froth stability is properly controlled. In an attempt to enhance the rejection of pyrite in middlings, various column circuits were experimentally examined and theoretically analyzed. The effect of circuit configuration on the overall circuit performance was evaluated by separation efficiency and separation curves. It has been shown that the overall separation efficiency of column flotation is rather insensitive to circuitry due to the unique characteristics of the unit flotation column, i.e., the addition of the wash water into the froth. / Ph. D.

M.S.

Thesis (MScEng) -- Stellenbosch University, 1986 / ENGLISH ABSTRACT: High turbulence, high shear forces and high centrifugal forces characterise the flow in hydrocyclones. These characteristics are employed advantageously in the air-sparged hydrocyclone so that a space time of only one second is necessary for effective flotation. Conventional flotation processes on the other hand require a few minutes. Flotation of pyrite from a coarse Witwatersrand ore (100% -300 micron; 92% +38 micron) produces a sulphur recovery of 90% with a sulphur grade of 40% in the concentrate when the content of solids of the slurry feed equals 10%. Higher recoveries up to 93% are possible when slurries with a higher content of solids (e.g. 30%) are used. However, the sulphur grade then decreases to about 35%. Flotation in a batch cell produces a recovery of sulphur of 95% with a sulphur grade of 40%, but in this case a flotation time of 5 minutes is required. A hydrocyclone with a diameter of 50 mm and a length of 410 mm produces optimum results at a slurry feed rate of 35 to 40 l/min and an air-flow rate of 200 l/min. An air-flow rate of about 150 l/min is adequate at slurry feed rates lower than 35 l/min. Flotation of particles finer than 38 micron is more successful at higher slurry feed rates. The optimum flotation of coarse particles (i.e. +106 micron) occurs at lower feed rates. The best flotation results are obtained in the size fraction between 38 and 75 micron, which produces a recovery and content of sulphur of 95% and 51% respectively. A collector concentrate of 160 g/ton, which is thrice the quantity used in conventional processes, is required. The best recoveries at slurry feed rates lower than 35 l/min are obtained when the frother concentrate is low (approximately 20 mg/l). A higher frother concentrate, i.e. between 50 to 60 mg/l, is required at higher feed rates. The air-sparged hydrocyclone may be used effectively for rougher flotation and especially for the flotation of ore finer than 150 micron. The use of a specially designed pedestal can minimize blockage of the underflow. Such a pedestal has been designed and tested successfully. / MINTEK

Ph.D. / Petrographic, chemical and multiple sulfur isotope analyses were conducted on pyrite from argillaceous, arenaceous and rudaceous sedimentary rocks from the Mesoarchean Witwatersrand Supergroup. Following detailed petrographic analyses, four paragenetic associations of pyrite were identified. These include: 1) Detrital pyrite (derived from an existing rock via weathering and/or erosion). 2) Syngenetic pyrite (formed at the same time as the surrounding sediment). 3) Diagenetic pyrite (formed in the sediment before lithification and metamorphism). 4) Epigenetic pyrite (formed during metamorphism and hydrothermal alteration). It was found that the distribution of the pyrite varies with respect to the stratigraphic profile of the Witwatersrand Supergroup and depositional facies within the Witwatersrand depository. In this regard, the four paragenetic associations of pyrite are either scarce or absent in marine-dominated depositional environments, which occur in the lower parts of the succession and in geographically distal parts of the depository. Conversely, the four paragenetic associations are well represented in fluvial-dominated depositional environments, which occur in the middle and upper parts of the succession and in geographically proximal parts of the depository. However, it is worth noting that diagenetic pyrite in the West Rand Group occurs as in situ segregations in carbonaceous shale, whereas syngenetic and diagenetic pyrite in the Central Rand Group occurs as reworked and rounded fragments in fluvial quartz-pebble conglomerates. The strong association between fluvial depositional environments and sedimentary pyrite (syngenetic and diagenetic pyrite) infers a continental source of the sulfur (sulfide weathering or volcanic activity), whereas the lack of pyrite in marine depositional environments is consistent with the model of a sulfate-poor Archean ocean. The connection between epigenetic pyrite and the fluvial-dominated depofacies is probably related to the elevated concentrations of precursor sulfides (i.e., remobilization of syngenetic and early diagenetic pyrite) and the presence of organic carbon (conversion of metal-rich early diagenetic pyrite into pyrrhotite and base metal sulfides). In support of the petrographic observations above, it was found that the trace element chemistry of each paragenetic association of pyrite yields a distinctive set of chemical compositions and interelement variations (Co, Ni and As contents). Regarding detrital pyrite, two chemical populations can be distinguished according to grain size: 1) small grains (tens of μm’s) with high levels of metal substitution (up to wt. %) and interelement covariation and iv 2) large grains (>100 μm) with low levels of metal substitution (≤200 ppm). These two populations are thought to represent pyrite derived from sedimentary and metamorphosed source areas, respectively (see below). The trace element chemistry of diagenetic pyrite varies relative to the Fe-content of the host rock. Diagenetic pyrite from Fe-rich host rocks, such as magnetic mudstone and banded iron formation (BIF), generally contain low Ni contents (<500 ppm), moderate As contents (<1500 ppm) and relatively high Co contents (up to a few wt. %). Elevated concentrations of As probably reflect desorption of As from clays and Fe-oxyhydroxides during diagenetic phase transformations, whereas anomalous concentrations of Co are tentatively linked to the reductive dissolution of Mn-oxyhydroxides.

Powell and Sakinaw Lakes are stably stratified ex-fjords, which became isolated from the Strait of Georgia approximately 11000 years ago by emerged sills due to postglacial isostatic rebound. Although both lakes contain highly sulphidic relict seawater (Powell 3.0 mM; Sakinaw 5.5 mM), they have distinct chemical differences, which may be due to Sakinaw receiving occasional inputs of seawater over the barely-emerged sill when strong onshore winds are coincident with spring tides. Powell Lake, now 50 m above sea level, has not received additional seawater since the sill originally emerged. Sakinaw has a very sharp chemocline located just below the oxic/anoxic interface, whereas in Powell, the interface is spread out over 200 m of the water column. Although both lakes have freshened, the ratios of major ion concentrations relative to chloride in the bottom saline waters are similar to those of present-day seawater. There are some differences, however, and these can be explained, in part, by the difference in molecular diffusivities for each of the ions. The bottom waters of Powell and Sakinaw Lakes are chemically similar to anoxic sediment porewaters. containing high concentrations of nutrients, DOC and alkalinity. Unlike Sakinaw, however, Powell Lake has very low concentrations of phosphate in its bottom waters, in spite of both lakes having similar particulate organic N:P ratios in their upper oxic waters. This may be attributable to more recent addition of sulphate to Sakinaw, allowing greater mineralization of phosphorus compared to the relatively oxidant-starved Powell Lake. High concentrations of reduced iron, hydrogen sulphide, and polysulphides result in formation of iron monosulphides and pyrite in the anoxic water columns of both lakes. The presence of these two minerals correlates well with their calculated saturation states. Pyrite precipitates directly with no monosulphide precursor at depths where sulphide concentrations are low; thus monosulphide phases are undersaturated. As sulphide levels increase with depth, iron monosulphides become saturated and are detected in the water column. Pyrite can then form via the slower reaction of elemental sulphur with monosulphide. The large separation of the oxic/anoxic interface and the chemocline in Sakinaw (∼10 m) and especially in Powell Lake (∼100 m) relative to that of sediment pore waters allows excellent resolution of these processes. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

La mangrove est un écosystème majeur des zones intertidales des côtes tropicales. En Nouvelle-Calédonie, les mangroves sont des zones tampon entre les massifs ultrabasiques, plus ou moins exploités, et un lagon inscrit au patrimoine mondial de l'UNESCO. Le long de l’estran, cet écosystème se décompose principalement en trois zones de végétation qui se développent parallèlement au trait de côte, avec du front de mer vers les terres : Rhizophora spp., Avicennia marina, tannes. Ce gradient botanique dépend de la durée d'immersion des marées, qui impose des gradients de teneur en eau, de salinité de l'eau interstitielle, d’oxygénation, et de teneur en matière organique dans les sédiments. L'objectif de cette thèse était d'améliorer notre compréhension du cycle biogéochimique du fer et du nickel dans les sédiments de mangrove, et de déterminer le rôle des paramètres édaphiques sur la mobilité de ces éléments métalliques. Ce travail est plus spécifiquement focalisé sur l’évolution de la cristallochimie de Fe et Ni en liant avec les (bio)transformations minéralogiques. Les analyses de spectroscopie d’absorption à rayons X montrent clairement que les spéciations de Fe et Ni évoluent en fonction des gradients redox qui marquent d’une part, la zone intertidale, et d’autre part, la profondeur. La goethite et les phyllosilicates, hérités des bassins versants latériques, sont les principaux hôtes du Fe et Ni dans la partie supérieure des sédiments de mangrove. Ces espèces minérales sont intégralement préservés en profondeur des sédiments des tannes, qui sont pauvres en matière organique et bien oxygénés. En revanche, sous les Rhizophora et les Avicennia, la goethite disparait rapidement avec la profondeur . Dans ces horizons inférieures anoxiques riches en matière organique, la sulphato-réduction se développe, et la pyrite et les complexes organiques sont les principales phases porteuses du Ni. A la limite entre les couches oxiques et anoxiques, une intense ré-oxydation du Fe (II) aqueux et des sulfures de fer conduit à la formation de ferrihydrite, lépidocrocite, et probablement de goethite. La proportion relative des oxyhydroxydes de fer néoformés et mal cristallisés, est plus élevé dans la mangrove à Rhizophora. En outre l’incorporation du Ni dans la pyrite est également moins prononcée dans cette zone. Une telle évolution latérale de la spéciation du Ni peut être liée à la réoxydation fréquente des pyrites porteuses de Ni en lien avec le balayage quotidien de cette zone par les marées. Ce cycle tidal, qui touche plus particulièrement la zone à Rhizophora du fait de sa position basse dans l’estran, peut être une cause majeure des cycles de réduction et d’oxydation des phases porteuses de Fe, et pourrait affecter de manière significative les bilans de masse du fer et du nickel dans les mangroves. En effet, le Ni tend à être immobile dans les tannes, à s'accumuler sous Avicennia, et à être partiellement libéré sous Rhizophora. Finalement, le comportement du Fe et du Ni dans des sédiments de mangrove subissant une oxydation intense, en réponse à un isolement de la mer, a été étudié. Au niveau du front d’oxydation, des concentrations très élevées de Ni en solution ont été mesurées , tandis que les concentrations en Ni dans la phase solide étaient quatre fois plus faible que dans l'horizon pyritisé, et 2,5 fois plus faibles que dans le sédiment supérieur. Ces résultats suggèrent que l'oxydation des sédiments de mangrove est une cause de perte en Ni pour l’écosystème. Cette thèse a permis une meilleure connaissance des processus minéralogiques qui conduisent à la fixation ou la libération des élémentes traces métalliques par les mangroves, et est donc utile pour la gestion des mangroves qui sont situés en aval de bassins versants latéritiques. / Mangrove forests are the dominant intertidal ecosystem of tropical coastlines. In New Caledonia, mangroves act as a buffer zone between Ni open-cast mines and a lagoon registered as a UNESCO World Heritage site. Across the intertidal zone, mangroves are composed of three main stands; with from the seaward side to the landward side: Rhizophora spp., Avicennia marina, salt-flat. This botanical gradient relies on the duration of tidal immersion, which imposes sedimentary gradients of water content, salinity, oxygenation, and organic content.The objective of this PhD thesis was to improve our understanding of the biogeochemistry of iron and nickel in mangrove sediments and to characterize the role of edaphic parameters on trace metals dynamic. Particular emphasis was focused on the mineralogical (bio)transformation of Fe and Ni bearing phases and on crystal chemistry. Both XANES and EXAFS data showed that Fe and Ni speciation strongly followed the redox boundaries marking the intertidal and depth zonations. Fe(III)-bearing goethite and phyllosilicates, herited from lateritic outcrops, were the major Fe and Ni hosts in the upper mangrove sediments. These mineral species were fully preserved at depth in the dry and oxic salt flat area. By contrast, beneath the vegetated Rhizophora and Avicennia stands, goethite rapidly disappeared with depth. In these anoxic horizons, sulfate reduction occurred, and pyrite and organic complexes became the dominant Ni-species. At the limit between oxic and anoxic layers, intense re-oxidation of aqueous Fe(II) and Fe-sulfides led to the formation of ferrihydrite, lepidocrocite and likely goethite. The relative proportion of the newly formed poorly ordered iron-oxyhydroxides was found to be higher in the Rhizophora mangrove stand. Moreover Ni incorporation in pyrite was less developed beneath Rhizophora stand. Such lateral evolution of Ni speciation may be related to reoxidation of Ni-bearing pyrites in the Rhizophora stand, which is subjected to periodic alternation of reducing and oxidizing events due to daily tidal fluctuations. The latter may be a major cause for continuous Fe reduction-oxidation cycles in the vegetated mangrove stands, and could significantly affect iron and nickel mass balances in mangroves. Indeed, Ni was found to be immobile in the salt flat, to accumulate beneath Avicennia and to be partially leached beneath Rhizophora. Eventually, Fe and Ni behavior in mangrove sediments currently oxidizing in response to isolation from the sea, was studied. In the layer of the oxidation front, really high concentrations of dissolved Ni were measured, while Ni concentrations in the solid phase were 4 times lower than in the pyritized horizon, and 2.5 times lower than in the upper sediment. These results suggest that mangrove sediment oxidation was a cause of Ni loss. This PhD thesis allows a better assesment of the mineralogical processes that lead to the fixation or the release of trace metals by mangroves, and is thus useful for the management of mangroves that are situated downstream lateritic watershed.

Pyrite porphyroblasts up to 300 mm in size are common in the polymetamorphosed, iron-rich, stratabound, massive sulfide ore at the Cherokee Mine, Ducktown, Tennessee. These porphyroblasts contain abundant inclusions of sphalerite, calcite, and micas that are used to determine the paragenesis of the porphyroblasts and the metamorphic history of the ore deposit. The ore mineralogy at the mine is: hexagonal pyrrhotite (60%), pyrite (30%), chalcopyrite (4%), sphalerite (3 %), and magnetite (3 %) with minor galena, molybdenum, tetrahedrite, bismuth, ilmenite, and rutile. The ore body is interpreted to have been syngenetic and to have contained both primary pyrrhotite and primary pyrite: additional pyrite may have formed as crusty accretions resulting from oxidation of primary pyrrhotite shortly after deposition. The early pyrites were later deformed and acted as seeds for the formation of the larger porphyroblasts by Ostwald ripening, and by the annealing of small pyrite grains into larger porphyroblasts, during isochemical metamorphism. Sphalerite geobarometry indicates initial growth of the pyrite porphyroblasts began at 6.8± 0.8 kilobars and that many sphalerite grains underwent some degree of re-equilibration at a later stage. Fluid inclusions formed during retrograde metamorphism have salinities near 12 % NaCl with a vapor phase rich in CO₂. Pyrrhotite-pyrite compositional profiles indicate at least partial re-equilibration of hexagonal pyrrhotite down to about 270°C. At lower temperatures further compositional re-equilibration was probably prevented, because the coexisting pyrite was too refractory to release the sulfur needed for the hexagonal pyrrhotite to react to monoclinic pyrrhotite. / Master of Science

The relative reactivities of pulverized samples (100-200 mesh) of 3 marcasite and 7 pyrite specimens from various sources were determined at 25°C and pH = 2.0 in ferric chloride solutions with initial ferric iron concentrations of 10⁻³ molal. The rate of the reaction: FeS₂ + 14Fe³⁺ + 8H₂O = 15Fe²⁺ + 2SO₄²⁻ + 16H⁺ was determined by calculating the rate of reduction of aqueous ferric ion from measured oxidation-reduction potentials. The reaction follows the rate law: -d m_Fe³⁺ / dt = k (A/M) m_Fe³⁺ where m_Fe³⁺ is the molal concentration of uncomplexed ferric iron, k is the rate constant and A/M is the surface area of reacting solid to mass of solution ratio. The measured rate constants, k, range from 1.0x10⁻⁴ to 2.7x10⁻⁴ sec⁻¹ ±5%, with lower-temperature/early diagenetic pyrite having the smallest rate constants, marcasite intermediate, and pyrite of higher-temperature hydrothermal and metamorphic origin having the greatest rate constants. Geologically, these small relative differences between the rate constants are not significant, so the fundamental reactivities of marcasite and pyrite are not appreciably different. The activation energy of the reaction for a hydrothermal pyrite in the temperature interval of 25 to 50°C is 92 kJ mol⁻¹. The BET-measured specific surface area for lower-temperature/ early diagenetic pyrite is an order of magnitude greater than that for pyrite of higher-temperature origin. Consequently, since the lower-temperature types have a much greater A/M ratio, they will appear to be more reactive per unit mass than the higher temperature types. / Master of Science