Pure Pyrite Research Articles

In situ study of the decomposition of pyrite in coal during hydropyrolysis

The thermal behaviours of pure pyrite, pyrite in coal and in situ formed Fe–S compounds under reductive atmospheres were studied over a wide temperature range (room temperature to 1050 °C), using a thermogravimetric analyser (TGA), temperature-programmed reduction coupled with online mass spectrometry (TPR–MS), X-ray diffraction (XRD) and scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM–EDS). The results suggest that the inherent hydrogen and oxygen in coal can promote the decomposition of pyrite at low temperature, while external H2 plays a principal role in sulfur removal in the hydropyrolysis (HyPy) of coal, where only very little pyrite in demineralized Yanzhou coal (YZ-Dem) is transformed to new organic sulfur forms, and the Fe–S compounds produced in the final products are thermally stable. In contrast, ferric iron can accelerate organic sulfur elimination in complex ways, and complete conversion from in situ formed Fe–S or Fe–C compounds to elemental iron occurs under a H2 atmosphere.

Pyrite thermal decomposition in source rocks

Sulphides, mainly pyrite, are very common minerals in source rocks for oil and natural gas. Pyrite thermal transformation is very complex process, depending on many factors affecting the decomposition reactions. Thermal decomposition of pyrite contained in bulk rock is even more complex due to the possible interactions with organic matter present in the source rock. In the paper the reactions of pure pyrite decomposition under different atmosphere were presented. Moreover the decomposition of pyrite in kerogen and bulk source rock were studied with TG/DSC methods. The evolved gases derived from combustion and pyrolysis processes were studied by FTIR and QMS methods. The results show that SO2 is the only gas detected during pyrite decomposition. Pyrite content in a rock sample is detectable over 10% contamination; below this value the sulphide presence can be masked by thermal decomposition of other rock components. In case of combustion process of source rock containing pyrite sulphur dioxide is released in temperature range of 400–430 °C. When the source rock is pyrolyzed, then pyrite decomposition, reflected by SO2 evolution, begins near temperature of 400 °C, and is extended in time. The release of sulphur dioxide from pyrolysed kerogen as a result of pyrite decomposition is detectable in the range of 400–480 °C.

A comparative study on the micro-surface characteristics at black shale initial oxidation stage

The pyrite oxidation is crucial to the overall black shale oxidation process. A. ferrooxidans was documented an effective oxidation ability on pure pyrite, but its role in black shale oxidation is unclear. In this study, a comparative study of acid solution and A. ferrooxidans on the micro-surface characteristics at the initial stage (7 days) was conducted on black shale slices, a comprehensive approach combining the micro-morphologies, micro-structures, micro-environmental pH and micro-surface elemental content were investigated by using polarizing microscopies, SEM, fluorescent staining and EDX line scan analysis. The pyrite oxidation rate was employed to the index for black shale oxidation degree, and analyzed by XRD, aqueous pH, oxidation-reduction potential (ORP), ferrous and ferric ions concentrations measurement. The results show that the micro-surface characteristics are different in acid solution and A. ferrooxidans groups, which significantly impact the pyrite oxidation rate. A. ferrooxidans promote the jarosite formation and elemental C accumulation on the rocks micro-surface, which is assumed to inhibit further reactions. Two reaction phases named “pyrite oxidized phase” and “jarosite formation phase” are proposed to occur in the initial stage of A. ferrooxidans oxidizing black shale. These findings provide experimental data to evaluate the micro-surface reactions during black shale oxidation process.

The state of Au and As in pyrite studied by X-ray absorption spectroscopy of natural minerals and synthetic phases

In many hydrothermal deposits arsenian pyrite contains economic concentrations of Au in an “invisible” form, which is structurally bound in pyrite or has grain size <0.1 μm. Here we use X-ray absorption spectroscopy (XAS) of natural minerals and synthetic phases to reveal the forms of Au occurrence in pyrite and determine the effect of physicochemical-compositional parameters on the concentration and state of “invisible” Au. A sample of natural arsenian pyrite with 300 ppm Au and 0.34 wt% As (average value) from the Samolazovskoe Au-sulfide deposit (Yakutia, Russia) was used to study the states of Au and As, and two arsenian pyrite samples from the Vorontsovka Carlin-type deposit (North Urals, Russia) were used to characterize the state of As. The synthesis experiments were performed employing the hydrothermal method at 300 °C/Psat and 450 °C/1000 bar, at contrasting redox states – in oxidized (H2S/H2SO4 redox buffer) and reduced (H2S predominates) fluids, in As-free and As-bearing systems. As a result, a series of samples of As-free and arsenian pyrites was obtained with Au concentrations from ca. 100 to 300 ppm. The concentration of “invisible” Au, which was homogeneously distributed within the samples, was independent of As concentration but decreased with increasing synthesis temperature. The concentration of Au dissolved in acidic sulfide hydrothermal fluid was not affected by the presence of As. The X-ray absorption XANES/EXAFS spectra were recorded simultaneously at Au L3-edge and As K-edge. The use of the high energy resolution fluorescence detection (HERFD-XAS) mode made possible to acquire data for the trace amounts of Au in As-rich samples. According to XANES spectroscopy Au occurs in pyrite in the 1 + oxidation state. Two forms of Au were detected in the pyrite samples: the solid solution Au and Au2S-like clusters/inclusions. In the solid solution state Au replaces Fe in the structure of pyrite and is coordinated only with sulfur atoms (NS = 6, RAu-S = 2.41 ± 0.01 Å) in all pyrite samples independently of the conditions of their formation/synthesis and As content, no As atoms were detected in the local atomic environment of Au. Thus, the Au-S distance in the 1st coordination shell increased by 0.15 Å relatively to the Fe-S distance in pure pyrite. The distant coordination shells of the solid solution Au correspond to the pyrite crystal structure. The presence of the Au2S-like clusters leads to a significant decrease of the calculated values of coordination number of Au and Au-S interatomic distance. In natural pyrite samples from the Vorontsovka deposit As1− replaces S in the anionic sites, increasing the 1st shell As-S and As-Fe distances by 0.07–0.1 Å relatively to the pure pyrite structure. A contribution of As3+ oxide was detected in pyrite from the Vorontsovka deposit. The synthetic pyrite samples contain As in the form of As1− solid solution and As3+ and As5+ oxides. Results of our study show that neither the concentration and speciation of As nor the redox state of the system affect the state of “invisible” Au in the studied pyrites.

DFT simulations of pyrite galvanic interactions with bulk, solid-solution and nanoparticle Au occurrences – Insights into gold cyanidation

Gold cyanidation is an important hydrometallurgical route central to which are the gold/mineral galvanic interactions which pose perennial challenges to understanding gold dissolution. In this study, Density Functional Theory simulations have been performed to unveil some important electronic features of gold/pyrite associations on gold cyanidation. Three typical gold/pyrite occurrences have been considered, namely, model_1) gold slab/pyrite slab association to emulate contactless liberated gold and pyrite with up to a few Å separations; model_2) gold-substituted pyrite solid-solution slab to emulate atomically dispersed gold in a sulfidic matrix; and model_3) pyrite slab with tethered gold nanocluster, in addition to isolated gold nanocluster and to pure pyrite slab. Gold/pyrite associations for all three studied models have been shown to induce electron transfer from gold to both pyrite’s sulfur and iron atoms. Model_1 revealed that galvanic interactions between liberated gold and pyrite can occur up to 10 Å separation distance between the metal and the sulfide slabs. However, the efficacy of galvanic interactions decreased with increasing the inter-slab separation distances. Model_2 divulged that S-substitutive incorporation of Au in the pyrite structure resulted in expanded crystal dimensions as well as changes in electron properties of gold, iron and sulfur atoms in the solid-solution structure. Occurrence of gold nanocluster on pyrite surface (model_3) led to the formation of metal-metal Au-Fe bond and covalent Au-S bond. Sorption interactions of cyanide with gold and pyrite surfaces in presence of dioxygen were investigated with the three model variants. The DFT simulations showed that CN− preferentially adsorbed on gold instead of pyrite sites provided gold/pyrite galvanic interactions deprived gold from electron density at the expense of pyrite. The cyanide adsorption energies were found to follow the sequence: model_1 with substituted Au+ > model_2 > model_3 with substituted Au+ > model_1 (short distance) > model_3 > pure pyrite (1 0 0) & model_1 (long distance contact) > isolated Au9 nanocluster. As Au electron deficiency via galvanic interactions or gold oxidation was enhanced, gold-cyanide interaction energy increased portending the promotion of gold dissolution in cyanidation processes.

New Insights into 99Tc(VII) Removal by Pyrite: A Spectroscopic Approach.

99Tc(VII) uptake by synthetic pure pyrite at 21 °Cwas studied in a wide pH range from 3.50 to 10.50 using batch experiments combined with scanning electron microscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and Raman microscopy. We found that pyrite removes Tc quantitatively from solution (log Kd = 5.0 ± 0.1) within 1 day at pH ≥ 5.50 ± 0.08. At pH < 5.50 ± 0.08, the uptake process is slower, leading to 98% Tc removal (log Kd = 4.5 ± 0.1) after 35 days. The slower Tc uptake was explained by higher pyrite solubility under acidic conditions. After 2 months in contact with oxygen at pH 6.00 ± 0.07 and 10.00 ± 0.04, Tc was neither reoxidized nor redissolved. XAS showed that the uptake mechanism involves the reduction from Tc(VII) to Tc(IV) and subsequent inner-sphere complexation of Tc(IV)-Tc(IV) dimers onto a Fe oxide like hematite at pH 6.00 ± 0.07, and Tc(IV) incorporation into magnetite via Fe(III) substitution at pH 10.00 ± 0.04. Calculations of Fe speciation under the experimental conditions predict the formation of hematite at pH < 7.50 and magnetite at pH > 7.50, explaining the formation of the two different Tc species depending on the pH. XPS spectra showed the formation of TcSx at pH 10.00 ± 0.04, being a small fraction of a surface complex, potentially a transient phase in the total redox process.

Phase-pure iron pyrite (FeS2) micro- and nano-sized crystals synthesized by simple one-step microwave-assisted hydrothermal method

Iron pyrite micro- and nano-sized crystals are desirable as active materials in lithium ion batteries and photovoltaics, and are particularly suitable for nanocrystal inks for roll-to-roll deposited or ink-jet printed solar cells. In this paper we report the synthesis of iron pyrite micro- and nano-sized crystals via simple and convenient green one-step microwave-assisted hydrothermal (M − H) process at relatively low growth temperatures, using commonly used precursors such as FeCl3, Na2S and S8 and. The structural, morphological and optical properties of the resulting nanostructured materials have been thoroughly investigated for two typical M − H growth temperatures. X-ray diffraction (XRD) pattern and Raman data revealed good crystalline quality for the as synthesized pyrite NCs. Typical XRD patterns show the dominant peaks which can be indexed as a pure cubic phase of FeS2, with lattice constant values close to the lattice parameters reported for FeS2, and in agreement with a stoichiometric pyrite phase. XRD and Raman analysis also confirm that no other impurities phases such as hexagonal FeS, marcasite, pyrrhotite, greigite, S or Fe–O compounds were detected, confirming the high purity of the synthesized iron pyrite nanocrystals. Various shapes of pyrite like quasi-cubic, cubic and flower-like FeS2 nanocrystals have been observed, which can be modulated by using different synthetic conditions. The sizes of the pyrite micro- and nanostructures were in the range of 150 nm to 1 μm as obtained. The present study indicates that the M − H method is a facile one-step way to obtain phase pure iron pyrite micro- and nano-sized crystals. Optical characterization confirms direct bandgap transitions (values of 2.61 eV and 2.55 eV for iron pyrite NCs samples hydrothermal grown at 130 °C and 160 °C, respectively), and indirect bandgap transitions (values of 1.19 eV and 1.52 eV for samples hydrothermal grown at 130 °C and 160 °C, respectively). Optical studies show high absorbance in the entire UV–Vis wavelength range making the as-synthesized pyrite nanocrystals potential candidate as absorber in nanoscale photovoltaic solar cells.

The State of Platinum in Pyrite Studied by X-Ray Absorption Spectroscopy of Synthetic Crystals

Abstract Pyrite (FeS2) is a typical container of Pt in ores of magmatic and hydrothermal origin and in some carbonrich ores of sedimentary-diagenetic origin. Knowledge of the state of Pt disseminated in the matrix of pyrite, including local atomic environment (type of atoms in the nearest and distant coordination shells, coordination numbers, interatomic distances) and oxidation state, is necessary for physical-chemical modeling of platinum group element mineralization and for the improvement of Pt ore extraction and processing technologies. Here we report results of an investigation of local atomic structure of synthetic Pt-bearing pyrites by means of X-ray absorption spectroscopy (XAS). Synthesis experiments, performed at 580° and 590°C in a Pt-saturated system by means of salt-flux method, yielded crystals of pyrite with concentrations of Pt up to 4 wt %. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) showed that the distribution of Pt within the pyrite grains is of zonal character, but within the distinct zones Pt is distributed homogeneously. Negative correlation between the concentrations of Pt and Fe was observed in the synthesized pyrite grains. The slope of the correlation line corresponds to the formation of the solid solution in the Pt-Fe-S system and/or to the formation of PtS2. The XAS experiments revealed the existence of two forms of Pt in pyrite. The main form is the solid solution Pt(IV), which isomorphically substitutes for Fe. The Pt-S distance in pyrite is ~0.1 Å longer than that of Fe-S in pure pyrite. The distortion of the pyrite crystal structure disappears at R &gt;2.5 Å. The second Pt-rich form was identified by means of high-resolution transmission electron microscopy (HRTEM) as nanosized inclusions of PtS2. Heating experiments with in situ registration of X-ray absorption spectra resulted in partial decomposition (dissolution) of PtS2 nanosized inclusions with the formation of the solid solution (Fe1–xPtx)S2. Therefore, the PtS2 nanosized particles can be considered as a quench product. Our data demonstrate that both Pt solid solution and PtS2 nanosized inclusions (at high Pt content) can exist in natural Pt-bearing pyrites.

DFT Study into the Influence of Carbon Material on the Hydrophobicity of a Coal Pyrite Surface.

From the macroscopic point of view, the hydrophilicity of symbiotic carbon pyrite is weakened overall compared to that of pure pyrite. It is very important to explain the impact of elemental carbon accreted on a pyrite surface on the surface’s hydrophobicity from the perspective of quantum chemistry. To study the influence of adsorbed carbon atoms on the hydrophilicity of a coal pyrite surface versus a pyrite surface, the adsorption of a single water molecule at an adjacent Fe site of a one-carbon-atom-covered pyrite surface and a carbon atom monolayer were simulated and calculated with the first-principles method of density functional theory (DFT). The water molecules can be stably adsorbed at the adjacent Fe site of the carbon-atom-covered pyrite surface. The hybridization of the O 2p (H2O) and Fe 3d (pyrite surface) orbitals was the main interaction between the water molecule and the pyrite surface, forming a strong Fe–O covalent bond. The water molecule only slightly adsorbs above a C atom on the carbon-atom-covered pyrite and the carbon atom monolayer surfaces. The valence bond between the water molecule and the pyrite surface changed from an Fe–O bond to an Fe–C–O bond, in which the C–O bond is very weak, resulting in a weaker interaction between water and the surface.

Glucose-assisted synthesis of metastable pyrite CuSe2 particles through hot injection method

CuSe2, a promising raw material for absorption layer of CuInSe2-based solar cell, has outstanding electronic and magnetic properties. Here, almost pure pyrite CuSe2 particles were successfully synthesized by hot injection method using glucose as a substitute for toxic adjuvant. Through XRD, SEM and TEM, samples are carried out microscopic analysis for studying the change of microscopic structure and composition of products obtained from different experiments. The results showed that glucose plays a driving role in the synthesis of CuSe2 and pure pyrite CuSe2 can be obtained only when the temperature is suitable.

Experimental Study of Pyrite Oxidation at 100 °C: Implications for Deep Geological Radwaste Repository in Claystone

The oxidation of pyrite is one of the near field processes of the chemical evolution of clay rock planned to host a deep geological radioactive waste repository during operation. Indeed, this process can lead to transitory acidic conditions in the medium (i.e., production of sulphuric acid, carbonic acid) which may influence the corrosion kinetics of the carbon steel components of some disposal cells. In order to improve the geochemical modelling of the long-term disposal, the oxidation of pyrite in contact with clays and carbonates at 100 °C must be evaluated. In this study, special attention was paid to the pyrite oxidation rate thanks to an original experimental set-up, involving several pyrite/mineral mixtures and a reactor coupled to a micro gas chromatograph (PO2 and PCO2 monitoring). Although thermodynamic modelling expects that hematite is the most stable phase in a pure pyrite heated system (low pH), experiments show the formation of native sulfur as an intermediate product of the reaction. In the presence of calcite, the pH is neutralized and drives the lower reactivity of pyrite in the absence of native sulfur. The addition of clay phases or other detrital silicates from the claystone had no impact on pyrite oxidation rate. The discrepancies between experiments and thermodynamic modelling are explained by kinetic effects. Two laws were deduced at 100 °C. The first concerns a pure pyrite system, with the following law: r P y = 10 − 4.8 · P O 2 0.5 · t − 0.5 . The second concerns a pyrite/carbonates system: r P y + C a = 10 − 5.1 · P O 2 0.5 · t − 0.5 where PO2 corresponds to the partial pressure of O2 (in bar) and t is time in seconds. Different mechanisms are proposed to explain the evolution with time of the O2 consumption during pyrite oxidation: (i) decrease of the specific or reactive surface area after oxidation of fine grains of pyrite, (ii) decrease of O2 pressure, (iii) growing up of secondary minerals (Fe-oxides or anhydrite in the presence of calcium in the system) on the surface of pyrite limiting the access of O2 to the fresh surface of pyrite, and (iv) change in the pH of the solution.

Controllable Synthesis of Zn-Doped α-Fe2O3 Nanowires for H2S Sensing.

One-dimensional Zn-doped α-Fe2O3 nanowires have been controllably synthesized by using the pure pyrite as the source of Fe element through a two-step synthesis route, including the preparation of Fe source solution by a leaching process and the thermal conversion of the precursor solution into α-Fe2O3 nanowires by the hydrothermal and calcination process. The microstructure, morphology, and surface composition of the obtained products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. It was found that the formation process of α-Fe2O3 is significantly influenced by the introduction of Zn2+. The gas sensing measurements indicated that the sensor based on 1% Zn-doped α-Fe2O3 nanowires showed excellent H2S sensing properties at the optimum operating temperature of 175 °C. Notably, the sensor showed a low H2S detection limit of 50 ppb with a sensor response of 1.5. Such high-performance sensing would be ascribed to the one-dimensional structure and high specific surface area of the prepared 1% Zn-doped α-Fe2O3 nanowires, which can not only provide a large number of surface active sites for the adsorption and reaction of the oxygen and H2S molecules, but also facilitate the diffusion of the gas molecules towards the entire sensing materials.

Oxidation reactivity of As(III)-containing pyrites: Differences between structurally-incorporated and adsorbed As(III)

Natural pyrites contain high contents of adsorbed and structurally-incorporated arsenic (As), which may in turn affect the oxidation rates of pyrite as well as the transformation and immobilization behaviors of As contaminants. However, the speciation and distribution of As on the oxidation reactivity of pyrites is still not clear yet. In this study, the oxidation reactivity of three different pyrites, namely, pure pyrite (Py-free), As(III)-adsorbed pyrite [Py*-As(III)ad] and As(III)-structurally-incorporated pyrite [Py-As(III)in, including Py-0.02 and Py-0.04] will be compared. In Py-As(III)in, As(III) replaces S(-I) and changes the relative ratio of Fe: S. The XPS results reveal that Fe(III)-S(-II) site densities on the pyrite surface increase with incorporated As, indicating that Py-As(III)in with more As carries more surface defects. The oxidation rates of pyrites are accelerated at pHs 7.0 and 9.0 by both structurally-incorporated and adsorbed As(III), which is more significant in structurally-incorporated As(III). At pH 2.5, structurally-incorporated As(III) promotes the oxidation of pyrite, whereas adsorbed As(III) inhibits its oxidation. The Tafel plots and the electrochemical impedance spectroscopy (EIS) analysis further support that As in Py-As(III)in decreases the charge transfer resistance on the pyrite surface and facilitates electron transfer between dissolved oxygen (DO) and the pyrite surface, thus enhancing the oxidation rates of pyrites. Variations of H2O2 in Py-As(III)in in the full pH range also indicate that As increases the two-electron transfer from pyrite to DO, resulting in the formation of more H2O2. At pH 2.5, the generated OH from H2O2 is the predominant species in pyrite oxidation, and at pHs 7.0 and 9.0, H2O2, Fe(IV) and OH all contribute to pyrite oxidation. Compared with Py-As(III)in, As in Py*-As(III)ad is more readily to be mobilized, with more As(V) being detected at pH 2.5. Subsequent decreases of As(III) and As(V) suggest re-immobilization of the released As via the adsorption or co-precipitation pathways. Solid characterizations further reveal that dissolved or adsorbed As(III) significantly inhibit the development of lepidocrocite, goethite and magnetite, whereas structurally-incorporated As(III) does not, triggering differences in the speciation of secondary Fe minerals. The present study reveals that the speciation and distribution of As(III) significantly affects the chemical and electronic properties of different pyrites, thus affecting the corresponding oxidation reactivity of different pyrites.

Influence of grinding media, pyrite mineralogy and water chemistry on the galvanic interaction between grinding media and pyrite

ABSTRACTThe purpose of this study was to investigate the role of grinding media type, water chemistry and pyrite mineralogy on the galvanic interaction between grinding media and pyrite during grinding of a gold ore. The galvanic current between pyrite and grinding media increased with the electrochemical activity of the grinding media, while it decreased as a function of increasing chromium content in the media. Furthermore, changing the pyrite-based electrode from pure pyrite to a gold-bearing pyrite electrode increased the galvanic current between the pyrite and the grinding media electrodes; therefore, the gold-bearing pyrite electrode is more active than the pure pyrite one. In addition, using process water instead of tap water increased the galvanic current density between pyrite and the high-chromium grinding media electrode (30% chromium). However, when the forged steel or the low-chromium grinding media electrodes were used, the galvanic current decreased compared to the tap water condition. In addition, the results obtained from SEM-EDX analysis showed that in both tap and process water conditions, oxide species were identified on the surface of the forged steel and low chromium electrodes. However, no oxygen was detected on the surface of the 30% chromium grinding media.

Hydration induced bandgap shift at pyrite-water interface

The practical application of earth abundant pyrite (FeS2) in photovoltaic devices is extremely limited by the low open-circuit voltage (OCV) (∼200 mV) induced low efficiency (&amp;lt;3%). As such, finding out the causes for the low OCV and the corresponding solutions has been widely concerned. Here, we report the hydration induced bandgap shift at the pyrite-water interface, which has been ignored in previous efforts. The bandgap shift may be one of the reasons responsible for the low OCV. Using ab initio calculations, we found that, compared to the pure pyrite surface in vacuum, the bandgap of the pyrite-water interfacial system possesses blue and red shifts at the water coverage of more and less than the mono-layer, respectively. The bandgap shift of the interfacial system could be explained by the adsorption symmetry and charge transfer between water and the substrate. These results reveal that the interfacial water could change the electronic structure of the pyrite surface, suggesting that hydration could be a highly probable stratagem to tune the photovoltage properties of pyrite-based materials.

Microbial assisted galvanic leaching of chalcopyrite concentrate in continuously stirred bioreactors

The objective of this study is to combine two proven technologies, namely galvanic and microbial assisted leaching of chalcopyrite in a novel way. Using a commercial chalcopyrite-rich flotation concentrate and pure natural pyrite samples from an Iranian copper mining complex as well as mixed cultures of moderately thermophilic microorganisms, continuous electrochemical bioleaching tests were performed in stirred bioreactors at 50 °C as a first step towards a commercial application of microbially assisted galvanic leaching of chalcopyrite.From the results of this study, the importance of oxidation reduction potential (ORP) on the catalytic interaction between chalcopyrite and pyrite could be pointed out as a main parameter for successful bioprocessing of chalcopyrite concentrates. Optimization of the particle size of feed (D80) and adjusting the ORP in the range between 400 and 500 mV are important criteria, governing the electrochemical bioleaching rate of chalcopyrite concentrates. It is believed that the main reason for increased copper recovery could be the control of chalcopyrite passivation resulting from improved galvanic interaction between chalcopyrite and pyrite in the selected ORP range and the bacterial assisted oxidation of Fe2+ to Fe3+. Under optimum conditions of this study (T = 50 ± 2 °C, pH = 1.1 ± 0.2, ORP = 400–500 mV, solid load = 10%, stirring rate = 300 rpm) the copper extraction from chalcopyrite flotation concentrate during continuous electrochemical bioleaching operations in stirred bioreactors was >95%, which should be high enough to justify the process economically.

Low temperature synthesis of iron pyrite (FeS2) nanospheres as a strong solar absorber material

Phase pure iron pyrite (FeS2) nanosphere morphology was synthesized by a solvothermal route an alternative to colloidal synthesis by utilizing an ethylene glycol and benzylamine as a solvent and structure directing ligand at 160 °C for 12 h in Teflon lined autoclave. The as obtained nanospheres were in average ∼50 nm diameter with a smooth surface and well mono-disperse as observed by the FESEM images. The XRD analysis shows a typical iron pyrite crystal phase with 2θ position at 28.23°, 32.76°, 36.84°, 40.48°, 47.32° and 56.01° without any impurity peaks. The Raman spectra further confirmed the phase pure pyrite structure. The UV-Vis and PL spectra shows excellent solar absorbance with a band gaps of 1.35 eV close to the direct band gap (1.38 eV) of pyrite materials, however the photoluminescence spectra shows a band gap of 1.39 eV close to the direct band gap. The obtained nanosphere morphology was highly promising as an excellent solar absorber material for the photovoltaic application.

An improved sequential extraction method to determine element mobility in pyrite-bearing siliciclastic rocks

ABSTRACTA sequential extraction method was developed for pyrite-bearing (FeS2) siliciclastic rocks. The focus of this study was to enhance the procedure by an improved oxidation step to completely dissolve not only organic matter but also microcrystalline pyrite. In the first experiment, four oxidation procedures were compared for pure pyrite at extraction temperatures of 25°C and 85°C with hydrogen peroxide (H2O2) as the main oxidant. It was found that pyrite dissolution was most effective by using a mixture of H2O2, ammonium acetate (NH4OAc) and nitric acid (HNO3) at 25°C. This procedure dissolved >90% pyrite, and detected >75% using solute iron measurements. The difference between these two results was explained by reprecipitation of secondary iron minerals. The procedure worked best at 25°C, since solvent evaporation at 85°C amplified iron oversaturation and precipitation. For the pyrite-bearing siliciclastic rocks, two sequential extraction schemes were compared to optimise solid–solvent ratio, extraction step order and type of solvent. Eventually, the most effective step order identified for siliciclastic rocks containing pyrite and little organic matter was to first (1) remove the exchangeable fraction, followed by (2) dissolution with acid and afterwards (3) with a reducing agent. The (4) oxidation step was performed last.

Improved cycling stability of the capping agent-free nanocrystalline FeS2 cathode via an upper cut-off voltage control

Transition metal chalcogenides such as FeS2 are promising electrode materials for energy storage. However, poor rate performance and low cycling stability hinder the practical application of FeS2 cathode in secondary batteries. In this study, highly pure pyrite FeS2 nanocrystals (NCs) with octahedral shape and 200–300 nm size have been synthesized via a facile and environmentally benign approach based on a surfactant-free aqueous reaction. Combined with a compatible ether electrolyte, the prepared FeS2 NCs, despite their dimension far beyond the quantum confined regime, could achieve high utilization and reversibility as a cathode active material due to the well-defined crystal structure and the uncapped rough surfaces. Furthermore, we find that the last charging voltage step of FeS2 only contributes a minor capacity but caused severe capacity fading due to the formation of soluble polysulfides. By suppressing this step through setting a proper upper cut-off voltage, the cycle life of the Li/FeS2 cell is dramatically improved. The Li/FeS2 cell running over a voltage window of 1.0–2.4 V at 1C delivers an initial capacity of 486.1 mA h g−1, slightly lower than that running over 1.0–3.0 V (561.1 mA h g−1), but outperforms the latter substantially after 500 cycles (367 mA h g−1 vs 315 mA h g−1), corresponding to a capacity decay rate as low as 0.048% per cycle. Our results provide a meaningful approach for the development of not only the advanced FeS2 material for long-life rechargeable batteries, but also other transition metal chalcogenide nanomaterials for a variety of potential applications.

On the origin of 'iron-cross' twins of pyrite from Mt. Katarina, Slovenia

AbstractIron-cross twins of pyrite are well known among mineralogists, however it is quite surprising that the conditions of their formation remain unexplored. To address this question we studied pyrite twins from the Upper Permian silts of Mt. Katarina near Ljubljana (Slovenia), which represent one of the most typical geological environments for twinned pyrite. Mineralization of pyrite starts with a reduction of the primary red-coloured hematite-rich sediment by sulfide-rich fluids that penetrated the strata. A short period of magnetite crystallization is observed prior to pyrite crystallization, which indicates a gradual reduction process. Sulfur isotope analysis of pyrite shows an enrichment in δ34S, suggesting its origin from the neighbouring red-bed deposit. Other sulfides, such as chalcopyrite and galena, formed at the end of pyrite crystallization. Remnants of mineralizing fluids trapped at the interfaces between the inclusions and host pyrite show trace amounts of Pb and Cu, indicating their presence in the solutions throughout the period of pyrite crystallization. An electron microscopy and spectroscopy study of twin boundaries showed that interpenetration twinning is accomplished through a complex 3D intergrowth of primary {110} Cu-rich twin boundaries, and secondary {100} boundaries that are pure. We show that approximately one monolayer of Cu atoms is necessary to stabilize the {110} twin structure. When the source of Cu is interrupted, the two crystal domains continue to form {100} interfaces, that are more favourable for pure pyrite.