Pyrite Particles Research Articles

The influence of fragmentation on the behavior of pyrite particles during pulverized coal combustion

Coal combustion is a complex series of reactions, dominated by transport mechanisms. A poorly understood aspect is the influence that fragmentation of excluded pyrite minerals has on the combustion thermal history and sulfur release. To explore this effect, fragmentation was incorporated into a mathematical model consisting of several stages, namely: heating up the particle, decomposition to pyrrhotite, fragmentation, and oxidization. A computational fluid dynamic (CFD) model was used to verify the heat and mass transfer predictions of the model. The model was then solved to simulate the particle heat and mass balance of a pyrite particle in a combustion environment.Fragmentation was found to increase particle temperature and sulfur release rate compared with a no fragmentation case, speeding up the particle temperature increase towards its melting point. Increased oxygen content in the bulk environment decreases the time required for a particle to reach its melting point and for complete sulfur release due to increased oxidation reaction rates. Model results also indicate that small pyrite particles need less time to reach both decomposition and melting temperatures; suggesting that smaller particles will spend more time in a molten state and react more completely.

Benchmarking flotation performance: Single minerals

Chalcopyrite, conditioned with sodium dicresyl dithiophosphate (DTP), was floated under standard and well-defined hydrodynamic conditions. The advancing contact angle values of the flotation feed and products were measured and the flotation response benchmarked against a calibration previously established for the chalcopyrite-amyl xanthate (KAX) system. Furthermore, the flotation response of pyrite, separately conditioned with KAX and DTP, was also evaluated under the same hydrodynamic conditions. When the advancing contact angle of chalcopyrite conditioned with DTP was the same (within 5°) as that of chalcopyrite conditioned with KAX, the flotation response was, within experimental error, the same. For both chalcopyrite and pyrite, heterogeneity of the advancing contact angle within the feed size fractions was demonstrated by significant differences in contact angle values measured on the flotation concentrate and tailings size fractions. The mean contact angle of the chalcopyrite and pyrite particles remained constant, within experimental error, through both flotation and sample preparation under the test conditions. The flotation response of chalcopyrite at 2% solids (w/w) was the same, within experimental error, as that at 30% solids (w/w) in the presence of silicate gangue, suggesting non-interaction of this gangue mineral with chalcopyrite under the test conditions. The operational advancing contact angles inferred for pyrite using the calibration established for the chalcopyrite-KAX system were, however, lower than the measured feed advancing contact angles, while the maximum recovery of pyrite was also lower than for chalcopyrite for the same feed advancing contact angle values, in the contact angle range less than 80°. The difference in flotation response between these two minerals for the same feed contact angle values was interpreted in terms of a difference in critical contact angle value for stable bubble–particle attachment. The critical advancing contact angle values for the pyrite size fractions were higher than values for chalcopyrite for size fractions above 20 μm. This difference in critical advancing contact angle was attributed to the difference in mineral specific gravity between chalcopyrite and pyrite.

Interfacial phenomena in the sulfur–polytetrafluoroethylene system under hydrothermal conditions

Sulfur occlusion of pyrite particles during oxidative pressure leaching of mineral sulfide concentrates at temperatures below 160 °C has a deleterious effect on pyrite oxidation kinetics. Existing techniques in mitigating sulfur occlusion involve the use of surfactants or operation at temperatures above 160 °C which are a burden to the process economy. This study looks at the suitability of polytetrafluoroethylene (PTFE) to act as a solid sorbent for the liquid sulfur generated during the pressure leach process. The suitability of PTFE as a solid sorbent was determined by comparing interfacial phenomena in the corresponding sulfur–PTFE–leach and sulfur–mineral–leach systems. The work of adhesion in a sulfur–PTFE system was calculated using the interfacial tension and contact angle measured in the relevant sulfur–PTFE system, which is comparable to published values in the pyrite–sulfur system.

Jarosite pseudomorph formation from arsenopyrite oxidation using Acidithiobacillus ferrooxidans

In this work, the oxidizing action of a native strain type A. ferrooxidans on a sulphide containing a predominance of arsenopyrite and pyrite has been evaluated. Incubation of the A. ferrooxidans strain in flasks containing 200 mL of T&K medium with the ore (particle size of 106 μm) at pulp density 8% (w/v) at 35 °C on a rotary shaker at 200 rpm resulted in preferential oxidation of the arsenopyrite and the mobilization of 88% of the arsenic in 25 days. Mineralogical characterization of the residue after biooxidation was carried out with FTIR, XRD and SEM/XEDS techniques. An in situ oxidation of the arsenopyrite is suggested on the basis of the frequent appearance of jarosite pseudomorph replacing arsenopyrite, in which the transformations Fe 2+ → Fe 3+, S − 2 → S + 6 and As − 1 → As + 3 → As + 5 occur for the most part without formation of soluble intermediates, resulting in a type of jarosite that typically contains high concentrations of arsenic (type A-jarosite). However, during pyrite oxidation, dissolution of the constituent Fe and S predominates, which is evidenced by corrosion of pyrite particles with formation of pits, generating a type of jarosite with high quantities of K (type B-jarosite). Lastly, a third type of jarosite (type C-jarosite) also precipitated forming a thin film that covered the grains of pyrite principally.

In situ calcite formation in limestone-saturated water leaching of acid rock waste

The result of leaching of a 75% acid rock/25% limestone column with limestone-saturated solution has shown that the pH of the effluent recovered from 2.5, after apparent loss of acid neutralizing capacity after 4 years with water leaching, to pH 7 in less than 3 years. Bulk assay results, XRD and SEM/EDS analyses of samples from the column at 384 weeks (pH 3.6) and 522 weeks (pH 6.9) during this recovery have suggested that this is due to formation in situ of fine calcite. Calcite, initially blended to the column material at 25 wt.% was not found in the XRD of the 384 week sample but is clearly found in the 522 week XRD. This increased calcite content appears to be derived from the limestone-saturated water as finely divided solid precipitated in the drying cycles in the column. This result is confirmed by assessment of the 522 week sample as non-acid forming. Loss of some reactive aluminosilicate minerals, formation of secondary, precipitated, surface-attached gypsum and loss of fine secondary jarosite occurs across this pH range but fine, surface-attached jarosite is still found in the 522 week sample implying relatively slow dissolution kinetics. In comparison with the 384 week sample, armouring of highly reacted pyrite particles by surface layers of iron oxyhydroxides and aluminosilicates has become more extensive at 522 weeks after return of the pH to neutral values. This is consistent with results from Freeport field samples from limestone blended test pads where pyrite armouring was also substantially increased at higher pH. The results suggest that it may be possible to effectively maintain neutral pH and passivate pyrite, reducing oxidation rates by more than an order of magnitude, using limestone-saturated solution dump feed rather than bulk limestone blending or covers.

Adenine oxidation by pyrite-generated hydroxyl radicals.

Cellular exposure to particulate matter with concomitant formation of reactive oxygen species (ROS) and oxidization of biomolecules may lead to negative health outcomes. Evaluating the particle-induced formation of ROS and the oxidation products from reaction of ROS with biomolecules is useful for gaining a mechanistic understanding of particle-induced oxidative stress. Aqueous suspensions of pyrite particles have been shown to form hydroxyl radicals and degrade nucleic acids. Reactions between pyrite-induced hydroxyl radicals and nucleic acid bases, however, remain to be determined. Here, we compared the oxidation of adenine by Fenton-generated (i.e., ferrous iron and hydrogen peroxide) hydroxyl radicals to adenine oxidation by hydroxyl radicals generated in pyrite aqueous suspensions. Results show that adenine oxidizes in the presence of pyrite (without the addition of hydrogen peroxide) and that the rate of oxidation is dependent on the pyrite loading. Adenine oxidation was prevented by addition of either catalase or ethanol to the pyrite/adenine suspensions, which implies that hydrogen peroxide and hydroxyl radicals are causing the adenine oxidation. The adenine oxidation products, 8-oxoadenine and 2-hydroxyadenine, were the same whether hydroxyl radicals were generated by Fenton or pyrite-initiated reactions. Although nucleic acid bases are unlikely to be directly exposed to pyrite particles, the formation of ROS in the vicinity of cells may lead to oxidative stress.

Kinetics of pyrite to pyrrhotite reduction by hydrogen in calcite buffered solutions between 90 and 180°C: Implications for nuclear waste disposal

Abstract The kinetics of abiotic redox reactions induced by hydrogen are poorly documented although it represents a growing area of interest in terms of both nuclear waste storage assessment and the comprehensive study of hydrogen-rich fluid in mid-ocean ridge hydrothermal systems. We present an experimental kinetics study of pyrite reduction into pyrrhotite under significant H2 pressure and mid-hydrothermal conditions. We describe the mechanism and kinetic behavior of this reaction by combining textural and solution analyses under various conditions of temperature, pyrite particles size, H2 pressure and pH. When pH is controlled by calcite, the reaction presents all the characteristics of a coupled dissolution–precipitation mechanism occurring at the pyrite–pyrrhotite interface. By considering the chemical affinity of the coupled reaction as a function of reaction extent, we demonstrate that the spatial coupling is induced both by pyrite as a substrate for pyrrhotite nucleation and by the role of fluid chemistry at the reaction front. Far from equilibrium with respect to pyrite, the kinetics of sulfide production associated with the reaction are linearly related to the square root of time with an activation energy of 53 kJ/mol. This value is higher than what is expected for a diffusion-controlled kinetic regime. We suggest that the reaction rate is controlled both by pyrite reductive dissolution and by sulfide diffusion through the porous pyrrhotite microstructure. We provide a simple sulfide production-rate expression on the basis of our measured rate constants that can be used in geochemical modeling to further evaluate the impact of hydrogen on pyrite under nuclear waste disposal conditions.

Tunnel ferroviaire de Vierzy : vieillissement, altération des maçonneries calcaires

The Vierzy railway tunnel was excavated in 1859-1860 in cuisan sands below 38 m of essentially lutetian limestones. The accident is due to the collapse of the roof of the tunnel on a 15 m long section and of about 4 m thick Lower lutetian formations. The initial cause of the collapse is a defect, at construction time, in the keystone zone of the masonry of the circular vault, at the joint between two sections of excavation. In 1914 a war destruction with explosives created a collapse and a « cathedral» 50 m long, 17 m high, and a cone of 10,000 m 3 debris at 20 m of the 1972 collapse. The shaking provoked the sinking of Lower lutetian formations on the masonry, at a distance, Middle Lutetian staying as a slab. Fumes of steam trains deposited soots with coal and sulphides (pyrite) particles in voids of the ground and of the mansory up to in cavernous mortar. This permitted a sulfuric acid alteration, increased by bacterial processes, changing carbonates into soluble sulfates (gypsum). It lowered the resistance of the vault and permitted the fall of quarry stones. At the inner surface freezing caused a progressive fall of debris and reduction of thickness of the masonry. Thin facing rolls of bricks or concrete were placed here and there as a protection. The removing of a section of such a roll of bricks, altered, to be substituted with concrete was the final cause. One limit of the removed section coincides with one limit of the collapse. On June 16, 1972, two passenger trains crossed into the tunnel at a small interval and collided with the cone of debris. There were 108 deaths. It is the third most important catastrophe in French railway history.

Application of Acidithiobacillus Ferrooxidans in Coal Flotation

Bioflotation is a potential method for removing pyritic sulphur from coal. Sodium cyanide is a well-known depressant for pyrite in flotation of sulphide minerals; however, for coal this reagent is unacceptable from the environmental point of view. This study investigates an alternate to sodium cyanide; Acidithiobacillus Ferrooxidans, a nonharmful bacterial reagent as a pyrite depressant. The flotation behavior of pyrite and other gangue particles using the sodium cyanide and the Ferrooxidans is compared by applying the general first-order flotation model. The kinetic parameters extracted from the model demonstrated that the modified flotation rate of pyrite was reduced, and the selectivity between coal and gangue was improved using the bacteria. These results indicate that Acidithiobacillus Ferrooxidans has potential in removing pyritic sulfur from coal.

The influence of pyrite grain size on the final oxygen isotope difference between sulphate and water in aerobic pyrite oxidation experiments†

Oxidation experiments with different pyrite grain sizes (63–100, 100–140, 140–180 μm) were carried out to investigate the oxygen and sulphur isotope composition of sulphate produced under aerobic acid conditions, which may help to understand oxidation mechanisms and to interpret data from natural sites. Long-term experiments with grain size 63–100 μm showed that constant δ 18OSO4 values were not achieved before 100 days. The final oxygen isotope difference between water and sulphate indicates that a small proportion of molecular oxygen is incorporated into sulphate even in the later course of the oxidation due to sulphite oxidation by molecular oxygen. However, most of the sulphate oxygen derives from water. Similar δ 18OSO4 values from experiments with grain sizes 63–100, 100–140, and 140–180 μ m indicate similar oxidation mechanisms for all three grain sizes. These results differed from previous results of identical experiments with grain size<63 μ m, where higher δ 18OSO4 values were obtained. We propose that the greater proportion of molecular oxygen in sulphate from oxidised fine-grained pyrite is caused by an intensified adsorption of molecular oxygen on sulphur sites of ultrafine pyrite particles. Hence, the formation of sulphate from the (initial) reaction on sulphur sites of pyrite and from sulphite oxidation should be more dominant if ultrafine material is present. The δ 34SSO4 values (2.0–2.7) obtained from experiments with the coarser grain sizes agreed with the δ 34S value of pyrite (2.4), whereas sulphur isotopes of sulphate obtained from previous experiments with fine-grained pyrite showed an initial 32S enrichment compared with pyrite. Due to the lack of δ 34SSO4 values from the beginning of the experiments with coarser grain sizes, it remains speculative that sulphur isotopes indicate at least initial differences in oxidation mechanisms between fine and coarser pyrite grain sizes.

Transformation behaviors of excluded pyrite during O2/CO2 combustion of pulverized coal

AbstractThe article was addressed to the transformation of excluded pyrite during O2/CO2 combustion of pulverized coal. Raw pyrite mineral was added to a pulverized coal sample, which was density fractionated to remove the excluded minerals, to simulate the excluded pyrite present in coal. The mixed sample was burned in a drop tube furnace in O2/CO2 and O2/N2 conditions to generate the residue ash, which was characterized by Mössbauer spectroscopic and size analyses. It was found that, in comparison with O2/N2 combustion at the same oxygen concentration, slightly less iron glass silicate was formed from excluded pyrite and silicates although the transformation of pyrite to oxides was slowed in O2/CO2 combustion, different from the behaviors of included pyrite in pulverized coal those were observed in previous study. Additionally, less fragmentation of excluded pyrite particles was also observed in O2/CO2 combustion. Copyright © 2009 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Nanosecond electrical discharges between semiconducting sulfide mineral particles in water

A model for developing electric discharges between sulfide mineral (pyrite) particles under high-voltage nanosecond pulses in a liquid medium (water) is considered. A possibility of electrical breakdowns of liquid gaps between particles under nanosecond pulses is shown. This probability and the energy released in discharge channels depend strongly on the sulfide conductivity.

Energy concentration in electric discharges between particles of semiconducting sulfide minerals under the action of high-power nanosecond pulses

A model for development of electric discharges between particles of sulfide minerals (pyrite) under the action of high-voltage nanosecond pulses is proposed. It is shown that through discharges in a layer of pyrite particles lead to energy concentration in small contact regions between particles; the concentrated energy is sufficiently high for local decomposition (disintegration) of mineral complexes.

Selenide retention onto pyrite under reducing conditions

Summary Pyrite (FeS2) is a mineral phase often present as inclusions in temperate soils. Moreover, it turns out to be a sorption sink for certain radionuclides in deep geological disposals. The present study was thus initiated to determine the capacity of pyrite to immobilize selenide (Se(-II)). Due to the fact that pyrite surface oxidizes readily, potentials were applied in order to minimise its surface evolution, and ensure the reducing conditions necessary for stabilizing Se(-II). The sorption experiments were carried out in NaCl electrolyte and were amperometrically controlled. After only several minutes of reaction, at least 97% of Se(-II) initially present in solution was disappeared. The K d values vary from 7–65 L/g and the isotherm curve shows site saturation at higher initial selenide concentrations and no pH-dependence. By means of several spectroscopic techniques, the reaction mechanism was investigated. The XRD and in situ XANES results indicate the presence of Se(0) on pyrite surface, which explain the rapid disappearance of Se observed in the sorption experiments. Moreover, XPS results obtained from Se-reacted pyrite particles reveal cleavage of S–S bonding which resulted in formation of S2− on pyrite surface. Thus, we conclude that Se(-II) can be immobilized by pyrite via surface redox reaction: ≡FeS2 + HSe− ⇔ ≡FeS + Se(0) + HS−.

Electrochemical study of hydrothermal and sedimentary pyrite dissolution

The dissolution of pyrite is of interest in the formation of acid mine drainage and is a complex electrochemical process. Being able to measure the rate of dissolution of particular pyrite samples under particular conditions is important for describing and predicting rates of AMD generation. Electrochemical techniques offer the promise of performing such measurements rapidly and with small samples. The oxidation of pyrite and the reduction of Fe3+ ions and/or O2 half reactions involved in the pyrite dissolution process were investigated by cyclic voltammetry and steady-state voltammetry using three pyrite materials formed in both sedimentary and hydrothermal environments. For each sample, two kinds of pyrite working electrodes (conventional constructed compact solid electrode, and carbon paste electrode constructed from fine-grained pyrite particles) were employed. Results indicated that for both the hydrothermal and sedimentary pyrite samples the oxidation and reduction half reactions involved in dissolution were governed by charge transfer processes, suggesting that hydrothermal and sedimentary pyrites obey the same dissolution mechanism despite their different formation mechanisms. In addition, the results showed that it is feasible to use a C paste electrode constructed from fine-grained or powdered pyrite to study the pyrite dissolution process electrochemically and to derive approximate rate expressions from the electrochemical data.

Carrier-microencapsulation using Si–catechol complex for suppressing pyrite floatability

Pyrite (FeS 2) is a common sulfide mineral associated with valuable metal minerals and coal, and it is rejected as a gangue mineral using physical separation techniques such as froth flotation and discharged into tailing pond. In the flotation, pyrite is frequently entrapped in the froth due to its hydrophobic nature. Formation of acid mine drainage due to the air-oxidation of pyrite in the tailing pond is also a serious problem. The authors have proposed carrier-microencapsulation (CME) as a method for suppressing both the floatability and oxidation of pyrite. In this method, pyrite is coated with a thin layer of metal oxide or hydroxide using catechol solution as a carrier combined with metal ions. The layer converts the pyrite surface from hydrophobic to hydrophilic and acts as a protective coating against oxidation. The present study demonstrates the effect of CME using Si–catechol complex to suppress the pyrite floatability: The bubble pick-up experiments showed that attachment of pyrite particles to air bubble is suppressed by the CME treatment at pH 4–10, Si–catechol complex concentration over 0.5 mol m −3 and treatment time within 2 min. The Hallimond tube flotation experiments showed that the pyrite floatability is suppressed by the CME treatment even in the presence of typical flotation collectors such as kerosene and xanthate. SEM-EDX analysis confirmed that Si present on the pyrite surface treated by Si–catechol complex, implying that SiO 2 or Si(OH 4) layer formed by the CME treatment convert the pyrite surface hydrophobic to hydrophilic.

Positron emission particle tracking as a method to map the movement of particles in the pulp and froth phases

Positron emission particle tracking (PEPT) is a method of detecting the position of particles in vessels over a period of time. It has been used effectively in a number of mixing applications, and this work introduces the concept to the froth flotation process. Using PEPT, it was possible to map the position of a pyrite particle as it mixed in the pulp phase of a Denver Cell batch flotation experiment. Once the air flow was switched on, it was then possible to observe the particle entering the froth after attaching to an air bubble, and observing the detachment from the froth, re-entry into the pulp phase and then re-attachment to another bubble leading to the particle entering the froth once again. From this work, PEPT has the potential to become a powerful tool in order to understand exactly what happens in flotation practice, and to enable more accurate mathematical models to be developed.

Thermoanalytical study on the oxidation of sulfide minerals at high temperatures

An experimental study on the oxidation of chalcopyrite (CuFeS2), pyrite (FeS2), covellite (CuS) and chalcocite (Cu2S) particles was conducted by means of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques. The oxygen concentration in the process gas was set to 40% and 70% (by volume) and a heating rate of 40°C/min was used. Response variables included the temperature of incipient reaction, the total exothermic heat of reaction, particle mass, morphology and mineralogy of the reacted particles. Based on the experimental data and phase stability calculations, reaction mechanisms were proposed to represent the behavior of the particles during oxidation. The reaction mechanisms were verified by X-ray diffraction (XRD) analyses of the oxidized particles and by thermo chemical and mass balance calculations to reproduce the DSC and TGA data. Overall kinetic models were developed to represent the evolution of the exothermic heat of reaction as a function of time.

A method for generating uniform size-segregated pyrite particle fractions

BackgroundStandardized sample preparation techniques allow comparison of pyrite dissolution experiments under diverse conditions. Our objective was to assess dry and wet sieving preparation methodologies, and to develop a reproducible technique that yields uniformly size-distributed material within a limited size range of interest.ResultsHere, we describe a wet sieving preparation method that successfully concentrates pyrite particles within a 44–75 μm diameter range. In addition, this technique does not require a post-processing cleanup step to remove adhering particles, as those particles are removed during the procedure. We show that sample preparation methods not only affect the pyrite size distribution, but also apparent dissolution rates.ConclusionThe presented methodology is non-destructive to the sample, uses readily available chemical equipment within the laboratory, and could be applied to minerals other than pyrite.

Computational fluid dynamics (CFD) simulation of flow in the rotary drum for pyrite bio-preoxidization

A two-dimensional algebric slip mixture (ASM) CFD model was developed to simulate the flow and mixing of liquid–gas-particle system in a rotary drum. The simulations show that there are several flow regimes in slurry phase, including anti-clockwise circulation flow of liquid, rising stream of bubbles, circumferential flow and eddy. The gas disperses mainly in a limited area in the vertical region of bubble stream. Pyrite particle mixing is mainly arised from the movement of baffles, and the distribution of particles is well. The increase of aeration rates would give rise to an increase of the volumetric averaged gas holdup. The increase of rotation speed has little influence in the averaged gas holdup. When three gas spargers were molded in the drum, there is not obvious improvement of gas holdup and particle mixing.