Coal Pyrite Research Articles

Role of inherent pyrite in coal on physicochemical structure of activated carbon and adsorption capacity

Pyrite is the most common inorganic iron-bearing mineral in coal. In order to reveal the influence of inherent pyrite on physicochemical structure and adsorption performance of AC, in this study, different content of pyrite (0.5%, 1%, 2%, 3%) were physically mixed with Yangcheng coal and briquetted, carbonized and steam activated for activated carbon (AC) preparation. As-obtained ACs’ morphology, pore texture, carbon crystallite structure and surface chemistry were characterized by SEM, BET, XRD and FTIR, respectively. The results show that pyrite (FeS2) transformed into hematite (Fe2O3) during AC preparation. Pyrite promotes the interaction between water vapor and carbon and enhances the rate of gasification reactivity by inhibiting the growth of heavy aromatic structures, catalyzing the graphitization process and enhancing the surface chemical properties. The total specific area and specific surface area of micropore of 2%FeS2/AC are up to 836 m2/g and 644 m2/g, which are 58.6% and 46.9% higher than AC, respectively. The 2 h sulfur capacity of 2%FeS2/AC is up to 65.2 mg/g, which is 18.8% greater than that of AC (54.9 mg/g), the mechanism of which was proposed: absorbed gaseous SO2 finally transformed into H2SO4 in macropores and forming an acid macro-environment where most Fe2O3 attacked by the generated H2SO4 on the AC’s pore surface to form Fe2(SO4)3. Fe3+ can promote the oxidation of SO3 to form H2SO4, and a redox cycle of Fe2+ and Fe3+ was established. Moreover, 1%FeS2/AC shows the highest Rhodamine B (RhB) adsorption, and the capacity of RhB is 168 mg/g, which is 41.2% higher than that of AC.

Kilometre-scale compartmentalization of fluid sources to a fossil hydrothermal system

The West Cumbria iron ore field exhibits kilometre-scale compartmentalization of fluid sources, as evidenced by sulphur isotope data. Barite accompanying haematite ore from 13 mines in an along-strike width of 5 km has δ34S(VCDT) isotopic compositions ranging from +7.9 to +23.6‰. The large variation is strongly controlled by NNW-SSE faulting, in which the heaviest compositions are to the north and lightest in the south. The heavy and light compositions are comparable with those of Carboniferous and Permian evaporites respectively, both of which occur in West Cumbria and are the assumed primary sources of the barite sulphur, though input from the oxidation of Carboniferous coal pyrite may have provided a minor component. The derivation of sulphur from stratigraphically distinct sources shows that faults isolated fluid flow in compartments on a local scale. Published data for barite from an adjacent fault block in a potential waste disposal site at Sellafield are further distinct, indicative of a primary pyrite source. These data emphasize that fluid sources can be highly compartmentalized in a faulted system and require supporting evidence to confirm if they can be correlated.

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.

The partitioning and environmental characteristics of trace elements in a coal-fired power plant: a case study at Huaibei, China

The trace elements (TEs) in coal are of great concern due to their toxicity and bioavailability. The coal, bottom ash, fly ash and gaseous species were sampled to study the partitioning and environmental behaviors of TEs during coal combustion. The mineral and phase characterizations of coal combustion residue were determined by X-ray powder diffraction (XRD) and fourier transform infrared spectroscopy (FTIR), respectively. The gaseous emission, risk assessment code and leaching experiment EN 12457 Part 2 procedure were employed to assess the potential environmental impacts. The concentrations of TEs in all the collected samples were determined by inductively coupled plasma mass spectrometry (ICP-MS). During combustion, the mineral compositions of coal (kaolinite, quartz, illite, pyrite and calcite) were changed to mullite, quartz, hematite, lime, magnetite and anhydrite. Arsenic, Cd, Cu, Pb and Zn were emitted mainly in fly ash and gaseous phases. In the current unconfined deposits, the TEs (As, Cu, Pb and Zn) in bottom ash and fly ash may pose a medium risk to the environment. High regularization and management countermeasures including immobilization/stabilization, re-utilization of combustion ash, and controlled disposal should be conducted to eliminate/minimize environmental and ecological risks in coal activities areas.

Coal mining‐derived ochres in the UK: a potential selenium trap

Thanks to the pioneering research of Paul Younger over the past 20 years, acid mine drainage in the UK has been recognized as a major environmental issue. Acid mine drainage and hydrous ferric oxide deposition are environmental hazards resulting from centuries of extensive coal mining activities across the UK. Oxidative weathering of pyrite in coal from spoil heaps and exposed bedrock can liberate trace elements, releasing them into local water systems. In addition to posing an environmental threat through water and ground contamination, ochres can also act as a remediation material, trapping elements such as selenium. Trace elements with a close association to iron oxides, such as selenium, may fix to the fine‐grained ochre materials, resulting in hyper‐enriched ochres. Selenium in coals has been known to cause an environmental issue in areas of North America but is also an important commodity for solar cells and nanotechnologies. Coal‐bearing areas of the UK, such as Northumberland, are known to contain a high selenium content, and coal‐derived ochres in these regions also contain significant selenium. The widespread occurrence of ochres in UK coal‐mining regions may therefore present a unique ‘E tech’ trace element source and prevent a toxicity problem for which they were once thought to be responsible.

Research on the Effect of Carbon Defects on the Hydrophilicity of Coal Pyrite Surface from the Insight of Quantum Chemistry.

To investigate the effect of carbon defects on the hydrophilicity of the whole surface of the coal pyrite, the adsorption of the single H2O molecule at different sites of the coal pyrite surface was studied with the DFT calculation. It was found that, like the ideal pyrite, the single H2O molecule can stably adsorb at the doping-position, the ortho-position and the meta-position of the coal pyrite. The covalent bond and anti-bond were formed between O (water molecule) and Fe (the coal pyrite) through the Fe 3d orbital and O 2p orbital. Meanwhile, the S–H bond was replaced by the C–H bond. But away from the carbon defect centre, the adsorption of the single H2O molecule increased gradually and the Fe–O covalent bond strength between the single H2O molecule and the pyrite strengthened, which eventually became close to that of the undoped coal pyrite surface.

Flotation behavior of pyrite in sub-bituminous and meta-bituminous coals with starch depressant in a microflotation cell

Removal of pyrite from coals is very important to reduce the emission of SO2 during coal combustion. The selective flotation of pyrite from coal is challenging due to the hydrophobic nature of both coal and pyrite particles. The surface properties of coal vary significantly with the degree of coalification. The present paper investigates the flotation behavior of pyrite from sub-bituminous and meta-bituminous coal which has different ranks and surface properties. XRD, XPS, contact angle, UV–visible spectroscopy, SEM and zeta potential measurements were used to characterize their differences in mineralogy and surface chemistry. The classical first-order model and the potential flow model were used for flotation kinetic studies and to back-calculate the induction times, respectively. Efficient selective flotation of pyrite from the sub-bituminous and meta-bituminous coal was observed in the presence of potassium amyl xanthate collector and starch depressant. The flotation recoveries of pyrite from the sub-bituminous coal and the meta-bituminous coal were over 95%, with pyrite grade of over 95% in the concentrates.

Mechanism of coal electrochemistry desulfurization under alkaline conditions

To study the desulfurization mechanism of coal-derived pyrite and organic sulfur in coal, the experiment used the solution of NaOH whose concentration is 2.0mol/L as electrolyte to dispose high sulfur coal samples whose content is 3.08% with the H type undivided electrolytic cell, and analyzed the before and after desulfurization of coal samples by scanning electron microscope, XRF and IR spectrum and the filtrate is from after electrolyzing coal sample by ion chromatograph. The results showed that the inorganic sulfur of coal sample mainly in the form of Pyrite that is a fine granular distribution in the coal and disseminates on the coal particle, then pyrite is removed by generating sulfate at the anode oxidation reaction, and finally, organic sulfur in coal samples mainly is removed by electrochemical reaction that can convert into the form of sulfate radicals.

Chemical Forms of Mercury in Pyrite: Implications for Predicting Mercury Releases in Acid Mine Drainage Settings.

Pyrite (cubic FeS2) is the most abundant metal sulfide in nature and also the main host mineral of toxic mercury (Hg). Release of mercury in acid mine drainage resulting from the oxidative dissolution of pyrite in coal and ore and rock resulting from mining, processing, waste management, reclamation, and large construction activities is an ongoing environmental challenge. The fate of mercury depends on its chemical forms at the point source, which in turn depends on how it occurs in pyrite. Here, we show that pyrite in coal, sedimentary rocks, and hydrothermal ore deposits can host varying structural forms of Hg which can be identified with high energy-resolution XANES (HR-XANES) spectroscopy. Nominally divalent Hg is incorporated at the Fe site in pyrite from coal and at a marcasite-type Fe site in pyrite from sedimentary rocks. Distinction of the two Hg bonding environments offers a mean to detect microscopic marcasite inclusions (orthorhombic FeS2) in bulk pyrite. In epigenetic pyrite from Carlin-type Au deposit, up to 55 ± 6 at. % of the total Hg occurs as metacinnabar nanoparticles (β-HgSNP), with the remainder being substitutional at the Fe site. Pyritic mercury from Idrija-type Hg deposit (α-HgS ore) is partly divalent and substitutional and partly reduced into elemental form (liquid). Divalent mercury ions, mercury sulfide nanoparticles, and elemental mercury released by the oxidation of pyrite in acid mine drainage settings would have different environmental pathways. Our results could find important applications for designing control strategies of mercury released to land and water in mine-impacted watersheds.

A novel high-sulfur fine coal clean desulfurization pretreatment：Microwave magnetic separation, high-gradient effect and magnetic strengthen

A new type of fine coal dry separation technology was investigated. Microwave energy was used to improve the magnetic properties of fine coal with a high sulfur content collected from different regions, and a magnetic medium was added to enhance the microwave energy absorption. The enhancement of magnetic desulfurization of −0.5 mm coal using microwave energy was studied, and the gradient effect of secondary magnetic separation of magnetic particle chain on the enhancement mechanism of desulfurization efficiency was evaluated. Microwave energy was used to strengthen the magnetic properties of coal pyrite, leading to high-gradient magnetic separation. In the X axis, demagnetizing field and background magnetic field of particles with different sizes interact near magnetic particles, producing a low magnetic field in the internal region of particles. Further, the change in the internal magnetic field of the entire particle is consistent with the background magnetic field in the positive direction along the X axis. The optimum sulfur content of clean coal in the two types of high-sulfur coal was obtained when 5% magnetic medium was added under a pretreatment time of 180 s, and the sulfur contents of clean coal were 3.12% and 2.08%, respectively. The clean coal yield of the two types of high-sulfur coal decreased by increasing the desulfurization rate, and the optimum media content used for the desulfurization of the two types of high-sulfur coal by rare-earth magnetic separator was 5%. A high-intensity magnetic separator was used for the separation of fine coal, and an effective way to achieve the clean utilization of pulverized coal before its combustion was investigated.

Thermal phase transition of pyrite from coal

The thermal phase transition of pyrite from coal was studied using thermogravimetry and derivative thermogravimetric, infrared spectroscopy and X-ray diffraction (XRD). According to the XRD results, the phase transition of the pyrite starts at 350 °C, pyrrhotite and magnetite are intermediate products of the oxidation from pyrite to hematite. It was proposed that the main gases and volatile products released during the thermal decomposition of the pyrite are water (H2O) and sulfur dioxide (SO2). The existence of sulfate species in the roasted samples also verified the emissions. These results proved the phase transition of pyrite and explain that they have a rational relationship. The environmental impact related to the thermal phase transition has been discussed, although the pyrolyzation in natural environment is really a very complicated process, it is still able to understand the thermal habits of pyrite in coal by studying pyrite from coal and control surrounding conditions so as to reduce and improve the environmental impact of sulfur pollution.

Alternate use of sulphur rich coals as solar photo-Fenton agent for degradation of toxic azo dyes

Abstract Low grade coals with high sulphur contents are not suitable as fuels in thermal power plants due to generation of high ash content and emission of toxic sulphur oxide gases. Here we have demonstrated an alternate use of pyrite rich coal as a source for sustainable generation of hydrogen peroxide and hydroxyl radicals in aqueous medium via solar light induced photo-Fenton process which were utilized for degradation of toxic azo-dyes. The textural, compositional and morphological features of pristine pyritic coals with varying sulphur concentrations are studied by an array of characterization techniques. The dye degradation capacities of pyrite rich coal are studied using model dye solutions e.g., Basic Blue 41 containing single azo group per unit and Trypan Blue containing two azo groups per unit. The rate of degradation (k) is faster for Basic Blue-41 (0.0192 min−1, ∼99% degradation) at optimized solar radiation exposure time, pH 6 and dose of 1 g/L pyritic coal. The dye degradation is presented as a measure of solar radiation accumulation energy per unit volume, and it is attributed to in-situ generation of hydrogen peroxide and hydroxyl radicals, and re-confirmed from scavenging studies. The dye degradation mechanism by pyritic coal is discussed in the light of solar photo-Fenton mechanism with respect to (1) qualitative identification of trace quantities of degradation products by gas chromatography-mass spectrometry technique; (2) leaching of 2700 mg/L iron in the reaction medium, estimated by inductive coupled plasma optical emission spectrometry, and (3) and changes in the chemical speciation of iron, sulphur and oxygen in the used coal by X-ray photoelectron spectroscopy.

High selenium in the Carboniferous Coal Measures of Northumberland, North East England

The Carboniferous Lower and Middle Coal Measures coals of the Northumberland Coalfield are anomalously rich in selenium (Se) content (up to 62 ppm) compared to the averages for the common UK and worldwide coals. As well as posing an environmental toxicity threat, Se is now regarded as an important resource for alloys, photovoltaic products and nanotechnologies, and high Se coals in North East England offer an opportunity to assess the Se means of occurrence, origins, transport mechanisms and enrichment in coals. At least two generations of pyrite host high Se in the sampled coal seams: microbial-formed disseminated pyrite (both cubic and framboidal in habit) and later cleat-filling pyrite, identified by petrographic observations, laser ablation methods and sulphur isotope compositions. There is a notable Se enrichment of up to 250 ppm in later formed cleat-filling pyrite. Trace element enrichment may have been sourced and influenced by seawater distribution during diagenesis, and localised dykes and deformation may have acted as an enrichment mechanism for sampled seams in the region. The high Se coals in Northumberland may provide a potential E tech element source and should be considered and carefully managed as coal mining and production are reduced in the area. The study also highlights the nature of Se enrichment in pyritic coals affected by cleat formation and multiple episodes of mineralisation, important as critical element demand continues to increase worldwide.

Studies on mercury occurrence in inorganic constituents of Polish coking coals

During the cokemaking process, a significant amount of mercury occurring in a coal blend is released to the atmosphere. One of the ways of reducing this emission is to reduce mercury content in a coal blend. This could be obtained through the coal washing process. The optimization of this process requires the knowledge of mercury occurrence in coal, especially in its inorganic constituents. A qualitative analysis of mercury occurrence in the inorganic constituents of Polish coking coals was performed using an electron probe microanalyzer (EPMA). For that purpose, selected samples of rejects and middling products derived from the washing process in dense media separators and jig concentrators were examined. The obtained results have confirmed a strong connection between mercury occurrence and the presence of sulfides (pyrite, marcasite, and chalcopyrite) in Polish coking coals. Significant amounts of mercury were also noticed for barite, siderite, and aluminosilicates. The highest value of mercury content, at the level of 0.100%, was obtained for marcasite. For the analyzed coals, the effectiveness of mercury removal in the washing process was determined by the forms of pyrite occurring in coal. The highest values of effectiveness of mercury removal were obtained in the case of coals for which the large framboidal pyrite aggregates with chalcopyrite overgrowths were noticed. It was also found that middling products were characterized by the occurrence of the Hg-rich overgrowths of pyrite on organic matter. To achieve a significant reduction in mercury content in clean coal, it is necessary to develop an effective method of removing this form of pyrite from hard coal.

Sequential Transformation Behavior of Iron-Bearing Minerals during Underground Coal Gasification

Detailed mineralogical information from underground coal gasification (UCG) is essential to better understand the chemical reactions far below the surface. It is of great scientific significance to study the mineral transformation and identify the typical minerals in certain process conditions, because it may help to ensure the stable operation of gasification processes and improve the utilization efficiency of coal seams. The transformation of iron-bearing minerals has the typical characteristics during the UCG process and is expected to indicate the process parameters. In this paper, UCG progress was subdivided into pyrolysis, reduction and oxidation stages, and the progressive coal conversion products were prepared. Two types of lignite with different iron contents, Ulankarma and Ulanqab coals, were used in this study. The minerals in the coal transformation products were identified by X-ray diffraction (XRD) and a scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic calculation performed using the phase diagram of FactSage 7.1 was used to help to understand the transformation of minerals. The results indicate that the transformation behavior of iron-bearing minerals in the two lignites are similar during the pyrolysis process, in which pyrite (FeS2) in the raw coal is gradually converted into pyrrhotite (Fe1−xS). In the reduction stage, pyrrhotite is transformed into magnetite (Fe3O4) and then changes to FeO. The reaction of FeO and Al2O3 in the low iron coal produces hercynite above 1000 °C because of the difference in the contents of Si and Al, while in the high iron coal, FeO reacts with SiO2 to generate augite (Fe2Si2O6). When the temperature increases to 1400 °C, both hercynite and augite are converted to the thermodynamically-stable sekaninaite.

Fine coal desulfurization and modeling based on high-gradient magnetic separation by microwave energy

Fine coal separation is one of the research focuses in the field of clean coal technology. A new type of fine coal dry separation technology was studied. Microwave energy was used to strengthen the magnetic properties of coal pyrite, leading to high- gradient magnetic separation. A high-intensity magnetic separator was used for the separation of fine coal, and an effective way to achieve the clean utilization of pulverized coal before its combustion was investigated. The R2 and corrected R2 values of the two-order model according to the sulfur content of clean coal were small. This shows that the recommended model has a higher fitting degree of response surface. Microwave pretreatment time T has a higher sensitivity to the sulfur content of clean coal than MC in a given range. The response surface has an optimal value, i.e., the highest desulfurization rate. With the increase in magnetic particle size, the desulfurization rate increased continuously. The evaluation index of the clean coal sulfur content of fine coal microwave-enhanced magnetic desulfurization test was optimized by a better two-order fitting model. The constraint conditions of the main influencing factors and the optimization indexes after the response were obtained. The sulfur content of clean coal was in the range of [1.98, 3.35].

Tellurium Enrichment in Jurassic Coal, Brora, Scotland

Mid-Jurassic pyritic coals exposed at the village of Brora, northern Scotland, UK, contain a marked enrichment of tellurium (Te) relative to crustal mean, average world coal compositions and British Isles Carboniferous coals. The Te content of Brora coal pyrite is more than one order of magnitude higher than in sampled pyrite of Carboniferous coals. The Te enrichment coincides with selenium (Se) and mercury (Hg) enrichment in the rims of pyrite, and Se/Te is much lower than in pyrites of Carboniferous coals. Initial pyrite formation is attributed to early burial (syn-diagenesis), with incorporation of Te, Se, Hg and lead (Pb) during later pyrite formation. The source of Te may have been a local hydrothermal system which was responsible for alluvial gold (Au) in the region, with some Au in Brora headwaters occurring as tellurides. Anomalous Te is not ubiquitous in coal, but may occur locally, and is detectable by laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS).

Coal pyrite microwave magnetic strengthening and electromagnetic response in magnetic separation desulfurization process

Coal desulfurization is imperative to creating clean coal and reducing harmful emissions during the burning process. Due to the lack of effective technology in the field of fine coal dry separation, a novel microwave-enhanced magnetic property processing method was demonstrated in this manuscript. Using representative simulated minerals, the response and absorption characteristics of coal pyrite were intuitively demonstrated while analyzing the selective absorption mechanisms of different minerals in fine coal. Results show that the coal pyrites from Inner Mongolia were reduced by approximately 20% after 60s of microwave treatment, and 60% after 90s. The binding energy and chemical shift were quite large when the oxidation state was high. Fe2+ and troilite FeS were oxidized to Fe2O3 after 60s microwave treatment, and to Fe3+ after 90s. Dynamic equilibrium was reached after 3min of microwave treatment. The conversion rate was lower for pyrite to pyrrhotite than for pyrrhotite to troilite. The conversion of pyrrhotite to troilite appeared to be sensitive to the amount of microwave treatment. The specific susceptibility dropped rapidly and was nearly an order of magnitude greater than the raw coal. This study indicates a high possibility for coal desulfurization through this dry method enhanced with microwave energy.

Removal of nitric oxide using combined Fe II EDTA and coal slurry in the presence of SO 2

Abstract Nitric oxide was removed by aqueous coal slurry containing FeIIEDTA in the presence of SO2 using a bubbling reactor. The effects of NO and O2 concentrations, temperature and pH on NO removal and coal pyrite leaching were studied. NO removal efficiency and coal pyrite leaching rate increased with the increase of NO concentration. The overall effect of high oxygen concentration is unfavorable to the removal of NO. The reduction rate of the complexed NO by coal pyrite increased with increasing temperature up to 60 °C, resulting in higher NO removal efficiency and coal pyrite leaching rate. Further increase in temperature was unfavorable to NO removal and coal pyrite leaching because the absorption of NO became the rate-determining step. The optimal pH was determined to be about 5.8. When pH was greater than 5.8, the reduction of FeIIEDTA-NO was inhibited, and therefore the NO removal efficiency decreased with further increase of pH. The reduction of FeIIEDTA-NO by coal pyrite played a significant part in coal pyrite leaching. Nevertheless, the leaching of coal pyrite was still controlled by chemical reaction.

Influence of hydrothermal dewatering on trace element transfer in Yimin coal

Abstract In this study Yimin lignite coal from Inner Mongolia was taken for investigating the influence of hydrothermal dewatering (HTD) on trace elements (Hg, As, Se) under different conditions. First, the coal samples were treated through HTD process at a temperature range of 200–300 °C, then proximate analysis was performed to check changes in the coal. Meanwhile, inductively coupled plasma-mass spectrometry and atomic fluorescence spectrometry were used to detect the concentrations of trace elements. Furthermore, minerals in samples were analyzed by powder X-ray diffraction (XRD). Sequential chemical extraction procedure (SCEP) was also performed to determine the occurrence modes of trace elements. The results showed that HTD treatment is effective in removing moisture and upgrading coal rank, and the operating temperature should be in good control at a certain range due to the loss of volatile mass. Minerals in this type of coal mainly include quartz and kaolinite, and HTD may have an effect on removing minor minerals muscovite and siderite. HTD can remove all three elements mentioned above. The highest removal rate detected is about 15% for As, 45% for Hg and 43% for Se. The experimental result shows that occurrence mode of Hg and As exist mainly in pyritic fraction while Se mainly in pyritic fraction and organic fraction. During HTD, all fractions of Hg and Se decrease obviously. However, for As, the decreasing amount in carbonate and pyritic fractions is much smaller than that in organic and silica bond fractions. It can be inferred that pyrite in coal may not decompose but only lose the bonds with trace elements. Pyrolysis behavior of coal and the strong solubility of sub-critical water should be responsible for the removal of trace elements.