Search Results (145)

Understanding the formation of high-total-dissolved-solids (TDS) groundwater is essential for mine-water source identification, treatment, and resource utilization in arid and semi-arid coal mining areas. However, previous studies have commonly focused on individual aquifers and have not adequately explained the hydrochemical differentiation and evolutionary relationships within shallow-to-deep multi-aquifer systems. Taking the Xiaojihan Coal Mine in northern Shaanxi as a case study, 90 surface-water and groundwater samples were analyzed using self-organizing maps (SOM), hydrochemical diagrams, major-ion ratios, chlor-alkali indices, mineral saturation indices, X-ray diffraction data, and permeability-TDS relationships. SOM identified three hydrochemical units broadly corresponding to shallow surface water and groundwater from the Quaternary and Luohe formations, groundwater from the Anding Formation, and deep groundwater dominated by the Zhiluo and Yan’an formations. Their mean TDS concentrations increased from 348.69 to 1341.80 and 2510.00 mg/L, respectively. Groundwater evolved from low-TDS, HCO₃-Ca-dominated shallow water to high-TDS, SO₄-Ca/Na-rich deep water. Shallow groundwater was mainly controlled by carbonate and silicate weathering, whereas deep groundwater was increasingly affected by prolonged water-rock interaction, gypsum and anhydrite dissolution, pyrite oxidation, and reverse cation exchange. The increase in deep-groundwater TDS was primarily associated with the enrichment of SO₄²⁻, Na⁺ + K⁺, and Ca²⁺. Lower permeability with depth slowed groundwater circulation, prolonged residence time, and enhanced mineralization. XRD data confirmed the occurrence of exchange-active clay minerals, while saturation indices showed that carbonate minerals were generally near saturation to supersaturated, whereas gypsum, anhydrite, and halite remained undersaturated and retained dissolution potential. These findings clarify the shallow-to-deep evolution mechanism of high-TDS groundwater and provide a scientific basis for mine-water source identification and targeted management in arid and semi-arid coal mining areas. Full article

In the Afşin–Elbistan coal seam in Türkiye, this study investigates the vertical distribution, enrichment features, and mineralogical controls of trace elements and rare earth elements (REEs). Thirty coal samples were collected vertically from the Kışlaköy open-pit mine. Major oxide concentrations were determined by XRF for 29 samples, whereas trace element concentrations were determined by ICP–MS for all 30 samples. Mineralogical and textural characteristics were investigated in 15 selected samples by XRD and SEM–EDS. The coal is abundant in quartz, pyrite, and gypsum, along with trace amounts of calcite, clay minerals, and feldspars, which demonstrate the combined effects of detrital input, reducing depositional conditions, and post-depositional alteration. The major oxide composition is dominated by SiO₂, CaO, Al₂O₃, and Fe₂O₃, showing notable contributions from carbonate, aluminosilicate, and Fe-bearing mineral phases. Most trace elements occur at levels close to the world coal average, whereas V, Ni, and U are significantly enriched, and Cr and Co are slightly enriched. Total REE contents range from 21 to 125 ppm, averaging 51 ppm, and display light REE enrichment over heavy REEs. Weak negative Ce anomalies and slight positive Eu anomalies indicate variable redox conditions and the influence of terrigenous mineral input. Vertical distribution patterns indicate that during peat accumulation and early diagenesis, variations in detrital input, mineral matter abundance, and depositional redox conditions regulated trace element and REE distributions. REEs are mainly associated with aluminosilicate and locally phosphate-bearing phases, whereas V, Ni, and U are linked to organic matter and/or sulfide-rich intervals. These results show that the main factors influencing trace-element and REE behaviour in the Afşin–Elbistan coal seam are the mineralogical composition and redox evolution. Full article

The severe slagging caused by high-alkali coals restricts the utilization of Xinjiang coal resources in China. This study investigates the mineral composition, geochemical characteristics, modes of occurrence, and controlling factors of alkali and alkaline earth metals in high-alkali coals from the Dananhu Coalfield, Turpan–Hami Basin. The investigated coals are classified as lignite, characterized by low ash yield, extra low sulfur, and medium–high alkali contents. Quartz and kaolinite are dominant, with accessory calcite, K-feldspar, pyrite, gypsum, siderite, celestite, and Ba-bearing celestite. Compared to average Chinese coals, Na, Mg, Ca, and Cl are enriched, with the shallowest No. 3 coal seam showing the highest enrichment. Based on the correlation analysis, K is likely associated with K-bearing aluminosilicates (e.g., K-feldspar and illite), while Ca, Mg, and Na probably exhibit both organic and inorganic affinities. Sequential extraction results indicate that Na is predominantly water-soluble and ion-exchangeable, whereas Ca and Mg are largely ion-exchangeable and HCl-soluble. The abundant cell cavities and oxygen-containing functional groups in lignite provide both binding sites and accommodation space for Ca, Mg, and Na. Despite a continental freshwater depositional setting, tectonic isolation and persistent arid conditions potentially promoted epigenetic enrichment of Na, Mg, and Ca. Closed hydrogeological units formed by tectonic movements restricted leaching and enhanced evaporation, concentrating these elements in the coal seams. Future research should focus on implementing economically feasible dealkalization pretreatment processes for the clean utilization of high-alkali coals. Full article

Coal plays a vital role in energy production, but its combustion releases SO₂, causing serious environmental problems. In this study, indigenous microorganisms with sulfur-metabolism potential were isolated from raw coal and evaluated for coal biodesulfurization. The isolated strain, Arthrobacter sp. DBW, degraded 50% of DBT within 100 h and reduced the total sulfur content of coal by 42%. Since pyrite remained after treatment, the sulfur removal was mainly attributed to organic sulfur removal. Mechanistic analysis indicated that Arthrobacter sp. DBW may remove organic sulfur through enzyme binding to DBT and subsequent C-S bond cleavage via the 4S pathway. In addition, partial kaolinite dissolution occurred during biodesulfurization, reducing the ash content by 1.06%. These results suggest that Arthrobacter sp. DBW has potential for organic sulfur removal from coal and may help reduce SO₂ emissions during subsequent combustion. Full article

Accurate identification of acid-producing layers is key to controlling acid mine drainage (AMD) in abandoned coal mines. This study collected 337 core samples from 34 boreholes in the Modigou mining area, Shanxi, China, and established a combined static–mineralogical–kinetic approach to evaluate the acid-generating and neutralization potentials of sulfur-bearing rocks. Three-stage net acid generation (NAG) tests identified the pyrite-bearing layer of the Benxi Formation and the No. 10 coal seam of the Taiyuan Formation as the main acid producers, with NAG values of 360.41 and 97.87 kg H₂SO₄/t, respectively, while the Taiyuan limestone showed a high neutralization capacity (ANC = 490 kg H₂SO₄/t). NAG pH was strongly negatively correlated with sulfur content (Pearson r = −0.75, p < 0.01). Sulfide oxidation acid production showed staged attenuation, with average decreases of 64.81% and 47.65% in the second and third stages. Humidity cell experiments demonstrated continuous acid production over 63 days under dry–wet cycles, with increased acid generation rates at higher flow velocities (Darcy flux: 3.54 × 10⁻³ cm/s for accelerated vs. 8.84 × 10⁻⁴ cm/s for standard conditions). Multi-dimensional flow-through simulations confirmed the AMD formation mechanism of “acid supply, buffer, and fracture conduction”. The identified acid-producing layers matched well with field discharge points. This multi-method coupling system provides a theoretical basis for source control of AMD in abandoned high-sulfur coal mines in the Yellow River Basin. This study did not account for microbial catalysis, which is a key limitation of the static chemical oxidation method used. Full article

This study focuses on the pollution assessment, potential ecological risks, influencing factors, and migration pathways of trace elements from slag heaps of coal gangue and pyrite to farmland in the mountainous areas of southeast China. Based on the pollution index and correlation analysis of trace elements, Cd, As, Pb, and Zn were identified as characteristic pollutants. In the solid waste, surrounding soil, and farmland soil, the mean concentrations of Cd, As, Pb, and Zn of pyrite slag heaps were generally 9.7–86.7 times higher than those of coal gangue dumps. In contrast, higher levels of As were found in coal gangue surrounding soil, while higher Cd and As concentrations existed in coal gangue-affected farmland soil. Mantel test results revealed significant statistical correlations between characteristic pollutants and environmental factors (geographic location, weather, and climate), particularly for pyrite slag heaps. The potential migration pathways from solid waste to the surroundings (soil and water) and then to farmland soil were finally revealed using partial least squares path modeling. This study demonstrated that the pyrite slag heaps were more heavily polluted than the coal gangue dumps. The pyrite slag heap was more susceptible to environmental factors, which could rapidly transfer trace elements to farmland soil via the surrounding soil and water. Therefore, this study offers a statistical framework to infer plausible trace element migration trends via multi-medium monitoring data. It also delivers comparative analytical references for risk assessment of two distinct types of slag heaps. Full article

Mechanically activated pyrite plays an important role in gold extraction and coal utilization, but its reactivity may change markedly during storage. This study investigates how air deactivation during storage affects the crystal structure and subsequent thermal decomposition behavior of mechanically activated pyrite. Pyrite was mechanically activated and then stored in air for 0, 7 and 180 days. X-ray diffraction (XRD) combined with Rietveld refinement was used to characterize variations in lattice parameters and unit-cell-related structural features, while non-isothermal thermogravimetric–differential scanning calorimetry (TG-DSC) under an argon atmosphere, together with the Flynn–Wall–Ozawa (FWO) method, was applied to evaluate the decomposition kinetics. Air deactivation induced a non-monotonic evolution of lattice parameters and unit-cell volume, which is attributed to combined effects of residual stress relaxation and air-induced surface-related modification during storage. All samples exhibited two mass-loss stages during heating, reflecting stepwise thermal decomposition, and their decomposition behavior varied systematically with deactivation time. The apparent activation energy depended on both conversion fraction and deactivation degree, and nucleation-and-growth-type mechanisms were found to dominate the decomposition process, with their relative contributions evolving with storage time. These results clarify how prior air-deactivation history influences the structural evolution and subsequent thermal decomposition behavior of mechanically activated pyrite and provide useful insight for its storage and utilization in related processes. Full article

The transitional shale of the Upper Permian Longtan Formation in the Sichuan Basin is characterized by high organic matter abundance, wide distribution, and significant resource potential. This study systematically analyzes the sedimentary environment and geochemical characteristics of the Longtan Formation, clarifying the evolutionary patterns of its sedimentary environment, detrital influx, paleoclimate, paleosalinity, and paleoredox conditions. The results indicate that the Longtan Formation can be divided into five Members. The Long 1 to Long 4 Members were predominantly deposited in littoral-marsh facies, whereas the Long 5 Member represents tidal flat–lagoon facies. Geochemical analyses indicate that the Long 4 Member exhibits the highest TOC content (3.37%–11.50%, avg. 6.39%), characterized by black shale interbedded with thin coal seams and pyrite bands. This interval corresponds to relatively low detrital input (Zr: 103–1124 ppm, avg. 697 ppm; Ti: 2589–3909 ppm, avg. 3408 ppm), a warm and humid paleoclimate (CIA: 88.4–96.8, avg. 93.6; Sr/Cu: 1.6–6.8, avg. 3.5), moderately elevated paleosalinity (Sr/Ba: 0.36–2.46, avg. 1.59), and oxic to intermittently reducing water conditions (U/Th: 0.21–0.68, avg. 0.37), making it the main interval for organic matter enrichment. From the Long 1 to Long 5 Members, the depositional environment evolved with progressively shallower water, decreasing terrigenous input, and increasingly oxic conditions. These changes, combined with high productivity and intermittently reducing conditions, collectively governed the accumulation and preservation of organic matter across the formation. Full article

In this study, the occurrence and leachability of rare earth elements and yttrium (REY) in medium-rank coal—meta-bituminous B coal from the southwestern part of the Upper Silesian Coal Basin in Poland—were investigated. The coal samples contained variable amounts of siderite, dolomite, calcite, kaolinite, illite, quartz, apatite, and pyrite in their mineral composition. A five-step sequential chemical leaching procedure was used, including deionized water, 3% HCl, 5% HNO₃, 10% HNO₃ with microwave assistance, and concentrated HCl–HF also with microwave assistance. The highest concentrations of ∑REY were observed in seam 404/1. Light REY (LREY) dominated the REY composition (>75%), while heavy REY (HREY) accounted for less than 10%. The chondrite-normalised REY patterns and total REY content indicate a clastic origin of REY-bearing minerals. The most efficient leaching occurred in stages IV and V. The solutions from stages I–III preferentially mobilised critical REY, while those from stages IV–V reflected the REY distribution in the coal. Based on the C_outl index, both coal and leachates from the later stages are classified as prospective REY resources. However, absolute REY concentrations should be considered when interpreting C_outl values. The positive correlation between apatite and kaolinite contents and ∑REE concentrations suggests their role in REY enrichment. Full article

Shearers and roadheaders are critical equipment in coal mining and roadway excavation, where the rock-breaking performance of cutting picks directly influences operational efficiency and economic outcomes. Complex geological conditions, such as hard coal seams and embedded inclusions like gangue or pyrite nodules, pose significant challenges to cutting efficiency and tool wear. This study presents a numerical investigation into the rotational cutting process of a single pick in heterogeneous coal seams using the Smoothed Particle Hydrodynamics (SPH) method integrated with a mixed failure model. The model combines the Drucker–Prager criterion for shear failure and the Grady–Kipp damage model for tensile failure, enabling accurate simulation of crack initiation, propagation, and coalescence without requiring explicit fracture treatments. Simulations reveal that cutting depth significantly influences the failure mode: shallow depths promote tensile crack-induced spallation of hard nodules under compressive stress, while deeper cuts lead to shear-dominated failure. The cutting pick exhibits periodic force fluctuations corresponding to stages of compressive-shear crack initiation, propagation, and spallation. The results provide deep insights into pick–rock interaction mechanisms and offer a reliable computational tool for optimizing cutting parameters and improving mining equipment design under complex geological conditions. A key finding is the identification of a critical transition in failure mechanism from tensile-dominated spallation to shear-driven fragmentation with increasing cutting depth, which provides a theoretical basis for practitioners to select optimal cutting parameters that minimize tool wear and energy consumption in field operations. Full article

The Carboniferous coal seams in Northeast Kazakhstan remain insufficiently investigated, with a lack of comprehensive mineralogical and geochemical assessments necessary to understand the geological processes controlling coal quality. This study examines 15 coal samples from the Karagandy Coal Formation (KCF) at the Saradyr and Bogatyr mines using proximate and ultimate analyses, FTIR, XRD, SEM–EDS, ED-XRF, and ICP-OES, providing the first detailed comparison of mineralogical and geochemical characteristics—including depositional signals and inorganic constituent distribution—between these mines within the KCF. The coals exhibit an average ash yield of 24.1% on a dry basis, volatile matter of 21.6% on a dry and ash-free basis, and low moisture content of 1.1% (air-dry), with low sulfur levels of 0.7% in whole coal across both mines. Mineralogical composition is dominated by quartz and clay minerals, with minor pyrite, apatite, chalcopyrite, and rutile. Major oxides in the coal ash average 68.2% SiO₂ and 19.5% Al₂O₃, followed by Fe₂O₃, K₂O, and TiO₂ (3–12.1%). Among the 24 identified trace elements, Sm is the most abundant at 6.3 ppm with slight enrichment (CC = 2.8), Lu remains at normal levels (CC < 1), and most other elements are depleted (CC < 0.5). The Al₂O₃/TiO₂ ratios (3.8–10.8) indicate contributions from intermediate to mafic parent materials. The detrital mineralogy, parting compositions, and elevated ash content indicate significant accommodation space development during or shortly after peat accumulation, likely within a vegetated alluvial plain depression. These findings provide new insights into the depositional environment and coal-forming processes of the KCF and contribute to regional assessments of coal quality and resource potential. Full article

The mineral matter in coal has great significance for geological evolution, and clean and fractional utilization. The Laochang mining area is one of the largest anthracite coal production bases in Southern China, and the most important coal energy base in Yunnan province, China. This study investigates the composition and mode of occurrence of mineral matter in the Laochang coals to reveal the sediment provenance, sedimentary environment, and hydrothermal fluids. The predominant minerals in the Laochang coals include oxide (quartz, anatase), clay (kaolinite, illite/smectite mixed layer), sulfide (pyrite, sphalerite), phosphate (xenotime, monazite, goyazite–gorceixite), and carbonate (calcite, dolomite, sideroplesite, siderite). The minerals in the Laochang coals are dominated by quartz (2.4~54.8%) and kaolinite (3.4~39.2%), followed by illite, smectite, muscovite, calcite, pyrite, and anatase. Quartz and dolomite in SB-7+8 coal have the highest proportions, reaching 54.8% and 17.3%. The modes of occurrence of minerals reflect that the Laochang coals are affected by the epigenetic hydrothermal fluids and seawater. The chalcophile elements Hg, Pb, Se, and Cr, and lithophile elements Li, Nb, Ta, Zr, Hf, and REY are slightly enriched in XB-3 coal, which is attributed to the intrusion of seawater and the supply of terrestrial detrital materials, respectively. REY is dominated by LREY, followed by MREY, and a lower level of HREY in the Laochang coals, which have a high fractionation degree. The REY enrichment H-type is influenced by the hydrothermal fluids. Based on the relationship between Al₂O₃ and TiO₂, Al₂O₃/TiO₂ and Nb/Yb, and the negative anomaly Eu, the detrital material in the erosion source area of the Laochang coal is derived from the Emeishan Large Igneous Province basalt and felsic–intermediate rocks. Full article

To address the issue of microstructure damage and stability deterioration of coal pillar dams in underground coal mine reservoirs caused by long-term exposure to highly mineralized mine water, this study conducts indoor simulation experiments to investigate the evolution of coal microstructures under immersion in salt solutions with varying mineralization degrees (1000–2000 mg/L). Changes in solution chemical parameters are monitored through dynamic water quality analysis, while the pore-fracture structure of coal is quantitatively characterized using scanning electron microscopy (SEM) and a pore-crack analysis system (PCAS). Results indicate that with increasing mineralization, solution pH initially decreases due to H⁺ release from pyrite and siderite oxidation, followed by a slight recovery in later stages owing to the buffering capacity of HCO₃⁻. The trends of TDS and EC are consistent, initially slightly decreasing due to ion adsorption, and then rising with the dissolution of minerals in the later stage. The continuous decline in ORP indicates a progressive enhancement in the solution’s reducing potential. Microstructural observations reveal that after immersion in highly mineralized solutions, the coal matrix undergoes more severe fragmentation, with increased pore quantity and irregular pore morphology, and the fracture network becomes more developed and interconnected. Quantitative analysis further demonstrates that, with increasing mineralization, the proportion of large pores (>24 μm) significantly rises; the fractal dimension first decreases and then increases; the rise in probability entropy reflects enhanced spatial disorder in pore arrangement; and the pore area ratio increases. The research results provide a theoretical basis for the long-term stability evaluation and safety control of coal pillars in highly mineralized mine water environments. Full article

The thermal transformation mechanism of pyrite in coal, which governs sulfur emissions and ash deposition, remains highly controversial. There are significant discrepancies in reported activation energies (E_a) (60–310 kJ/mol) and conflicting reaction pathways. To resolve these long-standing controversies, this study proposes a competition-coupling mechanism: pyrolysis and oxidation compete under local O₂ and temperature gradients, while coupling through microstructural evolution. Specifically, pyrolysis generates a porous Fe_1−XS that facilitates oxidation, which in turn can form a passivating oxide/sulfate layer that promotes further pyrolysis. This mechanism reconciles longstanding kinetic controversies by showing that the apparent activation energy is not a fixed value but instead a dynamic parameter, shifting along a continuous curve that bridges pyrolysis and oxidation-dominated regimes. Furthermore, we construct a unified phase diagram by incorporating the competition-coupling mechanism into classical thermodynamic equilibria. This diagram uses the molar ratio FeS₂/(FeS₂ + O₂) and temperature to categorize the transformation process into four distinct regions—pyrolysis-dominated, competition-coupling, oxidation-dominated, and melt-dominated. The key contribution of this work lies in the diagram which offers a practical framework for optimizing combustion and roasting systems, allowing for improved control over sulfur emissions and ash-related issues such as slagging and fouling. Full article

Density functional theory (DFT) calculations were employed to examine how carbon defects, symbiosis, and sulfur influence the wettability of coal pyrite by analyzing H₂O adsorption on distinct surface configurations. The comparison results of adsorption energy, Mulliken population, charge density, and electronic state density of water molecules on the surface of pyrite doped with carbon atoms show that the presence of carbon doping reduces the negative value of the adsorption energy of water molecules on the pyrite surface, the C atoms on the pyrite surface form weaker C-H bonds with the H atoms in the water molecules, the Fe-O bond strength weakens, and the thermodynamic trend weakens. And the bond of the pyrite surface with adsorbed carbon changes from an Fe-O bond to an Fe-C-O bond. The adsorption of water molecules on the pyrite surface is weakened, and there is a weaker thermodynamic trend. This is because the adsorption of carbon atoms changes from hydrophilic to nearly hydrophobic. The physical adsorption of sulfur atoms changes the adsorption energy of water molecules on the pyrite surface from negative to positive, and the bond changes from an Fe-O bond to an Fe-S-O bond, indicating that the adsorption intensity of water molecules on the pyrite surface with adsorbed sulfur is weakened, and there is no thermodynamic trend. The pyrite surface with adsorbed sulfur changes from hydrophilic to hydrophobic. Under the same impurity atom doping or adsorption concentration, the influence of sulfur on the adsorption of water molecules on the surface of pyrite is the greatest, followed by the adsorbed carbon, and the weakest is the carbon atom doping. Macroscopically, the overall hydrophobicity of the surface of coal-bearing pyrite covered with sulfur is greater than that of pyrite containing adsorbed carbon and even greater than that of coal-bearing pyrite doped with carbon atoms. Full article