Search Results (129)

Investigating the microstructural damage and mechanical properties of coal under deep mine hygrothermal conditions is essential for ensuring the safe and efficient extraction of coal resources. In this study, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and nanoindentation techniques were employed to examine the surface morphology and microscale mechanical properties of coal samples exposed to four environmental conditions, initial, humidified, heated, and coupled hygrothermal, under a peak indentation load of 70 mN. The results indicate that humidification led to the formation of dissolution pores and localized surface softening, resulting in a 15.9% increase in the peak indentation depth and reductions in the hardness and elastic modulus by 29.53% and 17.14%, respectively. Heating caused localized disintegration and the collapse of the coal surface, accompanied by surface hardening, with a slight 0.4% decrease in the peak indentation depth and increases in hardness and the elastic modulus by 1.32% and 1.56%, respectively. Under the coupled hygrothermal condition, numerous fine dissolution pores and microcracks developed on the coal surface, and the mechanical properties exhibited intermediate values between those observed in the humidified and heated states. Notably, the elevated temperature suppressed the moisture penetration into the coal matrix to some extent in the hygrothermal environment. A positive correlation was found between the hardness and elastic modulus, independent of the coal sample condition. The mineralogical composition significantly influenced the microscale mechanical behavior, with hard quartz minerals corresponding to lower peak indentation depths and a higher hardness, whereas soft kaolinite showed the opposite trend. Full article

Coal combustion generates significant volumes of ash, a technogenic by-product that poses a serious threat to regional environmental sustainability (environmental chemical contamination and air pollution). This study aims to assess the feasibility of utilizing this type of ash as a raw material component in the fabrication of refractory bricks and to investigate the fundamental properties of the resulting experimental products. Ash was incorporated into the batch composition at concentrations ranging from 10% to 40% by weight, blended with clay and water, then shaped through pressing and subjected to firing at 1000 °C and 1100 °C in an air atmosphere for 2 h. After complete cooling, the samples were subjected to compressive strength testing. Samples containing 40 wt% coal ash exhibited insufficient compressive strength and were therefore excluded from subsequent investigations. For the remaining samples, apparent density, open porosity and slag resistance were determined. The microstructural characterization was performed, and the phase composition of the samples was analyzed. The results revealed that the phase composition of the experimental samples differs significantly from that of the reference sample (ShA-grade chamotte brick in accordance with GOST 390-96, currently used as lining in metallurgical furnaces across the country), exhibiting a higher mullite content and the absence of muscovite. A small amount of kaolinite was detected in the experimental samples even after a 2-h firing process. This observation may be attributed to the effect of kaolinite crystallinity on the transformation process from kaolinite to metakaolinite. The mechanical strength of the experimental samples meets the relevant standards, while slag resistance demonstrated an improvement of approximately 15%. Open porosity was found to decrease in the experimental samples. In addition, a change in the pore size distribution was observed. Notably, the proportion of pores larger than 10,000 nm was significantly reduced. These findings confirm the feasibility of incorporating coal ash as a viable raw material component in the formulation of refractory materials. Full article

Coal mining and washing processes generate substantial amounts of coal gangue, posing significant environmental challenges. Coal gangue as a solid waste is rich in SiO₂ and Al₂O₃, with the SiO₂/Al₂O₃ molar ratio closely aligned with the ideal composition of 4A molecular sieves. In this study, through a synergistic pretreatment process involving low-temperature oxidation and hydrochloric acid leaching, the Fe₂O₃ content in coal gangue was reduced from 7.8 wt% to 1.1 wt%, markedly enhancing raw material purity. The alkali fusion–hydrothermal synthesis parameters were optimized via orthogonal experiments—calcination (750 °C, 2 h), aging (60 °C, 2 h), and crystallization (95 °C, 6 h) to maintain cubic symmetry, yielding highly crystalline 4A zeolite. Characterization via XRD, calcium ion adsorption capacity, SEM, and FTIR elucidated the regulatory mechanism of calcination on kaolinite phase transformation and the critical role of alkali fusion in activating silicon–aluminum component release. The as-synthesized zeolite exhibited a cubic morphology, high crystallinity, and sharp diffraction peaks consistent with the 4A zeolite phase. The pH of the zero point charge (pH_ZPC) of the 4A molecular sieve is 6.13. The 4A molecular sieve has symmetry-driven adsorption sites, and the adsorption of Cu²⁺ follows a monolayer adsorption mechanism (Langmuir model, R² = 0.997) with an average standard enthalpy change of 38.96 ± 4.47 kJ/mol and entropy change of 0.1277 ± 0.0148 kJ/mol, adhering to pseudo-second-order kinetics (R² = 0.999). The adsorption process can be divided into two stages. This study provides theoretical and technical insights into the high-value utilization of coal gangue. Full article

The pore–fracture structure of ultra-deep coal is critical for evaluating resource potential and guiding the exploration and development of deep coalbed methane (CBM). In this study, a coal sample was obtained from the Gaogu-4 well at a depth of 4369.4 m in the Shengli Oilfield of Shandong, China. A comprehensive suite of characterization techniques, including Field Emission Scanning Electron Microscopy (FE-SEM), X-ray diffraction (XRD), Mercury Intrusion Porosimetry (MIP), Low-temperature Nitrogen Adsorption (LT-N₂GA), and Low-pressure CO₂ Adsorption (LP-CO₂GA), were employed to investigate the surface morphology, mineral composition, and multi-scale pore–fracture characteristics of the ultra-deep coal. Based on fractal geometry theory, four fractal dimension models were established, and the pore structure parameters were then used to calculate the fractal dimensions of the coal sample. The results show that the ultra-deep coal surface is relatively rough, with prominent fractures and limited pore presence as observed under FE-SEM. Energy Dispersive Spectrometer (EDS) analysis identified the elements such as C, O, Al, Si, S, and Fe, thus suggesting that the coal sample contains silicate and iron sulfide minerals. XRD analysis shows that the coal sample contains kaolinite, marcasite, and clinochlore. The multi-scale pore–fracture structure characteristics indicate that the ultra-deep coal is predominantly composed of micropores, followed by mesopores. Macropores are the least abundant, yet they contribute the most to pore volume (PV), accounting for 70.9%. The specific surface area (SSA) of micropores occupies an absolute advantage, accounting for up to 97.46%. Based on the fractal model, the fractal dimension of the coal surface is 1.4372, while the fractal dimensions of the micropores, mesopores, and macropores are 2.5424, 2.5917, and 2.5038, respectively. These results indicate that the surface morphology and pore–fracture distribution of the ultra-deep coal are non-uniform and exhibit statistical fractal characteristics. The pore–fracture structure dominated by micropores in ultra-deep coal seams provides numerous adsorption sites for CBM, thereby controlling the adsorption capacity and development potential of deep CBM. Full article

Carbonaceous shale has garnered significant interest as a viable alternative source of rare earth elements (REEs) besides conventional REE-bearing ores. This study characterized rare earth element + Yttrium+ Scandium (REYs) enrichment in the 11 core samples of carbonaceous shale (7) and coal (4) collected from Arnot Mine. Major elements of the studied carbonaceous shale (CS) and coal showed high amounts of SiO₂, Al₂O₃, and Fe₂O₃, indicating a high content of aluminosilicate and iron-rich minerals. The plots Na₂O + K₂O against SiO₂ suggested alkali granite, granite, and granodiorite provenance sources for the studied shale and coal. The samples showed enrichment in low and heavy rare elements crystallized from a low potassium tholeiitic and medium calc-alkaline magma based on the plots of La_N/Yb_N and K₂O vs. SiO₂. The mineralogical and maceral analysis revealed the dominant presence of kaolinite (15%–45%), and it was suggested as the cation exchange site resulting from the isomorphous substitution of Al³⁺ for Si⁴⁺. Additionally, siderite was suggested as one of the REY hosts due to the Fe³⁺ site forming a complex with the REE³⁺ ions. Furthermore, the samples were classified as lignite to sub-bituminous coal category with dominant minerals including kaolinite, quartz, and siderite. The outlook coefficient (Coutl) of REY in CS revealed a promising area for economically viable, having two enrichment types, including low (La, Ce, Pr, Nd, and Sm) and heavy (Ho, Er, Tm, Yb, and Lu). The Eu_N/Eu_N* and Ce_N/Ce_N* ratio for the current studied samples exhibited a weak negative to no anomaly, and most of the studied samples were characterized by distinctive positive Gd anomalies derived from sediment source regions weathered from alkali granite, granite, and granodiorite provenance formed from a low potassium tholeiitic and medium calc-alkaline magma. Full article

In China’s northwest mining areas, shallow buried coal seams commonly feature double soft composite roof structures of mudstone and clay, resulting in poor roadway stabilization and proving challenging for effective roadway-surrounding rock (RSR) control. A mudstone–clay composite roof is particularly difficult to maintain due to the complex interactions between the soft rock layers and their sensitivity to moisture changes. Previous studies have investigated the properties of these soft rocks individually, but there is limited research on the behavior and control of double soft composite roofs. This study investigated the hydrophilic mineral composition and microstructure of mudstone and clay through X-ray diffraction (XRD) and scanning electron microscopy (SEM) experiments. Through an orthogonal experimental design, the influence of the clay layer thickness, number of layers, layer position, and relative moisture content on the stability of a mudstone–clay composite roof was studied. The results revealed the following: (1) Kaolinite, the primary hydrophilic component, constitutes a high proportion of clay, while both mudstone and clay exhibit abundant pores and cracks under SEM observation; (2) The relative moisture content emerged as the most significant factor affecting roadway deformation; and (3) A combined support of bolts, a short anchor cable, and a long anchor cable effectively controls RSR deformation in the case of a double soft composite roof. The methodology combining comprehensive material characterization and systematic parametric analysis can be extended to the study of other complex soft rock engineering problems, particularly those involving moisture-sensitive composite roof structures. Full article

The evaluation of reservoir properties and gas-bearing characteristics is critical for assessing shale gas accumulation. This study aimed to improve the precision of characterizing the properties and gas-bearing features of the Carboniferous and Permian shale reservoirs within the Qinshui Basin, Shanxi Province, China. It specifically focuses on the shale from the Late Carboniferous to Early Permian Shanxi and Taiyuan formations at Well Z1, located in the mid-eastern region of the basin. A comprehensive suite of analytical techniques, including organic geochemical analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), high-pressure mercury intrusion, low-temperature nitrogen adsorption, isothermal adsorption experiments, and gas content measurements, was used to systematically evaluate the reservoir properties and gas-bearing characteristics of the Carboniferous–Permian shale in Well Z1. The findings reveal the following. (1) The organic matter in the Shanxi and Taiyuan formations of Well Z1 is predominantly Type III humic kerogen, exhibiting high maturity and abundance. Specifically, 67.40% of the samples have TOC > 1.00%, classifying them as medium- to high-quality source rocks. The vitrinite reflectance (R_o) ranges from 1.99% to 2.55%, and T_max varies from 322.01 °C to 542.01 °C, indicating a high to over-mature stage. (2) The mineral composition of the shale is dominated by kaolinite, illite, and quartz, with a moderate brittleness index. The average clay mineral content is 52.12%, while quartz averages 45.53%, and the brittleness index averages 42.34. (3) The pore types in the shale are predominantly macropores, with varying peak intervals among different samples. (4) The surface area and specific pore volume of macropores show positive relationships with TOC, T_max, kaolinite, and the amount of desorbed gas, while they are negatively correlated with quartz. In contrast, mesopores exhibit positive correlations with TOC and illite. (5) Desorbed gas content exhibits a positive correlation with porosity, R_o, and illite. These insights enhance the comprehension of the reservoir’s properties, the characteristics of gas presence, and the determinant factors for the Carboniferous–Permian shale located in the Qinshui Basin, providing a robust practical procedure for the exploration and extraction of coal-measure shale gas resources within this area. Full article

The entrainment of gangue particles always increases the ash content of coal flotation concentrate. In the present work, the adjustment for entrainment of kaolinite in coal flotation was studied focusing on the parameters including solid concentration and frother dosage. The results indicated that the ash contents of concentrates were always higher in the early or late stage (flotation time: 0–30 s or 120–180 s) than that in the middle stage (flotation time: 30–120 s). This is mainly because of the mechanical entrainment in the early stage and the entrainment caused by water transportation in the later stage. It has been confirmed that reducing the feed solid concentration and frother dosage effectively decrease mechanical entrainment in the early stage. Furthermore, better flotation results can be obtained under a high solid concentration and frother dosage in the late stage of the flotation process. Therefore, a novel two-stage flotation process was proposed for the coal flotation. A better flotation performance (combustible recovery: 91.28%; ash content: 4.09%) can be achieved by two-stage flotation, compared to the flotation results (combustible recovery: 93.12%; ash content: 5.16%) of the one-stage flotation process. Full article

Investigation of the critical metal elements in coal and coal-bearing strata has become one of the hottest research topics in coal geology and coal industry. Coal-hosted Ga-Al-Li-REE deposits have been discovered in the Jungar and Daqingshan Coalfields of Inner Mongolia, China. Gallium, Al, and Li in the Jungar coals have been successfully extracted and utilized. This paper reviews the discovery history of coal-hosted Ga-Al-Li-REE deposits, including contents, modes of occurrence, and enrichment origin of critical metals in each coal mine, including Heidaigou, Harewusu, and Guanbanwusu Mines in the Jungar Coalfield and the Adaohai Coal Mine in the Daqingshan Coalfield, as well as the recently reported Lao Sangou Mine. Gallium and Al in the coals investigated mainly occur in kaolinite, boehmite, diaspore, and gorceixite; REEs are mainly hosted by gorceixite and kaolinite; and Li is mainly hosted by cholorite. Gallium, Al, and REEs are mainly derived from the sediment-source region, i.e., weathered bauxite in the Benxi Formation. In addition, REE enrichment is also attributed to the intra-seam parting leaching by groundwater. Lithium enrichment in the coals is of hydrothermal fluid input. The content of Al₂O₃ and Ga in coal combustions (e.g., fly ash) is higher than 50% and ~100 µg/g, respectively; concentrations of Li in these coals also reach the cut-off grade for industrial recovery (for example, Li concentration in the Haerwusu coals is ~116 µg/g). Investigations of the content, distribution, and mineralization of critical elements in coal not only provide important references for the potential discovery of similar deposits but also offer significant coal geochemical and coal mineralogical evidence for revealing the geological genesis of coal seams, coal seam correlation, the formation and post-depositional modification of coal basins, regional geological evolution, and geological events. Meanwhile, such investigation also has an important practical significance for the economic circular development of the coal industry, environmental protection during coal utilization, and the security of critical metal resources. Full article

The recycling of coal gangue has considerable potential to produce secondary environmental hazards, which significantly influence the high-end application of coal gangue in practical engineering. The present study investigates the effects of activation treatment on the physical, chemical properties and leaching behavior of coal gangue. The mineral composition, micro-pore structure and element leaching were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Thermogravimetry Analysis (TG), Low-Temperature Nitrogen Adsorption (LTNA) and Inductively Coupled Plasma (ICP). The results indicate that kaolinite and pyrite in coal gangue experienced reconstruction after 600 °C during thermal activation. The density of thermally activated coal gangue is increased with the calcination temperature as well as the alkalinity (from 4.8–7.1) due to the burning of organic and the oxidation of pyrite. The calcination treatment induced the reduction in macropore volume (>50 nm), and enhancement in mesopore (<5 nm) volume. Leachable Ni, Cd, Mn, Cu, Zn and Pd decreased by 99%, 67%, 86%, 40%, 99% and 93% after calcination at 800 °C, respectively. The Si and Al in 800 °C calcined coal gangue exhibited a high leaching ability in alkalinity solution; leachable Al reached 106.4 mg/kg, while leachable Si reached 86.1 mg/kg after 48 h of dynamic leaching. Full article

The Machala Mine, which is located in the Eastern Tibetan Plateau, was one of the highest coal mines in the world, with an elevation of 5200 m. The Machala coals are rich in a variety of critical and harmful elements. However, the occurrence and enrichment mechanisms of these elements are not well understood, which hinders the assessment of their environmental impacts and recycling potential. Furthermore, the enrichment mechanism identified in the Machala coals is expected to extend our understanding of the simultaneous enrichment of multiple elements in coal. A total of 18 coal samples were collected from 14 coal (or carbonaceous mudstone) seams at the Machala open-pit mine for systematic analyses, including macerals, coal quality, minerals, trace, and major elements of coals. The results suggested that the coal seams in the Machala Mine were dominated by low-sulfur (mainly less than 1%), medium-to-low-ash (average: 18.15%), low-volatile (average: 16.01%), and medium-high-to-high-calorific (average: 27.23 MJ/kg) coals. The coal macerals were predominantly vitrinite, with the collodetrinite being the most abundant, followed by collotelinite and vitroderinite, while telinite was present in low amounts. The mineral compositions were mainly quartz and kaolinite, with average contents of 37.8% and 48.2%, respectively, which were followed by illite and smectite-mixed layers, with average contents of 4.7% and 3.3%, respectively. Other minerals, including plagioclase, pyrite, siderite, chlorite, and potassium feldspar, were present in low quantities. However, C3 coal seam was an exception, with a high pyrite content of 23.2% (low-temperature ash sample). The Machala coal seams were characterized by varying degrees of enrichment in Li, As, Pb, Hg, and Sn. Correlation analyses and energy spectrum analyses indicated that clays were their major host minerals. The enrichment of Li in the Machala Coal Mine was mainly attributed to volcanic ash during the sedimentation period, while As, Pb, Hg, and Sn were primarily sourced from hydrothermal fluids in the late stages. Full article

This study analyzes 105 coal ash samples from Jurassic and Carboniferous coals from five mines in Kazakhstan, Lenin, Saradyr, Bogatyr, Maikuben, and Shubarkol, focusing on the inorganic elemental compositions, their occurrence, and industrial and environmental implications. Methods include coal ash yield and volatile matter analysis, mineralogical characterization via low-temperature ash X-ray diffraction (LTA-XRD), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). High-temperature ash (HTA) was analyzed using energy-dispersive X-ray fluorescence (EDXRF), highlighting XRF’s potential for rapid multi-elemental analysis. Nine major elements (Al, Si, P, S, Fe, K, Ca, and Ti) and eleven trace elements (As, Cu, Cr, Zn, Pb, V, Ga, Mn, Ni, Y, Yb, and Zr) were identified in HTA samples through EDXRF. SEM and dendrogram analysis confirm their co-occurrence with quartz, kaolinite, pyrite, and accessory minerals such as chalcopyrite, zircon, rutile, and REE-bearing apatite. The elemental content of samples enhances industrial suitability by reducing emissions. Only Yb shows slight enrichment for economic benefits, along with La, Ce, and Nd, while concentrations of potentially toxic elements indicate minimal environmental risk. EDXRF demonstrates its efficiency for large-scale investigations, with all samples analyzed in a few days using automated overnight measurements. This approach shows promise for future studies focusing on trace elements, including REE. Full article

Rare earth elements (REEs) have been identified as valuable and critical raw materials, vital for numerous technologies and applications. With the increasing demand for and supply gap in REEs, many research studies have focused on alternative sources of REEs. This study involved an elemental and mineralogical characterisation of discarded coal from a coal plant and coal fly ash (CFA) from a power station in South Africa for REE presence. XRD results revealed that the discard coal sample consisted mainly of kaolinite, pyrite, siderite, quartz, calcite, gypsum, and muscovite, whereas CFA contained abundant glassy amorphous phases, alumina silicates, quartz, gypsum, calcite, and minute levels of muscovite and hematite. SEM-EDAX showed REE-carrying grains containing phosphorus in both discard coal and CFA samples. This was followed by investigating the leaching potential of REEs using hydrochloric acid from discard coal and CFA. This research’s potential impact is possibly providing a new and sustainable source of REEs. For that purpose, multiple batch leaching tests were performed to investigate the effects of temperature and acid concentration on the leaching efficiencies of REEs from discard coal and CFA. The experimental results indicated that temperature strongly influences REE leaching efficiency, while acid concentration has a lesser impact. This study identifies the best leaching conditions for the total REE recovery as 1 M HCl and 80 °C for discard coal and CFA. Full article

This study aims to investigate the depositional environment, sediment sources, and elemental occurrence of Upper Paleozoic coal in the Renjiazhuang Mining District, Western Ordos Basin. Furthermore, SEM-EDX, optical microscope (OM), ICP-AES, ICP-MS, and AAS were used. Compared with hard coal of the world, M3 coals were enriched in Ga, Li, Zr, Be, Ta, Hf, Nb, Pb, and Th, M5 coals were enriched in Li (CC = 10.21), Ta (CC = 6.96), Nb (CC = 6.95), Be, Sc, Ga, Hf, Th, Pb, Zr, In, and REY, while M9 coals were enriched in Li (CC = 14.79), Ta (CC = 5.41), Ga, W, Hf, Nb, Zr, Pb, and Th. In addition, minerals were mainly composed of kaolinite, dolomite, pyrite, feldspar, calcite, and quartz, locally visible minor amounts of monazite, zircon, clausthalite, chalcopyrite, iron dolomite, albite, fluorite, siderite, galena, barite, boehmite, and rutile. In addition, maceral compositions of M3 coals and M9 coals were dominated by vitrinite (up to 78.50%), while M5 coals were the main inertite (up to 76.26%), and minor amounts of liptinite. REY distribution patterns of all samples exhibited light REY enrichment and negative Eu anomalies. The geochemistry of samples (TiO₂ and Al₂O₃, Nb/Y and Zr × 0.0001/TiO₂ ratios, and REY enrichment types) indicates that the sediment sources of samples originated from felsic igneous rocks. Indicator parameters (TPI, GI, VI, GWI, V/I, Sr/Ba, Th/U, and Ce_N/Ce_N*) suggest that these coals were formed in different paleopeat swamp environments: M3 coal was formed in a lower delta plain and terrestrial (lacustrine) facies with weak oxidation and reduction, and M5 coal was formed in a terrestrial and dry forest swamp environment with weak oxidation–oxidation, while M9 coal was formed in a seawater environment of humid forest swamps and the transition from the lower delta plain to continental sedimentation with weak oxidation and reduction. Statistical methods were used to study the elemental occurrence. Moreover, Li, Ta, Hf, Nb, Zr, Pb, and Th elements were associated with aluminosilicates, and Ga occurred as silicate. Full article

The flotation effect of lean coal is crucial for its clean utilization. Therefore, the flotation characteristics of difficult-to-float lean coal were studied. The analysis results of the feed properties showed that the ash content of the feed was high and the particle size was very fine. The minerals in the gangue mainly included sericite, kaolinite, quartz, white mica, and other substances. After flotation, the functional groups of the coal particles in the tailings decreased, and the absorption peak intensity weakened. Furthermore, the results of multi-factor flotation experiments showed that the dosages of the collector and the frother were significant factors affecting the yield of clean coal. The clean coal yield gradually increased with an increase in the two factors. The ash content of the clean coal increased with an increase in the frother dosage. Within the range of feed concentrations used in this work, the feed concentration was not a significant factor affecting the clean coal’s yield and ash content. Prediction models for the clean coal yield and ash content were proposed. Under optimized experimental conditions, the clean coal yield and the flotation perfection index were 72.15% and 46.63%, respectively, indicating a good flotation effect. Full article