Wenli Liu

Low-rank ultrafine flotation clean coal typically yields filter cake moisture above 20% due to abundant oxygen-containing functional groups and strong surface hydrophilicity. Conventional polyacrylamide (PAM) flocculants are hydrophilic and improve dewatering only by altering cake porosity, not by reducing particle surface hydrophilicity, so they remove little adsorbed water. In this study, hydrophobically modified anionic polyacrylamides (HMAPAM) were synthesized by grafting lauryl acrylate onto APAM. FTIR, ¹H NMR, XPS, and SEM confirmed the grafting and progressive enrichment of hydrophobic alkyl chains on the surface. Moderate hydrophobic modification markedly improved solid–liquid separation. HMAPAM-D (APAM/LA = 4.5:0.5) achieved a settling velocity of 0.817 cm/s at 9 mg/L, 50.2% higher than APAM, and reduced filter cake moisture to 16.64% at 1 mg/L under 0.6 MPa versus 19.39% for unmodified APAM. Excessive modification (HMAPAM-E, 4:1) promoted intramolecular self-association, producing heterogeneous flocs and higher filtration resistance that degraded dewatering efficiency. The performance gain stems from hydrophobic association combined with adsorption bridging. These results clarify how hydrophobic group content controls flocculation and dewatering, informing the design of better flocculants for this type of coal slurry.

China’s coking coal resources are scarce, and maximizing the recycling of these resources is the primary objective of coal processing and utilization. The embedding features of inorganic minerals within coking middling coal resources are an inherent factor influencing their liberation and separation efficiency. However, current research lacks a systematic investigation into how the embedding features of inorganic minerals in coking middling coal affect their liberation characteristics and flotation separation performance. This study examines three Chinese coking middling coal samples with distinct embedding features. Based on quantitative characterization of inorganic mineral embedding, grinding tests with varying durations and flotation separation tests on post-grinding products were conducted. Liberation and separation efficiencies were evaluated to explore the influence of inorganic mineral embedding on liberation degree and the subsequent impact of the liberation degree on flotation performance. The results indicate that the three coking middling coal samples with different embedding features exhibit significant differences in the dissociation behavior between inorganic minerals and organic matter. The particle size (D) of inorganic mineral phases is the primary factor influencing the liberation degree of inorganic minerals, while the complexity of intergrowth between inorganic minerals and organic matter (CIG) is a secondary factor. The CIG is the primary factor affecting the liberation of organic matter. As the liberation of inorganic minerals and organic matter increases, the separation efficiency improves for the Liuwan middling coal samples, whereas it deteriorates for the Shaqu and Changzhi samples.

The Three-Product Dense Medium Cyclone (TPDMC) has been widely applied in the coal preparation industry, yet the adaptive optimization of its parameters based on feed characteristics remains under-researched. This study utilizes a semi-industrial experimental platform with a JX300/240 TPDMC to investigate the influence of pump frequency (PF) and four second-stage structural parameters—cylindrical section length (L_2cy), overflow pipe insertion depth (Dep_2o), overflow pipe diameter (D_2o), and conical section length (L_2co)—on the separation performance of three feed materials with distinct washability characteristics. Experiments conducted with density tracer particles revealed a distinct hydrodynamic coupling effect: PF and D_2o were the only factors modulating inlet pressure (varying from 0.12 to 0.45 bar), which directly altered the clean coal yield. In contrast, L_2cy, Dep_2o, and L_2co primarily influenced the second-stage internal flow field and concentration effect, thereby affecting the yield and ash content of middling coal (gangue). To quantify feed-specific sensitivities, a new index, Near-Gravity-Range Material (NGRM), was proposed. Results demonstrated that Sample-3 exhibited the highest sensitivity to parameter variations, with its middling coal yield variation reaching 41.25% due to its high NGRM of 71%. Furthermore, statistical analyses were conducted to quantify the influence of each parameter on the heavy product partition ratio across different density fractions. Based on these findings, the following targeted optimization strategies are proposed: (1) for feeds rich in the 1.40–1.50 RD range, increasing PF or decreasing D_2o is recommended to enhance clean coal yield; (2) for materials dominated by the 1.7 ± 0.10 RD fraction, increasing D_2o, PF, or L_2cy maximizes middling coal recovery; and (3) for feeds high in the 1.90 ± 0.10 RD fraction, reducing Dep_2o, PF, L_2cy, or L_2co effectively minimizes middling coal contamination by high-density particles.

The utilization pathways of coal macerals exhibit a significant variability. The efficient separation of active and inert components in coal is critical for realizing their high value-added applications. The liberation efficiency of coal macerals directly influences the accuracy of subsequent separation processes. Investigating the influence of embedded properties of inertiniteincluding embedded granularity, embedded morphology, and concatenate particle typeson the liberation characteristics of coal macerals is essential for achieving separation efficiency. This study focuses on the relationship between the embedded properties of inertinite and the liberation characteristics of coal macerals. According to the development situation of coal in China and embedded characteristics of macerals, three sub-bituminous coal samples from different regions of China (Xinjiang Tianchi (TC), Xinjiang Qiuci (QC), and Inner Mongolia (Ordos)), each exhibiting distinct embedded properties, were selected for experimentation. Both ball and rod milling techniques were employed to conduct grinding tests, and the total liberation degree of the coal macerals (the weighted average of the liberation degree of vitrinite and inertinite) was measured at various particle sizes (-0.5 mm) of the ground products. The results demonstrate that coal samples with finer embedded granularity (expressed as equivalent diameter of the area) exhibited a lower total liberation degree at the same ground product size. The observed differences in the embedded morphology (aspect ratio) of coal macerals led to distinct responses to ball milling and rod milling. This research reveals that embedded granularity has the most pronounced influence on the total liberation degree of coal macerals followed by the embedded morphology. The concatenate particle types have the least impact on liberation. These findings provide a theoretical foundation for understanding the differences in liberation characteristics of macerals in subbituminous coals.

In this paper, Ordos low-rank coal was taken as the research object, methyl oleate and diethyl phthalate were used as test reagents, and the differences of coal slime flotation indexes under single collector and mixed collector conditions were compared and analyzed by the batch flotation testing of fine coal. Through DFT calculation, molecular dynamics simulation and Fourier transform infrared spectroscopy (FTIR) test, the mechanism of mixed collector improving the flotation index of low-rank coal was investigated. The results show that the flotation ability of single ester collector for low-rank coal is better than that of traditional oil collector, and the collector can further improve the flotation index of coal slime after mixing. There is a certain internal relationship between the frontier orbital energy and adsorption energy of collector molecules and their harvesting performance. When the single collector reaches adsorption equilibrium on the coal surface, the spatial orientation of the collector molecule is ‘horizontal’ on the coal surface, and the mixed collector is ‘vertical’ on the coal surface. Graphical abstract

An acid modified approach to enhance the adsorption capacity of low-rank coal (lignite) is proposed to reduce the risk of heavy metal ions within the wastewater. Adsorption kinetics, adsorption thermodynamics, adsorption coefficient and density functional theory DFT calculations were studied in this paper, respectively. The results indicate that the adsorption capacity of lignite was enlarged after HNO3 modification, and pseudo-second order kinetics model and Langmuir isothermal adsorption model can be used to describe the adsorption process. The surface chemical properties of lignite play a dominant role rather than the specific surface area and total pore volume in the Pb(II) cation adsorption process, and it is suggested that the adsorption of Pb(II) cation by raw lignite (RL) and modified lignite (ML) is mainly completed by chemical adsorption. The Fourier transform infrared spectroscopy (FTIR) characterization showed that the surface oxygen functional groups of lignite increased after modification. The results of interaction energies between the model molecule and Pb(II) cation show that Pb(II) cation and -C-O-C are most easily combined, followed by -COOH, and -C = O is the weakest.

The seepage and diffusion of acid mine drainage (AMD) generated from self-heating coal gangue tailings caused acid pollution to the surrounding soil and groundwater. Red mud derived from the alumina smelting process has a high alkali content. To explore the feasibility of co-disposal of coal gangue and red mud for prevention of AMD, coal gangue and red mud were sampled from Yangquan (Shanxi Province, China), and dynamic leaching tests were carried out through the automatic temperature-controlled leaching system under the conditions of different temperatures, mass ratios, and storage methods. Our findings indicated that the heating temperature had a significant effect on the release characteristics of acidic pollutants derived from coal gangue, and that the fastest rate of acid production corresponding to temperature was 150 °C. The co-disposal dynamic leaching tests indicated that red mud not only significantly alleviated the release of AMD but also that it had a long-term effect on the treatment of acid pollution. The mass ratio and stacking method were selected to be 12:1 (coal gangue: red mud) and one layer was alternated (coal gangue covered with red mud), respectively, to ensure that the acid-base pollution indices of leachate reached the WHO drinking-water quality for long-term discharge. The results of this study provided a theoretical basis and data support for the industrial field application of solid waste co-treatment.