Green Low-Carbon Technology for Metalliferous Minerals

Metalliferous minerals play a central role in the global economy. They will continue to provide the raw materials we need for industrial processes. Significant challenges will likely emerge if the climate-driven green and low-carbon development transition of metalliferous mineral exploitation is not managed responsibly and sustainably. Green low-carbon technology is vital to promote the development of metalliferous mineral resources shifting from extensive and destructive mining to clean and energy-saving mining in future decades. Global mining scientists and engineers have conducted a lot of research in related fields, such as green mining, ecological mining, energy-saving mining, and mining solid waste recycling, and have achieved a great deal of innovative progress and achievements. This Special Issue intends to collect the latest developments in the green low-carbon mining field, written by well-known researchers who have contributed to the innovation of new technologies, process optimization methods, or energy-saving techniques in metalliferous minerals development

Ecological and Health Risks Attributed to Rare Earth Elements in Coal Fly Ash

The occurrence and distribution of yttrium and rare earth elements (REYs), along with major elements and heavy metal(loid)s (HMs) in coal fly ash (CFA) from five coal-fired power plants (CFPPs), were analyzed, and the REY-associated ecological and health risks were assessed. The individual REYs in CFA were abundant in the following order: Ce > La > Nd > Y > Pr > Gd > Sm > Dy > Er > Yb > Eu > Ho > Tb > Tm > Lu. The total REY content ranged from 135 to 362 mg/kg, averaging 302 mg/kg. The mean light-to-heavy REY ratio was 4.1, indicating prevalent light REY enrichment in CFA. Significantly positive correlations between the REYs suggested that they coexist and share similar origins in CFA. REYs were estimated to pose low to moderate ecological risks, with risk index (RI) values ranging from 66 to 245. The hazard index (HI) and target cancer risk (TCR) of REYs from CFA, estimated to be higher for children (HIc = 0.15, TCRc = 8.4 × 10−16) than for adults (HIa = 0.017, TCRa = 3.6 × 10−16), were well below the safety limits (HI = 1, TCR = 1.0 × 10−6). However, the danger to human health posed by HMs in the same CFA samples (HIc = 5.74, TCRc = 2.6 × 10−4, TCRa = 1.1 × 10−4) exceeded the safe thresholds (excl. HIa = 0.63). The mean RI and HI attributed to REYs in CFA were 14% and 2.6%, respectively, of the total risks that include HMs

“Four Zones” control model and application for surface subsidence of bed separation grouting mining

The grouting technology of bed separations has been proved to be a new method which can meet the requirements of non-destructive mining and solid waste reduction. To effectively control the subsidence of ground structures caused by mining, the whole process of bed separation grouting mining is analyzed in a steady state based on the key stratum theory. For the first time, a “four zones” control model for surface subsidence under grouting bed separation was proposed, which includes natural zone, transition zone, warning zone, and protection zone, and the calculation formula for the “four zones” range was derived. Based on the engineering back-ground of controlling the subsidence of the ground coking plant at the 3501 panel, the proposed “four zones” model was validated by combining physical modelling and field measurement of subsidence. The results show that the surface subsidence curve of bed separation grouting in physical modelling shows an irregular “V” shape, and the surface subsidence of the panel first increases rapidly. After reaching the maximum subsidence, the surface subsidence first decreases rapidly, and then the reduction rate gradually slows down. The subsidence curve shows a clear “four zones” distribution, with a maximum subsidence of 1589 mm, appearing at the contact boundary between the natural zone and the transition zone. The subsidence of the contact boundary between the transition zone and the warning zone is 497.94 mm, and there is basically no subsidence within the protection zone. The predicted surface subsidence, horizontal deformation, slope, and curvature caused by mining under grouting conditions based on probability integral method are consistent with the field measured results, but significantly smaller than the predicted values under non-grouting conditions. It is determined that the surface deformation under grouting conditions meets the requirements of Grade I damage level for structures. Based on the practical engineering geology and observed data of the mine, the natural area is 261.19 m, the transition area is 246.09 m, the warning area is 655.25 m, and the protection area is 199.53 m. The proposed “four zones” control model provides a fundamental theoretical basis for studying the subsidence of bed separation grouting mining

Development and performance optimization of a new composite sealing material prepared by drilling cuttings

A highly efficient composite sealing material was prepared using drilling cuttings as the base material and a binder, a coagulant, and other additives as auxiliaries. A four-factor, three-level orthogonal test was designed based on the response surface method (RSM), and a response surface regression model was constructed using compressive strength, fluidity, expansion rate, and setting time as performance indexes to analyze the effects of each factor on material performance and optimize the material proportion. The samples were prepared by simulating the grouting process, the permeability of the samples was measured, and the sealability of the material was verified by analyzing the material microscopic morphology. Results showed that the regression model had a high level of confidence and accuracy and could predict the test results accurately within the range of the test. The effects of the interaction between factors on material performance were also examined. The low permeability of the sealing material samples verified the material’s feasibility. Gradual optimization of material performance revealed that the optimal proportion was 52.6% drill cuttings, 44.3% binder, 0.6% coagulant promoter, and 2.5% expansive agent. Under these conditions, the error between the predicted and test values of each material property was less than 5%, and the comprehensive performance was superior. These findings verify the accuracy of RSM and its applicability to the optimization of material performance. This work provides reasonable theoretical guidance for the preparation of drilling cuttings composite (DC) materials in practical engineering

High-temperature modification of steel slag using composite modifier containing silicon calcium slag, fly ash, and reservoir sediment

Steel slag (SS) is a kind of industrial solid waste, and its accumulation brings certain harm to the ecological environment. In order to promote the building material utilization of SS, high-temperature modification (HTM) of SS is performed using a composite modifier (CMSFR) containing silicon calcium slag (SCS), fly ash (FA), and reservoir sediment (RS). Then, the authors investigated the effect of CMSFR on the cementitious properties and volume soundness of SS mixture after HTM (SMHTM). After that, the mineral composition and microstructure of SMHTM were investigated through X-ray fluorescence analysis (XRF), X-ray diffraction (XRD), scanning electronic microscopy (SEM), energy dispersive spectrometry (EDS), and particle size analysis. It was found that the free CaO (f-CaO) content obviously decreased, and the cementitious properties improved in SMHTM. When the CMSFR content was 20% (SCS: FA: RS = 9:7:4), and the modification temperature (MT) was 1,250°C, the mass fraction of f-CaO in SMHTM dropped from 4.81% to 1.90%, down by 60.5%; the 28-day activity index of SMHTM increased to 85.4%, 14.3% higher than that of raw SS, which meets the technical requirement of Steel slag powder used for cement and concrete (GB/T 20491-2017): the activity index of grade I SS powder must be greater than or equal to 80%. As the mass fraction of CMSFR grew from 10% to 30%, new mineral phases formed in SMHTM, including diopside (CMS2), ceylonite (MgFe2O4), gehlenite (C2AS), tricalcium aluminate (C3A), and magnetite (Fe3O4). The HTM with CMSFR promotes the decomposition of RO phase (a continuous solid solution composed of divalent metal oxides like FeO, MgO, MnO, and CaO) in raw SS, turning the FeO in that phase into Fe3O4. The above results indicate that the SMHTM mixed with CMSFR can be applied harmless in cement and concrete, making low-energy fine grinding of SS a possibility

Mechanical Properties and Damage Characteristics of Coal-Based Solid Waste Paste Filling Materials with Different Moisture Content

It has been proven that it is a feasible treatment method to prepare paste filling material from coal-based solid waste to fill underground goaf. Based on the complexity of the goaf environment, especially the influence of humidity on paste filling materials, this paper prepared paste filling materials with a mass concentration of 80% by using coal gangue and fly ash, and carried out a uniaxial compression test of coal-based solid waste paste filling materials under four different water-bearing states. The experimental results show that: (1) The binary primary equation fits well the variation trend of paste filling strength with water content. With the increase of moisture content, the compressive strength of paste filling material gradually decreases, and the higher the moisture content, the more obvious the influence on the strength of paste filling. (2) The damage evolution equation and constitutive equation of paste filling materials with different moisture content were established. With the increase of strain, the influence of moisture content on the damage of paste filling material decreases gradually, and the moisture content can promote the damage development of paste filling material to a certain extent. (3) The influence mechanism of moisture content on coal based-solid waste paste filling material is discussed from the three aspects of physical effect, structural effect and chemical effect, which provides a direction for further research on the influence mechanism of moisture content on filling strength. The research on the failure mechanism of coal-based solid waste paste filling and the safety production of the mine can be used as a theoretical support

Experimental Study on Strength Development and Engineering Performance of Coal-Based Solid Waste Paste Filling Material

To explore the strength development characteristics and engineering performance of different coal-based solid waste filling materials cemented into filling body, coal gangue was used as coarse material, fly ash, desulfurization gypsum, gasification slag, and furnace bottom slag as fine material, and cement as a gelling agent. The uniaxial compressive strength (UCS) and bleeding rate of coal-based solid waste cemented backfill (CBSWCB) were tested by an orthogonal experiment, and the influencing factors of mechanical properties and strength development were analyzed. The multiple generalized linear model of strength and bleeding rate was established, and the optimal filling material ratio was determined. The engineering performance index of CBSWCB with the optimal ratio was tested. The results show the following points: (1) the concentration and content of desulfurization gypsum had a great influence on the early compressive strength of CBSWCB, while fly ash, gasification slag, and furnace bottom slag had little influence on the early compressive strength. (2) High concentration, high content of fly ash and furnace bottom slag, low content of desulfurization gypsum, and gasification slag can significantly improve the early strength. High concentration and high content of fly ash, low content of gasification slag, furnace bottom slag, and desulfurization gypsum are beneficial to the later strength increase. (3) Under the optimal ratio scheme, the bleeding rate of CBSWCB was 1.6%, the slump was 16.6 cm, the cohesion was general, the segregation resistance was good, the initial setting time was 5.42 h, the final setting time was 7 h, and the early strength after curing for 8 h reached 0.24 MPa

Study on the fluidity and mechanical properties of multi-source coal-based solid waste (MCSW) filling material

The backfilling mining technology for multi-source coal-based solid waste (MCSW) aligns with the key challenges and emerging paradigms in the coal industry, aiming at intelligent and cleaner coal mining, high efficiency, and low carbon utilization. This research area has gained considerable attention and significance for promoting high-quality coal mining, low environmental impact, and sustainable development. In this study, a response surface experiment was designed with desulfurization gypsum (DG), gasification slag (GS), and furnace bottom slag (FBS) as factors to analyze the performance of backfill materials with varying compositions. Parameters such as bleeding rate, compactness, viscosity, setting time, and age strength were tested. An optimal composition of backfill materials was obtained, with a MCSW content exceeding 90 %, 28-day strength exceeding 2 MPa, initial setting time between 240 and 480 min, and final setting time ranging from 300 to 600 min. The investigation of the optimal materials involved characterizing the pore structure, microstructure, composition, and silica gel structure through techniques such as X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). Additionally, the study explored micro-interface structures and enhancement mechanisms. After 28 days of curing, the backfill with a composition of DG 0.3, FBS 0.2, GS 0.1 exhibited a compressive strength of 5.18 MPa, with an initial setting time as short as 140 min. This can be attributed to the precipitation of silica gel on the grain surface, which mitigated the segregation effect. The silica gel promoted inter-particle bonding and rapid viscosity, preventing particle settling. This method not only ensures the safe disposal of coal-based solid waste but also contributes to the coordinated development of coal resource exploitation, environmental protection, and reduced carbon footprint

Eco-utilization of silicon-rich lye: Synthesis of amorphous calcium silicate hydrate and its application for recovering heavy metals

During sustainable high-value utilization of coal-based solid wastes, a kind of characteristic silicon-rich lye (SRL) generated, and its comprehensive recovery and recycling remains an important subject. Through a simple and facile strategy of mild caustic-crystallization method, SRL was favorably utilized through calcium silicate hydrate (C-S-H) synthesis with silicon conversion efficiency over 97.33%. The micro-morphology of C-S-H presented a typical "honeybee hive " like porous structure with numerous exchangeable activity cations and combination sites in the silicate chain structure. The amorphous C-S-H products possessed marvelous recovery abilities for typical heavy metals as Cu (II), Zn (II), and Cr (III) in industrial wastewater with recovery efficiency all above 99.6% in quite a short period. The recovery mechanism of C-S-H toward heavy metals was revealed as calcium ions exchange and interlayer structure combination with SiO4 and AlO4 tetrahedron through the analysis of characterization technologies and DFT simulation calculation. C-S-H synthesized in SRL contributes to achieving valuable resources conversion and recycling for guiding sustainable development of coal-based solid waste comprehensive utilization