Utilization of coal fly ash in China: a mini-review on challenges and future directions

Hydration and Hardening Properties of High Fly-Ash Content Gel Material for Cemented Paste Backfill Utilization

As more and more mines utilize the cemented paste backfill (CPB) mining method, the demand for reducing backfill cost and carbon footprint is increasing and becoming more critical. In this work, a new backfill gel binder made with 40 wt.% of low-quality Class F fly ash (FCM) is proposed to replace ordinary Portland cement (OPC). The binder hydration and gel hardening properties were experimentally investigated through X-ray diffraction, Mercury intrusion porosimetry, uniaxial compression, and thermogravimetric analysis. Three different mine tailings were used to verify the FCM's applicability. Results show that the strength performance of FCM-CPB is 72% of that of OPC-CPB, while FCM production cost is almost less than half of OPC. The hydration process of the FCM-CPB can be divided into five stages, and the main hydration products are ettringite and gel-like hydrates. The 31.2% porosity of FCM-CPB at 28-day curing is higher than that of 7-day curing, while the average pore size is lower, and the structure is denser. The FCM can meet the strength requirement of three different mine tailings regarding different subsequent filling and cut-and-fill mining methods. The proposed FCM provides a feasible alternative with economic and environmental benefits.

The influence of selected grain size fractions of coal fly ash on properties of clay-cement mortars used for the flood levees construction

This study examines the influence of different grain size fractions of coal fly ash on the properties of clay-cement mortars used in flood levee construction. Dry aerodynamic separation and mesh sieving were used to obtain ultrafine, fine, and medium fractions of high-calcium and silica fly ash. The experimental results reveal that the rheological properties of fresh mortars are significantly influenced by these fractions. High-calcium fly ash mortars exhibit high reactivity and rapid increase in viscosity, with finer fractions showing the highest reactivity. Silica ashes show increased reactivity in the later stages of suspension hardening. Their spherical shape contributes to reducing internal friction during flow in initial technological operations. Furthermore, the compressive strength of hardened mortars improves as the particle size decreases for both ashes, resulting in a dense and uniform microstructure. The separation and fractionation of fly ashes contribute to the obtaining of fractions that influence the parameters of clay-cement suspension application on different scales. The results show the potential benefits of ash separation, which can bring advantages in terms of economic viability, engineering performance, and ecological sustainability.

Thermal Studies of Fractionated Lignite and Brown Coal Fly Ashes

Coal fly ash (CFA), a by-product of coal combustion, is a valuable raw material for various applications. However, the heterogeneous nature of the composition and properties of CFA provides challenges to its effective usage and utilisation. This study investigates the thermal behaviour of the fly ashes of lignite (FA1) and brown coal (FA2) and their fractions obtained by dry aerodynamic separation. Thermal analysis techniques, including thermogravimetry (TG), differential scanning calorimetry (DSC), and evolved gas analysis (EGA), were used to characterise the behaviour of the fly ash fractions while heating up to 1250 °C. The results reveal distinct differences in the thermal behaviour between ash types and among their different size fractions. For the FA1 ashes, the concentration of calcium-rich compounds and the level of recrystallisation at 950 °C increased with the decrease in particle size. The most abundant detected newly formed minerals were anhydrite, gehlenite, and anorthite, while coarser fractions were rich in quartz and mullite. For the FA2 ashes, the temperature of the onset of melting and agglomeration decreased with decreasing particle size and was already observed at 995 °C. Coarser fractions mostly remain unchanged, with a slight increase in quartz, mullite, and hematite content. Recrystallisation takes place in less extension compared to the FA1 ashes. The findings demonstrate that the aerodynamic separation of fly ashes into different size fractions can produce materials with varied thermal properties and reactivity, which can be used for specific applications. This study highlights the importance of thermal analysis in characterising fly ash properties and understanding their potential for utilisation in various applications involving thermal treatment or exposure to high-temperature conditions. Further research on advanced separation techniques and the in-depth characterisation of fly ash fractions is necessary to obtain materials with desired thermal properties and identify their most beneficial applications.

Coal fly ash and bottom ash low-cost feedstocks for CO2 reduction using the adsorption and catalysis processes

Combustion of fossil fuels, industry and agriculture sectors are considered as the largest emitters of carbon dioxide. In fact, the emission of CO2 greenhouse gas has been considerably intensified during the last two decades, resulting in global warming and inducing variety of adverse health effects on human and environment. Calling for effective and green feedstocks to remove CO2, low-cost materials such as coal ashes "wastes-to-materials", have been considered among the interesting candidates of CO2 capture technologies. On the other hand, several techniques employing coal ashes as inorganic supports (e.g., catalytic reduction, photocatalysis, gas conversion, ceramic filter, gas scrubbing, adsorption, etc.) have been widely applied to reduce CO2. These processes are among the most efficient solutions utilized by industrialists and scientists to produce clean energy from CO2 and limit its continuous emission into the atmosphere. Herein, we review the recent trends and advancements in the applications of coal ashes including coal fly ash and bottom ash as low-cost wastes to reduce CO2 concentration through adsorption and catalysis processes. The chemical routes of structural modification and characterization of coal ash-based feedstocks are discussed in details. The adsorption and catalytic performance of the coal ashes derivatives towards CO2 selective reduction to CH4 are also described. The main objective of this review is to highlight the excellent capacity of coal fly ash and bottom ash to capture and selective conversion of CO2 to methane, with the aim of minimizing coal ashes disposal and their storage costs. From a practical view of point, the needs of developing new advanced technologies and recycling strategies might be urgent in the near future to efficient make use of coal ashes as new cleaner materials for CO2 remediation purposes, which favourably affects the rate of global warming.

Influence of Waste Glass Addition on the Fire Resistance, Microstructure and Mechanical Properties of Geopolymer Composites

Nowadays, humanity has to face the problem of constantly increasing amounts of waste, which cause not only environmental pollution but also poses a critical danger to human health. Moreover, the growth of landfill sites involves high costs of establishment, development, and maintenance. Glass is one of the materials whose recycling ratio is still insufficient. Therefore, in the presented work, the influence of the particle size and share of waste glass on the consistency, morphology, specific surface area, water absorption, setting time, and mechanical properties of geopolymers was determined. Furthermore, for the first time, the fire resistance and final setting time of such geopolymer composites were presented in a wide range. Based on the obtained results, it was found that the geopolymer containing 20% unsorted waste glass obtained a final setting time that was 44% less than the sample not containing waste glass, 51.5 MPa of compressive strength (135.2% higher than the reference sample), and 13.5 MPa of residual compressive strength after the fire resistance test (164.7% more than the reference sample). Furthermore, it was found that the final setting time and the total pore volume closely depended on the additive's share and particle size. In addition, the use of waste glass characterized by larger particle sizes led to higher strength and lower mass loss after exposure to high temperatures compared to the composite containing smaller ones. The results presented in this work allow not only for reducing the costs and negative impact on the environment associated with landfilling but also for developing a simple, low-cost method of producing a modern geopolymer composite with beneficial properties for the construction industry.

Isotopic Signatures and Outputs of Lead from Coal Fly Ash Disposal in China, India, and the United States

Despite extensive research and technology to reduce the atmospheric emission of Pb from burning coal for power generation, minimal attention has been paid to Pb associated with coal ash disposal in the environment. This study investigates the isotopic signatures and output rates of Pb in fly ash disposal in China, India, and the United States. Pairwise comparison between feed coal and fly ash samples collected from coal-fired power plants from each country shows that the Pb isotope composition of fly ash largely resembles that of feed coal, and its isotopic distinction allows for tracing the release of Pb from coal fly ash into the environment. Between 2000 and 2020, approx. 236, 56, and 46 Gg Pb from fly ash have been disposed in China, India, and the U.S., respectively, posing a significant environmental burden. A Bayesian Pb isotope mixing model shows that during the past 40 to 70 years, coal fly ash has contributed significantly higher Pb (∼26%) than leaded gasoline (∼7%) to Pb accumulation in the sediments of five freshwater lakes in North Carolina, U.S.A. This implies that the release of disposed coal fly ash Pb at local and regional scales can outweigh that of other anthropogenic Pb sources.

Effect of the Coal Fly Ash Blending Ratio on Biomass Slagging Structure Modification and Alkali Metal Migration

The analysis of ash deposition samples by electron microscopy (EM), scanning electron microscopy (SEM)/energy dispersive scanning (EDS), and X-ray diffraction (XRD) was carried out at three sampling points of a one-dimensional furnace (sampling temperatures were TL5 = 960 °C, TL6 = 855 °C, and TL7 = 820 °C, respectively). The improvement of ash deposition characteristics of biomass combustion with the different addition ratios of coal fly ash (CFA) (Mr = 1:1, 1.5:1 or 2:1) and the heterogeneous removal of gas phase alkali metal heterogeneous were studied. The results showed that under the condition of Mr = 1:1, the ash sample presented a three-dimensional porous loose structure like "coral reef" with the rice husk ash as the skeleton and the combustion products of CFA as the bonding point. The spherical and filamentous crystals mainly composed of SiO2, Al2O3, and silicates were distributed in the slag, and the surfaces of these crystals were accompanied by the eutectic composite salt of K-Al-Si with a high melting point, reflecting the heterogeneous curing effect of CFA on the gas phase migration of the alkali metal (K). With the decreasing addition ratio of CFA (Mr = 1.5:1 or 2:1), the spherical and filamentous crystals were increased in the slag. MgO, Fe2O3, and Al2O3 may be involved in the formation of high-temperature refractory Mg-Fe-aluminate, Ca- and Mg-silicate, which were the precursors of the rigid slag. The alkali metals mostly presented in the form of K2O and KCl in the gas-solid conversion under the temperature effect, serving as binders for bridging, agglomeration, and slagging of the above precursors. In addition, the kalium enrichment was accompanied by Al and S in the CFA, indicating that the Al- and S-active sites have obvious adsorption to kalium.

Review of the Preparation and Application of Porous Materials for Typical Coal-Based Solid Waste

The discharge and accumulation of coal-based solid waste have caused great harm to the ecological environment recently. Coal-based solid wastes, such as coal gangue and fly ash, are rich in valuable components, such as rare earth elements (REY), silicon dioxide, alkali metal oxides, and transition metal oxides, which can be used to synthesize various functional Si-based porous materials. This article systematically summarizes the physicochemical characteristics and general processing methods of coal gangue and fly ash and reviews the progress in the application of porous materials prepared from these two solid wastes in the fields of energy and environmental protection, including the following: the adsorption treatment of heavy metal ions, ionic dyes, and organic pollutants in wastewater; the adsorption treatment of CO2, SO2, NOx, and volatile organic compounds in waste gas; the energy regeneration of existing resources, such as waste plastics, biomass, H2, and CO; and the preparation of Li-Si batteries. Combining the composition, structure, and action mechanism of various solid-waste-based porous materials, this article points out their strengths and weaknesses in the above applications. Furthermore, ideas for improvements in the applications, performance improvement methods, and energy consumption reduction processes of typical solid-waste-based porous materials are presented in this article. These works will deepen our understanding of the application of solid-waste-based porous materials in wastewater treatment, waste gas treatment, energy regeneration, and other aspects, as well as providing assistance for the integration of new technologies into solid-waste-based porous material preparation industries, and providing new ideas for reducing and reusing typical Chinese solid waste resources.

Firing-Associated Recycling of Coal-Fired Power Plant Fly Ash

Coal-fired power plant fly ash is a global environmental concern due to its small particle size, heavy metal content, and increased emissions. Although widely used in concrete, geopolymer, and fly ash brick production, a large amount of fly ash remains in storage sites or is used in landfills due to inadequate raw material quality, resulting in a waste of a recoverable resource. Therefore, the ongoing need is to develop new methods for recycling fly ash. The present review differentiates the physiochemical properties of fly ash from two coal combustion processes: fluidized bed combustion and pulverized coal combustion. It then discusses applications that can consume fly ash without strict chemical requirements, focusing on firing-associated methods. Finally, the challenges and opportunities of fly ash recycling are discussed.

Influence of Various Coal Energy Wastes and Foaming Agents on Foamed Geopolymer Materials' Synthesis

The regularities of obtaining foamed alkali-activated geopolymer materials based on different wastes of coal power engineering (fly ash, fuel (boiler) slag, ash, and slag mixture) were considered. The phase composition of the studied waste showed the presence of a significant amount of the amorphous phase, as well as a crystalline phase. mostly in the form of high quartz. The microstructure of studied the waste showed that the fly ash consisted of monodisperse hollow aluminosilicate microspheres, the fuel slag was represented by polydisperse irregular particles, and the ash and slag mixture included both of these materials in different ratios. Blowing agents such as aluminum powder, hydrogen peroxide, and sodium hypochlorite were chosen to achieve the porous structure of the geopolymer materials. The calculations of the geopolymer precursor compositions were carried out. Samples were synthesized, and their physical and mechanical properties, such as density, strength, porosity, and thermal conductivity, were analyzed. The micro- and macrostructure of the samples, as well as the pore distribution of the obtained geopolymers were studied. Conclusions were made on the choice of the most-optimal foaming agent and the optimal coal combustion waste suitable for the synthesis of the geopolymer materials.

Reburning pulverized coal with natural gas/syngas upgrading: NO reducing ability and physicochemical structure evolution of coal char

The lifting gas activates the coal particles, which increases their ability to reduce NO. This technique overcomes the oxygen consumption of large pulverized coal in the early stages of re-firing during air/flue gas transport of pulverized coal. This study conducted experiments on a planar flame burner bench to analyze the physicochemical structure evolution of coal coke after natural gas and syngas activation using FTIR, XPS, and BET. The NO reduction capacity was tested on a micro fluidized bed reaction test bench. The results show that natural gas's upgrading effect is better than syngas. Hydrogen and hydrocarbon radicals generated by the reaction of natural gas with oxygen play a significant role in activation. After upgrading by natural gas, the specific surface area of carbon increased by about 54.2 %, the total pore volume increased by about 51.2 %, the whole oxygen-containing groups decreased by nearly 4.4 %, the total amount of alkyl complexes increased by about 3.6 %, and the nitric oxide reducing ability increased by almost 75 %. The technology minimizes expensive reactive gases while ensuring less reburned coal is used to reduce NOx emissions.

Immobilization of Pb and Zn in Contaminated Soil Using Alumina-Silica Nano-Amendments Synthesized from Coal Fly Ash

To apply coal fly ash to the remediation of heavy-metal-contaminated soil, an alumina-silica nano-amendment (ASNA) was synthesized from coal fly ash and was used for the immobilization of lead and zinc in contaminated soil. The investigation on the synthesis of the ASNA shows that the ASNA can be obtained under a roasting temperature of 700 °C, a ratio of alkali to coal fly ash of 1.2:1, and a molar ratio of silicon to aluminum of 1:1. The ASNA could increase the soil pH and cation exchange capacity (CEC) and decrease the bioavailability of Pb and Zn. When the ASNA addition increased from 0 to 2%, the bioavailability (extracted by CaCl₂) of Pb and Zn decreased by 47% and 72%, respectively. Moreover, the addition of the ASNA facilitated the transformation of Pb from a reducible fraction to oxidizable and residual fractions and Zn from an exchangeable fraction to a residual fraction. The correlation analysis and cluster analysis verify that the ASNA modulates the chemical speciation of heavy metals by increasing the soil's CEC and pH, thereby immobilizing heavy metals. It is expected that this study can provide a new method for the remediation of Pb- and Zn-contaminated soil.

Predicting the risk of nodular thyroid disease in coal miners based on different machine learning models

Background

Nodular thyroid disease is by far the most common thyroid disease and is closely associated with the development of thyroid cancer. Coal miners with chronic coal dust exposure are at higher risk of developing nodular thyroid disease. There are few studies that use machine learning models to predict the occurrence of nodular thyroid disease in coal miners. The aim of this study was to predict the high risk of nodular thyroid disease in coal miners based on five different Machine learning (ML) models.

Methods

This is a retrospective clinical study in which 1,708 coal miners who were examined at the Huaihe Energy Occupational Disease Control Hospital in Anhui Province in April 2021 were selected and their clinical physical examination data, including general information, laboratory tests and imaging findings, were collected. A synthetic minority oversampling technique (SMOTE) was used for sample balancing, and the data set was randomly split into a training and Test dataset in a ratio of 8:2. Lasso regression and correlation heat map were used to screen the predictors of the models, and five ML models, including Extreme Gradient Augmentation (XGBoost), Logistic Classification (LR), Gaussian Parsimonious Bayesian Classification (GNB), Neural Network Classification (MLP), and Complementary Parsimonious Bayesian Classification (CNB) for their predictive efficacy, and the model with the highest AUC was selected as the optimal model for predicting the occurrence of nodular thyroid disease in coal miners.

Result

Lasso regression analysis showed Age, H-DLC, HCT, MCH, PLT, and GGT as predictor variables for the ML models; in addition, heat maps showed no significant correlation between the six variables. In the prediction of nodular thyroid disease, the AUC results of the five ML models, XGBoost (0.892), LR (0.577), GNB (0.603), MLP (0.601), and CNB (0.543), with the XGBoost model having the largest AUC, the model can be applied in clinical practice.

Conclusion

In this research, all five ML models were found to predict the risk of nodular thyroid disease in coal miners, with the XGBoost model having the best overall predictive performance. The model can assist clinicians in quickly and accurately predicting the occurrence of nodular thyroid disease in coal miners, and in adopting individualized clinical prevention and treatment strategies.

Feasible synthesis of coal fly ash based porous composites with multiscale pore structure and its application in Congo red adsorption

Facing the great challenge that the increasing solid waste fly ash is difficult to treat and utilize properly, this paper reports a class of novel low-cost fly ash porous materials with high interconnected porosity fabricated by a facile foaming process. On this basis, composites with multiscale pore structures from the nanometer to macroscopic scale were designed and constructed by decorating layered double hydroxide (LDH) onto the inner channel surface. Such porous materials with 3D through-hole structures showed high interconnected porosity (up to 77.61%), suitable compressive strength (up to 23.79 MPa) and significant water permeation flux (549.86 m³∙m^-2∙h^-1 at 0.1 MPa). Moreover, the adsorption effect of dosage, initial concentration, pH, temperature and contact time on Congo red (CR) from simulated wastewater was investigated. The composites exhibited a good adsorption efficiency of ∼100% and adsorption capacities of 45.79 mg/g. The adsorption kinetic can be explained well by the pseudosecond-order kinetic equation and isotherm adsorption followed Langmuir isotherm model. This suggests that low-cost and eco-friendly fly ash composites have potential applications in industrial-scale wastewater treatment. This work also provides a general strategy to design and utilize fly ash porous materials for filtration and adsorption.

Conversion of Harmful Fly Ash Residue to Zeolites: Innovative Processes Focusing on Maximum Activation, Extraction, and Utilization of Aluminosilicate

Reuse of the solid residue from coal fly ash alumina extraction (FAAE) by acid leaching is problematic. Conversion of this solid residue into aluminum-rich zeolite (13X) and silicon-rich zeolite (ZSM-5) was investigated in this research. The FAAE residue was activated by alkali roasting with Na₂CO₃ powder (110% mass fraction) at 890 °C for 60 min. Silicon and aluminum were mainly present as two mineral phases, Na₂SiO₃ and NaAlSiO₄, respectively, in the product obtained after roasting. The roasted product was dissolved in water (liquid/solid ratio of 2) after 20 min at 100 °C. The water-leaching liquor was investigated for total conversion to aluminosilicate zeolites without external aluminum or silicon addition. Hydrothermal synthesis of aluminum-rich zeolite 13X was successful after fine tuning of the conditions, although the filtrate had an unusually high SiO₂/Al₂O₃ molar ratio. Production of 13X consumed a large amount of aluminum, which increased the Si/Al ratio to a level suitable for synthesis of ZSM-5. The synthesis of ZSM-5 from the mother liquor of 13X was proved feasible. The FAAE residue was transformed into high-value zeolite products by nearly 100%. Additionally, the tail liquid of this process, mainly containing Na₂CO₃, was completely recycled. This process could be used to realize high-efficiency and high-value utilization of similar aluminosilicate solid wastes.

Continuous Extraction and Continuous Backfill Mining Method Using Carbon Dioxide Mineralized Filling Body to Preserve Shallow Water in Northwest China

The exploitation and utilization of coal resources are not only prone to causing water table lowering, but also produce a large amount of CO2 and coal-based solid waste. A scientific concept that employs the CO2 and solid wastes to develop filling bodies and inject them into the mined-out area, to sequestrate CO2 and mitigate the overburden migration and thus preserve the overlying aquifer, is proposed. Continuous extraction and continuous backfill (CECB) mining was selected as the mining method to meet the aforementioned objectives. Additionally, carbon dioxide mineralized filling body (CMFB) under ambient temperature and pressure was developed, with fly ash as aggregate, and CO2 gas, silicate additives and cement as accessories. The uniaxial compressive strength (UCS) and tensile strength of CMFB with various curing times and fly ash contents were tested indoors. A physical analogue simulation and FLAC3D numerical calculation were then successively implemented on the premise of determining a similar material ratio of CMFB in analogue simulation and calibrating the parameters of the CMFB in numerical simulation. The deformation of aquifuge and water level lowering while using CECB and CMFB with various proportion of fly ash were obtained. When using the CMFB with 75% fly ash content and 28 d curing time, the maximum values of vertical displacement, horizontal displacement, inclination, horizontal deformation and curvature of aquiclude were 26 mm, 6.5 mm, 0.12 mm/m, 0.08 mm/m and 0.0015 mm/m2, respectively, and the water table decreased 0.47 m. The results show that the CMFB with 75% fly ash is the most appropriate ratio to realize water preservation mining, CO2 sequestration and harmless treatment of solid wastes, contributing to the green and sustainable development of coal areas.

Increasing mercury risk of fly ash generated from coal-fired power plants in China

The trend in mercury (Hg) content of fly ash, the primary solid Hg waste from coal-fired power plants (CFPPs), has not been well evaluated in China. This study integrated a national sampling campaign, a literature survey, and model predictions to investigate the trend in Hg content of fly ash and associated output nationwide. The results demonstrated that Hg content of fly ash from both our campaign conducted in 2020 and the literature survey met the Weibull distribution. The best estimate for the distribution of Hg content in our campaign (401.4 µg/kg) was about two-fold higher than that from the literature survey (236.6 µg/kg). The increasing trend was mainly attributed to the increasing installation of advanced air pollution control devices (APCDs) in CFPPs. The total national Hg output by fly ash was estimated to be 217.7 tons (64.1-501.8 tons) currently. Regarding the disposal of fly ash, a total of 16.1 tons and 105.8 tons of Hg were re-emitted into the air and immobilized in materials, respectively. The increasing trend in the Hg content of fly ash suggested the need for more stringent requirements for the disposal of solid Hg waste from CFPPs in China.

Improving the Properties of Porous Geopolymers Based on TPP Ash and Slag Waste by Adjusting Their Chemical Composition

The possibility of improving the properties of porous geopolymer materials based on ash and slag waste from thermal power plants by adjusting their chemical composition is considered. An X-ray phase analysis of ash and slag wastes was carried out, the geopolymers’ precursor compositions were calculated, and additives to correct their chemical composition were selected. The samples were synthesized and their physical and mechanical properties (density, porosity, compressive strength, thermal conductivity) were analyzed. The micro- and macro-structure of the samples and the pore distribution of the obtained geopolymers were studied and pore-distribution histograms were obtained. The influence of Si:Al ratio on structural changes was described. The geopolymers’ phase composition was studied, consisting of an amorphous phase and high quartz and mullite. A conclusion about the applicability of this method for obtaining high-quality porous geopolymers was made.

The Recycling of Coal Fly Ash: A Review on Sustainable Developments and Economic Considerations

The recycling and utilization opportunities for coal fly ash (CFA) have increased in the past two decades. However, limited commercialization of the material is still reported, while disposal and management remain major concerns. CFA utilization is currently commercially feasible in the building and construction industry. Other alternative uses that are being explored involve the extraction of valuable metals and the purification of wastewater. The CFA-produced adsorbent material utilized in wastewater purification processes should be able to generate water that meets the legal quality requirements for reutilization in alternative applications. On the other hand, in the recovery of metallic components such as smelter-grade alumina, high recovery and high purity products are only achievable through the processing of CFA using expensive and energy—intensive processes. Furthermore, most of the current CFA recycling processes tend to generate secondary solid residues (SSR), which can cause environmental pollution, thus requiring further downstream processing. In this context, this paper reviews and discusses current research on CFA recycling methods, challenges and opportunities associated with resource recovery from CFA, and the acceptability of the value-added products, and it therefore proposes sustainable processes for CFA utilization. This review further suggests that to successfully compete with bauxite for production of smelter-grade alumina, other saleable value-added products such as Ti, Fe and the REEs should be recovered by engineering an integrated process design. The generated SSR in each process must also be characterized, recycled and re-used to reduce waste production and advance the circular economy concept. The review concludes that for CFA to become considered as a more attractive commercial resource, there is need for its complete and holistic utilization in high volumes and in different applications to offset its low value.

Failure Criteria and Constitutive Relationship of Lightweight Aggregate Concrete under Triaxial Compression

This paper investigates the compression behavior and failure criteria of lightweight aggregate concrete (LAC) under triaxial loading. A total of 156 specimens were tested for three parameters: concrete strength, lateral confining pressure and aggregate immersion time, and their effects on the failure mode of LAC and the triaxial stress-strain relationship of LAC is studied. The research indicated that, as the lateral constraint of the specimen increases, the failure patterns change from vertical splitting failure to oblique shearing failure and then to indistinct traces of damage. The stress-strain curve of LAC specimens has an obvious stress plateau, and the curve no longer appears downward when the confining pressure exceeds 12 MPa. According to the experimental phenomenon and test data, the failure criterion was examined on the Mohr–Coulomb theory, octahedral shear stress theory and Rendulic plane stress theory, which well reflects the behavior of LAC under triaxial compression. For the convenience of analysis and application, the stress-strain constitutive models of LAC under triaxial compression are recommended, and these models correlate well with the test results.

A review on mineralogical speciation, global occurrence and distribution of rare earths and Yttrium (REY) in coal ash

Abstract

Rare earths and Yttrium (REY) are a group of critical metals essential for this electronic and digital era. China is the leading producer of REY with more than 90% of global export. Mines of REY are limited and the need for green and efficient energies have augmented the demand of REY and it is putting enormous pressure on global production. REY market is predicted to grow from USD 5.3 billion in 2021 to 9.6 billion by 2026, at a CAGR (Compound Annual Growth Rate) of 12.3%. The need for permanent magnets is propelling the demand of the critical group REY and is expected to rise gradually in the coming years. In the present review, we have summarized the minable REY resources and their applications. The requirement for alternative resource is pivotal to meet our future needs. We have extensively reviewed the studies of REY in coal fly ash (CFA). A comprehensive analysis has been done for the REY resources worldwide for the last several decades in coal ash (CFA and bottom ash) and divulged into the application, speciation and distribution for major coal-consuming countries like China, India, USA, Russia, UK, Poland, etc., individually. We have also made a comparative global study and inferred potential extractable coal ash resources using various parameters such as global average, critical percentage (C_p), outlook coefficient (C_out), etc., for a better understanding of economic exploitation.

Research highlights

We have put up enormous effort to synthesize rare earth elemental data of coal ash from different coal-consuming countries. Following are the major highlights of this review article.

We have compiled data on occurrence of Rare Earths and Yttrium (REY) in coal ash from 13 countries such as China, India, USA, UK, Poland, etc.

Up-to-date global data of mined REY resources and reserves have been compiled.

Broad characterization and classifications of REY have been introduced.

Comprehensive analysis of application, speciation and environmental impact of REY in coal ash have also been compiled.

Comparative study has been done using parameters such as global average, critical percentage, outlook coefficient, etc. These parameters would help in determining ideal candidates for beneficial extraction of REY.

This study would serve as a knowledge resource centre for new research related to REY.

Fast Screening of Coal Fly Ash with Potential for Rare Earth Element Recovery by Electron Paramagnetic Resonance Spectroscopy

Rare earth elements (REYs) are in increasing global demand, but their mining is costly and environmentally destructive. Coal fly ash (CFA) is a promising alternative source of REYs, but it is necessary to identify CFA with sufficiently high REY concentrations. This study proposes the use of electron paramagnetic resonance (EPR) spectroscopy as part of a simple method to identify CFAs with adequate REY concentrations. The EPR spectra of CFA samples taken from 186 Chinese commercial coal-fired power plants were analyzed. The results suggest that CFAs without evident 6-fold resonances are worth recycling (REY concentrations of 416 ± 108 mg/kg), while those with conspicuous 6-fold resonances are not worth recycling (REY concentrations of 55 ± 26 mg/kg). This is probably due to isomorphic substitution of Ca(II) for Mn(II) and REY(III), resulting in low concentrations of Mn(II) and REY(III) in Ca-rich CFAs. This EPR evaluation method does not require specialized sample preparation, professional skills, or secondary data analysis and has potential global significance in the fast screening of CFAs with REY-recycling potential.

Coal fly ash driven zeolites for the adsorptive removal of the ceftazidime drug

The overall cost and efficiency of an adsorbent material is a major issue in deriving a sorbent into commercial markets. In this study, efforts have been directed to produce adsorption-capable zeolites from the dispensable product of coal power plants, i.e., coal fly ash (CFA). In addition, coal mining water (CW) was used as a direct hydrothermal solvent. The mine water from China's coal mines was used in this experiment to substitute tap water (TP) for synthesizing zeolite from C-type fly ashes with different crystallization temperatures (45 to 95 °C). Here, CW led to the formation of X-type and A-type zeolites of comparable size. Regarding the proper utilization of waste products, i.e., coal fly ash and mine water, the study paves a simple yet extremely cost-effective approach to synthesize workable zeolitic materials for adsorption purposes. The detailed characterization justified the use of CW as a better solvent than TP to prepare zeolites based on their better granular size and fewer carbon impurities. The prepared zeolites were later used as an adsorbent for the trace removal of ceftazidime (CAZ), taken as a model pharmaceutical pollutant. The zeolites prepared using CW realised a higher adsorption capacity of 80 mg g⁻¹ during 20 min of agitation time. The pH, concentration, and external salt effects were also studied to achieve maximum removal efficiency. In general, the proposed approach enables the production of affordable yet efficient zeolite-based adsorbent materials without consuming any toxic and expensive reagents for practical application in environmental remediations.

Diagram showing a systematic approach to preparing zeolites using the hydrothermal approach with the coal mine water and tap water as dissolution solvents.

Estimating the Cost-Competitiveness of Recycling-Based Geopolymer Concretes

The cement industry is a major contributor to greenhouse gas emissions on a global scale. Consequently, there has been an increasing interest, in both academia and business, in low-carbon concretes in which Ordinary Portland Cement (OPC) is partially or fully replaced with industrial side streams. However, the realization of the environmental benefits of such materials depends on how competitive they are in the construction market, where low costs are a major competitive factor. This is not straightforward, as many types of concretes exist. Raw material prices vary, and costs can be influenced by governmental regulations via carbon pricing. This study presents a case study estimating the cost prices of four different geopolymer concretes with different material compositions and carbon footprints, considering the raw material price variability and the potential impact of carbon emissions regulation (carbon price). The case study demonstrates how material mix cost comparisons can be made openly and systematically. The results imply that carbon pricing, at the rates currently applied, does not significantly change the cost price difference between traditional and geopolymer concretes. Instead, cost-competitiveness of low carbon concretes depends heavily on the material mix type and the availability of critical side streams.

Activation Pretreatment and Leaching Process of High-Alumina Coal Fly Ash to Extract Lithium and Aluminum

Experiments were conducted to investigate the process of aluminum and lithium extraction from high-alumina coal fly ash (HCFA) generated from coal-fired power plants located in northern China. The presence of mullite and other aluminosilicates lead to low reactivity of coal fly ash. An activation pretreatment that destroys an inert composition of coal is necessary. The activation roasting of coal fly ash using sodium chloride and a subsequent leaching process were performed in this research. The results showed that almost no aluminum and lithium were dissolved under direct water leaching, while about 7% and 10% of those were leached into the acid solution respectively. Adding NaCl enhanced the atmospheric pressure leaching of aluminum and lithium with a leaching rate around 50%. Phase analysis and equilibrium calculations results showed that the roasting reaction between the HCFA and NaCl occurred, which led to generation of main new phase NaAlSi3O8. The pressure extraction efficiencies of aluminum and lithium were increased to about 93% and 98%, respectively. The implications of the findings provide an alternative process for recovering aluminum and lithium from readily available high-alumina coal fly ash.