Coal Fly Ash Samples Research Articles

Characterisation and Hydrochloric Acid Leaching of Rare Earth Elements in Discard Coal and Coal Fly Ash

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.

Investigating the Hydration, Mechanical Properties, and Pozzolanic Activity of Cement Paste Containing Co-Combustion Fly Ash

The heat of hydration, mechanical properties, pozzolanic activity, and microscopic characteristics of cement pastes incorporating co-combusted fly ash (CCFA) were investigated, and the disparities between the CCFA/cement system and the coal fly ash (CFA) binding system were also compared. The results indicate a decrease in the heat of hydration for both CFA and CCFA samples, with a more pronounced trend observed as the fly ash content increased from 10% to 30%. The distinction in the early hydration between CFA and CCFA samples primarily manifested in the rate of heat release, potentially correlated with variations in the active Al2O3 content in the fly ash. Neither CFA nor CCFA samples exhibited significant cementitious activity at 3 days, functioning solely as inert fillers in the cement paste. By 3 and 28 days, the mechanical properties of both CFA and CCFA samples were inferior to those of pure cement paste. However, by 180 days of hydration, the compressive strength of CCFA-blended mortar notably increased, with the highest strength observed in the 10% CCFA-blended sample. Both CFA and CCFA samples produced the secondary hydration product C-A-S-H and demonstrated comparable consumption of calcium hydroxide (CH). These findings underscore the potential of CCFA as a supplementary cementitious material (SCM) and lay a foundation for its widespread adoption.

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.

Assessment of enhanced strength and stiffness properties of bio-engineered coal fly ash

This study investigates the use of urease-producing bacteria for microbial induced calcite precipitation (MICP) to enhance the mechanical properties of coal fly ash (CFA). Sporosarcina pasteurii was employed to treat CFA with cementation solutions over 7, 14, and 28 days. Biotreated samples exhibited improved compressive strength, shear strength, stiffness, and cohesion. Calcite precipitation increased, and permeability decreased with treatment cycles, resulting in a 78% reduction in permeability and 8% calcite precipitation in a 28-day cycle. X-ray diffraction (XRD) and scanning electron microscope (SEM) investigations support the improved engineering properties of bioengineered coal fly ash (BCFA) samples.

Experimental and numerical techniques to evaluate coal/biomass fly ash blend characteristics and potentials

Fossil and renewable fuels are used by industrial units that produce energy-intensive products. Competitive fuel pricing encourages these fuel sources' usage globally, particularly in developing nations, which leads to large volumes of byproducts like fly ash among thermal power plant operators. The elements and compounds found in coal fly ash (CFA) and biomass fly ash (BFA) can be utilized through several engineering applications. This study aims to assess typical CFA and BFA samples quantitatively and qualitatively via techniques such as ultimate analysis (CH-S), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray fluorescence (XRF) elemental analysis, and ash fusion temperature (AFT), to anticipate the ideal ratios of coal to biomass blends for combustion applications while adhering to environmental regulations. The optimal blend, consisting of 75 % CFA and 25 % BFA, exhibited improved carbon (C%) and hydrogen (H%) percentages, increasing from 2.5 % to 4.67 % and from 0 % to 0.12 %, respectively. These improvements were further confirmed by the observed functional groups in FTIR, indicating a rising trend in both carbon and hydroxyl groups from BFA to CFA. XRF and XRD also confirmed it and TGA also showed optimum mass loss (ML%) behavior of 14.55 % for 75CFA + 25BFA. According to slagging and fouling indices, the values of RB/A, Rs, and Fu indicate a reduction in slagging and fouling issues through the blending of CFA with BFA. Simultaneously, the fusion temperature increased from 1181 °C to 1207 °C. CFA was found to increase the AFT of the BFA from 1197 °C to 1247 °C, mitigating their propensity. This suggests that a blend of 75CFA + 25BFA results in lower to medium range of slagging and fouling. However, AFI and BAI indicate a slightly higher range. AFT analysis further validates the conclusions drawn from the indices. The ternary phase diagram shows that the ash's melting point increases in the optimum blend. This is attributed to a reduced content of K2O (<15 %) and increased proportions of >50 % CaO and SiO2, effectively inhibiting slagging, agglomeration, and deposition. Meanwhile, the blend maintains a medium level of acidity and susceptively to corrosion, as observed in the case of 75CFA + 25BFA. The identification of optimal blend ratios can be anticipated to offer essential solutions for future research, aiming to ensure smooth industrial operations and regulatory compliance in power plants.

Assessment of Viability of Coal Mines in Tanzania for Extraction of Rare Earth Elements

This study aimed at determining the concentrations of rare earth elements (REEs) in coal and coal fly ash (CFA) from three coal mines in Tanzania: Kiwira, Ngaka and Rukwa. The goal was to assess if these resources could be commercially viable for extracting REEs. Coal and CFA samples were analysed using X-ray fluorescence (XRF) spectrometry and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The total concentrations of REEs in the coal samples ranged from 89.48 parts per million (ppm) to 196.30 ppm, while in CFA samples, ranged from 362.55 ppm to 475.77 ppm. Computed percentage of critical REEs (REYdef, rel%) and outlook coefficient (Coutl) values ranged from 27.41% to 50.74% and 0.62 to 2.00, respectively. Based on the evaluation criteria proposed for assessing coal and coal ash as sources of REEs, the results suggest that the studied coal and CFA samples have the potential to be used as sources of REEs for economic development. These findings have important implications for the Tanzanian government and other relevant authorities, as they provide valuable insights into the feasibility of investing in the coal and coal ash as promising sources of REEs. This is particularly significant considering the high global demands for REEs.

The early hydration and rheological characteristics of cement paste containing co-combustion fly ash

In this study, the early hydration and rheological characteristics of cement paste containing co-combustion fly ash (CCFA) were studied by using isothermal calorimetry, low-field NMR (LF-NMR), and rheological tests, and the differences between CCFA/cement system and the traditional binding system made with coal fly ash (CFA) were analyzed. The results indicate that co-combustion fly ash has a retarding effect on the early hydration of the cement. This effect becomes more pronounced as the dosage of fly ash increases from 10% to 30%. Additionally, the early hydration rate of samples containing CCFA was found to be faster compared to samples with CFA, even when the same admixture was used. The T2 spectrum analysis reveals that fly ashes primarily influence the early hydration characteristics by affecting the consumption of capillary water. As the admixture of fly ash increases, there is a reduction in moisture loss. The pattern of moisture evolution exhibits a strong correlation with the heat of hydration. At 72 h, the relaxation characteristics of both CCFA and CFA samples were found to be similar. Moreover, the rheological properties of the cement paste are influenced by factors such as the type and admixture amount of fly ash, as well as the degree of hydration. With an increase in the amount of fly ash blending, the rheological properties exhibit an increase as well. Specifically, CCFA samples display higher plasticity compared to CFA samples, even when the same admixture amount is used. The results of different analytical methods in the study were consistent.

Recovery of Rare Earth Elements from Coal Fly Ash with Betainium Bis(trifluoromethylsulfonyl)imide: Different Ash Types and Broad Elemental Survey

Previously, proof-of-concept studies have demonstrated that rare-earth elements (REEs) can be preferentially extracted from coal fly ash (CFA) solids using a recyclable ionic liquid (IL), betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]). When the suspension of aqueous solution—IL-CFA—is heated above 65 °C, the majority of REEs will separate from the bulk elements in the solids and partition to the IL phase. Acid stripping of the IL removes REEs and regenerates the IL for reuse in additional extraction cycles. The objective of this study is to showcase the applicability and effectiveness of the optimized method to recover REEs from various CFAs. Six CFA samples with different characteristics (feed coal basins, coal beds, and ash collecting points) and classifications (Class C and Class F) were examined. The process performance was evaluated for a broad range of elements (33 total), including 15 REEs, two actinides, six bulk elements, and 10 trace metals. Results confirmed good recovery of total REEs (ranging from 44% to 66% among the CFA samples) and the recovery process’ high selectivity of REEs over other bulk and trace elements. Sc, Y, Nd, Sm, Gd, Dy, and Yb consistently showed high leaching and partitioning into the IL phase, with an average recovery efficiency ranging from 53.8% to 66.2%, while the other REEs showed greater variability among the different CFA samples. Some amounts of Al and Th were co-extracted into the IL phase, while Fe co-extraction was successfully limited by chloride complexation and ascorbic acid reduction. These results indicated that the IL-based REE-CFA recovery method can maintain a high REE recovery efficiency across various types of CFA, therefore providing a promising sustainable REE recovery strategy for various coal ash wastes.

Studies on hydrometallurgical recovery of rare earth elements as mixed fluoride compound from coal fly ash using environmentally friendly organic acid

ABSTRACT Coal fly ash (CFA) emerged as one of the potential secondary source of rare earth elements (REEs). This paper discusses development of a process scheme for the recovery of REE from a CFA sample, assaying total REE of about 0.2% using environmentally benign lactic acid (LA) as leachant. The CFA is generated at a lignite coal-based thermal power plant. The REE occurs dominantly in an ion-exchangeable form and also as bound to iron oxides. Nearly 25% of total REE is constituted of heavy rare earth elements (HREEs). About 89% of REE values were found leachable with LA at 80°C in 2 h of reaction time at 10% solids content. Kinetic analysis of leaching process shows that the reaction between CFA and LA is chemical reaction controlled with activation energy of 42–45 kJ/mol. The dissolved REE values were extracted quantitatively from leachate using D2EHPA solvent diluted in kerosene and stripped back with 6 N HCl. The REE in strip liquor was precipitated as mixed REE-fluoride concentrate of purity 86% with precipitation efficiency of 98%. A scheme for quantitative recovery of LA from barren stream was developed for its recycle in leaching circuit.

Green Approach for Rare Earth Element (REE) Recovery from Coal Fly Ash.

Due to the growing demands of rare earth elements (REEs) and the vulnerability of REEs to potential supply disruption, there have been increasing interests in recovering REEs from waste streams such as coal fly ash (CFA). Meanwhile, CFA as a large industrial waste stream in the United States (U.S.) poses significant environmental and economic burdens. Recovery of REEs from CFA is a promising solution to the REE scarcity issue and also brings opportunities for CFA management. This study demonstrates a green system for REE recovery from Class F and C CFA that consists of three modules: REE leaching using citrate, REE separation and concentration using oxalate, and zeolite synthesis using secondary wastes from Modules I and II. In Module I, ∼10 and 60% REEs were leached from the Class F and C CFA samples, respectively, using citrate at pH 4. In Module II, the addition of oxalate selectively precipitated and concentrated REEs from the leachate via the formation of weddellite (CaC2O4·2H2O), while other trace metals remained in solution. In Module III, zeolite was synthesized using wastes from Modules I and II. This study is characterized by the successful recovery of REEs and upcycling of secondary wastes, which addresses both REE recovery and CFA management challenges.

Machine learning exploration of the mobility and environmental assessment of toxic elements in mining-associated solid wastes

Continuous development of the mining industry has led to the production of large volumes of solid wastes. Toxic elements (TEs) have been identified in mining-associated solid wastes, whose leaching potential poses a significant environmental risk. In this study, we introduce a synthesis framework titled Fast mobIlity Evaluation and environmentaL inDex (FIELD). The physicochemical properties of solid waste, element properties, and total TEs concentrations were used as input variables to construct the machine learning model and predict the TEs fractions. Novel environmental indexes were then proposed based on both the total TEs concentrations and corresponding chemical speciation. Applying the framework to a case study of coal fly ash (CFA) demonstrated robust performance of the deep neural network model with good generalization capabilities (R2 = 0.83 on the testing set), indicating it can rapidly and accurately predict TEs fractions. The TEs fractions were found to be primarily affected by the element properties. The results also demonstrate that the proposed environmental indexes are superior to current indexes for identifying the most environmentally hazardous TEs within a specific CFA sample, as well as for identifying the most dangerous CFA sample based on TEs fractions. Compared with traditional laboratory analysis and environmental risk assessment methods, the FIELD framework proposed in this study is more efficient and achieves robust results, with important reference significance for environmental pollution control.

Estimation of rare earth elements in Indian coal fly ashes for recovery feasibility as a secondary source

Huge amount of coal fly ash is being generated in India and the management of unutilized ash became an environmental issue due to its hazardous nature. Identification of new applications will increase its consumption which in turn reduces the environmental burden. Recently, fly ash was identified as potential secondary source to recover valuable rare earth elements. Considering this, the current study was initiated to quantify the rare earths in Indian coal fly ashes and the generated data was assessed for identifying suitable coal fly ash to recover rare earth elements. Accordingly, eight coal fly ash samples and two Indian coals were collected, acid digested and quantified for rare earths using Triple Quadruple-Inductively Coupled Plasma Mass Spectrometry. Chemical analysis results show that, Ce has highest abundance with concentration values ranging from 88±1.5 – 218±5.8 µg/g and the least concentration was observed for Lu with typical values ranging from 0.5 ± 0.02 – 0.8 ± 0.01 µg/g. Sum of all rare earth's content in analyzed samples was found in the range 234 µg/g to 533 µg/g. Coal fly ash samples, had critical REY content ≥30% (except few samples) and an outlook coefficient ≥0.7 which indicates that some samples can be used as secondary resource for rare earth elements extraction.

Characterization of trace elements in coal fly ash by extraction, micro-PIXE, TOF-SIMS, and XAFS

Coal fly ash (CFA) contains considerable amounts of potentially hazardous trace elements. Characterization of trace elements in CFA is essential for the safe disposal and recycle of CFA. The objectives of this study were i) to determine and predict the solubility of trace elements in CFA in relation to their chemical and mineralogical properties, and ii) to characterize trace elements using the surface chemical analysis including time-of-flight secondary ion mass spectrometry (TOF-SIMS) and accelerator-based micro particle induced X-ray emission (PIXE) analysis, in combination with X-ray absorption fine structure (XAFS) spectroscopy with a primary focus on As and Cr. The CFA samples from 12 thermal power plants contained B (ave. 203 mg kg−1), F (90 mg kg−1), Cr (63 mg kg−1), As (21 mg kg−1), and Se (3.2 mg kg−1), in which the water soluble fraction relative to the total concentration decreased in the order B (24 %) > Se (23 %) > F (20 %) > As (1.7 %) > Cr(IV) (0.71 %). A regression model indicated that water extractable As and Cr(VI) from CFA increased linearly with increasing SiO2 and CaO in CFA, respectively. The SIMS images showed that B was finely and heterogeneously distributed on CFA, whereas F was distributed homogeneously on CFA. The combined results from micro-PIXE and XAFS revealed that i) As was distributed on about 50-μm particles in the form of As(V) associated with Al and Ca, and ii) Cr was co-located with Fe and Ca on about 50-μm particles and was present as Cr(III). This study demonstrated that the combined results from TOF-SIMS, micro-PIXE, and XAFS techniques enable trace elements in CFA to be better characterized in terms of spatial distribution and chemical speciation.

Fouling and Slagging Investigation on Ash Derived from Sasol Coal Using ICP and XRF Analytical Techniques

During coal combustion in boilers, light fly ash particles are carried away along with the hot flue gases and the heavier bottom ash particles fall to the bottom of the boiler. The fly ash particles stick on the convective heat transfer surfaces and the furnace wall, causing fouling and slagging deposition problems during the boiler operation. The fouling and slagging effect reduces the boiler’s operational efficiency. This study was motivated by the decline in the operational efficiency of the installed boilers at Sasol synfuel operations in Secunda, Mpumalanga province in the Republic of South Africa. It was assumed that the drop in the boiler efficiency was caused by the coal ash deposition during the boiler operations. The rate of ash deposition and accumulation in the convective heat transfer tubes and furnace water walls during the boiler operation depends on the chemical composition of the coal ash produced during combustion. Coal fly and bottom ash samples were collected from the operational site for laboratory analysis to determine their chemical composition using induced coupled plasma optical emission spectroscopy, induced coupled plasma mass spectroscopy (ICP-OES, ICP-MS) and X-ray fluorescence (XRF) analytical methods. The major, minor and trace elements by mass (%) in the ash samples were obtained from the ICP-OES and ICP-MS, whereas the elemental composition in an oxidised atmosphere was obtained from the XRF analysis. The amount of unburnt coal particles within the ash samples was determined from the loss on ignition (LOI) test. The fouling and slagging prediction during Sasol boiler operation was evaluated using previously developed fouling and slagging indices as a guide using the analysed ash chemical composition results obtained in this study. It was concluded from the analysed results using the guided evaluated indices from the analysed coal ash chemistry that during the operations of Sasol boiler(s) there is a low to medium fouling prediction occurrence on the convective heat transfer tubes and a low slagging in the boiler furnace walls.

Determination of concentrations of non-volatile elements in fly ash released from coal combustion using EDXRF

ABSTRACT In this study, the concentration of thirteen non-volatile elements in coal fly ash samples obtained from the Kangal coal-fired thermal power plant located in Sivas province of Turkey was determined using an energy dispersive X-ray fluorescence (EDXRF) spectrometer. Non-volatile elements in coal fly ash samples were analysed in three groups: major (Al, Si, and Ca), rare earth (Y, La, Ce, Pr, and Nd), and other trace elements (Ga, Rb, Mo, Ba, and Hf). The average concentrations of Ca, Si and Al were found as 22.8, 10.9 and 5.4%, respectively. The average concentrations of Pr, Y and, Nd were found as 23.9, 19.1 and 78.2, respectively. La and Ce were observed below the detection limit of 2 mg/kg, except for two samples. The average concentrations of Ga, Rb, Mo, Ba and Hf were found as 18.6, 51.1, 177.3, 993.5 and 3.9 mg/kg, respectively. According to the average values of enrichment factor estimated for elements analysed in fly ash samples, Mo is found extremely enriched while Ca and Hf are significantly enriched.

Occurrences and patterns of major elements in coal fly ash under multi-acid system during microwave digestion processes

Coal fly ash (CFA) is a typical industrial solid waste generated from coal combustion. Microwave digestion is a common measure to determine the element compositions for complex samples like CFA, of which the reagents of digestion processes largely affect the results. In this work, to explore the behaviors of major elements during the digestion process and the effects of pre-treatments, the digestion patterns of Al, Ca, Fe, K, Mg and Ti in CFA samples were studied. The results showed that hydrofluoric acid greatly enhanced the digestion when the ratio of HF went up from 0 to 0.22, of which the digestion efficiencies increased from 21.79% to 59.55% for Al, from 82.23% to 88.23% for Ca, from 50.86% to 84.93% for Fe, from 16.94% to 76.95% for K, and from 9.94% to 95.90% for Ti. Perchloric acid functioned the same as nitric acid. The most suitable schemes for major elements in CFA was (7 mL HNO3 + 2 mL HCl + 2 mL HF) for 0.25 g of samples (Al: 100.00%, Ca: 93.05%, Fe:97.58%, K: 100.00%, Mg: 81.10%, Ti: 98.30%). According to the experiment of pre-treatments and the structure analysis, it could be concluded that Fe mostly existed in the amorphous phase while K and Ti mostly existed in the crystalline phase, and Ca and Mg were mostly embedded in the Al-related crystalline phase.

Analysis of coal ash samples from thermal power plants of India for their gallium content using NAA and EDXRF techniques

Coal fly ash (CFA) is an important secondary source for the recovery of gallium (Ga) which has a high potential for its wide applications in many strategic fields such as cellular communications and direct broadcast satellite.Various coal fly and bottom ash samples obtained from thermal power plants located in different parts of India were investigated for their gallium content using NAA and EDXRF techniques. The concentration of gallium in NIST SRM 1633b CFA is not available in NIST certificate and hence was established using k0 based IM-NAA method along with the other certified elements like As, Ce, Co, Eu, Fe, K etc. In addition, the gallium concentration in the NIST SRM was also obtained by relative NAA using Ga2O3 as a standard towards the validation of IM-NAA. Using the NIST SRM, the gallium content in the ash samples of Bituminous coal collected from South Central and Eastern India was found to be from 17.2 to 47.9 mg/kg whereas the same was 6.3–33.3 mg/kg for the ash samples of Lignite coal collected from the South-Central India and Western India. The gallium concentrations obtained by NAA in coal ash samples were compared with another non-destructive assay technique, i.e. EDXRF and the results are found to be in good agreement. These samples were proved to be a potential secondary source of gallium, available in India.

Direct Air Capture and Sequestration of CO2 by Accelerated Indirect Aqueous Mineral Carbonation under Ambient Conditions

Mineralization of gaseous carbon dioxide into solid carbonates using alkaline industrial residues such as coal fly ash has a dual advantage of reducing the carbon dioxide footprint of coal power plants and improving ash utilization. However, the slow mineral carbonation rate under atmospheric conditions is a major challenge, especially when using natural minerals or industrial residues for direct air capture (DAC) of CO2. In this study, using coal fly ash samples and concentrated alkali carbonate aqueous solutions as a recyclable solvent, we show the feasibility of coupling mineral carbonation with DAC under atmospheric conditions. Findings show that carbonation efficiency is best under alkaline conditions, achieving as high as ∼80% conversion to calcium carbonates within 1 h in a 1.9 M sodium carbonate solution. Based on the experimental results, a process coupling DAC and mineral carbonation that operates entirely under ambient conditions is proposed. The techno-economic and life cycle assessments for the proposed process project a levelized cost of $116–133/t-CO2-sequestered (US $2019) and process carbon emissions (GWP) in the range of 0.03–0.25 t-CO2e/t-CO2-sequestered. Considering the low cost, simplicity, and gigaton-scale sequestration potential, we believe that DAC based on alkaline industrial residue carbonation can be considered a “low-hanging fruit” in the pursuit of negative emissions to combat climate change.

Leaching of coal fly ash with sulphuric acid for synthesis of wastewater treatment composite coagulant

ABSTRACT The results of leaching of coal fly ash in H2SO4 solution are presented. The effects of leaching time, H2SO4 concentration, temperature, and solid/liquid ratio were explored for the dissolution process. Particle size distribution results showed that changes in particle size occurred during the dissolution process; XRD, XRF and FTIR results indicated the same characteristics of the mineral matter in the raw and treated coal fly ash samples; and SEM showed that the raw coal fly ash particles have smooth spherical surfaces while a pricked and corroded surface and a stem-like structure were formed after leaching. The dissolution rate of coal fly ash is controlled by mixed-controlled mechanism (diffusion and chemical reaction). The corresponding activation energies were calculated to be 59.8 kJ/mol for iron, 7.3 kJ/mol for aluminium and 5.8 kJ/mol for silicon. The reaction order was determined to be zero order kinetic model.

Rapid identification of reactivity for the efficient recycling of coal fly ash: Hybrid machine learning modeling and interpretation

As the main solid waste produced by coal combustion, the large accumulation of coal fly ash (CFA) causes serious environmental pollution and resource waste. Whether CFA can be recycled depends on its reactivity, which in turns can be represented by its amorphous content. This paper established random forest regression models optimized by artificial bee colony (ABC) for rapid screening of CFA, based on the correlation between chemical composition and amorphous phase. The study evaluated the model using correlation coefficient, r-square, root mean square error, and mean absolute error, giving results of testing set of 0.773, 0.477, 6.542, and 5.279. Feature importance and permutation importance were used to measure feature contribution. Partial dependence plots, Shapley additive explanations, and local interpretable model-agnostic explanations were also used to give global and local interpretation of the model performance. The overall results showed that Al2O3, MgO and CaO had the greatest influence on the amorphous content within CFA. When the mass fractions of Al2O3, MgO and CaO varied from 17.5% to 22.5%, 2%–4% and 10–15%, the mass fraction of amorphous phase reached the highest. For different CFA samples, chemical composition played a different role in determining the amorphous content. The post-analysis of the model provided some reference for promoting CFA recycling. The above results proved that the established model had good robustness and generalization capability, which can effectively determine the potential of CFA as supplementary cementitious materials to promote the cleaner production of cement or geopolymer resources.