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BACKGROUND: Nutrient recovery from wastewater is gaining attention in the frame of circular economy. In this study, Ca(OH)2‐treated zeolite (CaT‐Z), lignite fly ash (FA) and zeolitic fly ash (ZFA) were primarily used to adsorb phosphate phosphorus (PO4‐P) from anaerobically digested olive mill wastewater (ADOMW). The simultaneous recovery of potassium (K) was also examined based on adsorption and desorption data. The fractionation of adsorbed P was determined as an important parameter for its plant availability. RESULTS: More P was adsorbed from ADOMW on ZFA and FA (up to 4.35 and 5.21 mg g−1, respectively) than on CaT‐Z (2.62 mg g−1). An increased P adsorption on ZFA and FA was observed between incubation times of 7 and 14 days. The sequential desorption procedure verified the trend of P adsorption capacities and showed that the plant‐available P (NaHCO3‐P) amounted to 1.34, 2.34 and 1.69 mg g−1 CaT‐Z, ZFA and FA, respectively. After 14 days, CaT‐Z and ZFA adsorbed much more K (19.2 and 20.5 mg g−1, respectively) than FA (4.3 mg g−1). The desorption of exchangeable K confirmed this difference. Scanning electron microscopy–energy dispersive spectrometry analyses indicated P adsorption and surface precipitation as Ca–P phases as well as K+ for Ca2+ ion exchange on the loaded adsorbents. CONCLUSIONS: ZFA was more efficient for dual adsorption combining the properties of the zeolitic fraction (enhanced K adsorption) and pristine FA (P adsorption on Ca‐bearing phases). The sufficient concentrations of plant‐available P and K on the three adsorbents suggest their potential use as soil amendments. © 2022 Society of Chemical Industry (SCI). [ABSTRACT FROM AUTHOR]

In the current research, three industrial by-products containing useful iron and aluminum chemical components were introduced as potential alternative coagulants in marble processing wastewater treatment for possible water reuse. Specifically, the coagulation performance of lignite highly calcareous fly ash, siliceous fly ash (FAs) and electric arc furnace dust (EAFD) was compared with that of two commercial coagulants (Al2 (SO4 )3 ·18H2O and FeCl3 ·6H2 O). The kinetic studies were conducted at times up to 60 min by using coagulant dosages up to 4 mg/L. Turbidity (NTU), pH and conductivity were recorded during the kinetic studies. FAs and EAFD revealed enhanced coagulation performance, having similar turbidity removal efficiencies (%ΔNTU) to those of commercial coagulants, with short sedimentation times (~5 min). The pH values recorded for the three industrial by-products/coagulants were in the basic range of 7–9.7. Only the pH values for EAFD, at high dosages, were found to be close to neutral, while, for all commercial coagulants, neutral pH values were recorded at intermediate dosages. The experimental results presented may contribute to the formation of integrated and cost-effective strategies for marble wastewater management with low environmental footprint. [ABSTRACT FROM AUTHOR]

The amounts of ashes from circulating fluidized bed (CFB)-combustion steadily increase with the increasing rate of implementation of this environmentally-friendlier technology for firing solid fuels in power generation units. Therefore, beneficial uses of these powdery by-products, which possess particular characteristics, should be identified instead of landfill them at significant environmental and economic impact. Their valorization in the manufacturing of value-added ceramics is a challenging research area. In the present study, CFB-coal combustion fly ashes, mainly bearing Si-Al phases, were utilized as 100% raw materials for the synthesis of ceramics by powder metallurgy (PM) techniques. Test specimens were fabricated by powder cold pressing followed by sintering at 900, 1000 or 1100 °C. The produced materials were characterized by means of XRD and SEM-EDAX. Shrinkage upon sintering, apparent density, water absorption capability and Vickers microhardness were determined. The experimental results show that the recycling of CFB-fly ashes towards PM ceramics development is feasible. In fact, integral light-weight materials are obtained, exhibiting successfully-densified ceramic microstructures where quartz phases prevail. The physico-mechanical properties are proved to be influenced by the starting ash composition and the sintering temperature. The so-produced ceramics possess the potential to be tailored appropriately to meet requirements for specific applications of possible commercial interest. [ABSTRACT FROM AUTHOR]

Eight types of A356 Al-fly ash composites were produced by pressure infiltration of high-Ca lignite fly ash. This type of ash was used for the first time in Al-composites synthesis, and particularly by liquid metal infiltration techniques. After examining mineralogy and chemistry, specific, narrow ash size fractions were used for the synthesis of composites, and properties linked to microstructure and wear strength of the materials. The effect of using ground ash particles on the microstructure and tribological performance of the composites was also investigated. It was concluded that using fine, high-Ca ash particles can improve the properties of composites, and that using ash particles in a ground form can better facilitate the production process of MMCs. [ABSTRACT FROM AUTHOR]

Abstract: The mobility of inorganic pollutants is of key concern for a range of industrial and engineering applications of fly ash produced during the combustion of lignite in power generation. This paper investigates the role that the geochemical features of lignite, the ash composition and the partitioning of elements during combustion play in determining leaching properties of lignite fired by-products. The work is based on surveys on three lignite-fired power plants in Greece. Calcium-rich ashes show a high abatement potential for SO2 and other gaseous pollutants. For most elements, the concentrations in the parent lignite and the ashes follow the same trend. Relative enrichments in Cd, Co, Cr, Cu, Mo, Ni, Pb, U, V, W, Zn fingerprint the regional and local geological settings of the lignite basins. The total and leachable concentrations of highly volatile elements are strongly influenced by the interaction with ubiquitous free lime. A broad array of elements is highly insoluble in alkaline ash, while a few oxyanionic-forming elements display substantial mobility. Their mode of occurrence in the parent lignite plays a primary role in the leaching of combustion ashes. The outcomes of this study may assist in addressing the impact of co-firing high ash or high Ca alternative fuels on the leaching properties of combustion by-products. [Copyright &y& Elsevier]

Abstract: The present study investigates the heavy metal content of coal fly ash (FA) samples coming from three different sampling points (secondary cyclone, cooler and filter) of a pilot plant combustion facility. The combustion experiments were carried out in a 0.1MW Circulated Fluidized Bed (CFB) boiler using South African coal, with the addition of limestone for sulfur capture. FA was tested for the presence of selected heavy metals using inductively coupled plasma optical emission spectrometry (ICP-OES); batch leaching tests were conducted as well. The samples were also characterized in terms of their microstructure, chemical and mineralogical composition, total surface area and particle size distribution. Most of the studied metals (Cd, Cr, Cu, Ni, Mn, Zn) showed enrichment in the fine, filter FA particles, while Pb was mostly concentrated in the cooler sample. Regarding leaching characteristics of the examined samples, Cr was found to occur in considerable amounts. Although the use of CFB technology for the combustion of solid fuels steadily gets bigger worldwide, only a very limited number of studies have environmentally assessed CFB-coal FAs to date. Thus, the current study aims to contribute toward building a more integrated knowledge on the environmental impact of this abundant power production by-product. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of the study described in this paper was to determine the removal of Cr (total), Cr (VI), Cu, Ni, Pb, Zn and Cd from wastewater using different particle-size-fractions of highly calcareous and highly siliceous fly ashes (FAs). Three different Hellenic FAs (two calcareous and one siliceous) were tested for their capability of precipitating heavy metals from aqueous solutions. Each FA sample was separated into six different size-fractions with a grain diameter range of: [(0–25) (25–40) (40–90) (90–150) (150–400) and (>400)]μm. The different FA grain-fractions were evaluated in terms of their chemical composition, pH, Loss on Ignition (LOI) and CaOf (%). Batch adsorption experiments were then carried out, indicating that the various grain-fractions of the highly siliceous FA were more efficient in precipitating Cr (VI) but less capable of retaining Cd, Cu, Ni, Pb and Zn. On the other hand, the high-Ca fly ashes were proven to be more efficient in uptaking Cd, Cu, Ni, Pb and Zn, but less in hexavalent chromium. This particular tendency was also confirmed in the case of the different particle-size-fractions of same fly ashes. It was actually verified that FAs can be more effective in the field of industrial wastewater-remediation when separated into their size-fractions. [Copyright &y& Elsevier]

Abstract: Polish bituminous (PB) and South African (SA) coal fly ash (FA) samples, derived from pilot-scale circulated fluidized bed (CFB) combustion facilities, were utilized as raw materials for the synthesis of zeolitic products. The two FAs underwent a hydrothermal activation with 1M NaOH solution. Two different FA/NaOH solution/ratios (50, 100g/L) were applied for each sample and several zeolitic materials were formed. The experimental products were characterized by means of X-ray diffraction (XRD) and energy dispersive X-ray coupled–scanning electron microscope (EDX/SEM), while X-ray fluorescence (XRF) was applied for the determination of their chemical composition. The zeolitic products were also evaluated in terms of their cation exchange capacity (CEC), specific surface area (SSA), specific gravity (SG), particle size distribution (PSD), pH and the range of their micro- and macroporosity. Afterwards the hybrid materials were tested for their ability of adsorbing Cr, Pb, Ni, Cu, Cd and Zn from contaminated liquids. Main parameters for the precipitation of the heavy metals, as it was concluded from the experimental results, are the mineralogical composition of the initial fly ashes, as well as the type and the amount of the produced zeolite and specifically the mechanism by which the metals ions are hold on the substrate. [Copyright &y& Elsevier]

Abstract: The chemical and mineralogical composition of fly ash samples collected from laboratory scale circulating fluidised bed (CFB) combustion facility have been investigated. Three fly ashes were collected from the second cyclone in a 50kW laboratory scale boiler, after the combustion of different solid fuels. Characterisation of the fly ash samples was conducted by means of X-ray fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Quantitative analysis of the crystalline (mineral) and amorphous phases in each ash sample was carried out using the Rietveld-based Siroquant system, with an added spike of ZnO to evaluate the amorphous content. SiO2 is the dominant oxide in the fly ashes, with CaO, Al2O3 and Fe2O3 also present in significant proportions. XRD results show that all three fly ashes contain quartz, anhydrite, hematite, illite and amorphous phases. The minerals calcite, feldspar, lime and periclase are present in ashes derived from Polish coal and/or woodchips. Ash from FBC combustion of a Greek lignite contains abundant illite, whereas illite is present only in minor proportions in the other ash samples. [Copyright &y& Elsevier]

Abstract: The chemical and mineralogical composition of fly ash samples collected from different parts of a laboratory and a pilot scale CFB facility has been investigated. The fabric filter and the second cyclone of the two facilities were chosen as sampling points. The fuels used were Greek lignite (from the Florina basin), Polish coal and wood chips. Characterization of the fly ash samples was conducted by means of X-ray fluorescence (XRF), inductive coupled plasma-optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), particle size distribution (PSD) and X-ray diffraction (XRD). According to the chemical analyses the produced fly ashes are rich in CaO. Moreover, SiO2 is the dominant oxide in fly ash with Al2O3 and Fe2O3 found in considerable quantities. Results obtained by XRD showed that the major mineral phase of fly ash is quartz, while other mineral phases that are occurred are maghemite, hematite, periclase, rutile, gehlenite and anhydrite. The ICP-OES analysis showed rather low levels of trace elements, especially for As and Cr, in many of the ashes included in this study compared to coal ash from fluidised bed combustion in general. [Copyright &y& Elsevier]

Abstract: In this paper the mineralogy and geochemistry of Greek and Chinese coal fly ash are examined. Annual production of fly ash in China is around 160Mt while in Greece lignite fly ash accounts around 10Mt. Even though the mineralogical and chemical composition of the fly ashes coming from these two countries differs, there are common questions on the utilization of this material. The variation of the Greek fly ash’ chemical composition, from Ca-poor to Ca-rich fly ash, has resulted to applications such as dam construction, use in cement and possibly in concrete and road construction. The Chinese fly ash, which is rich in mullite, is broadly applied for brick making. [Copyright &y& Elsevier]

Fly ash from two lignite-fed power stations in Greece (Megalopolis and Meliti) has been utilized to synthesize zeolitic materials with upgraded adsorption capacity. Two different siliceous fly ash samples were subjected to hydrothermal treatment at fixed solid/liquid ratio and constant temperature. The zeolitic products have been characterized for their microstructure, chemical, and mineralogical composition by means of SEM, AAS, and XRD, respectively. The primary zeolitic crystals identified were phillipsite and thomsonite, in Megalopolis and Meliti fly ash, respectively. In light of their prospective utilization as liquid-phase sorbents, the specific surface area and porosity of materials were also determined, by means of N 2 -porosimetry. The zeolitic samples were tested for their actual heavy metal-removal capacity by water sampled from active lignite mines in Northern Greece. Artificial aquatic samples with known concentration of heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) were also tested, showing that the synthetic zeolitic materials are good to uptake a wide variety of potential pollutants with up to 100% efficiency. The efficiency of the synthetic zeolitic materials was comparatively assessed, showing that the more intense the presence of phillipsite in the synthetic materials, the greater the uptake rates for certain groups of trace elements. [ABSTRACT FROM AUTHOR]

Abstract: The chemical and mineralogical composition of lignite fly ash (FA) varies as a function of the prevalent conditions in both the processes of power production and lignite mining. The differentiation of the qualitative and quantitative composition of the highly-calcareous lignite fly ash, as a function of its particle size distribution, is verified in this paper. According to the results of the conducted research, a fine-grained fraction of considerable amount presents properties that obstruct the sustainable exploitation of calcareous lignite fly ash in cement industry applications. On the other hand, the same grain fraction (because of its hydraulic properties) can be utilized in other sort of applications, based on different criteria, i.e. in road constructions. The coarse-grained fraction (which reflects a low proportion to the total fly ash output) presents the same undesired characteristics as well. Rather, the intermediate grain fraction (75–150μm) presents the highly desirable properties when fly ash is utilized as a pozzolanic additive. In addition, the mechanism of the formation of the intermediate grain fraction strongly prevents the factors that cause the variation of fly ash-quality. It is therefore the optimum part of the whole amount of lignite FA, to be utilized as additive in cement manufacturing. The outcomes of this paper will hopefully contribute towards the crucial goal of the expansion of the utilization of calcareous lignite fly ash by proposing a more effective way of using this material, basically by taking advantage of its fundamental chemical and mineralogical properties. [ABSTRACT FROM AUTHOR]

Abstract Carbonation of industrial wastes rich in earth-alkali oxides is found to have a significant potential for CO 2 sequestration. This process opens new perspectives not only for carbon dioxide mitigation, but also for the valorization and new applications of industrial waste materials from coal-burning power plants. In this study, mineral carbonation of high–calcium fly ash is investigated under dry and moist conditions in a continuous flow reactor during up to 2 h, at temperatures ranging from 160 to 290 °C and CO 2 pressures between 1 and 6 bar. A comprehensive characterization of treated and untreated samples was carried out before and after carbonation using X-ray diffraction, X-ray fluorescence spectroscopy, thermogravimetric analysis, infrared spectroscopy and scanning electron microscopy. The maximum sequestration capacity achieved was 117.7 g CO 2 /kg fly ash (48.14% carbonation efficiency) under dry conditions. Results showed that increasing the pressure and temperature enhances the process of carbonation, as well as the presence of moderate amounts of water vapor in the CO 2 gas flow. Newly formed carbonates were always present in the treated samples. This study shows that about 21% of all CO 2 emissions of a coal-burning power plant could potentially be sequestered as carbonates. Highlights • This work presents carbonation of high-calcium fly ashes in dry and moist conditions. • Results show that carbonation occurred both with and without water vapor presence. • Higher carbonation efficiency is achieved with temperature and pressure increase. • SEM analysis showed the fly ash cenospheres evolution with growing calcite. • Maximum carbonation efficiency achieved is 48.14%. [ABSTRACT FROM AUTHOR]

Abstract: In the present paper, two Class C, lignite fly ashes (FAs) were utilized for the fabrication of Al/fly ash metal matrix composites (MMCs) using the powder metallurgy (PM) technique. Aluminum–fly ash and aluminum/silicon alloy (Al–12% Si)–fly ash mixtures, containing 5, 10 and 15wt.% of – both pozzolanic and hydraulic – FAs were prepared and compacted. The green products were tested for their thermal behavior in order for the sintering temperature to be determined. Then, they were sintered for 2 and 6h. The density of composites was decreasing with the increase of the percentage participation of fly ash particles into them. Apart from that, although FA clusters had been developing in the metal matrix, there was a significant enhancement in the hardness of MMCs when increasing FA content, which is mainly attributed to the intense calcareous nature of the reinforcement material. Volume and mass changes throughout calcination were also estimated as a function of increasing FA weight percent and it seems that CaO (%) of FAs plays a crucial role regarding the post-sintering raise in the mass and volume of composites. Microscopic studies of green and sintered compacts clearly indicated the effectiveness of the sintering process. Additionally, the composites were characterized in terms of their chemical composition and mineralogical structure. The rise of Ca–Si- and Si-mineral phases is intense, particularly for the composites with the highest percentage presence of CaO in FAs (15wt.%) and does not differentiate as a function of the sintering time. [Copyright &y& Elsevier]