Circulating Fluidized Bed Fly Ash Research Articles

Mechanical properties of solid waste-based composite cementitious system enhanced by CO2 modification

Utilizing CO2 to modify solid waste and produce high-performance cementitious material is a promising strategy. In this study, circulating fluidized bed fly ash (CFBFA), calcium carbide slag (CS) and flue gas desulphurization gypsum (FGDG) were prepared by mixing CO2 to produce cementitious materials. The impacts of CO2 on mechanical properties and carbonation reaction mechanism of composite materials were investigated. The results shown that the microstructure and mechanical properties of the gelling materials were improved by CO2 modification. When the CO2 mass transfer conditions as followed: CO2 flow rate was 1.5 L/min, stirring rate at 600 r/min, and stirring time at 20 min, compressive strength of modified material was 22 % higher than that of the non-CO2 modified material. Additionally, the density increased by 3.2 %, and porosity was reduced by 34.1 %. Among them, TG/DTG analysis found that the highest effective fixed CO2 rate was 11.1 %. The ultrasonic nondestructive testing also shown that the structure of the CO2-modified cementitious material was significantly better than that of the CFBFA-CS-FGDG composite cementitious system, and the ultrasonic loss rate was reduced by 9.4 %. Therefore, the use of CO2 to modify the composite system was of great significance for the realization of CO2 storage and the improvement of the properties of composite materials. One interesting future possibility is solid waste-based grouting materials production at energy plants: CFBFA, CS and FGDG can be used to capture CO2 from flue gases, thus improving the strength of produced grouting materials.

Revealing the intrinsic sintering mechanism of high-strength ceramsite from CFB fly ash: Focus on the role of CaO

The preparation of high-strength ceramsite from circulating fluidized bed (CFB) fly ash is an effective approach for commercial utilization. The role of CaO plays a key factor in the performance of ceramsite. The impact of CaO on the sintering mechanism was studied at the sintering temperature from 1200 °C to 1300 °C and the CaO addition ratio from 2 wt% to 8 wt%. The in-situ visual furnace equipped with a high-resolution microscope, incorporated with TG-DSC-FTIR, SEM, XRD, and the semi-quantitative analysis method of XRD, was conducted to investigate the sintering process of ceramsite. The results verified that during the high-temperature sintering, CaO reacted with SiO2 and Al2O3 in CFB fly ash to constitute low-melting minerals like anorthite, and the elevation of sintering temperature expedited the conversion of anorthite and eutectics into the molten liquid phase. The expansion gas was wrapped by the molten liquid phase and reached equilibrium, which contributed to the closure of the pores and the elevation of the shrinkage rate of ceramsite. In addition, the rise in sintering temperature dropped the amorphous phase content and increased the mullite content, further enhancing the compressive strength. When the sintering temperature was in the range of 1250 °C–1300 °C and the CaO content was in the range of 2 wt%-4 wt%, the high-strength ceramsite with excellent physicochemical properties could be obtained. It suggested that the synergistic impacts of sintering temperature and CaO improved the performance of ceramsite.

Formulation determination and performance study of high-volume fly ash-based grouting fire extinguishing materials

Considering the continued advancement of China's industrial sector, a substantial quantity of circulating fluidized bed fly ash (CFBFA) and calcium carbide slag (CS) is generated throughout the production process. Considering the rich calcium content inherent in CFBFA (circulating fluidized bed fly ash) and the inherently alkaline nature of CS(carbide slag), the amalgamation of these two constituents leads to a multitude of hydration reactions, culminating in the synthesis of polymeric grouting materials. This study undertaked a comprehensive examination and regulation of three factors within the binary system: water-cement ratio, CS content, and CS particle size. Through the utilization of scanning electron microscopy (SEM), X-ray diffraction (XRD), and (TG) thermogravimetric analysis, the investigation aims to elucidate the influence mechanism of the composite gelling system comprising fly ash and calcium carbide slag on the compressive strength, setting time, and other pertinent indicators of grouting materials. The data reveals that when the water-cement ratio is set at 0.85, CS content at 30%, and CS ball milling time at 1 h, the resulting grouting material exhibits a fluidity of 279 mm, a water separation ratio of 3.13%, and achieves a strength of 15.6 MPa at 28 days. It boasts a stable and easily transportable slurry, which possesses a certain degree of cementitious performance, effectively sealing off any air leakage channels. Moreover, by subjecting CS to appropriate pre-treatment measures, a suitable quantity of active silicon and aluminum elements can be thoroughly dissolved within the CFBFA (circulating fluidized bed fly ash) slurry system, giving rise to a staggered distribution and dense hydration product structure. Considering the requirements for gangue mountain fire extinguishing slurry, a CFBFA-CS based grouting material formula has been carefully selected. Additionally, simulation grouting fire extinguishing experiments have demonstrated that the injection of the slurry is highly effective in reducing the temperature of the combustion test block, isolating oxygen, and lowering the temperature within the combustion zone. After 5 h, the temperature drops to below 100 °C, and no re-ignition phenomena are observed. The outcomes of this research can provide valuable scientific guidance for the development of high-performance fire extinguishing grouting materials utilizing fly ash and carbide slag.

The effects and solidification characteristics of municipal solid waste incineration fly ash-slag-tailing based backfill blocks in underground mine condition

Currently, there is a limited understanding of the material performance effects and hydration mechanisms of solidified waste-based binder filling materials containing municipal solid waste incineration fly ash in the actual underground filling environment. This study utilized the Circulating Fluidized Bed (CFB) and Grate Furnace (GF) techniques to produce quadri-component uncalcined binder fill materials from municipal solid waste incineration fly ash (MSWI FA), blast furnace slag (BFS), flue gas desulfurization (FGD) gypsum, and tailings. The solidified test blocks were then cured in-situ in a void space of a certain iron underground mine in Hebei with conditions (temperature: 25-30℃, humidity: 18-19%, mine water influx: annual average of 150m3/h). The research results indicate that the mechanical performance and heavy metal solidification outcomes of these blocks surpassed those samples cured under laboratory conditions (temperature: (20±1)℃, humidity: not less than 90%), demonstrating the feasibility of using MSWI FA for underground binding and backfilling in mines. Building upon preliminary research, a binder mix proportion was established: 20wt.% MSWI FA, 70wt.% BFS, and 10wt.% FGD gypsum. Test blocks were fabricated using lead-zinc tailings sand and iron mine tailings as aggregates. All blocks met the 28-day strength requirements for typical mine backfills ranging from 1 to 6.5MPa, with the strength of the XB-M group reaching 24MPa (when cured underground). The leaching concentrations of heavy metals were all below the limits set for class III groundwater standards, with Pb, Zn, Cr, and As achieving a solidification rate of over 98% at 28 days. Microscopic hydration mechanisms indicate that the primary hydration products in the solidified body were ettringite, Friedel's salt, and C-S-H gel. The high content of reactive Al2O3 in the CFB fly ash enables the [AlO4]5- to substitute [SiO4]4-, leading to the formation of a long-chain, high polymerization C-A-S-H phase. Additionally, the abundant Cl- in GF fly ash readily reacted with [Al(OH)6]3−, forming Friedel's salt. The hydration products in the blocks cured under mine conditions significantly increased in their degree of polymerization. This provides a practical application approach for the future treatment and reuse of MSWI FA. Environmental implicationMSWI FA is classified as hazardous waste (HW18) due to its content of harmful components such as chlorides, dioxins, and soluble heavy metals, which pose potential environmental pollution risks. This study, for the first time, solidified test specimens were sent to an underground mining void in a certain iron mine in Hebei, China, for on-site curing (temperature range: 25-30°C, humidity: 18-19%, annual average mine water inflow rate: 150m3/h). The mechanical properties and the solidification performance of heavy metals in the test specimens were superior to those under laboratory curing conditions (temperature: (20±1)°C, humidity not less than 90%).

Mechanical properties and hydration behaviour of circulating fluidised bed fly ash- ground granulated blast furnace slag-lime ecofriendly cementitious material

As a major solid waste in coal-fired power plants, circulating fluidized bed fly ash (CFBFA) has a high content of SO3 and f-CaO, leading to a lack of avenues for its resource utilization. Using the characteristic of it, this study combined it with ground granulated blast furnace slag (GGBFS) to produce an ecofriendly cementitious material (CGL) without cement. At the CFBFA/GGBFS ratio of 3:2, the lime content of 20 wt% and the water–solid ratio of 0.45, the unconfined compressive strength of CGL mortar could achieve 33.44 MPa at 28 d. With the prolongation of curing age, high polymerized C-(A)-S-H gel with a foil-like morphology, as well as ettringite and hydrotalcite were continuously generated and filled the pores, resulting in a continuous decline of the porosity. Simultaneously, the proportion of small pores (<50 nm) increased continuously. In addition, CGL had significant environmental and economic advantages. Compared with Portland slag cement (32.5R), the energy consumption, carbon emissions, and production costs of CGL reduced by 73.51 %, 75.76 %, and 25.93 %, respectively. Therefore, this work offered a practical and novel way for resource utilisation of CFBFA.

Energy effective utilization of circulating fluidized bed fly ash to prepare silicon-aluminum composite aerogel and gypsum

To reduce the cost of Si-Al aerogels preparation, circulating fluidized bed fly ash (CFA) was developed to be as the alternative to synthetic precursors. High energy consumption of alkali-melting and secondary wastes production were the major challenges. Here, a technique characterized by effective energy consumption and non-secondary waste was developed to convert CFA into Si-Al aerogel. The process consists two stages, preparation of Si-Al sol by sintering of CFA and Na2CO3 followed by sulfuric acid leaching, and synthesis of Si-Al aerogel by so-gel with trimethyl chlorosilane modification and ambient pressure drying. The optimization results of proportion and sintering temperature showed that the optimal temperature of sintering of Na2CO3 and CFA with the mass ratio of 0.7 was 750 °C, 100 °C lower than that of most other waste aluminosilicate materials. CaSO4·0.5H2O which meet building gypsum requirement was obtained by specifying the drying temperature of acid-leached residue at 126 °C for 2 h. The modification procedure was explored to obtain Si-Al aerogel with a large specific surface area of 857 m2/g and hydrophobic angle of 139.3°. Thermal and mechanical properties tests indicated that the Si-Al aerogels and gypsum produced from CFA exhibited promising thermal insulation and the potential application in construction.

Insight into the synergic effects of circulating fluidized bed fly ash, red mud and blast furnace slag in preparation of ultrahigh-performance concrete: Reaction mechanism and performance optimization

Ultrahigh-performance concrete (UHPC) is a kind of special concrete with excellent mechanical properties and good flexibility that has attracted much attention. Partial replacement of cement with supplementary cementitious materials (SCMs) can significantly reduce costs and ensure UHPC performance. The composite preparation of cementitious materials using the physicochemical properties of different solid wastes has been proved to be effective. On this basis, the highly alkaline solid waste red mud (RM), highly pozzolanic activity solid waste containing sulfate circulating fluidized bed fly ash (CFA) and high pozzolanic activity solid waste blast furnace slag (BFS) were utilized to prepare low cement clinker UHPC. 20 groups of experiments were designed by response surface methodology (RSM). Then, the optimization and prediction of the performance of low cement UHPC was explored using simplex lattice method. In addition, the hydration degree and pore structure were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG), mercury intrusion porosimetry (MIP) and back scattered electron (BSE). In the case of 60% cement blending, the best compressive strength (128.15 MPa) occurred in 9% CFA, 28% RM and 3% BFS. The results demonstrated that the increasing strength existed with the addition of RM and a high content of CFA had a negative impact on performance development. In the cases of low cement systems (<45%), however, the compressive strength increased and then decreased as CFA content increased from 0 to 40% or RM content increased from 0 to 70%, which meant that a compound synergistic existed among the three solid wastes. Microstructure analysis demonstrates that the hydration degree and pore structure of UHPC were effectively improved due to synergies among CFA, RM and BFS. These results implied that the compound synergy is more significant in low clinker systems and beneficial to the development of strength. This paper provides a reference for low cement clinker and the low-cost preparation of UHPC.

Preparation, characterization, and rheological analysis of eco-friendly geopolymer grouting cementitious materials based on industrial solid wastes

Geopolymer grouting materials has been applied in underground engineering as a reinforcement material. At the same time, with the enhancement of human environmental awareness, low carbon environmental grouting materials have attracted widely attention. Therefore, this study used industrial solid wastes circulating fluidized bed fly ash (CFBFA), calcium carbide slag (CS), and flue gas desulfurized gypsum (FGDG) as the main raw materials to prepare eco-friendly geopolymer grouting materials. The compressive strength, setting time, fluidity and stone ratio analysis were systematically evaluated. Finally, the microscopic experiments, including X-Ray Diffraction, CT scanner, TG/DTG analysis, and Scanning Electron Microscopy (SEM), were conducted to investigate the mechanism of geopolymer grouting reinforcement materials based on CFBFA, CS and FGDG. The results showed that the solid-wasted grouting materials have a 28d compressive strength of 20.5 MPa, the stone ratio of 99.8%, the fluidity was 250 mm. And adding the 6% alkali activator may enhance the system's fluidity and setting time. In summary, this study analyzed the viability of producing high performance grouting materials for subterranean engineering utilizing industrial solid wastes CFBFA, CS, and FGDG, which are expected to be an alternative for cement as an eco-friendly grouting cementitious material.

The performance of bubble-mixed lightweight soil to relieve expansion of circulating fluidized-bed fly ash

The treatment of circulating fluidized bed (CFB) fly ash with bubble-mixed lightweight soil (BMLS) was proposed. Artificial air bubbles in BMLS were used to restrict the dilatancy of CFB fly ash mixed with cement. The effects of CFB fly ash on pore structure of BMLS with CFB fly ash (BMLS-CF) were investigated. The material composition of BMLS was evaluated by X-ray diffractometry, and the pore structure was characterized by scanning electron microscope and mercury intrusion porosimetry. The results show that sulfur compounds in CFB fly ash react with cement in BMLS-CF to form ettringite and gypsum, which adhere to pore sizes of various sizes. The pore sizes of 50 nm to 50 μm are the most affected by expansive substances. The pores in BMLS-CF can accommodate the formation of expansive substances, and the effect of the accommodation increases with the increases in volume and number of bubbles. When the CFB fly ash content is between 50% and 66.7% with the same air-corbubble content, the effect of the accommodation is the best. The unconfined compressive strength of BMLS-CF first increases, then decreases with the increase of the CFB fly ash content, and the peak arises at a CFB fly ash content of 66.7%. It is therefore recommended that the CFB fly ash content in BMLS-CF be 66.7–75%.

Hydration characteristics of cement with high volume circulating fluidized bed fly ash

Circulating fluidized bed fly ash (CFBFA) is a large industrial solid waste generated by clean circulating fluidized bed coal combustion technology. The coarse and porous particle structure and high content of f-CaO and II-CaSO4 are the main obstacles to the wide application of CFBFA as a supplementary cementitious material. In this study, the core–shell structure of f-CaO and II-CaSO4 were destroyed, and the pozzolanic activity of CFBFA was enhanced by ultrafine grinding. Physical activation can accelerate the mutual excitation of CFBFA and cement, promote the formation of ettringite (AFt) in the early stage of hydration, improve the microstructure, and the later strength of cement with 50 wt% UCFBFA can reach the level of pure cement. The hydration mechanisms of cements with pulverized fuel ash (PFA), CFBFA and ultrafine CFBFA (UCFBFA) were also comparatively analyzed and discussed. This study proves that ultra-fine grinding can effectively improve the reactivity of CFBFA, which provides ideas for the future research on the stability improvement of CFBFA-based cement and the large-scale resource application of CFBFA.

Utilization of coal gangue power generation industry by-product CFA in cement: Workability, rheological behavior and microstructure of blended cement paste

Circulating fluidized bed fly ash (CFA), by-product of coal gangue power generation industry based on circulating fluidized bed combustion technology, differs from ordinary fly ash (OFA) in its physical and chemical properties. This paper aims to investigate the influences of CFA with three particle sizes on the workability and rheological properties of cement paste. The results show that untreated raw CFA (RCFA) can significantly degrade many properties of fresh cement paste. Directly using CFA as a mineral admixture in cement-based materials is not suitable in terms of workability. However, CFA through grinding methods can significantly mitigate the detrimental effect of CFA on the flowability of cement paste. When 15% ultrafine CFA (UCFA) was added, the cement paste exhibited relatively favorable workability and rheological properties. The rheological behavior of cement paste containing CFA with three particle sizes follow the shear thinning, which is in line with the modified Bingham model. Furthermore, the reasons for the impact of CFA on the microscopic level of cement workability are analyzed and discussed. The addition of CFA reduces the amount of free solution in the cement paste, leading to the formation of a flocculated structure and an increase in the thixotropic loop area. Based on the analysis of TOC, BET and TEM, it was determined that numerous polycarboxylate superplasticizer (PCE) molecules are adsorbed onto the rough and porous surface structure of CFA particles, hence the destructive ability of PCE to the flocculation structure caused by cement hydration is weakened. Therefore, the mechanism that CFA affecting the working performance of cement paste can be determined as: the adsorption of free water and PCE molecules.

Preparation of CFB fly ash/sewage sludge ceramsite and the morphological transformation and release properties of sulfur

In order to improve the resource utilization of circulating fluidized bed (CFB) fly ash and sewage sludge, light-weight and high-strength CFB fly ash/sewage sludge ceramsite (CFBS ceramsite) was prepared by using CFB fly ash and original wet sludge as the main raw materials. The effects of dosages of CFB fly ash, sewage sludge, fluxing agent, and foaming agent and sintering temperature on the physical and mechanical properties of CFBS ceramsite were studied by single factor experiment method. Using SEM, XRD, sulfur analyzer, TG-DSC, flue gas analyzer, TG-MS and XPS analyses, the micro morphology and mineral composition of CFBS ceramsite were studied, and the morphological transformation and release properties of sulfur were also revealed. The results show that the optimal preparation conditions are: the mass ratio [CFB fly ash: sludge] = [65 %∼80 %: 35 %∼20 %], addition of 10 % glass powder, addition of 2.5 %∼10 % calcium carbonate, and sintering at 1200 ∼ 1250℃ for 15 ∼ 20 min. The resulting CFBS ceramsite can be obtained with bulk density of 750 ∼ 850 kg/m3, water absorption of 0.27 %∼0.94 % and cylinder compressive strength of 5.11 ∼ 7.86 MPa. Sulfur in the CFBS ceramsite is mainly derived from organic sulfur in the sewage sludge and inorganic sulfur in the CFB fly ash, and it exists in the form of sulfate and sulfide when the sintering temperature exceeds 400℃. The sulfur content of the final prepared CFBS ceramsite is 0.0217 wt%. Decomposition of SO2 from the anhydrite in CFB fly ash is the main reason for the swelling and foaming of the CFBS ceramsite. However, calcium carbonate, derived from either sulfur-fixing agent in CFB fly ash or auxiliary foaming agent, cannot play the role of foaming agent, while the CaO decomposed from calcium carbonate increases the content of fluxing component in the ceramsite and finally affects the properties of CFBS ceramsite.

A non-fired ceramsite construction material with enhanced lightweight high strength properties

The combustion of low calorific value coal in circulating fluidized bed boilers produces a large amount of fly ash with low activity and utilization. In this paper, the lightweight high strength ceramsite was prepared by a non-fired method using low-activity circulating fluidized bed fly ash (CFBFA) as raw material. And the effects of reinforcing agent and foaming agent on the properties of ceramsite were studied. The dosages of cement, quicklime, desulfurization gypsum and hydrogen peroxide were 15.0 wt%, 6.0 wt%, 6.0 wt% and 2.0 wt%, respectively. The hydration reaction in the system can be maximized by steam curing at 70 ℃ for 12 h. The bulk density and cylinder compressive strength of the obtained ceramsite were 695.34 kg/m3 and 5.5 MPa, respectively, which belonged to 700 grade high strength fly ash ceramsite. The corresponding apparent density and apparent porosity of the products were 1091.88 kg/m3 and 51.86 %, respectively. The admixture of cement, quicklime and desulfurization gypsum can all promote the hydration reaction in the system to produce denser needle-like and flocculent CSH. In addition, the admixture of hydrogen peroxide only enhanced the porosity of ceramsite and had no effect on the phase. The experimental process was simple, green and energy saving. In particular, the total solid waste utilization was up to 79 %.

Study on the composite gravel preparation and the synergistic absorption of CO2 by fly ash of CFB boiler

The cooperative treatment of fly ash and fuel gas of coal-fired plants is a promising way. In this study, circulating fluidized bed fly ash (CFBFA), gypsum, Portland Cement (PC) and water were mixed and pelleted, with carbonation curing by fuel gas to prepare the composite gravel. Experimental results showed that CSH gel and ettringite were the main hydration products and calcite was the main carbonation product. The highest strength of the composite gravel reached 11.11 MPa at 28d which was positively related to the absorption rate of CO2, and the CO2 absorption rate reached 4.789 %. Both hydration and carbonation reactions contributed positively to strength. The carbonation process caused a higher Si 2p binding energy, and the addition of gypsum led to a higher binding energy of Ca 2p. The addition of gypsum increased the porosity and main pore diameter. The porosity and main pore diameter determined the strength of composite gravel. It was observed that gypsum acted as a catalyst to promote the formation of hydration products and carbonation products. This study provided a theoretical support for the preparation of composite gravel which was a substitute for river sand and natural gravel.

Properties of Unburned Brick Produced by Entirely Waste-Stream Binder Activated by Desulfurization Gypsum

The massive accumulation of industrial solid wastes such as circulating fluidized bed fly ash (CFA), silicon-calcium slag (SCS), and desulfurization gypsum (FGD) occupy land resources and bring varying degrees of pollution to soil, water, and atmosphere. Unburned brick is a new construction material prepared from industrial waste residues such as fly ash and tailings without high-temperature calcination. It has excellent potential in consuming large quantities of industrial solid waste. In this paper, 70% of CFA and 30% of SCS are used as the primary raw materials, and the FGD is used as the activator to prepare unburned bricks by static pressure forming. The mechanical properties of the specimens at different curing ages were tested by compressive strength test. The hydration mechanism and microstructure of unburned brick were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), thermogravimetric (TG), Fourier transform infrared spectroscopy (FTIR), and inductively coupled plasma-optical emission spectrometry (ICP-OES). The results show that the compressive strength of the specimen increases first and then decreases with the increase of FGD content, and the compressive strength reaches the maximum when the FGD content is 5%. The microscopic test results show that the presence of FGD promoted a higher degree of CFA and SCS dissolution, increasing ettringite formation, which is responsible for strength increase, but extreme doses of FGD resulted in strength degradation. Meanwhile, the higher SiO2/Al2O3 ratio confirms the simultaneous formation of hydrated calcium silicate (C-S-H) gel and hydrated calcium aluminosilicate (C-A-S-H) gel within the hydrated product, while a low SiO2/Al2O3 ratio confirms the simultaneous formation of ettringite.

A Review on the Application of Circulating Fluidized Bed Fly Ash in Building Materials

Circulating fluidized bed (CFB)-based coal burning technology is a type of clean coal combustion technology. Owing to its characteristics of low temperature combustion and desulfurization in furnace, the CFB fly ash is quite different from ordinary fly ash. Based on the comprehensive understanding of physical and chemical characteristics of CFB fly ash, this review summarizes the research progress of the application of CFB fly ash in Portland cement, magnesium oxysulfate (MOS) cement, solid-waste–based cementitious material (SWCM), concrete, geopolymer, artificial aggregate, and other building materials. The study shows that CFB fly ash can improve the early physical and mechanical properties and reduce the drying shrinkage properties for cementitious materials. Its unique chemical composition endows it with good application performance in the MOS cement, geopolymer, and SWCM. However, unfavorable characteristics of CFB fly ash, such as loose and porous structure, self-hardening behavior, and expansibility greatly hinder its practical applications in building materials. Therefore, some measures, such as addition of rational water-reducing agent, ultrafine grinding, mixing of ordinary fly ash, controlling the dosage of CFB fly ash, and water-cement ratio, must be taken to improve the negative influence of CFB fly ash on the building materials, in order to expand its application range. This article summarizes the application and research status of CFB fly ash in building materials, aiming at further promoting the research studies on CFB fly ash resource utilization in building materials and improving the efficiency of CFB fly ash resource utilization.

High-efficiency leaching of Al and Fe from fly ash for preparation of polymeric aluminum ferric chloride sulfate coagulant for wastewater treatment

Preparation of inorganic polymer coagulants from circulating fluidized bed (CFB) fly ash is one of the most significant value-added and environment-friendly approaches. Nevertheless, CFB fly ash presents relatively low solubility in acidic systems, which limits its utilization efficiency. In this paper, enhanced metal leaching from CFB fly ash using combined HCl-H2SO4 leaching process for polymeric aluminum ferric chloride sulfate (PAFCS) preparation was proposed. PAFCS with the different molar ratios of Al3+/SO42− and Al3+/Fe3+ were prepared from as-obtained leaching liquors and characterized by SEM, XRD, FTIR, and coagulation experiments. The leaching efficiencies of Al and Fe in CFB fly ash were up to 96.75% and 97.64% by combined HCl-H2SO4 leaching process. The optimal molar ratios of Al3+/SO42− and Al3+/Fe3+ for the turbidity and UV254 removal were 10:1 and 10:1, respectively, due to the three-dimensional chain-net structure and the highest [Al + Fe]b content. When applied to biochemical effluent of coking wastewater treatment, PAFCS exhibited excellent removal performance on both humic acid-like and fulvic acid-like matters. The resultant flocs of PAFCS presented the largest recovery factor (Rf = 0.37) and strength factor (Sf = 0.79) in contrast with PAC and PAFC. This work provides a novel and high-efficient Al and Fe leaching strategy from CFB fly ash to prepare Al-Fe coagulant.

Effect of Phosphorus Slag Admixture on the Properties and Hydration Mechanism of Circulating Fluidized Bed Fly Ash-Based Multi-Solid Waste Cementitious Material.

This research aims to reveal the effect of phosphorus slag (PS) admixtures on the properties and hydration mechanism of circulating fluidized bed fly ash (CFA)-based multi-solid waste cementitious material (CWM). The results indicate that PS as an admixture is more helpful for improving the performance of CWM systems compared with blast furnace slag with a high specific surface area (HBFS) and gasification slag (GS). In this work, CWM2 is prepared with 30 wt.% CFA, 10 wt.% red mud (RM), 20 wt.% blast furnace slag (BFS), 10 wt.% PS, and 30 wt.% cement clinker (CC). The compressive strength and expansion value of CWM2 are the optimal (51.15 MPa and 0.70 mm) when the mass ratio of (Ca + Na)/(Si + Al) is 0.84, which can meet the requirements of 42.5 fly ash Portland cement. In addition, the polymerization degree of CWM2-28 days is the optimum (51.57%) because [PO4] and [SiO4] combine to improve its polymerization structure. The main hydration products are C-S-H gel, C/N-A-S-H gel, and ettringite in CWM, which are conducive to improve the compactness of the micromorphology. In addition, the consolidation of Na, As, Cd, and Hg is promoted in CWM2 by physical encapsulation and charge balance, which meet the drinking water requirements of the World Health Organization (WHO). Therefore, this work provides a new idea for the application of PS as an admixture in CFA-based multi-solid waste cementitious material.

Frost Resistance and Mechanism of Circulating Fluidized Bed Fly Ash-Blast Furnace Slag-Red Mud-Clinker Based Cementitious Materials.

The motivation of this work is to enhance the long-term frost resistance of circulating fluidized bed fly ash (CFA)-based multisolid waste cementitious material (CSM). In this research, CSM2 is prepared by 30 wt.% CFA, 20 wt.% blast furnace slag (BFS), 10 wt.% red mud (RM), 10 wt.% phosphorus slag (PS), and 30 wt.% cement clinker (CC). The strength and mass of CSM are detected by a press and electronic balance. The hydration products, polymerization degree, thermogravimetric, micromorphology, pore structure, and harmful element leaching are detected by XRD, MAS NMR, TG-DTG, SEM-EDX, MIP, and ICP-MS. The major findings indicate that the strength loss, mass loss, and strength of CSM2 after 25 freeze–thaw cycles (CSM2-25) are 2.35%, 0.36%, and 49.95 MPa, respectively, which is superior to other CSMs and still meets the performance requirements of fly ash Portland cement 42.5#. The main hydration products are C-S-H gel, C/N-A-S-H gel, and ettringite during the freeze–thaw cycle. The polymerization degree and thermogravimetric loss of hydration products in CSM2-25 are 50.65% and 12.82 wt.%, respectively, which are higher than those of other CSMs under the synergy of CFA, BFS, RM, and PS. In addition, the microscopic results show that the interface between the paste and aggregate, micromorphology, and pore structure of CSM2-25 are the densest when the mass ratio of Ca/(Si + Al) is 0.81. These characteristics are beneficial to the improvement of long-term frost resistance in CSM2. Finally, the leaching results of harmful elements in CSM2 after 25 freeze–thaw cycles still meet the WHO standard of drinking water. Therefore, this work provides a reliable reference for the preparation of green cementitious materials with great frost resistance by using CFA, BFS, RM, and PS.

Sustainable process for valuable-metal recovery from circulating fluidized bed fly ash through nitric acid pressure leaching

Circulating fluidized bed fly ash ( CFBFA ) is receiving widespread attention due to its threat to the environment and its high content of alumina (30–50%), gallium (30–100 g/t), etc. In this research, nitric acid pressure leaching technology was proposed to selectively extract aluminum and gallium from CFBFA. The E-pH diagram of the Al–Fe–H 2 O system and experiments proved that the leachate ranged within pH −0.5 to 1, which was not favorable for the generation of Al(OH) 3 colloids. Moreover, Al entered the leachate as Al 3+ , and the divalent iron was initially oxidized by NO 3 − and then hydrolyzed to form hematite . The lg c Fe3+ at equilibrium of Fe 3+ hydrolysis had a certain relationship with pH. Thus, at high acidity with increased temperature, more Al entered the solution and more Fe entered the residue. Based on experiments, the optimum conditions were found to be as follows: leaching temperature of 220 °C, liquid-to-solid ratio of 4:1 mL/g, HNO 3 concentration of 340 g/L, and leaching duration of 2 h. The leaching rate of Al, Fe, and Ga were approximately 80%, 6%, and 78% respectively. The basic separation of aluminum and iron could be realized during leaching. In the evaporation, concentration and crystallization process of the leachate, aluminum nitrate precipitated out at room temperature when leachate density exceeded 1.38 g/cm 3 . A greater density of the leachate was concentrated, and crystallization benefited the formation of aluminum nitrate crystals with less crystal water. • A feasible process for recovering aluminum from solid waste (CFBFA) was developed. • The E-pH diagram of Al–Fe–H 2 O system at high temperature was analyzed. • Aluminum and iron were basically separated during the leaching process. • The leaching agent could be regenerated and the process was sustainable.