High-alumina Coal Fly Ash Research Articles

Comprehensive analysis of thermochemical phase evolution and strategic recovery of valuable elements from high-alumina coal fly ash at different thermal conditions

The evaluation of the application potential of high-alumina coal fly ash (HACFA) resources relies on the assessment of the phase composition and leaching behavior of valuable elements. Herein, we systematically investigated the particle size, specific surface area, and morphology of high-alumina high-temperature coal ash (HAHCA) produced under laboratory conditions at different temperatures and quantitatively analyzed phase transformations using the Rietveld method. The results revealed that the HAHCA particle size gradually increased, specific surface area gradually decreased, and pores became lesser with the increasing coal combustion temperature of high-alumina coal (HAC) from 400 °C to 1300 °C. Furthermore, HAHCA phases sequentially changed across various temperatures, its main phases at a coal combustion temperature of 400 °C included 29.8 % kaolinite, 6.9 % boehmite, and 5.4 % calcite, but the crystalline phases were the most abundant and turned into 41.2 % mullite, 13.7 % corundum, and 11.6 % plagioclase when the coal combustion temperature reached 1300 °C. Additionally, the leaching behavior of 28 major and trace valuable elements of HAHCA under acidic and alkaline conditions was investigated. Most metal elements easily leached from HAHCA under acidic conditions at coal combustion temperatures of 600 °C–800 °C: the optimal leaching efficiencies of Al, Li, K, Rb, Mg, Ca, Sr, and Ba were 67.64 %, 92.71 %, 76.67 %, 73.46 %, 86.74 %, 98.87 %, 98.42 %, and 61.19 %, respectively. Si, a nonmetallic element, achieved a high leaching efficiency of 48.27 % under alkaline conditions at a coal combustion temperature of 1300 °C. Interestingly, only Mo exhibited a high leaching efficiency of approximately 98 % under acidic and alkaline conditions.

Catalytic Oxidation Activity of NO over Mullite-Supported Amorphous Manganese Oxide Catalyst.

Nitric oxide (NO) can pose a severe threat to human health and the environment. Many catalytic materials that contain noble metals can oxidize NO into NO2. Therefore, the development of a low-cost, earth-abundant, and high-performance catalytic material is essential for NO removal. In this study, mullite whiskers on a micro-scale spherical aggregate support were obtained from high-alumina coal fly ash using an acid-alkali combined extraction method. Microspherical aggregates and Mn(NO3)2 were used as the catalyst support and the precursor, respectively. A mullite-supported amorphous manganese oxide (MSAMO) catalyst was prepared by impregnation and calcination at low temperatures, in which amorphous MnOx is evenly dispersed on the surface and inside of aggregated microsphere support. The MSAMO catalyst, with a hierarchical porous structure, exhibits high catalytic performance for the oxidation of NO. The MSAMO catalyst, with a 5 wt% MnOx loading, presented satisfactory NO catalytic oxidation activity at 250 °C, with an NO conversion rate as high as 88%. Manganese exists in a mixed-valence state in amorphous MnOx, and Mn4+ provides the main active sites. The lattice oxygen and chemisorbed oxygen in amorphous MnOx participate in the catalytic oxidation of NO into NO2. This study provides insights into the effectiveness of catalytic NO removal in practical industrial coal-fired boiler flue gas. The development of high-performance MSAMO catalysts represents an important step towards the production of low-cost, earth-abundant, and easily synthesized catalytic oxidation materials.

Geochemistry and mineralogy of fly ash from the high-alumina coal, Datong Coalfield, Shanxi, China

The abundance and distribution of minerals and elements in high-alumina coal fly ash (HACFA) is the basis for evaluating the resource utilization potential of HACFA and the key to developing a rare element extraction process. The geochemistry and mineralogy of HACFA were investigated by using alkali desilication and particle size screening, combined with multiple analytical techniques. As fly ash particles become finer, the content of mullite decreases, the content of quartz, corundum, and glass increases, and the content of hematite decreases first and then increases. The molecular structure of the glass phase is silicon-oxygen tetrahedron Q4(0Al). Minerals containing REE appear in the shape of fish eggs or oolitic particles, mainly composed of Ai, Si, REE, O. The maximum contents of Al, Li, Ga, and REE in HACFA are 67.65%, 240.57 μg/g, 77.97 μg/g, and 672.95 μg/g, showing a high utilization potential. However, the high contents of harmful elements Cd and Cr need special attention. Most of the trace elements in HACFA mainly reside in the glass phase. The iron-rich minerals are the main carriers of the ferrophilic elements Fe, Zn, Cr, Co, Mn, and Mg. The oxyphilic elements Al, Sc, Ta, Nb, and Ga mainly occur in corundum or mullite.

The Influence of Hydrothermal Temperature on Alumina Hydrate and Ammonioalunite Synthesis by Reaction Crystallization

With the rapid development of the alumina industry and the shortage of bauxite, high-alumina coal fly ash (HACFA) has attracted more and more attention as a potential alternative alumina resource. In order to extract alumina from HACFA with newly developed technology, the investigation of the crucial step, the reaction between NH4Al(SO4)2·12H2O and NH3·H2O, is necessary and valuable. Thermodynamic analyses have shown that four kinds of alumina hydrate (boehmite, diaspore, gibbsite, and bayerite) might be formed at 120–200 °C, and ammonioalunite might be formed at temperatures over 180 °C. A hydrothermal reaction crystallization method was applied to this reaction. The experimental results showed that boehmite (AlOOH) could be formed at 150 °C and 200 °C after 12 h and NH4Al3(SO4)2(OH)6, an unstable intermediate, is formed during the initial stage and transformed into boehmite, eventually. The higher temperature (200 °C) was more energetically favorable for the formation of NH4Al3(SO4)2(OH)6, and the crystallinity of the products was better. More importantly, the sheet-like structure of boehmite (AlOOH) could be formed at 150 °C after 24 h of reaction time. The SEM results proved that the sheet-like structures evolutionary process of boehmite.

Mechanisms of Separation and Crystal Growth of Mullite Grains during Preparation of Mullite-Based Ceramics from High Alumina Coal Fly Ash

High-alumina coal fly ash (HAFA: 45% Al2O3 and 40% SiO2) is regarded as a special solid waste that is generated from power plants in northwestern China. It is regarded as an important substitute for bauxite and is applied to prepare mullite-based ceramics. In this work, a hydrometallurgy–pyrometallurgy synergistic process is proposed to resolve the lower Al2O3/SiO2 mass ratio (Al/Si) and lower degree of crystallinity that can promote the formation of compact mullite-based ceramics. During the activation–desilication process, the inert Al-O-Si is activated to form more active Si-O-H in acid activation, which can be decomposed easily in the desilication process, and the Al/Si mass ratio increases from 1.17 to 2.80, so the mullite grains and metastable phase can be exposed. During the sintering process, mullite grains and the metastable phase tended to axial growth, part of the metastable amorphous Q4(3,2,1Al) structure was transformed to Q4(4Al) structure (mullite), and then the staggered spatial structure was formed such that the bulk density and apparent porosity of the mullite-based ceramic reached 2.85 g/cm3 and 0.5%, respectively. This process not only consumes more HAFA but also helps alleviate the shortage of bauxite, which will promote the development of clean coal-fired power generation.

Molten salt synthesis of mullite whiskers entirely derived from fly ash for electronic packaging toughening ceramic applications

Preparation of mullite from high-alumina coal fly ash (CFA) has become one of the most important ways for high-value utilization. However, low Al2O3/SiO2 ratio, poor reaction activity and various impurities in fly ash led to poor quality of mullite products. Conventional sintering method requires addition of aluminum sources and sintering additives, and high synthesis temperature. This paper proposes a new technical route for preparation of mullite whiskers entirely from fly ash by molten salt method based on additive activation-acid leaching process. The growth process and synthesis mechanism of mullite whiskers in Na2SO4 molten salt from fly ash were systematically investigated, and the application of fly ash-derived mullite whiskers in electronic packaging toughening ceramic was also explored. The results show that leaching liquor containing Al3+, Na+, and SO42−, and amorphous silica slag can be separated and obtained after acid leaching of activated fly ash at 80 °C, and the fly ash-derived mullite whiskers with a length of about 1 μm can be synthesized from the mixture of crystalline products and silica slag by the proposed method under the optimal synthesis conditions. Using amorphous silica slag with higher dissolution activity as the silicon source, the nucleation of mullite whiskers with homogeneous nucleation process is promoted and highly monodispersed mullite whiskers derived from fly ash form. The fly ash-derived mullite whiskers can improve the mechanical strength, reduce the dielectric constant and dielectric loss of the composite ceramic substrates, and the dielectric constant and dielectric loss of the composite ceramic are the lowest 5.16 and 0.006, respectively, when the mullite addition content is 20 wt.%, indicating that they could be used as electronic packaging materials. This paper provides a novel pathway for synthesis of mullite whiskers and high-value utilization of fly ash.

A sustainable process to recycle aluminum from coal fly ash for simultaneous removal of iron: Solid waste management and evaluation

Coal fly ash (CFA) is one of the largest forms of global industrial waste, and continuous accumulation of CFA puts great pressure and poses potential risks to the natural environment. The objective of this study was to evaluate a sustainable process to recycle valuable elements in high-alumina coal fly ash (HACFA). In this study, an efficient and applicable approach was developed to recover Al selectively from HACFA, while most Fe recovery and removal were realized simultaneously in the leaching process. First, the thermodynamic stability of HACFA and the feasibility of Al leaching in HACFA were calculated. Then, HACFA was subjected to nitric acid pressure leaching to selectively extract Al. Under the optimal conditions (temperature of 220 °C, nitric acid concentration of 300 g/L, liquid-to- solid ratio of 4 mL/g, and duration of 2 h), 84.24% of the Al was extracted, and the leaching rate of Fe was 6.67%. The concentrations of Al and Fe in the leachate were 46.34 g/L and 0.52 g/L, respectively. The separation of Al and Fe was completed in the leaching process. The leach liquor was purified to further remove impurities, and Al2O3 was obtained after crystallization and pyrolysis of the aluminum-rich leachate. Leach residue was mainly composed of silica and Fe2O3. This process provides a feasible method for the recovery and utilization of valuable components in HACFA.

Mild Hydrothermal Synthesis of 11Å-TA from Alumina Extracted Coal Fly Ash and Its Application in Water Adsorption of Heavy Metal Ions (Cu(II) and Pb(II))

Non-biodegradable copper (Cu) and lead (Pb) contaminants in water are highly toxic and have series adverse effects. Therefore, it is very important to extract heavy metals from wastewater before it is discharged into the environment. Adsorption is a cost-effective alternative method for wastewater treatment. Choosing a low-cost adsorbent can help reduce the cost of adsorption. In this study, conversion of reside after extracting aluminum (REA) produced by sub-molten salt method transform high-alumina coal fly ash (CFA) into 11Å-tobermorite to adsorb Cu(II) and Pb(II) from aqueous solutions at room temperature. The synthesis of the adsorbent was confirmed using scanning electron microscope (SEM), X-ray diffractometer (XRD) and Brunauer–Emmett–Teller (BET) method surface analysis. To study the adsorption characteristics, factors such as initial Cu(II) and Pb(II) concentration, pH, contact time, adsorption characteristics and temperature were investigated in batch mode. The maximum adsorption capacity of Cu(II) and Pb(II) was 177.1 mg·g−1 and 176.2 mg·g−1, respectively. The Langmuir adsorption model was employed to better describe the isothermal adsorption behavior and confirm the monolayer adsorption phenomenon. The pseudo-second-order kinetic model was used to highlight Cu(II) and Pb(II) adsorption kinetics. Thermodynamic analysis indicated the removal Cu(II) and Pb(II) by TA-adsorbent was a nonspontaneous and exothermic reaction. The obtained results are of great significance to the conversion of industrial waste to low-cost adsorbent for Cu(II) and Pb(II) removal from water.

Improving the A/S ratio of pretreated coal fly ash by a two-stage roasting for Bayer alumina production

Coal fly ash has been considered as a potential resource for alumina production by Bayer process. However, the implementation is always limited by low alumina-silica mass ratio (A/S) of coal fly ash. Previously, we reported an approach to realize the desilication of coal fly ash and the A/S could reach 5.34. However, this A/S was still not high enough for Bayer process. Following the previous study, herein we systematically optimize the desilication conditions to further improve the A/S, and also evaluate the applicability of desilication residue for Bayer alumina production. First, the digestion properties of amorphous silicon dioxide and γ-alumina in the two-stage roasted calcines are analyzed by thermodynamic calculation. The results indicate that their alkali-digestion properties become remarkably different at relatively high temperature, thus amorphous silicon dioxide could be digested while γ-alumina is preserved in the residue by controlling the pH and digestion temperature. The desilication ratio and the A/S ratio could reach 82.27% and 7.46 at the optimal conditions of 116 g/L Na2O, 95 °C, 15 min digestion time and 1/10 solid-to-liquid ratio. The obtained desilication residue becomes acceptable and economical high grade raw material for Bayer process, which is confirmed by the production of standard alumina (AO-2, China). This research could provide an alternative economic process for recycling alumina from high-alumina coal fly ash.

Thermodynamics of the decomposition of aluminum chloride hexahydrate to prepare alumina

High-alumina coal fly ash is a high-quality alternative to produce alumina as one of the raw materials for the production of aluminum. The extraction of alumina from coal fly ash with hydrochloric acid is a common treatment method. With this method, coal fly ash is leached with hydrochloric acid to obtain an aluminum chloride solution which is then separated, purified, and concentrated to form aluminum chloride hexahydrate. Alumina is then prepared by calcining aluminum chloride hexahydrate. The decomposition processes of aluminum chloride hexahydrate significantly affect the physical and chemical properties of the alumina product. In this paper, the effects of temperature and decomposition atmosphere on the thermal decomposition reaction were studied by thermodynamic analysis. The crystal phase transformation and microscopic morphology of the decomposition products were studied by combining chemical analysis, XRD, and SEM techniques. The results showed that the chlorine content of the decomposition products obtained in the water vapor atmosphere was significantly lower than that obtained in the argon atmosphere.

Study on the correlation between Fe/Ti forms and reaction activity in high-alumina coal fly ash

High-alumina coal fly ash (HAFA) is an important aluminum and silicon resource. In the process of preparing aluminum–silicon materials from HAFA, the influence of impurity elements on its performance must be considered. In this work, the occurrence state of impurities in HAFA, micro morphology, and the bond energy of different impurity coordination were studied. Sulfuric acid leaching method and density functional theory were used to study the leaching behavior of impurities to verify the difficulty of removing different impurity elements. The results show that iron existed in the form of magnetic particles (34.78%), amorphous phase (49.24%), and crystalline phase (15.96%) in HAFA. Titanium mainly existed in amorphous phase (29.34%) and crystalline phase (69.4%). In sulfuric acid leaching, titanium was more difficult to leach, and the content of TiO2 decreased from 2.30% to 2.25%, whereas that of Fe2O3 decreased from 1.50% to 0.86%. The actual leaching behavior of impurity elements was consistent with the simulation results, with more energy required to remove Ti than Fe. These studies of impurity elements in HAFA will provide theoretical support for the preparation of aluminum–silicon materials.

Recycling of mullite from high-alumina coal fly ash by a mechanochemical activation method: Effect of particle size and mechanism research

High-alumina coal fly ash (HAFA) is a special solid waste since its alumina content can reach 40–50 wt%, which is seen as a potential resource for mullite material production. However, obtaining an ideal mullite material from HAFA is difficult because of its low Al2O3/SiO2 mass ratio. In this work, the microstructure characteristics of HAFA were systematically analyzed by combining multiple characterization techniques. It was found that HAFA had a core-shell structure with a mullite/corundum crystal core and a silica-rich amorphous phase shell. The novel mechanochemical activation–desilication process was used to remove amorphous phase from HAFA and elevate the Al2O3/SiO2 mass ratio. In particular, the effect of particle size after mechanical treatment and mechanism of the desilication process were extensively investigated. On decreasing the particle size, a high leaching rate of alumina was achieved during mechanochemical activation, thus generating a hydroxysodalite coating layer as desilication was suppressed, and the amorphous phase was effectively removed. The mineralogical phase of the desilicated HAFA is mainly mullite and corundum, and the Al2O3/SiO2 mass ratio was elevated from 1.29 to 3.02. Mullite refractory obtained from the desilicated HAFA exhibited excellent physical properties. This study provides insights into further high-valued utilization of HAFA.

Solvent extraction of lithium from hydrochloric acid leaching solution of high-alumina coal fly ash

The present work used solvent extraction method to extract lithium from hydrochloric acid leaching solution of high-alumina coal fly ash. In extraction stage, the optimum conditions were explored, at 20 ℃ and O/A = 1:4, using 70%(Vol.) tributyl phosphate-30%(Vol.) sulfonated kerosene-0.36 mol/L Fe3+ as extractant, extraction of lithium could reach 99% by two-stage extraction. In scrubbing stage, 95% of Al3+ in loaded organic phase was scrubbed with 6 mol/L HCl at O/A = 40:1, then 97% of Ca2+ in the loaded organic phase was scrubbed by three-stage countercurrent scrubbing with 5.5 mol/L LiCl + 0.5 mol/L HCl at O/A = 30:1. In stripping stage, 99% of Li+ in scrubbed organic phase was stripped by five-stage countercurrent stripping with 6 mol/L HCl at O/A = 10:1. In regeneration stage, 2 mol/L NaOH was used as regenerant to remove free acid and exchange H+ of extractive compound. There were no third phase, emulsification and solid deposition during the whole experiment. The structure of extractive compound was determined by slope method, and extracted complex ratio of Li+, Fe3+, and TBP was 1:1:1. The extraction process was found to be exothermic and spontaneous by thermodynamic study.Therefore, it’s a promising method to recover lithium from hydrochloric acid leaching solution of high-alumina coal fly ash by TBP-FeCl3-sulfonated kerosene extraction system.

Dependence on the distribution of valuable elements and chemical characterizations based on different particle sizes of high alumina fly ash

Understanding the influence of the particle size on the distribution of various elements of high alumina coal fly ash (HAFA) is of significance to the utilization of valuable elements in fly ash. The distribution of valuable elements and the chemical characterizations of various particle sizes of HAFA were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) analysis etc. in this work. It indicated that the contents of most of the metal oxides, including Al2O3, Fe2O3, CaO, Ga2O3, TiO2, and Li2O, decreased as the particle sizes of HAFA increasing. The contents of the nonmetal oxides, including SiO2 and organics, however, increased with the sizes increasing. For the phases distribution, the spherical mullite crystalline phase and the corundum phase were more abundant in the smaller particle sizes, while the amorphous glass phase and quartz were more abundant in the larger particle sizes. In addition, the desilication efficiency increased from 38.08% to 39.30% though mechanical pretreatment method. Interestingly, it was observed that the glass phase Al2O3 reacted with the sodium silicate solution to form zeolites, which accounted for a 22.85% of the mass after desilication. Classification may be an important pretreatment step in the recovery of valuable elements from HAFA due to the differences between the different granularities.

Mechanisms of mechanochemical activation during comprehensive utilization of high-alumina coal fly ash

High-alumina coal fly ash (HAFA) is an important aluminum and silicon resource. In terms of its comprehensive utilization, how to efficiently separate aluminum and silicon must be resolved. In this work, a mechanochemical activation method in the desilication process is adopted and investigated. Desilication ratio, efficient desilicated ratio, microscopic appearance, and coordination structure change in aluminum–silicon atoms are investigated during mechanochemical activation process. Results indicate that the Al-O-Si coordination structure in HAFA can be transformed from Q4 (3Al) and Q4 (2Al) to reactive Q2 (1Al) structure, and the radio of Q2 (1Al) with high activity increased from 6.93% to 14.47%, which can improve HAFA reaction activity obviously. During the desilication process, the aluminum–silicon mass ratio (A/S) can be elevated from 1.23 to 2.56 after the process optimization, which provides a high-quality raw material for further high-valued utilization of HAFA.

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.

A cleaner electrolysis process to recover alumina from synthetic sulfuric acid leachate of coal fly ash

The realization of clean extraction of aluminum hydroxide from high-alumina coal fly ash would be beneficial to achieve better overall coal utilization, and renders environmental and economic benefits. The current study mainly aimed to develop a nonhazardous approach to conduct electrolysis for alumina extraction from sulfuric acid leachate of high-alumina coal fly ash. The XRD, SEM, LPSA, FT-IR and ICP-AES results revealed that aluminum hydroxide was a main electrolysis product, with current efficiency of 83.25% under the optimized conditions as t = 1 h, J = 600 A/m2. The median particles of aluminum hydroxide after 1-h and 6-h electrolysis were 35.3 and 40.1 μm, respectively. The electrolysis mechanism was analyzed by cyclic voltammetry, which showed that the H2O gained electrons on the cathode to form OH– and H2. The OH– reacted with Al3+ to form Al(OH)3. In addition, the effluent of electrolysis was sulfuric acid and can be reused as a leaching reagent to promote zero pollution discharge. Hence, the electrolysis process provided an environment-friendly and efficient approach for alumina extraction from high-alumina coal fly ash.

Distribution and occurrence of lithium in high-alumina-coal fly ash

High-alumina-coal fly ash (HAFA) with a high Li content is regarded as a potential resource for Li production. To support the development of Li recovery technology from HAFA, the distribution and modes of occurrence of Li in HAFA were investigated. HAFA was separated into magnetic particles, glass, and mullite+corundum+quartz (MCQ) using an acid-alkali combination method. 79–94% of the Li was found in glass, with the remaining 5–16% and <5% of Li in MCQ and magnetic particles, respectively. Chemical state imaging of Li in HAFA, magnetic particles, and MCQ was obtained using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Lithium was found to be relatively uniformly distributed in the HAFA particles and strongly correlated with Al and Si. Possible formation pathways of Li in the glass phase were proposed. The energy differences between reactants and products (∆E) calculated using the generalized gradient approximation (GGA) method indicated that Li occurred in Q3(0Al) and Q3(1Al) structures by reacting with Q4(0Al) and Q4(1Al). Based on the experimental and simulation results, we propose extracting Li during the pre-desilication process by dissolving the glass phase.

Comprehensive Utilization of High Alumina Coal Fly Ash Under the Recyclable Economy Model

Comprehensive Utilization of High Alumina Coal Fly Ash Under the Recyclable Economy Model

Mechanism of mechanical–chemical synergistic activation for preparation of mullite ceramics from high-alumina coal fly ash

High-alumina coal fly ash (HAFA) is a special solid waste due to the existence of more than 45% alumina and 35% silica, which can be applied to prepare Al-Si series ceramics if the impurities can be removed and the Al/Si mass ratio can exceed 2.55 (Mullite: 3Al2O3.2SiO2). In this work, a new mechanical–chemical synergistic activation–desilication process is proposed, and the contents of different impurities can be lowered up to less than 1%, and the Al/Si mass ratio can be elevated from 1.26 to 2.71. Especially, the mechanism of this process is investigated in detail. The analysis of the mechanism shows that the decrease of Q4(3Al) and the increase of Q4(0Al), Q4(1Al), and Q4(2Al) improve amorphous silicate reactivity through synergistic activation, and the exposed amorphous Si-O-/Si-O-Si can be removed by OH- during the desilication process (desilicated ratio>55%), which help the fine mullite to exhibit excellent properties (bulk density > 2.85g/cm3, apparent porosity < 0.5%) during the sintering process. Finally, this process not only decreases the pollution but also alleviates the shortage of Al/Si resources and promotes the clean development of coal-fired power generation.