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High alkaline desilication solution (DSS), a high volume byproduct from the pretreatment of high‐alumina fly ash, was used as low‐cost mother liquor for the synthesis of calcium silicate hydrate (C‐S‐H). Through the combined analysis of X‐ray diffraction, thermogravimetric analysis, X‐ray fluorescence, 29Si MAS NMR, and Brunauer‐Emmett‐Teller, the relationship between chemical composition and structure of C‐S‐H synthesized under Ca/Si of 0.83:1 to 2.0:1 was investigated. Silicon conversion and yield of product have a positive correlation with Ca/Si. Sodium uptake in C‐S‐H is inhibited as Ca/Si increases. The formation of sodium in C‐S‐H transfers from "bound Na" to "mobile Na" and aluminum from tetrahedrally coordinated Al (IV) to octahedrally coordinated Al (VI). The increase of Ca/Si leads to shortening of silicate chain and formation of more dimers, which causes more water bound in C‐S‐H. The mechanism of calcium addition on silicate chain obtained from DFT calculation primarily results from more interlayer calcium occurrence to affect bridging tetrahedron and cationic bounding states reorganization. Reasonable control for Ca/Si momentously contributes to the adjustment for composition and structure of C‐S‐H synthesized in DSS. [ABSTRACT FROM AUTHOR]

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: 3Al 2 O 3 .2SiO 2 ). 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 Q 4 (3Al) and the increase of Q 4 (0Al), Q 4 (1Al), and Q 4 (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 > 2.85 g/cm 3 , 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. [ABSTRACT FROM AUTHOR]

Secondary pollution exists extensively in the utilization of industrial solid waste. To reduce environmental risks of building material products recycled from industrial solid waste, a framework of eco-design strategy is established based on the existing eco-design method and life cycle thinking. It focuses on controlling of chemical components by integrating tracking and controlling of toxic components introduced by industrial solid waste. The framework consists of four steps: analysis of raw material applicability, analysis of process control conditions, and safety analysis during product use and final disposal. Raw material requirements are designed including effective components, quality disturbance components, and environmental risk components. Operating condition requirements are given to control flows of selected components in key procedures. Environmental safety of products during use and final disposal under these requirements are tested by simulation experiments. Then a series of eco-design requirements are proposed. The framework is applied in a case study of mullite recycled from coal fly ash and the result shows that the established framework could be feasible for products recycled from industrial solid waste. Nevertheless, the method needs expansion with industrial solid waste properties and product requirements, and steps need optimization if applied in other wastes. [ABSTRACT FROM AUTHOR]

In the alumina extraction process of high-alumina fly ash, alkali was used to remove amorphous silica and then recovered through causticization. Focusing on the causticization process, the coupling control of CO 3 2 − conversion and CaCO 3 crystallization is important to secondary-pollution reduction and high-value utilization. Influences of temperature, stirring rate, feeding time, Ca(OH) 2 concentration, and calcium/carbonate (Ca 2 + /CO 3 2 − ) ratio on the conversion and CaCO 3 morphology were investigated. Feeding time and Ca 2 + /CO 3 2 − ratio showed crucial effects on the conversion and crystallization of CaCO 3 particles. Under optimized conditions, the conversion of CO 3 2 − can reach 95.67%, and the alkali can be well recycled with the generation of superfine calcium carbonate. Furthermore, the morphology evolution of CaCO 3 was investigated in the reaction process. At the beginning of causticization, a combination of amorphous and crystalline structures can be observed. Subsequently, cubic and homogeneous morphologies were formed. At the end of the reaction, the edges of the cubic particles were dissolved, and the large particles with irregular shapes were formed by the joint of small ones in the alkaline system. [ABSTRACT FROM AUTHOR]

Alumina was leached from high-alumina coal fly ash with a NH 4 HSO 4 + H 2 SO 4 mixed solution. The leaching behavior was systematically investigated, as were factors such as mixed solution composition and concentration, reaction time and temperature, and liquid-to-solid ratio. In optimal conditions, the alumina extraction ratios can reach 87.8%, 91.1%, and 87.5% for the three different sources of coal fly ash, respectively. The phases and morphology of the raw materials and residues were also analyzed through X-ray diffraction and scanning electron microscopy–energy dispersive spectroscopy. Results showed that mullite, which is the main form taken by alumina in high-alumina coal fly ash, can be effectively decomposed to release the alumina and induce a new amorphous silica phase in mixed solution. [ABSTRACT FROM AUTHOR]

High alumina fly ash (HAFA), a main industrial solid waste produced in increasing amounts in western China, is a type of alumina-rich resource that can replace bauxite in the preparation of alumina. A new process of alumina extraction from HAFA is proposed in this paper. This method involved the raw HAFA being treated first by a low concentration alkali solution to form desilicated HAFA and then alumina was effectively extracted from desilicated HAFA in mixed alkali solution (NaOH + Ca(OH) 2 ). The effects of the reaction conditions on alumina leaching from desilicated HAFA were investigated. The extraction efficiency of alumina could reach 92.0% under the following optimal conditions: alkali to ash ratio of 6, calcium to silicate ratio of 1.0, and reaction temperature of 280 °C. After alumina extracted from desilicated HAFA, the solid residues were collected and characterized to better understand the mechanism of mineral phase transition of desilicated HAFA with alumina extraction in mixed alkali solution. By changing the calcium to silica ratio or alkali to ash ratio, the mineral phases could be transformed from 1.2Na 2 O·0.8CaO·Al 2 O 3 ·2SiO 2 ·H 2 O to NaCaHSiO 4 . A relatively high alkali to ash ratio or calcium to silicate ratio was preferred for the extraction of alumina from desilicated HAFA. [ABSTRACT FROM AUTHOR]

Synthesis of calcium silicate hydrate (C-S-H) was conducted over the range of 50°C-90°C and C/S ratio of 0.86-2.14 in the highly alkaline Na2O-CaO-SiO2-H2O system for silicon utilization in high alumina fly ash. Structural change in C-S-H formed in the highly alkaline system was investigated using XRD and 29Si MAS NMR spectra. X-ray photoelectron spectroscopy was used to confirm the amount of sodium ions in C-S-H. Conversion of Si may reach 99% under optimum conditions. A higher degree of polymerization of silicate was obtained at lower temperature and C/S ratio. Na+ was confirmed to exist as Na-OSi and Na-OH. The amount of Na+ is the least at C/S ratio of 1.43, which conform to the prediction of topological constraint theory. High Ca/Si ratio leads to the increasing in Na+ combined in the interlayer. Increasing in the Na+ concentration in the system also increases the amount of Na+ combined in the interlayer and reduces the polymerization. Ion exchange was proven to be an effective way to remove Na+ combined in the interlayer of C-S-H. [ABSTRACT FROM AUTHOR]

This study is mainly focused on the direct adsorption of low concentration gallium from the feed solution in pre-desilication soda-lime sintering process from coal fly ash. The adsorption kinetics, mechanism, and the influence of impurities, cyclic times, and eluant content are systematically researched. Results showed that the adsorption capacity was 2.89 mg/g resin with gallium concentration of 50 mg/L. The adsorption mechanism could be explained by the interaction between the oxygen atoms and nitrogen atoms of amidoxime group. Gallium was eluted efficiently by NaOH and Na2S mixed solution and the concentration could be reached to 2400 mg/L. [ABSTRACT FROM PUBLISHER]

In the removal of silicate and impurities in the acid treatment of high-alumina fly ash, the transition from silica sol to gel in the presence of metal cations has a major role in filtration and product quality. In this work, an online physical analysis method that can continuously measure zeta potential, pH, and viscosity is established for low-concentration colloidal systems. The change in zeta potential can be explained by Derjaguin•Landau•Verwey•Overbeek (DLVO) theory and model of electric double layer, which is verified by the morphology of colloids in different periods. The influences of metal cations with different valences and contents on the transition from silica sol to gel are investigated in detail. Results show that trivalent ions have a positive role in the delayed coagulation because of their hydrolysis and complexation in relatively low concentrations. By contrast, divalent and monovalent ions have a negative role in the transition from sol to gel because of the bridging effect. In terms of concentration, divalent ions have greater influence on the transition from sol to gel than monovalent ions. This established method is adaptable for the analysis of a low-concentration colloidal system and could be applied in the acid treatment of silicon-containing solid wastes. [ABSTRACT FROM AUTHOR]

Sodium was removed from alumina-extracted coal fly ash (CFA) by using diluted NaOH solution. The effects of reaction temperature, reaction time, and NaOH concentration on sodium removal were studied. Result shows that the sodium (Na 2 O) content of the alumina-extracted CFA was decreased from 20 wt.% to 0.6 wt.% under optimal conditions: temperature of 185 °C, NaOH concentration of 50 g/L, residence time of 2 h, and liquid-to-solid ratio of 40 for water washing. The possible mechanism of sodium removal was also investigated using Fourier transform infrared spectroscopy, X-ray diffraction analysis, and nuclear magnetic resonance spectroscopy. The results indicated that NaCaHSiO 4 in the residue was decomposed by NaOH to form NaSiO 3 and Ca(OH) 2 , and then converted into tobermorite [Ca 5 Si 6 O 16 (OH) 2 ·nH 2 O] and NaOH. The sodium obtained through the reaction in the residue was mainly combined with the silanol group and then washed with water. [ABSTRACT FROM AUTHOR]

• The utilization of HAFA is accomplished by mechanochemical synergistic activation. • The inert Q4 (3Al)/Q4 (2Al) is changed to reactive Q2 (1Al) during this process. • The desilication ratio is elevated from 30% to above 50% by dilute alkali solution. 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. [ABSTRACT FROM AUTHOR]

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 Q 3 (0Al) and Q 3 (1Al) structures by reacting with Q 4 (0Al) and Q 4 (1Al). Based on the experimental and simulation results, we propose extracting Li during the pre-desilication process by dissolving the glass phase. [ABSTRACT FROM AUTHOR]

High performance mullite ceramics were prepared from high-aluminum fly ash without any extra materials via a series of steps. The original fly ash was pretreated with alkali and acid to adjust the chemical composition. Furthermore, the physical properties and phase transformation of the as-prepared samples at different sintering temperatures were systematically investigated. The results showed that by pretreated process, Al 2 O 3 content can reach up to 65.35 wt%, and most of the impurities were removed. The beneficiated sintering sample exhibited a mullite content of 88.33%, an apparent porosity of 1.20%, a bulk density of 2.78 g/cm 3 and a compressive strength of 169 MPa, which was comparable with previous reports. All results indicate that high-aluminum fly ash, pretreated by an effective and feasible method, is suitable to prepare high performance mullite ceramics. [ABSTRACT FROM AUTHOR]

[Display omitted] • A stepwise process for selective recovery of Ga(III) from coal fly ash acid leach liquor was proposed. • The concentration of Cl− plays the key role in the adsorption of Ga(III). • The coexisting Fe(III) in the liquor is the main influencing factor. • Ga(III) is coordinated in the form of GaCl 3 with the O atoms of the polyurethane foam. The interest in gallium (Ga) recovery from coal fly ash has been driven by its rapidly growing demand for manufacturing and its extraction can facilitate the effective use of coal fly ash, which is conducive to reducing the soil degradation and environment danger. In this study, polyurethane foam (PUF) as an adsorbent for Ga(III) recovery was found to exhibit a good structural stability and a high selectivity in a strongly acidic solution. Adsorption measurements show that PUF has a good adsorption ability and its maximum adsorption capacity is 62.53 mg/g. Furthermore, its adsorption behavior can be described well by the pseudo-second-order model. The concentrations of Cl(I) played the key role in the adsorption. Meanwhile, the co-existed Fe(III) in the coal fly ash hydrochloric acid leach liquor (CFAHL) exhibits significant interference to the selective adsorption of Ga(III). Therefore, a stepwise process involving Fe(III) removal and Ga(III) adsorption is proposed for the recovery Ga from CFAHL. At an equilibrium pH of 0, the Fe(III) in the CFAHL can be removed by 1 mol/L P507. Then, the Ga(III) can be quantitatively adsorbed from the CFAHL using PUF with limited co-adsorption of Al(III), Mg(II), and Ca(II). The adsorption capacity of Ga(III) remains steady after PUF is used for more than 10 cycles. Based on the results of IR, XPS and DFT, it is speculated that the Ga(III) is probably coordinated in the form of GaCl 3 with the donor O atoms of the urethane group (NHCOO) on the PUF surface. The overall results demonstrate the potential use of PUF for Ga recovery from acidic solutions. [ABSTRACT FROM AUTHOR]

• A approach for Ga(III) recovery from CFA sulfuric acid leach liquor is developed. • Ga(III) can be recovered by stepwise separation using P507 and Cyanex 272. • The extraction of Ga(III) with Cyanex 272 follows an ion exchange mechanism. • The Ga(III) complexes with Cyanex 272 include three coordination compounds. For the ever-growing demand of gallium (Ga) resources in electronic applications, the recovery of Ga from the mining residues or waste has received considerable interest. The present paper is focused on the selective recovery of Ga from the sulfuric acid leach liquor of coal fly ash (CFA) by stepwise separation using acidic extractant. The results show that Ga(III) can be effectively separated by controlling the equilibrium pH values and meanwhile employing appropriate extractant based on the different ionic potential and electronic configuration of metal ions. At equilibrium pH ≤ 0.8, the impurities Ti(IV) and Fe(III) in the liquor can be preferably removed by a two-stage cross current extraction with 1 mol/L P507. Then Ga(III) can be selectively separated against Al(III) using 0.5 mol/L Cyanex 272 at equilibrium pH of 2.4–2.6. The separation factors of Fe/Ga, Fe/Al and Ga/Al can reach 145, 133 and 40, respectively. The extraction mechanism of Ga(III) as well as the Ga(III) complexes with Cyanex 272 were investigated using slope analysis and spectroscopic methods including FT-IR, NMR and ESI-MS, indicating that the extraction of Ga(III) with Cyanex 272 follows an ion exchange mechanism. The Ga(III) complexes with Cyanex 272 (H 2 L 2) contains three coordination compounds, GaL 3 , Ga(HL)L 3 and Ga(HL) 3 L 3. Both the P-O and P = O groups of Cyanex 272 play the roles in the coordination reaction of Ga(III). The study provides a feasible way for the selective recovery of Ga from coal fly ash. [ABSTRACT FROM AUTHOR]