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Mullite is becoming increasingly important for use in refractory lining, structural ceramic and catalyst products. The kaolinite-mullite process is a simple method to prepare mullite. Under conventional heating kaolinite has to be heated at 1400 °C for 1 h to form well-ordered orthorhombic mullite (3Al2O3·2SiO2) that is accompanied by the formation...
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... eliminate errors that could result from free water, mass change measurements were obtained using TG-DSC testing, in a temperature range of 105-1400 C. Under a heating rate of 5 C min À1 , the thermal weight loss of the kaolin (Fig. 5) was 14.24%, close to the LOI of the kaolin. An endothermic peak due to dehydroxylation appeared at 507 C. Analysis of TG-DSC and XRD results showed a broad exothermic peak at 838 C that was associated with the formation of gamma alumina. Two exothermic peaks appeared at 940 and 985 C due to the formation of pseudo-tetragonal mullite ...
Context 2
... a conventional heating rate of 5 C min À1 , the weight losses of kaolin heated to 500 and 600 C were 6.79% and 0.55%, respectively, and at 40 C min À1 , were 11.92% and 3.94% (Fig. 5). The mismatch between heat supply rate and heat transfer rate under the conventional heating rate of 40 C min À1 led to the higher weight loss of heated kaolin. Under microwave heating, the kaolin specimen was heated from 200 to 600 C in 60 s, and the microwave heating rate was much faster than the conventional heating rate of 40 C min ...
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Aluminum oxycarbide (Al2OC) is known as a promising material for refractory and abrasive uses. However, Al2OC is synthesized at high temperatures with a long reaction time by an electric furnace. Microwaves are known as a low energy heating method due to its heating mechanism; rapid heating and cooling, selective and internal heating, etc. Therefor...
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... 21 However, the biggest problem of these methods is that the precursor composition and sol homogeneity are difficult to control, and it is difficult to achieve large-scale industrial applications. 22,23 Fly ash contains a large amount of SiO 2 and is a potential raw material for preparation of porous silica materials. 24 The typical process for synthesis of mesoporous silica using y ash as raw material is based on alkali leaching or alkali fusion. ...
Extraction of valuable metals besides silica from high-alumina fly ash is one of the most important high-value utilization pathways. However, it is difficult to realize high-efficiency extraction due to the stable structure e.g. of quartz and mullite. In this paper, mineral phase transformation for valuable metal recovery and mesoporous silica in situ preparation from fly ash by a selective acid leaching method was proposed. The mineral phase transformation, dissolution behavior of each metal, and pore structure of fly ash derived mesoporous silica were systematically investigated. The results show that the co-activation of fly ash by Na2CO3–K2CO3 formed the phases of kalsilite and (Na, K)AlSiO4. During the acid leaching process, Al, Li, and Ga could be leached with the efficiencies of 86.17%, 89%, and 80% in the FK system. In the FN system, the efficiencies of Al, Li, and Ga are 92.38%, 95%, and 83%, respectively. The crystal plane (002) was destroyed for kaliophilite while all the crystal planes were destroyed for nepheline. With the increase of HCl solution concentration, the porous silica exhibited the same change order of pore shape. The pore structure of as-prepared porous silica was type IV and the hysteresis loop was type H3, and the specific surface areas could be 565.54, 448.02, and 746.76 m² g⁻¹, respectively. Finally, the leaching liquors can be used to produce crystal aluminum chloride, lithium carbonate and gallium. This paper might provide technical support for full recycling of high-value resources from fly ash.
... However, controlling the composition and homogeneity of precursors is challenging when synthesizing mesoporous SiO 2 on a large scale via the sol-gel process, the composition and homogeneity of the precursors are difficult to effectively control, which also hinders the development of an industrialized process for the preparation of mesoporous silica [75]. To address this challenge, Li et al. [76] proposed to successfully prepare and synthesize mesoporous silica from CFA by controlled acid leaching. ...
Coal fly ash (CFA) is the primary solid waste generated by the coal-fired industry, and the predominant treatment methods include accumulation, landfill, and the production of cement-based building materials. However, the availability of these methods is relatively limited, and there is a need for technological upgrades. The extensive accumulation not only leads to environmental pollution but also has detrimental effects on human health. With its loose structure and main chemical components of Al2O3 and SiO2, CFA is conducive to the synthesis of promising nanoporous materials for applications in adsorption. The use of CFA as a raw material can significantly reduce preparation costs, improve comprehensive utilization efficiency, and manufacture more valuable products, aligning with the current strategy of sustainable development. Currently, the most widely used synthesis method is hydrothermal synthesis. This review focuses on the principles, methods, and influencing factors of synthesis, with particular emphasis on CFA-based mesoporous silica, zeolites, and metal–organic frameworks (MOFs), which have not been systematically reviewed previously. The quality of these synthesized nanoporous materials can be finely adjusted through the synthesis process. A comparison of the advantages and disadvantages of each process will be made, and the impact of the synthesis conditions on the synthetic products will be analyzed. Additionally, a brief discussion on the latest research advances in their applications in adsorption will be provided. Finally, relevant challenges and issues have been proposed.
... In addition, there are few characteristic peaks of quartz, albite and muscovite. The FT-IR pattern of kaolin is shown in Fig. 1b, the absorption band is mainly concentrated in the range of 400-1500/ Fig. 1b [38,39]. The active sites such as Al-OH, Si-O-Si, Al-O, Si-O contained in the kaolin are the basis of kaolin modification. ...
... The metakaolin also had a large number of mesopores and a small amount of macropores, which had little effect on the pore properties. The specific surface area was 14 m 2 /g [38,39]. ...
... The peaks at 773/cm and 700/cm corresponded to the symmetrical stretching vibration on the outside and inside of the Si-O tetrahedron, respectively. The peak at 461/cm corresponded to the internal bending vibration of the Si-O bond, and the absorption peak at 569/cm corresponded to the external bending vibration of the Si-O bond, and these peaks were consistent with the literatures [39]. ...
Y Zeolite plays an irreplaceable role in many important applications due to its unique physicochemical properties, which has attracted extensive attention from researchers. Response surface methodology (RSM) was used to optimize the process parameters for the hydrothermal synthesis of Y zeolite using kaolin activated by microwave heating. The best relative crystallinity was 108.17% could be achieved at the condition: the aging temperature of guiding agent was 65 ℃, the crystallization temperature was 100 ℃, the crystallization time was 24 h. In addition, the aging temperature of guiding agent was the most important influence factor. Under the optimal condition, the SiO2/Al2O3 ratio of synthesized Y zeolite was 4.54, 90% of the particle size was less than 60 μm, and the specific surface area of Y zeolite was 479 m²/g. The successful preparation of Y zeolite with high crystallinity is expected to provide reference for the application of wastewater treatment, CO2 sequestration and fluid catalytic cracking for petroleum.
... The peak at 536 cm − 1 is associated with the Si-O-Al vibration, which indicates the presence of muscovite. Furthermore, the presence of the finger regions at 797 cm − 1 , 468 cm − 1 and 429 cm − 1 is attributed to the Si-O deformation vibration due to the quartz position [30,31]. The FTIR results are in good agreement with the XRD data in Fig. 4a. ...
Expanded clay as a high-quality raw material to produce ceramsite is rapidly consumed in the past decades. A novel and green process to prepare the ultra-lightweight ceramsite using non-expanded clay and waste sawdust as raw materials was proposed in the present study, and the physical properties and phase transition mechanism of the ultra-lightweight ceramsite under different raw material formulas and sintering parameters were investigated. The synergistic effects of sawdust and Fe2O3 have a great influence on the expansion properties of ceramsite. The bulk density of ceramsite decreases with the increasing Fe2O3 content and sintering temperature, and it decreases first and then increases with the increase of sawdust content. The generation of gas and vitreous phase by sawdust burning and Fe2O3 reduction can promote the expansion of ceramsite. The typical core-cortex structure of ceramsite is formed, and the compact structure of the external layer significantly reduces the water absorption of ceramsite. The ultra-lightweight ceramsite was obtained with a bloating index of 321%, water absorption of 12.5%, bulk density of 280 kg/m³, and compressive strength of 1.0 MPa under the optimum conditions.
... For temperatures higher than 1200 • C [5,22,23], as the liquid phase content depends on the increase of the alkali ratio (Na 2 O/K 2 O), its viscosity decreases [24,25] having an enhanced effect on mass transport and crystal growth. The reaction between the scaly mullite, referred to as primary mullite, previously formed through a solid-state reaction [5,9,23,26], and the liquid phase (rich in alkali ions) leads to its recrystallization and growth to granular and elongated (>1 μm) mullite crystals, referred to as sec- ...
... [32,[36][37][38][39][40], including aspects such as the thermal runaway effect [41]. No matter the material, ceramic or other, the literature refers to several advantages of microwave heating: shorter time and lower processing temperatures, improved properties, and economical, energy and environmental savings/gains [30,31,[42][43][44][45]. Reported smaller grain sizes can be attributed to the fast microwave sintering process, allowing higher densifications as there is insufficient time for the grains to grow [26,43,46]. ...
... The majority of the studies on microwave firing involves small samples, requiring only single-mode applicators (microwave furnaces/ resonators), usually equipped with controllable frequency and power sources, circulators, stab-tuners and adjustable short-circuit, allowing a fine-tuning of the maximum electromagnetic field at the position of the samples [26,[47][48][49][50]. Likewise, in multi-mode applicators, microwave firing requires a homogeneous electromagnetic field over the sample's volume, to avoid its deformation and cracks formation [51]. ...
Microwave heating technology is presented as an alternative to conventional stoneware firing, with focus on the crystallochemical transformations. Scanning electron micrographs of microwave fast-fired samples show similar features as those of conventionally fired in a natural gas kiln, taken as reference samples. Electrically fast-fired samples present incipient mullite type-II crystals, while in the microwave and conventionally (gas) fired samples it is fully formed. X-ray diffraction pattern show that the crystallochemical transformations occur at lower temperatures when samples are microwave fired, particularly for temperatures lower than the vitrification temperature. Kaolinite dehydroxylation is fully accomplished at 500 °C when the samples are microwave fired, being still incomplete at 800 °C in electrically fired samples. Stoneware crystallochemical and microstructural transformations point to lower microwave firing temperatures (up to 100 °C lower) than that required by electric firing. These observations are in good agreement with what is reported in the literature, explaining at the microscopic level what happens at the macroscopic level.
... For temperatures higher than 1200 • C [5,22,23], as the liquid phase content depends on the increase of the alkali ratio (Na 2 O/K 2 O), its viscosity decreases [24,25] having an enhanced effect on mass transport and crystal growth. The reaction between the scaly mullite, referred to as primary mullite, previously formed through a solid-state reaction [5,9,23,26], and the liquid phase (rich in alkali ions) leads to its recrystallization and growth to granular and elongated (>1 μm) mullite crystals, referred to as sec- ...
... [32,[36][37][38][39][40], including aspects such as the thermal runaway effect [41]. No matter the material, ceramic or other, the literature refers to several advantages of microwave heating: shorter time and lower processing temperatures, improved properties, and economical, energy and environmental savings/gains [30,31,[42][43][44][45]. Reported smaller grain sizes can be attributed to the fast microwave sintering process, allowing higher densifications as there is insufficient time for the grains to grow [26,43,46]. ...
... The majority of the studies on microwave firing involves small samples, requiring only single-mode applicators (microwave furnaces/ resonators), usually equipped with controllable frequency and power sources, circulators, stab-tuners and adjustable short-circuit, allowing a fine-tuning of the maximum electromagnetic field at the position of the samples [26,[47][48][49][50]. Likewise, in multi-mode applicators, microwave firing requires a homogeneous electromagnetic field over the sample's volume, to avoid its deformation and cracks formation [51]. ...
This work reports and compares the structural crystallochemical transformations occurring during the microwave and conventional porcelain manufacture. Batches of greenware (just dried) porcelain pieces are microwave and electrically fired at increasing temperatures, from 420 °C up to 1100 °C. Crystallochemical transformations are identified by XRD analysis, and compared the results from samples microwave and conventionally fired. Microwave fired samples show the full and rapid collapse of kaolinite structure for firing temperatures just above 500 °C, whereas the collapse of kaolinite structure of the electrically fired samples is progressive, from about 500 °C up to 950 °C. Muscovite structure totally collapses at about 950 °C for microwave fired greenware samples, whereas muscovite structure total collapse only occurs at about 1050 °C for electrically fired greenware samples. Microwave and electric firing lead to appreciable differences in the sanidine – orthoclase – microcline structural transformations. Mullite formation could be identified in the microwave fired samples at temperatures 50 °C lower than in the electrically fired ones, especially for the conventional firing temperatures above 1050 °C, the same temperature reported in the literature.
... than when conventional heating is used. 28,32 One explanation for these observations is that microwave radiation gives rise to an additional driving force, referred to as ponderomotive force. [33][34][35][36] Rybakov et al 33 explains that the presence of macroscopic structural nonuniformities in the material, such as pores surfaces and/or grains boundaries, or even vacancies, with a significantly higher electric susceptibility when comparing with an average zone in the crystalline lattice, is a necessary condition to promote the ponderomotive effects. ...
... This may be due to organic matter burnout and/or kaolinite dehydroxylation. This was also observed in the microwave firing of earthenware tiles, 52 of kaolin, 32 and of bi-fired porcelain, 30 more particularly during the first firing of porcelain, that is, in greenware to biscuit firing, when residual moisture, organic matter, and whose primary mineral transformations have not yet took place. 30 In previous research conducted by Reinosa et al 53 the dielectric loss (ε′′) and the transformations occurring during microwave firing of kaolin, at the frequency of ~2.45 GHz, are correlated. ...
... It is to be noted that in the research conducted by Reinosa et al 53 those presented by others. 28,30,32,52 Microwave firing of low impurities kaolin 32 shows a reduction in the heating rate between 400°C and 600°C. Microwave firing of earthenware tiles 52 shows a plateau in the same temperature range. ...
Industrial competition and environmental concerns lead to the exploration of alternative and energy‐efficient technologies for ceramic materials processing. The main objective of this work was to present microwave heating as a viable option for stoneware processing. Stoneware functional properties are presented and discussed, with emphasis on impact strength, water absorption, porosity, and color. Microstructure analyses show that microwave‐ and gas‐fired samples have higher densifications than electrically fired samples. A relevant finding for processing conditions is that microwave firing requires temperatures approximately 100°C lower than those required by conventional firing. Microwave‐fired samples’ rupture energies are approximately twice (0.57 ± 0.06 (J)) those of the reference samples (0.26 ± 0.03 (J)), and their water absorptions are approximately one‐half (1.5% at 1170°C and 0.8% at 1190°C) of those of the reference samples (2.0%), whereas the water absorption of electrically fired samples at 1180°C has been estimated to be 7.5%. Color analysis also evidences a shift to lower microwave firing temperatures, what is attributed to the enhanced transformations promoted by microwave heating when comparing with the transformations promoted by conventional (gas or electric) heating.
... It is described that the formation mechanism of mullite starts with two prereactions which consist of the formation of pseudo-tetragonal mullite (at~700°C) in a solid-liquid phase and its posterior transformation to orthorhombic mullite at~800°C as a solid state reaction. (Zhang et al., 2014) These processes could explain the increase of the heating ratio from 700°C in Fig. 1b and the existence of variations in the band width curve of Fig. 1c at the mentioned temperatures. ...
The thermal evolution of kaolin clay under microwave radiation shows an unexpected large heating rate up to 500 °C/min for temperatures>650 °C. Such heating rate is associated with a resistivity drop of >103 Ω·m observed after the dehydroxylation process of the kaolin structure. The high efficiency of the microwave heating
effect is correlated with the presence of surface carriers that absorbs microwaves electromagnetic field. The layered structure of the clay-based materials allows the appearance of charge carriers at the surface of the crystal lattice that is electromagnetically activated. This effect represents a breakthrough in the efficient use of microwaves
energy in order to produce efficient thermal treatments in large volume of non-metallic minerals with a drastic reduction of the greenhouse gasses for mass production industries.
... It was possible that alumina in albite may be little responsible for mullite formation (in this time range) because there was some of residual albite observed in the MWS sample. A similar result reported that microwave can enhance formation of mullite because a selective heating, and accelerated the nucleation and growth of mullite and changed the mullite formation route of kaolinite [33]. Moreover, the chemical analysis result shows that mullite in MWS samples has more Al +3 than that in CS sample suggesting that mullite should firstly forms via clays decomposition rather than in the liquid glass melt because clays have more molar fraction of alumina com-pared with the alumina in glass melt. ...
Microstructural evolution on sintering of porcelain powder compacts using microwave radiation was compared with that in conventionally sintered samples. Using microwaves sintering temperature was reduced by ~ 75 °C and dwell time from 15 min to 5 min while retaining comparable physical properties i.e. apparent bulk density, water absorption to conventionally sintered porcelain. Porcelain powder absorbed microwave energy above 600 °C due to a rapid increase in its loss tangent. Mullite and glass were used as indicators of the microwave effect: mullite produced using microwaves had a nanofibre morphology with high aspect ratio (~32±3:1) believed associated with a vapour-liquid-solid (VLS) formation mechanism not previously reported. Microwaves also produced mullite with different chemistry having ~63 mol% alumina content compared to ~60 mol% alumina in conventional sintered porcelain. This was likely due to accelerated Al⁺³ diffusion in mullite under microwave radiation. Liquid glass was observed to form at relatively low temperature (~900–1000 °C) using microwaves when compared to conventional sintering which promoted the porcelains ability to absorb them.
... The intensified E-field was large enough to ionize air to melt copper. Based on the simulation results the same size of CFA cylinder and copper cylinder specimens were prepared and irradiated in the TE 103 single mode microwave applicator equipped with IR thermal imaging system that detects only the surface temperature of specimen 13,34 . When the specimens satisfied the E-field intensification requirements, large amount of fused phase formed in the gap of two CFA cylinder specimens (Fig. 4d). ...
... Experimental apparatus. The illustration of experimental apparatus was shown in previous investigation 34 . ...
The nature of microwave sintering cannot be explained in the past and has been generally called microwave effect. Here we show that the E-field intensification is the reason of microwave fast sintering of solid state inorganic compounds. The intensification degree varied with dielectric constant of compound, distance between two particles, angle between the direction of E-field and the normal to the surface at the adjacent point of two spheres. Ultra-high temperature caused by E-field intensification leads to fusing of solid materials at contact zone and enhances the mass transportation. The key to develop a microwave energy-saved sintering method is to control the distance between particles and uniformity of particles instead of the particle size.
