Recycling of coal fly ash for fabrication of elongated mullite rod bonded porous SiC ceramic membrane and its application in filtration

Mullite bonded SiC ceramic membranes were synthesized by a facile solid-state reaction process, using SiC, solid waste fly ash as raw materials and MoO₃ as catalyst for growth of mullite at 1000 °C. The effect of MoO₃ catalyst on mullitization reaction and mullite morphology was investigated. Different pore formers were used to enhance the porosity and to observe its effects on the permeability parameters and filtration characteristics. At room temperature Darcian (k₁) and non-Darcian (k₂) in both water and air flow were measured and clean water flux was determined. The porous SiC ceramics with addition of 5 wt.% MoO₃ exhibited a flexural strength of 38.4 MPa at porosity 36.4 vol% and showed 92% oil removal efficiency from oily wastewater. This technique, combining low-cost materials and the co-sintering at low temperature, can serve as a cost-effective method for the production of high-performance porous SiC ceramic membrnaes for filtration application.     

Design and preparation of high permeability porous mullite support for membranes by in-situ reaction

The natural mineral kaolin combined with alumina additives Al(OH)₃,α-Al₂O₃ and AlF₃ was used to prepare porous mullite ceramic membrane supports using an in-situ reaction. The effects of composition and sintering temperature on the sintering behavior, pore structure, permeability and microstructure of the resulting porous mullite supports were extensively investigated. The experimental results showed that excess SiO₂ in kaolin can be consumed by adding alumina precursors, which resulted in a stiff skeleton of interlinked needle-like mullite crystals in-situ during the sintering. The needle-like mullite crystals touched each other and formed a short network, which acted as a porous skeletal network structure. This network resulted in a highly permeable porous structure. The resulting support is suitable for the preparation of asymmetric ceramic membranes. The densification and pore structure of the support can be effectively adjusted by control of the quantity of alumina precursors in the composition and the sintering temperature. Sintering the subject mullite compositions at 1500 °C for two hours resulted in support structures with an average porosity of 45.9%, an average pore size of 1.3 µm and a penetrating porosity of 35.9%.     

Porous mullite ceramics derived from coal fly ash using a freeze-gel casting/polymer sponge technique

The objective of this study was to prepare highly porous mullite ceramics with relatively large-sized pores and improved compressive strength using a freeze/gel casting route combined with polymer sponge for recycling of coal fly ash into high value-added ceramics. In this work, a tertiary-butyl alcohol /coal fly ash slurry system with an appropriate addition of Al₂O₃ was used. A reticulated structure with large pore size of 220–300 μm, which formed on burnout of polyurethane was obtained; then, the skeletons consisted mainly of more dense crystalline phases together with a few fine pores (<3 μm). The rod-shaped mullite crystals with an aspect ratio of >3.7 (~4 μm in diameter) seen to have grown within the silicate melts existed. The compressive strength of the sintered porous materials increased in the reverse order of the degree of porosity, i.e. low porosity gave a high compressive strength. The porous materials with an average porosity of 61.6 %, sintered at 1600 °C with 70 wt.% solid loading showed the maximum average compressive strength (~45 MPa).     

Microstructural evolution and densification behavior of porous kaolin-based mullite ceramic added with MoO3

Microstructural evolution and densification behavior of porous kaolin-based mullite ceramic added with MoO₃ were investigated. The results indicated that MoO₃ addition not only lowered the secondary mullitization temperature to below 950?°C, but also facilitated effectively the anisotropic growth of mullite grains. Fine mullite whiskers grew and interlocked with one another in the pre-existing pore regions, in-situ forming a stiff 3D skeleton structure of mullite whiskers, which arrested further densification of the sample. On the other hand, due to the great capillary attraction of small pores, the liquid phase tended to spread over small grains, which favored the growth from small mullite grains into whiskers at the expense of the liquid phase. Consequently, competitive mechanisms of sintering and crystal growth of mullite functioned, which further limited the sample densification. As a result, the total linear shrinkage of the sample added with MoO₃ after firing at 1400?°C was only ??2.75%, and its porosity was retained at as high as 67%.     

High-porosity whisker-mullite/corundum membrane support prepared from recycled industrial waste coal cinder

Recycled coal cinder was utilized as a raw material for the preparation of a high-porosity and low-cost whisker-mullite/alumina membrane support via in situ synthesis of mullite whiskers with MoO₃ and AlF₃ as additives. The effects of the sintering temperature and additives on the physical properties (porosity, shrinkage, phase composition, microstructure, pore size, nitrogen permeation, and tortuosity factor) of the porous mullite-based membrane supports were investigated in detail. Based on the synergistic effect of AlF₃ and MoO₃, the porosity of the as-synthesized membrane supports was as high as 60%. This is attributed to the reduction in the viscosity of the solid phase reaction interface between the mullite precursors by the MoO₃ additives, thus promoting the formation of mullite grains, while the AlF₃ additive effectively promoted the anisotropic growth of whisker-mullite via vapor deposition. This whisker-mullite membrane support with a cross-interlocking structure has an inherently unimodal pore-size distribution and low tortuosity factor (~0.4).     

Recycling of waste fly ash for production of porous mullite ceramic membrane supports with increased porosity

Low-cost porous mullite ceramic membrane supports were fabricated from recycling coal fly ash with addition of natural bauxite. V₂O₅ and AlF₃ were used as additives to cause the growth of mullite crystals with various morphologies via an in situ reaction sintering. Dynamic sintering, microstructure and phase evolution of the membrane supports were characterized in detail and open porosity, pore size, gas permeation and mechanical properties were determined. It showed the membrane support with 3 wt.% V₂O₅ and 4 wt.% AlF₃ addition exhibits an open porosity of ∼50%, mechanical strength of 69.8 ± 7.2 MPa, an interlocking microstructure composed of anisotropically grown mullite whiskers with an aspect ratio of 18.2 ± 3.6 at 1300 °C. Addition of more V₂O₅ lowered the secondary mullitization temperature, resulting in more mullite formation at lower temperatures. The fabricated membrane supports feature high porosity without mechanical strength degradation, possible strengthening mechanism of the mullite whiskers was further discussed.     

Realizable recycling of coal fly ash from solid waste for the fabrication of porous Al2TiO5-Mullite composite ceramic

As the environment deteriorates, recycling of solid waste has become increasingly important. This study aimed to optimize the use of the Fe₂O₃, SiO₂, and CaO components in coal fly ash and to convert coal fly ash into stable porous Al₂TiO₅-mullite (AT–M) composite ceramic by sintering with AlOOH and TiO₂ additives at high temperatures. The phase composition, microstructure, apparent porosity, corrosion resistance, and mechanical properties of porous AT–M composite ceramics were systematically investigated. Results indicated that the sintered samples exhibited pore size distributions within the 0.16-2.9 μm, apparent porosities of approximately 52.8%, and flexural strength of 29.6 MPa. Corrosion resistance data revealed quality losses in the aqueous NaOH and H₂SO₄ solutions for 10 hours at 0.42% and 2.19%, respectively. After corrosion for 8 hours, the average flexural strength of the samples remained at 21.6 ± 0.53 and 20.84 ± 0.6 MPa, respectively. These findings show that these porous AT–M ceramics may provide enhanced corrosion resistance under alkaline conditions. The porous AT–M composite ceramics may fabricate high-performance composite membrane supports for the high temperature flue gas filtration.     

Porous fibrous ZrO2-mullite ceramics prepared via tert-butyl alcohol-based gel-casting

Mullite fiber was used to fabricate ZrO₂-mullite based porous ceramic via tert-butyl alcohol (TBA)-based gel-casting process using zirconite and bauxite as raw materials. Phase compositions, microstructure, pore size distribution, linear shrinkage, bulk density, apparent porosity, thermal conductivity, and compressive strength were analyzed to investigate influences of mullite fiber content and added Y₂O₃ on prepared porous ceramics. Results show that bird nest-like three-dimensional fibrous reticular skeleton structure was constructed with mullite fibers that evenly enwrapped rod-like mullite and ZrO₂ grains. Prepared porous fibrous ZrO₂-mullite ceramics had narrow pore size distribution that consisted of mullite and m-ZrO₂. With an increase in mullite fiber content, linear shrinkage and bulk density decreased, apparent porosity increased, and relatively good thermal conductivity was obtained. In addition, added Y₂O₃ reacted with Al₂O₃ and SiO₂ to form Y-Al-Si-O glass phase, which promoted sintering and densification of the ceramic, thus improving its compressive strength.     

Near-net-size preparation of porous mullite matrix ceramics with in situ formed 3D mullite whisker network

Porous mullite matrix ceramics have excellent thermal and mechanical properties suitable for applications such as in thermal insulation. However, their applications are limited by processing defects from nonuniform sintering shrinkage and the trade-off between high porosity (preferred for low thermal conductivity) and high mechanical strength. Herein, we seek to minimize the sintering shrinkage by near-net-size preparation and improve the strength by in situ formed whisker network structure. Gelcasting forming technology and pressureless sintering were used to prepare porous mullite matrix ceramics using kyanite and α-Al₂O₃ powders as the starting materials and using MoO₃ to promote the growth of mullite whiskers. The results showed that the sintering shrinkage could be compensated by the volume expansion from solid-state reaction during reaction sintering. The in situ formed three-dimensional (3D) whisker network further reduced sintering shrinkage and effectively improved the strength of the ceramics. An ultralow sintering shrinkage of .78% was achieved. The near-net-shape porous mullite matrix ceramics strengthened by 3D whisker network had a high porosity of 63.9%, a high compressive strength of 83.8 MPa and a high flexural strength of 53.5 MPa.     

Fabrication and characterization of anorthite–mullite–corundum porous ceramics from construction waste

In this work, anorthite–mullite–corundum porous ceramics were prepared from construction waste and Al₂O₃ powders by adding AlF₃ and MoO₃ as mineralizer and crystallization catalyst, respectively. The effects of the sintering temperature and time on open porosity, mechanical properties, pore size distribution, microstructure, and phase composition were characterized in detail. The results showed that the formation of the mullite whiskers and the properties of the anorthite–mullite–corundum porous ceramics depended more on the sintering temperature than the holding time. By co-adding 12 wt% AlF₃ and 4 wt% MoO₃, mullite whiskers were successfully obtained at sintering temperatures upon 1350 °C for 1 h. Furthermore, the resultant specimens exhibited excellent properties, including open porosity of 66.1±0.7%, biaxial flexural strength of 23.8±0.9 MPa, and average pore size of 1.32 µm (the corresponding cumulative volume percent was 37.29%).     

Porous mullite ceramics with enhanced compressive strength from fly ash-based ceramic microspheres: Facile synthesis,structure, and performance

Porous mullite ceramics are widely used in heat insulation owing to their high temperature and corrosion resistant properties. Reducing the thermal conductivity by increasing porosity, while ensuring a high compressive strength, is vital for the synthesis of high-strength and lightweight porous mullite ceramics. In this study, ceramic microspheres are initially prepared from pre-treated high-alumina fly ash by spray drying, and then used to successfully prepare porous mullite ceramics with enhanced compressive strength via a simple direct stacking and sintering approach. The influence of sintering temperature and time on the microstructure and properties of porous mullite ceramics was evaluated, and the corresponding formation mechanism was elucidated. Results show that the porous mullite ceramics, calcined at 1550 °C for 3 h, possess a porosity of 47%, compressive strength of 31.4 MPa, and thermal conductivity of 0.775 W/(m?K) (at 25 °C), similar to mullite ceramics prepared from pure raw materials. The uniform pore size distribution and sintered neck between the microspheres contribute to the high compressive strength of mullite ceramics, while maintaining high porosity.     

原位反应合成多孔莫来石质支撑体及其显微结构

采用粉煤灰、烧铝矾七和球黏土为原料,通过原位反应烧结合成球形多孔莫来石质支撑体.考察了烧结温度对多孔莫来石质支撑体相转变、晶粒尺寸、孔径分布和气孔率的影响,并对其莫来石化的机理进行了探讨.研究表明:在1 300℃烧结温度下,二次莫来石在硅铝酸盐液相中通过溶解-沉淀机制开始成核,并于1 400℃烧结发育成针状的晶粒;当烧结温度升至1 500℃时,溶解在液相中的莫来石进行结构调整和生长,并最终在1 550℃发生原位反应并形成刚性交错网络结构的柱状莫来石.在低温(<1 400℃)烧结阶段,二次莫来石的成核和生长引起体积膨胀,导致气孔率随温度的升高而增大;在高温(≥1 500℃)烧结阶段,大量玻璃相的存在使液相烧结占主导,气孔率因体积收缩呈下降趋势.支撑体的平均孔径随烧结温度的升高而增大,其原因是在高温烧结时莫来石生长消耗过多的SiO2使大量微孔联接形成大孔.     

Impact of inorganic waste fines on structure of mullite microspheres by reaction sintering

The impact of waste composition and morphology on the properties of the mullite microstructure were studied by reaction sintering two types of siliceous waste powders with calcined alum sludge. Stoichiometric mixtures of fly ash or rice husk ash with the alum sludge were shaped into mullite microspheres by droplet coagulation. The mullitization reaction was followed by HT-XRD with a distinct difference in onset temperature. The specific powder composition of fly ash (presence of primary mullite) shifted the onset temperature to higher values as compared to the rice husk ash mixture. After sintering at 1600?°C, the mullitization reaction resulted in the same yield for both samples. Although the mullite content in the microspheres was equal, distinct differences on the microstructure could be observed. Reaction sintering of the fly ash mixture resulted in lower total porosity with smaller pore sizes.     

Novel cordierite-acicular mullite composite for diesel particulate filters

Cordierite-acicular mullite composites containing 0, 25, 50, 75 and 100 wt% of mullite were fabricated from waste MoSi₂ and commercial powders of Al₂O₃ and spinel (MgAl₂O₄). Careful oxidation of pulverised waste MoSi₂ rendered a precursor mixture of MoO₃ and amorphous SiO₂, which served as pore forming agent and SiO₂ source, respectively. Evaporation of MoO₃ at ～750 °C allowed production of highly porous cordierite-mullite ceramic composite after sintering in air at 1350 °C for 4 h. The combination of equiaxed cordierite grains and elongated (prism-like) mullite grains, resulted in unique microstructure with open porosity between 53.3 and 55.6 vol% which makes the obtained composite convenient for application as diesel particulate filter material. The presence of mullite affected four key thermo-mechanical properties which determine the thermal shock resistance of cordierite-mullite composite. The best thermal shock resistance was measured in composite containing 75 wt% of mullite. It was a result of improved thermal conductivity (1.081 W/mK) and bending strength (3.62 MPa) and relatively low values of coefficient of thermal expansion (3.8 × 10^?6 K^?1) and elastic modulus (2.27 GPa).     

Preparation and characterization of porous anorthite ceramics from red mud and fly ash

The porous anorthite ceramics with high porosity, good mechanical strength and low heat conductivity were prepared using red mud and fly ash as raw materials via the pore forming method. The effects of sintering temperature and fly ash on phase evolution, densification, compressive strength, thermal conductivity and microstructure of the ceramic materials were investigated. The results showed that the compressive strength of the porous ceramics had an obvious improvement with the increase in fly ash, and the densification and heat conductivity decreased firstly and then increased. In particular, specimen S2 containing 30 wt% red mud and 40 wt% fly ash sintered at 1150°C had the better performances. It had the water absorption of 18.18%, open porosity of 38.52%, bulk density of 1.29 g/cm³, compressive strength of 42.46 MPa, and heat conductivity of 1.24 W/m·K. X-ray diffraction analysis indicated that mullite, anorthite, α-quartz, and diopside ferrian were the dominant phases in the specimens. Scanning electron microscopy micrographs illustrated that plenty of open pores with strip shape and closed pores with axiolitic shape existed in the specimens. Furthermore, the existence of mullite could prevent crack propagation to enhance the energy of inter-granular fracture. It endowed the porous anorthite ceramics with high porosity, good compressive strength, and low heat conductivity.     

Substitution of quartz and clay with fly ash in the production of architectural ceramics: A mechanistic study

Quartz and clay are substituted gradually by fly ash using a triaxial ceramic formulation under simulated industrial conditions and the effects of fly ash substitution on the macroscopic properties and microstructures of the sintered ceramics are evaluated systematically. With the substitution of 35 wt% (1250 °C), the ceramic sample exhibited optimal properties, including linear shrinkage of 15.61%, bulk density of 2.39 g cm^-3, water absorption of 0.62% and flexural strength of 41.70 MPa, due to the accelerated densification and fly ash-spurred needle-shaped mullite. The microstructure analysis shows that the sintered matrix consists of three types of particles, quartz-, clay- and feldspar-like particles showing sintering behavior with respect to filling the glassy matrix with preserved morphology, precipitating mullite crystals, and fusing with the surrounding glassy matrix, respectively. The strength of the fly ash - containing ceramics is analyzed by the dispersion-strengthening mechanism and porosity and the results indicate that the fly ash particles affect the mechanical strength due to Griffith flaws when the total porosity is less than 25% and pores at higher total porosity. This study provides a viable strategy to recycle industrial fly ash in the production of architectural ceramics.     

In-situ growth of mullite whiskers and their effect on the microstructure and properties of porous mullite ceramics with an open/closed pore structure

Porous mullite ceramics with an open/closed pore structure were prepared by protein foaming method combined with fly ash hollow spheres. Both the open porosity and total porosity of samples were enhanced by increasing the hollow sphere content. Mullite whiskers with a diameter of 0.2–4 μm were grown in-situ in the porous mullite ceramics with an AlF₃ catalyst, conforming to a vapor-solid growth mechanism. The pore structure of the porous mullite ceramics was significantly affected by the mullite whiskers which increased the open porosity and total porosity. Moreover, the median pore size was reduced from 65.05 μm to 36.92 μm after the introduction of mullite whiskers. The flexural strength and the thermal conductivity of the samples decreased with increasing total porosity. The porosity dependence of the thermal conductivity was well described by the universal model, providing a reference for the prediction of thermal conductivity of porous ceramics with open/closed pores.     

Application of mullite-zeolite-alumina microfiltration membranes coated by SiO2 nanoparticles for separation of oil-in-water emulsions

The main objective of this work is the manipulation of hydrophilic materials in support, intermediate and selective layer to synthesize a novel nano-tubular ceramic membrane for treatment of oily wastewater. First, porous mullite-alumina-zeolite composite membranes were prepared by an extrusion method. Changes in porosity, pore size, shrinkage, and mechanical strength of the support membranes were investigated as function of percentage composition and sintering temperature in order to obtain the optimal conditions. According to the results, the most favorable condition set was determined to be a support membrane with a weight percent of 50, 30, and 20 for mullite, alumina, and zeolite, respectively, and a porosity of 38%, a pore size of 0.39 µm, and a shrinkage of 10.2% sintered at 1250 °C and with good mechanical properties at 24.6 MPa. The cross-flow filtration technique was employed to coat the natural zeolite on the inner surface of the support membrane to achieve a narrower pore size distribution. Finally, a thick layer of nano-SiO₂ was coated on the membrane by utilizing the dip-coating method to develop a hydrophilic membrane while avoiding defects. Moreover, scanning electron microscopic (SEM) analysis of the SiO₂ membranes showed that the natural zeolite and nano-SiO₂ layer is homogeneous and demonstrates high adhesion to the support membrane. Besides, the result of COD rejection showed that the SiO₂ membranes have an undeniable capability in rejection of oil droplets with a reasonable permeation flux. Therefore, the obtained membranes are highly promising for practical applications and environmental remediation in sensitive Persian Gulf zone.     

Fabrication and characterization of porous mullite ceramics with ultra-low shrinkage and high porosity via sol-gel and solid state reaction methods

Porous mullite ceramics with ultra-low shrinkage and high porosity were prepared by solid state reaction between MoO₃ and mullite precursor powders which were synthesized from tetraethylorthosilicate and aluminium nitrate nonahydrate via sol-gel methods. The synthetic process of mullite precursor powder and effects of MoO₃ amount on the phase composition, microstructure, physical properties such as firing shrinkage, open porosity, bending strength, water absorption and bulk density of porous mullite ceramics were investigated. The results indicated that the addition of MoO₃ not only lowered the mullite forming temperature from 985.4 to 853.3 °C, but also restrained densification behavior of samples due to the formation of mullite and Al₂O₃–MoO₃ solid solution, besides, MoO₃ also improves the formability, open porosity and bending strength of samples. The optimal amounts of MoO₃ is 8 wt%, and the resultant samples exhibit outstanding properties, including a low shrinkage rate of 1.86 ± 0.07%, an open porosity of 61.91 ± 0.16% and a bending strength of 9.35 ± 1.11 MPa.     

粉煤灰泡沫陶瓷的制备及结构表征

以粉煤灰为主要原料,添加少量的高岭土及微量的添加剂为辅料,采用泡沫浸渍法制备粉煤灰泡沫陶瓷。采用X射线衍射、场发射扫描电镜研究了泡沫陶瓷的生成相及其分布,以及泡沫陶瓷的形貌：并对泡沫陶瓷的孔隙率、抗压强度等进行了表征。结果显示,泡沫陶瓷的生成相主要为莫来石,通过扫描电镜观察到莫来石为细长的针状,大量存在于孔隙位置。从泡沫陶瓷孔隙率的测试结果可知,随着粉煤灰含量的升高,泡沫陶瓷的平均孔隙率下降,抗压强度升高;同一组分试样,当烧结温度升高时,试样的平均气孔率下降,抗压强度升高到一定值后会下降。