Effects of microwave irradiation on the crystalline phase of zeolite synthesized from fly ash by hydrothermal treatment

Coal fly ash was dissolved into NaOH aqueous solution and the mixture of rice husk ash as a silica source with the solution separated from the insoluble fly ash was treated hydrothermally at 373 K with microwave heating and conventional heating. Through this experiment, we investigated effects of microwave irradiation on the crystalline phase of zeolites synthesized from fly ash and clarified what process is intensively affected by the microwave irradiation. As a result, it was found that the product powder synthesized with microwave heating has a relatively higher hydroxysodalite phase than that with conventional heating, even if the synthesis conditions were the same. It was also clarified that promotion of hydroxysodalite generation by microwave irradiation is not induced by the enhancement of the crystalline phase transition from phillipsite to hydroxysodalite, but by the acceleration of hydroxysodalite formation from the precursor, aluminosilicate gel, directly. Moreover, it was found that promotion of hydroxysodalite formation is not due to the heat spot on the surface of the carbon caused by microwave irradiation. We revealed that the sensitivity, which is defined as the ratio of the quantity of generated heat to the energy of the microwave generated by the magnetron, of hydroxysodalite is almost 10 times as high as that of phillipsite. From these results, the following enhancement mechanism of hydroxysodalite crystal generation by microwave irradiation was proposed. When zeolite nuclei grow to the zeolite crystal on the interface between zeolite nuclei and aluminosilicate gel, the crystal growth of hydroxysodalite is promoted selectively as the surface temperature of hydroxysodalite is much higher than that of phillipsite in the case of microwave heating.     

Sintering of MSWI fly ash by microwave energy

This study presents the sintering of municipal solid waste incineration (MSWI) fly ash assisted by microwave energy. The composition of fly ash was investigated by chemical sequential extraction and modified microwave digestion method. Effects of process time, container materials, aging time and salt contents were also discussed. The major elements of fly ash are Ca, Cl, Na, Si, K, Al, Mg, and Zn, and the metal species, Zn, Cr, Pb, Ca, and Cu, are mainly in the oxide phase. Under microwave processing, the fly ash was sintered into a glass-ceramics and the leaching concentrations of heavy metals were restrained. The stabilization efficiency increased with an increase in processing time in most of the cases. Better stabilization efficiency of fly ash was discovered by using the SiO(2) or Al(2)O(3) container than by using the graphite plate/SiC plate. The presence of salt in the fly ash could enhance the sintering and stabilization of fly ash. During the aging time of 0-30 days, negligible Pb in the sintered fly ash was leached out, and the leaching concentration was lower than the criterion.     

Extraction of metals from municipal solid waste incinerator fly ash by hydrothermal process

This work examined the extraction properties of metallic elements from municipal incinerator fly ash under hydrothermal conditions. The ash was firstly pre-washed by distilled water, then subjected to hydrothermal treatments. The pre-washing process was effective for Na, K, Ca extraction with extraction percentages of 67%, 76% and 48%, respectively. The optimum contact time was 30 min for the pre-washing process. Five types of acids were tested for the extraction experiments and hydrochloric acid was found to be most effective for metal extraction from the ash. Compared to room condition, hydrothermal treatment accelerated the dissolution of the ash, thus promoted the reaction of acid with hazardous metals such as Cr, Cd, Pb, and furthermore, the consumption speed of acid was slowed down under hydrothermal condition. The acid simultaneously reacted with all the metal in the ash under hydrothermal condition but preferentially reacted with Ca at room condition. The optimum hydrothermal treatment temperature, time and liquid/solid ratio were 150 degrees C, 5h and 10:1 (ml:g), respectively.     

关于水热技术处理垃圾焚烧飞灰的研究探讨

生活垃圾焚烧飞灰含有重金属和有毒有机物,对环境危害十分严重。水热法被认为是一种有效的无害化处理技术,能将飞灰改性,降低飞灰毒性,还可以合成沸石类物质,提高产物的应用价值。本文基于垃圾焚烧飞灰理化特性,对现有的水热处理技术进行了较为全面的总结。根据水热法的机理,系统地介绍了传统水热法、微波水热法、熔融预处理的微波水热法和超声波水热法,并通过对比分析指出各方法在处理效率、处理效果等方面存在的问题。此外,还简要介绍了水热处理后飞灰的主要应用领域。最后结合近几年水热技术的研究现状,提出了新型水热技术在工业化应用上存在的大型化的局限性以及未来发展方向。     

Zeolite from fly ash: synthesis and characterization

Coal fly ash was used to synthesize X-type zeolite by alkali fusion followed by hydrothermal treatment. The synthesized zeolite was characterized using various techniques such as X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, BET method for surface area measurement etc. The synthesis conditions were optimized to obtain highly crystalline zeolite with maximum BET surface area. The maximum surface area of the product was found to be 383 m²/g with high purity. The crystallinity of the prepared zeolite was found to change with fusion temperature and a maximum value was obtained at 823 K. The cost of synthesized zeolite was estimated to be almost one-fifth of that of commercial 13X zeolite available in the market.     

Solidification of coal fly ash using hydrothermal processing method

Solidification of Coal Fly-ash (CFA) has been carried out using a hydrothermal processing method. In the hydrothermal processing, the CFA was first compacted in a mold at 20 - 50 MPa, and then hydrothermally cured in an autoclave. The hydrothermal curing was performed at 150 – 250°C for 15 – 60 h. The experimental results showed that NaOH solution, Ca(OH)₂ content, compaction pressure, autoclave curing temperature and time significantly affected the strength of solidified bodies. The most important strength-producing constituent in the solidified bodies produced with CFA was tobermorite, or tobermorite-like calcium silicate hydrate. When the CaO/SiO₂ ratio of the starting material was close to 0.83, tobermorite readily formed and the formed tobermorite enhanced the strength of solidified bodies. The tensile strength determined by the Brazilian test reached more than 10 MPa under the hydrothermal processing. As such, the hydrothermal processing method may provide a high potential for recycling CFA on a large scale.     

Selective formation of Na-X zeolite from coal fly ash by fusion with sodium hydroxide prior to hydrothermal reaction

Hydrothermal treatment of fly ash with alkali gives various types of zeolites such as Na-Pl, Na-A and hydroxysodalite, where the zeolite zone was formed like an egg white, covering the central core of fly ash particles, as evinced in the previous paper. By fusion with sodium hydroxide, most of the fly ash particles were converted into sodium salts such as silicate and aluminate, from which hydrothermal reaction without stirring favourably resulted in the formation of Na-X zeolite. Crystallinity of Na-X zeolite as high as 62% was attained at the optimum condition of NaOH/fly ash = 1.2 and a fusion temperature of 823 K. Fly ash contains 14 wt% mullite (3Al₂O₃·2SiO₂), which was revealed to be a less-active crystalline component for zeolite formation. Aluminium-enriched fly ash gave Na-A in place of Na-X zeolite. Scanning electron microscope images of cubic and octahedral crystals characteristic of Na-A and Na-X zeolite, respectively, obtained from fly ash, are given.     

Sonochemical treatment of fly ash for dye removal from wastewater

Fly ash samples modified by NaOH solution and sonochemical treatment were tested for a basic dye (methylene blue) adsorption in aqueous solution. It is found that sonochemical treatment of fly ash can significantly increase the adsorption capacity depending on the concentration of NaOH and treatment time. The untreated FA and the sonochemically treated sample exhibits adsorption capacity at 6 x 10(-6)mol/g and 1.2 x 10(-5)mol/g at 30 degrees C, respectively. The adsorption tests show that solution pH and adsorption temperature also influence the adsorption behaviour. The adsorption isotherms can be fitted by Langmuir and Freudlich models, while the two-site Langmuir heterogeneous model will present the best result.     

微波消解-AAS测定垃圾焚烧飞灰中重金属

采用微波消解法对城市生活垃圾焚烧飞灰样品进行预处理,在此基础上优化了城市生活垃圾焚烧飞灰的微波消解酸体系及消解程序.优化后的微波消解酸体系及用量比例为HNO3:HF:HClO4=3:4:1;消解程序的四个工步分别为:3min(5×101 kPa),3min(10×101 kPa),4min(15×101 kPa),6min(20×101 kPa).该方法的相对标准偏差为1.2%～4.5%,加标回收率为95.9%～103.7%,应用于实际的城市生活垃圾焚烧飞灰样品取得了良好的效果.     

A comparative study using direct hydrothermal and indirect fusion methods to produce zeolites from coal fly ash utilizing single-mode microwave energy

This paper investigates the effects of microwave irradiation on the synthesis of zeolite Na-A from coal fly ash at atmospheric pressure. Microwave irradiation has shown to accelerate zeolitization from several hours to several minutes. A single-mode laboratory-scale microwave was employed, which enabled control of irradiated power and temperature of the reaction mixture, while the pressure was controlled by affixing a condenser on the 40 mL Teflon reactor vessel with a working volume 20 mL. Design of Experiment was used to compare two methods of converting CFA to zeolite-A, direct hydrothermal method and indirect fusion method. Experiments conducted were two level four factorial designs. The first factor considered was the categorical factor of conversion method (hydrothermal vs fusion), while the other three factors investigated were power (P), time (θ), and aluminate concentration (δ). Zeolite produced from CFA (CFAZA) was characterized using XRD, FTIR, SEM, TGA, BET, and cation exchange capacity. It was observed that the crystallinity of the product was influenced by factors such as, conversion method, power, time, and aluminate concentration. The hydrothermal and fusion products were comparable to each other in their characteristics, however, hydrothermal CFAZA performed better at immobilizing heavy metal ions and showed better crystalline structure, whereas fusion CFAZA had a higher BET surface area and a slightly higher CEC. Regardless of the performance of the categorical factors the other factors, i.e., power, time, and aluminate concentration followed the same trend for both types of CFAZA.     

Kaliophilite from fly ash: synthesis, characterization and stability

Kaliophilite was synthesized by fusion method using fly ash as starting material. In this method, at first, alkaline fusion of fly ash with KOH occurs, followed by hydrothermal treatment in KOH medium. Scanning electron microscopy (SEM) observations revealed that the synthesized kaliophilite (S-KAL) was a plate-like crystal. X-ray diffraction (XRD) results showed three characteristic diffraction peaks located at 2? = 19·56°, 20·78° and 28·71°, respectively. The thermal analysis indicated that the S-KAL had remarkable thermal stability when heated to 1000°C. Leaching test confirmed the high retention rate of potassium for S-KAL in boiling water for 10 h.     

Utilization of coal fly ash in the glass-ceramic production

Manufacturing the glass-ceramic has been proposed as a useful choice to recycle coal fly ash from power plants. In this work, a glass-ceramic of SiO2-Al2O3-Fe2O3-CaO family was synthesized by mixing 90 wt% of coal fly ash, from a power plant in west of China, with Na2O, and then melted at 1350 degrees C. The ceramization of the obtained glass was carried out at 770 degrees C for 2h. Esseneite and nepheline were found present as major crystal phases. The produced glass-ceramic exhibited good chemical durability as well as good mechanical properties. The toxicity characteristic leaching procedure (TCLP) method found that the glass-ceramic was non-hazardous.     

Sewage sludge hydrothermal treatment by microwave irradiation combined with alkali addition

This article focuses on the effects of microwave treatment on sewage sludge with alkali addition. Both the solubilization of organic matters and the settleability of sludge were investigated. It was found that the combined treatment provided a rapid and efficient process to release organics from sludge flocs. Anaerobic digested sludge with low organic content (37%) obtained better results as compared to fresh sludge with high organic content (56%). For digested sludge, 50–70% of VSS was dissolved into a solution at 120–170 °C with a dose 0.2 g NaOH/g-DS within 5 min, and 80% of the total COD was released to supernatants. The liquor soluble COD concentration increased up to 20 g/L. Nearly 60% of nitrogen dissolved at 170 °C. For fresh sludge, solo microwave heating reduced 40% of VSS at 170 °C within 1 min. Adding 0.05 g NaOH/g-DS increased the VSS dissolution ratio to 50% and got 35% of SS reduction. After 1 min microwave treatment with alkali, the treated digested sludge provided a significant settleability improvement. Nevertheless, the high organic content of fresh sludge lessened the improvement of settleability behavior.     

Role of microwave radiation in curing the fly ash geopolymer

Fly ash geopolymer requires rather long heat curing to obtain reasonable strength development at an early age. However, the long heat curing period limits the application of the fly ash geopolymer. High strength development and a reduction in heat curing duration have been considered for energy saving. Therefore, this research proposed a process using 90-W microwave radiation for 5 min followed by conventional heat curing for high-calcium fly ash geopolymer. Results showed that the compressive strengths of geopolymer with microwave radiation followed by conventional heat curing were comparable to those of the control cured at 65 °C for 24 h. Microwave radiation gave the enhanced densification. In addition, SEM images showed that the gels formed on the fly ash particles owing to the promoted dissolution of amorphous phases from fly ash. This method accelerated the geopolymerization and gave the high compressive strength comparable to the conventional curing.     

Laboratory compaction of fly ash and fly ash with cement additions

The use of power-industry wastes as a material for earthen structures depends on its compactibility. It has been confirmed that a fly ash/bottom ash mix compacted several times in Proctor's moulds are not representative. The relationship between dry density of solid particles and water content for re-used waste samples was determined. The re-compaction effect on grain-size distribution, density of solid particles, specific surface and sand equivalent of wastes was investigated. Tests were conducted on fly ash samples compacted by the Standard and Modified Proctor methods. Another aim of the paper was to investigate the influence of cement additions on the compactibility of a fly ash/bottom ash mix. Waste samples in the natural state and with different percentages of cement additions (2, 5 and 10%) were compacted by both impact compaction methods to obtain compactibility curves rhod(w). It was found that cement addition resulted in an increased rhod max value, while wopt decreased. Linear regression relationships for changes in compaction parameters after cement stabilisation are also given.     

Non-hydrothermal synthesis of mesoporous materials using sodium silicate from coal fly ash

Siliceous mesoporous materials with pores of ordered 2-D hexagonal structure were successfully prepared without hydrothermal treatment from condensation–polymerization of various concentration of quaternary ammonium salt as structure directing agents and silica precursor from the supernatant of coal fly ash (CFA) in the presence of catalyst. The synthesized materials had high surface area of ca. 740 m² g⁻¹ and pore volume of ca. 0.42 mL g⁻¹. The synthesized material exhibited a narrow size pore distribution and the mean pore diameter as measured by Dollimore–Heal method was about 2.3 nm. The formation of ammonium salt that act as precipitant during the synthesis enable the hydrolysis and co-condensation of the sodium silicate at room temperature.     

Removal of chlorophenols from aqueous solution by fly ash

Fly ash from coal-fired thermal power plants can be used for the removal of 2-chlorophenol (2-CP) and 2,4-dichlorophenol (2,4-DCP) with enthalpy changes of about -3 kcal/mol. The amounts of 2-CP and 2,4-DCP removed are affected by the pH value of the solution. The efficiency of removal improves when the pH value is less than the pK(a) values of 2-CP and 2,4-DCP, respectively. The adsorbed amount of chlorophenol by fly ash is also affected by particle diameter, carbon content, and the specific surface area of the ash used in this study. As expected, more adsorption takes place with fly ash of higher carbon content and larger specific surface area. Moreover, the adsorbed amount of chlorophenol is not influenced by the matrix in the wastewater, as shown by studying the removal of 2-CP and 2, 4-DCP in wastewater from a synthetic fiber plant. Chlorophenols in the wastewater were also removed efficiently through a fly ash column, with breakthrough times being inversely proportional to flow rates.     

Hydrothermal synthesis of NaA zeolite membrane together with microwave heating and conventional heating

Uniform and dense NaA zeolite membrane was prepared by hydrothermal synthesis method together with microwave heating and conventional heating. The properties of the as-synthesized zeolite membrane were investigated by XRD, SEM and pervaporation evaluation for dehydration of 95 wt.% isopropanol/water mixture at 343 K, respectively. After microwave heating, the α-Al₂O₃ support surface was covered with homogeneous zeolite nuclei, which facilitated to form uniform, pure and dense NaA zeolite membrane in the following conventional heating process. High quality NaA zeolite membrane, i.e., with a separation factor (water/isopropanol) of 10,000 and a flux of 1.44 Kg/(m² h), could be hydrothermal synthesized together with microwave heating and conventional heating.