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Submicron zeolite has become the focus of research because of its high thermal stability, high catalytic activity, and uniform size. A rapid method for synthesizing submicron zeolite Y by using microwave heating is introduced in this work. The effect of directing agent, synthesis temperature, crystallization time, and alkali ratio on the synthesis of submicron zeolite Y is systematically investigated. XRD, SEM, and particle size investigations are carried out to characterize the obtained product. The experimental results show that adding directing agent, increasing the alkali ratio, increasing the temperature or prolonging the microwave radiation time are advantageous to the formation and growth of zeolite Y. However, the high alkali ratio leads to the dissolution of the generated crystal, while excessively increasing the temperature or crystallization time easily generates uneven-sized particles or causes crystal transformation. Submicron zeolite Y of particle size 0.5–0.8 μm accounting for 80% is synthesized at 110°C in just 2 h using alkali ratio of 3 under microwave heating. [ABSTRACT FROM AUTHOR]

In this work, the kinetic reactions between NaOH and the Al-bearing minerals were investigated, when the slurry mixture of diasporic bauxite and alkaline solutions were roasted by microwave (MWH) and conventional heating (CH). The XRD patterns reveal that the MWH method effectively promotes the reactions between NaOH with Al-bearing minerals to form NaAlO 2 , and inhibits the dehydration of AlOOH. Compared to CH, MWH greatly decreases the roasting temperature nearly 400 °C and shortens the reaction time by 3 times to completely transform Al-bearing minerals from raw bauxite into NaAlO 2 . The model fitting results show that the kinetics of the reactions between NaOH and the Al-bearing minerals for both heating methods can be described by the Jander's model. The value of apparent activation energy for the reactions between NaOH and Al-bearing minerals by MWH is 31.31 kJ/mol, which is 23% less than that of the reactions in CH. In addition, the SEM-EDS analysis results show that the reacted layer gradually expands outward when the microwave roasting time prolonged. Moreover, the influence of MWH on the kinetics of the reaction between NaOH and Al-bearing minerals explained in detail by a specific activation mechanism of H 2 O vapor and the differences between dielectric loss of reactants and products. [ABSTRACT FROM AUTHOR]

A novel process involving microwave roasting of slurry mixtures of diasporic bauxite and alkaline solutions followed by dilute alkaline leaching, is proposed for extracting alumina from low grade diasporic bauxite ores. It was found that when the mole ratio of m NaOH / ( m Al + m Fe ) and m Ca / m Si in the slurry mixture was close to 1 and 2, the Al and Si contained in the bauxite were completely liberated into soluble NaAlO 2 and insoluble CaSiO 4 after roasting using a 1.0 kW microwave source at 800 °C for 20 min. The roasting process significantly improved the performance of the downstream leaching processing. The extraction of Al reached 97% while that of Si was as low as 7%, on leaching in low-concentration alkali solution at 80 °C under atmospheric pressure for 20 min. A new mechanism involving flashing, layering and homogeneous reaction is proposed for the new microwave roasting process. The key steps proposed are: creation of a hot core at the peak location of the electric field, preheating of the outer layer facilitated by heat conduction and vapor from the hot core, and finally microwave heating of the outer layer due to increasing dielectric loss and decreasing penetration depth. [ABSTRACT FROM AUTHOR]

Spent CuCl residue produced from zinc hydrometallurgy is a typical toxic and hazardous solid waste as well as a secondary resource owing to its high contents of chlorine and valuable metals (Pb, Cu, Zn). The removal of chlorine from the CuCl residue and recycling of valuable metals are critical issues for the copper slag resource treatment process. In this study, a novel method of microwave sulfation-roasting under an oxygen atmosphere is used to improve the dechlorination process, and various parameters, such as roasting temperature, duration, sulfuric acid concentration, and liquid/solid ratio, are optimized. First, according to thermodynamic calculations, the addition of H 2 SO 4 can significantly decrease both the roasting temperature and time needed for microwave oxidation-roasting. The dechlorination experimental results reveal that the dechlorination rate of the CuCl residue is as high as 91.91% under a temperature of 250 °C, holding time of 120 min, sulfuric acid concentration of 70%, and liquid-solid ratio of 1.42. The microstructure, chemical composition, and phase changes of the CuCl residue before and after roasting are characterized using scanning electron microscopy coupled with energy-dispersive spectroscopy, X-ray fluorescence and X-ray diffraction; further, the mass contents of the main elements in the roasting product and tail gas absorption solution are determined by chemical analysis, Evidently, the selective heating property of microwaves produces more holes and cracks in the copper slag, which can provide a new reaction channel for H 2 SO 4 and CuCl slag reactions, and almost all CuCl in the copper slag is completely removed by transformation into CuO, CuSO 4 and Cu 2 O(SO 4). Simultaneously, a large number of gas by-products produced in the dechlorination process, such as SO 2 and Cl 2 , can be absorbed by the NaOH solution. Therefore, no pollution is generated in the environment. These findings illustrate that microwave sulfation-roasting is an effective method for promoting the treatment and recycling of CuCl waste residues. [Display omitted] • Microwave heating was proposed to realize the chlorine removal from CuCl residue. • The thermodynamic analysis demonstrated the viability of dechlorination reaction. • Sulfating roasting can reduce roasting temperature and time by half. • The dechlorination rate of CuCl residue under optimal conditions is 91.91%. • CuCl in residue was completely removed by transforming into CuO and CuSO 4. [ABSTRACT FROM AUTHOR]

Water is a kind of strongly microwave absorbing material. However, water is generally believed that it cannot enhance the effect of heat transfer and mass transfer of reactions at high temperature in the microwave field, due to its performance of easy drying. In this work, the intensifying effect of water on the high temperature reactions in the microwave field is proved. Specifically, the effects of different mixing methods of bauxite and alkaline, including wet mix and dry mix, on phase constitution, microstructure and leaching effect of roasting products in microwave heating process are compared. The results show that compared with the dry mixing, the wet mixing method is more benefit for forming NaAlO2and Ca2SiO4. The roasted product of wet mix is looser and more homogeneous, which is more conducive to the subsequent leaching process. The leaching rate of Al2O3is calculated to be 96.68% with wet mix at 800 °C and 92.58% with dry mix at 1000 °C. In addition, the mechanism of intensifying action of water on the high temperature reactions in the microwave field is discussed. [ABSTRACT FROM AUTHOR]

• Microwave flash evaporation(MFE) process could be utilized to concentrate sulfuric acid. • Power consumption and heat transfer of concentration process were analyzed. • Clean production mode averts the corrosion and the pollution of products. The ameliorative microwave flash evaporation system (MFE) was utilized to concentrate and distillate waste sulfuric acid, the system may overcome the problems of equipment corrosion, product pollution, high energy consumption, long process flow, and low heat transfer rate during recovery process of waste sulfuric acid. The dilute sulfuric acid with mass fraction of 75 wt% was used as experimental medium, 5 experimental cases was generated to verify the effects of MFE with the conditions of different volume, microwave power and depressurization rate. Experimental results demonstrated that the sulfuric acid could be concentrated to mass fraction of 85–95%, and the heating efficiency was among 30% to 80%, heat transfer volume coefficient was among 100 J/K·L-1 to 500 J/K·L-1 with the combination of vacuum and microwave heating during flash evaporation process. Thermal process analysis indicated that the interaction among experimental factors affected concentration and distillation process. At last, we gave the modeling and process optimization method for concentrating sulfuric acid through MFE process based on analyzing those experimental cases. [ABSTRACT FROM AUTHOR]

• Polyphosphoric acid (PPA) preparation though microwave intensification flash evaporation. • Over 92% fluorine separation and recycling for reduced environmental impact. • PPA with 75 wt.% P 2 O 5 content has only 89.98% operation cost in contrast to traditional method. • The influencing mechanism of the PPA preparation and fluorine recycling was studied. Traditional evaporators for the concentration of wet process phosphoric acid (WPA) suffer from concentration, corrosion, and low fluorine yield (less than 40 wt.%). Based on the excellent heating and permittivity properties of phosphoric acid in microwave fields, a microwave intensification flash evaporation (MFE) system is proposed to concentrate highly corrosive WPA for preparing polyphosphoric acid (PPA) and efficiently recovering valuable fluoride resources. A polytetrafluoroethylene tank in a microwave resonant cavity coupled with a vacuum pump is used to facilitate boiling and evaporation. This study reports the evolution of the PPA preparation process by flash evaporation, defluorination, and the boiling phenomenon. In addition, the influencing mechanism of pressure and temperature on PPA preparation and fluorine recovery are investigated. The experimental results indicate the successful preparation of PPA with a P 2 O 5 content higher than 75 wt.% and the cost is considerably much lower than the traditional method. Meanwhile, the recovery yield of fluorine exceeds 90 wt.%, alleviating the depletion of strategic natural resources and potential risk of environmental pollution. This study provides a basis for the efficient preparation of PPA and recovery of fluorine resource from WPA. It is also potentially significant to industries such as metallurgy and sewage treatment. [Display omitted] [ABSTRACT FROM AUTHOR]

Investigating the microwave-absorbing characteristics of secondary copper smelting fly ash (SCFA), which is both a hazardous waste and valuable secondary resource, can be expected to expedite the efficient application of microwave heating technology in the comprehensive treatment of fly ash. Here, we have examined SCFA and its pure components, namely PbCl 2 , ZnO, and ZnS phases, with regard to thermal behavior and microwave-absorbing characteristics. Our results indicate that SCFA has excellent microwave-absorbing performance in the temperature range 30–800 °C, with a dielectric loss coefficient ε ″ of 5.68 × 10−2 F m−1 at ambient temperature and a maximum value of 36.29 × 10−2 F m−1 at 550 °C. As a consequence, the SCFA could be heated to 700 °C within 12.6 min. The heating process of SCFA can be divided into three main stages: removal of adsorbed water and bound water (<260 °C), oxidation reactions of Pb, PbO, ZnS, and PbS (260–437 °C), and deoxygenation reactions of PbO 2 , Pb 3 O 2 , and Pb 3 O 4 (437–1000 °C). The dielectric properties and thermal characteristics of SCFA are closely related, with PbCl 2 exerting a significant influence. We propose herein a microwave-heating mechanism of SCFA based on analysis of its thermogravimetric characteristics, dielectric properties, and heating behavior in a microwave field. • The dielectric properties and thermal behavior of SCFA were systematically investigated. • SCFA endowed excellent microwave-absorbing ability with increasing temperature. • PbCl 2 dominated the dielectric properties of SCFA. • Dielectric and heating properties studies suggest the microwave heating mechanism. [ABSTRACT FROM AUTHOR]

• A microwave flash evaporation system (MFE) is demonstrated. • MFE process is more efficient than CFE process. • Microwave heating raises liquid temperature and intensifies heat transfer during MFE. • Dimensionless analysis is used in heat transfer characterization of MFE process. • Heat transfer intensity increases with superheating degree in MFE process. The evaporation efficiency of conventional flash evaporation was limited by the superheated degree of liquid, process intensification was one profitable method to enhance evaporation efficiency of flash evaporation. In this article, microwave was used as external energy field to enhance flash evaporation process, the proprietary equipment was developed to validate the performance of microwave flash evaporation (MFE) process, and series experiments were carried out under the conditions of the microwave power varied from 0.81 to 1.35 kW, liquid flow rate from 10 to 40 L/h and initial pressure from 4.0 to 8.0 kPa. Results indicated the productivity of microwave flash evaporation (MFE) was more than conventional flash evaporation (CFE) due to the heating effect of microwave. The temperature of flash evaporation chamber was among 19 to 50 °C and pressure of flash chamber during experiment was among 4 to 12 kPa, according to the variation of temperature and pressure of flash evaporation chamber, the flash evaporation was divided into two stage, i.e., rapid flash evaporation stage and equal flash evaporation stage. The dimensionless numbers of non-equilibrium fraction (NEF) and Jakob number (Ja) was applied to assess the heat transfer process, further analysis indicated the variation of NEF and Ja for MFE was influenced by flow rate, superheating degree and microwave power, and the Ja increased with the increase of superheating degree. This mechanism of microwave enhancing flash evaporation process was inferred as microwave fed into vessel in situ to replenish thermal loss of liquid and be absorbed to induce temperature raise instantaneously, which led to the transient flash evaporation or direct boiling evaporation of liquid. [ABSTRACT FROM AUTHOR]

A rapid microwave heating method is proposed for the liquid phase synthesis of Y-type zeolite under high-temperature conditions. The effect of microwave heating temperatures, crystallization time and silica to alumina ratio on the synthesis of Y-type zeolite is systematically investigated. The synthesized samples are characterized by XRD, SEM, FT-IR, N 2 sorption and particle size analysis. Microwave treatment was found to promote the high-temperature synthesis of Y-type zeolite with the advantages of shorter crystallization time and product properties of the produced Y-type zeolite. Specifically, under the optimized conditions, such as input ratio of Si/Al of 4, microwave heating temperature of 150 °C, crystallization time of 0.5 h, pure Y-type zeolite is synthesized with BET specific surface area of 645 m2/g and the total pore volume is 0.322 cm3/g. The evolution order of zeolites with crystallization temperature is as follows: amorphous phase, Y-type zeolite, and P2 zeolite. An extended crystallization time or increased ratio of Si/Al facilitated the formation of P2 zeolite and analcime-C. The synthesis process promoted by microwave is explained by the high-speed rotation of polar water molecules, the formation of isolated water molecules and the dispersion of initial aluminosilicate gel in the microwave field. Image 1 • Microwave is used for the synthesis of zeolite Y under high-temperature conditions. • The BET specific surface area of zeolite Y synthesized at 150 °C for 0.5 h is 645 m2/g. • The evolution order of zeolites with crystallization temperature is obtained. • The influence of microwave on the synthesis of zeolite is explained. [ABSTRACT FROM AUTHOR]

Mercuric chloride supported on activated carbon was widely used as the catalyst for vinyl chloride monomer (VCM) production especially in Asia. The resultant depleted mercury-containing catalyst (DMC) was a hazardous solid waste due to the high mercury toxicity. Microwave heating (MWH) was used to decontaminate DMC. Effects of microwave power and treatment time on mercury removal efficiency were studied. Up to 99.98% of mercury could be recovered under the optimum treatment conditions of 700 W for 20 min with microwave energy consumption being 15.56 kW h kg −1 (DMC); meanwhile the treated DMC was a clean regenerated activated carbon (RAC) without mercury leaching toxicity. The obtained RAC was a satisfactory mercury adsorbent with high specific surface area (632.9 m 2 /g) and abundant surface functional groups. Then the RAC was used to absorb mercury from HgCl 2 solution. Effects of pH, adsorbent dose, agitation time and initial mercury concentration on mercury sorption were investigated. Reduction and ion exchange were mainly responsible for mercury sorption mechanisms. Adsorption process conforms to pseudo-second-order kinetic model and Langmuir adsorption isotherm model. The calculated adsorption capacity of RAC was 109.05 mg/g. The absorbed mercury was efficiently desorbed by MWH. High sorption capacity of RAC maintained after five times of cyclic sorption/desorption. This new process realized the recycling of secondary resource, providing a new idea for cleaner production of VCM in engineering applications. [ABSTRACT FROM AUTHOR]

A microwave flash evaporation system is designed and applied to research the intensification effect of microwave on flash evaporation, in present work, a mathematic model of the energy transfer and efficiency of microwave flash evaporation (MFE) process is presented, and series of experiments used water as medium are conducted. Production rate, thermal efficiency of phase transition, thermal proportion of steam, electro-thermal conversion efficiency, and indicator parameters such as, gain output ratio (GOR), specific thermal energy consumption (STEC), and total volumetric heat transfer coefficient are calculated to evaluate energy transfer of MFE process. As a result, the production rate, thermal conversion efficiency and thermal proportion of steam increase with microwave power but decrease with flow rate, and the maximum value of electro-thermal conversion efficiency achieved 76.66%. GOR value and total volumetric heat transfer coefficient of MFE increase with microwave power but decrease with flow rate, the specific thermal energy consumption increases with flow rate but decrease with microwave power. In addition, the economy of MFE process is also discussed. This work may provide the theoretical support for the feasibility of the utilization of microwave heating in flash evaporation. • Mathematical model of microwave flash evaporation is presented. • Adopted experiments to verify performance and production ratio of MFE system • Discussed energy transfer and efficiency of MFE process • The maximum value of electro-thermal conversion efficiency is 76.66%. [ABSTRACT FROM AUTHOR]