Microwave Heating Characteristics Research Articles

Multi-physical field coupling modeling of microwave heating and reduction behavior of zinc oxide

Microwave heating provides a cleaner pyrometallurgical method for separating zinc from electric arc furnace dust (EAFD, a solid waste generated during steel production with excellent dielectric properties). Despite its undisputed heating efficiency, the impact of microwave heating characteristics on the zinc removal process from EAFD remains unclear. This paper focuses on ZnO, one of the primary components of EAFD, and develops an electromagnetic-thermal-chemical reaction model to analyze the effects of microwave power and graphite addition on heating behavior, reduction reactions, field distributions, and reaction kinetics. The results indicate that the heat generated from the interaction between microwaves and materials exhibits inherent non-uniformity, leading to the localized thermal effect and making the error of general temperature measurement methods. Increasing microwave power and adding graphite enhance heating efficiency and promote the reduction reaction but also result in significant internal-external temperature disparity and worse temperature distribution. At 1000 W, with 1.2 times the stoichiometric amount of graphite addition, the temperature measurement error reaches approximately 200 °C, potentially affecting the kinetic results to a certain degree. The non-isothermal kinetics results from simulations indicate that the localized thermal region exhibits a lower average activation energy of 47.5 kJ/mol compared to experimental results, suggesting that the lower activation energy of the ZnO reduction reaction during microwave heating is primarily caused by the localized thermal effect rather than the non-thermal effect. Additionally, in the energy distribution, the higher proportion of return loss during heating underscores the importance of a well-designed microwave heating system.

Effect of microwave curing on the properties of aggregate-mastic interfacial transition zones in cement emulsified bitumen mixture containing steel slag

The poor properties of the aggregate-mastic interfacial transition zones (ITZ) are the core factor for the low structural strength of cement emulsified bitumen mixture (CEBM), while incorporating steel slag with high microwave responsiveness into CEBM and followed by microwave curing may be an effective method to enhance ITZ properties. To verify this hypothesis, the ITZ properties of CEBM containing steel slag were characterised and analysed by a series of techniques such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and nanoindentation. The results showed that the ITZ properties increased continuously with microwave curing time. Specifically, the thickness of ITZ continuously decreased, while the elastic modulus and hardness continuously increased. When the microwave curing time was less than 7 min, the growth in cement hydrates and temperature simultaneously enhanced the ITZ properties. When the microwave curing time exceeded 7 min, the cement hydrates gradually decreased due to the water content falling below the minimum required for cement hydration, and the improvement of ITZ properties mainly depended on the temperature rise. Remarkably, the microwave heating characteristics of steel slag resulted in a tighter bond with bitumen, which led to consistently better surface ITZ properties than limestone. However, it should be avoided that the microwave curing time is too long, as this would result in the steel slag temperature being too high and aging its surface bitumen. Additionally, the indirect tensile strength of CEBM affected by the ITZ properties was still increased even after the decrease in cement hydrates.

Numerical analysis of microwave-enhanced oil shale pyrolysis by rotation turntable based on the Arbitrary Lagrangian-Eulerian method

Microwave heating technology has great potential for efficient pyrolysis of oil shale. However, the inhomogeneous heating characteristics of microwave heating can lead to secondary cracking of pyrolysis products, thus reducing the overall oil yield. In order to address this issue, a rotational movement of oil shale under microwave heating has been proposed to obtain a uniform temperature distribution of the oil shale. Then, a new coupled three-dimensional electromagnetic-thermal-chemical-hydraulic model incorporating the Arbitrary Lagrangian-Eulerian (ALE) method was developed to investigate the temperature distribution and pyrolysis characteristics of oil shale under the rotational motion during microwave pyrolysis. The effects of rotational speed, rotational direction, and microwave power on the pyrolysis process of oil shale were explored through this model. The results showed that the introduction of rotational movement effectively addressed the uneven temperature distribution within the oil shale. Compared with the static condition, the rotational condition reduced the secondary reactions of oil products during microwave pyrolysis and increased the oil yield of oil shale to 50.12 %. As the rotation speed increased, the temperature distribution within the oil shale became more uniform, but the total oil yield reached its peak at a moderate speed of 180 s/r. Research on microwave power indicated that rotation increased the maximum usable power and highlighted that reducing the residence time of pyrolysis products within the oil shale was key to increasing oil yield. This study provides a novel approach and numerical simulation method for enhancing the microwave pyrolysis efficiency of oil shale, and the findings of this study are applicable to both the ex-situ and in-situ exploitation of oil shale.

Microwave heating characteristics of cement mortar containing carbonyl iron powder applied to airport pavement deicing

To improve the microwave deicing efficiency of airport concrete pavement, carbonyl iron powder (CIP) was used as an absorbing material to prepare the microwave-absorbing cement mortar. Firstly, to characterize the microwave absorption ability of the mortar, its electromagnetic (EM) parameters and microwave reflection loss (RL) with different CIP content were investigated by using the coaxial vector network analyzer (CVNA) and arching reflected method, respectively. Then, the microwave heating rate and deicing efficiency of the mortar coated with ice was investigated under different icing temperature and ice layer thickness conditions with a microwave heating frequency of 2.45 GHz. Finally, the mechanical strength of mortar after deicing was tested to analyze the effect of an icing-deicing cycle on its engineering performance. The results showed that at the total frequency band, the mean RL and effective bandwidth of 10CCM was −2.8 dB lower and 1.6 GHz wider than that of the control group. Meanwhile, at 2.45 GHz, the RL of 10CCM was 1.31 times lower than that of the control group. When the icing temperature and ice layer thickness were − 10 °C and 1 cm, the heating rate of 10CCM was 1.34 times higher than that of the control group. Compared to the icing temperature, the changes in ice layer thickness on the mortar surface had less effect on its heating rate. After deicing, the CIP mortar still had favorable mechanical strength. Hence, the CIP was a favorable absorbing material that could be applied to cement mortar to improve the microwave deicing efficiency of the concrete pavement during long-term service.

Thermal behaviors of cement and mortar under microwave treatment and the influencing factors: An experimental study

Microwave-assisted breakage of cementitious materials has emerged as a promising green and low-carbon method for dismantling cementitious structures. To understand the mechanism of microwave-induced cracking in structures, it is essential to study the thermal behaviors of cement and mortar under microwave treatment. In this paper, the effects of mixture ratio, water content, and microwave power on the microwave heating characteristics of cement and mortar specimens were investigated through real-time temperature and mass monitoring. In particular, the influence of microwave power on the bursting effect of cement and mortar specimens was analysed, and the mechanism of specimen bursting was explained. The results show that moisture content, water-cement ratio, and microwave power significantly impact the heating rate and mass loss of cement and mortar during microwave treatment. Based on the heating rate, the heating process of cement and mortar under microwave treatment can be divided into three to four stages, depending on the moisture content. The primary reasons for variations in the heating rate are water evaporation and the decomposition of hydrolysis products. The thin-wall ball model based on steam pressure theory can explain the bursting of cement and mortar specimens. Our findings suggest that increasing microwave power and water content in mortar specimens can lead to microwave-induced cracking.

Microwave heating and devolatilization characteristics of polyethylene particles in a fixed‐bed devolatilizer

AbstractA new method of microwave‐enhanced devolatilization has been proposed to solve the problem that volatiles of polyethylene particles are difficult to remove. Using water as microwave susceptor in a fixed‐bed devolatilizer, microwave heating characteristics were studied by optical fiber thermometry and infrared thermal imaging, and particle devolatilization characteristics were investigated by headspace gas chromatography. The results showed that bed temperature rose and particle devolatilization efficiency increased with heating time, moisture content, and input microwave power. The non‐uniform devolatilization at different positions existed due to the non‐uniform distribution of bed temperature in both radial and axial directions. The effective diffusion coefficient of volatiles under microwave irradiation increased with moisture content and input microwave power, and the microwave‐enhanced coefficient of water also increased with input microwave power. Microwave heating and devolatilization mechanism of moist polyethylene particles were proposed, in which the modes of mass and energy transfer were clarified. In view of the above, the feasibility of microwave‐enhanced polyethylene devolatilization has been confirmed.

Effect of silicon carbide powder on temperature field distribution characteristics and microwave deicing efficiency of cement concrete containing magnetite (Fe3O4) powder

To improve the temperature field distribution characteristics and overall deicing efficiency of magnetite (Fe3O4) powder concrete during microwave heating at the extremely low-temperature condition, the silicon carbide (SiC) powder with favorable heat transfer ability was added to the concrete system. The microwave heating characteristics and thermal conductivity of concrete were studied to analyze its heating rate and heat transfer ability, respectively. Meanwhile, the thermal response changes of the concrete surface were recorded during the microwave heating, and the temperature difference (TD) and standard deviation (St.d) were used to characterize its surface temperature field distribution characteristics. In addition, the ice layer shedding time (ILST) of the concrete surface was used to evaluate the overall deicing efficiency of the concrete coated with ice. The results showed that in the microwave-absorbing concrete, the Fe3O4 content was the main factor affecting its microwave heating rate. The SiC could effectively improve the thermal conductivity of concrete. When the SiC content was 6 wt%, the heat transfer ability of concrete was improved by 69.2%. Therefore, after adding the SiC, the temperature field distribution characteristics of the concrete surface were tending to be uniform during microwave heating. After heating 120 s, the SiC content with 6 wt% could effectively reduce the TD by 23 °C and the St.d by 5.6 °C of surface temperature field of concrete containing Fe3O4. When the mass percentage of Fe3O4 and SiC was 1:1, the ILST of concrete coated with ice was the shortest at the icing temperature of −30 °C and the ice layer thickness of 20 mm. It was 55.5 and 24.3 s shorter than that of concrete without absorbing materials and thermal conductivity materials, respectively, and its deicing efficiency was 1.51 and 1.22 times higher than theirs.

Microwave heating characteristics and energy utilisation efficiency of the double-layer magnetite cement concrete (DLMCC)

ABSTRACT In order to save and improve the utilisation of magnetite meanwhile rise the road microwave deicing efficiency, double-layer cement concrete specimens with different magnetite contents were designed. The heating efficiency and heat distribution of the surface and heat conduction process and electromagnetic loss of internal of the double-layer magnetite cement concrete (DLMCC) were studied. In addition, the relative absorptivity based on energy conservation was used to evaluate the energy utilisation efficiency. The results show that the double-layer structure could save 80% of magnetite and make utilisation of magnetite reach 93.39%. The surface temperature rise rate of DLMCC was improved greatly while was obtained through a low surface temperature/magnetite ratio. The optimum heat distribution of DLMCC was in the range of −2.24°C to 1.66°C of surface temperature. The influence regions of internal heat on the surface temperature were constant. The maximum electromagnetic loss of DLMCC within 10 mm of the surface was 27.43%, and the maximum energy utilisation efficiency of DLMCC increased by 14.35%.

Can the container-dielectrics control heating patterns for microwave assisted material processing? A finite element based introspection

This work investigates the effect of acrylic and glass containers on microwave heating of a representative low lossy dielectric material enclosed in the container and exposed to uniform plane microwaves from one side. Microwave heating characteristics are determined via detailed mathematical models for the electric field and temperature distributions within the semi-infinite sample–container assembly. An in-house FORTRAN based algorithm involving Galerkin finite element method is developed to solve the governing equations and determine the heating characteristics (power and temperature distributions) for various sample dimensions as a function of container thickness. It has been found that the acrylic container can be safely used without any significant alteration of the heating characteristics of the dielectric samples. On the other hand, the glass container can lead to a significant alteration of the heating characteristics as a function of container thickness. The power absorption, heating pattern (location of the heating fronts) and spatial heating uniformity within dielectric samples are found to vary significantly with the thickness of the glass container. The results indicate that the glass container can play the critical role to alter the heating characteristics based on the processing requirement (uniform heating or targeted heating at varying locations). These results are expected to play important roles for material processing and the role of the glass containers towards optimal microwave processing applications is revealed for the first time. These results are also important for further investigations on the containers (materials, shapes etc.) and their potential applications for the enhanced, controlled or targeted microwave processing of materials.

Microwave heating mechanism and self-healing performance of asphalt mixture with basalt and limestone aggregates

Traditional asphalt mixtures can’t absorb microwave energy efficiently, which limits the development of microwave heating technology in the field of road maintenance. Based on the microwave heating characteristics of basalt aggregates, the overall microwave self-healing rate of the asphalt mixture can be enhanced. The basalt was tested by XRF, XPS, XRD and electromagnetic parameters to reveal its microwave heating mechanism. Through the heating rate test, SCB test and fatigue test of asphalt mixture, its heating characteristics, flexural strength, fatigue resistance and self-healing performance were studied. The results showed that the excellent wave-absorbing properties of basalt are highly correlated with the elements of Si, Fe and Al. Its TanδM was slightly larger than TanδE, which indicated that basalt can absorb microwave energy through dielectric loss and magnetic loss. The aggregate type and particle size both affected the microwave heating rate of the aggregates. After microwave heating, the flexural strength and fatigue resistance of asphalt mixture with basalt and limestone aggregates can recover at least 65% and 23% respectively.

Microwave heating mechanism and Self-healing performance of scrap tire pyrolysis carbon black modified bitumen

Conventional asphalt mixture has poor microwave absorbing performance and microwave heating efficiency. Based on the characteristics of dielectric loss of scrap tire pyrolysis carbon black (PCB), it is proposed to improve the microwave absorbing performance and self-healing rate of bitumen. The phase composition and electromagnetic parameters of PCB were tested to reveal its microwave heating mechanism. The preparation parameters, heating characteristics and self-healing properties of PCB modified bitumen were studied through the dispersion uniformity test, microwave heating test and SCB test. The main phases of PCB are microwave absorbing carbon and silicon, indicating that PCB is a good electric loss microwave absorbing material. 40 min is the recommended mixing time of 15% PCB modified bitumen. PCB's dosage, microwave frequency and microwave heating time have significant effects on the microwave heating characteristics of PCB modified bitumen. PCB can improve high-temperature stability, thermal conductance, heat storage capacity and self-healing rate of bitumen.

Dependence of composition of SiC–Al2O3 composite absorbers on microwave heating characteristics

Spark plasma sintering technology was used to prepare SiC–Al2O3 composite absorbers with a range of compositions. Scanning electron microscopy showed that of the components was uniform in all compositions. The influences of composition, heating time, and microwave power of the composite absorbers on their heating characteristics were analyzed. As the microwave power was increased from 500 W to 1000 W, the temperature achieved by the absorber increased. After being heated for 24 min at a microwave power of 1000 W, all samples maintained a constant temperature. The absorber with 40mass% Al2O3 content reached the highest temperature (952 °C); the absorber with 95 mass%Al2O3 content achieved the lowest temperature (313 °C). These findings have importance as a reference for precise temperature control in microwave heating of composite absorber.

A review about microwave regeneration technology of waste activated carbon

Activated carbon (AC) can concentrate and transfer pollutants using its own adsorption capacity. Waste AC is a kind of solid waste, its adsorption capacity of pollutants is greatly reduced after adsorption saturation. If handled improperly, it will cause secondary pollution to the environment. Therefore, it is necessary to regenerate the waste AC to restore its adsorption performance and reduce environmental pollution and resource waste. Microwave regeneration technology is of great research value because of its low energy consumption, time saving and high efficiency. Based on the characteristics of microwave heating and regeneration mechanism, this paper summarizes the advantages of microwave regeneration compared with conventional heating regeneration, and the application status of microwave technology in the regeneration of waste AC was reviewed. The influence of different types of pollutants adsorbed by AC, the change of control conditions for microwave regeneration of AC and the characteristics of AC itself on microwave regeneration were briefly described. And the development direction of microwave regeneration of AC in the future is also prospected.

Enhanced Leaching of Zinc from Zinc-Containing Metallurgical Residues via Microwave Calcium Activation Pretreatment

Given the shortage of zinc resource, the low utilisation efficiency of secondary zinc resource, and the crucial problem that the synchronous dissolution of zinc from different mineral phases, an activation pretreatment method merged with calcium activation and microwave heating approach was proposed to enhance the zinc leaching from complex encapsulated zinc-containing metallurgical residues (ZMR). Results indicated that under the optimal pretreatment conditions, including microwave activation temperature of 400 °C, CaO addition of 25% and activation time of 20 min, the zinc leaching rate reached 91.67%, which was 3.9% higher than that by conventional roasting pretreatment. Meanwhile, microwave heating presents excellent treatment effects, manifested by the zinc leaching rates, all exceeding that of conventional roasting under the same conditions, while the process temperature is decreased by 200 °C. In addition, XRD and SEM-EDS analysis denoted that microwave calcification pretreatment can effectively promote the transformation of the refractory zinc minerals like Zn2SiO4 and ZnFe2O4 into the easily leachable zinc oxides. The distinctive selective heating characteristics of microwave heating strengthened the dissociation of mineral inclusion, and the generated cracks increased the interfacial reaction area and further enhancing the leaching reaction of zinc from ZMR.

Study on the microwave extraction process and product distribution of essential oils from citrus peel

Citrus peel wastes containing a series of very beneficial active compounds are produced in large quantities around the world every year, which are usually extracted by traditional methods, requiring high energy costs, long extraction times and additional reagents. Based on the unique heating characteristics of microwave heating, this paper uses microwave heating to extract the essential oils of citrus peel. Exploring the content change rule and the composition distribution situation of citrus peel product with different reaction time and microwave power, and comparing the microwave extraction and conventional extraction methods of citrus peel essential oils, infer the mechanism of microwave extraction. Considering the efficiency and energy consumption, optimum microwave power and time were selected as 450 W and 10 min. At microwave power of 450 W, 29.22% liquid product was obtained with 37.82% limonene yield in the liquid. The limonene yield per unit energy consumption in case of microwave heating was found to be 18.6 times that of traditional heating, and the extraction mechanism based on microwave heating effect and pumping effect was proposed.

Decomposition Process of Nonoxidative Microwave Radiation Roasting of a Mixed Rare Earth Concentrate with Sodium Carbonate.

This article introduces an efficient decomposition process that uses sodium carbonate (Na2CO3) and activated carbon (C) as additives to decompose Bayan Obo mixed rare earth concentrate (hereinafter to be referred to as RE concentrate) by nonoxidative microwave radiation roasting. The roasting temperature, holding time, and contents of Na2CO3 and activated carbon are investigated. The optimum process parameters for decomposition are 800 °C and 30 min. The ratio of m(Na2CO3)/m(RE concentrate) is 0.5, and the ratio of m(C)/m (ER concentrate and Na2CO3) is 0.2 based on experimental data. Under the above conditions, the decomposition rate (shortened to DR) of RE concentrate is 98.58%, and the removal rates (shortened to CRs) of fluorine (F) and phosphorus (P) reached 80.35 and 46.75%, respectively. These rates are higher than traditional oxidation roasting under the same conditions. The three reasons for the result are the unique microwave heating characteristics, the overall efficient reaction of the mixture (RE concentrate, Na2CO3, and activated carbon), and the high nonoxidation rate of cerium. For these reasons and large experimental data, the reaction rate of the mixture is improved, and the efficiency of dilute hydrochloric acid for leaching rare earth elements is enhanced. In this article, the valence of rare earth elements in the roasted ore is all in the form of trivalence. Importantly, this nonoxidative roasted product can avoid the generation of chlorine in hydrochloric acid leaching. Moreover, such a short holding time is scarce in traditional roasting. When the mixture was roasted by utilizing microwave heating, the sinter phenomenon of the roasted product was avoided at high-temperature roasting. Finally, the surface morphology of RE concentrate at different conditions was observed by scanning electron microscopy (SEM) analysis, which can be used to compare the specific differences of roasting methods. According to these results, this process is beneficial for the decomposition of RE concentrate in terms of Na2CO3 roasting and is helpful for improving the clean and green technology method of hydrometallurgy.

An Experimental Study on the Dielectric Properties of Rubber Materials.

According to specific formulas, the mixing of rubber samples occurs by two methods: open mixing and internal mixing. The effects of frequency, mixing process, carbon black (CB) content, zinc oxide (ZnO) content, and stearic acid (SA) content on the dielectric properties of rubber materials were studied. The results showed that the effects of the mixing process on the dielectric properties of the rubber samples cannot be ignored, and the appropriate mixing process should be selected when preparing the required rubber materials. The dielectric constant and loss factor of the rubber samples vary depending on the frequency. The dielectric constant had a peak and valley value, while the loss factor only had a peak. The dielectric constant and loss factor of rubber samples were significantly affected by the content of CB, ZnO, and SA. The peak frequency decreased with the increase in CB content, however, the dielectric constant increased with an increase in CB content. The higher the ZnO content, the lower the peak frequency. In addition, the dielectric constant and loss factor increased with an increase in ZnO content. The higher the SA content, the greater the peak frequency. In addition, the dielectric constant and loss factor decreased with an increase in SA content. It is hoped that the experimental results obtained can provide guidance for the study of the dielectric properties, microwave absorption properties, and microwave heating characteristics of rubber polymers.

Behaviour of Microwave-Heated Al4SiC4 at 2.45 GHz.

The ongoing development of high-temperature processes with the use of microwaves requires new microwave absorbers that are useful at these temperatures. In this study, we propose Al4SiC4 powders as important and efficient microwave absorbers. We investigated both the behavioural microwave heating and electrical permittivity characteristics of Al4SiC4 powders with various particle sizes at 2.45 GHz. The TE103 single-mode cavity indicated that Al4SiC4 powder samples yielded different heating behaviours and dielectric constants for each particle size compared with SiC. By microwave heating ∅50 mm × 5 mm disks of Al4SiC4 and SiC, we demonstrate that for specific sizes, Al4SiC4 can be heated at a higher temperature than SiC. Finally, the results of the two-dimensional two-colour thermometer show that an energy concentration appears at the interface of the microwave-heated Al4SiC4. These phenomena, which are inconsistent in individual physical property values, can be explained without contradicting microwave heating physics.

Strategy to scale up microwave synthesis with insight into the thermal and non-thermal effects from energy-based perspective

A strategy to industrialize microwave synthesis from the perspectives of process optimization, scale up and reactor design was devised for the synthesis of glycerol carbonate (GC). The strategy is based on the application of absorbed energy density (AED) and absorbed power density (APD) parameters. Here, the microwave synthesis was conducted using a constant-power heating mode. The mapping of microwave heating characteristics based on heating power, reaction temperature and AED was first performed for the microwave system. Then, the optimization of APD and AED was conducted and the intensive optimum conditions at 175 J/g AED and 1.9 W/g APD was used to scale up the microwave synthesis from 0.3 mol (27 g) to 3 mol (267 g). The resulting syntheses were highly reproducible and consistent. Based on APD and AED, the thermal and non-thermal microwave effects can be characterized and they are useful in the design of continuous-flow microwave reactor.

Microwave demulsification characteristics and product analysis of oily sludge

ABSTRACT Oily sludge is a kind of industrial hazardous waste that contains a lot of harmful substances. It has high water content and oil content. Demulsification is the key link for its resource utilization. Based on the unique heating characteristics of microwave heating, this paper uses microwave heating method to demulsify oily sludge in refineries. The effects of microwave power, temperature, stirring time, and additives on the demulsification of oily sludge are studied. The composition of each layer of the oil phase is analyzed to explore the possibility of oil reuse. The orthogonal test results show that the order of the influence of various factors on the dehydration rate of sludge is: temperature>power>stirring time; microwave power 700 W, temperature 55°C, and stirring time 10 min, the dehydration rate of sludge reaches the maximum value of 69.3%; adding sludge pyrolysis residue can significantly improve the efficiency of sludge dehydration; based on the results of microwave primary demulsification experiments, a deep demulsification and dehydration method for sludge is proposed. Based on the carbon number method, the relative contents of the liquid phase product were corrected, and it was found that the three layers of liquid phase product composition after demulsification and centrifugation were significantly different. The upper layer was mainly composed of gasoline (69.54%), and the middle layer was mainly composed of diesel (89.55%), the lower layer was mainly heavy oil components (75.87%).