Dielectric Properties and Microwave Heating of Molybdenite Concentrate at 2.45 G

Yonglin Jiang, Bingguo Liu,, Jinhui Peng and Libo Zhang

(1.National Local Joint Engineering Laboratory of Engineering Applications of Microwave Energy and Equipment Technology, Kunming 650093, China; 2.Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, Kunming 650093, China; 3.Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China)

Molybdenum trioxide is an important multiphase crystals[1]that occurs in various nanostructure[2]with a very wide range of uses in areas such as oxidation catalysis[3], photo degradation[4], gas sensors[5], battery electrodes, smart windows[6], ion conductors[7], imaging devices[8]and lubricants[9]. Traditionally, molybdenum trioxide is produced for industrial use through sublimation and calcination[10]after ammonia leaching of raw molybdenite concentrate. Molybdenite however, high energy consumption and short equipment life are major downsides of the sublimation[11]of molybdenite concentrate by conventional heating. Additionally, the hydrometallurgy process used during ammonia leaching is not efficient as it requires a longer production procedure.

Microwave technology is a highly efficient, low energy consuming, rapid and uniform, clean and easy-controlling heat source because of its two main advantages compared to traditional thermal processing. Firstly, the microwave energy can be directly generated from the intensive exercises among inner molecules under microwave irradiation. Secondly, heat diffusing from the interior to the surface of the material can enhance heat transfer effect[12]. In a dielectric material, there is a close relationship between the microwave absorption property and its dielectric permittivity (dielectric property)[13-14]. Many studies have been done on the basic applications of dielectric property.Maurya[15]described the dielectric behaviors of multiplayer ceramic configurations for a design of high-performance capacitors. Rujun Tang[16]investigated the dielectric permittivity of Sr3Co2Fe24O41Z-type hexaferrite to determine the performance of a magneto electronic device operating in the microwave region. Boreddy[17]reported the egg white powder with a temperature-dependent dielectric property can be treated by microwave in order to improve its functional properties.

Therefore, it is essential to understand the relational physical parameters in several corresponding change factors to acquire the actual process conditions.In this work, dielectric properties of molybdenite concentrate with different apparent densities were measured using the cavity perturbation method. The temperature rising behavior was investigated under the different conditions to provide a theoretical foundation to prepare high purity MOO3in fields of microwave.

1 Experimental

1.1 Experimental materials

The molybdenite concentrate with a particle size of 50-300 mesh was provided by Jiangsu Hengxing Tungsten & Molybdenum Co., Ltd. of China. Its chemical composition and XRD analysis are shown in Tab. 1 and Fig. 1 respectively.

Tab.1 Main chemical compositions of molybdenite concentrate

Tab.1 shows the elements molybdenum and sulphur occupy a large proportion in the molybdenite concentrate without any treatments. The main impurities are copper, oxides of calcium and lead, etc.

Fig.1 XRD spectrum of molybdenite concentrate sample

In Fig. 1, the sharp diffraction peaks of MoS2indicate that MoS2is the main morphology for this molybdenite concentrate sample. Some weak diffraction peaks corresponding to WS2as well as CdBrCl were also detected.

From Fig. 2, it can be seen that molybdenite concentrate particle has a layer structure and a wide grain size distribution ranging from several microns to more than 100 μm.

Fig.2 SEM micrograph of molybdenite concentrate sample

1.2 Dielectric property measurements

The resonant cavity perturbation method is widely adopted for microwave dielectric property measurements since it has a high measurement accuracy and simple requirements[18]for preparing the desired specimen and operating in measurement procedure.To take this measurement, a dielectric resonator was centered on a Teflon support positioned in the center of a resonant cylindrical cavity (Fig. 3). The cavity was excited using a coaxial probe where the electromagnetic wave was incident into the dielectric resonator at the desired frequency. The complex permittivity was determined from a calculation that was performed by a computer based on those reflection coefficients measured using a vector net-work analyzer.

1—vertical cylindrical cavity; 2—vector network analyser (E5071C); 3—laptopFig.3 Schematic of the developed system for the dielectric measurement

According to perturbation theory, the fundamental concept of the perturbation technique is that the appearance of a small piece of the dielectric specimen in the resonant cavity will cause a subtle change in the resonant frequency and a decrease in the quality factor of the cavity. This assures that the change of electromagnetic field,when the sample is introduced, will be small.The amended formulas derived for the measurements of dielectric permittivity and loss tangent are expressed as[19-20]

(1)

(2)

(3)

(4)

wherefcandfsare the resonant frequencies,VcandVsare the volumes of cavity and the sample,QcandQsare the measured quality factors of the cavity without and with a lossy sample inside the cavity respectively.Q′cis the modification fromQcbased on the fact that the quality factor of the cavity will increase for the specimen that is considered to be lossless (Q′c>Qc) because of the direct proportion in quality factor to dielectric constantε′.Qdis also a quality factor that calculates the value of the loss tangent by its reciprocal. The dielectric constant and loss factor were calculated based on the reflection coefficient and resonance frequency variation under the empty sensor and filled with the sample, respectively.

1.3 Temperature curves

The microwave reactor employed in the present study is made by the Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, and has the ability to alter the power intensity in the range of 0-3 kW at a frequency of 2.40 GHz (Fig.4). The temperature was measured through a K-type thermocouple placed inside a thermos well. The crucible that held the sample material is made of ceramic materials. Using the microwave power, sample mass and thickness required, temperature values were recorded per second by the temperature-display panel that was connected to the thermocouple. In the process of roasting, plenty of air was blown into the heating chamber.

Fig.4 Microwave high-temperature material treatment equipment (linking cooling device in operation)

2 Results and Discussion

2.1 Dielectric properties

The three dielectric parameters represent specific features of the dielectric material undergoing microwave irradiation.ε′ is the measure of the ability of a material to store electric energy(or the polarization ability of molecules under electric field).ε″ (also called loss factor) represents how much energy is lost during the interaction of the material with radiation. Loss tangent (tanδ) is a measure of how much energy coming from the material can be converted into heat under microwave stimulation at the specific frequency and particular temperature. The dielectric parameters of molybdenite concentrate as a fitting function of apparent density under required measurement condition were shown in Fig.5; Tab. 2 shows each regression curve equation and the corresponding correlation coefficient (R2).

Fig.5 Effect of apparent density on dielectric permittivity

From this plot, we can clearly observe the shifting trends of dielectric permittivity under room temperature. The three parameters are proportional to the apparent density of samples that increase monotonically with the increase inapparent densities within the experimental condition. The possible reason is that the air between material particles was continuously discharged with the increase of apparent density resulting in the significant change of data trend of dielectric characteristics (ε′,ε″ and tanδ). Additionally, clearance between material particles is reduced as the apparent density increases and the sufficient contacti between the particles lead to a greater contact proportion, which leads to a strengthening of space charge polarization, interfacial polarization and dipole polarization[21](these three types of polarization can be equivalent to the orientation polarization)generating in the microwave frequency range. Therefore, within a microwave penetration depth there is more microwave energy that can be absorbed by dielectric material, hence the material will be heated more efficient.

2.2 Microwave penetration depth

Penetration depth (Dp) is defined as the distance from the surface of the material to the inner part where microwave power is reduced to 1/e of its surface value.It is the measure of how deep a microwave radiation can penetrate into a material and a critical criterion for designing any microwave heating system. Penetration depth is affected by temperature, characteristics of the heating material and the frequency of microwave incident. Penetration depth can be calculated as[22]

(5)

whereλ0=c/frefers to the wavelength of microwave in free space,cis the velocity of light in free space andfis the frequency of microwave radiation. The microwave penetration depth of the molybdenite concentrate was obtained from the derivation relative to series of values of apparent density, and determined the heating uniformity for the material by microwave at 2.45 GHz (λ=12.24 cm). The effect of apparent density on microwave penetration depth is illustrated in Fig. 6.

Fig.6 Effect of apparent density on microwave penetration depth for molybdenite concentrate

As seen clearly in Fig. 6, with an increase in the apparent density,Dpdecreases inversely. Similarly, a quadratic polynomial regression written in Tab. 2 has been used to model the data which shows a high value of correlation coefficientR2(=0.999 3) that indicates the fitting equation can accurately predict the relationship between penetration depth and density. Through the foregoing analysis, it is known that with the increase in the apparent density, the material can absorb more microwave energy. The energy density at the surface of material is maximum when microwave is fed into a material. While the microwave energy would be converted into heat, the microwave field strength and power are constantly being attenuated as the microwave permeates into the interior of material. The attenuation state determines the material penetration. This finding correlates with the study done by Peng[23]. When microwave penetration depth is greater than the size of the specimen heated, the influence was negligible. Conversely, when the penetration depth is less than the size of specimen heated, microwave energy penetration will be limited resulting in anon-uniform heating of the specimen. On the other hand, as the apparent density increases, the dielectric loss factors of material may increase greatly. If the value of the dielectric loss is too high, the microwave energy will attenuate quickly on the surfaces of the material and will not be heated internally. However, a material with low dielectric loss and smaller particle size can be heated wholly and evenly because of a larger microwave penetration depth. Therefore, the investigation for the microwave penetration depth is necessary. Appropriate thickness of material or microwave intensity can be caught from the size of penetration depth of the material treated by microwave.

Tab.2 Regression equations for dielectric parameters and penetration (ρ: apparent density)

2.3 Temperature rising behavior

2.3.1Effect of sample mass on temperature rising behavior

Fig. 7 shows the time-dependent temperature plots for various sample masses of molybdenite concentrate at microwave power 0.5 kW and sample thickness 4.0 cm.

Fig.7 Temperature rising behaviors for molybdenite concentrate for different sample masses

From Fig.7, the temperature trends among different sample masses can be clearly compared in any same time intervals until the temperature is raised to 800 ℃. Since the oxidation roasting temperature was usually set between 550-650 ℃, it is possible to select the experimental temperature upper limit of 800 ℃ for a better preparation selection. The microwave power was set to a lower value of 0.5 kW in order to achieve a slower temperature increase rate so that changing trends would be observed clearly. The sample thickness was set as 4.0 cm to ensure uniform heating and precise measurement of temperature. Results show that the slope of the curve is different within the same time period, indicating that different materials possess different heating rates. The time required for temperature up to 800 ℃ increased when the materials mass increased. The average heating rates of molybdenite concentrate at the sample mass of 70 g, 90 g, 110 g, and 130 g were 97 ℃/min, 70.5 ℃/min, 55.4 ℃/min and 43.1 ℃/min respectively. Hence a smaller mass of sample indicates a faster apparent heating rate. In fact in a microwave field, the effect of the specimen mass of molybdenite concentrate on the heating rate can be calculated as[24]

(6)

whereTis the material heating temperature,T0is the material initial temperature,Eis the electric field strength,τis the time,mis the mass of material,cpis the specific heat capacity of material,fis the frequency of microwave andε0is the dielectric permittivity in a vacuum.

As shown in Eq.(6), there is an inversely proportional relationship between the heating rate and the specimen mass.The greater the concentrate mass, the smaller the heating rate is. This correlates with the experimental testing results. This is because the increase of specimen mass can lead to a decrease of microwave power density and a larger contact area between the sample particles, thus resulting in increasing heat dissipation to the external environment. Furthermore, when apparent density is constant, a larger amount of molybdenite concentrate indicates a thicker sample and the need for more microwave power. Consequently with increasing specimen mass, the power density of the specimen decreases, which leads to an insufficient microwave energy supply, and a slower increase rate of temperature during the microwave heating.

2.3.2Effect of thickness on temperature rising behavior

Fig. 8 shows how temperature varies with thickness of concentrate sample filled in the corundum cylinder crucible when the sample mass and microwave power were set to 100 g and 0.5 kW respectively.

Fig.8 Temperature rising behaviors of molybdenite concentrate at different specimen thickness

As expected, the time required to increase temperature up to 800 ℃ became longer with the increase in thickness of molybdenite concentrate. This means that heating rate was gradually reduced. In addition, in Fig.8, the intersection and displacement of curves may be caused by non-uniformly heating due to the presence of large particles and lumps in the material.According to the transmission line theory[25], the reflection loss (RL) based on the complex permittivity and permeability can be calculated. A larger specimen thickness will match a smaller RL peak[26]that represents a larger value of RL, which thus indicates a lower absorption rate on microwave energy for the test samples (when other variables are constant). On the other hand, as the sample thickness increases, the microwave penetration resistance increases as well. Moreover, when the microwave is gradually penetrated into the interior of material, the energy density will appear to exponentially decay[27]along with the diffusion depth. Simultaneously, part of the microwave energy is absorbed by the material and transformed into other forms of energy. Thus in a thicker sample, delivering the same heat needs more microwave energy(microwave power). In summary, within the same microwave power, a thicker specimen can only absorb less microwave power thus exhibiting a relatively slow heating rate.

2.3.3Effect of microwave power on temperature rising behavior

The temperature rising behavior of molybdenite concentrate pertaining to microwave power is shown in Fig. 9.

Fig.9 Temperature rising behaviors for molybdenite concentrate under different microwave power

From Fig. 9, it is clear that the heating rates of molybdenite concentrate increases with the increase in microwave power. The average heating rates successively increase from 43.1 ℃/min to 96.9 ℃/min against an increase in microwave power. The increasing microwave power infers that increasing electric field strength when other conditions were unchanged. Microwaves could penetrate the deeper interior of the material and thus would result in stronger heating uniformity. Because the molybdenite concentrate specimen absorbed more microwave energy with the increase inE, this results in the increase of temperatures. Therefore, increasing the microwave power properly can reduce the heating time and improve the apparent average heating rate of molybdenite concentrate, as same as the heating situation of pyrite, hematite, galena, rutile and coal, etc. Chen[28]also proved this experimentally.

2.4 Characterization of products

Fig.10 SEM images of products of molybdenite concentrate samples boasted in air atmosphere

After the molybdenite concentrate sample was roasted in the field of microwave for five minutes at 800 ℃, the products obtained were examined by scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). As seen in Fig. 10, the products present a compact plate-shape structure with a smooth surface, which correlates with the results of molybdenum trioxide nanobelts from a preparation by thermal evaporation technique[29]. The XRD pattern of the plate-shape crystal is shown in Fig.11. The positions of strong diffraction peaks at (0 2 0), (1 1 0), (0 4 0), (0 6 0) and (0 10 0) planes are in good agreement with those reported in literature for the orthorhombicα-MoO3crystalline phase. Nevertheless, some weak peaks of impurities such as unseperated BaSi4O9, unreacted MoS2and the MoO2, which formed as an intermediate product, are also detected. The occurrence of these constituents may be explained by a limited local supply of O2during the oxidation process, or an unavailable adequate reaction time. The X-ray fluorescence (XRF) results of roasting products are presented in Tab.3. MoO3accounts for a major proportion (>90%). The presence of other elements in the product produced infers that this presented work still needs to be improved in order to achieve MoO3product in a better quality. For example, considering the melting point of MoO3crystals of 795, the production for high-purity MoO3would be feasible through sublimating the liquefied MoO3crystals at a higher temperature and then collecting them using a condensation process. Since the melting points of those impurities are much higher they will remain in the solid phase while the purified MoO3is collected. This perspective can be illustrated through further research. In any case, the findings have provided a solid foundation to produce high purity molybdenum trioxide by using microwave energy.

Fig.11 X-ray diffractogram of products after roasting of molybdenite concentrate

Tab.3 XRF test results of products obtained from microwave roasting for the concentrate sample at corresponding conditions %

3 Conclusion

The dielectric properties and temperature rising behavior were investigated to illustrate the feasibility of preparing high-purity MOO3from molybdenite concentrate through microwave heating. The results show that the dielectric constant, dielectric loss, and loss tangent are proportional to the apparent density of molybdenite concentrate in the range 0.9-1.4 g/cm3and the apparent heating rate of the molybdenite concentrate increases with the increase in microwave power and decreases with the increase in the sample mass and thickness. The temperature of the samples reach approximately 800 ℃ after microwave treatment 100 g of the sample for 5 min at 0.5 kW. The products obtained from microwave roasting for molybdenite concentrate at air atmosphere were examined by XRD, SEM and XRF characterization techniques, Thed MOO3crystals in good quality have been confirmed to be prepared. Molybdenite concentrate can be heated up to a high temperature by microwave, it is feasible to prepare high purity MOO3from molybdenite concentrate via microwave energy technology.

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