Advances in Condensed Matter Physics

Advances in Condensed Matter Physics / 2021 / Article

Research Article | Open Access

Volume 2021 |Article ID 5544835 | https://doi.org/10.1155/2021/5544835

Wei Wei, Keyou Shi, Yupeng Xie, Shoufu Yu, Jiawei Li, Min Chen, Zengming Tang, Ailian Zhu, Qiucai Zhang, Yong Liu, "Microwave Vitrification of Uranium Tailings: Microstructure and Mechanical Property", Advances in Condensed Matter Physics, vol. 2021, Article ID 5544835, 7 pages, 2021. https://doi.org/10.1155/2021/5544835

Microwave Vitrification of Uranium Tailings: Microstructure and Mechanical Property

Academic Editor: Sefer Bora Lisesivdin
Received29 Jan 2021
Revised15 Mar 2021
Accepted27 Mar 2021
Published19 Apr 2021

Abstract

In this work, the dense glass matrix of uranium tailings was successfully fabricated via microwave sintering process with Na2CO3 as a sintering aid. The effects of Na2CO3 additive and sintering temperature on the microstructure and mechanical properties of as-prepared solids were systematically investigated. XRD results confirmed the vitrified forms can be achieved at 1200°C within 30 min with 20 wt.% Na2CO3 addition. Importantly, the Na2CO3 additive significantly reduced the firing temperature from 1500°C to 1200°C and promoted densification. FT-IR analysis demonstrated that the main characteristic peaks of the sintered samples were attributed to the vibration of Si-O-Si. Microstructural studies presented the homogeneous distribution of glass phases. The results of mechanical properties of the sintered forms show that bulk density and Vickers hardness increased with increasing Na2CO3 content as well as sintering temperature, and the highest bulk density (2.45 ± 0.01 g/cm3) and Vickers hardness (823 ± 25 HV) were obtained at the temperature of 1300°C with 20 wt.% Na2CO3 addition, the heating rate of 20°C/min, and the soaking time of 30 min. It implied that the combination of microwave sintering with the appropriate addition of Na2CO3 would provide an efficient method for the immobilization of radionuclides in uranium tailings.

1. Introduction

With the development of the nuclear industry, the demand for uranium resources increases dramatically, which resulted in a significant increase in uranium tailings [1]. The main pollution in uranium tailings is actinide nuclides and other radionuclides, whose disposal methods are necessary for preventing their migration into water or air [2, 3]. The migration of radioactive contamination into the surrounding would pollute the environment and pose a great threat to human health [1, 47]. Although the average grade of radionuclides content is slightly low, the potential harm to human health cannot be ignored [8, 9]. Therefore, it is urgent to find an efficient and reliable immobilization method to safely dispose of radionuclides in uranium tailings.

For the remediation of the uranium tailings pond, physical remediation, chemical remediation, microbial remediation, and phytoremediation have been considered [10]. The physical remediation [11, 12] method requires simple equipment at a low cost, which can be considered as efficient treatment. The chemical remediation [9, 13, 14] method has expensive costs for large numbers of chemical reagents, which could result in secondary pollution. The microbial remediation and phytoremediation method [1519] have low cost, while the strong biological selectivity limits their application. Meanwhile, the posttreatment of microorganisms and plants might also cause secondary pollution.

In situ vitrification (ISV) technology is commonly considered as an effective alternative to physical remediation to immobilize radionuclides in radioactively contaminated soil with high chemical durability [2023]. It used Joule heating to convert radioactively contaminated soil into the glass in which most radionuclides can be fixed [2426]. Conventional pressureless sintering, spark plasma sintering (SPS), and microwave sintering are the most commonly used methods employed for heating materials [27]. The traditional joule heating method requires a long heating time, and the uniformity of the solidification is hard to obtain [25]. SPS method could achieve high-density parts of materials at lower sintering temperatures, but it needs to be carried out by applying pressure [28]. Compared with other sintering methods, microwave sintering is a highly efficient heating method without pressure in which the material is heated by the dielectric loss of the material itself, rather than gradually transferring heat to the inner of the material by heating the surface of the material [29]. Therefore, a rapid vitrification method and good homogeneity of the cured matrix are the key challenges for the solidified uranium tailings.

In this study, microwave sintering has been employed to immobilize the radionuclides in uranium tailings. The Na2CO3 was introduced as a sintering additive to lower the sintering temperature and promote densification. Effects of Na2CO3 addition and sintering temperature on phase composition, microstructure, density, and Vickers hardness were investigated systematically. As a result of this work, high-density and hardness vitrified forms with homogeneously distributed amorphous glass phases were successfully fabricated. The outcomes provide a theoretical basis for the researchers to solidify radionuclides and have important guiding significance for the engineering application of the beach surface of the uranium tailings reservoir in the later stage.

2. Experimental

2.1. Preparation

The raw uranium tailings sample was collected from a uranium tailings pond in Hunan Province, China, ranging from −10 cm to −30 cm in depth, and the cladding was taken from the surface varying from 0 cm to −10 cm in depth. After being pretreated at 105°C for 24 h to remove the absorptive water, the raw uranium tailings and surface cladding were thoroughly mixed in an agate mortar in a 1 : 1 mass ratio and sifted through a 200-mesh sieve. The chemical compositions of the uranium tailings and surface cladding determined by X-ray fluorescence (XRF, Axios, Netherlands) are listed in Table 1.


CompoundSiO2Al2O3K2OFe2O3SO3Na2OFCaOMgOTiO2P2O5U3O8

Uranium tailings79.0210.113.561.961.930.830.830.540.370.270.210.0045
Surface cladding47.9731.682.7413.700.250.160.341.021.490.20

In order to reduce the sintering temperature, the sodium carbonate powder (AR grade) was proposed as a sintering aid. The doping gradient of Na2CO3 was set to 5%, from 0% to 20%. Each sample was weighted at 6.000 g, and then, the sample consisting of sodium carbonate, uranium tailings, and surface cladding was further mixed in a mortar using alcohol (AR grade) as a medium. Microwave sintering was carried out for all samples. During this process, the sample held in a 10 mL alumina crucible was placed on some SiC plates which were used to act as a preheater for auxiliary heating [3032]. Samples with varying Na2CO3 contents were sintered at 1000°C, 1100°C, 1200°C, and 1300°C, respectively, for 30 min in air atmosphere. The heating rate was set to 30°C/min, while the temperature was below 600°C. Then, the heating rate was set to 20°C/min in the temperature range of 600°C to 900°C. After 900°C, the target firing temperature was reached with the heating rate of 10°C/min. The sintered specimens were naturally cooled to room temperature. The entire microwave sintered process real-time power and temperature output varied with time are depicted in Figure 1. From the sintered sample in Figure 2, we can see that, without adding Na2CO3, the sample contracted with the increase of temperature. When the sintering temperature is 1000°C, a small amount of amorphous phase appears with the increase of Na2CO3 content. When the sintering temperature is increased to accelerate the material reaction, it can be seen that the samples of 1200°C and 1300°C with 20 wt.% Na2CO3 addition are completely glass.

2.2. Characterization

The phase structure of the sintered compacts was examined by X-ray diffraction (XRD, Ultima IV, Japan) using Cu-Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA in the 10°–90° (2θ) range with a scan speed of 5°/min. The detailed structure information was collected by Fourier transform infrared spectrometer (FT-IR, IRPrestige-21, American). The microstructure of the vitrified forms was observed by scanning electron microscopy (SEM, EVO 18, Germany). The bulk density of samples was measured using a high-precision solid-liquid dual-use density tester (DE-200T, China) by the Archimedes method with distilled water as the liquid medium. The Vickers hardness of the samples was obtained by using a standard microindentation device (HVS-1000AV, China) with 200 g load.

3. Result and Discussion

3.1. XRD Analysis

Figure 3(a) shows the XRD patterns of samples with a series of Na2CO3-doping contents (0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.%) calcined at 1200°C for 30 min. It can be seen that the main phases of pure uranium tailings cladding complexes are quartz and NaAlSi3O8. With the Na2CO3 dosage amount increasing, the intensity of diffraction peaks of SiO2 and NaAlSi3O8 gradually decreased. Furthermore, the Na2CO3 contents amount to 20 wt.%, and no obvious crystal diffraction peaks can be observed. It implies that the introduction of Na2CO3 promoted the glass phase transformation.

To further confirm the optimal sintering temperature, a series of samples dopped 20 wt.% were cured at different temperatures (1000°C, 1100°C, 1200°C, and 1300°C) for 30 min. Figure 3(b) presents the XRD pattern of the above samples. As can be seen, the glass with the presence of crystalline phases can be found when the firing temperatures are in the range of 1000°C to 1100°C. When the temperature is 1000°C, the two main phases of the samples are also quartz and slight NaAlSi3O8. As the temperature increased, the intensity of NaAlSi3O8-related diffraction peaks decreased rapidly, while that of quartz reduced gradually. At 1100°C, the phase of NaAlSi3O8 disappeared, and until 1200°C, no SiO2-related peaks were observed. It indicates that the samples with 20 wt.% Na2CO3-doping had been vitrified almost completely at 1200°C for 30 min.

3.2. FT-IR Analysis

To further detect the detailed structure of sintered samples, the FT-IR technique was utilized. Figure 4(a) exhibits the FT-IR spectra of the sintered samples with various contents of Na2CO3 obtained at 1200°C for 30 min. It can be intuitively seen that the main absorption bands of the solidified samples were concentrated in the range of 400–1800 cm−1 [33]. Generally, the absorption peaks near 474 cm−1 and 786 cm−1 were attributed to the bending vibration of Si-O-Si and the symmetrical telescopic vibration peak of Si-O bonds, respectively [34]. The absorption peak near 584 was due to the Al-O bond. The strong and wide absorption peaks in the range of 800 cm−1–1200 cm−1 were attributed to the antisymmetric stretching vibration of the Si-O-Si bond, which indicates that the glass network structure exists [35]. As the doping amount of Na2CO3 increased, the intensity of these absorption peaks increased, implying that the amorphous degree of sintered samples increased, which was in accordance with the XRD result. It is noteworthy that, with the dosage amount of Na2CO3 increasing, the absorption peaks near 1080 cm−1 became much flatter and shifted towards the lower wavenumber, indicating that the enhancement of the Si-O bond and amorphous degree. In addition, the weak absorption peaks at 1380 cm−1 and 1536 cm−1 were assigned to the bending vibration peak of –CH3 and stretching vibration and bending vibration peak of –OH bonds, respectively [1]. These peaks of –CH3 and –OH may be caused by the introduction of alcohol during the sample preparation just before characterization. On the other hand, Figure 4(b) demonstrates the FT-IR spectra of the samples 20 wt.% Na2CO3-doped sintered at the temperature range of 1000°C–1300°C. With the increase of temperature, the position of the main absorption peak of the solidified body has not changed, and the intensity of the absorption peak has decreased. This may be due to the gradual increase in the content of Na2CO3 decomposition into Na2O, which promotes the fracture of Si-O bonds and thus plays the role of melting.

3.3. SEM Analysis

Figure 5 shows the SEM and EDS of uranium tailings doped 20% Na2CO3 solidified by microwave sintering at 1000°C and 1200°C. It can be seen from Figure 5(a) that when the microwave sintering temperature is 1000°C, the surface of uranium tailings doped with Na2CO3 was relatively uniform and smooth, but there were some voids on the surface, indicating that the solidified body is not dense at this time. Figure 5(c) shows the macromorphology of samples with 20 wt.% Na2CO3-doped before and after sintering at 1200°C. We can see that the loose uranium tailings mixture was transformed into dense glass and the volumes of samples reduced significantly after sintering. Compared with Figure 5(a), we can see that the surface of the microwave-cured body at 1200°C was smoother and there was no obvious void. In consequence, combined with the analysis results of XRD, the cured body at this time was vitreous. On the other hand, as displayed in Figures 5(b) and 5(d). The contents of the main elements (O, Na, Mg, Al, Si, and K) distributed in these figures were almost the same, and the analysis results are consistent with the XRF analysis results of uranium tailings. To sum up, when the doping amount of Na2CO3 is 20 wt.% and the microwave sintering temperature is 1200°C, uranium tailings can be sintered into dense vitreous surface.

3.4. Density and Porosity Analysis

Density and porosity are also important parameters to judge the quality of materials. Figure 6 studies the effects of temperature and Na2CO3 addition on the density and porosity of microwave-cured uranium tailings. As displayed in Figure 6(a), when the doping amount of Na2CO3 was 20 wt.%, the density of the microwave-cured body gradually increases with the increase in temperature. When the temperature was 1300°C, the density was 2.45 g/cm3. In addition, the porosity gradually decreases with the increase in temperature. When the temperature was 1200°C, the porosity will basically be 0. As can be seen from Figure 6(b), when the temperature is 1200°C, the density of the microwave-curable body gradually increases with the increase of the doping amount of Na2CO3, while the porosity gradually decreases with the increase of the doping amount. When the doping amount of Na2CO3 is 20, the density is 2.40 g/cm3 and the porosity is 0. Combining the analysis results of XRD and SEM-EDS, it can be seen that when the microwave sintering temperature was 1200°C and the Na2CO3 doping amount was 20 wt.%, uranium tailings can be sintered into a dense glass solidified body. Therefore, considering both the excellent performance of the microwave solidified body and the energy saving, the temperature of 1200°C and the Na2CO3 doping amount of 20 can be used as the best sintering process.

3.5. Vickers Hardness Analysis

As demonstrated in Figure 7, the Vickers hardness of samples sintered at various temperatures versus Na2CO3 doping amount was plotted to investigate the effect of Na2CO3 doping on the mechanical properties of the solidified matrix. At least five Vickers indentations were performed on different positions of each polished sample surface. The final values of hardness were based on their average [32]. The Vickers hardness exhibited a positive correlation with the density, as we recognized [36, 37]. The Vickers hardness of the sintered samples increased with the sintering temperature increasing. The hardness of samples had a slowdown growth trend from 1200°C to 1300°C, consistent with the trend of the density. Moreover, the hardness reached a relatively high value of 781 HV and 823 HV for the samples with the addition of 20 wt.% Na2CO3 sintered at 1200°C and 1300°C, respectively. Based on the above analysis, it can be concluded that the uranium tailings mixture doped 20 wt.% Na2CO3 sintered at 1200°C showed excellent performance, regardless of the density or Vickers hardness.

4. Conclusion

The uranium tailings mixtures (1 : 1 mass ratio) were successfully vitrified by microwave sintering at 1200°C within 30 min with the addition of Na2CO3. The effects of Na2CO3 dopant on the microstructure and densification behavior of as-prepared solids were systematically studied. Using 20 wt.% Na2CO3 doping, the vitrification temperature can be reduced to 1200°C. Under this condition, amorphous glass phases were homogeneously distributed in the sintered samples. As the Na2CO3 content increased, the density of the sample sintered at 1200°C increased from 1.39 g/cm3 to 2.24 g/cm3 initially and then increased to 2.40 g/cm3 with elevated Na2CO3 content. The density increase tendency gradually became slow from 1200°C to 1300°C. The Vickers hardness exhibited a similar tendency with the density. The hardness reached a relatively high value of 781 HV and 823 HV for the samples with the addition of 20 wt.% Na2CO3 sintered at 1200°C and 1300°C, respectively. The uranium tailings mixture doped 20 wt.% Na2CO3 sintered at 1200°C showed excellent performance, regardless of the density or Vickers hardness. It can be concluded that the introduction of Na2CO3 was verified to be instrumental to reduce the sintering temperature and enhance the densification of the vitrified forms. It indicated that the combination of microwave sintering with the appropriate addition of Na2CO3 would provide an efficient method for the immobilization of radionuclides in uranium tailings.

Data Availability

The data used to support the findings of this study are included within the supplementary information file.

Additional Points

This paper is an experimental study on glass solidified uranium tailings based on microwave sintering. Uranium tailings, surface cladding, Na2CO3, and alcohol were mixed and stirred well in an agate mortar. The glass matrix with a smooth surface and good mechanical properties was prepared by a microwave muffle furnace.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Authors’ Contributions

First author Wei Wei and second author Keyou Shi contribute equally to the article.

Acknowledgments

The authors appreciate the financial supports from the Research and Development Program in Key Areas of Hunan Province (no. 2019SK2011), Hunan Provincial Innovation Foundation For Postgraduate (no. CX20190711), the National Natural Science Foundation of China (no. 11875164), the Decommissioning of Nuclear Facilities and Radioactive Waste Treatment Project of National Defense Science and Industry Administration (no. JSZL2019403C001), “Thirteenth Five-Year Plan” Technology Basic Research Project of National Defense Science and Industry (nos. JSZL2017403B008 and JSZL2018403B001), Equipment Pre-Research Project of the Equipment Development Department of the Central Military Commission (no. 32104060202), and Military and Civil Integration Project in Hunan Province (no. 200GJM001).

Supplementary Materials

The document “graphical abstract.doc” is a graphical abstract of the manuscript. “XRF.txt” is the original data file of Table 1. “Power and Temperature.txt” is the raw data file of Figure 1. “XRD.txt” is the raw data file of Figure 3. “FTIR.txt” is the raw data file of Figure 4. “SEM and EDS.doc” is the raw data file of Figure 5. “Density and porosity.txt” is the raw data file of Figure 6. “Vickers hardnessd.txt” is the raw data file of Figure 7. (Supplementary Materials)

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