Microwave Vitrification of Uranium Tailings: Microstructure and Mechanical Property

School of Resource & Environment and Safety Engineering, University of South China, Hengyang 421001, China School of Nuclear Science and Technology, University of South China, Hengyang 421001, China Hunan Province Engineering Technology Research Center of Uranium Tailings Treatment, Hengyang 421001, China Hunan Province Engineering Research Center of Radioactive Control Technology in Uranium Mining and Metallurgy, Hengyang 421001, China


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]. e 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]. e migration of radioactive contamination into the surrounding would pollute the environment and pose a great threat to human health [1,[4][5][6][7]. Although the average grade of radionuclides content is slightly low, the potential harm to human health cannot be ignored [8,9]. erefore, 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]. e physical remediation [11,12] method requires simple equipment at a low cost, which can be considered as efficient treatment. e chemical remediation [9,13,14] method has expensive costs for large numbers of chemical reagents, which could result in secondary pollution. e microbial remediation and phytoremediation method [15][16][17][18][19] 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 [20][21][22][23]. It used Joule heating to convert radioactively contaminated soil into the glass in which most radionuclides can be fixed [24][25][26]. Conventional pressureless sintering, spark plasma sintering (SPS), and microwave sintering are the most commonly used methods employed for heating materials [27]. e 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]. erefore, 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. e Na 2 CO 3 was introduced as a sintering additive to lower the sintering temperature and promote densification. Effects of Na 2 CO 3 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. e 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.

Preparation.
e 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. e chemical compositions of the uranium tailings and surface cladding determined by X-ray fluorescence (XRF, Axios, Netherlands) are listed in Table 1.
In order to reduce the sintering temperature, the sodium carbonate powder (AR grade) was proposed as a sintering aid. e doping gradient of Na 2 CO 3 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 [30][31][32]. Samples with varying Na 2 CO 3 contents were sintered at 1000°C, 1100°C, 1200°C, and 1300°C, respectively, for 30 min in air atmosphere. e heating rate was set to 30°C/min, while the temperature was below 600°C. en, 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. e sintered specimens were naturally cooled to room temperature. e entire microwave sintered process realtime 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 Na 2 CO 3 , 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 Na 2 CO 3 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.% Na 2 CO 3 addition are completely glass.

Characterization.
e phase structure of the sintered compacts was examined by X-ray diffraction (XRD, Ultima IV, Japan) using Cu-Kα radiation (λ �1.5406Å) at 40 kV and 40 mA in the 10°-90°(2θ) range with a scan speed of 5°/min. e detailed structure information was collected by Fourier transform infrared spectrometer (FT-IR, IRPrestige-21, American). e microstructure of the vitrified forms was observed by scanning electron microscopy (SEM, EVO 18,Germany). e 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. e Vickers hardness of the samples was obtained by using a standard microindentation device (HVS-1000AV, China) with 200 g load. Figure 3(a) shows the XRD patterns of samples with a series of Na 2 CO 3 -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 NaAlSi 3 O 8 . With the Na 2 CO 3 dosage amount increasing, the intensity of diffraction peaks of SiO 2 and NaAlSi 3 O 8 gradually decreased. Furthermore, the Na 2 CO 3 contents amount to 20 wt.%, and no obvious crystal diffraction peaks can be observed. It implies that the introduction of Na 2 CO 3 promoted the glass phase transformation.

XRD Analysis.
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 NaAlSi 3 O 8 . As the temperature increased, the intensity of NaAlSi 3 O 8 -related diffraction peaks decreased rapidly, while that of quartz reduced gradually. At 1100°C, the phase of NaAlSi 3 O 8 disappeared, and until 1200°C, no SiO 2 -related peaks were observed. It indicates that the samples with 20 wt.% Na 2 CO 3 -doping had been vitrified almost completely at 1200°C for 30 min.

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 Na 2 CO 3 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]. e absorption peak near 584 was due to the Al-O bond. e 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 Na 2 CO 3 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 Na 2 CO 3 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 -CH 3 and stretching vibration and bending vibration peak of -OH bonds, respectively [1]. ese peaks of -CH 3 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.% Na 2 CO 3 -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. is may be due to the gradual increase in the content of Na 2 CO 3 decomposition into Na 2 O, which promotes the fracture of Si-O bonds and thus plays the role of melting. Figure 5 shows the SEM and EDS of uranium tailings doped 20% Na 2 CO 3 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 Na 2 CO 3 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.% Na 2 CO 3 -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 microwavecured 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).

SEM Analysis.
e 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 Na 2 CO 3 is 20 wt.% and the microwave sintering temperature is 1200°C, uranium tailings can be sintered into dense vitreous surface.

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 Na 2 CO 3 addition on the density and porosity of microwave-cured uranium tailings. As displayed in Figure 6(a), when the doping amount of Na 2 CO 3 was 20 wt.%, the density of the microwavecured body gradually increases with the increase in temperature. When the temperature was 1300°C, the density was 2.45 g/ cm 3 . 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 microwavecurable body gradually increases with the increase of the doping amount of Na 2 CO 3 , while the porosity gradually decreases with the increase of the doping amount. When the doping amount of Na 2 CO 3 is 20, the density is 2.40 g/cm 3 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 Na 2 CO 3 doping amount was 20 wt.%, uranium tailings can be sintered into a dense glass solidified body. erefore, considering both the excellent performance of the microwave solidified body and the energy saving, the temperature of 1200°C and the Na 2 CO 3 doping amount of 20 can be used as the best sintering process. Figure 7, the Vickers hardness of samples sintered at various temperatures versus Na 2 CO 3 doping amount was plotted to investigate the effect of Na 2 CO 3 doping on the mechanical properties of the solidified matrix. At least five Vickers indentations were performed on different positions of each polished sample surface. e final values of hardness were based on their average [32]. e Vickers hardness exhibited a positive correlation with the density, as we recognized [36,37]. e Vickers hardness of the sintered samples increased with the sintering temperature increasing. e 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.% Na 2 CO 3 sintered at 1200°C and 1300°C, respectively. Based    Advances in Condensed Matter Physics on the above analysis, it can be concluded that the uranium tailings mixture doped 20 wt.% Na 2 CO 3 sintered at 1200°C showed excellent performance, regardless of the density or Vickers hardness.

Conclusion
e uranium tailings mixtures (1 : 1 mass ratio) were successfully vitrified by microwave sintering at 1200°C within 30 min with the addition of Na 2 CO 3. e effects of Na 2 CO 3 dopant on the microstructure and densification behavior of as-prepared solids were systematically studied. Using 20 wt.% Na 2 CO 3 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 Na 2 CO 3 content increased, the density of the sample sintered at 1200°C increased from 1.39 g/cm 3 to 2.24 g/cm 3 initially and then increased to 2.40 g/cm 3 with elevated Na 2 CO 3 content. e density increase tendency gradually became slow from 1200°C to 1300°C. e Vickers hardness exhibited a similar tendency with the density. e hardness reached a relatively high value of 781 HV and 823 HV for the samples with the addition of 20 wt.% Na 2 CO 3 sintered at 1200°C and 1300°C, respectively. e uranium tailings mixture doped 20 wt.% Na 2 CO 3 sintered at 1200°C showed excellent performance, regardless of the density or Vickers hardness. It can be concluded that the introduction of Na 2 CO 3 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 Na 2 CO 3 would provide an efficient method for the immobilization of radionuclides in uranium tailings.

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

Additional Points
is paper is an experimental study on glass solidified uranium tailings based on microwave sintering. Uranium tailings, surface cladding, Na 2 CO 3 , and alcohol were mixed and stirred well in an agate mortar. e glass matrix with a smooth surface and good mechanical properties was prepared by a microwave muffle furnace.

Conflicts of Interest
e 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.

Supplementary Materials
e 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)