2017 Volume 57 Issue 12 Pages 2107-2114
The lack of phase diagrams and other related thermodynamic information for the silicate slag system with additions Nb and REE seriously restrict the comprehensive utilization of REE-Nb-Fe ore deposit resources in China. In this study, the equilibrium phase relations in the CaO–SiO2–Nb2O5–La2O3 quarternary system at 1273 K and 1473 K were investigated experimentally using the high-temperature equilibrium experiment followed by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). Subsolidus phase relations in the CaO–SiO2–Nb2O5–La2O3 quarternary system and La2O3–SiO2–Nb2O5 ternary system were determined and presented in the form of independent tetrahedron regions and triangle regions, respectively, according to the Gibbs Phase Rule, the isothermal section for 5 wt%La2O3 and 10 wt%La2O3 at 1273 K were also constructed respectively.
Bayan Obo REE-Nb-Fe ore deposit in China’s Inner Mongolia is a super large complex ore resource. However, the beneficiation of valuable elements is very difficult because of the multiple-element symbiotic specificity of the REE-Nb-Fe ore deposit. Until now, so much rare earth and niobium resources are wasted as tailings in the process of ore dressing, which led to a huge waste of resources.1) For these tailings, many processes and techniques have been proposed to enriching rare earth resources and smelting metal niobium. However, the uncertainty of thermodynamic property especially the phase diagram data for the tailings which contain rare earth and niobium severely restrict the development of above mentioned processes.2) The CaO–SiO2–Nb2O5 ternary system phase diagram investigated by Wilkins A L3) is a relatively mature one up to now, but REE as important component for the tailings was not contained in this ternary system. Therefore the phase relations for slag system with REE and Nb remain unclear, it is necessary to update the fundamental thermodynamic information for the development and application of treating tailings.
The high-temperature equilibrium experiment followed by quenching technique were widely used in the investigation of the silicate system phase diagram as the most scientific and common method.4,5,6,7,8,9,10,11,12) In the present work, the equilibrium phase relations in the CaO–SiO2–Nb2O5–La2O3 system within the specified region were experimentally determined using the high-temperature equilibrium experiment followed by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscope analysis (EDS). Based on the results, the subsolidus phase relations in the CaO–SiO2–Nb2O5–La2O3 and La2O3–SiO2–Nb2O5 phase diagram system at 1273 K were determined and presented in the form of independent tetrahedron regions and triangle regions. Isothermal section for 5 wt%La2O3 and 10 wt%La2O3 at 1273 K were also constructed respectively in the CaO–SiO2–Nb2O5–La2O3 system.
Reagent grade oxides powders of CaO (99.99 mass fraction pure), SiO2 (99.99 mass fraction pure), Nb2O5 (99.99 mass fraction pure), and La2O3 (99.99 mass fraction pure) were used to prepare the slags, which were calcined at 1273 K for 4 hours to evaporate the moisture and impurities, then carefully weighted, fully mixed and pre-melted in air atmosphere using MoSi2 furnace. The mixtures were placed inside platinum crucibles which placed inside the furnace at 1873 K for 6 hours to completely homogenize the slags.6) The samples were then quenched into ice-water, dried, crushed, and grinded to 200 meshes for further utilization. The furnace temperature was monitored by a B-type thermocouple placed next to the samples with an overall temperature accuracy estimated to be ±2 K.
The pre-melt slag was analyzed by X-Ray diffraction (XRD) and scanning electron microscope (SEM) to ensure the result of pre-melted, and the energy dispersive spectrometer (EDS) result was used as the initial composition of pre-melted slag, as listed in Table 1. Quenching by ice-water ensured that the quenching pre-melted slags showed glassy phase, as shown in Figs. 1 and 2. The pre-melt results showed that 1873 K achieved the homogenization of slag samples. The measured compositions of pre-melt slag are projected on the CaO-SiO2-Nb2O5-10wt%La2O3 pseudo-ternary phase diagram, as shown in Fig. 3.
| Slag No. | CaO | SiO2 | Nb2O5 | La2O3 | |
|---|---|---|---|---|---|
| 1# | Designed | 4.82% | 3.32% | 81.86% | 10.00% |
| EDS | 4.23% | 3.42% | 81.06% | 11.29% | |
| 2# | Designed | 29.11% | 38.89% | 22.00% | 10.00% |
| EDS | 26.89% | 38.53% | 22.42% | 12.16% | |
| 3# | Designed | 20.55% | 12.03% | 57.42% | 10.00% |
| EDS | 20.08% | 12.87% | 56.97% | 10.08% | |
| 4# | Designed | 31.54% | 20.46% | 38.00% | 10.00% |
| EDS | 30.74% | 20.94% | 38.55% | 9.77% | |
| 5# | Designed | 15.78% | 22.22% | 52.00% | 10.00% |
| EDS | 15.38% | 19.72% | 53.48% | 11.42% | |
| 6# | Designed | 28.92% | 25.08% | 36.00% | 10.00% |
| EDS | 28.36% | 25.04% | 36.27% | 10.34% |
mass percent
XRD result of typical pre-melt slag.
Backscattered electron image of typical pre-melt slag.
The projection of pre-melt compositions on CaO-SiO2-Nb2O5-10 wt%La2O3 phase diagram.
The MoSi2 furnace used for the pre-melt process was also used for the equilibrium experiment. The pre-melt slag (1.5 g) was placed in the platinum crucible and placed into the hot zone of the MoSi2 furnace. The curve of controlled temperature is shown in Fig. 4. In order to avoid non-equilibrium phase exists during the experiment, the sample was heated to 1873 K to ensure that it completely melted again, and then cooling to the equilibrium temperature, this process can improve the accuracy of the experiment. The equilibrium temperature involved in the experiment was 1273 K and 1473 K, the equilibrium time lasted 24 hours based on experiences reported by previous authors,13,14,15) repeat experiments with longer equilibrium time were performed for some samples to check whether the equilibrium was achieved. After the equilibrium, the sample was rapidly taken out from the furnace and quenched to 273 K by ice-water, the quenching process was completed in 2 seconds to ensure that all samples maintained the high temperature equilibrium phase composition. Quenched samples were then dried and embedded in epoxy resin and polished for analysis. Scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to identify and analyze the composition of equilibrium phase in each sample.
Curve of controlled temperature in high-temperature equilibrium experiment.
The microstructure and compositions of the equilibrium phases at 1273 K are shown in Table 2 and Fig. 5, respectively. The compositions of the equilibrium phase are the average values calculated from six different analysis points in the samples. In Fig. 5(a), four phases were detected by SEM for sample 1#, based on the XRD result of 1#, it is easy to confirm that the black phase is SiO2, the deep gray phase is CaO·Nb2O5, the white phase is La2O3·6Nb2O5 and the light gray phase is Nb2O5. Base on the EDS results, element calcium was found in the composition of La2O3·6Nb2O5, and element lanthanum was found in the composition of CaO·Nb2O5 and 2CaO·Nb2O5. It was also found by Frolov,18) that element calcium can dissolved in La2O3·6Nb2O5 and element lanthanum can dissolved in CaO·Nb2O5 and 2CaO·Nb2O5 within a certain composition range. In Fig. 5(b), four phases were detected by SEM for sample 2#, we confirm that the light gray with strip shape is CaO·Nb2O5, the black phase is SiO2, the white phase with block shape is La2O3·Nb2O5 and the deep gray phase is CaO·SiO2. The equilibrium phases determined by the XRD result of sample 2#, as shown in Fig. 6, were consistent with the result of SEM. In Fig. 5(c), four phases were detected by SEM for sample 3#, based on the XRD result, we confirm that the black phase is CaO·SiO2, the gray phase with strip shape is CaO·Nb2O5 and 2CaO·Nb2O5, the white phase is La2O3·Nb2O5. There is no difference in morphology and contrast between CaO·Nb2O5 and 2CaO·Nb2O5 in SEM microphotographs. In Fig. 5(d), four phases were detected by SEM for sample 4#, based on the XRD result of 4#, it is easy to confirm that the gray phase with strip shape is 2CaO·Nb2O5, the black phase is CaO·SiO2, the white phase with block shape is La2O3·Nb2O5 and the deep gray phase is 10CaO·6SiO2·Nb2O5. The CaO–SiO2–Nb2O5 ternary compound, known as “Niocalite”, was first discovered by Nickel.16) Element lanthanum was detected in the 10CaO·6SiO2·Nb2O5 by EDS in present work.
| Slag No. | Contrast in the SEM Microphotograph | Phase | Composition, Mass percent | |||
|---|---|---|---|---|---|---|
| CaO | SiO2 | Nb2O5 | La2O3 | |||
| 1# | black | SiO2 | – | 100% | – | – |
| deep gray | CaO·Nb2O5 | 9.87% | – | 81.92% | 8.20% | |
| white | La2O3·6Nb2O5 | 1.18% | – | 84.19% | 14.63% | |
| light gray | Nb2O5 | – | – | 100.00% | – | |
| 2# | light gray | CaO·Nb2O5 | 13.96% | – | 86.04% | – |
| deep gray | CaO·SiO2 | 41.79% | 58.21% | – | – | |
| black | SiO2 | – | 100.00% | – | – | |
| white | La2O3·Nb2O5 | 5.41% | – | 42.28% | 52.31% | |
| 3# | black | CaO·SiO2 | 48.82% | 51.18% | – | – |
| gray | CaO·Nb2O5 | 14.29% | – | 82.81% | 2.90% | |
| gray | 2CaO·Nb2O5 | 21.09% | – | 70.91% | 8.01% | |
| white | La2O3·Nb2O5 | 2.84% | – | 45.46% | 51.69% | |
| 4# | gray | 2CaO·Nb2O5 | 22.96% | – | 72.10% | 4.94% |
| black | CaO·SiO2 | 46.68% | 53.32% | – | – | |
| white | La2O3·Nb2O5 | 7.22% | – | 43.81% | 48.97% | |
| deep gray | 10CaO·6SiO2·Nb2O5 | 41.44% | 29.99% | 25.30% | 3.27% | |
SEM microphotographs of equilibrium phases at 1273 K.
XRD result of sample 2# at 1273 K.
Under the constant pressure conditions, the Gibbs Phase Rule for the system can be expressed as formula (1).
| (1) |
The terms F, C and P in formula (1) are the number of degrees of freedom, components and equilibrium phases,17) respectively. As the constant pressure condition, the completely crystallized temperature for the system is Ts. As shown in Table 3, when T is less than Ts and the solid phase decomposition was not appeared in the system, the degree of freedom could be as the uniform function of the temperature, which means the number of components and the number of equilibrium phases are same. It can be seen that four subsolidus equilibrium phases will co-exist in the CaO–SiO2–Nb2O5–La2O3 quarterary system at T<Ts and the four phases should constitute an independent tetrahedron region in three-dimensional composition space. Three equilibrium solid phases will co-exist in the La2O3–SiO2–Nb2O5 system at T<T’s and the three phases should constitute an independent triangle region in two-dimensional composition plane.
| C | Single-Component | Binary | Ternary | Quarternary | n- Component |
|---|---|---|---|---|---|
| Phase Rule | F=1−P+1 | F=2−P+1 | F=3−P+1 | F=4−P+1 | F=n−P+1 |
| P | P=1 | P=2 | P=3 | P=4 | P=n |
According to the high-temperature equilibrium experiment results and the compatibility of phase diagram, subsolidus phase relations in the CaO–SiO2–Nb2O5–La2O3 quarternary system were determined and presented in six independent tetrahedron regions as shown in Table 4 and Fig. 7. The tetrahedrons ⑤ and ⑥ were determined by the tetrahedron ① and ② according to the existing phase information.18,19,20,21)
| Tetrahedron No. | Four solid phases involved in each tetrahedron |
|---|---|
| ① | SiO2–CaO·Nb2O5–Nb2O5–La2O3·6Nb2O5 |
| ② | La2O3·Nb2O5–CaO·SiO2–SiO2–CaO·Nb2O5 |
| ③ | La2O3·Nb2O5–CaO·SiO2–CaO·Nb2O5–2CaO·Nb2O5 |
| ④ | La2O3·Nb2O5-CaO·SiO2-10CaO·6SiO2·Nb2O5-2CaO·Nb2O5 |
| ⑤ | La2O3·3Nb2O5–CaO·Nb2O5–SiO2–La2O3·Nb2O5 |
| ⑥ | La2O3·3Nb2O5–CaO·Nb2O5–SiO2–La2O3·6Nb2O5 |
The positions of the tetrahedrons region in the spatial quaternary phase diagram.
Due to the consistency between the CaO–SiO2–Nb2O5–La2O3 quarternary phase diagram and its sub-system phase diagram, related independent triangle regions in CaO–SiO2–Nb2O5, CaO–La2O3–Nb2O5 and La2O3–SiO2–Nb2O5 ternary system at 1273 K were constructed, respectively, at the same time. The equilibrium sections of CaO–SiO2–Nb2O5 and CaO–La2O3–Nb2O5, as shown in Figs. 8(a) 8(b), are consistent with the research of Wilkins A L2) and Frolov A M,18) respectively. The equilibrium section in special region of La2O3–SiO2–Nb2O5 ternary system, as shown in Fig. 8(c) is unreported in the previous study. Solid phase equilibrium relations in La2O3–SiO2–Nb2O5 system within specific composition range could be confirmed by this section.
The positions of the triangle region in the ternary sub-system of CaO–SiO2–Nb2O5–La2O3.
The microstructure and compositions of the equilibration phases at 1473 K are shown in Table 5 and Fig. 9, respectively.
| Slag No. | Contrast in the SEM Microphotograph | Phase | Composition, Mass percent | |||
|---|---|---|---|---|---|---|
| CaO | SiO2 | Nb2O5 | La2O3 | |||
| 3# | light gray | Liquid | 25.04% | 27.71% | 30.62% | 16.62% |
| gray | 2CaO·Nb2O5 | 21.54% | – | 70.39% | 8.07% | |
| gray | CaO·Nb2O5 | 14.71% | – | 85.29% | – | |
| white | La2O3·Nb2O5 | 3.64% | – | 45.04% | 51.32% | |
| 4# | light gray | Liquid | 23.92% | 27.25% | 31.99% | 16.84% |
| gray | 2CaO·Nb2O5 | 24.03% | – | 70.61% | 5.36% | |
| white | La2O3·Nb2O5 | 2.23% | – | 44.02% | 53.75% | |
| black | 10CaO·6SiO2·Nb2O5 | 42.49% | 29.37% | 25.41% | 2.74% | |
| 5# | deep gray | Liquid | 20.38% | 35.23% | 25.82% | 18.57% |
| black | SiO2 | – | 100.00% | – | – | |
| gray | CaO·Nb2O5 | 15.20% | – | 84.80% | – | |
| white | La2O3·Nb2O5 | 5.57% | – | 42.79% | 51.64% | |
| 6# | light gray | Liquid | 23.37% | 27.77% | 31.42% | 17.44% |
| gray | 2CaO·Nb2O5 | 23.54% | – | 70.82% | 5.64% | |
| white | La2O3·Nb2O5 | 4.39% | – | 42.67% | 52.94% | |
| deep gray | 10CaO·6SiO2·Nb2O5 | 42.30% | 28.21% | 26.35% | 3.15% | |
SEM microphotographs of equilibrium phases at 1473 K.
In Fig. 9(a), four phases were detected by SEM for sample 3#, based on the XRD result of 3#, it is easy to confirm that the gray phase with block shape is CaO·Nb2O5, the gray phase with strip shape is 2CaO·Nb2O5, the white phase with block shape is La2O3·Nb2O5 and the matrix light gray phase is liquid. As for sample 4# in Fig. 9(b), the white phase is La2O3·Nb2O5, the black phase is 10CaO·6SiO2·Nb2O5, the gray phase with strip shape is 2CaO·Nb2O5 and the matrix light phase is liquid. Element lanthanum was still detected by EDS in 10CaO·6SiO2·Nb2O5 at 1473 K. As for sample 5# in Fig. 9(c), the white phase with block shape is La2O3·Nb2O5, the black phase is SiO2, the gray phase is CaO·Nb2O5 and the matrix deep gray phase is liquid. And the XRD result of sample 5# is shown in Fig. 10. As for sample 6# in Fig. 9(d), the white phase is La2O3·Nb2O5, the deep gray phase is 10CaO·6SiO2·Nb2O5, the gray phase is 2CaO·Nb2O5 and the matrix light gray phase is liquid.
XRD result of sample 5# at 1473 K.
The equilibrium solid phases in the slag samples at 1473 K are included in the equilibrium solid phases in previous independent tetrahedron regions determined at 1273 K. Because of the liquid exists at 1473 K disappears at 1273 K, it can be confirmed that the completely crystallization temperature belong to related independent tetrahedron region is between 1273 K and 1473 K.
3.4. Isothermal Section for 5 wt%La2O3 and 10 wt%La2O3After six independent tetrahedron regions, as shown in Fig. 7, in the CaO–SiO2–Nb2O5–La2O3 system were determined. Equilibrium sections for 5 wt% and 10 wt% La2O3 for afore mentioned system at 1273 K were constructed, respectively, as shown in Figs. 11(a) and 12(a). And their relative positional relations in the three-dimensional quarternary phase diagram were as shown in Figs. 11(b) and 12(b), respectively. Solid phase equilibrium relations in the CaO–SiO2–La2O3–Nb2O5 system within specific composition range could be confirmed by these sections.
Equilibrium section of CaO-SiO2-Nb2O5-5 wt%La2O3 system.
Equilibrium section of CaO-SiO2-Nb2O5-10 wt%La2O3 system.
High-temperature equilibrium and quench technique have been applied to investigate equilibrium phase relations in the CaO–SiO2–Nb2O5–La2O3 system at 1273 K and 1473 K According to the Gibbs Phase Rule, six independent tetrahedron regions in CaO–SiO2–Nb2O5–La2O3 system and four triangle regions in La2O3–SiO2–Nb2O5 system were determined. Equilibrium sections for 5 wt% and 10 wt% La2O3 in the CaO–SiO2–Nb2O5–La2O3 system at 1273 K were also constructed, respectively, which could be used to confirm the solid phase equilibrium relations for the specified region in the CaO–SiO2–Nb2O5–La2O3 system.
This work was financially supported by National Key R&D Program of China (No. 2017YFC0805105), the National Natural Science Foundation of China. (No. 51304042) and the Fundamental Research Funds for the Central Universities China (N 162506002).
