Subsolidus Phase Relations in the CaO–SiO2–Nb2O5–La2O3

ISIJ International, Vol. 57 (2017), No. 12

© 2017 ISIJ2107

ISIJ International, Vol. 57 (2017), No. 12, pp. 2107–2114

* Corresponding author: E-mail: [email protected]: http://dx.doi.org/10.2355/isijinternational.ISIJINT-2017-324

1. Introduction

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.4–12) In the present work, the equilib-rium phase relations in the CaO–SiO2–Nb2O5–La2O3 system within the specified region were experimentally determined using the high-temperature equilibrium experiment followed

Subsolidus Phase Relations in the CaO–SiO2–Nb2O5–La2O3 Quarternary System at 1 273 K

Jiyu QIU1,2) and Chengjun LIU1,2)*

1) Key Laboratory for Ecological Metallurgy of Multimetallic Ores (Ministry of Education), Northeastern University, Shenyang, 110819 P.R. China. 2) Northeastern University School of Metallurgy, Shenyang, 110004 P.R. China.

(Received on June 6, 2017; accepted on August 4, 2017)

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 1 273 K and 1 473 K were investigated experimentally using the high-temperature equilibrium experiment followed by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy disper-sive 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 tetra-hedron regions and triangle regions, respectively, according to the Gibbs Phase Rule, the isothermal sec-tion for 5 wt%La2O3 and 10 wt%La2O3 at 1 273 K were also constructed respectively.

KEY WORDS: phase diagram; high-temperature equilibrium experiment; CaO–SiO2–Nb2O5–La2O3; SiO2–Nb2O5–La2O3; subsolidus phase relation.

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 1 273 K were determined and presented in the form of independent tetrahedron regions and tri-angle regions. Isothermal section for 5 wt%La2O3 and 10 wt%La2O3 at 1 273 K were also constructed respectively in the CaO–SiO2–Nb2O5–La2O3 system.

2. Experiment

2.1. Sample PreparationReagent grade oxides powders of CaO (99.99 mass frac-

tion 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 1 273 K for 4 hours to evaporate the moisture and impuri-ties, 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 1 873 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 utili-zation. 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 spec-trometer (EDS) result was used as the initial composition of pre-melted slag, as listed in Table 1. Quenching by ice-


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water ensured that the quenching pre-melted slags showed glassy phase, as shown in Figs. 1 and 2. The pre-melt results showed that 1 873 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.

2.2. Equilibration ExperimentsThe 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 1 873 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 1 273 K and 1 473 K, the equilibrium time lasted 24 hours based on experiences reported by previous authors,13–15) repeat experiments with longer equilibrium time were per-formed 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.

Table 1. The Comparison of designed and EDS measured compo-sitions of pre-melt slags.

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

Fig. 1. XRD result of typical pre-melt slag.

Fig. 2. Backscattered electron image of typical pre-melt slag.

Fig. 3. The projection of pre-melt compositions on CaO-SiO2-Nb2O5-10 wt%La2O3 phase diagram.

Fig. 4. Curve of controlled temperature in high-temperature equi-librium experiment.


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3. Results and Discussion

3.1. Equilibrium Phase Relations at 1 273 KThe microstructure and compositions of the equilibrium

phases at 1 273 K are shown in Table 2 and Fig. 5, respec-tively. 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

Table 2. Experimental determined phase compositions of equilibrium phase at 1 273 K in the CaO–SiO2–Nb2O5–La2O3 system.

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%

Fig. 5. SEM microphotographs of equilibrium phases at 1 273 K.


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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 consis-tent 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 morphol-ogy 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 discov-ered by Nickel.16) Element lanthanum was detected in the 10CaO·6SiO2·Nb2O5 by EDS in present work.

3.2. Determination of Subsolidus Phase RelationsUnder the constant pressure conditions, the Gibbs Phase

Rule for the system can be expressed as formula (1).

F C P= − +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 com-pletely 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

Fig. 6. XRD result of sample 2# at 1 273 K.

Table 3. The relationship between C and P in silicate system with constant pressure and T< Ts.

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

Table 4. Subsolidus phase relations in the CaO–SiO2–Nb2O5–La2O3 quarternary system.

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

of freedom could be as the uniform function of the tem-perature, 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 indepen-dent triangle region in two-dimensional composition plane.

According to the high-temperature equilibrium experi-ment 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.18–21)

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 1 273 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.

3.3. Equilibrium Phase Relations at 1 473 KThe microstructure and compositions of the equilibra-

tion phases at 1 473 K are shown in Table 5 and Fig. 9, respectively.

In Fig. 9(a), four phases were detected by SEM for sam-


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Fig. 7. The positions of the tetrahedrons region in the spatial quaternary phase diagram.

Fig. 8. The positions of the triangle region in the ternary sub-system of CaO–SiO2–Nb2O5–La2O3.


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Table 5. Experimental determined phase compositions of equilibrium phase at 1 473 K in CaO–SiO2–Nb2O5–La2O3 system.

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%


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%


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%

Fig. 9. SEM microphotographs of equilibrium phases at 1 473 K.

ple 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


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detected by EDS in 10CaO·6SiO2·Nb2O5 at 1 473 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.

The equilibrium solid phases in the slag samples at 1 473 K are included in the equilibrium solid phases in previous independent tetrahedron regions determined at 1 273 K. Because of the liquid exists at 1 473 K disappears at 1 273 K, it can be confirmed that the completely crystallization temperature belong to related independent tetrahedron region is between 1 273 K and 1 473 K.

Fig. 10. XRD result of sample 5# at 1 473 K.

Fig. 11. Equilibrium section of CaO-SiO2-Nb2O5-5 wt%La2O3 system.

Fig. 12. Equilibrium section of CaO-SiO2-Nb2O5-10 wt%La2O3 system.


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3.4. Isothermal Section for 5 wt%La2O3 and 10 wt%La2O3

After six independent tetrahedron regions, as shown in Fig. 7, in the CaO–SiO2–Nb2O5–La2O3 system were deter-mined. Equilibrium sections for 5 wt% and 10 wt% La2O3 for afore mentioned system at 1 273 K were constructed, respectively, as shown in Figs. 11(a) and 12(a). And their relative positional relations in the three-dimensional quar-ternary 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 composi-tion range could be confirmed by these sections.

4. Conclusion

High-temperature equilibrium and quench technique have been applied to investigate equilibrium phase relations in the CaO–SiO2–Nb2O5–La2O3 system at 1 273 K and 1 473 K According to the Gibbs Phase Rule, six independent tet-rahedron 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 1 273 K were also constructed, respectively, which could be used to confirm the solid phase equilibrium relations for the speci-fied region in the CaO–SiO2–Nb2O5–La2O3 system.

AcknowledgementsThis 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).

REFERENCES

1) J. Ren, G. Y. Xu and W. M. Wang: Technology and Application of Niobium Mining and Metallurgy, Nanjing University Press, Nanjing, (2001), 1.

2) S. Y. Li, Y. S. Zhou and H. Y. Du: Development and Application Technology of Niobium Resources, Metallurgical Industry Press, Beijing, (1992), 1.

3) A. Jongejan and A. L. Wilkins: J. Alloys Compd., 19 (1969), 203.4) J. J. Shi, L. F. Sun, B. Zhang, X. Q. Liu, J. Y. Qiu, Z. Y. Wang and

M. F. Jiang: Metall. Mater. Trans. B, 47 (2016), 425.5) J. J. Shi, L. F. Sun, J. Y. Qiu, Z. Y. Wang, B. Zhang and M. F. Jiang:

ISIJ Int., 56 (2016), 1124.6) J. J. Shi, L. F. Sun, J. Y. Qiu, B. Zhang and M. F. Jiang: J. Alloys

Compd., 699 (2017), 193.7) L. F. Sun, J. J. Shi, B. Zhang, J. Y. Qiu, Z. Y. Wang and M. F. Jiang:

J. Cent. South Univ., 24 (2017), 48.8) Y. B. Kang, I. H. Jung, S. A. Decterov, A. D. Pelton and H. G. Lee:

ISIJ Int., 44 (2004), 975.9) Y. B. Kang and H. G. Lee: ISIJ Int., 45 (2005), 1552.

10) M. Tanahashi, N. Furuta, C. Yamauchi and T. Fujisawa: ISIJ Int., 41 (2001), 1309.

11) S. Jahanshahi and S. Wright: ISIJ Int., 33 (1993), 195.12) Y. B. Kang and H. G. Lee: ISIJ Int., 45 (2005), 1543.13) H. M. Henao, F. Kongoli and K. Itagaki: Mater. Trans., 46 (2005),

812.14) H. M. Henao, C. Pizarro, J. K. Front, A. Moyano, P. C. Hayes and

E. Jak: Metall. Mater. Trans. B, 46 (2010), 1186.15) S. S. Pandit and K. T. Jacob: Metall. Mater. Trans. B, 26 (1995), 397.16) E. H. Nickel: Am. Mineral., 41 (1956), 785.17) C. J. Liu: Phase Rule and Phase Diagram Thermodynamics, Higher

Education Press, Beijing, (1995), 1.18) A. M. Frolov and A. A. Evdokimov: Russ. J. Inorg. Chem., 32

(1987), 1771.19) M. Ibrahim, N. F. H. Bright and J. F. Rowland: J. Am. Ceram. Soc.,

45 (1962), 329.20) M. Ibrahim and N. F. H. Bright: J. Am. Ceram. Soc., 45 (1962), 221.21) N. A. Toropov, I. A. Bondar and F. J. Galakhov: Trans. 8th Int.

Ceramic Cong., Organizing Committee of 8th ICC, Copenhagen, (1962), 85.

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Subsolidus Phase Relations in the CaO–SiO2–Nb2O5–La2O3