Phase equilibria in medium basicity region of CaO-SiO2-Nb2O5-(5%, 10%, 15%) La2O3 system

Author’s Accepted Manuscript Phase Equilibria in Medium Basicity region of CaO-SiO2-Nb2O5-(5%, 10%, 15%) La2O3 System Jiyu Qiu, Chengjun Liu, Zhengyue Liu, Zhe Yu

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To appear in: Ceramics International Received date: 25 September 2018 Revised date: 16 October 2018 Accepted date: 17 October 2018 Cite this article as: Jiyu Qiu, Chengjun Liu, Zhengyue Liu and Zhe Yu, Phase Equilibria in Medium Basicity region of CaO-SiO2-Nb2O5-(5%, 10%, 15%) La2O3 System, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.142 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Phase Equilibria in Medium Basicity region of CaO-SiO2-Nb2O5-(5%, 10%, 15%) La2O3 System

Authors: Jiyu Qiu1, 2), Chengjun Liu1, 2)*, Zhengyue Liu1, 2), Zhe Yu1, 2)

Affiliations: 1

Key Laboratory for Ecological Metallurgy of Multimetallic Ores (Ministry of

Education), Shenyang, Liaoning Province, 110819, China 2

School of Metallurgy, Northeastern University, Shenyang, Liaoning Province, 110819,

China

Detailed information for authors: *Corresponding author: Chengjun Liu E-mail: [email protected] Full address: Mailbox 288#, Northeastern University, No.11, Alley 3, Wenhua Road, Heping District, Shenyang, Liaoning Province, China, 110819 Phone number:+86-13898850333 Fax number: +86-024-83681589 First-author: Jiyu Qiu E-mail: [email protected] Full address: Mailbox 288#, Northeastern University, No.11, Alley 3, Wenhua Road, Heping District, Shenyang, Liaoning Province, China, 110819 Co-author: Zhengyue Liu E-mail: [email protected] Full address: Mailbox 288#, Northeastern University, No.11, Alley 3, Wenhua Road, Heping District, Shenyang, Liaoning Province, China, 110819 Co-author: Zhe Yu E-mail: [email protected] Full address: Mailbox 288#, Northeastern University, No.11, Alley 3, Wenhua Road, 1

Heping District, Shenyang, Liaoning Province, China, 110819

Abstract The thermodynamic information, such as primary phase fields and liquidus temperatures, in the CaO-SiO2-Nb2O5-La2O3 quaternary system phase diagram is important for the comprehensive utilization of niobium and rare earth resources. In the current work, the phase equilibria of CaO-SiO2-Nb2O5-(0-15%)La2O3 system within medium basicity region at 1373 K-1673 K were experimentally studied. X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS) were used to identify the type and composition of equilibrium phases in the quenched samples after thermodynamic equilibrium experiment. According

to

the

experimental

results,

the

univariate

line

between

CaNb2O6-Ca2Nb2O7-LaNbO4 primary phase fields, the bivariate interface between CaNb2O6-Ca2Nb2O7

primary

phase

fields,

the

bivariate

interface

between

CaNb2O6-LaNbO4 primary phase fields, and isothermal liquidus surfaces in CaNb2O6 and Ca2Nb2O7 primary phase fields were determined, respectively. Finally, CaO-SiO2-Nb2O5-La2O3 phase diagram within specific region was spatially constructed, and the results were also presented in the form of CaO-SiO2-Nb2O5 pseudo-ternary system phase diagram with w(La2O3)=5%, 10% and 15%. The research results are important for the comprehensive utilization of Nb and RE resources, meanwhile, they 2

are also important for the improvement of related phase diagrams.

Key words: phase equilibria; silicate(D); niobates(D); SiO2(D)

3

1 Introduction The niobium (Nb) and rare earth (RE) elements are widely used in many materials. They need to be smelted and extracted from related mineral. China’s Bayan Obo mineral is a typical multiple-element polymetallic symbiotic mineral, wherein the niobium and rare earth reserves are occupying the front rank in the world[1]. However, in the early years, related utilization was only focused on iron resource, which means that a large amount of Nb and RE resources were discarded into the tailings, this is a huge waste of resources[2]. Until now, related pyrometallurgy extraction process for the Nb and RE resources in these tailings is still in the development stage. However, the uncertainty of high-temperature thermodynamic property of correlative slag system have restrict the development of utilization technology of Nb and RE resources from Bayan Obo tailings[3-7]. The fundamental slag system involved in related extraction process is CaO-SiO2-Nb2O5-La2O3 quaternary system, whose sub-systems phase diagrams had been partly studied, including CaO-La2O3[8], La2O3-SiO2[9], La2O3-Nb2O5[10], CaO-SiO2-Nb2O5[11] and CaO-SiO2-La2O3[12] systems. However, the simultaneous presence of La and Nb in CaO-SiO2-Nb2O5-La2O3 system makes the thermodynamic property very complex, it means that the phase equilibria inside this system cannot be simply determined by the sub-systems. At the current stage, the lack of thermodynamic data makes it difficult to directly calculate the phase diagram of this quaternary system. In author's previous work[13], sub-solidus phase relations within specific region in this

4

quaternary system were identified. And recently, phase equilibria in low basicity region in this system, which is mainly about SiO2 primary phase field, were studied. However, the phase equilibria in higher basicity region, especially about the primary phase fields of Nb or La containing phases, are still need to be studied. Thermodynamic equilibrium experiment is a scientific and accurate measurement method recognized in the field of silicate phase diagram, it has been widely used in different systems[14-20]. In the current work, phase equilibria in the CaO-SiO2-Nb2O5-La2O3 system at 1373 K-1673 K were experimentally studied by the thermodynamic equilibrium experiment. X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS) were used to identify the equilibrium phases in the samples. The primary phase fields of CaNb2O6, Ca2Nb2O7 and LaNbO4, and the liquidus temperatures of CaNb2O6 and Ca2Nb2O7 primary phase fields within specific region were determined. Finally, the CaO-SiO2-Nb2O5-La2O3 quaternary system phase diagram within medium basicity region was constructed and presented in the form of spatial tetrahedron and CaO-SiO2-Nb2O5-(5%,10%,15%)La2O3 pseudo-ternary triangle, respectively. 2 Experimental procedure The slag samples used in the thermodynamic equilibrium experiment need to be pre-melted first to achieve the homogeneous of composition. The main procedure of pre-melt experiment was described in previous work[12]. The starting compositions of these pre-melt slag samples were measured by EDS, as listed in Table 1. In this paper,

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the starting compositions of the slag samples are mostly located in the SiO2-CaNb2O6-LaNbO4-CaSiO3 and Ca2Nb2O7-CaNb2O6-LaNbO4-CaSiO3 sub-solidus tetrahedron regions which had been identified in previous work[13], as shown in Figure 1. The slag samples without any precipitates were used in the subsequent thermodynamic equilibrium experiment. The related temperature control curve is shown in Figure 2. After that, the samples were rapidly quenched to 273 K by ice-water to maintain the phase equilibria. SEM(EVO18 type that produced by Zess), XRD(X’ pert PRO type that produced by PANalytical), and EDS(X-Max20 type that produced by Oxford) were used to identify the equilibrium phase and measure the composition of each equilibrium phase in the quenched slag samples. The relative error of EDS was estimated within 1%. The experimental temperatures were 1673 K, 1573 K, 1473 K, and 1373 K. In addition, Verification experiment 1 and 2, as shown in Figure 2, were also performed for some samples in the pre-experiment to check whether the equilibrium was achieved. There was no significant difference in these samples, which means that the current experimental procedure was effective. 3 Results and discussion 3.1 Typical phase equilibria in spatial quaternary phase diagram Based on the Gibbs Phase Rule[21] in a quaternary system, the compositions of liquid phase equilibrium with different kinds of solid phase will located in different and specific spatial trajectory. Table 2 and Figure 3 show the rules they should follow (Here

6

we chose a simple eutectic quaternary phase diagram as an example). In Figure 3, typically , EAB is the eutectic point of A-B binary systems, EABC is the eutectic point of A-B-C ternary system, E is the quaternary eutectic point, EAD-EABD-E-EACD is an interface between B and D primary phase fields, the gray surfaces are the liquidus surfaces at different temperatures in the C primary phase field of A-B-C-D quaternary system. It is worth mentioning that the thermodynamic information in multi-components phase diagram is continuous and self-consistent, which means that the boundaries of liquidus surface can end in the sub-system, the bivariate interface, or the univariate line. 3.2 Phase Equilibria in experiment According to the SEM, EDS and XRD results of the quenched slag samples after the thermodynamic equilibrium experiment, the phase equilibria of CaO-SiO2-Nb2O5-La2O3 system within specific region at different temperatures were identified and listed in Table 3. It can be seen that there are five kinds of phase equilibria relations at experimental temperatures, they are ①liquid phase+CaNb2O6, ②liquid phase+Ca2Nb2O7, ③liquid phase+CaNb2O6+LaNbO4, ④liquid phase+CaNb2O6+Ca2Nb2O7, and ⑤liquids phase +CaNb2O6+Ca2Nb2O7+LaNbO4. The details of these phase equilibria in typical slag samples will be introduced as follows. Sample A7 shows typical two-phase co-existing (①) at 1573 K, as shown by the SEM result in Figure 4a. The white phase is CaNb2O6 and the gray phase is liquid phase, which is also proved by XRD result (Figure 4b). Sample B2 shows another kind of

7

two-phase co-existing (②) at 1573 K, as shown by the SEM and XRD results in Figure 4c and 4d. The white phase is Ca2Nb2O7 and the gray phase is liquid phase. All the corresponding compositions of liquid phase in Table 3 were measured by EDS and given in Table 4. None solid solution of any other elements was found in all the CaNb2O6 and LaNbO4 phase in experiment, and their compositions were almost identical to the stoichiometric ratio, thus they were not included in related description. However, obvious La was detected in the Ca2Nb2O7 at different temperatures, as shown in Table 5. According to aforementioned discussion, the composition of liquid phase equilibrium with one solid phase will be used in construction of isothermal liquidus surface in corresponding primary phase field. Sample A7 shows typical three-phase co-existing (③) at 1373 K, as shown by SEM result in Figure 5a. The light gray phase is CaNb2O6, the white phase is LaNbO4 and the dark gray phase is liquid phase, it was also proved by related XRD result (Figure 5b). Sample A5 shows another kinds of three-phase co-existing (④) at 1473 K, as shown by SEM result in Figure 5c, wherein the dark gray phase is liquid phase, the white block phase is Ca2Nb2O7 and the white phase with strip shape is CaNb2O6. Ca2Nb2O7 and CaNb2O6 phases in the sample cannot be distinguished only from their contrast in SEM image, but the related EDS and XRD results can help to identify these two phases. According to the discussion in section 3.1, each composition of liquid phase co-exist with two solid phases will be used in construction of public surface (bivariate interface) of corresponding primary phase fields.

8

Sample A8 shows typical four-phase co-existing (⑤) at 1473 K, as shown in SEM results in Figure 5d. Wherein the dark gray phase is liquid phase, the white phase is LaNbO4, and the light gray phases are CaNb2O6 and Ca2Nb2O7, respectively. According to discussion in section 3.1, each composition of liquid phase co-exist with three solid phases will be used in construction of corresponding public line (univariate line) of three primary phase fields. 3.3 Spatial phase diagram of low w(La2O3) and medium basicity region in CaO-SiO2-Nb2O5-La2O3 system By combing with the discussion in section 3.1 and the experimental results, the existence of following thermodynamic information can be determined: bivariate interface between Ca2Nb2O7-CaNb2O6 primary phase fields and LaNbO4-CaNb2O6 primary phase fields, univariate line of Ca2Nb2O7-CaNb2O6-LaNbO4, and isothermal liquidus surface in CaNb2O6 and Ca2Nb2O7 primary phase fields. The existence of these different kinds of phase equilibria are also consistent with the existence of Ca2Nb2O7-CaNb2O6-LaNbO4-CaSiO3 sub-solidus field which had been already identified [13]. It is worth to mention that due to the continuous of the phase diagram, both corresponding phase equilibria in sub-systems and in adjacent region of the quaternary system can be used in the construction of isothermal liquid surface. In the current work, the available phase equilibria information in sub-systems include the Ca2Nb2O7-CaNb2O6 eutectic line and corresponding liquidus lines in Ca2Nb2O7 and

9

CaNb2O6 primary phase fields in the CaO-SiO2-Nb2O5[11] system. While the available information in adjacent region of the quaternary system include the CaNb2O6-SiO2-LaNbO4 univariate line and CaNb2O6-SiO2 bivariate interface in the low basicity region determined by author in the previous work, as shown in Figure 6. According to the CaNb2O6-SiO2-LaNbO4 univariate line in the quaternary system (Figure 6), compositions of liquid phases equilibrium with CaNb2O6+LaNbO4(Table 4), and the compositions of liquid phases equilibrium with LaNbO4+CaNb2O6+Ca2Nb2O7 (Table 4), we constructed the bivariate interface between CaNb2O6 and LaNbO4 primary phase fields(green surface in Figure 7), wherein the pink, green and blue points are corresponding liquid compositions in the experiment at 1573 K, 1473 K and 1373 K, respectively. Similarly, according to the Ca2Nb2O7-CaNb2O6 eutectic line in CaO-SiO2-Nb2O5 system[11], compositions of liquid phases equilibrium with CaNb2O6+Ca2Nb2O7 (Table 4), and compositions of liquid phases equilibrium with LaNbO4+CaNb2O6+Ca2Nb2O7 (Table 4), we constructed the bivariate interface between CaNb2O6 and Ca2Nb2O7 primary phase fields(yellow surface in Figure 7). Based on the Gibbs Rule, the bivariate interface between two adjacent primary phase fields is composed of boundary lines of the different-temperature isothermal liquidus surfaces. According to related isothermal lines on the three bivariate interfaces in Figure 7, the compositions of liquid phases equilibrium with CaNb2O6 in the quaternary system (Table 4), and liquidus lines in the CaNb2O6 primary phase field in CaO-SiO2-Nb2O5 system[11], we constructed part of isothermal liquidus surfaces in CaNb2O6 primary

10

phase field at 1673 K, 1573 K, 1473 K and 1373 K, respectively . They are shown by the red, pink, green and blue surfaces in Figure 8. Similarly, according to the isothermal lines on the bivariate interface in Figure 7, the compositions of liquid phase equilibrium with Ca2Nb2O7 in the quaternary system (Table 4), liquidus lines in the Ca2Nb2O7 primary phase field in CaO-SiO2-Nb2O5 system[11], we constructed the isothermal liquidus surfaces in Ca2Nb2O7 primary phase field at 1673 K, 1573 K and 1473 K, respectively. They are shown by the red, pink and green surfaces in Figure 9. Finally, the spatial phase diagram of CaO-SiO2-Nb2O5-La2O3 quaternary system within low w(La2O3) and medium basicity region was constructed according to the experimental phase equilibria, as shown in Figure 10. It can be seen that four primary phase fields were identified in the experimental composition region, they were SiO2, LaNbO4, CaNb2O6 and Ca2Nb2O7. In the low w(La2O3) region (less than 20%), the primary crystal phase changes from SiO2→CaNb2O6→Ca2Nb2O7 as the SiO2 decreases. With the increase of w(La2O3) content, the corresponding primary crystal phase changes to LaNbO4. According to the current experimental results and previous work on sub-solidus phase equilibria[13], it can be inferred that 10CaO·6SiO2·Nb2O5 primary phase field will be involved in the higher basicity region, while a large LaNbO4 primary phase field will be involved in the higher w(La2O3) region (more than 20%), which will be concerned in subsequent study. 3.4 Pseudo-ternary phase diagram of CaO-SiO2-Nb2O5-(5%, 10%, 15%)La2O3 system The spatial quaternary phase diagram can accurately and intuitively show the phase

11

equilibria, but it is difficult to directly obtain thermodynamic data from it. Therefore, according to the spatial phase diagram in Figure 10, CaO-SiO2-Nb2O5 pseudo-ternary phase diagrams with w(La2O3)=5%, 10%, and 15% were also constructed, as shown in Figure 11(a)-(c), respectively. It can be seen that the trend of liquidus lines and boundary line between primary phase fields are consistent with that in the aforementioned spatial phase diagram, that is, the liquidus temperature decreases as the w(Nb2O5) decreases. In the primary phase field of CaNb2O6, the liquidus temperature slightly increases with the decrease of the basicity, while in the Ca2Nb2O7 primary phase field, the liquidus temperature decreases with the decrease of the basicity. 4 Conclusions In the current work, the phase equilibria of CaO-SiO2-Nb2O5-(0-15%)La2O3 system within medium basicity region at 1373 K-1673 K were studied experimentally by thermodynamic equilibrium experiment followed by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). According to the experimental results, the univariate line of CaNb2O6-Ca2Nb2O7-LaNbO4 primary phase fields, bivariate interface between CaNb2O6 and Ca2Nb2O7 primary phase fields, bivariate interface between CaNb2O6 and LaNbO4 primary phase fields, and isothermal liquidus surfaces at different temperatures in the CaNb2O6 and Ca2Nb2O7 primary phase fields were determined, respectively. Finally, CaO-SiO2-Nb2O5-La2O3 phase diagram was constructed and presented in the form of spatial tetrahedron and CaO-SiO2-Nb2O5-(5%,10%,15%)La2O3 pseudo-ternary triangle.

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Acknowledgements This work was supported by the National Key R&D Program of China (No. 2017YFC0805100), National Natural Science Foundation of China (NSFC, No. 51774087), the Fundamental Research Funds for the Central Universities China (No.N162506002). References [1] D L Lin, C L Li, H L Wu, Key research and technical progress of mining and smelting technology in Bayan Obo special mineral, 1st ed., Metallurgical Industry Press, Beijing, 2007, 1-27. [2] J Ren, G Y Xu, W M Wang, Technology and application of niobium mining and metallurgy, 1st ed., Nanjing University Press, Nanjing, 2001, 1-40. [3] S Y Li, Y S Zhou, H Y Du, Development of application technology of Nb resource, 1st ed., Metallurgical Industry Press, Beijing, 1992, 1-53. [4] K Sakuraya, S Furuyama, S Yoshimatsu, Behavior of gaseous reduction of molten slag containing niobium oxide, Tetsu-to-Hagane, 74(1988) 794-800. https://doi.org/10.2355/tetsutohagane1955.74.5_794 [5] A Sato, G Aragane , A Kashara , K Muneyuki, Y Shiro, Selective Recovery of Silicon, Niobium, or Manganese from Pig Iron with Fe2O3-based Fluxes by Flux Extraction Method, Trans. Iron Steel Inst. Jpn., 26(1986) 942-948. https://doi.org/10.2355/isijinternational1966.26.942 [6] R Inoue , X P Zhang, H Li, S Hideaki, Trans. Iron Steel Inst. Jpn., 27(1987) 946-950.

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https://doi.org/10.2355/isijinternational1966.27.946 [7] H Qiyong, D Jian, H Shiliang, Iron Steel. 19(1984)12-17. [8] T L Barry, V S Stubican , R Roy, Phase Equilibria in the System CaO-Yb2O3, J Am Ceram Soc, 49(1966) 667-670. https://doi.org/10.1111/j.1151-2916.1966.tb13196.x [9] N A Toropov, I A Bondar, F J Galakhov, Trans Intern Ceram Congr 8th Copenhage, (1962) 85-103. [10] E P Savchenko, N A Godina, E K Keler, Solid-phase reactions between pentoxides of niobium and tantalum and oxides of the rare-earth elements, Chem High Temp Mater, (1969) 108-113. [11] A Jongejan, A L Wilkins, Phase relationships in the high-lime part of the system CaO-Nb2O5-SiO2, J Less Common Met, 19(1969)203-208. https://doi.org/10.1016/0022-5088(69)90096-4 [12] J Y Qiu, C J Liu, Z Yu, Isothermal section of CaO-SiO2-La2O3 system within specific region at 1673-1473  K, Ceram Int, 44(2018) 12564-12572. https://doi.org/10.1016/j.ceramint.2018.04.053 [13] J Y Qiu, C J Liu, Solid Phase Equilibrium Relations in the CaO-SiO2-Nb2O5-La2O3 System at 1273 K, Metall Mater Trans B, 49(2018) 69-77. https://doi.org/10.1007/s11663-017-1144-0 [14] J J Shi, L F Sun, J Y Qiu, B Zhang, M F Jiang, Phase equilibria of CaO-SiO2-5wt% MgO-10wt%Al2O3-TiO2 system at 1300 °C and 1400 °C relevant to Ti-bearing furnace slag, J Alloys Compd, 699(2017) 193-199.

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https://doi.org/10.1016/j.jallcom.2016.12.328 [15] J J Shi, L F Sun, J Y Qiu, M F Jiang, Phase equilibria of CaO-SiO2-5wt.%MgO-30 wt.%Al2O3-TiO2 system at 1400 °C and 1450 °C relevant to high Al2O3 Ti-bearing blast furnace slag system, J Alloys Compd, 722(2017) 25-32. https://doi.org/10.1016/j.jallcom.2017.06.058 [16] D.H. Woo, H.G. Lee, Phase equilibria of the MnO-SiO2-Al2O3-MnS system, J. Am. Ceram. Soc. 93 (2010) 2098-2106. [17] Y. Yokogawa, M. Yoshimura, High-temperature phase relations in the system Y2O3-Ta2O5, J. Am. Ceram. Soc. 74 (1991) 2077-2081. https://doi.org/10.1111/j.1151-2916.1991.tb08262.x [18] M. Chen, B. Zhao, Phase equilibrium studies of “Cu2O”-SiO2-Al2O3 system in equilibrium with metallic copper, J. Am. Ceram. Soc. 96 (2013) 3631-3636. https://doi.org/10.1111/jace.12573 [19] V Grover, A K Tyagi, Ternary phase relations in CeO2-DyO1.5-ZrO2 system, Ceram Int, 39(2013)7563-7569. [20] J J Shi, L F Sun, J Y Qiu, Z Y Wang, B Zhang, M F Jiang, Experimental determination of the phase diagram for CaO-SiO2-MgO-10%Al2O3-5%TiO2 system, ISIJ. Int. 56 (2016) 1124-1131. https://doi.org/10.2355/isijinternational.ISIJINT-2016-045 [21] S M Hao, M Jiang, X H Li. Materials thermodynamics, 2nd ed., Chemical Industry Press, Beijing, 2013, 39-89.

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Figure captions Figure 1 Starting compositions in the CaO-SiO2-Nb2O5-La2O3 phase diagram, mass% Figure 2 Temperature control curve during pre-melt and equilibria experiment Figure 3 Schematic diagram of spatial diagram of a eutectic quaternary system Figure 4 SEM and XRD results of typical two-phase co-existing samples (a)A7-1573 K-SEM

(b) A7-1573 K-XRD (c) B2-1573K-SEM

(d) B2-1573

K-XRD Figure 5 SEM results of typical three-phase and four-phase co-existing samples (a) A7-1373 K-SEM (b) A7-1373 K-XRD (c) A5-1473 K-SEM (d) A8-1473 K-SEM Figure 6 CaNb2O6-SiO2-LaNbO4 univariate line and CaNb2O6-SiO2 bivariate interface in the quaternary system, mass% Figure 7 The CaNb2O6-LaNbO4 and CaNb2O6-Ca2Nb2O7 bivariate interfaces in the quaternary system, mass% Figure 8 Isothermal liquidus surfaces at 1673 K, 1573 K, 1473 K and 1373 K in CaNb2O6 primary phase field, mass% Figure 9 Isothermal liquidus surfaces at 1673 K, 1573 K and 1473 K in Ca2Nb2O7 primary phase field, mass% Figure 10 Spatial phase diagram of CaO-SiO2-Nb2O5-La2O3 quaternary system within

16

specific region, mass% Figure 11 Phase diagrams of CaO-SiO2-Nb2O5-(5%, 15%, 20%)La2O3 pseudo-ternary systems, mass%, (a)CaO-SiO2-Nb2O5-5%La2O3 pseudo-ternary system CaO-SiO2-Nb2O5-10%La2O3 pseudo-ternary system

(b)

(c) CaO-SiO2-Nb2O5-15%La2O3

pseudo-ternary system

Tables Table 1 Starting compositions of slag samples, mass% No.

CaO

SiO2

Nb2O5

La2O3

No.

CaO

SiO2

Nb2O5

La2O3

A1

30.55

33.50

30.19

5.76

A10

11.80

2.35

72.50

13.36

A2

28.91

30.84

33.44

6.81

A11

10.66

1.52

73.86

13.96

A3

25.56

33.23

36.16

5.05

B1

28.70

27.64

32.36

11.31

A4

23.42

24.42

42.51

9.65

B2

32.22

21.32

36.00

10.45

A5

22.88

19.78

47.38

9.96

B3

32.00

20.11

38.22

9.67

A6

25.69

16.97

52.67

4.67

B4

33.35

14.08

46.30

6.27

A7

19.88

14.41

56.02

9.69

B5

25.82

16.05

47.55

10.59

A8

20.35

12.94

56.99

9.71

B6

23.48

7.54

65.64

3.34

A9

18.80

14.64

61.74

4.82

B7

19.21

5.73

66.08

8.98

Table 2 Trajectory and degree of freedom of different kinds of phase equilibria in quaternary system

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Constant temperature Phase equilibria

Degree of

Variable temperature Degree of

Trajectory freedom

Trajectory freedom

Liquid

3

area

4

-

Liquid+Solid1

2

surface

3

area

Liquid+Solid1+Solid2

1

spatial line

2

surface

Liquid+Solid1+Solid2+Solid3

0

point

1

spatial line

Liquid+Solid1+Solid2+Solid3+Solid4

-

-

0

point

Table 3 Phase equilibria in slag samples at different temperature No.

1573 K

1473 K

1373 K

No.

1673 K

1573 K

A1

L+CN

A10

L+CN+LN

A2

L+CN

A11

L+CN+LN

A3

L+CN

B1

A4

L+CN

B2

L+C2N

A5

L+CN+C2N

B3

L+C2N

A6

L+CN+C2N

A7

L+CN

A8 A9

B4 L+CN+LN L+CN+C2N+LN

L+CN

L+CN+LN

L+C2N

L+C2N

B5 B6 B7

L+C2N L+CN+C2N L+CN+C2N+LN

L=Liquid, CN=CaNb2O6, C2N=Ca2Nb2O7, LN=LaNbO4 Table 4 Compositions of liquid phases in samples at different temperatures, mass%

18

1473 K

No.

T, K

CaO

SiO2

Nb2O5

La2O3

No.

T, K

CaO

SiO2

Nb2O5

La2O3

A1

1473

31.49

35.89

26.45

6.17

A9

1573

23.48

32.24

34.40

9.89

A2

1473

30.41

34.93

26.91

7.74

A10

1573

17.35

25.92

38.61

18.13

A3

1473

28.89

38.32

25.38

7.40

A11

1573

15.89

26.30

39.04

18.77

A4

1473

25.92

33.36

27.62

13.10

B1

1473

28.84

30.92

27.61

12.64

A5

1473

27.06

29.99

28.25

14.70

B2

1573

33.29

23.76

30.32

12.63

A6

1573

27.03

25.89

38.39

8.68

B3

1573

32.62

23.87

32.01

11.50

A7

1573

21.63

23.50

39.45

15.42

B4

1673

33.71

18.75

39.71

7.84

1473

23.01

30.16

28.37

18.46

B5

1473

28.62

28.49

29.58

13.31

1373

27.04

38.27

18.59

16.09

B6

1673

23.47

12.56

58.40

5.57

1473

25.09

29.06

28.74

17.12

B7

1573

21.77

22.78

39.56

15.89

A8

Table 5 composition of CaNb2O7 in samples at different temperatures, mass% No.

T

Phase

CaO

Nb2O5

La2O3

A5

1473

Ca2Nb2O7

21.91

70.66

7.43

A6

1573

Ca2Nb2O7

21.41

74.04

4.55

A8

1473

Ca2Nb2O7

21.54

70.39

8.07

B1

1473

Ca2Nb2O7

22.33

70.60

7.07

B2

1573

Ca2Nb2O7

24.64

70.41

4.95

B3

1573

Ca2Nb2O7

23.32

70.30

6.37

B4

1673

Ca2Nb2O7

24.61

70.80

4.59

B5

1473

Ca2Nb2O7

22.54

70.78

6.68

19

B6

1673

Ca2Nb2O7

23.26

23.26

5.78

B7

1573

Ca2Nb2O7

19.03

70.49

10.48

20

21

22

23

24

25

26

27

28

29

30