Phase Transition Behavior of Rare Earth Oxide Ce₂O₃ in CaO–SiO₂–5wt.% Al₂O₃ System at 1673–1873 K

Abstract

Based on previous phase diagram studies on the CaO-SiO₂-Ce₂O₃-5wt.% Al₂O₃ system at 1673–1773 K. In this study, the equilibrium phase diagram experiment at 1873 K was extended. The phase composition and phase ratio were analyzed and statistically calculated by FE-SEM, XRD, EPMA, and XRF, so the equilibrium phase diagram of CaO-SiO₂-Ce₂O₃-5wt.% Al₂O₃ at 1873 K was obtained. At this high temperature, the phases coexisting with the liquid phase are CaO·2Ce₂O₃·3SiO₂, SiO₂, Ca₂SiO_4, and Ce₂Si₂O₇. In addition, the liquid phase region and the initial crystal point at the temperature of 1673–1873 K in CaO-SiO₂-Ce₂O₃-5wt.% Al₂O₃ system were plotted. The precipitation and phase transformation behavior of rare earth oxide Ce₂O₃ in CaO-SiO₂-Al₂O₃ are explained in detail, which provides a theoretical basis for the value-added utilization of rare earth oxides.

Introduction

The formation of rare earth-bearing phases in pyrometallurgical processes can be estimated and evaluated using the phase equilibrium of the system. The experimental measurement method of the phase diagram can be divided into dynamic method and the static method according to the difference in the experimental measurement principle [1]. The dynamic method is that when the components of the experimental point cross the phase boundary, the physicochemical properties of the system on the phase boundary will also change due to the change of the phase structure. Static method, also known as isothermal method, a series of samples with known component content at a specific temperature to heat treatment, so that the sample reaches a stable equilibrium state. The representative method of the static method is the quenching method, also known as the high temperature equilibrium experimental method [2]. The research method used in this study is the high temperature equilibrium method.

Recently, with the development of science and technology and the maturity of the application of detection methods, many scholars have carried out more and more research on rare earth-containing oxide systems, which greatly enriched and supplemented the basic information of relevant thermodynamics. In order to understand the role of Ce₂O₃ in sulfur dissolution behavior, Jeong et al. [3] determined the sulfide capacity of MnO-SiO₂-Al₂O₃-Ce₂O₃ system at 1600°C by gas-residue equilibrium method, and analyzed its structure by Raman microscopy. Tas et. al [4, 5] conducted a systematic study on the thermodynamic behavior of Ce₂O₃-Ce₂Si₂O₇ system from 1150 °C to 1970 °C, and found that a total of three eutectic reactions occurred in the system at 1664 °C ± 5 °C, 1870 °C ± 25 °C, 1762 °C ± 10 °C, and a relatively complete phase diagram of the Ce₂O₃-Ce₂Si₂O₇ system was measured for the first time.

In the production of rare earth ceramics, rare earth glass, rare earth steel, and other related materials, CaO-SiO₂-RE₂O₃ system is the basic slag system, and the phase transition is the necessary thermodynamic basis for practical applications. There are many studies on the role and advantages of rare earths in material applications, but there are few studies on rare earth oxide phase diagrams, and only some binary phase diagrams have been completely constructed. However, the thermodynamic property of complex systems involved in practical applications of rare earths cannot be obtained from these simple binary systems, but there are very few studies containing rare earth elements, such as CaO-SiO₂-Ce₂O₃ [6]. Qiu et al. conducted a large number of experimental studies on La₂O₃-containing systems [7,8,9,10,11,12,13], and determined the existence of the solid solution phase of the ternary compound CaO·3SiO₂·2La₂O₃, and obtained the isothermal cross-sectional diagram of CaO-SiO₂-Nb₂O₅-La₂O₃ ternary system at specific temperature, which enriched the thermodynamic data of La₂O₃-containing system. Lan et al. [14,15,16] presented the thermodynamic data of CaO-SiO₂-Ce₂O₃ ternary system, CaO–SiO₂–CaF₂–Ce₂O₃ quaternary system, and CaO–SiO₂–CaF₂–P₂O₅–Ce₂O₃ quinary system at specific temperatures, and studied the crystallization behavior of REE phase (Ce_9.33-xCax(SiO₄)₆O_2-0.5x). The temperature range of REE phase crystallization separately in REE-containing slag of selected components, as well as the crystallization and growth kinetics of cerium fluorosilica (Ce_9.33-_xCa_x(SiO₄)₄O_5-0.5xF₂) and the formation mechanism of cerium apatite (Ca_5-xCe_x[(Si, P)O₄]₃F) were obtained, which provided a theoretical basis for the sustainable utilization of rare earth elements in rare earth slag in ceramics and rare earth resource recovery. On this basis, the equilibrium phase composition of SiO₂-Ce₂O₃-CaO-5wt.% Al₂O₃ at 1673–1873 K was studied in this paper, which provided basic phase transition data for the recycling of rare earth waste residue in ceramics and other fields.

Methodology

Rare Earth Slag Samples Preparation

To ensure the accuracy of the experimental results, all the experimental slag samples were pre-melted. In the pre-melting test, an electronic balance with an accuracy of 0.0001 g was used to weigh the required oxide powder according to the design composition and mix evenly. Then, A platinum crucible was used to contain the oxide mixture powder and placed in the constant temperature zone of a high-frequency induction heating furnace. The temperature was raised to 1923 K for 4 h under the atmosphere of Ar + 10% H₂ to obtain a liquid phase slag with uniform composition. After the pre-melting time was finished, the slag sample was quickly put into the ice-water mixture and quenched.

In order to confirm whether the pre-slag is a uniform glass phase due to the crystal precipitation easy occur during the quenching process. It is necessary to test whether the pre-slag is a uniform glass phase. The pre-slag was ground in an agate mortar, and part of the sample was taken for XRD detection (X-ray Diffraction, Empyrean 2, PANalytical), and Fig. 1 shows the micromorphology and XRD detection pattern of the homogeneous glass phase of N1 sample. In addition, it is also necessary to detect the valence state of Ce in the pre-slag sample, because this study mainly investigates the equilibrium phase of Ce with Ce₂O₃ when it is stable in the cerium-containing slag system. Therefore, the remaining slag sample after grinding was detected by XPS (X-ray Photoelectron Spectroscopy, ESCALAB250Xi, ThermoScientific). For instance, the XPS spectrum of 3d_5/2 and 3d_3/2 of Ce³⁺ ions of N1 sample are shown in Fig. 2. Due to the transition behavior between the two different energy levels of Ce³⁺ ions, the emergence of 3d_5/2 and 3d_3/2 was found in Fig. 2. This result indicates that only Ce³⁺ is present and not Ce⁴⁺ in the pre-melted sample. For slag samples with XRD detection determined to be glass phase and XPS detection determined that the valence state of Ce is completely + 3 valence, the XRF instrument (X-ray Fluorescence Spectrometer, AxiosmAX, PANalytical) was used to determine the pre-slag composition.

Fig. 1

Microstructure a and XRD patterns b of pre-melted slag sample N1

Fig. 2

XPS detection map of quenching slag sample N1

Due to the burning loss and weighing error of the test, the composition of the pre-melted slag may be different from the design. Therefore, it is necessary to detect the composition of quenching slag. The XRF was used to analyze the composition of the pre-melted slag samples, and the results are listed in Table 1. It can be found that there is only a slight deviation in the measured value compared to the design component. In order to more intuitively see the composition distribution of the pre-melted slag sample, the detection point is placed in the pseudo-ternary phase diagram of the SiO₂-Ce₂O₃-CaO-5wt.% Al₂O₃ system, as shown in Fig. 3.

Table 1 Comparison of design and tested composition of samples for CaO-SiO₂-Ce₂O₃-5wt.% Al₂O₃

Fig. 3

Pre-melting component points of SiO₂-Ce₂O₃-CaO-5wt. % Al₂O₃ system at 1673–1873 K

Fig. 4

Schematic diagram of a box-type furnace and b gas purification unit

Equilibration Experiments

The chamber furnace was used to perform a phase equilibrium experiment at high temperature, as shown in Fig. 4a. The experimental temperature is measured by a type B thermocouple placed at the bottom of the furnace. Temperature control accuracy up to ± 1 K. In addition, the equilibrium time, experimental atmosphere, and quenching method are the main factors affecting the accuracy of high temperature equilibrium experimental results. Therefore, the atmosphere in the experiment consists of 10% H₂ in high-purity Ar and purified by a gas purification device, as shown in Fig. 4b. To ensure that the slag sample is not oxidized and to isolate other possible sources of contamination. The deaeration unit was composed of heating device and copper, sponge titanium, and magnesium deaeration chamber, which can keep the partial pressure of oxygen in argon below 10^–16. In the experiment, about 2 g of homogenized glass phase slag sample was weighed into a platinum crucible and placed in the constant temperature zone of the chamber furnace. All samples are heated again to 1923 K and held warm for 30 min, then cooled at a faster rate (10 K/min) to the target equilibrium temperature (1873 K). Based on the previous experimental results and the experimental experience of predecessors [17], the equilibrium time was set at 24 h.

After the equilibrium time was reached, the sample was quickly placed into the ice-water mixture for quenching. The quenched sample was then dried at 120 °C. Electron Probe Microanalysis (EPMA, JXA-8230) was employed to perform compositional analysis of each sample and XRD was used to detect the precipitated phase contained in the slag sample. In addition, the coexistence phase was observed using a Scanning Electron Microscope (SEM, JSM-7900F). The equilibrium micromorphology of the sample was obtained in BSE mode. The SEM test conditions: the beam current is 13 mA, the accelerating voltage is 20 kV. The EPMA measurement conditions: the beam current is 10^–8 A, the accelerating voltage is 20 kV and the probe diameter is 1 μm. The CaSiO₃, Al₂O_3, and CeP₅O₁₄ were used as the standard to analyze CaO, SiO₂, Al₂O_3, and Ce₂O₃, respectively. For each equilibrium phase, more than 10 different points were analyzed and averages calculated. Combined with the test results of XRD, FE-SEM, and EPMA, the phase composition of the sample in equilibrium was obtained.

Results and Discussion

Equilibrium Phase Composition of SiO₂-Ce₂O₃-CaO-5wt.% Al₂O₃ System at 1873 K

Since the phase equilibrium results of CaO-SiO₂-Ce₂O₃-5wt.% Al₂O₃ system at 1673–1773 K have been studied in the previous work [17], this part only shows the phase equilibrium results at 1873 K. When the temperature of CaO-SiO₂-Ce₂O₃-5wt.% Al₂O₃ system increased to 1873 K, the phase equilibrium relationship became simpler, most of the samples were displayed as pure liquid phase after 24 h of equilibrium, only a few samples appeared precipitated phase, and the microscopic morphology of the precipitated phase did not change much, as shown in Fig. 5.

Fig. 5

Microstructure of the equilibrium phase of the CaO-SiO₂-Ce₂O₃-5wt.%Al₂O₃ system at 1873 K and the corresponding XRD detection patterns (a Microstructure of sample N14, b XRD detection pattern of sample N14, c Microstructure of sample N31, d XRD detection pattern of sample N31, e Microstructure of sample N7, f XRD pattern of sample N7, g Microstructure of sample N26, and h XRD pattern of sample N26)

Figure 5 shows the micromorphology of sample N14 and XRD test results, it can be seen that SiO₂ with black rod and dot equilibrium phase coexists with liquid phase in the gray area (Fig. 5a), and the samples N11, N16, N25, and N32 are the same. When the sample components are adjusted to the same as N31, the micromorphology of the high temperature equilibrium phase is shown in Fig. 5c, combined with the XRD test results, it can be found that the gray-white sword-like is the Ce₂Si₂O₇ phase, and the EPMA detection results show that Ca²⁺ does not replace Ce³⁺ in Ce₂Si₂O₇. In samples N5–N8, N17, and N19, a regular hexagonal CaO·2Ce₂O₃·3SiO₂ was detected, and its microscopic morphology is shown in Fig. 5e. The EPMA test results shows that there were both Ca elements and Ce elements in the precipitated phases of N26 and N29, and their microscopic morphology is shown in Fig. 5g, combined with the XRD test results, it could be judged that the gray-white dots were (Ca, Ce)₂SiO₄ phase. Table 2 lists the composition of all phases in each sample under investigation, similarly, in samples with ‘*’, the precipitated phase Ca₂SiO₄ is obtained by converting Ce³⁺ to Ca²⁺. All measured compositional data are normalized and used to construct phase diagrams.

Table 2 Equilibrium phase composition of the CaO-SiO₂-Ce₂O₃-5wt.%Al₂O₃ system at 1600 °C

Determination of the Isotherms at 1673–1873 K

During the preparation of the sample, the content of Al₂O₃ was set to 5 wt.%. However, due to the crystal precipitation in the slag, the Al₂O₃ content in the quenched liquid phase cannot be accurate at 5 wt.%. Therefore, the liquid composition after quenching must be standardized in order to use the phase diagram to represent the phase relationship of CaO-SiO₂-Ce₂O₃-5wt.% Al₂O₃ system [18, 19]. As shown in Fig. 6, the liquid phase isotherms of SiO₂-Ce₂O₃-CaO-5wt.% Al₂O₃ system at 1673–1873 K are obtained based on the above experimental results.

Fig. 6

Liquid phase isotherms of SiO₂-Ce₂O₃-CaO-5wt.% Al₂O₃ system at 1673–1873 K

It can be seen that when the content of Al₂O₃ is constant at 5%, with the increase of temperature, the liquid line moves closer to the Ce₂O₃-SiO₂ boundary line. When the temperature reaches 1873 K, a large liquid region appears on the Ce₂O₃-SiO₂ boundary line. In addition, when the temperature is 1673 K, it is necessary to add at least 13 wt.% CaO to form the liquid phase region. When the temperature rises to 1773 K, it is necessary to add at least 8 wt.% CaO to form the liquid phase region. When the temperature rises to 1873 K, it is only necessary to adjust the ratio of SiO₂/Ce₂O₃ to form the liquid phase region. It shows that in the SiO₂-Ce₂O₃-5wt.% Al₂O₃ system, when the ratio of SiO₂/Ce₂O₃ is moderate, the addition of an appropriate amount of CaO can significantly reduce the melting point of the mixed oxide. This also provides a new idea for solving the problem of nozzle clogging caused by Ce₂O₃-type high-melting-point fine inclusions produced in the smelting process of rare earth steel, that is, the high-melting-point rare earth enrichment phase can be modified into a low-melting-point phase through the synergistic effect of calcium-silicon or calcium-aluminum, so as to fundamentally solve the clogging problem caused by the enrichment of high-melting-point fine inclusions at the nozzle, which is also a focus of our subsequent research.

In the above research system, the basicity range in the liquid phase region increases with the decrease of Ce₂O₃ content. At a fixed basicity, the liquidus temperature increases with the increase of Ce₂O₃ content. At the same time, obvious knob phenomenon was observed on each isotherm, and the composition of knob moved to higher basicity and higher Ce₂O₃ content with the increase of temperature.

Construction of SiO₂-Ce₂O₃-CaO-5wt.% Al₂O₃ System Phase Diagram

Phase diagram can be used to quantitatively describe the phase equilibrium of systems. It can help to determine the direction of a reaction and avoid the resource consumption during the research process. Combining the isotherms of SiO₂-Ce₂O₃-CaO-5wt.% Al₂O₃ system at 1673–1873 K with the above equilibrium phase structure, a phase diagram related to the system containing rare earth was constructed, as shown in Fig. 7. According to the trend of phase boundary, the intersection point of phase boundary is found. Meanwhile, several points on the CaO-SiO₂ phase boundary are data points obtained according to the known phase diagram [20].

Fig. 7

Phase diagram of SiO₂-Ce₂O₃-CaO-5wt.% Al₂O₃ system

Combining the current results with the phase diagram information of CaO-SiO₂-Ce₂O₃ [6], the isotherms and phase relationships of specific compositions related to rare earth element Ce were constructed in Fig. 7. It is worth noting that the liquid phase data points on the SiO₂-CaO line, that is, when the content of Ce₂O₃ is 0, the data obtained by the experiment in this study are basically consistent with the known CaO-SiO₂-Al₂O₃ system phase diagram [20] data points, which again proves that the experiment has high accuracy. In the region from 1 to 6, according to the law of the adjacent phase region and the trend of the phase boundary line shown by the experimental results, the phase relationship between region 2 and region 3, region 3 and region 4 is deduced, that is, the three-phase equilibrium between Ca_2-xCe_xSiO_4+δ, CaO·2Ce₂O₃·3SiO₂ and liquid phase, and the two-phase equilibrium between Ce₂Si₂O₇ and liquid phase. In addition, the phase relationship in the solid phase region with SiO₂ content less than 20 wt.% and the dotted region is not studied in this work.

The equilibrium state detected from 1673 K and the change trend of the primary crystal phase at 1773 K can be summarized as follows. The liquid phase region is mainly in the range of SiO₂ content in 50 wt.%-75 wt.%. In the low Ce₂O₃ content and low basicity region, the primary phase is SiO₂. In the region of high basicity and low Ce₂O₃ content, the precipitated phase is Ca₂SiO₄. With the increase of Ce₂O₃ content, the main precipitated phase is transformed into CaO·2Ce₂O₃·3SiO₂. It can be found that the liquidus at 1673 K is very close to the liquidus at 1773 K in the large area near the phase boundary between Ca_2-xCe_xSiO_4+δ and CaO·2Ce₂O₃·3SiO₂. This indicates that the area of the primary crystal zone of CaO·2Ce₂O₃ 3SiO₂ in the system of this study is less affected by temperature in the range of 1673 K to 1773 K, and is greatly affected by basicity.

Conclusions

In the current study, the phase equilibrium relationship of CaO-SiO₂-Ce₂O₃-5wt.% Al₂O₃ phase diagram system was studied. By using XRD, XRF, EPMA, FE-SEM, and EDS analysis, the equilibrium phase at 1673–1873 K was determined. The main results can be summarized as follows:

• (1) The compositions of liquid phase (L), CaO·2Ce₂O₃·3SiO₂ solid solution phase, Ca_2-xCe_xSiO₄ + δ (Ca₂SiO₄, as shown in Fig. 9, by converting Ce³⁺ to Ca²⁺) solid solution phase and Ce₂Si₂O₇ phase were determined by high temperature equilibrium technique at 1673–1873 K.

• (2) The isothermal liquid phase of CaO-SiO₂-Ce₂O₃-5wt.% Al₂O₃ system at 1673–1873 K was determined by high temperature equilibrium experiment. At the same time, the location of the primary crystal zone of the four precipitated phases was determined.

• (3) According to the experimental results, the isothermal section diagram of CaO-SiO₂-Ce₂O₃-5wt.% Al₂O₃ system at 1673–1873 K was obtained.