Effect of CeO₂ Content on Melting Performance and Microstructure of CaO-Al₂O₃-SiO₂-MgO Refining Slag

Abstract

CeO₂ can be applied to refining slag to minimize the size of inclusions, speed up the deoxidization process, and adsorb Al₂O₃ inclusions. The impact through which CeO₂ content affects slag’s melting efficiency is still uncertain. The thermal analyzer was used to measure the thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) curves of the slag melting process. According to the study results, with the increase in CeO₂ content, the melting temperature of slag decreased first and then increased. The slag’s melting point fell from 1364 °C to 1324 °C and then rose to 1503 °C. XRD and SEM were used to analyze the CaO-Al₂O₃-SiO₂-MgO-CeO₂ slag’s microstructure. The mineral-phase structure of CeO₂-containing refining slag was primarily composed of Ca₂SiO₄ and 3CaO·Al₂O₃, MgO, SiO₂, CaO·Al₂O₃, and Ca₈Ce₆Al₆O₂₆. The proportion of 3CaO·Al₂O₃, CaO·Al₂O₃, and Ca₂SiO₄ decreased as the rare-earth-oxide content increased, while the proportion of Ca₈Ce₆Al₆O₂₆ increased. FactSage was used to estimate the equilibrium-phase compositions of slags with various compositions, and a model for predicting melting points was carried out by a linear regression model. Results were obtained through the analysis of equilibrium-phase composition and crystal structure transformation. The main reasons for the melting point decrease were the change of degree of polymerization and the decrease in contents and complete melting temperature of high-melting-point Ca₃Al₂O₆ and Ca₂SiO₄ compounds. The latter increase in melting point was due to the number of Ca₈Ce₆Al₆O₂₆ compounds and precipitation temperature increases and the complexity of the structural-network increases.

1. Introduction

The rare-earth cerium element belongs to the third subgroup, has a high atomic number, and has electronegativity between Mg and Ca. As a result, it is frequently used in the metallurgical industry as a microalloying material to achieve the goals of deep purification of molten steel, inclusion modification, and grain refinement [1,2,3,4], thereby improving the corrosion resistance, wear resistance, and heat resistance of steel [5,6,7]. The core of rare-earth steel process control is to obtain low oxygen and low sulfur conditions of molten steel. Therefore, the aluminum deoxidation process is the best choice, and production of Al₂O₃ inclusions are unavoidable. It will result in the burning loss of rare-earth elements if it is not floated in time. At the moment, the CaO-Al₂O₃-SiO₂-MgO refining slag that can reduce the content of oxygen, sulfur, and inclusions in molten steel is widely used. According to the study in [8,9,10,11,12,13,14], Al₂O₃ inclusions can be well absorbed after adding rare-earth oxides to the refining slag system, and the deoxidation rate and inclusion grade reduction rate can reach more than 15%. Furthermore, cerium oxides can raise the activity of Ce₂O₃, which inhibit the oxidation of rare-earth elements in molten steel, which can significantly improve its solid-solution–rare-earth ratio. However, rare-earth oxides belong to alkaline oxides, and too-high basicity will affect the melting temperature of refining slag, but the impact of rare-earth oxides on slag melting temperature is still not clear.

Researchers have made some progress in understanding the effect of rare-earth oxides on various slag melting performances. According to Xi’s research [15], the melting point of converter slag containing CeO₂ and La₂O₃ decreased first and then increased. The slag temperature was lowest when the mass fraction of rare-earth oxides was 6%. Long [16,17] discovered that when the m(CaO)/m(Al₂O₃) mass ratio in refining slag is 1, the melting temperature increases first and then decreases, reaching a maximum of 1516 °C when the addition amount is 10%, but the influence mechanism is still unknown. Qi [9,10] discovered that adding rare-earth oxides could reduce not only the critical cooling rate of mold flux but also the initial crystallization temperature. Rare-earth oxides can be used in various types of slag, but the effect and mechanism of rare-earth oxides on slag differ due to the differences in slag composition.

Some research has been done to determine the effect of Ce₂O₃ content on the melting performance of refining slag. Ueda and Wang [18,19] discovered that adding Ce₂O₃ produced some high-melting-point mineral-phase structures (3CaO·5Al₂O₃·Ce₂O₃ and 8CaO·3Ce₂O₃·3Al₂O₃), increasing the melting temperature of slag, but no quantitative relationship between the addition amount and the melting temperature was found. Chen [20] discovered that adding 10% of CeO₂ increases the liquid phase ratio of slag at 1600 °C, owing to the formation of a low-melting-point composite-mineral-phase structure between CeO₂ and a high-melting-point calcium aluminate compound (CaO·6Al₂O₃), lowering the melting temperature of slag. The thermodynamics of the CaO-Al₂O₃-Ce₂O₃ slag system were evaluated by Liu [21]. When the Ce₂O₃ addition is less than 48%, the slag can be melted at 1600 °C, and the thermodynamic data of the slag phase structure are optimized. Long [16,17] believed that the content of high-melting-point calcium aluminate had a large influence on the melting temperature, whereas Xi [15] believed that the formation of rare-earth silicide (CeSiO₄) had a large influence on the melting temperature. According to scholar research summaries, the research conclusions on the influence of rare-earth oxides on the melting performance of refining slag are not unified, and the studies are all about Ce₂O₃. However, CeO₂ is the most stable state of cerium oxide at room temperature, and Ce₂O₃ has significant practical-application limitations. As a result, studying the effect of CeO₂ on the melting performance of refining slag is more systematic and comprehensive.

The phase structure is closely related to the melting temperature of slag. Slag melting temperature is bound to be affected by the formation of high-melting-point compounds [21,22,23]. The melting temperature of the slag was measured using a thermal analyzer in this paper, and the effect of CeO₂ content on the melting temperature of the slag was investigated. Then FactSage was used to calculate the equilibrium-mineral-phase structure of the slag, and the influence of CeO₂ content in the slag on the mineral-phase structure is summarized to provide a theoretical basis for optimizing the rare-earth refining slag.

2. Experimental Materials and Methods

2.1. Experimental Materials

The raw materials used in this experiment are refining slag, 99.9% pure CeO₂ reagent, 99% pure MgO crucible, and graphite crucible. The slag weighs 250 g in total.

Table 1. Composition of refining slag (mass fraction/%).

Compositions	Al2O3	CaO	MgO	TFe	SiO2	MnO	F	S	Na2O	TiO2	R(CaO/SiO2)
Contents	26.79	53.75	6.84	0.34	11.22	0.21	0.47	0.17	0.07	0.14	4.79

shows the specific composition of refining slag.

2.2. Experimental Method

In order to let CeO₂ react fully with the refining slag, the pre-melting operation of the slag was performed first. Six groups of experiments were conducted. Refining slag’s compositions are as shown in Table 1. Refining slag was grinded fully with CeO₂ reagent in an agate crucible before being melted in a tubular resistance furnace. According to the composition of refining slag and the content of CeO₂ added, the approximate composition of slag was calculated, and the results are shown in

Table 2. Experimental slag-composition ratios.

Experimental Group	Slag Composition/%
	Al2O3	CaO	MgO	SiO2	CeO2
0	27.17	54.51	6.94	11.37	0
1	25.81	51.79	6.59	10.81	5
2	25.00	50.15	6.39	10.46	8
3	24.46	49.06	6.25	10.24	10
4	23.10	46.34	5.90	9.67	15
5	21.74	43.61	5.55	9.10	20

. The following were the slag-preparation steps: (1) The mixed reagent (refining slag and CeO₂) was loaded into the MgO crucible, which was then placed in the tubular resistance furnace. (2) In an argon atmosphere, the heating rate was set to 10 °C/min, the slag was heated to 1600 °C and held for 1 h, and then it was slowly cooled to room temperature in an argon atmosphere.

The mineral-phase structure of the prepared slag was analyzed. The characterization of phase structure of slag was determined using an X-ray diffractometer (D8 Advance, AXS, Brook, Germany). The target material was Cu, and the radiation type was monochromatic X-ray at a 1.54e wavelength. The light microscope (DSX 510, Olympus, Fangu, Japan) was used to observe the distribution morphology of phase in slag. The scanning electron microscope (TESCAN VEGA3 XMH, Brno, Czech Republic) was used to analyze the composition information of slag phase. The imaging mode was secondary electron and backscattered electron, and the accelerating voltage was 20 kV.

Subsequently, the melting-point test experiment of the pre-melted slag was carried out. Firstly, the high-temperature confocal microscope was used to observe whether the slag could be completely melted at 1600 °C. Then, the TG-DSC thermal analyzer (DTA449C) of the NETZSCH Company, Bavaria, Germany, was used to determine the melting point of slag. The instrument has a temperature range of up to 1650 °C, a sample measurement range of 0~50 mg, and a sensitivity of 0.01 μg. Alumina crucible was used in the experiment. (Because refining slag is pre-melted slag with a stable slag phase and the structure of the crucible is denser, the alumina-crucible reaction is not considered). To improve the uniformity of samples, a ball crusher was used to powder the sintered slag, which was then ground for 10 min with an agate grinding dish to less than 300 meshes. A 10 mg sample was weighed and placed in a crucible. It was then placed in a hot balance room and purged with 20 mL/min of high-purity argon gas while protected by 80 mL/min of high-purity argon gas. The experiment’s heating rate was 20 °C/min, rising from room temperature to 1600 °C. The instrument automatically recorded the data of weights and heats.

3. Experimental Results

3.1. Microstructure of Refining Slag

When CeO₂ is added in amounts less than 10%, the slag is bluish green in color, has little porosity, and has a compact structure. The color of the slag changes to black with a considerable local porosity when CeO₂ is added in amounts greater than 10%. Epoxy resin was used to embed the slag sample, which was then examined under a light microscope. Figure 1 shows the mineral-phase structure of slag. The mineral-phase structure shows that the microstructure of the slag without rare-earth-oxide treatment is bulk calcium aluminate and doped with rod-like calcium silicate mineral-phase structure. When the CeO₂ content is less than 15%, the slag is dominated by small crystals, with no obvious dendritic crystals. Dendritic crystals dominate the slag phase structure with a CeO₂ content of more than 15%.

SEM was used to examine the slag, and an energy-dispersive spectrometer was used to determine the specific composition of the mineral-phase structure. Figure 2 depicts the results. It can be seen from the element-distribution map that there are some desulfurization products (CaS) in the slag, which are dispersed in the slag without aggregation. In the No.0 slag, the matrix phase structure is 3CaO·Al₂O₃, CaO·Al₂O₃, Ca₂SiO₄ has a dispersed distribution, and there are more developed MgO crystals. After the addition of CeO₂, the Ca-Al-Ce-O compound appears (as a white-striped structure in the backscattered electron mode), and its elemental composition conforms to the Ca₈Ce₆Al₆O₂₆ structure by EDS analysis, as shown in Figure 3.

The prepared slag was crushed and sieved to a size of less than 0.0074 mm, and the phase contained was detected and analyzed using XRD, the results of which are shown in Figure 4. The control group’s mineral-phase structure was primarily composed of Ca₂SiO₄ and 3CaO, Al₂O₃, MgO, SiO₂, and CaO. The mineral-phase structure of CeO₂-containing refining slag was primarily composed of Ca₂SiO₄ and 3CaO·Al₂O₃, MgO, SiO₂, CaO, and Ca₈Ce₆Al₆O₂₆. The proportion of 3CaO·Al₂O₃ decreased as the rare-earth-oxide content increased, while the proportion of Ca₈Ce₆Al₆O₂₆ increased.

The surface composition scanning and XRD quantitative phase analysis method were used to count the few fields of view of slag, and the comparison shown in

Table 3. Slag’s mineral-phase ratios.

ID	xCaO·Al2O3	Ca2SiO4	Ca8Ce6Al6O26	MgO
(a)	65~75	33	-	3
(b)	54~56	32	6–8	4
(c)	52~53	34	11~13	2
(d)	44~50	35	15~18	3
(e)	41~43	31	23~25	3
(f)	34~36	33	28~30	3

was obtained. MgO is converted by Mg element content, Ca₈Ce₆Al₆O₂₆ is converted by Ce element content, and xCaO·Al₂O₃ is the residual phase fraction. The proportion of xCaO·Al₂O₃ decreased as CeO₂ content increased, while Ca₈Ce₆Al₆O₂₆ increased, and MgO and Ca₂SiO₄ content did not change significantly. Through the quantitative analysis of mineral-phase structure, low-melting-point substances such as calcium aluminate and calcium silicate accounted for a large proportion, indicating that the melting-point peak can represent the slag stably flow temperature.

3.2. Measurement Results of Melting Point

To explore whether the slag can be melted at 1600 °C, the melting of slag containing 15% of CeO₂ during heating and the crystallization process during cooling was observed by a high-temperature confocal microscope. First used was a heating rate of 5 °C/s to 1200 °C, then a heating rate of 0.5 °C/s to 1600 °C, then the same cooling rate of 0.5 °C/s from 1600 °C to 1200 °C, and finally a 5 °C/s cooling rate to room temperature. The results are shown in Figure 5.

It can be seen from the high-temperature rock phase that the slag powder was not melted at 1311.4 °C and was in a granular state. At 1351.6 °C, the low-melting-point compounds of slag began to melt, and the solid particles were wrapped by liquid slag. At 1360 °C, the low-melting-point material was completely melted, and there were some high-melting-point compounds between the liquid slags. With the further increase in temperature, reaching 1397.8 °C, the number of high-melting-point substances became fewer but still existed; when the temperature reached 1500.3 °C, the slag completely melted, and there was no solid material. At 1600.2 °C, there was also no homogeneous liquid phase, indicating that the slag can be completely melted at 1600 °C. With the gradual cooling of the slag, at 1284.7 °C, some precipitates began to appear; at 1112.6 °C, most of the liquid phase began to transform into a solid and, at 1076.0 °C, transformed completely into a solid.

Figure 6 shows the melting-point test results of each group of refining slag. According to the ICTA Standardization Committee, when the DSC and TG curves are used to characterize the melting temperature of a substance, the melting point is defined as the intersection of the front baseline (the line segment with little change in heat before the melting-point peak) extension line and the tangent at the melting peak’s maximum slope. However, the DSC curve shows that there is no obvious front baseline before the melting-point peak, making the initial melting temperature impossible to measure. As a result, the melting-point peak is regarded as the initial melting temperature.

The endothermic peaks of CaCO₃ and Ca(OH)₂ appear in the DSC-TG curve because some of the powder after the preparation reacted with H₂O and CO₂ in the air. During the heating process, there is an obvious endothermic peak, which is in the range of 1300~1500 °C, and the mass changes little, so it can be judged to be the phase transition temperature of slag. It can be seen from the curve that with the increase in CeO₂ content, the temperature of the endothermic peak first decreases and then increases. It can be concluded that CeO₂ content has a great influence on the melting temperature of slag. By integrating the area of the endothermic peak (Hf), it is found that the heat absorption increases first and then decreases, from 74.87 J/g to 116.3 J/g, and then decreases to 94.85 J/g. It indicates that the proportion of low-melting-point minerals in the structure of slag increases and the number of production increases.

Several experiments were repeated to make the results more rigorous, and the measured melting of refining slag is shown in Figure 7, and the blue line indicates the downward trend of the melting point, and the red line indicates the upward trend of the melting point. When the CeO₂ content is 5~15%, the melting point of refining slag is between 1320 °C and 1380 °C, which can meet the requirements of refining. When the CeO₂ content exceeds 15%, the melting point increases rapidly, and it cannot be completely melted during the refining process, affecting the refining rhythm. The melting point of unpre-melted refining slag with 18% of CeO₂ content was tested to confirm the trend’s correctness. It was discovered that the melting point of the refining slag with 18% of CeO₂ content was higher than 15%, implying that the melting point decreased when the CeO₂ content was less than 15%. When the CeO₂ content exceeds 15%, the melting point rises. The melting temperature of slag is closely related to the mineral-phase structure. The formation of high-melting-point compounds is bound to affect the melting temperature of slag [20,21]. Therefore, it is necessary to study the law of slag mineral-phase structure.

4. Analysis and Discussion

4.1. Effect of CeO₂ Content on Equilibrium Mineral Phase of Slag

The most stable valence state of cerium at room temperature is +4, but under the reducing condition of refining slag, the process of CeO₂ converting into Ce₂O₃ will occur, as shown in Equation (1). The activity of FeO is the most intuitive index to measure the oxidizability of slag. The activity of FeO is 2.35 × 10⁻¹⁴~4.71 × 10⁻¹⁰ under the composition shown in Table 2 by coexistence theory. By querying the distribution coefficient of oxygen between slag and molten steel, the oxygen concentration w[O] % in slag at equilibrium can be calculated to be 5.38 × 10⁻¹⁵~1.08 × 10⁻¹⁰. According to the equilibrium of Equation (1), the equilibrium constant K₁ at 1600 °C can be calculated to be 5.28 × 10⁻⁸, and then w(Ce₂O₃)%/w(CeO₂)% = 9.82 × 10⁶~1.12 × 10². Therefore, it is considered that CeO₂ can be completely converted to Ce₂O₃ under the current low-oxygen conditions.

2CeO₂ = Ce₂O₃ + [O] △G₁^θ = −205.8T + 646400

(1)

lg(w[O]%) = lg(a(FeO)) − (6320/T − 2.734)

(2)

where G₁^θ is the Gibbs free energy of the reaction from CeO₂ to Ce₂O₃ and K₁ is the equilibrium constant of the reaction. Additionally, a(FeO) is the activity of FeO in slag, and w[O] % is the oxygen content in slag.

FactSage thermodynamic software (v8.2, GTT-Technologies, Achen, Germany) was used to calculate the mineral-phase structure that may be formed by the slag compositions in Table 2. In the thermodynamic software simulation process, the database was FToxid, the cooling method was Normal Equilibrium Cooling, the temperature range was 1000~1600 °C, the calculation step was 50 °C, the pressure was set to 1 atm, and the equilibrium oxygen partial pressure was set to P_O2 = 10⁻¹⁰. Figure 8 shows the calculation results of equilibrium precipitates in the finished product.

At 1600 °C, the MgO phases were first formed in the liquid slag, followed by the α-Ca₂SiO4 phase at 1475~1500 °C, the β-Ca₂SiO₄ phase at 1418~1422 °C, the 3CaO·Al₂O₃ phase at 1353~1378 °C, the Ca₈Ce₆Al₆O₂₆ phase at 1370~1502 °C, and the CaO·Al₂O₃ phase at 1275 °C. The phase equilibrium results show that as the CeO₂ content increased, the production of MgO did not change significantly. The β-Ca₂SiO₄, CaO·Al₂O₃, and 3CaO·Al₂O₃ phases production decreases while Ca₈Ce₆Al₆O₂₆ production increases. The following are the main reactions:

3CaO(s) + Al₂O₃(s) = Ca₃Al₂O₆(l)    ΔG₃^θ = −252400 + 52.64T

(3)

CaO(s) + Al₂O₃(s) = CaAl₂O₄(l)    ΔG₄^θ = −49060 − 6.628T

(4)

8CaO(s) + 3Ce₂O₃(s) + 3Al₂O₃(s) = 8CaO·3Ce₂O₃·3Al₂O₃(s)    ΔG₅^θ = −733100 + 43.76T

(5)

According to Equations (3)–(5), at the experimental temperature of 1600 °C, ΔG₃^θ = −153,805 J/mol, ΔG₄^θ = −59,722.9 J/mol, ΔG₅^θ = −651,137 J/mol, so the three compounds can be completely generated, and the source of Ce₂O₃ in slag is CeO₂ transformed into Ce₂O₃.

With the increase in CeO₂ content, the precipitation temperature of Ca₃Al₂O₆ and β-Ca₂SiO₄ decreases, and the precipitation temperature of 8CaO·3Ce₂O₃·3Al₂O₃ increases. That is, at the same temperature, the melting ratio of slag increases, which reduces the melting point of slag.

4.2. Effect Mechanism of CeO₂ Content on Slag Melting Performance

The melting temperature of five-membered refining slag decreases first and then increases as the CeO₂ content increases. The CeO₂ content primarily influences the melting process of high-melting-point oxides. The effect of CeO₂ on the melting point of slag is divided into early and later, two processes. During the initial stage, the melting temperature decreases as the content of CeO₂ increases from 0% to 15%. CeO₂ mainly reduces the equilibrium production of high-melting-point Ca₃Al₂O₆ and Ca₂SiO₄ by reacting with Al₂O₃ and CaO, while the number of Ca₈Ce₆Al₆O₂₆ composite compounds increases little, and meanwhile, CeO₂ promotes the reduction of complete melting temperature of Ca₃Al₂O₆ and β-Ca₂SiO₄. Ca₈Ce₆Al₆O₂₆ is also a high-melting-point compound. During the later stage, the number of Ca₈Ce₆Al₆O₂₆ compounds and precipitation temperature increases as CeO₂ content increases, resulting in an increase in slag melting temperature.

At the same time, the melting point is related to the crystal’s resistance to the temperature effect, which is determined by the crystal lattice energy and the Coulomb attraction between ions. The higher the melting point of slag, the greater the lattice energy. According to Qi’s [24] research, in the range of 0~20%wt, Ce₂O₃ plays an opposing role in two different phases. With the increase of Ce₂O₃ mole fraction in the silicate slag, the total proportion of QSi₃ and QSi₂ structural units(shown at Figure 9) with a high degree of polymerization will increase, and the complexity of the network will increase. With the increase of the Ce₂O₃ mole fraction in the aluminate slag, the relative proportion of [AlO₄]-tetrahedron as the network-forming body decreases first and then stabilizes, and the network-modifier [AlO₆]-octahedron’s relative proportion rises. The degree of polymerization of the slag showed a trend of decreasing first and then becoming stable. With the CaAl₂O₄ having an [AlO₄]-hexahedral structure, the composite compounds Ca₃Al₂O₆ and Ca₈Ce₆Al₆O₂₆ have an [AlO₆]-octahedron structure. The total amount of the Ca₃Al₂O₆ and Ca₈Ce₆Al₆O₂₆ is increasing, as is the proportion of octahedron structure. The compactness of the hexahedral structure is greater than that of the octahedral structure. As a result, the melting point of slag is decreasing due to the change of degree of polymerization. Simultaneously, the content of the silicate mineral phase does not change much with the content of CeO₂, but the complexity of the structural-network increases, resulting in the trend of slag melting point decreasing first and then increasing.

The reasons for the melting point decrease are the change of degree of polymerization and the quantity and precipitation temperature decrease of high-melting-point Ca₃Al₂O₆ and Ca₂SiO₄ compounds. The latter increase in melting point is due to the number of Ca₈Ce₆Al₆O₂₆ compounds and precipitation temperature increases and the complexity of the structural-network increases.

4.3. A Model for Predicting Melting Points Using CeO₂ Refining Slag

There are many uncontrollable factors in the measurement process of slag, so it is necessary to establish a prediction model of slag melting point. However, there is no accurate model of the melting point at present. Lolja [25] found that when a single oxide content was used as a variable, it was impossible to correlate with the melting point, so the combination form containing multiple components was used for fitting and prediction. Therefore, the multiple linear regression method is selected to establish the melting-point prediction model.

$$y={\beta }_0+{\beta }_1{x}_1+{\beta }_2{x}_2+{\beta }_3{x}_3+\cdots +{\beta }_p{x}_p+\epsilon$$

(6)

As defined, ${\beta }_0,{\beta }_1,{\beta }_2,{\beta }_3,\cdots ,{\beta }_p$ are independent of unknown parameters, and $\epsilon$ is an unobservable random variable.

Table 4. Model regression results.

Model	Variables and Parameters	Non-Standardized Coefficients		Normalized Coefficients	R2
		B	Standard Error	Beta
CeO2 addition ≤15%	Constant	1237.45	-	-	0.879
	MgO	5.9022	157.5922	0.0059
	CaO·Al2O3	1.7448	15.0276	0.0201
	3CaO·Al2O3	0.3798	7.3960	0.0934
	Ca8Ce6Al6O26	−0.277	0.6375	−0.1303
	Ca2SiO4	1.2146	301.3412	0.0285
CeO2 addition >15%	Constant	42	-	-	1
	CeO2	73	-	-

is the regression result of model parameters.

In order to make the model more accurate, the model is divided into two kinds of CeO₂ addition amounts, ≤15% and >15%. Because the amount of data is less when the addition amount is more than 15%, the melting temperature and CeO₂ content are used to establish the model. When the addition amount is less than 15%, the regression model is established by using the equilibrium-mineral-phase composition and melting temperature. The results are shown in Table 4.

Through C regression analysis, it can be concluded that the fitting accuracy R² of the model is 0.879, indicating strong applicability. As a result, the goal of predicting the melting point of CeO₂-bearing refining slag by equilibrium-phase structure can be accomplished, providing a theoretical basis for the rare-earth steel refining process.

5. Conclusions

In this paper, the effect of CeO₂ content on the melting properties of the CaO-Al₂O₃-SiO₂-MgO slag system is systematically analyzed by thermodynamic calculation and experiment. The following conclusions are obtained:

(1) The melting temperature of slag was measured by the thermal analyzer. With the increase in CeO₂ content, the melting temperature of slag decreased first and then increased, and the heat absorption increased first and then decreased, from 74.87 J/g to 116.3 J/g, and then decreased to 94.85 J/g. It indicates that the proportion of low-melting-point minerals in the structure of slag increased and the number of productions increased.

(2) The influence of CeO₂ content on the mineral-phase structure was analyzed by XRD characterization and an energy spectrometer. The results show that with the increase in CeO₂ content in slag, the proportion of 3CaO·Al₂O₃, CaO·Al₂O₃, and β-Ca₂SiO₄ decreased, and the proportion of Ca₈Ce₆Al₆O₂₆ increased. In the argon environment, CeO₂ first transformed to Ce₂O₃, and rare-earth elements mostly existed in the form of rare-earth compounds.

(3) The equilibrium-phase composition of slag with different compositions was calculated by FactSage, and a model for predicting melting points was carried out by a linear regression model. The fitting accuracy of the model was above 0.879, which can achieve the purpose of accurately predicting the melting point of slag.

(4) The reasons for the melting-point decrease are the transformation of calcium aluminate crystal structure and the decrease in contents and complete melting temperature of high-melting-point Ca₃Al₂O₆ and Ca₂SiO₄ compounds. The latter increase in melting point is due to the number of Ca₈Ce₆Al₆O₂₆ compounds and precipitation temperature increases and the crystal transformation of calcium silicate.