Influence of TiO2, Al2O3, and Basicity on Viscosity and Structure of High Titanium-Bearing Blast Furnace Slag

The viscosity of high-titanium blast furnace slag with different TiO2 content, Al2O3 content, and basicity was measured at 1653–1773 K using the rotational cylinder method. The phase composition of the slag is measured by XRD. Phase diagram of the slags is calculated by FactSage software. Ionic network structure of the slags is analyzed by FT–IR. Results show that TiO2 depolymerizes the silicate network structure, reducing viscosity at high temperature, while increasing Al2O3 content generates a more complicated silicate, increasing viscosity. Basicity affects viscosity, with higher basicity resulting in lower viscosity above 1733 K. Perovskite significantly affects the viscosity of slag. This study provides an in-depth understanding of the relationship between the composition and viscosity of high-titanium blast furnace slag, which is very important for improving production efficiency.


Introduction
Viscosity is an important physical property of slag to determine the stability and productivity of the blast furnace iron-making. Conventional blast furnace slag mainly contains CaO, SiO 2 , MgO, and Al 2 O 3 . When vanadium-titanium ore is used as a raw material for blast furnace iron-making, the softening and melting zone of the furnace changed [1][2][3][4][5][6]. Most of the TiO 2 enters the blast furnace slag, making it sticky and causing difficulties in separating slag and iron in the furnace hearth. The viscosity and structure of the blast furnace slag changed due to the addition of TiO 2 [7][8][9].
High titanium-bearing blast furnace slag is the main type of blast furnace slag produced by vanadium and titanium ore in the Panzhihua area during blast furnace smelting, which is mainly composed of oxides such as CaO, SiO 2 , MgO, Al 2 O 3 , and TiO 2 , and when the temperature is constant, the viscosity of the slag mainly depends on these oxide components [10,11]. Some investigations on the viscosity of titanium-bearing slag have been carried out. It is generally believed that the viscosity decreases with the increase in TiO 2 content [12,13]. However, there are still different opinions about the mechanism of the influence of TiO 2 . One opinion is that TiO 2 as the network former forms the simpler structure of TiO 4 4− monomers. So, the strength of the network structure is weakened [11,14,15]. Another opinion is that TiO 2 acting as a basic oxide in the slag depolymerizes the silicate network structure [16,17].
Suitable basicity plays a crucial role in the blast furnace smelting process. Excessive basicity leads to an increase in high-melting-point compounds in the slag, resulting in poor fluidity of blast furnace slag and difficulty in separating slag and iron. If the basicity is too low, the desulfurization capacity of blast furnace slag decreases, and at the same time, the blast furnace slag erodes the furnace lining. On the other hand, it is well-known that Al 2 O 3 is a typical amphoteric oxide. In the CaO-MgO-SiO 2 -Al 2 O 3 slag system, whether Al 2 O 3 acts as an acidic oxide or as an alkaline oxide depends on the slag composition. In some slags, Al 2 O 3 is more inclined to act as a network former, and the viscosity increases with

Materials and Methods
Preparing slag by mixing pure oxides (MgO, SiO 2 , Al 2 O 3 , TiO 2 ) and pure CaCO 3 as the source of CaO. First, 200 g of slag powder was pre-melted at 1773 K for 1 h, then the sample was removed and cooled to room temperature by water cooling to obtain a water-quenched slag as an experimental sample. About 20 g slag samples were used to carry out the qualitative FT-IR and XRD characterization of the slags and an additional slag sample was used to measure the viscosity. The calculated chemical composition of the slag is shown in Table 1. The design composition of the slag is based on the actual composition of the Pangao steel plant [8][9][10]. In this study, the slag viscosity was measured using the rotating-cylinder method [22]. The experimental equipment used is a high temperature melt property tester (RT-3, Mingjian Hi-Tech Industrial Co., Suzhou, China). Figure 1a shows the experimental apparatus, which consists of an electric resistance furnace equipped with U-shaped MoSi 2 heating elements Materials 2023, 16, 2575 3 of 13 for system heating. A Mo crucible with a height of 80 mm and an inner diameter of 40 mm was used to hold a 140 g sample, which was heated at a rate of 5 K/min up to 1773 K with a constant flow of Ar gas (500 mL/min). Viscosity measurement was conducted at every 40 K interval during cooling, with an equilibration time of 30 min at each temperature. The rotating spindle, as detailed in Figure 1b, was set to rotate at a speed of 300 rev/min, and five viscosity measurements were taken at the same temperature. The viscosity reported in this paper is an average of these five measurements. 18 24.62 22.38 10.00 18.00 25.00 1.10 In this study, the slag viscosity was measured using the rotating-cylinder method [22]. The experimental equipment used is a high temperature melt property tester (RT-3, Mingjian Hi-Tech Industrial Co., Suzhou, China). Figure 1a shows the experimental apparatus, which consists of an electric resistance furnace equipped with U-shaped MoSi2 heating elements for system heating. A Mo crucible with a height of 80 mm and an inner diameter of 40 mm was used to hold a 140 g sample, which was heated at a rate of 5 K/min up to 1773 K with a constant flow of Ar gas (500 mL/min). Viscosity measurement was conducted at every 40 K interval during cooling, with an equilibration time of 30 min at each temperature. The rotating spindle, as detailed in Figure 1b, was set to rotate at a speed of 300 rev/min, and five viscosity measurements were taken at the same temperature. The viscosity reported in this paper is an average of these five measurements.

Results and Discussion
The viscosity of the CaO-SiO2-MgO-Al2O3-TiO2 slags with different basicity, TiO2 content, and Al2O3 content are shown in the Table 2. The viscosity increases with the decrease in temperature. When the temperature decreases to 1653 K, the viscosity shows a large increasing trend in some slag systems.

Results and Discussion
The viscosity of the CaO-SiO 2 -MgO-Al 2 O 3 -TiO 2 slags with different basicity, TiO 2 content, and Al 2 O 3 content are shown in the Table 2. The viscosity increases with the decrease in temperature. When the temperature decreases to 1653 K, the viscosity shows a large increasing trend in some slag systems.  Figure 2 shows the effect of TiO 2 content on viscosity of the CaO-SiO 2 -8 mass% MgO-14 mass% Al 2 O 3 -TiO 2 slag with R = 1.10. The viscosity decreases with the increase in TiO 2 content from 10 mass% to 30 mass% when the temperature is greater than 1693 K, which is consistent with previous study [11,15]. However, at 1653 K, the viscosity first increases and then decreases with the increase in TiO 2 content, and it shows a maximum when TiO 2 content is 20 mass%. This is different from the previous studies, where it was found that the viscosity of CaO-SiO 2 -TiO 2 slag is the highest when the TiO 2 content is 25% by mass [15]. 18 1.13 0.24 0.21 0.19 Figure 2 shows the effect of TiO2 content on viscosity of the CaO-SiO2-MgO-14 mass% Al2O3-TiO2 slag with R = 1.10. The viscosity decreases with th in TiO2 content from 10 mass% to 30 mass% when the temperature is greater tha which is consistent with previous study [11,15]. However, at 1653 K, the visc increases and then decreases with the increase in TiO2 content, and it shows a m when TiO2 content is 20 mass%. This is different from the previous studies, wh found that the viscosity of CaO-SiO2-TiO2 slag is the highest when the TiO2 cont by mass [15].  Figure 3 shows the isotherm phase diagram calculated by FactSage [23]. At tures above 1693 K, the composition of the slag is entirely in liquid phase, and its is controlled by the network structure of the liquid phase. As the TiO2 content i increases, the viscosity exhibits a decreasing trend. This can be attributed to th the ionic radius of Ti 4+ is about 1.5 times larger than that of Si 4+ , resulting in a wea between Ti 4+ and O 2− compared to that between Si 4+ and O 2− [8]. Consequently mation of TiO4 4− monomers weakens the network structure [15]. On the other effect of Al2O3 content on viscosity is different from that of TiO2, despite the la radius of Al 3+ compared to that of Si 4+ in the slag [18,24]. Therefore, the reason addition leading to a decrease in viscosity is not yet clear, although it is possible may exist as the basic oxide and depolymerize the network structure in the pr systems.  Figure 3 shows the isotherm phase diagram calculated by FactSage [23]. At temperatures above 1693 K, the composition of the slag is entirely in liquid phase, and its viscosity is controlled by the network structure of the liquid phase. As the TiO 2 content in the slag increases, the viscosity exhibits a decreasing trend. This can be attributed to the fact that the ionic radius of Ti 4+ is about 1.5 times larger than that of Si 4+ , resulting in a weaker bond between Ti 4+ and O 2− compared to that between Si 4+ and O 2− [8]. Consequently, the formation of TiO 4 4− monomers weakens the network structure [15]. On the other hand, the effect of Al 2 O 3 content on viscosity is different from that of TiO 2 , despite the larger ionic radius of Al 3+ compared to that of Si 4+ in the slag [18,24]. Therefore, the reason for TiO 2 addition leading to a decrease in viscosity is not yet clear, although it is possible that TiO 2 may exist as the basic oxide and depolymerize the network structure in the present slag systems.  It can be seen from Figure 3 that liquidus temperature of the slags is greater than 1653 K. Perovskite is present in the slag at 1653 K, according to the XRD curves of the slags in Figure 4. So, the viscosity should be affected by the solid phase precipitated from the slag at low temperatures. As can be seen from Figure 3, the 1653 K isothermal curve is much further away from the composition points of slags containing 15 mass% to 25 mass% TiO2 than from slags containing 10 mass% and 30 mass%. It can be considered that the former have more solid phase in the slag than the later. The content of the solid phase plays a key role in the slag viscosity at 1653 K. Hence, the viscosity increases first and then decreases with TiO2 content from 10 mass% to 30 mass%.  Figure 5 illustrates how basicity affects the viscosity of the CaO-SiO2-Al2O3-MgO-25 mass% TiO2 slag. As shown, the viscosity of the slag decreases with increasing basicity when the temperature is greater than 1733 K. This can be attributed to the network structure, as indicated in Figure 3, where the liquidus temperature of the slag with a basicity ranging from 1.00 to 1.20 is lower than 1733 K. CaO serves as a network modifier, which It can be seen from Figure 3 that liquidus temperature of the slags is greater than 1653 K. Perovskite is present in the slag at 1653 K, according to the XRD curves of the slags in Figure 4. So, the viscosity should be affected by the solid phase precipitated from the slag at low temperatures. As can be seen from Figure 3, the 1653 K isothermal curve is much further away from the composition points of slags containing 15 mass% to 25 mass% TiO 2 than from slags containing 10 mass% and 30 mass%. It can be considered that the former have more solid phase in the slag than the later. The content of the solid phase plays a key role in the slag viscosity at 1653 K. Hence, the viscosity increases first and then decreases with TiO 2 content from 10 mass% to 30 mass%. It can be seen from Figure 3 that liquidus temperature of the slags is greater than 165 K. Perovskite is present in the slag at 1653 K, according to the XRD curves of the slags Figure 4. So, the viscosity should be affected by the solid phase precipitated from the sla at low temperatures. As can be seen from Figure 3, the 1653 K isothermal curve is muc further away from the composition points of slags containing 15 mass% to 25 mass% TiO than from slags containing 10 mass% and 30 mass%. It can be considered that the form have more solid phase in the slag than the later. The content of the solid phase plays a ke role in the slag viscosity at 1653 K. Hence, the viscosity increases first and then decreas with TiO2 content from 10 mass% to 30 mass%.  Figure 5 illustrates how basicity affects the viscosity of the CaO-SiO2-Al2O3-MgO-2 mass% TiO2 slag. As shown, the viscosity of the slag decreases with increasing basici when the temperature is greater than 1733 K. This can be attributed to the network stru ture, as indicated in Figure 3, where the liquidus temperature of the slag with a basici ranging from 1.00 to 1.20 is lower than 1733 K. CaO serves as a network modifier, whic  Figure 5 illustrates how basicity affects the viscosity of the CaO-SiO 2 -Al 2 O 3 -MgO-25 mass% TiO 2 slag. As shown, the viscosity of the slag decreases with increasing basicity when the temperature is greater than 1733 K. This can be attributed to the network structure, as indicated in Figure 3, where the liquidus temperature of the slag with a basicity ranging Materials 2023, 16, 2575 6 of 13 from 1.00 to 1.20 is lower than 1733 K. CaO serves as a network modifier, which depolymerizes the network structure, whereas SiO 2 acts as a network former, which polymerizes the network structure. The rise in basicity corresponds to an increase in CaO content and a decrease in SiO 2 content. In other words, an increase in basicity reduces both the size and number of the silicate network structure, leading to a decrease in viscosity. Figure 6 shows the XRD results of the CaO-SiO 2 -16 mass% Al 2 O 3 -10 mass% MgO-25 mass% TiO 2 slag system at different basicity levels at 1653 K. The solid phase of the slag contains calcium titanate and spinel phases. As the basicity increases from 1.00 to 1.20, the content of the solid phase in the slag increases significantly, leading to an increase in viscosity at 1653 K. In particular, although there are solid phase points with an basicity of 1.00 in the slag at 1653 K, the viscosity is still low. This may be because the solid phase is mainly dispersed in the liquid phase, and when the amount of solid phase is small, it does not form large aggregates. The formation of large aggregates by the solid phase has almost no effect on the viscosity. depolymerizes the network structure, whereas SiO2 acts as a network former, which polymerizes the network structure. The rise in basicity corresponds to an increase in CaO content and a decrease in SiO2 content. In other words, an increase in basicity reduces both the size and number of the silicate network structure, leading to a decrease in viscosity.  Figure 6 shows the XRD results of the CaO-SiO2-16 mass% Al2O3-10 mass% MgO-25 mass% TiO2 slag system at different basicity levels at 1653 K. The solid phase of the slag contains calcium titanate and spinel phases. As the basicity increases from 1.00 to 1.20, the content of the solid phase in the slag increases significantly, leading to an increase in viscosity at 1653 K. In particular, although there are solid phase points with an basicity of 1.00 in the slag at 1653 K, the viscosity is still low. This may be because the solid phase is mainly dispersed in the liquid phase, and when the amount of solid phase is small, it does not form large aggregates. The formation of large aggregates by the solid phase has almost no effect on the viscosity. depolymerizes the network structure, whereas SiO2 acts as a network former, which po ymerizes the network structure. The rise in basicity corresponds to an increase in Ca content and a decrease in SiO2 content. In other words, an increase in basicity reduces bot the size and number of the silicate network structure, leading to a decrease in viscosity.  Figure 6 shows the XRD results of the CaO-SiO2-16 mass% Al2O3-10 mass% MgO-2 mass% TiO2 slag system at different basicity levels at 1653 K. The solid phase of the sla contains calcium titanate and spinel phases. As the basicity increases from 1.00 to 1.20, th content of the solid phase in the slag increases significantly, leading to an increase in vi cosity at 1653 K. In particular, although there are solid phase points with an basicity o 1.00 in the slag at 1653 K, the viscosity is still low. This may be because the solid phase mainly dispersed in the liquid phase, and when the amount of solid phase is small, it doe not form large aggregates. The formation of large aggregates by the solid phase has almo no effect on the viscosity.    Figure 7 shows the effect of Al 2 O 3 on viscosity of the CaO-SiO 2 -Al 2 O 3 -MgO-25 mass% TiO 2 slag with R = 1.10. The viscosity increases with the increase of Al 2 O 3 content in the slag at 1733 and 1773 K, and the viscosity decreases at 1653 K. At 1693 K when the Al 2 O 3 content increases, the viscosity of the slag with 8 mass% MgO decreases and the viscosity of the slag with 10 mass% MgO increases, with the viscosity of the two contents changing in opposite patterns. It is widely believed that Al 2 O 3 acts as an amphoteric oxide in the slag [25][26][27][28]. The amphoteric oxide may show different properties in different slag system [21,26]. In the present study, Al 2 O 3 may be expected, as a network former, to cause an increase in the viscosity at high temperature.  Figure 7 shows the effect of Al2O3 on viscosity of the CaO-SiO2-Al2O3-MgO-25 TiO2 slag with R = 1.10. The viscosity increases with the increase of Al2O3 content slag at 1733 and 1773 K, and the viscosity decreases at 1653 K. At 1693 K when the content increases, the viscosity of the slag with 8 mass% MgO decreases and the vi of the slag with 10 mass% MgO increases, with the viscosity of the two contents ch in opposite patterns. It is widely believed that Al2O3 acts as an amphoteric oxide slag [25][26][27][28]. The amphoteric oxide may show different properties in different slag [21,26]. In the present study, Al2O3 may be expected, as a network former, to ca increase in the viscosity at high temperature. The viscosity of the sample undergoes a significant increase when the temp drops to 1653 K, which can be attributed to the solid phase appearing, as indic Figure 8 calculated by FactSage [23]. As illustrated in Figure 8a, at 1653 K, the li temperature decreases with the increase in Al2O3 content, when the MgO conte mass%. The reduction in solid phase content may be the primary reason for the vi decrease with the addition of Al2O3 content. When the Al2O3 content increases f mass% to 18 mass% at 10 mass% MgO content, the primary crystal zone under change, as observed in Figure 8b. The XRD results depicted in Figure 9 show that th phase is converted from perovskite to perovskite and spinel when the Al2O3 con added. In some instances, the intensity of the perovskite peak reduces with the int tion of the spinel. When the content of spinel in the slag is low, the effect of perovs viscosity is potentially greater than that of spinel. Hence, at 1653 K and 10 mass% content, the addition of Al2O3 content induces a decrease in viscosity. The viscosity of the sample undergoes a significant increase when the temperature drops to 1653 K, which can be attributed to the solid phase appearing, as indicated in Figure 8 calculated by FactSage [23]. As illustrated in Figure 8a, at 1653 K, the liquidus temperature decreases with the increase in Al 2 O 3 content, when the MgO content is 8 mass%. The reduction in solid phase content may be the primary reason for the viscosity decrease with the addition of Al 2 O 3 content. When the Al 2 O 3 content increases from 14 mass% to 18 mass% at 10 mass% MgO content, the primary crystal zone undergoes a change, as observed in Figure 8b. The XRD results depicted in Figure 9 show that the solid phase is converted from perovskite to perovskite and spinel when the Al 2 O 3 content is added. In some instances, the intensity of the perovskite peak reduces with the introduction of the spinel. When the content of spinel in the slag is low, the effect of perovskite on viscosity is potentially greater than that of spinel. Hence, at 1653 K and 10 mass% MgO content, the addition of Al 2 O 3 content induces a decrease in viscosity.

FT−IR Spectra of the Slag
In order to further understand the influence mechanism of the slag compositions on the viscosity, FT−IR spectra of the quenched slag at 1773 K were obtained. Figure 10

FT-IR Spectra of the Slag
In order to further understand the influence mechanism of the slag compositions on the viscosity, FT-IR spectra of the quenched slag at 1773 K were obtained. Figure 10 shows the FT-IR spectra of the slag containing different TiO 2 content. The FT-IR spectrum of the slag can be divided into three crucial ranges according to the wave number. These ranges represent [SiO 4 ]-tetrahedra between 1200 cm −1 and 800 cm −1 , [AlO 4 ]-tetrahedra between 730 cm −1 and 630 cm −1 , and [Al-O-Si]-rocking between 480 cm −1 and 410 cm −1 , respectively [29,30]. dra between 730 cm −1 and 630 cm −1 , and [Al-O-Si]-rocking between 480 cm −1 and 410 respectively [29,30]. The depth of the FT−IR spectra between 1200 cm −1 and 800 cm −1 decreases wit increase in TiO2 content, which indicates that the network structure of [SiO4]-tetrah becomes simpler with TiO2 content addition. The depth of the peak representing the O-Si]-rocking is also decreased with the increase in TiO2 content. The effect of TiO2 con on [AlO4]-tetrahedra is insignificant. According to previous study, the existent form TiO2 in the slag include two types [12,31]. TiO2 as a basic oxide provides Ti 4+ to dep merize network structure or it as an acidic oxide forms simple network structure of T In the present study, TiO2 might be more inclined to existing as a network modifier depolymerizes the silicate network structure. So, the viscosity decreases with TiO2 a tion.  The depth of the FT-IR spectra between 1200 cm −1 and 800 cm −1 decreases with the increase in TiO 2 content, which indicates that the network structure of [SiO 4 ]-tetrahedra becomes simpler with TiO 2 content addition. The depth of the peak representing the [Al-O-Si]-rocking is also decreased with the increase in TiO 2 content. The effect of TiO 2 content on [AlO 4 ]-tetrahedra is insignificant. According to previous study, the existent forms of TiO 2 in the slag include two types [12,31]. TiO 2 as a basic oxide provides Ti 4+ to depolymerize network structure or it as an acidic oxide forms simple network structure of TiO 4 4− . In the present study, TiO 2 might be more inclined to existing as a network modifier and depolymerizes the silicate network structure. So, the viscosity decreases with TiO 2 addition. Figure 11 shows the FT-IR spectra of the slag with different basicity. The depth of the FT-IR spectra between 1200 cm −1 and 800 cm −1 decreases with the increase in basicity. Sometime, the depth of FT-IR spectra representing the [AlO 4 ]-tetrahedra has hardly changed. It is similar to the effect of TiO 2 on the network structure. The network structure of [SiO 4 ]-tetrahedra should become simpler with the increase in basicity, which is consistent with the description in Figure 5. The FT−IR spectra of the slag with different Al2O3 contents are shown in Figure 12.
The depth of the FT−IR spectra characterizing the [SiO4]-tetrahedra has hardly changed and it characterizing [AlO4]-tetrahedra significantly increases with the Al2O3 content from 14 mass% to 16 mass%. As Al2O3 content continues to 18 mass%, the depth of the FT−IR spectra characterizing the [SiO4]-tetrahedra does not only increase but also the central position of the peak of the FT−IR spectra has changed. Sometimes, the depth of the FT−IR spectra characterizing [AlO4]-tetrahedra decreases. It is easily understandable that the network structure of [AlO4]-tetrahedra in the slag system increases as the Al2O3 content serves as an acidic oxide. However, when the Al2O3 content reaches 18 mass%, the network structure of [AlO4]-tetrahedra is reduced and the network structure of [SiO4]-tetrahedra becomes more complicated. This is because some of the Si 4+ in [SiO4]-tetrahedra are replaced by Al 3+ , and Al 3+ still exist in the form of [SiO4]tetrahedra. As a result, a more intricate silicate network structure is formed while the network structure of [AlO4]-tetrahedra is reduced. It is evident that the effect of the silicate network structure on viscosity should be more pronounced than that of [AlO4]-tetrahedra. According to Figures 2 and 12, the increase in viscosity with Al2O3 content from 14 mass% to 16 mass% is attributed to an increased proportion of [AlO4]-tetrahedra in the network structure, whereas the increase in viscosity with Al2O3 content from 16 mass% to 18 mass% is attributed to the more complex silicate network structure generated.

Conclusions
In this study, the viscosity of CaO-SiO2-MgO-Al2O3-TiO2 slag with different TiO2 content (10-30 mass%), Al2O3 content (14-16 mass%), and basicity (1.00-1.20) was measured. FactSage was used to draw the slag phase diagram and X-ray diffraction analysis was It is easily understandable that the network structure of [AlO 4 ]-tetrahedra in the slag system increases as the Al 2 O 3 content serves as an acidic oxide. However, when the Al 2 O 3 content reaches 18 mass%, the network structure of [AlO 4 ]-tetrahedra is reduced and the network structure of [SiO 4 ]-tetrahedra becomes more complicated. This is because some of the Si 4+ in [SiO 4 ]-tetrahedra are replaced by Al 3+ , and Al 3+ still exist in the form of [SiO 4 ]tetrahedra. As a result, a more intricate silicate network structure is formed while the network structure of [AlO 4 ]-tetrahedra is reduced. It is evident that the effect of the silicate network structure on viscosity should be more pronounced than that of [AlO 4 ]-tetrahedra. According to Figures 2 and 12, the increase in viscosity with Al 2 O 3 content from 14 mass% to 16 mass% is attributed to an increased proportion of [AlO 4 ]-tetrahedra in the network structure, whereas the increase in viscosity with Al 2 O 3 content from 16 mass% to 18 mass% is attributed to the more complex silicate network structure generated.

Conclusions
In this study, the viscosity of CaO-SiO 2 -MgO-Al 2 O 3 -TiO 2 slag with different TiO 2 content (10-30 mass%), Al 2 O 3 content (14-16 mass%), and basicity (1.00-1.20) was measured. FactSage was used to draw the slag phase diagram and X-ray diffraction analysis was performed to understand the effect of calcium titanate and spinel solid phases on slag viscosity, and the relationship between network structure and viscosity was studied through FT-IR spectra.
(1) TiO 2 might be more inclined to existing as a network modifier, and depolymerizes the silicate network structure in liquid slag. So, the viscosity decreases with the TiO 2 content from 10 mass% to 30 mass% at high temperature. (2) The viscosity of the slag increases as the content of Al 2 O 3 increases from 14 mass% to 16 mass%. This is due to the formation of a network structure of [AlO 4 ]-tetrahedra. However, when the Al 2 O 3 content increases from 16 mass % to 18 mass %, there is a partial replacement of Si 4+ in [SiO 4 ]-tetrahedra with Al 3+ , while some Al 3+ still exists in the form of [SiO 4 ]-tetrahedra. As a result, the increase in viscosity can be attributed to the generation of a more complex silicate structure. (3) The basicity of slag affects its viscosity, with higher basicity resulting in lower viscosity at temperatures above 1733 K. The decrease in viscosity is due to the reduction in size and number of silicate network structure caused by an increase in CaO content and a decrease in SiO 2 content. The increase in solid phase content with increasing basicity results in an increase in viscosity at 1653 K, but when the amount of solid phase is small, it has almost no effect on the viscosity. (4) At 1653 K, the solid phase in the current slag system includes perovskite and spinel, with perovskite playing an important role in viscosity.
Author Contributions: W.Z., D.L. and C.S. contributed to the material synthesis, performed the experiments, material characterization, data analysis, and paper writing; T.L. and S.Y. contributed to the data analysis, paper writing, and design of the experiment. All authors have read and agreed to the published version of the manuscript.
Funding: This research was financially supported by the Programs of the National Natural Science Foundation of China (Nos. 52174319 and 51474125).

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.