Effect of Erosion Behavior of FeO-CaO-SiO 2 -MgO-Al 2 O 3 Blast Furnace Primary Slag on Al 2 O 3 Substrate

: Al 2 O 3 substrate is widely used as a lining refractory material throughout the blast furnace (BF) process. Accordingly, the erosion of Al 2 O 3 refractory by molten slag has a negative inﬂuence on the running cost and smooth operation of BFs. The effect of the erosion behavior of BF primary slag containing FeO-CaO-SiO 2 -MgO-Al 2 O 3 on Al 2 O 3 substrate refractory was fundamentally investigated using the high-temperature contact angle method and FactSage thermodynamic software based on the composition of BF primary slag in a typical iron and steel enterprise of China. The results showed that the primary slag mentioned above was easily wetted with Al 2 O 3 substrate, and the observed contact angles were 24.5 ◦ and 22.0 ◦ , when the FeO mass fraction ( w (FeO)) was maintained at 10% and 15% of the primary slag, respectively. Moreover, the starting melting temperature of the primary slag with high FeO content, of 1263 ◦ C, was lower. The erosion thickness between the slag and Al 2 O 3 substrate increased from 19.23 to 23.17 µ m as the added w (FeO) increased from 10% to 15%. In addition, it was observed via SEM-EDS analysis that the interface layer was formed, and high-melting-point compounds were generated during the wetting process. This was attributed to the interaction between the molten slag and Al 2 O 3 existing in the substrate, which may have inhibited the continuous dissolution of the Al 2 O 3 in the substrate into slag. Good surface wettability and the dissolution of the Al 2 O 3 substrate refractory into the primary slag of the BF are two dominant factors leading to the erosion of the refractory. through the measurement of the observed contact angle and melting temperature during the wetting process, in addition to the line scanning results of SEM-EDS. The results of this study will provide guidance for reducing the consumption of refractory materials and effectively controlling the operating costs of the BF.


Introduction
In modern BF technology, due to the dual pressures of economic benefits and environmental protection, in addition to the lack of high-quality resources and the need to control operating costs [1], numerous enterprises have been required to increase their use of low-grade iron ore and poor-quality fuel. This inevitably leads to fluctuations in the composition of the BF primary slag, and thus aggravates the erosion by slag of refractory materials [2][3][4]. To effectively overcome these issues, higher requirements have been proposed for the precise control of the composition of ironmaking slag. The majority of previous studies have focused on the final slag of the CaO-SiO 2 -Al 2 O 3 -MgO slag system. Compared with the final slag, the slag system formed in the lower part of the BF, which is composed of primary slag and bosh slag, has distinct characteristics and different slag behavior [5][6][7][8][9], which has not been fully elucidated to date. In the BF process, it is generally believed that the BF primary slag has more research significance than the final slag [10].
BF primary slag is composed of iron oxide FeO and gangue (mainly SiO 2 and Al 2 O 3 ) in the ore, in addition to artificially added MgO, CaO, and other ingredients. The content of FeO can reach up to 20-30% [11][12][13][14]. At the cohesive zone, the BF primary slag produces wetting phenomena with substrate refractory materials, and also erodes the refractory. At elevated temperature conditions, the slag exists in the form of a melt or liquid phase, and the liquid phase slag also reacts with the oxides in the refractory after erosion. Therefore, the essence of slag erosion is the erosion of the liquid phase slag, that is, the dissolution of refractory into the slag [15,16], and the penetration of the slag into refractory materials. It is well known that slag erosion has a significant impact on the service life of refractory materials and is an important cause of refractory damage. Moreover, the consumption of refractory materials is one of the main costs of BF smelting. Thus, inhibiting the interaction between slag and refractory materials is expected to be an effective means to reduce the consumption of refractory materials [17].
Wettability is related to the ability or propensity of a liquid to spread on a solid surface. The important application of wettability and its significant economic benefits have prompted the acceleration of scientific research in related fields. It is well known that there multiple solid-liquid two-phase interfaces occur during the BF ironmaking process. The solid and liquid phases undergo physical and chemical reactions at the interface, leading to the occurrence of element migration. The contact angle is an important parameter that reflects the wettability of a solid surface. By adjusting the contact angle between the two phases, not only is the consumption of refractories is reduced, but smooth operation of the BF and the quality of the hot metal are also ensured [18][19][20].
The results obtained from the literature regarding the erosion of slag and refractories are inconsistent because of different application backgrounds and experimental conditions. The effect of the corrosion behavior of the FeO-CaO-SiO 2 -MgO-Al 2 O 3 converter slag on Al 2 O 3 substrate refractories was investigated by Song et al. [21]. The research results revealed that the sample was easily wetted with Al 2 O 3 substrate when the temperature increased up to 1400 • C. The erosion process could be defined in terms of three main stages: melting, dissolution and diffusion, and cooling and crystallization. The erosion phenomenon was mainly attributed to the dissolution of Al 3+ in the substrate into the slag. Sarkar et al. [22] discussed the interaction between FeO and alumina small plates under the condition of flash ironmaking. The results demonstrated that FeAl 2 O 4 was found in the interface layer via XRD and SEM-EDS technology, and Fe 2+ and Al 3+ cations underwent reverse diffusion through the FeAl 2 O 4 layer. The diffusion ability mainly depends on the temperature, and the effective diffusion coefficient (Deff) and the average thickness of the spinel layer increased when the temperature rose. Wang et al. [23] examined the transient interaction between molten iron, the Al 2 O 3 lining refractory, and the slag phase. The transient shape and thickness of the iron-slag-lining three-phase interface was measured and discussed by the authors. Their findings suggested that the erosion phenomenon was primarily caused by the molten iron entering the Al 2 O 3 refractory. The reaction layer included two sub-layers: an Fe-rich layer adjacent to the iron with more FeO and less Al 2 O 3 , and an Al 2 O 3 -rich layer, which contained more Al 2 O 3 and less FeO. FeO was generated due to the re-oxidation of molten iron, and entered the space between the refractory and molten iron. The thickness of the reaction layer increased approximately linearly with increasing time, indicating that the presence of FeO enhanced the erosion of molten iron on the lining refractory. In summary, despite significant previous research efforts regarding the erosion of the slag and Al 2 O 3 refractory [24,25], there remains a scarcity of studies on the influence of w(FeO) in the BF primary slag containing FeO-CaO-SiO 2 -MgO-Al 2 O 3 on the erosion of Al 2 O 3 substrate. Therefore, it is of great significance to investigate this issue.
In this context, the influence of w(FeO) in the BF primary slag containing FeO-CaO-SiO 2 -MgO-Al 2 O 3 on the erosion thickness of Al 2 O 3 substrate was fundamentally investigated using the high-temperature contact angle method and FactSage thermodynamic software. The action mechanism of FeO on the interface between the slag and Al 2 O 3 substrate was determined through the measurement of the observed contact angle and melting temperature during the wetting process, in addition to the line scanning results of SEM-EDS. The results of this study will provide guidance for reducing the consumption of refractory materials and effectively controlling the operating costs of the BF.

Materials
The composition of the mixed slag in the experiment was selected according to the composition of the BF primary slag of a local iron and steel enterprise, and was composed of analytical-grade pure reagents CaO, SiO 2 , MgO, and Al 2 O 3 . The mass fractions of FeO in slags A and B were 10% and 15% respectively, and the basicity (C/S) was 1.4, as shown in Table 1. The pure oxides (CaO, SiO 2 , MgO, and Al 2 O 3 ) were thoroughly mixed in an agate mortar according to the predetermined proportion, then placed in a molybdenum crucible (ϕ = 52 mm*H = 84 mm, 100 g), and calcined in a muffle furnace at 1500 • C for 6 h. During the calcination process, 1 L/min of high purity nitrogen (99.99%) was injected. The pre-melted slag was crushed and then ground in a ball mill machine for subsequent use. The commercially available ferrous oxalate dihydrate (FeC 2 O 4 ·2H 2 O) was placed in a molybdenum crucible, heated to 1000 • C, maintained at this temperature for 1 h, and then cooled to room temperature to obtain FeO powder. After drying, it was sealed and stored for later use or used immediately. Among the whole reaction process, 600 mL/min high purity nitrogen was continuously applied to prevent oxidation of FeO. A smooth surface was crucial for the accuracy of the wetting experiment. To obtain a smooth surface, the Al 2 O 3 substrate was sanded using sandpaper of No. 800, 1000, and 1200 mesh in turn, polished with a polishing cloth, cleaned, and dried. The pre-melted slag and FeO powder were homogeneously mixed in a predetermined proportion (Table 1), and 0.8 g of the mixture was weighed using a high-precision electronic balance and then placed into a mold. The mixture was pressed for 3 minutes using a uniaxial hydraulic presser with a pressure of 10 MPa to obtain a cylindrical pellet (ϕ = 8 mm*H = 6 mm). About 0.8 g of the mixture was placed on the Al 2 O 3 substrate for the wetting experiment.

Experimental Setup and Methods
The wetting behavior of the slag and Al 2 O 3 substrate was investigated using the high-temperature contact angle measurement instrument in the laboratory. A schematic diagram of the device is shown in Figure 1. The temperature was measured with a B-type thermocouple wrapped in alumina and placed near the sample, with an error of ±2 K. The heating rate was 10 • C/min throughout the experiment. At the beginning of the experiment, the slag sample and Al 2 O 3 substrate were first placed in the low-temperature zone of the furnace tube to prevent the sample from instantaneously breaking when contacting the high temperature, and then moved slowly to the high-temperature zone when the furnace temperature reached 1400 • C. High-purity nitrogen (1 L/min) was continuously injected throughout the experiment to prevent FeO from being oxidized at a high temperature. The drop image was continuously captured by a digital camera, and the contact angle between the slag droplets and the Al 2 O 3 substrate was measured by the image processing software to determine the wettability of the slag and Al 2 O 3 substrate. 1500 mesh in turn), and then polished, dried, and sprayed with gold. The cross-sectional microstructure and element distribution of the sample were analyzed by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS, sigma 500, Zeiss, Germany). The images were processed with Origin. The quinary equilibrium phase diagram of slag A (CaO-SiO2-Al2O3-8 wt%MgO-10 wt%FeO) and the saturated solubility of Al2O3 in the liquid phase were calculated using FactSage 7.3 thermodynamic simulation software at 1400 °C and 1 atmospheric pressure (1 atm).  Figure 2 presents the schematic profiles of slags A and B droplets on the Al2O3 substrate at different temperatures. The start melting temperature, hemisphere point temperature, and complete melting temperature of the sample correspond to 5/6 H, 1/2 H, and 1/3 H (H-height of slag sample at 1100 °C), respectively, of the initial height of the sample [26,27]. It can be observed from the figure that the height of the slag samples decreases significantly with increasing temperature, which indicates that both types of slag diffuse rapidly along the Al2O3 substrate. When the slag sample reached 1400 °C, the contact angles θ1 and θ2 between the slag and Al2O3 substrate in Figure 2e were measured, and the arithmetic mean of the contact angles was defined as the observed contact angle θ, as shown in Equation (1).

Wetting Behavior of Slag and Al2O3 Substrate
For primary slag with 10% and 15% FeO, the observed contact angles at 1400 °C were approximately 24.5° and 22°, respectively, indicating that slags A and B had better wettability with the Al2O3 substrate at this temperature. The better wettability may aggravate the erosion by slag of the Al2O3 refractory materials due to the interaction with the interface.  When the sample temperature reached 1400 • C, the sample was moved from the central high temperature zone of the furnace tube to the initial position, and the slag sample was quenched. To gain more insight into the mineral composition of the slag, some partially dried slag samples were analyzed by X-ray diffraction (XRD, Rigaku Ultima IV, Japan) under the following conditions: voltage: 40 mA; 40 kV; scanning range: 10-80 • (2θ); step size: 0.02 • (2θ); and scanning speed: 5 • /min. The sample was embedded in epoxy resin and sliced so that the slag droplets and Al 2 O 3 substrate were in the same horizontal plane. The treated sample surface was sanded using sandpaper (400, 800, 1000, 1200, and 1500 mesh in turn), and then polished, dried, and sprayed with gold. The cross-sectional microstructure and element distribution of the sample were analyzed by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS, sigma 500, Zeiss, Germany). The images were processed with Origin. The quinary equilibrium phase diagram of slag A (CaO-SiO 2 -Al 2 O 3 -8 wt%MgO-10 wt%FeO) and the saturated solubility of Al 2 O 3 in the liquid phase were calculated using FactSage 7.3 thermodynamic simulation software at 1400 • C and 1 atmospheric pressure (1 atm). Figure 2 presents the schematic profiles of slags A and B droplets on the Al 2 O 3 substrate at different temperatures. The start melting temperature, hemisphere point temperature, and complete melting temperature of the sample correspond to 5/6 H, 1/2 H, and 1/3 H (H-height of slag sample at 1100 • C), respectively, of the initial height of the sample [26,27]. It can be observed from the figure that the height of the slag samples decreases significantly with increasing temperature, which indicates that both types of slag diffuse rapidly along the Al 2 O 3 substrate. When the slag sample reached 1400 • C, the contact angles θ 1 and θ 2 between the slag and Al 2 O 3 substrate in Figure 2e were measured, and the arithmetic mean of the contact angles was defined as the observed contact angle θ, as shown in Equation (1). resin and sliced so that the slag droplets and Al2O3 substrate were in the same horizontal plane. The treated sample surface was sanded using sandpaper (400, 800, 1000, 1200, and 1500 mesh in turn), and then polished, dried, and sprayed with gold. The cross-sectional microstructure and element distribution of the sample were analyzed by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS, sigma 500, Zeiss, Germany). The images were processed with Origin. The quinary equilibrium phase diagram of slag A (CaO-SiO2-Al2O3-8 wt%MgO-10 wt%FeO) and the saturated solubility of Al2O3 in the liquid phase were calculated using FactSage 7.3 thermodynamic simulation software at 1400 °C and 1 atmospheric pressure (1 atm).  Figure 2 presents the schematic profiles of slags A and B droplets on the Al2O3 substrate at different temperatures. The start melting temperature, hemisphere point temperature, and complete melting temperature of the sample correspond to 5/6 H, 1/2 H, and 1/3 H (H-height of slag sample at 1100 °C), respectively, of the initial height of the sample [26,27]. It can be observed from the figure that the height of the slag samples decreases significantly with increasing temperature, which indicates that both types of slag diffuse rapidly along the Al2O3 substrate. When the slag sample reached 1400 °C, the contact angles θ1 and θ2 between the slag and Al2O3 substrate in Figure 2e were measured, and the arithmetic mean of the contact angles was defined as the observed contact angle θ, as shown in Equation (1).

Wetting Behavior of Slag and Al2O3 Substrate
For primary slag with 10% and 15% FeO, the observed contact angles at 1400 °C were approximately 24.5° and 22°, respectively, indicating that slags A and B had better wettability with the Al2O3 substrate at this temperature. The better wettability may aggravate the erosion by slag of the Al2O3 refractory materials due to the interaction with the interface.   For primary slag with 10% and 15% FeO, the observed contact angles at 1400 • C were approximately 24.5 • and 22 • , respectively, indicating that slags A and B had better wettability with the Al 2 O 3 substrate at this temperature. The better wettability may aggravate the erosion by slag of the Al 2 O 3 refractory materials due to the interaction with the interface.

Effect of w(FeO) on Melting Temperature
The experimental results regarding the melting temperature of slag samples at different w(FeO) are shown in Figure 3. The starting melting temperature, hemispherical temperature, and complete melting temperature of the slag samples decrease as w(FeO) increases from 10% to 15%. The starting melting temperature is reduced from 1271 to 1263 • C. As a consequence of the melting point of FeO (1370 • C) being lower than that of the other oxides in the slag system, more FeO reacts with SiO 2 to form increasingly complex low-melting point compounds, such as FeO·SiO 2 , thereby reducing the melting temperature of the slag. Figure 2 shows the lower temperature corresponding to the slag with high w(FeO) under the same proportion of the residual height ratio. Li et al. [28] studied the influence of w(FeO) on the melting characteristics of the primary slag under various slag basicity values. The results revealed that the melting temperature of the primary slag is significantly influenced by w(FeO) regardless of the slag basicity. Moreover, the melting temperature of the primary slag significantly declines with the increase in w(FeO). It can be seen from the above that the research results also show this trend.

Effect of w(FeO) on Melting Temperature
The experimental results regarding the melting temperature of slag samples at different w(FeO) are shown in Figure 3. The starting melting temperature, hemispherical temperature, and complete melting temperature of the slag samples decrease as w(FeO) increases from 10% to 15%. The starting melting temperature is reduced from 1271 to 1263 °C. As a consequence of the melting point of FeO (1370 °C) being lower than that of the other oxides in the slag system, more FeO reacts with SiO2 to form increasingly complex low-melting point compounds, such as FeO·SiO2, thereby reducing the melting temperature of the slag. Figure 2 shows the lower temperature corresponding to the slag with high w(FeO) under the same proportion of the residual height ratio. Li et al. [28] studied the influence of w(FeO) on the melting characteristics of the primary slag under various slag basicity values. The results revealed that the melting temperature of the primary slag is significantly influenced by w(FeO) regardless of the slag basicity. Moreover, the melting temperature of the primary slag significantly declines with the increase in w(FeO). It can be seen from the above that the research results also show this trend.

Mineral Composition of Slags
To further understand the mineral composition of the slag with different w(FeO), the samples of slags A and B were characterized by power XRD. The XRD pattern in Figure 4 shows the mineral composition of slags A and B. The obvious irregular diffraction peaks are mainly caused by the rapid cooling after the wetting experiment. According to the figure, the mineral phases formed in slags A and B are essentially identical, and mainly include akermanite (Ca2MgSi2O7), gehlenite (Ca2Al2SiO7), merwinite (Ca3MgSi2O8), fayalite (Fe2SiO4), dicalcium silicate (Ca2SiO4), and (Mg, Fe) O. The results show that slag B contains more Fe2SiO4 and (Mg, Fe) O than slag A, which is likely due to the higher w(FeO). More Fe2SiO4 phase is the main reason for the decrease in the melting temperature in the previous section. In the current study, because the interface layer is very narrow and difficult to separate from the substrate, the XRD patterns show the mineral composition without the interface layer. The chemical composition of the interface layer is analyzed and discussed according to the quinary equilibrium phase diagram in Section 3.6.2.

Mineral Composition of Slags
To further understand the mineral composition of the slag with different w(FeO), the samples of slags A and B were characterized by power XRD. The XRD pattern in Figure 4 shows the mineral composition of slags A and B. The obvious irregular diffraction peaks are mainly caused by the rapid cooling after the wetting experiment. According to the figure, the mineral phases formed in slags A and B are essentially identical, and mainly include akermanite (Ca 2 MgSi 2 O 7 ), gehlenite (Ca 2 Al 2 SiO 7 ), merwinite (Ca 3 MgSi 2 O 8 ), fayalite (Fe 2 SiO 4 ), dicalcium silicate (Ca 2 SiO 4 ), and (Mg, Fe) O. The results show that slag B contains more Fe 2 SiO 4 and (Mg, Fe) O than slag A, which is likely due to the higher w(FeO). More Fe 2 SiO 4 phase is the main reason for the decrease in the melting temperature in the previous section. In the current study, because the interface layer is very narrow and difficult to separate from the substrate, the XRD patterns show the mineral composition without the interface layer. The chemical composition of the interface layer is analyzed and discussed according to the quinary equilibrium phase diagram in Section 3.6.2.

Element Distribution and Microstructure
The microstructure and elemental composition in the cross-section of the slag and Al2O3 substrate after the wetting experiment were analyzed using SEM-EDS, and the mapping results are visualized in Figures 5 and 6. The graphs indicate that the microstructure and element distribution of slags A and B are approximately the same, and the main elements include Fe, Ca, Si, Mg, Al, and O. The cross-section is mainly composed of a uniform gray phase area, a bright phase area, and a small dark gray phase area. The results of the energy spectrum at different positions of slags A and B are presented in Table 2. The dark gray area contains a large quantity of Al2O3 and CaO, in addition to a trace amount of SiO2, which corresponds with the results achieved by scanning shown in Figures 5 and 6. It is speculated that there is Ca2Al2SiO7 in calcium aluminosilicate minerals. It is also noted that, for the gray phase area, the composition proportions of CaO, MgO, Al2O3, and SiO2 are close to those of the original slag, which is a homogeneous liquid phase formed at a high temperature. Moreover, the relative mass percentage of FeO in slag B is higher than that in slag A, which coincides with the experimental proportion of FeO in Table 1. The bright phase area mainly contains FeO and MgO. Combined with the scanning results of Figures 5 and 6, it can be seen that there are more Mg elements in the Ferich region, which is a continuous solid solution of (Mg, Fe) O. This is because Mg 2+ in MgO can partially replace the vacancy of Fe 2+ in the FeO crystal lattice and form (Mg, Fe) O solid solution. Moreover, the amount of solid solution increases with the increase in w(FeO), which can lead to the grains aggregating in pieces. This is consistent with the above-mentioned mapping results.

Element Distribution and Microstructure
The microstructure and elemental composition in the cross-section of the slag and Al 2 O 3 substrate after the wetting experiment were analyzed using SEM-EDS, and the mapping results are visualized in Figures 5 and 6. The graphs indicate that the microstructure and element distribution of slags A and B are approximately the same, and the main elements include Fe, Ca, Si, Mg, Al, and O. The cross-section is mainly composed of a uniform gray phase area, a bright phase area, and a small dark gray phase area. The results of the energy spectrum at different positions of slags A and B are presented in Table 2. The dark gray area contains a large quantity of Al 2 O 3 and CaO, in addition to a trace amount of SiO 2 , which corresponds with the results achieved by scanning shown in Figures 5 and 6. It is speculated that there is Ca 2 Al 2 SiO 7 in calcium aluminosilicate minerals. It is also noted that, for the gray phase area, the composition proportions of CaO, MgO, Al 2 O 3 , and SiO 2 are close to those of the original slag, which is a homogeneous liquid phase formed at a high temperature. Moreover, the relative mass percentage of FeO in slag B is higher than that in slag A, which coincides with the experimental proportion of FeO in Table 1. The bright phase area mainly contains FeO and MgO. Combined with the scanning results of Figures 5 and 6, it can be seen that there are more Mg elements in the Fe-rich region, which is a continuous solid solution of (Mg, Fe) O. This is because Mg 2+ in MgO can partially replace the vacancy of Fe 2+ in the FeO crystal lattice and form (Mg, Fe) O solid solution. Moreover, the amount of solid solution increases with the increase in w(FeO), which can lead to the grains aggregating in pieces. This is consistent with the above-mentioned mapping results.

Erosion Thickness of Slag and Al2O3 Substrate
Based on the SEM-EDS analysis, the influence of the slag on the erosion thickness of the Al2O3 substrate after the wetting experiment was studied. The line scanning results of slags A and B in different regions are shown in Figures 7 and 8, respectively. At hightemperature conditions, Al2O3 will dissolve into the slag, and different Al element content gradients form an interface layer. In the distance range of 0-250 μm, the average Al content in the slag is taken as the reference and the parallel line with the X axis is called the "average baseline" (see Figure 7b (Al) and Figure 8b (Al)). This intersects with the gradually increasing Al content change curve, and the vertical line between the intersection point and the X axis is defined as the starting position of erosion. Because the Al2O3 substrate contains almost no elements such as Fe, Ca, and Si, the vertical line between the point at which the element content approaches 0 and the X axis is the end position of erosion. The distance between the two vertical lines is regarded as the thickness of the interface layer. Because the diffusion rates of elements are not comparable, the measurement errors can be reduced to the maximum extent based on the average value of Al content. According to Figures 7b and 8b, the cross-section is largely categorized into three layers: the sample layer (layer I), the interface layer (layer II), and the original Al2O3 substrate layer (layer III). Within layer II, Ca content in slags A and B increases first and then tends to decrease, indicating that the Ca element diffuses into the interface layer. Moreover, it can also be observed that the content of the Fe element in slag A decreases gradually, whereas the same element in slag B initially increases and then decreases, and the content of Fe in slag B is relatively higher. The above-mentioned phenomena are caused by two possible factors: one is that the w(FeO) in slag B is higher than that in slag A; the other is that there are some Fe-rich regions above the slag B interface layer, which leads to the migration of the Fe element to the interface layer. The thickness of the interface layer of slags A and B can be observed to be 19.23 and 23.17 μm, respectively, indicating that increasing the w(FeO) in the slag can increase the thickness of the interface layer.
The chemical compositions of energy spectrum analysis results in different regions are presented in Table 3. It can be observed that the composition of the oxide corresponding to the weight percentage of spot 1 (layer I) in Figures 7 and 8 is close to that of the original slag. According to the results of Section 3.3, the slag may be composed of Ca2MgSi2O7, Ca2Al2SiO7, Ca3MgSi2O8, Fe2SiO4, Ca2SiO4, and (Mg, Fe) O. The chemical

Erosion Thickness of Slag and Al 2 O 3 Substrate
Based on the SEM-EDS analysis, the influence of the slag on the erosion thickness of the Al 2 O 3 substrate after the wetting experiment was studied. The line scanning results of slags A and B in different regions are shown in Figures 7 and 8, respectively. At hightemperature conditions, Al 2 O 3 will dissolve into the slag, and different Al element content gradients form an interface layer. In the distance range of 0-250 µm, the average Al content in the slag is taken as the reference and the parallel line with the X axis is called the "average baseline" (see Figure 7b (Al) and Figure 8b (Al)). This intersects with the gradually increasing Al content change curve, and the vertical line between the intersection point and the X axis is defined as the starting position of erosion. Because the Al 2 O 3 substrate contains almost no elements such as Fe, Ca, and Si, the vertical line between the point at which the element content approaches 0 and the X axis is the end position of erosion. The distance between the two vertical lines is regarded as the thickness of the interface layer. Because the diffusion rates of elements are not comparable, the measurement errors can be reduced to the maximum extent based on the average value of Al content. According to Figures 7b and 8b, the cross-section is largely categorized into three layers: the sample layer (layer I), the interface layer (layer II), and the original Al 2 O 3 substrate layer (layer III). Within layer II, Ca content in slags A and B increases first and then tends to decrease, indicating that the Ca element diffuses into the interface layer. Moreover, it can also be observed that the content of the Fe element in slag A decreases gradually, whereas the same element in slag B initially increases and then decreases, and the content of Fe in slag B is relatively higher. The above-mentioned phenomena are caused by two possible factors: one is that the w(FeO) in slag B is higher than that in slag A; the other is that there are some Fe-rich regions above the slag B interface layer, which leads to the migration of the Fe element to the interface layer. The thickness of the interface layer of slags A and B can be observed to be 19.23 and 23.17 µm, respectively, indicating that increasing the w(FeO) in the slag can increase the thickness of the interface layer.
The chemical compositions of energy spectrum analysis results in different regions are presented in Table 3. It can be observed that the composition of the oxide corresponding to the weight percentage of spot 1 (layer I) in Figures 7 and 8 is close to that of the original slag. According to the results of Section 3.3, the slag may be composed of Ca 2 MgSi 2 O 7 , Ca 2 Al 2 SiO 7 , Ca 3 MgSi 2 O 8 , Fe 2 SiO 4 , Ca 2 SiO 4 , and (Mg, Fe) O. The chemical composition of spot 2 (layer II) in the figure is mainly composed of Al 2 O 3 , CaO, and SiO 2 . As the w(FeO) in the slag increases from 10% to 15%, the values of FeO/(Al 2 O 3 +CaO+SiO 2 +MgO) in the interface layer of slags A and B are 0.073 and 0.123, respectively. It can be clearly observed that the value of slag A is relatively low. Moreover, FeO easily reacts with SiO 2 to generate a low-melting-point Fe 2 SiO 4 , which has good fluidity and is interpenetrated with the generated high-melting-point slag. It can act as a cosolvent and increase the diffusion of Al 2 O 3 to the interface layer, so the slag containing more FeO can aggravate the erosion thickness on the Al 2 O 3 substrate.
As the w(FeO) in the slag increases from 10% to 15%, the values of FeO/(Al2O3+CaO+SiO2+MgO) in the interface layer of slags A and B are 0.073 and 0.123, respectively. It can be clearly observed that the value of slag A is relatively low. Moreover, FeO easily reacts with SiO2 to generate a low-melting-point Fe2SiO4, which has good fluidity and is interpenetrated with the generated high-melting-point slag. It can act as a cosolvent and increase the diffusion of Al2O3 to the interface layer, so the slag containing more FeO can aggravate the erosion thickness on the Al2O3 substrate.       Figure 9. The presented graph indicates that the content of Al 2 O 3 in the liquid phase first shows an increasing trend and then decreases with a higher Al 2 O 3 content. When the slag contains 27% Al 2 O 3 , the maximum saturation of Al 2 O 3 in the liquid phase is found to be relatively high, up to 21.21% and 26.15%, respectively. The content of Al 2 O 3 in the original slag samples is 12%. From Figure 9, it can be observed that the content of Al 2 O 3 in slags A and B does not reach saturation, which illustrates that Al 2 O 3 dissolves and migrates to the slag. Figure 9. The presented graph indicates that the content of Al2O3 in the liquid phase first shows an increasing trend and then decreases with a higher Al2O3 content. When the slag contains 27% Al2O3, the maximum saturation of Al2O3 in the liquid phase is found to be relatively high, up to 21.21% and 26.15%, respectively. The content of Al2O3 in the original slag samples is 12%. From Figure 9, it can be observed that the content of Al2O3 in slags A and B does not reach saturation, which illustrates that Al2O3 dissolves and migrates to the slag.

Crystallization Process of the Interface Layer
When the temperature reached 1400 °C, the rapid cooling was carried out, and the crystallization process of layer II can be explained by the quinary phase diagram in Figure  10. The point "O" in Figure 10 is the original composition of slag A and is located in the primary crystal region of Ca3MgSi2O8. We hypothesize that the dissolution of Al2O3 into the slag at a high temperature only changes the chemical composition of the interface layer, and the composition of the slag will move along a straight line in the O→N direction. This component point transfers to the primary crystal region of Ca2Al2SiO7 or CaMg2Al16O27, and Ca2Al2SiO7 and CaMg2Al16O27 phases with a high melting temperature will be generated in the interface layer after cooling. As can be seen from the mass percentage of the energy spectrum in Figures 7 and 8, at point 2, the interface layer mainly consists of Ca, Si, and Al elements, in addition to a small amount of Mg and Fe. The results demonstrate that Ca and Si diffuse to the interface layer and Al2O3 substrate dissolves into the slag during wetting. Sarkar et al. [29] discussed the interaction of Al2O3 refractories with FeO-SiO2 and FeO-SiO2-CaO slag, and found that the effective diffusion coefficients of Fe 2+ and Al 3+ were reduced significantly due to the formation of high-melting-point compounds, such as hercynite (FeAl2O4), mullite (3Al2O3·2SiO2), and anorthite (CaAl2Si2O8), in the interface layer, indicating that the high-melting-point compounds may further inhibit the diffusion of Al 3+ ion into the slag. According to our experimental results, the Ca2Al2SiO7 and CaMg2Al16O27 phases can hinder the diffusion of Al 3+ .
In summary, the erosion mechanism of slag on Al2O3 substrate can be explained by the schematic diagram in Figure 11. Upon heating, the proportion of residual height of the

Crystallization Process of the Interface Layer
When the temperature reached 1400 • C, the rapid cooling was carried out, and the crystallization process of layer II can be explained by the quinary phase diagram in Figure 10. The point "O" in Figure 10 is the original composition of slag A and is located in the primary crystal region of Ca 3 MgSi 2 O 8 . We hypothesize that the dissolution of Al 2 O 3 into the slag at a high temperature only changes the chemical composition of the interface layer, and the composition of the slag will move along a straight line in the O→N direction. This component point transfers to the primary crystal region of Ca 2 Al 2 SiO 7 or CaMg 2 Al 16 O 27 , and Ca 2 Al 2 SiO 7 and CaMg 2 Al 16 O 27 phases with a high melting temperature will be generated in the interface layer after cooling. As can be seen from the mass percentage of the energy spectrum in Figures 7 and 8 With the dissolution of Al2O3 and the interaction between CaO, MgO, SiO Al2O3, the phase with a high melting temperature precipitates at the interface layer. over, a higher amount of FeO can easily generate a higher number of low-melting compounds with SiO2 and coexist with high-melting-point slag, which increases th fusion of Al2O3 and leads to the increase in erosion thickness. By comparison, the melting-point products are gradually covered between the slag and the Al2O3 sub which further represses the dissolution of Al2O3. Based on the points discussed ab can be concluded that a lower w(FeO) can reduce the erosion degree of slag on the substrate.  In summary, the erosion mechanism of slag on Al 2 O 3 substrate can be explained by the schematic diagram in Figure 11. Upon heating, the proportion of residual height of the solid slag decreases and gradually fuses into a liquid state. The good wettability of molten slag and Al 2 O 3 substrate is the first step leading to substrate erosion as the temperature reaches 1400 • C. Furthermore, the dissolution of Al 2 O 3 in the substrate into the slag is another significant factor of substrate erosion. The line scanning results in Figures 7 and 8 also prove the distribution of Ca, Si, and Al elements from the slag layer to the substrate layer. With the dissolution of Al 2 O 3 and the interaction between CaO, MgO, SiO 2 , and Al 2 O 3 , the phase with a high melting temperature precipitates at the interface layer. Moreover, a higher amount of FeO can easily generate a higher number of low-melting-point compounds with SiO 2 and coexist with high-melting-point slag, which increases the diffusion of Al 2 O 3 and leads to the increase in erosion thickness. By comparison, the high-melting-point products are gradually covered between the slag and the Al 2 O 3 substrate, which further represses the dissolution of Al 2 O 3 . Based on the points discussed above, it can be concluded that a lower w(FeO) can reduce the erosion degree of slag on the Al 2 O 3 substrate.
fusion of Al2O3 and leads to the increase in erosion thickness. By comparison, the highmelting-point products are gradually covered between the slag and the Al2O3 substrate, which further represses the dissolution of Al2O3. Based on the points discussed above, it can be concluded that a lower w(FeO) can reduce the erosion degree of slag on the Al2O3 substrate.

Discussion
The effect of the erosion behavior of BF primary slag containing FeO-CaO-SiO2-MgO-Al2O3 on Al2O3 substrate was investigated using the high-temperature contact angle method and FactSage thermodynamic software. The main results are presented below.
(1) For primary slag with 10% and 15% FeO, the observed contact angles at 1400 °C were approximately 24.5° and 22°, respectively, indicating that slags A and B had good wettability with the Al2O3 substrate. Moreover, the starting melting temperature of slag with 15% FeO, of 1263 °C, was lower.
(2) The mineral phases formed in slags A and B were essentially identical, and more Fe2SiO4 was observed in slag B. The thickness of the interface layer of slags A and B was Figure 11. The schematic diagram of erosion mechanism.

Discussion
The effect of the erosion behavior of BF primary slag containing FeO-CaO-SiO 2 -MgO-Al 2 O 3 on Al 2 O 3 substrate was investigated using the high-temperature contact angle method and FactSage thermodynamic software. The main results are presented below.
(1) For primary slag with 10% and 15% FeO, the observed contact angles at 1400 • C were approximately 24.5 • and 22 • , respectively, indicating that slags A and B had good wettability with the Al 2 O 3 substrate. Moreover, the starting melting temperature of slag with 15% FeO, of 1263 • C, was lower.
(2) The mineral phases formed in slags A and B were essentially identical, and more Fe 2 SiO 4 was observed in slag B. The thickness of the interface layer of slags A and B was found to be 19.23 and 23.17 µm, respectively, indicating that a higher FeO in the slag can aggravate the erosion degree of Al 2 O 3 substrate.
(3) Good wettability and dissolution of Al 2 O 3 into slag are two important factors leading to substrate erosion. In addition, the formation of high-melting-point products can further inhibit the dissolution of Al 2 O 3 substrate.