Effect of Main Composition on the Viscosity and Thermal Stability of BaO-Containing Slag

Abstract

The authors of this study systematically investigate the influence of the main components of BaO-containing slag on its viscosity and thermal stability. The results indicate that the viscosity of slag significantly increases with the mass fraction of the crystalline phase. Increasing the slag basicity from 1.00 to 1.10 and the MgO content from 5.0% to 9.5% is an effective way to weaken the influence of crystallization on slag viscosity and improve the stability of BaO-containing slag. Al₂O₃ content over 11% is necessary to maintain a higher slag temperature, which enhances the ability of BaO-containing slag to resist the influence of crystallization on relative viscosity. When the heat of BaO slag is greatly reduced, increasing the slag basicity from 1.05 to 1.15 and MgO content to over 6.5% can help maintain the thermal stability of slag. The effect of Al₂O₃ content on the viscosity of slag under conditions of fixed heat is greater than that under constant temperature conditions. As the heat decreases, the Al₂O₃ content increase from 9% to 13% has a more significant effect on the viscosity and temperature of slag. A lower Al₂O₃ content in slag is advantageous for reducing the influence of heat fluctuation on slag viscosity.

1. Introduction

The ironmaking industry is responsible for the national economy [1,2]. A blast furnace is the largest single piece of equipment in the smelting process, and its main products are molten iron and slag [3]. The behavior of molten slag is of significant importance because the viscosity of slag affects the gas and liquid permeabilities of the hearth, which in turn influences the steady production and fuel consumption of the blast furnace [4,5,6]. Current studies mainly focus on the influence of composition on the viscosity of common slag systems, such as CaO-MgO-Al₂O₃-SiO₂, at a fixed temperature [7,8,9,10]. However, with the continuous deterioration of raw material conditions, iron and steel enterprises have started to use lower-grade ores, leading to inevitable changes in the composition of slag. BaO is one of the oxides that appears in the slag after the ore grade drops, and BaO-containing slag has also become the main type of slag in some blast furnace smelting [11,12,13,14]. The composition of slag is usually adjusted to ensure the effective separation of slag and iron [15]. Therefore, the influence of the main composition on the performance of BaO-containing slag is key to adjusting its performance.

Controlling the furnace condition to be stable is one of the keys to blast furnace smelting. In the actual smelting process, the supply of raw materials and fuels generally determines the thermal condition [16]. On the one hand, changes in the slag composition caused by the fluctuation in charge significantly impact the thermal stability of the hearth, making it necessary to investigate the relationship between slag composition and viscosity under fixed heat conditions. On the other hand, heat fluctuation caused by changes in the fuel state can affect the mobility of slag. Hence, it is also necessary to study thermal condition changes in the BaO-containing slag system.

In this study, a rotating cylinder viscometer was used to measure the viscosity of the CaO-MgO-Al₂O₃-SiO₂-BaO slag system with different compositions. Meanwhile, the heat capacity of CaO-MgO-Al₂O₃-SiO₂-BaO slag with different compositions at high temperatures was studied, and thermodynamic data were obtained using FactSage software, which provided a reference for optimizing the metallurgical properties of BaO-containing slag.

2. Material and Methods

2.1. Materials Preparation

Experimental samples were prepared using pure chemical reagent grade powders of CaO, SiO₂, Al₂O₃, MgO, and BaO. BaO was obtained by roasting and decomposing BaCO₃. The powders were calcined at 1273 K for 6 h in a muffle furnace to decompose carbonates or hydroxides. Subsequently, calcined reagents were accurately weighed to 140 g, mixed, and placed in a graphite crucible with an inner diameter of 40 mm and height of 80 mm. To prevent interaction between the melt and the crucible, the graphite crucible was lined with a molybdenum crucible with an inner diameter of 39 mm and height of 60 mm. The crucible was then placed in a resistance furnace for viscosity testing. The composition of each sample is presented in

Table 1. Chemical composition of slag (wt%).

No.	CaO/SiO2 (R2)	CaO/%	SiO2/%	MgO/%	Al2O3/%	BaO/%
A-1	1.05	41.49	39.51	8.00	9.00	2.00
A-2	1.05	40.98	39.02	8.00	10.00	2.00
A-3	1.05	40.46	38.54	8.00	11.00	2.00
A-4	1.05	39.44	37.56	8.00	13.00	2.00
B-1	1.00	40.00	40.00	8.00	10.00	2.00
B-2	1.05	40.98	39.02	8.00	10.00	2.00
B-3	1.10	41.90	38.10	8.00	10.00	2.00
B-4	1.15	42.79	37.21	8.00	10.00	2.00
M-1	1.05	43.02	40.98	5.00	9.00	2.00
M-2	1.05	42.26	40.24	6.50	9.00	2.00
M-3	1.05	41.49	39.51	8.00	9.00	2.00
M-4	1.05	40.72	38.78	9.50	9.00	2.00

. Among them, B1–B4 represent a different basicity, A1–A4 represent a different Al₂O₃ content, and M1–M4 represent a different MgO content.

2.2. Viscosity Measurements

The viscosity of the experimental slag was measured using the rotating cylinder method, and the experimental setup is depicted in Figure 1. A molybdenum silicon rod was utilized as the heating element, and the viscometer’s molybdenum spindle was calibrated with castor oil before the experiment. The slag sample was placed in the molybdenum crucible and heated to 1773 K in a high-purity argon protective gas environment at a flow rate of 1.5 L/min. The sample was kept at 1773 K for 120 min to ensure the complete homogenization of slag. Subsequently, a molybdenum rotor was inserted into the slag sample, with the tip of the rotor positioned approximately 10 mm above the crucible’s bottom. The crucible and rotor should be aligned along the viscometer’s axis during viscosity measurements, as even slight deviations can result in substantial experimental errors. Viscosity measurements were performed at a cooling rate of 3 K/min.

2.3. Thermodynamic Calculations

The Equilib module in FactSage software was used to calculate the phase change in BaO-containing slag systems with varying slag alkalinity during the cooling process from 1773 K to 1273 K, with a temperature interval of 15 K. According to the first law of thermodynamics, the enthalpy change in slag at a specific temperature under constant pressure conditions is equivalent to the heat absorbed by slag. However, it should be noted that during the melting process, slag may be heated due to crystal transition, a phase change, and a chemical reaction. To enhance the accuracy of the calculations, Equations (1)–(4) were utilized to perform calculations using FactSage thermodynamic software, which has been widely applied to forecast thermodynamic data for multi-component slag systems [17,18,19].

$${\mathrm{C}}_{\mathrm{pi}}=\left(\mathrm{A}+\mathrm{B}\mathrm{t}+\mathrm{C}{\mathrm{t}}^2+\mathrm{D}{\mathrm{t}}^3+\mathrm{E}/{\mathrm{t}}^2\right)/{\mathrm{M}}_{\mathrm{i}}$$

(1)

$${\mathrm{C}}_{\mathrm{p}}={\displaystyle \sum {\mathrm{m}}_{\mathrm{i}}{\mathrm{C}}_{\mathrm{pi}}}$$

(2)

$$\Delta {\mathrm{H}}_{\mathrm{i}}={\displaystyle {\int }_{298}^{{\mathrm{T}}_{\mathrm{tr}}}{\mathrm{C}}_{\mathrm{pi}\left(\mathrm{s}\right)}}\mathrm{d}\mathrm{T}+{\Delta }_{\mathrm{tr}}{\mathrm{H}}_{\mathrm{i}}+{\displaystyle {\int }_{{\mathrm{T}}_{\mathrm{tr}}}^{{\mathrm{T}}_{\mathrm{M}}}{{\mathrm{C}}^{\prime }}_{\mathrm{pi}(\mathrm{s})}}\mathrm{dT}+{\Delta }_{\mathrm{s}}^{\mathrm{l}}{\mathrm{H}}_{\mathrm{i}}+{\displaystyle {\int }_{{\mathrm{T}}_{\mathrm{M}}}^{\mathrm{T}}{{\mathrm{C}}^{\prime }}_{\mathrm{pi}(\mathrm{l})}\mathrm{d}\mathrm{T}}$$

(3)

$$\Delta {\mathrm{H}}_{\mathrm{T}}={\displaystyle \sum {\mathrm{m}}_{\mathrm{i}}{\mathrm{H}}_{\mathrm{i}}}$$

(4)

C_pi stands for the specific heat capacity, J/g K. i stands for the composition of the slag. A, B, C, D and E stand for the corresponding thermodynamic parameters. T stands for temperature (K)/1000. M_i stands for relative molar mass. ΔH_i stands for the enthalpy change at a certain temperature of the slag, J/g. Δ_trH_i and Δ^l_sH_i stand for the enthalpy of crystal phase transformation and the enthalpy of the melting of blast furnace slag in terms of unit mass during heating up, respectively, J/g. ΔH_T stands for the enthalpy change in a slag sample at a given target temperature, J. T_tr stands for the crystal transition temperature, K. T_M stands for slag melting temperature, K.

During the calculation process for determining the heat capacity and enthalpy changes in the slag system, the initial temperature and pressure of the component slags were set to 298 K and 101.325 kPa, respectively. The heat capacity and enthalpy of the slag system were then calculated at temperatures of 1773 K, 1823 K and 1873 K. The enthalpy change in slag at a given temperature was approximately equal to the heat absorbed by slag under constant pressure in terms of only volume work, which is in accordance with the first law of thermodynamics. The average enthalpy change in the slag samples at 1773 K was determined based on the given heat input.

3. Results and Discussion

3.1. The Characteristic of Slag Viscosity and Equilibrium Phase

3.1.1. Effect of Basicity on Slag Viscosity and Phase Evolution

The effect of basicity on the viscosity and melting temperature of CaO-SiO₂-8%MgO-10%Al₂O₃-2%BaO (from samples B1 to B4) slag is presented in Figure 2. As shown in Figure 2a, the viscosity tends to decrease with increasing basicity above 1753 K, and the viscosity decreases with increasing basicity when the basicity is below 1.10. The activation energy of molten slag typically represents the energy barrier [20], and therefore, the temperature dependence of liquid viscosity is commonly expressed by the Arrhenius function:

$$\eta =\mathrm{Aexp}(E\eta /RT)$$

where A represents the pre-exponential factor, R represents the universal gas constant, and Eη represents apparent the activation energy for viscous flow, which can be evaluated by the plotting of natural logarithm of the viscosity (lnη) and reciprocal temperature curve (1/T). The calculated activation energy is shown in Figure 2b. The decrease in activation energy with increasing basicity below 1.10 suggests that the structural units of the slag become less complex and more easily flowable. This phenomenon can be attributed to the presence of CaO, a basic oxide that provides free O²⁻ and Ca²⁺ ions, which can depolymerize complex structures in molten slag, as suggested in previous studies [21,22,23]. As a result, viscosity initially decreases with increasing basicity. However, with further increases in basicity up to 1.10, especially at lower temperatures, the viscosity actually increases with the increase in basicity. These observations suggest that changes in basicity can have a complex effect on the structural units and flow properties of molten slag [24,25].

The equilibrium phases that form during the cooling process at different basicity levels were analyzed using FactSage 8.1 software, and the results are presented in Figure 3a. The main precipitated phases are the melilite phase, enstatite (CaSiO₃), merwinite (Ca₃MgSi₂O₈), anorthite (CaAl₂Si₂O₈) and celsian (BaAl₂Si₂O₈). This finding aligns with the results obtained from previous literature [26,27]. Merwinite is the first phase to precipitate at 1588 K, followed by the melilite phase. In terms of the amounts of precipitated phases, the melilite phase, merwinite and enstatite are the three main equilibrium phases during the cooling process. After the complete crystallization of slag, as shown in Figure 3b, the mass fraction of the melilite phase decreases from 71.3% to 52.76%, the mass fraction of enstatite decreases from 16.96% to 12.76%, and the mass fraction of merwinite increases from 3.93% to 30.22% as the basicity changes from 1.00 to 1.15, respectively. Higher basicity favors the precipitation of merwinite, and the initial precipitation temperature of merwinite also increases with increasing basicity. In contrast, the content and precipitation temperature of melilite decrease with increasing basicity. The liquidus temperature and melting temperature, as summarized according to the phase precipitation temperature, are shown in Figure 3b. The liquidus temperature of CaO-SiO₂-8%MgO-10%Al₂O₃-2%BaO (B1–B4) slag increases with increasing basicity, while the melting temperature increases significantly when the basicity reaches 1.10. Therefore, as the basicity of the slag increases, crystallization is more likely to occur at lower temperatures.

3.1.2. Effect of Al₂O₃ on Slag Viscosity and Phase Evolution

As the Al₂O₃ content increases from 9.0% to 10.0% and from 11.0% to 13.0%, the viscosity of the CaO-SiO₂-8%MgO-Al₂O₃-2%BaO (A1–A4) slag gradually increases, as presented in Figure 4. Conversely, when the Al₂O₃ content increases from 10.0% to 11.0%, the viscosity of slag decreases. With the increase in Al₂O₃ content, the melting temperature of slag does not change much, indicating that an appropriate increase in Al₂O₃ content will not have a large negative impact on the fluidity of slag in the current slag system. The influence of Al₂O₃ in previous studies has yielded similar conclusions [28].

With an increasing Al₂O₃ content, the mass fraction of melilite in slag first decreases, and then increases after full crystallization, while the mass fractions of merwinite and anorthite phases both increase, and then decrease. As shown in Figure 5, the crystallization temperature of the primary crystal phase of slag gradually decreases with an increasing Al₂O₃ content. The crystallization temperatures of melilite, enstatite and celsian all exhibit a trend of decreasing first, and then increasing. With an increasing Al₂O₃ content, the influence of crystal phase mass fraction on slag viscosity also increases. When the Al₂O₃ content reaches 13.0%, the mass fraction of the crystalline phase of slag is only about 3%, while the viscosity value of slag has reached above 1.0 Pa·s. Therefore, controlling the Al₂O₃ content in slag is beneficial to improve the ability of BaO-containing slag to resist crystallization and relative viscosity.

3.1.3. Effect of MgO on Slag Viscosity and Phase Evolution

When the slag is in the high-temperature zone, as shown in Figure 6, different MgO contents have a minimal effect on slag viscosity. However, when the temperature of slag is constant, the viscosity tends to decrease with an increase in MgO content, especially when the content is at 9.5%, as shown in Figure 6. The melting temperature of slag initially and noticeably increases with MgO, but with further increases in the MgO content, the rate of temperature increase slows down. Therefore, for slag with a basicity of 1.05 and an Al₂O₃ content of 9.0%, the MgO content can be considered to be between 6.5% and 9.5%.

As the MgO content increases from 5.0% to 9.5%, the mass fraction of each phase after the slag sample has been completely crystallized is shown in Figure 7. The content of the melilite phase increased from 46.79% to 76.75%, while the content of the enstatite phase decreased from 36.3% to 8.47%. The content of the celsian phase also decreased from 2.23% to 1.68%, while the content of the merwinite phase and anorthite phase increased first, and then decreased. These results indicate that the increase in the MgO content is conducive to the precipitation of the mellow feldspar phase.

3.2. The Characteristic of Heat Capacity and Thermal Stability of Slag

3.2.1. Effect of Basicity on Heat Capacity and Thermal Stability of Slag

The heat capacity of slag is an important physical and chemical property that affects its behavior during high-temperature processing. Slag with a higher heat capacity requires more heat to be absorbed to increase the temperature by 1 K, making it more difficult to vary the slag temperature with the heat supply. At the target basicity, the heat capacity of slag increases with temperature, with a more significant increase observed at higher basicities, as depicted in Figure 8a. However, at the target temperature, an increase in slag basicity leads to an initial decrease in heat capacity, followed by a gradual increase. It is important to note that the final slag temperature typically reaches around 1773 K [29,30]. Therefore, increasing the basicity of BaO-containing slag beyond 1.1 is likely to increase the heat consumption of the blast furnace.

Figure 8b illustrates the influence of basicity on slag viscosity under fixed heating conditions. The results indicate that as the basicity of slag increases, its viscosity decreases, while the temperature tends to increase. The impact of basicity on slag viscosity is more significant under constant heating conditions than it is under a constant temperature (1773 K). This phenomenon is attributed to the fact that as the basicity of slag increases, CaO provides more free oxygen ions (O²⁻) that react with bridging oxygen, O° [23,31]. This simplifies the network structure of aluminosilicate and reduces the slag viscosity. Under fixed heating conditions, the increase in basicity not only reduces the degree of network polymerization, but also elevates the temperature of slag. The rise in temperature strengthens the thermal vibration between ions in slag and increases the inter-ion spacing, which weakens the inter-ion force, thereby increasing the anti-adhesion of ion migration and reducing the viscosity of slag. Therefore, the effect of basicity on the viscosity of slag under constant heating conditions is the combined result of chemical depolymerization and thermal depolymerization.

In Figure 9a, the variation in slag temperature with basicity content is depicted for different heat reductions: 10%, 7.5%, 5%, and 2.5%. The results show that, under a fixed heat decrement, the slag temperature increases as the slag basicity increases. However, the temperature of slag gradually decreases as the percentage of heat reduction increases. When the basicity of slag changes from 1.05 to 1.15, the temperature fluctuations caused by the heat reduction of slag become insignificant.

Figure 9b shows the variation in slag viscosity with basicity in different heat reduction tests. The results demonstrate that under a constant heat reduction, the slag viscosity decreases as the basicity increases, especially when the basicity is lower than 1.10. Furthermore, the viscosity fluctuations decrease with increasing basicity. Apart from the direct effect of the increased alkalinity, there is also a significant increase in the slag temperature, which results in an excess of heat energy within slag [32,33]. This, in turn, significantly reduces the slag viscosity due to the combined effect of the chemical and thermal depolymerization of the network structure. Therefore, it is apparent that increasing the basicity is advantageous for improving the fluidity and thermal stability of slag.

Moreover, as the basicity further increases, the network structure of slag may have largely depolymerized into simpler polymer types, and the depolymerization of free oxygen ions (O²⁻) may have a limited effect on viscosity [34,35]. Consequently, both the slag viscosity and its fluctuations become relatively insignificant at this stage. Therefore, for the current slag system, slag exhibits good resistance to temperature fluctuations when the basicity ranges from 1.05 to 1.10.

3.2.2. Effect of Al₂O₃ on Heat Capacity and Thermal Stability of Slag

With an increase in Al₂O₃ content, the heat capacity of slag gradually increases, as depicted in Figure 10a. Furthermore, as the temperature of slag rises, its heat capacity also increases gradually. The heat storage capacity of slag is improved with an increase in Al₂O₃ content. Therefore, to maintain slag temperature stability, while smelting iron ore with a high Al₂O₃ content, it is necessary to increase the heat supply of the blast furnace and increase the coke ratio or fuel ratio.

As shown in Figure 10b, under fixed heat conditions, with an increase in Al₂O₃ content, the viscosity of slag exhibits a significant increasing trend, and the temperature of slag gradually decreases. Compared to the constant temperature of 1773 K, the change in slag viscosity is more significant with an increase in Al₂O₃ content under fixed heat conditions.

Figure 10c,d shows the effect of Al₂O₃ content on slag temperature and viscosity when the heat is reduced. As the heat input decreases, the temperature of slag continuously decreases. When the heat loss is small, the viscosity of the slag sample tends to increase with an increase in Al₂O₃ content, but the difference in viscosity values is small. With an increase in thermal loss, the influence of Al₂O₃ content on slag viscosity significantly increases.

3.2.3. Effect of MgO on Heat Capacity and Thermal Stability of Slag

Based on Figure 11a, it can be observed that the heat capacity of slag increases almost linearly as the MgO content in slag increases, while the slag temperature remains constant. Additionally, when the MgO content is constant, the heat capacity of slag increases with the temperature. Figure 11b also indicates that the viscosity of slag increases, and then decreases as the MgO content increases, while the heat remains fixed. However, when the temperature is kept constant at 1773 K, the viscosity of slag generally decreases.

Figure 11c demonstrates that the temperature of slag reduces gradually as the heat input decreases. When the heat reduction values are 7.5% and 10%, the temperature of slag decreases initially, and then increases as the MgO content increases. Furthermore, Figure 11d illustrates the variation in slag viscosity with MgO content under heat fluctuation conditions. For minor heat loss, the MgO content has a minimal effect on slag viscosity. As the heat loss increases, the viscosity of slag first increases, and then decreases, and the maximum viscosity of slag is observed when the MgO content is 6.5%.

4. Conclusions

(1) In the case of BaO-containing slag, the viscosity of slag increases considerably with an increase in the mass fraction of the crystalline phase. Increasing the basicity of slag appropriately can improve the flowability of slag. A higher MgO content can weaken the impact of crystallization on slag viscosity and enhance the stability of barium-containing slag. For slag with a high Al₂O₃ content, maintaining a high slag temperature or controlling a specific Al₂O₃ content can improve the slag sample’s ability to resist the influence of crystallization and relative viscosity.

(2) The types of slag crystallization precipitates are not affected by the change in basicity. As the MgO content increases, the crystallization temperature of slag increases gradually, which makes it easier for the crystalline phase to precipitate when the slag temperature decreases. With an increase in Al₂O₃ content, the mass fraction of the melilite phase in slag increases gradually, while the mass fraction of the enstatite phase decreases. The crystallization temperature and the precipitated mass fraction of the celsian phase and anorthite phase remain relatively stable.

(3) When there is a significant reduction in heat input, a slag sample with a higher MgO content experiences a smaller increase in viscosity. Increasing the content of MgO in slag can help maintain its thermal stability. The effect of Al₂O₃ content on the viscosity of slag is more pronounced under conditions of fixed heat than it is under constant temperature conditions. As the heat input decreases, an increase in Al₂O₃ content has a more significant impact on the viscosity and temperature of slag. A lower Al₂O₃ content in slag can reduce the influence of heat fluctuation on slag viscosity.