Effect of CaO/SiO₂ and MgO/Al₂O₃ on the Metallurgical Properties of Low Boron-Bearing High-Alumina Slag

Abstract

For optimizing the operational efficiency and productivity within blast furnace processes, a profound understanding of the viscous flow characteristics of CaO–SiO₂–MgO–Al₂O₃–B₂O₃ slag systems is of paramount importance. In this study, we conducted a comprehensive investigation into the influence of the CaO/SiO₂ and MgO/Al₂O₃ ratios on the viscosity, break point temperature (T_Br), and activation energy (E_η) of low boron-bearing high-alumina slag. Concurrently, we elucidated the underlying mechanisms through which these ratios affect the viscous behavior of the slag by employing a combination of analytical techniques, including X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and thermodynamic modeling using the Factsage software. The experimental findings reveal that, as the CaO/SiO₂ ratio increases from 1.10 to 1.30, the slag viscosity at 1773 K decreases from 0.316 Pa·s to 0.227 Pa·s, while both the T_Br and E_η exhibit an upward trend, rising from 1534 K and 117.01 kJ·mol⁻¹ to 1583 K and 182.86 kJ·mol⁻¹, respectively. Conversely, an elevation in the MgO/Al₂O₃ ratio from 0.40 to 0.65 results in a reduction in slag viscosity at 1773 K from 0.290 Pa·s to 0.208 Pa·s, accompanied by a decrease in T_Br from 1567 K to 1542 K. The observed deterioration in slag flow properties can be attributed to an enhanced polymerization degree of complex viscous structural units within the slag matrix. Ultimately, our study identifies that an optimal viscous performance of the slag is achieved when the CaO/SiO₂ ratio is maintained at 1.25 and the MgO/Al₂O₃ ratio is maintained at 0.55, providing valuable insights for the rational design and control of blast furnace slag systems.

1. Introduction

The cost-reduction and economic–efficiency-enhancement strategies for ironmaking enterprises frequently involve the utilization of low-cost bentonite and high-alumina ores in blast furnace smelting processes. However, excessive Al₂O₃ content in slag can induce detrimental effects, including elevated slag viscosity, impaired permeability in the cohesive zone, and compromised slag–iron separation efficiency. While B₂O₃ incorporation has been validated to improve slag viscous behavior, the addition of CaO and MgO mitigates slag polymerization through network depolymerization, thereby enhancing flowability [1]. Excessive CaO/MgO, however, escalates slag volume and energy consumption, conflicting with energy conservation and emission reduction goals. Consequently, systematic investigation of CaO/SiO₂ and MgO/Al₂O₃ ratios on viscous flow characteristics in low-boron high-alumina slag systems is imperative [2,3].

Recent studies have advanced the understanding of boron-containing slag rheology. Li et al. [4] demonstrated that, in CaO–MgO–Al₂O₃–SiO₂-10 wt% FeO systems (C/S = 1.4), MgO promotes silicate network depolymerization and viscosity reduction, whereas Al₂O₃ > 10 wt% induces counteracting polymerization effects. Liang et al. [5] reported that increasing CaO/SiO₂ from 1.10 to 1.30 reduces slag polymerization degree, viscosity (η), activation energy (E_η), and break temperature (T_Br), while MgO content elevation (6.0–12.0 wt%) exhibits non-monotonic effects on η, T_Br, and E_η. Gao et al. [6] observed that viscosity in SiO₂–CaO–MgO-9 wt% Al₂O₃ systems (basicity 0.4–1.0; MgO 13–19 wt%) depends on the synergistic action of basic oxides, with basicity elevation being more effective than MgO increase in viscosity reduction at lower temperatures. Huang et al. [7] confirmed that B₂O₃ incorporation in SiO₂-30 wt% Al₂O₃–B₂O₃-12 wt% Na₂O–CaO systems reduces polymerization degree, viscosity, and T_Br. Despite these advances, the combined effects of CaO/SiO₂ and MgO/Al₂O₃ ratios on low-boron high-alumina slag viscous behavior remain underexplored.

This study systematically investigates the influence of CaO/SiO₂ and MgO/Al₂O₃ ratios on the viscous flow characteristics of low-boron high-alumina slag, with a focus on viscosity, break temperature, and viscous flow activation energy. Experimental viscosity measurements were conducted across a range of slag compositions. Mechanistic insights were elucidated through X-Ray Diffraction (XRD) and Fourier-Transform Infrared Spectroscopy (FTIR) analyses, while Factsage (version 7.2) thermodynamics software was employed to determine liquidus temperatures. The optimal slag composition identified herein provides theoretical guidance for industrial practice in ironmaking enterprises.

2. Results and Discussion

Different CaO/SiO₂ and MgO/Al₂O₃ slag were obtained through experiments. The η–T curve is shown in Figure 1. The viscosity increases with decreasing temperature, and there is a clear turning point on every η–T curve.

2.1. Effects of CaO/SiO₂ and MgO/Al₂O₃ on the Viscous Behaviors of the Slag

2.1.1. Effects of CaO/SiO₂ on the Viscous Behaviors of the Slag

The effects of different CaO/SiO₂ on slag viscosity are shown in Figure 2a. When the temperature increases from 1733 to 1773 K, with an increase in CaO/SiO₂ from 1.10 to 1.30, the η of the slag decreases significantly first and then slows down. When CaO/SiO₂ is 1.25, the minimum of η_1773K is 0.227 Pa·s.

The viscosity of slag is mainly affected by the internal network structure. The Si_xO_y^z− and Al_xO_y^z− tetrahedral structures are the main structural units of the slag. The complex network structure inside the slag can be depolymerized to reduce viscosity. As CaO/SiO₂ increases in the slag, the viscosity will decrease. The reason is that the free oxygen ion O²⁻ dissociated from basic oxide CaO can interact with bridging oxygen O in the network structure of aluminosilicate to form non-bridging oxygen O⁻ [8,9,10], resulting in the aluminate silicate network structure being depolymerized into smaller network units. By the viscosity module of FactSage prediction, the theoretical viscosities at 1773 K are 0.314, 0.296, 0.281, 0.268, and 0.256 Pa·s, respectively. The trend of change is the same as that of the experiment, providing theoretical support for the experimental results.

In order to elucidate the relationship between slag viscosity and internal structure, FTIR is employed to analyze the slag of different CaO/SiO₂. The FTIR of silicate aluminate slag is generally divided into three regions, and the wave number range is 400–600 cm⁻¹, 600–800 cm⁻¹, and 800–1200 cm⁻¹, which respectively corresponds to T–O–T (T represents Si or Al) bending vibration, [AlO₄]⁵⁻ tetrahedral asymmetric tensile vibration, and [SiO₄]⁴⁻ tetrahedral symmetric tensile vibration [11,12,13]. The FTIR analysis results of different CaO/SiO₂ are shown in Figure 3a. As CaO/SiO₂ increases from 1.10 to 1.30, the transmitted wave valley of the [SiO₄]⁴⁻ tetrahedral symmetric stretching vibration band shifts towards lower wave numbers, and the bandwidth widens, indicating the simplification of the aluminosilicate network structure. Gaussian deconvolution was performed on Si–O axisymmetric vibration bands with CaO/SiO₂ ratios of 1.10 and 1.30, and the corresponding areas of each peak are used to characterize the corresponding amount of Q_i (i = 0~3). Q₀, Q₁, Q₂, and Q₃ represent the structures of SiO₄⁴⁻, Si₂O₇⁶⁻, Si₂O₆⁴⁻, and Si₂O₅²⁻ [14,15,16]. The smaller the value of i, the simpler is the slag structure, and the higher is the degree of polymerization. In brief, when CaO/SiO₂ increases from 1.10 to 1.30, the silicate and aluminate network structure in the slag are depolymerized, resulting in the reduction in the η in the slag.

2.1.2. Effects of MgO/Al₂O₃ on the Viscous Behaviors of the Slag

The effects of different MgO/Al₂O₃ on slag viscosity are shown in Figure 2b. When the temperature increases from 1733 to 1773 K, with an increase in CaO/SiO₂ from 0.40 to 0.65, the η of the slag decreases significantly first and then slows down. When MgO/Al₂O₃ is 0.55, the η_1773K is 0.226 Pa·s.

The effect mechanism of MgO/Al₂O₃ on slag viscosity is similar to the CaO/SiO₂. Both MgO and CaO are basic oxide, and the dissociated O²⁻ ions promote the depolymerization of complex structures. By the viscosity module of FactSage prediction, when MgO/Al₂O₃ increases from 0.40 to 0.65, the theoretical viscosities at 1773 K are 0.281, 0.272, 0.263, 0.254, 0.247, and 0.239 Pa·s, respectively. The trend of change is the same as that of the experiment, providing theoretical support for the experimental results.

In order to elucidate the relationship between slag viscosity and internal structure, FTIR is employed to analyze the slag of different MgO/Al₂O₃. The FTIR analysis results of different MgO/Al₂O₃ slag are shown in Figure 3b. When MgO/Al₂O₃ increases from 0.40 to 0.65, the depth of [SiO₄]⁴⁻ tetrahedral symmetric tensile vibration becomes shallower, and the bandwidth becomes wider, indicating an increase in the distance between Si–O bonds and the disintegration of the slag silicate network structure into smaller network units; the depth of the [AlO₄]⁵⁻ tetrahedron asymmetric tensile vibration band gradually becomes shallow and finally almost disappears, indicating the aluminate network structure in the slag is depolymerized; and the groove depth of the Si–O–Al bending vibration band is slightly weakened, indicating a decrease in the number of Si–O–Al structures used to connect [AlO₄]⁵⁻ and [SiO₄]⁴⁻ tetrahedra [17,18,19]. In conclusion, the silicon aluminate network structure in the slag is depolymerized, resulting in the reduction in the η in the slag.

2.2. Effects of CaO/SiO₂ and MgO/Al₂O₃ on the Break Point Temperature of Slag

2.2.1. Effects of CaO/SiO₂ on the Break Point Temperature of Slag

The effect of different CaO/SiO₂ on the break point temperature is shown in Figure 4a. When CaO/SiO₂ increases from 1.10 to 1.30, the T_Br shows an uptrend, increasing from 1534 K to 1583 K.

The phase diagram of the five-component slag system CaO–SiO₂–MgO-17.00%Al₂O₃-3.83%B₂O₃ as plotted by the phase diagram module in the FactSage is shown in Figure 5. The different CaO/SiO₂ components are located in the crystalline region of the pyrochlore. With the increase in CaO/SiO₂, the liquid temperature of the slag increases, and the ability to crystallize at high temperatures becomes stronger, leading to an increase in T_Br. The liquid temperatures were 1613.92, 1623.05, 1631.00, 1637.96, and 1643.76 K, respectively. The liquidus temperature of the slag increased. Thus, the crystallization capacity of the slag is enhanced, and the T_Br also increases. These results are in agreement with the trend of the measurements of T_Br.

The XRD analysis results of different CaO/SiO₂ slag are shown in Figure 6a. The basic phase in different CaO/SiO₂ slag is melilite. When CaO/SiO₂ increases from 1.10 to 1.30, the diffraction peak intensity of melilite, spinel, and Ca₂B₂O₅ phases increases. When CaO/SiO₂ is 1.15, the Mg₃(BO₃)₂ phase disappears, and the pyroxene phase appears in the slag. The number of high melting point phases in the slag increases relatively, and the crystallization ability of the slag increases under high temperature conditions, resulting in an increase in the T_Br and a decrease in fluidity.

2.2.2. Effects of MgO/Al₂O₃ on the Break Point Temperature of Slag

The effect of different MgO/Al₂O₃ on the break point temperature is shown in Figure 4b. With the increasing of MgO/Al₂O₃ from 0.40 to 0.65, the T_Br of the slag shows a downward trend, decreasing from 1570 K to 1542 K.

The phase diagram of the CaO–SiO₂-7.98%MgO–Al₂O₃-0.47%B₂O₃ slag system is calculated by the Phase Diagram module in FactSage. As shown in Figure 7, with the continuous decrease in MgO/Al₂O₃, the composition of the slags is located in the area of the melilite phase, and the liquidus temperature of melilite is relatively sparse, which demonstrates that the phase is stable. According to FactSage, the liquidus temperature of slag with different MgO/Al₂O₃ was calculated as 1345.83, 1349.95, 1354.95, 1360.93, 1367.64, and 1374.78 °C, and the liquidus temperature of the slag increased. Thus, the crystallization capacity of the slag is enhanced, and the T_Br also increases. These results are in agreement with the trend of the measurements of T_Br.

The XRD analysis results of different MgO/Al₂O₃ slag are shown in Figure 6b. There are melilite, spinel, pyroxene, Ca₂B₂O₅, and Mg₃(BO₃)₂ in the slag, and the melilite is the basic phase. When MgO/Al₂O₃ increases from 0.40 to 0.65, the diffraction peak intensity of melilite, spinel, pyroxene, and Mg₃(BO₃)₂ gradually weakens, while the diffraction peak intensity of Ca₂B₂O₅ and pyroxene slightly increases. This indicates that the Ca₂B₂O₅ and pyroxene in the slag are relatively increased, while the number of high melting point phases is relatively reduced, resulting in a decrease in the crystallization ability of the slag, a decrease in the T_Br, and an improvement in fluidity under high temperature conditions.

2.3. Effects of CaO/SiO₂ and MgO/Al₂O₃ on Activation Energy of Slag Viscous Flow

Viscous flow activation energy is a crucial viscosity characteristic of slag. The E_η reflects the sensitivity of slag viscosity to temperature, representing the thermostability of slag [20,21,22]. The calculation of viscous flow activation energy in this article adopts the modified Weymann–Frenkel equation by Urban, as shown in Formula (1). Formula (2) can be obtained by taking the logarithm of both sides of Formula (1). The viscosity data measured in the experiment are calculated using the linear regression method, and the slope is E_η. The linear fitting results and the trend of E_η with different CaO/SiO₂ and MgO/Al₂O₃ are shown in Figure 8. The results indicate that there is a fine linear relationship between ln(η/T) and 1/T. The linear correlation coefficients are all greater than 0.99 [23,24].

$$\eta =ATexp\left({\displaystyle \frac{E_{\eta }}{RT}}\right)$$

(1)

$$\ln \left({\displaystyle \frac{\eta }{T}}\right)=\ln A+{\displaystyle \frac{E_{\eta }}{R}}\times {\displaystyle \frac{1}{T}}$$

(2)

where η is the viscosity, Pa·s; A is the proportionality constant; T is the temperature, K; and R is the gas constant, 8.314 J·(mol·K)⁻¹.

2.3.1. Effects of CaO/SiO₂ on Activation Energy of Slag Viscous Flow

As shown in Figure 9a, when CaO/SiO₂ increases from 1.10 to 1.30, the E_η increases from 117.01 to 182.86 kJ·mol⁻¹. This indicates that the sensitivity of slag viscosity to temperature is weakened. On the premise of ensuring better slag stability, the CaO/SiO₂ value of 1.25 is reasonable. From the perspective of slag structure, the complex slag structure is decomposed into a simpler structure, the activation energy of slag is increased, and the stability is improved [25,26,27]. The stability of the slag can also be characterized by the density of the isotherm in the phase diagram. The thinner the contour lines temperature and related subjects, the less the temperature affects the slag composition, and the better the slag stability is.

2.3.2. Effects of MgO/Al₂O₃ on Activation Energy of Slag Viscous Flow

As shown in Figure 9b, when MgO/Al₂O₃ increases from 0.40 to 0.50, the E_η changes inconspicuously. The E_η increases from 126.20 to 205.86 kJ·mol⁻¹. When MgO/Al₂O₃ increases from 0.50 to 0.65, the E_η significantly increases. This indicates that the thermostability of the slag to temperature is enhanced. The complex slag structure is decomposed into a simpler structure, and the activation energy of the slag is increased.

As shown in Figure 6 and Figure 8, within the experimental value range, the isotherm becomes sparse with the increase in CaO/SiO₂ and MgO/Al₂O₃, indicating a better stability of the slag, which is consistent with the experimental fitting results [28,29].

Briefly, when CaO/SiO₂ is 1.25, η_1773K has a minimum of 0.227 Pa·s, the T_Br is lower at 1570 K, E_η is stable at a lower level, and the slag has a good thermal stability performance. When MgO/Al₂O₃ is 0.55, the decreasing trend of η_1773K begins to slow down to 0.226 Pa·s, and T_Br and E_η are 1570 K and 161.99 KJ·mol⁻¹, respectively. Overall, when CaO/SiO₂ is 1.25 and MgO/Al₂O₃ is 0.55, good metallurgical properties of the low boron-bearing high-alumina slag system can be obtained, providing a good reference basis for blast furnace operation.

3. Experimental

3.1. Raw Material

Based on the on-site BF slag compositions, the slag samples for the experiments were synthesized with the analytical reagent oxides of CaO, SiO₂, MgO, and Al₂O₃. The basicity of the BF slag is 1.25, the Al₂O₃ is 17.00%, the B₂O₃ is 0.47%, and the MgO/Al₂O₃ is 0.47, which belongs to the boron-bearing high-alumina blast furnace slag. The chemical compositions of the slag samples are listed in

Table 1. Chemical composition of base blast furnace slag (mass fraction/%).

CaO	SiO2	MgO	Al2O3	B2O3
38.01	30.37	7.98	17.00	0.47

. The experimental slag samples were synthesized by adding the analytical-grade oxides, with the site blast furnace slag as the reference slag. In order to improve the accuracy of the experiment, the furnace slag and the analytical-grade oxides were roasted, mixed evenly, put into the molybdenum crucible, and then pre-melted under an argon atmosphere to stabilize the temperature and homogenize the compositions. The pre-melted slag samples were used for the determination of viscous flow behaviors [30]. The experimental scheme is shown in

Table 2. Experimental scheme (mass fraction/%).

No.	CaO	SiO2	MgO	Al2O3	B2O3	CaO/SiO2	MgO/Al2O3
1	35.03	31.85	7.98	17.00	0.47	1.10	0.47
2	35.77	31.11	7.98	17.00	0.47	1.15	0.47
3	36.48	30.40	7.98	17.00	0.47	1.20	0.47
4	37.15	29.72	7.98	17.00	0.47	1.25	0.47
5	37.80	29.08	7.98	17.00	047	1.30	0.47
6	37.81	30.25	6.80	17.00	0.47	1.25	0.40
7	37.74	29.87	7.65	17.00	0.47	1.25	0.45
8	36.87	29.49	8.50	17.00	0.47	1.25	0.50
9	36.39	29.11	9.35	17.00	0.47	1.25	0.55
10	35.92	28.74	10.20	17.00	0.47	1.25	0.60
11	35.45	28.36	11.05	17.00	0.47	1.25	0.65

. In series-1, MgO was kept at 7.98%, Al₂O₃ was kept at 17.00%, and B₂O₃ was kept at 0.47% in the slag, and CaO/SiO₂ was increased from 1.10 to 1.30. In series-2, CaO/SiO₂ ratio was kept at 1.25 in the slag, and MgO/Al₂O₃ was increased from 0.40 to 0.65.

3.2. Experimental Procedure

The viscosity–temperature (η–T) curves of the slag were acquired on an RTW-10 melt property tester by the rotating cylinder method. Figure 10 shows the schematic diagram of the experimental device, which consisted of a heating system, a rotating system, a measuring system, a control system, and an atmosphere system.

The crucible containing the experimental slag sample was placed on a graphite base and heated to 1500 °C with the furnace temperature, and then, the temperature was held for 30 min. During the constant temperature, a molybdenum probe was used to stir the slag to homogenize the chemical compositions. When the temperature stabilized, the viscosity was measured at a speed of 200 r/min. In the process of viscosity measurement, argon gas was injected into the furnace tube at a flow rate of 1.5 L/min to maintain an inert atmosphere. The measurement results were recorded by the control system. When the viscosity reached about 3.5 Pa·s, the measurements were ended. The experiment was repeated twice. The quenched experimental slags and the natural cooling slags were crushed and grinded to facilitate the analysis by FTIR and XRD (Bruker D8 ADVANCE, Karlsruhe, Gemany).

This article defines the viscosity of slag at 1773 K as high-temperature viscosity (η_1773K). Making 135° straight line is tangent to the viscosity–temperature curve(η–T). The temperature corresponding to the tangent point is defined as the T_Br of the slag, as shown in Figure 11.

4. Conclusions

(1)

With CaO/SiO₂ increasing from 1.10 to 1.30, viscosity first decreased significantly and then slowed down. When CaO/SiO₂ is 1.25, η_1773K is 0.227 Pa·s. T_Br shows an increasing trend, increasing from 1534 K to 1583 K. E_η increases from 117.01 to 182.86 kJ·mol⁻¹, and the thermal stability of the slag deteriorates first and then improves. At this point, the slag system has a better performance.

(2)

With MgO/Al₂O₃ increasing from 0.40 to 0.65, viscosity first decreased significantly and then slowed down. When MgO/Al₂O₃ is 0.55, η_1773K is 0.226 Pa·s. T_Br decreases from 1570 K to 1542 K. The E_η increases from 126.20 to 205.86 kJ·mol⁻¹, and the thermal stability of the slag first improves and then deteriorates. At this point, the slag system has a better performance.

(3)

For comprehensive considerations, when CaO/SiO₂ is 1.25 and MgO/Al₂O₃ is 0.55, the η_1773K, T_Br, and E_η are at a reasonable value. The low boron-bearing high-alumina slag system has the best metallurgical performance at this value.