Evaluating the Effect of MgO/Al₂O₃ Ratio on Thermal Behaviors and Structures of Blast Furnace Slag with Low Carbon Consumption

Abstract

In order to clarify the effect of the MgO/Al₂O₃ ratio on the fluidity of a low-alumina blast furnace slag system, the influence law of slag fluidity with different MgO/Al₂O₃ ratios was studied based on the composition of blast furnace slag through a viscosity experiment and themodynamic software. By studying the effect of the MgO/Al₂O₃ ratio on the activation energy of viscous flow of slag combined with FT-IR, the effect of the MgO/Al₂O₃ ratio on the thermal-stability of low-aluminum slag was interpreted from the microstructure level. Results indicated that the viscosity and the melting temperature of slag both showed a gradual downward trend due to the increase of the MgO/Al₂O₃ ratio. Besides, the temperature stability of the low aluminum slag became more stable due to the depolymerization of the complex structure of slag. Considering the actual operating conditions of blast furnace, the MgO/Al₂O₃ ratio of slag was suggested to be controlled to 0.60 and the basicity to be no higher than 1.20 under the conditions of this investigation. Industrial test results showed that the coke rate could be saved as 3.49 kg/t when the MgO/Al₂O₃ ratio decreased from 0.70 to 0.58.

1. Introduction

Since the middle of the 20th century, CO₂ as a typical greenhouse gas has become the chief culprit of global warming. China’s iron and steel industry has always been a large consumer of coal resources due to its long and multiple processes, accounting for 17% of the country’s total coal consumption. The dominant ironmaking route, blast furnace, is an especially energy-intensive process based on fossil fuel consumption; the steel sector is thus responsible for about 7% of all anthropogenic CO₂ emissions. The CO₂ emissions of the blast furnace ironmaking process in the iron and steel industry account for approximately 80% of the total CO₂ emissions of all processes [1,2,3,4]. Therefore, the pressure on CO₂ emission reduction for the blast furnace ironmaking process is increasing. The carbon in the blast furnace is from coke and pulverized coal injection. For reducing CO₂ emissions in blast furnace ironmaking process, it is necessary to ensure the supporting role of coke and maintain its chemical reaction function of each section in the blast furnace to reduce the coke rate (K) at the maximum. Meanwhile, high gangue content of iron ore has been widely used in blast furnaces due to the shortage of high-grade ores in recent years. For improving the slag fluidity, charging burdens with high MgO content are essential for the smooth operation of the blast furnace. Therefore, the determination of the proper MgO/Al₂O₃ ratio of blast furnace slag becomes the study focus. Generally, the decrease of the MgO/Al₂O₃ ratio will increase the viscosity of slag, which affects the metallurgical properties of slag and iron tapping [5]. Therefore, it is necessary to systematically study the relationship between the MgO/Al₂O₃ ratio and the metallurgical properties of slag, and then explore the feasibility of reducing the MgO/Al₂O₃ ratio of slag. Previous research has demonstrated that the proper MgO/Al₂O₃ ratio could improve the fluidity of blast furnace slag [6,7,8,9,10,11]. Among them, Talapaneni [8], Shankar [10], and Feng [11] et al., have all proven that the viscosity of slag decreases with the increase of the binary basicity (w(CaO/SiO₂)) and the mass fraction of MgO and TiO₂ in slag. In addition, Seok and Chang et al. have found that when the content of MgO and w(CaO/SiO₂) remains unchanged, the slag viscosity increases with the increase of Al₂O₃ content by structural analysis [12,13]. However, there are few studies on the thermal stability of blast furnace slag and the effects of the MgO/Al₂O₃ ratio on the microstructure of slag. In this study, the spindle method and thermodynamic software FactSage7.0 (ThermFact LTD., Montreal, QC, Canada) were used to analyze the effect of the MgO/Al₂O₃ ratio and w(CaO/SiO₂) on the viscosity and thermal stability of slag in detail. Then the microstructure of slag with different MgO/Al₂O₃ ratios was characterized by FT-IR detection. The structural role of each component in the polymerization or depolymerization reactions of silicates was then clarified. Additionally, the feasibility of reducing the MgO/Al₂O₃ ratio of slag to reduce the amount of slag and coke rate was discussed, which can help to realize the green and sustainable development of the blast furnace.

2. Materials and Methods

2.1. Materials

Table 1. Chemical composition of slag.

CaO/%	SiO2/%	Al2O3/%	MgO/%	R2	MgO/Al2O3
42.72	36.21	12.11	8.96	1.18	0.74

shows the chemical composition of slag in a commercial blast furnace. The content of Al₂O₃ in slag was less than 14%, which is a typical low-aluminum slag.

The experimental slag samples (140 g) were synthesized with the analytical-grade oxides of CaO, SiO₂, MgO, and Al₂O₃. The mass fraction of Al₂O₃ in each group of slag samples was fixed at 12% based on the approximate compositions of this low-aluminum slag obtained in the practical production process. The experimental scheme and chemical compositions of low-aluminum slag with different MgO/Al₂O₃ ratios and binary basicity are shown in

Table 2. Experimental schemes of different MgO/Al₂O₃ ratio and binary basicity.

NO.	MgO/Al2O3	CaO/%	SiO2/%	Al2O3/%	MgO/%	R2
1	0.55	44.40	37.00	12.00	6.60	1.20
2	0.60	44.07	36.73	12.00	7.20	1.20
3	0.65	43.75	36.45	12.00	7.80	1.20
4	0.70	43.42	36.18	12.00	8.40	1.20
5	0.75	43.09	35.91	12.00	9.00	1.20
6	0.60	41.39	39.41	12.00	7.20	1.05
7	0.60	42.32	38.48	12.00	7.20	1.10
8	0.60	43.22	37.58	12.00	7.20	1.15
9	0.60	44.07	36.73	12.00	7.20	1.20
10	0.60	44.89	35.91	12.00	7.20	1.25

.

In order to eliminate the influence of moisture on the tests, the pure reagent was dried in a stoving chest at 378 K for 12 h before the experiment. A RTW-10 melt physical property comprehensive tester was used to measure the viscosity of slag at different temperatures under argon atmosphere. The viscous flow activation energy was calculated to investigate the influence of different MgO/Al₂O₃ ratios and binary basicity on the thermal stability of slag.

2.2. Experimental Apparatus and Method

The viscosity measurements of experimental slags were carried out by the rotating cylinder method with a RTW-10 melt physical property comprehensive tester. The schematic diagram of the apparatus is shown in Figure 1.

Using Si–Mo bar as the heating element, the furnace tube was made of corundum, the bottom of the furnace was equipped with a refractory base and there was a circulating cooling water device around the furnace shaft. The temperature was controlled by changing the furnace current and voltage through the control cabinet.

During the experiment, the well-mixed slag sample (140 g) was filled in a graphite crucible. The crucible was then placed at the corundum tube of the apparatus. When the experimental slags were heated to 1873 K in an argon atmosphere (gas flow rate 1.5 L/min), it was kept for 1 h to homogenize the compositions of the molten slag phase. After the slag sample completely melted, the molybdenum rotating spindle was slowly immersed in the molten slag to rotate and stir, the speed was set to 300 r/min, and then the slag viscosity was measured at a cooling rate of 300 K/min. The measurements were ended when the viscosity of the slag reached 4 Pa·s. In order to facilitate the removal of the molybdenum rotating spindle and the subsequent testing of the experimental slag sample, the furnace temperature was raised to 1873 K after the viscosity measurement was completed. The taken out slag was cooled by the water quenching method and milled with the particle size lower than 0.074 mm. The NEXUS-470 FT-IR infrared spectrometer was used to detect the microstructure of the slag with different MgO/Al₂O₃ ratios and binary basicity. Approximately 10 g of the slag sample was used to measure the FT-IR spectra. The spectra range was from 1600 cm⁻¹ to 200 cm⁻¹ with a resolution less than 2 cm⁻¹. The precision of the infrared transmittance was less than 0.1% T and the signal-to-noise ratio was higher than 3600:1.

3. Results and Discussions

3.1. Effect of MgO/Al₂O₃ Ratio and Binary Basicity on Slag Viscosity

Figure 2 displays the viscosity-temperature curves of low-aluminum slags with different MgO/Al₂O₃ ratios and binary basicity, respectively.

Figure 2 indicates that as the MgO/Al₂O₃ ratio and the binary basicity of slag increased, the viscosity of slag showed a decreasing trend. This was mainly because increasing the MgO/Al₂O₃ ratio and binary basicity of slag increased the mass fraction of MgO and CaO, both of which were basic oxides that could provide free oxygen ions (O²⁻) to the slag. The interaction of the O²⁻ and bridging oxygen in the slag destroyed the Si–O–Si structure. The complex silicon-oxygen composite anion group (Si_xO_y^z−) in the slag was disintegrated, and the polymerization degree of the slag was reduced, which led to a macroscopic phenomenon of the viscosity reduction of the slag. In addition, the increase of the MgO/Al₂O₃ ratio could provide more Mg²⁺ to the slag as a network modifier and reduce the degree of slag polymerization. Through regression analysis of the viscosity data of different MgO/Al₂O₃ ratios and binary basicity of slag, it was found that each increase of 0.1 in the MgO/Al₂O₃ ratio decreased the viscosity of the slag by about 0.42~0.45 Pa·s. For every 0.1 increase in the binary basicity of low-aluminum slag, the slag viscosity decreased by about 0.53~0.79 Pa·s, which showed that the binary basicity of slag had a stronger influence on the slag viscosity than the MgO/Al₂O₃ ratio. Taking into account the temperatures of slag in the actual production process, the slag viscosity of different MgO/Al₂O₃ ratios and binary basicity were all lower than 0.40 Pa·s in the high temperature region of 1733 K~1773 K in this study, which can meet the needs of actual production. Therefore, the MgO/Al₂O₃ ratio of slag could be reduced to 0.60 under the premise of ensuring the stability of the furnace temperature.

Figure 3 shows the quaternary phase diagram of the CaO–SiO₂-12%massAl₂O₃–MgO system. Symbol ‘◆’ in the figure stands for the phase composition of the slag sample from NO.1 to NO.5. The red line displays the change trend of the slag component under different MgO/Al₂O₃ ratios. Symbol ‘●’ in the figure stands for the phase composition of the slag sample from NO.6 to NO.10. The blue line displays the change trend of the slag component under different binary basicity.

Previous studies have shown that under the actual production conditions of the blast furnace, the optimal mineral composition of slag should be maintained in the primary crystal region of melilite in the phase diagram [14,15,16,17]. The isotherm of this region was sparse, indicating an appropriate melting temperature and good fluidity of the slag. In this research, the MgO/Al₂O₃ ratio of slag was set to increase from 0.55 to 0.75. The phase point of the slag in the phase diagram gradually shifted to the left and close to the center of the melilite primary crystal region, indicating that the proportion of melilite with a low-melting point in the slag increased as the MgO/Al₂O₃ ratio increased. The high melting point substances of slag were relatively reduced, resulting in a decrease in the viscosity of slag. With the binary basicity of slag increasing from 1.05 to 1.25 and the MgO/Al₂O₃ ratio fixed at 0.60, the phase point of slag moved upward and gradually deviated from the center of the melilite primary crystal region to the edge, and then finally transited to the high melting point phase dicalcium silicate (Ca₂SiO₄, melting point 2403 K) primary crystal region. The slag phase point was out of the liquid phase region when the slag basicity reached 1.25. It showed that the exorbitantly high binary basicity of slag inhibited the precipitation of melilite in the slag, which made the high melting point substances in the slag relatively increased, the melting temperature of the slag greatly increased, and the slag viscous flow was restricted, resulting in the deterioration of fluidity. Therefore, the higher the basicity, the smaller the decreasing range of slag viscosity. This showed that overly high basicity was not conducive to actual production. In summary, it was recommend that the MgO/Al₂O₃ ratio of the low-aluminum slag should be controlled at about 0.60 and the basicity should not be higher than 1.20 in the actual production process.

3.2. Effects of MgO/Al₂O₃ Ratio and Binary Basicity on Slag Thermal Stability

The viscous flow activation energy of blast furnace slag can reflect the strength of the frictional resistance experienced by the slag flow and the change of the slag microstructure. In addition, the sensitivity of the viscosity of the slag to the temperature is the thermal stability of the slag, which can be expressed by its viscous flow activation energy. The higher the activation energy, the more sensitively the temperature will affect the viscosity of the slag (the viscosity of the slag changes greatly with temperature fluctuations), resulting in poor thermal stability of the slag. Due to the difference in structural unit characteristics of slag with different composition content, the value of activation energy is also different. Therefore, it is necessary to study the influence of different MgO/Al₂O₃ ratios and basicity on the thermal stability of slag. The viscous flow activation energy of slag can be obtained by the Wayman-Frankel formula [18]:

$$\eta =A_W\times T\times e^{\frac{E_W}{RT}}$$

(1)

where η is the viscosity of slag, Pa·s; A_w is the proportionality constant; E_w is the viscous flow activation energy, J·mol⁻¹; R is the gas constant (8.314 J·(mol·K)⁻¹); T is the temperature, K.

Take the logarithm of both sides of Equation (1) to get Equation (2):

$$\mathit{ln}\eta =\mathit{ln}{A}_W+\mathit{ln}T+\frac{E_W}{RT}$$

(2)

The fitting results of lnη and 1/T for different MgO/Al₂O₃ ratios and basicity of low-aluminum slag are shown in Figure 4a and Figure 5a. The product of the slope of each line and R was the viscous flow activation energy of each component slag. The change of viscous flow activation energy of low-aluminum slag with different MgO/Al₂O₃ ratios and basicity is shown in Figure 4.

It can be seen from Figure 4 that the viscous flow activation energy of the low-aluminum slag showed a gradual decrease overall with the continuous increase of the MgO/Al₂O₃ ratio in the low-aluminum slag system. Therefore, the increase of the MgO/Al₂O₃ ratio will weaken the sensitivity of the slag viscous flow to temperature changes, indicating that the increase of the MgO/Al₂O₃ ratio enhanced the thermal stability of slag.

Figure 5 displays that the viscous flow activation energy of the slag decreased first, and then rose as the basicity increased during the slag basicity changes in the range of 1.05 to 1.25, indicating that the thermal stability of the slag tended to deteriorate. When the basicity of slag was lower than 1.20, the slag viscous flow activation energy showed a downward trend as the basicity increased, which indicated that the slag viscosity gradually reduced to temperature sensitivity and the slag thermal stability gradually improved. However, the viscous flow activation energy of the slag displayed a contrary tendency, as it increased when the basicity was higher than 1.20. The viscosity of the slag became sensitive to temperature and the thermal stability of the slag tended to deteriorate.

In summary, the increase in the basicity of low-aluminum slag improved the fluidity and thermal stability of slag, but the overly high value of basicity will have an adverse effect. This suggests that the slag basicity should not be higher than 1.20 in actual production.

3.3. The Influence of MgO/Al₂O₃ Ratio and Basicity on the Thermal Stability of Blast Furnace Slag

The change of the blast furnace slag viscous flow activation energy was mainly caused by the microstructure variation of slag, which indicated that the thermal stability of slag was related to the slag microstructure. Figure 6 shows the infrared transmission spectrum of the slag with a binary basicity of 1.20 and different MgO/Al₂O₃ ratios.

Studies have shown that different microstructures in the slag have differences in the corresponding wavebands of the infrared transmission spectrum. Because the blast furnace slag and silicate crystals are highly similar, the silicate structure theory is also applicable to blast furnace slag, so the infrared spectrum of the slag is mainly distributed in the wave number range of 1200~400cm⁻¹ [18]. Park et al. [19] qualitatively analyzed the microstructure of the slag and believed that the 1200~400cm⁻¹ wavenumber band in the spectrum can be roughly divided into [SiO₄]⁴⁻ axisymmetric vibration at 1200~750cm⁻¹, [AlO₄]⁵⁻ reverse bending vibration at 720~630 cm⁻¹, [AlO₆]⁻ vibration, and Si–O–Al bending vibration. Mysen et al. [20] found that the [SiO₄]⁴⁻ axisymmetric vibration band is mainly composed of four non-bridge oxygen symmetric stretching vibrations, Q₃, Q₂, Q₁, and Q₀. The degree of polymerization of slag gradually increased from Q₀ to Q₃.

As shown in Figure 6, when the MgO/Al₂O₃ ratio of the slag changed from 0.55 to 0.75, the end of the [SiO₄]⁴⁻ axisymmetric vibration band gradually expanded to the low wave number region, moving from 780 cm⁻¹ to 760 cm⁻¹. It showed that the expansion of the [SiO₄]⁴⁻ axisymmetric vibration band reflected the increase in the distance between Si and O in the slag system [21,22,23,24], which meant that the degree of polymerization of the slag network structure decreased with the MgO/Al₂O₃ ratio of slag. The groove center of the Si–O–Al bending vibration section gradually shifted to the right with the increase of the MgO/Al₂O₃ ratio, and the strength gradually weakened, indicating that the Si–O–Al structure in the slag decreased. The [AlO₄]⁵⁻ reverse bending vibration did not change significantly with the MgO/Al₂O₃ ratio. It can be seen that the increase in the MgO/Al₂O₃ ratio of slag promoted the simplification of the microstructure. However, it was difficult to judge the changes of the four non-bridge oxygen symmetric stretching vibrations (Q₀~Q₃) in the [SiO₄]⁴⁻ axisymmetric vibration in Figure 6. Therefore, according to Lambert Beer’s law, the peak separation fitting processing was performed on the 1300~700 cm⁻¹ wavenumber band in the infrared spectrum of different MgO/Al₂O₃ ratios. The relative content of each characteristic peak area was then calculated to characterize the amount of Q₀~Q₃ in the slag to achieve the purpose of quantitatively analyzing the change of Q₀~Q₃ in the slag with different MgO/Al₂O₃ ratios. The peak separation and calculation results are shown in Figure 7 and Figure 8.

As shown in Figure 7f, the content of (Q₀ + Q₁) in the slag continued to increase while the content of (Q₂ + Q₃) gradually decreased as the MgO/Al₂O₃ ratio of slag increased. This was mainly because the increase of the MgO/Al₂O₃ ratio will further increase the amount of free oxygen ions in the slag, which will depolymerize the main viscous flow unit Si_xO_y^z− in the slag, and thus, Q₂ and Q₃ in the slag can gradually be transformed into Q₀ and Q₁. The latter two had relatively low energy barriers to overcome when participating in viscous flow, which meant that as the slag MgO/Al₂O₃ ratio increased, the fluidity of the slag gradually improved and the slag viscous flow activation energy continued to decrease. The heat required for the movement of particles in the slag reduced, resulting in the sensitivity of the slag viscosity to temperature decreasing. Therefore, the slag could maintain a certain fluidity under the condition of temperature fluctuations, and its thermal stability had further improved, which also confirmed the accuracy of the calculation results of viscous flow activation energy in Section 3.2.

Figure 8 shows the infrared transmission spectra of low-aluminum slag under the conditions of different R₂ values.

As the slag basicity increased from 1.05 to 1.25, the wavenumber was in the range of 720~630 cm⁻¹. The [AlO₄]⁵⁻ antisymmetric vibration grooves gradually became shallower when the basicity was lower than 1.15. However, when the basicity reached to 1.20, the groove of the [AlO₄]⁵⁻ antisymmetric vibration began to deepen, which reflected that the amount of Al₂O₃ in the slag polymerized into a complex aluminate structure in the form of four-coordinated Al. This is mainly because that Al₂O₃ is an amphoteric oxide. When the molar ratio of Al₂O₃ to alkaline oxide was less than 1, [AlO₄]⁵⁻ would be formed by the substitution of Al³⁺ for Si⁴⁻ in the slag structure. With the increase of binary basicity, more oxygen ions could be provided by CaO, which indicated the existence of a small quantity of complex network structures. The center position of the Si–O–Al vibration groove at the wavenumber of 500 cm⁻¹ moved to the right with the increase of basicity, indicating that the Si–O–Al structure in the slag was reduced. In addition, the [SiO₄]⁴⁻ axisymmetric vibration band was located in the 1200~750 cm⁻¹ wave number range, which played a decisive role in the degree of polymerization of the slag microstructure. The width of the vibration band increased with the increase in basicity, indicating that the distance between the Si and O of the slag increased. The increase of slag basicity led to a continuous decrease in the degree of slag polymerization. Similarly, it could not evaluate the change of the non-bridge oxygen symmetric stretching vibration band with basicity in this region quantitatively. It was necessary to perform peak fitting processing on the Fourier curve of this region and calculate the area ratio of each characteristic peak.

Figure 9 shows the results of the quantitative analysis of Q_i in the slag.

With the basicity of slag increasing, the simple structure (Q₀ + Q₁) in the slag gradually increased, and the complex structure (Q₂ + Q₃) continued to decrease. When the basicity was lower than 1.20, Q₂ showed little change and Q₃ gradually decreased when the basicity increased. However, when the basicity exceeded 1.20, the Q₂ content in the slag decreased sharply, and Q₃ changed from gradually decreasing to beginning to increase. The growth rate of the total amount of slag (Q₁ + Q₀) slowed down significantly. This was mainly because the increase in slag basicity increased the number of free oxygen ions provided by the basic oxide CaO in the slag. The interaction between free oxygen ions and the bridging oxygen of the slag caused the disintegration of the viscous flow unit in the slag. The relatively simple structure (Q₀ + Q₁) in the slag further increased so that the content of the complex structure (Q₂ + Q₃) and the polymerization degree of the slag relatively reduced. The energy barrier required to overcome the movement of the viscous flow unit in the slag gradually decreased, and the sensitivity of the slag viscosity to temperature decreased. Therefore, the fluidity of the slag was not greatly weakened and the thermal stability gradually enhanced with temperature fluctuations. The relatively complex Q₃ content in the slag increased sharply, and the growth rate of (Q₀ + Q₁) in the slag slowed down significantly when the basicity reached to 1.20. Combined with the analysis of the phase diagram in Section 3.1, when the slag basicity was greater than 1.20, the phase point of the slag was located at the boundary between the melilite primary crystal region and Ca₂SiO₄. The isotherms in this region were dense and the chemical stability of the slag was poor. It showed that overly high basicity would inhibit the precipitation of melilite in the slag, resulting in the formation of a high melting point of Ca₂SiO₄ in the slag, which led to the internal structure of the slag becoming complicated. The slag viscosity was more sensitive to temperature and the thermal stability of the slag deteriorated. This was consistent with the results of the viscous flow activation energy of slag in Section 3.2.

3.4. Industrial Test Results

This section explored the feasibility of reducing the MgO/Al₂O₃ ratio of blast furnace slag based on the results of laboratory research. Industrial tests were carried out in a 2580 m³ blast furnace in three stages. During the standard period of industrial tests in 2020, the slag amount of the blast furnace was 343 kg/t. After the implementation of decreasing the MgO/Al₂O₃ ratio of slag in 2021, the slag amount reduced to 318 kg/t. With the MgO/Al₂O₃ ratio dropping from 0.70 at the initial stage to 0.58 in the industrial tests, the operation conditions of the blast furnace remained stable. In addition, through the analysis of the heat balance of the blast furnace, the slag enthalpy, which is a heat expenditure item, is reduced due to the decrease of the MgO/Al₂O₃ ratio. The slag enthalpy change can be obtained by Equation (3) [25]:

$$\Delta q_{slag}=\Delta u\cdot Q_u=\left(343-318\right)\cdot 1937\mathrm{kJ}/\mathrm{kg}=48425\mathrm{kJ}$$

(3)

where Δu is the value of slag amount before and after the industrial test, kg/t; Q_u is the enthalpy per kilogram of slag, kJ/kg. The heat income of the blast furnace mainly comes from the effective physical heat brought by coke combustion and hot air, of which the former accounts for about 60%. The amount of heat released by coke combustion in the blast furnace hearth is 9791 kJ/kg, and the fixed carbon content in the coke is calculated as 85%. After reducing the MgO/Al₂O₃ ratio, the amount of coke saved can be calculated according to Equation (4):

$$\Delta K=\frac{\Delta q_{slag}\cdot 0.6}{9791\times 0.85}=\frac{48425\times 0.6}{9791\times 0.85}\mathrm{kg}/\mathrm{t}=3.49\mathrm{kg}/\mathrm{t}$$

(4)

where ΔK is the amount of coke saved, kg/t; 9791 is the heat released when the carbon in the coke burning into CO when the graphitization degree of coke is 50%.

Reducing the MgO/Al₂O₃ ratio of the blast furnace slag can effectively reduce the amount of slag and the K ratio of the blast furnace, which helps to reduce the CO₂ emissions effectively in the blast furnace smelting process.

4. Conclusions

• The increase of the MgO/Al₂O₃ ratio and binary basicity in the low-aluminum slag can help reduce the degree of slag polymerization, resulting in a decrease of slag viscosity and viscous flow activation energy, which improves the fluidity and thermal stability of slag.
• When the binary basicity is higher than 1.20, the precipitation of melilite in the slag is inhibited, and the proportion of the high melting point ore phase Ca₂SiO₄ increases relatively. The slag structure tends to be more complicated, which slows down the growth rate of (Q₀ + Q₁) in the slag, the Q₃ content in the slag increases sharply, and the thermal stability of the slag becomes worse.
• In combination with actual operating conditions and requirements for slag, the MgO/Al₂O₃ ratio of blast furnace slag should be controlled to 0.60 and the basicity should be no higher than 1.20 under the conditions of this investigation.
• Reducing the MgO/Al₂O₃ ratio of slag can effectively reduce the amount of blast furnace slag and K based on the results of the industrial tests. The coke rate can be saved as 3.49 kg/t according to the theoretical calculation, which would be a benefit for reducing CO₂ emissions and promoting the sustainable development of the ironmaking industry.