Effect of MnO on Crystallization Behavior and Rheological Characteristics of Blast Furnace Slag during Gas Quenching into Beads

Abstract

The preparation of glass microbeads by gas quenching of blast furnace slag is an effective way to achieve efficient recovery and resource utilization of the residual heat from blast furnace slag. However, due to the high viscosity and easy crystallization of blast furnace slag, there are problems such as low bead formation rate, opaque glass microbeads, and poor chemical stability. MnO tempering agent was developed, and the influence of MnO on the crystallization behavior and rheological properties of the tempered slag was analyzed. The evolution law of the crystallization phase of the tempered slag was clarified, and a viscosity-crystallization coupling control method conducive to the bead formation of blast furnace slag was proposed. The results show that during the isothermal process, when the MnO content in the tempered slag is in the range of 0.17–12.24%, with the increase of its content, the initial crystallization temperature and the amount of crystal precipitation gradually decrease, effectively inhibiting the precipitation of crystals. However, when the MnO content exceeds 12.24%, the excessive MnO increases the activity of the easily precipitated phase, and the initial crystallization temperature increases instead. Therefore, when the MnO content in the tempered slag is 12.24%, the crystallization ability is the weakest, and the glass phase content is the highest. In the continuous cooling process, when the MnO content in the tempered slag is 12.24% and the cooling rate exceeds 3°C/s, the tempered slag completely solidifies into a glassy state.

1. Introduction

The efficient recycling of industrial waste and the minimal exploitation of natural resources are two important aspects of a circular economy and sustainable development. BF slag is the most important solid by-product in the iron and steel industry, with high annual output. For each ton of pig iron smelted can produce 300–400 kg of BF slag, and BF slag contains rich sensible heat, the slag discharge temperature is as high as 1500°C, and the sensible heat per ton of slag is 1675MJ, equivalent to 60 kgce/t standard coal, which is a very high-quality energy.^1,2) However, the current treatment methods for BF slag mainly use the water quenching method, which not only consumes a large number of water resources, produces harmful gases, and has a low value in terms of product resource utilization, but also fails to recover sensible heat of BF slag.^3,4,5) Therefore, it is necessary to develop a new resource utilization way for BF slag.

In recent years, domestic and foreign scholars and enterprises began to realize the unreasonable disposal of existing BF slag, and began to study the rational utilization of BF slag, such as the use of BF slag to produce slag bricks and slag glass-ceramics in China,^6,7) although the added value of BF slag has been improved, sensible heat has not been utilized; At present, the main methods used in foreign countries are wind tunnel wind quenching method and rotating cup and dish method,⁸⁾ which can not only make use of its sensible heat, but also granulated BF slag can be used to produce cement and concrete, but its product value is low. Therefore, it is very urgent to develop a new comprehensive utilization technology for BF slag, which combines efficient waste heat recovery with high value-added resource products.

Glass microbeads are a new type of filling material with excellent properties such as lightweight and high strength, which have been developed in recent years, and are widely used in aviation, aerospace, railway, building materials, chemical industry, and other fields.⁹⁾ At present, the production of glass microbeads in China has problems of poor product quality, high energy consumption, and high cost, mainly because the production of glass microbeads often takes chemical products as raw materials, and the production process is complex, resulting in high cost and high energy consumption. BF slag is a silicate material dominated by oxides of Si, Ca, Mg, and Al. Under the condition of gas quenching, most of the molten BF slag can be condensed into a glass body, which meets the basic requirements for the preparation of glass microbeads and can realize efficient recovery of residual heat from BF slag, which significantly improves the resource value of BF slag.^10,11,12) The gas quenching process is shown in Fig. 1.

Fig. 1. Schematic diagram of gas quenching granulation method. (Online version in color.)

By utilizing the impact kinetic energy of transverse supersonic airflow, the slag forms a flat liquid film under the combined action of air impact, reflux zone, and pressure gradient. Then, the axial and spanwise waves make the liquid film produce vortices and pore structures, and the liquid film is torn into liquid bands. Finally, due to the aerodynamic force, RT unstable waves appear on the surface of the liquid strip of molten slag, causing the liquid strip to break into droplets, which are cooled to obtain solid slag particles,^13,14,15) as shown in Fig. 2.

Fig. 2. Schematic diagram of slag crushing process in blast furnace.

This method has certain requirements for the viscosity of the raw materials, and the unconditioned BF slag has a relatively high viscosity, which is prone to the generation of glass fibers during the gas quenching process. Moreover, the generated glass microbeads have a large particle size, poor sphericity, and low yield, which is unfavorable for the production of glass microbeads.⁹⁾ Therefore, it is necessary to add alkaline substances to the BF slag for conditioning to reduce its viscosity.

However, the alkaline modifier will inevitably promote the crystal precipitation while reducing the viscosity, resulting in glass microbead devitrification and chemical stability deterioration, thus affecting the quality of glass microbeads. Therefore, it is very important to develop suitable tempering agent and obtain crystallization control technology of tempering slag under the condition of reducing viscosity.

At present, there are few reports on the decreasing viscosity and inhibiting crystallization of BF slag, which mainly focus on the crystallization mechanism and crystal phase evolution mechanism of BF slag, coal ash slag, and alloy slag.

Xuan et al.,^16,17) studied the crystallization characteristics of the SiO₂–Al₂O₃–CaO–Fe₂O₃–MgO slag system based on DSC and determined that alkalinity can promote the crystallization of coal ash slag. Tian et al.,¹⁸⁾ studied the crystallization behavior of BF slag and modified BF slag by using the single wire thermocouple method. The crystallization of BF slag could be inhibited by increasing the acidity coefficient, and the viscosity of BF slag would be increased by increasing the acidity coefficient. Ren et al.,¹⁹⁾ added fly ash to the BF slag to control the crystallization of the BF slag, but the viscosity of the BF slag was greatly improved. Zhang et al.,²⁰⁾ studied the influence of Al₂O₃ on the viscosity and crystallization properties of iron-nickel alloy slag, and the results showed that the change of viscosity was mainly related to the precipitation and growth of crystals, and was also affected by the type and number of crystals.

Wang et al.,²¹⁾ studied the effects of CaO on the crystallization properties and viscosity of silicomanganite slag in the fiber forming process, and the results showed that the addition of CaO increased the viscosity of silicomanganite slag, and the precipitation of crystals in the slag was the main reason for the increase in slag viscosity. Cai et al.,²²⁾ studied the crystallization behavior of modified BF slag by using the single wire thermocouple method. In the isothermal process, the initial crystallization temperature gradually decreased with the increase of the acidity coefficient of modified BF slag. In the continuous cooling process, when the acidity coefficient increased, the critical cooling rate decreased, indicating that the crystal growth rate was controlled by the diffusion driving force and the subcooling driving force.

To this end, by adding suitable MnO as a tempering agent, the relationship among MnO content, initial crystallization temperature, and mineral crystallization phase was established using FactSage thermodynamic simulation software, and the influence of MnO on crystallization behavior and rheological properties of blast slag was analyzed. It is of great significance for the green and low-carbon production of the glass microbeads industry and the goal of achieving carbon neutrality and carbon peak.

2. Raw Materials and Research Methods

2.1. Raw Materials

To study the crystallization behavior and rheological properties of BF slag after conditioning, part of the BF slag was collected from a steel plant in Tangshan, and its raw material composition is shown in Table 1.

Table 1. Chemical composition of BF slag/%.

Raw materials	SiO2	CaO	MgO	Al2O3	K2O	Na2O	Fe2O3	TiO2	MnO
BF slag	30.32	38.16	9.06	15.47	0.24	0.34	3.68	1.64	0.17

In the BF slag, analytically pure reagent MnO₂ (MnO₂ decomposes into MnO at high temperatures) is added to prepare the tempered slag, in which the content ratio of the tempering agent is 0%, 5%, 10%, 15%, and 20%, respectively. Then, a certain amount of BF slag and MnO₂ are ground and mixed evenly through a mortar. Subsequently, the corundum crucible containing the tempered slag is placed in a tube furnace and heated to 1500°C for hot remelting. After maintaining a constant temperature for 1 hour, the tube furnace is turned off for cooling to room temperature for later use. The tempering scheme is shown in Table 2, and the composition of the tempered slag is shown in Table 3.

Table 2. Experimental scheme.

ID	option	Quality/g
		BF slag	MnO2
1#	w(MnO2)=0%	200.00	0.00
2#	w(MnO2)=5%	190.40	9.60
3#	w(MnO2)=10%	180.38	19.62
4#	w(MnO2)=15%	170.36	29.64
5#	w(MnO2)=20%	160.34	39.66

Table 3. Chemical composition of BF slag/%.

ID	Chemical composition/%
	SiO2	CaO	MgO	Al2O3	K2O	Na2O	Fe2O3	TiO2	MnO
1#	30.32	38.16	9.06	15.47	0.24	0.34	3.68	1.64	0.17
2#	29.38	36.98	8.78	14.99	0.23	0.33	3.56	1.59	4.08
3#	28.09	35.35	8.39	14.33	0.23	0.32	3.41	1.52	8.16
4#	26.77	33.68	8.00	13.66	0.21	0.30	3.24	1.45	12.24
5#	25.43	32.00	7.59	12.97	0.20	0.28	3.09	1.37	16.32

2.2. Research Methods

2.2.1. Thermodynamic Simulation

Equilib module and Phase Diagram module of thermodynamic simulation software are used to simulate the addition of modifier MnO into BF slag with CaO–SiO₂–MgO–Al₂O₃–MnO five-element slag system as the research object and database as FToxid. In the process of high-temperature cooling at 0.17–16.32% of MnO, the initial crystallization temperature and the type of crystal phase were determined, and the influence of modifier MnO on the crystallization of BF slag was determined.

2.2.2. Determination of Crystallization Property of Slag

The crystallization performance of the BF slag was measured by the SHTT device. The experimental device is shown in Fig. 3. First, take a small amount of BF slag with a small spoon into the surface dish, add a small amount of anhydrous ethanol to make a paste, take 2–3 mg of slag sample on the thermocouple wire tip; Then adjust the microscope focal length, so that the observed image is the clearest; Then input the temperature control curve through the program on the computer, so that the BF slag is heated to 1500°C at a speed of 6°C/s and maintained at this temperature for 350 s, so that it is fully melted, uniform and eliminates bubbles. During the isothermal research process, cool at a rate of 100°C/s to different temperatures and maintain a constant temperature for 1000 seconds. Observe and record the image of the tempering slag changing over time. In the study of the continuous cooling process, the cooling rates of 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0°C/s were used to cool to room temperature, and the crystallization image of the tempered slag was observed and recorded.

Fig. 3. Slag crystallization property tester (MTLQ-BQ-3). (Online version in color.)

2.2.3. Viscosity Test

First, the BF slag is ground to less than 200 mesh by the ball mill. A corundum crucible containing 140 g of tempered BF slag was heated using a melt physical properties comprehensive tester (Equipment Company: Chongqing University, Equipment Model: VWR-1800) and waited for the temperature to rise to 1500°Cconstant temperature for 30 min so that the slag was fully melted and uniform; Then, the molybdenum rotor connected with the corundum rod is immersed in the molten slag for rotation at a speed of 12 r/min; Finally, the melt physical properties comprehensive tester was cooled at a cooling rate of 3°C/min to measure the temperature-viscosity curve.

2.2.4. XRD Analysis

Firstly, 150 g of BF slag sample is added to the corundum crucible, and the corundum crucible is heated in a tube furnace. After the furnace temperature rises to 1500°C, the constant temperature is 120 min, so that the BF slag is fully melted and uniform. The temperature is lowered to 1400°C, 1300°C, 1200°C and 1100°C at a cooling rate of 10°C/min, and the temperature was constant for 30 min. Finally, the BF slag was quickly put into water and cooled to room temperature. The BF slag was analyzed by XRD with a D/MAX2500PC X-ray diffractometer.

3. Experimental Results and Analysis

3.1. FactSage Analysis

According to FactSage, the crystallization behavior of BF slag with different MnO content was simulated, and the results are shown in Fig. 4.

Fig. 4. Mineral phase of modified BF slag with different MnO content in FactSage. (a) w(MnO)=0.17%; (b) w(MnO)=4.08%; (c) w(MnO)=8.16%; (d) w(MnO)=12.24%; (e) w(MnO)=16.32%. (Online version in color.)

It can be seen from Fig. 4 that the main mineral phases of BF primary slag (w(MnO)=0.17%) are melilite, merwinite, calcium-magnesia olivine, and spinel. After the original BF slag was quenched and tempered by adding MnO, the mineral phase also changed, and the content of melilite kept decreasing, while the content of merwinite, calcium-magnesia olivine, and spinel kept rising, mainly because the increase of MnO content in BF slag causes free Ca²⁺ to combine with melilite to form merwinite and the increase of Mn²⁺ ion. The manganese olivine phase is formed with Si⁴⁺ ion at high temperatures.

Figure 4(a) shows that when the MnO content is 0.17%, the slag gradually changes from liquid phase to solid phase with the decrease of slag temperature. At 1441.7°C, the Melilite phase began to precipitate, at 1413.4°C, the Ca₃MgSi₂O₈ phase precipitated, at 1425.1°C and 1360.6°C precipitated spinel (Spinel) and Olivine respectively.

Figures 4(b)–4(d) shows that when the content of MnO increases, the precipitation temperature of the melilite phase decreases from 1441°Cto 1220°C, and the maximum precipitation amount decreases from 73.7% to 26.7%; the precipitation temperature of the merwinite phase decreases from 1413°C to 1347°C, and the maximum precipitation amount increases from 19.5% to 33.5%. The precipitation temperatures of spinel and olivine decreased from 1425°C and 1360°C to 1350°C and 1164°C respectively, and the maximum precipitation amounts increased from 3.3% and 2.8% to 7.3% and 19.9%, respectively. The initial crystal phase of blast slag changes from the feldspar phase to the spinel phase with the increase of MnO content. When MnO content is 8.2%, the manganese oxide phase appears. With the increase of MnO content, the initial crystallization temperature of the Mn oxide phase increases from 1171°C to 1363°C, and the maximum precipitation rate increases from 2.2% to 6.3%.

Figure 4(e) shows that when MnO content is 16.32%, the initial crystallization phase changes from spinel phase to manganese oxide phase, and the initial crystallization temperature increases from 1363.3°C to 1450.9°C. The effect of MnO content in BF slag on the initial crystallization temperature is shown in Fig. 5.

Fig. 5. Effect of MnO content on the initial crystallization temperature of BF slag.

As can be seen in Fig. 5, when MnO is added to the BF slag, the initial crystallization temperature of the slag decreases with the increase of MnO content. When the MnO content is 12.24%, the initial crystallization temperature of the tempered BF slag reaches the minimum (1363.3°C). When the content of MnO exceeds 12.24%, the initial crystallization temperature of slag increases instead of decreasing. When the content of MnO is 16.32%, the initial crystallization temperature of slag rises to 1450.9°C.

In conclusion, with the increase of MnO content in BF slag, the initial crystallization temperature of the BF slag decreases, which is conducive to the production of amorphous glass beads. However, when the content of MnO is too much in the BF slag, the initial crystallization temperature will increase, which is easy to causes the crystallization phenomenon of glass beads in production, and the product pass rate will become worse. Therefore, the use of MnO as a tempering agent to improve the performance of glass microbeads in the process of gas quenching of BF slag is not suitable for the addition of more than 12.24%.

3.2. Crystallization Behavior of Tempered Slag during the Isothermal Process

3.2.1. Crystallization Behavior of Tempered Slag with Different MnO Content during the Isothermal Process

As can be seen from Fig. 6, when the MnO content varies from 0.17 to 12.24%, the TTT curve gradually moves to the right with the increase of MnO content in the BF slag, indicating that the crystallization ability of the BF slag is weakened and the crystal incubation time is gradually extended. However, when the MnO content exceeds 12.24%, the TTT curve of the BF slag changes abruptly. Crystal incubation time is obviously shortened. In addition, the TTT curve of the BF slag showed a double C-type curve, indicating that at least two kinds of crystals were precipitated in the slag.

Fig. 6. Isothermal cooling curves of BF slag with different MnO content. (Online version in color.)

When the MnO content varied in the range of 0.17–12.24%, the corresponding constant temperature cooling temperature of the two nasal tips of the BF slag was 1310°C and 1190°C, respectively. The incubation time was the shortest, and with the increase in MnO content, the incubation time was extended by 32 s and 40 s, respectively. In addition, when the content of MnO is 12.24%, the BF slag is still vitreous when it is above 1340°C, that is, the crystallization range gradually narrows. Therefore, in the process of using BF slag gas to quench into beads, the MnO content should not exceed 12.24%, otherwise it will have a greater impact on the quality and output of glass beads and should avoid the temperature at the nose tip, which can improve the degree of glass beads for glass transition and enhance field controllability.

3.2.2. XRD Analysis

It can be seen from Fig. 7 that the main mineral phase structure of the original BF slag (MnO content is 0.17%) is chrysoclase (Ca₂MgSi₂O₇, Ca₂Al₂SiO₇), spinel (MgAl₂O₄), olivine (Mg₂SiO₄) and melilite (Ca₃Mg(SiO₄)₂). After MnO is added to the original BF slag to adjust its mineral phase structure, its main mineral phase is the same as that of the original BF slag, but at the same time, due to the combination of Mn²⁺, Si⁴⁺ and Mg²⁺, manganese olivine phase (Mn₂SiO₄) and manganese oxide phase (MnSiO₃, MnO_0.305MgO_0.605) are formed. At different temperatures, when the content of MnO is 4.08%, a small amount of manganese olivine phase and manganese oxide phase precipitate. When the content of MnO is 12.24%, the amount of ore phase precipitation increases. When the content of MnO is 16.32%, the precipitation is further enhanced, and the feldspar phase is greatly inhibited. In addition, when the melt is cooled to 1400°C and the MnO content is 12.24%, the BF slag is almost completely transformed into a vitreous body.

Fig. 7. XRD pattern of tempered BF slag with different MnO content. (a) 1100°C; (b) 1200°C; (c) 1300°C; (d) 1400°C. (Online version in color.)

At 1100°C, 1200°C, 1300°C and 1400°C, the diffraction peak intensity of all mineral phases of the BF slag with high MnO content is lower than that of the original BF slag, indicating that the increase of MnO content in the BF slag will reduce the precipitation amount of crystals. The main reason that MnO can inhibit its crystallization and promote the formation of a vitreous body is that Mn is easy to form anion groups with O²⁻, which reduces O²⁻ in the slag system, and increases the polymerization degree of aluminoxy and siloxy anion groups. Therefore, due to the strengthening of polymerization, the network structure becomes complicated, resulting in the addition of MnO to the slag is conducive to improving the formation of the vitreous body and weakening the crystallization ability of the BF slag during the cooling process.²³⁾

At 1400°C, the diffraction peak intensity of the BF slag with MnO content of 16.32% is higher than that of the BF slag with MnO content of 12.24%, which indicates that after the content of MnO in the BF slag increases to 16.32%, there are easily crystallized substances precipitated. This is because the increase of MnO content in slag not only makes the network structure of the slag system become complicated and weakens the crystallization ability but also increases the component activity of the crystal and reduces its free energy, resulting in excessive MnO will promote the crystal phase. For example, the following chemical reactions exist:

  

$$\begin{array}{l}a({\text{R}}_{\text{2}}\text{O})+b(\text{RO})+c({\text{R}}_{\text{2}}{\text{O}}_{\text{3}})+d({\text{SiO}}_{\text{2}})\\ =a{\text{R}}_{\text{2}}\text{O}\cdot b\text{RO}\cdot c{\text{R}}_{\text{2}}{\text{O}}_{\text{3}}\cdot d{\text{SiO}}_{\text{2}}\end{array}$$(1)

  

$$\mathrm{\Delta }{G}_T=\mathrm{\Delta }{G}_T^0+RTln\frac{1}{a_{{\text{R}}_2\text{O}}^a\cdot a_{\text{RO}}^b\cdot a_{{\text{R}}_2{\text{O}}_3}^c\cdot a_{{\text{SiO}}_2}^d}$$(2)

In the formula, R represents the gas constant, T represents the temperature and represents the activity of each component of the compound. The activity of the main components of the BF slag was calculated according to FactSage, as shown in Table 4.

Table 4. The activity of the main components of the slag was reduced by different MnO content/%.

MnO content/%	SiO2	CaO	MgO	Al2O3	MnO
12.24	0.004935	0.002547	0.008271	0.063685	0.122010
16.32	0.003371	0.001916	0.009345	0.067336	0.189330

It can be seen in Table 4 that the activity of SiO₂ and CaO in the components decreases with the increase of MnO content in the BF slag, while the activity of MgO and Al₂O₃ increases. Then, the precipitated material is calculated in FactSage, which can be calculated according to (Eq. (2)).

At 1400°C, the free energy of Mn₂SiO₄ decreases from −24907 kJ/mol to −29766 kJ/mol, and the free energy of MnSiO₃ decreases from −16839 kJ/mol to −21698 kJ/mol in the conditioned slag with MnO content increasing from 12.24% to 16.32%. The free energy of MnO_0.305MgO_0.605 decreases from −22240 kJ/mol to −27152 kJ/mol, and the free energy of other mineral phases increases gradually. According to the results of XRD analysis, although the increase of MnO content can reduce the total amount of crystal phase in the BF slag, the activity in the slag also increases with the increase of its component content, and the initial crystallization temperature of the melt increases. That is, increasing the MnO content can reduce the amount of crystal phase, but excessive will lead to an increase in the initial crystallization temperature.

3.3. Crystallization Behavior of Tempered Slag during Continuous Cooling

As can be seen from Fig. 8, with the increase in MnO ratio, the initial crystallization temperature of the tempered slag decreases, and the crystal incubation time increases. When the MnO content in the BF slag ranges from 0.17% to 4.08%, the BF slag completely solidifies into a glassy state when the cooling rate exceeds 6°C/s, and when the MnO content is 12.24% and the cooling rate exceeds 3°C/s, the BF slag completely solidifies into a glassy state. However, when the MnO content continued to increase to 16.32%, the crystallization temperature increased, and when the cooling rate was 6°C/s, the crystallization time was rapidly shortened to 19 s, and the crystallization temperature was increased to 1386°C. This is mainly due to the increase of MnO, which increases the activity of MnO in the BF slag and precipitates crystals containing MnO. Therefore, adding MnO to BF slag can inhibit the precipitation of its crystal phase, which is conducive to the production of glass microbeads under the gas quenching process, but it should be noted that the content of MnO should not exceed 12.24%.

Fig. 8. Continuous cooling curves of tempered slag with different MnO content. (Online version in color.)

3.4. Influence of MnO Content on Viscosity of BF Slag

As can be seen from Figs. 9(a) and 9(b), after adding MnO to the BF slag for tempering, the viscosity and melting temperature of the tempered slag first decrease and then increase with the increase of MnO content. When the MnO content is 12.24%, the viscosity and melting temperature of the tempered slag are the lowest. When the MnO content is 16.32%, the temperature of the tempered slag is lower than the melting temperature of 1406°C, the viscosity curve becomes steeper, and the viscosity increases significantly. This is mainly because when the MnO content is 16.32%, the precipitated phase of the tempered slag changes, and the high melting point MnO_0.305MgO_0.605 mineral phase begins to precipitate, resulting in the presence of crystals in the melt at high temperatures, so the viscosity increases. This is consistent with the initial crystallization results, TTT curves, and XRD results simulated by the Factsage software. However, since the Factsage software is in an equilibrium state, the initial crystallization temperature and melting temperature of the tempered slag are somewhat different.

Fig. 9. Effect of different MnO content on viscosity of BF slag. a) Viscosity of tempered slag with different MnO contents, b) Melting temperature of tempered Slag with different MnO contents, c) Temperature at 1 Pa·s for tempered slag with different MnO contents. (Online version in color.)

As can be seen from Fig. 9(c), the viscosity required for the bead formation process of the general tempered slag should be less than 1 Pa·s. When the dosage of MnO is 12.24%, the temperature range of the tempered slag with a viscosity less than 1 Pa·s is significantly the largest, and the viscosity change in the high-temperature section is also relatively gentle, with a wider temperature adaptation range. This improves the stability and operability of the gas quenching treatment of the tempered slag, and at the same time reduces the internal bonding force on the surface of the liquid film after the molten slag is gas quenched, causing the enhancement of the conversion of aerodynamic kinetic energy into liquid disturbance kinetic energy, reducing the wavelength of the liquid film, promoting the easier dispersion and granulation of the melt and enhancing the heat transfer efficiency of the gas-liquid, thereby obtaining glass microbeads with a smaller average diameter, reducing the content of fibers, improving the quality of recovered air waste heat, and further improving the yield and quality of glass microbeads. Therefore, when the content of MnO added to the BF slag is 12.24%, its rheological properties are the best, which can improve the yield and quality of glass microbeads.

4. Conclusion

(1) The TTT curve shows that with the increase of MnO content in the tempered slag, the initial crystallization temperature of the tempered slag decreases first and then increases, while the crystallization amount gradually decreases, and when the MnO content is 12.24%, the initial crystallization temperature is the lowest. In addition, the TTT curve of the BF slag presents a double C-type curve, that is, at least two kinds of crystals are precipitated in the BF slag.

(2) When the content of MnO varies in the range of 0.17–12.24%, the corresponding constant temperature cooling temperature of the two nose tips of the tempered slag are 1310°C and 1190°C, respectively, and the incubation time is the shortest. Therefore, in the process of quenching the BF slag into beads, the temperature at the nose tip should be avoided, which can improve the degree of glass transition and enhance the field controllability.

(3) When the content of MnO varies from 0.17 to 16.32%, with the increase of MnO content in the tempered slag, the mineral phases of the tempered slag gradually change from feldspar, magnesia rosinite, spinel and olivine to feldspar, magnesia rosinite, spinel, and, olivine (containing MnO) and manganese oxide phase, and the precipitation of feldspar significantly decreases. This is mainly because the increase of MnO leads to the decrease of Gibbs free energy of formation of Mn-containing mineral phases, so the addition of excessive MnO will promote the crystal phase.

(4) The CCT curve shows that when the MnO content of the tempered slag increases from 0.17 to 16.32%, the critical cooling rate decreases first and then increases, and when the MnO content is 12.24%, the critical cooling rate reaches the minimum value of 3°C/s.

(5) The addition of an appropriate amount of MnO to the tempered slag not only inhibits crystallization but also reduces its viscosity, thereby improving the disturbance wavelength of the liquid film after gas quenching of the molten slag, further enhancing the dispersed granulation of the liquid film and improving the quality of the glass microbeads.

Statement for Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors give sincere thanks for the financial support from the National Natural Science Foundation of China (U20A20271); Central Guided Local Science and Technology Development Fund Project of Hebei Province (246Z4002G).

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