# Influence of Al2O3, CaO/SiO2, and B2O3 on Viscous Behavior of High Alumina and Medium Titania Blast Furnace Slag

Authors: Lingtao Bian; Yanhong Gao
Journal: Journal of Chemistry
 (2017)

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## Abstract

The effect of Al2O3, CaO/SiO2, and B2O3 on the viscosity of high alumina and medium titania blast furnace slag was analyzed. An increase in CaO/SiO2 ratio from 1.14 to 1.44 resulted in higher slag viscosity and break point temperature. They also increased with increasing Al2O3 content but decreased with adding B2O3 and Al2O3 simultaneously at a fixed CaO/SiO2 ratio of 1.14, which suggested that the effect of B2O3 on viscosity and break point temperature is predominant compared to Al2O3. Apparent activation energies of CaO–SiO2–MgO–Al2O3–TiO2–B2O3 slag were found to be between 74 and 169 kJ/mol.


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## Body

## 1. Introduction

As high-quality iron ore resources in the world decrease, vanadium–titanium magnetite ores from china or iron ores with high Al2O3 from Australia and India have become an alternative choice in the blast furnace (BF) smelting process [1–3]. As a result, more alumina (exceeding 15% in many enterprises) or titania (more than 10%) occur in the BF slag, resulting in higher viscosity, worse fluidity, and poor operational stability [4–10]. So it is essential to studying the viscous behavior of BF slag with more alumina (above 15%) or titania (above 10%) for optimizing the iron-making operation. It has been found that the little amount of B2O3 remarkably improved the properties of slag [11–15]. Influence of B2O3 on high titanium (above 20%) BF slag [11–13, 16], medium titanium (10%–20%) BF slag [14], and high alumina (above 15%) BF slag [17, 18] has been studied in previous reports. However, it is hard to find investigations on the viscous behavior of high alumina (above 15%) and medium titania (15%–20%) BF slag (HAMT BF slag) and the influence of B2O3, Al2O3, and basicity on viscous behavior. This research work appears just in this background.In this study, the viscous behavior of the CaO–SiO2–(13%–19%) Al2O3–MgO–(17%–20%) TiO2 slags was measured to clarify the effect of Al2O3 and C/S. In addition, B2O3 was added to the slag in order to improve its fluidity. A series of slags containing different Al2O3, B2O3 content, and C/S were designed and the viscosity, break point temperature (the critical temperature at which the measured viscosity changes abruptly during the cooling cycle), and apparent activation energy were measured and analyzed.


## 2. Experimental

All samples were prepared by adding analytical-grade reagents CaO, SiO2, Al2O3, and B2O3 to basic slag obtained from BF and analyzed by chemical processing method. Three slag series were designed with binary basicity (C/S) range of 1.14–1.44, Al2O3 content of 13%–19%, and B2O3 content of 1%–4%. The chemical compositions of the designed slag are listed in Table 1. Slag samples were put inside a molybdenum crucible and melted in a high temperature furnace.Table 1
Chemical compositions of slag samples (wt.%).
Number
CaO
SiO2
MgO
Al2O3
TiO2
FeO
V2O5
MnO
B2O3
C/S
Basic slag
28.33
24.85
8.64
14.24
19.48
1.11
0.20
0.65
—
1.14
R1
29.76
24.04
9.27
13.77
18.84
1.07
0.19
0.63
—
1.24
R2
31.46
23.45
9.06
13.44
18.38
1.05
0.19
0.62
—
1.34
R3
33.02
22.91
8.87
13.13
17.95
1.02
0.18
0.60
—
1.44
A1
27.45
24.08
9.28
15.98
18.87
1.08
0.19
0.63
—
1.14
A2
27.12
23.79
9.18
17.00
18.64
1.06
0.19
0.63
—
1.14
A3
26.78
23.50
9.08
18.01
18.41
1.05
0.19
0.62
—
1.14
B1
27.12
23.79
9.18
16.00
18.64
1.06
0.19
0.63
1.00
1.14
B2
26.45
23.20
8.98
17.02
18.18
1.04
0.19
0.61
2.00
1.14
B3
25.80
22.64
8.78
17.97
17.74
1.01
0.18
0.60
3.00
1.14
B4
25.13
22.05
8.58
19.00
17.28
0.98
0.18
0.58
4.00
1.14The slag viscosity was measured by rotating cylinder method and recorded during the cooling cycle and the experiment was not ended until a steep increase in the viscosity value. The experimental setup and experimental procedure in detail can be found in our earlier studies [19, 20].


## 3. Results and Discussion

### 3.1. Effect of CaO/SiO2 on Viscous Behavior

The effect of C/S on viscosity at different temperatures is shown in Figure1. As clearly observed, the viscosity strongly depends on the temperature for each sample. The viscosities of all slag samples are under 0.5 Pa·s at above 1460°C. As temperature reduces to 1440°C, the viscosity of sample R3 (C/S = 1.44) goes up to 0.92 Pa·s and the viscosities of sample R2 (C/S = 1.34) and sample R3 both exceed 1.0 Pa·s with temperature continuous decreasing to 1420°C. When temperature further drops to 1400°C, the viscosity of sample R1 (C/S = 1.24) increases sharply to 3.38 Pa·s and that of sample R2 or R3 is even higher. For basic slag sample (C/S = 1.14), its viscosity does not reach 0.91 Pa·s until 1360°C because of low basicity. It illustrates that these slags are short slags. When the temperature is lower than break point temperature, the viscosity increases sharply in a narrow temperature range. Meanwhile, the fluidity and the stability of slags become worse. Besides, with an increase of the CaO/SiO2, the change of viscosity is much sharper, and the viscosity becomes more sensitive to the temperature change, resulting from the precipitation of phases with the high melting point [21, 22].Figure 1
Viscosity isotherms for slag series R and basic slag.Figure2 indicates that break point temperature raises with increasing C/S and shows relatively gentle change at the stage where C/S varies from 1.24 to 1.34. As we all know, although CaO can modify the melt structure effectively by providing additional free oxygen ions (O2−) as a typical basic oxide, more addition of CaO exerts a negative effect on the viscosity and the break point temperature because of its high melting temperature.Figure 2
Break point temperature for slag series R and basic slag.


### 3.2. Effect of Al2O3 on Viscous Behavior

Figure3 shows the effect of Al2O3 on the viscosity of CaO–SiO2–MgO–Al2O3–(18%-19%) TiO2 slag at various temperature and a fixed C/S of 1.14. It is noted that the viscosity increases with increasing Al2O3 content from 14.24% to 18.01%. When temperature is 1360°C, the viscosity goes up rapidly, especially as Al2O3 content is more than 16%. Viscosity of sample A1 is more than 1.5 Pa·s and the value of sample A3 goes up to 5.5 Pa·s. When temperature decreases to 1340°C, viscosity of basic slag reaches up to 3.5 Pa·s. It is inferred that these slags also take on the characteristics of short slag and more Al2O3 responds to larger viscosity. With an increase of the Al2O3 content, viscosity varies more quickly in a narrow temperature range and the fluidity deteriorates rapidly, attributing to the precipitation of phases with the high melting point [21, 22]. The break point temperature shows a similar tendency, as marked in Figure 4. When Al2O3 content in slag exceeds 17%, it dramatically increases. It is reported that Al2O3 behaves as an amphoteric oxide and initially Al3+ cations can replace Si4+ to form [AlO4]5− tetrahedral units [23–25]. However, after further addition of Al2O3, it behaves as a network modifier and exists in the [AlO6]9− octahedral configuration [3]. Thus complex structure corresponds to higher break point temperature and larger viscosity. In this investigation, Al2O3 behaves as a network former and increases the slag viscosity, agreeing with previous study [3].Figure 3
Viscosity isotherms for slag series A and basic slag.Figure 4
Break point temperature for slag series A and basic slag.


### 3.3. Combined Effect of B2O3 and Al2O3 on Viscous Behavior

The viscosity-temperature curves of the slag samples are shown in Figure5. Viscosity is closely related to temperature for each sample and B2O3 addition decreases the viscosity and improves the fluidity of slags regardless of Al2O3 content in slag. The viscosity of CaO–13% Al2O3–SiO2–MgO–18% TiO2–B2O3 slags decreases with increasing B2O3 addition in our previous investigation [14]. Similar results are obtained by Sun et al. [11], Fox et al. [26], and Wang et al. [27] despite different compositions. Besides, the break point temperature of slag was decreased by the addition of B2O3, as shown in Figure 6. The more B2O3 in slag, the faster break point temperature drops.Figure 5
Viscosity changes of the slag with temperature at different B2O3 content.Figure 6
Break point temperature of the slag with B2O3 content.According to some studies on boron bearing multicomponent slag [28, 29], B2O3 exists mainly in the form of [BO3]− triangle and [BO4]− tetrahedral units; B2O3 additions could decrease the [AlO4]− tetrahedral structural units and transformed the 3D network structures such as pentaborate and tetraborate into 2D network structures of boroxol and boroxyl rings by breaking the bridged oxygen atoms (O0) to produce nonbridged oxygen atoms (O−) leading to a decrease in the slag viscosity.The isoviscosity curves of boron containing HAMT BF slag are constructed in Figure7 and its iso-break point temperature curves are plotted in Figure 8. As can be seen, viscosities and break point temperatures decrease gradually with the addition of B2O3 and Al2O3, which suggests that the B2O3 effect is predominant compared to the Al2O3 additions. The isoviscosity curves become closer and closer as the B2O3 content decreases and the Al2O3 content increases, which indicates that the thermal stability of slag starts to deteriorate. However, increase of Al2O3 must be accompanied by decrease of B2O3 to maintain constant viscosity (in Figure 7) in several domains where B2O3 is greater than 3.5 or Al2O3 is more than 18.5%. Furthermore, these domains where increase of Al2O3 should be accompanied by decrease of B2O3 for constant break point temperature are more widespread in Figure 8. Significantly, in present study, break point temperatures are all under 1350°C after adding B2O3, which can meet the requirement of BF operation.Figure 7
Effects of Al2O3 and B2O3 content on viscosity.Figure 8
Effects of Al2O3 and B2O3 content on break point temperature.


### 3.4. Effect of B2O3 on the Apparent Activation Energy for Viscous Flow

Temperature dependence of viscosity (η) is given by the Arrhenius equation (see (1)), from which the apparent activation energy can be derived.(1)ln⁡η=ln⁡A+EaRT,where ηis viscosity of the slag; A is constant; Ea is the apparent activation energy; R is molar gas constant; T is absolute temperature. Variations in Ea can reveal changes in the frictional resistance of viscous flow and suggest a change in the structure of the molten slag. The variation of apparent activation energy is constructed in Figure 9 based on (1). It decreases with an increase of B2O3 content and Al2O3 content, which indicates that the resistance for viscous flow becomes smaller. So the slag structure becomes simpler and less complex. This is attributed to a weakening of the bond energy by increasing of B–O bonds in the network structure of BO4 and BO3 units [30]. In other words, although increasing Al2O3 can make the network of slag melts complex, it was modified by adding B2O3.Figure 9
Relationship of apparent activation energy and B2O3 and Al2O3 content in slag.


## 3.1. Effect of CaO/SiO2 on Viscous Behavior

The effect of C/S on viscosity at different temperatures is shown in Figure1. As clearly observed, the viscosity strongly depends on the temperature for each sample. The viscosities of all slag samples are under 0.5 Pa·s at above 1460°C. As temperature reduces to 1440°C, the viscosity of sample R3 (C/S = 1.44) goes up to 0.92 Pa·s and the viscosities of sample R2 (C/S = 1.34) and sample R3 both exceed 1.0 Pa·s with temperature continuous decreasing to 1420°C. When temperature further drops to 1400°C, the viscosity of sample R1 (C/S = 1.24) increases sharply to 3.38 Pa·s and that of sample R2 or R3 is even higher. For basic slag sample (C/S = 1.14), its viscosity does not reach 0.91 Pa·s until 1360°C because of low basicity. It illustrates that these slags are short slags. When the temperature is lower than break point temperature, the viscosity increases sharply in a narrow temperature range. Meanwhile, the fluidity and the stability of slags become worse. Besides, with an increase of the CaO/SiO2, the change of viscosity is much sharper, and the viscosity becomes more sensitive to the temperature change, resulting from the precipitation of phases with the high melting point [21, 22].Figure 1
Viscosity isotherms for slag series R and basic slag.Figure2 indicates that break point temperature raises with increasing C/S and shows relatively gentle change at the stage where C/S varies from 1.24 to 1.34. As we all know, although CaO can modify the melt structure effectively by providing additional free oxygen ions (O2−) as a typical basic oxide, more addition of CaO exerts a negative effect on the viscosity and the break point temperature because of its high melting temperature.Figure 2
Break point temperature for slag series R and basic slag.


## 3.2. Effect of Al2O3 on Viscous Behavior

Figure3 shows the effect of Al2O3 on the viscosity of CaO–SiO2–MgO–Al2O3–(18%-19%) TiO2 slag at various temperature and a fixed C/S of 1.14. It is noted that the viscosity increases with increasing Al2O3 content from 14.24% to 18.01%. When temperature is 1360°C, the viscosity goes up rapidly, especially as Al2O3 content is more than 16%. Viscosity of sample A1 is more than 1.5 Pa·s and the value of sample A3 goes up to 5.5 Pa·s. When temperature decreases to 1340°C, viscosity of basic slag reaches up to 3.5 Pa·s. It is inferred that these slags also take on the characteristics of short slag and more Al2O3 responds to larger viscosity. With an increase of the Al2O3 content, viscosity varies more quickly in a narrow temperature range and the fluidity deteriorates rapidly, attributing to the precipitation of phases with the high melting point [21, 22]. The break point temperature shows a similar tendency, as marked in Figure 4. When Al2O3 content in slag exceeds 17%, it dramatically increases. It is reported that Al2O3 behaves as an amphoteric oxide and initially Al3+ cations can replace Si4+ to form [AlO4]5− tetrahedral units [23–25]. However, after further addition of Al2O3, it behaves as a network modifier and exists in the [AlO6]9− octahedral configuration [3]. Thus complex structure corresponds to higher break point temperature and larger viscosity. In this investigation, Al2O3 behaves as a network former and increases the slag viscosity, agreeing with previous study [3].Figure 3
Viscosity isotherms for slag series A and basic slag.Figure 4
Break point temperature for slag series A and basic slag.


## 3.3. Combined Effect of B2O3 and Al2O3 on Viscous Behavior

The viscosity-temperature curves of the slag samples are shown in Figure5. Viscosity is closely related to temperature for each sample and B2O3 addition decreases the viscosity and improves the fluidity of slags regardless of Al2O3 content in slag. The viscosity of CaO–13% Al2O3–SiO2–MgO–18% TiO2–B2O3 slags decreases with increasing B2O3 addition in our previous investigation [14]. Similar results are obtained by Sun et al. [11], Fox et al. [26], and Wang et al. [27] despite different compositions. Besides, the break point temperature of slag was decreased by the addition of B2O3, as shown in Figure 6. The more B2O3 in slag, the faster break point temperature drops.Figure 5
Viscosity changes of the slag with temperature at different B2O3 content.Figure 6
Break point temperature of the slag with B2O3 content.According to some studies on boron bearing multicomponent slag [28, 29], B2O3 exists mainly in the form of [BO3]− triangle and [BO4]− tetrahedral units; B2O3 additions could decrease the [AlO4]− tetrahedral structural units and transformed the 3D network structures such as pentaborate and tetraborate into 2D network structures of boroxol and boroxyl rings by breaking the bridged oxygen atoms (O0) to produce nonbridged oxygen atoms (O−) leading to a decrease in the slag viscosity.The isoviscosity curves of boron containing HAMT BF slag are constructed in Figure7 and its iso-break point temperature curves are plotted in Figure 8. As can be seen, viscosities and break point temperatures decrease gradually with the addition of B2O3 and Al2O3, which suggests that the B2O3 effect is predominant compared to the Al2O3 additions. The isoviscosity curves become closer and closer as the B2O3 content decreases and the Al2O3 content increases, which indicates that the thermal stability of slag starts to deteriorate. However, increase of Al2O3 must be accompanied by decrease of B2O3 to maintain constant viscosity (in Figure 7) in several domains where B2O3 is greater than 3.5 or Al2O3 is more than 18.5%. Furthermore, these domains where increase of Al2O3 should be accompanied by decrease of B2O3 for constant break point temperature are more widespread in Figure 8. Significantly, in present study, break point temperatures are all under 1350°C after adding B2O3, which can meet the requirement of BF operation.Figure 7
Effects of Al2O3 and B2O3 content on viscosity.Figure 8
Effects of Al2O3 and B2O3 content on break point temperature.


## 3.4. Effect of B2O3 on the Apparent Activation Energy for Viscous Flow

Temperature dependence of viscosity (η) is given by the Arrhenius equation (see (1)), from which the apparent activation energy can be derived.(1)ln⁡η=ln⁡A+EaRT,where ηis viscosity of the slag; A is constant; Ea is the apparent activation energy; R is molar gas constant; T is absolute temperature. Variations in Ea can reveal changes in the frictional resistance of viscous flow and suggest a change in the structure of the molten slag. The variation of apparent activation energy is constructed in Figure 9 based on (1). It decreases with an increase of B2O3 content and Al2O3 content, which indicates that the resistance for viscous flow becomes smaller. So the slag structure becomes simpler and less complex. This is attributed to a weakening of the bond energy by increasing of B–O bonds in the network structure of BO4 and BO3 units [30]. In other words, although increasing Al2O3 can make the network of slag melts complex, it was modified by adding B2O3.Figure 9
Relationship of apparent activation energy and B2O3 and Al2O3 content in slag.


## 4. Conclusions

In the present study, viscous behavior of HAMT BF slag was analyzed when Al2O3 content and B2O3 content in slag varied. The important results were summarized as follows:(1)
The thermal stability of the slag is better at higher temperature. Viscosity begins to deteriorate rapidly when temperature is below 1420°C and C/S varies from 1.14 to 1.44. For HAMT BF slag with fixed C/S of 1.14, the temperature is 1360°C.(2)
Break point temperature increases faster with Al2O3 content in high aluminum and medium titanium slag; Al2O3 content especially exceeds 17%. Therefore, Al2O3 content in slag should be under 17% during iron-making process.(3)
B2O3 addition can improve fluidity of HAMT BF slag and decline significantly break point temperature regardless of Al2O3 content.(4)
Apparent activation energy decreases with an increase of B2O3 content and Al2O3 content.



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