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Influence of [Al.sub.2][O.sub.3], CaO/Si[O.sub.2], and [B.sub.2][O.sub.3] on Viscous Behavior of High Alumina and Medium Titania Blast Furnace Slag.

1. Introduction

As high-quality iron ore resources in the world decrease, vanadium-titanium magnetite ores from china or iron ores with high [Al.sub.2][O.sub.3] 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 [B.sub.2][O.sub.3] remarkably improved the properties of slag [11-15]. Influence of [B.sub.2][O.sub.3] 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 [B.sub.2][O.sub.3], [Al.sub.2][O.sub.3], and basicity on viscous behavior. This research work appears just in this background.

In this study, the viscous behavior of the CaO-Si[O.sub.2]-(13%-19%) [Al.sub.2][O.sub.3]-MgO-(17%-20%) Ti[O.sub.2] slags was measured to clarify the effect of [Al.sub.2] [O.sub.3] and C/S. In addition, [B.sub.2] [O.sub.3] was added to the slag in order to improve its fluidity. A series of slags containing different [Al.sub.2][O.sub.3], [B.sub.2][O.sub.3] 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, Si[O.sub.2], [Al.sub.2][O.sub.3], and [B.sub.2][O.sub.3] 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, [Al.sub.2][O.sub.3] content of 13%-19%, and [B.sub.2][O.sub.3] 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.

The 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/Si[O.sub.2] on Viscous Behavior. The effect of C/S on viscosity at different temperatures is shown in Figure 1. 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[degrees]C. As temperature reduces to 1440[degrees]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[degrees]C. When temperature further drops to 1400[degrees]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[degrees]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/Si[O.sub.2], 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 2 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 ([O.sup.2-]) as a typical basic oxide, more addition of CaO exerts a negative effect on the viscosity and the breakpoint temperature because of its high melting temperature.

3.2. Effect of [Al.sub.2][O.sub.3] on Viscous Behavior. Figure 3 shows the effect of [Al.sub.2][O.sub.3] on the viscosity of CaO-Si[O.sup.2-]MgO-[Al.sub.2][O.sub.3]-(18%-19%) Ti[O.sub.2] slag at various temperature and a fixed C/S of 1.14. It is noted that the viscosity increases with increasing [Al.sub.2][O.sub.3] content from 14.24% to 18.01%. When temperature is 1360[degrees]C, the viscosity goes up rapidly, especially as [Al.sub.2][O.sub.3] 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 x s. When temperature decreases to 1340[degrees]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 [Al.sub.2][O.sub.3] responds to larger viscosity. With an increase of the [Al.sub.2][O.sub.3] 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 breakpoint temperature shows a similar tendency, as marked in Figure 4. When [Al.sub.2][O.sub.3] content in slag exceeds 17%, it dramatically increases. It is reported that [Al.sub.2][O.sub.3] behaves as an amphoteric oxide and initially [Al.sup.3+] cations can replace [Si.sup.4+] to form [[Al[O.sub.4]].sup.5-] tetrahedral units [23-25]. However, after further addition of [Al.sub.2][O.sub.3], it behaves as a network modifier and exists in the [[Al[O.sub.6]].sup.9-] octahedral configuration [3]. Thus complex structure corresponds to higher break point temperature and larger viscosity. In this investigation, [Al.sub.2][O.sub.3] behaves as a network former and increases the slag viscosity, agreeing with previous study [3].

3.3. Combined Effect of [B.sub.2][O.sub.3] and[Al.sub.2][O.sub.3] on Viscous Behavior. The viscosity-temperature curves of the slag samples are shown in Figure 5. Viscosity is closely related to temperature for each sample and [B.sub.2][O.sub.3] addition decreases the viscosity and improves the fluidity of slags regardless of [Al.sub.2][O.sub.3] content in slag. The viscosity of CaO-13% [Al.sub.2][O.sub.3]-Si[O.sup.2-]MgO-18% Ti[O.sup.2-][B.sub.2][O.sub.3] slags decreases with increasing [B.sub.2][O.sub.3] 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 breakpoint temperature of slag was decreased by the addition of [B.sub.2] [O.sub.3], as shown in Figure 6. The more [B.sub.2][O.sub.3] in slag, the faster break point temperature drops.

According to some studies on boron bearing multicomponent slag [28, 29], [B.sub.2][O.sub.3] exists mainly in the form of [[B[O.sub.3]].sup.-] triangle and [[B[O.sub.4]].sup.-] tetrahedral units; [B.sub.2][O.sub.3] additions could decrease the [[Al[O.sub.4]].sup.-] 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 ([O.sup.0]) to produce nonbridged oxygen atoms ([O.sup.-]) leading to a decrease in the slag viscosity.

The isoviscosity curves of boron containing HAMT BF slag are constructed in Figure 7 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 [B.sub.2][O.sub.3] and [Al.sub.2][O.sub.3], which suggests that the [B.sub.2][O.sub.3] effect is predominant compared to the [Al.sub.2][O.sub.3] additions. The isoviscosity curves become closer and closer as the [B.sub.2] [O.sub.3] content decreases and the [Al.sub.2] [O.sub.3] content increases, which indicates that the thermal stability of slag starts to deteriorate. However, increase of [Al.sub.2][O.sub.3] must be accompanied by decrease of [B.sub.2][O.sub.3] to maintain constant viscosity (in Figure 7) in several domains where [B.sub.2][O.sub.3] is greater than 3.5 or [Al.sub.2][O.sub.3] is more than 18.5%. Furthermore, these domains where increase of [Al.sub.2][O.sub.3] should be accompanied by decrease of [B.sub.2][O.sub.3] for constant break point temperature are more widespread in Figure 8. Significantly, in present study, break point temperatures are all under 1350[degrees]C after adding [B.sub.2][O.sub.3], which can meet the requirement of BF operation.

3.4. Effect of [B.sub.2][O.sub.3] on the Apparent Activation Energy for Viscous Flow. Temperature dependence of viscosity ([eta]) is given by the Arrhenius equation (see (1)), from which the apparent activation energy can be derived.

ln [eta] = ln A + [Ea/RT], (1)

where [eta] is 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 [B.sub.2][O.sub.3] content and [Al.sub.2][O.sub.3] 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 B[O.sub.4] and B[O.sub.3] units [30]. In other words, although increasing [Al.sub.2][O.sub.3] can make the network of slag melts complex, it was modified by adding [B.sub.2][O.sub.3].

4. Conclusions

In the present study, viscous behavior of HAMT BF slag was analyzed when [Al.sub.2][O.sub.3] content and [B.sub.2][O.sub.3] 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[degrees]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[degrees] C.

(2) Break point temperature increases faster with [Al.sub.2][O.sub.3] content in high aluminum and medium titanium slag; [Al.sub.2][O.sub.3] content especially exceeds 17%. Therefore, [Al.sub.2][O.sub.3] content in slag should be under 17% during iron-making process.

(3) [B.sub.2][O.sub.3] addition can improve fluidity of HAMT BF slag and decline significantly breakpoint temperature regardless of [Al.sub.2][O.sub.3] content.

(4) Apparent activation energy decreases with an increase of [B.sub.2][O.sub.3] content and [Al.sub.2][O.sub.3] content.

https://doi.org/10.1155/2017/6895928

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Authors' Contributions

The authors contributed equally to this work.

Acknowledgments

This research was funded by the National Natural Science Foundation of China (no. 51374267), Chongqing Research Program of Basic Research and Frontier Technology (no. cstc2017jcyjAX0236 and no. cstc2016jcyjA0142), and the Scientific and Technological Research Program of Chongqing Municipal Education Commission (no. KJ1713326).

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Lingtao Bian (1) and Yanhong Gao (2)

(1) Chongqing College of Electronic Engineering, Chongqing 401331, China

(2) School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China

Correspondence should be addressed to Yanhong Gao; gyh3636@hotmail.com

Received 19 June 2017; Revised 28 September 2017; Accepted 29 October 2017; Published 31 December 2017

Academic Editor: Sedat Yurdakal

Caption: Figure 1: Viscosity isotherms for slag series R and basic slag.

Caption: Figure 2: Break point temperature for slag series R and basic slag.

Caption: Figure 3: Viscosity isotherms for slag series A and basic slag.

Caption: Figure 4: Break point temperature for slag series A and basic slag.

Caption: Figure 5: Viscosity changes of the slag with temperature at different [B.sub.2][O.sub.3] content.

Caption: Figure 6: Break point temperature of the slag with [B.sub.2][O.sub.3] content.

Caption: Figure 7: Effects of [Al.sub.2][O.sub.3] and [B.sub.2][O.sub.3] content on viscosity.

Caption: Figure 8: Effects of [Al.sub.2][O.sub.3] and [B.sub.2][O.sub.3] content on break point temperature.

Caption: Figure 9: Relationship of apparent activation energy and [B.sub.2][O.sub.3] and [Al.sub.2][O.sub.3] content in slag.
Table 1: Chemical compositions of slag samples (wt.%).

Number         CaO    Si[O.sub.2]    MgO     [Al.sub.2][O.sub.3]

Basic slag    28.33      24.85       8.64           14.24
R1            29.76      24.04       9.27           13.77
R2            31.46      23.45       9.06           13.44
R3            33.02      22.91       8.87           13.13
A1            27.45      24.08       9.28           15.98
A2            27.12      23.79       9.18           17.00
A3            26.78      23.50       9.08           18.01
B1            27.12      23.79       9.18           16.00
B2            26.45      23.20       8.98           17.02
B3            25.80      22.64       8.78           17.97
B4            25.13      22.05       8.58           19.00

Number        Ti[O.sub.2]    FeO     [V.sub.2][O.sub.5]    MnO

Basic slag       19.48       1.11           0.20           0.65
R1               18.84       1.07           0.19           0.63
R2               18.38       1.05           0.19           0.62
R3               17.95       1.02           0.18           0.60
A1               18.87       1.08           0.19           0.63
A2               18.64       1.06           0.19           0.63
A3               18.41       1.05           0.19           0.62
B1               18.64       1.06           0.19           0.63
B2               18.18       1.04           0.19           0.61
B3               17.74       1.01           0.18           0.60
B4               17.28       0.98           0.18           0.58

Number         [B.sub.2][O.sub.3]    C/S

Basic slag             --            1.14
R1                     --            1.24
R2                     --            1.34
R3                     --            1.44
A1                     --            1.14
A2                     --            1.14
A3                     --            1.14
B1                    1.00           1.14
B2                    2.00           1.14
B3                    3.00           1.14
B4                    4.00           1.14
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Title Annotation:Research Article
Author:Bian, Lingtao; Gao, Yanhong
Publication:Journal of Chemistry
Date:Jan 1, 2018
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