Abstract
The effects of B2O3 and basicity (CaO/SiO2) on the viscous behavior and structure of medium titanium bearing blast furnace slag (MTBBFS) were investigated. High temperature viscosimeter was applied to measure the viscosities of CaO-SiO2-MgO-TiO2-Al2O3-B2O3 slag system and X-ray diffraction (XRD), NBO/T ratio, and structure parameter were employed to analyze its network structure. The results showed that the viscosity decreased and break point temperature increased with increasing basicity to 1.20. However B2O3 addition gave rise to a decrease in slag viscosity and break point temperature inspite of basicity. The more B2O3 content leads to the more pronounced variation, especially for the slag with larger basicity. The conventional NBO/T formula was revised to predict the structure variation of relatively complicated medium Ti bearing slag based on the work of Yanhong Gao and other researchers. The increase of B2O3 content in slag made parameter turn from to , suggesting that network structure became simpler. It was also noticed that the addition of B2O3 could suppress the formation of perovskite.
1. Introduction
Titanium resources are comparatively abundant in China, and most of them exist as vanadium-titanium magnetite ore and locate in the southwest area [1]. TiO2 content in blast furnace slag gradually increases with the ore addition. Blast furnace slag is significant in the process of iron-making, especially titanium bearing slag, which exerts a deep influence on the reduction of ore, separating efficiency of slag-metal, gas permeability, and so on [2–10]. As one of the most important physical properties of slag, the viscosity is closely related to temperature and slag composition. So understanding the viscosity behaviors of slag with different B2O3 additions and basicity is beneficial for optimizing operation for blast furnace process. Although B2O3 is a typical acidic oxide, it can be used to modify BF slag. It has been found that the little amount of B2O3 remarkably enhanced the fluidity of slag with relatively high TiO2 content and low basicity [11–13]. However, few studies have reported the effect of B2O3 and large basicity on the viscosity of MTBBFS. Therefore the present study was motivated. The viscosity variation of slag with B2O3 content from 1% to 4% and a basicity range of 1.06–1.20 was investigated. Meanwhile the melt structure was analyzed by XRD and structure parameter .
It is generally known that structure of metallurgical slag is very complex and physical properties are very dependent on the level of polymerization of silicate network. We prefer the parameter, , to be an evaluation of the polymerization of the melt, which can be calculated by the following [14–16]:
NBO/T is denoted as the ratio of nonbridging oxygen to tetragonal ions and is a measure of the depolymerization of the melt. Silicates are the basis of most metallurgical slags, where the types of O bonds are classified as bridging oxygen (denoted as BO or O0), nonbridging oxygen (denoted as NBO or O−), and free oxygen (denoted as O2−), as shown in Figure 1. The structure of a slag can thus be represented by the mole fractions () of O0, O−, and O2− present. Consequently, it is customary to divide various constituents into either network formers (e.g., SiO2) or network breakers (CaO, MgO, etc.).
In this study, a conventional NBO/T formula has been revised to describe polymerization variation of MTBBFS, which would appear to be exceedingly useful in predicting properties since it can underline contributions of different oxides to properties.
2. Experimental
In this investigation, analytical reagents CaO and B2O3 were added to normal slag obtained from the blast furnace in desired proportions. Composition of slag samples is listed in Table 1. The slag viscosity was measured by rotating cylinder method and the viscosity of each sample was recorded during the cooling cycle above break point temperature of the sample. The break point temperature is the critical temperature at which the measured viscosity changes abruptly during the cooling cycle as described by Sridhar et al. [17]. A viscometer is connected to a molybdenum spindle through a molybdenum rod. The graphite crucible (50 mm in internal diameter, 100 mm in height, and 20 mm in thickness of wall and base) containing the slag sample (about 120 g of slag sample) was placed inside the furnace. The schematic diagram of the experiment apparatus and experimental procedure can be found in our previous paper [11, 18, 19].
After completing the viscosity measurements, the mineral compositions of quenched slags were acquired by XRD (X-ray diffraction). Diffraction conditions were as follows: Cu target K alpha rays, voltage of 35 kV, current of 25 mA, continuous scanning, the scanning range of 10~90°, and scanning speed of 0.06.
3. Results and Discussion
3.1. Effect of Basicity on Viscosity of MTBBFS
The viscosities have been presented in Figure 2 to evaluate the effects of basicity and temperature. It can be observed that increasing basicity resulted in an increase in viscosity at lower temperatures. Especially under 1623 K, the values rise sharply with basicity. However, no obvious changes are found at high temperatures (over 1623 K in this experiment). Besides, break point temperature rises linearly with basicity in Figure 3. For every 0.01 increase in basicity, it goes up around 1.4°C.
3.2. Effect of B2O3 on Viscosity of Medium Ti Bearing Slag
Figure 4 presents viscosity of slag containing boron or boron free slag as a function of temperature. The shapes of viscosity curves are similar for both slags. There are slight differences between viscosities of both slags at high temperatures. However, the value increased sharply under break point temperature. Break point temperature lowered quickly with increasing boron content from 1% to 4%. More boron in slag leads to more decrease in break point temperature. It decreases by about 20°C at 1% increase of boron content in slag, as shown in Figure 5.
As an acidic oxide with lower melting temperature, B2O3 can combine with basic oxides to form eutectics, such as MgOB2O3 (988°C) and CaOB2O3 (below 1150°C). So superheat degree of slag was increased at a fixed temperature. Ultimately, the viscosity dropped because of weak interaction among flow units.
3.3. Effect of B2O3 and Basicity on Viscosity of MTBBFS
The effects of temperature on the viscosity of the CaO-SiO2-MgO-Al2O3-TiO2 slags at different basicities and B2O3 contents are plotted in Figure 6, where the slag viscosity decreases with increasing temperature. All of the viscosities are within 0.2 Pas at a given temperature (>1603 K), which are less than slag without boron. Break point temperature decreases from 1593 K to 1575 K with increasing B2O3 content from 1% to 4% and basicity from 1.06 to 1.20 (in Figure 5). The relationships of viscosity, break point temperature and basicity, and B2O3 content are constructed in Figures 7 and 8, respectively.
Figure 7 exhibits that slag viscosity declines with B2O3 addition regardless of basicity, indicating the stronger effect of B2O3 compared to basicity. Moreover, the larger the basicity, the more intensive the curve, suggesting that B2O3 has more sensitive effect on slag viscosity with large basicity.
In Figure 8, break point temperature increases with basicity. At a fixed B2O3 content, the larger the basicity, the higher the break point temperature. Influence of B2O3 is exactly the opposite. When B2O3 content in slag is lower, in order to make break point temperature go up 10°C, basicity needs to increase by about 0.02 in low basicity areas. But it only needs to increase by about 0.01 in high basicity areas. But it only needs ~0.01 increase in high basicity areas. For higher B2O3 content in slag, different results can be found by increasing the same value of break point temperature. In low basicity areas, basicity needs to rise to ~0.03 and in high basicity areas it is ~0.015. It proves that more B2O3 content in slag leads to faster reduction in break point temperature and greater influences of B2O3 content happen for slag with large basicity.
Larger viscosity and more intensive curve after increasing basicity can be caused by gradual formation of compounds with high melting point, such as 2CaOSiO2. Some new compounds with low melting point were produced by adding B2O3. Fluidity was effectively improved, especially to blast furnace slag with large basicity.
3.4. Effect of Basicity and B2O3 on Structure of MTBBFS
For a CaO-SiO2-TiO2-A12O3-MgO-B2O3 slag, the conventional NBO/T ratio is given by (2) where is the mole fraction [20–23]. The NBO/T ratio and are calculated based on Table 2 and (2). Relationship of basicity and is displayed in Figure 9. Consider
(a) Before revision
(b) After revision
As observed from Figure 9(a), increasing basicity tends to make reduce and network turn from Si2 (, chain) to Si2 (, polyhedra). It can be concluded that silicate network becomes less polymerized. A slower drop in the parameter can be seen with increasing basicity after B2O3 addition; it suggests the opposite effect of B2O3 comparing with basicity, which is contrary to the facts. A reasonable explanation is that the influences of different oxides on the structure are different, for example, CaO and MgO. Bond strength is used to measure destructive capacities of alkaline oxide as an important parameter. Destructive capacity of other cations is described as the ratio of bond strength of CaO and MeO () assuming destructive capacity of CaO to be equal to 1 [24, 25].
This study presumes that the dominant structure associated with Al-O arrangements is AlO4 tetrahedra based on the report from McMillan [26, 27]. B2O3 is thought to be a network breaker for its function of improving slag fluidity. Therefore, (2) is corrected as follows:
The results of parameter versus CaO/SiO2 and B2O3 are shown in Figure 9(b). As clearly observed, decreases with increasing CaO/SiO2 from 1.06 to 1.20 and adding B2O3 from 0% to 4%, which encourages change in slag structure from complexity to simpleness. Furthermore, all values fall off remarkably after revising (2), indicating that the dominant structural unit associated with B-O arrangements can be BO3 triangular, which is a simpler and less structural two-dimension (2D) structure compared to three-dimension (3D) structures, BO4 tetrahedra [13]. Although B-O bond in BO3 triangular is stronger than Si-O bond in a layer, layer and adjacent one link by weaker molecular bonds. The viscosity and melting temperature of the slag are intimately associated with its structure, whereas structure depends upon its composition. SiO2 forms framework for sample as the network former. The increase of B2O3 content makes large tetrahedron clusters break. As a result, its melting temperature is low and chemical stability is poor. It is easy to form irregular network structure when cooling, implying that glass phase tends to precipitate more easily and suppress the formation of perovskite (melting temperature: 2243 K, in agreement with results from Figures 10 and 11). In addition, boron can capture oxygen from silicate-oxygen chains to produce boron clusters. Thus silicate-oxygen chains were broken and the network structure became simpler. Therefore, viscosity and break point temperature reduced. However a larger basicity corresponds to a higher O/Si ratio, which gives rise to a change in the connecting mode of oxygen-silicon tetrahedron from layer (O/Si = 2.5) or chain (O/Si = 3.0) to individual silica tetrahedra (O/Si = 4.0). That is to say, parameter decreases. Thus viscosity of slag reduces. However, dicalcium silicate (melting point: 2614°C) or other compounds with high melting point are produced steadily due to excessive CaO, which leads to a rise in viscosity and break point temperature.
(a)
(b)
(a)
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4. Conclusions
The role of basicity and B2O3 in CaO-SiO2-MgO-TiO2-Al2O3-B2O3 slag system was studied by measuring viscosity combined with structure parameter and XRD results. The experimental results were summarized as follows:(1)Slag viscosity and break point temperature both decrease with increasing B2O3 at each basicity. The more B2O3 content leads to the more pronounced variation, especially for the slag with larger basicity.(2)Increasing basicity from 1.06 to 1.20 makes viscosity and break point temperature of MTBBFS rise.(3)It is found that B2O3 addition changes network structure of MTBBFS from complex to simple based on the revised NBO/T formula and XRD results.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
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. cstc2013jcyjA90007), and Research Foundation of Chongqing University of Science & Technology (no. CK2014B16).