Relationship between structure and viscosity of CaO–SiO2–Al2O3–MgO–TiO2 slag
Introduction
Viscosity is one of the key properties of slags which have a prominent effect on the metallurgical processes such as the kinetics of reactions and transport phenomena in metal-slag systems in high temperature [1], [2]. Additionally, the viscosity of slags is very sensitive to the temperature and composition which influence the structure of the slags in nature. Therefore, the viscosities of various slags have been measured by lots of metallurgists [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Over recent years, numerous mathematical models have been developed to describe the viscosities of metallurgical slags as functions of composition and temperature due to time consumption and difficulty in viscosity measurement at high temperature [9], [10], [15], [16], [17], [18], [19], [20].
It's known that the slags in the fields of pyrometallurgy are based on the silicate melts and the viscosity of silicate melts is a direct result of the composition and temperature. Nevertheless, several oxides exist in the slags resulting in the complexity of the structure, which contributes to the difficulty in the prediction for viscosities in comprehensive ranges of composition, in particular for blast furnace (BF) slags containing high TiO2. Hence, viscosities of slags containing TiO2 have also been investigated by steelmakers for understanding the relationship between composition and properties [3], [4], [5], [14], [21], [22], [23], [24], [25], [26]. TiO2 has been regarded as a basic oxide which can decrease the polymerization of the framework in slags by various viscosity measurements in CaO–SiO2–TiO2 systems by Dingwell [27], CaO–SiO2–Al2O3–TiO2 slag system by Saito et al. [5] as well as the CaO–SiO2–MgO–TiO2–Al2O3 quinary slag systems carried out by Sohn et al. [21] and Liao et al. [3].
It's accepted that the viscosity of silicate melts is directly related to the structure such as the degree of polymerization. Therefore, structure analysis of different slags was investigated by some researchers [7], [8], [21], [22], [28]. Moreover, Park et al. [22], investigated the effect of TiO2 on the silicate structure in CaO–SiO2–17 mass% Al2O3–10 mass% MgO slags by the FT-IR and Raman spectrum. Results obtained by FT-IR and Raman spectrum are generally utilized for estimating the polymerization of Si and Al in this quinary system. Nevertheless, detailed structure especially the local ordering of the slags with both high basicity and TiO2 content in CaO–SiO2–Al2O3–MgO–TiO2 slag, based on the industrial composition in blast furnace, has not been studied systematically and viscosity of this slag was measured relatively less. It's accepted that molecular dynamics (MD) simulation is an excellent tool for studying the microstructure with classical dynamics, which has been successfully used in the metallurgical melt for decades such as CaO–Al2O3 melts by Belashchenko et al. [29], CaO–SiO2–Al2O3 system by Zheng et al. [30] and CaO–SiO2–MgO–Al2O3 slag by Shimoda [31]. However, the structure and properties of slag bearing high TiO2, which was produced by blast furnace (BF) in Panzhihua Iron & Steel Corporation, are very different from that by general BF. Therefore, a deeply understanding of structure for this type of slag is essential. Besides, investigation of structure in CaO–SiO2–TiO2 system has been performed with different additions of TiO2 at a fixed CaO/SiO2 ratio in our previous work, and the role of TiO2 acting as a basic oxide was verified in terms of structure [32].
Consequently, in the present work, the MD simulation was extended to this quinary slag based on the industrial composition. In order to investigate the structure, the effect of basicity and TiO2 additions were considered with TiO2 ranging from 0 to 30 mass% and basicity fixed at 0.8, 1.0 and 1.2, respectively. Furthermore, viscosity with basicity of 1.0 and 1.2, barely investigated along with TiO2 up to 30 mass%, was also measured to quantitatively investigate the relationship between viscosity and microstructure within this system.
Section snippets
Simulation method
MD simulation was carried out with the Born–Mayer–Huggins (BMH) interatomic potentials which has been generally used in the research of structure of glasses or slags and has been proved successfully by the comparison with the experiment results using XRD, NMR, Raman spectrum and the simulated results from MD [29], [33], [34], [35], [36]. The BMH interatomic potential function is composed of the coulombic interaction, the repulsion interaction and the van der Waals force. However, the van der
Experimental procedure
The viscosity of this 200 g of slag placed in a Mo crucible was measured by a rotating spindle cylinder viscometer, which consists of a rotating system, a heating system, and a measuring system. The diagram of the experimental setup is presented in Fig. 1. The crucible and the bob, both made of Mo, were applied for the measurement and the dimensions of which are demonstrated in Fig. 2. Six U-shape MoSi2 heating elements were used for heating and melting the slags. The temperature was calibrated
Pair distribution function and coordination number
The pair distribution function (PDF), gij(r) is generally used to assess the short-range order in the metallurgical slags at high temperature which belongs to an amorphous systems. The PDF can be calculated by Eq. (2), where Ni and Nj are the total numbers of ions i and j, respectively, V is the volume of the system, and n(r) denotes the average number of the ions j surrounding the ion i in a spherical shell within r ± Δr/2. The average coordination number (CN), given by Eq. (3), can be evaluated
Conclusion
Structure analysis of the CaO–SiO2–14 mass% Al2O3–8 mass% MgO–TiO2 slag was performed by MD methods with a potential function of BMH, considering two factors, basicity and TiO2 additions. Besides, viscosities of composition at basicities of 1.0 and 1.2 were measured with TiO2 addition ranging from 0 to 30 mass% for quantitatively establishing the relationship between structure and viscosity. The results show that TiO2 act as a basic oxide analogous to CaO and MgO in terms of their average
Acknowledgments
This work is supported by the Major Program of National Natural Science Foundation of China (grant no. 51090383) and the Fundamental Research Funds for the Central Universities (grant no. CDJZR12130054).
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