Relationship between structure and viscosity of CaO–SiO₂–Al₂O₃–MgO–TiO₂ slag

Highlights

• The linked ways between the [SiO₄] and [AlO₄] tetrahedrons were clarified in detail.

• The charge-balancing capability of basic oxides was quantified for [AlO₄].

• The relation between structure and viscosity in Ti-bearing slag was established.

Abstract

Structure of the CaO–SiO₂–Al₂O₃–MgO–TiO₂ slag was studied by molecular dynamics simulation at 1773 K. Additionally, part of viscosities were measured for quantitatively establishing the relationship between the structure and viscosity with varying basicities and TiO₂ additions. Role of TiO₂ as basic oxide was verified and it can be inferred that the structure of TiO₂ is relatively similar to MgO compared to CaO. In addition, distribution of three different types of O was presented from which it can be obtained that nonbridging oxygen preferentially localized beside Si rather than Al. Moreover, parameter α_M was proposed to estimate charge-balancing capability of M for [AlO₄] (where M denotes Ca^2 +, Mg^2 +, Ti^4 +) along with the average values of α_Ca, α_Mg and α_Ti equaling to 1.731, 1.282 and 1.092, respectively, suggesting that Ca^2 + is preferentially compensated for [AlO₄] than Mg^2 + and Ti^4 +. Therefore, TiO₂ and MgO have a prominent effect on depolymerizing the framework of the slags than CaO in slags containing high Al₂O₃. Finally, the natural logarithm of the measured viscosities exhibit a linear dependence on the fraction of Q⁴ for Si, indicating that the degree of polymerization for Si is still the principal factor which affects the viscosity despite the complicated composition.

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.