Coupling effect of TiO2 and Al2O3 on the structure of CaO–SiO2–MgO–xwt%Al2O3–ywt%TiO2 slag systems

Abstract

This study analyzes the influence of TiO2 and Al2O3 contents on the microstructure of CaO–SiO2–MgO–xwt%Al2O3–ywt%TiO2 (14 ≤ x ≤ 22, 0 ≤ y ≤ 10) blast furnace slag systems based on the change of slag viscosity, Raman spectroscopy, and molecular dynamics. The Raman spectroscopy results indicate that an increase in TiO2 content leads to the gradual depolymerization of complex silicate structures ( and ) into simpler structures ( and ) in the slag. At the same time, the Al–O–Al bonds in the aluminate structures of the slag also depolymerize into simpler Al–O− forms, resulting in a decrease in the degree of polymerization of both silicates and aluminates. In contrast, an increase in Al2O3 content generally results in an increased degree of polymerization for the silicates and aluminates. Molecular dynamics simulations of the polymerization and depolymerization processes in the microstructure of the blast furnace slag reveal that Si and Al mainly exist in tetrahedral [SiO4]4− and [AlO4]4−, while Ti mainly exists in the form of simple pentacoordinate [TiO5]6− and hexacoordinate [TiO6]8−. TiO2 exhibits basic properties in this system, whereas Al2O3 demonstrates acidic behavior. The addition of TiO2 introduces free oxide ions into the system, causing the bridging oxygens to break into non-bridging oxygens, leading to the depolymerization of complex structures and , which simplifies the slag structure. On the other hand, an increase in Al2O3 content tends to capture or share the oxide ions within the system to form [AlO4]4−, resulting in the polymerization of free oxygens into non-bridging oxygens, which further polymerize into bridging oxygens and lead to the consolidation of simple structures and , resulting in a more complex slag structure. Both Raman spectroscopy analysis and molecular dynamics simulation results indicate that the degree of polymerization of [SiO4]4− and [AlO4]4− in the slag network structure is a crucial factor determining the fluidity of the slag.

1. Introduction

In high-alkalinity silicate melts, Al2O3 exhibits acidic behavior, acting as a network former to create aluminum oxide tetrahedra, which leads to a polymerized network structure and increases the degree of network polymerization. Conversely, in acidic environments, Al2O3 behaves as a basic substance, serving as a network modifier to form octahedral [AlO6]9− structures, resulting in the depolymerization of the network and a decrease in polymerization degree [1–2]. Therefore, Al2O3 in silicate melts is usually considered to be an amphoteric oxide in terms of structure formation. Similarly, TiO2, as an amphoteric oxide, also has a variable coordination form. The tetra-coordinated [TiO4]4− and penta-coordinated [TiO5]6− are acidic acting as network formers, and the hexa-coordinated [TiO6]8− octahedron is alkaline as the network modifier [3–4]. Therefore, Al2O3 and TiO2 in blast furnace slag show typical amphoteric oxide characteristics.

In recent years, with the increasing content of Al2O3 in iron ore and the use of inexpensive iron-containing raw materials, Al2O3 content in blast furnace slag has been steadily rising. A significant portion of the slag now contains more than 16% Al2O3 [5–6], and some furnaces even reaches 17%, which has led to a series of problems in blast furnace smelting, particularly a marked deterioration in slag fluidity [7]. At the same time, numerous studies have shown that an appropriate amount of TiO2 in the slag can reduce its viscosity. Ohno and Ross [8], through experimental testing and Raman spectroscopy analysis, found that TiO2 can effectively reduce the viscosity of the CMAST (CaO–MgO–Al2O3–SiO2–TiO2) system in a neutral atmosphere. TiO2 acts as a network modifier by disrupting the silicate network structure, thereby reducing the degree of polymerization of the slag structure. Similar conclusions have been reached by researchers such as Park et al. [9], Sohn et al. [10], Yan et al. [11], Handfield and Charette [12], Kato and Minowa [13], and Pang et al. [14], who confirmed that the proper addition of TiO2 can improve the fluidity of the slag.

In this paper, the effect of TiO2 content on the viscosity of high-alumina blast furnace slag (Al2O3 ≥ 16wt%) under reduction conditions was studied based on the modification of TiO2 in the slag. The microstructure of CaO–SiO2–MgO–xwt%Al2O3–ywt%TiO2 slag system, the effect of TiO2 and Al2O3 on the structure, and the coupling effect are investigated using Raman spectroscopy and molecular dynamics simulations.