Structural characteristics and viscous behaviors of Al2O3–CaO–SiO2–Fe2O3 slags

Abstract

The high-temperature properties of the Al2O3–CaO–SiO2–Fe2O3 basic slag had significant influences on steelmaking operations and waste slag utilization. To further clarify the structural characteristics and properties of Al2O3–CaO–SiO2–Fe2O3 slags, the structures and viscosities of the slags were researched. The slag liquidus temperature was determined, which decreased from 1365 to 1287°C after 4.16wt%–8.52wt% Al2O3 was added to the slags and then increased to 1356°C after 17.07wt% Al2O3 was added. Structure analysis indicated that increasing temperature depolymerized the structure of the 4.16wt%Al2O3–CaO–SiO2–Fe2O3 slag by decreasing the amount of complex AlO4 units and promoting the formation of simplified silicate monomers. The addition of Al2O3 to slags could promote the polymerization of the slag structure by increasing the quantities of complex AlO4 tetrahedral and complicated Si–O units. Variations in the degree of structure polymerization showed similar trends at the same superheat degree and the same quenching temperature, and both samples could be used for analyzing the impact of Al2O3 on slag structures. Finally, the viscous behavior of the present slag system was evaluated. Increasing Al2O3 content could increase slag viscosity, and the apparent activation energy increased from 132.13 to 174.83 kJ/mol as the content of Al2O3 increased from 4.16wt% to 17.07wt%.

1. Introduction

Al2O3–CaO–SiO2–Fe2O3 systems are the basic components of steelmaking slags in basic oxygen furnaces [1–2], and Fe2O3-containing silicate systems are applied to the preparation of glass ceramics and further utilization of waste slags [3–4]. The high-temperature physicochemical properties of these basic slag systems significantly influence the operations of steelmaking processes and waste slag utilization. Furthermore, Al2O3 is usually added to steelmaking slags to change slag properties for the further enrichment and extraction of valuable elements, such as V and P [5]. As an amphoteric oxide, Al2O3 could play different roles depending on its content and the other components of the slag [6]. When the content of the basic oxides, such as CaO and Na2O, in slags is high, which could provide Ca2+ and Na+ ions as compensation ions to promote the formation of AlO4 units, Al2O3 would act as an acidic oxide to polymerize slag structures. If basic oxides in slags are inadequate to compensate Al3+ ions for the formation of AlO4 units, then excess Al2O3 would act as a basic oxide and form AlO6 units to depolymerize slag structures. Hence, understanding variations in the properties of Al2O3–CaO–SiO2–Fe2O3 slags with various Al2O3 contents is important for the control of smelting operations in metallurgical processes.

Temperature and slag components have a decisive impact on slag properties, and variations in slag properties are often the external manifestations of microstructural changes in slags. Understanding the effects of temperatures and composition on slag structures is conducive to clarifying relationships between the macroscopic properties and microscopic structures of slags. Thus, uncovering the structure characteristic of Al2O3–CaO–SiO2–Fe2O3 molten slags is crucial to revealing the properties of slags.

The properties and structures of slags containing iron oxides have been studied. As a basic oxide, FeO could depolymerize structure and reduce the viscosity of slags, such as Al2O3–FeO–SiO2–CaO [7], CaO–MgO–Al2O3–MnO–FeO–SiO2 [8], and CaO–MgO–Al2O3–FeO–SiO2 [9]. As the content of Fe2O3 increases from 24wt% to 44wt%, the viscosity of the CaO–MgO–SiO2–Fe2O3 slag decreases [10]. Yang et al. [11] investigated the influence of Al2O3 on the thermal conductivity of Al2O3–CaO–MgO–SiO2 slags with the hot-wire method, indicating that adding 9mol% Al2O3 could enhance the formation of Al–O–Si units and promote thermal conductivity. Furthermore, a viscosity prediction model of a CaO–Al2O3–FexO–SiO2 system was developed by Kondratiev and Jak [12], Wu et al. [13], and Zhang et al. [14], providing a useful tool for estimating slag viscosity. The viscosity model could provide information about the effect of Al2O3 on slag viscosity through theoretical calculations. Kim and Park [15] found that increasing Al2O3 could polymerize the structure and increase the viscosity of the FetO–SiO2–MgO–CaO–Al2O3 slag. Song et al. [16] indicated that Al2O3 polymerized a CaO–FeOt–SiO2–BaO-based slag structure and therefore increased slag viscosity. Wang et al. [17] found that Al2O3 increased the viscosities of CaO–MgO–SiO2–FeO–Fe2O3–Al2O3 slags.