The Structure and Properties of Silicate Slags

Abstract

The physicochemical properties of silicate melts are dependent upon the structure of the melts. In this chapter, the structure of silicates containing various kinds of oxides and fluoride, that is, aluminosilicates, borosilicates, titanosilicates as well as silicate melts containing iron oxide and calcium fluoride was thoroughly reviewed. The basic unit of silicate structure is the SiO4-tetrahedron, in which the bridging oxygen (BO) and nonbridging oxygen (NBO) are joined to adjacent SiO4-tetrahedron and cations such as Ca2+, Na+, etc., respectively. The network-breaking cations are normally arranged in a sixfold coordination, that is, MO6-octahedron unit, but it can occasionally deviate depending on the melt systems. The structural parameters such as Qn(=4-NBO/T, n is the number of BO) value as well as cation field strength (z/r2) representing the degree of polymerization of the melts, where several Qnunits are distributed, that is, coexisting according to melt compositions. In aluminosilicate melts, Al3+ in the AlO4-tetrahedron unit requires a charge-balancing cation (Na+ or 0.5Ca2+, etc.). Alternatively, in the lean-silica (<10% SiO2) calcium-aluminosilicate melts, the Al atoms become localized preferentially in Q4 [Al], whereas the Si atoms have no site preference and thus are distributed in various Qn[Si] units. Therefore, the Qn[Si] units are localized at the boundary of Q4[Al] units. Not only the physicochemical properties such as phase equilibria and thermodynamic properties (activity, enthalpy, entropy, heat capacity, density, and molar volume) but also the transport properties such as viscosity, electrical resistivity, diffusivity, acoustic property, and thermal properties including thermal expansion, thermal conductivity, and optical properties are affected by the structure of silicate melts. The miscibility gap in silica-rich systems, liquidus temperature of binary silicates, activity coefficient of SiO2, enthalpy of solution, and entropy of fusion are affected by the cation field strength (z/r2). The viscosity, electrical resistivity, and diffusivity are affected by the resistance to movement posed by the large silicate anions as well as the number and mobility of cations. The temperature dependencies of viscosity, resistivity, and diffusivity can be mainly represented by the Arrhenius equation. But the Weymann equation, the Vogel–Fulcher–Tamman equation, and the Adam–Gibbs equation are also considered. Due to several complex limitations, such as phonon or lattice conduction, convection, and radiation conduction of the silicate systems, there are some uncertainties in measured thermal properties. The surface tension and interfacial tension are influenced by the surface structure of the silicate melts rather than the bulk structure. The surface tension for the metal phase tends to be three or four times higher than the surface tension of silicate melts so the interfacial tension is mostly controlled by the surface tension of the metal phase and hence is sensitive to the soluble O and S contents in the metal. The modeling studies to link the thermophysical properties and the degree of polymerization of the silicate melts are still challenging. © 2024 Elsevier Ltd. All rights reserved.