Composition and Mobility of Ionic Ensembles in Slags for Steel Refining in the Ladle–Furnace Unit

Abstract

High-lime synthetic slags for refining steels in the ladle–furnace unit are investigated. The content of the slag mixtures is as follows: 60 wt % CaO, 7 and 8 wt % MgO, 7–23 wt % Al2O3, and 9–18 wt % SiO2, with additions of 8 wt % CaF3 and 5–15 wt % Na2O. Polymer theory is used to calculate the composition of the anionic subsystem in the slag melts. The log-mean polymerization constants K p * for multicomponent melts are calculated from the known polymerization constants in binary systems. It is found that K p * ≈ 10–3–10–2 in the range 1500–1600°C. In that range, the melt’s degree of polymerization is 3 × 10–4–8 × 10–3. In the most polymerized melt, the ionic content of the dimers Si2O 7 6- and Al2O 7 8- is no more than 0.1 and 1.5% of the values for the corresponding monomers. Therefore, we assume, with an error of about 2%, that the structural units of the anionic subsystem are monomers AlO 4 5- and SiO 4 4- simple O2– and F– ions (slag 7). The cationic subsystem consists of Ca2+, Mg2+, Na+, and Al3+ ions in octahedral coordination with oxygen (less than 3% of all the Al atoms). In all the melts, the concentrations of free oxygen ions O2– and Ca2+ ions are similar. In half the cases, the content of O2– ions is greater than the content of Ca2+ ions. The mean mobility U and self-diffusion coefficient D for all the cations are calculated from data for the electrical conductivity and the density. With increase in temperature from 1500 to 1600°C, U and D increase by 50 and 60%, respectively, in all the slags. With increase in the mutual substitution of the components in the slag mixtures M = n(Na2O, CaF2)/n(Al2O3 + SiO2), mol/mol, at 1600°C, U increases from 1.14 × 10–8 to 1.46 × 10–8 m2/(V s) for slags 1–6 (0 ≤ M ≤ 1.1) and from 1.01 × 10–8 to 1.66 × 10–8 m2/(V s) for slags 7–10 (0.25 ≤ M ≤ 0.65). Correspondingly, D increases from 9.2 × 10–10 to 12.8 × 10–10 m2/s for slags 1–6 and from 8.2 × 10–10 to 14.3 × 10–10 m2/s for slags 7–10. The temperature dependence of U and D may be approximated by an Arrhenius equation with activation energies E U and E D . With increase in M in the given ranges, E U declines from 146 to 100 kJ/mol (slags 1–6) and from 124.5 to 109 kJ/mol (slags 7–10). Likewise, E D declines from 159 to 116.5 kJ/mol (slags 1–6) and from 139.5 to 124 kJ/mol (slags 7–10). The mean values of E U and E D correlate with the mean distance between the cations in the melts. On the basis of the proposed alternative model of the conductivity, the O2– ions may also transfer electric charge. Preliminary estimates show that the oxygen transport number at 1600°C may exceed 0.1 in some slags.