Thermodynamics and constitution of silicate melts. The system PbO + PbF2+ SiO2

Abstract

Activities of PbF2 in PbO + PbF2 and PbO + PbF2+ SiO2 melts with up to 40 mol% PbF2 were determined at 1073–1173 K by means of an electrochemical cell with CaF2 as solid electrolyte. The results, together with previous measurements of the activity of PbO, were interpreted in terms of a theory in which the anionic constitution of the melts is represented by O2–, F– and an equilibrium array of silicate and fluorosilicate ions of formula SinO3n+1–mF(2n+2–m)–m. The equilibrium constant k′ for the formation of fluorosilicate ions by the reaction F–+ SinO3n+1–mF(2n+2–m)–m= SinO3n–mF(2n+1–m)–m+1+ O2– was estimated to be 0.4 ± 0.025. This allowed the anionic distribution to be evaluated for basic melts. In addition to O2– and F–, the main anionic constituents in melts with XPbO/XSiO2 > 2.5 and XPbF2⩽ 0.25 are SiO4–4, SiO3F3–, SiO2F2–2, SiO3F–, Si2O6–7 and Si2O6F5–, with decreasing amounts of more highly fluorinated species. The volatile constituent SiF4 is also present in small amount and becomes increasingly significant as XSiO2/XPbO is increased. Substitution of PbO by PbF2 causes a net decrease in the mean chain length of the anions.