Redox dynamics in multicomponent, iron-bearing silicate melts and glasses: Application to the float-glass processing of high-temperature silicate glassmelts

Processing high-strain-point glasses by the float process is challenged by the relative thermochemical properties of glassmelts and the liquid-metal float medium. As the chemical reaction between the glassmelt and the float metal involves dynamic reduction of the glassmelt, this research has examined the constraints on high-temperature float processing of glassmelts by combining metal-alloy/oxide reaction thermodynamics and Wagnerian kinetic models for redox reactions in silicate melts.; The dynamic response of Fe-bearing, p-type (polaronic) semiconducting amorphous silicates to a chemical potential gradient of oxygen has been shown to be rate-limited by the chemical diffusion of network-modifying cations. The persistence of this mechanism to very low Fe concentrations in Fe-doped magnesium aluminosilicate glasses was proven with Rutherford backscattering spectroscopy. Three glasses, with 0.1, 0.5, and 1.25 mol. % FeO were reacted with air at temperatures from 710-845{dollar}spcirc{dollar}C. For all compositions and temperatures, oxidation was dominated by network modifier diffusion; an activation energy of 475 kJ{dollar}cdot{dollar}mol{dollar}sp{lcub}-1{rcub}{dollar} characterized the process.; Chemical dynamics in a high-temperature float environment were characterized on liquid-liquid reaction couples between two low-Fe sodium-aluminoborosilicate (NABS) glassmelts (0.01 and 0.08 mol. % FeO) and Au-30Sn and Au-28Ge (atomic basis) alloys. Experiments were performed in the temperature range 1250-1450{dollar}spcirc{dollar}C for 30 min; wavelength-dispersive and Rutherford backscattering spectroscopies were employed. These exothermic liquid-metal alloys display large negative deviations from ideal solution behavior, with significantly depressed chemical activities. Diffusion of Sn or Ge in the NABS glassmelts (depth and concentration) was limited at all temperatures to levels comparable to conventional soda-lime (NCS) float glass ({dollar}sim{dollar}2 min on pure Sn at 1100{dollar}spcirc{dollar}C). Incorporation of Sn or Ge was reduced significantly in the higher-Fe-content NABS glassmelt. Based on this finding, combined with the substantial database on conventional float glass, a kinetic model for the reaction is presented. The metal/silicate couples are well characterized as a cation-diffusion rate-limited reaction of serial steps that include two structural reactions in the interior of the glassmelt: one involving oxidation of the incorporated (i.e., from the float alloy) cation from the divalent to tetravalent state and the second (related) involving the reduction of Fe{dollar}sp{lcub}3+{rcub}{dollar} to Fe{dollar}sp{lcub}2+{rcub}.{dollar}...