Viscosity of transient glass-forming melt and its relation to foaming during batch-to-glass conversion

Abstract

Primary foam, which affects the heat transfer into the glass batch and the final glass quality, occurs when a sufficient quantity of transient glass-forming melt evolves with viscosity low enough to close the open porosity of the reacting glass batch. To better understand how the fraction of transient melt and its viscosity affect the primary foam temperature range, we determined, with x-ray diffraction, the fraction and composition of the transient glass-forming melt in a heated waste glass melter feed as a function of temperature. Then we prepared a set of transient melts that occurred within the foaming temperature interval and measured their viscosities with spindle and falling sphere viscometers. The results agree with the Adam-Gibbs and VFT viscosity-composition models, even for transient melt compositions outside of the models' compositional validity range. As silica and other refractory particles dissolved in the predominantly borate transient melt while temperature increased, viscosity increased from the initial value of ∼ 500 Pa s at the onset of foaming (∼650 °C) to a maximum of ∼ 770 Pa s when silica dissolution was almost complete (∼700 °C). As temperature increased further, transient melt viscosity decreased to ∼ 220 Pa s (∼850 °C) when the primary foam collapsed.