Viscosity of glass‐forming melt at the bottom of high‐level waste melter‐feed cold caps: Effects of temperature and incorporation of solid components

Abstract

During the final stages of conversion of melter feed (glass batch) to molten glass, the glass‐forming melt becomes a continuous liquid phase that encapsulates dissolving solid particles and gas bubbles that produce primary foam at the bottom of the cold cap (the reacting melter feed in an electric glass‐melting furnace). The glass‐forming melt viscosity plays a dominant role in primary foam formation, stability, and eventual collapse, thus affecting the rate of melting (the glass production rate per cold‐cap area). We have traced the glass‐forming melt viscosity during the final stages of feed‐to‐glass conversion as it changes in response to changing temperature and composition (resulting from dissolving solid particles). For this study, we used high‐level waste melter feeds—taking advantage of the large amount of data available to us—and a variety of experimental techniques (feed expansion test, evolved gas analysis, thermogravimetric analyzer‐differential scanning calorimetry, X‐ray diffraction, and viscometer). Starting with a relatively low value at the moment when the melt connects, melt viscosity reached maximum within the primary foam layer and then decreased to its final melter operating temperature value. At the cold‐cap bottom—the boundary between the primary foam layer and the thermal boundary layer—where physicochemical reactions of a melter feed influence the driving force of the heat transfer from the melt to the cold cap, the melt viscosity affects the rate of melting predominantly through its effect on the temperature at which primary foam is collapsing.

Melting rate correlation with batch properties and melter operating conditions during conversion of nuclear waste melter feeds to glasses

“…In molten glasses, viscosity is the main factor determining the foam stability. In our previous study, 50 we evaluated the transient melt composition of HLW melter feeds using fractions of undissolved solids from x-ray diffraction data, and estimated melt viscosity using a model, [53][54][55] concluding that the melt viscosity at T B for HLW melter feeds was 25-85 Pa s. 50 It can be assumed that viscosity, which depends on the melt composition, presence of solid inclusions, and melt homogeneity, would not exceed a certain value at the cold-cap bottom.…”

“…The foaming interval, T FO < T < T B , is marked as a shaded strip. Gas evolution virtually stops within the foaming interval for moderately foaming feeds, for which the temperature of the maximum volume is well defined and indicates the point at which melt viscosity decreases to a value at which the foam becomes unstable 50 . Small samples of vigorously foaming feeds release gas during volume expansion as well as during gradual volume decrease, during which the decay of primary foam is controlled by gas evolution rate rather than by foam stability.…”

Visual Observation of Foaming at the Batch-Melt Interface During Melting of Soda-Lime-Silica Glass

“…In our recent studies [28][29][30], we suggest that the dominant factor for the heat transfer into the batch is the so called batch bottom temperature, T B , because for most feeds that do not foam excessively, the main resistance to the heat transfer is on the melt side; the foam thickness, d F , is a variable that "adjusts" itself to accommodate the heat incoming from the melt. However, we argued that the T B estimated using common laboratory techniques, such as FET, is far from precise, especially because of the differences between in the foam behavior in a heated laboratory sample and at the bottom of the batch in a large glass-melting furnace [12].…”

Effect of material properties on batch‐to‐glass conversion kinetics

“…This transformation occurs within the cold cap, a layer of batch that floats on the pool of molten glass. During the passage through the cold cap, the bound water is released and evaporated [11][12][13] ; inorganic salts melt [14][15][16] ; gases evolve from nitrates, nitrites, carbonates, sulfates, and organics [17][18][19] ; refractory particles dissolve in glass-forming melt [20][21][22][23][24][25] ; and evolving gases trapped in glass-forming melt produce primary foam. [26][27][28][29] The batch melting progress is investigated, typically at a constant heating rate, to determine the evolution of the chemical and mineralogical composition and the material properties as functions of temperature.…”

“…This transformation occurs within the cold cap, a layer of batch that floats on the pool of molten glass. During the passage through the cold cap, the bound water is released and evaporated 11–13 ; inorganic salts melt 14–16 ; gases evolve from nitrates, nitrites, carbonates, sulfates, and organics 17–19 ; refractory particles dissolve in glass‐forming melt 20–25 ; and evolving gases trapped in glass‐forming melt produce primary foam 26–29 …”