Glass Melting Technology: A Technical and Economic ... - OSTI

The presence of some distinct solid particles at high temperatures is beneficial. Since the
solubility of gases decreases as silica content increases, solid sand grains provide sites for
nucleation of gas bubbles and set up localized convective flow. Also, for efficient
diffusion and convection melting reactions and destabilization of foam conditions, it is
desirable that the viscosity of the intermediate liquid material phases be as low as
possible. This is achieved by delaying the complete dissolution of silica grains until all
other processes except refining are finished.
Chemical homogenization
Glass-forming melt is either present from the beginning in the form of cullet added as a
batch component or is generated when or as the early borate, borosilicate or silicate
eutectic melts. This high-viscosity melt slows down melting reactions, which continue
via diffusion. Melt homogenization proceeds mainly through the growth and motion of
bubbles, whether in the batch blanket or during fining. The viscous glass-forming melt
traps gases; large numbers of growing bubbles tend to expand the reacting mixture into
foam (so-called primary foam). Bubbles stretch the membranes of the viscous melt until
they burst and shrink back. This is an efficient homogenization mechanism.
Both homogenization and segregation occur within the batch blanket. Low-viscosity
molten salts can drain through open pores between refractory particles, enriching the
lower layers of batch with fluxes. However, in a steady state, local enrichments do not
affect the homogeneity of the final product. An example of local enrichment is refluxing.
For example, SOx from deeper and hotter layers of the batch condenses in the colder
upper layers, where SOx turns into sodium salts that drain down to be partly absorbed in
the glass and partly decomposed. This is particularly true for cold-top electric melters.
Batch segregation can lead to different glass compositions. Only minor levels of
demixing can be overcome via homogenizing mechanisms in the glass fusion process.
When recycled cullet has a different composition than the target glass composition, it is
necessary to rely upon a variety of homogenization mechanisms to obtain a chemically
consistent final glass.
Convection currents within the bulk melt generally operate on a larger scale than bubbles
in batch piles. For these currents to be effective, high velocity gradients are necessary.
These gradients stretch and thin melt inhomogeneities, thus helping their attenuation by
diffusion. However, high-viscosity inclusions (such as alumina-rich inhomogeneities)
should be avoided because high-viscosity inhomogeneities are difficult to stretch and the
diffusion coefficients of their components are small. Some compositions rely upon
bubblers in the melter to accentuate mixing and assimilation of inhomogeneities.
The batch pile, or the cold mixture of raw materials, is melted not only at the surface, but
also from the underside by the molten glass bath. Relatively cold, bubbly glass forms
below the bottom layer of batch material and sinks to the bottom of the tank. Appropriate
convection currents must bring this material to the surface, since fining occurs in tank
furnaces primarily at the surface of the melt, where bubbles need to rise only a short
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