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

surface and escape from the melt. Some of the trapped gas dissolves in the glass, but
other portions form discrete gaseous inclusions. These bubbles must be eliminated from
the glass melt as they can cause defects in the finished product, affecting mechanical
strength and appearance. Small bubbles remaining in the finished glass are called seeds.
The concentration acceptable in the final product varies by glass segment. Processes to
remove or reduce the level of seeds to commercial standards is known as refining or
fining.
Most gases in the newly formed glass are remnants of raw material decomposition and
entrained gas from air or combustion products. The upward movement of bubbles
contributes to the physical mixing of the melt necessary to obtaining a homogenous
material with optimal physical properties. The bubbles rise at speeds determined by their
size and the viscosity of the glass. Large bubbles rise quickly and contribute to mixing,
while small bubbles move slowly at speeds that may be slow with respect to the largerscale
convection currents in the furnace, and they are thus more difficult to eliminate.
The refining stage to remove objectionable gaseous inclusions relies upon a number of
mechanisms. Bubbles rise to the surface roughly according to Stokes’s law. Each bubble,
during its trip through the glass melt, attracts new quantities of gas by diffusion from
neighboring layers and by coalescence with other bubbles. Increasing the temperature of
the glass will reduce its viscosity and expand the volume of the gas. This will allow the
inclusion to rise by buoyancy to the surface and escape to the furnace atmosphere, which
speeds up the fining process greatly. Another mechanism is absorption into the glass by
solubility. Historically, these time and temperature mechanisms were relied upon for
refining, and the furnaces were significantly rate limited.
High temperatures are conventionally provided in the refining zone to expedite the rise
and escape of the gaseous inclusions by reducing the viscosity of the melt and by
enlarging the bubble diameters. The energy required for the high temperatures employed
in the refining stage and the large melting vessel required to provide sufficient residence
time for the gaseous inclusions to escape from the melt are major expenses of a
glassmaking operation. Therefore, it would be desirable to improve the refining process
to reduce these costs.
The general principle of chemical fining is to add materials that, when in the melt will
release gases with the appropriate solubility in the glass. Depending on the solubility of
the gas in the glass melt (which is generally temperature dependent) the bubbles may
increase in size and rise to the surface or be completely reabsorbed. Small bubbles have a
high surface-to-volume ratio, which enables better exchange between the gas contained in
the bubbles and the glass. Carbon dioxide and the components of air have limited
solubility in the glass melt and it is usually necessary to use chemical fining agents to
effectively eliminate the small bubbles generated by the melting process.
The release of gas at high temperatures, from fining agents or redox reactions of
components, is beneficial for melt homogenization and fining, but can produce
undesirable effects when gas bubbles accumulate under the floating batch. Excessively
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