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

ubbly or foamy layers under the batch blanket (particularly in all-electric melting)
insulate the batch by blocking heat transfer. As a result, the melting rate is slowed down,
or the melting process can be destabilized.
The most frequently used fining agent in the glass industry is sodium sulfate, sometimes
called “salt cake,” followed by sodium or potassium nitrates in combination with arsenic
or antimony trioxides. These last two are more expensive, have associated environmental
and health issues, and tend to be used mainly for the production of special glass. At
approximately 2642°F (1450°C) (2192°F [1200°C] if reducing agents are present)
sodium sulfate decomposes to produce sodium oxide (which is incorporated into the
glass), gaseous oxides of sulfur, and oxygen. The oxygen bubbles combine with or absorb
other gases, particularly carbon dioxide and air, thereby increasing in size and rising to
the surface. The gaseous oxides of sulfur are absorbed into the glass, or join the furnace
waste gas stream. In flat glass and container glass production, sodium sulfate is by far the
most common fining agent. The predominance of sodium sulfate as the fining agent is
due to its parallel action as an oxidizing agent for adjusting the redox state of the coloring
elements in the glass. It is also the least expensive effective fining agent for massproduced
glass.
Nitrates are used to force trioxides to convert to pentoxides as low batch-charging
temperatures. Arsenic trioxide is used for higher melting glasses, 2660–2732°F (1460–
1500°C), and antimony for lower melting glasses, 2372–2552°F (1300–1400°C).
Increasing glass temperatures revert the pentoxides back to trioxides with a release of
oxygen, which provides very effective refining. As the glass cools, dissolution fining may
occur; that is, oxygen bubbles are removed by reaction with the arsenic or antimony
trioxide to form the pentoxide. Sodium nitrate can also be used as a fining/oxidizing
agent, particularly if a high degree of oxidation is required. Calcium sulfate and various
nitrates are sometimes used for colored glasses. Unfortunately, nearly all the NO2
produced goes up the stack and is a significant contributor to the NOx produced.
Gases relatively soluble in glass include water, SO3 and O2. Nitrogen (N2) and SO2 are
relatively insoluble at glass-processing temperatures and do not dissolve easily. CO2 and
argon (from air entrapment) are somewhat soluble, depending upon the time and
temperature of exposure. Each glass composition has a relative solubility constant for
different gas species that is proportional to temperature. The extent to which
thermodynamic equilibrium conditions occur will also influence solubility. This is
because their solubility in glass is sufficiently high for resorption at lower temperatures
where the glass is too viscous to allow bubble dissolution in a reasonable time.
Homogenizing/Conditioning
Batch segregation, melt segregation, volatilization, and temperature fluctuations, as well
as refractory corrosion from tank lining material, cause compositional differences (cord,
striae) within the melt. These inhomogeneities must be removed by diffusion and flow
before the glass is cooled in the forehearth prior to delivery and forming. Vigorous fining
action and convection current mixing help break up cord. Mechanical and static mixers
continuously shear the glass to help active homogeneity.
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