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

Primary-melt liquids usually wet and coat solid particle surfaces, which enhances
precipitation of solid products. This precipitation keeps the fraction of glass-forming melt
in the mixture at a low level, so the diffusion distances through liquid films separating
solid particles are short. Motion of melt is hindered by a dense network of precipitated
crystals and is promoted by the evolution of gaseous products and surface tension
gradients. As the amount of liquid phase finally increases, drops and films of liquid
become connected into larger and larger areas, resulting in the interconnecting of all
liquid phases.
The degree of melt conducted affects batch permeability and heat conductivity. Gases
generated by the reactions can initially escape freely through open pores. When the melt
becomes interconnected, batch porosity becomes closed, and the mixture is blown into
foam that can exceed the melt volume many times. Thermal conductivity of the melting
batch strongly depends on both melt fraction and total gas content.
Gas evolution
There are two kinds of batch components, those essentially preserved in the glass as
oxide forms and those lost due to their volatility. Decomposition of the raw materials
results in the liberation of gases (CO2, H2O), refining gases (SO2, O2), and volatiles.
These gases are inherent components of glass-forming raw materials and are introduced
to contribute to the melting process, but only a small fraction remains in the final product.
Soda ash, acting as a flux, melts at 1560°F (849°C). This is a very significant stage in the
melting process as it is followed by rapid and violent reaction in the melt due to the
release of carbon dioxide. The agitation from gas evolution acts as a stirring mechanism
and thus develops a homogenizing and refining effect.
Minor refining-agent ingredients are deliberately introduced into batches to produce
bubbles at high temperatures. They over saturate the glass with gases and force the small
carbon dioxide or bubbles from air trapped within the melt to grow and quickly leave the
melt. Refining agents themselves produce bubbles, usually nucleated on solid surfaces
that stir and effectively homogenize the melt.
If gas evolution is too vigorous or melt viscosity relatively high, foam occurs. In
commercial glass batches containing carbonates and fining agents, the carbonate foam is
produced at 1832–2102°F (1000–1150°C). This foam can be reduced or even suppressed
by sulfate or other salts that do not mix with molten silicates. The sulfate foam is
generally produced at 2597–2732°F (1425–1500°C). Carbon is a minor component of
some sulfate-containing batches that functions to control the redox state of the melt.
Solubility of sulfates in glass varies, with redox reactions controlling SO2 and SO3 .
Using fine grains is more effective than using melting agents because melting agents do
not affect the end of decomposition of carbonates. The last CO2 leaving the mixture
causes carbonate foam and gaseous inclusions that must be subsequently refined.
Decarbonation of batches and silica–sodium carbonate reaction products are also
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