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

The thickness of concentration layers (boundary layers) around refractory grains depends
on the melting history. If the history is such that the melt is homogeneous, the
concentration layer will be thin and dissolution will proceed rapidly. If, on the other
hand, a single grain of silica is surrounded by an almost-saturated melt and the silica
content decreases over a long distance, dissolution will be slow. Either of these situations
can occur in glass melting. A mechanism of imposing shear forces on these layers can
significantly increase reaction rates.
Carbonates are the most common salts used, although other salts are added as melting
agents. The combining of mutually soluble salts promotes reactions at lower
temperatures, but they do not affect the temperature at which the last carbon dioxide
molecule is driven off. This temperature can be reduced if finely ground materials are
used or if the heating rate is slow. The atmosphere is important, mainly because of its
effect on CO2 removal. As carbonates disappear, silicate melt is formed. Silica is usually
the last solid phase to dissolve. If its dissolution is slow, segregation may prevent
completion of the conversion.
The quality of the resulting glass, based on the number of undissolved sand grains in the
bulk, the surface of the melt, and the number of seeds, increases as the temperature at the
end of CO2 release decreases. At a slow heating rate, sodium carbonate reacts
preferentially with silica and moves away from lime; at a high heating rate, sodium
carbonate reacts preferentially with lime.
Different temperature histories lead to different conditions for the final dissolution of
refractory grains, bubble removal, and homogenization. In the extreme case of a very
slow temperature-increase rate, the batch will attain a relatively high degree of
conversion as soon as the temperature is reached at which the reactions are rapid enough.
This will lead to a high silica content in the initial melt and, hence, high liquid-phase
viscosity at low temperatures.
At the other extreme of a very fast temperature increase rate, all solid reactions and early
melting reactions will be skipped. The carbonate melt will be present at high
temperatures, at which the reactions are vigorous. After full decomposition of carbonate,
residual silica grains will be embedded in a melt of a relatively low silica content. This
process is preferred for high production rates.
Buoyancy moves bubbles and solids through the melt, further enhancing diffusion and
stirring the melt. Though convection, whether buoyancy- or surface forces–driven,
greatly enhances diffusion, the diffusion-controlled dissolution is still relatively slow. Up
to 90% of the sand-grain volume is consumed within the batch blanket, yet dissolving the
remaining 10% in the viscous melt close to saturation takes most of the residence time to
dissolve. Also, the motion of slowly dissolving solids can lead to partial segregation.
Finally, small grains readily agglomerate. Agglomeration causes a local increase of the
refractory component to a level above the solubility limit. Agglomerated and segregated
grains tend to persist, and these limit the conversion rate in the final stages of melting.
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