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

promoted by the addition of fine-grained refractory raw materials other than silica, such
as aluminum hydroxide or alkali-aluminosilicates.
The evolution of the glass-forming melt composition (an element of the reaction path)
depends on the batch makeup. For example, fine silica dissolves early, producing a highviscosity
melt. Such a melt traps a large number of small bubbles that are difficult to
remove, and is slow in dissolving refractory components (alumina, zirconia, etc.). Larger
grains of silica sand, on the other hand, allow the glass-forming melt to retain low
viscosity even at lower temperatures. This melt releases gas more rapidly, homogenizes
more easily, and dissolves the remaining solids (from the batch or intermediately formed)
more efficiently (because of higher diffusion coefficients and better stirring by escaping
gases). Only silica grains survive to high temperatures, where they provide nuclei for
fining bubbles.
Refractory component solution
By the time the temperature reaches 1832–1994°F (1000–1090°C), the melting reactions
between all except the most refractory raw materials (i.e., silica, feldspar, chromites, etc.)
have proceed very rapidly. Refractory particles are defined as those with a melting point
higher than the maximum temperature used to produce glass. These are mainly silica and
alumina source grains in the melt (about 85% of which will have already reacted and
dissolved). The final stages of the melting process consist of the dissolution of these
remaining refractory materials.
These reactions take place much more slowly, since by this time the newly formed glass
has become much more viscous (rich in silica) and the agitation from gas-liberating
reactions have completed. For this reason, compositions that are more difficult to melt
will require much finer particle-size specifications. The stage at which all the crystalline
materials have disappeared and been incorporated into the melt is known as the batchfree
time.
Some refractory components dissolve at early stages of melting because their particle size
is small (for example, MgO produced by decomposition of dolomite or CaO from
decomposition of limestone). In most commercial furnaces, silica grains dissolve slowly
in molten glass unless they can contact other fluxing components. Some batch
components, such as sulfates, reduce the surface tension and allow more wetting to
promote quicker reactions.
The time to dissolve depends on the original grain size, the dissolution rates of silica in
carbonate and silicate melts, and the grain fraction dissolved in the carbonate melt. Most
silica reacts during the stage of vigorous reactions with the molten carbonates. When a
relatively unreacted silica grain exits the melting system, it is usually a result of
segregation. The agglomeration of sand particles is another unfavorable condition for
grain dissolution.
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