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

quartz. If limestone and sodium carbonate are in preferential contact, they form a double
carbonate, Na2Ca(CO3)2, at 752–932°F (400–500°C).
If the batch particles are fine and well mixed and if there is sufficient time for the
processes to progress, solid-state reactions may consume large portions of silica and
decompose most of the carbonates, and thus can be a significant factor in promoting
higher melting rates. This emphasizes the importance of optimum mixing for all
components to remain in close contact with complementary batch constituents. They are
affected by pelletizing, briquetting, preheating, prereacting or presintering. They can be
controlled by heat transfer, volume or surface diffusion, or the rate of gas removal. This
is an avenue to accelerate melting rates. Some efforts to agglomerate the batch have
confirmed this principle.
Liquid phase/interactions with solids
Liquid phases are formed from the melting of batch constituents and various eutectic
mixtures. Typical eutectic mixtures for soda-lime-silica glasses are those between sodium
and calcium carbonates, formed at 1427°F (775°C), and that between sodium disilicate
and silica at about 1472°F (800°C), which reacts with additional soda to precipitate
metasilicates at temperatures below 1544°F (840°C).
During the first permanent liquid stage, vigorous melting reactions occur. This stage of
the fusion process is characterized by the presence of molten salts of low viscosity, glassforming
melts, and intermediate crystalline compounds such as double carbonate or
silicates. Unreacted batch constituents, intermediate crystalline reaction products, and
liquid melt phases are all involved in a complex mutual interaction.
Inorganic salts melt, forming low-temperature eutectic liquid phases. Generally, oxoanionic
salts are perfectly miscible in molten state. These melts have a low viscosity and
wet the solid particles, but also tend to segregate by drainage. Their presence accelerates
sintering and the reactions that take place between individual batch particles. The primary
melt participates at the formation of intermediate crystalline forms, which often
precipitate from it, to be later dissolved in the glass-forming melt. The primary melt
reacts with solid components, releasing gases such as COx, NOx, O2, and SOx. A fraction
of inorganic salts remains dissolved in glass; thus glass can contain some CO2 and up to 1
mass% SO3. The gas phase may also survive in the form of intermediate solids.
Molten inorganic salts and intermediate glass forming melts, which are partly mutually
soluble, assist reactions with solids by dissolving them and transferring constituents into
the molten mixture more rapidly. If two salts are present together, they mutually dissolve.
This has two effects: the melt forms at a lower temperature than the melting point of
either salt or the salts mutually dilute each other.
These low-melting-temperature inorganic salts, such as Na2CO3, dissolve some batch
components are wet refractory grains, thus enhancing reactions at temperatures at which
otherwise only slow solid-state reactions would occur. For example, K2CO3 and Li2CO3
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