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

intimately in contact with each other and also in the correct proportion. Homogeneity can
be assured by agglomeration into pellets or application of pressure to form briquettes.
1.2. Technical description of glass fusion/refining
Commercial glass is based upon a fusion reaction between a number of oxide
components at high temperatures that yields a non-crystalline product when cooled to a
rigid state. Glasses are characterized by their atomic structure, which lacks long-ranged
periodic order. At the atomic level, then, glass resembles a liquid, but it retains the same
molecular structure at room temperature. For this reason, glass is referred to as a
“supercooled liquid.” Unlike crystals, glasses do not have a sharp melting point.
In today’s glass melting furnaces, a well-mixed batch of raw materials formulated to
yield the required glass chemistry is continuously charged to the furnace. For soda-lime
glasses (container and flat), these raw materials are silica sand, feldspar, soda ash,
limestone, dolomite and salt cake (sodium sulfate). Borosilicate and lead glasses would
also use borates and lead oxides. The batch floats on the glass melt and is heated by the
radiation of the flames in the combustion chamber and the transfer of heat from the hot
glass melt. Several chemical and physical changes occur during the heating of the raw
materials. Solid-state reactions between particles of the raw materials result in the
formation of eutectic melts. The batch particles dissolve in this melt, often accompanied
by dissociation reactions, which result in the formation of gaseous components such as
carbon dioxide and water vapor. The dissolution of all solid particles, homogenization,
and the removal of gaseous products are the producer’s main concern in melting
commercial glass quality.
Evaluating alternative glass melting technologies must include a review of the industry’s
perspective on the basic nature of commercial glassmaking mechanisms. Two levels of
detail, one simplified version and the second more detailed, are provided to explain the
glass fusion process.
The maximum temperatures encountered in refractories or on the glass surface in glass
furnaces are container glass, 2912°F (1600°C); flat glass, 2948°F (1620°C); special glass,
3002°F (1650°C); continuous filament, 3002°F (1650°C); and glass wool, 2552°F
(1400°C). The mean residence time of the molten glass in industrial furnaces varies from
20 to 60 h. The quality of the glass product is highly dependent on the glass melter
temperature, the residence time distribution, the mean residence time, and the batch
composition. In general, three processes are distinguished in the melting tank: the melting
process, the refining process, and the homogenization process (both chemical and
thermal). These three essential glass melting processes may partly overlap with each
other inside the furnace.
Traditional glass formation involves placing properly formulated and prepared raw
materials onto the surface of previously formed molten glass. Additional thermal energy
is applied to drive a series of basic mechanisms to produce more molten glass into the
system. Most commercial glass produced on a large scale is melted in continuous
furnaces, either in fossil fuel–fired tanks or in electric furnaces with a cold cap or
“blanket.”
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