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

Chapter III Traditional Glass Melting
III.1. Current practice
Understanding basic mechanisms of commercial glassmaking is essential for evaluating current and innovative
technologies. Most commercial glass is melted on a large scale in continuous furnaces, either fossil fuel-fired
tanks, oxy-fuel fired tanks, electric furnaces or mixed fuel furnaces. Three basic processes occur in the furnace
tank: the melting process, the refining process, and the homogenization process, both chemical and thermal.
These three processes can occur simultaneously within the melter.
Traditional glass formation involves placing raw materials, properly formulated and prepared, on the surface of
previously formed molten glass. Additional thermal energy is applied to facilitate a series of basic mechanisms
and produce more molten glass.
Commercial glass is a non-crystalline product that results from a fusion reaction between a number of oxide
components at high temperatures. When cooled to a rigid state, the atomic structure of glasses resembles that of
a liquid but in fact retains the same molecular structure at room temperature. Therefore, glass is referred to as a
super cooled liquid and, unlike crystalline materials, has no sharp melting point.
Glass types are classified by chemical composition into four main groups: soda-lime glass, lead crystal and
crystal glass, borosilicate glass and special glasses. Over 95 percent of all glass produced is soda lime, lead and
crystal, or borosilicate composition. Special glass formulations are produced mainly in small amounts to
account for 5 percent of glass produced commercially. Most commercial glasses are silicate-based with the
main component being silicon dioxide (SiO2).
In traditional glass melting furnaces, a well-mixed batch of raw materials is formulated to yield desired glass
chemistry. As the batch is continuously charged into the furnace, it floats on top of the glass melt and is heated
by the radiation of flames in the combustion chamber and the transfer of heat from the hot glass melt in which
chemical and physical changes are occurring. Solid-state reactions between particles of the raw materials result
in formation of eutectic melts. As the batch particles dissolve in the melt, reactions can occur to form gaseous
components such as carbon dioxide and water vapor. In producing commercial quality glass, the glass
producers’ main concerns are dissolution of all solid particles, homogenization, and removal of gaseous
products.
Quality of the glass product results from the temperature in the glass melter, the residence time distribution, the
mean residence time, and the batch composition. Residence time of the molten glass in industrial furnaces
varies from 20 to 60 hours. The maximum temperatures encountered on refractories or on the glass surface in
the furnaces vary for the type of glass produced: 2912°F (1600°C) for container glass; 2948°F (1620°C) for flat
glass; 3002°F (1650°C), for special glass; 3002°F (1650°C) for continuous filament; 2552°F (1400°C) for glass
wool.
The conventional method of providing heat to melt glass is to burn fossil fuels above a batch of continuously
fed batch material and to withdraw the molten glass continuously from the furnace. Glass is melted and refined
at a temperature of 2372 to 2822°F (1300 to 1550°C) at which heat transfer occurs by radiative transmission
from the refractory superstructure that has been heated by the flames to 3002°F (1650°C), and from the flames
themselves.
A glass furnace is designed so that the heat input is arranged to cause convective currents to recirculate within
the melted batch materials and to ensure consistent homogeneity of the finished glass that is fed into the
forming process. The mass of molten glass is held constant in the furnace for a mean residence time of 24
hours for container glasses or a mean residence time of up to 72 hours for some float glass furnaces.
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