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

Dissolution of the more refractory (higher-melting) grains, such as sand, is accelerated by
fluxes (lower-melting materials). The decomposition of alkali carbonates (Li2CO3,
Na2CO3, K2CO3) or alkaline earth carbonates (MgCO3, CaCO3, SrCO3, BaCO3) results in
a similar fluxing action on sand and other minerals, notably alumina-containing ones
such as nepheline syenite and feldspars. Undissolved grains (stones) can be introduced
either when the refractory grains fail to react completely or because the reactants are not
intimately mixed. Increasing the melting temperature aids in dissolving stones and
compensates for minor deviations from an ideally prepared and delivered batch.
Eventually, the final glass chemistry is reached when all raw materials have reacted and
intermediate compounds have combined into the end product.
Gas is evolved during the first stages of melting because of (1) the decomposition of the
carbonates or sulfates, or both; (2) air trapped between the grains of the fine grained
batch materials; (3) water evolved from the hydrated batch materials; and (4) the change
in oxidation state of some of the batch materials. Hot gases evolving from batch melting
and refining reactions percolate through the pile and bubble through the upper molten
layer. As this mixture approaches the maximum temperature, fining agents, which are
minor components deliberately added to the batch, begin to release large volumes of gas
that diffuse into existing bubbles and nucleate new bubbles on undissolved grains.
Bubbles rapidly increase in volume and quickly leave the melt, stirring and
homogenizing it vigorously.
Some of the bubbles remain trapped in the final melt and have to be removed by refining
agents or by using temperatures several hundred degrees higher than the temperature
necessary for melting. The process of clarification of the molten glass is called refining.
Refining mechanisms include both thermal and physical means. Increasing temperatures
expand the volume of gases, making the glass more fluid, and bubbles rise to the surface
more rapidly. Chemical fining agents are materials intentionally added to the batch that
promote the evolution of gases generated during the fusion process at elevated
temperatures. Some processes make the bubbles larger in size, which can rise more
quickly out of the melt. Others promote more physical agitation of the less soluble types
of gases.
Chemical compositions of the individual raw materials and their grain characteristics are
variables that can influence intermediate chemical compositions leading to a final glass
oxide analysis. Seed conditions can be related to batch redox formulation, batch
preparation and handling, furnace temperature control, and even glass reactions to
refractories. Each area of the operation needs to be standardized and optimized by
measurements and control procedures.
Other quality aspects of the glass can deteriorate in this stage due to refractory corrosion,
volatilization, segregation, and reboil. For the production of good-quality glass, it is
important that the batch is well mixed and properly treated, that the maximum melting
temperature is as high as possible, that refining agents that liberate gases at a maximum
temperature are present, and that homogeneity-deteriorating processes are prevented.
When the homogenized melt cools down, the chemical solubility and partial pressures of
the various gases present can cause very small remnant bubbles to be quickly adsorbed
into the glass network, and thus disappear. This process is usually enhanced by the
release of oxygen through redox control mechanisms, usually via the addition of sulfates
into the batch.
117