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

of soda. Other raw materials include boron, generally from deposits in California or
Turkey. Either anhydrous borax or boric acid is used, but their consumption is decreasing
because of the energy required to produce them. Mineral ores such as colemanite, rasorite
or ulexite are now used in addition to boric acid or penta-aquo borax (Na2B4O75H2O)
when possible. Feldspar and nepheline syenite are common mineral sources of alumina.
Litharge is used as a source of lead oxide for the manufacture of lead glasses. Sulfates or
nitrates oxidize other oxides in the glass and control fining reactions.
Powdered anthracite coal is a common reducing agent in glass manufacture. Fining
agents remove the bubbles in the molten glass and include sulfates, halides, peroxides,
chlorates, perchlorates, CeO2, MnO2, As2O3 and Sb2O3. They react by release of oxygen
or sulfur trioxide, or by vaporization as in the case of halides. Controlled decomposition
of sodium sulfate with powdered coal is used to fine many soda-lime compositions.
Common colorants for glass include iron, chromium, cerium, cobalt, nickel and selenium.
Small amounts of iron are sometimes desirable for color and controlled radiant heat
transfer during melting. Ferrous sulfide or iron pyrites produce amber-colored glass used
in the container industry. Iron chromite is a source of chromium for green container
glasses, whereas small amounts of cobalt and nickel oxides added to flint glass decolorize
the yellow-green color that results from iron contamination. Selenium with iron and
cobalt yields a bronze color. Ceria is used to increase ultraviolet absorption in optical or
colorless soda-lime glass and to protect glasses from x-ray browning.
Cullet, or broken glass, is used as a batch material to enhance glass melting and to reduce
the amount of dust and other particulate matter that often accompanies a batch made
exclusively from raw materials. Some forming operations, for example, the ribbon
machine, generate as much as 70% waste glass, which must be recycled as cullet. More
efficient operations, such as those used in the container industry, may require the
purchase of cullet from recycled glass distributors or other sources. Typically, 10–50% of
a glass batch comprises cullet.
Melting and fining depend on the batch materials interacting with each other at the proper
time and in the proper order. Therefore, care must be taken to obtain materials of
optimum grain size, to weigh them carefully, and to mix them together intimately. The
efficiency of the melting operation and the uniformity and quality of the glass product are
often determined by the batch preparation. Batch handling systems vary widely
throughout the industry, ranging from manual to fully automatic. Each batch material is
stored in its own bin and the correct amount, determined and controlled by a computer, is
weighed directly from the bin onto a conveyor belt that feeds directly into a mixer.
Alternatively, sometimes only the gross weighing is made automatically and the final
dribble feed is controlled manually. For small tanks and pot furnaces where the
composition changes frequently, the batches are sometimes weighed into a bin or hopper.
Often the weighing step is the most crucial in small tank operations, which are the most
sensitive to fluctuations and often melt the types of glasses that must meet the most rigid
specifications.
Wet mixing and batch agglomeration (pelletizing or briquetting) are coming into vogue
for several reasons. A wet batch (3–4% water) prevents dusting, controls air pollution,
and ensures homogeneity, therefore increasing melting efficiency and glass quality.
Especially in batches with raw materials of varying grain size, a wet batch prevents
particle segregation because of fine particles settling to the bottom of the bin. The
homogeneous batch melts more efficiently because all of the batch materials are more
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