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

distance to escape. If thermal currents flow too fast, they inhibit fining by bringing the
glass to the conditioning zone too soon. Guiding walls or weirs can be built into the inner
tank structure to create ideal glass flow paths.
Redox state
The oxidation-reduction (redox) state of glass is defined as its partial pressure of oxygen.
It can be measured directly using electrochemical methods, or indirectly through the
analysis of redox couples such as Fe 2+ /Fe 3+ . If transition metal oxides are glass
components, redox influences glass color. Redox is also important for batch melting
reactions and fining. Between the stage of vigorous reactions and the final stage, there is
a temperature gap during which redox or other gas-liberating reactions can produce
oxygen or other gases and generate foam. For example, the reaction of nitrates and
sulfates with other glass components is accelerated (and proceeds at lower temperatures)
when reducing agents such as carbon, are present. Arsenic, antimony and many
transition-metal oxides can function as fining agents that release oxygen at elevated
temperatures; their functioning depends on the presence of oxidizing agents, such as
nitrates, in the batch.
Each of these reactions is intended to promote the evolution of refining gases at
appropriate stages of the melting process. Varying the redox (oxidizing and reducing)
components within the formulated batch will have a varying impact upon gaseous
evolution within the melting process.
Excessive quantities of bubbles produce foam. Foaming can interfere with the melting
process, usually by blocking heat transfer and upsetting the steady state. Also, foam
covering the free surface of glass retards heat transfer to the melt. The amount of foam
depends on the gas generation rate and the factors that influence foam stability. These
factors are not well understood, although it is known that glass films easily rupture when
subjected to mechanical, thermal, or chemical shocks.
With conversion to oxy-fuel from air-fuel and with no modification to the fining package,
many glass manufacturers have noticed increased thickness of foam and difficulties in
surface combustion penetrating through the foam. The change to oxy-fuel increases the
water vapor above the glass from typically 14% with air fuel to greater than 60% with
oxy-fuel. The quantity of water chemically absorbed as hydroxyls in the glass structure is
nearly doubled as the relation follows a square root relationship. Quadrupling the water
content doubles the chemically dissolved water. This doubling in water or hydroxyls is
equivalent to the refining potential of 33% of the sodium sulfate introduced in a container
glass batch. Reducing fining agents by 10–15% has been shown to reduce surface foam
and to have no negative impact upon glass quality.
Refining
In the melting of glass, substantial quantities of gas are produced as a result of the
decomposition of batch materials and to a much lesser extent from air trapped in the raw
materials. Other gases are physically entrained by the batch materials or are introduced
into the melting glass from combustion heat sources. Most of the gas escapes during the
initial phase of melting, but some becomes entrapped in the melt and must rise to the
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