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

Batch melting in combustion furnaces
In typical fossil fuel–fired furnaces, batch is fed into the furnace on the top of the pool of
molten glass in piles that melt from both above and below. Overcoming the low heat
conductivity of batches is the major obstacle to rapid heat transfer. The heat absorbed in a
thin upper layer of the pile converts the batch into a liquid mixture of melt, undissolved
silica grains and gas bubbles, all of which flow down along the inclined surface. The hot
molten glass under the pile contributes some heat to the batch at its lower interface.
Cold top electric melting
In all-electric furnaces, a batch covers the entire melter’s glass surface with a blanket that
is heated from within the molten glass. A typical batch particle moves vertically, first
entering the upper zone where it is heated by percolating reaction gases. Thus its
temperature increases only slightly above that at which it was charged. It loses free water
(batch is usually charged wet to prevent dusting) and absorbs volatile components rising
from the lower reaction zone, which are only several centimeters thick. In this zone, most
of the heat is absorbed and the temperature rises sharply to that of molten glass. The
extent to which evolving gases can freely rise through the molten layer and not build up a
foam layer will determine one important heat transfer efficiency for the system.
SiO2 is the major glass-forming oxide for all container, flat, fiber and tableware glasses
produced commercially. Crystalline silica melts above 3100°F (1704°C). By adding
fluxing ingredients, such as alkali (Na2O, K2O) or borates (B2O3), the melting point drops
significantly due to eutectic melting. Stabilizers (CaO, MgO, Al2O3) are added to
improve chemical durability and forming properties. Raw materials economically
available to provide these modifying oxides are formulated into a batch. The resultant
material is converted from a crystalline structure to a vitreous state glass. The specific
combination of all oxide species in the final glass defines its physical properties.
Thus, glass formation involves a number of key steps, starting with properly formulating
specific raw materials to contribute required oxides. The properly prepared batch
ingredients must be kept in close proximity as they are heated. A series of chemical
reactions and physical processes initiate some components’ melting and convert others
into new, intermediate compounds. Melting can be divided into several stages: heating,
primary melting, grain solution, fining and homogenization, and conditioning, all of
which require very close control.
The process includes a variety of transfer modes: fluid flow, heat transfer, and mass
transfer. Before the batch totally melts, it undergoes a number of processes, such as
drying, removing chemically bonded water, hydrothermal reactions, crystalline
inversions, and solid-state reactions. Preheating raises the batch as rapidly as possible to
the melting temperature where significant reactions occur that generally result in a
distinct change in the flow characteristics of the batch.
Some of the raw materials with lower melting temperatures or fluxes begin to melt first
(1382–2192°F or 750–1200°C), then the sand dissolves into these melted fluxing agents.
The silica from the sand combines with the sodium oxide from the soda ash and with
other batch materials to form silicates. At the same time, large amounts of gases escape
through the decomposition of the hydrates, carbonates, nitrates, and sulfates, giving off
water, carbon dioxide, oxides of nitrogen, and oxides of sulfur. The glass melt finally
becomes transparent and the melting phase is completed.
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