Material: Recycled Glass Issue: Fiberglass insulation manufacturing has grown to be one of the largest
uses for recycled glass. Because fiberglass does
not have glass’s clarity, there is an assumption
among some that fiberglass raw materials can tolerate
higher levels of contaminants than container manufacturing. In fact, metal, organic, and ceramic contaminants
can be costly to fiberglass quality and production
equipment. Best Practice: During fiberglass production,
raw batch materials and glass cullet are continuously
added to the furnace.
After melting and “fining,” the molten glass
is spun into fibers by a process called fiberizing.
To ensure production of consistent fibers, cullet
must meet specifications for major and minor oxide
chemical composition, color consistency, and contaminant
levels.
Major and Minor Oxide Chemical Composition. The chemical composition of cullet as raw batch material for fiberglass
should fall within the ranges listed in the table. Variations
in major oxide concentration can affect both the
viscosity and emissivity of the glass melt.
Viscosity is a measure of flow resistance,
while emissivity is the ability of a material to
emit heat through radiation.
Both of these properties affect the attenuation
ability of the molten glass during fiber formation.
In addition, a shift in a major oxide concentration
can cause fluctuations in the electrical resistivity
of the melt. This
requires an adjustment of transformer settings controlling
the power input to the furnace, as well as creating
changes in heat flow patterns, current paths, and
temperature distribution, all of which are undesirable
during stable furnace operations.
Minor and trace oxide composition of the
batch cullet is also very important. Oxides such as CoO, Cr2O, and FeO
can influence infrared heat transmission, heat transfer
during melting, and rate of fiber cooling.
These factors will affect fiber attenuation,
leading to variability in the fiber diameters and
lengths. Color Consistency. Each of the color groups of container glass - flint (clear), green,
and amber - is characterized by a different oxidation
state. Flint is highly oxidized, green varies from
slightly oxidized to slightly reduced, and amber
is highly reduced. Therefore, color changes in the cullet supply
can shift the bulk glass melt oxidation state in
the furnace. The melting process can operate within a wide
range of oxidation states, but stability within
that range is absolutely necessary.
A sudden shift in the oxidation state will
result in mild to serious furnace upset because
the solubility of SO2 gas in the glass
melt varies with the oxidation state.
In general, SO2 is highly soluble
in an well-oxidized or well-reduced melt. When the oxidation state shifts and SO2
is released, a glassy foam forms on the surface
of the melt, insulating the reaction zone of the
batch material from the heat of the molten glass
beneath. More
power must then be applied to the furnace to melt
the incoming batch material, which will otherwise
form a crust on the surface of the melt.
Increasing the temperature of the melt changes
the viscosity of the glass, and the rate of fiberization
must be adjusted. Metal Contamination. Metal contaminants found in cullet feedstock are not oxidized in
the fiberglass furnace, and thus will not dissolve. These contaminants instead sink and form pools of molten metal on
the furnace floor, causing corrosion and subsequent
glass leaks, shortening the life of the furnace.
Or, the metals can work their way through
the furnace refractory, contact support steel, and
cause a ground, shutting off the furnace power and
threatening worker safety. Organics. Cullet
feedstock used for fiberglass raw batch material
should also be free of organics including paper
labels, plastic caps, or even wood chips. The presence of organic material can affect
the oxidation state of the melt much like a change
in the color-mix. Ceramics. Cullet feedstock must be free
of coarse ceramic contaminants, or should be fine-sized
prior to melting. Ceramic contaminants in cullet feedstock include
fragments of dishware (saucers, cups) cookware (Visionware,
Pyrex), as well as bricks, rock, and concrete.
Ceramic particles larger than No. 12 mesh
(1.7mm) do not melt in the fiberglass furnace, and
clog the furnace spinners. Spinners are rotating, flat-bottomed bowls with perforated sidewalls
that are used to produce the glass fibers during
fiberization. Implementation: Cullet feedstock processing
should include color sorting, removal of metals
and organics, and removal or fine-sizing of coarse
ceramics. For
information on processing steps, refer to the Best
Practices Typical Contaminants in
Recycled Glass, Automated
Color-Sorting of Recycled Glass, Magnetic Separation Technologies,
Non-Ferrous
Separation Technologies, Removal of Ceramics From
Recycled Glass, and Fine-Sizing of Recycled
Glass.
To maintain chemical consistency in the cullet
feedstock and glass melt, the fiberglass manufacturer
or MRF should periodically complete a comprehensive
chemical analysis for representative cullet samples.
Acceptance standards of individual fiberglass
manufacturers for chemical consistency, color distribution,
and contaminant levels should be considered prior
to cullet preparation and delivery. Benefits: Ensuring
the chemical and color-mix consistency of the batch
material optimizes the glass melting and fiberizing
stages of fiberglass production. In addition, eliminating cullet contaminants
will help minimize damage to the furnace and fiberizing
equipment. Application
Sites: Fiberglass
manufacturing facilities, material recovery facilities. Contact: For more information about this Best Practice, contact
CWC, mailto:info@cwc.org
References: Apotheker,
S., “Fiberglass Manufacturers Revisit Cullet,” Resource Recycling,
June 1980, pp. 22-24. Methods for Sampling and Testing
Glass Cullet, ReTAP, Clean Washington Center, 1996. Guter,
E., “Quality
Cullet is Required for Fiberglass, Too,”
Glass Industry, January 1996, pp. 13-35. Guter, Ernest,
Owens-Corning Science and Technology Center, Granville,
OH. Purchase
Acceptance Standards, 1990, Owens-Corning Fiberglas
Corporation, No. CRM.59.03.312. Issue Date / Update: November 1996
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