Material: Recycled
Glass
Issue: Fine-sizing of crushed glass
below 1/8-inch typically requires that the glass be dried prior
to the use of sizing screens.
Drying is especially important when seeking a tight gradation
of finer material from No. 16 mesh down to No. 200 mesh.
Moisture content in post-consumer container glass can cause
the glass to clump together and adhere to screening equipment,
inhibiting proper screening, and resulting in glass that is out
of size specifications for the application. Understanding efficient glass drying technologies
and protocols is essential in meeting fine-sizing requirements
when processing recycled glass.
Best
Practice: Glass
bottles and containers typically contain moisture when they arrive
at the material recovery facility (MRF).
The moisture is from food residue and exposure to precipitation
during collection or stockpiling.
Although coarse glass generally drains and flows very easily,
with little apparent cohesion, reducing the particle size of the
cullet increases the overall surface area, resulting in much greater
apparent cohesion. It
may seem like a paradox that while coarse glass used as construction
aggregate has been shown to be very permeable, finely crushed
glass retains moisture extremely well.
This may be because the large particles of coarse glass
have no pockets to hold moisture, and drain freely.
In finer grades, however, the close packing of the particles
results in the surface tension of the water holding the particles
together in a similar manner to that seen when water is between
two sheets of glass. Increasing
the apparent cohesion causes the cullet particles to clump together
and adhere to processing machinery and sizing screens.
Piles of finely ground glass on impermeable surfaces have
been found to retain water literally for years.
Therefore, only movement and heat, not time, can dry finely-crushed
glass.
As explained above, excess moisture is especially detrimental
to glass fine-sizing operations.
This best practice illustrates some of the techniques used
to dry the glass during the fine-sizing process.
For additional information on fine-sizing, refer to the
Fine-sizing of Recycled Glass Best Practice.
The most common dryer is an inclined tumbling rotary unit, which in aggregate processing
can be larger than 30 feet long and 6 feet in diameter. Much smaller units have been built for small
volume glass processing. These
units are usually placed after a pre-crusher but before the pulverizing
stage in the glass processing scheme.
Material tumbles by gravity from one end to the other as
the unit spins. Heat is generated with a natural gas burner
and blown through the unit with a fan.
Information on appropriate flow rates and dryer temperatures
are best obtained from manufacturers or by performing trial runs. Dryers of this type can also be paired with
air cyclones to remove airborne dust during processing.
The biggest challenge with wet glass in a fine-sizing
operation is its tendency to clog the sizing screens. This phenomenon is called blinding. Some glass processors combine drying with screening.
When using a trommel screen, the trommel cylinder is usually
inclined to facilitate flow of the glass from one end of the screen
to the other (see the Screening Technologies for Recycled Glass Best Practice).
The heat source can be positioned at the low end of the
trommel with a fan to cause the heat to flow up the trommel screen
and dry the glass as it moves downward.
Heat sources can also be used
in conjunction with vibratory screens.
Vibratory screens using an elliptical vibration motion
have been found to be efficient for generating tight grades of
fine-sized glass. The elliptical vibration keeps the glass more
“lively,” enhancing screening speed and minimizing blinding. Two types of heat sources are common options
for vibratory screens. A
heat source with a fan can be positioned so that the heat is projected
up through the screen, drying and removing dust to a baghouse. Alternatively, vibratory screens are often available with heated
screens. Heated screens
are not as effective with very wet glass, but may not require
a bag house because they do not generate as much dust as a fan.
Generation of fugitive dust should always be considered
when adding dryers to the screening process (see Dust Control Strategies for Glass Processing Best
Practice). If possible, the screens and dryers should
be enclosed, which will also help conserve energy. The enclosed screens and heater may then be paired with a baghouse
or air cyclone to help remove airborne dust. When designing the
glass processing system, the end-user should also remember that
dryers will require additional space, energy, and costs.
To help minimize the need for dying, glass cullet stockpiles
should be stored inside or covered during wet weather. The glass should be protected from additional
moisture exposure during processing, especially if processing
occurs outside. Wet fine-sized
glass can also be difficult to transport, especially during loading
and unloading. Vibration during shipment can cause the wet
cullet to densify. The
moisture serves as a lubricant to help the relative movement of
the particles in becoming a denser material, causing clumping.
For additional moisture considerations in the processing
and shipment of cullet, refer to the Moisture Considerations in Processing and Distribution of
Glass Cullet Best Practice.
Implementation: Reducing glass moisture through
drying increases the production rate of glass fine-sizing operations
by reducing blinding. It
also helps the recycling facility meet the size specifications
for the intended application.
Benefits:
Reducing
glass moisture through drying increases the production rate of
glass fine-sizing operations by reducing blinding.
It also helps the recycling facility meet the size specifications
for the intended application.
Application
Sites: Glass
processing facilities
Contact: for more information about this Best Practice, contact
CWC mailto:info@cwc.org.
References:
(1)
Small-Scale Recycled Glass-to-fines Processing
System, Clean Washington Center, 1996.
(2)
Weiser,
S., 1995, Fine-Grind Technology,
Part 2: Results of Plant Production Trials Using Fine-Grind
Cullet, Ceramic Engineering Science Procedures, No. 16, Vol.
2, pp. 101-104. Freas, Don, TriVitro
Corporation, Seattle, WA Cadwalader, Kevin, REMco Inc., Livermoore, CA
Issue
Date / Update: November
1996
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