Fine-Sizing of Recycled Glass Material: Recycled
Glass
Issue: Ceramic contaminants in recycled
glass cullet can result in inclusions in finished glass products.
Ceramic inclusions can lead to eventual fracturing or breakage of
the glass container, especially if the container is filled with a
pressurized liquid like soda or beer.
Providing a cullet supply free of ceramic contaminants is generally
the responsibility of the materials recovery facility (MRF).
Many MRF’s physically remove ceramic contaminants from the
cullet, either manually or with automated systems.
An alternative to complete ceramic removal is to reduce the
entire cullet supply to No.12 mesh or smaller. Called “fine-sizing”,
this size reduction enables most ceramics to melt in the furnace.
This Best Practice describes some prevalent technologies for
fine-sizing cullet.
Best
Practice: This best practice considers
two approaches to fine-sizing glass cullet: vertical shaft grinding
and flexible impact pulverizing.
Moisture control is also discussed.
For more information on ceramic removal and moisture considerations,
refer to the Best Practices Removal of Ceramics From Recycled Glass and Moisture Considerations in Processing and Distribution of Glass
Cullet.
Vertical Shaft
Impact Grinders: A vertical shaft grinder (VSG) uses a multiport, cullet lined rotor
to fine-size cullet through autogenic particle-to-particle attrition. A steady stream of cullet is fed into the top
of a VSG system and accelerated through a turning rotor. The accelerated cullet stream is discharged
into a comminution (size reduction) chamber where particle-to-particle
bombardment reduces the cullet size.
This process continuously replenishes the cullet lining while
simultaneously fine-sizing the glass.
The resulting cullet is then removed through a high frequency
screening system. Coarse particles
that do not pass through the screen are automatically recirculated
to undergo further attrition. Studies
show that velocities in excess of 100m/s are required to produce a
glass powder smaller than No. 12 mesh by this method.
Machines of this type can be adjusted to produce cullet sizes
from No. 12 to No. 40 mesh, at rates as high as 100 metric tons per
hour. The VSG’s avoid much
of the wear caused by glass abrasion by using this “glass on glass”
attrition strategy.
While vertical shaft grinders provide effective size
reduction for most ceramics, some material may not disintegrate smaller
than No. 12 mesh. This is
because many ceramics (and other contaminants) may be lighter and
more ductile than glass cullet, and thus less susceptible to attrition. Unable to pass through the screening system, these contaminants
will slowly collect in the comminution chamber. VSG’s typically utilize
a periodic purging cycle designed to eject these coarse contaminants.
Flexible Impact
Pulverizers. Flexible impact pulverizers (FIP’s)
are similar in morphology to a hammermill crusher with free swinging
arms. These systems pulverize
glass through a combination of arm impact and particle-to-particle
attrition. The cullet is fed into the first of two contiguous,
internally joined barrel housings.
Within each housing is a belt driven horizontal shaft. Mounted on these shafts in a spiral pattern
are numerous flexible impactors, also called “hammer/link assemblies.” The shafts, hammer/link assemblies, and housing
liners are generally constructed of abrasion resistant steel to reduce
machinery wear and tear. While
the shafts spin, the cullet is fed through the chambers and reduced
in size. The spiral arrangement of the hammers on the
shafts drives the cullet through the chambers.
The resulting fine-sized cullet is then conveyed to a rotating trommel separator
which removes any coarse material.
As FIP’s do not have pinch points through which all material
must pass, ceramics and other contaminants which aremore ductile than
cullet may not break down, and can be removed by the trommel.
Machines of this type can typically produce a 3/8-inch to a
No. 12 mesh minus cullet fraction.
Processing rates vary from 1 to 20 metric tons of cullet per
hour, depending on the size of the FIP.
Moisture Field studies have shown that wet fine-sized cullet can be difficult
to process and handle. With a rise in moisture, the surface tension
at the cullet particle interface increases, thereby increasing its
apparent cohesion. An increase
in the apparent cohesion causes the cullet particles to clump together,
as well as adhere to processing machinery.
Wet fine-sized cullet is notorious for clogging up screening
equipment. MRF’s using fine-sizing equipment may find
it necessary to check the moisture content of the incoming cullet. If the moisture is too high, the facility may
then need to dry the cullet, possibly with a rotary drier, before
fine-sizing. In wet climates
or periods of wet weather, stockpiles of glass should be covered or
stored inside prior to and during fine-sizing to protect from additional
weather exposure. The resulting fine-sized cullet should also
be stored and transported under dry conditions.
It should be noted that fine-sized cullet which is
too dry can also be difficult to handle, and may result in dust control
problems. In these instances
it may be necessary to add
moisture to the cullet. This
can be done with a hose or with storage facility sprinklers. In general,
fine-sized cullet should have a moisture content of at least 0.2 to
0.3% to inhibit dusting. Cullet dust can also be controlled by the installation
of baghouses, or enclosures around glass processing equipment.
Limiting the fine-sized cullet to less than 10% by weight material
finer than No. 140 mesh will also help mitigate the dust problem.
Implementation: Fiberglass plants and an increasing
number of container manufacturing plants use fine-sized material in
their furnaces. In addition,
increasing quantities of glass are crossing state borders and traveling
longer distances to be recycled. It is important for recyclers to
understand the needs and quality requirements of container manufacturers
and fiberglass plants.
Benefits: Size reduction strategies are important for both conventional
and innovative uses for recycled glass. Any business considering processing glass needs to be aware of the
existing technologies.
Application
Sites: Glass
bottle manufacturers, material recovery facilities.
Contact: for more information about this Best Practice, contact
CWC mailto:info@cwc.org. References:
(2)
Zippe, B, and Drescher, H.,
1996, Advances in Cullet Treatment
Technology, Glass Machinery Plants & Accessories, Information
Service, No. 25, pp. 67-71;
(3)
Rodriguez, D., Application of Differential Grinding For Fine
Cullet Production and Contaminant Removal, Ceramic Engineering Science
Procedures, No.16, Vol. 2, pp. 96-100, 1995;
(4)
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; (4) How Cullet
Can Be Used Effectively As a Raw Material, Glass Industry, December, 1994, pp. 27-28. Cadwalader, Kevin, REMco Inc., Livermoore, CA Andela, Cynthia, Andela Tool and Machine, Richfield Springs,
NY Issue Date / Update: November 1996
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