Material: Recycled
Glass
Issue: Recycled glass generally contains
two types of moisture.
First, some containers have residual moisture from
the original food content. Second, piles of glass stored outdoors retain
some amount of dew and rainwater.
When cullet is either too wet or too dry, processing
and shipment can be difficult.
Therefore, moisture control strategies may be necessary.
Best
Practice: Depending
on the gradation, glass cullet is free-draining compared
with most natural aggregates.
When cullet is used as construction aggregate, its
free-draining characteristics result in a relative insensitivity
to moisture content during compaction. However, the moisture content of glass cullet can affect its processing
efficiency, ease and cost of distribution, and remelting
characteristics. This
best practice illustrates the effects of moisture on processing
and shipping. These considerations can impact the operation
at both the materials recovery facility (MRF) which generates
the cullet, and the end-user.
For additional information, refer to the following
Best Practices: Fine-sizing of Recycled
Glass; Selecting Recycled Glass
Crushing Equipment; Dust Control with Glass
Aggregate at Construction Sites; and Dust Control
Strategies for Processing.
Glass bottles and containers typically contain moisture
from food residue and exposure to precipitation during collection
or storage when they arrive at the MRF.
As the moisture content in cullet increases, the
surface tension at the particle interface increases, and
the apparent cohesion within the cullet mass increases.
This increase in apparent cohesion will cause the
cullet particles to clump together and adhere to processing
machinery. Because
glass cullet is abrasive due to its highly angular particle
shape, this adhesion to machinery can result in unexpected
machine wear.
High moisture content can also cause the cullet to
clog screening devices used for sizing and contaminant removal,
as well as baghouses used for dust control.
Reducing the particle size of the cullet increases
the overall surface area of the cullet, which increases
the surface tension and results in greater apparent cohesion.
Therefore, excess moisture is especially detrimental
to glass fine-sizing operations that produce a cullet equivalent
to a No. 12 mesh or smaller. MRF’s using fine-sizing equipment may find it necessary to check
the moisture content of incoming cullet.
If the moisture is too high, the facility may need
to dry the cullet before fine sizing.
To avoid excessive moisture content, stockpiles of
glass should be covered or stored inside prior to and during
cullet processing. This
is especially necessary in wet climates or during periods
of wet weather. In cold weather, moisture content in glass
piles can result in piles of solid, frozen glass.
The moisture content of glass cullet should also be
controlled prior to and during shipment.
Wet cullet fractions smaller than ½-inch can be difficult
to transport because they do not flow freely during loading
and unloading. Trucks that have been loaded with glass containing
too much moisture have been fined for leaking organics-contaminated
water. In addition,
finely graded cullet can retain up to 15% by weight of moisture,
adding a significant cost to the user who pays by the pound.
Cullet containing too much moisture has been known
to clog the hoppers of freight cars, making removal very
difficult and costly. Vibration during shipment can cause the wet cullet to densify, resulting
in increasing handling time and difficulty. (The moisture facilitates the densification by serving as a lubricant,
helping the relative movement of the particles in becoming
a denser material.) Cullet that is free of excess moisture
is otherwise easy to transport, even in subzero temperatures.
Cullet that is too dry can also be difficult to handle,
as dry cullet will have minimal apparent cohesion. The material can flow relatively easily and
become difficult to stockpile.
Dry cullet may also have dust control problems.
In these instances it may be necessary to add moisture to the cullet. The
dust can also be controlled by the installation of baghouses,
or enclosures around glass processing equipment.
In general, fine-sized cullet powder should have
a moisture content of at least 0.2 to 0.3% to inhibit dust.
See the Dust
Control with Glass Aggregate at Construction Sites Best
Practice.
Implementation: Over the years, glass processors
have learned strategies for dealing with moisture in glass. Today, however, there are an increasing number
of processors of glass for alternative uses and an evolution
of existing processors toward fine grind cullet.
Both of these developments require a knowledge of
moisture issues in glass processing.
Benefits:
This information will help facilitate cullet
processing, and improve the quality of the material supplied
to end-users.
Application
Sites:
Recyclers,
glass container manufacturers, construction sites.
Contact: For more information about this Best Practice, contact
CWC, mailto:info@cwc.org.
References:
Cadwalader, Kevin, REMco (Rock Engineered Machinery Co.) Inc., Livermore,
CA.
Kneas, Don, National Association
of Plastic Container Recovery (NAPCOR),
Seattle, WA, telephone conversation, 10/14/96.
Zimmerman, Don, Recycle America, Seattle, WA.
Issue
Date / Update: November 1996
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