Material: Recycled
Glass
Issue: Glass container manufacturers
are using increasing quantities of recycled glass in the manufacture
of new containers. Increasing
percentages of recycled glass bring increasing amounts of contaminants.
Ceramic contaminants in recycled glass include fragments of
dishware (saucers, cups) cookware (Visionware, Pyrex), mirror
glass, window glass, as well as bricks, rock, and concrete.
Ceramic contaminants are more difficult to detect and
remove than ferrous or non-ferrous metals or organics because
they cannot be detected electro-magnetically.
This best practice details the protocol and limitations
of the methods for ceramic removal.
Best
Practice: Although
small chips of many ceramic contaminants will actually melt
with the glass at glass furnace temperatures, ceramic contaminants
larger than No. 12 mesh do not melt during the standard furnace
cycle (1 to 1.5 days), resulting in inclusions in finished glass
products. These inclusions
can lead to eventual breakage of the glass product, especially
if the product is filled with a pressurized liquid like soda
or beer. To avoid these impurities, the cullet supply
can be fine-sized to No. 12 mesh or smaller (See the Fine-sizing of Recycled Glass Best Practice),
or the ceramic contaminants can be physically removed, or both. The methods of ceramic removal include source
reduction, manual sorting, and automated ceramic removal.
Source Reduction The most efficient means of avoiding ceramic contamination of recycled
glass supplies is through increased public awareness. The Glass Packaging Institute has been proactive
for many years, working to educate the public on which types
of glass can be recycled into new containers.
However, a certain amount of ceramics is inevitably collected. Consequently, MRFs need to spend resources
to remove these contaminants in-house.
The removal of ceramics generally precedes the color-sorting
stage in the processing of recycled glass.
Manual Ceramic
Removal In-house manual extraction of ceramics typically
begins with the removal of all 2-inch minus material, which
is too small to be sorted by hand.
This can be accomplished by passing the glass over a
two-inch screen. The two-inch minus fraction that passes through
the screen may be used for alternative applications such as
construction aggregate. (Additional
crushing will be required for the production of cullet aggregate,
which typically has a maximum particle size of 3/4 to one inch.) The glass remaining on the screen is then fed
on a conveyer past a number of plant personnel, who are trained
to identify the various types of ceramics.
These personnel will follow either a negative sorting
protocol, which removes the undesirable material (ceramics),
or a positive sorting protocol, which removes and saves the
glass. These personnel
may also be responsible for removing other debris, such as labels,
caps, plastic, metal, etc.
Speed of processing depends mainly on the number and
efficiency of the personnel used. Automated Ceramic
Removal Automated ceramics removal generally
requires that glass be crushed and screened to a specific size,
typically 3/8-inch minus. The
resulting glass fraction must be vacuumed and then fed into
an automatic ceramic removal unit by a vibrating conveyer belt,
which keeps the glass in a thin layer. As it enters the unit, the cullet passes over
a plate embedded with fiber optic cables.
A fast pulsing light (usually visible light) is projected
through the glass to the fiber optic cables, which detect the
position of any opaque material.
The unit then directs one of a series of “air knives”
to remove the opaque material with a burst of air. Machines of this type can screen up to 20 tons
of cullet per hour. However,
this equipment is relatively expensive, and typically cannot
distinguish cullet from clear cookware, mirrors, or window glass,
which must be removed by hand.
Implementation: Increased public awareness
of ceramic contaminants is the first step and most efficient
way to minimize the degree of screening necessary for recycled
glass supplies. Technology is improving constantly to remove
the contaminants that inevitably get into the recycled glass
stream.
Benefits:
This
information helps illustrate the problem of ceramic contamination
faced by the recycling industry.
Application
Sites: Glass
bottle manufacturers, material recovery facilities.
Contact: For more information about
this Best Practice, contact CWC, mailto:info@cwc.org.
References: Bickman, Josh, Magnetic Separation Systems (MSS), Inc.,
Nashville, TN
Matheson, Greg, BFI, Bellevue,
WA, telephone conversation, 10/11/96.
Strom, Hank, BFI, Bellevue,
WA, telephone conversation, 10/10/96.
Zimmerman, Don, Recycle America,
Seattle, WA, telephone conversation, 10/14/96.
Issue
Date / Update: November 1996
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