Typical
Contaminants in Recycled Glass
Material: Recycled Glass
Issue: This Best Practice summarizes contaminants found in
cullet feedstock and describes contaminant tolerances in some applications.
Best Practice: Contaminants originating at the point of collection
or introduced into the cullet stream during shipment and processing
can generally be separated into three categories:
Ceramics Ceramic contamination is a broad category including dishware,
porcelain caps, pottery, heat resistant cookware (pyrex™, visionware™),
mirror glass, laboratory glass, light bulbs, crystal, window glass,
bricks, concrete, as well as stones and dirt. Ceramics can be removed
manually or with automated systems. For container and fiberglass manufacturers,
the cullet supply can be sufficiently reduced in size so that ceramic
contaminants in small amounts do not adversely impact the manufacturing
process.
Metal Metal contaminants are generally in the form of container lids
or seals. Typical ferrous metals include iron and steel, which are
magnetic, and can be removed through magnetic separation techniques.
Non-ferrous metal contamination includes brass, aluminum, lead, and
stainless steel. This material is not magnetic, and must be removed
with non-ferrous electrical detection/physical removal separation
strategies.
Organics Organic contamination includes paper and plastic labels,
plastic caps, cork, paper bags, wood debris, plants, food residue
(e.g., sugar), and any other combustible or degradable material. Organic
material can be removed by washing or passing the cullet through a
size-specific screening device.
One good characteristic of both metal and organic contaminants is
that they are less friable than glass. Therefore, they do not fracture
as easily in glass crushers and can be removed with a properly sized
screen.
Hazardous or Toxic There should be NO hazardous or toxic materials
in recycled glass supplies. It is the responsibility of the collector
to insure that such materials are kept out of the system.
The following is a list of cullet applications and their specifications
for contaminant removal:
Glass Container Manufacturing and Fiberglass Insulation Manufacturing
Glass container manufacturers and fiberglass manufacturers require
batch cullet to be free of coarse ceramics, and ferrous and non-ferrous
metals. Ceramic contaminants larger than No. 12 mesh typically do
not melt in the furnace, which can result in ceramic inclusions in
the finished glass containers and clogged fiber forming equipment.
Ferrous and non-ferrous metals melt but do not dissolve, and can cause
corrosion damage to the furnace and glass forming equipment. Too much
organic material in the cullet can affect the oxidation state of the
melt, requiring modifications in temperature control.
Glassphalt Cullet used to supplement natural aggregate in asphalt
must be free of organic material prior to mixing. The decay of these
organics can result in odor and glasphalt settlement problems.
Portland Cement Concrete When used as aggregate in non-structural
Portland Cement Concrete (PCC), cullet should be cleaned of all organic
residue, particularly sugar. When mixed in PCC, sugar causes an increase
in setting time, and a decrease in the ultimate strength of the concrete.
Slow Rate Water Filter Medium Cullet should be free of organics, which
can cause excessive effluent turbidity. Ferrous and some non-ferrous
(e.g. lead) metals are detrimental to effluent quality.
Construction Applications Construction-related cullet applications
can include general backfill, roadways, utility backfill, as well
as landfill cover and underground storage tank backfill. Cullet used
for these applications should generally have no more than 5% to 10%
debris as determined using a visual inspection method. The decay of
these organics can cause potential settlement of the engineered fill.
The following table summarizes the impact of contaminants on the various
cullet applications.
Notes on Contaminants:
X : May have an adverse impact in generally expected amounts
O : May not have an adverse impact in generally expected amounts
Many applications and contaminant removal strategies described above
are detailed in other Best Practices.
Implementation: Cullet users should carefully evaluate the
potential effects of each type of contaminant on the intended application,
and develop specifications for contaminant tolerance and removal.
Benefits: Awareness of typical recycled glass contaminants
will help end-users develop application-specific standards for cullet
grades. Understanding these standards will enable material recycling
facilities to implement processing strategies for cullet production.
Application Sites: Glass processors, glass end-users.
Contact: for more information about this Best Practice, contact
the CWC info@cwc.org.
References:
(1) Don Zimmerman, Recycle America, 7901 First Avenue South, Seattle,
WA, telecon, 10/14/96.
(2) Hank Strom, BFI, 1533 120th Avenue NE, Bellevue, WA, telecon,
10/10/96.
(3) Day, D.E., and Schaffer, R., Glasphalt Paving Handbook, University
of Missouri-Rolla, p. 53.
(4) Guter, E., Quality Cullet is Required for Fiberglass, Too, Glass
Industry, January, pp.13-35, 1996.
(5) Evaluation of Crushed Recycled
Glass as a Filtration Medium in Slow Rate Sand Filtration, 1995,
prepared for The Clean Washington Center by Gray & Osborne, Inc.,
Seattle WA.
(6) 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.
(7) Glass Feedstock Evaluation Project: Engineering Suitability Evaluation.
Report GL-93-3 Evaluation of Cullet As A Construction Aggregate, Report
GL-93-3, Clean Washington Center, 1994.
Issue Date / Update: January 1997
