Material: Recycled
Glass Issue: Achieving efficient recovery of post-consumer glass depends greatly on
maintaining glass quality all the way from collection through
processing. The primary market for post-consumer recycled
glass in most parts of the country is the manufacture of new
containers. Specifications
for glass container manufacturing require that glass be beneficiated,
a process of size reduction and contaminant removal from color
sorted cullet (see Cullet Specifications for Container Manufacturing
Best Practice). Breakage during handling
at material recovery facilities (MRFs) decreases the ability
to effectively identify and remove contaminants.
Breakage also reduces the ability to sort for color,
which works most effectively with glass greater than 2 inches
square. Sorting and removing contaminants are also important to other applications
besides container manufacturing, such as fiberglass insulation
or niche product applications. Best Practice: To reduce breakage and increase the value of collected
glass, it is important to note that breakage occurs at several
points in the collection and processing of post-consumer glass
containers, and varies depending on whether glass has been
color sorted prior to arriving at the MRF.
For information on controlling breakage in collection,
see Controlling Breakage in the Collection
of Recycled Glass Best Practice. This best practice addresses strategies for
reducing breakage within material recovery facilities. Major points of breakage after collection include
“tipping” activity, where glass containers are unloaded at
the MRF, at feeding and conveying points within processing
lines, and whenever containers are handled with heavy equipment.
Studies have shown that modifications
in glass handling at recovery facilities can reduce breakage
rates, lowering production of mixed residuals.
The modifications include changes in design such as
lowered tipping heights, the addition of deflection ramps,
and the installation of rubber baffles at impact points.
Procedural changes that may be implemented include
tipping and handling speeds, compaction strategies, and loader
operation. Minimization
of total handling steps may be the most important strategy
for achieving efficient recovery of glass containers. It is important to note that many programs use transfer stations
distinct from MRFs, meaning additional loading and transportation
steps for recovered containers.
The practices discussed here apply to such transfer
stations as well. With a large percentage of
glass being broken at the unloading stage, and as the material
moves through the line dropping from one level to another,
simple and inexpensive means of reducing impact forces are
critical. Equipment
additions or modifications including deflection ramps, drop
chutes, and rubber baffles are all designed to reduce impact
forces. Deflection ramps placed below tipping points
effectively reduce drop distances and can be faced with abrasive-resistant,
shock-absorbent plastic to further reduce impact. Drop chutes also reduce drop distances and encourage containers
to “roll” rather than break on each other, and can be implemented
at infeed hoppers or conveyor ends.
Ramps and chutes should be angled between 30 and 45
degrees to achieve impact reduction with enough slope to prevent
hang-ups.
Rubber or synthetic baffles
represent one of the most effective and least expensive methods
of reducing glass breakage.
Baffles reduce impact speed and deflect drop angles,
and can be installed at nearly any impact point within a processing
line. Such points
include between the in-feed conveyor and trommel or bar screens,
and any other conveyor interface.
Baffles should be made of durable abrasive resistant
material. Success
has also been achieved with bumper guards installed on steel
shaker tables and conveyors to soften the impact of containers
on the steel edges. Procedural changes include
using greater care in handling, including slow dumping and
gentle in-feed loading by both manual handlers and heavy equipment
such as end-loaders. The recommendations for loaders focus on operating
speeds, encouraging operators to slow loading or “scooping”
and avoiding running over the piles.
While seemingly contrary to throughput objectives,
studies suggest that a small reduction in speed can yield
a net increase in recovered material throughput, by reducing
breakage. Additionally,
some transfer station programs intentionally compact material
for volume reduction. Given
the weight of glass containers, transportation efficiencies
from volume reduction are limited and not likely to offset
material loss. Procedural modifications require worker education
regarding the need to reduce breakage and their role in improving
efficiency. Implementation: Materials recovery facilities
sometimes get so consumed with increasing production rates
that they give little attention to quality.
An analysis of handling procedures to minimize breakage
should be part of a total quality management program, ideally
including statistical process control, to optimize facility
revenue from recovered materials. Benefits: Modification of glass handling practices in material
recovery facilities can significantly increase marketable
recovery rates for collected glass containers.
Reduction in breakage means lower production of unsortable
mixed color glass. Material that cannot be sorted is called
residual and must be removed in processing and handled for
disposal, or channeled to lower value applications. Inexpensive design and procedure modifications
can boost program revenues from the sale of cullet to manufacturers.
A study by SWANA (1) showed that programs have been able to
reduce breakage by as much as 33%. Application Sites: Material recovery facilities, glass processors,
transfer stations. Contact: For
more information about this Best Practice, contact CWC, mailto:info@cwc.org. References: Glass Container Market Recovery Study, prepared by SWANA, with the EPA
and GPI, Dec. 1995. “Glass Composition and Breakage
in a Commingled System for Curbside Handling,” Consumers Glass. “Optimizing the Collection
of Glass Containers,” Shaan Kervis Hamilton, Resource Recycling,
July 1995. Paradiso, Joe,
Consumers Glass,
Toronto, Ontario, Canada. Reducing Breakage of Container Glass in Commingled Curbside Recycling
Programs: Strategies for Collection, Sorting, and Processing
Systems, Rhode Island Solid Waste Management
Corporation, and the Rhode Island Department of Environment
Management, October 1991. “What Happens to Glass After
it’s Collected?” Joe Paradiso, Resource Recycling, July 1995,
pp.70-75. Issue
Date / Update: November 1996
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