Material: Recycled
Glass
Issue: Processing any type of glass
for recycling requires size reduction (crushing), an activity
that creates fine particles of glass that can become airborne
dust. All types of dust have come under increasing regulatory control
in the U.S. because of the potential health hazards they
pose toward workers (see Analysis of Glass Dusts Best Practice). Controlling dust in an industrial
processing facility can be a costly proposition. Knowledge of effective dust control strategies
for glass processing facilities is essential to create
a safe working environment and to mitigate liability concerns.
Best
Practice: In
designing a dust control system for a glass processing
facility, it is important to consider the entire processing
system layout. This includes everything from where material
is received, to where finished product is loaded for shipment. One design strategy to minimize airborne dust
generation involves enclosing equipment at any point where
dust is likely to be created, particularly where the glass
drops from one material handling device onto another,
such as the discharge from the crushing mechanism, conveyors,
and screens. Most
material handling equipment is now available with enclosures
from manufacturers. These products are a recent development by
equipment manufacturers in response to growing health
concerns related to airborne dusts.
In designing enclosures on equipment such as conveyor
belts, access to the processing lines should be considered,
particularly at points where blockage of material flow
may occur, or where maintenance may be required.
A complete dust control system must include negative
pressure ducting to all processing system enclosures. The ducting would go first to a “drop-out” box, or place where the
airflow slows enough to allow large particles and pieces
of paper to settle. Then
the airflow would go into a cyclone, where the coarser
airborne particles would be removed, then to a baghouse
for the finest dust.
Finally, the dust-free air can be discharged through
a fan.
Care should be taken to assure that airflow systems
are designed to pull air, never push it.
Ideally, the fan should be pulling from the baghouse
exit. This is important for two reasons: (1) positive
pressure ducting will leak air (and dust) through the
seams, and (2) if the fan is in the glass dust-bearing
air stream, fan blades will wear out quickly from abrasion.
Baghouses are a common type of final dust control used
with airborne dust in industrial settings. Most baghouses are preceded by cyclones that remove coarser particles
from the airflow prior to the baghouse, increasing the
time between bag cleanings.
The bags are typically supported by metal cages,
where the dust enters from outside the bag, with clean
air exiting from the inside through a plenum. The bags can be cleaned by a short burst or pulse of clean gas injected
into the top of the bag, instantaneously reversing the
flow of air through the bag, thereby removing fine particles
trapped between the fibers. Recent technology developments in baghouses include the use of smaller
bags and improved filter materials that allow greater
airflow, facilitate cleaning, and require less horsepower
to operate.
The configuration described above creates three potential
products for a glass processing facility: a somewhat contaminated
fine-grained aggregate, and two grades of clean glass
dust. Markets can then be investigated for the dust.
Water is perhaps the simplest and most intuitive
tool that can be used in controlling dust. A fine mist of water applied to glass before
processing reduces dust to undetectable levels. However, moisture creates a number of other issues in handling glass
for recycling, including difficulties in screening, storing,
and bagging. Also,
moist dust can clog baghouses.
Adding moisture can be an effective means of dust
control for coarse crushing and screening systems, but
water atomizers should only be used strategically before
dryers in higher quality processing systems.
Implementation: During system design is the
best time to consider integrating dust control into the
processing system. There are many excellent manufacturers of dust
control equipment listed in the Thomas
Register. For
small-scale, budget operations, a negative-pressure system
can be built using several barrels for drop-out boxes,
hoses for ducting, filter bags, and a fan.
Benefits:
Effective
dust control has the direct benefit to processors of mitigating
environmental liabilities.
This relates to both worker health issues and air
emission compliance under regulations from air pollution
control authorities.
An additional benefit may be found by recovering
fines from baghouse collection for industrial mineral
applications.
Application
Sites: Glass
Processing Facilities, Material recovery facilities.
Contact: For more information about
this Best Practice, contact CWC, mailto:info@cwc.org.
References:
Freas, Don, TriVitro Corporation.
Industrial Exposure and Control Technologies for OSHA Regulated Hazardous
Substances, Occupational Safety and Health Administration, March 1989.
Product Literature, Aeropulse Incorporated, Bensalem, PA 19020, 1996.
Respiratory Health Aspects
of Ground Glass vs. Ground Silica, ReTAP Fact Sheet GL-94-1, Clean
Washington Center, 1995.
Issue
Date / Update: November 1996
|
