Material: Recycled Glass Issue: Screening equipment can play an important role in contaminant removal
and aggregate sizing (see Contaminants in Recycled
Glass and Fine Sizing of Crushed Glass
Best Practices). Understanding screening technology
is a critical part of glass processing system design. Best Practice: There are two principal types
of screens that have been used to effectively process recycled
glass. Trommel screens are rotary cylindrical screens used
mainly for coarse sizing and contaminant removal. Vibratory
screens are flat or inclined surfaces that separate material
by vibration. Vibratory screens are especially effective
with finely crushed glass to meet tight sizing requirements.
Both types of screens use gravity to achieve separation
by passing material over a surface with calibrated openings. Trommel screens use either wire mesh or perforated plates as
the screening surface.
Trommels are usually inclined to facilitate the flow
of glass from one end to the other.
The rotation means the screening surface changes
continuously. A
glass particle will fall through the screen if it is smaller
than the screen openings.
Typically used in high volume processing applications,
trommels are especially durable. They are often used to remove contaminants
such as pieces of caps and labels.
Because pieces of paper and metal are less friable
than glass, they generally remain in larger pieces after
passing through a crusher. Therefore, if properly sized, most glass falls
through the screen while most contaminants pass through
the trommel and out the end into a disposal bin. Trommels are
used with a wide range of commodities.
They typically range in size from 2 to 10 feet in
diameter and 4 to 50 feet in length.
Internal flights can be incorporated to prevent sliding
of the material over the screen. They are similar in configuration to drum dryers,
and given a heat source, can serve multiple functions. Two-stage trommels use two screens in sequence
to achieve differential sizing. Trommel screens
are less prone to plugging with wet material than vibratory
screens because most wet material will fall off of the screen
when it turns upside down.
Trommels are sometimes equipped with brushes to clean
the screen openings at the top of the arc, or air jets to
blow them out. Vibratory screens can employ perforated plates, but predominantly
use wire mesh of various types to separate materials based
on size and density. In
screening granular materials, the vibratory amplitude (measure
of vertical height of vibration wave) is an important element
in facilitating the flow of materials through the screen.
For vibratory screens to work effectively with finely
crushed glass, it is necessary that the glass be sufficiently
dry to prevent clumping of material which will yield false
sizing and may clog screens (see Drying Glass for Fine Sizing Best Practice). It is important
to match physical screen design to the characteristics of
glass and desired size objectives in order to assure efficient
operation. Glass fractures differently from other types
of granular material, and typically has very square edges,
even in very fine sizes, emphasizing the need to configure
screens to prevent blinding (particles obstructing the mesh).
Vibratory screens with drive mechanisms that provide
high speed elliptical vibration have been found to perform
well with crushed glass, because they keep loads “lively,”
enhancing the separation speed and preventing clogging. Screens that provide reversible directions of
rotation can be useful in handling glass because the direction
of rotation affects the lateral rate of flow across the
screen. Vibratory screens
come in several configurations, including rectangular and
round configurations. With
inclined rectangular deck vibratory screens, material flows
from top to bottom of the screen, with oversize material
falling off the end. With round screens, material flows in a spiral pattern, with the
oversize passing out through a pipe on the side. With little or no incline to aid flow, round screens size more accurately
but also are more prone to plugging with wet material. Multiple screen
decks are used to achieve differential sizing. In multi-deck units, the largest screen opening size is on top.
The vertical stratification allows any number of
sizes of granular material to be produced at once. Screen tension is important to maintain operational
efficiency. End-tension
screens allow quick adjustments and permit the use of square-opening
and harp-type screens.
Some models automatically cycle the screen tension
to allow the screen to flex and clean itself.
Bouncing ball decks can also be employed for continuous
cleaning. The finer
the desired particle size, the lower amplitude and higher
frequency of vibration required. Implementation: When choosing any screen, it
is important to consider volumes, relative density, moisture
contents, and desired sizing.
Trommel screens can be configured to fit the characteristics
of incoming materials by selecting the screen surface type,
size and number of openings, rotational speed of the screen,
length, and inclination.
Trommels are recommended for coarse sizing and contaminant
removal, and may be used prior to final size reduction.
With a heat source, trommels may also be used for
drying. Vibratory screens are most useful for fine
sizing. The highly
abrasive nature of glass dictates the selection of screening
mesh for durability. Generation of fugitive dust from screens should
be considered in system design (see Dust Control Strategies for Glass Processing Best Practice). Most screens are available in enclosed models
with air discharge ports attached to a baghouse for dust
control. Benefits: Mechanical screening devices
offer potential operational efficiencies that cannot be
achieved through manual sorting.
Effective sizing and contaminant removal can dramatically
improve the economic viability of any glass recycling facility.
Accurate sizing also allows access to new applications
for recycled glass, including 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: Andela, Cynthia, Andela
Tool & Machine, Richfield Springs, NY Collins, David or Hess, Larry,
Thurlow-Collins, Inc., Edmonds, WA FMC Corporation, Mark Virus, Homer City, PA 15748, phone conversation October 1996. Product Literature - Andela Tool & Machine, 1996 Product Literature - Midwestern Industries, Inc,
1996. Issue Date / Update: November 1996
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