Ferrous Metals Contaminant Removal Material: Recycled
Glass
Issue: Ferrous metal contaminants
are common in post-consumer glass waste streams. However, end-use
specifications frequently call for near zero levels of ferrous metals
(see Specifications for Container Manufacturing
and Specifications for Fiberglass Manufacturing
Best Practices). Ferrous metals can cause damage
to glass furnaces and production equipment.
Also, many alternative uses for recycled glass cannot tolerate
ferrous metals because of chemical incompatibilities or because the
ferrous metals will oxidize over time.
In addition to these manufacturing problems, larger pieces
of ferrous metal may also damage crushing and screening equipment.
Therefore, efficient and economical ferrous metal removal is
critical in glass processing.
Best
Practice: The
positive characteristic of ferrous contamination is that these materials
are magnetic and therefore can be detected and removed with the strategic
use of magnetic separation technologies.
Magnetic separation has been used in many material handling
applications for decades. Magnetic
separators can be configured in numerous ways to meet the processing
needs of any commodity.
Two basic types of magnetic devices are electromagnets,
which generate magnetic fields when power is applied, and permanent
magnets, which self-generate a magnetic field. Magnets are configured for use in two primary
ways: rotating, which work within a material stream to detect and
segregate ferrous contaminants by diverting their flow; and as stationary
magnets, which physically pull ferrous metals out of the material
stream. Rotating magnet configurations include magnetic
head pulleys which function in conjunction with conveyor systems (see Conveyor Technologies for Glass Handling Best Practice),
and magnetic drum separators.
Magnetic head
pulleys
are often the least expensive technology for achieving removal of
ferrous metals, and are self cleaning by their configuration.
With head pulleys, the magnet operates under the conveyor belt
and generates a magnetic pull which retains any ferrous metals long
enough to cause them to fall in a different trajectory from the other
materials off the end of the belt.
A carefully located splitter plate segregates the discharge
of the material flow based on those different trajectories.
Magnetic drums are another type of all-purpose
magnetic separator used to purify streams of granular material.
Magnetic drums can be used to protect grinders, crushers, and
other processing equipment against tramp iron damage.
Drums employ either permanent or electro-magnets.
Permanent magnet drums can be installed inside closed chutes, at outlets of
chutes and hoppers, or where materials discharge from feeders and
conveyors. Drums usually have
a non-magnetic surface cover and cams to prevent build-up. Ferrous metal flow is diverted in a manner
similar to that described with head pulleys, as illustrated in the
graphic.
Overhead and
cross belt magnets lift ferrous contaminants from material streams and deposit them in discharge
hoppers. These configurations
also use both permanent and electro-magnets.
The magnets can be positioned in several fashions, including
directly above or just beyond the end of a conveyor, where they function
similarly to head pulleys in affecting the trajectory of the ferrous
materials coming of the belt. Cross
belt magnets employ a moving belt containing the magnet that attracts
ferrous metals out of the stream and carries them to the discharge.
Wipers or brushes can be used to facilitate cleaning.
Implementation: Magnets can be configured in
several different ways to control operating efficiencies based on
process flow characteristics. The
key elements in the design of magnetic separation systems include
the strength of the magnetic field, the size of the contaminants to
be removed, the mass and thickness of the material flow, and the speed
of the belt. Designers should
consider conveyor layout and the configuration of other ancillary
equipment. Magnetic separation
will not remove non-ferrous metals (which do not contain iron) such
as aluminum, brass and other alloys (see Non-Ferrous Separation Technologies Best Practice).
Benefits:
Ferrous
metal contamination can cause significant damage to manufacturing
equipment and processing systems.
Magnetic separation technologies are necessary to prevent costly
repairs and downtime. Failure to effectively remove contaminants
can result in lost revenue from load rejection and jeopardize market
access for processors. The
benefits of quality separation can be quantified in the market value
of processed glass and reduced maintenance and equipment costs.
Application
Sites:
Glass processors, Material recovery facilities.
Contact: For more information about
this Best Practice, contact CWC mailto:info@cwc.org
References:
(1)
Don Freas, TriVitro Corp., 351 Elliott Avenue W, Seattle,
WA 98119, phone: 206-301-0181, fax: 206-301-0183.
(2)
Product Literature,
Dings Co. Magnetic Group, 4740 W. Electric Avenue, Milwaukee, WI 53219. (414) 672-7830.
(3)
Product Literature,
Magnetic Separation Systems, Inc., 624 Grassmere Park Drive, Suite
8, Nashville, TN 37211 Issue
Date / Update: November
1996 |
