Non-Ferrous Metal Contaminant Removal Material: Recycled
Glass
Issue: Non-ferrous metals such as brass, some stainless steel, lead, and aluminum
cause quality control concerns in recycled glass processing. Depending on the end-use application, undetected
metal contaminants can cause costly problems with equipment and products.
Non-ferrous metals can cause damage to furnaces and glass forming
equipment in container and fiberglass manufacturing.
Non-ferrous metals also may cause compatibility problems where
processed recycled glass is used in composition materials.
Finally, the presence of non-ferrous metals can cause a disclosure
problem if not detected and quantified in Materials Safety Data Sheets
for specialty processed glass.
Magnetic removal of ferrous metals
is straightforward because ferrous metals physically react to
magnetic fields, facilitating their removal
(see Magnetic Separation Technologies Best Practice). Non-ferrous
metals and ceramic contaminants pose greater challenges because the
technology for their effective removal is more complicated. These contaminants also become increasingly difficult to remove
in proportion to the fineness of the processed material being cleaned.
Best
Practice: The
two steps for non-ferrous contaminant screening are detection and
removal. Several manufacturers produce equipment for
non-ferrous metal separation. The
most prevalent technology is known as an “eddy-current” detection
system. An eddy current is an electric current induced
within a conductor (in this case the non-ferrous metal) when that
conductor either moves through a non-uniform magnetic field or is
subjected to a change in magnetic flux.
When non-ferrous metals in a glass stream are exposed to an
alternating magnetic field, an eddy-current system can detect their
presence with a proximity sensor.
This technology is well suited to identify non-ferrous metals
within a commingled stream of material such as recycled glass.
After the non-ferrous material is detected it must
be removed from the stream. This
is done with a diverter mechanism to reject contaminants. The diverter may be either a mechanical gate
or a pneumatic blast. Diverter
mechanisms must function very rapidly to remove metal particles and
to prevent diverting an excess quantity of glass.
High sensitivity detectors can sense particles down to less
than ¼ inch in size. The removal
mechanism is timed to reject a small portion of the contaminated cullet
including the non-ferrous metal.
The rejected materials are discharged through a different port
to ensure segregation. Some systems include cascading secondary and
tertiary detection conveyor systems to compound contaminate removal
efficiency.
Implementation: The efficiency of an eddy-current
system should be tailored to the characteristics of a given material
stream, including size of material and speed to mass ratios. The magnetic field size and length of time
the material is exposed to detection are set to the physical characteristics
of recycled glass. Today’s
automated eddy current systems have sophisticated controls allowing
several dozen contaminant particles to be independently identified
simultaneously.
Benefits:
The
high costs of non-ferrous contaminants in glass manufacturing warrant
significant investment in removal technologies. Eddy-currents are the leading technology used in removing non-ferrous
metal contaminants in cullet. The
amount of cullet that is rejected along with metal contaminants varies
depending on system sensitivity and redundancy.
Application
Sites: Glass
processing plants, Material recovery facilities.
Contact: For more information about
this Best Practice, contact CWC, mailto:info@cwc.org.
References: Economic Development Through Scrap Based Manufacturing, M.R. Lewis, The Institute for Local Self-Reliance, 1992. How to Make Good Quality Cullet, Don Schendel, Grason Equipment Co., Glass Industry,
February 1990. Product Literature, Dings Co. Magnetic Group, 4740 W. Electric Avenue, Milwaukee, WI 53219. (414)
672-7830.
Issue
Date / Update: November 1996
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