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| The Custom Services Group
of Solvay Solexis cryogenically screens ground plastic resins
at temperatures of -50° to -275°F (-46° to -170°C), placing
extreme demands on its centrifugal screening equipment. |
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Cantilevered shaft and hinged
door of
centrifugal screener allow screens to be replaced before icing
can prevent restarting of the frigid unit, and permit rapid
wash-downs between varied production runs,
preventing cross-contamination. |
High capacity screening of resin powder at
-275°F (-170°C)
HILLSBOROUGH, NJ — Among varied techniques to pulverize materials
on a toll basis, the Custom Services Group of Solvay Solexis cryogenically
grinds and screens plastic resins such as fluoropolymers, nylons,
polypropylenes, engineering resins, polyurethanes and polyesters.
The grinding processes involve screening at custom sizes, which
can place extreme demands on screening equipment, particularly at
cryogenic temperatures from -50° to -275°F (-46° to -170°C).
"Sifter screens are more susceptible to breakage under cryogenic
conditions due to embrittlement from the cold," says Craig
Davis, sales and marketing manager. "It is critical to minimize
downtime when this happens because system components can freeze,
and material left in the system can become wet."
He continues, "The choice of sifter is crucial to a grinding
operation in terms of its ability to keep up with the grinding process
at a variety of screen sizes, and its impact on the operation from a maintenance standpoint."
Solvay Solexis found circular vibratory separators easy to clean
and maintain, but blinding occurred at sizes finer than 300 microns
with powder containing as much as 30-50% of oversized material.
To achieve a respectable throughput rate, the separator needs to
be large in diameter, which can pose a problem if space is at a
premium.
Rectangular gyratory screeners, Solvay Solexis concluded, have
adequate screen area to keep pace with grinding operations, but
their service in cryogenic
operations requires a considerable amount of upkeep due to frame
warpage and wear and tear on gaskets and wire cloth. When a problem
occurs with a rectangular screener, Davis says it requires a minimum
of one hour for an experienced person to remove, comb through, repair,
and replace 8-12 screens. The long downtime can lead to startup
difficulties in a cryogenic operation.
Tough screening problem solved centrifugally
"Centrifugal screeners provide a compromise because they keep
up with grinding processes with the same efficiency as a rectangular
screener, but are easy to clean and maintain like circular vibratory
screeners," he says. Solvay Solexis uses centrifugal screeners
in many of its cryogenic processes Centrifugal separators contain
a stationary, horizontally-oriented wire cylinder, inside of which
are rotating helical paddles that continuously propel and accelerate on-size material
through the cylinder walls, ejecting oversize particles through
the downstream end of the cylinder into a discharge spout.
Davis found centrifugal screeners to meet his capacity requirements
and — due to the action of the rotating paddles, which do not contact
the screen — to aid in the break down of loose agglomerates,
increasing yield while reducing the incidence of screen breakage
per ton of polymer screened.
Solvay Solexis recently added the "Quick-Clean Centri-Sifter"
separator from Kason Corporation. This unit is equipped with three
roller bearings located outside of the screening chamber instead
of the normal two.
The bearings at the motor end of the shaft and on a hinged cover
at the discharge end provide the support needed to handle the high
speeds and loads of high capacity screening without vibration. When
the end cover is hinged open, the shaft becomes a cantilever supported
by the two bearings located between the motor and Centri-Sifter
body, allowing the screens and/or paddles to be removed rapidly
for replacement or wash-down.
In low temperature applications, rapid screen changes not only
minimize interruptions, but allow restarting of the frigid unit
before the formation of ice can hamper or prevent restarting. As
a toll processor, Solvay Solexis also benefits from rapid wash-downs
it frequently performs to prevent cross-contamination of custom
production runs.
Solvay Solexis also micronizes, screens and blends resins and other
materials at ambient temperatures in conjunction with 15 milling
operations, and packages liquids and solids in containers ranging
from bulk bags to small sample packs. Products handled include additives
for paints, inks, polymers, and nutritional products (nutraceuticals),
compounds, adhesives, and specialty resins suited for sintering,
composites, and fabric coating applications. |
