01-06-2012, 02:02 PM
DMS cyclone separation processes for optimization of plastic wastes recycling and their implications
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Introduction
Reduction of the high volumes of plastics in waste materials
destined to landfills presents a challenge for waste management.
This situation is expected to deteriorate as the demand
for disposal of such wastes is generally forecast to increase. It
is estimated that within the 27 members of the European
Economic Community (EEC), Norway and Czech
Republic, some 12.4 million tonnes were disposed of in landfills,
while 5.0 million tonnes was recycled and 7.2 million
tonnes were used in energy recovery (PlasticsEurope 2008).
Energy recovery from plastic waste is widely promoted in
some sectors and purported to be a means of reducing
fossil fuel CO2 emissions. It is suggested that being derived
from fossil fuels, the combustion of plastic wastes should not
be considered as a means of reducing greenhouse gas emissions.
Furthermore, the combustion of certain types, especially
those containing halogens, increases the potential
environmental risks of producing toxic or carcinogenic
gases.
Objectives
The present study was conducted as a preliminary demonstrative
evaluation of the potential of cylindrical-type DMS
cyclone technology for the recovery and recycling of plastics
based on differences in densities.
As DMS cyclone technology is yet to be applied industrially
for recycling of plastic waste materials, this study was
conducted to indicate the precision of density separations
obtainable, maximum and minimum treatable particle sizes,
through-put capacity, product purities and recoveries that
might reasonably be anticipated. Several potential DMS
cyclone methods for optimizing product purities and recoveries
were considered based on conventional theory of DMS
processes. Preliminary tests were conducted to derive a
reliable indication of the potential precision in density separation,
and the combined purity and recovery of high- and
low-density products generated of the two most basic versions
of these optimizing methods.
Introduction
Results of published investigations based on DMS cyclones
have been reported among others by Gent et al. (2009b),
Pascoe (2006), Bevilacqua et al. (2000), Ferrara et al. (1999)
and Pascoe & Hou (1999). Of these, the best reported results
for separating plastics with slight density differences are those
of Gent et al. (2009b) and Pascoe and Hou (1999). Both of
these investigations were conducted with the LARCODEMS
version of the cylindrical-type DMS cyclone. No comparable
research has been published for the cylindroconical-type DMS
cyclones even though used much more extensively in industry.
However, a number of investigations have been conducted to
attempt the separation of plastics by density (Petty et al. 1993,
MBA Polymers 1998, Bevilacqua et al. 2000, Pascoe 2006)
using density medias in hydrocyclones which have a form
that is identical to cylindroconical-type DMS cyclones.
Conclusions and recommendations
The results of investigations for separation by density of
plastics indicate that cyclone DMS has the potential for substantially
reducing plastic wastes such as ASR, WEEE and
cable sheathing presently being disposed of in landfills. Large
amounts of plastic waste materials as fine as 0.5mm and as
coarse as 120mm and with a difference in densities of 0.068
to 0.094 g cm3 or less can be processed effectively by density
into fractions with purities >99.9%. DMS cyclone separations
of plastics at a several densities and the reprocessing
procedures required to achieve high purities in both the lowand
high-density products along with total recoveries of a
broad range of waste plastic types implies a scale of operation
that has yet to implicated anywhere by the recycling
industry.