Separation of plastic materials

Separation of a mixture of plastic materials by separating the plastic mixture according to particle size, and introducing plastic of similar size particles into a vertical fluidized bed column having an upwardly flowing gaseous stream therein. The flow rate of the gaseous stream is controlled to provide a relatively low density fraction of the plastic mixture exiting at the upper end of the column, and a relatively high density fraction of the plastic mixture exiting at the lower end of the column. In one embodiment, electrostatic charges are induced on the particles of the plastic mixture prior to introducing the plastic of similar size particles into the column, and the column is charged to attract thereto the plastic particles having the highest electrostatic charge.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and a system for sorting 
particles of different plastic materials. More particularly, the present 
invention relates to a method and a system for separating and sorting 
plastic materials by particle size and density, as well as with selective 
electrostatic charges. 
2. Description of Prior Art 
Disposal of solid waste is an increasing problem that has reached crisis 
levels in some parts of the country. An obvious solution is to recycle 
materials that normally are landfilled. With a commercially viable 
separation process, a category of readily recyclable material would be 
waste plastics. Further, other industries, e.g. wire & cable and 
automotive, have a number of materials that would have value if they were 
reclaimable. A major obstacle to the reclamation of these materials is the 
presence of commingled plastic materials which are extremely difficult to 
separate into discrete components. 
It is very difficult to cost effectively recycle commingled plastics 
because failure to separate the plastics completely may substantially 
reduce the properties of one or more of the plastic material. For example, 
PET and PVC materials are not compatible. PET melts at about 500.degree. 
F. while PVC will thermally degrade at approximately 400.degree. F. Upon 
degrading, PVC gives off hydrochloric acid due to dehydrochlorination 
which destroys desirable properties of the PET material. Specifically, 
discoloration, voids, and black specs may be found in the PET product as a 
result of the PVC which will downgrade the quality of the recycled 
product. Further, the evolving HCl may be harmful to personnel, and can 
cause corrosive wear of extrusion or other equipment. 
In the plastic packaging industry, typical rigid household plastic 
packages, such as bottles and other containers, are primarily made of 
polyvinylchloride (PVC), polyethylene terephthalate (PET), polystyrene 
(PS), polypropylene (PP), and polyethylene (PE), including high density 
polyethylene (HDPE) and low density polyethylene (LDPE). 
Plastics used in packaging containers are currently labeled with a recycle 
triangular logo formed by three arrows aligned head-to-tail and having a 
number within the triangle identifying the plastic in accordance with the 
following Table. 
TABLE 
______________________________________ 
No Plastic 
______________________________________ 
1 PET (PETE) 
2 HDPE 
3 PVC 
4 LDPE 
5 PP 
6 PS 
7 Other (co-extruded, mixed 
plastics, etc.) 
______________________________________ 
Thus, plastic bottles, for example, are manually separable by the user 
and/or the collector. However, waste plastic packaging materials generally 
are commingled during collection, and the bottles and containers are 
either compacted or comminuted for space conservation. The primary known 
separation methods are water flotation or hydrocyclone processes based on 
density differences. These wet methods are expensive and time consuming 
due to costly drying. Further, water is of particular concern when the 
plastic is destined for extrusion pelletization. 
The water flotation process generally is a float/sink operation where the 
lighter fractions, (e.g. PE and PP) float on the water and are culled off 
while the heavier fractions (e.g. PVC, PET and PS) sink. The density 
ranges of these materials are very close and may overlap. For example, 
typical PVC densities range from 1.25 to 1.36 g/cc while PET densities 
range from 1.32 to 1.39 g/cc. 
With this overlap in densities, obtaining a satisfactory separation of PVC 
and PET with the water floatation method will be very difficult. Further, 
the water flotation process requires a relatively high amount of energy 
and water utilization, as well as a need to purify the water effluent. It 
should be noted that the use of hydrocyclones can shift the density where 
separation occurs so that PS can generally be recovered with the PE and PP 
fractions. 
SUMMARY OF THE INVENTION 
In accordance with a broad aspect of the present invention, there is 
provided a method of separating a mixture of plastic materials comprising 
the steps of separating the plastic mixture according to particle size, 
and introducing plastic of similar size particles into a vertical 
fluidized bed column having an upwardly flowing gaseous stream therein. 
The flow rate of the stream is controlled to provide a relatively low 
density fraction of the plastic mixture exiting at the upper end of the 
column, and a relatively high density fraction of the plastic mixture 
exiting at the lower end of the column. 
In accordance with a specific aspect of the invention, there is provided 
the further step of comminuting the plastic mixture into particles having 
a size equal to or less than 1.0 inch, and preferably less than 0.3 inch 
prior to introducing similar size plastic particles into the fluidized bed 
column. 
In accordance with another specific aspect of the present invention, the 
low density fraction is typically equal to or less than about 1.1 g/cc, 
and the high density fraction is typically greater than about 1.1 g/cc. In 
accordance with still another specific aspect of the invention, the low 
density fraction includes at least one of high density polyethylene, low 
density polyethylene, polypropylene and polystyrene; and the high density 
fraction includes at least one of polyvinylchloride and polyethylene 
terephthalate. 
In accordance with yet another specific aspect of the present invention, 
there is provided the further steps of inducing electrostatic charges on 
the particles of the plastic mixture prior to introducing similar size 
plastic particles into the vertical fluidized bed column, and charging the 
column for attracting plastic particles having the higher electrostatic 
charge and a polarity opposite to that of the column. 
In accordance with another broad aspect of the present invention, there is 
provided a system for separating a mixture of plastic materials comprising 
means for separating the plastic mixture according to particle size, a 
vertical fluidized bed column, and means for providing an upwardly flowing 
gaseous stream in the column. The system also includes means for feeding 
plastic of similar size particles into the column, and means for 
controlling the flow rate of the stream to provide a relatively low 
density fraction of the plastic mixture exiting at the upper end of the 
column and a relatively high density fraction of the plastic mixture 
exiting at the lower end of the column. 
In accordance with another specific aspect of the present invention, the 
system also comprises means upstream of the feeding means for comminuting 
the plastic mixture into particles having a size equal to or less than 1.0 
inch, and preferable less than 0.3 inch. As used herein, comminuting is a 
generic term for size reduction. The term includes grinding, cutting, 
dicing, crushing and the like. 
In accordance with yet another specific aspect of the present invention, 
the system also includes means upstream of the feeding means for inducing 
electrostatic charges on the particles in the plastic mixture, and means 
for charging the column to attract the plastic particles having the higher 
opposite polarity electrostatic charge. 
There are several advantages for this process. The process does not include 
wetting and drying of the material and is therefore much less expensive 
than flotation. Also, there is no waste water to be concerned with, and no 
hazardous by-products in the waste water.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
With reference to the FIGURE, there is shown a flow diagram of a system for 
separating a mixture of plastic materials wherein the mixture is fed from 
a hopper or other storage means 8 by a line 9 to a granulator or grinder 
10 for comminuting the plastic mixture into particles, preferably having a 
particle size equal to or less than 1.0 inch and preferably less than 0.3 
inch. The ground plastic particles are then supplied by a line 11 to a 
storage container 12. From the storage container 12, the ground plastic is 
passed by a line 13 to a filter screen apparatus 14 which provides 
vibratory motion to the mixed plastic particles. From the lower level of 
the screen apparatus 14, dust-like particles are separated and sent by 
line 16 to a waste storage container 18 for further processing or other 
treatment. The intermediate level of the screen apparatus 14 separates out 
particles of less than 1.0 inch, and preferably from about 0.2 inch to 
about 0.3 inch, which are sent by a line 20 through valves 21 and 23 to 
the input end of a vertical fluidized bed 22. 
The output from the upper level of the screen apparatus 14 is recycled by a 
line 24 back to the input feed line 9 of the granulator or grinder 10. In 
a preferred embodiment, this recycle line 24 will contain particles of 
greater than 0.3 inch. 
Means including a gas source 30 sends a gas stream, e.g. air, by lines 
32,34 to a diffuser plate 35 in the lower input end of the column 22 for 
providing an upwardly flowing gaseous stream in the column 22. A valve 36 
is located in the line 34 to control or adjust the flow rate of the gas 
stream such that a desired relatively low density fraction of the plastic 
mixture fed to the column by the line 20 exits at the upper end of the 
column 22, and a relatively high density fraction of the plastic mixture 
exits at the lower end of the column 22. The high density fraction of the 
plastic mixture exits the column 22 through a funnel-like structure 37 at 
the lower end thereof, and is fed or removed by a line 38 for further 
separation or for other recycle operation. 
In a preferred embodiment of the present invention, the high density 
fraction is greater than 1.1 g/cc such that it includes at least one of 
polyvinylchloride and polyethylene terephthalate. In this embodiment the 
low density fraction is equal to or less than 1.1 g/cc and therefore 
includes polypropylene, high and low density polyethylene, and 
polystyrene. Specifically, The densities of plastic materials discussed 
herein are listed in the following Table I. 
TABLE I 
______________________________________ 
Plastic Density, g/cc 
______________________________________ 
HDPE 0.955 g/cc (0.935-0.965) 
LDPE 0.920 g/cc (0.914-0.935) 
PP 0.900 g/cc (0.902-0.910) 
PVC 1.300 g/cc (1.250-1.360) 
PET 1.350 g/cc (1.320-1.390) 
PS 1.000 g/cc (0.980-1.100) 
______________________________________ 
One way of adjusting the air flow in the column 22 is to provide a 
calibrating sample of known plastic materials of similar size particles 
and having contrasting colors such as orange colored HDPE and green 
colored PVC. This known calibration sample is supplied from a calibrating 
source 40 through a line 42, the valve 23 and the feed stream line 20 to 
the column 22. An operator would visually observe the flow of the 
calibrating sample in the column 22 and adjust the control valve 36 to 
cause the orange HDPE particles to flow upwardly and the green PVC 
particles to fall downwardly in the column 22. At this stage the column 22 
and air flow therein would be calibrated to provide a desired fractional 
split. Calibration could be carried out initially, intermittently and/or 
continuously with the colored known particles acting as tracers in the 
column 22 during the separation process. Further, the calibrating sample 
may include any number of plastic materials, each having its own 
distinctive color. The control valve 36 is adjusted to provide a desired 
density break as discussed above. 
The light fraction output at the upper end of the column 22 may be sent to 
another column 44 via a valve 46, a line 48 and a column input line 50. In 
this instance, a valve 52 would be adjusted to control air flow at the 
lower input end of the column 44 such that the high density fractional 
break would occur at a lower density value. A different high density 
fraction would fall downwardly through a funnel-like output 54 and a line 
55 for further handling, and a new low density fraction including for 
example HDPE and/or LDPE would rise and exit at the upper end of the 
column 44 via a line 56. This light end fraction could then be fed through 
a valve 59 to yet another column 58 operating in a similar manner as 
columns 22 and 44 for further treatment either at the same fractional 
break or at a new break level. 
Also in like manner, the high density fractional output of columns 22 and 
44 can be sent to another column 60 operating similarity to columns 22 and 
44 to further separate the high density fractions either at the same 
fractional break or at a new break level. 
In accordance with another aspect of the present invention, separation of a 
mixture of plastic material including PVC and PET can be further enhanced 
by selectively charging the plastic particles prior to feeding them to the 
column. Specifically, means 66 are between the valve 21 and the line 20 
feeding the column 22 for inducing an electrostatic charge on the 
particles of the plastic mixture being feed to the column 22. A charge 
source 68 is provided for charging the surface of the column 22 to a 
polarity opposite to the polarity of the plastic material sort to be 
attracted thereto. The particle charging means 66 may be any device for 
causing particle motion or particles rubbed together such as mechanical 
means of fluidization or agitation. For example, a device such as a 
cyclone or mechanical agitator may be used to generate the electrostatic 
charge. The device 66 may also be an electrical generator such as a corona 
discharge. Alternatively, the charging may occur without a device 60 by 
taking place in the columns by the fluidized action of the plastic 
particles. 
In one embodiment of the present invention, the feed of plastic is 
essentially composed of PVC having a density in the order of 1.3 g/cc 
(1.25-1.36 g/cc) and HDPE having a density of about 0.955 g/cc which is 
feed though the grinder 10 and the separator 14. The intermediate level 
output having a particle size range of from about 0.20 inches to about 
0.30 inches is then sent to the particle charging device 66 which applies 
electrostatic charges to the plastic particles in the charge mix, or the 
charge may be generated by the particles rubbing together while traveling 
in the lines to the column. The charge on the PVC will be about 
-12,639.times.10.sup.-10 coulombs/cc, while the HDPE will have a charge of 
about -1,908.times.10.sup.-10 coulombs/cc. This plastic mix is fed by the 
line 20 to the column 22 with the column having been calibrated by feeding 
a calibration mixture of PVC and HDPE to the column from the calibration 
source 40 and adjusting the flow rate by valve 36 to provide for the HDPE 
to pass upwardly through the upper end of the column 22 and the PVC to 
fall downwardly and exhaust out the exit and 36 of the column 22. The 
column 22 has a positive charge applied thereto by the charge source 68 
such that PVC particles entrained in the HDPE stream moving upwardly in 
the column 22 are attracted to the inner surface of the column 22. Also, 
in one embodiment where the column is plexiglass the flow of the particles 
acts to charge the column positively. 
The PVC particles adhering to the inner surface of the column 22 may be 
removed by one of several methods. One method would be to have a collar 70 
positioned within and abutting the inner surface of the column 22 with a 
vertical driving arm 72. A motor 74 periodically drives the arm 72 to run 
the collar 70 down the inner surface of the column 22 to scrape the PVC 
particles off the wall and permit such removed particles to fall 
downwardly through to the outlet 37 of the column 22. 
The particles of PVC also may be removed from the interior surface of the 
column 22 by turning off the valve 36 and the feed valve 21 such that no 
new feed is being feed to the column 22, and then reversing the charge on 
the column 22 by the charge source 68 from a positive voltage to a 
negative voltage to repel the negatively charged PVC particles from the 
inner surface. The repelled PVC particles fall by gravity to the lower end 
of the column 22 and out through the funnel like bottom 37. A variation of 
this method would be to remove the charge applied by the source 68 to the 
column 22 such that the applied voltage is effectively zero. 
In another embodiment, the inner surface of the column 22 may be cleared of 
PVC by initially turning off the valve 21 to stop the feed to the column 
22, and changing the valve 46 such that the PVC is fed by line 74 to PVC 
exhaust line 38. The valve 36 is then opened further to increase the air 
fed by the line 34 to the column 22 to a level sufficient to cause PVC 
particles to exhaust upwardly through the column 22. The polarity of the 
charge applied by the charge source 68 to the column 22 is then reversed 
such that the surface of the column 22 is negative to repel the negatively 
charged PVC particles therefrom. The repelled PVC particles then exhaust 
upwardly under the increased pressure of the air flow in the column 22 out 
through the valve 46, the line 64 to the PVC exhaust line 38. 
As discussed above, PVC and PET have overlapping densities in that PVC has 
a density in the range of 1.25 to 1.36 g/cc and PET has a density in the 
range of 1.32 to 1.39 g/cc. Separation of such a mixture of PVC and PET 
can be obtained by selectively charging the plastic particles prior to 
feeding them to the column 60. Specifically, means 78 are located before a 
valve 83 in a line 82 feeding the column 60 for inducing an electrostatic 
charge on the PVC and PET particles being feed to the column 60. A charge 
source 80 is provided for charging the surface of the column 60 to a 
polarity opposite to the polarity of the PVC. As described in U.S. Pat. 
No. 5,118,407, PET may be charged positively and PVC negatively charged, 
In this instance the column would be charged positively. The PVC is 
attracted to the column while the PET exits the top of the column 80 by a 
line 84. 
The PVC is removed from the interior surface of the column 60 by any one of 
the methods described above with reference to column 22, including wiping 
the interior surface with a collar driven by a motor 88. 
EXAMPLE 1 
In Example 1, a mixture of resins was prepared by blending 75% unpigmented, 
ground HDPE milk bottles with 25% pigmented, ground PVC recycle resin. The 
two samples had different particle size distributions as shown in the 
following Table II. 
TABLE II 
______________________________________ 
Particle Size PVC HDPE 
(mm) (wt %) (wt %) 
______________________________________ 
&gt;2.0 0.05 3.9 
1-2 16.85 56.4 
0.706-1.000 40.20 32.5 
0.590-0.706 12.60 1.3 
0.355-0.590 27.93 4.68 
&lt;0.355 2.53 1.22 
______________________________________ 
100 grams of the mixture was placed in a fluidized bed column and the 
fluidizing air stream was turned on until 0.66 grams was blown from the 
bed and collected. The fluidizing velocity was increased, and a second cut 
was collected consisting of 0.325 grams. Eleven additional cuts were 
collected, each containing an average of 4.9 grams. A total of 54.92 grams 
was removed in these twelve cuts. The PVC content of the last four was 
determined. It was observed that the PVC consisted of much smaller 
particles than the HDPE in each cut that was blown from the bed. Each of 
these four samples was classified through a stack of screens. In this way 
the smaller PVC particles separate from the HDPE. The resin remaining in 
the column contained 68.4% of the original PVC resin at 37.9% 
concentration. The last four cuts were analyzed with the results shown in 
the following Table III. 
TABLE III 
______________________________________ 
Weight PVC &gt;0.706 mm 
&lt;0.706 mm 
Sample (g) (%) (% PVC) (% PVC) 
______________________________________ 
10 3.307 9.90 0.65 40.6 
11 7.721 12.40 0.30 60.6 
12 12.735 13.65 0.50 72.5 
13 8.190 9.50 0.80 80.7 
______________________________________ 
The above example shows that fluidization alone will not provide sufficient 
separation of PVC and HDPE resin when they are of different particle size 
distributions. 
EXAMPLE 2 
Ground samples of HDPE milk bottle resin and recycled PVC were sized 
classified by screening to obtain samples of similar size in the range of 
between 0.706 mm and 0.590 mm cuts of each. One sample was prepared from 
these cuts combining 10% PVC and 90% HDPE. The sample was continuously 
injected into the side of a fluidized bed column with the air rate such 
that some resin was continuously blown out the top to be collected. At the 
conclusion of test, 96.2% of the original HDPE fed was recovered overhead 
at a purity of 99.3%. The resin remaining in the bottom of the column 
contained 93.6% of the PVC fed at a purity of 87%. 
EXAMPLE 3 
A second sample of 20% PVC was prepared as in the last example and 
similarly injected and separated but at a slightly greater air rate. The 
resin collected overhead contained 99.6% of the HDPE fed at 95% purity. 
The bottoms contained 79% of the PVC fed at 98.4% purity. 
Examples 2 and 3 show that size classification and fluidization can be 
combined to separate plastics having a density difference as illustrated 
by HDPE (0.955 g/cc) and PVC (1.3 g/cc). Further, fluidizations can be 
conducted in a single pass or in a train. 
In another embodiment, an electrostatic charge is induced on the ground 
particles of plastic materials in the mixture. The particle motion causing 
the charge build up can be generated via mechanical means such as 
fluidization, and/or agitation. Other devices for generating a charge 
build up are a cyclone or mechanical agitators. The separation takes place 
as highly electrostatically charged particles settle on the interior 
surface holding an opposite charge. This process was demonstrated in a 
fluidization column where HDPE and PVC were separated. 
In another example screening and fluidizing a mixture of HDPE and PVC was 
used to separate these two materials. It was observed that PVC collects a 
negative charge due to its highly electronegative chlorine atoms present 
in the chains and generates an opposite charge on the walls of a 
plexiglass fluidization column. Although both PVC and HDPE held a negative 
charge, PVC had about six times the charge of HDPE per unit volume. Thus, 
in this embodiment the plastics to be separated are charged to the same 
polarity, i.e. negative, while the column is charged to the opposite 
polarity, i.e. positive. 
EXAMPLES 4-9 
Experimental data was collected from a 3.5" plexiglass fluidization column 
having a height of 80.25", and having an air supply of 100 psi. The 
material consisted of thin wall PVC and HDPE bottles. Both PVC and HDPE 
bottles were ground in a NELMOR RG1012M1 Granulator and sent through a 
1/4" screen. Several blends of 500 g mixture of the two materials were 
made with 15% and 20% PVC. Each blend was carefully introduced to the 
column and tested. 
Separation using several air velocities were investigated, with conditions 
of the column at atmospheric pressure and temperature. Table IV shows the 
results for a 15% PVC blend. 
TABLE IV 
__________________________________________________________________________ 
Separation by Fluidization at Different Air Velocities 
INITIAL RECOVERY 
EXAMPLE 
LOAD, 500 G 
MATERIAL 
AIR % INITIAL 
HDPE 
NUMBER TOTAL BLOWN OUT 
VELOCITY 
HDPE PURITY, % 
__________________________________________________________________________ 
4 15% PVC HDPE 3.04 m/s 
29.41% 99.99% 
5 15% PVC HDPE 3.42 m/s 
47.06% 99.75% 
6 15% PVC HDPE 3.80 m/s 
68.26% 99.28% 
7 15% PVC HDPE 4.18 m/s 
75.29% 98.16% 
8 15% PVC HDPE 4.56 m/s 
80.85% 97.01% 
__________________________________________________________________________ 
The experiments confirm that good separation may be obtained by 
fluidization alone, and that the electrostatic attractive forces between 
the PVC particles and the walls of the plexiglass column greatly enhanced 
the separation. Note from the following Table V the high purity of the PVC 
retrieved from the column walls due to a significant positive charge build 
up on the plexiglass surface. 
TABLE V 
__________________________________________________________________________ 
Electrostatic Separation of PVC 
PVC Retrieved from Column Walls 
INITIAL RECOVERY 
EXAMPLE 
LOAD, 500 G 
MATERIAL 
AIR % INITIAL 
PVC 
NUMBER TOTAL ONWALL VELOCITY 
PVC PURITY, % 
__________________________________________________________________________ 
6 15% PVC PVC 3.80 m/s 
45.07% 97.41% 
7 15% PVC PVC 4.18 m/s 
20.00% 96.77% 
9 20% PVC PVC 3.80 m/s 
30.00% 97.40% 
__________________________________________________________________________ 
Also note from Table IV that HDPE purity decreases as the air rate 
increases. Thus, both the air flow rate and the surface charge on the 
column should be adjustable and controllable, as described hereinabove 
with reference to the Figure. 
Both ground resins were tested for electrostatic charge density after 
prolonged agitation: HDPE -1.908.times.10.sup.-10 Coulombs/cc, PVC 
-12.639.times.10.sup.-10 Coulombs/cc. The electrostatic charges can also 
be induced by electrostatic charge generation. Charged drums, plates, 
screens, and columns will also be effective in attracting and separating 
PVC from HDPE. 
Although the columns in the Figure are cylindrical, they may have any 
desired cross-sectional shape, for example elliptical, square, 
rectangular, etc. Further, the plastic particles are movable between the 
various components in the Figure by any combination of blowers, pumps, 
screw or belt conveyors, gravity, and the like as is well known in the 
art. 
While the invention has been described in conjunction with specific 
embodiments thereof, it is evident that many alternatives, modifications, 
and variations will be apparent to those skilled in the art in light of 
the foregoing description. According, it is intended to embrace all such 
alternatives, modification, and variations as fall within the spirit and 
broad scope of the appended claims.