Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF SEPARATING POLYOLEFINIC SYNTHETIC MIXTURES
Description
to
The ilzvention concerns a method for separating plastic material mixtures. The
invention
is particularly useful for the separation of plastic material mixtures from
consumer waste
recycling systems, such as the Dualen System Deutschland (Der grune Punkt)
[The Dual
System Germany (The Green Dot System)], in other words, of post-consumer
plastic
material waste mixtures.
US patent number 5,198,471 describes a method by which polymers are separated
from a
physical batch of different solid plastic materials. The batch is suspended in
a solution-
heating vessel in a solvent at a first Low temperature at wluch a first
plastic material type
dissolves while the other plastic material types remain solid. After a certain
dissolving
period, the resulting solution is extracted from the heating vessel. New
solvent is added
which will be at a temperature that will dissolve the next plastic material
type in the
batch. In this manner, these solution steps are continued until all types of
plastic material
have been dissolved. Finally, the plastic material types are recovered from
each
separated solution using the conventional flash evaporation technique. The
dissolving
period for this process is at Least one hour for each dissolving cycle but
neither heating
periods nor solution removal periods are mentioned in this patent. Usually,
these periods,
at a minimum, fall within the same time frame.
This means that when implementing this prior art process in an industrial
setting, if the
2 o plastic material batch contain 3 types of plastic material, the through-
put of a separation
heating vessel would only be 1/3 that of the heating vessel in which the
plastic material
batch would dissolve 100% right away.
Furthermore, this prior method has the disadvantage that each plastic material
component
requires a large interim storage tank so the plastic material preparation can
continually
drain at the end of the process.
Furthermore, this prior method has the disadvantage that as the different
solutions are
removed from the solvent reactor (heating vessel), a relatively large amount
of solution
remains, an amount which will be larger the finer the plastic material batch
has been
grinded (to shorten dissolving times). This remaining solution contains
dissolved plastic
material
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from the plastic material type previously dissolved and pollutes the
subsequent plastic
material solutions and therefore the plastic material type. To avoid this
disadvantage the
remaining batch must be washed with new solvent, which has the disadvantage
that it
takes more time and requires an additional solvent preparation.
Additionally, this prior art process has the disadvantage that when removing
solution
from the first solutions, the solution always also contains fine-grained solid
plastic
material parts from the not yet dissolved plastic material types, which, when
using a flash
evaporation technique in the preparation step, inevitably appear as impurities
in the
newly generated plastic material unless, prior to subsequent processing, an
intermediary
solution filtration occurs. In an industrial setting, in order to avoid
further impurities a
separate filtration step would have to be set up for each plastic material
type in the batch,
in other words for each solution type. The solid matter generated would have
to be
returned as suspension to the solvent heating vessel with solvent.
It becomes clear that when implementing the method described in US patent
5,198,471 in
an industrial setting, the investments necessary would become very expensive
and that
for these reasons, no implementation into an actual industrial installation
has occurred.
EP 0 790 277 A1 describes a polymer separation method in which the plastic
material
batches are consecutively suspended in different solvents such as toluene,
THF, xylene
and ethylene benzene at room temperature or, as the case may be, at
135° C to dissolve
the plastic material types PS, PVC and polyolefin and to recover each of them.
In this
manner, the plastic material is recovered from the solution using
precipitation with
methanol. To recover each polyolefin component, the batch is not dissolved in
xylene
only at 135° C but also at temperatures of 75° C (LDPE
dissolving temperature), 105° C
(remailzing LDPE and HDPE) and 118° C (PP dissolving temperature}. This
process has
the disadvantage, in addition to the ones already described above with respect
to patent
US-A-5,198,471, that the different solvents within the same facility will come
into
contact with parts of the facility and therefore an industrial implementation
of the process
would require an expensive solvent preparation.
With all the described methods of the prior art, when a polyolefin batch
composed of
LDPE, HDPE and PP is used as the starting material, the result will always be
an LDPE
or HDPE blend with a PP content of > 5% or, a PP blend with an HDPE content of
> 5%,
due to the different sources of contamination.
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A polypropylene content of >_ 5% by weight in HDPE can be harmful for many
purposes
since polypropylene is not soluble in polyethylene and the polymer blend
embrittles with
increasing polypropylene content, in other words, the impact strength
decreases
significantly, the weld strength is lost and the proness to stress cracking
increases.
The German patent application number 198 06 355.5 describes a thermal
separation
method for mixed polymers in which with the use of temperature increases two
fluid
phases are generated one of which is richer in solvent and another of which is
richer in
polymers. In particular, in a first phase separation step, a polyethylene-rich
phase is
generated, which is separated into an LDPE-rich phase and an HDPE-rich phase
in a
second phase separation step.
Following this phase formation, it is the task of the phase separation to
combine the
droplets from both phases in "closed" phases. In reality, this phase
separation is difficult
to implement.
Finally, in this method, the polymers must be recovered from each solution.
This
recovery of the polymers from the solutions is not described in patent
application number
198 06 355.5.
The separation method described in patent number US-A-5,198,471 using a flash
evaporation technique and final vacuum extrusion has the disadvantage that the
additives
found in the plastic material waste mixtures such as waxes, anti-statics and
stabilizers
remain in the recovered polymers in unknown amounts. Should the polymers be
recovered from the solution by lowering the temperature and using
precipitation/crystallization and the correct precipitation temperature is
chosen, most of
the wax, polymer chain fragments and additives remain in the solution;
however, a
smaller undetermined amount is precipitated with the polymer.
Furthermore, a serious quality issue arises when simply lowering the
temperature since
the polymer will be precipitated in a fine powder form so that the final
filtration of the
filter cake will contain approximately 50% to 60% residual moisture.
This high level of residual moisture contains the undesirable additives
mentioned above
in proportional amounts, which adversely affects the quality of the polymer
blend. The
polymer mixture recovered from plastic material waste has the additional
problem that
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the cracked coloring agents from packaging could be concentrated in the
polymer phase
involved. In the case of a polyolefm mixture, these accumulate during the
first phase
separation step in the polyethylene-rich phase while the polypropylene phase
remains
relatively clean; during the second step - during the separation of LDPE and
HDPE -
they concentrate in the HDPE rich phase.
Therefore, it is the purpose of the invention to identify a method that will
avoid the
above-described disadvantages when recovering polymers from a mufti-type
plastic
material batch. It is the goal to recover polymer blends with a purity level
higher than
95%, preferably higher than 97%, using starting material that consists of non-
compatible
plastic material type mixtures (for example PP and HDPE). Other objectives
include:
- The recovery of polymer blends with the lowest possible wax and additive
content
which stems from the used starting material;
- The recovery of polymer blends with proportions of plastic material types
that are
highly reproducible, in order to safeguard constant technological
characteristics;
and
- The simplest technology with a high yield.
As described below, the method of the present invention achieves these
objectives.
The invention suggests a method for the separation of mixed plastic materials
using a
starting material that consists of a polyolefm plastic fraction or another
plastic material
mixture. The starting material is brought into contact with a solvent, and the
temperature
of the solution and preferably also the proportion of solvent to volume of
plastic material
is adjusted in such a manner that at least one of the polymer types and
preferably several
of the polymer types from the plastic material batch dissolve and the solution
as a whole
has sufficiently low viscosity for the final solid-liquid separation. Finally,
at least the
single dissolved polymer type is sheared and precipitated from the solution to
separate
the polymer type from all other components in the solution, including the
additional types
of polymers contained in the solution.
To separate each polymer type, the solution passes through one or multiple
precipitation
steps. Each precipitation step could include several cooling steps to cool the
solution to a
transportation temperature at which no polymer will be precipitated and
finally, to
CA 02376488 2001-12-14
cool the solution in the next or, as the case may be, last, cooling step, to a
precipitation
temperature at wluch each specific polymer type is sheared and precipitated.
Preferably, the solution is passed through as many precipitation steps as it
contains
dissolved polymer types whereby, in each precipitation step, the solvent is
broken down
into to a polymer type as well as the solvent with the remaining dissolved
polymer types,
polymer fragments, waxes, additives, coloring agents and insoluble materials.
Analysis of the method according to the invention has revealed that a
particularly good
separation of the components from a plastic material mixture and particularly
clean end
products are achieved if, in a first precipitation step, two polymer types
together, namely
polypropylene and HD-PE are sheared and precipitated, the HD-PE/PP fibers are
separated from the remaining LD-PE solution using solid-liquid separation and,
finally,
the LD-PE, on the one hand, is recovered in the conventional manner from the
LD-PE
and the PP and HI~-PP fibers, on the other hand, are separated in the
conventional
manner.
Tn this, as well as in other embodiments of the invention, one or more of the
shearing and
precipitation steps of the entire process could be replaced by a separation
step for one or
more of the polymer types to be separated. In the replacement step(s), a
separation of the
polymer types in question occurs in the liquid phase taking advantage of a
lack of
miscibility. In a method similar to the method described in patent application
number
198 06 355.5 as seen above, for example, two liquid phases are formed, each of
which
contains an elevated concentration of different polymer types and these liquid
phases are
separated in a separating funnel, centrifuge or a coalescense separator in
which, in
accordance with the invention, the resulting separated liquid polymer blend
solution
could be prepared in an additional shearing and precipitation step or shearing
crystallization.
This alternative embodiment in which a precipitation step for a specific
polymer type is
replaced by a separation step by the formation of several liquid phases, can
be technically
advantageous as is described in further detail below.
Even though the shearing and precipitation of polymers from solution to
produce fiber
structures is basically well known in the prior art (as shown in, for example,
patent DE-
A-196 18 330) this method has never yet been used for the separation of
several polymer
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types from a plastic material mixture and in particular, from a plastic
material waste
mixture. However, the invention has surprisingly shown that the separation of
polymers
using shearing and precipitation yields a fiber structure, which has very low
residual
moisture and a high level of purity that is superior to the products generated
using the
conventional methods.
Preferably, the solution is thoroughly cleaned of inorganic contaminants,
additives,
coloring agents, undissolved plastic material and the like prior to the
separation of the
dissolved polymer types in a single or a multiple stepped mechanical solid-
liquid
separation system. After the separation of the different polymer types, each
polymer type
is cleaned again as a polymer blend in corresponding washing steps using a
solid-liquid
separation technique and is recovered using, for example, sequential degassing
extrusion
or vacuum drying with sequential extrusion. The low molecular weight polymer
fragments and the waxes dissolved in the solvent are recovered from the
solution in a
separate solvent preparation system using distillation and could be settled as
wax using
conventional preparation methods..
Other characteristics and advantages of the invention are described in the
attached patent
claims.
The invention is described in further detail below with reference to best
modes and to the
drawings. The Figures show:
Fig. 1 a schematic representation of an apparatus for the implementation of
the method
in accordance with the invention;
Fig. 2 a close-up representation of a possible embodiment of a shearing head
which
could be used in the apparatus shown in Figure 1;
Fig. 3 an alternative embodiment of a shearing head which could be used in the
apparatus shown in Figure 1;
Fig. 4 a schematic representation of an additional embodiment of an apparatus
for the
implementation of the method in accordance with the invention;
Fig. 5 a flow chart explaining the method in accordance with the invention;
and
Fig. 6 a flow chart explaining a modified embodiment of the method in
accordance with
the invention; and
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Fig. 7 a flow chart explaining yet another embodiment of the method in
accordance with
the invention.
The method in accordance with the invention is described below with reference
to the
preparation of plastic material mixtures by the Dualen System Deutschland [The
Green
Dot System Germany], in particular, with reference to the separation of the
polyolefin
portion of the mixed plastic material fraction. However, the invention is
equally
applicable to other kinds of plastic material mixtures.
The polyolefin share of the Dualen System Deutschland [The Green Dot System
Germany] mixed plastic material consists of in total approximately.20 to 30%
polypropylene and approximately 35 to 55% LDPE and HDPE from packaging in
changing proportions. Of that amount, between 15 and 35 wt. % is HDPE and
between
and 35 wt. % is LDPE.
A preferred embodiment of the invention is shown in Figure 5. In the method in
accordance with the invention, a sequence of operations of which is
schematically shown
in Figure 5, a mixture of PP, LDPE, LLDPE and HDPE is used as starting
material, as
shown in step 10. This starting material is put into contact with a solvent,
such as
petroleum spirits or n-hexane, and is completely dissolved at an elevated
temperature, for
example approximately 140° C, as shown in step 12. Instead of petroleum
spirits or n-
hexane, decalin or xylene could also be used as a solvent. An advantageous
value for the
adjustment of the polymer concentration in the solvent is approximately 20%.
Finally,
the solution is cleaned of undissolved compounds in one or several steps Using
filtration,
centrifuging or other mechanical separation techniques, as shown in step 14.
In this
particular case, when used plastic material packaging is employed, the
insoluble
compounds are usually inorganic contaminants, undissolved cellulose parts,
PVC, PET or
PS packaging materials, paper fibers, non-polyolefin packaging and inorganic
filling and
the like. Following this mechanical cleaning step the solution consists 99% or
more of
the solvent and the dissolved polyolefin plastic materials PP, HDPE, LDPE and
LLDPE
(hereafter collectively called LDPE).
In accordance with the first embodiment of the invention shown in Figure S
each polymer
type is precipitated one after the other from the solution using
crystallization under
simultaneous shearing action to separate each polymer type and to keep waxes,
polymer
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chain fragments and as many coloring agents and filling materials as possible
in the
solution. Additionally, the solution cleaned in step 14 is sheared and
precipitated
consecutively at three different temperatures, which are established
empirically. In a first
precipitation step shown in step 16, the solution is cooled to a temperature
T1 and HDPE
is sheared and precipitated. In a solid-liquid separation step, the HDPE is
separated from
the solution so that the fibrous precipitated HI~PE, on the one hand, and the
remaining
HDPE solution on the other hand, will be available separately for the next
processing
steps as shown in to step 18.
The fibrous, precipitated HDPE is degassed in a bypass extruder in step 20 so
that in step
22 a polymer blend with an HDPE content of 95% and a PP content of <_ 3% will
result.
The suspension remaining following the separation of the HI~PE is cooled to a
second
lower precipitation temperature T2 to shear and precipitate PP, as shown in
step 24. In
step 26, similarly to step 18, the solution containing the precipitated PP
fibers, is exposed
to a solid-liquid separation to receive the PP fibers, on the one hand, and
the remaining
suspension, from which the PP was filtered, on the other hand, as shown in
step 26.
The precipitated PP is again degassed in a bypass extruder, as shown in step
28, so a
polymer blend with a PP content of 95% and an HDPE content of 5 3% will
result, as
shown in step 30.
The suspension remaining following the separation of the PP is then cooled in
a third step
to a third even lower temperature T3 to shear and precipitate the LDPE, as
shown in step
32. The solution with the precipitated LDPE undergoes a solid-liquid
separation, as
shown in step 34 and the recovered LDPE fibers are degassed in a bypass
extruder, as
shown in step 36, whereby a polymer blend results that contains approximately
95%
LDPE. The remaining solvent is reprocessed to clean it of waxes, additives and
other
impurities in step 40.
Figure 5 shows the essential features of the invention, the characteristics of
which could
be modified and adjusted in numerous ways.
When shearing and precipitating it must be considered that the choice of
solvent to be
used and the shearing rate will strongly influence the exact precipitation
temperature of
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each polymer type. It should be noted that the precipitation temperatures for
the different
polymer types must be sufficiently different to ensure definite separation of
the different
polymer. In the case of the polyolefin plastic material fraction consisting of
LDPE,
HDPE and PP, it was surprisingly discovered that a separation method using
crystallization and simultaneous shearing provides a better separation of the
plastic
fractions into each different component than the selective dissolving method
from the
above mentioned patent number US-A-5,198,471, as can easily be inferred from
the table
below:
Solvent Dissolving temperature in Precipitation temperature in
method according to patent method according to the
number US 5,198,471 ~C) invention (° C~
LDPE HDPE PP LDPE HDPE PP
Petroleum 70-75 96-103 100-11367-70 95-100 78-86
s irits
Decalin 80-90 115-130_130-14050-60 90-100 70-80
N-hexane >100 >100 100 70-80 T100-lI080-110
It is clear from the table above that the temperature ranges for dissolving
HDPE and PP
overlap in the selective dissolving method from patent number US-A-5,198,471,
so that
selective dissolving is virtually impossible. However, the precipitation
temperatures for
the different polymer types HDPE and PP in the method according to the
invention at
hand diffei by approximately 9 to 10° C while the precipitation
temperatures for polymer
types PP and LDPE using the same solvents would show a difference of
approximately 8
to 9° C.
Not only does the invention demonstrate that a complete separation of
precipitation
temperatures for each polymer type is possible at which point a true selective
separation
of the polymer types can be realized, but in contrast to the dissolving method
of the prior
art in which the dissolving temperature of PP is higher than the temperature
necessary for
HDPE, the precipitation temperatures for PP, when using the solvents mentioned
here,
petroleum spirits and decalin, are lower than the temperature necessary for
HDPE. In
contrast to the prior art, shearing crystallization in the method in
accordance with the
invention not only provides a substantially better selective separation of the
different
polymer types contained in plastic material mixtures but this separation
occurs using
completely different temperature ranges and it occurs in a different order
than seen in
prior art.
The shearing and precipitation in accordance with the invention yields a
polymer powder
with a fiber-like structure that could be shaped like shish kebab. This fiber
structure,
which occurs during crystallization, has the additional advantage that the
separated
polymer following filtration (solid-liquid separation in steps 18, 26 or, as
the case may
be, 34) has very low residual moisture, which could even be under 10% by
weight.
Similar to this low residual moisture, the level of waxes and other additives
in the
recovered polymer powder is very low, which means that the purity of the
recovered
polymer powder is high and the level of remaining waxes, additives and the
like could be
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further reduced in simple washes of the polymer powder with pure solvents and
filtration
of the resulting blend.
Past experience shows that the amount and type of waxes and other additives
present in
the plastic material mixture as well as the molecular weight distribution of
each polymer
type have only little influence on the exact temperature setting and the
relationship of
each precipitation temperature to the other and thus the sharpness of
separation or, as the
case may be, clarity with which each polymer type can be separated from each
other and
recovered. Significant deciding factors are the choice of solvent, the
temperature control
and the shearing rate as well as distribution.
A variation of the method in accordance with the invention is schematically
represented
in the flow chart of Figure 6.
Basically, as shown in Figure 5, it is possible to precipitate and thus to
separate each
polymer type one after the other using the sequential shearing and
precipitation steps:
due to the contamination of the starting material polymer from different
coloring agents,
pigments and the like, it would be more advantageous from a processing and
technical
viewpoint to modify the described process. In an alternative embodiment of the
invention as shown in Figure 6 it is therefore suggested that, firstly, the
polypropylene is
dissolved from the plastic material batch consisting of PP, HI7PE and LDPE
using
another method and only afterwards separate the remaining PE types, LDPE and
HDPE
using shearing and precipitation. The method described in patent application
number
198 06 355.5 is suitable for the initial separation of PP.
The inventor realizes that when separating the PP phase from the PE phase
using
solubility gaps at higher temperatures, the coloring agents and pigments
accumulate in
the PE phase resulting in an almost pure PP phase. Should it not only be
necessary to
separate each polymer or polyolefin type with highest possible polymer purity
from a
plastic fraction, but also to remove most coloring agents, pigments and the
like with a
high level of purity, the modified invention method as shown in Figure 5 with
a two-step
separation method is suggested.
In the first step of this method (Figure 6), steps 10 to 14 proceed as
described above in
connection with Figure 5. In step 42, the solution is adjusted to a
temperature that allows
a separation phase to occur due to the lack of miscibility that results at a
higher
temperature. This method is described in further detail in patent application
198 06
355.5, which is hereby incorporated by reference.
When, for example, the solvent n-hexane is used, the solution, which, for
example, could
have a polymer concentration of 20% by weight, is heated in step 42 to a
temperature
higher than 170° C, preferably between 180° C and 210° C.
At this temperature, after
approximately 40 minutes, a polypropylene-rich phase, which can simply be
extracted,
settles in the upper part of a separating funnel that is sequenced to follow
the dissolving
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heating vessel, as shown in step 44, whereby the high pressure of the solvent
n-hexane
contributes the necessary driving force. The extracted PP rich solution or
phase contains
besides the polymer polypropylene also small amounts of LDPE and HDPE, (a
total of
approximately 4.7%, when using a plastic material mixture of approximately 51%
PP,
43% LDPE and 16% HDPE as the starting material) as well as waxes and other
additives
(approximately 1.6% when using the above mentioned combination as the starting
material). The result in the lower phase when using the same plastic material
mixture as
the starting material is a polymer blend with approximately 3.2% PP and 37.1%
LDPE
and HI~PE as well as 1.5% waxes and other additives, as shown in step 46.
This modification of the method in accordance with the invention of the
polypropylene
rich phase from step 44 is mechanically cleaned using centrifuging, filtration
or the like,
as shown in step 48, to remove coloring agents and remaining insolubles, as
shown in
step 50. Preferably, the cleaned polypropylene rich phase is not recovered
using a flash
evaporation technique and degassing extrusion, as is seen in the prior art,
but rather is
processed further, in accordance with the invention, using shearing and
precipitation so
that polymer types still contained in the solution are precipitated as polymer
powder with
a shish kebab-like fiber structure.
In the embodiment example shown in Figure 6 the cleaned PP rich phase is,
firstly,
cooled to the first precipitation temperature T1 to shear and precipitate the
remaining
HDPE in the solution, as shown in step 52. The resulting suspension is
separated using a
solid-liquid separation method in step 54 into HDPE fibers (moisture-
containing HDPE),
as shown in step 56 and into the PP rich solvent, as shown in step 58.
The PP rich solution resulting from the solid-liquid separation in step 54 is
cooled to a
second precipitation temperature T2 at which polypropylene is sheared and
precipitated,
as shown in step 60. The resulting suspension is again separated in a solid-
liquid
separation step into its components polypropylene and solvent, as shown in
step 62.
The above described steps 52 to 62 for the separation of HDPE and PP using
shearing
and precipitation at precipitation temperatures T1 or, as the case may be, T2
essentially
corresponds to steps 16 to 26 from the method sequence as shown in Figure 5.
The
APE or, as the case may be, PP fibers resulting from steps 54 or, as the case
may be, 62
using the solid-liquid separation are finally degassed in a bypass extruder to
remove the
residual moisture from the polymer powder as also occurs in the method from
Figure 5.
The polyethylene-rich phase recovered in the phase separation in step 42 is
further
processed essentially as already described with reference to Figure 5. First,
the
polyethylene rich phase 46 is once more enriched with solvent to generate
approximately
a 20% polymer content in the solution. To separate coloring agents, remaining
insolubles
and the like, the solution generated in step 64 is, with the addition of new
solvent,
exposed to a subsequent mechanical cleaning, for example a centrifuge in step
66. The
~
CA 02376488 2001-12-14
12
cleaned solution is cooled to the first precipitation temperature T1 in step
68 and I-~PE
is precipitated using shearing. The suspension with HDPE fibers is separated
in step 70
resulting in, on the one hand, HDPE fibers 72 and, on the other hand, a
remaining
solution which in step 74 is cooled to the third temperature T3 and LDPE is
precipitated
using shearing. The foregoing steps essentially correspond to steps 16 to 18
as well as
step 32 of the process described above in reference to the method described in
Figure 5.
The LDPE suspension generated using shearing and precipitation in step 74 is
subsequently processed as in steps 34 to 40 in Figure 5. Steps 56 or, as the
case may be,
step 72 from Figure 6 correspond to method sequence steps 20 and 22 in Figure
5.
Method step 62 in Figure 6 corresponds to method steps 26 to 30 from Figure 5.
In other
words, the modified method shown in Figure 6 is essentially identical to the
method in
Figure 5 except for the fact that the starting solution first is separated
into a PP rich phase
and an HDPE rich phase.
Besides the excellent selectivity of the method in accordance with the
invention, it is
clear that the separation of different polymer types, in accordance with the
method of the
invention, keeps the residual moisture of the polymer fibers under I O% while
the residual
moisture in the prior art is approximately 60% by weight. To increase the
purity of the
product, the recovered polymer fibers (filter-cake) could be washed with pure
solution
and dehumidified. Since this is a method found in prior art it will not be
described in
further detail here. Because of the clearly improved removal of moisture and
increased
purity of the shearing precipitated polymer achieved with the method in
accordance with
the invention, the additional washing step is not necessary, since
particularly the
recovered polymer blends contain only minimal amounts of foreign polymers.
The modified method shown in Figure 6, in which in general terms the
separation of at
least one certain polymer type occurs using phase separation instead of
shearing and
precipitation, has the additional advantage that most coloring agents and
other impurities
remain in the PE rich phase in the phase separation, while the PP rich phase
essentially is
free of such impurities.
An additional particular aspect of the method in accordance with the invention
is that the
precipitation crystallization occurs in two steps, initially cooling the
solution to the
coolest possible, but safe, transportation temperature at which no polymer is
precipitated
and the shearing and precipitation then occurs at a pre-determined
precipitation
temperature in order to carry out the separation of the polymer type using as
little energy
as possible and while precisely adjusting the precipitation temperatures.
Figure 7 again shows an additional modification of the method according to the
invention
for the separation of polyolefin plastic material mixtures.
CA 02376488 2001-12-14
13
Although, the results from analysis of the method according to the invention
are
preliminary, they have shown that at least during shearing and precipitation
using a multi-
stage impulse counter-flow mixer in a heating vessel (as described in further
detail
below) at shearing rates that allow the precipitation temperatures of each
polymer type
HDPE, PP and LDPE to reach correspondingly approximately 100° C,
80° C and 75° C,
in accordance with the table above, not all plastic material powder is
precipitated under
all conditions allowing it to easily be filtered but rather that gels could
form which have a
high solvent content and which are difficult to process further. To avoid such
gel
formation, the shearing rate must be increased at which point the
precipitation
temperatures of PP and HDPE are so similar that their separation into polymer
blends
with very high purity (greater or equal to 95%) is difficult to achieve.
Therefore, in the
case of the gel formation, an alternative separation method in accordance with
Figure 7 is
suggested and explained in further detail below referring to Figure 7.
In the embodiment shown in Figure 7, firstly, the plastic starting material,
which in
particular contains PP, LDPE and HDPE is precipitated, as was the case in the
above
mentioned implementations in accordance with the invention. In a first
mechanical
separation step using a filter, decanter, centrifuge or the like, insoluble
compounds, heavy
materials and the like are separated.
Then the solution is led through a first precipitation step, as occurred in
the
implementation in Figure 5, and at a precipitation temperature of
approximately 60-70° C
polypropylene and HDPE is sheared and precipitated together so that only the
LDPE
remains dissolved in the solution. In this manner, an LDPE solution containing
PP/HDPE fibers is generated which could be separated using a solid-liquid
separation.
The remaining solution which now mainly contains dissolved LDPE can be further
processed in the conventional manner to recover the LDPE from it in particular
using
degassing or shearing crystallization as shown in Figures 5 or 6.
Finally, the PP and HDPE fibers are also separated as described in, for
example, Figures
or 6, in other words, for example, using a solid-liquid separation in
accordance with the
German patent application number 198 06 355.5 or using shearing
crystallization as
described above.
The remaining degassing steps, solvent processing, separation of waxes,
solvents,
additives and so forth could occur as described above.
The implementation in Figure 7, as has akeady been discussed, has the
advantage that
under all currently known processing conditions the gel formation can be
prevented using
higher shearing rates whereby the components PP and HDPE that were
precipitated
CA 02376488 2001-12-14
14
together, preferably, are separated using liquid-liquid separation.
Additionally, the fibers
are dissolved again at approximately 140° C and are finally separated
in a centrifuge at
approximately 170-200° C.
The method described is reliable and generates polymer blends with high
purity.
Figure 1 shows a precipitation heating vessel which is suitable for the
implementation of
the method in accordance with the invention.
Figure 1 shows the precipitation heating vessel 100 fitted with an agitation
device 102
which is implemented as a multi-stage impulse counter-flow mixer, a feeding
pipe 104
for solvents and a shearing head 106 that is moved with the aid of a driving
shaft 108 and
a motor 110. In the precipitation heating vessel 100 is a suspension or
solution 112 made
up of solvent, polymers, dissolved waxes and polymer fragments as well as
impurities.
The solution is led to a heat exchanger 1 I4 that is fitted with an inlet and
an outlet for the
coolant fluid. A second heat exchanger 116 serves to adjust a pre-determined
constant
temperature in the precipitation heating vessel 100. Preferably, this second
heat
exchanger 116 is a vapor condenser with a pressure controlled cooling water
volume
control with a connection to a vacuum system.
The processing sequence for the precipitation heating vessel, shown in Figure
1, is
described below using as an example the separation of PP from a polypropylene
rich
solution using petroleum spirits as the solvent. The same configuration could
naturally
also be used for the separation of every polymer type with every suitable
solvent
providing that the right combination of parameters, in particular the
temperature and the
shearing rate is chosen.
In the example at hand, a polypropylene rich solution with a temperature
higher or equal
to 170° C is led over the first heat exchanger 114 using petroleum
spirits as the solvent
and in a first processing step is cooled to a temperature that is somewhat
higher than the
first precipitation temperature, for example to approximately 130° C so
that no plastic
material polymer is precipitated which could cause the heat exchanger 114 to
clog. The
solution cooled to this temperature is transported in the pipe 104 under the
suspension
liquid gauge 112. In one embodiment, the pipe 104 leads into the lower area of
the
heating vessel; in another embodiment, it opens into the shearing head 106,
resulting in
two different implementations of the shearing head in Figures 2 and 3.
In both Figures, it can be seen that the pipe 104 opens into shearing gap 120
or, as the
case may be, 122 of shearing head 106' or, as the case may be, 106". This
shearing gap
120 or, as the case may be, 122 can be conical or flat as shown in Figures 2
and 3. The
shearing
. CA 02376488 2001-12-14
head 106, 106', 106" can have a smooth or a structured surface whereby the
surface
structure will be knobby to safely transport the precipitated polymer powder
out of the
gap 120, 122.
In this two-step process, the solution that has been pre-cooled to
approximately 130° C
abruptly cools to the set precipitation temperature as it leaves the pipe I04
and enters the
precipitation heating vessel 105. For the precipitation crystallization of the
polypropylene rich solution using petroleum spirits as the solvent, this
precipitation
temperature lies between 78° C and 86° C.
The additional heat that entered the precipitation heating vessel 100 with the
solution is
led out of the system using vapor condensation in the second heat exchanger
116,
assuring that the pre-determined constant temperature for the precipitation
crystallization
of the desired polymer type is always maintained with adequate precision of
about t 2° C
in the precipitation heating vessel even on an industrial scale. This
precision is achieved
as the heating vessel pressure is used as the reference variable for the
temperature
adjustment, which in this system is in thermodynamic equilibrium with the
liquid
temperature. At temperatures, which are below the boiling temperature for the
solvent at
standard pressure, low pressure evaporation cooling is generated with the help
of vacuum
pumps.
If in the feeding pipe 104, enough excess pressure relative to the pressure of
the
suspension in the precipitation heating vessel 100 can be generated, the
shearing in
accordance with the invention could also be achieved using a correspondingly
shaped
valve (not shown) at the end of the pipe 104 so that the solution has the
desired shearing
rate at entry into the precipitation heating vessel 100. In that case, the
shearing head 106
shown in the drawing is not necessary.
In Figure 1 a suspension outlet 118 is shown through which the suspension with
the
precipitated polymer type can be removed for the solid-liquid separation.
Figure 4 shows an additional embodiment of the precipitation heating vessel
for
implementation of the method in accordance with the invention, in which the
corresponding components have the same designation. Basically, the
precipitation
heating vessel is identical to the precipitation hcating vessel in Figure 1,
in which the
solution feeding through the pipe 104 also has a mixing nozzle 130 and a solid-
liquid
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16
separation step 132. This alternative embodiment allows a part of the
suspension in the
precipitation heating vessel 100 to be led to the mixing nozzle 130 via a wet
grinder 134
and a pump 136 and to be sheared and mixed in the mixing nozzle 130 with an
approximately equal amount of solution that was added via the heat exchanger
114. The
advantage of this alternative implementation is described in more detail
below.
A polypropylene rich solution is led to the heat exchanger 114 and cooled
there to
approximately 114° C and led on to the mixing nozzle 130. An equal
amount of
suspension from the precipitation heating vessel 100 with a temperature of
approximately
78° C is also led to the mixing nozzle 130 so that a mixing temperature
of approximately
96° C is generated in the mixing nozzle. Petroleum spirits is used as
the solvent. As
described above, since HDPE dissolves at temperatures between 95° C to
100° C when
using petroleum spirits as the solvent and the mixing nozzle 130 (which also
can be a
dispersing device) generates a shearing effect, the remaining HDPE is
dissolved in the PP
rich solution in the mixing nozzle 130 while being sheared and can be
separated from the
PP rich solution using the solid-liquid separation step 132 prior to being
measured out for
precipitation in the precipitation heating vessel 100.
These measures have several advantages. Firstly, the temperature difference
between the
suspension in the precipitation heating vessel 100 and suspension led past the
heat
exchanger 114 can be taken advantage of to separate remaining HDPE from the
solution
in a preliminary step, namely the mixing nozzle 130. Additionally, the
resulting repeated
pre-cooling of the solution prior to reaching the precipitation heating vessel
100,
additionally, help keep the temperature constant in the precipitation heating
vessel 100
since the differences in temperatures to be equalized are smaller.
In the precipitation heating vessel itself 100 the temperature is easily kept
constant within
~ 2° C of the lowest precipitation temperature of 78° C for PP
via the second heat
exchanger 116 using vapor condensing. The excess heat can be cooled using the
condensation cooling water.
In the embodiment shown in Figure 4, the desired shearing can be set in the
precipitation
heating vessel 100 using mufti-stage impulse counter-flow mixing blades 138.
The
suspension generated in the precipitation heating vessel 100 is either led
back to the
mixing nozzle 130 via the wet grinder 134 and the pump 136 or via the
suspension outlet
140 to a solid-liquid separation step.
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t
17
The invention characteristics disclosed above and in the drawings as well as
in the patent
claims could be significant both individually and in any chosen combination
for the
different embodiments of the invention.