Note: Descriptions are shown in the official language in which they were submitted.
ZOOlX02
MET~OD FOR EXTRACTION OF IMPURITIES
FROM POWDER MATERIAL
FIELD OF THE INVENTION
S The present invention relates to a method for
- highly removing impurities or the like contained in
powder or particle polymers, rubbers, etc. thereinafter
referred to as polymers).
BACKGROUND OF THE INVENTION
Conventionally, use has been made of, as a
method for removing impurities (e.g., solvents, monomers,
oligomers, and by-products) remaining in powder or
particles, or the like, of polymers, such means as
evaporation by heating and vacuum evaporation. However,
small amounts of impurities still remain, and some of the
impurities deteriorate the quality of the product or are
harmful to the human body. If the polymer is heated in
order to reduce the contents of impurities, the polymer
will be, for example, changed in quality or softened, so
that it is impossible to lower the remaining amounts of
impurities to certain degrees or below.
Therefore, recently, high-pressure extraction
processes using, for example, supercritical fluids have
been suggested. For example, ~apanese Patent Publication
2001~02
No. 46Y72/1984 discloses a process wherein cyclic
oligomers in a polymer are decreased by the use of
supercritical fluids.
However, when it is tried to remove impurities
highly from a liquid polymer using the high-pressure
extraction process described in Japanese Patent
Publication No. 46972/1384, as mentioned in Example 1
therein, there is a limit to high-degree extraction to
decrease impurities to a certain degree or below.
Further, when a polymer in the form of powder
or particles, or the like is subjected to the above high-
pressure extraction process in order to remove impurities
highly, in some cases a change, such as softening or
melting of the polymer, which is caused under extraction
conditions, causes the particles of the polymer to stick
to each other. Furthermore, if the temperature of the
extraction conditions is at or below the temperature at
which a change, such as softening of the polymer, will
occur, depending on the particle size and the particle
size distribution of the polymer in the form of powder or
particles, or the like, the volume of the polymer
increases due to absorption of the extractant when the
polymer is subjected to the ~xtraction, which sometimes
causes the particles of the polymer to stick to each
other.
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That is, in the prior high-pressure extraction
processes, when a polymer is subjected to extraction, the
particles of the polymer stick to each other along with
the progress of the extraction process, theraby forming a
mass of the polymer, which raises a qualitative problem
with the polymer product, is unfavorable when the product
is to be removed from the extractor, and raises the
necessity of a new process, such as grinding of the
product.
Therefore, it is considered to lower the
temperature so that such sticking or formation of a mass
may not take place. However, when the temperature is
lowered, the extraction speed becomes low, and the
extraction requires a longer period of time, which is
economically disadvantageous, because the device must be
made large and the amount of the extractant must be
increased.
In particular, when the polymer is a powder or
particles having small particle sizes and a broad
particle size distribution, sticking and the formation of
a mass are more liable to take place, and extraction to a
high degree is impossible.
BRIEF SUMMARY OF THE INVENTION
Therefore, the first object of the present
~00120~
invention is to provide a method for removing impurities
or the like contained in a polymer in the form of a
powder or particles by extraction without causing
sticking or the formation of a mass of the powder or
particles.
A second object of the present invention is to
provide a method for refining a powder or particles of a
polymer wherein impurities contained in the polymer can
be extracted efficiently, and the degree of the
extraction of the impurities is very high.
The above and other objects, features, and
advantages of the invention will become apparent in the
following detailed description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a flow sheet showing an embodiment of
the present invention.
Fig. 2 shows the particle size distribution of
a powder or particles to which the method of the present
invention can be suitably applied.
DETAILED DESCRIPTION OF THE INVENTION
The inventors have conducted research
intensively to overcome the above prior problems involved
in the removal of impurities in powder or particle
2001202
products, and have found that in charging a powder or 4
particle product of a polymer into an extractor, when the
powder or particle product is brought in contact with an
extractant in the supercritical state or a liquid form,
with the paxticle size distribution of the powder or
particle product and the void volume at the time of
charging thereof into the extractor being restricted
within respectively prescribed ranges, the particles can
be prevented from sticking to each other and impurities
can be removed efficiently, leading to the present
invention.
That is, the present invention provides (1~ in
a method for refining powdery or particulate solid
material by extracting impurities contained therein with
an extractant, the improvement comprising charging an
extractor with the solid material with the voidage
(fractional free volume) being 0.7 to 0.95 (hereinafter
referred to as first invention), and (2) in a method for
refining powdery or particulate solid material by
2G extracting impurities contained therein with an
extractant, the improvement comprising charging an
extractor with the solid material, whose particle
diameter is made substantially uniform, or the solid
material in which the weight of all particles whose
particle diameter is smaller than the particle diameter
ZO~)lZi[~2
indicating the maximum frequency of the particle size
distribution is made to be 40 % or below of the total
weight, and extracting the charged solid material with
the voidage being 0.36 to 0.95 (hereinafter referred to
as second invention).
An embodiment of the present invention will be
described with reference to the drawings.
Fig. 1 is a flow sheet showing an embodiment of
the present invention that uses a fixed-bed-type
extractor. In Fig. 1, an extractant is fed from a
storage tank (not shown), such as an extractant tank or
bomb, through a line 1 into a pressure booster 2 for the
extractant, where the pressure of the extractant is
increased to a pressure suitable for the extraction. The
pressure booster is a compressor when the extractant from
line 1 is a gas, or a pump when the extractant is a
liquid. The pressure booster may comprise plural
boosters that may be arranged in series or in parallel.
The extractant, whose pressure has been boosted to a
pressure for extraction, is fed through a line 3 into a
heat exchanger 4, where the temperature of the extractant
is controlled to a temperature for extraction by a fluid
from a line 5, and the extractant is fed through a line 6
to an extractor 7. A dispersing device 8 for the
extractant locaied in an upper section of extractor 7 is
~001;~02
composed of, for example, a sintered metal plate. The
extractant fed through line 6 into extractor 7 is
dispersed uniformly through dispersing device 8.
A support plate 14 made of a fine wire mesh or
a porous metal is situated at a lower portion in
extractor 7.
In Fig. 1, although the extractant is fed from
an upper portion of extractor 7, the extractant may be
fed from a lower portion of extractor 7. In the latter
case, the support plate 14 i5 placed at a lower portion
in extractor 7 so that it can also serve as a dispersing
device for the extractant. Further, a suitable device
may be placed at an upper portion in extractor 7 to
prevent the powder or particles from scattering.
A prescribed amount of powder or particles of a
solid polymer containing impuxities, which is to be
sub~ected to extraction, is charged below the dispersing
device 8 of the extractor from an inlet 12.
The extractant that has extracted impurities is
passed from a line 9 into a pressure controlling valve 10
and is discharged or recovered through a line 11. If the
extractant is to be recovered and reused, the impurities
contained in the extractant are separated and removed by
usual means of applying a pressure reduction and/or
elevating the temperature of the extractant, or, for
zoo~xoz
example, by means of adsorption separation.
The powder or particles that have been
subjected to ex~raction are removed from an outlet 13.
In the present invention, the term '~voidage
()" means the ratio of the volume of the void to the
bulk volume (V) of the powder or particles. The volume
of the void can be determined as the difference (V - Vp),
wherein V stands for the bulk volume and Vp stands for
the true volume of the powder or particles. The true
volume (Vp) can be determined as a product of the ratio
of the bulk density (~) to the true density (~p) and the
bulk volume (V). The bulk density (~) can be determined
by filling a container of a certain volume (V) with the
powder or particles and then measuring the weight (N)
thereof.
In the present first invention, when the
voidage is made greater than 0.7, extraction can be
carried out without being affected by the particle size
distribution and without causing sticking or forming a
mass. If the voidage is 0.95 or over, the volume
efficiency of the extractor will be lowered, which is
uneconomical.
Now the second invention will be described.
When the voidage is lowered below 0.7, the charge in the
extractor can be increased, thereby increasing the
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processing capacity favorably, but sticking or the
formation of a mass sometimes takes place, depending on
the state of the particle size distribution of the powder
or particles. However, for powder or particles that are
substantially uniform in particle diameter, if the
voidage is at least 0.36, s~icking or the formation of a
mass will not take place. Herein the term "substantially
uniform in particle diameter" means that 95 % or over of
the total weight of the powder or particles falls within
the range of + 10 % of an average diameter.
Further, for a powder or particles that are
ununiform in particle diameter and which therefore have a
particle size distribution, if the powder or particles
have such a particle size distribution that part of the
small particle size has been subtracted, and the voidage
is 0.36 or over, sticking or the formation of a mass will
not take place. In this case, it is essential that, in
the particle size distribution diagram of the particular
powder or particles, as shown in Fig. 2, wherein ~he
distribution of the region in which larger particle sizes
are involve~, which is located to the right of the
center, indicating the maximum frequency of the size
distribution, and the distribution of the region in which
smaller particle sizes are involved, which is located to
the left of said center, are assumed approximately equal,
~001202
from the region in which smaller particle sizes are
involved is subtracted a part A, which is at least 10 %
(namely, the weight of all particles having particle
sizes smaller than the particle size indicating the
maximum frequency is less than 40 ~ of the total weight),
more preferably 50 %, of the assumed whole distribution
of the powder or particles.
The adjustment to such voidage and particle
size distribution can be attained by controlling
processing conditions for, for example, pelletizing,
grinding, and sieving in the production steps of the
polymer.
In the present invention, the term ~'powder or
particles~- is not particularly restricted with respect to
the shape, and, as is apparent from above, it means not
only powders, but also particles, spheres, pellets, etc.,
which may be mixed.
In the present invention, although there is no
particular restriction on the particle diameter of the
powder or'particles, the average particle diameter is,
for example, 0.1 mm or over, and preferably 0.3 mm or
over. Further it is added that particles having a size
of 10 mm can be refined by the method of the present
invention not being limited to it as to upper limit.
In the present invention, the smaller the
200~2Q~
particle diameter of the powder or particles is, the
faster the diffusion of the extractant into the inside
thereof is, thereby increasing the extraction effect, to
quicken the extraction rate. The powder or particles
include a body that is powdery by nature and a solid
substance that has been finely ground, for example as
mentioned above. In the prior art, such a powder or
particles easily formed a mass when extraction was
carried out, whereas in the present invention such
formation of a mass is prevented, ieading to the increase
of the extraction effect and the extraction rate.
Preferable powders or particles that are
subject to the present invention are polymers (including
rubbers), such as polyethylene, polypropylene,
polystyrene, polyvinyl chloride, polyoxymethylene, and
polyacrylamide, and their copolymers, as well as
chlorinated products, rubbers, and chlorinated rubbers.
Impurities to be extracted include monomers, oligomers,
etc., remaining in such polymers, for example remaining
formaldehyde, styrene, trioxymethylene, water, and
organic solvents.
Preferable powder or particles suitable for the
extraction treatment of the method of the present
invention are ones having appropriate swellability for
the particular extractant, and an extractant that can
~OO~Z02
appropriately dissolve or swell such powder or particles
is used to carry out the extraction treatment.
The extractant used in the present invention is
suitably selected for the particular powder or particles
to be subjacted to extraction treatment, and as specific
examples of the extractant can be mentioned (aj carbon
dioxide, nitrous oxide, carbon disulfide, hydrocarbons
such as ethane, ethylene, propane and propylene, and
halogenated hydrocarbons, (b) a mixture of two or more of
the above gases, and (c) a mixture made up of the above
material and a second extractant for impurities (e.g.,
usual organic solvents such as methanol, ethanol, and
to~uene).
The extractant used in the present invention is
liquid or in the supercritical state, although it is
preferable that the extractant is used in the
supercritical state. In this case, although the pressure
and the temperature of the extractor may vary depending
on the powder or particles, generally it is preferable
20 that the pressure is 40 to 500 kg/cm2G and the
temperature is 0 to 150C. If the pressure is below 40
kg/cm2G, the extraction capacity of the extractant is low
and an adequate extraction effect cannot be obtained,
and, on the other hand, if the pressure is over 500
kg/cm2G, it is not economical in terms of the increased
~OOlX02
cost of the device that must be designed to be pressure
resistant and the power required to compress the
extractant. Further, if the temperature is below 0C,
the extraction rate lowers, and, on the other hand, if
S the temperature is over 150C, for example softening,
aggregation, forming of a mass, or deterioration of
quality of the powder or particles to be subjected to
extraction will take place.
In the present invention, plural extractors may
be arranged in parallel to perform the extraction
continuously, or they may be arranged in series.
According to the present invention, extraction
treatment can be carried out without allowing powder or
particles tG stick or form a mass. According to the
present invention, not only impurities can be extracted
effectively, but also refining of powder or particles can
be carried out with the extraction of impurities being
quite high.
Now the present invention will be further
described in detail with reference to Examples and
Comparative Examples.
Example 1
Chlorinated rubber (residual solvent
concentration: 6.9 %; residual solvent: carbon
tetrachloride) was subjected to extraction based on the
200~Z~)2
flow sheet shown in Fig. 1. The extractor was charged
with 68 g of the chlorinated rubber, and the voidage was
0.7. Carbon dioxide was supplied as an extractant from
the top of the extractor at a rate of 2.5 Q/min (at NTP
condition). After the pressure and the temperature in
the extractor were respectively kept at 100 kg/cm2G and
30C for 6 hours, the supply of carbon dioxide was
stopped. After the pressure in the extractor reached
atmospheric pressure, the chlorinated rubber was taken
out and analyzed. Formation of a mass was not observed
in the powder, and the concentration of the solvent in
the powder was reduced from 6.9 % to 0.5 ~.
Comparative Exam~le l
Extraction was carried out under the same
conditions as those in Example 1 based on Fig. 1. The
extractor was charged with lO9 g of chlorinated
polypropylene, and the voidage was measured and found to
be 0.6. The chlorinated polypropylene powder used for
the extraction was such that the weight of particles
having particle sizes smaller than the particle size
indicating the maximum frequency in the particle size
distribution of the chlorinated polypropylene powder was
50 % of the total weight. After the extraction, residual
concentration of the solvent was 0.7 %, but the powder in
the extractor formed a large mass.
14
; :00120;~
Example 2
Powdery polys~yrene (HIPS; residual monomer
concentration: about 2,000 ppm) was sieved out with a
48-mesh sieve, and part having larger particle sizes (the
weight of particles having particle sizes smaller than
the particle size indicating the maximum frequency in the
particle size distribution was 10 % of the total weight)
was subjected to extraction with carbon dioxide based on
the flow sheet of Fig. 1. The extractor was charged with
20 g of the powdery raw material, and the voidage was
measured and found to be 0.4. The pressure and the
temperature in the extractor were kept at 245 kg/cm2G and
80C respectively. After the extraction, no particular
formation of a mass was observed in the powder, and the
residual monomer concentration was 500 ppm or below.
COmRarative ExamPle 2
Powdery polystyrene (~IPS; residual monomer
concentration: about 2,000 ppm) was subjected to
extraction with carbon d oxide based on the flow sheet of
Fig. 1. Without being sieved with a 48-mesh sieve, 20 g
of the powdery polystyrene was charged into the
extractor, and the voidage was measured and found to be
0.35. The pressure and the temperature in the extractor
were kept at 245 kg/cm2G and 80~C respectively. After
the extraction, residual monomer concentration was about
~001~02
500 ppm, but the powder in the extractor formed a large
mass.
Ha~ing described the invention as related to the
embodiment, it will be obvious tha~ the invention not be
limited by any of the details of the description, unless
otherwise specified, but rather be construed broadly
within its spirit and scope as set out in the
accompanying claims.
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