CA2001202A1

CA2001202A1 - Method for extraction of impurities from powder material

Info

Publication number: CA2001202A1
Application number: CA002001202A
Authority: CA
Inventors: Hisashi Miyakawa; Naoki Mizutani; Kazuoki Urabe; Kageo Yoshida; Osamu Nittani; Koji Hirano
Original assignee: Hisashi Miyakawa; Naoki Mizutani; Kazuoki Urabe; Kageo Yoshida; Osamu Nittani; Koji Hirano; Yoshida, Sumiko; Toyo Engineering Corporation; Asahi Denka Kogyo Kabushiki Kaisha
Current assignee: Adeka Corp; Toyo Engineering Corp
Priority date: 1988-10-26
Filing date: 1989-10-23
Publication date: 1990-04-26
Also published as: FR2638098A1; JPH02117904A; DD286297A5; FR2638098B1; DE3935405A1

Abstract

ABSTRACT OF THE DISCLOSURE

METHOD FOR EXTRACTION OF IMPURITIES
FROM POWDER MATERIAL
There is disclosed a method for extraction of impurities from powdery or particulate solid material charged in a extractor. According to the disclosure, not only the extraction of impurities can be carried out efficiently without causing aggregation or the formation of a mass of the solid material, but also refinement of a solid material can be carried out, with the degree of the extraction of impurities being very high.

Description

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.

200~02 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 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 zoo~o~

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.

; :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.

Claims

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 being 0.7 to 0.95.

2. 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 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 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.

3. The method as claimed in claim 1, wherein the solid material is a polymer or a rubber, selected from the group consisting of polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyoxymethylene, polyacrylamide, and their copolymers and chlorinated products, rubbers, and chlorinated rubbers.

4. The method as claimed in claim 2, wherein the solid material is a polymer or a rubber, selected from the group consisting of polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyoxymethylene, polyacrylamide, and their copolymers and chlorinated products, rubbers, and chlorinated rubbers.

5. The method as claimed in claim 1, wherein the solid material has an average particle diameter of 0.1 mm or over.

6. The method as claimed in claim 2, wherein the solid material has an average particle diameter of 0.1 mm or over.

7. The method as claimed in claim 1, wherein the extractant is selected from the group consisting of carbon dioxide, nitrous oxide, carbon disulfide, a hydrocarbon and a halogenated hydrocarbon, a mixture of two or more thereof, and a mixture made up thereof and a second extractant for impurities.

8. The method as claimed in claim 2, wherein the extractant is selected from the group consisting of carbon dioxide, nitrous oxide, carbon disulfide, a hydrocarbon and a halogenated hydrocarbon, a mixture of two or more thereof, and a mixture made up thereof and a second extractant for impurities.

9. The method as claimed in claim 1, wherein the extractant is used in liquid state.

10. The method as claimed in claim 2, wherein the extractant is used in liquid state.

11. The method as claimed in claim 1, wherein the extractant is used in the supercritical state.

12. The method as claimed in claim 2, wherein the extractant is used in the supercritical state.

13. The method as claimed in claim 1, wherein the pressure in the extractor is in a range of 40 to 500 kg/cm2G.

14. The method as claimed in claim 2, wherein the pressure in the extractor is in a range of 40 to 500 kg/cm2G.

15. The method as claimed in claim 1, wherein the temperature in the extractor is in a range of 0 to 150°C.

16. The method as claimed in claim 2, wherein the temperature in the extractor is in a range of 0 to 150°C.

17. The method as claimed in claim 2, wherein 95 % or over of the total weight of the solid material falls within the range of ? 10 % of an average diameter.