Patent Publication Number: US-11661482-B2

Title: Separation and purification method of polyarylene sulfide

Description:
The present application is a National Phase entry pursuant to 35 U.S.C. § 371 of International Application No. PCT/KR2019/013771 filed on Oct. 18, 2019, and claims priority to and the benefit of Korean Patent Applications No. 10-2018-0125505 filed on Oct. 19, 2018 and No. 10-2019-0129387 filed on Oct. 17, 2019 with the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference in their entirety. 
     FIELD 
     The present disclosure relates to a method of more efficiently separating and purifying a polyarylene sulfide exhibiting excellent strength, heat resistance, flame retardancy, and processability when processed into a molded product after polymerization. 
     BACKGROUND 
     Polyarylene sulfide (PAS), which is represented by polyphenylene sulfide (PPS), has been widely used in automobiles, electrical and electronic products, machinery and the like to replace metals, especially die casting metals such as aluminum and zinc, due to its excellent strength, heat resistance, flame retardancy, and processability. Particularly, since the PPS resin has excellent flowability, it is suitable to use it as a compound by kneading with a filler such as glass fiber or a reinforcing agent. 
     Generally, PAS is prepared by polymerizing a sulfur source and a dihalogenated aromatic compound in the presence of an amide-based compound such as N-methyl pyrrolidone (NMP). Since the polymerization reaction of this method is a desalination polycondensation mechanism, a large amount of by-products such as sodium chloride are produced. Therefore, a process of removing the by-products is necessary after the polymerization reaction. However, it is difficult to completely remove the by-products in ordinary treatment, and commercially available general-purpose PPS products contain about thousands ppm of alkali metals. When alkali metal salts remain in the resulting polymer, deterioration of physical properties such as electrical properties may occur. Therefore, when a molded product prepared by using this PAS as a raw material is applied to electrical and electronic components, deterioration of electrical properties by the alkali metal in PAS is a big obstacle. 
     Accordingly, various methods are currently known for removing by-products such as sodium chloride from the PAS component. For example, Japanese Patent Publication No. 1986-220446 discloses a method of repeatedly washing a reaction product containing PPS with water and hot water, and then washing with an acidic aqueous solution, followed by washing with ion-exchanged water, wherein the PPS is obtained by reacting sodium hydrosulfide, sodium hydroxide and p-dichlorobenzene in N-methyl-2-pyrrolidone. Specifically, in order to remove the metal compound (by-product) generated in the PAS manufacturing process, a mixture containing solid PAS and a metal compound is contacted with water and the metal compound is dissolved in water and removed from the PAS. However, this method requires a long washing time using a large amount of water to remove metal impurities, and thus a very large and complicated process is required. In addition, there is a problem in that it is also necessary to treat a large amount of aqueous wastewater generated in the washing process. Process costs and the emission of environmental pollutants are high. Recently, a method of removing by-products such as sodium chloride from the PAS component has been known, in which the PAS component is separated by filtration using a vibrating mesh and sodium chloride is passed together with waste slurries. However, in this case, mesh holes are blocked by the sodium chloride fine powders, thereby reducing separation efficiency after several filtration processes. 
     In addition, even after PAS is polymerized, remaining unreacted substance is removed by washing with an organic solvent such as N-methyl-2-pyrrolidone (NMP) and acetone, or water. The organic solvent such as N-methyl-2-pyrrolidone as described above is not only expensive but also known to be a major cause of environmental pollution when discharged in the form of an aqueous solution. Therefore, it is generally recovered, purified and reused. However, since the organic solvent such as N-methyl-2-pyrrolidone has high solubility in organics, it has excellent compatibility with water and it is mixed with water infinitely. In addition, when a large amount of an inorganic salt is dissolved such as an effluent from a PAS manufacturing process, it is difficult to distill as-is, so various separation and purification methods have been tried. However, when water-washing is performed to remove by-products such as sodium chloride from the PAS component, the organic solvent used to wash the PAS is mixed with water, making it expensive to recover the organic solvent. 
     Accordingly, there is a continuing demand for development of a process for more efficiently separating and purifying a polyarylene sulfide without reducing separation efficiency, even if applied several times in succession, while increasing a recovery rate of the organic solvent with minimal energy consumption and washing costs of the entire process. 
     SUMMARY 
     The present disclosure provides a method of more efficiently separating and purifying polyarylene sulfide exhibiting excellent strength, heat resistance, flame retardancy, and processability when processed into a molded product after polymerization. 
     The present disclosure is also to provide a preparation method of a polyarylene sulfide including the separation and purification process as described above. 
     According to one embodiment of the present disclosure, there is provided a separation and purification method of a polyarylene sulfide, including the steps of: separating polyarylene sulfide particles by sedimentation, and then removing alkali metal halide particles with an organic solvent by sedimentation from a mixed solution containing polyarylene sulfide particles, alkali metal halide particles and an organic solvent using a decanter centrifugal separator at 1000 rpm to 2500 rpm; and drying the separated polyarylene sulfide particles at 100° C. to 200° C. before washing with water. 
     According to another embodiment of the present disclosure, there is provided a preparation method of a polyarylene sulfide, including the steps of: preparing a sulfur source including a sulfide of an alkali metal and a mixed solvent of water and an amide-based compound by dehydrating a hydrosulfide of an alkali metal and a hydroxide of an alkali metal in a mixed solvent of water and the amide-based compound; preparing polyarylene sulfide particles together with alkali metal halide particles by adding a dihalogenated aromatic compound and an amide-based compound to a reactor containing the sulfur source, and performing a polymerization reaction; washing the polymerization product containing the polyarylene sulfide particles and the alkali metal halide particles with an organic solvent; separating the polyarylene sulfide particles by sedimentation, and then removing the alkali metal halide particles with the organic solvent by sedimentation from a mixed solution containing the polyarylene sulfide particles, the alkali metal halide particles and the organic solvent by using a decanter centrifugal separator at 1000 rpm to 2500 rpm; and drying the separated polyarylene sulfide particles at 100° C. to 200° C. before washing with water. 
     As described above, in the present disclosure, polyarylene sulfide particles may be first separated by sedimentation depending on the difference in particle size without degrading separation efficiency due to the alkali metal halide particles, and then dried under optimum conditions to perform water-washing with no organic solvent remaining, thereby increasing a recovery rate of the organic solvent and significantly reducing recovery costs. Therefore, the present disclosure may efficiently separate and purify polyarylene sulfide particles with high separation efficiency even if separated several times in succession. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view of a process for separating and purifying a polyarylene sulfide in Example 1 according to one embodiment of the present disclosure. 
         FIG.  2    is a schematic view illustrating a process of removal of alkali metal halide particles by sedimentation after separating polyarylene sulfide particles by sedimentation in a decanter centrifugal separator according to Example 1. 
         FIG.  3    is a schematic view of a process for separating and purifying a polyarylene sulfide in Comparative Example 1 according to the prior art. 
         FIG.  4    is a schematic view illustrating a process of filtering polyarylene sulfide particles in a vibrating mesh to separate and purify them on the upper part, followed by removing alkali metal halide particles by passing them downward according to Comparative Example 1. 
     
    
    
     DETAILED DESCRIPTION 
     In the present disclosure, the terms “the first”, “the second”, and the like are used to describe a variety of components, and these terms are merely employed to distinguish a certain component from other components. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “include”, “have”, or “possess” when used in this specification, specify the presence of stated features, numbers, steps, components, or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, components, or combinations thereof. 
     As the present invention can be modified and have various forms, specific embodiments thereof are shown by way of examples and will be described in detail. However, it is not intended to limit the present invention to the particular form disclosed and it should be understood that the present invention includes all modifications, equivalents, and replacements within the idea and technical scope of the present invention. 
     Hereinafter, the present disclosure will be described in detail. 
     The present disclosure provides a method for efficiently separating and purifying polyarylene sulfide particles from a mixed solution containing polyarylene sulfide particles, alkali metal halide particles and an organic solvent. 
     In particular, the present disclosure is to efficiently separate polyarylene sulfide particles from a waste liquid obtained after washing the polyarylene sulfide particles produced in a polyarylene sulfide (PAS) manufacturing process with an organic solvent. Specifically, the polyarylene sulfide particles are first separated by sedimentation depending on the difference in particle size using a decanter centrifugal separator, and then dried under optimum conditions to perform water-washing with no organic solvent remaining, thereby increasing a recovery rate of the organic solvent and significantly reducing recovery costs. 
     In addition, in the present disclosure, the polyarylene sulfide particles produced after polymerization are first separated by sedimentation depending on the difference in particle size using a decanter centrifugal separator, thereby removing the organic solvent and alkali metal halide particles with the waste liquid by sedimentation. Therefore, the present disclosure may efficiently separate and recover polyarylene sulfide particles without degrading separation efficiency even if separated several times in succession. 
     According to one embodiment of the present disclosure, the separation and purification method of a polyarylene sulfide separates polyarylene sulfide particles and an organic solvent using a decanter centrifugal separator from a mixed solution containing polyarylene sulfide particles, alkali metal halide particles and an organic solvent, and then removes the organic solvent by an optimized drying process before washing with water. 
     Specifically, the separation and purification method of a polyarylene sulfide includes a step of separating polyarylene sulfide particles with an organic solvent by sedimentation, and then removing alkali metal halide particles by sedimentation from a mixed solution containing polyarylene sulfide particles, alkali metal halide particles and an organic solvent using a decanter centrifugal separator at 1000 rpm to 2500 rpm. 
     In the separation and purification method of a polyarylene sulfide, a decanter centrifugal separator is used instead of a conventional vibrating mesh, so that a problem of poor separation efficiency due to blocked mesh holes can be solved. That is, when the polyarylene sulfide particles are separated and purified by filtration, the mesh holes are blocked by the alkali metal halide particles after several filtration processes, thereby reducing the separation efficiency. However, the present disclosure is characterized in that the separation efficiency does not decrease even if separated several times in succession. 
     In particular, the present disclosure uses the decanter centrifugal separator at 1000 rpm to 2500 rpm, and does not perform separation by sedimentation depending on the difference in specific gravity of particles which is commonly known. Specifically, the present disclosure is characterized in that, despite a high specific gravity of alkali metal halide particles such as sodium chloride, polyarylene sulfide particles are first separated and purified by the difference in sedimentation velocity depending on the difference in particle size. 
     The decanter centrifugal separator may be used at 1000 rpm to 2500 rpm to make a difference in sedimentation velocity depending on the difference in particle size. When the decanter centrifugal separator is used at higher than 2500 rpm, the sedimentation of alkali metal halides is accelerated, resulting in coming out with polyarylene sulfide (PAS), and thereby lowering the separation efficiency. When the decanter centrifugal separator is used at lower than 1000 rpm, the sedimentation takes a long time, thereby increasing an amount of waste liquid in the polyarylene sulfide (PAS). 
     At this time, the decanter centrifugal separator may be used at a temperature of room temperature to 180° C., and a pressure of normal pressure to 5 bar. Herein, the room temperature refers to an ambient temperature at normal pressure, and may be about 20° C. to about 28° C., or about 22° C. to about 26° C. In addition, the normal pressure refers to an atmospheric pressure without a separate reduced pressure or pressurized condition, and may be about 0.95 atm to about 1.1 atm, or about 0.95 bar to about 1.1 bar. 
     The specific kind of the decanter centrifugal separator that can be used in the separation and purification process of a polyarylene sulfide according to the present disclosure is not particularly limited. For example, the decanter centrifugal separator is generally commercially available, and a commercially available one can be used in the present disclosure. 
     The decanter centrifugal separator is a device capable of separating a liquid-liquid mixture or a liquid-solid mixture by sedimentation, dehydration, concentration, etc. using centrifugal force. Generally, the decanter centrifugal separator is known to sediment and separate particles having a large specific gravity depending on the difference in specific gravity, that is, density of particles. However, the present disclosure is characterized in that the decanter centrifugal separator is operated under optimum conditions to separate polyarylene sulfide particles first by sedimentation and then remove alkali metal halide particles by sedimentation depending on the difference in particle size, not depending on the difference in specific gravity. 
     The present disclosure is to efficiently separate polyarylene sulfide particles from a waste liquid containing polyarylene sulfide particles produced in a polyarylene sulfide (PAS) manufacturing process, various inorganic salts and impurities. Accordingly, the mixed solution introduced into the decanter centrifugal separator may be a waste liquid or a mixed liquid thereof produced in a synthesizing or washing process of a polyarylene sulfide, and may further contain at least one selected from the group consisting of sodium hydroxide (NaOH), sodium acetate (NaOAc), sodium sulfide (Na 2 S), sodium hydrosulfide (NaSH), and p-dichlorobenzene in addition to polyarylene sulfide particles which are products of the polymerization process, alkali metal halide particles which are by-products, and an organic solvent and water which are used for washing. 
     Herein, specific examples of the polyarylene sulfide particles include polyphenylene sulfide (PPS). 
     The polyarylene sulfide particles may have a particle size of 100 micrometers (μm) to 2000 micrometers (μm), or 150 μm to 1800 μm. 
     For example, the particle size of polyarylene sulfide may be measured using standard mesh sieve having various sieve sizes such as 100 μm, 40 μm, or 10 μm. 
     Specific conditions for measuring the particle size of polyarylene sulfide are not particularly limited. For example, the particle size may be measured after the polyarylene sulfide particles are stored at about 22° C. to 26° C. under a relative humidity of 60% to 78% for at least about 3 hours, preferably at least about 6 hours, at least about 9 hours, at least about 15 hours, or at least about 24 hours. 
     In addition, the polyarylene sulfide particles may have a density of 1 g/cm 3  to 1.5 g/cm 3 , or 1.1 g/cm 3  to 1.45 g/cm 3 . 
     For example, the density may be measured by the method of the American Society for Testing and Materials, ASTM D 1505. 
     Meanwhile, specific examples of the alkali metal halide particles produced as a by-product together with the polyarylene sulfide particles in the polyarylene sulfide (PAS) manufacturing process include sodium chloride (NaCl), sodium iodide (NaI), and the like, and any one or a mixture of two or more thereof may be used. 
     The alkali metal halide particles may have a particle size of 5 μm to 30 μm, or 6 μm to 28 μm. 
     In addition, the alkali metal halide particles may have a density of 1.9 g/cm 3  to 3 g/cm 3 , or 2 g/cm 3  to 2.8 g/cm 3 . 
     Herein, the size and density of the alkali metal halide particles may be measured in the same manner as the polyarylene sulfide particles. 
     In addition, the organic solvent contained with the polyarylene sulfide particles produced in the polyarylene sulfide (PAS) manufacturing process and the alkali metal halide particles which are by-products is used in the washing process, and may be at least one selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, and isopropyl alcohol (IPA). 
     According to one embodiment of the present disclosure, after separating the polyarylene sulfide particles and the organic solvent using a decanter centrifugal separator as described above, a step of removing the organic solvent, etc. is performed by a first drying process before washing the separated polyarylene sulfide particles with water. 
     In particular, the first drying process before washing the polyarylene sulfide particles is performed at about 100° C. to about 200° C. prior to water-washing, so that the water-washing can be performed with no organic solvent remaining, thereby increasing a recovery rate of the organic solvent and reducing recovery costs. 
     The first drying process may be performed at about 100° C. to about 200° C., preferably at about 120° C. to about 190° C., or at about 135° C. to about 180° C. In particular, the first drying process may be performed at about 100° C. or more, preferably at about 120° C. or more, or at about 135° C. or more to evaporate the organic solvent used for polymerization or washing. In addition, the first drying process may be performed at about 200° C. or less, preferably at about 190° C. or less, or at about 180° C. or less, in order to prevent cross-linking of the polyarylene sulfide (PAS) caused by contact with hot air. 
     In addition, the first drying process may be performed under normal pressure or reduced pressure. The normal pressure is as described above, and the reduced pressure refers to a pressure below the normal pressure. For example, the first drying process may be performed under a pressure of 4×10 −14  bar to 1.1 bar, 4×10 −1 ° bar to 1.1 bar, or 4×10 −6  bar to 1.1 bar. In particular, it may be performed under reduced pressure in order to facilitate the evaporation of the organic solvent and to prevent cross-linking. 
     In the present disclosure, water-washing with no organic solvent remaining can be performed by the above first drying process before washing the polyarylene sulfide particles, and the polyarylene sulfide particles obtained after the drying process may have a residual amount of the organic solvent of 0.5 wt % or less, 0.3 wt % or less, or 0.1 wt % or less. 
     Subsequently, after removing the organic solvent, etc. by the first drying process, water-washing of the polyarylene sulfide particles may be further performed once or twice or more. In addition, after washing the dried polyarylene sulfide particles with water, a second drying process may be further performed to remove a solvent such as water. The second drying process may be performed at about 100° C. to about 200° C., preferably at about 130° C. to about 190° C., or at about 140° C. to about 180° C. In particular, the second drying process may be performed at about 100° C. or more, preferably at about 130° C. or more, or at about 140° C. or more to evaporate the solvent such as water after the washing process. In addition, the second drying process may be performed at about 200° C. or less, preferably at about 190° C. or less, or at about 180° C. or less, in order to prevent cross-linking of the polyarylene sulfide (PAS) caused by contact with hot air. 
     In addition, the second drying process may be performed under normal pressure or reduced pressure. The normal pressure is as described above, and the reduced pressure refers to a pressure below the normal pressure. For example, the second drying process may be performed under a pressure of 4×10 −14  bar to 1.1 bar, 4×10 −10  bar to 1.1 bar, or 4×10 −6  bar to 1.1 bar. In particular, it may be performed under reduced pressure in order to facilitate the evaporation of the solvent such as water and to prevent cross-linking. 
     In the separation and purification method of a polyarylene sulfide as described above, polyarylene sulfide particles are first separated by sedimentation depending on the difference in particle size using a decanter centrifugal separator, and then dried under optimum conditions to perform water-washing with no organic solvent remaining, thereby increasing a recovery rate of the organic solvent and significantly reducing recovery costs. 
     In particular, a recovery yield of polyarylene sulfide (PAS) particles separated and purified through the separation and purification process of the present disclosure may be about 90% or more, about 95% or more, or about 98% or more. And a residual amount of the organic solvent in washing water obtained from the water-washing process may be about 0.5 wt % or less, about 0.1 wt % or less, or 0.05 wt % or less. 
     Meanwhile, according to another embodiment of the present disclosure, there is provided a preparation method of a polyarylene sulfide including the separation and purification process as described above. 
     The preparation method of a polyarylene sulfide includes the steps of: preparing a sulfur source including a sulfide of an alkali metal and a mixed solvent of water and an amide-based compound by dehydrating a hydrosulfide of an alkali metal and a hydroxide of an alkali metal in a mixed solvent of water and an amide-based compound (first step); preparing polyarylene sulfide particles together with alkali metal halide particles by adding a dihalogenated aromatic compound and an amide-based compound to a reactor containing the sulfur source, and performing a polymerization reaction (second step); washing the polymerization product containing the polyarylene sulfide particles and the alkali metal halide particles with an organic solvent (third step); separating the polyarylene sulfide particles by sedimentation, and then removing the alkali metal halide particles with the organic solvent by sedimentation from a mixed solution containing the polyarylene sulfide particles, the alkali metal halide particles and the organic solvent using a decanter centrifugal separator at 1000 rpm to 2500 rpm (fourth step); and drying the separated polyarylene sulfide particles at 100° C. to 200° C. before washing with water (fifth step). 
     The preparation method of a polyarylene sulfide according to another embodiment of the present disclosure will be described in each step. 
     The above-described first step is preparing a sulfur source. 
     The sulfur source is prepared by dehydrating a hydrosulfide of an alkali metal, and a hydroxide of an alkali metal in a mixed solvent of water and an amide-based compound. Therefore, the sulfur source may include the mixed solvent of water and an amide-based compound remaining after the dehydration, together with a sulfide of an alkali metal prepared by the reaction of the hydrosulfide of an alkali metal with the hydroxide of an alkali metal. 
     Meanwhile, the sulfide of an alkali metal may be determined depending on the type of the hydrosulfide of an alkali metal used in the reaction. Specific examples thereof include lithium sulfide, sodium sulfide, potassium sulfide, rubidium sulfide, and cesium sulfide, and any one or a mixture of two or more thereof may be used. 
     Specific examples of the hydrosulfide of an alkali metal that can be used in the preparation of the sulfur source by reacting the hydrosulfide of an alkali metal with the hydroxide of an alkali metal include lithium hydrogen sulfide, sodium hydrosulfide, potassium hydrogen sulfide, rubidium hydrogen sulfide, cesium hydrogen sulfide, and the like. Any one or a mixture of two or more thereof may be used, and an anhydride or a hydrate thereof may be used. 
     Specific examples of the hydroxide of an alkali metal include lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide and the like, and any one or a mixture of two or more thereof may be used. The hydroxide of an alkali metal may be used in an amount of 0.90 to 2.0 equivalents, more specifically 1.0 to 1.5 equivalents, and more particularly 1.0 to 1.2 equivalents, based on 1 equivalent of the hydrosulfide of an alkali metal. 
     In the present disclosure, the equivalent refers to molar equivalent (eq/mol). 
     Further, in the preparation of the sulfur source by the reaction of the hydrosulfide of an alkali metal with the hydroxide of an alkali metal, an organic acid salt of an alkali metal capable of promoting the polymerization reaction and raising the degree of polymerization of a polyarylene sulfide in a short period of time may be added as a polymerization assistant. Specific examples of the organic acid salt of an alkali metal include lithium acetate, sodium acetate, and the like, and any one or a mixture of two or more thereof may be used. The organic acid salt of an alkali metal may be used in an amount of about 0.01 to 1.0 equivalent, specifically about 0.01 to 0.8 equivalent, and more specifically about 0.05 to 0.5 equivalents, based on 1 equivalent of the hydrosulfide of an alkali metal. 
     The reaction between the hydrosulfide of an alkali metal and the hydroxide of an alkali metal may be carried out in a mixed solvent of water and an amide-based compound. Specific examples of the amide-based compound include amide compounds such as N,N-dimethylformamide or N,N-dimethylacetamide, pyrrolidone compounds such as N-methyl-2-pyrrolidone (NMP) or N-cyclohexyl-2-pyrrolidone; caprolactam compounds such as N-methyl-ε-caprolactam, imidazolidinone compounds such as 1,3-dialkyl-2-imidazolidinone, urea compounds such as tetramethyl urea; phosphoric acid amide compounds such as hexamethylphosphoric acid triamide; and the like, and any one or a mixture of two or more thereof may be used. Among them, the amide-based compound may preferably be N-methyl-2-pyrrolidone (NMP), considering a reaction efficiency and a cosolvent effect as a polymerization solvent for preparing a polyarylene sulfide. 
     The water may be used in an amount of about 1 to 8 equivalents, specifically about 1.5 to 5 equivalents, and more specifically about 2.5 to 5 equivalents, based on 1 equivalent of the amide-based compound. 
     Meanwhile, in the first step, a sulfide of an alkali metal may be prepared by dehydrating reactants containing a hydrosulfide of an alkali metal, a hydroxide of an alkali metal and the like. Herein, the dehydration reaction may be performed by stirring at about 100 rpm to 500 rpm at a temperature of about 130° C. to 205° C. More specifically, the dehydration reaction may be performed by stirring at about 100 rpm to 300 rpm at a temperature of about 175° C. to 200° C. The dehydration reaction may be performed for about 30 minutes to 6 hours, or for about 1 hour to 3 hours. 
     During the dehydration reaction, the solvent such as water in the reactants may be removed by distillation or the like, and some of the amide-based compound may be discharged together with the water. In addition, some of the sulfur contained in the sulfur source may react with water by heat during the dehydration reaction, and may be volatilized as hydrogen sulfide gas. 
     As a result of the reaction of the hydrosulfide of an alkali metal, the hydroxide of an alkali metal and the alkali metal salt, a sulfide of an alkali metal is precipitated in a solid phase in a mixed solvent of water and an amide-based compound. Accordingly, when the sulfur source prepared by reacting the hydrosulfide of an alkali metal with the hydroxide of an alkali metal is used as a sulfur source in the preparation of a polyarylene sulfide according to the present disclosure, the molar ratio of the sulfur source refers to a molar ratio of the hydrosulfide of an alkali metal added during the reaction. 
     Subsequently, in order to remove the solvent such as water from the reaction product containing the sulfide of an alkali metal produced as a result of the above reaction, a dehydration process is performed. The dehydration process may be carried out according to a method well known in the art. The conditions are not particularly limited, and the specific process conditions are as described above. 
     Further, during the dehydration reaction, the sulfur contained in the sulfur source reacts with water to produce hydrogen sulfide and a hydroxide of an alkali metal, and the generated hydrogen sulfide is volatilized. Therefore, the amount of sulfur in the sulfur source remaining in the system after the dehydration reaction may be reduced by the hydrogen sulfide which is volatilized out of the system during the dehydration reaction. For example, when using the sulfur source mainly containing a hydrosulfide of an alkali metal, the amount of sulfur remaining in the system after the dehydration reaction is equal to the molar amount of sulfur in the introduced sulfur source minus the molar amount of hydrogen sulfide volatilized out of the system. Therefore, it is necessary to quantify the amount of effective sulfur contained in the sulfur source remaining in the system after the dehydration reaction from the amount of hydrogen sulfide volatilized out of the system. Specifically, the dehydration reaction is performed until the molar ratio of water to 1 mol of effective sulfur is 1 to 5, specifically 1.5 to 4, more specifically 1.75 to 3.5. When the water content in the sulfur source is excessively decreased by the dehydration reaction, water may be added to adjust the water content before the polymerization process. 
     Accordingly, the sulfur source prepared by the reaction of the hydrosulfide of an alkali metal with the hydroxide of an alkali metal and the dehydration as described above may include a mixed solvent of water and an amide-based compound together with a sulfide of an alkali metal, and the water may be included in a molar ratio of 1.75 to 3.5 based on 1 mol of sulfur contained in the sulfur source. In addition, the sulfur source may further include a hydroxide of an alkali metal prepared by the reaction of sulfur with water. 
     According to one embodiment of the present disclosure, the second step is polymerizing the sulfur source with a dihalogenated aromatic compound to prepare a polyarylene sulfide. 
     The dihalogenated aromatic compound usable for the preparation of the polyarylene sulfide is a compound in which two hydrogen atoms of an aromatic ring are substituted with halogen atoms. Specific examples thereof include o-dihalobenzene, m-dihalobenzene, p-dihalobenzene, dihalotoluene, dihalonaphthalene, dihalobiphenyl, dihalobenzoic acid, dihalodiphenyl ether, dihalodiphenylsulfone, dihalodiphenylsulfoxide, and dihalodiphenylketone, and any one or a mixture of two or more thereof may be used. In the dihalogenated aromatic compound, the halogen atom may be fluorine, chlorine, bromine or iodine. Among them, p-dichlorobenzene (p-DCB) may preferably be used in order to increase reactivity and suppress side reactions in the preparation of a polyarylene sulfide. 
     The dihalogenated aromatic compound may be added in an amount of about 0.8 to 1.2 equivalents based on 1 equivalent of the sulfur source. When the dihalogenated aromatic compound is added within the above range, a polyarylene sulfide having excellent physical properties may be obtained without lowering melting viscosity of the prepared polyarylene sulfide and increasing the content of chlorine present in the polyarylene sulfide. Considering the excellent effect of controlling the addition amount of the sulfur source and the dihalogenated aromatic compound, the dihalogenated aromatic compound may be added in an amount of about 0.9 to 1.1 equivalents. 
     Further, a step of lowering the temperature of the reactor containing the sulfur source to a temperature of about 150° C. to 200° C. may be further included before the second step to prevent vaporization of the dihalogenated aromatic compound. 
     Further, the polymerization reaction of the sulfur source and the dihalogenated aromatic compound may be carried out in a solvent of an amide-based compound which is a polar aprotic organic solvent and stable to an alkali at a high temperature. 
     Specific examples of the amide-based compound are as described above, and pyrrolidone compounds such as N-methyl-2-pyrrolidone (NMP) or N-cyclohexyl-2-pyrrolidone are preferable considering the reaction efficiency. 
     Since the amide-based compound contained in the sulfur source in the first step functions as a co-solvent, the amide-based compound may be added in the second step in a molar ratio of water (H 2 O) to the amide-based compound present in the polymerization system (a ratio of water/an amide-based compound) to be about 0.85 or more. 
     During the polymerization reaction, other additives such as a molecular weight regulator, a cross-linking agent and the like for controlling the polymerization reaction or the molecular weight may be further added in an amount not lowering physical properties and the yield of the polyarylene sulfide to be finally prepared. 
     The polymerization reaction of the sulfur source and the dihalogenated aromatic compound may be performed at about 200° C. to 300° C. Alternatively, the polymerization reaction may be performed in multiple steps, varying the temperature within the above-mentioned temperature range. Specifically, after the first polymerization reaction at about 200° C. or more and less than 250° C., the second polymerization reaction may be performed at a temperature higher than that of the first polymerization reaction, specifically at about 250° C. to 300° C. 
     The polyarylene sulfide produced as a result of the polymerization reaction is in the form of particles, and alkali metal halide particles are produced as a by-product together with the polyarylene sulfide particles. 
     The features related to the polyarylene sulfide particles and the alkali metal halide particles thus produced are as described above, and a detailed description thereof will be omitted. 
     According to one embodiment of the present disclosure, the third step is washing the reaction product produced as a result of the polymerization reaction with an organic solvent in order to remove impurities such as oligomers. 
     Specific examples of the organic solvent include N-methyl-2-pyrrolidone (NMP), acetone, isopropyl alcohol (IPA), and the like, and any one or a mixture of two or more thereof may be used. 
     This washing process with an organic solvent may be carried out according to a method well known in the art, and the conditions are not particularly limited. 
     According to one embodiment of the present disclosure, the fourth step is separating and purifying the polyarylene sulfide particles by sedimentation from the reaction product produced as a result of the polymerization reaction. 
     For separating and purifying the polyarylene sulfide particles, a mixed solution containing polyarylene sulfide particles, alkali metal halide particles and an amide-based compound is introduced into a decanter centrifugal separator, and the decanter centrifugal separator is operated at 1000 rpm to 2500 rpm to separate the polyarylene sulfide particles by sedimentation first, followed by sedimentation and removal of the alkali metal halide particles. 
     In another embodiment of the present disclosure, the features related to the sedimentation and separation process of polyarylene sulfide particles using a decanter centrifugal separator are as described above, and a detailed description thereof will be omitted. 
     According to one embodiment of the present disclosure, the fifth step is removing the organic solvent, etc. by the first drying process before washing the separated polyarylene sulfide particles with water, after the polyarylene sulfide particles and the organic solvent are separated using a decanter centrifugal separator as described above. 
     In particular, the first drying process before washing the polyarylene sulfide particles is performed at about 100° C. to about 200° C. prior to water-washing, so that the water-washing can be performed with no organic solvent remaining, thereby increasing a recovery rate of the organic solvent and reducing recovery costs. 
     In another embodiment of the present disclosure, the features related to the first drying process of polyarylene sulfide particles before washing with water are as described above, and a detailed description thereof will be omitted. 
     Thereafter, the separated and purified polyarylene sulfide particles may be optionally further washed with water, etc., filtered or dried according to a conventional method. 
     As a specific preparation method of the polyarylene sulfide, following examples may be referred to. However, the preparation method of the polyarylene sulfide is not limited to this description. The preparation method may further include a step which is usually carried out in the technical field of the present invention, and the step(s) of the preparation method may be changed by the step(s) usually changeable. 
     Hereinafter, the present invention will be described in more detail with reference to the following preferred examples. However, these examples are for illustrative purposes only, and the invention is not intended to be limited by these examples. 
     EXAMPLES 
     Example 1 
     1-1. Preparation of Polyphenylene Sulfide 
     Sodium sulfide was prepared by mixing 70% sodium hydrosulfide (NaSH) and sodium hydroxide (NaOH) in an equivalent ratio of 1:1.05 to prepare a PPS polymer. At this time, 0.4 equivalent of sodium acetate (CH 3 COONa) powder, 1.65 equivalents of N-methyl-2-pyrrolidone (NMP), and 4.72 equivalents of deionized water (DI water) were added to the reactor. Herein, the equivalent refers to molar equivalent (eq/mol). At this time, the solid reagent was added first, followed by NMP, DI water. Then, the reactor was stirred at about 150 rpm and heated to about 195° C. for 1 hour and 40 minutes to dehydrate. Thereafter, the temperature of the reactor was lowered to about 175° C., and 1.02 times more equivalent of para-dichlorobenzene (p-DCB) than sodium hydrosulfide and 1.35 equivalents of N-methyl-2-pyrrolidone (NMP) were added to the reactor. Thereafter, the reaction mixture was heated to about 230° C. and reacted for about 2 hours as a front reaction, and further heated to about 255° C. and reacted for about 2 hours as a rear reaction, followed by addition of 3 equivalents of distilled water and stirring for about 5 minutes. As a result, 500 g of a slurry containing PPS polymer particles was obtained. The slurry contained polyarylene sulfide particles, alkali metal halide particles, and an amide-based compound. 
     1-2. Separation and Purification of Polyphenylene Sulfide 
     100 g of the slurry produced in step 1-1 was aliquoted and washed once using 100 g of NMP. The obtained mixed solution was subjected to filtration using a decanter centrifugal separator to separate a polyarylene sulfide as a PPS wet cake as shown in  FIG.  1   . At this time, the decanter centrifugal separator was operated at 1000 rpm under room temperature and normal pressure conditions. 
     The PPS wet cake thus obtained was subjected to a first vacuum drying process under a pressure of about 0.15 bar or less at a temperature of 150° C. prior to washing with water to make a dry PPS. A residual amount of the organic solvent in the dry PPS was 0.5 wt % or less. Thereafter, washing was repeated four times using 50 g of water, and then a second vacuum drying process was performed under a pressure of about 0.15 bar or less at a temperature of 150° C. 
     The time taken to filter 100 g of the PPS slurry produced by repeating the above process was 0.3 hour including additional time for adding, etc., and a total amount of the PPS particles separated and purified was about 15.2 g. PPS fine powders remaining in the wastewater after recovery were further recovered using a fine filter, and the amount recovered was about 0.31 g. Accordingly, it was confirmed that a recovery yield of the PPS particles separated and purified with respect to a total amount of the PPS particles recovered was about 98%. In addition, it was confirmed that NMP content in washing water collected through the water-washing process performed before the last drying step was 500 ppm (0.05 wt %). 
     Comparative Example 1 
     The separation and purification process of a polyarylene sulfide was performed in the same manner as in Example 1, except that a PPS wet cake was obtained by performing filtration at room temperature and normal pressure using a vibrating mesh (see  FIG.  4   ) instead of the decanter centrifugal separator shown in  FIG.  3    and washed with water without a separate first drying process, in performing the separation and purification process of the mixed solution obtained by aliquoting 100 g of the slurry produced in step 1-1 of Example 1 and washing once using 50 g of NMP. 
     The time taken to filter 100 g of the PPS slurry produced by repeating the above process was 0.5 hour. A total amount of the PPS particles separated and purified was 14.7 g and a recovery yield was about 94.8%. In addition, NMP content in washing water collected through the water-washing process performed before the last drying step was 4 wt %, from which it can be seen that separation efficiency was greatly reduced. 
     Comparative Example 2 
     The separation and purification process of a polyarylene sulfide was performed in the same manner as in Example 1, except that the drying process was performed on a tray at 90° C. under normal pressure instead of vacuum drying, in performing the separation and purification process of the mixed solution obtained by aliquoting 100 g of the slurry produced in step 1-1 of Example 1 and washing once using 50 g of NMP. 
     The time taken to filter 100 g of the PPS slurry produced by repeating the above process was 0.3 hour including additional time for adding, etc., and a total amount of the PPS particles separated and purified was about 15.4 g. PPS fine powders remaining in the wastewater after recovery were further recovered using a fine filter, and the amount recovered was about 0.11 g. Accordingly, it was confirmed that a recovery yield of the PPS particles separated and purified with respect to a total amount of the PPS particles recovered was about 98%. In addition, it was confirmed that NMP content in washing water collected through the water-washing process performed before the last drying step was 4000 ppm (0.4 wt %).