Process for treating a polymer slurry

A process for treating a polyarylene sulfide polymer slurry which has been provided by reactions of an alkali sulfide and a dichloroaromatic compound in N-methylpyrrolidone, the process comprising the following steps of (i) through (iv): (i) a step of separating a polymer slurry which contains polyarylene sulfide particles, alkali chloride as a by-product in the form of crystals and a solution and an arylene sulfide oligomer and has a liquid component which is principally N-methylpyrrolidone into polyarylene sulfide particles and a slurry containing the crystalline alkali chloride, the oligomer, and unreacted organic material by screening the slurry without diluting with water; (ii) a step of subjecting the slurry containing the crystalline alkali chloride to solid-liquid separation to obtain the crystalline alkali chloride; (iii) a step of washing the polyarylene sulfide particles with an organic solvent and water; and (iv) a step of recovering the organic solvent from a filtrate obtained by solid-liquid separation.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates to a process for obtaining a polyarylene sulfide 
(referred to hereinafter as PAS) in the form of particles. More 
particularly, this invention relates to a process for treating a polymer 
slurry in order to prepare a particulate PAS in the form of a slurry 
containing in addition a crystalline alkali chloride by reacting an alkyl 
sulfide with a dichloroaromatic compound in a medium of water and 
N-methylpyrrolidone (referred to hereinafter as NMP), characterized in 
that the PAS, the chrystalline alkali chloride as a by-product and NMP are 
recovered in good yields. 
2. Prior Art 
Hitherto, there have been proposed many methods for producing PAS, among 
which methods a typical one involves the reaction of dichlorobenzene and 
an alkali sulfide in a polar solvent. There have also been reported many 
proposals with reference to specific reaction conditions and the like 
therefor. 
However, none of these proposals have been successful in isolating the 
resulting PAS from the medium containing an arylene sulfide oligomer as a 
by-product of polymerization, and therefore the PAS has been 
conventionally recovered in the form of powder from the reaction mixtures 
by the solvent flashing method. Furthermore, the PAS in the form of powder 
brings about problems such as that when it becomes a product it is easily 
blown off as dust upon drying because it is a resin having a small 
particle size (fine powder) with a relatively low bulk density. 
For the purpose of avoiding such problems, there have been developed 
recovering methods of PAS as a polymerization product to separate and 
recover the PAS more readily. One of such methods is disclosed, for 
example, in Unexamined Published Japanese Patent Application No. 1536/84. 
This method comprises polymerizing an alkali sulfide and a dihaloaromatic 
compound in NMP as a medium, then, for the purpose of separating PAS from 
the polymer slurry and recovering the medium NMP, adding first a large 
amount of water to the polymer slurry to dissolve therein most of soluble 
salts, then filtering PAS, and finally recovering NMP from the aqueous 
solution of NMP remaining after an arylene sulfide oligomer has been 
removed. However, the method uses water in a large amount during the 
separating of PAS, and thus it is not economical to recover NMP by the 
direct distillation of NMP containing a large amount of water. For this 
reason, a process for recovering NMP comprising once extracting it with an 
extracting agent such as n-hexanol and recovering it again by separating 
from the extracting agent is commonly used. Such a process makes the 
recovery of NMP complicated and brings about problems such as the 
treatment of waste water in a large amount. 
SUMMARY OF THE INVENTION 
An object of this invention is to improve the economical efficiency of the 
production of PAS by reducing the use of water or other solvents as far as 
possible and simplifying the procedure of separating PAS or an oligomer 
and recovering NMP as a solvent from a polymer slurry which is produced 
during the process of producing PAS by polymerization using NMP as a 
solvent. 
The process for treating a polyarylene sulfide polymer slurry according to 
this invention which has been provided by reaction of an alkali sulfide 
and a dichloroaromatic compound in N-methylpyrrolidone, the process 
comprising the following steps of (i) through (iv): 
(i) a step of separating a polymer slurry which contains polyarylene 
sulfide particles, alkali chloride as a by-product in the form of crystals 
and a solution and an arylene sulfide oligomer and has a liquid component 
which is principally N-methylpyrrolidone into polyarylene sulfide 
particles and a slurry containing the crystalline alkali chloride, the 
oligomer, and unreacted organic material by screening the slurry without 
diluting with water; 
(ii) a step of subjecting the slurry containing the crystalline alkali 
chloride to solid-liquid separation to obtain the crystalline alkali 
chloride, 
(iii) a step of washing the polyarylene sulfide particles with an organic 
solvent and water, and 
(iv) a step of recovering the organic solvent from the liquid obtained by 
the solid-liquid separation. 
According to a preferred embodiment of this invention screening with a 
sieve in step (i) of the polyarylene sulfide particles from the polymer 
slurry can be carried out while adding the liquid component obtained in 
the step (ii). 
According to another preferred emobdiment of this invention, methanol or 
acetone can also be used as the organic solvent for washing the 
polyarylene sulfide particles. 
According to a further preferred embodiment of this invention, water is 
added to the reaction mixture during the polymerization in order to 
produce the polymer slurry, the temperature then being raised to carry out 
the residual polymerization, and the mixture is cooled after 
polymerization to ensure that 90% or more of the precipitated polymer has 
a particle size of 100 .mu.-5 mm and most of the aforementioned crystals 
have a particle size of 1-70.mu.. 
According to another embodiment of this invention, the sieve for screening 
and separating the polyarylene sulfide particles is set to have a mesh of 
50-500.mu.. 
According to still another embodiment of this invention, the process 
comprises washing the polyarylene sulfide particles in a washing machine 
consisting of a stirring zone and a filtration zone with a filtering plate 
for dividing the zones therebetween, washing twice or more the polyarylene 
sulfide particles with a small amount of an organic solvent and water in 
the stirring zone provided with a stirrer to remove N-methylpyrrolidone, a 
polyarylene sulfide oligomer and an alkali chloride present within the 
pores of the polymer particles, and taking out the filtrate through the 
filtering plate from the stirring zone to remove it from the filtering 
zone. 
According to this invention, the aforementioned problems in the prior art 
are solved. 
In other words, the process for treating the polymer slurry according to 
this invention is generally applied to the treatment of the PAS particles 
having a larger particle size than that of the crystalline alkali chloride 
as a by-product. According to this invention, it is advantageous that the 
PAS particle is screened without dissolving the alakli chloride as a 
by-product. Moreover, the oligomer and unreacted organic materials which 
are dissolved in the solvent NMP can be separated from the polymer 
particles. 
Separation of the PAS can be carried out without dissolving the crystalline 
alkali chloride, so that no water other than that required for the 
polymerization is introduced into the solvent NMP, which may be easily 
recovered and reused. Furthermore, NMP and the crystals of the deposited 
salt can be readily separated. A great saving in the energy required for 
recovering solvents by the distillation of the used NMP, methanol or 
acetone or by the extraction of NMP from the NMP/alkali chloride/arylene 
sulfide oligomer mixture with an organic solvent such as methanol, acetone 
or the like can be achieved in comparison with those of other known 
methods. 
Furthermore, most of the alkali chloride contained in the filtered PAS 
polymer particles on completion of the polymerization have been removed 
during the previous step, so that the alkali chloride or the like can be 
readily removed by washing twice or more with the amount of water in the 
washing machine provided with a stirrer and a filtering plate for dividing 
the two zones therebetween.

DETAILED DESCRIPTION OF THE INVENTION 
Polymer Slurry 
The PAS polymer slurry to which this invention is applied is the one that 
has such a large particle size that the PAS can be separated from a 
crystalline alkali chloride by screening with sieves. The PAS particles 
preferably have a particle size of 100 micron or more. 
An example of the method of preparing a PAS having such a particle size is 
disclosed, for example, in the specification of Japanese Patent 
Application No. 126725/84, which we have previously filed. This method is 
the method of polymerizing an alkali sulfide and a dihaloaromatic compound 
in two steps in NMP as a solvent and comprises reacting the dihaloaromatic 
compound in a conversion of 50-98 mole % with the charged alkali sulfide 
in a solvent consisting of 0.2-5 liters of NMP and 0.5-2.4 moles of water 
per mole of the charged alkali at a temperature of 180.degree.-235.degree. 
C., adding water to ensure that water is present in an amount of 2.5-7.0 
moles per mole of the charged alkali sulfide, and carrying out reaction at 
a temperature of 245.degree.-290.degree. C. to obtain PAS in the form of 
particles. According to this method, there is provided a polymer slurry 
containing almost no PAS having a particle size of less than 100.mu. and 
containing the alkali chloride as a by-product, most of which has been 
deposited as fine crystals of 1-70.mu.. 
The PAS polymer slurry obtained on completion of polymerization contains in 
addition to PAS, a crystalline alkali chloride, a dissolved alkali 
chloride, an arylene sulfide oligomer and an unreacted dihaloaromatic 
compound, so that it is not always easy to obtain PAS from the slurry by 
direct solid-liquid separation, but it is possible to separate the PAS 
particles and the crystalline alkali chloride as fine crystals by 
screening with sieves. 
Treatment of the Slurry 
It is possible to separate the PAS particles from the polymer slurry by 
screening with a screening apparatus having a mesh size of 50-500.mu., 
particularly preferably 70-200.mu., in which the PAS particles are 
retained on the screen and the crystalline alkali chloride is transferred 
to the liquid phase. For the purpose of carrying out the screening more 
readily, the screening according to this invention can be carried out 
while the liquid produced in the later steps is added. 
In the PAS particles separated by screening is still contained NMP in which 
the arylene sulfide oligomer and the alkali chloride are dissolved, and 
thus the solvent is removed by washing. The washing is first of all 
carried out with an organic solvent and then with water. As the organic 
solvent for washing in this case, a solvent which does not affect PAS and 
has compatibility with water is used. It is preferable that a solvent 
dissolves the alkali chloride only slightly and has a low latent heat in 
evaporation. Preferred solvents are acetone and methanol. The organic 
solvent used for the washing of PAS can be used repeatedly by recovering 
it by distillation, part or all of which can be used as a liquid poured on 
the polymer slurry in the separation of the PAS particles therefrom. 
The arylene sulfide oligomer and NMP are removed from the PAS particles 
thus separated, and the alkali chloride is also removed by washing with 
water. Still small amount of substances are remain in the fine pores of 
the particles, so that it is desirable to carry out, not washing once with 
a large amount of a solvent, but washing repeatedly with a small amount of 
a solvent to contact with a fresh solvent and to pass the solvent through 
the aforementioned pores compulsorily. 
Therefore, it is preferred to use a batch-wise system to repeat washing and 
extraction of liquid in a washing machine. In order to carry out washing 
more efficiently, it is possible to use an ultrasonic apparatus to promote 
washing. As a washing machine to carry out such washing, a washing machine 
comprising a stirring zone and a filtering zone with a filtering plate 
provided for dividing the two zones therebetween can be used. The washing 
process comprises washing twice or more the polyarylene sulfide particles 
with a small amount of an organic solvent and water in the stirring zone 
provided with a stirrer to remove N-methylpyrrolidone, a polyarylene 
sulfide oligomer and an alkali chloride which are present in the pores of 
the polymer particles, and taking out the filtrate through the filtering 
plate from the stirring zone to remove it from the filtering zone. The PAS 
particles do not pass through the filtering plate, and liquid extraction 
is carried out under reduced pressure, pressurized condition or 
atmospheric condition. 
On the other hand, the slurry which is obtained by separating the PAS 
particles from the polymer slurry and thus contains the crystalline alkali 
chloride is first subjected to centrifugal separation method or the like 
to separate the crystalline alkali chloride. The filtrate is then 
subjected to distillation to recover NMP for reuse. Part of the filtrate 
prior to distillation can be used as a pouring liquid on the separation by 
screening. 
An organic solvent such as methanol separated from a later step which will 
be described is used as a pouring liquid in the separation of PAS. 
The PAS particles washed with an organic solvent such as methanol and the 
like and further washed with water is dried to obtain a product. 
As described above, it is possible to obtain efficiently and inexpensively 
with a simple process a product which does not contain NMP, an oligomer, 
an unreacted dihaloaromatic compound or an alkali chloride. 
This invention will now be described more specifically with reference to 
the appended drawing. An alkali sulfide and a dichloroaromatic compound 
dissolved in N-methylpyrrolidone are polymerized in a polymerization 
vessel to prepare a polymer slurry. The polymer slurry contains 
polyarylene sulfide particles, crystalline and dissolved alkali chlorides 
and an arylene sulfide oligomer and comprises N-methylpyrrolidone as a 
main liquid component. The polymer slurry is separated into the 
polyarylene sulfide particles and a slurry containing the crystalline 
alkali chloride, the oligomer and an unreacted organic material by 
screening with a polymer slurry separator 2. The polyarylene sulfide 
particles thus separated are first washed with an organic solvent (acetone 
or methanol) in a washing vessel 3 to separate the polyarylene sulfide 
particles from filtrate. The polyarylene sulfide particles are further 
washed with water and acetone (or methanol) in a washing machine 5. The 
washed polyarylene sulfide particles are dehydrated in a separator 6 to 
obtain a product A. 
The filtrate B is taken out from the other outlet of the washing machine 5. 
The slurry which is separated in the polymer slurry separator 2 and 
contains the crystalline alkali chloride is subjected to solid-liquid 
separation in a separator 7. 
The crystalline alkali chloride thus separated is then washed with acetone 
or methanol in a washing vessel 8 and the crystalline alkali chloride E is 
discarded through a separator 9, and the separated acetone or methanol is 
preferably recycled to the separator 2. 
The organic solvent (acetone or methanol) recovered from the liquid after 
solid-liquid separation is put into a column 10 and then purified for 
re-use (D), also C is discharged from the bottom of the column 10 and then 
discarded.