JPH08259689A - Production of high-purity polycyanoaryl ether - Google Patents

Abstract

PURPOSE: To obtain a polycyanoaryl ether useful for electronic and precision machines, etc., with a low frequency of washing and a small amount of waste water according to a method for producing the polycyanoaryl ether gentle to the environment by separating and removing a by-product salt from a polymer solution prepared by carrying out the polycondensation of a dihalogenobenzonitrile with a dihydric phenol while the polymer solution is hot. CONSTITUTION: The polycondensation of (C) a dihalogenobenzonitrile (e.g. 2,6- dichlorobenzonitrile) with (D) a dihydric phenol (e.g. resorcin or hydroquinone) is carried out in the presence of (A) an alkali metallic salt [e.g. sodium (hydrogen)carbonate or potassium carbonate] and (B) a polymerization solvent (e.g. N-methyl-2-pyrrolidone). A by-product salt in the resultant high-molecular weight polycyanoacryl ether solution at 50-200g/l polymer concentration and 100-210 deg.C solution temperature is separated and removed from the solution by filtration, separation by allowing to stand still or centrifugation to afford the objective polymer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing high-purity polycyanoaryl ether, and more particularly to a method for producing high-purity polycyanoaryl ether which is useful in electronic machinery, precision machinery, aircraft and other high-tech fields. Is.

[0002]

2. Description of the Related Art Polycyano aryl ethers are widely used as engineering plastics having high heat resistance and as materials for electronic machines and precision parts. In the production of this polycyanoaryl ether, a poor solvent method and a solvent distillation method are known as precipitation methods for the resulting polymer (JP-A-4-31433, etc.). Here, the poor solvent method is a method in which water, alcohols, ketones, and the like, which are soluble in the polymerization solvent and are poor solvents for the polymer, and a mixture thereof are added to the polymerization liquid while stirring. Further, the solvent evaporation method is a method of evaporating a solvent from the polymerization liquid, concentrating it, and precipitating it. However, the polymers obtained by any of the methods produce a large amount of an inorganic salt as a by-product in the production of polycyanoaryl ether.
It was difficult to obtain a high-purity polycyanoaryl ether in which the concentration of the inorganic salt was lowered to a range where there was no problem in using the product, usually to a range of 50 ppm or less. On the other hand, Japanese Patent Publication Sho 6
JP-A-3-62532 or JP-A-4-31433 discloses that a powder obtained by precipitating a polymer from a polar solvent solution of a polymer obtained by a reaction of a dihalogenobenzonitrile and a dihydric phenol is washed with water to obtain an inorganic substance. Although a method of removing salts is disclosed, since the precipitated polymer is porous and bulky, it requires a large amount of water and a large number of repeated operations to remove it up to a specified residual inorganic salt content. I needed it. Further, the large amount of water discharged by using a large amount of water in the washing step is not preferable for the environment. Therefore, it has been strongly desired to simplify the washing step for obtaining high-purity polycyanoaryl ether, reduce the number of washing times by half, and establish an environmentally friendly manufacturing method.

[0003]

SUMMARY OF THE INVENTION The object of the present invention is to remove most of the inorganic salts in the polymerization liquid, thereby reducing the number of washings and reducing the amount of waste water. It is to develop a method for producing a cyanoaryl ether.

[0004]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to achieve the above object. As a result, before the polymer is made into particles, in a state where the polymer is dissolved,
It was found that by removing the by-product salt (inorganic salt) from the polymerization solution by hot separation such as filtration, stationary separation or centrifugation, the washing process can be simplified and the number of washings can be greatly reduced. The present invention has been completed based on such findings.

That is, according to the present invention, after the condensation polymerization of dihalogenobenzonitrile and dihydric phenol in the presence of an alkali metal salt and a polymerization solvent, the concentration of the obtained high molecular weight polycyanoaryl ether is 50 to 200 g / From a solution having a liquid temperature of 100 to 210 ° C.
Disclosed is a method for producing a high-purity polycyanoaryl ether, which comprises removing a by-product salt in a solution by static separation or centrifugation. Hereinafter, the method for producing a high-purity polycyanoaryl ether according to the present invention will be described in more detail.

First, in the present invention, dihalogenobenzonitrile and dihydric phenol are subjected to condensation polymerization in the presence of an alkali metal salt and a polymerization solvent. In the present invention, the dihalogenobenzonitrile used for the polymerization is 2,6
-Dichlorobenzonitrile; 2,6-difluorobenzonitrile; 2,4-dichlorobenzonitrile; 2,4-
Examples thereof include difluorobenzonitrile; 2-chloro-6-fluorobenzonitrile; 2-fluoro-6-chlorobenzonitrile, and the like, with 2,6-dichlorobenzonitrile and 2,6-difluorobenzonitrile being particularly preferred.

As the dihydric phenol, 1,2-
Dihydroxybenzene, 1,3-dihydroxybenzene, 1,4-dihydroxybenzene, 2-methyl-1,
4-dihydroxybenzene, 2,6-dimethyl-1,4
-Dihydroxybenzenes such as dihydroxybenzene and 2-methoxy-1,4-dihydroxybenzene;
4'-dihydroxybiphenyl, 3,5'-dihydroxybiphenyl, 3,5-dihydroxybiphenyl, 3-
Methyl-4,4'-dihydroxybiphenyl, 2,2 '
-Dihydroxybiphenyls such as dimethyl-4,4'-dihydroxybiphenyl; 1,2-dihydroxynaphthalene, 1,3-dihydroxynaphthalene, 1,4-dihydroxynaphthalene, 1,5-dihydroxynaphthalene, 1,6-dihydroxynaphthalene , 1,7-dihydroxynaphthalene, 1,8-dihydroxynaphthalene,
2,3-dihydroxynaphthalene, 2,6-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 4,
Examples thereof include dihydroxynaphthalene such as 8-dimethyl-2,6-dihydroxynaphthalene; dihydroxydiphenyl ether such as 4,4′-dihydroxydiphenyl ether. Particularly preferred dihydroxyaryl compounds are 1,3 and 1,4-dihydroxybenzene (resorcinol, hydroquinone).

Further, examples of the alkali metal salt include inorganic bases such as sodium hydrogen carbonate, sodium carbonate, potassium carbonate and potassium hydrogen carbonate, and sodium hydrogen carbonate, sodium carbonate and potassium carbonate are particularly preferable. In the method of the present invention, the dihalogenobenzonitrile and the dihydric phenol and the alkali metal salt as described above are appropriately mixed, usually, the dihalogenobenzonitrile and the dihydric phenol are mixed in an approximately equimolar ratio, and the alkali metal salt is In general, the molar ratio to the starting material dihalogenobenzonitrile is 1.0 to 3.0, specifically, sodium hydrogen carbonate is 2.05 to 2.40 (molar ratio), sodium carbonate and potassium carbonate are It is 1.03 to 1.30 (molar ratio).

The solvent used in the polymerization (polycondensation), that is, the polymerization solvent, is N-methylformamide,
N-alkyl carboxylic acid amides such as ethylacetamide; N, N-dimethylformamide, N, N-diethylformamide, N, N-dimethylacetamide and other N,
N-dialkylcarboxylic acid amides; N-alkyllactams such as N-methylpyrrolidone and N-ethylpyrrolidone; N-alkylcarboxamides such as N-methylpyrrolidinone; sulfoxides such as dimethyl sulfoxide and diethyl sulfoxide; dimethyl sulfone , Sulfone such as diethyl sulfone and diphenyl sulfone; cyclic sulfone such as sulfolane; nitriles such as acetonitrile, propionnitrile, butyronitrile, succinonitrile, benzonitrile, benzophenone; and mixtures thereof. Among these solvents, dimethyl sulfoxide, dimethyl sulfone, diethyl sulfone, diphenyl sulfone, sulfolane, N, N
-Dimethylformamide, N-methylpyrrolidone, N-
Aprotic strongly polar organic solvents such as methylpyrrolidinone and mixtures thereof are preferred, sulfolane, N-methylpyrrolidone, especially N-methyl-2-pyrrolidone (NM
P) is preferred.

In the polymerization, dihalogenobenzonitrile as a raw material and dihydric phenol are dissolved in the polymerization solvent. For example, when resorcinol is used as the dihydric phenol and N-methylpyrrolidone is used as the polymerization solvent, the concentration is reduced. 0.5-2.5 (resorcinol mol / N
-Methylpyrrolidone liter), preferably 0.7-1.
5 (resorcinol mol / N-methylpyrrolidone liter) is preferably used as a solution. The polycondensation reaction is usually 140 to 350 ° C, preferably 1
It may be carried out at a temperature range of 60 to 250 ° C. for 1 to 10 hours, preferably 2 to 5 hours. Furthermore, this reaction may be carried out under normal pressure or under a slight pressure. Also,
It is more effective to carry out the reaction in an atmosphere of an inert gas such as argon gas or nitrogen gas.

A molecular weight modifier may be added, if necessary, in the above polycondensation reaction. Examples of the molecular weight regulator that can be used include monohalogenobenzonitrile and dihalogenobenzonitrile. Also,
The amount to be used may be appropriately determined based on the relationship with the molecular weight of the target polymer. More specific conditions for the above polymerization are described in JP-A-59-206433 and 61-162.
No. 523, No. 61-192732, No. 62-
No. 223226, No. 63-189435,
This is as described in Japanese Patent Laid-Open No. 63-270733.

Next, the by-product salt in the solution is removed from the solution containing the high molecular weight polycyanoaryl ether obtained by the above-mentioned condensation polymerization by hot separation such as filtration, stationary separation or centrifugation. The method will be described. Here, before thermal separation, dilution is performed so that the concentration of the obtained high molecular weight polycyanoaryl ether is usually 50 to 200 g / liter, preferably 80 to 150 g / liter. If the concentration is less than 50 g / liter, the amount of liquid to be filtered is increased and the working efficiency is lowered, and if it exceeds 200 g / liter, the filtration rate is lowered and productivity is lowered, which is not preferable. . As for the method of dilution, the dilution solvent was preheated to 50 to 210 ° C., and the solution was added to the solution containing the high molecular weight polycyanoaryl ether while stirring. If the diluting solvent is less than 50 ° C, the polymer may precipitate, which is not preferable.
If it exceeds 10 ° C, the polymer may be decomposed, which is not preferable. When the stationary separation is carried out as the hot separation, it is necessary to dilute the polymer within the above-mentioned polymer concentration range in order to enhance the sedimentation property of the salt. Further, if the polymerization initiation concentration exceeds 200 g / liter, one hour or more is required for sedimentation, which is not efficient. further,
If the concentration is less than 50 g / liter by dilution, not only is the amount of solvent used large and it is uneconomical, but the bulk density of the product decreases, which is not preferable.

In the present invention, by using a filtration method, a stationary separation method or a centrifugal separation method as the hot separation,
The by-product salt in the solution is removed. Conventionally, the polymer was precipitated from the obtained polymerization solution containing the polymer, and then the by-product salt contained in the polymer was removed by performing many washing steps and the like. On the other hand, in the present invention,
The above-mentioned diluted solution is treated to separate and remove the precipitated salts without precipitating the polymer from the polymerization solution.

When the stationary separation method is used as the thermal separation,
Even if the static separation is performed in the same tank where the above dilution was performed,
It may be moved to another tank. Also, the temperature control of the tank is
The temperature difference between the inside and the outside of the tank is within 20 ° C, preferably within 10 ° C. If the temperature difference between the inside and the outside of the tank exceeds 20 ° C., polymer precipitation or convection of the liquid in the tank may occur, which is not preferable. In the thermal separation, it is necessary to keep the liquid temperature at 210 ° C. or lower in order to prevent the polymer from being cut and decomposed, and it is necessary to limit the standing time in the range where the liquid temperature is high. That is, when the liquid temperature is 210 to 205 ° C, it is within 0.5 hours, and when the liquid temperature is 195 to 205 ° C, it is within 1 hour, and 185 to 195.
At ℃, it is preferable to let the standing time within 2 hours, 1
If the temperature is lower than 85 ° C, it is possible to stand for 2 hours or more. Also,
The liquid temperature needs to be 100 ° C. or higher from the viewpoint of preventing polymer precipitation. When the stationary separation method is used, when the supernatant liquid is withdrawn after stationary, the supernatant liquid is withdrawn into the granulation tank from the extraction pipe at the upper portion of the salt depositing layer by pressurizing with nitrogen. At this time, in order to prevent blockage in the pipe, it is necessary to keep the pipe warm or warm. The salt may be washed to recover the residual polymer adhering to the salt. The salt is washed by using a solvent that is approximately equal in amount and isothermal to the above-mentioned liquid to be extracted, and similarly to the above, after the stationary separation, the supernatant liquid is extracted into the particle forming tank.

When the centrifugal separation method is used as the hot separation,
In the decanter type, either a general vertical type or a horizontal type can be used. As the separation operation, the same operation as the above-mentioned static separation method is performed.

When the filtration method is used as the hot separation, various types such as a single plate type, a multi-layer type and a centrifugal type can be generally used. Note that, when the liquid temperature is cooled, precipitation of the polymer occurs, so that it is usually necessary to perform heat retention and heating operations. Further, the driving force of the filtration is preferably that of the pressure type, because the pressure of the pressure type may lower the liquid temperature on the filtration surface and cause polymer precipitation. In the filtration method, the opening of the filter is usually 100 μm
The following is preferably 10 μm or less. Material of filter board is S
There are US materials, synthetic fibers, natural fibers, etc., which are used depending on the type of solvent and temperature. Also, as the separation operation,
The same operation as in the above static separation method is performed.

From the desalted solution obtained by the above-mentioned hot separation, the polymer is made into particles by the poor solvent method (Japanese Patent Publication No. 63-62532) or the solvent distillation method (Japanese Patent Laid-Open No. 4-31433). To do. The poor solvent method is a method in which water, alcohols, ketones, and the like, which are soluble in the polymerization solvent and are poor solvents for the polymer, and a mixture thereof are added to the polymerization liquid while stirring. Further, the solvent evaporation method is a method of evaporating a solvent from the polymerization liquid, concentrating it, and precipitating it.
The particle size of the powder thus obtained is 1 to 1000 μm.
m.

In the present invention, in the above-mentioned polymerization, a cyclic oligomer is by-produced depending on the combination of the monomers, and if it is contained in the product, the performance of the product is impaired. is necessary. As a method of removal, in the powder obtained above, there is a difference in solubility between the produced polymer and the by-produced oligomer, and the fact that the oligomer is usually more soluble is used for separation and removal in a mixed solvent. It can be performed. For example, the volume ratio of the polymerization solvent / water is 70 to 90/3.
A mixed solvent of 0 to 10 can be used, and alcohols such as methanol and ketones such as acetone can be used instead of water. In such removal of the by-product cyclic oligomer, the solid-liquid separation operation is performed in the same manner as the salt removal operation, and by repeating this operation, the degree of by-product removal can be enhanced.

The powder obtained as described above is
3 to 5 times more solvent than polymer and usually 1000 to 50
It contains 00 ppm residual salt. Therefore, it is necessary to remove the solvent or the residual salt. When removing the solvent mainly, it is preferable to wash using a washing liquid such as methanol, ethanol, acetone, methyl ethyl ketone, and when mainly removing the residual salt,
Use water such as ion-exchanged water or distilled water, or use inorganic acids such as hydrochloric acid or organic acids such as oxalic acid to improve efficiency.
It is preferable to perform cleaning using a cleaning liquid in which H is adjusted to about 2 to 4. In such solvent removal or residual salt removal, the degree of by-product removal can be increased by repeating the operation according to the purity of the intended product and the like.

Finally, the polymer obtained through the above operation is dried. Since the obtained high-purity polycyanoaryl ether has high heat resistance, no special care is required in drying, and either vacuum system or normal pressure system, batch system or continuous system can be used, and the temperature is usually 50 A wide range of up to 250 ° C is possible. Further, the dryer is not particularly limited, but when the above cleaning and removal are carried out in an acidic region, a corrosion problem may occur, so it is preferable to use a corrosion resistant material. As a model of the dryer, for example, a paddle dryer, a ribbon dryer, a Nauta-type conical dryer, a fluid dryer, a vibration dryer, a rotary dryer, a disk dryer, a conical dryer or the like can be used.

[0021]

EXAMPLES The present invention will be described in more detail by way of examples, which should not be construed as limiting the invention thereto.

Example 1 16.5 g of resorcinol, 25.7 g of 2,6-dichlorobenzonitrile and sodium carbonate were placed in a 1 liter separable flask equipped with a rectification device, a stirring device, a Dean Stark trap and an inert gas blowing tube. 17.5 g, 150 ml of N-methyl-2-pyrrolidone (NMP) was added, and the temperature was raised to 195 ° C. while blowing argon gas,
After that, toluene was added for 1 hour to reflux and dehydrate.
Then, the mixture was reacted at 200 ° C. for 75 minutes. Next, a solution of 2,6-difluorobenzonitrile 0.3 g / NMP 5 ml was added and reacted for 75 minutes to increase the molecular weight.
A solution of 2.0 g of 2,6-difluorobenzonitrile / 5 ml of NMP was added and reacted for 30 minutes for terminal treatment. The agitator at this time is SUS with a span of 60 mm.
A 316 half-moon wing was used.

In the stationary separation, the oil bath for heating was turned off when the end treatment was completed. 180 ° C in advance
Then, 150 ml of N-methylpyrrolidone that had been heated to was added and the stirring was stopped. The polymer concentration at this time was 105 g / liter. After leaving still for 15 minutes, it was confirmed that the salt separation was good, and the supernatant liquid was extracted into another 1-liter separable flask heated to 110 ° C. by pressurizing with nitrogen. In salt washing, 150 ml of N-methylpyrrolidone preheated to 180 ° C. was added, and the mixture was stirred for 5 minutes and then stopped. The polymer concentration at this time was 11 g / liter. After leaving still for 15 minutes, it was confirmed that the separation of the salt was good, and the salt was extracted into the separable flask in the same manner. In the granulation, 160 ml of N-methylpyrrolidone and 40 ml of ion-exchanged water were mixed in advance at room temperature and added dropwise with stirring.

For the removal of oligomers, 400 cm with a diameter of 10 cm
The particle forming liquid was put into a pressure filter having a capacity of 1 liter and having a SUS316L screen of × 40 mesh (twill weave), and filtration was performed under nitrogen pressure. N-methylpyrrolidone 3
20 ml and 80 ml of ion-exchanged water were mixed in advance, the liquid heated to 110 ° C. was charged into the above filter, and after stirring for 5 minutes, pressure filtration was similarly performed. In washing with water,
400 ml of ion-exchanged water at 60 ° C. was charged into the above filter, stirred for 10 minutes, and filtered under pressure to obtain a polymer cake. Repeat this operation 6 more times, 2
The second time, 2.5 g of oxalic acid was added.

For drying, the cake is taken out and SUS is used.
It was placed in a vat made of a resin and dried in a vacuum dryer at 180 ° C. for 6 hours to obtain 29.3 g of the polymer of the present invention. The yield of the obtained polymer was 93.5%, and the reduced viscosity was 60.
C, concentration in p-chlorophenol solution is 0.2 g / dl
At 1.28 dl / g. The residual sodium in the obtained polymer was 8 ppm, the residual oligomer in the polymer was 1.7%, and the residual NMP in the polymer was 0.4%.
The bulk density was 0.15 g / ml. The method for measuring the residual amount of each component is as follows. Na, K: Atomic absorption spectrophotometer Oligomer: High performance liquid chromatograph method (dimethylformamide extraction) NMP: Gas chromatograph method (chloroform extraction)

Example 2 The same raw material as in Example 1 was used to carry out a reaction and end treatment to obtain a polymer. In filtration separation, a filter made of SUS316L having an opening of 10 μm was set in 1 liter of pressure filtration, and heated to 200 ° C. in an oil bath.
The above polymerization solution was charged into the filter, pressurized with nitrogen, and salts were filtered. The filtrate was taken in a 1 liter separable flask. In the salt washing, 200 ml of N-methylpyrrolidone preheated to 180 ° C. was added to wash the deposits, and the deposits were likewise put in a pressure filter, stirred for 5 minutes, pressurized with nitrogen and filtered. did. The filtrate was taken in the separable flask described above. In the granulation, N-methylpyrrolidone 16 was placed in a separable flask containing the above filtrate.
The particle-forming liquid at room temperature, in which 0 ml and 40 ml of ion-exchanged water were mixed in advance, was added dropwise with stirring.

For the removal of oligomers, 400 cm with a diameter of 10 cm
The particle forming liquid was put into a pressure filter having a capacity of 1 liter and having a SUS316L screen of × 40 mesh (twill weave), and filtration was performed under nitrogen pressure. N-methylpyrrolidone 3
20 ml and 80 ml of ion-exchanged water were mixed in advance, the liquid heated to 110 ° C. was charged into the above filter, and after stirring for 5 minutes, pressure filtration was similarly performed. In washing with water,
400 ml of ion-exchanged water at 60 ° C. was charged into the above filter, stirred for 10 minutes, and filtered under pressure to obtain a polymer cake. Repeat this operation 6 more times, 2
The second time, 2.5 g of oxalic acid was added. In the drying, the above cake was taken out, placed in a SUS vat and dried in a vacuum dryer at 180 ° C. for 6 hours to obtain 29.8 g of the polymer of the present invention. The yield of the obtained polymer is 95.1.
%, And the reduced viscosity is 1.33 dl / g at 60 ° C. and the concentration in the p-chlorophenol solution is 0.2 g / dl. The residual sodium content in the obtained polymer was 7 p.
pm, the residual oligomer in the polymer was 1.4%, the residual NMP in the polymer was 0.4%, and the bulk density was 0.18 g / ml.

Comparative Example 1 In the static separation of Example 1, the polymerization liquid obtained by the reaction (polymer concentration 209 g / liter) was allowed to stand still for 90 minutes without dilution with N-methylpyrrolidone. No purified polymer was obtained because there was no precipitation of salt from the polymerization solution after standing.

Comparative Example 2 In the stationary separation of Example 1, N-methylpyrrolidone 1
Inject 50 ml to give a polymer concentration of 209 g /
Instead of using liter, N-methylpyrrolidone 50
The same procedure as in Example 1 was carried out except that 0 ml was added and the polymer concentration was set to 48 g / liter. As a result, a polymer was obtained, but the bulk density was 0.09 g / ml. Therefore, the obtained polymer is inferior in moldability due to its low bulk density, and in addition, this method is economically disadvantageous because the amount of solvent consumption increases. First result
Shown in the table.

Comparative Example 3 The same procedure as in Example 2 was carried out except that the same amount of N-methylpyrrolidone used in the salt washing was added to remove the oligomer without filtering and salt washing. did. However, regarding the number of washings with water, residual sodium is 10 ppm
This was done 11 times in order to: The results are shown in Table 1.

Comparative Example 4 In Example 2, 50 ml of 20 ° C. N-methylpyrrolidone was added for dilution, and the polymerization solution temperature was 80 ° C.
However, the polymer was precipitated and the salt and the polymer could not be separated. First result
Shown in the table.

Comparative Example 5 In Example 2, 50 ml of 180 ° C. N-methylpyrrolidone was added for dilution, and the polymerization solution was heated and pressurized to 220 ° C. and separated by filtration. Although a purified polymer was obtained, the reduction in molecular weight due to the decomposition of the polymer was confirmed from the measurement result of the reduced viscosity. The results are shown in Table 1.

[0033]

[Table 1]

[0034]

According to the method for producing a high-purity polycyanoaryl ether of the present invention, before polymerizing into particles,
By removing the by-product salt from the polymerization liquid by hot separation such as filtration, stationary separation or centrifugation, the washing process can be simplified and the number of washings can be greatly reduced. More specifically, the Na content remaining in the polycyanoaryl ether is adjusted to 20%.
In the case of washing so as to reduce the concentration to ppm or less, conventionally, it was necessary to wash 10 to 15 times with about 10 times as much water as the weight of the polymer in one washing. Similar high-purity polycyanoaryl ethers can be produced. Therefore, the present invention is a method for producing a high-purity polycyanoaryl ether that is less wasteful due to cleaning and is environmentally friendly. The high-purity polycyanoaryl ether obtained by the present invention can be used for electronic machines, precision machines, aircrafts and others. It is preferably used as a useful material in the high-tech field.

Claims (1)

[Claims] 1. In the presence of an alkali metal salt and a polymerization solvent,
After condensation polymerization of dihalogenobenzonitrile and dihydric phenol, the obtained high molecular weight polycyanoaryl ether has a concentration of 50 to 200 g / liter and a liquid temperature of 100 to 210 ° C. A method for producing a high-purity polycyanoaryl ether, which comprises removing a by-product salt in a solution by static separation or centrifugation.