WO2016008175A1 - Method for separating and purifying p-dichlorobenzene - Google Patents

Abstract

Provided is a high-purity, high-yield, low-energy consumption and pollution-free method for separating and purifying p-dichlorobenzene, which mainly includes solventing-out precipitation and fractional crystallization steps, wherein the solventing-out precipitation is as follows: putting a p-dichlorobenzene mixture in an ethanol solution, then adding distilled water and controlling the temperature at 5°C-8°C, and standing to obtain p-dichlorobenzene with a purity of 80%-88%; and then fractionally crystallizing (i.e. two-stage and six-section melt crystallizing and refining) the p-dichlorobenzene with the above-mentioned concentration to obtain the p-dichlorobenzene product with a purity of more than 99.9% and a yield of more than 90%. The composite separation and purification method uses ethanol as a main solvent and water as a precipitation agent, and is low in solvent costs, free from pollution, mild in process conditions, and convenient in operation; and in each step of the fractional crystallization process, a DCS system is used to realize the continuous production, reduce the sublimation and loss of the raw material, and then improve the handling capacity and product quality.

Description

Method for separating and purifying p-dichlorobenzene Method for separating and purifying p-dichlorobenzene

Technical field

The invention relates to a method for purifying dichlorobenzene, belonging to a method for separating and purifying an organic mixture.

Background technique

p-Dichlorobenzene (PDCB) is a white crystal with camphor odor and sublimation at room temperature. Its relative density is

1.4581 g/cm3, boiling point 174 ° C, melting point 53 ° C. It is soluble in ethanol, ether, benzene, chloroform, carbon disulfide, but almost insoluble in water. P-dichlorobenzene is

An important organic chemical product, as a highly effective and low-toxic civilian anti-mite and anti-mildew product, for the safety of use, the purity of dichlorobenzene must be greater than

99.5%. Dichlorobenzene is an important organic synthetic raw material for the synthesis of fuels and pesticides and for the production of engineering plastics such as polyphenylene sulfide. Industrial production

The content of p-dichlorobenzene in the dichlorobenzene obtained by catalytic chlorination is only 77-85%. Therefore, in order to obtain high-purity dichlorobenzene, the product must be separated and purified.

Para-dichlorobenzene is a widely used fine chemical that has been studied abroad for over 100 years. Domestically, due to the limitations of separation technology, the development of dichlorobenzene and

The application is still in the development stage. The production methods of p-dichlorobenzene commonly used in the industry are: cooling method, extraction method, washing method, distillation method, rectification method, recrystallization

Method, fractional crystallization method and emulsification method, and isomerization method. The cooling method in the above method is a method that is applied more. The above method generally has a problem of product purity.

Not high; therefore, in order to obtain high purity p-dichlorobenzene, it is necessary to develop a new separation and purification technology to further purify the product.

According to the difference in melting point between o- and p-dichlorobenzene (PDCB53°C, ODCB-17°C ), the industry generally uses multi-stage melt crystallization and sweat crystallization method to divide

from. However, in the process of melt crystallization, it is difficult to obtain a high-purity product due to the entanglement and adhesion of the mother liquid in the crystal. In order to solve this problem, people

A lot of research work has been done from the perspective of engineering and technology, and some new and efficient crystallization equipments have been developed, as well as new separations such as adsorption separation and washing crystallization.

method.

Solvent crystallization is the difference in the interaction between the separated material and the solvent molecule. By changing the properties of the solvent to selectively dissolve the impurities, the target component is the most

It is crystallized from the solvent to a large extent. The specific process is: the solute is first dissolved in a solvent (called the main solvent) to form a saturated solution, and then passed through another

a second solvent (called a precipitant) that is completely miscible with the main solvent and is immiscible with the solute to significantly reduce the solubility of the solute, allowing the solute to crystallize and separate.

the goal of. In this process, the main solvent acts to selectively dissolve impurities to improve the purity of the product, while the role of the precipitant is to reduce the solubility of the product.

To increase its yield. Therefore, as long as the solvent is properly selected, the dual effects of high purity and high yield can be achieved.

The rectification method is a method in which the boiling point of each component is different, and the separation is performed in a distillation still having a higher number of plates, and the difference in boiling points of the separated substances is more

The easier it is to separate. The boiling point of p-dichlorobenzene and o-dichlorobenzene is only about 5 °C, so there is a defect that the separation is difficult and the energy consumption is high.

The recrystallization method generally uses two doctor blade surface crystallizers. In the first clean zone, the crude dichlorobenzene mixture is cooled to 40 ° C ~ 45 ° C and pumped to the crystallizer.

The material is further cooled to 35 ° C, and the separated parallel body is separated by a centrifuge, reheated and melted, and enters the second clean zone, and is cooled to a lower temperature in the second clean zone.

temperature. In order to obtain a high-purity counterpart, the crystal can be ground and sieved, the fine powder is remelted again and recycled to the second clean zone, which will contain two parts of the orthodontic body.

The mother liquors are combined and sent to a vacuum distillation column to obtain industrial grade o-dichlorobenzene (content 87%) at the top of the column, and if necessary, heavy distillation can be carried out to obtain high purity.

Degree of o-dichlorobenzene. The bottom liquid residue of the vacuum distillation column is recycled to the crystallizer to recover a trace amount of the parallax. High purity products can be obtained by recrystallization, but this requires

For a variety of equipment such as centrifuge pumps, the energy consumption is relatively large.

The fractional crystallization method is based on the difference in melting point of each composition, and the temperature is controlled to be different, and the crystallization is separated stepwise. Since the melting point of o-dichlorobenzene and p-dichlorobenzene differs by 70.6 ° C,

Therefore, the fractional crystallization method is easy to separate, and the latent heat of fusion is lower than the latent heat of vaporization (the latent heat of o-dichlorobenzene is 12.29 J/g and 271.7, respectively). J/g), energy consumption

Rectification is low and operation at lower temperatures also avoids decomposition of the product. Therefore, many countries in the world generally use fractional crystallization to separate the pairs and neighbors of dichlorobenzene.

Isomer. At present, the commonly used methods for the purification of p-dichlorobenzene at home and abroad are crystallization and emulsification. Emulsification and purification method is to use surfactant aqueous solution and pair

The oily o-dichlorobenzene in the crystal lattice of dichlorobenzene or wrapped on the surface of the crystal lattice forms a stable emulsifier. After cooling, filtration, centrifugation and drying, the content is

99.5% of p-dichlorobenzene, o-dichlorobenzene content is less than 0.5%.

The invention mainly integrates according to the advantages and disadvantages of the above various methods, and tries to find a purification separation method which is composed of the above two or more methods.

And can achieve low energy consumption, no pollution, high purity, high yield function.

Summary of the invention

 The invention provides a method for separating and purifying p-dichlorobenzene with high purity, high yield, low energy consumption and no pollution, and the main purpose is to achieve the above technical purpose.

The following technical solutions are implemented:

The method for separating p-dichlorobenzene according to the present invention is characterized in that it comprises a step of elution precipitation and a stepwise crystallization, wherein the step of elution and precipitation comprises:

A1, 40~78wt% of p-dichlorobenzene mixture is placed in 70~95% ethanol solution and mixed to form a solution, wherein the quality of p-dichlorobenzene and ethanol solvent

The ratio is 1:2.5~7.1;

B1, gradually adding distilled water to the above solution, wherein the added weight of the distilled water is 0.35 to 0.45 times the weight of the ethanol; controlling in the process of adding distilled water

The temperature of the solution is from 5 ° C to 8 ° C, and is allowed to stand for 20 to 40 minutes to obtain p-dichlorotoluene having a purity of 80 to 88% and a yield of more than 77%;

The stepwise crystallization is carried out by using the above-mentioned 80-88 wt% p-dichlorobenzene as a raw material, and purifying by using two-stage six-stage melt crystallization, and the specific steps are as follows:

A2, the above-mentioned purity of 80~88% of p-dichlorotoluene is purified by the first stage 1A cooling crystallization to obtain 95~97wt% of p-dichlorobenzene product and mother liquor, and the mother liquor is sent to the adjacent two

Chlorobenzene refining process;

The p-dichlorobenzene product of b2 and 1A is purified by the 1B stage melt crystallization to obtain 98~99wt% of p-dichlorobenzene product and mother liquor. The mother liquor is sent to the raw material tank, and the 1A stage crystallization is repeated.

process;

The c2, 1B-para-dichlorobenzene product is melted in the 1C stage and sent to the second-stage crystallization section for further purification;

After the molten product of d2 and 1C is cooled by the second stage 2A, the product is crystallized and refined to produce 99.5 wt% of p-dichlorobenzene product and mother liquor, and the mother liquor is returned to the raw material tank.

Continue to repeat 1A cooling crystallization;

After the product of e2 and 2A is melted and crystallized in section 2B, more than 99.9 wt% of p-dichlorobenzene product and mother liquor are purified, and the mother liquor returns to section 2A to continue cooling and crystallization;

F2. Finally, the product obtained in paragraph 2B is melted in 2C section and sent to the sliced and packaged finished product.

Here, the mixture of the first raw materials contains impurities such as o-dichlorobenzene and low boilers, and further, the low boilers are chlorinated benzene and m-dichlorobenzene.

Here, the concentration of the ethanol solution in the above-described elution and precipitation step was 90% by weight.

The mass ratio of the p-dichlorobenzene to the ethanol solution in the above-described elution and precipitation step is 1:3 to 5.

Among them, the amount of distilled water added is preferably 0.4 times the mass of ethanol.

Among them, in the temperature control process of the b step in the dissolution and precipitation, the temperature is gradually increased from 5 ° C to 8 ° C, wherein the heating rate is 0.075 ~ 0.15 ° C / min.

The solubility of o- and para-dichlorobenzene is sensitive to the change of the solvent ratio, and the p-dichlorobenzene gradually precipitates with the addition of water. This is mainly due to water molecules and

Hydrogen bonding between the molecules of ethanol, water molecules surround the alcohol molecules, so that the interaction between ethanol molecules and dichlorobenzene is gradually weakened. At the same time, by lowering the temperature,

The p-dichlorobenzene is gradually precipitated from the mixed solvent. The method combines extraction, crystallization and washing into one body, and utilizes the selectivity of the main solvent ethanol to impurities (ODCB).

The precipitation of water on the target product (PDCB) and the washing action of the mixed solvent on the crystal, so as to effectively solve the adhesion and wrap of the impurity in the crystal

The problem is to ensure the yield and purity of the product.

Specifically, the cooling temperature of the 1A section in the step a2 to c2 of the stepwise crystallization is lowered from 50 ° C to 10 ° C, wherein the cooling rate is 0.15 ° C / min, and the melting of the 1 B section

The temperature was raised from 10 ° C to 48 ° C, the heating rate was 0.1 ° C / min, the melting temperature of the 1 C stage was raised from 48 ° C to 80 ° C, and the heating rate was 1 ° C / min.

Specifically, the cooling temperature of the 2A section in the step d2 to f2 of the stepwise crystallization is: from 70 ° C to 15 ° C, the cooling rate is 0.2 ° C / min, and the melting temperature of the 2B section

The temperature is raised from 15 ° C to 50 ° C, the heating rate is 0.08 ° C / min, and the melting temperature of the 2 C section is: from 50 ° C to 80 ° C, and the heating rate is 1 ° C / min.

Wherein, in the b step of the elution and precipitation in the present invention and the step a2 to f2 in the stepwise crystallization, the DCS system is used to realize continuous operation, and the whole p-dichlorobenzene separation and purification step is realized.

Accurate control ensures the purity and yield of the finished product and reduces energy consumption.

Among them, the stepwise crystallization step of the present application finally yields a yield of p-dichlorobenzene of more than 90%.

The beneficial effects of the present invention are as follows: (1) using ethanol as a main solvent and water as a precipitating agent, separating and purifying from a mixture of o- and p-dichlorobenzene by elution crystallization

Dichlorobenzene This is a simple and effective separation method with low solvent cost and no pollution. (2) The above-mentioned dissolution crystallization equipment is simple, the process conditions are mild, and the operation

Very flexible. (3) The secondary six-stage melt crystallization process is adopted to ensure precise control of temperature and materials at any time, thereby improving the yield of the product. (4) Adopt DCS system

Continuous production, automatic control operation, reducing raw material sublimation loss, ensuring the continuity and stability of the dichlorobenzene refining process, and improving the product

The processing volume and product quality have obtained greater economic benefits. (5) The mass fraction of the raw material p-dichlorotoluene in the elution precipitation step of the present invention is 40 to 78%, and the broadening is widened.

The range of purification of p-dichlorobenzene is high, and the purity of the finished product of the present invention is high and the yield is high.

DRAWINGS

 BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram showing the refining process of the step of crystallization of dichlorobenzene in the present invention.

detailed description

In order to enable the public to fully understand the technical essence and beneficial effects of the present invention, the applicant will describe in detail the specific embodiments of the present invention in conjunction with the accompanying drawings below.

The Applicant does not limit the technical solutions to the embodiments, and any form according to the concept of the present invention, rather than a substantial change, should be regarded as the protection of the present invention.

range.

Example 1

First, 10 kg of a mixture of 48 wt% of p-dichlorobenzene was placed in 25 kg of a 90 wt% ethanol solution and mixed to form a solution. Then gradually add 8.75kg of distilled water

Into the above solution, the process of adding distilled water will be 5 ° C, the temperature is gradually increased to 8 ° C at a heating rate of 0.075 ° C / min, and allowed to stand for 20 min to obtain a purity of

88% of p-dichlorotoluene 8.6 kg. The second raw material is separated by a two-stage melt crystallization, and the raw materials are added to the storage tank A, and the raw materials are sent to the step-wise crystallizer C by a pump.

In the first stage crystallization process, the materials are cooled and partially melted by using different temperatures of the medium to remove the non-condensable liquid, and the finished product is sent to the second stage section for treatment.

After three stages of treatment, the finished product is finally obtained. The specific two-stage six-stage treatment steps are as follows: (1) The above raw materials are separated by a first-stage cooling crystallization to separate 97 wt% of the pair.

Dichlorobenzene product and mother liquor, the mother liquor is sent to the o-dichlorobenzene refining process, the operating conditions are: the temperature is reduced from 50 ° C to 10 ° C, the cooling rate is 0.15 ° C / min; (2)

Part 1A 97% by weight of p-dichlorobenzene product is subjected to melt crystallization in Section 1B to separate 98.1% by weight of p-dichlorobenzene product and mother liquor. The mother liquor is sent to the raw material tank, and the 1A stage crystallization is repeated.

The specific operating conditions are as follows: the melting temperature is raised from 10 ° C to 48 ° C, and the heating rate is 0.1 ° C / min; (3) 1B segment 98.1 wt % of p-dichlorobenzene product

After the 1C stage is melted, it is sent to the second stage crystallization and further refined. The operating conditions are: the melting temperature is raised from 48 ° C to 80 ° C, wherein the heating rate is 1 ° C / min; (4) 1 C

After the melted product, the product is cooled and crystallized in the second stage 2A to refine 99.2% by weight of the p-dichlorobenzene product and the mother liquor. The mother liquor is returned to the raw material tank, and the 1A segment is repeated.

Crystallization process, operating conditions are: cooling temperature from 70 ° C to 15 ° C, cooling rate of 0.2 ° C / min; (5) 2A section of the product through 2B melt crystallization separation quality

The fraction of 99.9 wt% of p-dichlorobenzene product and mother liquor, the mother liquor returned to the 2A section to continue cooling and crystallization, the operating conditions are: the melting temperature is raised from 15 ° C to 50 ° C,

The heating rate is 0.08 ° C / min; (6) Finally, the product obtained in stage 2B is melted in 2C section and sent to the slicer D slice and packaged to obtain the finished product. The operating conditions are: melting

The temperature is: from 50 ° C to 80 ° C, the heating rate is 1 ° C / min, wherein the purity of the product is 99.96%.

Example 2

A mixture of 12 kg of 40 wt% p-dichlorobenzene was placed in 85.2 kg of a 70 wt% ethanol solution and mixed to form a solution. Then gradually add 38.4kg of distilled water

In the above solution, the temperature was 5 ° C in the process of adding distilled water, and the temperature was gradually raised to 8 ° C at a heating rate of 0.12 ° C / min, and allowed to stand for 30 min to obtain a purity of

80% of 0.25 kg of p-dichlorotoluene. Then, separation by two-stage six-stage melt crystallization is carried out: the above raw material is subjected to the first stage cooling crystallization to separate 97 wt% of p-dichloro

The benzene product and the mother liquor, the mother liquor is sent to the o-dichlorobenzene refining process, wherein the temperature is gradually reduced from 50 ° C to 10 ° C, the cooling rate is 0.15 ° C / min; after the 1 B section melting

The crystallization is separated to obtain 98.1% by weight of the p-dichlorobenzene product and the mother liquor, and the mother liquor is sent to the raw material tank, and the crystallization process of the 1A stage is repeated, and the temperature is raised from 10 ° C during the melting process.

The temperature is up to 48 ° C, wherein the heating rate is 0.1 ° C / min; then into the 1C section and then sent to the second stage of crystallization to continue refining, wherein the melting temperature is gradually increased from 48 ° C

The temperature is raised to 80 ° C at 1 ° C / min; then the second stage 2A cooling crystallization is used to purify 99.2% by weight of dichlorobenzene product and mother liquor, the mother liquor returns to the raw material tank, continue

Repeat the 1A crystallization process, in which the operating temperature is reduced from 70 ° C to 15 ° C, the cooling rate is 0.2 ° C / min, and then the mass fraction is separated by 2B melt crystallization.

The number is 99.9 wt% of the p-dichlorobenzene product and the mother liquor, and the mother liquor is returned to the 2A section to continue cooling and crystallization, wherein the melting temperature is operated: from 15 ° C to 50 ° C, liter

The temperature rate is 0.08 ° C / min; finally, the product obtained in stage 2B is melted in 2C section and sent to the slicer D slice, and the finished product is packaged, wherein the melting temperature is from 50 ° C.

The temperature was raised to 80 ° C, the heating rate was 1 ° C / min, and the purity of the finally obtained product was 99.94%.

Example 3

A mixture of 20 kg of 70 wt% p-dichlorobenzene was placed in 55 kg of an 80 wt% ethanol solution and mixed to form a solution. Then gradually add 22 kg of distilled water to the above

In the solution, the process of adding distilled water is 5 ° C, gradually increasing to 8 ° C at a heating rate of 0.10 ° C / min, and standing for 30 min to obtain a purity of 86.5%.

Dichlorotoluene 46.3kg. Then, separation by two-stage six-stage melt crystallization is carried out: the above raw materials are subjected to the first stage cooling crystallization to separate 96.5 wt% of p-dichlorobenzene.

The mother liquor and the mother liquor are sent to the o-dichlorobenzene refining process, wherein the temperature is gradually reduced from 50 ° C to 10 ° C, the cooling rate is 0.15 ° C / min; after 1 B melt crystallization

Separating 98.0 wt% of p-dichlorobenzene product and mother liquor, the mother liquor is sent to the raw material tank, and the crystallization process of 1A is repeated, and the temperature is raised from 10 ° C during the melting process to

48 ° C, wherein the heating rate is 0.1 ° C / min; then into the 1C section melted and sent to the second stage of crystal break to continue refining, wherein the melting temperature is gradually increased from 48 ° C to 1 ° C

/min is heated to 80 ° C; and then through the second stage 2A cooling crystallization to refine 99.2% by weight of dichlorobenzene product and mother liquor, the mother liquor returns to the raw material tank, continue to repeat

1A crystallization process, in which the operating temperature is reduced from 70 ° C to 15 ° C, the cooling rate is 0.2 ° C / min, and then the mass fraction is determined by 2B melt crystallization.

99.91wt% of p-dichlorobenzene product and mother liquor, the mother liquor returns to the 2A section to continue cooling and crystallization, wherein the melting temperature is operated: from 15 ° C to 50 ° C, heating rate

The rate is 0.08 ° C / min; finally, the product obtained in stage 2B is melted in 2C section and sent to the slicer D slice to package the finished product, wherein the melting temperature is raised from 50 ° C.

The temperature was raised to 1 ° C / min to 80 ° C, and the purity of the final product was 99.91%.

Example 4

8 kg of a mixture of 65 wt% p-dichlorobenzene was placed in 40 kg of a 95 wt% ethanol solution and mixed to form a solution. Then gradually add 16kg of distilled water to the above solution.

In the liquid, the process of adding distilled water will be 5 ° C, gradually increase to 8 ° C at a heating rate of 0.15 ° C / min, and let stand for 40 min to obtain p-dichloro with a purity of 85%.

Toluene 6.5 kg. Then, separation by two-stage six-stage melt crystallization is carried out: the above raw materials are subjected to first-stage cooling crystallization to separate 96 wt% of p-dichlorobenzene product and mother.

Liquid, mother liquor is sent to the o-dichlorobenzene refining process, wherein the temperature is gradually reduced from 50 ° C to 10 ° C, the cooling rate is 0.15 ° C / min; after 1B melt crystallization separation

98.0 wt% of p-dichlorobenzene product and mother liquor are obtained, and the mother liquor is sent to the raw material tank, and the crystallization process of 1A is repeated, and the temperature is raised from 10 ° C to 48 ° C during the melting process.

The heating rate is 0.1 ° C / min; then it is melted in the 1C section and sent to the second-stage crystal break to continue the refining, wherein the melting temperature is gradually increased from 48 ° C to 1 ° C / min.

Warm to 80 ° C; then through the second stage 2A cooling crystallization to refine 99.1% of the dichlorobenzene product and mother liquor, the mother liquor returned to the raw material tank, continue to repeat the 1A knot

Crystallization process, in which the operating temperature is lowered from 70 ° C to 15 ° C, the temperature drop rate is 0.2 ° C / min, and then the mass fraction is 99.9 wt % after 2B melt crystallization.

The p-dichlorobenzene product and the mother liquor, the mother liquor is returned to the 2A section to continue cooling and crystallization, wherein the melting temperature is operated: from 15 ° C to 50 ° C, the heating rate is 0.08

°C/min; Finally, the product obtained in stage 2B is melted in 2C stage and sent to the slicer D slice and packaged to obtain the finished product, wherein the melting temperature is raised from 50 ° C to 80 ° C.

The heating rate was 1 ° C / min, and the purity of the finally obtained product was 99.92%.

The process index of the p-dichlorobenzene product obtained by the stepwise crystallization of the present invention is as follows:

project	result
Finished product purity	> 99.9wt%
Freezing point	52.9~53.0 °C
Production capacity	8400t/a
Process yield	Leaching precipitation: ≥ 77%; step-by-step crystallization: > 90%
Steam energy consumption	0.8kg steam / 1kg finished product
Power consumption	0.22kW · h/1kg finished product

Claims (7)

1. A method for separating and purifying p-dichlorobenzene, which comprises the steps of elution precipitation and fractional crystallization, wherein the precipitation precipitation comprises: a1 will contain

   40~78wt% of p-dichlorobenzene mixture is put into 70~95wt% ethanol solution to form a solution, wherein the mass ratio of p-dichlorobenzene to ethanol solvent is 1:

   2.5~7.1;

   B1, gradually adding distilled water to the above solution, wherein the added weight of the distilled water is 0.35 to 0.45 times the weight of the ethanol; controlling in the process of adding distilled water

   The temperature of the solution is from 5 ° C to 8 ° C, and is allowed to stand for 20 to 40 minutes to obtain p-dichlorotoluene having a purity of 80 to 88 wt % and a yield of more than 77%;

   The stepwise crystallization comprises: using the above-mentioned 80-88 wt% p-dichlorobenzene as a raw material, and purifying by using two-stage six-stage melt crystallization, and the specific steps are as follows:

   A2, the above-mentioned purity of 80~88% of p-dichlorotoluene is purified by the first stage 1A cooling crystallization to obtain 95~97wt% of p-dichlorobenzene product and mother liquor, and the mother liquor is sent to the adjacent two

   Chlorobenzene refining process;

   The p-dichlorobenzene product of b2 and 1A is purified by the 1B stage melt crystallization to obtain 98~99wt% of p-dichlorobenzene product and mother liquor. The mother liquor is sent to the raw material tank, and the 1A stage crystallization is repeated.

   process;

   The c2, 1B-para-dichlorobenzene product is melted in the 1C stage and sent to the second-stage crystallization section for further purification;

   After the molten product of d2 and 1C is cooled by the second stage 2A, the product is crystallized and refined to produce 99.5 wt% of p-dichlorobenzene product and mother liquor, and the mother liquor is returned to the raw material tank.

   Continue to repeat 1A cooling crystallization;

   After the e2 and 2A products are melted and crystallized in the 2B stage, the p-dichlorobenzene product and the mother liquid are refined to 99.9 wt% or more, and the mother liquid is returned to the 2A stage to continue cooling and crystallization;

   F2. Finally, the product obtained in paragraph 2B is melted in 2C section and sent to the sliced and packaged finished product.
2. The method for separating and purifying p-dichlorobenzene according to claim 1, wherein the concentration of the ethanol solution in the elution and precipitation is 90% by weight.
3. The method for separating and purifying p-dichlorobenzene according to claim 1, wherein the mass ratio of p-dichlorobenzene to ethanol solution in the elution and precipitation is=1:

   3~5.
4. The method for separating and purifying p-dichlorobenzene according to claim 1, wherein the amount of distilled water added in the precipitation is 0.4 times the weight of the ethanol.
5. The method for separating and purifying p-dichlorobenzene according to claim 1, wherein the temperature control in the step b2 of the elution and precipitation is to gradually increase the temperature from 5 ° C.

   The temperature is up to 8 ° C, wherein the heating rate is 0.075 ~ 0.15 ° C / min.
6. The method for separating and purifying p-dichlorobenzene according to claim 1, wherein the cooling temperature of the step 1A in the step a2 to c2 of the stepwise crystallization is

   : from 50 ° C to 10 ° C, the cooling rate is 0.15 ° C / min, the melting temperature of the 1B section is: from 10 ° C to 48 ° C, the heating rate is 0.1 ° C / min, 1C section

   The melting temperature is: 48 ° C to 80 ° C, the heating rate is 1 ° C / min.
7. The method for separating and purifying p-dichlorobenzene according to claim 1, wherein the cooling temperature of the step 2A in the step d2~f2 of the stepwise crystallization is

   : from 70 ° C to 15 ° C, the cooling rate is 0.2 ° C / min, the melting temperature of the 2B section is: from 15 ° C to 50 ° C, the heating rate is 0.08 ° C / min, 2C

   The melting temperature is: from 50 ° C to 80 ° C, the heating rate is 1 ° C / min.