A method for purifying p-phenylenediamine

Abstract

This invention relates to the field of fine chemical production technology, and specifically discloses a method for purifying p-phenylenediamine. The method is characterized by: feeding crude p-phenylenediamine into a static melt crystallizer; heating and melting the material, then slowly cooling it to the crystallization temperature; during crystallization, a purging atmosphere is blown in and maintained at a slight positive pressure until crystallization is complete; after crystallization, the pressure is switched to a slight negative pressure; then, the crystallized layer is slowly heated to induce sweating, and during sweating, a pressure-switching atmosphere is used for purging; after sweating, the crystallized layer is heated to complete melting, and the molten liquid is fed to a decoking tower for purification to obtain high-purity p-phenylenediamine product. This invention eliminates the need for additional complex equipment such as centrifuges and filters, fully utilizes the low-cost advantage of static crystallizers, is easy to operate, shortens separation time, and greatly improves separation efficiency and the economics of the process.

Description

Technical Field

[0001] This invention relates to the field of fine chemical production technology, and in particular to a method for purifying p-phenylenediamine by removing impurities. Background Technology

[0002] p-Phenylenediamine is an important fine chemical product widely used in the synthesis of dyes and rubber and plastic additives. High-purity p-Phenylenediamine is an important monomer for the preparation of high-performance fiber materials—para-aramid (poly(p-phenylene terephthalamide), aramid 1414), which plays a vital role in key fields such as national defense, military industry, and aerospace. The high purity requirement (above 99.99%) for polymerization grade required for the preparation of para-aramid also poses new challenges to the production of ultra-high-quality p-Phenylenediamine.

[0003] p-Phenylenediamine is chemically reactive and easily oxidized and deteriorates. During preparation and separation, it readily generates other impurities, affecting product quality. The current mainstream production process for p-phenylenediamine uses chlorobenzene as a starting material, proceeding through nitration, ammonolysis, and catalytic hydrogenation reduction. However, some trace impurities and byproducts in the raw materials have boiling points close to p-phenylenediamine. Obtaining high-purity p-phenylenediamine entirely through distillation requires a very high number of theoretical plates and a large reflux ratio, resulting in high equipment investment and energy consumption. Furthermore, the impurity removal rate is not high, leading to limited purification efficiency.

[0004] Utilizing the difference in crystallization points between the target product and impurities, melt crystallization offers significant advantages over distillation methods in purifying crude p-phenylenediamine in terms of equipment investment and operational energy consumption. However, in traditional melt crystallization processes, the mother liquor discharged after sweating often fails to completely separate from the crystallized layer, resulting in intergranular encapsulation and poor purification. To achieve the required product purity, existing technologies typically employ additional "sweat-crystallization" cycles, drastically reducing separation efficiency and increasing production costs. Some reports also suggest using mechanical methods such as centrifugation during crystallization to improve the separation of the mother liquor from the crystallized layer; however, this not only increases the complexity of the separation operation but also significantly raises equipment investment due to the introduction of centrifugation equipment, reducing the economic viability of the process. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, this invention provides a method for purifying p-phenylenediamine that is based on a low-cost static crystallizer, is simple to operate, and has low production costs.

[0006] This invention is achieved through the following technical solution:

[0007] A method for purifying p-phenylenediamine, using crude p-phenylenediamine obtained by pre-purification in a primary distillation column as the treatment target, includes the following steps:

[0008] (1) The crude p-phenylenediamine is fed into a static melt crystallizer. The material is heated and melted, and then slowly cooled to the crystallization temperature. The set crystallization temperature is maintained for a certain period of time to allow the material to crystallize fully. During this process, the purge atmosphere is blown in after the crystallization plate appears, and the back pressure valve controls the crystallizer to maintain a slight positive pressure until the crystallization is completely finished. The purge atmosphere is blown in and slowly depressurized until it is converted to a slight negative pressure by the vacuum pump. After the purge is continued for a period of time, the mother liquor in the crystallizer is drained.

[0009] (2) Slowly heat the crystal layer obtained in step (1) to make it sweat. During the sweating process, use the same pressure-switching atmosphere as in step (1) to blow it so that the sweat is completely separated from the crystal layer. After the sweating is finished, drain the sweat.

[0010] (3) The crystalline layer obtained in step (2) is heated to complete melting, and the molten liquid is transported to the decoking tower for refining to obtain high-purity p-phenylenediamine product.

[0011] In their long-term research on the separation and purification of p-phenylenediamine chemicals, the inventors discovered that during the melting and crystallization process in a static crystallizer, problems such as incomplete separation of the mother liquor and sweat excreted through perspiration from the crystal layer, resulting in intercrystalline encapsulation, mainly occurred because the mother liquor and sweat adhered to the surface of the crystal layer, and the crystal layer formed during the crystallization process was too dense, causing some of the mother liquor and sweat to be trapped in the crystal layer and unable to be fully excreted.

[0012] This invention creatively employs a "pressure-switching crystallization" coupled with "localized purging" processing strategy to specifically overcome the aforementioned problems. The specific principle is as follows: During the crystallization layer formation stage, a purging atmosphere with a weak dissolving effect on the crystallized layer is introduced into the crystallizer, and a positive pressure condition is maintained continuously, allowing some of the purging atmosphere to penetrate into the crystal layer and simultaneously etching out tiny voids. After crystallization is complete, the pressure inside the crystallizer is adjusted from positive to negative, extracting the purging atmosphere embedded within the crystal layer. The continuously introduced purging atmosphere with a weak dissolving effect thoroughly washes away the mother liquor and sweat adhering to the crystal layer's interior and surface. This process requires only a simple gas distribution and vacuum pump system in conjunction with a static crystallizer, eliminating the need for complex equipment such as centrifuges. It is easy to operate and requires low equipment investment.

[0013] A further preferred embodiment of the present invention is as follows:

[0014] The purity of p-phenylenediamine in the crude p-phenylenediamine is 99%-99.5%.

[0015] In step (1), the material is cooled at a rate of 1-3℃ / h after melting, and the set crystallization temperature is 115-130℃. The crystallization temperature is maintained for 1-6h.

[0016] More preferably, the purging atmosphere is nitrogen saturated with methanol, nitrogen saturated with water, or a mixture of the two; even more preferably, the purging atmosphere is preheated before being blown into the crystallizer, and the preheating temperature is kept consistent with the temperature of the crystallized layer inside the crystallizer.

[0017] More preferably, the pressure inside the crystallizer during the crystallization process is controlled at 0.1-300 kPa (gauge pressure), and the vacuum degree inside the crystallizer after crystallization is controlled at 0.05-30 kPa.

[0018] In step (2), the sweating temperature is 131-141℃.

[0019] The mother liquor discharged in step (1) and the sweat discharged in step (2) are collected and returned to the distillation pre-purification section.

[0020] The process of this invention can obtain p-phenylenediamine with a purity of over 99.99% through only one "crystallization-sweating" operation, which fully meets the requirements for polymerization-grade purity in the production of para-aramid.

[0021] The beneficial effects of this invention are as follows: it eliminates the need for additional complex equipment such as centrifugation and filtration, greatly reducing equipment investment and the complexity of separation operations, fully leveraging the low-cost advantage of static crystallizers, simplifying operation, allowing for the separation of higher purity p-phenylenediamine products with fewer sweating cycles, shortening separation time, and significantly improving separation efficiency and the economics of the process. Attached Figure Description

[0022] The invention will now be further described with reference to the accompanying drawings.

[0023] Figure 1 This is the gas chromatogram of the crude p-phenylenediamine obtained by pre-purification by distillation in the primary distillation column in Example 1;

[0024] Figure 2 The gas chromatogram of p-phenylenediamine obtained in Example 1 through a single "melt crystallization-sweating" process;

[0025] Figure 3 This is the gas chromatogram of p-phenylenediamine obtained by further simple distillation and purification in Example 1 using a decoking tower;

[0026] Figure 4 The gas chromatogram of p-phenylenediamine obtained in Example 2 through a single "melt crystallization-sweating" process;

[0027] Figure 5 The gas chromatogram of p-phenylenediamine obtained in Example 3 through a single "melt crystallization-sweating" process;

[0028] Figure 6The gas chromatogram of p-phenylenediamine obtained in Example 4 through a single "melt crystallization-sweating" process is shown.

[0029] Figure 7 The gas chromatogram of p-phenylenediamine obtained by one "melt crystallization-sweating" process in Example 5 (Comparative Example 1);

[0030] Figure 8 The gas chromatogram of p-phenylenediamine obtained by one "melt crystallization-sweating" process in Example 6 (Comparative Example 2);

[0031] Figure 9 The gas chromatogram of p-phenylenediamine obtained by two "melt crystallization-sweating" processes in Example 6 (Comparative Example 2);

[0032] Figure 10 The gas chromatogram of p-phenylenediamine obtained in Example 7 (Comparative Example 3) through two "melt crystallization-sweating" processes is shown. Detailed Implementation

[0033] The technical solution of the present invention will be described in detail below with reference to specific embodiments to facilitate understanding of the present invention, but this is not intended to limit the present invention.

[0034] Example 1: A method for purifying p-phenylenediamine, comprising the following steps:

[0035] (1) p-Phenylenediamine was pre-purified by distillation in a primary distillation column to obtain crude p-Phenylenediamine. The p-Phenylenediamine content in the crude product was analyzed by gas chromatography. The purity of p-Phenylenediamine was 99.383% (quantitative analysis by area normalization method, see attached table for detailed analysis results). Figure 1 );

[0036] Take 1 kg of the above-mentioned crude p-phenylenediamine and add it to a static melt crystallizer. After heating the material to a molten state, control the cooling rate to slowly cool down and crystallize at 1℃ / h. After cooling down to the set crystallization temperature of 120℃, maintain it for 1h to allow the material to fully crystallize. During this process, observe through the sight glass on the crystallizer that when a crystal layer appears on the crystallization plate, start blowing in a purge atmosphere. The purge atmosphere is N2 saturated with methanol. The purge atmosphere is preheated by a preheater before being blown into the crystallizer. The preheating temperature is 120℃. Control the internal pressure of the crystallizer to maintain at 50 kPa (gauge pressure) through the back pressure valve until the crystallization is completely finished. Continue to blow in the purge atmosphere and slowly depressurize until the inside of the crystallizer is converted to negative pressure by a vacuum pump. The vacuum degree is controlled at 10 kPa. After purging for a period of time, drain the mother liquor in the crystallizer.

[0037] (2) The crystal layer obtained in step (1) is slowly heated to induce sweating. The sweating temperature is set at 132°C. During the sweating process, the same variable pressure atmosphere as in step (1) is used to purge the sweat so that the sweat is completely separated from the crystal layer. After the sweating is finished, the sweat is drained.

[0038] The mother liquor discharged in step (1) and the sweat discharged in step (2) are collected and returned to the distillation pre-purification section.

[0039] (3) The crystalline layer obtained in step (2) is heated to completely melt it. The molten liquid is collected, sealed in an airtight container, and a sample is taken. The p-phenylenediamine content is analyzed by gas chromatography. The analysis results show that the purity of p-phenylenediamine is 99.990% (see Appendix for detailed analysis results). Figure 2 The collected liquid was further purified by simple distillation using a decoking tower to obtain p-phenylenediamine product with a purity of 99.995% (details of the analysis are attached). Figure 3 ).

[0040] The quantitative analysis method for p-phenylenediamine involved in the above embodiments is an optimized version of GB / T25789-2010, and the specific method is as follows:

[0041] Gas chromatograph: Agilent 7820A; Detector: Flame Ionization Detector (FID); Column: DB-1701 (30m × 0.32mm × 0.25μm); Detector temperature: 300℃; Vaporization chamber temperature: 300℃; Combustion gas (hydrogen) flow rate: 30mL / min; Combustion gas (air) flow rate: 300mL / min; Compensation gas (nitrogen) flow rate: 20mL / min; Split ratio: 25:1; Injection volume: 1μL.

[0042] Programmed temperature conditions: initial column temperature 150℃, hold for 4 min; increase temperature to 300℃ at a rate of 30℃ / min, hold for 5 min.

[0043] Under the above chromatographic conditions, the retention time of p-phenylenediamine peak is approximately 2.1 min.

[0044] Example 2: A method for purifying p-phenylenediamine, comprising the following steps:

[0045] (1) p-Phenylenediamine was pre-purified by distillation in the same pre-distillation column as in Example 1 to obtain crude p-Phenylenediamine with a purity of 99.383%;

[0046] Take 1 kg of the above-mentioned crude p-phenylenediamine and add it to a static melt crystallizer. After heating the material to a molten state, control the cooling rate to slowly cool down and crystallize at 2℃ / h. After cooling down to the set crystallization temperature of 125℃, maintain it for 2 hours to allow the material to fully crystallize. During this process, observe through the sight glass on the crystallizer that when a crystal layer appears on the crystallization plate, start blowing in a purge atmosphere. The purge atmosphere is water-saturated N2. Before blowing in the crystallizer, the purge atmosphere is preheated by a preheater at a preheating temperature of 125℃. Control the internal pressure of the crystallizer to maintain at 100 kPa (gauge pressure) through the back pressure valve until the crystallization is completely finished. Continue to blow in the purge atmosphere and slowly depressurize until the inside of the crystallizer is converted to negative pressure by a vacuum pump. The vacuum degree is controlled at 20 kPa. After purging for a period of time, drain the mother liquor in the crystallizer.

[0047] (2) The crystal layer obtained in step (1) is slowly heated to induce sweating. The sweating temperature is set at 135°C. During the sweating process, the same variable pressure atmosphere as in step (1) is used to purge the sweat so that the sweat is completely separated from the crystal layer. After the sweating is finished, the sweat is drained.

[0048] The mother liquor discharged in step (1) and the sweat discharged in step (2) are collected and returned to the distillation pre-purification section.

[0049] (3) The crystalline layer obtained in step (2) is heated to completely melt it. The molten liquid is collected, sealed in an airtight container, and a sample is taken. The p-phenylenediamine content is analyzed by gas chromatography. The analysis results show that the purity of p-phenylenediamine is 99.991% (see Appendix for detailed analysis results). Figure 4 The collected liquid was further purified by distillation using a decoking tower to obtain p-phenylenediamine with a purity of 99.996%.

[0050] The quantitative analysis method for p-phenylenediamine in the above examples is the same as that in Example 1.

[0051] Example 3: A method for purifying p-phenylenediamine, comprising the following steps:

[0052] (1) The crude p-phenylenediamine obtained by pre-purification using the same primary distillation tower operation as in Example 1 was used as the feed for the melt crystallizer, and the purity of p-phenylenediamine was 99.383%;

[0053] Take 1 kg of the above-mentioned crude p-phenylenediamine and add it to a static melt crystallizer. After heating the material to a molten state, control the cooling rate to slowly cool down and crystallize at 3℃ / h. After cooling down to the set crystallization temperature of 130℃, maintain it for 3h to allow the material to fully crystallize. During this process, observe through the sight glass on the crystallizer that when a crystal layer appears on the crystallization plate, start blowing in a purge atmosphere. The purge atmosphere is a mixture of two atmospheres: N2 saturated with methanol and N2 saturated with water. The volume flow ratio of the two atmospheres is 1:1. The purge atmosphere is preheated by a preheater before being blown into the crystallizer. The preheating temperature is 130℃. The pressure inside the crystallizer is controlled to be 200KPa (gauge pressure) by the back pressure valve until the crystallization is completely finished. Continue to blow in the purge atmosphere and slowly depressurize until the crystallizer is converted to negative pressure by a vacuum pump. The vacuum degree is controlled to be 30KPa. After purging for a period of time, drain the mother liquor in the crystallizer.

[0054] (2) The crystal layer obtained in step (1) is slowly heated to induce sweating. The sweating temperature is set at 140°C. During the sweating process, the same variable pressure atmosphere as in step (1) is used to purge the sweat so that the sweat is completely separated from the crystal layer. After the sweating is finished, the sweat is drained.

[0055] The mother liquor discharged in step (1) and the sweat discharged in step (2) are collected and returned to the distillation pre-purification section.

[0056] (3) The crystalline layer obtained in step (2) is heated to completely melt it. The molten liquid is collected, sealed in an airtight container, and sampled. The p-phenylenediamine content is analyzed by gas chromatography. The analysis results show that the purity of p-phenylenediamine is 99.992% (see Appendix for detailed analysis results). Figure 5 ).

[0057] The quantitative analysis method for p-phenylenediamine in the above examples is the same as that in Example 1.

[0058] Example 4: A method for purifying p-phenylenediamine, comprising the following steps:

[0059] (1) The crude p-phenylenediamine obtained by pre-purification using the same primary distillation tower operation as in Example 1 was used as the feed for the melt crystallizer, and the purity of p-phenylenediamine was 99.383%;

[0060] Take 1 kg of the above-mentioned crude p-phenylenediamine and add it to a static melt crystallizer. After heating the material to a molten state, control the cooling rate to slowly cool down and crystallize at 2℃ / h. After cooling down to the set crystallization temperature of 125℃, maintain it for 3 hours to allow the material to fully crystallize. During this process, observe through the sight glass on the crystallizer that when a crystal layer appears on the crystallization plate, start blowing in a purge atmosphere. The purge atmosphere is a mixture of two atmospheres: N2 saturated with methanol and N2 saturated with water. The volume flow ratio of the two atmospheres is 1:1. The purge atmosphere is preheated by a preheater before being blown into the crystallizer. The preheating temperature is 125℃. The pressure inside the crystallizer is controlled to be 5 kPa (gauge pressure) by the back pressure valve until crystallization is completely finished. Continue to blow in the purge atmosphere and slowly depressurize until the crystallizer is converted to negative pressure by a vacuum pump. The vacuum degree is controlled to be 1 kPa. After purging for a period of time, drain the mother liquor in the crystallizer.

[0061] (2) The crystal layer obtained in step (1) is slowly heated to induce sweating. The sweating temperature is set at 135°C. During the sweating process, the same variable pressure atmosphere as in step (1) is used to purge the sweat so that the sweat is completely separated from the crystal layer. After the sweating is finished, the sweat is drained.

[0062] The mother liquor discharged in step (1) and the sweat discharged in step (2) are collected and returned to the distillation pre-purification section.

[0063] (3) The crystalline layer obtained in step (2) is heated to completely melt it. The molten liquid is collected, sealed in an airtight container, and a sample is taken. The p-phenylenediamine content is analyzed by gas chromatography. The analysis results show that the purity of p-phenylenediamine is 99.993% (see Appendix for detailed analysis results). Figure 6 ).

[0064] The quantitative analysis method for p-phenylenediamine in the above examples is the same as that in Example 1.

[0065] Example 5 (Comparative Example 1):

[0066] (1) The crude p-phenylenediamine obtained by the same distillation pre-purification method as in Example 1 (the purity of p-phenylenediamine was 99.383% according to gas chromatography) was used as the feed for the melt crystallization purification unit.

[0067] Take 1 kg of the above-mentioned crude p-phenylenediamine and add it to a static melt crystallizer. After heating the material to a molten state, control the cooling rate to slowly cool down and crystallize at 1℃ / h. After cooling down to the set crystallization temperature of 120℃, maintain it for 1h to allow the material to fully crystallize. During this process, observe through the sight glass on the crystallizer that when a crystal layer appears on the crystallization plate, start blowing in pure N2 to purge. The N2 is preheated by a preheater before being blown into the crystallizer. The preheating temperature is 120℃. Control the internal pressure of the crystallizer to maintain at 50KPa (gauge pressure) through the back pressure valve until the crystallization is completely finished. Continue to blow in the purging atmosphere and slowly depressurize until the inside of the crystallizer is converted to negative pressure by a vacuum pump. The vacuum degree is controlled at 10KPa. After purging for a period of time, drain the mother liquor in the crystallizer.

[0068] (2) The crystal layer obtained in step (1) is slowly heated to induce sweating. The sweating temperature is set to 132°C. During the sweating process, the same variable pressure atmosphere as in step (1) is used to purge the sweat so that the sweat is completely separated from the crystal layer. After the sweating is finished, the sweat is drained.

[0069] The mother liquor discharged in step (1) and the sweat discharged in step (2) are collected and returned to the distillation pre-purification section.

[0070] (3) The crystalline layer obtained in step (2) is heated to completely melt it. The molten liquid is collected, sealed in an airtight container, and a sample is taken. The p-phenylenediamine content is analyzed by gas chromatography. The analysis results show that the purity of p-phenylenediamine is 99.824% (see Appendix for detailed analysis results). Figure 7 ).

[0071] Example 6 (Comparative Example 2):

[0072] (1) The crude p-phenylenediamine obtained by pre-purification by distillation in the same column as in Example 1 was used as the feed for the melt crystallizer. The purity of p-phenylenediamine was 99.383%.

[0073] Take 1 kg of the above-mentioned crude p-phenylenediamine and add it to a static melt crystallizer. After purging the air in the crystallizer with N2, stop the gas supply. Heat the material to a molten state and control the cooling rate to 1℃ / h to slowly cool down and crystallize. After cooling down to the set crystallization temperature of 120℃, maintain it for 1 hour to allow the material to fully crystallize. Then drain the mother liquor in the crystallizer.

[0074] (2) Slowly heat the crystallized layer obtained in step (1) to induce sweating. Set the sweating temperature to 132°C. After sweating, drain the sweat.

[0075] The mother liquor discharged in step (1) and the sweat discharged in step (2) are collected and returned to the distillation pre-purification section.

[0076] (3) The crystalline layer obtained in step (2) is heated to completely melt it. The molten liquid is collected, sealed in an airtight container, and sampled. The p-phenylenediamine content is analyzed by gas chromatography. The analysis results show that the purity of p-phenylenediamine is 99.763% (see Appendix for detailed analysis results). Figure 8 ).

[0077] The p-phenylenediamine obtained in step (3) was returned to the crystallizer and the same "crystallization-sweating" operation of steps (1)-(3) was repeated to obtain a sample after secondary crystallization. The sample was taken and the p-phenylenediamine content was analyzed by gas chromatography. The analysis results showed that the purity of p-phenylenediamine was 99.844% (see Appendix for detailed analysis results). Figure 9 ).

[0078] Example 7 (Comparative Example 3):

[0079] (1) The crude p-phenylenediamine obtained by pre-purification by distillation in the same column as in Example 1 was used as the feed for the melt crystallizer. The purity of p-phenylenediamine was 99.383%.

[0080] Take 1 kg of the above-mentioned crude p-phenylenediamine and add it to a static melt crystallizer. After heating the material to a molten state, control the cooling rate to slowly cool down and crystallize at 1℃ / h. After cooling down to the set crystallization temperature of 120℃, maintain it for 1h to allow the material to crystallize fully. During this process, observe through the sight glass on the crystallizer that when a crystal layer appears on the crystallization plate, start blowing in a purging atmosphere. The purging atmosphere is N2 saturated with methanol. The purging atmosphere is preheated by a preheater before being blown into the crystallizer. The preheating temperature is 120℃. The crystallizer is kept at atmospheric pressure throughout the crystallization process. After the crystallization is completed, continue purging for a period of time and then drain the mother liquor from the crystallizer.

[0081] (2) The crystal layer obtained in step (1) is slowly heated to induce sweating. The sweating temperature is set at 132°C. During the sweating process, the same atmospheric pressure atmosphere as in step (1) is used to purge the sweat so that the sweat is completely separated from the crystal layer. After the sweating is finished, the sweat is drained.

[0082] The mother liquor discharged in step (1) and the sweat discharged in step (2) are collected and returned to the distillation pre-purification section.

[0083] (3) The crystalline layer obtained in step (2) is heated to completely melt it. The molten liquid is collected, sealed in an airtight container, and a sample is taken. The p-phenylenediamine content is analyzed by gas chromatography. The analysis results show that the purity of p-phenylenediamine is 99.842% (see Appendix for detailed analysis results). Figure 10 ).

[0084] The preferred embodiments of the present invention have been described in the above examples. It is obvious that many variations can be made within the inventive concept of the present invention. It should be noted that any changes made within the inventive concept of the present invention will fall within the protection scope of the present invention.

Claims

1. A method for purifying p-phenylenediamine, using crude p-phenylenediamine obtained by pre-purification in a primary distillation column as the treatment target, characterized in that, Includes the following steps: (1) The crude p-phenylenediamine is fed into a static melt crystallizer. The material is heated and melted, and then slowly cooled to the crystallization temperature. The set crystallization temperature is maintained and a crystal layer appears on the crystallization plate. A purging atmosphere is blown in. The back pressure valve controls the crystallizer to maintain a slight positive pressure until the crystallization is completely finished. The purging atmosphere is blown in and slowly depressurized until it is converted to a slight negative pressure by a vacuum pump. After continuous purging, the mother liquor in the crystallizer is drained. During the crystallization process, the pressure inside the crystallizer is controlled at 0.1-300 kPa, and after crystallization, the vacuum degree inside the crystallizer is controlled at 0.05-30 kPa. (2) Slowly heat the crystal layer obtained in step (1) to make it sweat. During the sweating process, use the same pressure-switching atmosphere as in step (1) to purge. After the sweating is finished, drain the sweat. (3) The crystalline layer obtained in step (2) is heated to complete melting, and the molten liquid is transported to the decoking tower for refining to obtain p-phenylenediamine product.

2. The method for purifying p-phenylenediamine as described in claim 1, characterized in that: The purity of p-phenylenediamine in the crude p-phenylenediamine is 99%-99.5%.

3. The method for purifying p-phenylenediamine as described in claim 1, characterized in that: In step (1), the material is cooled at a rate of 1-3℃ / h after melting, and the set crystallization temperature is 115-130℃. The crystallization temperature is maintained for 1-6h.

4. The method for purifying p-phenylenediamine as described in claim 1, characterized in that: In step (1), the purging atmosphere is nitrogen saturated with methanol, nitrogen saturated with water, or a mixture of the two.

5. The method for purifying p-phenylenediamine as described in claim 1, characterized in that: In step (2), the sweating temperature is 131-141℃.

6. The method for purifying p-phenylenediamine as described in claim 4, characterized in that: The purging atmosphere is preheated before being blown into the crystallizer, and the preheating temperature is kept consistent with the temperature of the crystallized layer inside the crystallizer.

7. The method for purifying p-phenylenediamine as described in claim 1, characterized in that: The mother liquor discharged in step (1) and the sweat discharged in step (2) are collected and returned to the distillation pre-purification section.

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