Method for recovering and reusing sodium 2, 4-dichlorophenolate

Abstract

The invention provides a method for recovering and reusing sodium 2, 4-dichlorophenate, which comprises the following steps: carrying out neutralization reaction on chloroacetic acid, caustic soda liquid and water to obtain a sodium chloroacetate solution; mixing the sodium chloroacetate solution with the sodium 2, 4-dichlorophenolate solution, and heating to carry out condensation reaction; carrying out suction filtration on the obtained condensation reaction liquid to obtain condensation mother liquor of 2, 4-D sodium salt and phenolic wastewater; adding acid into the condensation mother liquor of the phenol-containing wastewater, and extracting to obtain a phenol-containing solution; alkali is added into the phenol-containing solution for reverse extraction, and phenol-containing recovery liquid is obtained; and adding the phenol-containing recovery liquid into chloroacetic acid, adding alkali, carrying out a neutralization reaction, standing for layering, and separating to obtain 2, 4-dichlorophenol and a sodium chloroacetate solution, thereby realizing recovery and reutilization of sodium 2, 4-dichlorophenate. According to the technical scheme, reaction conditions do not need to be changed or a catalyst does not need to be added, so that energy consumption increased due to change of the reaction conditions or use of the catalyst can be avoided. Meanwhile, the waste liquid treatment cost is reduced, and the production cost is further reduced.

Description

Technical Field

[0001] The technical scheme of the present invention belongs to the technical field of chemical production, and relates to a method for synthesizing 2,4-dichlorophenoxyacetic acid and recycling a recovered liquid. Background Art

[0002] 2,4-D (2,4-Dichlorophenoxyacetic acid, abbreviated as 2,4-D), chemical name: 2,4-dichlorophenoxyacetic acid, is a broad-spectrum herbicide belonging to the phenoxyacetic acid class of compounds. It is mainly used to control a variety of broadleaf weeds, especially in farmland and lawns. 2,4-D interferes with the growth process of weeds by simulating the effects of plant hormones, ultimately leading to their abnormal growth and death. Compared with other herbicides, 2,4-D is safe, low-toxic, easy to degrade, and has low cost of use. It is currently a very popular herbicide.

[0003] The production of chemical pesticides has to face the problem of environmental protection. 2,4-D will also encounter environmental problems during the production process. 2,4-D phenolic waste liquid is one of the wastewaters generated during the production of 2,4-dichlorophenoxyacetic acid (2,4-D). Its main components include phenolic compounds (such as chlorophenol) and other organic pollutants. The existing process flow is as follows: Figure 1 shown. Summary of the invention

[0004] Compared with the prior art, this technical solution mainly solves the following technical problems: 1. It solves the problem of phenol-containing wastewater treatment, thereby realizing the recycling of resources and improving resource utilization. Using ethyl acetate and n-butyl acetate as extraction solvents can make the recycling rate as high as 85%, reducing production costs and avoiding the waste of raw materials. 2. It solves the pollution of phenol-containing wastewater to the environment, which meets the current requirements of green chemistry and sustainable development.

[0005] In order to achieve the above purpose, the technical means and steps mainly adopted by this technical solution are as follows: A method for recovering and reusing sodium 2,4-dichlorophenol comprises the following steps: (1) neutralizing chloroacetic acid with liquid alkali and water to obtain a sodium chloroacetate solution; (2) Mixing the sodium chloroacetate solution and the sodium 2,4-dichlorophenol solution, and then heating them to carry out a condensation reaction; (3) filtering the obtained condensation reaction liquid to obtain a condensation mother liquor of 2,4-D sodium salt and phenol-containing wastewater; (4) adding acid to the condensed mother liquor of the phenol-containing wastewater for extraction to extract a phenol-containing solution; (5) adding alkali to the phenol-containing solution for stripping to obtain a phenol-containing recovery liquid; (6) adding the phenol-containing recovery liquid to chloroacetic acid, adding alkali, performing a neutralization reaction, and then standing to separate the layers to obtain a 2,4-dichlorophenol sodium recovery liquid and a sodium chloroacetate solution; (7) heating the sodium 2,4-dichlorophenolate solution, adding the sodium 2,4-dichlorophenolate recovery solution described in step (6), and dropping the sodium chloroacetate solution while continuing to heat to carry out a condensation reaction; (8) The condensation mother liquor is subjected to the cycle of steps (3) to (7) above again to achieve the recovery and reuse of sodium 2,4-dichlorophenol.

[0006] In step (2), the sodium chloroacetate solution is added dropwise to the sodium 2,4-dichlorophenol solution for condensation reaction. The dropping time is 2-4 hours, the condensation reaction temperature is 101-110° C., and during the condensation reaction, the pH is 10-11 and the reaction time is 2-3 hours.

[0007] In step (4), the condensed mother liquor is first added to the solvent before extraction, the temperature is raised to 50-70°C, and then acid is added to adjust the pH to 6-7 for extraction and stratification. The supernatant is taken and acid is added to adjust the pH to 0.1-1 for extraction and stratification to obtain a phenol-containing solvent solution.

[0008] The phenol-containing solvent is selected from any one of dichloromethane, toluene, ethyl acetate and n-butyl acetate; the acid is a hydrochloric acid solution with a mass concentration of 20-32%.

[0009] The acid used is hydrochloric acid with a mass fraction of 20-30%, and the volume ratio of the solvent to the phenol-containing waste liquid is 0.4-0.5:1.

[0010] The pH value of the phenol-containing waste liquid is adjusted to 6-7 and pH=1 in sequence, and 2,4-dichlorophenol is extracted using an organic solvent: first, the pH value is adjusted to 6-7 by pretreatment to separate other impurities; then, the pH value of the condensation mother liquor of 2,4-dichlorophenoxyacetic acid sodium is adjusted to 1, so that the 2,4-dichlorophenol sodium in the condensation mother liquor is converted into 2,4-dichlorophenol and dissolved in the organic phase, and the 2,4-dichlorophenol solution is obtained by extraction and separation.

[0011] In step (6), the volume ratio of the phenol-containing recovery liquid and chloroacetic acid is 1:1-5; the pH is adjusted to 10-11 by adding alkali, and the temperature is raised to 25-50°C for neutralization reaction. The obtained phenol-containing recovery liquid is used in the process of neutralizing chloroacetic acid with liquid alkali to obtain sodium chloroacetate, thereby realizing the recycling of resources, improving resource utilization, and also reducing production costs.

[0012] The mass concentration of chloroacetic acid is 70-85%, preferably 80% chloroacetic acid. The concentration of liquid caustic soda in the neutralization process is 15-35%.

[0013] The volume ratio of chloroacetic acid: recovery liquid: NaOH is 1.7:1:0.6-0.8, and the reaction temperature is 25-50°C for neutralization for 0.5-1 hour.

[0014] The alkali described in steps (5) and (6) is liquid alkali with a mass concentration of 15-35%.

[0015] The liquid alkali used in the stripping process is 19% NaOH. The volume ratio of the solution containing the phenolic solvent to the liquid alkali is 1:0.14-0.18.

[0016] Liquid alkali is used to strip out the 2,4-dichlorophenol solution in the organic phase, so that the 2,4-dichlorophenol dissolved in the organic solvent is converted into an aqueous solution of sodium 2,4-dichlorophenol, which can be recycled to produce 2,4-D. The organic phase after stripping can be repeatedly recycled to complete the above operation. The use of ethyl acetate or n-butyl acetate can achieve a recycling efficiency of 85%-90%, which has the characteristics of a high recycling rate, reduces production costs, and also avoids the pollution of the environment by organic solvents, which meets the current requirements of green chemistry and sustainable development.

[0017] In step (7), the sodium 2,4-dichlorophenol solution is heated to 90-100° C., 2,4-dichlorophenol is added, and the temperature is again raised to 102-108° C., and sodium chloroacetate solution is added dropwise to carry out a condensation reaction. The dropping time is 2-4 hours. During the condensation reaction, the pH is 10-11. After the reaction is completed, the temperature is kept for 2-4 hours. After the reaction is completed, the pH is adjusted to be greater than 11.

[0018] During the condensation reaction, the molar ratio of sodium chloroacetate:sodium 2,4-dichlorophenol (i.e. the total amount of newly prepared sodium 2,4-dichlorophenol and recovered sodium 2,4-dichlorophenol) is 1.00-1.12:1.

[0019] During the condensation reaction, the pH regulator is liquid alkali, and the mass concentration of the liquid alkali is 15-35%.

[0020] The technical solution of the present invention reduces the amount of raw material sodium 2,4-dichlorophenol by optimizing the condensation process flow, and obtains sodium 2,4-dichlorophenoxyacetate (abbreviated as: 2,4-D sodium salt). Due to the resource utilization of recovered phenol, according to the optimized feed ratio, the original feed amount of sodium 2,4-dichlorophenol is reduced in proportion. This process not only completes the application of recovered phenol and realizes the recycling of resources, but also reduces the waste of raw materials and reduces the production cost.

[0021] After condensation, 2,4-D sodium salt is obtained by suction filtration. After the 2,4-D sodium salt is dissolved in water, acid is added to adjust the pH to 0.5-1.0, the temperature is raised to 90-115° C. and kept for reaction for 0.5-2 hours. After the reaction is completed, 2,4-dichlorophenoxyacetic acid is obtained by filtration, washing and drying.

[0022] Technical Effects Compared with the existing technology, the beneficial effects of the present technical solution are as follows: 1. Effective recovery of phenol, high recycling rate: The present technical solution generates 2,4-dichlorophenol salt and haloacetic acid salt by neutralization reaction of alkaline solution with 2,4-dichlorophenol and haloacetic acid, and then obtains 2,4-dichlorophenoxyacetic acid by condensation reaction and acidification treatment. In this process, there is always unreacted phenol in the condensation process, which can be recovered and applied to the next batch of reactions, thereby improving the reaction conversion rate of phenol and reducing the waste of phenol. 2. Reduce production costs: Since the present technical solution can effectively recover unreacted phenol, the consumption of phenol can be reduced, thereby reducing production costs. At the same time, since the present technical solution does not require changing the reaction conditions or adding a catalyst, it can also avoid the increased energy consumption caused by changing the reaction conditions or using a catalyst. At the same time, the cost of waste liquid treatment is also reduced, further reducing the production cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 For existing process flow.

[0024] Figure 2 The present invention is a process flow chart for the application of phenol-containing recovery liquid. DETAILED DESCRIPTION

[0025] In order to further illustrate the present invention, the following embodiments are used to further illustrate the essential contents of the present invention, but the present invention is not limited thereto: Example 1 (1) Take 85 mL of chloroacetic acid (0.957 mol) and place it in a 500 mL three-necked flask, then add 80 mL of 32% liquid caustic soda and 15 mL of pure water dropwise, control the neutralization reaction temperature below 50 °C, stir for 30 minutes and then stop stirring to obtain a sodium chloroacetate solution; (2) 189.3 mL of sodium 2,4-dichlorophenol solution (phenol content of 56.19%, density of 1.37 g / mL) was measured and added to a 1000 mL four-necked flask, the temperature was raised to 102°C, and the sodium chloroacetate solution prepared in the above neutralization step was added dropwise. When the sodium chloroacetate solution was added to the 85th mL, the pH value of the reaction solution was lower than 10, and about 5 mL of 32% liquid alkali was required. During the reaction, the pH value of the reaction system was adjusted to about 10 by adding alkali. The sodium chloroacetate solution was added in about 2.5 hours, and the condensation reaction was carried out by heat preservation for 2 hours. After the heat preservation was completed, the temperature was lowered to 50°C, and alkali was added to adjust the pH value to greater than 11. A total of 15 mL of 32% liquid alkali was consumed during the alkali addition process. 2,4-D sodium salt and condensation mother liquor were obtained by suction filtration; (3) Add water in a mass ratio of 2,4-D sodium salt solid to water of 1:2 and stir until dissolved, add 92.4 g of 30% hydrochloric acid, keep warm at 90-115° C. for 0.5-2 hours, maintain pH at about 0.5-1.0 after the reaction, filter, wash with water 3 times, and dry to obtain 192 g of 2,4-dichlorophenoxyacetic acid solid with a purity of 98.4% and a percentage yield of 95.9%.

[0026] Example 2 (1) 330 mL of the condensation mother liquor obtained in Example 1 and 150 mL of ethyl acetate were placed in a 1000 mL three-necked flask, heated to 55-65° C., and the pH was adjusted to 6-7 with about 11 mL of 30% hydrochloric acid in advance. The mixture was separated into three layers, and about 9 mL of impurities in the bottom layer were extracted. 15 mL of hydrochloric acid was added until the pH value was 1, and 160 mL of ethyl acetate solution containing phenol was extracted; (2) Place 160 mL of ethyl acetate solution containing phenol and 28 mL of 19% liquid caustic soda in a 500 mL flask, stir for 30 minutes, extract and separate the layers using a separatory funnel, and complete stripping to obtain 51 mL of phenol-containing recovery liquid (phenol content: 21.3%, density: 1.26 g / mL); (3) Take 85 mL of chloroacetic acid (0.957 mol) and place it in a 500 mL three-necked flask. Measure 51 mL of the above-mentioned recovered liquid and slowly pour it into the chloroacetic acid solution. Then, drop 70 mL of 32% liquid alkali. Control the neutralization reaction temperature below 50°C. Stir for 30 minutes and then stop stirring. After standing, stratification occurs. The lower layer is 2,4-dichlorophenol sodium solution, and the upper layer is sodium chloroacetate solution. (4) Take a 1000mL four-necked flask, calculate the required amount of 171.9mL of sodium 2,4-dichlorophenol solution (phenol content is 56.19%, density is 1.37g / mL) according to the phenol content of the recovered liquid, add the sodium 2,4-dichlorophenol solution into the four-necked flask, heat it to 98°C, add the lower phenol solution (i.e., sodium 2,4-dichlorophenol solution) in the above neutralization process into the four-necked flask, continue to heat it to 102°C, and drop the upper sodium chloroacetate solution in the neutralization process. When the sodium chloroacetate solution is added to the 60th mL, the pH value of the reaction solution is lower than 10, and about 10mL of 32% liquid alkali needs to be added. During the reaction, the pH value of the reaction system needs to be adjusted to 10±0.5 by adding alkali. The sodium chloroacetate solution is dripped in about 2.5 hours, and the reaction is kept warm for 2 hours. After the insulation is completed, the temperature is lowered to 50°C, and alkali is added to adjust the pH value to greater than 11. A total of 24mL is consumed in the alkali addition process. 32% liquid caustic soda, after filtration, 2,4-D sodium salt and condensation mother liquor are obtained; (5) Add water in a mass ratio of 2,4-D sodium salt solid to water of 1:2 and stir until dissolved, add 91.2 g of 30% hydrochloric acid, keep warm at 90-115°C for 0.5-2 hours, maintain pH at about 0.5-1.0 after the reaction, filter, wash with water 3 times, and dry to obtain 190 g of 2,4-dichlorophenoxyacetic acid solid with a purity of 98.1% and a percentage yield of 94.6%; (6) The condensed mother liquor in step (4) was again introduced into the above extraction (i.e., step (1)), and the phenol was separated by stripping, which was then used in the neutralization step. After three times, 168 g of 2,4-dichlorophenoxyacetic acid solid was obtained with a purity of 93.2%.

[0027] Embodiment 3: (1) 330 mL of the condensation mother liquor obtained in Example 1 and 150 mL of n-butyl acetate were placed in a 1000 mL three-necked flask, heated to 55-65° C., and the pH was adjusted to 6-7 with about 12 mL of 30% hydrochloric acid in advance. The solution was separated into three layers, and about 8 mL of impurities in the bottom layer were extracted. 16 mL of hydrochloric acid was added until the pH value was 1, and 162 mL of phenolic n-butyl acetate solution was extracted; (2) Place 162 mL of phenol-containing n-butyl acetate solution and 29 mL of 19% liquid caustic soda in a 500 mL flask, stir for 30 minutes, extract and separate the layers using a separatory funnel, and complete stripping to obtain 52 mL of phenol-containing recovery liquid (phenol content: 22.1%, density: 1.26 g / mL); (3) Take 85 mL of chloroacetic acid (0.957 mol) and place it in a 500 mL three-necked flask. Measure 50 mL of the above-mentioned recovered liquid and slowly pour it into the chloroacetic acid solution. Then, drop 70 mL of 32% liquid alkali. Control the neutralization reaction temperature below 50 °C. Stir for 30 minutes and then stop stirring. After standing, stratification occurs. The lower layer is 2,4-dichlorophenol and the upper layer is sodium chloroacetate solution. (4) Take a 1000mL four-necked flask, calculate the required amount of 171.2mL of sodium 2,4-dichlorophenol solution (phenol content is 56.19%, density is 1.37g / mL) according to the phenol content of the recovered liquid, add the sodium 2,4-dichlorophenol solution into the four-necked flask, heat it to 98°C, add the lower phenol solution in the above neutralization process into the four-necked flask, continue to heat it to 102°C, and drop the upper sodium chloroacetate solution in the neutralization process. When the sodium chloroacetate solution is added to the 60th mL, the pH value of the reaction solution is lower than 10, and about 10mL of 32% liquid alkali needs to be added. During the reaction, the pH value of the reaction system needs to be adjusted to about 10 by adding alkali. The sodium chloroacetate solution is dripped in about 2.5 hours, and the reaction is kept warm for 2 hours. After the insulation is completed, the temperature is lowered to 50°C, and alkali is added to adjust the pH value to greater than 11. A total of 23mL is consumed in the alkali addition process. 32% liquid caustic soda, after filtration, obtain 2,4-D sodium salt and condensation mother liquor; (5) Add water in a mass ratio of 2,4-D sodium salt solid to water of 1:2 and stir until dissolved, add 92.4 g of 30% hydrochloric acid, keep warm at 90-115° C. for 0.5-2 hours, maintain pH at about 0.5-1.0 after the reaction, filter, wash with water 3 times, and dry to obtain 191 g of 2,4-dichlorophenoxyacetic acid solid with a purity of 98.3% and a percentage yield of 95.3%.

[0028] (6) The condensed mother liquor in step (4) was again introduced into the above extraction (i.e., step (1)), and the phenol was separated by stripping, which was then used in the neutralization step. After four times, 176 g of 2,4-dichlorophenoxyacetic acid solid was obtained with a purity of 94.7%.

[0029] Embodiment 4: (1) 330 mL of the condensation mother liquor obtained in Example 1 and 150 mL of ethyl acetate were placed in a 1000 mL three-necked flask, heated to 55-65° C., and the pH was adjusted to 6-7 with about 12 mL of 30% hydrochloric acid in advance. The mixture was separated into three layers, and about 8 mL of impurities in the bottom layer were extracted. 15 mL of hydrochloric acid was added until the pH value was 1, and 161 mL of ethyl acetate solution containing phenol was extracted; (2) Place 162 mL of ethyl acetate solution containing phenol and 27 mL of 19% liquid caustic soda in a 500 mL flask, stir for 30 minutes, extract and separate the layers using a separatory funnel, and complete stripping to obtain 52 mL of phenol-containing recovery liquid (phenol content: 19.8%, density: 1.24 g / mL); (3) Take 85 mL of chloroacetic acid (0.957 mol) and place it in a 500 mL three-necked flask. Measure 50 mL of the above-mentioned recovered liquid and slowly pour it into the chloroacetic acid solution. Then, drop 70 mL of 32% liquid alkali. Control the neutralization reaction temperature below 50 °C. Stir for 30 minutes and then stop stirring. After standing, stratification occurs. The lower layer is 2,4-dichlorophenol and the upper layer is sodium chloroacetate solution. (4) Take a 1000mL four-necked flask, calculate the required amount of 173.3mL of sodium 2,4-dichlorophenol solution (phenol content is 56.19%, density is 1.37g / mL) according to the phenol content of the recovered liquid, add the sodium 2,4-dichlorophenol solution into the four-necked flask, heat it to 98°C, add the lower phenol solution in the above neutralization process into the four-necked flask, continue to heat it to 102°C, and drop the upper sodium chloroacetate solution in the neutralization process. When the sodium chloroacetate solution is added to the 60th mL, the pH value of the reaction solution is lower than 10, and about 10mL of 32% liquid alkali needs to be added. During the reaction, the pH value of the reaction system needs to be adjusted to about 10 by adding alkali. The sodium chloroacetate solution is dripped in about 2.5 hours, and the reaction is kept warm for 2 hours. After the insulation is completed, the temperature is lowered to 50°C, and alkali is added to adjust the pH value to greater than 11. A total of 23mL is consumed in the alkali addition process. 32% liquid caustic soda, after filtration, 2,4-D sodium salt and condensation mother liquor are obtained; (5) Add water in a mass ratio of 2,4-D sodium salt solid to water of 1:2 and stir until dissolved, add 93.2 g of 30% hydrochloric acid, keep warm at 90-115° C. for 0.5-2 hours, maintain pH at about 0.5-1.0 after the reaction, filter, wash with water 3 times, and dry to obtain 193 g of 2,4-dichlorophenoxyacetic acid solid with a purity of 97.8% and a percentage yield of 95.8%.

[0030] (6) The condensed mother liquor in step (4) was again introduced into the above extraction (i.e., step (1)), and the phenol was separated by stripping, which was then used in the neutralization step. After three times, 172 g of 2,4-dichlorophenoxyacetic acid solid was obtained with a purity of 93.7%.

[0031] Embodiment 5: The steps are the same as those in Example 2, except that the solvent is ethylene dichloride. During the recycling process of the condensation mother liquor, after two times of recycling, 109 g of 2,4-dichlorophenoxyacetic acid solid is obtained with a purity of 84.5%.

Claims

1. A method for recovering and reusing sodium 2,4-dichlorophenol, characterized in that: The steps include: (1) neutralizing chloroacetic acid with liquid alkali and water to obtain a sodium chloroacetate solution; (2) Mixing the sodium chloroacetate solution and the sodium 2,4-dichlorophenol solution, and then heating them to carry out a condensation reaction; (3) filtering the obtained condensation reaction liquid to obtain a condensation mother liquor of 2,4-D sodium salt and phenol-containing wastewater; (4) adding acid to the condensed mother liquor of the phenol-containing wastewater for extraction to extract a phenol-containing solution; (5) adding alkali to the phenol-containing solution for stripping to obtain a phenol-containing recovery liquid; (6) adding the phenol-containing recovery liquid to chloroacetic acid, adding alkali, performing a neutralization reaction, and then standing to separate the layers to obtain a 2,4-dichlorophenol sodium recovery liquid and a sodium chloroacetate solution; (7) heating the sodium 2,4-dichlorophenol solution, adding the 2,4-dichlorophenol described in step (6), and adding dropwise the sodium chloroacetate solution while continuing to heat the solution to carry out a condensation reaction; (8) The condensation mother liquor is subjected to the cycle of steps (3) to (7) above again to achieve the recovery and reuse of sodium 2,4-dichlorophenol.

2. The method for recovering and reusing sodium 2,4-dichlorophenol according to claim 1, characterized in that: In step (2), the sodium chloroacetate solution is added dropwise to the sodium 2,4-dichlorophenol solution for condensation reaction. The dropping time is 2-4 hours, the condensation reaction temperature is 101-110° C., and during the condensation reaction, the pH is 10-11 and the reaction time is 2-3 hours.

3. The method for recovering and reusing sodium 2,4-dichlorophenol according to claim 1, characterized in that: In step (4), the condensed mother liquor is first added to the solvent before extraction, the temperature is raised to 50-70°C, and then acid is added to adjust the pH to 6-7 for extraction and stratification. The supernatant is taken and acid is added to adjust the pH to 0.1-1 for extraction and stratification to obtain a phenol-containing solvent solution.

4. The method for recovering and reusing sodium 2,4-dichlorophenol according to claim 3, characterized in that: The phenol-containing solvent is selected from any one of dichloromethane, toluene, ethyl acetate and n-butyl acetate; the acid is a hydrochloric acid solution with a mass concentration of 20-32%.

5. The method for recovering and reusing sodium 2,4-dichlorophenol according to claim 1, characterized in that: In step (6), the volume ratio of chloroacetic acid: recovered liquid: liquid alkali is 1.7:1:0.6-0.8; the pH is adjusted to 10-11 by adding alkali, and the temperature is raised to 25-50°C for neutralization reaction.

6. The method for recovering and reusing sodium 2,4-dichlorophenol according to claim 1, characterized in that: The alkali described in steps (5) and (6) is liquid alkali with a mass concentration of 15-35%.

7. The method for recovering and reusing sodium 2,4-dichlorophenol according to claim 1, characterized in that: In step (7), the sodium 2,4-dichlorophenol solution is heated to 90-100° C., and then the sodium 2,4-dichlorophenol recovery liquid is added. When the temperature is again raised to 102-108° C., a sodium chloroacetate solution is added dropwise to carry out a condensation reaction. The dropping time is 2-4 hours. During the condensation reaction, the pH is 10-11. After the reaction is completed, the temperature is kept for 2-4 hours. After the reaction is completed, the pH is adjusted to be greater than 11.

8. The method for recovering and reusing sodium 2,4-dichlorophenol according to claim 2 or 7, characterized in that: During the condensation reaction, the pH regulator is liquid alkali, and the mass concentration of the liquid alkali is 15-35%.

9. The method for recovering and reusing sodium 2,4-dichlorophenol according to claim 1, 2 or 7, characterized in that: After condensation, 2,4-D sodium salt is obtained by suction filtration. After the 2,4-D sodium salt is dissolved in water, acid is added to adjust the pH to 0.5-1.0, the temperature is raised to 90-115° C. and kept for reaction for 0.5-2 hours. After the reaction is completed, 2,4-dichlorophenoxyacetic acid is obtained by filtration, washing and drying.

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