A method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid

Abstract

The present application provides a method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid, comprising the following steps: S10, preparing a solution A with a pH value greater than 10; S20, adding hydrogen peroxide to solution A and stirring for 1 hour to obtain solution B; S30, circulating filtration to obtain a filtrate C; S40, decolorizing to obtain a solution D; S50, fully absorbing the solution D after the decolorization is completed; S60, acidifying; and S70, cooling, filtering, washing, and drying to obtain 3,3'-dithiodipropionic acid. The method has the advantages of recycling the recyclable filter cake, avoiding the problem of being unable to recycle and directly discharging the filter cake in the prior art, improving the waste utilization rate, and effectively saving resources. The method is applicable to the technical field of waste utilization during the preparation of 3-mercaptopropionic acid.

Description

Technical Field

[0001] The present application relates to the field of waste utilization in the process of preparing 3-mercaptopropionic acid, and specifically to a method for extracting 3,3′-dithiodipropionic acid in the process of preparing 3-mercaptopropionic acid. Background Art

[0002] 3-Mercaptopropionic acid is an intermediate in the pharmaceutical product Fenalide and is also used as a stabilizer for polyvinyl chloride. It can be used in transparent products and exhibits excellent thermal stability. It is also used as an antioxidant, catalyst, and biochemical reagent.

[0003] The patent application document with publication number CN111848458B proposes a method for preparing 3-mercaptopropionic acid and a process for extracting dicyandiamide during the preparation process. During the vacuum distillation in step (9), 3-mercaptopropionic acid is distilled out, and the distillation residue of the vacuum distillation is discharged as waste. However, some 3-mercaptopropionic acid in the distillation residue cannot be distilled out. If it is treated as waste, it will cause serious waste of resources. Summary of the Invention

[0004] In order to solve one of the above technical defects, the present application provides a method for extracting 3,3′-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid.

[0005] The present invention provides a method for extracting 3,3′-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid, comprising the following steps:

[0006] S10, preparing solution A with a pH value greater than 10;

[0007] S101, adding alkali solution and water into a reactor and stirring evenly;

[0008] S102, crushing the recyclable filter cake into a fine powder and adding it to the reactor, stirring until the recyclable filter cake is completely dissolved to obtain solution A;

[0009] S103, when the color of solution A is jet black, add alkali solution to make the pH value of solution A greater than 10;

[0010] S20, adding hydrogen peroxide to solution A, adding 20 kg of hydrogen peroxide at a time, stirring for 1 hour until all the hydrogen peroxide is added, to obtain solution B;

[0011] S30, circulating filtration: Solution B is filtered through a filter to remove impurities and obtain filtrate C;

[0012] S40, decolorization: After the temperature of the filtrate C is reduced to room temperature, it passes through a filter again, and then passes through at least three decolorization tanks in sequence for 1 hour to obtain a solution D;

[0013] S50, after the decolorization is completed, the solution D in the decolorization tank is fully sucked out by using negative pressure;

[0014] S60, acidification: slowly adding hydrochloric acid to solution D to acidify to obtain solution E;

[0015] S70, cooling, filter pressing, washing, and drying: After solution E is cooled to room temperature, it is filtered using a filter press. The filter cake obtained by the filtration is washed three times and dried to obtain 3,3'-dithiodipropionic acid.

[0016] Preferably, in step S102, the recyclable filter cake is crushed into fine powder and then added into the reactor, and stirred to dissolve the recyclable filter cake completely to obtain solution A; wherein: when the temperature in the reactor exceeds 75°C, the cooling water needs to be adjusted to maintain the temperature in the reactor below 75°C.

[0017] Preferably, in the acidification step S30, hydrochloric acid is slowly added to solution D for acidification to obtain solution E; wherein the time for slowly adding hydrochloric acid is not less than 1 hour.

[0018] More preferably, the method for preparing the recyclable filter cake comprises the following steps:

[0019] (1) Preparing a synthesis solution: hydrochloric acid, thiourea, and acrylic acid are sequentially added to a synthesis reactor to prepare a synthesis solution at a pH of 1 to 2;

[0020] (2) Hydrolysis reaction: Add the synthesis liquid and dilute alkali to the hydrolysis kettle in sequence;

[0021] (3) Filtration: Filtration obtains filtrate 1 and filter cake 1;

[0022] (4) Cooling and filtration:

[0023] Add filtrate 1 into the cooling tank, and use the cooling unit to reduce the temperature in the cooling tank to below 0°C;

[0024] The material in the cooling tank is then filtered through a filter press to obtain filtrate 2 and filter cake 2;

[0025] (5) preparing a mercaptopropionic acid solution: adding hydrochloric acid dropwise to the filtrate 2 to obtain a solution containing 3-mercaptopropionic acid;

[0026] (6) Filtration: obtaining filtrate 3 and filter cake 3;

[0027] (7) Extraction: The filtrate 3 is extracted with an organic solvent to obtain an organic solution containing 3-mercaptopropionic acid as the lower layer and a waste liquid as the upper layer;

[0028] (8) distilling off the organic solvent: distilling off the organic solvent in the organic solution containing 3-mercaptopropionic acid;

[0029] (9) vacuum distillation to obtain a recyclable filter cake: the material after the organic solvent is evaporated is subjected to vacuum distillation to distill off 3-mercaptopropionic acid, and the distillation residue is the recyclable filter cake;

[0030] (10) Condensation and packaging: The distilled 3-mercaptopropionic acid is condensed, collected into a product homogenizing kettle, and then packaged.

[0031] Preferably, the method for preparing the synthetic liquid in step (1) specifically comprises the following steps: adding hydrochloric acid and thiourea to a synthetic kettle in sequence, mixing and stirring evenly, slowly raising the temperature in the synthetic kettle to 40-90°C, and keeping warm for 0.5-2.5 hours; then dropwise adding acrylic acid to the synthetic kettle, and then slowly raising the temperature to 70-90°C, keeping warm and reacting for 0.5-3 hours, thereby preparing the synthetic liquid.

[0032] Preferably, the hydrolysis reaction in step (2) specifically includes the following steps: adding the synthesis liquid to the hydrolysis kettle, lowering the temperature in the hydrolysis kettle to 30-40°C; adding dilute alkali to the hydrolysis kettle, slowly raising the temperature in the hydrolysis kettle to 80-110°C, and keeping the temperature for reaction for 0.5-2h.

[0033] Preferably, the preparation of the mercaptopropionic acid solution in step (5) specifically includes the following steps: slowly lowering the temperature in the hydrolysis kettle to 20-60°C; then adding the filtrate 2 to the hydrolysis kettle, and then adding hydrochloric acid dropwise to a pH value of 1-2; to obtain a mixed solution containing 3-mercaptopropionic acid.

[0034] Preferably, the extraction in step (7) specifically includes the following steps: adding the filtrate 3 to the extraction kettle, adding the organic solvent to the extraction kettle, and stirring to fully dissolve the mercaptopropionic acid in the organic solvent; standing for 20 minutes to separate the layers, the lower layer is the organic solution containing mercaptopropionic acid, and the organic solution containing mercaptopropionic acid is placed in a storage tank for standby use; the upper layer is the extracted waste liquid, and extraction is performed three more times according to the above method; the extracted waste liquid is placed in a recovery tank and allowed to stand to recover part of the organic solvent.

[0035] Preferably, the step (8) of distilling out the organic solvent comprises the following steps: slowly adding the organic solution containing mercaptopropionic acid into a solvent distillation kettle, raising the temperature to 55° C., and distilling out the organic solvent; raising the temperature in the solvent distillation kettle to 120° C.; and until the organic solvent is completely distilled out.

[0036] Preferably, the reduced pressure distillation in step (9) comprises the following steps: adding the material after the solvent is evaporated into the steam product kettle, raising the temperature to 130° C., and performing reduced pressure distillation under a vacuum degree of 7 mmHg; collecting the front fraction; and continuing to perform reduced pressure distillation on the remaining part until the 3-mercaptopropionic acid is completely evaporated.

[0037] This application recycles the recyclable filter cake from the preparation of 3-mercaptopropionic acid, avoiding the problem of direct disposal in existing technologies. This improves waste utilization and effectively conserves resources. The prepared 3,3′-dithiodipropionic acid is a high-purity organic reagent that can be used primarily as an antioxidant and in the production of thioester antioxidants. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0039] Figure 1 This is a flow chart of a method for extracting 3,3′-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid provided in an embodiment of the present application. DETAILED DESCRIPTION

[0040] In order to make the technical solutions and advantages of the embodiments of the present application more clearly understood, the exemplary embodiments of the present application are further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present application, and are not an exhaustive list of all the embodiments. It should be noted that the embodiments and features in the embodiments of the present application can be combined with each other unless they conflict.

[0041] The present invention provides a method for extracting 3,3′-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid, comprising the following steps:

[0042] S10, preparing solution A with a pH value greater than 10;

[0043] S101, adding alkali solution and water into a reactor and stirring evenly;

[0044] In step S102, the recyclable filter cake is crushed into a fine powder and added to the reactor. Stirring is performed until the recyclable filter cake is completely dissolved to obtain Solution A. When adding the recyclable filter cake, care should be taken to protect the stirring rod to prevent damage. Gas may overflow during this process. This can be absorbed by turning on a gas absorption device connected to the reactor to prevent large amounts of gas from overflowing and causing air pollution. The gas overflowing in step S102 is generated by the reaction of ammonia nitrogen compounds in the recyclable filter cake with the alkali solution.

[0045] S103, when the color of solution A is jet black, add alkali solution to make the pH value of solution A greater than 10;

[0046] S20, adding hydrogen peroxide to solution A, adding 20 kg of hydrogen peroxide at a time, stirring for 1 hour until all the hydrogen peroxide is added, to obtain solution B;

[0047] During the addition of hydrogen peroxide, attention should be paid to gas overflow and temperature changes to prevent safety accidents. If the temperature rises rapidly, the rate of hydrogen peroxide addition should be slowed down. If the temperature rises too quickly, some hydrogen peroxide will overflow and be unable to participate in the reaction, resulting in a waste of raw materials. The gas overflowing in step S20 is ammonia generated by the reaction of the strong oxidant hydrogen peroxide with the ammonia nitrogen compound.

[0048] S30, circulating filtration: Solution B is filtered through a filter to remove impurities and obtain filtrate C;

[0049] In this process, two filter kettles are used, and the solution B is circulated and filtered between the two filter kettles through the filter, which can easily remove impurities.

[0050] S40, decolorization: After the temperature of the filtrate C drops to room temperature, it passes through the filter again, and then passes through at least three decolorization tanks in sequence for decolorization for 1 hour to obtain solution D; after each decolorization treatment in the decolorization tank, a sample is taken for colorimetric analysis to prevent the color from being too dark.

[0051] S50, after the decolorization is completed, the solution D in the decolorization tank is fully sucked out by using negative pressure; fully sucking out the solution D can prevent the loss of materials.

[0052] S60, acidification: slowly adding hydrochloric acid to solution D to acidify to obtain solution E;

[0053] S70, cooling, filter pressing, washing, and drying: After solution E is cooled to room temperature, it is filtered using a filter press. The filter cake obtained by the filter pressing is washed three times and dried to obtain 3,3′-dithiodipropionic acid;

[0054] During the filtration process, the solution E in the filter press is filtered through a filter press. According to the viscosity of the solution in the filter press, part of the filtrate is added back into the filter press to dilute the solution in the filter press, thereby preventing the solution in the filter press from being too thick to flow out and affecting the yield of 3,3′-dithiodipropionic acid. During the washing process, the washing water can be recycled.

[0055] The use of alkali solution and hydrogen peroxide in this application fully reacts the ammonia nitrogen compounds in the recyclable filter cake, and can fully extract the 3,3′-dithiodipropionic acid in the recyclable filter cake. In this application, the recyclable filter cake in the process of preparing 3-mercaptopropionic acid is recycled, avoiding the problem of being unable to recycle it and directly discharging it in the existing technology, improving the waste utilization rate and effectively saving resources. The prepared 3,3′-dithiodipropionic acid is a high-purity organic reagent that can be mainly used as an antioxidant and used to produce thioester antioxidants.

[0056] Specifically, in steps S10-S70, the reactants added are: 1700 kg of recyclable filter cake, 120-180 kg of hydrogen peroxide, 1500 kg of alkali solution, 1000 kg of water, and 1350 kg of hydrochloric acid. The recyclable filter cake contains 30% water, the alkali solution is a 30% sodium hydroxide solution, and the hydrochloric acid has a concentration of 31%. The reactants provided in this application are used to recycle the recyclable filter cake in the process of preparing 3-mercaptopropionic acid to produce 3,3'-dithiodipropionic acid. The raw materials are readily available and do not contain highly toxic substances, making the preparation process more convenient and safe.

[0057] Furthermore, in step S102, the recyclable filter cake is crushed into a fine powder and then added to the reactor, and stirred until the recyclable filter cake is completely dissolved to obtain solution A. When the temperature in the reactor exceeds 75°C, the cooling water needs to be adjusted to maintain the temperature in the reactor below 75°C. Specifically, after the jacket of the reactor is filled with cooling water, the water inlet valve is closed to maintain a small amount of water flowing into the jacket to maintain the temperature in the reactor below 75°C.

[0058] Furthermore, in step S30, acidification is performed by slowly adding hydrochloric acid to solution D to obtain solution E; wherein the slow addition of hydrochloric acid is performed for no less than 1 hour. In the present application, the addition of hydrochloric acid is performed for no less than 1 hour to prevent the problem of rapid crystallization and easy coagulation of the solution into a single mass due to excessive addition of hydrochloric acid.

[0059] Furthermore, the method for preparing the recyclable filter cake comprises the following steps:

[0060] (1) Preparing a synthesis solution: hydrochloric acid, thiourea, and acrylic acid are sequentially added to a synthesis reactor to prepare a synthesis solution at a pH of 1 to 2;

[0061] Add hydrochloric acid and thiourea to the synthesis kettle in sequence, mix and stir evenly, slowly raise the temperature in the synthesis kettle to 40-90°C, and keep warm for 0.5-2.5 hours; then add acrylic acid dropwise to the synthesis kettle, and then slowly raise the temperature to 70-90°C, keep warm and react for 0.5-3 hours to prepare the synthesis liquid.

[0062] (2) Hydrolysis reaction: Synthesis liquid and dilute alkali are sequentially added to the hydrolysis kettle; the dilute alkali solution is a sodium hydroxide solution with a concentration of 20-30%; the treatment effect of using a sodium hydroxide solution with a concentration of 20-30% is good, and the waste liquid generated can be separated by simple separation and distillation.

[0063] Add the synthetic liquid to the hydrolysis kettle and lower the temperature in the hydrolysis kettle to 30-40°C; add dilute alkali to the hydrolysis kettle and slowly raise the temperature in the hydrolysis kettle to 80-110°C, and keep the temperature for reaction for 0.5-2h.

[0064] (3) Filtration: Filtration obtains filtrate 1 and filter cake 1;

[0065] (4) Cooling and filtration:

[0066] Add filtrate 1 into the cooling tank, and use the cooling unit to reduce the temperature in the cooling tank to below 0°C;

[0067] The material in the cooling tank is then filtered through a filter press to obtain filtrate 2 and filter cake 2;

[0068] The purification process for extracting dicyandiamide comprises the following steps: adding water to the filter cake 2 and hot-melting the mixture at a temperature of 60-80° C.; then lowering the temperature to below 20° C. for cooling and crystallization; and drying and packaging to obtain dicyandiamide. In the drying step, hot air at 100° C. is used for hot air drying. The hot air drying can dry the dicyandiamide obtained by cooling and crystallization by hot air, completely remove moisture in the dicyandiamide, and obtain dry dicyandiamide powder.

[0069] Water recycling: After the dicyandiamide is extracted through purification, the water is pumped through a vacuum of 0.5-0.6 mmHg and then recycled for the next purification process. This water recycling reduces water waste.

[0070] (5) preparing a mercaptopropionic acid solution: adding hydrochloric acid dropwise to the filtrate 2 to obtain a solution containing 3-mercaptopropionic acid;

[0071] The temperature in the hydrolysis kettle is slowly lowered to 20-60° C.; the filtrate 2 is then added to the hydrolysis kettle, and hydrochloric acid is then added dropwise until the pH value reaches 1-2; a mixed solution containing 3-mercaptopropionic acid is obtained.

[0072] (6) Filtration: obtaining filtrate 3 and filter cake 3;

[0073] (7) Extraction: The filtrate 3 is extracted with an organic solvent to obtain an organic solution containing 3-mercaptopropionic acid as the lower layer and a waste liquid as the upper layer;

[0074] Add filtrate 3 to the extraction kettle, add the organic solvent to the extraction kettle, and stir to fully dissolve the mercaptopropionic acid in the organic solvent; let it stand for 20 minutes to separate the layers, the lower layer is the organic solution containing mercaptopropionic acid, and the organic solution containing mercaptopropionic acid is placed in a storage tank for standby use; the upper layer is the extracted waste liquid, and it is extracted three more times according to the above method; the extracted waste liquid is placed in a recovery tank and allowed to stand to recover part of the organic solvent.

[0075] The organic solvent is one or more of chloroform, benzene, ethyl acetate, carbon tetrachloride, and dichloromethane; the organic solvent is used to extract 3-mercaptopropionic acid from a solution containing 3-mercaptopropionic acid, thereby separating the 3-mercaptopropionic acid from other components in the solution.

[0076] (8) distilling off the organic solvent: distilling off the organic solvent in the organic solution containing 3-mercaptopropionic acid;

[0077] Slowly add the organic solution containing mercaptopropionic acid into the solvent evaporation kettle, raise the temperature to 55°C, and evaporate the organic solvent; raise the temperature in the solvent evaporation kettle to 120°C; until the organic solvent is completely evaporated.

[0078] The evaporated organic solvent is condensed into liquid by a condenser and then collected in an organic solvent recovery tank for recycling. The recovered organic solvent can be used again to extract 3-mercaptopropionic acid, thus reducing the waste of organic solvent.

[0079] (9) vacuum distillation to obtain a recyclable filter cake: the material after the organic solvent is evaporated is subjected to vacuum distillation to distill off 3-mercaptopropionic acid, and the distillation residue is the recyclable filter cake;

[0080] The material after the solvent is evaporated is added to the distillation product kettle, the temperature is raised to 130°C, and reduced pressure distillation is performed under a vacuum degree of 7 mmHg; the front fraction is collected; the remaining fraction is further distilled under reduced pressure until the 3-mercaptopropionic acid is completely evaporated.

[0081] The collected front fraction is added to the product still of the next vacuum distillation to fully utilize the intermediate product and ensure the purity of the distilled 3-mercaptopropionic acid.

[0082] (10) Condensation and packaging: After the distilled 3-mercaptopropionic acid is condensed, it is collected in a product homogenizer and packaged. When no more liquid is generated in the condenser, it means that the 3-mercaptopropionic acid has been completely distilled.

[0083] In the present application, dicyandiamide is extracted from the filter cake 2 in step (4) and water is recovered. In step (8), the organic solvent is recovered. In step (9), the recyclable filter cake and the fore-fraction are recycled. The above operations recycle most of the waste, reduce waste emissions, and protect the environment. New products, dicyandiamide and 3,3′-dithiodipropionic acid, are extracted, improving economic efficiency. Extracting dicyandiamide also increases the yield of 3-mercaptopropionic acid, resolving the problem of low yield of 3-mercaptopropionic acid produced when thiourea is used as a raw material. The purity of the dicyandiamide extracted in the process of preparing 3-mercaptopropionic acid in the present application reaches 98.0-99.3%, and the yield of dicyandiamide reaches 65-85%. Simultaneously, the purity of the prepared 3-mercaptopropionic acid reaches 98.0-99.5%, and the yield of 3-mercaptopropionic acid reaches 82-85%.

[0084] This application extracts 65-85% of the dicyandiamide produced by the reaction to prepare dicyandiamide, and the remaining dicyandiamide is prepared into urea. The waste liquid after extraction in the present invention includes: raw materials acrylic acid and thiourea that did not participate in the reaction, and the intermediate products urea, sodium chloride, and water. The substances in the waste liquid can be separated and recovered through distillation and filtration steps. The raw acrylic acid can be further used as a raw material to prepare 3-mercaptopropionic acid after recovery, thiourea and urea can be used to synthesize urea-formaldehyde resin, sodium chloride can be used as a raw material for industrial sodium chloride, and water can be used for boiler water. The separated and recovered waste liquid is further treated to meet emission standards, with minimal pollution to the environment.

[0085] Specifically, in steps (1) to (10), the amount of reactants added is calculated in molar ratio as follows:

[0086] In step (1), hydrochloric acid: thiourea: acrylic acid = 1.0-1.5: 1: 1.1-1.5;

[0087] In step (2), the ratio of dilute alkali to thiourea is 2.1-3.0:1;

[0088] In step (5), hydrochloric acid:thiourea=1.2-1.5:1.

[0089] Specifically, in the purification treatment of step (4), the ratio of water to dicyandiamide is 0.75:0.5 to 0.75.

[0090] In the present application, the above reactants are used to extract dicyandiamide during the preparation of 3-mercaptopropionic acid, and the raw materials are easily available and do not include highly toxic substances.

[0091] The relevant reaction formulas involved in the reaction in the method for preparing 3-mercaptopropionic acid provided in this application are:

[0092] Step (1):

[0093]

[0094] Step (2):

[0095]

[0096] Step (4):

[0097] NH2CN+NH2CN→C2H4N4;

[0098] Step (5):

[0099]

[0100]

[0101] Side effects:

[0102]

[0103] The relevant reaction formulas involved in the method for extracting 3,3′-dithiodipropionic acid provided in this application are:

[0104] Step S10:

[0105]

[0106] Step S60:

[0107] C6H8S2O4Na2+2HCl→C6H 10 O4S2+2NaCl.

[0108] The structural formula of 3,3′-dithiodipropionic acid prepared in this application is:

[0109]

[0110] According to the above reaction formula, the present application extracts dicyandiamide and 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid. While extracting dicyandiamide, the yield of 3-mercaptopropionic acid can be increased. 3,3'-dithiodipropionic acid is extracted from the recyclable filter cake without affecting the preparation of dicyandiamide and 3-mercaptopropionic acid. The recyclable filter cake is directly recycled, reducing waste emissions, conserving resources, and minimizing environmental pollution.

[0111] Although the preferred embodiments of the present application have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present application.

[0112] Obviously, those skilled in the art may make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application is intended to include these modifications and variations.

Claims

1. A method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid, characterized in that: The following steps are involved: S10, preparing solution A with a pH value greater than 10; S101, adding alkali solution and water into a reactor and stirring evenly; S102, crushing the recyclable filter cake into a fine powder and adding it to the reactor, stirring until the recyclable filter cake is completely dissolved to obtain solution A; S103, when the color of solution A is jet black, adding alkali solution to make the pH value of solution A greater than 10; S20, adding hydrogen peroxide to solution A, adding 20 kg of hydrogen peroxide at a time, stirring for 1 hour until all the hydrogen peroxide is added, to obtain solution B; S30, circulating filtration: Solution B is filtered through a filter to remove impurities and obtain filtrate C; S40, decolorization: After the temperature of the filtrate C is reduced to room temperature, it passes through a filter again, and then passes through at least three decolorization tanks in sequence for 1 hour to obtain a solution D; S50, after the decolorization is completed, the solution D in the decolorization tank is fully sucked out by using negative pressure; S60, acidification: slowly adding hydrochloric acid to solution D to acidify to obtain solution E; S70, cooling, filter pressing, washing, and drying: After solution E is cooled to room temperature, it is filtered using a filter press. The filter cake obtained by the filter pressing is washed three times and dried to obtain 3,3'-dithiodipropionic acid.

2. The method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid according to claim 1, characterized in that: In step S102, the recyclable filter cake is crushed into a fine powder and then added into the reactor, and stirred to dissolve the recyclable filter cake completely to obtain solution A. When the temperature in the reactor exceeds 75° C., the cooling water needs to be adjusted to maintain the temperature in the reactor below 75° C.

3. The method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid according to claim 1, characterized in that: The acidification step S30 is as follows: hydrochloric acid is slowly added to solution D to acidify the solution to obtain solution E; wherein the time for slowly adding hydrochloric acid is not less than 1 hour.

4. The method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid according to any one of claims 1 to 3, characterized in that: The method for preparing the recyclable filter cake comprises the following steps: (1) Preparing a synthesis solution: adding hydrochloric acid, thiourea, and acrylic acid to a synthesis reactor in sequence to prepare a synthesis solution at a pH of 1 to 2; (2) Hydrolysis reaction: Add the synthesis liquid and dilute alkali to the hydrolysis kettle in sequence; (3) Filtration: Filtration obtains filtrate 1 and filter cake 1; (4) Cooling and filtration: Add filtrate 1 into the cooling tank, and use the cooling unit to reduce the temperature in the cooling tank to below 0°C; The material in the cooling tank is then filtered through a filter press to obtain filtrate 2 and filter cake 2; (5) preparing a mercaptopropionic acid solution: adding hydrochloric acid dropwise to the filtrate 2 to obtain a solution containing 3-mercaptopropionic acid; (6) Filtration: obtaining filtrate 3 and filter cake 3; (7) Extraction: The filtrate 3 is extracted with an organic solvent to obtain an organic solution containing 3-mercaptopropionic acid as the lower layer and a waste liquid as the upper layer; (8) distilling off the organic solvent: distilling off the organic solvent in the organic solution containing 3-mercaptopropionic acid; (9) vacuum distillation to obtain a recyclable filter cake: the material after the organic solvent is evaporated is subjected to vacuum distillation to distill off 3-mercaptopropionic acid, and the distillation residue is the recyclable filter cake; (10) Condensation and packaging: The distilled 3-mercaptopropionic acid is condensed, collected into a product homogenizing kettle, and then packaged.

5. The method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid according to claim 4, characterized in that: The method for preparing the synthetic liquid in step (1) specifically comprises the following steps: Add hydrochloric acid and thiourea to the synthesis kettle in sequence, mix and stir evenly, slowly raise the temperature in the synthesis kettle to 40-90°C, and keep warm for 0.5-2.5h; Then, acrylic acid is added dropwise to the synthesis kettle, and the temperature is slowly raised to 70-90° C., and the reaction is carried out at this temperature for 0.5-3 hours to prepare a synthesis liquid.

6. The method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid according to claim 4, characterized in that: The hydrolysis reaction in step (2) specifically comprises the following steps: Add the synthetic liquid to the hydrolysis kettle and reduce the temperature in the hydrolysis kettle to 30-40°C; Add dilute alkali to the hydrolysis kettle, slowly raise the temperature in the hydrolysis kettle to 80-110°C, and keep the temperature to react for 0.5-2h.

7. The method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid according to claim 4, characterized in that: The preparation of the mercaptopropionic acid solution in step (5) specifically comprises the following steps: The temperature in the hydrolysis kettle is slowly lowered to 20-60° C.; the filtrate 2 is then added to the hydrolysis kettle, and hydrochloric acid is then added dropwise until the pH value reaches 1-2; a mixed solution containing 3-mercaptopropionic acid is obtained.

8. The method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid according to claim 4, characterized in that: The extraction in step (7) specifically comprises the following steps: Add the filtrate 3 to the extraction kettle, add the organic solvent to the extraction kettle, and stir to fully dissolve the mercaptopropionic acid in the organic solvent; The mixture was allowed to stand for 20 minutes to separate layers. The lower layer was an organic solution containing mercaptopropionic acid. The organic solution containing mercaptopropionic acid was placed in a storage tank for later use. The upper layer was the waste liquid from the extraction. The extraction was repeated three times according to the above method. The extraction waste liquid is placed in a recovery tank and allowed to stand to recover part of the organic solvent.

9. The method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid according to claim 4, characterized in that: The step (8) of distilling off the organic solvent comprises the following steps: Slowly add the organic solution containing mercaptopropionic acid into the solvent steamer, raise the temperature to 55°C, and steam out the organic solvent; The temperature in the solvent distillation kettle was raised to 120°C until the organic solvent was completely distilled off.

10. The method for extracting 3,3'-dithiodipropionic acid during the preparation of 3-mercaptopropionic acid according to claim 4, characterized in that: The reduced pressure distillation described in step (9) comprises the following steps: After the solvent is evaporated, the material is added into the steam product kettle, the temperature is raised to 130℃, and the vacuum degree is 7mmHg under reduced pressure distillation; The front fractions were collected; the remaining fractions were distilled under reduced pressure until 3-mercaptopropionic acid was completely distilled off.

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