Treatment method for preparing alpha-hydroxy carboxylic ester and byproduct ammonium bisulfate mixed salt by acetone cyanohydrin method

Abstract

The invention relates to the technical field of chemical production, and particularly discloses a treatment method for preparing alpha-hydroxy carboxylic ester and byproduct ammonium bisulfate mixed salt through an acetone cyanohydrin method. According to the method, unreacted sulfuric acid in the mixed salt is used, water is dropwise added at a high temperature to be matched with the methanol-water mixture at the same time, so that monomethyl sulfate is fully hydrolyzed into sulfuric acid and methanol, a large amount of extra waste salt generated by decomposing monomethyl sulfate with strong base at a high temperature is avoided, and the dropwise adding water amount is controlled, so that monomethyl sulfate can be completely hydrolyzed, and the production cost is reduced. After ammonia gas is introduced into the saline water, concentration treatment is not needed, fertilizer-grade I-type standard ammonium sulfate can be obtained through direct centrifugation, the centrifuged ammonium sulfate mother liquor is not directly discharged out of the system, but continues to be circulated back to the system to be used for dissolving mixed salt obtained after high-temperature acidolysis, ammonium sulfate in the mixed salt continues to be recycled, resource utilization of the ammonium sulfate mother liquor is achieved, and the utilization rate of the ammonium sulfate mother liquor is increased. And no liquid hazardous waste is generated in the whole system, so that the method is a green, energy-saving and recycling treatment method, and has higher popularization and application values.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of chemical production, and particularly relates to a treatment method of by-product ammonium bisulfate mixed salt in preparation of alpha-hydroxycarboxylic acid ester by acetone cyanohydrin method. BACKGROUND

[0002] In the chemical field, the process of preparing carboxylic acid ester by cyanohydrin alcoholysis reaction of sulfuric acid and cyan compound in methanol solvent is widely applied, and the method of producing 2-hydroxyisobutyric acid methyl ester by reaction of acetone cyanohydrin and concentrated sulfuric acid in methanol system is the core scheme for synthesizing 2-hydroxyisobutyric acid methyl ester, 2-hydroxyisobutyric acid and polyacrylic acid methyl ester in industry. The above process will inevitably produce by-product ammonium bisulfate mixed salt in the production process, and the main components of the mixed salt include ammonium bisulfate, sulfuric acid monomethyl ester, unreacted sulfuric acid and a small amount of organic impurities. Among them, the content of sulfuric acid monomethyl ester is about 25% to 35%, the existence of the substance not only seriously interferes with the recovery and purification of subsequent ammonium sulfate salt, but also may cause equipment corrosion or secondary pollution risk in subsequent processing steps, so the sulfuric acid monomethyl ester must be converted or decomposed before the recovery of ammonium sulfate salt.

[0003] In the current research, although there is a report that the sulfuric acid monomethyl ester in wastewater is converted into sodium methyl sulfate by controlling the pH of the system, and then the sodium methyl sulfate is used to synthesize nitromethane, p-toluenesulfone and benzyl ether, etc., but this process needs to introduce a large amount of sodium hydroxide, and needs to go through a series of operations such as high-temperature reaction, distillation, rectification, etc., which greatly increases the difficulty of wastewater treatment, and the industrial application is difficult.

[0004] Currently, there are mainly two treatment paths in industry: one is to convert sodium methyl sulfate into sodium sulfate by atmospheric reflux hydrolysis with water as the solvent under the condition of sodium hydroxide, and then sodium sulfate is obtained by concentration and dehydration separation. Although this method has relatively simple process steps, a large amount of sodium hydroxide is consumed, and a mixed salt of sodium sulfate and ammonium sulfate is produced as a by-product, which is difficult to separate, so the added value of the obtained sodium sulfate product is low, resulting in low industrial economic value of this path. The second is to add excess ammonia water to the waste salt system, and convert the sulfuric acid monomethyl ester into ammonium sulfate by high-pressure alkaline hydrolysis, but this scheme needs to discharge excess ammonia gas from the system, and the ammonium sulfate product can be obtained only by high-energy consumption concentration operation, so the overall energy consumption and operation cost are high, which cannot meet the economic requirements of industrial large-scale application.

[0005] Therefore, it has become a technical requirement to be urgently solved in the field to develop a process which is more simple and economical in operation, and can effectively treat sulfuric acid monomethyl ester and realize efficient recovery of ammonium sulfate salt. SUMMARY

[0006] In view of the problems that the reagent consumption is large, the energy consumption is high, the product separation is difficult or the economic value is low in the existing treatment method of by-product ammonium bisulfate mixed salt in preparation of alpha-hydroxycarboxylic acid ester by acetone cyanohydrin method, and the high-efficiency and low-cost treatment and resource recovery of the mixed salt cannot be realized, the present application provides a treatment method of by-product ammonium bisulfate mixed salt in preparation of alpha-hydroxycarboxylic acid ester by acetone cyanohydrin method.

[0007] To solve the above technical problems, the technical scheme provided by the present application is: A treatment method of by-product ammonium bisulfate mixed salt in preparation of alpha-hydroxycarboxylic acid ester by acetone cyanohydrin method, comprising the following steps: Step a, the by-product ammonium bisulfate mixed salt is heated and melted, water is added dropwise to the molten by-product ammonium bisulfate mixed salt for hydrolysis reaction, at the same time, methanol-water mixture is collected at normal pressure, and after the dropwise addition is completed, the temperature is kept until the hydrolysis is complete, to obtain ammonium bisulfate and ammonium sulfate mixed salt; Step b, water is added to the ammonium bisulfate and ammonium sulfate mixed salt for dissolution, to obtain a mixed salt solution; Step c, activated carbon is added to the mixed salt solution for decolorization, and solid-liquid separation is performed, to obtain a decolorized mixed salt solution; Step d, ammonia gas is introduced into the decolorized mixed salt solution, and solid-liquid separation is performed, to obtain ammonium sulfate solid and ammonium sulfate mother liquor.

[0008] The ester bond in the molecule of monomethyl sulfate is affected by the spatial steric hindrance of methyl and the strong electron-withdrawing effect of sulfate, and compared with ordinary carboxylic acid ester (such as methyl acetate), the hydrolysis activity is significantly reduced, and it is difficult to break under conventional conditions; and the content of monomethyl sulfate in the mixed salt is as high as 25% to 35%, and local mass transfer resistance is easily formed under high concentration, which further inhibits the hydrolysis reaction, and the existing technology generally needs to introduce additional acid and alkali reagents, depends on high pressure or high temperature operation, but still cannot realize complete hydrolysis.

[0009] In view of the core bottleneck that the monomethyl sulfate in the by-product ammonium bisulfate mixed salt in preparation of alpha-hydroxycarboxylic acid ester by acetone cyanohydrin method is difficult to hydrolyze efficiently, the present application breaks through the conventional idea of introducing additional acid and alkali reagents in the existing technology, and creatively uses the unreacted sulfuric acid in the by-product ammonium bisulfate mixed salt as an acidolysis reagent, water is added dropwise to the molten mixed salt to initiate the hydrolysis reaction, and at the same time, the methanol-water mixture generated in the reaction is removed in real time by using normal pressure distillation, so that the high-content monomethyl sulfate contained in the mixed salt is completely hydrolyzed, and finally the mixed salt of ammonium bisulfate and ammonium sulfate is obtained, which lays a foundation for the subsequent efficient recovery of ammonium sulfate.

[0010] After the hydrolysis of monomethyl sulfate is completed, the salt is dissolved in water to obtain a mixed salt solution, then the colored impurities are removed by decolorization to obtain a colorless clear liquid, then ammonia gas is introduced into the decolorized mixed salt solution to neutralize the generated ammonium bisulfate, and then the ammonium sulfate is obtained by filtration, the quality of the ammonium sulfate can reach the standard of fertilizer grade I (GB / T 535-2020), and the high-value utilization of the by-product mixed salt is realized, which provides a feasible technical path for the greenization and industrialization upgrading of the alpha-hydroxycarboxylic acid ester industry prepared by the acetone cyanhydrin method.

[0011] The application does not need to introduce additional acid and alkali reagents, and does not need the complex operations such as high-pressure alkaline hydrolysis and rectification separation in the traditional method, and does not need to be equipped with high-energy consumption equipment such as high-pressure reaction kettle and rectification tower, but can complete the hydrolysis of monomethyl sulfate and the recovery of methanol through conventional heating melting and atmospheric distillation, greatly reducing the processing cost and production safety risk, and facilitating the realization of industrialized production application.

[0012] It should be noted that the alpha-hydroxycarboxylic acid ester in the application is prepared by hydrolysis and esterification of cyanogen in an alcohol solvent. Specifically, for the preparation of 2-hydroxyisobutyric acid ester by the acetone cyanhydrin method, the remaining distillation kettle residue, i.e. the by-product ammonium bisulfate mixed salt, is obtained after distillation.

[0013] Taking the preparation of 2-hydroxyisobutyric acid ester by the acetone cyanhydrin method as an example, acetone cyanhydrin first reacts with sulfuric acid in water to form alpha-hydroxy-isobutyramide hydrogen sulfate, and then esterification is carried out to obtain methyl 2-hydroxyisobutyrate and ammonium bisulfate. In this process, sulfuric acid reacts with methanol to form monomethyl sulfate. After distillation, the main components contained in the remaining kettle residue include ammonium bisulfate, monomethyl sulfate, unreacted sulfuric acid and a small amount of organic impurities, wherein the content of monomethyl sulfate is about 25%~35%.

[0014] Further, in step a, the temperature of the heating melting is 120℃~140℃.

[0015] Further, in step a, the molar ratio of the amount of water added to the monomethyl sulfate in the by-product ammonium bisulfate mixed salt is 2:1~5:1, and the temperature of the added water and the temperature of the hydrolysis are both 120℃~140℃.

[0016] The preferred amount of water added can ensure that the water and monomethyl sulfate are in sufficient contact, and the subsequent hydrolysis under heat can promote the complete hydrolysis of monomethyl sulfate. Combined with a small amount of dissolved water matching the amount of mixed salt in step b, the concentration of the final mixed salt solution can be just in the range of ammonium sulfate saturation precipitation after ammonia is introduced, so that after the introduction of ammonia gas to neutralize ammonium bisulfate to generate ammonium sulfate, the concentration of ammonium sulfate in the system reaches saturation instantaneously, without any concentration treatment, ammonium sulfate solid can be obtained directly by centrifugal separation, and the processing efficiency of the whole process is improved.

[0017] Further, in step a, the water dropping time is 1h-3h; and the water hydrolysis time after the water dropping is completed is 2h-3h.

[0018] The water dropping time of 1h-3h can make the water melt into the molten salt at a slow and continuous rate, form a uniform water-mixed salt reaction environment, and avoid the problems of local water excess or deficiency, and match the hydrolysis rate of monomethyl sulfate with the methanol extraction capacity at normal pressure, thereby improving the hydrolysis rate of monomethyl sulfate.

[0019] Further, in step b, the mass ratio of the water to the by-product ammonium bisulfate mixed salt in step a is 0.5:1-1.5:1.

[0020] Further, in step c, the activated carbon is an acidic wood carbon, and the mass ratio of the activated carbon to the mixed salt solution is 0.001:1-0.01:1.

[0021] Further, in step c, the waste activated carbon obtained through the solid-liquid separation can be regenerated through anaerobic high-temperature calcination, and the regenerated activated carbon can be reused to reduce the cost.

[0022] For example, the anaerobic high-temperature calcination temperature is 700℃-900℃, and the calcination time is 1h-3h.

[0023] Further, in step d, the end point of the ammonia gas introduction is that the pH of the feed liquid is 6-7.

[0024] Further, in step d, the ammonia gas introduction time is 2h-3h, and the ammonia gas introduction temperature is 20℃-50℃.

[0025] The preferred ammonia gas introduction amount and introduction time can make the ammonium bisulfate in the system be fully converted into ammonium sulfate, and avoid the problem of excessive free ammonia in the system, which cannot meet the standard of GB / T 535-2020 Fertilizer Grade Ammonium Sulfate.

[0026] Further, in step d, the ammonium sulfate mother liquor is also used to dissolve the ammonium bisulfate and ammonium sulfate mixed salt in step b.

[0027] Further, in step d, the solid-liquid separation method is filtration, and the filtration temperature is 20℃-30℃.

[0028] The application provides a treatment method of by-product ammonium bisulfate mixed salt in preparation of alpha-hydroxycarboxylic acid ester by acetone cyanohydrin method, which utilizes unreacted sulfuric acid in the mixed salt, and through the way of adding water drop by drop at high temperature and simultaneously recovering methanol-water mixture, the monomethyl sulfate is fully hydrolyzed into sulfuric acid and methanol, the problem of generating a large amount of additional waste salt by using strong base and high-temperature decomposition of monomethyl sulfate is avoided, and the amount of water drop is controlled, so that the monomethyl sulfate is completely hydrolyzed, and after ammonia gas is passed through the salt water, the salt water does not need to be concentrated and treated, and direct centrifugation can obtain ammonium sulfate of fertilizer grade I standard (GB / T 535-2020), and the ammonium sulfate mother liquor after centrifugation is not directly discharged from the system, but is recycled back to the system to dissolve the mixed salt after high-temperature acidolysis, and the ammonium sulfate in the mixed salt is continuously recovered, the ammonium sulfate mother liquor is resourceized, and no liquid hazardous waste is generated in the whole system, which is a green, energy-saving and resourceful treatment method, and has high popularization and application value. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The application provides a treatment method of by-product ammonium bisulfate mixed salt in preparation of alpha-hydroxycarboxylic acid ester by acetone cyanohydrin method, which utilizes unreacted sulfuric acid in the mixed salt, and through the way of adding water drop by drop at high temperature and simultaneously recovering methanol-water mixture, the monomethyl sulfate is fully hydrolyzed into sulfuric acid and methanol, the problem of generating a large amount of additional waste salt by using strong base and high-temperature decomposition of monomethyl sulfate is avoided, and the amount of water drop is controlled, so that the monomethyl sulfate is completely hydrolyzed, and after ammonia gas is passed through the salt water, the salt water does not need to be concentrated and treated, and direct centrifugation can obtain ammonium sulfate of fertilizer grade I standard (GB / T 535-2020), and the ammonium sulfate mother liquor after centrifugation is not directly discharged from the system, but is recycled back to the system to dissolve the mixed salt after high-temperature acidolysis, and the ammonium sulfate in the mixed salt is continuously recovered, the ammonium sulfate mother liquor is resourceized, and no liquid hazardous waste is generated in the whole system, which is a green, energy-saving and resourceful treatment method, and has high popularization and application value. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical scheme and advantages of the application clearer, the application will be further described in detail below with examples. It should be understood that the specific examples described herein are only used to explain the application, and are not used to limit the application.

[0031] In order to better illustrate the application, the following examples are further illustrated by examples.

[0032] The following examples and comparative examples take the treatment of waste salt containing ammonium bisulfate, monomethyl sulfate, sulfuric acid and a small amount of organic impurities generated in the process of synthesizing 2-hydroxyisobutyric acid methyl ester as an example for detailed description.

[0033] The 2-hydroxyisobutyric acid methyl ester waste salt described in the following examples and comparative examples is derived from the production process of synthesizing 2-hydroxyisobutyric acid methyl ester by taking acetone cyanohydrin, sulfuric acid and methanol as raw materials, and the 2-hydroxyisobutyric acid methyl ester waste salt contains 25% to 35% monomethyl sulfate.

[0034] Preparation of ammonium bisulfate mixed salt: After mixing concentrated sulfuric acid, water and acetone cyanohydrin, the temperature is raised to 40-50 DEG C and kept for 1 h, methanol is added, the temperature is raised to reflux and kept for 12 h, after the end of the incubation, the methanol and 2-hydroxyisobutyric acid methyl ester are distilled out under reduced pressure, and the remaining material is by-product ammonium bisulfate mixed salt. The indicators of the by-product ammonium bisulfate mixed salt are detected: SO4 2- : 52.54%, total nitrogen 11.20% (calculated as N), acidity 0.52% (calculated as sulfuric acid), monomethyl sulfate: 27.72%.

[0035] The following examples directly take the above-mentioned mixed salt for treatment.

[0036] Example 1 The present example provides a treatment method for the by-product ammonium bisulfate mixed salt of methyl 2-hydroxyisobutyrate, which specifically comprises the following steps: Step a: 200g of the above-prepared by-product ammonium bisulfate mixed salt is melted at 130℃, then 44.5g of water is added dropwise to the molten by-product ammonium bisulfate mixed salt, and a methanol-water mixture is collected simultaneously during the dropwise addition. The dropwise addition is performed for 3h, and after the dropwise addition is completed, the temperature is maintained at 130℃ for 2h to obtain a brownish yellow ammonium bisulfate and ammonium sulfate mixed salt; Step b: 100g of water is added to the ammonium bisulfate and ammonium sulfate mixed salt obtained in step a, and the salt is dissolved by stirring to obtain a brownish yellow mixed salt solution; Step c: The above mixed salt solution is heated to 75℃, 0.32g of acidic wood charcoal is added, and stirring and heat preservation are performed for 4h. Filtration is performed to obtain a colorless and transparent decolorized mixed salt solution 307g and waste acidic wood charcoal 1.03g; Step d: The decolorized mixed salt solution is cooled to 20℃, and at this temperature, 24.96g of ammonia gas is introduced into the solution until the pH is 6-7. The gas introduction is performed for 2h, and filtration is performed to obtain ammonium sulfate wet product 129.3g (after drying, the nitrogen (N) content is 20.72%, and the sulfur (S) content is 24.10%), and yellowish ammonium sulfate mother liquor 198.42g, which is used for the next batch of dissolved and acid-decomposed ammonium bisulfate and ammonium sulfate mixed salt.

[0037] Example 2 The present example provides a treatment method for the by-product ammonium bisulfate mixed salt of methyl 2-hydroxyisobutyrate, which specifically comprises the following steps: Step a: 200g of the above-prepared by-product ammonium bisulfate mixed salt is melted at 130℃, then 44.5g of water is added dropwise to the molten by-product ammonium bisulfate mixed salt, and a methanol-water mixture is collected simultaneously during the dropwise addition. The dropwise addition is performed for 3h, and after the dropwise addition is completed, the temperature is maintained at 130℃ for 2h to obtain a brownish yellow ammonium bisulfate and ammonium sulfate mixed salt; Step b: 100g of water is added to the ammonium bisulfate and ammonium sulfate mixed salt obtained in step a, and the salt is dissolved by stirring to obtain a brownish yellow mixed salt solution; Step c: The above mixed salt solution is heated to 75℃, 0.32g of acidic wood charcoal is added, and stirring and heat preservation are performed for 4h. Filtration is performed to obtain a colorless and transparent decolorized mixed salt solution 307g and waste acidic wood charcoal 1.03g; Step d, the decolorized mixed salt solution was cooled to 50℃, and ammonia gas was introduced into the solution until the pH reached 6-7, which took 3h. The solution was filtered to obtain 153g of ammonium sulfate wet product (the nitrogen (N) content was 20.65% and the sulfur (S) content was 24.18% after drying), and 257g of yellowish ammonium sulfate mother liquor, which was used for dissolving the by-product ammonium bisulfate and ammonium sulfate mixed salt in the next batch.

[0038] Example 3 The present example provides a method for treating the by-product ammonium bisulfate and ammonium sulfate mixed salt of 2-hydroxyisobutyric acid methyl ester, which specifically comprises the following steps: Step a, 200g of the by-product ammonium bisulfate and ammonium sulfate mixed salt prepared above was melted at 140℃, and then 35.64g of water was added dropwise into the molten by-product ammonium bisulfate and ammonium sulfate mixed salt. The methanol-water mixture was collected during the dropwise addition, which took 1h. After the dropwise addition was completed, the solution was kept at 140℃ for 2h to obtain a brownish yellow ammonium bisulfate and ammonium sulfate mixed salt; Step b, 287g of the ammonium sulfate mother liquor obtained in step d of Example 2 was added into the ammonium bisulfate and ammonium sulfate mixed salt obtained in step a, and the salt was dissolved by stirring to obtain a brownish yellow mixed salt solution; Step c, the mixed salt solution was heated to 70℃, and 2.55g of acidic wood charcoal was added. The solution was stirred and kept at 70℃ for 2h, and then filtered to obtain 493g of a colorless and transparent decolorized mixed salt solution and 4.55g of waste acidic wood charcoal; Step d, the decolorized mixed salt solution was cooled to 30℃, and ammonia gas was introduced into the solution until the pH reached 6-7, which took 2h. The solution was filtered to obtain 222g of ammonium sulfate wet product (the nitrogen (N) content was 20.56% and the sulfur (S) content was 24.07% after drying), and 270g of yellowish ammonium sulfate mother liquor, which was used for dissolving the by-product ammonium bisulfate and ammonium sulfate mixed salt in the next batch.

[0039] Example 4 The present example provides a method for treating the by-product ammonium bisulfate and ammonium sulfate mixed salt of 2-hydroxyisobutyric acid methyl ester, which specifically comprises the following steps: Step a, 200g of the by-product ammonium bisulfate and ammonium sulfate mixed salt prepared above was melted at 130℃, and then 35.64g of water was added dropwise into the molten by-product ammonium bisulfate and ammonium sulfate mixed salt. The methanol-water mixture was collected during the dropwise addition, which took 3h. After the dropwise addition was completed, the solution was kept at 130℃ for 2h to obtain a brownish yellow ammonium bisulfate and ammonium sulfate mixed salt; Step b, 270g of the ammonium sulfate mother liquor obtained in step d of Example 3 was added into the ammonium bisulfate and ammonium sulfate mixed salt obtained in step a, and the salt was dissolved by stirring to obtain a brownish yellow mixed salt solution; Step c, the waste acidic wood charcoal obtained in step c of Examples 1-3 was calcined at 800℃ for 2h in an oxygen-free environment to obtain 5.32g of regenerated acidic wood charcoal; The mixed salt solution was heated to 72°C, 5 g of the regenerated acidic wood charcoal was added, stirred and kept for 4 h, and then filtered to obtain 477 g of colorless and transparent decolorized mixed salt solution and 7.02 g of waste acidic wood charcoal; Step d, the decolorized mixed salt solution was cooled to 40°C, and ammonia gas was bubbled into the solution until the pH reached 6-7, with a bubbling time of 3 h. The solution was filtered to obtain 213 g of ammonium sulfate wet product (the nitrogen (N) content was 20.55% and the sulfur (S) content was 24.03% after drying), and 286 g of yellowish ammonium sulfate mother liquor, which was used for dissolving the ammonium bisulfate and ammonium sulfate mixed salt in the next batch.

[0040] Example 5 The present example provides a method for treating the by-product ammonium bisulfate mixed salt of methyl 2-hydroxyisobutyrate, which specifically comprises the following steps: Step a, 200 g of the by-product ammonium bisulfate mixed salt prepared above was melted at 130°C, and then 44.55 g of water was added dropwise into the melted by-product ammonium bisulfate mixed salt. The methanol-water mixture was collected during the dropwise addition, and the dropwise addition was kept for 3 h. After the dropwise addition was completed, the mixture was kept at 130°C for 3 h to obtain brownish yellow ammonium bisulfate and ammonium sulfate mixed salt; Step b, 100 g of water was added into the ammonium bisulfate and ammonium sulfate mixed salt obtained in step a, and the salt was dissolved by stirring to obtain a brownish yellow mixed salt solution; Step c, the mixed salt solution was heated to 75°C, 0.32 g of acidic wood charcoal was added, stirred and kept for 4 h, and then filtered to obtain 303 g of colorless and transparent decolorized mixed salt solution and 1.56 g of waste acidic wood charcoal; Step d, the decolorized mixed salt solution was cooled to 20°C, and ammonia gas was bubbled into the solution until the pH reached 6-7, with a bubbling time of 2 h. The solution was concentrated under reduced pressure to remove water to obtain ammonium sulfate, with a nitrogen (N) content of 20.59% and a sulfur (S) content of 24.13%.

[0041] Comparative Example 1 The present comparative example provides a method for treating the by-product ammonium bisulfate mixed salt of methyl 2-hydroxyisobutyrate, which is different from Example 5 only in that the acidolysis temperature is reduced to 90°C, and the water used for acidolysis is added at one time. The method specifically comprises the following steps: Step a, 200 g of the by-product ammonium bisulfate mixed salt prepared above was melted at 90°C, and then 44.5 g of water was added into the melted by-product ammonium bisulfate mixed salt at one time, and kept for 3 h to obtain brownish yellow ammonium bisulfate and ammonium sulfate mixed salt; Step b, 100 g of water was added into the ammonium bisulfate and ammonium sulfate mixed salt obtained in step a, and the salt was dissolved by stirring to obtain a brownish yellow mixed salt solution; Step c, the mixed salt solution was heated to 75℃, 0.32g of acidic wood charcoal was added, and stirred for 4h. Filtration was performed to obtain a colorless and transparent decolorized mixed salt solution 321g, and 1.23g of waste acidic wood charcoal; Step d, the decolorized mixed salt solution was cooled to 20℃, and ammonia gas was introduced into the solution until the pH reached 6-7. The gas introduction time was 2h. The solution was concentrated under reduced pressure to remove water to obtain ammonium sulfate. The nitrogen (N) content was 17.47%, and the sulfur (S) content was 17.62%. The ammonium sulfate obtained in this example did not meet the standard requirements of fertilizer grade I type.

[0042] Example 2 This example provides a treatment method for the by-product ammonium bisulfate mixed salt of methyl 2-hydroxyisobutyrate. The method uses an alkaline neutralization and hydrolysis method, which specifically includes the following steps: 200g of the by-product ammonium bisulfate mixed salt prepared above was taken, 135g of water was added, and the salt was dissolved by stirring. 0.5% of acidic wood charcoal based on the weight of the material was added, and the solution was decolorized at 75℃ for 1h. Filtration was performed, and the filtrate was adjusted to a pH of 13-14 using 32% liquid alkali. The solution was heated to reflux and maintained for 5h. During this period, liquid alkali was continuously added to maintain the pH of the solution at 13-14. After the maintenance period, the pH of the solution was adjusted to 6-7 using sulfuric acid to obtain a sodium sulfate salt solution. The solution was concentrated under reduced pressure to remove water, and dried to obtain sodium sulfate. The sodium sulfate content was 98.53%, and the ammonium sulfate content was 0.52%.

[0043] Example 3 This example provides a treatment method for the by-product ammonium bisulfate mixed salt of methyl 2-hydroxyisobutyrate. The method uses an ammonia gas high-pressure alkaline hydrolysis method, which specifically includes the following steps: 200g of the by-product ammonium bisulfate mixed salt prepared above was taken, 135g of water was added, and the salt was dissolved by stirring. 0.5% of acidic wood charcoal based on the weight of the material was added, and the solution was decolorized at 75℃ for 1h. Filtration was performed, and the filtrate was adjusted to a pH of 13-14 using 32% liquid alkali. The solution was heated to reflux and maintained for 5h. During this period, liquid alkali was continuously added to maintain the pH of the solution at 13-14. After the maintenance period, the pH of the solution was adjusted to 6-7 using sulfuric acid to obtain a sodium sulfate salt solution. The solution was concentrated under reduced pressure to remove water, and dried to obtain sodium sulfate. The sodium sulfate content was 98.53%, and the ammonium sulfate content was 0.52%.

[0044] In summary, the application uses unreacted sulfuric acid in the mixed salt as an acidolysis reagent, heats the by-product ammonium bisulfate mixed salt to a molten state, adds water to hydrolyze the monomethyl sulfate while recovering methanol-water mixture (methanol can be reused in the main reaction) under normal pressure; by precisely controlling the amount of water, the mixed salt after hydrolysis is dissolved, decolorized by activated carbon, neutralized by ammonia gas and directly centrifuged to obtain ammonium sulfate meeting the GB / T 535-2020 Fertilizer Grade I Standard, and the ammonium sulfate mother liquor after centrifugation is recycled for the mixed salt dissolution process to recover residual ammonium sulfate. This method does not require additional introduction of strong alkali, does not require rectification, evaporation and concentration process, realizes efficient conversion of by-product monomethyl sulfate in the mixed salt and resource utilization of methanol and ammonium sulfate, the whole process does not produce liquid or solid hazardous waste, has the advantages of energy saving, environmental protection and economy, can promote the green upgrading of the α-hydroxycarboxylic acid ester industry prepared by the acetone cyanohydrin method, and has significant application value.

[0045] The above merely describes preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement or improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A process for the treatment of ammonium bisulphate mixed salt as a by-product of the preparation of α-hydroxy carboxylic acid esters by the acetone cyanohydrin method, characterised in that, The method comprises the following steps: Step a, heating and melting the by-product ammonium bisulfate mixed salt, adding water dropwise to the molten by-product ammonium bisulfate mixed salt for hydrolysis reaction, meanwhile, the methanol-water mixture is extracted under normal pressure, and after the dropwise addition is completed, the hydrolysis is kept until completion to obtain ammonium bisulfate and ammonium sulfate mixed salt; Step b, dissolving the ammonium bisulfate and ammonium sulfate mixed salt in water to obtain a mixed salt solution; Step c, adding activated carbon to the mixed salt solution for decolorization, and then performing solid-liquid separation to obtain a decolorized mixed salt solution; Step d, passing ammonia gas into the decolorized mixed salt solution, and then performing solid-liquid separation to obtain ammonium sulfate and ammonium sulfate mother liquor.

2. The process according to claim 1, characterized in that, in the acetone cyanohydrin process for the production of α-hydroxy carboxylic acid esters, the ammonium bisulphate by-product mixed salt is treated by, The α-hydroxy carboxylic acid ester is prepared by cyanohydroxylation and esterification of acetone cyanohydrin and sulfuric acid in an alcohol solvent.

3. The process according to claim 1, wherein the ammonium bisulfate mixed salt byproduct of the acetone cyanohydrin process for the production of α-hydroxy carboxylic acid esters is treated by, In step a, the temperature of the heating and melting is 120-140°C.

4. The process according to claim 1, wherein the ammonium bisulfate mixed salt byproduct of the acetone cyanohydrin process for the production of α-hydroxy carboxylic acid esters is treated by, In step a, the molar ratio of the amount of water to be added to the sulfuric acid monomethyl ester in the by-product ammonium bisulfate mixed salt is 2:1-5:1, and the temperature of the water to be added and the temperature of the hydrolysis after keeping warm are both 120-140°C.

5. The process according to claim 4, characterized in that, in the acetone cyanohydrin process for the production of α-hydroxy carboxylic acid esters, the ammonium bisulphate by-product mixed salt is treated by, In step a, the time for adding water is 1-3 hours, and the time for keeping warm after the addition is completed is 2-3 hours.

6. The process according to claim 1, wherein the ammonium bisulfate mixed salt byproduct of the acetone cyanohydrin process for the production of α-hydroxy carboxylic acid esters is treated by, In step b, the mass ratio of water to the by-product ammonium bisulfate mixed salt in step a is 0.5:1-1.5:

1.

7. The process according to claim 1, wherein the ammonium bisulfate mixed salt byproduct of the acetone cyanohydrin process for the production of α-hydroxy carboxylic acid esters is treated by, In step c, the activated carbon is acidic wood carbon, and the mass ratio of the activated carbon to the mixed salt solution is 0.001:1-0.01:1; and / or In step c, the temperature for decolorization is 70-80°C, and the decolorization time is 1-4 hours.

8. The process according to claim 1, wherein the ammonium bisulfate mixed salt byproduct of the acetone cyanohydrin process for the production of α-hydroxy carboxylic acid esters is treated by, In step d, the end point of the ammonia gas passing is that the pH of the material liquid is 6-7.

9. The process according to claim 8, characterized in that, in the acetone cyanohydrin process for the production of α-hydroxy carboxylic acid esters, the ammonium bisulphate by-product mixed salt is treated by, In step d, the time for passing ammonia gas is 2-3 hours, and the temperature for passing ammonia gas is 20-50°C.

10. The process according to claim 1, wherein the ammonium bisulfate mixed salt byproduct of the acetone cyanohydrin process for the production of α-hydroxy carboxylic acid esters is treated by, In step d, the ammonium sulfate mother liquor is also used for dissolving the ammonium bisulfate and ammonium sulfate mixed salt in step b.

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