A method for treating nitromethane tail gas and a method for producing nitromethane

Abstract

The present application provides a kind of nitromethane tail gas treatment method, comprising the following steps: sodium hydroxide or potassium hydroxide aqueous solution and low-grade fatty diol and organic base miscible to form mixed solution after preheating, then nitromethane tail gas is passed into mixed solution and reacts, reaction end product is separated and purified to obtain corresponding sodium nitrite or potassium nitrite product.The present application also provides a kind of nitromethane production method, including the sodium nitrite or hydrogen potassium obtained above as raw material to recycle production nitromethane.The method of the present application solves the problem of nitromethane tail gas treatment, also can turn waste into treasure, reduce production cost, conducive to nitromethane production and environmental protection.

Description

Technical Field

[0001] This invention relates to the field of chemical process technology, specifically to a method for treating nitromethane tail gas and a method for producing nitromethane. Background Technology

[0002] Nitromethane (CH3NO2) is an organic compound, a colorless, oily liquid, slightly soluble in water, and soluble in ethanol, ether, and dimethylformamide. It is used in organic synthesis to produce pesticides such as chloropicrin and nitro alcohols, as well as explosives, rocket fuel, pharmaceuticals, dyes, pesticides, and gasoline additives. It can also be used as an organic solvent. Domestically, nitromethane is mainly produced through a substitution reaction between dimethyl sulfate and sodium nitrite. Due to the similar nucleophilicity of the two oxygen atoms and one nitrogen atom on the nitro group, a large amount of the isomer methyl nitrite—a nitromethane—is inevitably generated during the process. Methyl nitrite is the main harmful component in nitromethane tail gas. Methyl nitrite is a toxic, flammable, and explosive gas; it is almost insoluble in water and easily decomposes into nitrogen oxides when heated or exposed to light. Due to the instability and low solubility of methyl nitrite in water, its industrial treatment has been a major challenge hindering the sustainable, stable, and healthy development of the nitromethane industry. Current technologies primarily employ alkaline aqueous solutions for nitromethane tail gas treatment. However, this method suffers from very low water solubility of methyl nitrite, resulting in a limited effective reaction rate and a small amount of methyl nitrite being absorbed and reacted. A significant portion of the methyl nitrite remains discharged with the tail gas, leading to the production of large amounts of nitrogen oxides from the decomposition of methyl nitrite, making it difficult to meet national standards for industrial nitrogen oxide emissions and thus detrimental to environmental protection. Therefore, properly treating nitromethane tail gas will be beneficial for the sustainable industrial production of nitromethane. Summary of the Invention

[0003] This invention provides a method for treating nitromethane tail gas, which solves the problem of purifying and absorbing nitromethane tail gas in the prior art, reduces the emission of nitrogen oxides in the tail gas, and is beneficial to environmental protection.

[0004] Specifically, the above-mentioned method for treating nitromethane tail gas includes the following steps:

[0005] Step 1: Mix sodium hydroxide or potassium hydroxide aqueous solution with lower aliphatic glycol and organic base to form a mixed solution, and then preheat it;

[0006] Step 2: Pass the nitromethane tail gas into the mixed solution described in Step 1 for reaction;

[0007] Step 3: Separate and purify the reaction products to obtain the corresponding sodium nitrite or potassium nitrite products.

[0008] Specifically, the aforementioned nitromethane tail gas can be the tail gas generated by reacting sodium nitrite or potassium nitrite with dimethyl sulfate to prepare nitromethane.

[0009] Specifically, the above-mentioned method for treating nitromethane tail gas involves reacting methyl nitrite, the main component of the nitromethane tail gas, with sodium hydroxide, thereby removing methyl nitrite, the primary source of excessive nitrogen oxide emissions in the nitromethane tail gas. The reaction between methyl nitrite and sodium hydroxide is shown in the following formula:

[0010]

[0011] Specifically, the organic base mentioned in step 1 is selected from triethylamine, 4-dimethylaminopyridine, or hexamethylenetetramine. The organic base acts as a catalyst in this reaction system, accelerating the treatment speed of nitromethane tail gas and shortening the time required for nitromethane tail gas to meet emission standards. The hydrolysis reaction of methyl nitrite with sodium hydroxide is similar to a nucleophilic substitution reaction. The methyl group in methyl nitrite combines with the hydroxide ion in sodium hydroxide to form methanol. However, the reaction rate of methyl nitrite with the alkaline solution is very low. Triethylamine, 4-dimethylaminopyridine, or hexamethylenetetramine are tertiary amines, lacking hydrogen atoms on nitrogen, making them less prone to other side reactions. They can also increase the nucleophilic reaction rate of hydroxide ions in the nucleophilic substitution reaction, thus accelerating the reaction rate of methyl nitrite with sodium hydroxide.

[0012] Specifically, the reaction temperature in step 2 is 75-90℃, such as 75℃, 80℃, 85℃, 90℃, etc. In this invention, nitromethane tail gas is directly passed into an aqueous solution of sodium hydroxide to react with lower aliphatic diols and organic bases to form a mixed solution. The reaction temperature should not be too high, as excessively high temperatures can cause methyl nitrite to decompose and produce toxic gases.

[0013] Specifically, the lower aliphatic diol mentioned in step 1 is selected from ethylene glycol, 1,2-propanediol, or 1,3-propanediol. This invention does not use methanol, ethanol, or diethyl ether, which are readily soluble in methyl nitrite, as solvents; instead, it chooses lower aliphatic diols. On the one hand, this invention reacts at a heating temperature of 75-90°C, at which temperature methanol, ethanol, and diethyl ether are gaseous and unsuitable as organic solvents for dissolving methyl nitrite. On the other hand, this invention produces methanol as a byproduct in the treatment of nitromethane tail gas. Lower aliphatic diols such as ethylene glycol, 1,2-propanediol, or 1,3-propanediol have significantly different boiling points from methanol and are higher than methanol, making it easier to remove and recover the methanol. Specifically, a water separator is used to remove the generated methanol. As methanol is removed from the reaction system, the reaction proceeds in the forward direction.

[0014] Specifically, in step 1, the amount of sodium hydroxide or potassium hydroxide aqueous solution, lower aliphatic diol, and organic base can be adjusted according to the amount of nitromethane tail gas being treated.

[0015] Specifically, in step 1, the mass concentration of the sodium hydroxide aqueous solution is 30%-50%, such as 30%, 40%, or 50%. A certain mass concentration of sodium hydroxide aqueous solution not only increases the solubility of sodium hydroxide in lower aliphatic diols, which is beneficial for the contact reaction between methyl nitrite and sodium hydroxide, but also allows it to be mixed with liquid triethylamine, or to dissolve solid 4-dimethylaminopyridine or hexamethylenetetramine.

[0016] Specifically, in step 1, the mass ratio of the aqueous solution of sodium hydroxide to the lower aliphatic diol and the organic base is 1:(0.5-1):(0.01-0.2).

[0017] The mixed solution described in step 1 can be single-stage or multi-stage. When the nitromethane tail gas emission does not meet the standards, the mixed solution is multi-stage, specifically two-stage, three-stage, four-stage, etc. The nitromethane tail gas is passed through the first stage reaction before being introduced into the next stage, until the nitromethane tail gas emission meets the standards. When the nitromethane tail gas emission meets the standards, the introduction into the next stage is stopped. This continuous treatment of nitromethane tail gas ensures that nitromethane production is not interrupted due to tail gas issues.

[0018] The emission standards for nitromethane tail gas can be referenced in GB16297-1996 Integrated Emission Standard for Air Pollutants, such as the nitrogen oxide emission concentration limit of 240 mg / m³. 3 .

[0019] Specifically, the flow rate of the nitromethane tail gas introduced in step 2 can be selected by those skilled in the art based on engineering adaptation.

[0020] Specifically, to accelerate the treatment of nitromethane tail gas and promote uniform reaction, stirring can be employed depending on the actual situation, such as magnetic stirring or mechanical stirring. For example, with magnetic stirring, the stirring speed can be controlled at 100-300 r / min. If the stirring speed is too low, it will not accelerate the reaction between the nitromethane tail gas and the mixed solution; if the stirring speed is too high, the nitromethane tail gas will overflow, which is detrimental to its treatment. Therefore, a suitable stirring speed ensures sufficient contact and reaction between the nitromethane tail gas and the mixed solution without overflowing.

[0021] Specifically, step 3 involves directly cooling the reaction solution or first evaporating and concentrating it before cooling it to 0-10°C (e.g., 0°C, 5°C, 8°C, 10°C, etc.), and then filtering it to obtain the corresponding product. Specifically, the filtrate obtained in step 3 can be used as a mother liquor and returned to step 1 to be mixed with sodium hydroxide or potassium hydroxide aqueous solution, lower aliphatic diols, and organic bases to form a mixed solution for reuse in nitromethane tail gas treatment. This mother liquor recycling significantly reduces the amount of mixed solution used and lowers tail gas treatment costs.

[0022] The present invention also provides a method for preparing nitromethane by reacting sodium nitrite or potassium hydroxide, the product obtained by the above-mentioned treatment method, with dimethyl sulfate as a raw material for the production of nitromethane by the nitrite replacement method.

[0023] Beneficial effects

[0024] 1. This invention solves the problem of purifying and absorbing nitromethane tail gas in the prior art, reduces the emission of nitrogen oxides in the tail gas, and the reaction is carried out at normal pressure without the need for additional pressurization, so the equipment requirements are not high. The products of the reaction, sodium nitrite or potassium hydroxide, can also be used as raw materials for the production of nitromethane, realizing the treatment of nitromethane tail gas and also allowing for the recycling of nitromethane, which is beneficial to environmental protection.

[0025] 2. This invention solves the problem of low efficiency and poor effect of traditional sodium hydroxide absorption of nitromethane tail gas. The nitromethane tail gas treatment method of this invention is simple in process, low in investment cost, and can continuously treat nitromethane tail gas, thus fully ensuring the continuous production of nitromethane.

[0026] 3. This invention realizes the production of nitromethane by recycling nitromethane tail gas, which saves the purchase cost of raw materials and reduces the emission of nitromethane tail gas, turning nitromethane tail gas into a valuable resource and realizing the resource reuse of tail gas. Detailed Implementation

[0027] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0028] In this invention, nitromethane tail gas refers to the tail gas generated in the process of producing nitromethane by the replacement method of dimethyl sulfate and nitrite (such as sodium nitrite or potassium hydroxide), and the tail gas contains methyl nitrite.

[0029] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0030] Example 1

[0031] (1) Prepare multiple four-necked flasks connected in series. Add 20g of 40% sodium hydroxide aqueous solution, 20g of ethylene glycol, and 2g of 4-dimethylaminopyridine to each four-necked flask. Equip the four-necked flasks with condensers, water separators, and thermometers. Stir the magnetically at 300r / min. Mix evenly, preheat to 90℃, keep warm, and set aside.

[0032] (2) The tail gas produced from the production of nitromethane via dimethyl sulfate and sodium nitrite is introduced into a first-stage four-necked flask at a flow rate of 5 L / min. The nitrogen oxide emissions of the treated tail gas are measured. If the nitrogen oxide emissions are higher than or equal to 240 mg / m³, the reaction is considered complete. 3 The gas is then passed into a series of four-necked flasks for the reaction. If the nitrogen oxide emissions are below 240 mg / m³, the reaction proceeds. 3 Then it is unnecessary to pass the reaction into the next stage;

[0033] (3) After a two-stage series reaction in a four-necked flask, the nitrogen oxide emissions are 240 mg / m³. 3 the following;

[0034] (4) After ventilating for 5 hours, collect all the reaction products from the four-necked flasks, cool them to 0°C, and then filter them. After drying the filter residue, 10.5g of sodium nitrite product was obtained. The sodium nitrite content was 99.3% of the product. The filtrate was returned to step (1) as mother liquor for reuse.

[0035] (5) Add 7g of sodium nitrite, 0.3g of sodium carbonate and 10mL of water recovered in step (4) to the reaction vessel. Add 6.3g of dimethyl sulfate dropwise within half an hour. Stir and heat, and control the temperature at about 60℃. After distilling for 2 hours, distill again to obtain 3.4g of nitromethane with a purity of 99.3%. The nitromethane tail gas can be returned to step (2) for recycling.

[0036] Example 2

[0037] (1) Prepare multiple four-necked flasks connected in series. Add 20g of sodium hydroxide aqueous solution with a mass concentration of 40%, 20g of ethylene glycol, and 2g of hexamethylenetetramine to each four-necked flask. Equip the four-necked flasks with condensers, water separators, and thermometers. Stir the magnetically at a speed of 200r / min. Mix evenly, preheat to 90℃, keep warm, and set aside.

[0038] (2) The nitromethane tail gas, as in Example 1, was introduced into the first-stage four-necked flask via a conduit at a flow rate of 5 L / min for reaction. The nitrogen oxide emissions of the treated tail gas were measured. If the nitrogen oxide emissions were higher than or equal to 240 mg / m³, the reaction was considered successful. 3 The gas is then introduced sequentially into a series of four-necked flasks for the reaction. If the nitrogen oxide emissions are below 240 mg / m³, the reaction proceeds. 3 Then it is unnecessary to pass the reaction into the next stage;

[0039] (3) After a two-stage series reaction in a four-necked flask, the nitrogen oxide emissions are 240 mg / m³. 3 the following;

[0040] (4) After ventilating for 6 hours, collect all the reaction products from the four-necked flasks, cool them to 0°C, and then filter them. After drying the filter residue, 10.2g of sodium nitrite product was obtained, and the sodium nitrite content was found to be 98.8%.

[0041] Example 3

[0042] (1) Prepare multiple four-necked flasks connected in series. Add 20g of sodium hydroxide aqueous solution with a mass concentration of 40%, 20g of ethylene glycol, and 2g of triethylamine to each four-necked flask. Equip the four-necked flasks with condensers, water separators, and thermometers. Stir the magnetically at a speed of 200r / min. Mix evenly, preheat to 90℃, keep warm, and set aside.

[0043] (2) The nitromethane tail gas, as in Example 1, was introduced into the first-stage four-necked flask via a conduit at a flow rate of 5 L / min for reaction. The nitrogen oxide emissions of the treated tail gas were measured. If the nitrogen oxide emissions were higher than or equal to 240 mg / m³, the reaction was considered successful. 3 The gas is then introduced sequentially into a series of four-necked flasks for the reaction. If the nitrogen oxide emissions are below 240 mg / m³, the reaction proceeds. 3 Then it is unnecessary to pass the reaction into the next stage;

[0044] (3) After a two-stage series reaction in a four-necked flask, the nitrogen oxide emissions are 240 mg / m³. 3 the following;

[0045] (4) After ventilating for 7 hours, collect all the reaction products from the four-necked flasks, cool them to 0°C, and then filter them. After drying the filter residue, 10.1g of sodium nitrite product was obtained. The sodium nitrite content was found to be 98.9%.

[0046] Example 4

[0047] (1) Prepare multiple four-necked flasks connected in series. Add 20g of 30% sodium hydroxide aqueous solution, 10g of 1,2-propanediol, and 0.2g of 4-dimethylaminopyridine to each four-necked flask. Equip the four-necked flasks with condensers, water separators, and thermometers. Stir the flasks magnetically at 100r / min. Mix thoroughly, preheat to 80℃, keep warm, and set aside.

[0048] (2) The nitromethane tail gas, as in Example 1, was introduced into the first-stage four-necked flask via a conduit at a flow rate of 5 L / min for reaction. The nitrogen oxide emissions of the treated tail gas were measured. If the nitrogen oxide emissions were higher than or equal to 240 mg / m³, the reaction was considered successful. 3 The gas is then introduced sequentially into a series of four-necked flasks for the reaction. If the nitrogen oxide emissions are below 240 mg / m³, the reaction proceeds. 3 Then it is unnecessary to pass the reaction into the next stage;

[0049] (3) After the three-stage series reaction in a four-necked flask, the nitrogen oxide emissions were 240 mg / m³.3 the following;

[0050] (4) After ventilating for 6 hours, collect all the reaction products from the four-necked flasks, cool them to 5°C, and then filter them. After drying the filter residue, 9.8g of sodium nitrite product was obtained, and the sodium nitrite content was found to be 99.1%.

[0051] Example 5

[0052] (1) Prepare multiple four-necked flasks connected in series. Add 20g of 50% sodium hydroxide aqueous solution, 15g of 1,3-propanediol, and 4g of 4-dimethylaminopyridine to each four-necked flask. Equip the four-necked flasks with condensers, water separators, and thermometers. Stir the flasks magnetically at 100r / min. Mix thoroughly, preheat to 75℃, keep warm, and set aside.

[0053] (2) The nitromethane tail gas, as in Example 1, was introduced into the first-stage four-necked flask via a conduit at a flow rate of 5 L / min for reaction. The nitrogen oxide emissions of the treated tail gas were measured. If the nitrogen oxide emissions were higher than or equal to 240 mg / m³, the reaction was considered successful. 3 The gas is then introduced sequentially into a series of four-necked flasks for the reaction. If the nitrogen oxide emissions are below 240 mg / m³, the reaction proceeds. 3 Then it is unnecessary to pass the reaction into the next stage;

[0054] (3) After the three-stage series reaction in a four-necked flask, the nitrogen oxide emissions were 240 mg / m³. 3 the following;

[0055] (4) After ventilating for 8 hours, collect all the reaction products from the four-necked flasks, cool them to 10°C, and then filter them. After drying the filter residue, 9.3g of sodium nitrite product was obtained, and the sodium nitrite content was found to be 98.6%.

[0056] Example 6

[0057] (1) Prepare multiple four-necked flasks connected in series. Add 28g of 40% potassium hydroxide aqueous solution, 16g of ethylene glycol, and 1g of 4-dimethylaminopyridine to each four-necked flask. Equip the four-necked flasks with condensers, water separators, and thermometers. Stir the magnetically at 300r / min until the mixture is homogeneous. Preheat the mixture to 90℃ and keep it warm for later use.

[0058] (2) The tail gas generated from the production of nitromethane via dimethyl sulfate and potassium nitrite is introduced into a first-stage four-necked flask at a flow rate of 5 L / min. The nitrogen oxide emissions of the treated tail gas are measured. If the nitrogen oxide emissions are higher than or equal to 240 mg / m³, the reaction is considered complete. 3 The gas is then introduced sequentially into a series of four-necked flasks for the reaction. If the nitrogen oxide emissions are below 240 mg / m³, the reaction proceeds. 3 Then it is unnecessary to pass the reaction into the next stage;

[0059] (3) After a two-stage series reaction in a four-necked flask, the nitrogen oxide emissions are 240 mg / m³. 3 the following;

[0060] (4) After ventilating for 5 hours, the reaction products of all four-necked flasks were collected, evaporated and concentrated at 80°C, crystallized at 0°C, filtered and dried to obtain 13.3g of potassium nitrite product, which was found to contain 98.6% potassium nitrite.

[0061] Comparative Example 1

[0062] The nitromethane tail gas was treated in the same manner as in Example 1, passing through a conduit at a flow rate of 5 L / min sequentially through a three-stage sodium hydroxide aqueous solution with a pH of 11 and a three-stage sodium hydroxide aqueous solution with a mass concentration of 40% and a pH of 12. The nitrogen oxide concentration was still found to be greater than 240 mg / m³. 3 .

[0063] Comparative Example 2

[0064] (1) Prepare multiple four-necked flasks connected in series. Add 20g of sodium hydroxide aqueous solution with a mass concentration of 40% and 20g of ethylene glycol to each four-necked flask. Equip the four-necked flasks with condensers, water separators and thermometers. Stir the magnetically at a speed of 300r / min. Mix evenly, preheat to 90℃, keep warm and set aside.

[0065] (2) The nitromethane tail gas, as in Example 1, was introduced into the first-stage four-necked flask via a conduit at a flow rate of 5 L / min for reaction. The nitrogen oxide emissions of the treated tail gas were measured. If the nitrogen oxide emissions were higher than or equal to 240 mg / m³, the reaction was considered successful. 3 The gas is then introduced sequentially into a series of four-necked flasks for the reaction. If the nitrogen oxide emissions are below 240 mg / m³, the reaction proceeds. 3 Then it is unnecessary to pass the reaction into the next stage;

[0066] (3) After a six-stage series reaction in four-necked flasks, the nitrogen oxide emissions are 240 mg / m³. 3 the following;

[0067] (4) After ventilating for 7 hours, collect all the reaction products from the four-necked flasks, cool them to 0°C, and then filter them. After drying the filter residue, 8.5g of sodium nitrite product was obtained. The sodium nitrite content was found to be 96.7%.

[0068] Comparative Example 3

[0069] (1) Prepare multiple four-necked flasks connected in series. Add 20g of 40% sodium hydroxide aqueous solution and 2g of 4-dimethylaminopyridine to each four-necked flask. Equip the four-necked flasks with condensers, water separators and thermometers. Stir the magnetically at 300r / min. Mix evenly, preheat to 90℃, keep warm and set aside.

[0070] (2) The tail gas produced by the production of nitromethane from dimethyl sulfate and sodium nitrite was introduced into a series of four-necked flasks at a flow rate of 5 L / min via a conduit, and the nitrogen oxide emissions of the treated tail gas were measured.

[0071] (3) After a six-stage series reaction in a four-necked flask, the nitrogen oxide emissions are still greater than 240 mg / m³. 3 .

[0072] Comparative Example 1 illustrates an existing multi-stage alkaline treatment method for nitromethane tail gas. Despite multi-stage alkaline treatment, the nitrogen oxide concentration remained high. Comparative Example 2, without the presence of organic bases, while able to treat the tail gas, required more stages of treatment, resulting in a longer time to meet emission standards. This demonstrates that the presence of a certain amount of organic base significantly shortens the reaction time, indicating a significant catalytic effect of organic bases in this reaction system. Commonly used solvents such as methanol and ethanol are gaseous at the reaction temperature of 75-90℃ in this invention, making them unsuitable as liquid-phase reaction solvents. Comparative Example 3, without the addition of lower aliphatic diols, still exhibited a high nitrogen oxide concentration after multi-stage treatment. As can be seen from Examples 1-5, nitromethane tail gas is continuously produced during nitromethane production. Under heating conditions, it undergoes a purification reaction with a mixed solution formed by sodium hydroxide aqueous solution, lower aliphatic diols, and organic bases. The main reaction involves the reaction of methyl nitrite in the nitromethane tail gas with sodium hydroxide to remove methyl nitrite. The organic base acts as a catalyst, significantly reducing nitrogen oxide emissions. The method of this invention for treating nitromethane tail gas produces sodium nitrite which can be recycled to produce nitromethane, effectively reducing nitrogen oxide emissions, meeting environmental emission standards, and realizing the resource utilization of nitromethane tail gas.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0074] The foregoing embodiments and methods described in this invention may vary based on the capabilities, experience, and preferences of those skilled in the art.

Claims

1. A method for treating nitromethane tail gas, characterized in that, Includes the following steps: Step 1: Mix sodium hydroxide or potassium hydroxide aqueous solution with lower aliphatic glycol and organic base to form a mixed solution, and then preheat it; Step 2: Pass the nitromethane tail gas into the mixed solution described in Step 1 for reaction; Step 3: After the reaction is complete, separate and purify the reaction products to obtain the corresponding sodium nitrite or potassium nitrite products; The organic base mentioned in step 1 is selected from triethylamine, 4-dimethylaminopyridine, or hexamethylenetetramine; The reaction temperature in step 2 is 75-90°C.

2. The method according to claim 1, characterized in that, The lower aliphatic diol mentioned in step 1 is selected from ethylene glycol, 1,2-propanediol, or 1,3-propanediol.

3. The method according to claim 1, characterized in that, In step 1, the mass concentration of the sodium hydroxide aqueous solution is 30%-50%.

4. The method according to claim 1, characterized in that, In step 1, the mass ratio of the aqueous solution of sodium hydroxide to the lower aliphatic diol and the organic base is 1:(0.5-1):(0.01-0.2).

5. The method according to claim 1, characterized in that, The mixed solution described in step 1 may be single-stage or multi-stage; When the nitromethane tail gas emission does not meet the standard, the mixed solution is multi-stage. The nitromethane tail gas is passed through the first stage reaction and then passed into the next stage until the nitromethane tail gas emission meets the standard. When the nitromethane tail gas emissions meet the standards, stop feeding it into the next stage.

6. The method according to claim 1, characterized in that, Step 2 also includes the step of removing methanol by evaporation.

7. The method according to claim 1, characterized in that, Step 3 involves directly cooling the reaction solution or first evaporating and concentrating it before cooling it to 0-10°C, and then filtering it to obtain the corresponding product.

8. The method according to claim 7, characterized in that, Step 3 also includes the process of returning the filtrate obtained from the filtration to step 1.

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