Methyl isocyanate preparation method based on dimethyl sulfate route

Abstract

The invention relates to the technical field of methyl isocyanate, in particular to a methyl isocyanate preparation method based on a dimethyl sulfate route, which comprises the following steps: raw material preparation: raw materials comprise sodium cyanate, orthodichlorobenzene and dimethyl sulfate, the mass ratio of sodium cyanate to dimethyl sulfate is 1: (0.6), and the mass of orthodichlorobenzene is 3-5 times that of sodium cyanate; the method comprises the following steps: raw material pretreatment: respectively pretreating sodium cyanate, o-dichlorobenzene and dimethyl sulfate for later use; according to the method, uniform slurry is formed through grinding, the specific surface area of sodium cyanate particles is increased, full contact between the sodium cyanate particles and dimethyl sulfate is promoted, the effect of improving the reaction efficiency is achieved, after heating and negative-pressure dehydration treatment, hydrolysis of dimethyl sulfate and hydrolysis side reaction of methyl isocyanate can be prevented through reduction of moisture, and therefore generation of impurities is reduced, and the product quality is improved. The product purity is improved, the product is prevented from being decomposed or subjected to secondary reaction at high temperature due to lower rectification temperature, and the product stability is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of methyl isocyanate, and specifically provides a method for preparing methyl isocyanate based on the dimethyl sulfate route. Background Art

[0002] Methyl isocyanate is an important organic synthesis intermediate. It is a colorless transparent liquid at room temperature, with a pungent odor, highly toxic and flammable. It is easily soluble in organic solvents and has active chemical properties. It is a key raw material for the production of polyurethane foam and pesticides.

[0003] In the prior art, methyl isocyanate is usually prepared by the phosgene method. The phosgene method relies on highly toxic phosgene. The reaction easily generates by-products such as hydrogen chloride, which is difficult to separate from the product, and it is easy to leave residual chloride ions, affecting the product purity. Based on this, the present invention provides a method for preparing methyl isocyanate based on the dimethyl sulfate route. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing methyl isocyanate based on the dimethyl sulfate route. The methyl isocyanate prepared by the present invention not only has good purity performance but also has excellent yield performance.

[0005] To achieve the above purpose, the present invention provides the following technical solution: A method for preparing methyl isocyanate based on the dimethyl sulfate route, comprising the following steps: S1: Raw material preparation. The raw materials include sodium cyanate, o-dichlorobenzene, and dimethyl sulfate. The mass ratio of sodium cyanate to dimethyl sulfate is 1:(0.6), and the mass of o-dichlorobenzene is 3 to 5 times the mass of sodium cyanate; S2: Raw material pretreatment. Sodium cyanate, o-dichlorobenzene, and dimethyl sulfate are respectively pretreated for later use; S3: Reaction preparation. The pretreated sodium cyanate, o-dichlorobenzene, and dimethyl sulfate are input into a reaction kettle for mixing treatment to obtain methyl isocyanate.

[0006] Further, the pretreatment method of sodium cyanate is as follows: Sodium cyanate is added into an oven, the oven temperature is set at 40 - 44°C, and it is dried for 20 - 30 minutes. Then, the sodium cyanate is taken out and cooled to room temperature. The above drying step is carried out again, and the drying and cooling treatments are repeated 4 - 6 times. The obtained product and the soaking solution are added into a water bath, the water bath is set to heat up to 50 - 60°C, and a magnetic stirrer is connected at the same time. The magnetic stirrer is set at a rotation speed of 120 - 160 r / min, and the constant temperature stirring treatment is carried out for 10 - 20 minutes. The product obtained by constant temperature stirring is added into a centrifuge, and the centrifuge is set to carry out centrifugation at 3000 - 5000 r / min for 6 - 10 minutes. The precipitate is taken, and the precipitate is added into an oven, and the oven is set to dry at 60 - 70°C for 40 - 50 minutes. The dried product is ground, and the grinding particle size is 2 - 6 μm, thus completing the pretreatment of sodium cyanate.

[0007] Further, the mass of the soaking solution is 40 - 60% of the mass of sodium cyanate.

[0008] Further, the soaking solution is prepared by mixing disodium ethylenediaminetetraacetate and deionized water, and the mass ratio of disodium ethylenediaminetetraacetate to deionized water is 1:(6 - 8).

[0009] Further, the pretreatment method of o-dichlorobenzene is as follows: O-dichlorobenzene is added into a mixer, the mixer is set at a rotation speed of 30 - 40 r / min, an ultraviolet lamp is set above the mixer, the wavelength of the ultraviolet lamp is 200 - 300 nm, and the distance from the liquid level is 20 - 40 cm. The stirring and irradiation treatment is carried out for 20 - 30 minutes, thus completing the pretreatment of o-dichlorobenzene.

[0010] Further, the pretreatment method of dimethyl sulfate is as follows: Dimethyl sulfate and a treating agent are added into a water bath, the water bath is set at a temperature of 46 - 48°C, and a magnetic stirrer is connected at the same time. The magnetic stirrer is set at a rotation speed of 60 - 80 r / min, and the constant temperature stirring treatment is carried out for 12 - 16 minutes. The obtained product is added into a centrifuge, and the centrifuge is set to carry out centrifugation at 3000 - 5000 r / min for 8 - 12 minutes. The precipitate is taken, and the precipitate is added into an oven, and the oven is set to dry at 52 - 54°C for 30 - 40 minutes. The dried product is ground, and the grinding particle size is 4 - 10 μm, thus completing the pretreatment of methyl isocyanate.

[0011] Further, the mass of the treating agent is 4 - 6% of the mass of dimethyl sulfate.

[0012] Further, the treating agent is prepared by the following method: Triethylamine and phosphoric acid are added into a reaction kettle, the set temperature of the reaction kettle is 0-2°C, and it is subjected to constant-temperature stirring for 10-20 minutes at a rotation speed of 100-120 r / min. Then, triphenyl phosphite and 2,6-di-tert-butyl-p-cresol are added, and it is continuously subjected to constant-temperature stirring for 6-12 minutes to obtain the treating agent. Among them, the mass ratio of triethylamine to phosphoric acid is 1:(0.1-0.3), the mass of triphenyl phosphite is 6-10% of the mass of triethylamine, and the mass of 2,6-di-tert-butyl-p-cresol is 4-8% of the mass of triethylamine.

[0013] Further, the method for the reaction preparation is as follows: The sodium cyanate and ortho-dichlorobenzene that have completed pretreatment are subjected to mixed grinding treatment, and then subjected to heating and negative-pressure dehydration treatment, and then sent into a reaction kettle. The dimethyl sulfate that has completed pretreatment is sent into the reaction kettle. The set temperature of the reaction kettle is 172-180°C, and the reaction is carried out under normal pressure for 4-10 hours. The obtained product is subjected to rectification treatment to obtain methyl isocyanate.

[0014] Further, the temperature of the rectification treatment is 64-66°C.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In the preparation process of methyl isocyanate in the present invention, by grinding to form a uniform slurry, the specific surface area of sodium cyanate particles is increased, promoting its full contact with dimethyl sulfate, playing a role in improving the reaction efficiency. After heating and negative-pressure dehydration treatment, the reduction of water can prevent the hydrolysis of dimethyl sulfate and the hydrolysis side reaction of methyl isocyanate, thereby reducing the generation of impurities and improving the product purity. The lower rectification temperature avoids the decomposition of the product at high temperature or the occurrence of secondary reactions, ensuring the stability of the product.

[0016] 2. In the present invention, through the pretreatment of each raw material in the preparation process of methyl isocyanate, after sodium cyanate is subjected to multiple low-temperature drying and then treated with disodium ethylenediaminetetraacetate, metal ion impurities can be chelated and removed. After ortho-dichlorobenzene is irradiated with ultraviolet light, peroxide impurities can be decomposed. Dimethyl sulfate uses triethylamine and phosphoric acid to neutralize acidic impurities. Triphenyl phosphite and 2,6-di-tert-butyl can inhibit oxidation and decomposition, balancing the dispersibility and reaction activity, achieving the purpose of improving the preparation purity and yield. Specific Embodiments

[0017] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work shall fall within the protection scope of the present invention.

[0018] It should be noted that all raw materials used in the following examples are commercially available raw materials.

[0019] Example 1: A method for preparing methyl isocyanate based on the dimethyl sulfate route, comprising the following steps: S1: Raw material preparation. The raw materials include sodium cyanate, o-dichlorobenzene, and dimethyl sulfate. The mass ratio of sodium cyanate to dimethyl sulfate is 1:0.6, and the mass of o-dichlorobenzene is 3 times the mass of sodium cyanate. S2: Raw material pretreatment. Sodium cyanate, o-dichlorobenzene, and dimethyl sulfate are respectively pretreated for later use. S3: Reaction preparation. The pretreated sodium cyanate, o-dichlorobenzene, and dimethyl sulfate are input into a reaction kettle for mixing treatment to obtain methyl isocyanate.

[0020] The pretreatment method of sodium cyanate is as follows: Sodium cyanate is added into an oven, the oven temperature is set at 40 °C, and it is dried for 20 min. Then, the sodium cyanate is taken out and cooled to room temperature, and the above drying step is carried out again. The drying and cooling treatments are repeated 4 times. The obtained product and the soaking solution are added into a water bath kettle, the water bath kettle is set to heat up to 50 °C, and at the same time, a magnetic stirrer is connected. The magnetic stirrer is set at a rotation speed of 120 r / min, and the constant temperature stirring treatment is carried out for 10 min. The product obtained by constant temperature stirring is added into a centrifuge, the centrifuge is set at 3000 r / min for centrifugation for 6 min, and the precipitate is taken. The precipitate is added into an oven, the oven is set at 60 °C for drying for 40 min, and the dried product is ground. The grinding particle size is 2 μm, and the pretreatment of sodium cyanate is completed.

[0021] The mass of the soaking solution is 40% of the mass of sodium cyanate.

[0022] The soaking solution is prepared by mixing disodium ethylenediaminetetraacetate and deionized water, and the mass ratio of disodium ethylenediaminetetraacetate to deionized water is 1:6.

[0023] The pretreatment method of o-dichlorobenzene is as follows: o-dichlorobenzene is added into a mixer, the mixer is set at a rotation speed of 30 r / min, an ultraviolet lamp is set above the mixer, the wavelength of the ultraviolet lamp is 200 nm, and the distance from the liquid level is 20 cm. The stirring and irradiation treatment is carried out for 20 min, and the pretreatment of o-dichlorobenzene is completed.

[0024] The pretreatment method of dimethyl sulfate is as follows: Dimethyl sulfate and a treating agent are added into a water bath kettle, the water bath kettle is set at a temperature of 46 °C, and at the same time, a magnetic stirrer is connected. The magnetic stirrer is set at a rotation speed of 60 r / min, and the constant temperature stirring treatment is carried out for 12 min. The obtained product is added into a centrifuge, the centrifuge is set at 3000 r / min for centrifugation for 8 min, and the precipitate is taken. The precipitate is added into an oven, the oven is set at 52 °C for drying for 30 min, and the dried product is ground. The grinding particle size is 4 μm, and the pretreatment of methyl isocyanate is completed.

[0025] The mass of the treating agent is 4% of the mass of dimethyl sulfate.

[0026] The treating agent is prepared by the following method: Triethylamine and phosphoric acid are added into a reaction kettle, the set temperature of the reaction kettle is 0 °C, and it is stirred at a constant temperature for 10 min with a rotation speed of 100 r / min. Then triphenyl phosphite and 2,6-di-tert-butyl-p-cresol are added, and it is continuously stirred at a constant temperature for 6 min to obtain the treating agent. Among them, the mass ratio of triethylamine to phosphoric acid is 1:0.1, the mass of triphenyl phosphite is 6% of the mass of triethylamine, and the mass of 2,6-di-tert-butyl-p-cresol is 4% of the mass of triethylamine.

[0027] The reaction preparation method is as follows: Sodium cyanate and o-dichlorobenzene after pretreatment are mixed and ground, then heated under negative pressure for dehydration, and sent into a reaction kettle. Dimethyl sulfate after pretreatment is sent into the reaction kettle. The set temperature of the reaction kettle is 172 °C, and the reaction is carried out at normal pressure for 4 h. The obtained product is subjected to rectification treatment to obtain methyl isocyanate.

[0028] The temperature of the rectification treatment is 64 °C.

[0029] Example 2: A method for preparing methyl isocyanate based on the dimethyl sulfate route, including the following steps: S1: Raw material preparation. The raw materials include sodium cyanate, o-dichlorobenzene and dimethyl sulfate. The mass ratio of sodium cyanate to dimethyl sulfate is 1:0.6, and the mass of o-dichlorobenzene is 4 times the mass of sodium cyanate. S2: Raw material pretreatment. Sodium cyanate, o-dichlorobenzene and dimethyl sulfate are respectively pretreated for later use. S3: Reaction preparation. Sodium cyanate, o-dichlorobenzene and dimethyl sulfate after pretreatment are input into a reaction kettle for mixing treatment to obtain methyl isocyanate.

[0030] The pretreatment method of sodium cyanate is as follows: Sodium cyanate is added into an oven, the oven temperature is set at 42 °C, and it is dried for 25 min. Then sodium cyanate is taken out and cooled to room temperature, and the above drying step is carried out again. The drying and cooling treatments are repeated 5 times. The obtained product and the soaking solution are added into a water bath kettle. The water bath kettle is set to heat up to 55 °C, and at the same time, a magnetic stirrer is connected. The magnetic stirrer is set at a rotation speed of 140 r / min and stirred at a constant temperature for 15 min. The product obtained by stirring at a constant temperature is added into a centrifuge. The centrifuge is set at 4000 r / min for centrifugation for 8 min. The precipitate is taken, and the precipitate is added into an oven. The oven is set at 65 °C for drying for 450 min. The dried product is ground, and the grinding particle size is 4 μm, thus completing the pretreatment of sodium cyanate.

[0031] The mass of the soaking solution is 50% of the mass of sodium cyanate.

[0032] The soaking solution is prepared by mixing disodium ethylenediaminetetraacetate and deionized water, and the mass ratio of disodium ethylenediaminetetraacetate to deionized water is 1:7.

[0033] The pretreatment method of o-dichlorobenzene is as follows: o-dichlorobenzene is added to a mixer, the mixer speed is set to 35 r / min, an ultraviolet lamp with a wavelength of 250 nm is set above the mixer, the distance from the liquid level is 30 cm, and the mixing and irradiation treatment is carried out for 25 minutes to complete the pretreatment of o-dichlorobenzene.

[0034] The pretreatment method of dimethyl sulfate is as follows: dimethyl sulfate and a treating agent are added to a water bath, the temperature of the water bath is set to 47°C, and a magnetic stirrer is connected at the same time. The speed of the magnetic stirrer is set to 70 r / min, and constant temperature stirring is performed for 14 minutes. The resulting product is added to a centrifuge, and the centrifuge is set to 4000 r / min for centrifugation for 10 minutes. The precipitate is taken and added to an oven, and the oven is set to 53°C for drying for 35 minutes. The dried product is ground and the ground particle size is 7 μm, thereby completing the pretreatment of methyl isocyanate.

[0035] The mass of the treating agent is 5% of the mass of dimethyl sulfate.

[0036] The treating agent is prepared by the following method: triethylamine and phosphoric acid are added to a reactor, the reactor temperature is set at 1°C, constant temperature stirring is performed for 15 minutes, and the rotation speed is 110 r / min. Then, triphenyl phosphite and 2,6-di-tert-butyl-p-cresol are added, and constant temperature stirring is continued for 9 minutes to prepare the treating agent, wherein the mass ratio of triethylamine to phosphoric acid is 1:0.2, the mass of triphenyl phosphite is 8% of the mass of triethylamine, and the mass of 2,6-di-tert-butyl-p-cresol is 6% of the mass of triethylamine.

[0037] The reaction preparation method is as follows: the pretreated sodium cyanate and o-dichlorobenzene are mixed and refined, and then heated and dehydrated under negative pressure, and then fed into a reactor. The pretreated dimethyl sulfate is fed into the reactor, the reactor temperature is set at 176°C, the reaction is carried out under normal pressure for 7 hours, and the obtained product is distilled to obtain methyl isocyanate.

[0038] The temperature of the distillation treatment was 65°C.

[0039] Example 3: A method for preparing methyl isocyanate based on a dimethyl sulfate route, comprising the following steps: S1: Raw material preparation, including sodium cyanate, o-dichlorobenzene and dimethyl sulfate, the mass ratio of sodium cyanate to dimethyl sulfate is 1:0.6, and the mass of o-dichlorobenzene is 5 times that of sodium cyanate; S2: Raw material pretreatment: sodium cyanate, o-dichlorobenzene and dimethyl sulfate are pretreated separately and set aside; S3: Reaction preparation. The pre-treated sodium cyanate, o-dichlorobenzene, and dimethyl sulfate are input into a reaction kettle for mixing to produce methyl isocyanate.

[0040] The pre-treatment method for sodium cyanate is as follows: Sodium cyanate is added to an oven, the oven temperature is set at 44 °C, and it is dried for 30 min. Then, the sodium cyanate is taken out and waited to cool to room temperature. The above drying step is carried out again, and the drying and cooling treatments are repeated 6 times. The obtained product and the soaking solution are added to a water bath kettle, and the water bath kettle is set to heat up to 60 °C. At the same time, a magnetic stirrer is connected, and the magnetic stirrer is set at a rotation speed of 160 r / min for constant temperature stirring for 20 min. The product obtained by constant temperature stirring is added to a centrifuge, and the centrifuge is set at 5000 r / min for centrifugation for 10 min. The precipitate is taken, and the precipitate is added to an oven, and the oven is set at 70 °C for drying for 50 min. The dried product is ground, and the grinding particle size is 6 μm to complete the pre-treatment of sodium cyanate.

[0041] The mass of the soaking solution is 60% of the mass of sodium cyanate.

[0042] The soaking solution is prepared by mixing disodium ethylenediaminetetraacetate and deionized water, and the mass ratio of disodium ethylenediaminetetraacetate to deionized water is 1:8.

[0043] The pre-treatment method for o-dichlorobenzene is as follows: o-Dichlorobenzene is added to a mixer, the mixer is set at a rotation speed of 40 r / min, an ultraviolet lamp is set above the mixer, the wavelength of the ultraviolet lamp is 300 nm, and the distance from the liquid level is 40 cm, and it is stirred and irradiated for 30 min to complete the pre-treatment of o-dichlorobenzene.

[0044] The pre-treatment method for dimethyl sulfate is as follows: Dimethyl sulfate and a treating agent are added to a water bath kettle, the water bath kettle is set at a temperature of 48 °C, and at the same time, a magnetic stirrer is connected, and the magnetic stirrer is set at a rotation speed of 80 r / min for constant temperature stirring for 16 min. The obtained product is added to a centrifuge, and the centrifuge is set at 5000 r / min for centrifugation for 12 min. The precipitate is taken, and the precipitate is added to an oven, and the oven is set at 54 °C for drying for 40 min. The dried product is ground, and the grinding particle size is 10 μm to complete the pre-treatment of methyl isocyanate.

[0045] The mass of the treating agent is 6% of the mass of dimethyl sulfate.

[0046] The treating agent is prepared by the following method: Triethylamine and phosphoric acid are added into a reaction kettle. The temperature of the reaction kettle is set at 2°C, and it is subjected to constant-temperature stirring for 20 min at a rotation speed of 120 r / min. Then, triphenyl phosphite and 2,6-di-tert-butyl-p-cresol are added, and it is continuously subjected to constant-temperature stirring for 12 min to obtain the treating agent. Among them, the mass ratio of triethylamine to phosphoric acid is 1:0.3, the mass of triphenyl phosphite is 10% of the mass of triethylamine, and the mass of 2,6-di-tert-butyl-p-cresol is 8% of the mass of triethylamine.

[0047] The reaction preparation method is as follows: Sodium cyanate and o-dichlorobenzene that have completed pretreatment are mixed and ground into a slurry, and then subjected to heat and negative pressure dehydration treatment, and sent into a reaction kettle. Dimethyl sulfate that has completed pretreatment is sent into the reaction kettle. The temperature of the reaction kettle is set at 180°C, and the reaction is carried out under normal pressure for 10 h. The obtained product is subjected to rectification treatment to obtain methyl isocyanate.

[0048] The temperature of the rectification treatment is 66°C.

[0049] Comparative Example 1. The difference between this comparative example and Example 1 is that: In this comparative example, sodium cyanate was not pretreated.

[0050] Comparative Example 2. The difference between this comparative example and Example 1 is that: In this comparative example, dimethyl sulfate was not pretreated.

[0051] Comparative Example 3. The difference between this comparative example and Example 1 is that: In this comparative example, the treating agent was not added during the pretreatment of dimethyl sulfate.

[0052] Comparative Example 4. The difference between this comparative example and Example 1 is that: In this comparative example, none of the raw materials were pretreated.

[0053] Performance test: Methyl isocyanate prepared in Example 1, Example 2, Example 3, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 was subjected to performance testing, and the obtained test data was recorded in the following table:

[0054] It can be seen that the purity performance and yield performance of methyl isocyanate prepared in Comparative Examples 1, 2, 3, and 4 are lower than those in Examples 1, 2, and 3; this shows that: During the preparation of methyl isocyanate, by grinding to form a uniform slurry, the specific surface area of sodium cyanate particles is increased, promoting its full contact with dimethyl sulfate, playing a role in improving the reaction efficiency. After heat and negative pressure dehydration treatment, the reduction of water can prevent the hydrolysis of dimethyl sulfate and the hydrolysis side reaction of methyl isocyanate, thereby reducing the generation of impurities and improving the product purity. The lower rectification temperature avoids the decomposition of the product at high temperature or the occurrence of secondary reactions, ensuring the product stability; By pretreating each raw material in the preparation process of methyl isocyanate, sodium cyanate is dried at low temperature for multiple times and then treated with disodium ethylenediaminetetraacetate, which can chelate and remove metal ion impurities. After o-dichlorobenzene is irradiated with ultraviolet light, peroxide impurities can be decomposed. Dimethyl sulfate neutralizes acidic impurities with triethylamine and phosphoric acid. Triphenyl phosphite and 2,6-di-tert-butyl can inhibit oxidation and decomposition, balance dispersibility and reactivity, and achieve the purpose of improving the preparation purity and yield.

[0055] By comparing and analyzing the relevant data in the table, it can be seen that the methyl isocyanate prepared by the present invention not only has good purity performance but also excellent yield performance. This shows that the method for preparing methyl isocyanate based on the dimethyl sulfate route provided by the present invention has a broader market prospect and is more suitable for popularization.

[0056] In the description of this specification, the description with reference to terms such as "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0057] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. Obviously, many modifications and changes can be made according to the content of this specification. These embodiments are selected and specifically described in this specification to better explain the principle and practical application of the present invention, so that those skilled in the art can understand and utilize the present invention well. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A method for preparing methyl isocyanate based on the dimethyl sulfate route, characterized in that: It includes the following steps: S1: Raw material preparation. The raw materials include sodium cyanate, o-dichlorobenzene, and dimethyl sulfate. The mass ratio of sodium cyanate to dimethyl sulfate is 1:(0.6), and the mass of o-dichlorobenzene is 3 - 5 times the mass of sodium cyanate; S2: Raw material pretreatment. Sodium cyanate, o-dichlorobenzene, and dimethyl sulfate are pretreated separately for later use; S3: Reaction preparation. The pretreated sodium cyanate, o-dichlorobenzene, and dimethyl sulfate are input into a reaction kettle for mixing treatment to obtain methyl isocyanate.

2. The method for preparing methyl isocyanate based on the dimethyl sulfate route according to claim 1, wherein The pretreatment method of the sodium cyanate is as follows: Sodium cyanate is added into an oven, the oven temperature is set at 40 - 44°C, and it is dried for 20 - 30 min. Then the sodium cyanate is taken out and waited to cool to room temperature. The above drying step is carried out again, and the drying and cooling treatments are repeated 4 - 6 times. The obtained product and the soaking solution are added into a water bath kettle, the water bath kettle is set to heat up to 50 - 60°C, and at the same time, a magnetic stirrer is connected. The magnetic stirrer is set at a rotation speed of 120 - 160 r / min, and it is stirred at a constant temperature for 10 - 20 min. The product obtained by stirring at a constant temperature is added into a centrifuge, the centrifuge is set at 3000 - 5000 r / min for centrifugation for 6 - 10 min, and the precipitate is taken. The precipitate is added into an oven, the oven is set at 60 - 70°C for drying for 40 - 50 min, and the dried product is ground, and the grinding particle size is 2 - 6 μm, thus completing the pretreatment of sodium cyanate.

3. The method for preparing methyl isocyanate based on the dimethyl sulfate route according to claim 2, wherein, The mass of the soaking solution is 40 - 60% of the mass of sodium cyanate.

4. The method for preparing methyl isocyanate based on the dimethyl sulfate route according to claim 2, wherein, The soaking solution is prepared by mixing disodium ethylenediaminetetraacetate and deionized water, and the mass ratio of disodium ethylenediaminetetraacetate to deionized water is 1:(6 - 8).

5. The method for preparing methyl isocyanate based on the dimethyl sulfate route according to claim 1, wherein The pretreatment method of the o-dichlorobenzene is as follows: o-dichlorobenzene is added into a mixer, the mixer is set at a rotation speed of 30 - 40 r / min, an ultraviolet lamp is set above the mixer, the wavelength of the ultraviolet lamp is 200 - 300 nm, and the distance from the liquid level is 20 - 40 cm. It is stirred and irradiated for 20 - 30 min to complete the pretreatment of o-dichlorobenzene.

6. The method for preparing methyl isocyanate based on the dimethyl sulfate route according to claim 1, wherein, The pretreatment method of the dimethyl sulfate is as follows: Dimethyl sulfate and a treating agent are added into a water bath kettle, the water bath kettle is set at a temperature of 46 - 48°C, and at the same time, a magnetic stirrer is connected. The magnetic stirrer is set at a rotation speed of 60 - 80 r / min, and it is stirred at a constant temperature for 12 - 16 min. The obtained product is added into a centrifuge, the centrifuge is set at 3000 - 5000 r / min for centrifugation for 8 - 12 min, and the precipitate is taken. The precipitate is added into an oven, the oven is set at 52 - 54°C for drying for 30 - 40 min, and the dried product is ground, and the grinding particle size is 4 - 10 μm, thus completing the pretreatment of methyl isocyanate.

7. The method for preparing methyl isocyanate based on the dimethyl sulfate route according to claim 6, characterized in that, The mass of the treating agent is 4 - 6% of the mass of dimethyl sulfate.

8. The method for preparing methyl isocyanate based on the dimethyl sulfate route according to claim 6, characterized in that, The treatment agent is prepared by the following method: Triethylamine and phosphoric acid are added into a reaction kettle. The temperature of the reaction kettle is set at 0 - 2 °C, and it is subjected to constant-temperature stirring treatment for 10 - 20 min at a rotation speed of 100 - 120 r / min. Then, triphenyl phosphite and 2,6-di-tert-butyl-p-cresol are added, and the constant-temperature stirring treatment is continued for 6 - 12 min to obtain the treatment agent. Among them, the mass ratio of triethylamine to phosphoric acid is 1:(0.1 - 0.3), the mass of triphenyl phosphite is 6 - 10% of the mass of triethylamine, and the mass of 2,6-di-tert-butyl-p-cresol is 4 - 8% of the mass of triethylamine.

9. The method for preparing methyl isocyanate based on the dimethyl sulfate route according to claim 1, characterized in that, The method for the said reaction preparation is as follows: Sodium cyanate and o-dichlorobenzene after completing pretreatment are subjected to mixing and grinding treatment, and then subjected to heating and negative-pressure dehydration treatment, and sent into a reaction kettle. Dimethyl sulfate after completing pretreatment is sent into the reaction kettle. The temperature of the reaction kettle is set at 172 - 180 °C, and the reaction is carried out for 4 - 10 h under normal pressure. The obtained product is subjected to rectification treatment to obtain methyl isocyanate.

10. The method for preparing methyl isocyanate based on the dimethyl sulfate route according to claim 9, characterized in that, The temperature of the rectification treatment is 64 - 66 °C.

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