Washing method of crude DNT

Abstract

The invention provides a washing method of crude DNT (Dinitrotoluene). The washing method comprises the following steps: 1) carrying out primary washing on the crude DNT by using water or low-concentration acidic water until the content of sulfuric acid in the DNT is 300-1000ppm; (2) carrying out secondary washing on the DNT obtained in the step (1) by adopting an ammonia water solution, controlling the concentration of free ammonia in secondary washing water to be 300-1000ppm according to the amount of the added ammonia water solution, and calculating the theoretical adding amount of the ammonia water solution based on a formula; 3) performing concentration treatment on the primary washing water to obtain mixed acid, recycling the mixed acid to a nitration device, and recycling generated low-concentration acidic water to primary washing; and 4) extracting the secondary washing water by using an organic solvent to recover DNT in the secondary washing water, stripping the water obtained after extraction, discharging the water, condensing the gas phase generated by stripping, and returning to the extraction process. According to the washing method of the crude DNT, the harmless treatment of the wastewater is realized while the stable operation in the washing process of the crude DNT is ensured.

Description

Technical Field

[0001] The present invention belongs to the technical field of DNT (dinitrotoluene) production, and relates to a method for washing crude DNT. Background Art

[0002] Dinitrotoluene (DNT) is an important raw material for preparing explosives, fuels, and polyurethane foams. In the current industrial DNT production process, an isothermal mixed-acid nitration process is adopted, using toluene as the raw material, sulfuric acid as the catalyst, and concentrated nitric acid as the nitrating agent. Through a two-stage nitration reaction process of toluene, a crude DNT product is obtained. There are two main impurities in the crude DNT product, namely nitric acid dissolved in DNT and nitrocresol substances. In order to obtain a refined DNT product, the crude DNT needs to be washed to remove the above impurities. The crude DNT usually undergoes two washes. The process of first using low-concentration acidic water to wash away nitric acid is called the first wash. The wastewater generated from the first wash generally contains 5% - 15% dilute nitric acid, which is called yellow water in the industry; after the first wash, the DNT needs to be further washed with ammonia water for the second wash to wash away the nitrocresol therein, and this process is called alkali wash. The alkali wash wastewater is called red water in the industry.

[0003] Yellow water and red water are the main wastewater generated in the DNT production process and are also the most difficult to treat. Due to its high nitric acid content, yellow water causes waste of nitric acid during wastewater treatment on the one hand and easily leads to excessive total nitrogen in the treated wastewater on the other hand. CN205616822U discloses a yellow water concentration and organic matter recovery device, which can recover nitric acid and organic matter in yellow water and is an efficient means for treating yellow water. The nitrocresol salt in red water has high biochemical toxicity. In the industry, industrialization generally adopts Fenton oxidation treatment or high-pressure pyrolysis process for treatment, and the treatment cost is high. To solve this problem, CN102516082B uses the wastewater after nitrobenzene ammonia washing to prepare water coal slurry, which can efficiently and environmentally treat the wastewater, but is limited by the gasification capacity of the water coal slurry; CN103359865B crystallizes and recovers the nitrophenolate salt in the wastewater, realizing the harmless treatment of the wastewater. However, the structure of the nitrophenolate salt is complex, and it is difficult to achieve effective resource utilization. At the same time, the nitrophenolate salt has poor thermal stability and is prone to safety accidents during storage (see the 3·21 explosion accident of Tianjiayi Company in Xiangshui, Jiangsu).

[0004] In addition, the DNT washing process involves multiple mixing and separation processes. Since the impurity composition in the crude DNT is complex, once there are parameter fluctuations in the nitration reaction process, it is extremely easy to cause emulsification in the subsequent mixing and separation processes, thus affecting the normal production operation.

[0005] Therefore, how to achieve efficient disposal of the crude DNT washing wastewater while ensuring the stable operation of the production process, especially avoiding emulsification, is a key technical issue in the DNT production process. Summary of the Invention

[0006] The technical problem to be solved by the present invention is how to achieve the stable operation of the crude DNT washing process and simultaneously achieve the harmless treatment of wastewater.

[0007] Based on the above technical problems, the present invention proposes a method for washing crude DNT. In this method, on the one hand, to ensure the stable operation of the crude DNT washing process, the key parameters and impurity content of the primary washing and secondary washing are controlled. Specifically, during the primary washing process, by controlling the sulfuric acid content in the DNT after washing, the emulsification phenomenon during the primary washing process is prevented; during the secondary washing process, by controlling the free ammonia concentration, the washing effect is ensured, and at the same time, the ammonia accumulation problem during the subsequent wastewater extraction and stripping process is avoided. On the other hand, to achieve the harmless treatment of wastewater, the primary washing wastewater with a high acid content is concentrated and recovered; for the secondary washing wastewater containing nitrocresol salts, after removing the dissolved DNT therein through extraction - separation - stripping, it is sent to the gasification coal blending treatment. Since the free ammonia in the secondary washing water is controlled, during the extraction - separation - stripping process, the unstable interface phenomenon caused by excessive free ammonia content during the separation process after extraction is avoided.

[0008] To achieve the above object, the technical solutions adopted by the present invention are as follows:

[0009] A method for washing crude DNT, which comprises the following steps:

[0010] 1) Primary washing: Use water or low - concentration acidic water to mix with crude DNT for primary washing to obtain primary washing water and DNT after primary washing. Among them, multi - stage washing is carried out until the sulfuric acid content in the DNT after primary washing is 300 - 1000 ppm, preferably 500 - 800 ppm;

[0011] 2) Secondary washing: Use an ammonia aqueous solution to carry out secondary washing on the DNT obtained in step 1) to obtain secondary washing water and DNT after secondary washing. The free ammonia concentration in the secondary washing water is controlled to be 300 - 1000 ppm, preferably 600 - 800 ppm, by the amount of the added ammonia aqueous solution.

[0012] Among them, the theoretical addition amount of the ammonia aqueous solution is calculated based on the following formula:

[0013] Free ammonia concentration (ppm) = (amount of ammonia aqueous solution added (t) × ammonia water concentration (wt%) - mass of DNT after secondary washing (t) × nitrocresol content in DNT (wt%) / 153 * 17 - mass of DNT after secondary washing (t) × content of inorganic acid and small - molecule acid in DNT (wt%) / 63 * 17) / (amount of ammonia aqueous solution added) (t)) * 1000000

[0014] The content of nitrocresol in DNT, calculated according to 4500 ppm, is 0.45 wt%.

[0015] The content of inorganic acids and small molecule acids in DNT, calculated according to 500 ppm, is 0.05 wt%.

[0016] Measure the concentration of free ammonia in the secondary washing water. If it is not within the above range, adjust it by adding water and ammonia water to make the free ammonia concentration within the above range.

[0017] 3) Concentrate the primary washing water obtained in step 1). After concentration, the mixed acid is recycled to the nitration unit, and the low-concentration acidic water generated is recycled to the primary washing of DNT.

[0018] 4) Extract the DNT in the secondary washing water obtained in step 2) with an organic solvent, and then strip the water obtained after extraction and discharge it. The gas phase generated by stripping is condensed and returned to the extraction process.

[0019] In a preferred embodiment, in the primary washing process of step 1), a multi-stage countercurrent washing method is adopted. The number of washing stages is 2 - 4, and the pH value of the mixed system in the last washing stage is controlled to be 0.1 - 2.

[0020] "Countercurrent washing" herein refers to countercurrent washing in a multi-stage process, that is, the crude DNT to be washed is added in the first washing stage. The washing water in the first washing stage is the water obtained by layer separation from the second washing stage. The DNT obtained after the first washing is then used in the second washing with the water layer separated from the third washing stage (if any) as the washing water. Fresh washing water (water or low-concentration acidic water) is added in the last washing stage, and the water obtained by layer separation is used as the washing water for the previous washing stage. Overall, it is a countercurrent washing process, but each washing stage is a process of stirring and mixing and layer separation.

[0021] During countercurrent washing, primary washing water is obtained after the first washing stage, and DNT after the first washing is obtained after the last washing stage.

[0022] In a preferred embodiment, in the primary washing process of step 1), in each washing stage, first mix the washing water with DNT, and then let it stand for layer separation under the action of gravity. The stirring and mixing process is carried out in a stirring kettle, and the layer separation process is carried out in a tank. Preferably, the stirring and mixing time is 3 - 10 min, preferably 4 - 6 min, and the layer separation time is 10 - 20 min, preferably 13 - 16 min.

[0023] In a preferred embodiment, during the first washing process in step 1), the temperatures of both the stirring and mixing process and the stratification process are controlled at 65 - 70 °C.

[0024] In a preferred embodiment, during the first washing process in step 1), the low - concentration acidic water is acidic water with a concentration of nitric acid + sulfuric acid ≤ 5 wt%, preferably nitric acid + sulfuric acid ≤ 3 wt%. There is no special limitation on the content of nitric acid and sulfuric acid here, and it can also contain only one of the acids, that is, nitric acid or sulfuric acid, as long as the amounts of the two acids meet the above range.

[0025] In a preferred embodiment, during the first washing process in step 1), the volume ratio of the aqueous phase to the oil phase in each stage of the washing process is controlled at 0.5 - 5:1, preferably 0.5 - 3:1.

[0026] In a preferred embodiment, during the second washing process in step 2), a multi - stage counter - current washing method is adopted, and the number of washing stages is 2 - 5, preferably 3 - 4. An ammonia water solution is added to the system in the last stage of the washing process.

[0027] In a preferred embodiment, during the second washing process in step 2), in each stage of the washing process, the ammonia water solution is mixed with DNT, and then separated by stratification under the action of gravity. The stirring and mixing process is carried out in a stirring kettle, and the stratification process is carried out in a tank. Preferably, the mixing and stirring time is 3 - 10 min, more preferably 4 - 6 min; the stratification time is 10 - 20 min, more preferably 13 - 16 min.

[0028] In a preferred embodiment, during the second washing process in step 2), the concentration of the ammonia water solution is 0.1 wt% - 15 wt%.

[0029] In a preferred embodiment, during the second washing process in step 2), the temperatures of the stirring, mixing and stratification are controlled at 60 - 70 °C.

[0030] In a preferred embodiment, during the second washing process in step 2), the volume ratio of the aqueous phase to the oil phase in each stage of the washing process is controlled at 0.5 - 3:1, preferably 1 - 2:1.

[0031] In a preferred embodiment, in step 3), the concentration treatment of the first washing water is carried out in a distillation column. Preferably, the feed plate of the distillation column is located on the 7th - 12th theoretical plate, the top pressure of the column is 25 - 35 kpaa, the number of theoretical plates is 16 - 25, preferably 18 - 20, and the reflux ratio is 0.2 - 0.4; low - concentration acidic water is drawn from the top of the column, and concentrated mixed acid is obtained at the bottom of the column.

[0032] In a preferred embodiment, in step 4), the extraction process is carried out in a stirred tank, and after extraction, the organic solvent and the secondary washing water are separated in a storage tank. Preferably, the volume ratio of the organic phase to the aqueous phase is 0.5 to 3:1, preferably 1 to 2:1. Preferably, the stirring time of the extraction process is 3 to 6 min, more preferably 4 to 5 min, and the separation time is 10 to 20 min, more preferably 10 to 13 min.

[0033] In a preferred embodiment, in step 4), after extraction and separation, the water obtained after extraction and separation is subjected to stripping treatment. The stripping ratio is 0.05 to 0.2. The stripping process is carried out inside a stripping column. Water is fed from the top of the column. The stripping column has 5 to 10 theoretical plates, the temperature at the bottom of the column is 98 to 100 °C, and the operation is at atmospheric pressure.

[0034] In a preferred embodiment, in step 4), after extraction and stripping, the secondary washing water is cooled to 30 to 60 °C and mixed with raw coal to form a water-coal slurry for gasification treatment.

[0035] In a preferred embodiment, in step 4), the organic solvent is toluene and / or mononitrotoluene (MNT).

[0036] Beneficial effects

[0037] According to the method for washing crude DNT of the present invention, on the one hand, to ensure the stable operation during the washing process of crude DNT, the key parameters and impurity contents of the primary washing and secondary washing are controlled to prevent the emulsification phenomenon during the primary washing process, which is beneficial to the rapid separation of the aqueous phase and the oil phase; during the secondary washing process, the washing effect is ensured by controlling the free ammonia concentration, and at the same time, the ammonia accumulation problem during the subsequent extraction and stripping of wastewater is avoided. On the other hand, to achieve the harmless treatment of wastewater, the primary washing wastewater with a high acid content is concentrated and recovered; for the secondary washing wastewater containing nitrocresolates, after removing the dissolved DNT therein by extraction-separation-stripping, it is sent to the gasification coal blending treatment. Since the free ammonia in the secondary washing water is controlled, during the extraction-separation-stripping process, the unstable interface phenomenon caused by the excessive free ammonia content during the separation process after extraction is avoided. Specific embodiments

[0038] The present invention will be further described below through specific examples. The examples described in the present invention are only for the illustration of the present invention and do not limit the scope of the present invention.

[0039] The raw material used in the present invention is the crude product of dinitrotoluene (crude DNT) produced by Wanhua Chemical Group Co., Ltd.

[0040] The nitric acid concentration analysis is carried out using a Metrohm 905 Titrando potentiometric titrator, a pH electrode, and titrated with 0.5 mol / l sodium hydroxide solution.

[0041] The sulfuric acid in DNT was first extracted with equal masses of water and dichloromethane, and then the aqueous phase was separated and the sulfuric acid concentration was determined by a Wantong TH-980C ion chromatograph. The obtained value was the sulfuric acid concentration in DNT.

[0042] The free ammonia in the solution was analyzed by using a Metrohm 905 Titrando potentiometric titrator, a pH electrode, and a 0.1 mol / l sulfuric acid solution.

[0043] Example 1

[0044] The crude DNT is washed using the following process

[0045] 1) One-stage washing, using acidic water with a nitric acid content of 0.1wt% to wash the crude DNT once, the one-stage washing process is a two-stage countercurrent washing, wherein the nitric acid content in the crude DNT is 3wt%. In the first-stage washing process, the crude DNT is washed with washing water from the second-stage washing process. In the second-stage washing process, the DNT obtained after the first-stage washing is washed with acidic water with a nitric acid content of 0.1wt%. The stirring time of DNT and washing water in the single-stage washing process is 3min, and then the stratification is carried out, and the stratification time is 10min. The pH value of the mixed system in the last-stage washing process is controlled to be 0.1. The sulfuric acid content in the obtained DNT after the one-stage washing is 1000ppm, and there is no need to repeat the one-stage washing step. The volume ratio of washing water to DNT in each stage of washing is controlled to be 1:1 by the circulation amount of washing water after stratification. The washing water obtained in the first-stage washing process is the one-stage washing water, and the DNT obtained in the second-stage washing process is the one-stage washing DNT.

[0046] 2) Secondary washing, the DNT washed once was mixed with an ammonia solution and water with a concentration of 10 wt% (a low concentration was formed for secondary washing, and the secondary washing process was a two-stage countercurrent washing, and the amount of ammonia solution added was calculated based on a free ammonia concentration of 600 ppm. In the first stage washing process, the crude DNT was washed with an ammonia solution from the second stage washing process. In the second stage washing process, the DNT obtained after the first stage washing was washed with an ammonia solution. The volume ratio of washing water to DNT in each stage of washing was controlled to be 1:1 by the circulation amount of washing water after standing and stratifying. The stirring and mixing time of DNT and washing water in each stage of washing was 3 minutes, and the stratification time of washing water and DNT after stirring was 10 minutes. The free ammonia content in the secondary washing water obtained by measurement was 300 ppm. The washing water obtained in the first stage washing process was the secondary washing water, and the DNT obtained in the second stage washing process was the secondary washing DNT.

[0047] 3) Feed the primary washing water obtained in step 1) into a concentration tower for concentration. The theoretical number of plates in the concentration tower is 20, the feed plate is the 10th theoretical plate, the top pressure of the tower is 32 kpaa, and the reflux ratio is 0.34. After concentration, the nitric acid concentration is 44.4 wt%, and the nitric acid concentration of the low-concentration nitric acid withdrawn from the top of the tower is 0.1 wt%. The low-concentration nitric acid withdrawn from the top of the tower is recycled to the washing process of crude DNT, and the concentrated nitric acid is recycled to the nitration reaction.

[0048] 4) Mix the secondary washing water obtained in step 2) with toluene to extract DNT in the secondary washing water. The volume ratio of toluene to the secondary washing water is controlled at 1:1. Toluene and the secondary washing water are stirred and mixed for 3 min, and the static separation time after mixing is 10 min. The wastewater after extraction is subjected to stripping treatment. The stripping ratio is 0.05. The wastewater is fed into the top of the stripping tower. The number of plates in the stripping tower is 5, and it operates at atmospheric pressure. The bottom temperature of the tower is 99 °C. After stripping, the wastewater is cooled to 30 °C and sent to the gasification coal grinding process.

[0049] During the stripping process of the wastewater, part of the ammonia returns to the extraction process with the condensate, resulting in an increase in free ammonia during the wastewater extraction process. The final equilibrium value is 800 ppm. Under this condition, the wastewater / toluene extraction process operates normally.

[0050] Example 2

[0051] The crude DNT is washed using the following process

[0052] 1) Primary washing: Wash the crude DNT with acidic water containing 1 wt% nitric acid. The primary washing process is a three-stage countercurrent washing, and the nitric acid content in the crude DNT is 10 wt%. In the first-stage washing process, wash the crude DNT with the washing water from the second-stage washing process. In the second-stage washing process, wash the DNT obtained after the first-stage washing with the washing water from the third-stage washing process. In the third-stage washing process, wash the DNT obtained after the second-stage washing with acidic water containing 1 wt% nitric acid. The stirring time of DNT and the washing water in the single-stage washing process is 4 min, and then static separation is carried out. The static separation time is 13 min. Control the pH value of the last-stage washing process to be 0.5. The sulfuric acid content in the DNT after the primary washing is 800 ppm, and there is no need to repeat the primary washing step. Control the volume ratio of the washing water to DNT in each stage of the washing process to be 0.5:1 by controlling the circulation amount of the washing water after static separation. The washing water obtained in the first-stage washing process is the primary washing water, and the DNT obtained in the third-stage washing process is the DNT after the primary washing.

[0053] 2) Secondary washing: Mix the DNT from the primary washing with an aqueous ammonia solution at a concentration of 12 wt% and water (to form a low-concentration aqueous ammonia solution) for secondary washing. The secondary washing process is a three-stage countercurrent washing. Calculate the addition amount of the aqueous ammonia solution based on a free ammonia concentration of 800 ppm. In the first-stage washing process, use the aqueous ammonia solution from the second-stage washing process to wash the crude DNT. In the second-stage washing process, use the aqueous ammonia solution from the third-stage washing process to wash the DNT obtained from the first-stage washing. In the third-stage washing process, use the aqueous ammonia solution to wash the DNT obtained from the second-stage washing. Control the volume ratio of washing water to DNT at 1:1 in each stage of the washing process by the circulation amount of the washing water after static separation. The stirring and mixing time of DNT and washing water in each stage of the washing process is 4 min, and the separation time of the washing water and DNT after stirring is 13 min. The free ammonia content in the finally obtained secondary washing water is 1200 ppm, and it is diluted with water to a free ammonia content of 800 ppm. The washing water obtained from the first-stage washing process is the secondary washing water, and the DNT obtained from the third-stage washing process is the DNT after secondary washing.

[0054] 3) Feed the primary washing water obtained in step 1) into a concentration tower for concentration treatment. The theoretical number of plates in the concentration tower is 25, the feed plate is the 7th theoretical plate, the top pressure of the tower is 25 kpaa, and the reflux ratio is 0.3; the concentration of nitric acid after concentration is 50 wt%, and the low-concentration nitric acid concentration at the top of the tower is 1 wt%. The low-concentration nitric acid drawn from the top of the tower is recycled to the crude DNT washing process, and the concentrated nitric acid is recycled to the nitration reaction.

[0055] 4) Mix the secondary washing water obtained in step 2) with toluene to extract DNT in the secondary washing water. Control the volume ratio of toluene to secondary washing water at 2:1. Stir and mix toluene and the secondary washing water, and the mixing time is 4 min. The static separation time after mixing is 13 min. The wastewater after extraction is subjected to stripping treatment. The stripping ratio is 0.1, the wastewater is fed into the top of the stripping tower, the number of plates in the stripping tower is 7, and it is operated at atmospheric pressure. The bottom temperature of the tower is 98 °C. After stripping, the wastewater is cooled to 60 °C and sent to the gasification coal grinding process.

[0056] During the wastewater stripping process, part of the ammonia returns to the extraction process with the condensate, resulting in an increase in the free ammonia in the wastewater extraction process. The final equilibrium value is 1500 ppm. Under this condition, the wastewater / toluene extraction process operates normally.

[0057] Example 3

[0058] The crude DNT is washed using the following process

[0059] 1) Primary washing: The crude DNT is washed once with acidic water containing 5 wt% nitric acid. The primary washing process is a four-stage countercurrent washing, with the nitric acid content in the crude DNT being 2 wt%. In the first-stage washing process, the crude DNT is washed with the washing water from the second-stage washing process. In the second-stage washing process, the DNT obtained after the first-stage washing is washed with the washing water from the third-stage washing process. In the third-stage washing process, the DNT obtained after the second-stage washing is washed with the washing water from the fourth-stage washing process. In the fourth-stage washing process, the DNT obtained after the third-stage washing is washed with acidic water containing 1 wt% nitric acid. The stirring time of the crude DNT and the washing water in a single-stage washing process is 6 min, and then static settling is carried out for 16 min. The pH value of the last-stage washing process is controlled to be 1. The sulfuric acid content in the DNT after primary washing is 500 ppm, and there is no need to repeat the primary washing step. The volume ratio of the washing water to the DNT in each stage of the washing process is controlled to be 2:1 by the circulation volume of the washing water after static settling. The washing water obtained in the first-stage washing process is the primary washing water, and the DNT obtained in the fourth-stage washing process is the DNT after primary washing.

[0060] 2) Secondary washing: The DNT after primary washing is mixed with an ammonia water solution with a concentration of 15 wt% and water (to form a low-concentration ammonia water solution) for secondary washing. The secondary washing process is a five-stage countercurrent washing, and the addition amount of the ammonia water solution is calculated based on the free ammonia concentration of 400 ppm. In the first-stage washing process, the crude DNT is washed with the washing water from the second-stage washing process. In the second-stage washing process, the DNT obtained after the first-stage washing is washed with the washing water from the third-stage washing process. In the third-stage washing process, the DNT obtained after the second-stage washing is washed with the washing water from the fourth-stage washing process. In the fourth-stage washing process, the DNT obtained after the third-stage washing is washed with the washing water from the fifth-stage washing process. In the fifth-stage washing process, the DNT obtained after the fourth-stage washing is washed with the ammonia water solution. The volume ratio of the washing water to the DNT in each stage of the washing process is controlled to be 2:1 by the circulation volume of the washing water after static settling. The stirring and mixing time of the DNT and the washing water in each stage of the washing process is 6 min, and the stratification time of the washing water and the DNT after stirring is 16 min. The free ammonia content in the secondary washing water finally obtained is 600 ppm. The washing water obtained in the first-stage washing process is the secondary washing water, and the DNT obtained in the fifth-stage washing process is the DNT after secondary washing.

[0061] 3) The primary wash water obtained in step 1) is fed into a concentration tower for concentration. The theoretical number of plates in the concentration tower is 16, the feed plate is the 12th theoretical plate, the top pressure is 35 kpaa, and the reflux ratio is 0.2. After concentration, the nitric acid concentration is 30 wt%, and the low-concentration nitric acid withdrawn from the top of the tower has a concentration of 5 wt%. The low-concentration nitric acid withdrawn from the top of the tower is recycled to the crude DNT washing process, and the concentrated nitric acid is recycled to the nitration reaction.

[0062] 4) The secondary wash water obtained in step 2) is mixed with toluene to extract DNT in the secondary wash water. The volume ratio of toluene to secondary wash water is controlled at 2:1. Toluene and secondary wash water are stirred and mixed for 6 min, and the static separation time after mixing is 20 min. The wastewater after extraction is subjected to stripping treatment. The stripping ratio is 0.2. The wastewater is fed into the top of the stripping tower. The number of plates in the stripping tower is 10, and it operates at atmospheric pressure. The bottom temperature of the tower is 100 °C. After stripping, the wastewater is cooled to 40 °C and sent to the gasification coal grinding process.

[0063] During the wastewater stripping process, part of the ammonia returns to the extraction process with the condensate, resulting in an increase in free ammonia in the wastewater extraction process. The final equilibrium value is 1000 ppm. Under this condition, the wastewater / toluene extraction process operates normally.

[0064] Example 4

[0065] The crude DNT is washed using the following process

[0066] 1) Primary washing: Acidic water with a nitric acid content of 1.8 wt% is used for primary washing. The primary washing process is a three-stage countercurrent washing, and the nitric acid content in the crude DNT is 0.1%. In the first-stage washing process, the wash water from the second-stage washing process is used to wash the crude DNT. In the second-stage washing process, the wash water from the third-stage washing process is used to wash the DNT obtained after the first-stage washing. In the third-stage washing process, acidic water with a nitric acid content of 1.8 wt% is used to wash the DNT obtained after the second-stage washing. The stirring time of the crude DNT and the wash water in the single-stage washing process is 10 min, and then static separation is carried out. The static separation time is 20 min. The pH value of the last-stage washing process is controlled at 2. The sulfuric acid content in the DNT after the primary washing is 300 ppm, and there is no need to repeat the primary washing step. The circulation amount of the wash water after static separation is used to control the volume ratio of the wash water to DNT in each stage of the washing process at 3:1. The wash water obtained in the first-stage washing process is the primary wash water, and the DNT obtained in the third-stage washing process is the DNT after the primary washing.

[0067] 2) Secondary washing: The DNT from the primary washing is subjected to secondary washing with an aqueous ammonia solution at a concentration of 14% and water (forming a low-concentration aqueous ammonia solution). The secondary washing process is a five-stage countercurrent washing. The amount of the aqueous ammonia solution added is calculated based on a free ammonia concentration of 400 ppm. During the first-stage washing process, the crude DNT is washed with the washing water from the second-stage washing process. During the second-stage washing process, the DNT obtained after the first-stage washing is washed with the washing water from the third-stage washing process. During the third-stage washing process, the DNT obtained after the second-stage washing is washed with the washing water from the fourth-stage washing process. During the fourth-stage washing process, the DNT obtained after the third-stage washing is washed with the washing water from the fifth-stage washing process. During the fifth-stage washing process, the DNT obtained after the fourth-stage washing is washed with the aqueous ammonia solution. The volume ratio of the washing water to DNT in each stage of the washing process is controlled at 3:1 by controlling the circulation amount of the washing water after static separation. The stirring and mixing time of DNT and the washing water in each stage of the washing process is 10 min, and the stratification time of the washing water and DNT after stirring is 20 min. The free ammonia content in the finally obtained secondary washing water is 300 ppm. The washing water obtained from the first-stage washing process is the secondary washing water, and the DNT obtained from the fifth-stage washing process is the DNT after secondary washing.

[0068] 3) The primary washing water obtained in step 1) is fed into a concentration tower for concentration treatment. The theoretical number of plates in the concentration tower is 18, the feed plate is the 10th theoretical plate, the top pressure of the tower is 30 kpaa, and the reflux ratio is 0.4; the concentration of nitric acid after concentration is 35 wt%, and the concentration of the low-concentration nitric acid drawn from the top of the tower is 1.8 wt%. The low-concentration nitric acid drawn from the top of the tower is recycled to the crude DNT washing process, and the concentrated nitric acid is recycled to the nitration reaction.

[0069] 4) The secondary washing water obtained in step 2) is mixed with toluene to extract DNT in the secondary washing water. The volume ratio of toluene to the secondary washing water is controlled at 3:1. Toluene and the secondary washing water are stirred and mixed for 5 min, and the static separation time after mixing is 15 min. The wastewater after extraction is subjected to stripping treatment. The stripping ratio is 0.15. The wastewater is fed into the top of the stripping tower. The number of plates in the stripping tower is 8, and it is operated at atmospheric pressure. The bottom temperature of the tower is 99 °C. The wastewater after stripping is cooled to 40 °C and sent to the gasification coal grinding treatment.

[0070] During the wastewater stripping process, part of the ammonia returns to the extraction process with the condensate, resulting in an increase in the free ammonia in the wastewater extraction process. The final equilibrium value is 800 ppm. Under this condition, the wastewater / toluene extraction process operates normally.

[0071] Comparative Example 1

[0072] The crude DNT is washed using the following process

[0073] Perform one washing, and one washing process is a three-stage countercurrent washing. The nitric acid content in the crude DNT is 0.1 wt%. In the first-stage washing process, the crude DNT is washed with the washing water from the second-stage washing process. In the second-stage washing process, the DNT obtained after the first-stage washing is washed with the washing water from the third-stage washing process. In the third-stage washing process, the DNT obtained after the second-stage washing is washed with pure water. The stirring time of the crude DNT and the washing water in the single-stage washing process is 3 min, and then static settling is carried out, resulting in emulsification. Measure the sulfuric acid content in the DNT after one washing to be 30 ppm. Since the sulfuric acid content in the DNT is low, the acidity in the last-stage washing process is weak, the ionization of organic acids in the DNT increases, reducing the interfacial tension and resulting in emulsification.

[0074] The amount of fresh washing water is large, the acidity is weak, and the number of washing stages is also large, resulting in too low sulfuric acid content in the crude DNT, leading to emulsification.

[0075] Comparative Example 2

[0076] The crude DNT is washed by the following process

[0077] 1) For one washing, the crude DNT is washed once with acidic water with a nitric acid content of 1 wt%, and one washing process is a three-stage countercurrent washing, where the nitric acid content in the crude DNT is 10 wt%. In the first-stage washing process, the crude DNT is washed with the washing water from the second-stage washing process. In the second-stage washing process, the DNT obtained after the first-stage washing is washed with the washing water from the third-stage washing process. In the third-stage washing process, the DNT obtained after the second-stage washing is washed with acidic water with a nitric acid content of 1 wt%. The stirring time of the crude DNT and the washing water in the single-stage washing process is 4 min, and then static settling is carried out, with a static settling time of 13 min. Control the pH value of the last-stage washing process to be 0.5, and the sulfuric acid content in the DNT after one washing is 800 ppm. Control the volume ratio of the washing water to the DNT in each stage of the washing process to be 0.5∶1 by the circulation amount of the washing water after static settling. The washing water obtained in the first-stage washing process is the primary washing water, and the DNT obtained in the third-stage washing process is the DNT after one washing.

[0078] 2) Secondary washing: Mix the DNT from the primary washing with an ammonia water solution at a concentration of 13% and water (to form a low-concentration ammonia water solution) for secondary washing. The secondary washing process is a three-stage countercurrent washing. Calculate the addition amount of the ammonia water solution based on a free ammonia concentration of 2500 ppm. During the first-stage washing process, use the ammonia water solution from the second-stage washing process to wash the crude DNT. During the second-stage washing process, use the ammonia water solution from the third-stage washing process to wash the DNT obtained after the first-stage washing. During the third-stage washing process, use the ammonia water solution to wash the DNT obtained after the second-stage washing. Control the volume ratio of washing water to DNT at 0.5:1 in each stage of the washing process by the circulating amount of the washing water after static separation. The stirring and mixing time of DNT and washing water in each stage of the washing process is 4 min, and the stratification time of the washing water and DNT after stirring is 8 min. The free ammonia content in the finally obtained secondary washing water is 3000 ppm. The washing water obtained in the first-stage washing process is the secondary washing water, and the DNT obtained in the third-stage washing process is the DNT from the secondary washing.

[0079] 3) Feed the primary washing water obtained in step 1) into a concentrator for concentration treatment. The number of theoretical plates in the concentrator is 25, the feed plate is the 7th theoretical plate, the top pressure is 25 kpaa, and the reflux ratio is 0.3; the concentration of nitric acid after concentration is 50%, and the low-concentration nitric acid concentration at the top of the column is 1 wt%. The low-concentration nitric acid taken from the top of the column is recycled to the crude DNT washing process, and the concentrated nitric acid is recycled to the nitration reaction.

[0080] 4) Mix the secondary washing water obtained in step 2) with toluene to extract DNT from the wastewater. Control the volume ratio of toluene to wastewater at 2:1. Stir and mix toluene and wastewater for 4 min. After extraction, the wastewater is subjected to stripping treatment. The stripping ratio is 0.1. The wastewater is fed into the top of the stripping column. The number of plates in the stripping column is 7, and it operates at atmospheric pressure. The bottom temperature of the column is 98 °C. After stripping, the wastewater is cooled to 60 °C and sent to the gasification coal grinding process. As the operation time increases, the free ammonia concentration in the aqueous phase during the extraction process gradually rises to 18000 ppm. The excessive free ammonia affects the interfacial tension between the two phases, making it difficult to separate the wastewater / toluene. The wastewater and toluene form an emulsion phase and enter the stripping column, resulting in abnormal production.

Claims

1. A method for washing crude DNT, comprising the following steps: 1) Primary washing: A primary washing is carried out by mixing water or low-concentration acidic water with crude DNT to obtain primary washing water and DNT after primary washing, where Performing multi-stage washing until the sulfuric acid content in DNT after primary washing is 300 - 1000 ppm; 2) Secondary washing: using an ammonia water solution to perform secondary washing on the DNT obtained in step 1) to obtain secondary washing water and DNT after secondary washing, and controlling the free ammonia concentration in the secondary washing water to be 300 - 1000 ppm by the amount of ammonia water solution added; Wherein, the theoretical addition amount of the ammonia water solution is calculated based on the following formula: Free ammonia concentration (ppm) = ((amount of ammonia water solution added (t) × ammonia water concentration (wt%) - mass of DNT after secondary washing (t) × content of nitrocresol in DNT (wt%) / 153 * 17 - mass of DNT after secondary washing (t) × content of inorganic acid and small molecule acid in DNT (wt%) / 63 * 17) / (amount of ammonia water solution added) (t)) * 1000000, The content of nitrocresol in DNT is calculated as 4500 ppm, 0.45 wt%; The content of inorganic acid and small molecule acid in DNT is calculated as 500 ppm, 0.05 wt%; Measuring the free ammonia concentration in the secondary washing water, and if it is not within the above range, adjusting it to be within the above range by adding water and ammonia water; 3) Concentrating the primary washing water obtained in step 1), and after concentration, the mixed acid is recycled to the nitration unit, and the low-concentration acidic water generated is recycled to the primary washing of DNT; 4) Subjecting the secondary washing water obtained in step 2) to organic solvent extraction to recover the DNT therein, and then subjecting the water obtained after extraction to stripping and discharging it, and the gas phase generated by stripping is condensed and returned to the extraction process.

2. The method for washing crude DNT according to claim 1, wherein, The sulfuric acid content in DNT after primary washing is 500 - 800 ppm; and / or The free ammonia concentration in the secondary washing water is 600 - 800 ppm; and / or In the primary washing process of step 1), a multi-stage countercurrent washing method is adopted, the number of washing stages is 2 - 4, and the pH value of the mixed system in the last washing stage is controlled to be 0.1 - 2; the primary washing water is obtained after the first stage of washing, and the DNT after primary washing is obtained after the last stage of washing.

3. The method for washing crude DNT according to claim 1 or 2, wherein, In the primary washing process of step 1), in each stage of washing, first mix the washing water and DNT by stirring, and then let it stand and separate by gravity; the stirring and mixing process is carried out in a stirring kettle, and the separation process is carried out in a tank; and / or In the primary washing process of step 1), the temperatures of both the stirring and mixing process and the separation process are controlled at 65 - 70 °C; and / or In the primary washing process of step 1), the low-concentration acidic water has a nitric acid + sulfuric acid concentration ≤ 5 wt%; and / or In the primary washing process of step 1), the volume ratio of the water phase to the oil phase in each stage of washing is controlled to be 0.5 - 5:

1.

4. The method for washing crude DNT according to claim 3, wherein, In the primary washing process of step 1), during each stage of the washing process, the stirring and mixing time is 3 to 10 min; the layering time is 10 to 20 min; and / or In the primary washing process of step 1), the low-concentration acidic water is acidic water with a nitric acid + sulfuric acid concentration ≤ 3 wt%; and / or In the primary washing process of step 1), control the volume ratio of the water phase to the oil phase in each stage of the washing process to be 0.5 to 3:

1.

5. The washing method of crude DNT according to claim 4, wherein, In the primary washing process of step 1), during each stage of the washing process, the stirring and mixing time is 4 to 6 min and / or; the layering time is 13 to 16 min.

6. The washing method of crude DNT according to any one of claims 1 to 5, wherein, In the secondary washing process of step 2), a multi-stage countercurrent washing method is adopted, the number of washing stages is 2 to 5, and an ammonia water solution is added to the system in the last stage of the washing process; and / or In the secondary washing process of step 2), during each stage of the washing process, mix the ammonia water solution with DNT, and then separate by layering under the action of gravity. The stirring and mixing process is carried out in a stirring kettle, and the layering process is carried out in a tank; and / or In the secondary washing process of step 2), the concentration of the ammonia water solution is 0.1 wt% to 15 wt%; and / or In the secondary washing process of step 2), control the stirring and mixing and layering temperature at 60 to 70 °C; and / or In the secondary washing process of step 2), control the volume ratio of the water phase to the oil phase in each stage of the washing process to 0.5 to 3:

1.

7. The washing method of crude DNT according to claim 6, wherein, In the secondary washing process of step 2), the number of washing stages is 3 to 4; and / or In the secondary washing process of step 2), control the volume ratio of the water phase to the oil phase in each stage of the washing process to 1 to 2:1; and / or In the secondary washing process of step 2), the stirring and mixing time is 3 to 10 min; the layering time is 10 to 20 min.

8. The washing method of crude DNT according to claim 7, wherein, In the secondary washing process of step 2), the stirring and mixing time is 4 to 6 min; and / or, the layering time is 13 to 16 min.

9. The washing method of crude DNT according to any one of claims 1 to 8, wherein, In step 3), the primary washing water is concentrated in a distillation column; and / or In step 4), the extraction process is carried out in a stirring kettle, and after extraction, the organic solvent and the secondary washing water are separated by layering in a storage tank; and / or In step 4), after extraction and separation, the water obtained after extraction and separation is subjected to stripping treatment, the stripping ratio is 0.05 to 0.2, the stripping process is carried out inside a stripping column, water is fed from the top of the column, the number of theoretical plates of the stripping column is 5 to 10, the temperature at the bottom of the column is 98 to 100 °C, and the operation is carried out at atmospheric pressure; and / or In step 4), after the secondary washing water is subjected to extraction and stripping, it is cooled to 30 to 60 °C and mixed with raw coal to form a water-coal slurry for gasification treatment; and / or In step 4), the organic solvent is toluene and / or mononitrotoluene (MNT).

10. The washing method of crude DNT according to claim 9, wherein, the feed plate of the rectification column is located at the 7th to 12th theoretical plate, the top pressure of the column is 25 to 35 kpaa, the number of theoretical plates is 16 to 25, preferably 18 to 20, and the reflux ratio is 0.2 to 0.4; low-concentration acidic water is withdrawn from the top of the column, and concentrated mixed acid is obtained at the bottom of the column; and / or in step 4), the stirring time during the layering process of the organic solvent and the secondary washing water in the storage tank after extraction is 3 to 6 min, preferably 4 to 5 min; the layering time is 10 to 20 min, preferably 10 to 13 min.