A method for recovering nitric acid from crude DNT

Abstract

The present invention provides a method for recovering nitric acid in crude DNT, the method comprising the following steps: (1) mixing crude DNT, a metal salt solution and an additive in a reactor to obtain a mixed system, performing two-phase separation after the reaction, and obtaining a first crude nitric acid product and a first nitrate; (2) returning the first crude nitric acid product obtained in step (1) to the reactor in step (1) for washing, and obtaining a crude nitric acid product and washed DNT after two-phase separation; (3) ultrasonically treating the crude nitric acid product obtained in step (2) to obtain a nitric acid solution. The method for recovering nitric acid in crude DNT provided by the present invention realizes the cracking of nitrobenzoic cresol in crude DNT while recovering nitric acid in crude DNT, thereby reducing the amount of red water generated in the subsequent washing process and reducing the cost of wastewater treatment.

Description

Technical Field

[0001] The invention belongs to the technical field of DNT (dinitrotoluene) production, and in particular relates to a method for recovering nitric acid in crude DNT. Background Art

[0002] Dinitrotoluene (DNT) is an important chemical raw material, widely used in the production of explosives, fuels, and polyurethane foams. Industrial DNT production is mainly an isothermal mixed acid nitration process, using sulfuric acid as a catalyst, concentrated nitric acid as a nitrating agent, and toluene as a raw material. Through a two-stage nitration reaction process, crude DNT products are obtained.

[0003] Crude DNT generally contains two main impurities, namely nitric acid dissolved in DNT and nitrocresols. In order to obtain refined dinitrotoluene products, the above impurities need to be removed separately in the subsequent crude DNT washing process. The process of first using low-concentration acidic water to remove nitric acid is called acid washing. The wastewater produced by acid washing generally contains 5-15% dilute nitric acid, which is called yellow water in the industry. After acid washing, DNT needs to be further washed with ammonia water to remove nitrocresols. This process is called alkaline washing, and the alkaline washing wastewater is called red water in the industry.

[0004] Yellow water and red water are the main wastewaters generated during the DNT production process and are also the most difficult wastewaters to treat. Due to its high nitric acid content, yellow water not only leads to a waste of nitric acid during wastewater treatment, but also easily leads to excessive total nitrogen in the treated wastewater.

[0005] CN 205616822U discloses a yellow water concentration and organic matter recovery device, the recovery device comprises a yellow water rectification tower, the yellow water rectification tower is provided with yellow water and dilute nitric acid inlets, the tower top outlet is connected to the primary condenser feed port, the primary condenser upper gas outlet is connected to the secondary condenser feed port, the secondary condenser upper gas outlet is connected to the vacuum pump, the secondary condenser lower outlet is connected to the upper reflux port of the yellow water rectification tower; the primary condenser lower outlet is connected to the tower top condensate separation tank inlet, the tower top condensate separation tank bottom outlet is connected to the DNT first washing kettle through pipeline one and pump, the tower top condensate separation tank upper outlet is connected to the reuse condensate storage tank inlet, the reuse condensate storage tank outlet is connected to the DNT first washing kettle through pipeline two and reuse condensate pump; pipelines one and two are provided with liquid level control valves. The recovery device provided by the patent can be used to recover nitric acid and organic matter in yellow water, which is an efficient yellow water treatment method. However, nitrocresol salts in red water have high biochemical toxicity. The industry generally uses Fenton oxidation treatment or high-pressure thermal cracking process for industrial treatment, which has high treatment costs.

[0006] In view of the above problems, CN 102516082A uses wastewater after washing with nitrobenzene and ammonia water to prepare water-coal slurry, which can treat wastewater efficiently and environmentally friendly, but is limited by the gasification capacity of water-coal slurry; CN 103359865A recovers nitrophenolate crystals in wastewater to achieve harmless treatment of wastewater, but nitrophenolate has a complex structure and is difficult to achieve effective resource utilization. At the same time, nitrophenolate has poor thermal stability and safety accidents are prone to occur during storage.

[0007] Therefore, how to achieve the recovery of nitric acid in crude DNT while minimizing the generation of wastewater containing nitrocresol salts is a key technical field in crude DNT washing.

[0008] In addition, in the mixed acid method DNT production process, the waste acid generated needs to be further concentrated. Due to the continuous corrosion of equipment in the actual production process, the waste acid contains Fe, Cr, and Ni ions, which are continuously precipitated to form metal salts during the waste acid concentration process. This part of the metal salt is generally treated as waste, resulting in a waste of resources.

[0009] In summary, providing a treatment method for reducing the amount of red water generated while recovering nitric acid in crude DNT has been one of the problems that urgently need to be solved in the art. Summary of the invention

[0010] In view of the shortcomings of the prior art, the object of the present invention is to provide a method for recovering nitric acid in crude DNT, which can recover the nitric acid in the crude DNT while achieving the cracking of nitrobenzophenol in the crude DNT, thereby reducing the amount of red water generated in the subsequent washing process and reducing the cost of wastewater treatment.

[0011] To achieve this object, the present invention adopts the following technical solutions:

[0012] The present invention provides a method for recovering nitric acid in crude DNT, the method comprising the following steps:

[0013] (1) mixing crude DNT, a metal salt solution and an additive in a reactor to obtain a mixed system, and performing two-phase separation after the reaction to obtain a first crude nitric acid product and a first nitration product;

[0014] (2) returning the first crude nitric acid obtained in step (1) to the reactor in step (1) for washing, and obtaining crude nitric acid and washed DNT after two phase separation;

[0015] (3) Ultrasonic treatment of the crude nitric acid obtained in step (2) to obtain a nitric acid solution.

[0016] The crude DNT of the present invention mainly contains nitric acid, nitrocresols and part of sulfuric acid, wherein the content of nitric acid is 0.1-10wt%, the content of nitrocresols is 0.3-0.5wt%, and the content of sulfuric acid is 0.05-0.2wt%.

[0017] The present invention utilizes the dilute nitric acid environment formed during the reaction of crude DNT, provides metal ions through a metal salt solution, and then catalyzes the dilute nitric acid to oxidize nitrocresols in the crude DNT, so that the nitrocresols in the crude DNT are decomposed into small molecules such as NOx, oxalic acid, and acetic acid, thereby greatly reducing the nitrocresol content in the DNT after pickling, and reducing the generation of red water. At the same time, the small molecules such as oxalic acid and acetic acid generated by the decomposition enter the dilute nitric acid phase after stratification, and are ultrasonically treated with the dilute nitric acid, and the small molecules are further decomposed into CO2 and H2O, thereby achieving harmless treatment of nitrocresols. The yellow water after ultrasonic treatment is concentrated, and the nitric acid in the yellow water is recovered, thereby achieving the recovery of nitric acid in the crude DNT.

[0018] The metal salt solution of the present invention is formed during the concentration process of waste acid. The low-concentration waste acid produced in the DNT production process is generally concentrated and reused in the nitration reaction process. Generally, the waste acid contains a small amount of nitric acid (0.5-1.5%), organic matter (MNT or DNT, about 0.5-1%), and a sulfuric acid concentration of 70-75%. During the waste acid concentration process, nitric acid and organic matter in the waste acid are removed, and the sulfuric acid concentration is concentrated to 90-96%. After the sulfuric acid concentration is increased, the solubility of Fe, Cr, and Ni ions in sulfuric acid decreases, forming salts that adhere to the inside of the equipment, affecting heat transfer and clogging pipes. These salt substances are generally removed by stopping the machine for cleaning, and the cleaning liquid discharged after cleaning is the metal salt solution.

[0019] Preferably, the reactor in step (1) comprises a reaction kettle.

[0020] After the two-phase separation in step (1) of the present invention, the solid is still placed in the reactor, and the separated liquid is returned to the reactor for countercurrent washing.

[0021] As a preferred technical solution of the present invention, the content of nitric acid in the crude DNT in step (1) is 0.1 to 10 wt%, for example, it can be 0.1 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt% or 10 wt%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0022] Preferably, the metal salt solution in step (1) contains sulfate.

[0023] Preferably, the concentration of sulfate in the metal salt solution is 5-10%, for example, 5%, 6%, 7%, 8%, 9% or 10%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably, it is 6-8%.

[0024] Preferably, the sulfate includes Fe2(SO4)3, NiSO4 and CoSO4.

[0025] Preferably, the additive comprises water or low concentration nitric acid.

[0026] Preferably, the concentration of the low-concentration nitric acid is 0.1-5wt%, for example, it can be 0.1wt%, 1wt%, 8wt%, 3wt%, 4wt%, or 5wt%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably, it is 0.1-2wt%.

[0027] Preferably, the concentration of nitric acid in the mixed system in step (1) is 2 to 25 wt%, for example, 2 wt%, 5 wt%, 8 wt%, 10 wt%, 14 wt%, 18 wt%, 22 wt% or 25 wt%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably, it is 5 to 20 wt%.

[0028] The concentration of nitric acid in the mixed system formed in step (1) of the present invention is 2 to 25 wt %. If the concentration of nitric acid in the mixed system is too high, the nitric acid washing in the crude DNT will not be thorough, resulting in increased alkali consumption in the subsequent alkali washing process. If the concentration of nitric acid in the mixed system is too low, the flow rate of the washing nitric acid obtained by washing will be too large, and the steam consumption in the subsequent concentration process will increase.

[0029] Preferably, the content of Fe, Cr and Ni elements in the mixed system of step (1) is 200-1000 ppm, for example, it can be 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm or 1000 ppm, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0030] The metal ion concentration in the mixed system in step (1) of the present invention is obtained by OES (atomic emission spectroscopy) testing method using an Agilent 700 series ICP-OES instrument. During the test, the Fe ion wavelength is 259.4 nm, the Cr ion wavelength is 267.7 nm, and the Ni ion wavelength is 231.6 nm.

[0031] The content of metal ions in the mixed system of step (1) of the present invention depends on the amount of metal salt solution added, wherein too much metal salt solution added will lead to too high a concentration of metal ions in the nitric acid after washing. These metal ions will be recycled with the nitric acid for nitration reaction and will enter the waste acid again, resulting in an increase in the frequency of salt washing during the waste acid concentration process. If the amount of metal salt solution added is too low, the catalytic effect will be poor, the decomposition of nitrocresol will be insufficient, and the optimal effect cannot be achieved.

[0032] Preferably, the reaction temperature in step (1) is 65-90°C, for example, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably 65-80°C.

[0033] Preferably, the reaction time of step (1) is 5 to 20 min, for example, 5 min, 9 min, 13 min, 17 min or 20 min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably 8 to 15 min.

[0034] During the reaction process of step (1) of the present invention, metal ions catalyze nitrocresols in crude DNT in an acidic environment, so that nitrocresols in crude DNT are decomposed into small molecules such as NOx, oxalic acid, acetic acid, etc., thereby greatly reducing the nitrocresol content in DNT after pickling and reducing the generation of red water.

[0035] During the reaction process, the temperature will affect the catalytic reaction efficiency and corrosion. Too high a temperature will increase the corrosion rate of the stainless steel container, while too low a temperature will reduce the catalytic reaction efficiency and cause DNT solidification.

[0036] Preferably, the two-phase separation in step (1) comprises static separation or centrifugal separation.

[0037] Preferably, the washing in step (2) comprises countercurrent washing.

[0038] Preferably, the countercurrent washing is performed 1 to 4 times, for example, 1 time, 2 times, 3 times or 4 times.

[0039] Preferably, the volume ratio of the first crude nitric acid to crude DNT in the countercurrent washing process is (0.5-2):1, for example, it can be 0.5:1, 0.8:1, 1:1, 1.2:1, 1.4:1, 1.6:1, 1.8:1 or 2:1, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0040] Preferably, the temperature of the countercurrent washing is 30-90°C, for example, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C or 90°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably 50-80°C.

[0041] Preferably, the residence time of the countercurrent washing is 5 to 20 min, for example, 5 min, 9 min, 13 min, 17 min or 20 min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably 8 to 15 min.

[0042] The reaction principle in the countercurrent washing process of the present invention is the same as the reaction described in step (1). The first crude nitric acid obtained in step (1) is mixed with crude DNT to form a new dilute nitric acid environment, and the nitrocresols in the crude DNT will continue to be decomposed.

[0043] The purpose of the countercurrent washing of the present invention is to further reduce the content of nitrocresol in the crude DNT and further reduce the generation of red water.

[0044] Preferably, the frequency of the ultrasonic treatment in step (3) is 25 to 30 kHz, for example, 25 kHz, 26 kHz, 27 kHz, 28 kHz, 29 kHz or 30 kHz, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0045] Preferably, the residence time of the ultrasonic treatment in step (3) is 5 to 10 min, for example, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0046] Preferably, the temperature of the ultrasonic treatment in step (3) is 60-80°C, for example, 60°C, 64°C, 68°C, 72°C, 76°C or 80°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0047] Preferably, the pressure of the ultrasonic treatment in step (3) is 50-150 kPaa, for example, 50 kPaa, 70 kPaa, 90 kPaa, 110 kPaa, 130 kPaa or 150 kPaa, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0048] The purpose of the ultrasonic treatment of the present invention is to treat impurities in crude nitric acid, so that the impurities therein are further decomposed into CO2 and H2O, thereby achieving harmless treatment of nitrotoluene.

[0049] Preferably, the ultrasonic treatment in step (3) further includes concentrating the nitric acid solution.

[0050] Preferably, the concentrated nitric acid can be reused in the nitration reaction.

[0051] Preferably, the concentrating comprises performing distillation in a distillation column.

[0052] Preferably, the top pressure in the distillation is 25-35 kPa, for example, 25 kPa, 27 kPa, 29 kPa, 31 kPa, 33 kPa or 35 kPa, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0053] Preferably, the number of theoretical plates in the distillation is 16 to 25, for example, 16, 18, 20, 22 or 25, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0054] Preferably, the reflux ratio in the distillation is 0.2-0.4, for example, it can be 0.2, 0.22, 0.24, 0.26, 0.28, 0.3, 0.32, 0.34, 0.36, 0.38 or 0.4, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0055] During the distillation process of the present invention, nitric acid with a concentration of 0.2-5% is extracted from the top of the tower, and the nitric acid can be used in the reaction process described in step (1).

[0056] As a preferred technical solution of the present invention, the method for recovering nitric acid from crude DNT provided by the present invention comprises the following steps:

[0057] (1) mixing crude DNT, a metal salt solution and an additive in a reaction kettle to obtain a mixed system with a nitric acid concentration of 2 to 25%, reacting at a temperature of 30 to 90° C. for 5 to 20 minutes, and then separating the two phases to obtain a first crude nitric acid product and a first nitration product;

[0058] The content of nitric acid in the crude DNT is 0.1-10 wt %; the concentration of sulfate in the metal salt solution is 5-10%; the content of metal elements in the mixed system is 200-1000 ppm;

[0059] (2) returning the first crude nitric acid obtained in step (1) to the reactor in step (1) for countercurrent washing 1 to 3 times, and obtaining crude nitric acid and washed DNT after two phase separation;

[0060] In the countercurrent washing, the volume ratio of the first crude nitric acid to the crude DNT is (0.5-2):1; the temperature of the countercurrent washing is 30-90° C., and the residence time is 5-20 min;

[0061] (3) ultrasonically treating the crude nitric acid obtained in step (2) to obtain a nitric acid solution;

[0062] The ultrasonic treatment has a frequency of 25 to 30 kHz, a residence time of 5 to 10 min, a temperature of 60 to 80° C., and a pressure of 50 to 150 kPaa.

[0063] (4) distilling the nitric acid solution obtained in step (3) in a distillation tower, and the concentrated nitric acid can be reused in the nitration reaction;

[0064] The number of theoretical plates in the distillation is 16 to 25, the tower top pressure is 25 to 35 kPa, and the reflux ratio is 0.2 to 0.4.

[0065] The numerical range described in the present invention not only includes the point values ​​listed above, but also includes any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0066] Compared with the prior art, the present invention has the following beneficial effects:

[0067] (1) The recovery method provided by the present invention recovers the nitric acid dissolved in the crude DNT, while achieving the harmless treatment of nitrophenols in the crude nitration product and the resource utilization of the metal salts produced by the waste acid concentration;

[0068] (2) The recovery method provided by the present invention significantly reduces the amount of nitrophenol-containing wastewater generated, thereby reducing the cost of wastewater treatment;

[0069] (3) The recycling method provided by the present invention is simple, safe and environmentally friendly. DETAILED DESCRIPTION

[0070] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0071] The raw materials involved in the present invention are crude dinitrotoluene (crude DNT) produced by Wanhua Chemical Group Co., Ltd. and metal salts produced in the waste acid concentration process.

[0072] The present invention adopts a Metron 905 Titrando potentiometric titrator to analyze the concentration of nitric acid, and adopts an Agilent Cary 60 ultraviolet visible spectrophotometer to analyze the nitrophenols.

[0073] Example 1

[0074] This embodiment provides a method for recovering nitric acid in crude DNT, the recovery method comprising the following steps:

[0075] (1) mixing 22.5 t / h of crude DNT, 66 kg / h of a metal salt solution, and 5 t / h of a low-concentration nitric acid having a concentration of 0.1% in a reactor to obtain a mixed system having a nitric acid concentration of 11.8%, reacting at a temperature of 69° C. for 8 min, and then performing two-phase separation to obtain a first crude nitric acid product and a first nitration product;

[0076] The content of nitric acid in the crude DNT is 3wt%, and the content of nitrocresol is 4200ppm; the concentration of sulfate in the metal salt solution is 5%; the content of metal elements in the mixed system is 200ppm;

[0077] (2) returning the first crude nitric acid obtained in step (1) to the reactor in step (1) for countercurrent washing once, and obtaining crude nitric acid and washed DNT after two phase separation;

[0078] The volume ratio of the first crude nitric acid to the crude DNT is 1:1; the temperature of the countercurrent washing is 69° C., and the residence time is 8 min;

[0079] (3) ultrasonically treating the crude nitric acid obtained in step (2) to obtain a nitric acid solution;

[0080] The ultrasonic treatment was carried out at a frequency of 25 kHz, a residence time of 7 min, a temperature of 70°C and a pressure of 50 kPaa.

[0081] (4) distilling the nitric acid solution obtained in step (3) in a distillation tower, and the concentrated nitric acid can be reused in the nitration reaction;

[0082] The feed plate is the 10th theoretical plate, the number of theoretical plates in the distillation is 20, the tower top pressure is 32 kPa, and the reflux ratio is 0.34.

[0083] The concentration of concentrated nitric acid after distillation in this embodiment is 44.4%, and the concentration of low-concentration nitric acid produced from the top of the tower is 0.1%;

[0084] The nitrocresol content in the washed DNT obtained after the two-phase separation in step (2) of this embodiment is reduced to 2000 ppm, and 52% of the nitrocresol is decomposed during the washing process.

[0085] Example 2

[0086] This embodiment provides a method for recovering nitric acid in crude DNT, the recovery method comprising the following steps:

[0087] (1) mixing 22.5 t / h of crude DNT, 260 kg / h of a metal salt solution and 7 t / h of 1% low-concentration nitric acid in a reactor to obtain a mixed system with a nitric acid concentration of 25%, reacting at 60° C. for 5 min, and then performing two-phase separation to obtain a first crude nitric acid product and a first nitrate;

[0088] The content of nitric acid in the crude DNT is 10wt%, and the content of nitrocresol is 4400ppm; the concentration of sulfate in the metal salt solution is 5%; the content of metal elements in the mixed system is 1000ppm;

[0089] (2) returning the first crude nitric acid obtained in step (1) to the reactor in step (1) for countercurrent washing three times, and obtaining crude nitric acid and washed DNT after two phase separation;

[0090] The volume ratio of the first crude nitric acid to crude DNT is 0.5:1; the temperature of the countercurrent washing is 65° C., and the residence time is 5 min;

[0091] (3) ultrasonically treating the crude nitric acid obtained in step (2) to obtain a nitric acid solution;

[0092] The ultrasonic treatment was carried out at a frequency of 30 kHz, a residence time of 5 min, a temperature of 65°C and a pressure of 150 kPaa.

[0093] (4) distilling the nitric acid solution obtained in step (3) in a distillation tower, and the concentrated nitric acid can be reused in the nitration reaction;

[0094] The feed plate is the 7th theoretical plate, the number of theoretical plates in the distillation is 25, the tower top pressure is 25 kPa, and the reflux ratio is 0.3.

[0095] In this embodiment, the concentration of concentrated nitric acid after distillation is 50%, and the concentration of low-concentration nitric acid produced from the top of the tower is 1%;

[0096] The nitrocresol content in the washed DNT obtained after the two-phase separation in step (2) of this embodiment was reduced to 1600 ppm, and 63.6% of the nitrocresol was decomposed during the washing process.

[0097] Example 3

[0098] This embodiment provides a method for recovering nitric acid in crude DNT, the recovery method comprising the following steps:

[0099] (1) mixing 22.5 t / h of crude DNT, 260 kg / h of a metal salt solution, and 7 t / h of a low-concentration nitric acid having a concentration of 2% in a reactor to obtain a mixed system having a nitric acid concentration of 10.2%, reacting at a temperature of 90° C. for 20 min, and then performing two-phase separation to obtain a first crude nitric acid product and a first nitration product;

[0100] The content of nitric acid in the crude DNT is 2wt%, and the content of nitrocresol is 3800ppm; the concentration of sulfate in the metal salt solution is 6%; the content of metal elements in the mixed system is 740ppm;

[0101] (2) returning the first crude nitric acid obtained in step (1) to the reactor in step (1) for countercurrent washing twice, and obtaining crude nitric acid and washed DNT after two phase separation;

[0102] The volume ratio of the first crude nitric acid to the crude DNT is 2:1; the temperature of the countercurrent washing is 90° C., and the residence time is 20 min;

[0103] (3) ultrasonically treating the crude nitric acid obtained in step (2) to obtain a nitric acid solution;

[0104] The ultrasonic treatment was carried out at a frequency of 25 kHz, a residence time of 10 min, a temperature of 80°C and a pressure of 75 kPaa.

[0105] (4) distilling the nitric acid solution obtained in step (3) in a distillation tower, and the concentrated nitric acid can be reused in the nitration reaction;

[0106] The feed plate is the 12th theoretical plate, the number of theoretical plates in the distillation is 16, the tower top pressure is 35 kPa, and the reflux ratio is 0.2.

[0107] In this embodiment, the concentration of concentrated nitric acid after distillation is 30%, and the concentration of low-concentration nitric acid produced from the top of the tower is 5%;

[0108] The nitrocresol content in the washed DNT obtained after the two-phase separation in step (2) of this embodiment is reduced to 1520 ppm, and 60% of the nitrocresol is decomposed during the washing process.

[0109] Example 4

[0110] This embodiment provides a method for recovering nitric acid in crude DNT, the recovery method comprising the following steps:

[0111] (1) mixing 22.5 t / h of crude DNT, 260 kg / h of a metal salt solution and 7 t / h of a low-concentration nitric acid with a concentration of 1.8% in a reactor to obtain a mixed system with a nitric acid concentration of 2%, reacting at a temperature of 30° C. for 10 min, and then performing two-phase separation to obtain a first crude nitric acid product and a first nitration product;

[0112] The content of nitric acid in the crude DNT is 0.1wt%, and the content of nitrocresol is 4500ppm; the concentration of sulfate in the metal salt solution is 8%; the content of metal elements in the mixed system is 665ppm;

[0113] (2) returning the first crude nitric acid obtained in step (1) to the reactor in step (1) for countercurrent washing 4 times, and obtaining crude nitric acid and washed DNT after two phase separation;

[0114] The volume ratio of the first crude nitric acid to the crude DNT is 1:1; the temperature of the countercurrent washing is 30° C., and the residence time is 10 min;

[0115] (3) ultrasonically treating the crude nitric acid obtained in step (2) to obtain a nitric acid solution;

[0116] The ultrasonic treatment was carried out at a frequency of 25 kHz, a residence time of 8 min, a temperature of 60°C and a pressure of 75 kPaa.

[0117] (4) distilling the nitric acid solution obtained in step (3) in a distillation tower, and the concentrated nitric acid can be reused in the nitration reaction;

[0118] The feed plate is the 10th theoretical plate, the number of theoretical plates in the distillation is 18, the tower top pressure is 30 kPa, and the reflux ratio is 0.4.

[0119] The concentration of concentrated nitric acid after distillation in this embodiment is 35%, and the concentration of low-concentration nitric acid produced from the top of the tower is 1.8%;

[0120] The nitrocresol content in the washed DNT obtained after the two-phase separation in step (2) of this embodiment was reduced to 2025 ppm, and 55% of the nitrocresol was decomposed during the washing process.

[0121] Example 5

[0122] This embodiment provides a method for recovering nitric acid from crude DNT. The difference between the recovery method and the embodiment 1 is that:

[0123] In this example, the metal element content in the mixed system in step (1) is changed to 1000 ppm.

[0124] By using the recovery method provided in this embodiment, the concentration of concentrated nitric acid after distillation is 44.4%, and the concentration of low-concentration nitric acid extracted from the top of the tower is 0.1%; the content of nitrocresol in the washed DNT obtained after the two-phase separation in step (2) is reduced to 1470 ppm, and 65% of nitrocresol is decomposed during the washing process.

[0125] Compared with Example 1, this example changes the amount of metal salt solution added, thereby changing the metal ion content in the mixed system. As the metal ion content increases, the catalytic effect increases.

[0126] Example 6

[0127] This embodiment provides a method for recovering nitric acid from crude DNT. The difference between the recovery method and the embodiment 1 is that:

[0128] In this example, the metal element content in the mixed system in step (1) is changed to 100 ppm.

[0129] By using the recovery method provided in this embodiment, the concentration of concentrated nitric acid after distillation is 44.4%, and the concentration of low-concentration nitric acid extracted from the top of the tower is 0.1%; the content of nitrocresol in the washed DNT obtained after the two-phase separation in step (2) is reduced to 3150ppm, and 25% nitrocresol is decomposed during the washing process.

[0130] Compared with Example 1, this example changes the amount of metal salt solution added, thereby changing the metal ion content in the mixed system. As the metal ion content decreases, the catalytic effect weakens and the decomposition rate of nitrocresol decreases significantly.

[0131] Example 7

[0132] This embodiment provides a method for recovering nitric acid from crude DNT. The difference between the recovery method and the embodiment 2 is that:

[0133] In this example, the metal element content in the mixed system in step (1) is changed to 1500 ppm.

[0134] By using the recovery method provided in this embodiment, the concentration of concentrated nitric acid after distillation is 50%, and the concentration of low-concentration nitric acid extracted from the top of the tower is 1%; the content of nitrocresol in the washed DNT obtained after the two-phase separation in step (2) is reduced to 1700 ppm, and 61.3% of nitrocresol is decomposed during the washing process.

[0135] Compared with Example 2, this example changes the amount of metal salt solution added, thereby changing the metal ion content in the mixed system. As the metal ion content increases, the catalytic effect does not increase significantly; however, excessive metal ion content will increase the metal content in the concentrated nitric acid, reducing its recycling value.

[0136] Example 8

[0137] This embodiment provides a method for recovering nitric acid from crude DNT. The difference between the recovery method and the embodiment 1 is that:

[0138] In step (1) of this embodiment, the amount of low-concentration nitric acid added is increased to change the nitric acid concentration in the mixed system to 30%.

[0139] By using the recovery method provided in this embodiment, the concentration of concentrated nitric acid after distillation is 44.4%, and the concentration of low-concentration nitric acid extracted from the top of the tower is 0.1%; the content of nitrocresol in the washed DNT obtained after the two-phase separation in step (2) is reduced to 2000 ppm, and 52% of nitrocresol is decomposed during the washing process.

[0140] Compared with Example 1, the present example changes the acidity in the mixed system. The increase in acidity increases the nitric acid content in the washed DNT from 50 ppm to 560 ppm, and the nitric acid removal effect in the crude DNT becomes poor.

[0141] Comparative Example 1

[0142] This comparative example provides a method for recovering nitric acid from crude DNT, and the difference between the recovery method and Example 1 is only that:

[0143] In this comparative example, the metal salt solution in step (1) is replaced with a sodium sulfate solution.

[0144] By using the recovery method provided in this comparative example, the concentration of concentrated nitric acid after distillation is 44.4%, and the concentration of low-concentration nitric acid extracted from the top of the tower is 0.1%; the content of nitrocresol in the washed DNT obtained after the two-phase separation in step (2) is reduced to 4100 ppm, and 2.4% nitrocresol is decomposed during the washing process.

[0145] Comparative Example 2

[0146] This comparative example provides a method for recovering nitric acid from crude DNT, and the difference between the recovery method and Example 1 is only that:

[0147] In this comparative example, the ultrasonic treatment described in step (3) was omitted.

[0148] By using the recovery method provided in this comparative example, the concentration of concentrated nitric acid after distillation is 44.4%, and the concentration of low-concentration nitric acid extracted from the top of the tower is 0.1%; the content of nitrocresol in the washed DNT obtained after the two-phase separation in step (2) is reduced to 2000ppm, and 52% of nitrocresol is decomposed during the washing process. However, the content of small molecular acids in the concentrated nitric acid after distillation increases from 105ppm to 1500ppm. Complete decomposition cannot be achieved.

[0149] In summary, the method for recovering nitric acid in crude DNT provided by the present invention not only recovers nitric acid in crude DNT, but also realizes the cracking of nitrobenzoic acid in crude DNT, thereby reducing the amount of red water generated in the subsequent washing process and reducing the cost of wastewater treatment.

[0150] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for recovering nitric acid from crude DNT, characterized in that: The recovery method comprises the following steps: (1) mixing crude DNT, a metal salt solution and an additive in a reactor to obtain a mixed system, and performing two-phase separation after the reaction to obtain a first crude nitric acid product and a first nitrate; The crude DNT has a nitric acid content of 0.1-10wt%, a nitrocresol content of 0.3-0.5wt%, and a sulfuric acid content of 0.05-0.2wt%; The metal salt solution contains sulfate with a concentration of 5-10wt%, and the sulfate is Fe2(SO4)3, NiSO4 and CrSO4; The additive is selected from water or low-concentration nitric acid with a concentration of 0.1 to 5 wt%; (2) returning the first crude nitric acid obtained in step (1) to the reactor in step (1) for countercurrent washing, and separating the two phases to obtain the crude nitric acid and washed DNT; (3) ultrasonically treating the crude nitric acid obtained in step (2) to obtain a nitric acid solution; The frequency of the ultrasonic treatment is 25-30 kHz, the residence time is 5-10 min, the temperature is 60-80° C., and the pressure is 50-150 kPaa; (4) distilling the nitric acid solution obtained in step (3) in a distillation tower, and the concentrated nitric acid can be reused in the nitration reaction; The tower top pressure in the distillation is 25-35 kPa, the number of theoretical plates is 16-25, and the reflux ratio is 0.2-0.

4.

2. The recycling method according to claim 1, characterized in that: The reactor in step (1) is a reaction kettle.

3. The recycling method according to claim 1, characterized in that: The concentration of sulfate in the metal salt solution is 6-8wt%.

4. The recycling method according to claim 1, characterized in that: The concentration of the low-concentration nitric acid is 0.1-2 wt %.

5. The recycling method according to claim 1, characterized in that: The concentration of nitric acid in the mixed system in step (1) is 2-25 wt%.

6. The recycling method according to claim 5, characterized in that: The concentration of nitric acid in the mixed system in step (1) is 5-20wt%.

7. The recycling method according to claim 1, characterized in that: The content of Fe, Cr and Ni elements in the mixed system of step (1) is 200-1000 ppm.

8. The recycling method according to claim 1, characterized in that: The reaction temperature in step (1) is 65-90°C.

9. The recycling method according to claim 8, characterized in that: The reaction temperature in step (1) is 65-80°C.

10. The recycling method according to claim 1, characterized in that: The reaction time of step (1) is 5 to 20 minutes.

11. The recycling method according to claim 10, characterized in that: The reaction time of step (1) is 8 to 15 minutes.

12. The recycling method according to claim 1, characterized in that: The two-phase separation in step (1) is selected from static separation or centrifugal separation.

13. The recycling method according to claim 1, characterized in that: The number of countercurrent washing is 1 to 4 times.

14. The recycling method according to claim 1, characterized in that: The volume ratio of the first crude nitric acid to the crude DNT in the countercurrent washing is (0.5-2):

1.

15. The recycling method according to claim 1, characterized in that: The temperature of the countercurrent washing is 30-90°C.

16. The recycling method according to claim 15, characterized in that: The temperature of the countercurrent washing is 50-80°C.

17. The recycling method according to claim 1, characterized in that: The residence time of the countercurrent washing is 5 to 20 minutes.

18. The recycling method according to claim 17, characterized in that: The residence time of the countercurrent washing is 8 to 15 minutes.

19. The recycling method according to any one of claims 1 to 18, characterized in that: The recovery method comprises the following steps: (1) mixing crude DNT, a metal salt solution and an additive in a reactor to obtain a mixed system with a nitric acid concentration of 2 to 25 wt%, reacting at a temperature of 30 to 90° C. for 5 to 20 minutes, and then separating the two phases to obtain a first crude nitric acid product and a first nitrate; The content of nitric acid in the crude DNT is 0.1-10wt%, the content of nitrocresols is 0.3-0.5wt%, and the content of sulfuric acid is 0.05-0.2wt%; the concentration of sulfate in the metal salt solution is 5-10%; the content of metal elements in the mixed system is 200-1000ppm; (2) returning the first crude nitric acid obtained in step (1) to the reactor in step (1) for countercurrent washing 1 to 3 times, and obtaining crude nitric acid and washed DNT after two phase separation; The volume ratio of the first crude nitric acid to the crude DNT in the countercurrent washing is (0.5-2):1; the temperature of the countercurrent washing is 30-90° C., and the residence time is 5-20 min; (3) ultrasonically treating the crude nitric acid obtained in step (2) to obtain a nitric acid solution; The ultrasonic treatment has a frequency of 25-30 kHz, a residence time of 5-10 min, a temperature of 60-80 °C, and a pressure of 50-150 kPaa. (4) distilling the nitric acid solution obtained in step (3) in a distillation tower, and the concentrated nitric acid can be reused in the nitration reaction; The number of theoretical plates in the distillation is 16 to 25, the top pressure is 25 to 35 kPa, and the reflux ratio is 0.2 to 0.4.