Green preparation of liquid phase sodium salicylate and associated high COD wastewater treatment method

Abstract

The present application provides a kind of green preparation of liquid phase sodium salicylate and associated high COD wastewater treatment method, belong to fine chemical product preparation and industrial wastewater resource processing technical field.The method obtains first sodium salicylate solution by reacting salicylanilide crystal with sodium hydroxide;High COD salicylanilide wastewater is removed toluene by negative pressure flash evaporation, after cryogenic enrichment, it is sent into reaction extractor, under alkaline condition, sodium hydroxide solution and toluene are added to carry out reaction extraction, salicylanilide is converted into sodium salicylate into aqueous phase, organic impurities migrate to toluene phase, and second sodium salicylate solution is obtained;Two solution is combined and adjusted to obtain liquid phase sodium salicylate product, impurity toluene phase is distilled to recover toluene for recycling;The present application integrates product liquid phase preparation and product recovery of salicylanilide in wastewater in the same process, realizes wastewater organic load source head reduction and closed circuit utilization of solvent and heat.

Description

Technical Field

[0001] This invention belongs to the technical field of fine chemical product preparation and industrial wastewater resource utilization treatment, and relates to a green preparation method of liquid-phase sodium salicylate and a method for the co-treatment of associated high COD wastewater. Background Technology

[0002] Salicylate (also known as 2-cyanophenol) is an important fine chemical intermediate widely used in pharmaceuticals, pesticides, dyes, and other fields. Currently, the mainstream industrial production route for salicylate is to use salicylamide as a raw material, undergoing a phosgene dehydration reaction to obtain salicylate, followed by neutralization, layering, and cooling crystallization to obtain the solid salicylate product. However, producing and selling salicylate in solid powder form has several drawbacks. First, solid salicylate powder has a strong, pungent odor, exposing operators to continuous odor exposure during crystallization, centrifugation, drying, and packaging. Second, solid salicylate is prone to oxidation and discoloration during storage and transportation, affecting the product's appearance. Furthermore, downstream users typically need to re-dissolve or prepare a solution before adding solid salicylate to the reaction, increasing the operational steps involved. To address these issues, existing technologies have proposed a device that directly adds sodium hydroxide solution to the crystallization vessel to convert salicylate into an aqueous sodium salicylate solution, improving the product's form to some extent.

[0003] Meanwhile, the production of salicylaniline inevitably generates high-COD organic wastewater. This wastewater contains a certain amount of salicylaniline, toluene, and other organic byproducts, resulting in a high organic load. Direct discharge into the end-of-pipe treatment system puts significant pressure on wastewater treatment facilities. Current treatment methods for this type of wastewater typically include cryogenic crystallization to recover solid salicylaniline, filtration separation, and returning the solid to the main production system for re-neutralization and crystallization. However, this route of "first precipitating salicylaniline from the wastewater into a solid, then returning the solid to the main process for dissolution and utilization" involves unnecessary phase change steps and a large circulating load. Furthermore, the toluene entrained in the high-COD wastewater easily forms a stable emulsion layer at low temperatures, interfering with subsequent solid-liquid separation and extraction phase separation operations. In addition, the wastewater itself usually carries a certain temperature; if cryogenic cooling is performed directly without pretreatment, the sensible heat carried is directly consumed during the cooling process, leading to high refrigeration energy consumption. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to provide a green preparation method for liquid-phase sodium salicylate and a method for the co-treatment of associated high-COD wastewater. The method includes: reacting salicylate crystals with an aqueous sodium hydroxide solution to obtain a first liquid-phase sodium salicylate solution; removing and recovering toluene from the high-COD wastewater containing salicylate generated during the production process via negative pressure flash evaporation, followed by cryogenic enrichment and feeding into a reaction extractor; adding sodium hydroxide solution and toluene under alkaline conditions for reaction extraction, converting the salicylate in the wastewater into sodium salicylate which enters the aqueous phase, while organic impurities migrate to the toluene phase, resulting in a second liquid-phase sodium salicylate solution; merging and adjusting the two solutions to obtain the finished liquid-phase sodium salicylate product; and recovering the toluene-containing phase by distillation for recycling. This invention integrates the liquid-phase preparation of the product with the product recovery of salicylate from the wastewater into the same process, achieving source reduction of organic load in wastewater and closed-loop utilization of solvents and heat, which helps reduce the pressure on end-of-pipe wastewater treatment and the material and energy consumption of the production process.

[0005] To achieve this objective, the present invention adopts the following technical solution:

[0006] This invention provides a green preparation method for sodium salicylate in liquid phase and a method for co-treatment of associated high-COD wastewater. The method includes:

[0007] S1: The salicylnitrile organic phase obtained after neutralization and separation during the industrial production of salicylnitrile is cooled and crystallized. After the toluene mother liquor above the crystal is extracted, toluene is added to the crystallization vessel for displacement and cleaning. The salicylnitrile crystal is reacted with sodium hydroxide aqueous solution, and the mixture is allowed to stand and separate into layers to obtain the first liquid phase sodium salicylnitrile solution.

[0008] S2: The high COD salicylates wastewater with high water content generated during the production process is subjected to negative pressure flash evaporation, so that the toluene in the wastewater is vaporized and condensed for recovery, and the flash evaporation wastewater is obtained.

[0009] S3: After flash evaporation, the wastewater is deeply enriched by cryogenic treatment and then sent to a reaction extractor. Sodium hydroxide solution and toluene are added at the same time to carry out reaction extraction, so that the salicylaniline in the wastewater is converted into sodium salicylaniline under alkaline conditions and enters the aqueous phase. Organic impurities migrate to the toluene phase. After separation, a second liquid phase of sodium salicylaniline solution and a toluene phase containing impurities are obtained.

[0010] S4: Combine the first liquid-phase sodium salicylate solution with the second liquid-phase sodium salicylate solution, adjust and obtain the finished liquid-phase sodium salicylate product; distill the impurity-containing toluene phase to recover toluene, and recycle it for S3 and / or S1.

[0011] The method is as follows: S1: The salicylate-containing organic phase obtained after neutralization and stratification during the industrial production of salicylate is cooled and crystallized to precipitate salicylate crystals; after extracting the toluene mother liquor above the crystals, toluene is added to the crystallization vessel for displacement cleaning, and the toluene mother liquor and cleaning toluene are collected and sent for distillation recovery or returned to the toluene storage tank; then sodium hydroxide aqueous solution is directly added to the crystallization vessel for reaction; after the reaction is completed, the mixture is allowed to stand and separate into layers, the upper residual toluene phase is extracted and sent to the solvent recovery system, and the lower aqueous phase is the first liquid phase sodium salicylate solution;

[0012] S2: The high COD salicylates wastewater generated during the production process is first subjected to negative pressure flash evaporation pretreatment, so that the toluene in the wastewater is preferentially vaporized and condensed for recovery, resulting in recovered toluene and flash-evaporated wastewater; the heat released by the flash steam during the condensation process is recovered through a heat exchanger and used to preheat the sodium hydroxide solution used in the subsequent reaction extraction.

[0013] S3: The flash-evaporated wastewater is further cooled and cryogenically enriched. The cryogenically enriched wastewater is then sent to a reaction extractor. Sodium hydroxide solution and recovered toluene are added simultaneously to carry out the extraction reaction. After standing and separating, a second liquid phase of sodium salicylate solution and a toluene phase containing impurities are obtained.

[0014] S4: After confirming the quality of the second liquid-phase sodium salicylate solution obtained in S3, it is combined with the first liquid-phase sodium salicylate solution. After adjusting the concentration and free alkali content, the finished liquid-phase sodium salicylate product is obtained. At the same time, the toluene phase containing impurities is distilled and recovered. The purity of toluene at the top of the column is controlled to be no less than 98%, and it is returned to S3 as a reaction extractant and / or returned to S1 as toluene for crystallization cleaning. The residual liquid at the bottom of the column is sent to the post-treatment system.

[0015] As a preferred technical solution of the present invention, in step S1, the number of replacement cleaning cycles is 1-2 times.

[0016] In some optional embodiments, the concentration of the sodium hydroxide aqueous solution is 10-15 wt.%, for example, it can be 10 wt.%, 10.5 wt.%, 11 wt.%, 11.5 wt.%, 12 wt.%, 12.5 wt.%, 13 wt.%, 13.5 wt.%, 14 wt.%, 14.5 wt.%, or 15 wt.%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0017] In some optional embodiments, the molar ratio of the salicylonitrile crystals to sodium hydroxide is 1:(1-1.2), for example, it can be 1:1.00, 1:1.02, 1:1.04, 1:1.06, 1:1.08, 1:1.10, 1:1.12, 1:1.14, 1:1.16, 1:1.18 or 1:1.20, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0018] In some alternative embodiments, the reaction temperature is 20-45°C, for example, 20.0°C, 22.5°C, 25.0°C, 27.5°C, 30.0°C, 32.5°C, 35.0°C, 37.5°C, 40.0°C, 42.5°C, or 45.0°C, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0019] In some alternative embodiments, the reaction time is 0.5-2 hours, for example, 0.50 hours, 0.65 hours, 0.80 hours, 0.95 hours, 1.10 hours, 1.25 hours, 1.40 hours, 1.55 hours, 1.70 hours, 1.85 hours, or 2.00 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0020] As a preferred technical solution of the present invention, in step S2, the feed temperature of the wastewater is 65-75℃, for example, it can be 65℃, 66℃, 67℃, 68℃, 69℃, 70℃, 71℃, 72℃, 73℃, 74℃ or 75℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0021] In some alternative embodiments, the flash pressure is 20-40 kPa, for example, 20 kPa, 22 kPa, 24 kPa, 26 kPa, 28 kPa, 30 kPa, 32 kPa, 34 kPa, 36 kPa, 38 kPa or 40 kPa, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0022] In some optional embodiments, the temperature of the flash-evaporated wastewater is 35-45°C, for example, it can be 35°C, 36°C, 37°C, 38°C, 39°C, 40°C, 41°C, 42°C, 43°C, 44°C or 45°C, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0023] In some optional embodiments, the temperature of the preheated sodium hydroxide solution is 30-40°C, for example, 30°C, 31°C, 32°C, 33°C, 34°C, 35°C, 36°C, 37°C, 38°C, 39°C or 40°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0024] As a preferred technical solution of the present invention, in step S3, the temperature of the cryogenic enrichment is 0-5℃, for example, it can be 0.0℃, 0.5℃, 1.0℃, 1.5℃, 2.0℃, 2.5℃, 3.0℃, 3.5℃, 4.0℃, 4.5℃ or 5.0℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] In some optional embodiments, the concentration of the sodium hydroxide solution is 8-15 wt%, for example, it may be 8.0 wt%, 8.7 wt%, 9.4 wt%, 10.1 wt%, 10.8 wt%, 11.5 wt%, 12.2 wt%, 12.9 wt%, 13.6 wt%, 14.3 wt%, or 15.0 wt%, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0026] In some alternative embodiments, the pH of the reaction extraction system is 9.5-10.5, for example, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0, 10.1, 10.2, 10.3, 10.4 or 10.5, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0027] In some optional embodiments, the mass ratio of toluene to enriched wastewater is (0.3-1):1, for example, it can be 0.30:1, 0.37:1, 0.44:1, 0.51:1, 0.58:1, 0.65:1, 0.72:1, 0.79:1, 0.86:1, 0.93:1 or 1.00:1, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0028] In some alternative embodiments, the temperature of the reaction extraction is 5-15°C, for example, 5°C, 6°C, 7°C, 8°C, 9°C, 10°C, 11°C, 12°C, 13°C, 14°C or 15°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0029] In some optional embodiments, the reaction extraction time is 10-120 min, for example, 10 min, 21 min, 32 min, 43 min, 54 min, 65 min, 76 min, 87 min, 98 min, 109 min or 120 min, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0030] As a preferred technical solution of the present invention, in step S4, the purity of the toluene at the top of the column is not less than 98%, and it is returned to S3 as a reaction extractant and / or returned to S1 as toluene for crystallization cleaning.

[0031] In some optional embodiments, in step S4, the concentration and free alkali content of the combined liquid-phase sodium salicylate solution are adjusted to meet the product specifications; preferably, the free alkali content is adjusted to a range that can maintain the sodium salicylate in a stable ionized state during storage and does not significantly affect downstream use; more preferably, the mass fraction of free sodium hydroxide in the finished liquid-phase sodium salicylate product is 0.1-5 wt%, more preferably 0.5-3 wt%.

[0032] Salicylic nitrile, also known as 2-hydroxybenzonitrile, contains a phenolic hydroxyl group directly attached to a benzene ring in its molecular structure. This phenolic hydroxyl group is weakly acidic and can undergo an acid-base neutralization reaction with sodium hydroxide to form the corresponding sodium salt—sodium salicylic nitrile. Compared to the parent compound salicylic nitrile, sodium salicylic nitrile is an ionic compound with higher water solubility, allowing it to dissolve stably in the aqueous phase at higher concentrations. Furthermore, because the phenolic hydroxyl group is converted to a phenol oxide anion and no longer exists as a free phenol, this structural form of salicylic nitrile exhibits higher chemical stability, with a reduced tendency for oxidation and discoloration under normal storage conditions. In addition, the volatile surface area of ​​the liquid product is much smaller than that of the solid powder, resulting in a correspondingly reduced diffusion of pungent odors.

[0033] In section S1, after the salicylonitrile crystals obtained from crystallization are extracted with toluene mother liquor and washed by displacement, the toluene content remaining on the crystal surface and between crystals is significantly reduced. At this point, an aqueous sodium hydroxide solution is directly added to the crystallization vessel. The salicylonitrile crystals dissolve in the aqueous phase and undergo an acid-base reaction with sodium hydroxide to form sodium salicylonitrile, which dissolves in the aqueous phase. Since the phenolic hydroxyl groups of salicylonitrile are weakly acidic, using a slightly excess of sodium hydroxide helps to fully promote the reaction towards salt formation. Simultaneously, a small excess of alkali maintains a weakly alkaline environment in the product solution, which is beneficial to the stability of sodium salicylonitrile in solution. The reaction temperature is selected at 20-45℃. This temperature range ensures sufficient dissolution rate and reactivity of salicylonitrile in water while avoiding potential hydrolysis side reactions of the nitrile group at excessively high temperatures. The reaction time is set at 0.5-2 hours to ensure complete dissolution of the crystals and near-complete salt formation. After the reaction, a small amount of toluene that may remain in the system will separate into layers on the upper layer after settling, as it is immiscible with the sodium salicylonitrile aqueous solution. This layer can be extracted and sent to the solvent recovery system. The lower aqueous phase is the first liquid phase, sodium salicylate solution.

[0034] The toluene displacement cleaning of salicylnitrile crystals before adding sodium hydroxide aqueous solution has practical technological significance. After cooling crystallization, mother liquor components usually remain on the crystal surface and between crystals, which may contain condensation byproducts and other organic impurities. By extracting the original toluene mother liquor and adding fresh toluene for 1-2 displacement cleaning cycles, impurities adhering to the crystal surface can be dissolved and carried away by the cleaning solution, utilizing toluene's excellent dissolving ability for non-polar impurities, without changing the product crystallization conditions. The cleaned crystals produce a higher purity product solution in the subsequent alkaline reaction stage, which is beneficial to the stability of the final product quality.

[0035] Furthermore, the product liquefaction operation in section S1 not only adjusts the product form but also provides a unified product form interface for the sodium salicylate product liquid from the wastewater recovery line in section S3. Since both the main product line and the wastewater recovery line ultimately output sodium salicylate aqueous solution, they can be directly merged in section S4 without undergoing an additional solid-liquid conversion step.

[0036] High-COD wastewater generated during salicylates production typically contains dissolved and entrained toluene. Toluene's boiling point at atmospheric pressure is approximately 110.6℃, but its apparent boiling point decreases significantly under reduced pressure. When the flash pressure is reduced to the range of 20-40 kPa, the boiling point of toluene can drop significantly below the actual temperature of the wastewater. When high-COD wastewater at 65-75℃ is fed into a negative-pressure flash tank, the toluene in the wastewater is in a superheated state under low pressure, allowing it to rapidly vaporize and separate from the liquid phase as steam. Simultaneously, the saturated vapor pressure of water remains much lower than that of toluene under the same temperature and pressure conditions, and water has a higher latent heat of vaporization. Therefore, under the selected operating conditions, the vaporization of toluene exhibits significant selectivity, and the evaporation loss of water is relatively limited.

[0037] The flash evaporation process can be considered thermodynamically as an approximately adiabatic flash evaporation process: the latent heat required for the vaporization of toluene in the wastewater is mainly provided by the sensible heat of the wastewater itself, thus the temperature of the wastewater naturally decreases after flash evaporation. By controlling the wastewater temperature after flash evaporation within the range of 35-45℃, the toluene content in the wastewater can be significantly reduced, and the wastewater temperature can also be reduced from the original 65-75℃ to a mesophilic level.

[0038] The pre-treatment desolventizing section has multiple synergistic effects on subsequent sections. Firstly, the pre-removal of toluene eliminates the material basis for the formation of a stable emulsion layer in the toluene-water system under low-temperature conditions during the subsequent cryogenic stage, which is beneficial for improving the clarity of the phase interface in subsequent liquid-liquid separation. Secondly, the reduction in wastewater temperature means that the subsequent cryogenic system only needs to handle the remaining cooling load from the intermediate temperature to the target low temperature, reducing refrigeration energy consumption compared to directly cryogenically cooling high-temperature wastewater. Furthermore, the latent heat of vaporization released when toluene vapor condenses in the condenser is a moderately low-temperature heat source. This heat can be recovered through a heat exchanger and used to preheat the sodium hydroxide solution used in section S3 to 30-40°C, enabling the cascade utilization of heat within the process. The toluene recovered after condensation has high purity and can be directly reused in the subsequent reaction extraction section or the upstream crystallization and cleaning stage.

[0039] The toluene content in the wastewater after negative pressure flash evaporation and desolvation has been significantly reduced, but it still contains dissolved salicylnitrile and other organic impurities. In the S3 section, the flash-evaporated wastewater is first subjected to cryogenic treatment, lowering its temperature to 0-5℃. The solubility of salicylnitrile in water decreases with decreasing temperature; cryogenic treatment can bring the concentration of salicylnitrile in the wastewater close to or reach a supersaturated state, thereby achieving enrichment of salicylnitrile in the liquid phase. This enrichment effect is beneficial to improving the conversion and recovery efficiency of salicylnitrile in subsequent reaction extraction steps.

[0040] The wastewater after cryogenic enrichment is not subjected to solid-liquid separation, but is directly fed into a reactive extractor. This process design avoids the multi-step operation of "crystallization-filtration-return to the main system for dissolution" in the traditional route. In the reactive extractor, two streams of sodium hydroxide solution and toluene are added to the system simultaneously, serving as the reaction reagent and the extraction organic phase, respectively.

[0041] The selective separation mechanism of reactive extraction is based on the acid-base properties of salicylonitrile molecules and the differences in the partitioning behavior of each component between the two phases. The phenolic hydroxyl group in the salicylonitrile molecule possesses dissociable weak acidity, and its acid dissociation constant causes it to exist primarily in molecular form under neutral to weakly acidic conditions. It has limited solubility in water and exhibits some lipophilicity. When the pH of the system is adjusted to the range of 9.5-10.5, the proton on the phenolic hydroxyl group of the salicylonitrile is removed by the base, and the molecule is converted into sodium salicylonitrile ions. This ionization process fundamentally changes the hydrophilicity of the molecule: from the limited water solubility of the neutral molecular state to the high water solubility of the ionic state. Therefore, sodium salicylonitrile tends to remain stably in the aqueous phase.

[0042] Meanwhile, condensation byproducts, residual neutral organic matter, and trace amounts of toluene not completely removed by flash evaporation in the wastewater, because their molecular structures do not contain acidic groups capable of dissociation within the specified pH range, remain in a neutral molecular state under alkaline conditions. These neutral organic substances tend to partition into the organic phase in the water-toluene two-phase system. Toluene, as a non-polar organic solvent, exhibits good similarity solubility with these neutral impurities, effectively extracting them from the aqueous phase and enriching them in the organic phase. Some weakly polar byproducts, such as salicylamide, also partition into the organic phase, further reducing the impurity content in the aqueous phase.

[0043] The choice of pH range has a significant impact on the selectivity of the reaction extraction and the quality of the product. If the system pH is too low, the deprotonation of the phenolic hydroxyl group of salicylonitrile is insufficient, and a considerable proportion of molecular salicylonitrile will still tend to enter the organic phase, leading to a decrease in the recovery rate of sodium salicylonitrile in the aqueous product solution. If the system pH is too high, in addition to increasing the consumption of alkali, the excessively alkaline environment may promote the hydrolysis of nitrile groups, generating byproducts such as salicylamide or sodium salicylate, affecting product purity. Furthermore, excessively high pH may also cause some weakly acidic impurities that should enter the organic phase to dissociate and remain in the aqueous phase, reducing the selectivity and purity of the product solution. Therefore, controlling the pH of the reaction extraction system in the range of 9.5-10.5 is an appropriate choice between ensuring sufficient conversion of salicylonitrile to sodium salt form, maintaining a high aqueous phase recovery rate, and controlling the degree of side reactions.

[0044] The sodium hydroxide solution concentration is selected as 8-15 wt.%, which provides sufficient alkalinity to maintain the target pH without triggering undesirable side reactions such as nitrile hydrolysis due to excessively high local alkalinity. The mass ratio of toluene to enriched wastewater is controlled within the range of (0.3-1):1. This ensures sufficient organic phase volume for thorough impurity extraction while avoiding excessive toluene consumption and unnecessary increases in subsequent distillation load due to an excessively high solvent ratio. The reaction extraction temperature is set at 5-15℃. This is to connect with the discharge temperature of the cryogenic enrichment section, reducing unnecessary heating operations. Furthermore, the appropriate low temperature helps reduce the partition coefficient of salicylaniline in the organic phase, causing the ionized sodium salicylaniline to remain more likely in the aqueous phase, thus increasing the enrichment degree of the target product in the aqueous product solution. The reaction extraction time is set at 10-120 min to ensure that the acid-base reaction and liquid-liquid mass transfer processes tend to reach equilibrium under different operating scales and equipment types.

[0045] After settling and stratification, the aqueous phase is the second liquid product rich in sodium salicylate, which can be directly combined with the first liquid sodium salicylate solution obtained from section S1. The organic phase is a toluene phase containing impurities, which is sent for distillation and recovery. After the remaining aqueous phase with low organic content is removed by the target organic matter and major impurities during the reaction extraction process, the COD load is reduced compared to the original high COD wastewater, and it can be discharged into the end-of-pipe wastewater treatment system for subsequent treatment to meet standards.

[0046] In section S4, the first liquid-phase sodium salicylate solution from section S1 and the second liquid-phase sodium salicylate solution from section S3 are combined. The concentration and free alkali content are adjusted to meet the product specifications, yielding the finished liquid-phase sodium salicylate. Concentration adjustment ensures the effective sodium salicylate content in the product solution is within the target range, while free alkali content adjustment maintains a suitable alkalinity environment, ensuring the sodium salicylate remains in a stable ionized state during storage and preventing excessive free alkali from affecting downstream applications.

[0047] The toluene phase containing impurities obtained after stratification in section S3 is distilled to recover high-purity toluene at the top of the column. The purity of the recovered toluene is controlled to be no less than 98%, meeting the quality requirements for use as a solvent for reaction extraction of organic phases and crystallization cleaning. It can be recycled back to section S3 and / or section S1. The small amount of high-boiling-point impurities remaining at the bottom of the column are sent to the appropriate post-treatment system for disposal according to their properties and hazardous waste management requirements. Through the closed-loop circulation of toluene, the solvent is shared between the product preparation line and the wastewater treatment line, reducing the amount of fresh toluene replenished.

[0048] Throughout the entire process, toluene not only serves as the mother liquor for crystallization and displacement cleaning in section S1, and as the organic phase for reaction extraction in section S3, but also acts as a priority target for removal and recovery from wastewater in section S2. The toluene used in each section is collected in the distillation recovery system, purified, and redistributed to various points of use, forming a closed-loop solvent circulation throughout the entire process. Simultaneously, the flash condensation heat recovery in section S2 is used to preheat the alkali solution in section S3, achieving thermal coupling between process units. This interconnection of material and energy flows means that each section no longer operates as an isolated unit, but rather constitutes a holistic process with intrinsic connections at the material and energy levels.

[0049] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0050] This invention directly reacts salicylonitrile with sodium hydroxide to prepare a liquid-phase sodium salicylonitrile solution, avoiding the odor exposure and oxidation discoloration problems associated with solid powder form, while also providing convenience for downstream feedstock use. Simultaneously, the sodium salicylonitrile product liquid from the wastewater treatment line can be directly incorporated into the same product system, achieving unified output of product preparation and wastewater recovery at the liquid phase stage.

[0051] This invention utilizes a negative-pressure flash evaporation pre-desolventization section to preferentially vaporize and remove toluene from high-COD wastewater under reduced pressure, leveraging the wastewater's inherent process heat. This achieves efficient toluene recovery while simultaneously lowering the wastewater temperature and reducing the cooling load on subsequent cryogenic systems. The latent heat released during the flash steam condensation process is further recovered and utilized via a heat exchanger to preheat the alkali solution used in subsequent stages, thus realizing the cascade utilization of heat within the process.

[0052] This invention employs alkaline reaction extraction to treat cryogenically enriched wastewater. It utilizes the chemical property that the phenolic hydroxyl groups in salicylnitrile molecules selectively deprotonate under alkaline conditions to generate water-soluble sodium salts, thereby achieving the directional transfer of the target product to the aqueous phase. Simultaneously, the toluene organic phase is used to extract neutral impurities, enabling the wastewater treatment process to simultaneously complete both the productization of the target substance and the separation of impurities, avoiding the multi-step phase transition cycle of "crystallization-filtration-reflux-redissolution" in traditional routes.

[0053] In this invention, toluene is used as both the organic phase in the reaction extraction and the solvent for crystallization cleaning, and is recycled between different stages of the process. After distillation and recovery, it is returned to the reaction extraction section and the product preparation section, respectively, forming a closed-loop solvent cycle. The overall process integrates product preparation, wastewater resource utilization, and solvent recovery into a unified flow, which helps reduce the organic load of the end-of-pipe wastewater treatment system and the overall operating cost. Detailed Implementation

[0054] The technical solution of the present invention will be described in detail below with reference to specific embodiments. The embodiments described herein are specific implementations of the present invention and are used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary and should not be construed as limiting the implementation of the present invention or the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can also adopt other obvious technical solutions based on the content disclosed in the claims and the specification of this application. These technical solutions include technical solutions that employ any obvious substitutions and modifications made to the embodiments described herein.

[0055] The chemical reagents used in the embodiments and comparative examples of this invention are all commercially available products and have not undergone further purification or processing.

[0056] Example 1

[0057] This embodiment provides a green preparation method for sodium salicylate in liquid phase and a method for co-treatment of associated high-COD wastewater. The specific steps of the green preparation method for sodium salicylate in liquid phase and the method for co-treatment of associated high-COD wastewater are as follows:

[0058] S1: The organic phase containing salicylate obtained after neutralization and separation is cooled and crystallized to precipitate salicylate crystals; after the toluene mother liquor above the crystals is extracted, toluene is added to the crystallization vessel for two displacement cleanings. The toluene mother liquor and the cleaning toluene are collected and sent for distillation recovery or returned to the toluene storage tank; then, a 10 wt.% sodium hydroxide aqueous solution is directly added to the crystallization vessel to make the molar ratio of salicylate to sodium hydroxide 1:1.1, and the reaction is stirred at 30°C for 1.5 h; after the reaction is completed, the mixture is allowed to stand and separate into layers. The upper residual toluene phase is extracted and sent to the solvent recovery system, and the lower aqueous phase is the first liquid phase sodium salicylate solution.

[0059] S2: The high-COD salicylates and acrylonitrile wastewater generated during the production process is first subjected to negative pressure flash evaporation pretreatment. The wastewater feed temperature is controlled at 70℃ and the flash evaporation pressure is 35kPa, so that the toluene in the wastewater is preferentially vaporized and condensed for recovery, resulting in recovered toluene and flash-evaporated wastewater. The temperature of the flash-evaporated wastewater is controlled at 40℃. The heat released by the flash steam during the condensation process is recovered through a heat exchanger and used to preheat the sodium hydroxide solution used in the subsequent reaction extraction to 38℃.

[0060] S3: The flash-evaporated wastewater is further cooled to 2℃ for cryogenic enrichment. The cryogenically enriched wastewater is then fed into a reaction extractor. Simultaneously, sodium hydroxide solution and recovered toluene are added, wherein the sodium hydroxide solution concentration is 12wt%, the system pH is controlled at 10.2, the mass ratio of toluene to enriched wastewater is 0.8:1, the reaction extraction temperature is 12℃, and the reaction extraction time is 100min. After standing and separating, a second liquid phase of sodium salicylate solution and a toluene phase containing impurities are obtained.

[0061] S4: After confirming the quality of the second liquid-phase sodium salicylate solution obtained in S3, it is combined with the first liquid-phase sodium salicylate solution. After adjusting the concentration and free alkali content, the finished liquid-phase sodium salicylate product is obtained. At the same time, the toluene phase containing impurities is distilled and recovered. The purity of toluene at the top of the column is controlled to be no less than 98%, and it is returned to S3 as a reaction extractant and / or returned to S1 as toluene for crystallization cleaning. The residual liquid at the bottom of the column is sent to the post-treatment system.

[0062] Example 2

[0063] This embodiment provides a green preparation method for sodium salicylate in liquid phase and a method for co-treatment of associated high-COD wastewater. The specific steps of the green preparation method for sodium salicylate in liquid phase and the method for co-treatment of associated high-COD wastewater are as follows:

[0064] S1: The organic phase containing salicylate obtained after neutralization and separation is cooled and crystallized to precipitate salicylate crystals; after extracting the toluene mother liquor above the crystals, toluene is added to the crystallization vessel for one displacement cleaning; the toluene mother liquor and the cleaning toluene are collected and sent for distillation recovery or returned to the toluene storage tank; then, a 15 wt.% sodium hydroxide aqueous solution is directly added to the crystallization vessel to make the molar ratio of salicylate to sodium hydroxide 1:1, and the reaction is stirred at 45°C for 0.5 h; after the reaction is completed, the mixture is allowed to stand and separate into layers, the upper residual toluene phase is extracted and sent to the solvent recovery system, and the lower aqueous phase is the first liquid phase sodium salicylate solution;

[0065] S2: The high-COD salicylates and acrylonitrile wastewater generated during the production process is first subjected to negative pressure flash evaporation pretreatment. The wastewater feed temperature is controlled at 65℃ and the flash evaporation pressure is 20kPa, so that the toluene in the wastewater is preferentially vaporized and condensed for recovery, resulting in recovered toluene and flash-evaporated wastewater. The temperature of the flash-evaporated wastewater is controlled at 35℃. The heat released by the flash steam during the condensation process is recovered through a heat exchanger and used to preheat the sodium hydroxide solution used in the subsequent reaction extraction to 30℃.

[0066] S3: The flash-evaporated wastewater is further cooled to 5°C for cryogenic enrichment. The cryogenically enriched wastewater is then fed into a reaction extractor. Simultaneously, sodium hydroxide solution and recovered toluene are added, wherein the sodium hydroxide solution concentration is 8wt%, the system pH is controlled at 9.5, the mass ratio of toluene to enriched wastewater is 0.3:1, the reaction extraction temperature is 5°C, and the reaction extraction time is 10 min. After standing and separating, a second liquid phase of sodium salicylate solution and a toluene phase containing impurities are obtained.

[0067] S4: After confirming the quality of the second liquid-phase sodium salicylate solution obtained in S3, it is combined with the first liquid-phase sodium salicylate solution. After adjusting the concentration and free alkali content, the finished liquid-phase sodium salicylate product is obtained. At the same time, the toluene phase containing impurities is distilled and recovered. The purity of toluene at the top of the column is controlled to be no less than 98%, and it is returned to S3 as a reaction extractant and / or returned to S1 as toluene for crystallization cleaning. The residual liquid at the bottom of the column is sent to the post-treatment system.

[0068] Example 3

[0069] This embodiment provides a green preparation method for sodium salicylate in liquid phase and a method for co-treatment of associated high-COD wastewater. The specific steps of the green preparation method for sodium salicylate in liquid phase and the method for co-treatment of associated high-COD wastewater are as follows:

[0070] S1: The organic phase containing salicylate obtained after neutralization and separation is cooled and crystallized to precipitate salicylate crystals; after extracting the toluene mother liquor above the crystals, toluene is added to the crystallization vessel for one displacement cleaning; the toluene mother liquor and the cleaning toluene are collected and sent for distillation recovery or returned to the toluene storage tank; then, a 12wt.% sodium hydroxide aqueous solution is directly added to the crystallization vessel to make the molar ratio of salicylate to sodium hydroxide 1:1.15, and the reaction is stirred at 25°C for 1 hour; after the reaction is completed, the mixture is allowed to stand and separate into layers; the upper residual toluene phase is extracted and sent to the solvent recovery system, and the lower aqueous phase is the first liquid phase sodium salicylate solution;

[0071] S2: The high-COD salicylates and acrylonitrile wastewater generated during the production process is first subjected to negative pressure flash evaporation pretreatment. The wastewater feed temperature is controlled at 72℃ and the flash evaporation pressure is 25kPa, so that the toluene in the wastewater is preferentially vaporized and condensed for recovery, resulting in recovered toluene and flash-evaporated wastewater. The temperature of the flash-evaporated wastewater is controlled at 42℃. The heat released by the flash steam during the condensation process is recovered through a heat exchanger and used to preheat the sodium hydroxide solution used in the subsequent reaction extraction to 32℃.

[0072] S3: The flash-evaporated wastewater is further cooled to 4°C for cryogenic enrichment. The cryogenically enriched wastewater is then fed into a reaction extractor. Simultaneously, sodium hydroxide solution and recovered toluene are added, wherein the sodium hydroxide solution concentration is 10wt%, the system pH is controlled at 9.8, the mass ratio of toluene to enriched wastewater is 0.5:1, the reaction extraction temperature is 8°C, and the reaction extraction time is 60 min. After standing and separating, a second liquid phase of sodium salicylate solution and a toluene phase containing impurities are obtained.

[0073] S4: After confirming the quality of the second liquid-phase sodium salicylate solution obtained in S3, it is combined with the first liquid-phase sodium salicylate solution. After adjusting the concentration and free alkali content, the finished liquid-phase sodium salicylate product is obtained. At the same time, the toluene phase containing impurities is distilled and recovered. The purity of toluene at the top of the column is controlled to be no less than 98%, and it is returned to S3 as a reaction extractant and / or returned to S1 as toluene for crystallization cleaning. The residual liquid at the bottom of the column is sent to the post-treatment system.

[0074] Example 4

[0075] This embodiment provides a green preparation method for sodium salicylate in liquid phase and a method for co-treatment of associated high-COD wastewater. The specific steps of the green preparation method for sodium salicylate in liquid phase and the method for co-treatment of associated high-COD wastewater are as follows:

[0076] S1: The organic phase containing salicylate obtained after neutralization and separation is cooled and crystallized to precipitate salicylate crystals; after the toluene mother liquor above the crystals is extracted, toluene is added to the crystallization vessel for two displacement cleanings. The toluene mother liquor and the cleaning toluene are collected and sent for distillation recovery or returned to the toluene storage tank; then, a sodium hydroxide aqueous solution with a concentration of 14 wt.% is directly added to the crystallization vessel to make the molar ratio of salicylate to sodium hydroxide 1:1.2, and the reaction is stirred at 20°C for 2 hours; after the reaction is completed, the mixture is allowed to stand and separate into layers. The upper residual toluene phase is extracted and sent to the solvent recovery system, and the lower aqueous phase is the first liquid phase sodium salicylate solution.

[0077] S2: The high-COD salicylates and acrylonitrile wastewater generated during the production process is first subjected to negative pressure flash evaporation pretreatment. The wastewater feed temperature is controlled at 75℃ and the flash evaporation pressure is 40kPa, so that the toluene in the wastewater is preferentially vaporized and condensed for recovery, resulting in recovered toluene and flash-evaporated wastewater; the temperature of the flash-evaporated wastewater is controlled at 45℃; the heat released by the flash steam during the condensation process is recovered through a heat exchanger and used to preheat the sodium hydroxide solution used for subsequent reaction extraction to 40℃;

[0078] S3: The flash-evaporated wastewater is further cooled to 0℃ for cryogenic enrichment. The cryogenically enriched wastewater is then fed into a reaction extractor. Simultaneously, sodium hydroxide solution and recovered toluene are added, wherein the sodium hydroxide solution concentration is 15wt%, the system pH is controlled at 10.5, the mass ratio of toluene to enriched wastewater is 1:1, the reaction extraction temperature is 15℃, and the reaction extraction time is 120min. After standing and separating, a second liquid phase of sodium salicylate solution and a toluene phase containing impurities are obtained.

[0079] S4: After confirming the quality of the second liquid-phase sodium salicylate solution obtained in S3, it is combined with the first liquid-phase sodium salicylate solution. After adjusting the concentration and free alkali content, the finished liquid-phase sodium salicylate product is obtained. At the same time, the toluene phase containing impurities is distilled and recovered. The purity of toluene at the top of the column is controlled to be no less than 98%, and it is returned to S3 as a reaction extractant and / or returned to S1 as toluene for crystallization cleaning. The residual liquid at the bottom of the column is sent to the post-treatment system.

[0080] Comparative Example 1

[0081] This comparative example provides a green preparation method for liquid-phase sodium salicylate and a method for the co-treatment of associated high-COD wastewater. The difference from Example 1 is that negative pressure flash evaporation is not performed, and the high-COD salicylate wastewater generated during the production process is directly sent to the cryogenic section. Furthermore, flash evaporation condensation heat recovery is not performed to preheat the sodium hydroxide solution. Other operating steps and process parameters are exactly the same as in Example 1.

[0082] Comparative Example 2

[0083] This comparative example provides a green preparation method for liquid-phase sodium salicylate and a method for the co-treatment of associated high-COD wastewater. The difference from Example 1 is that in S3, after the flash-evaporated wastewater is cooled to 2°C for cryogenic crystallization, no reaction extraction is performed. Instead, the suspension after crystallization is filtered to obtain wet salicylate and mother liquor. The wet salicylate is returned to the crystallization kettle and added to the sodium hydroxide aqueous solution along with the next batch of salicylate crystals according to the operation in S1 to prepare sodium salicylate solution. The mother liquor is discharged into the terminal wastewater treatment system. Other operation steps and process parameters are exactly the same as in Example 1.

[0084] Comparative Example 3

[0085] This comparative example provides a green preparation method for liquid-phase sodium salicylate and a method for the co-treatment of associated high COD wastewater. The difference from Example 1 is that cryogenic enrichment is not performed in S3. Instead, the flash-evaporated wastewater (temperature about 40°C) is directly fed into the reaction extractor for reaction extraction at a temperature of 40°C. Other operating steps and process parameters are exactly the same as in Example 1.

[0086] Comparative Example 4

[0087] This comparative example provides a green preparation method for liquid-phase sodium salicylate and a method for the co-treatment of associated high COD wastewater. The difference from Example 1 is that sodium hydroxide solution is not added during the reaction extraction in S3. Instead, only recovered toluene is added to carry out neutral physical extraction under the original pH conditions of the wastewater. Other operation steps and process parameters are exactly the same as in Example 1.

[0088] Comparative Example 5

[0089] This comparative example provides a green preparation method for liquid-phase sodium salicylate and a method for the co-treatment of associated high-COD wastewater. The difference from Example 1 is that in S1, the salicylate crystals are directly packaged as solid salicylate products after centrifugation, filtration, and drying, without adding sodium hydroxide aqueous solution to prepare sodium salicylate solution; in S2-S3, the high-COD salicylate-containing wastewater is directly cooled to 2°C for cryogenic crystallization without negative pressure flash evaporation, and the wet solid salicylate obtained by filtration is returned to the next batch crystallization section for recrystallization, centrifugation, and drying, and the mother liquor is discharged into the terminal wastewater treatment system. Other operation steps and process parameters are exactly the same as in Example 1.

[0090] The performance of the green preparation method of sodium salicylate by liquid phase and the co-treatment method of associated high COD wastewater in Examples 1-4 and Comparative Examples 1-5 was tested. The specific process is as follows:

[0091] Wastewater COD reduction rate test: The final effluent phase before entering the terminal wastewater treatment system in each example and comparative example was taken, and its COD value was determined according to the dichromate method of GB 11914-1989; at the same time, the COD value of the original high-COD salicylates-nitriles wastewater before treatment was measured, and the wastewater COD reduction rate was calculated.

[0092] COD reduction rate (%) = (COD value of wastewater before treatment - COD value of effluent after treatment) / COD value of wastewater before treatment × 100%.

[0093] Total recovery rate test of salicylnitrile: The content of salicylnitrile in the original high-COD wastewater and the content converted to salicylnitrile in the final liquid phase sodium salicylnitrile product were determined by high performance liquid chromatography (HPLC). The total recovery rate was calculated according to the following formula:

[0094] Total recovery rate of salicylaniline (%) = (Converted amount of salicylaniline from wastewater in the final product / Total amount of salicylaniline in the original wastewater) × 100%.

[0095] Product purity test: The mass fraction of sodium salicylate obtained from each example and comparative example was determined by HPLC.

[0096] Phase separation effect test: Record the time required for the aqueous phase and organic phase to reach a clearly distinguishable interface during the settling process after the S3 reaction extraction (or physical extraction) is completed. This time is recorded as the phase separation time.

[0097] Cryogenic section refrigeration energy consumption test: Record the refrigeration energy consumed in each embodiment and comparative example to cool the wastewater from its actual temperature when entering the cryogenic section to 2°C, expressed as the refrigeration energy required to treat a unit mass of wastewater (kJ / kg).

[0098] The test results are shown in Table 1.

[0099] Table 1. Performance test results of the green preparation of sodium salicylate by liquid phase and the co-treatment of associated high COD wastewater in Examples 1-4 and Comparative Examples 1-5.

[0100]

[0101] As shown in Table 1, the test results of Example 1 and Comparative Example 1 indicate that after omitting the negative pressure flash evaporation step, the toluene entrained in the wastewater was not removed beforehand. During the subsequent cryogenic cooling process, toluene formed a stable emulsion intermediate layer with the aqueous phase, making it difficult to separate the phases after the reaction extraction and prolonging the separation time. Due to the presence of the emulsion layer, the interfacial mass transfer between the aqueous and organic phases was hindered, reducing the conversion efficiency of salicylonitrile to the aqueous phase. At the same time, some sodium salicylonitrile was entrained into the organic phase by the emulsion layer, resulting in a decrease in the total recovery rate of salicylonitrile. The emulsion entrainment caused a small amount of organic impurities to mix into the aqueous product liquid, resulting in a decrease in product purity. Since flash evaporation pre-cooling was not performed, the wastewater entered the cryogenic section at a high temperature of about 70°C. The cryogenic section had to bear the entire cooling load from high temperature to the target temperature, resulting in a significant increase in refrigeration energy consumption. Under the combined effect of the above factors, the organic load in the wastewater was not fully transferred to the product phase and the organic recovery phase, resulting in a significant decrease in the COD reduction rate of the final discharged aqueous phase.

[0102] As shown in Table 1, the test results of Example 1 and Comparative Example 2 indicate that after replacing the reaction extraction with the traditional cryogenic crystallization-filtration-solid reflux route, although the solubility of dispersed salicylates in the wastewater decreased under cryogenic conditions, the complex composition of the wastewater system resulted in small crystals that were easily encapsulated by organic impurities. During filtration, a large number of fine crystals were lost with the mother liquor, leading to a lower overall salicylates recovery rate than in Example 1. A significant amount of unprecipitated salicylates and other organic matter remained in the filtered mother liquor, which was directly discharged into the end-of-pipe wastewater treatment system, resulting in a decrease in the COD reduction rate. Although the product purity was relatively less affected by this route, impurities carried in the refluxed wet solids were introduced into the aqueous phase in the next batch of salt formation reaction, still causing a slight decrease in product purity. This route uses solid-liquid separation rather than liquid-liquid reaction extraction, and does not involve a phase separation step; therefore, the phase separation time is not applicable. The refrigeration energy consumption in the cryogenic section was the same as in Example 1, as the flash evaporation and cryogenic operating conditions remained unchanged.

[0103] As shown in Table 1, the test results of Example 1 and Comparative Example 3 reveal that, omitting the cryogenic enrichment step, the flash-evaporated wastewater directly enters the reaction extractor at approximately 40°C and with a low salicylate concentration. Because the lack of cryogenic cooling reduces the solubility and volume concentration of salicylate, the concentration of salicylate in the wastewater is low, resulting in insufficient mass transfer driving force for the alkaline salt formation reaction. This reduces the efficiency of the conversion of salicylate to sodium salicylate, leading to a decrease in the overall recovery rate of salicylate. Simultaneously, at higher temperatures, the partition coefficient of salicylate between the aqueous and organic phases changes, with some salicylate becoming more concentrated. The organic impurities remained in the organic phase and failed to effectively form salts to enter the aqueous phase, further reducing the recovery rate. Due to the high extraction temperature, the solubility of organic impurities in the aqueous phase also increased accordingly, resulting in a decrease in product purity. Unconverted and unrecovered salicylates and other organic substances in the wastewater were discharged with the aqueous phase, reducing the COD reduction rate of the wastewater. Under these conditions, the phase separation effect of the reaction extraction was acceptable. Since most of the toluene was removed by flash evaporation, there was no obvious emulsification interference, and the phase separation time was close to that of Example 1. Since no cryogenic operation was performed, the energy consumption of the cryogenic section was not applicable.

[0104] As shown in Table 1, the test results of Example 1 and Comparative Example 4 indicate that when neutral physical extraction is performed without the addition of sodium hydroxide solution during reaction extraction, the salicylnitrile molecules in the wastewater contain phenolic hydroxyl groups and remain in their molecular state under neutral conditions. Their solubility in toluene is higher than their solubility in water. Therefore, during toluene extraction, salicylnitrile preferentially migrates to the organic phase rather than remaining in the aqueous phase, failing to form sodium salicylnitrile product in the aqueous phase, resulting in an extremely low overall recovery rate of salicylnitrile. Furthermore, because a large amount of salicylnitrile flows out with the organic phase and fails to be productized in the aqueous phase, the aqueous phase lacks effective water... The sodium salicylates product could not be combined with the first product liquid according to the standard, and the product purity could not be properly evaluated. Although the salicylates in the wastewater were removed by toluene extraction, they were only transferred to the organic phase rather than converted into the product. The salicylates in the organic phase need to be separated and treated separately, and the wastewater COD reduction rate was lower than that in Example 1. Under neutral conditions, there was no change in interfacial activity caused by salt formation reaction. The density difference and interfacial tension between the aqueous phase and the organic phase were conducive to rapid stratification, and the phase separation time was short. The refrigeration energy consumption of the cryogenic section was the same as that in Example 1 because the flash evaporation and cryogenic operation conditions were not changed.

[0105] From the test results of Example 1 and Comparative Example 5 in Table 1, it can be seen that after adopting the traditional separation route of solid product discharge and independent wastewater treatment, the product is produced in the solid form of salicylonitrile instead of liquid sodium salicylonitrile. Since the product forms are different, no direct comparison of purity is made. In terms of wastewater treatment, neither negative pressure flash evaporation pre-solventization nor alkaline reaction extraction for productization is carried out. Toluene entrained in the wastewater forms emulsion interference during the cryogenic process, which limits the crystallization efficiency and filtration effect. A large amount of salicylonitrile is lost with the mother liquor, and the total recovery rate of salicylonitrile is lower than that of Example 1. The salicylonitrile, toluene and other organic by-products remaining in the mother liquor are directly discharged into the terminal wastewater treatment system, resulting in a decrease in the COD reduction rate of the wastewater. The wastewater enters the cryogenic section directly from high temperature without flash evaporation pre-cooling, which increases the refrigeration energy consumption of the cryogenic section. This route adopts solid-liquid separation operation instead of liquid-liquid reaction extraction, and does not involve a phase separation step, so the phase separation time is not applicable.

[0106] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A green preparation method of liquid phase sodium salicylanilate and a method for co-processing of associated high COD wastewater, characterized in that, Includes the following steps: S1: The salicylnitrile organic phase obtained after neutralization and separation during the industrial production of salicylnitrile is cooled and crystallized. After the toluene mother liquor above the crystal is extracted, toluene is added to the crystallization vessel for displacement and cleaning. The salicylnitrile crystal is reacted with sodium hydroxide aqueous solution, and the mixture is allowed to stand and separate into layers to obtain the first liquid phase sodium salicylnitrile solution. S2: The high COD salicylates wastewater with high water content generated during the production process is subjected to negative pressure flash evaporation, so that the toluene in the wastewater is vaporized and condensed for recovery, and the flash evaporation wastewater is obtained. S3: After flash evaporation, the wastewater is deeply enriched by cryogenic treatment and then sent to a reaction extractor. Sodium hydroxide solution and toluene are added at the same time to carry out reaction extraction, so that the salicylaniline in the wastewater is converted into sodium salicylaniline under alkaline conditions and enters the aqueous phase. Organic impurities migrate to the toluene phase. After separation, a second liquid phase of sodium salicylaniline solution and a toluene phase containing impurities are obtained. S4: Combine the first liquid-phase sodium salicylate solution with the second liquid-phase sodium salicylate solution, adjust and obtain the finished liquid-phase sodium salicylate product; distill the impurity-containing toluene phase to recover toluene, and recycle it for S3 and / or S1.

2. A green process for the preparation of liquid phase sodium salicylanilate and simultaneous treatment of associated high COD wastewater as claimed in claim 1, wherein, In S1, the molar ratio of salicylic acid crystals to sodium hydroxide is 1:(1-1.2).

3. The method for green preparation of liquid-phase sodium salicylate and synergistic treatment of associated high-COD wastewater according to claim 1, characterized in that, In S1, the reaction temperature is 20-45℃; the reaction time is 0.5-2h.

4. The method for green preparation of liquid-phase sodium salicylate and synergistic treatment of associated high-COD wastewater according to claim 1, characterized in that, In S2, the feed temperature of the wastewater is 65-75℃, the pressure of the flash evaporation is 20-40kPa, and the temperature of the wastewater after flash evaporation is 35-45℃.

5. The method for green preparation of liquid-phase sodium salicylate and co-treatment of associated high-COD wastewater according to claim 1, characterized in that, In S2, the heat released during the condensation of flash steam is recovered through a heat exchanger and used to preheat the sodium hydroxide solution used in S3 to 30-40℃.

6. The method for green preparation of liquid-phase sodium salicylate and synergistic treatment of associated high-COD wastewater according to claim 1, characterized in that, In S3, the temperature of the cryogenic enrichment is 0-5℃.

7. The method for green preparation of liquid-phase sodium salicylate and synergistic treatment of associated high-COD wastewater according to claim 1, characterized in that, In S3, the concentration of the sodium hydroxide solution is 8-15 wt.%, and the pH of the reaction extraction system is 9.5-10.

5.

8. The method for green preparation of liquid-phase sodium salicylate and co-treatment of associated high-COD wastewater according to claim 1, characterized in that, In S3, the mass ratio of toluene to enriched wastewater is (0.3-1):1, the reaction extraction temperature is 5-15℃, and the reaction extraction time is 10-120 min.

9. The method for green preparation of liquid-phase sodium salicylate and co-treatment of associated high-COD wastewater according to claim 1, characterized in that, In S4, the purity of the toluene recovered by distillation is not less than 98%.

10. The method for green preparation of liquid-phase sodium salicylate and co-treatment of associated high-COD wastewater according to claim 1, characterized in that, In S4, the concentration and free alkali content of the combined liquid-phase sodium salicylate solution are adjusted.

11. The method for green preparation of liquid-phase sodium salicylate and co-treatment of associated high-COD wastewater according to claim 1, characterized in that, The remaining aqueous phase after the reaction extraction and stratification in S3 is discharged into the terminal wastewater treatment system.

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