A multi-stage negative pressure distillation system and method for recovery and purification of coking desulfurization waste liquid

Abstract

The application discloses a multistage negative pressure distillation system and method for recovery and purification of coking desulfurization waste liquid, and relates to the technical field of coking industrial waste liquid treatment and resource utilization. The system comprises a pretreatment unit, an organic solvent mixing unit, a multistage negative pressure distillation separation unit, a solvent recovery unit and a product refining unit. After the pretreated waste liquid is mixed with an ethanol-glycol complex solvent, the mixture is sequentially treated in a first light component removal column, a second salt separation column and a third refining column; ethanol, water and low-boiling organic substances form a volatile solvent-containing stream, which is returned to the organic solvent mixing unit after condensation, rectification and composition adjustment; glycol mainly enters the subsequent salt separation and product refining process in the liquid phase, and is returned to the second salt separation column and / or the third refining column with at least part of the mother liquor after target salt crystallization and solid-liquid separation, thereby forming a volatile solvent recovery path and a high-boiling liquid phase circulation path, respectively, and stably realizing continuous step-by-step recovery of ammonium thiosulfate and ammonium thiocyanate.

Description

Technical Field

[0001] This invention relates to the field of coking industry wastewater treatment and resource utilization technology, specifically to a multi-stage negative pressure distillation system and method for the recovery and purification of coking desulfurization wastewater. This system and method are suitable for the recovery and purification of coking desulfurization wastewater generated from wet oxidative desulfurization processes. Background Technology

[0002] During the wet oxidation desulfurization process of coke oven gas, coking plants generate a certain amount of coking desulfurization wastewater. This wastewater typically contains inorganic salt components such as ammonium thiocyanate, ammonium thiosulfate, and ammonium sulfate, as well as suspended sulfur, tar-like substances, pyridine, quinoline, catalyst residues, and other recalcitrant organic impurities. It is characterized by high salt content, complex composition, strong corrosiveness, and difficulty in treatment. Direct discharge or only simple treatment can easily cause water, soil, and air pollution, and also waste recyclable resources such as thiocyanate and thiosulfate.

[0003] Currently, the main treatment methods for coking desulfurization wastewater include evaporation and concentration, cooling crystallization, stepwise salt separation, chemical conversion, and organic solvent-assisted extraction. Among these, evaporation and concentration and cooling crystallization processes can achieve salt enrichment to a certain extent. However, coking desulfurization wastewater contains multiple salts, and the separation behavior of components such as thiocyanate and thiosulfate affects each other. Relying solely on single-stage evaporation or conventional crystallization methods makes it difficult to obtain high-purity single salt products. The resulting products often contain sulfates, ammonium salts, and organic impurities, limiting the subsequent utilization value of the recovered products.

[0004] Some existing processes employ negative pressure evaporation or multi-stage evaporation to lower the evaporation temperature and improve treatment efficiency. However, these processes typically focus on waste liquid concentration and water removal, failing to adequately address the stepwise separation of different salt components. Furthermore, suspended sulfur, colloidal particles, and high-boiling-point organic impurities in coking desulfurization wastewater easily deposit and scale in evaporators, distillation towers, or heat exchange components, causing equipment blockage, reduced heat transfer efficiency, and frequent shutdowns for cleaning, thus affecting the continuous and stable operation of the unit.

[0005] Some treatment routes introduce organic solvents to assist in the extraction or purification of salt components. However, existing organic solvent treatment methods mostly involve solvent extraction or recrystallization based on concentrated, crystallized or dried materials. The interaction stage between the solvent and the target components in the waste liquid is relatively delayed, making it difficult to form a continuous synergy with the front-end negative pressure distillation separation process. Furthermore, if the solvent addition, mixing, separation and recovery path is not designed properly, problems such as uneven mixing, large solvent loss, low recovery efficiency and high operating costs may occur.

[0006] Therefore, existing technologies for recovering and purifying coking desulfurization wastewater still suffer from problems such as insufficient product purity, low resource recovery rate, high energy consumption, significant solvent loss, easy equipment clogging, and insufficient stability in continuous operation. Especially when treating high-salt, high-impurity coking desulfurization wastewater generated by wet oxidation desulfurization processes such as HPF and PDS, there is an urgent need for a recovery and purification system and method that organically combines front-end pretreatment, organic solvent-assisted regulation, multi-stage negative pressure stepwise separation, solvent recovery and recycling, and product refining processes to improve the recovery purity and operational stability of useful components such as thiocyanate and thiosulfate. Summary of the Invention

[0007] The purpose of this invention is to provide a multi-stage negative pressure distillation system and method for recovering and purifying coking desulfurization waste liquid, so as to improve the recovery purity of thiocyanate and thiosulfate, reduce solvent loss and equipment blockage risk, and realize continuous resource recovery of waste liquid.

[0008] To achieve the above-mentioned technical objectives, the present invention adopts the following technical solution: A multi-stage negative pressure distillation system for the recovery and purification of coking desulfurization wastewater, comprising: The pretreatment unit, organic solvent mixing unit, multi-stage negative pressure distillation and separation unit, and product refining unit are connected sequentially along the material flow direction, as well as a solvent recovery unit connecting the multi-stage negative pressure distillation and separation unit and the organic solvent mixing unit; The pretreatment unit includes a precision filtration assembly, a decolorization assembly, and a buffer stirring tank connected in sequence. The buffer stirring tank is used to output the pretreated waste liquid that has undergone impurity removal, decolorization, homogenization, and preheating. The organic solvent mixing unit includes a metering pump, a static mixer, and a mixing buffer tank. The metering pump is used to send the ethanol-ethylene glycol compound solvent into the static mixer, so that the compound solvent is mixed with the pretreated waste liquid before it enters the multi-stage negative pressure distillation and separation unit to form a mixed liquid. The volume ratio of ethanol to ethylene glycol is 3:1, and the volume ratio of the compound solvent to the pretreated waste liquid is 1:4 to 1:6. The multi-stage negative pressure distillation and separation unit includes a first-stage light-removal tower, a second-stage salt separation tower, and a third-stage purification tower connected in series, with the absolute value of the operating negative pressure increasing step by step and the temperature of the tower bottom increasing step by step. The primary light-light removal tower has its top outlet connected to the solvent recovery unit and its bottom outlet connected to the secondary salt separation tower. The secondary salt separation tower has a first product stream outlet that outputs a first product stream containing ammonium thiosulfate, and its bottom outlet connected to the tertiary purification tower. The tertiary purification tower has its top outlet connected to the solvent recovery unit and a bottom product stream outlet that outputs a second product stream containing ammonium thiocyanate. Both the first product stream outlet and the bottom product stream outlet are connected to the product purification unit. The solvent recovery unit includes a solvent condenser, a solvent distillation tower, a solvent storage tank, and a transfer pump connected in sequence. The transfer pump recovers the solvent-containing stream and returns it to the static mixer.

[0009] Optionally, the precision filtration assembly includes a first-stage filtration structure and a second-stage filtration structure connected sequentially along the waste liquid flow direction. The first-stage filtration structure is used to remove solid particles and suspended sulfur from the coking desulfurization waste liquid, and the second-stage filtration structure is used to remove fine suspended matter and colloidal impurities from the coking desulfurization waste liquid. The decolorization assembly is used to remove colored impurities, organic matter, and catalyst residues from the coking desulfurization waste liquid.

[0010] Optionally, the first-stage filtration structure is a stainless steel mesh filter with a pore size of 1.0 μm, and the second-stage filtration structure is a polytetrafluoroethylene microfilter with a pore size of 0.2 μm; the decolorization component includes an activated carbon adsorption tower; the buffer mixing tank is equipped with a conical wall scraper agitator, and the bottom of the buffer mixing tank is equipped with a drain port.

[0011] Optionally, the static mixer is provided with multiple layers of staggered guide vanes, the mixing buffer tank is used to temporarily store and homogenize the mixed liquid output by the static mixer, and the mixing buffer tank is provided with a pH monitoring component for monitoring the pH value of the mixed liquid.

[0012] Optionally, the operating negative pressure of the primary light component removal tower is -0.07MPa to -0.08MPa, and the reboiler temperature is 70℃ to 80℃; the operating negative pressure of the secondary salt separation tower is -0.08MPa to -0.09MPa, and the reboiler temperature is 85℃ to 95℃; the operating negative pressure of the tertiary refining tower is -0.09MPa to -0.095MPa, and the reboiler temperature is 100℃ to 110℃.

[0013] Optionally, the primary light-removal tower, the secondary salt separation tower, and the tertiary refining tower are all packed towers. Each tower is equipped with anti-fouling perforated corrugated packing, and each tower is equipped with a liquid level sensor, a temperature sensor, and a pressure sensor. Each tower has a drain outlet at the bottom for discharging high-boiling-point impurities and sediment residues.

[0014] Optionally, the product refining unit includes a crystallizer, a centrifuge, a dryer, and a product storage tank. The crystallizer is connected to both the first product stream outlet and the bottom product stream outlet. The centrifuge separates the crystals and mother liquor after crystallization. The dryer vacuum dries the crystals. The product storage tank stores the dried ammonium thiosulfate and ammonium thiocyanate products. The centrifuge has a crystal outlet and a mother liquor outlet. The mother liquor outlet is connected to the secondary salt separator and / or the tertiary refining tower to return the mother liquor obtained from solid-liquid separation to the secondary salt separator and / or the tertiary refining tower. The mother liquor is used to contain ethylene glycol that enters the product refining unit with the first and second product streams.

[0015] To achieve the above-mentioned technical objectives, the present invention also adopts the following technical solution: A method for recovering and purifying coking desulfurization wastewater, comprising: The coking desulfurization waste liquid is subjected to impurity removal, decolorization, homogenization and preheating to obtain pretreated waste liquid; Before the pretreated waste liquid enters the multi-stage negative pressure distillation treatment, the pretreated waste liquid is quantitatively mixed with an ethanol-ethylene glycol compound solvent to form a mixed liquid, wherein the volume ratio of ethanol to ethylene glycol in the ethanol-ethylene glycol compound solvent is 3:1, and the volume ratio of the ethanol-ethylene glycol compound solvent to the pretreated waste liquid is 1:4 to 1:6. The mixed liquid is subjected to a first-stage negative pressure desulfurization treatment, a second-stage negative pressure desalination treatment, and a third-stage negative pressure refining treatment in sequence, with the absolute value of the operating negative pressure and the treatment temperature increasing step by step in the first-stage negative pressure desulfurization treatment, the second-stage negative pressure desalination treatment, and the third-stage negative pressure refining treatment. In the first-stage negative pressure desulfurization process, a low-boiling-point solvent-containing stream is separated from the mixed liquid to form a desulfurized concentrate that enters the second-stage negative pressure desalination process. In the second-stage negative pressure desalination process, a first product stream containing ammonium thiosulfate is separated from the desulfurized concentrate to form an ammonium thiocyanate-rich feed solution that enters the third-stage negative pressure refining process. In the third-stage negative pressure refining process, a solvent-containing stream is separated from the ammonium thiocyanate-rich feed solution to form a second product stream containing ammonium thiocyanate. The solvent-containing stream generated from the first-stage negative pressure light removal treatment and the third-stage negative pressure refining treatment is condensed and distilled to obtain recovered solvent, and the recovered solvent is returned to the step of mixing with the pretreated waste liquid. The first product stream and the second product stream were subjected to crystallization, solid-liquid separation and drying, respectively, to obtain ammonium thiosulfate product and ammonium thiocyanate product.

[0016] Optionally, the operating negative pressure of the first-stage negative pressure de-lightening treatment is -0.07MPa to -0.08MPa, and the treatment temperature is 70℃ to 80℃; the operating negative pressure of the second-stage negative pressure desalination treatment is -0.08MPa to -0.09MPa, and the treatment temperature is 85℃ to 95℃; the operating negative pressure of the third-stage negative pressure refining treatment is -0.09MPa to -0.095MPa, and the treatment temperature is 100℃ to 110℃.

[0017] Optionally, when the first product stream and the second product stream are subjected to crystallization, solid-liquid separation and drying, the crystallization temperature is 20°C to 30°C and the crystallization time is 4h to 6h, the drying temperature is 60°C to 70°C and the drying time is 2h to 3h, and the mother liquor obtained from solid-liquid separation is returned to the secondary negative pressure salt separation treatment and / or the tertiary negative pressure purification treatment.

[0018] The main advantages of this invention compared to existing technologies are as follows: This invention, by sequentially arranging a pretreatment unit, an organic solvent mixing unit, a multi-stage negative pressure distillation and separation unit, and a product refining unit along the material flow direction, and connecting a solvent recovery unit between the multi-stage negative pressure distillation and separation unit and the organic solvent mixing unit, allows the coking desulfurization wastewater to be quantitatively mixed with an ethanol-ethylene glycol compound solvent before entering the multi-stage negative pressure distillation and separation unit. The wastewater is then continuously and stepwise processed in a primary light-light removal tower, a secondary salt separation tower, and a tertiary refining tower with progressively increasing negative pressure and tower bottom temperature. This improves upon existing coking desulfurization wastewater treatment methods that suffer from insufficient precision in single-stage evaporation or ordinary crystallization separation, excessive product inclusions, high solvent loss, and easy equipment blockage. It also enhances the recovery purity and continuous processing stability of ammonium thiosulfate and ammonium thiocyanate.

[0019] In this invention, ethanol and ethylene glycol are added at an initial feed ratio of 3:1 before the mixed liquid enters the primary light component removal tower. After the primary light component removal treatment, ethanol mainly enters the condensation and distillation recovery path with the volatile solvent-containing stream at the top of the tower, while ethylene glycol mainly enters the subsequent salt separation and product purification processes with the liquid phase. After the target salt crystallization and solid-liquid separation, it is recycled back with at least a portion of the mother liquor, thereby forming a staged migration and circulation path adapted to the volatility characteristics of ethanol and ethylene glycol.

[0020] In this invention, the primary light component removal tower, the secondary salt separation tower, and the tertiary refining tower are connected in series, and the absolute value of the operating negative pressure and the bottom temperature of the towers are increased step by step. This allows the mixed liquid to sequentially complete the removal of low-boiling-point components, the separation of the first product stream containing ammonium thiosulfate, and the formation of the second product stream containing ammonium thiocyanate. This facilitates segmented control of the treatment process of different components in coking desulfurization wastewater, reduces the single-stage treatment load, and improves the controllability of system operation.

[0021] This invention enables the solvent-containing streams output from the primary light solvent removal tower and the tertiary purification tower to be recovered through a solvent condenser, a solvent distillation tower, and a solvent storage tank before returning to the static mixer, forming an organic solvent recycling path. This reduces the consumption of fresh organic solvents, lowers solvent emissions and operating costs, and allows the solvent-assisted separation process to adapt to continuous operation requirements.

[0022] This invention uses a precision filtration assembly, a decolorization assembly, and a buffer stirring tank to remove impurities, decolorize, homogenize, and preheat coking desulfurization wastewater. It can remove suspended sulfur, solid particles, colloidal impurities, and some organic impurities at the front end, reducing the risk of packing blockage, bottom deposition, and equipment wear in subsequent distillation towers, and helping to extend the continuous operation cycle of the unit.

[0023] This invention uses a product refining unit to crystallize, separate solids and liquids, and dry the first product stream containing ammonium thiosulfate and the second product stream containing ammonium thiocyanate, respectively. This allows the product streams after distillation and separation to be further converted into recyclable salt products, which is beneficial for the resource recovery of useful components in coking desulfurization wastewater and reduces the pressure of subsequent harmless treatment of the wastewater. Attached Figure Description

[0024] Figure 1 This is a system architecture diagram of the multi-stage negative pressure distillation system of the present invention; Figure 2 This is a schematic diagram illustrating the steps of the coking desulfurization waste liquid recovery and purification method of the present invention. Detailed Implementation

[0025] The present invention will be further described below with reference to specific embodiments. It should be understood that the following embodiments are used to illustrate the technical solution of the present invention and should not be construed as limiting the scope of protection of the present invention. Without departing from the core concept of the present invention, those skilled in the art can make adaptive adjustments to the specifications of local equipment, tower dimensions, pumping capacity and control parameters according to the actual composition, treatment scale and recovery product requirements of coking desulfurization wastewater.

[0026] like Figure 1 As shown, this invention provides a multi-stage negative pressure distillation system 100 for the recovery and purification of coking desulfurization wastewater. The coking desulfurization wastewater can be desulfurization wastewater generated by wet oxidation desulfurization processes such as HPF and PDS. This type of wastewater typically contains ammonium thiocyanate, ammonium thiosulfate, ammonium sulfate, water, suspended sulfur, pyridine, quinoline, catalyst residues, tar impurities, and other organic impurities.

[0027] The multi-stage negative pressure distillation system of the present invention includes a pretreatment unit 110, an organic solvent mixing unit 120, a multi-stage negative pressure distillation separation unit 130, and a product refining unit 150 connected sequentially along the material flow direction, as well as a solvent recovery unit 140 connected between the multi-stage negative pressure distillation separation unit 130 and the organic solvent mixing unit 120. Coking desulfurization wastewater first undergoes pretreatment to remove solid particles, suspended sulfur, colloidal impurities, colored impurities, and some organic impurities. It is then quantitatively mixed with an ethanol-ethylene glycol compound solvent to form a mixed liquid, which subsequently enters a primary light-duty removal tower 131, a secondary salt separation tower 132, and a tertiary refining tower 133 for multi-stage negative pressure distillation separation. The solvent-containing streams output from the primary light-duty removal tower and the tertiary refining tower enter the solvent recovery unit, and after condensation, distillation, and temporary storage, are returned to the organic solvent mixing unit for recycling. The salt-containing product streams output from the secondary salt separation tower and the tertiary refining tower enter the product refining unit, where, after crystallization, solid-liquid separation, and drying, ammonium thiosulfate and ammonium thiocyanate products are obtained.

[0028] The ethanol to ethylene glycol volume ratio of 3:1 is the initial feed ratio before the mixed liquid enters the primary light-duty removal tower 131. It does not mean that the ethanol to ethylene glycol ratio remains the same after the primary negative pressure light-duty removal treatment. After the mixed liquid enters the primary light-duty removal tower 131, ethanol, water, and low-boiling-point organic matter mainly enter the volatile solvent-containing stream at the top of the tower, while ethylene glycol, due to its lower volatility, is mainly retained in the light-duty removal concentrate at the bottom of the tower, and then sequentially enters the secondary salt separation tower 132 and the tertiary purification tower 133 along with the light-duty removal concentrate.

[0029] The pretreatment unit includes a precision filter assembly 111, a decolorization assembly 112, and a buffer mixing tank 113 connected in sequence. The precision filter assembly is used to remove solid particles, suspended sulfur, fine suspended solids, and colloidal impurities from the coking desulfurization wastewater, thereby reducing the risk of clogging, scaling, or packing contamination in the subsequent static mixer and distillation tower.

[0030] In one embodiment, the precision filtration assembly includes a first-stage filtration structure and a second-stage filtration structure connected sequentially along the wastewater flow direction. The first-stage filtration structure is a stainless steel mesh filter with a pore size of 1.0 μm, used to remove larger solid particles, suspended sulfur, and other particulate matter. The second-stage filtration structure is a polytetrafluoroethylene (PTFE) microfilter with a pore size of 0.2 μm, used to further remove fine suspended solids and colloidal impurities. The PTFE microfilter has good corrosion resistance and can adapt to the treatment environment where salt and organic impurities coexist in coking desulfurization wastewater.

[0031] The decolorization component can be an activated carbon adsorption tower. The activated carbon adsorption tower is filled with powdered or granular activated carbon, which adsorbs colored impurities, some organic matter, and catalyst residues from the waste liquid. Specifically, No. 3 powdered activated carbon with an iodine value of approximately 900 can be used. The activated carbon filling amount is 60%-70% of the effective volume of the adsorption tower, and the residence time of the waste liquid in the activated carbon adsorption tower can be controlled to be 30 to 40 minutes. After decolorization, the waste liquid enters a buffer mixing tank.

[0032] The buffer mixing tank is used for homogenization, buffering, and preheating of decolorized coking desulfurization wastewater. The tank is equipped with a conical scraper agitator, with a stirring speed controllable from 300 rpm to 500 rpm. The scraper agitator reduces sediment buildup on the tank walls during mixing, ensuring uniform dispersion of salt components and residual suspended solids in the wastewater. The buffer mixing tank can also be equipped with a jacketed or internal coil preheating structure to preheat the wastewater to 40°C to 50°C, reducing temperature fluctuations during subsequent mixing with compound solvents. A drain port is located at the bottom of the buffer mixing tank for periodically removing deposited impurities.

[0033] The organic solvent mixing unit includes a metering pump 121, a static mixer 122, and a mixing buffer tank 123. The metering pump is used to quantitatively feed the ethanol-ethylene glycol compound solvent into the static mixer, so that the compound solvent is mixed with the pretreated waste liquid to form a mixed feed solution before the pretreated waste liquid enters the multi-stage negative pressure distillation separation unit. This pre-mixing method allows ethanol and ethylene glycol to participate in the composition adjustment of the mixed feed solution before entering the first-stage light solvent removal tower 131. During the subsequent negative pressure treatment, ethanol and ethylene glycol form different migration paths according to their respective volatility characteristics. Ethanol is mainly discharged with the volatile solvent-containing stream at the top of the tower, while ethylene glycol is mainly retained in the liquid phase at the bottom of the tower.

[0034] The ethanol-ethylene glycol compound solvent is prepared by mixing ethanol and ethylene glycol at a volume ratio of 3:1. Industrial ethanol or anhydrous ethanol can be used, and industrial-grade ethylene glycol can be used; both can be obtained through conventional chemical raw material channels. During preparation, ethanol is first added to an organic solvent storage tank, followed by ethylene glycol, and the mixture is stirred until homogeneous. The volume ratio of the compound solvent to the pretreated waste liquid is controlled at 1:4 to 1:6. In other words, when the feed volume flow rate of the pretreated waste liquid is Q, the added volume flow rate of the compound solvent can be controlled at Q / 4 to Q / 6; in the compound solvent, the volume flow rate of ethanol accounts for 3 / 4 of the total volume flow rate, and the volume flow rate of ethylene glycol accounts for 1 / 4. This method clearly defines the composition and amount of the compound solvent, ensuring a stable mixing ratio in different batches of treatment.

[0035] The 3:1 ratio refers to the mixing ratio and initial feed ratio when the compound solvent is added to the pretreated waste liquid. This ratio is not required to be maintained in the liquid after the first-stage light removal treatment.

[0036] The static mixer is equipped with multiple layers of staggered guide vanes. Pretreated waste liquid and ethanol-ethylene glycol compound solvent enter the static mixer simultaneously, where they are repeatedly split, rotated, and recombined under the action of the guide vanes, thereby improving the mixing uniformity of the two-phase or multi-component liquid. The mixing time of the static mixer can be controlled between 10 and 15 minutes. The mixed liquid output from the static mixer enters a mixing buffer tank. The mixing buffer tank may be equipped with a stirring device to temporarily store and homogenize the mixed liquid, preventing localized solvent concentration fluctuations or liquid stratification. The mixing buffer tank may also be equipped with a pH monitoring component, allowing the pH value of the mixed liquid to be controlled between 6.5 and 7.5 to reduce the risk of instability of salt components or equipment corrosion under excessively acidic or alkaline conditions.

[0037] The multi-stage negative pressure distillation separation unit comprises a primary light-light residue removal tower 131, a secondary salt separation tower 132, and a tertiary purification tower 133 connected in series. All three towers can utilize packed tower structures, with anti-fouling perforated corrugated packing inside. The tower bodies can be made of 316L stainless steel. Each tower is equipped with a level sensor, a temperature sensor, and a pressure sensor to monitor the liquid level, bottom temperature, top temperature, and operating negative pressure in real time. Each tower may also have a drain outlet at the bottom to discharge high-boiling-point impurities, sediment residue, or small amounts of solid deposits.

[0038] The primary light-boiling component removal tower is used to remove low-boiling-point components from the mixed feed liquid. The operating negative pressure of the primary light-boiling component removal tower is -0.07 MPa to -0.08 MPa, the bottom temperature is 70°C to 80°C, the top temperature can be 45°C to 55°C, and the reflux ratio can be 2:1 to 3:1. After the mixed feed liquid enters the primary light-boiling component removal tower 131, under negative pressure and heating conditions, ethanol, water, and low-boiling-point organic matter mainly form a volatile solvent-containing stream, which is discharged from the top outlet of the primary light-boiling component removal tower 131 to the solvent recovery unit 140; ethylene glycol, ammonium thiocyanate, ammonium thiosulfate, and other non-volatile or high-boiling-point components are mainly retained in the bottom liquid phase, forming a light-boiling component removal concentrate, which enters the secondary salt separation tower 132 from the bottom outlet of the primary light-boiling component removal tower 131. The primary light component removal tower can reduce the moisture load of the subsequent secondary salt separation tower by removing low-boiling-point components in advance, and bring the concentration of the salt-containing liquid into a range suitable for salt separation treatment.

[0039] The primary light-light removal process does not aim to completely remove ethanol from the mixed feed liquid. The endpoint can be determined based on the ethanol content in the overhead condensate, the water content in the bottom light-light removal concentrate, and the liquid level within the column. After the primary light-light removal process, the ratio of ethanol to ethylene glycol in the light-light removal concentrate can differ from the initial feed ratio of 3:1.

[0040] The 3:1 volume ratio of ethanol to ethylene glycol is used to define the initial solvent composition of the mixed liquid before it enters the primary light-duty removal tower 131, not to define the composition ratio of the liquid phase medium in the secondary salt separation tower 132 and the tertiary purification tower 133. After the primary negative pressure light-duty removal treatment, ethanol is mainly discharged with the volatile solvent-containing stream at the top of the tower, while ethylene glycol is mainly retained in the light-duty removal concentrate at the bottom of the tower. The secondary negative pressure salt separation treatment and the tertiary negative pressure purification treatment do not operate under the condition of maintaining a 3:1 volume ratio of ethanol to ethylene glycol. In subsequent treatments, the stepwise enrichment of different salt components is mainly achieved by utilizing the changes in the composition of the liquid formed by the staged negative pressure concentration. Ethylene glycol, as a high-boiling-point liquid phase component, continues to flow with the liquid.

[0041] The secondary salt separator is used to separate the first product stream containing ammonium thiosulfate from the light-duty solvent concentrate, forming an ammonium thiocyanate-rich feed solution that enters the tertiary purification tower. The operating negative pressure of the secondary salt separator is -0.08 MPa to -0.09 MPa, the bottom temperature is 85°C to 95°C, the top temperature can be 60°C to 70°C, and the reflux ratio can be 3:1 to 4:1. The secondary salt separator has a first product stream outlet for discharging the first product stream containing ammonium thiosulfate. The first product stream can be a feed solution, concentrate, or slurry with relatively concentrated ammonium thiosulfate after secondary negative pressure salt separation; its specific form can be determined based on the salt concentration, solvent content, and subsequent crystallization conditions within the tower. The bottom outlet of the secondary salt separator is connected to the tertiary purification tower, and the feed solution output from the bottom is rich in ammonium thiocyanate.

[0042] The first product stream output from the secondary salt separator 132 is a liquid product stream with relatively enriched ammonium thiosulfate, and the bottom feed of the secondary salt separator 132 is a liquid product stream with relatively enriched ammonium thiocyanate. Ethylene glycol, as a high-boiling-point liquid component, can continue to flow with the first product stream and the bottom feed. The ammonium thiosulfate product and the ammonium thiocyanate product are formed through subsequent cooling crystallization, solid-liquid separation, and drying, respectively.

[0043] After the light-boiling-point concentrate enters the secondary salt separation tower 132, it undergoes further removal of moisture and residual low-boiling-point components under negative pressure and heating conditions, gradually increasing the salt concentration in the feed solution. The secondary salt separation process controls the temperature, pressure, reflux state, and feed concentration within the tower to ensure that the feed solution with a relatively high concentration of ammonium thiosulfate forms the first product stream and exits from the first product stream outlet. Simultaneously, ammonium thiocyanate is relatively concentrated in the feed solution that continues to enter the tertiary purification tower 133. Ethylene glycol mainly remains in the feed solution as a high-boiling-point liquid phase component during this process, and ethanol is not required to continue participating in the secondary salt separation process.

[0044] After the ammonium thiocyanate-rich feed solution enters the three-stage refining tower 133, residual moisture and volatile components are further removed under conditions of increased absolute negative pressure and processing temperature. This further enriches the ammonium thiocyanate in the feed solution at the bottom of the tower, forming the second product stream. Thus, the first-stage light component removal, the second-stage salt separation, and the third-stage refining respectively perform the functions of removing low-boiling-point components, relatively enriching ammonium thiosulfate, and further enriching ammonium thiocyanate. Each stage of the process relies on the continuous changes in the composition and concentration of the feed solution to form a step-by-step treatment process that is seamlessly connected.

[0045] The three-stage refining column is used for negative pressure refining of ammonium thiocyanate-rich feed solutions. The operating negative pressure of the three-stage refining column is -0.09 MPa to -0.095 MPa, the bottom temperature is 100°C to 110°C, the top temperature can be 75°C to 85°C, and the reflux ratio can be 4:1 to 5:1. The top outlet of the three-stage refining column is connected to the solvent recovery unit for outputting the solvent-containing stream formed by residual organic solvents and low-boiling-point impurities. The three-stage refining column has a bottom product stream outlet for outputting a second product stream containing ammonium thiocyanate. The second product stream can be a relatively enriched concentrate or crystal slurry of ammonium thiocyanate, and is sent to the product refining unit for subsequent crystallization, solid-liquid separation, and drying. The second product stream output from the three-stage refining column 133 may contain ethylene glycol. After entering the product refining unit 150, the second product stream undergoes cooling and crystallization to form crystals of ammonium thiocyanate. Ethylene glycol is mainly retained in the crystallization mother liquor and is discharged with the mother liquor after solid-liquid separation.

[0046] The solvent recovery unit includes a solvent condenser 141, a solvent distillation column 142, a solvent storage tank 143, and a transfer pump 144 connected in sequence. The solvent-containing streams output from the top outlet of the primary light-light removal column and the top outlet of the tertiary purification column first enter the solvent condenser. The solvent condenser is used to condense, temporarily store, and homogenize the solvent-containing streams, and the condensate then enters the solvent distillation column. Since ethanol, ethylene glycol, and water may have varying degrees of miscibility, this invention does not rely on simple static stratification as the sole solvent recovery method. Instead, it uses the solvent distillation column to distill the condensate to remove water and low-boiling-point impurities. The volatile solvent-containing streams output from the primary light-light removal column 131 and the tertiary purification column 133 enter the solvent condenser 141. The condensate mainly contains ethanol, water, and low-boiling-point organic matter, and may contain a small amount of ethylene glycol carried by the gas flow or entering the gas phase under negative pressure. After the condensate enters the solvent distillation column 142, water and low-boiling-point impurities are removed by distillation to obtain a recovered solvent with ethanol as the main reusable component.

[0047] The solvent distillation column can operate at a negative pressure of -0.08 MPa, with a bottom temperature of 80°C to 90°C, a top temperature of 65°C to 75°C, and a reflux ratio of 2:1 to 3:1. The recovered solvent after distillation enters a solvent storage tank. The solvent storage tank is connected to a static mixer via a transfer pump, which returns the recovered solvent to the static mixer, allowing it to be mixed again with the pretreated waste liquid. The ethanol and ethylene glycol content in the recovered solvent is tested, and ethanol and / or ethylene glycol are added based on the test results to adjust the volume ratio of ethanol to ethylene glycol in the compounded solvent fed into the static mixer 122 to 3:1 before mixing with the pretreated waste liquid. Thus, the solvent recovery unit and the organic solvent mixing unit form a closed loop, reducing the consumption of fresh solvent.

[0048] The product refining unit includes a crystallizer 151, a centrifuge 152, a dryer 153, and a product storage tank 154. The crystallizer is connected to the outlet of the first product stream of the secondary salt separation tower and the outlet of the bottom product stream of the tertiary refining tower, respectively, to receive the first product stream containing ammonium thiosulfate and the second product stream containing ammonium thiocyanate. The first and second product streams can be processed in separate crystallizers or by switching between batches within the same crystallizer.

[0049] The crystallization process employs a cooling crystallization method, with a crystallization temperature of 20°C to 30°C and a crystallization time of 4 to 6 hours. The first and second product streams respectively enter crystallizer 151 for cooling crystallization. During the cooling process and while maintaining the predetermined crystallization time, ammonium thiosulfate or ammonium thiocyanate gradually forms solid-phase crystals, while ethylene glycol, water, uncrystallized target salt, and other soluble components are mainly retained in the liquid phase, thereby forming a crystal slurry composed of target salt crystals and ethylene glycol-containing mother liquor.

[0050] After crystallization, the crystal slurry enters a centrifuge 152 for solid-liquid separation. The centrifuge 152 separates the target salt crystals from the ethylene glycol-containing mother liquor through centrifugation. The ethylene glycol-containing mother liquor is discharged through the mother liquor outlet, while the wet target salt crystals are discharged through the crystal outlet. By performing thorough solid-liquid separation of the crystal slurry, the ethylene glycol that enters the product refining unit 150 with the first and second product streams is primarily retained in the mother liquor and discharged with it, thereby reducing the amount of ethylene glycol-containing liquid phase entrained in the wet target salt crystals.

[0051] After centrifugation, the wet crystals of the target salt are placed in dryer 153 for vacuum drying at a temperature of 60°C to 70°C for 2 to 3 hours. Vacuum drying is used to further reduce the liquid phase components and moisture entrained in the wet crystals. After drying, the purity of the resulting ammonium thiosulfate and ammonium thiocyanate products is tested, and the residual ethylene glycol in the dried products is detected by gas chromatography. Once the product purity and residual ethylene glycol meet the corresponding product quality control requirements, the ammonium thiosulfate and ammonium thiocyanate products are stored in product storage tank 154.

[0052] The ethylene glycol-containing mother liquor obtained from centrifugation enters the subsequent mother liquor recycling process. The ethylene glycol content and high-boiling-point impurity content in the mother liquor are tested, and the amount of mother liquor returned and removed from the recycling system are determined based on the test results. At least a portion of the ethylene glycol-containing mother liquor meeting the recycling conditions is returned to the secondary salt separation tower 132 and / or the tertiary purification tower 133 to continue recovering the uncrystallized target salt and utilizing the ethylene glycol; the remaining mother liquor is removed from the recycling system for subsequent processing to avoid the continuous accumulation of ethylene glycol, uncrystallized salts, and high-boiling-point impurities during continuous recycling.

[0053] In one embodiment, the present invention may further include a control system. The control system may be a PLC control system, electrically connected to sensors, metering pumps, transfer pumps, heating devices, condensing devices, and valves in the pretreatment unit, organic solvent mixing unit, multi-stage negative pressure distillation separation unit, solvent recovery unit, and product refining unit. The control system can automatically adjust the flow rate of the compound solvent added based on the feed flow rate of the pretreatment waste liquid, maintaining the volume ratio of the compound solvent to the pretreatment waste liquid at 1:4 to 1:6; it can also adjust the heating amount, vacuum degree, and reflux ratio based on the pressure, temperature, and liquid level signals of the primary light component removal tower, secondary salt separation tower, and tertiary refining tower to maintain stable operation of the multi-stage negative pressure distillation separation process.

[0054] The control system is also used to control the return and discharge of ethylene glycol-containing mother liquor. After the mother liquor output from centrifuge 152 enters the mother liquor storage tank, the ethylene glycol content and high-boiling-point impurity content in the mother liquor are detected. When the composition of the mother liquor is within the normal operating control range, the mother liquor return pipeline is opened, allowing the mother liquor to return to the secondary salt separation tower 132 and / or the tertiary purification tower 133 for further processing; when continuous circulation leads to the accumulation of high-boiling-point impurities or other non-target components in the mother liquor, the mother liquor return flow rate is reduced and the mother liquor discharge flow rate is increased, allowing part of the mother liquor to enter the mother liquor treatment equipment.

[0055] After the mother liquor is discharged and treated, the return flow rate of the mother liquor is readjusted according to the liquid level of the mother liquor storage tank, the ethylene glycol content, and the operating liquid levels of the secondary salt separation tower 132 and the tertiary purification tower 133. This ensures that the mother liquor circulation volume, discharge volume, and system replenishment volume are coordinated, thereby preventing the unlimited accumulation of ethylene glycol and high-boiling-point impurities during continuous operation.

[0056] like Figure 2 As shown, the present invention also provides a method for recovering and purifying coking desulfurization wastewater. This method can be implemented using the aforementioned multi-stage negative pressure distillation system, or using equivalent equipment with the same material flow direction and separation function.

[0057] Specifically, the method includes the following steps.

[0058] First, S210: The coking desulfurization wastewater is subjected to impurity removal, decolorization, homogenization, and preheating to obtain pretreated wastewater. Impurity removal can be carried out using a two-stage filtration method, where the first stage of filtration removes larger solid particles and suspended sulfur, and the second stage of filtration removes fine suspended solids and colloidal impurities; decolorization can be carried out using activated carbon adsorption; homogenization and preheating can be carried out in a buffer stirred tank, and the preheating temperature can be 40℃ to 50℃.

[0059] Then, in step S220: before the pretreated waste liquid enters the multi-stage negative pressure distillation process, the pretreated waste liquid is quantitatively mixed with an ethanol-ethylene glycol compound solvent to form a mixed solution. The volume ratio of ethanol to ethylene glycol in the ethanol-ethylene glycol compound solvent is 3:1, and the volume ratio of the compound solvent to the pretreated waste liquid is 1:4 to 1:6. The mixing process can be carried out in a static mixer for 10 to 15 minutes. After mixing, the solution enters a mixing buffer tank for homogenization, and the pH is controlled at 6.5 to 7.5.

[0060] The volume ratio of ethanol to ethylene glycol is 3:1, which is the initial feed ratio of the mixed liquid before it enters the first-stage negative pressure delighting treatment. After the first-stage negative pressure delighting treatment, the ratio of ethanol to ethylene glycol in each stage of the liquid is not required to be maintained.

[0061] Next, the mixed liquid is subjected to a first-stage negative pressure light-weight removal treatment, a second-stage negative pressure desalination treatment, and a third-stage negative pressure refining treatment. The operating negative pressure for the first-stage negative pressure light-weight removal treatment is -0.07 MPa to -0.08 MPa, and the treatment temperature is 70℃ to 80℃; the operating negative pressure for the second-stage negative pressure desalination treatment is -0.08 MPa to -0.09 MPa, and the treatment temperature is 85℃ to 95℃; the operating negative pressure for the third-stage negative pressure refining treatment is -0.09 MPa to -0.095 MPa, and the treatment temperature is 100℃ to 110℃. The absolute value of the operating negative pressure and the treatment temperature increase progressively with each stage of the first-stage negative pressure light-weight removal treatment, the second-stage negative pressure desalination treatment, and the third-stage negative pressure refining treatment.

[0062] S230: In the first-stage negative pressure light-duty removal process, ethanol, water, and low-boiling-point organic matter are mainly formed into volatile solvent-containing streams, while ethylene glycol and target salt components are mainly retained in the light-duty removal concentrate; the volatile solvent-containing streams are sent to condensation and distillation processes, and the light-duty removal concentrates are sent to the second-stage negative pressure salt separation process.

[0063] S240: In the secondary negative pressure salt separation process, the first product stream containing ammonium thiosulfate and ethylene glycol is separated from the light-removal concentrate and forms an ammonium thiocyanate-rich feed solution that enters the tertiary negative pressure refining process.

[0064] S250: In a three-stage negative pressure refining process, a volatile solvent-containing stream is separated from an ammonium thiocyanate-rich feed solution to form a second product stream containing ammonium thiocyanate and ethylene glycol.

[0065] S260: The volatile solvent-containing stream generated from the primary negative pressure light removal process and the tertiary negative pressure refining process is condensed and distilled to remove moisture and low-boiling-point impurities, yielding a recycled solvent with ethanol as the main reusable component.

[0066] S270: Detect the ethanol and ethylene glycol content in the recovered solvent, add ethanol and / or ethylene glycol according to the detection results to adjust the volume ratio of ethanol to ethylene glycol in the compound solvent to 3:1, and return the adjusted compound solvent to the step of mixing with the pretreated waste liquid.

[0067] During solvent recovery, water and low-boiling-point impurities can be removed by solvent distillation column, and the solvent can be replenished and adjusted according to the ratio of ethanol to ethylene glycol in the recovered solvent to maintain the volume ratio of ethanol to ethylene glycol of 3:1 in the compound solvent returned to the static mixer.

[0068] Finally, in S280: the first product stream and the second product stream are cooled and crystallized to form crystals of ammonium thiosulfate and ammonium thiocyanate respectively; the crystallized slurry is subjected to solid-liquid separation to separate the target salt crystals from the ethylene glycol-containing mother liquor; the crystals are dried; at least a portion of the ethylene glycol-containing mother liquor is returned to the secondary negative pressure salt separation treatment and / or the tertiary negative pressure purification treatment; and the mother liquor that is not returned is removed from the recycling path for subsequent processing.

[0069] To monitor the migration of ethanol and ethylene glycol in a multi-stage negative pressure distillation system, sampling locations can be set at the outlet of the static mixer 122, the top condensate outlet and bottom outlet of the first-stage light-weight removal column 131, the first product stream outlet and bottom outlet of the second-stage salt separation column 132, the top condensate outlet and bottom product stream outlet of the tertiary purification column 133, and the mother liquor outlet of the centrifuge 152. After the system enters a stable operating state, samples can be collected from each sampling location at predetermined time intervals to obtain the compositional changes of each material stream.

[0070] The contents of ethanol and ethylene glycol in each sample can be determined by gas chromatography, the moisture content by Karl Fischer method, and the contents of ammonium thiocyanate, ammonium thiosulfate, and ammonium sulfate by ion chromatography or titration analysis. By comparing the contents of ethanol and ethylene glycol in the condensate at the top of the first-stage light-light removal tower 131 and the concentrated light-light removal liquid at the bottom, the distribution state of ethanol and ethylene glycol in the first-stage negative pressure light-light removal process can be determined. By detecting the ethylene glycol content in the centrifuged mother liquor and the dried product, the state of ethylene glycol discharged with the crystallization mother liquor and recycled back can be determined.

[0071] During continuous operation, the control system can adjust the amount of ethanol and ethylene glycol added, the feed rate of the first-stage negative pressure light removal treatment, the mother liquor return rate, and the feed rate of the second-stage salt separation tower 132 and the third-stage purification tower 133 based on the ethanol content in the condensate at the top of the first-stage light removal tower 131, the ethylene glycol content in the concentrated light removal liquid at the bottom of the first-stage light removal tower 131, the ethylene glycol content in the centrifugal separation mother liquor, and the liquid level changes of each stage of the tower, so as to keep the feed and discharge of each stage of the treatment process balanced.

[0072] The purity of ammonium thiosulfate and ammonium thiocyanate products can be determined by ion chromatography, titration, or quantitative analysis methods suitable for ammonium salt products. The residual ethylene glycol in the products can be determined by gas chromatography. Organic solvent recovery rate is calculated as the ratio of the mass of recovered solvent returned to static mixer 122 to the total mass of organic solvent added during the corresponding operating cycle. Product recovery rate is calculated as the ratio of the mass of the target salt in the obtained product to the mass of the corresponding target salt in the original coking desulfurization wastewater. Equipment operational stability can be evaluated based on continuous operating time, pressure drop changes within the tower, bottom discharge volume, packing blockage, and shutdown cleaning cycle.

[0073] Example 1: A coking plant uses the HPF desulfurization process, generating coking desulfurization wastewater. The treatment capacity of this coking desulfurization wastewater is 10 m³ / d. The wastewater contains 8.5% ammonium thiocyanate, 6.2% ammonium thiosulfate, 1.3% ammonium sulfate, and 82.5% water. The remainder consists of impurities such as pyridine, quinoline, suspended sulfur, and catalyst residue.

[0074] The waste liquid is treated using the multi-stage negative pressure distillation system of this invention. In the pretreatment unit, the precision filtration component adopts a two-stage filtration structure, the residence time in the activated carbon adsorption tower is 35 min, and the buffer stirring tank preheats the waste liquid to 45°C with a stirring speed of 400 r / min.

[0075] In the organic solvent mixing unit, the volume ratio of ethanol to ethylene glycol in the ethanol-ethylene glycol compound solvent is 3:1, the volume ratio of the compound solvent to the pretreated waste liquid is 1:5, the static mixer mixing time is 12 min, and the pH of the mixed liquid in the mixing buffer tank is controlled at 7.0.

[0076] In the multi-stage negative pressure distillation separation unit, the first-stage light component removal tower operates at a negative pressure of -0.075 MPa, with a bottom temperature of 75°C, a top temperature of 50°C, and a reflux ratio of 2.5:1; the second-stage salt separation tower operates at a negative pressure of -0.085 MPa, with a bottom temperature of 90°C, a top temperature of 65°C, and a reflux ratio of 3.5:1; and the third-stage purification tower operates at a negative pressure of -0.092 MPa, with a bottom temperature of 105°C, a top temperature of 80°C, and a reflux ratio of 4.5:1.

[0077] In the solvent recovery unit, the solvent distillation column operates at a negative pressure of -0.08 MPa, a bottom temperature of 85°C, a top temperature of 70°C, and a reflux ratio of 2.5:1. In the product refining unit, the crystallization temperature is 25°C, the crystallization time is 5 hours, the vacuum drying temperature is 65°C, and the drying time is 2.5 hours.

[0078] The unit operated continuously and stably for 72 hours, processing a total of 30 m³ of desulfurization wastewater, yielding 2.52 t of ammonium thiocyanate with a purity of 98.3% and 1.83 t of ammonium thiosulfate with a purity of 98.1%. The organic solvent recovery rate was 95.7%, and the total product recovery rate was 96.2%. Compared with conventional distillation units, energy consumption was reduced by 32.5%. During operation, no significant blockage or corrosion was observed, and the low-boiling-point wastewater met discharge standards after subsequent biochemical treatment.

[0079] During operation, samples are taken from the top condensate outlet and bottom outlet of the primary light component removal tower 131, the first product stream outlet of the secondary salt separation tower 132, the bottom product stream outlet of the tertiary purification tower 133, and the mother liquor outlet of the centrifuge 152, and the composition of ethanol, ethylene glycol, and target salt in the samples are analyzed. Based on the ethanol content and recovered solvent composition in the top condensate of the primary light component removal tower 131, ethanol and / or ethylene glycol are added to the solvent storage tank 143 to maintain the ethanol to ethylene glycol volume ratio in the compound solvent returned to the static mixer 122 at 3:1. The flow rate of the crystallization mother liquor returned to the secondary salt separation tower 132 and / or the tertiary purification tower 133 is adjusted according to the ethylene glycol content in the crystallization mother liquor and the liquid level changes in each tower. Thus, ethanol mainly circulates through the condensation and distillation path of the volatile solvent-containing stream at the top of the towers, while ethylene glycol mainly circulates through the mother liquor return path after product purification.

[0080] During continuous operation, the centrifugal separation mother liquor first enters a mother liquor storage tank. The return and discharge rates of the mother liquor are adjusted based on the detection results of ethylene glycol and high-boiling-point impurities. Under normal operating conditions, at least a portion of the ethylene glycol-containing mother liquor is returned to the secondary salt separation tower 132 and / or the tertiary purification tower 133. As the circulation time increases, when the accumulation of non-target high-boiling-point components in the mother liquor is detected, the mother liquor discharge rate is increased, and the discharged mother liquor is sent to a mother liquor treatment device for vacuum concentration. The recovered ethylene glycol-containing liquid phase is returned to the system for reuse, and the concentrated residue is discharged from the system. Through the above dynamic return and discharge treatment of the mother liquor, no salt separation abnormalities caused by the continuous accumulation of ethylene glycol or high-boiling-point impurities occurred during the 72-hour continuous operation of the unit.

[0081] Example 2: A coking plant uses the PDS desulfurization process, generating coking desulfurization wastewater. The treatment capacity of this coking desulfurization wastewater is 8 m³ / d. The wastewater contains 10.2% ammonium thiocyanate, 5.8% ammonium thiosulfate, 1.5% ammonium sulfate, and 81.3% water. The remainder consists of organic impurities and suspended sulfur.

[0082] The multi-stage negative pressure distillation system of this invention is used for treatment. Compared with Example 1, this example adjusts some operating parameters based on the high ammonium thiocyanate content in the PDS desulfurization wastewater. The volume ratio of ethanol to ethylene glycol in the ethanol-ethylene glycol compound solvent remains 3:1, the volume ratio of the compound solvent to the pretreated wastewater is adjusted to 1:4.5, and the static mixer mixing time is 14 min.

[0083] The primary light component removal tower operates at a negative pressure of -0.072 MPa and a reboiler temperature of 72°C; the secondary salt separation tower operates at a negative pressure of -0.082 MPa and a reboiler temperature of 88°C; the tertiary refining tower operates at a negative pressure of -0.09 MPa and a reboiler temperature of 102°C. In the product refining unit, the crystallization temperature is 28°C, the crystallization time is 4.5 h, and the vacuum drying temperature is maintained within the range of 60°C to 70°C.

[0084] During continuous processing, the same sampling and detection methods as in Example 1 were used to monitor the composition of ethanol, ethylene glycol, and target salts in the condensate at the top of the first-stage light-light ...

[0085] The unit operated continuously and stably for 72 hours, processing a total of 24 m³ of desulfurization wastewater, yielding 2.38 t of ammonium thiocyanate with a purity of 98.5% and 1.37 t of ammonium thiosulfate with a purity of 98.2%. The organic solvent recovery rate was 96.1%, and the total product recovery rate was 96.5%. Compared with conventional distillation units, energy consumption was reduced by 31.8%. During operation, the unit operated stably without any significant equipment failures, and the recovered products met industrial-grade reuse requirements.

[0086] As can be seen from the above embodiments, in this invention, before the pretreated waste liquid enters the multi-stage negative pressure distillation and separation unit, ethanol and ethylene glycol are added in a predetermined ratio, and the mixed liquid is sequentially passed through a first-stage negative pressure light component removal, a second-stage negative pressure salt separation, and a third-stage negative pressure purification. Ethanol mainly enters the condensation and distillation recovery path with the volatile solvent-containing stream generated by the first-stage light component removal tower and the third-stage purification tower, while ethylene glycol mainly enters the subsequent salt separation and product purification process with the liquid phase. After the target salt crystallization and solid-liquid separation, it is returned to the second-stage salt separation tower and / or the third-stage purification tower with the mother liquor, thereby forming a cyclic treatment path adapted to volatile components and high-boiling-point liquid phase components respectively, realizing the continuous stepwise recovery of ammonium thiosulfate and ammonium thiocyanate in coking desulfurization waste liquid.

[0087] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A multi-stage negative pressure distillation system for the recovery and purification of coking desulfurization wastewater, characterized in that, include: The pretreatment unit, organic solvent mixing unit, multi-stage negative pressure distillation and separation unit, and product refining unit are connected sequentially along the material flow direction, as well as a solvent recovery unit connecting the multi-stage negative pressure distillation and separation unit and the organic solvent mixing unit; The pretreatment unit includes a precision filtration assembly, a decolorization assembly, and a buffer stirring tank connected in sequence. The buffer stirring tank is used to output the pretreated waste liquid that has undergone impurity removal, decolorization, homogenization, and preheating. The organic solvent mixing unit includes a metering pump, a static mixer, and a mixing buffer tank. The metering pump is used to send the ethanol-ethylene glycol compound solvent into the static mixer, so that the compound solvent is mixed with the pretreated waste liquid before it enters the multi-stage negative pressure distillation and separation unit to form a mixed liquid. The volume ratio of ethanol to ethylene glycol is 3:1, and the volume ratio of the compound solvent to the pretreated waste liquid is 1:4 to 1:

6. The organic solvent mixing unit also includes a solvent composition detection component and a replenishment port. The solvent composition detection component is used to detect the ethanol and ethylene glycol content in the recovered solvent. The replenishment port is used to add ethanol and / or ethylene glycol according to the detection results, so that the volume ratio of ethanol to ethylene glycol fed into the static mixer is adjusted to 3:

1. The multi-stage negative pressure distillation and separation unit includes a first-stage light-removal tower, a second-stage salt separation tower, and a third-stage purification tower connected in series, with the absolute value of the operating negative pressure increasing step by step and the temperature of the tower bottom increasing step by step. The primary light-removal tower has a top outlet for outputting a volatile solvent-containing stream and connected to the solvent recovery unit, and a bottom outlet for outputting a concentrated light-removal solution retaining high-boiling-point solvent components and connected to the secondary salt separation tower. The secondary salt separation tower has a first product stream outlet outputting a first product stream containing ammonium thiosulfate, and a bottom outlet connected to the tertiary purification tower. The top outlet of the tertiary purification tower is connected to the solvent recovery unit, and a bottom product stream outlet outputting a second product stream containing ammonium thiocyanate. Both the first product stream outlet and the bottom product stream outlet are connected to the product purification unit. The solvent recovery unit includes a solvent condenser, a solvent distillation tower, a solvent storage tank, and a transfer pump connected in sequence. The transfer pump recovers the solvent-containing stream and returns it to the static mixer. The product refining unit includes a crystallizer and a solid-liquid separator. The crystallizer is used to form crystals of target salts in the first product stream and the second product stream. The solid-liquid separator is used to separate the crystals from the mother liquor. The mother liquor outlet of the solid-liquid separator is connected to the secondary salt separation tower and / or the tertiary refining tower so that at least a portion of the mother liquor obtained from the solid-liquid separation is returned to the secondary salt separation tower and / or the tertiary refining tower.

2. The multi-stage negative pressure distillation system according to claim 1, characterized in that, The precision filtration assembly includes a first-stage filtration structure and a second-stage filtration structure connected sequentially along the waste liquid flow direction. The first-stage filtration structure is used to remove solid particles and suspended sulfur from the coking desulfurization waste liquid, and the second-stage filtration structure is used to remove fine suspended matter and colloidal impurities from the coking desulfurization waste liquid. The decolorization assembly is used to remove colored impurities, organic matter, and catalyst residues from the coking desulfurization waste liquid.

3. The multi-stage negative pressure distillation system according to claim 2, characterized in that, The first-stage filtration structure is a stainless steel mesh filter with a pore size of 1.0 μm, and the second-stage filtration structure is a polytetrafluoroethylene microfilter with a pore size of 0.2 μm; the decolorization component includes an activated carbon adsorption tower; the buffer mixing tank is equipped with a conical wall scraper agitator, and the bottom of the buffer mixing tank is equipped with a drain port.

4. The multi-stage negative pressure distillation system according to claim 1, characterized in that, The static mixer is equipped with multiple layers of staggered guide vanes. The mixing buffer tank is used to temporarily store and homogenize the mixed liquid output from the static mixer. The mixing buffer tank is equipped with a pH monitoring component for monitoring the pH value of the mixed liquid.

5. The multi-stage negative pressure distillation system according to claim 1, characterized in that, The operating negative pressure of the first-stage light component removal tower is -0.07MPa to -0.08MPa, and the bottom temperature is 70℃ to 80℃; the operating negative pressure of the second-stage salt separation tower is -0.08MPa to -0.09MPa, and the bottom temperature is 85℃ to 95℃; the operating negative pressure of the third-stage refining tower is -0.09MPa to -0.095MPa, and the bottom temperature is 100℃ to 110℃.

6. The multi-stage negative pressure distillation system according to claim 1, characterized in that, The primary light-removal tower, the secondary salt separation tower, and the tertiary refining tower are all packed towers. Each tower is equipped with anti-fouling perforated corrugated packing, and each tower is equipped with a liquid level sensor, a temperature sensor, and a pressure sensor. Each tower has a drain outlet at the bottom for discharging high-boiling-point impurities and sediment residues.

7. The multi-stage negative pressure distillation system according to claim 1, characterized in that, The product refining unit includes a crystallizer, a centrifuge, a dryer, and a product storage tank. The crystallizer is connected to the first product outlet and the bottom product outlet of the tower, respectively. The centrifuge is used to separate the crystals and mother liquor after crystallization. The dryer is used to vacuum dry the crystals. The product storage tank is used to store the dried ammonium thiosulfate product and ammonium thiocyanate product. The centrifuge has a crystal outlet and a mother liquor outlet. The mother liquor outlet is connected to the secondary salt separation tower and / or the tertiary refining tower so that at least a portion of the mother liquor obtained from solid-liquid separation is returned to the secondary salt separation tower and / or the tertiary refining tower.

8. A method for recovering and purifying coking desulfurization wastewater, characterized in that, include: The coking desulfurization waste liquid is subjected to impurity removal, decolorization, homogenization and preheating to obtain pretreated waste liquid; Before the pretreated waste liquid enters the multi-stage negative pressure distillation treatment, the pretreated waste liquid is quantitatively mixed with an ethanol-ethylene glycol compound solvent to form a mixed liquid, wherein the volume ratio of ethanol to ethylene glycol in the ethanol-ethylene glycol compound solvent is 3:1, and the volume ratio of the ethanol-ethylene glycol compound solvent to the pretreated waste liquid is 1:4 to 1:

6. The mixed liquid is subjected to a first-stage negative pressure desulfurization treatment, a second-stage negative pressure desalination treatment, and a third-stage negative pressure refining treatment in sequence, and the absolute value of the operating negative pressure and the treatment temperature increase step by step in the first-stage negative pressure desulfurization treatment, the second-stage negative pressure desalination treatment, and the third-stage negative pressure refining treatment. In the first-stage negative pressure desulfurization process, ethanol, water, and low-boiling-point organic compounds are mixed to form a volatile solvent-containing stream, resulting in a desulfurization concentrate retaining ethylene glycol. This volatile solvent-containing stream then undergoes condensation and distillation. The desulfurization concentrate then enters a second-stage negative pressure salt separation process. In the second-stage negative pressure salt separation process, a first product stream containing ammonium thiosulfate is separated from the desulfurization concentrate, forming an ammonium thiocyanate-rich feed solution that enters the third-stage negative pressure refining process. In the third-stage negative pressure refining process, a solvent-containing stream is separated from the ammonium thiocyanate-rich feed solution, forming a second product stream containing ammonium thiocyanate. The volatile solvent-containing streams generated from the first-stage negative pressure light removal treatment and the third-stage negative pressure refining treatment are condensed and distilled. The ethanol and ethylene glycol contents in the recovered solvent are detected. Based on the detection results, ethanol and / or ethylene glycol are added to adjust the volume ratio of ethanol to ethylene glycol to 3:1 before returning it to the step of mixing with the pretreated waste liquid. The first product stream and the second product stream are subjected to crystallization and solid-liquid separation, respectively, so that the ammonium thiosulfate crystals and ammonium thiocyanate crystals are separated from the mother liquor. The separated crystals are dried, and at least a portion of the mother liquor is returned to the secondary negative pressure salt separation treatment and / or the tertiary negative pressure purification treatment.

9. The method for recovering and purifying coking desulfurization wastewater according to claim 8, characterized in that, The operating negative pressure for the first-stage negative pressure desulfurization treatment is -0.07MPa to -0.08MPa, and the treatment temperature is 70℃ to 80℃; the operating negative pressure for the second-stage negative pressure desalination treatment is -0.08MPa to -0.09MPa, and the treatment temperature is 85℃ to 95℃; the operating negative pressure for the third-stage negative pressure refining treatment is -0.09MPa to -0.095MPa, and the treatment temperature is 100℃ to 110℃.

10. The method for recovering and purifying coking desulfurization wastewater according to claim 8, characterized in that, When the first product stream and the second product stream are subjected to crystallization, solid-liquid separation and drying respectively, the crystallization temperature is 20°C to 30°C and the crystallization time is 4h to 6h, the drying temperature is 60°C to 70°C and the drying time is 2h to 3h, and at least a portion of the mother liquor obtained from solid-liquid separation is returned to the secondary negative pressure salt separation treatment and / or the tertiary negative pressure purification treatment.

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