A lithium iron phosphate wastewater closed loop fractional salt treatment equipment and treatment method

Abstract

The application discloses a lithium iron phosphate wastewater closed-loop fractional salt treatment equipment and a treatment method. The equipment comprises: an original solution preheating system; an evaporation and concentration system, a feed inlet of which is connected with a discharge outlet of the original solution preheating system; a hot crystallization system, the hot crystallization system comprising a gas-liquid separator internally provided with a forced circulation heating unit and a steam compressor connected with a secondary steam outlet of the gas-liquid separator; a flash cooling crystallization system, the flash cooling crystallization system comprising a flash crystallizer, a condensing device connected with a steam outlet of the flash crystallizer and a vacuum pumping device; the flash cooling crystallization system is provided with a phosphorus ammonium salt clear mother liquor outlet, the phosphorus ammonium salt clear mother liquor outlet is connected with a reflux liquid inlet of the hot crystallization system through a reflux pipeline; the phosphorus ammonium salt clear mother liquor outlet forms a unique liquid passage to the hot crystallization system through the reflux pipeline. The lithium iron phosphate wastewater closed-loop fractional salt treatment equipment can reduce the fouling and blocking problems in the cooling crystallization process to a certain extent.

Description

Technical Field

[0001] This invention relates to the field of industrial wastewater treatment technology, specifically to a closed-loop desalination treatment device and method for lithium iron phosphate wastewater. Background Technology

[0002] Lithium iron phosphate (LiFePO4) is used as the cathode material in lithium-ion batteries. During its production and cleaning processes, it generates large amounts of wash water and mother liquor containing ammonium sulfate and monoammonium phosphate. Traditional evaporation methods can only evaporate this wastewater to obtain a mixed salt of ammonium sulfate and monoammonium phosphate, resulting in low economic value and difficulty in resource utilization. Based on the difference in solubility of ammonium sulfate and monoammonium phosphate at different temperatures, ammonium sulfate can be obtained first through thermal crystallization, followed by cooling crystallization to obtain monoammonium phosphate, thus achieving the separation and purification of the two salts.

[0003] In existing technologies, to achieve the aforementioned salt separation objective, some solutions employ a processing system comprising a preheating unit, a falling film evaporation unit, a forced circulation evaporation crystallization unit, and a continuous vacuum flash crystallization unit. However, because this system cannot effectively control the purity of the upstream crystallization mother liquor, an additional impurity salt evaporation unit and insoluble matter removal unit must be installed after the flash crystallization unit to separate and remove impurities and insoluble matter generated during system operation. This not only increases equipment investment and operating costs but also makes the system structure more complex, and the purity of the monoammonium phosphate product is difficult to guarantee.

[0004] Furthermore, conventional cooling crystallization processes commonly employ cooling crystallizers with jacketed or coiled water-cooled heat exchangers. Because monoammonium phosphate solution readily crystallizes on the heat exchanger walls during cooling, this leads to scaling and blockage inside the cooler, causing a sharp decline in heat exchange efficiency. This severely impacts continuous production, necessitating frequent shutdowns for cleaning and hindering improvements in production efficiency. Summary of the Invention

[0005] This invention aims to at least partially address one of the problems of the prior art. To this end, this invention proposes a closed-loop salt separation treatment device for lithium iron phosphate wastewater. This device has a compact structure and can achieve efficient separation and recovery of ammonium sulfate and monoammonium phosphate without the need for a separate miscellaneous salt discharge unit. Simultaneously, it can reduce scaling and clogging problems during the cooling and crystallization process to a certain extent.

[0006] The present invention also provides a processing method.

[0007] According to one aspect of the present invention, a closed-loop desalination treatment device for lithium iron phosphate wastewater includes: Original liquid preheating system; An evaporation and concentration system, wherein the inlet of the evaporation and concentration system is connected to the outlet of the raw liquid preheating system, and the evaporation and concentration system includes a falling film evaporation unit and a steam compressor connected to the secondary steam outlet of the falling film evaporation unit; A thermal crystallization system, wherein the inlet of the thermal crystallization system is connected to the outlet of the concentrated liquid of the evaporation and concentration system, and the thermal crystallization system includes a gas-liquid separator with a forced circulation heating unit inside, and a steam compressor connected to the secondary steam outlet of the gas-liquid separator. A flash cooling crystallization system, wherein the feed inlet of the flash cooling crystallization system is connected to the ammonium salt clear mother liquor outlet of the hot crystallization system, and the flash cooling crystallization system includes a flash crystallizer, a condensing device connected to the steam outlet of the flash crystallizer, and a vacuuming device; The flash cooling crystallization system is provided with a phosphate ammonium salt clear mother liquor outlet, which is connected to the reflux inlet of the hot crystallization system through a reflux pipeline. The outlet of the ammonium phosphate mother liquor, via the reflux pipeline, forms the only liquid passage to the thermal crystallization system.

[0008] This invention connects an evaporation and concentration system, a thermal crystallization system, and a flash cooling crystallization system in series in a specific order. A single liquid passage is established between the flash cooling crystallization system and the thermal crystallization system, from the ammonium phosphate mother liquor outlet through a reflux pipeline to the reflux liquid inlet. This allows the ammonium phosphate mother liquor separated after thermal crystallization to directly enter the flash cooling crystallization system for cooling and crystallization. All the ammonium phosphate mother liquor separated after flash cooling crystallization is then returned to the thermal crystallization system for circulation, forming a complete closed-loop circulation circuit. Because the material circulates continuously within the closed loop, there is no need for additional impurity removal units such as a mixed salt evaporation unit or an insoluble matter removal unit. Ammonium sulfate crystals can be produced from the thermal crystallization system, and monoammonium phosphate crystals can be produced from the flash cooling crystallization system. Furthermore, since the flash cooling crystallization system uses a flash crystallizer combined with a condenser and a vacuum device, cooling and crystallization of the solution are achieved through vacuum flash evaporation. This eliminates the need for traditional water-cooled heat exchangers, structurally reducing the problem of scale buildup and blockage in heat exchangers during the cooling crystallization process.

[0009] In some embodiments, the stock solution preheating system includes a stock solution tank, a stock solution pump, a distilled water preheater, a non-condensable gas preheater, and a fresh steam preheater connected sequentially by pipelines, wherein the distilled water preheater is connected to a distilled water tank via a distilled water pump. The distilled water tank is also connected to the fresh steam preheater via a pipeline to collect the distilled water generated by steam condensation in the evaporation and concentration system and the thermal crystallization system, and to pump the water into the distilled water preheater to exchange heat with the original liquid.

[0010] In some embodiments, the falling film evaporation unit is a double-effect falling film evaporation unit, comprising a first-effect falling film evaporator and a second-effect falling film evaporator connected in series. The first-effect falling film evaporator includes a first-effect falling film heat exchanger and a first-effect falling film separator. The second-effect falling film evaporator includes a second-effect falling film heat exchanger and a second-effect falling film separator. The steam compressor is a concentrated steam compressor, the inlet of which is connected to the secondary steam outlet of the second-effect falling film separator, and the outlet of which is connected to the heat source inlet of the first-effect falling film heat exchanger. A concentrated gas scrubbing tower is also provided between the double-effect falling film separator and the concentrated steam compressor; The evaporation and concentration system also includes a falling film transfer pump, which is used to transport the concentrated liquid that has reached the required concentration in the double-effect falling film evaporator to the thermal crystallization system.

[0011] In some embodiments, the forced circulation heating unit of the thermal crystallization system includes: At least one stage of forced circulation pump and forced circulation heat exchanger, wherein the inlet of the forced circulation pump is connected to the liquid outlet of the gas-liquid separator, the outlet is connected to the feed inlet of the forced circulation heat exchanger, and the discharge outlet of the forced circulation heat exchanger is connected to the gas-liquid separator; An ammonium salt discharge pump, the inlet of which is connected to the crystal slurry outlet of the gas-liquid separator; An ammonium salt centrifuge, the inlet of which is connected to the outlet of the ammonium salt discharge pump, and the clear mother liquor outlet of the ammonium salt centrifuge constitutes the clear mother liquor outlet of the ammonium salt in the thermal crystallization system; A crystallization scrubbing tower is also provided between the gas-liquid separator and the steam compressor.

[0012] In some embodiments, the flash cooling crystallization system further includes: A phosphate ammonium salt discharge pump, the inlet of which is connected to the crystal slurry outlet of the flash crystallizer; The inlet of the ammonium phosphate centrifuge is connected to the outlet of the ammonium phosphate discharge pump, and the clear mother liquor outlet of the ammonium phosphate centrifuge constitutes the clear mother liquor outlet of the flash cooling crystallization system.

[0013] In some embodiments, the thermal crystallization system further includes an ammonium salt mother liquor tank and an ammonium salt mother liquor pump, wherein the inlet of the ammonium salt mother liquor tank is connected to the mother liquor outlet of the ammonium salt centrifuge, and the outlet of the ammonium salt mother liquor pump is connected to the return port of the gas-liquid separator. The flash cooling crystallization system also includes a phosphate ammonium salt mother liquor tank and a phosphate ammonium salt mother liquor pump. The inlet of the phosphate ammonium salt mother liquor tank is connected to the mother liquor outlet of the phosphate ammonium salt centrifuge, and the outlet of the phosphate ammonium salt mother liquor pump is connected to the return port of the flash crystallizer.

[0014] According to another aspect of the present invention, a closed-loop desalination treatment method for lithium iron phosphate wastewater includes the following steps: S1. Pass the lithium iron phosphate wastewater into the raw liquid preheating system and preheat it to the evaporation temperature; S2. The preheated material is fed into the evaporation and concentration system for evaporation and concentration to obtain a saturated solution; S3. The saturated solution is introduced into the thermal crystallization system for forced circulating thermal crystallization to precipitate ammonium sulfate crystals. The ammonium sulfate crystals and ammonium salt mother liquor are obtained by centrifugation. S4. The ammonium salt mother liquor obtained in step S3 is fed into the flash cooling crystallization system. The monoammonium phosphate crystals are precipitated by vacuum flash cooling. The monoammonium phosphate crystals and ammonium phosphate mother liquor are obtained by centrifugation. The material fed into the flash cooling crystallization system is composed of the ammonium salt mother liquor. S5. All the phosphate ammonium salt mother liquor obtained in step S4 is transported back to the thermal crystallization system for cyclic thermal crystallization.

[0015] By sequentially evaporating and concentrating lithium iron phosphate wastewater, followed by thermal crystallization and flash cooling crystallization, and then recirculating all the mother liquor separated from ammonium phosphate after flash cooling crystallization back to the thermal crystallization system, a complete closed-loop salt separation process is formed. This eliminates the need for additional steps in the process to evaporate impurities and remove insoluble substances, achieving efficient separation and separate recovery of ammonium sulfate and monoammonium phosphate. The process flow is simpler and more compact, significantly reducing equipment investment and operating costs. Furthermore, the flash cooling crystallization step utilizes vacuum flash evaporation to cool the ammonium salt mother liquor and precipitate monoammonium phosphate. The material fed into the flash cooling crystallization system consists solely of the ammonium salt mother liquor, without the addition of other materials. The dilution process simplifies operation, reduces material throughput, and fundamentally avoids scaling and clogging issues caused by water-cooled heat exchangers in traditional cooling crystallization processes, ensuring long-term continuous and stable operation. Thermal crystallization and flash cooling crystallization produce ammonium sulfate and monoammonium phosphate crystals respectively, achieving resource recovery of these two salts from wastewater without generating any impurities, resulting in significant economic and environmental benefits. Furthermore, the secondary steam generated in the evaporation and concentration and thermal crystallization steps is compressed by a steam compressor and recycled as the evaporation heat source for each step. During normal continuous operation, virtually no external fresh steam is consumed, significantly reducing operating energy consumption.

[0016] In some embodiments, in step S2, the evaporation and concentration system is a double-effect falling film evaporation system, the secondary steam generated by the first-effect falling film separator is used as the heat source of the second-effect falling film heat exchanger, and the secondary steam generated by the second-effect falling film separator is pressurized and heated to 110°C and 143.3 kPa by the concentrated steam compressor and then used as the heat source of the first-effect falling film heat exchanger.

[0017] In some embodiments, in step S3, the gas-liquid separator maintains a slight negative pressure of 70.1 kPa, and the generated secondary steam has a temperature of 90°C. After being pressurized and heated to 110°C and 143.3 kPa by the crystallization steam compressor, it is sent to the forced circulation heat exchanger as a heat source.

[0018] In some embodiments, in step S4, the flash temperature for flash cooling crystallization is 45°C; During normal continuous operation, the entire heat source for evaporation of the evaporation concentration system and the thermal crystallization system is provided by their respective steam compressors, without consuming external fresh steam. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A schematic diagram of a closed-loop salt separation treatment device for lithium iron phosphate wastewater provided in an embodiment of this application.

[0021] Figure 2 for Figure 1 An enlarged schematic diagram of part A in the middle.

[0022] Figure 3 for Figure 1 Enlarged diagram of part B.

[0023] Figure 4 for Figure 1 An enlarged schematic diagram of section C.

[0024] Figure label: 10-Crude oil preheating system; 111-Crude oil tank; 112-Crude oil pump; 113-Distilled water preheater; 114-Non-condensable gas preheater; 115-Fresh steam preheater; 20-Evaporation and concentration system; 21-Single-effect falling film heat exchanger; 22-Single-effect falling film separator; 23-Single-effect falling film circulating pump; 24-Double-effect falling film heat exchanger; 25-Double-effect falling film separator; 26-Double-effect falling film circulating pump; 27-Falling film transfer pump; 30-Hot crystallization system; 31-Gas-liquid separator; 32-First-stage forced circulation heat exchanger; 33-First-stage forced circulation pump; 34-Second-stage forced circulation heat exchanger; 35-Second-stage forced circulation pump; 36-Ammonium salt discharge pump; 37-Ammonium salt centrifuge; 38-Ammonium salt mother liquor tank; 39-Ammonium salt mother liquor pump; 301-Ammonium salt clear mother liquor pump 40-Flash cooling crystallization system; 41-Flash crystallizer; 42-Flash condenser; 43-Flash vacuum system; 44-Phosphorus ammonium salt discharge pump; 45-Phosphorus ammonium salt centrifuge; 46-Phosphorus ammonium salt mother liquor tank; 47-Phosphorus ammonium salt mother liquor pump; 48-Phosphorus ammonium salt clear mother liquor tank; 49-Phosphorus ammonium salt clear mother liquor pump; 410-Flash propeller; 51-Concentrating steam compressor; 52-Concentrating scrubbing tower; 53-Concentrating scrubbing circulating pump; 54-Concentrating liquid collection pump; 55-Crystallizing steam compressor; 56-Crystallizing scrubbing tower; 57-Crystallizing scrubbing circulating pump; 58-Crystallizing liquid collection pump; 59-Spray water pump; 510-Evaporating vacuum pump; 511-Evaporating vacuum pump condenser; 61-Distilled water tank; 62-Distilled water pump; 63-Ammonium salt mother liquor tank. Detailed Implementation

[0025] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0026] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0027] In the description of this invention, "several" means one or more, "more than" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0028] In the description of this invention, unless otherwise explicitly defined, terms such as "setting," "installing," and "connecting" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0029] In the production and cleaning processes of lithium iron phosphate cathode materials, wash water and mother liquor containing ammonium sulfate and monoammonium phosphate are continuously generated. Traditional evaporation processes for treating this wastewater only yield a mixed salt of ammonium sulfate and monoammonium phosphate, resulting in low added value and difficulty in achieving economically viable resource recovery. Direct discharge causes severe environmental pollution and resource waste. Utilizing the difference in solubility of the two salts at different temperatures—ammonium sulfate's solubility changes relatively little with temperature, while monoammonium phosphate's solubility decreases significantly with decreasing temperature—ammonium sulfate can be preferentially precipitated through evaporation crystallization at a higher temperature. The remaining solution can then be cooled and crystallized to separate monoammonium phosphate, thus achieving separate recovery of both and transforming the original waste into a marketable industrial-grade product.

[0030] Existing technologies have proposed a processing system to achieve the aforementioned salt separation target, comprising a preheating unit, a falling film evaporation unit, a forced circulation evaporation crystallization unit, and a continuous vacuum flash crystallization unit. However, in actual operation, this scheme struggles to effectively control the impurity content of the upstream crystallization mother liquor. With long-term system operation, impurities continuously accumulate in the mother liquor, necessitating the addition of a mixed salt evaporation unit and an insoluble matter discharge unit at the downstream end of the flash crystallization unit to divert and dispose of the accumulated mixed salts and insoluble matter. This not only increases equipment investment and operation and maintenance costs but also makes the entire system more complex, with intricate piping connections and a larger footprint. Furthermore, due to impurities interfering with the crystallization process, the purity of the monoammonium phosphate product is difficult to reliably guarantee.

[0031] On the other hand, conventional cooling crystallization processes typically employ cooling crystallizers equipped with jacketed or coiled water-cooled heat exchangers. Because monoammonium phosphate solutions readily precipitate crystal layers on the heat exchanger walls during cooling, the continuous accumulation of these crystal layers leads to scaling and blockage inside the cooler, drastically reducing heat exchange efficiency. This severely disrupts continuous production, forcing frequent equipment shutdowns for cleaning. This scaling problem not only increases the workload of manual cleaning and maintenance but also reduces equipment utilization due to frequent start-ups and shutdowns, hindering further improvements in production efficiency and becoming a major technical challenge for those skilled in the art.

[0032] To address the problems of complex system structure, scale buildup and clogging in coolers, and low product purity in existing technologies, the present invention aims to provide a closed-loop salt separation treatment device for lithium iron phosphate wastewater. This device connects an evaporation and concentration system, a thermal crystallization system, and a flash cooling crystallization system in a specific sequence. A reflux pipeline connects the phosphate ammonium salt mother liquor outlet of the flash cooling crystallization system to the reflux liquid inlet of the thermal crystallization system, forming a single liquid pathway to the thermal crystallization system, thus creating a closed-loop circulation of materials throughout the system. Based on this structure, the system can achieve efficient separation and separate recovery of ammonium sulfate and monoammonium phosphate without the need for separate salt evaporation and insoluble matter discharge units. The overall structure of the equipment is more compact, the pipeline connections are simplified, and the equipment investment and operation and maintenance costs are significantly reduced. Meanwhile, the flash cooling crystallization system uses a flash crystallizer in conjunction with a condenser and a vacuum device. It achieves cooling of the solution and crystallization by vacuum flash evaporation, completely eliminating the water-cooled heat exchanger in the traditional cooling crystallization process. This eliminates the clogging problem caused by scaling on the heat exchange wall from the equipment structure, ensuring long-term continuous and stable operation of the equipment and effectively improving the purity of the monoammonium phosphate crystallized product.

[0033] Specifically, the beneficial effects of the present invention are as follows: by connecting the evaporation and concentration system, the thermal crystallization system, and the flash cooling crystallization system in a specific order, and setting up a unique liquid passage from the ammonium phosphate clear mother liquor outlet through the reflux pipeline to the reflux liquid inlet, a closed-loop material circulation is formed. It is possible to achieve effective separation of ammonium sulfate and monoammonium phosphate without setting up a miscellaneous salt evaporation unit and an insoluble matter discharge unit in the system. The equipment structure is more compact, and the investment and maintenance costs are significantly reduced.

[0034] Meanwhile, the flash cooling crystallization system uses a flash crystallizer in conjunction with a condenser and a vacuum device. It utilizes vacuum flash evaporation to cool the solution and precipitate crystals, completely eliminating the water-cooled heat exchanger in the traditional cooling crystallization process. This eliminates the problem of scale buildup and blockage in the cooler from the equipment structure, ensuring long-term stable operation of the equipment. The thermal crystallization and flash cooling crystallization produce ammonium sulfate crystals and monoammonium phosphate crystals, respectively, realizing the resource recovery of the two salts in the wastewater without producing impurities, resulting in significant economic and environmental benefits. In addition, both the evaporation concentration system and the thermal crystallization system are equipped with steam compressors. During normal continuous operation, the high-temperature and high-pressure steam after secondary steam compression can be used as the entire heat source for evaporation, significantly reducing dependence on external fresh steam and resulting in low operating energy consumption.

[0035] like Figure 1As shown, this embodiment provides a closed-loop salt separation treatment device for lithium iron phosphate wastewater, including a raw liquid preheating system 10, an evaporation and concentration system 20, a thermal crystallization system 30, and a flash cooling crystallization system 40. The inlet of the evaporation and concentration system 20 is connected to the outlet of the raw liquid preheating system 10. The evaporation and concentration system 20 includes a falling film evaporation unit and a steam compressor connected to the secondary steam outlet of the falling film evaporation unit. The inlet of the thermal crystallization system 30 is connected to the concentrated liquid outlet of the evaporation and concentration system 20. The thermal crystallization system 30 includes a gas-liquid separator 31 with an internal forced circulation heating unit and a steam compressor connected to the secondary steam outlet of the gas-liquid separator 31. The inlet of the flash cooling crystallization system 40 is connected to the ammonium salt clear mother liquor outlet of the thermal crystallization system 30. The flash cooling crystallization system 40 includes a flash crystallizer 41, a flash condenser 42 connected to the steam outlet of the flash crystallizer 41, and a flash vacuum system 43. The flash cooling crystallization system 40 is provided with a phosphate ammonium salt clear mother liquor outlet. This phosphate ammonium salt clear mother liquor outlet is connected to the reflux liquid inlet of the thermal crystallization system 30 through a reflux pipeline, and the phosphate ammonium salt clear mother liquor outlet forms the only liquid passage to the thermal crystallization system 30 through the reflux pipeline.

[0036] The four systems described above are connected sequentially according to the material flow direction, forming a continuous wastewater treatment and salt separation production line. Specifically, the evaporation and concentration system 20 receives the preheated raw liquid and concentrates it through falling film evaporation to obtain a near-saturated concentrate; the thermal crystallization system 30 receives the concentrate from the evaporation and concentration system 20 and causes ammonium sulfate to crystallize out due to supersaturation through forced circulation heating and flash evaporation; the flash cooling crystallization system 40 receives the ammonium salt mother liquor separated from the thermal crystallization system 30 and lowers the solution temperature through vacuum flash cooling and water evaporation, causing monoammonium phosphate to crystallize out due to reduced solubility. Since the ammonium phosphate mother liquor outlet of the flash cooling crystallization system 40 forms the only liquid passage to the thermal crystallization system 30 via a return pipeline, the material forms a complete closed-loop circulation circuit within the system. No miscellaneous salts requiring external discharge are generated during system operation. Therefore, there is no need to set up miscellaneous salt evaporation units and insoluble matter discharge units in the system; ammonium sulfate crystals can be produced from the thermal crystallization system 30, and monoammonium phosphate crystals can be produced from the flash cooling crystallization system 40. Compared with existing technologies, the equipment structure is more compact, the pipeline connection is simplified, and the equipment investment and operation and maintenance costs are significantly reduced. At the same time, since the flash cooling crystallization system 40 uses a flash crystallizer 41 in conjunction with a flash condenser 42 and a flash vacuum system 43 to achieve solution cooling and crystal precipitation through vacuum flash evaporation, the water-cooled heat exchanger in the traditional cooling crystallization process is completely eliminated. This eliminates the clogging problem caused by scaling on the heat exchange wall from the equipment structure, ensuring long-term continuous and stable operation of the equipment and effectively improving the purity of the monoammonium phosphate crystallized product.

[0037] As a further definition of the raw liquid preheating system 10, the raw liquid preheating system 10 includes a raw liquid tank 111, a raw liquid pump 112, a distilled water preheater 113, a non-condensable gas preheater 114, and a fresh steam preheater 115, which are connected sequentially by pipelines. The distilled water preheater 113 is connected to a distilled water tank 61 via a distilled water pump 62. The distilled water tank 61 is also connected to the fresh steam preheater 115 via pipelines. It is used to collect distilled water generated by steam condensation in the evaporation concentration system 20 and the thermal crystallization system 30, and sends it to the distilled water preheater 113 for heat exchange with the raw liquid via the distilled water pump 62. The raw liquid tank 111 is used to temporarily store the lithium iron phosphate washing water and mother liquor to be treated. The raw liquid pump 112 pumps the material in the raw liquid tank 111 sequentially into the distilled water preheater 113, the non-condensable gas preheater 114, and the fresh steam preheater 115. After being heated to an evaporation temperature of approximately 100°C through staged heat exchange, it enters the evaporation concentration system 20. The distilled water preheater 113, non-condensable gas preheater 114, and fresh steam preheater 115 all employ plate heat exchangers, which offer high heat exchange efficiency and a compact structure. The distilled water tank 61 collects high-temperature distilled water (approximately 105°C) generated by steam condensation in the system. The distilled water pump 62 pumps this high-temperature distilled water to the hot side of the distilled water preheater 113 to exchange heat with the raw liquid, utilizing the system's residual heat to preheat the raw liquid. The distilled water after heat exchange is discharged from the system, thereby reducing the system's consumption of external energy. Saturated fresh steam at a pressure of 0.1 MPaG and a temperature of 120°C is introduced to the hot side of the fresh steam preheater 115 as a heat source for system startup and replenishment. Non-condensable gas preheater 114 is introduced to the hot side of the non-condensable gas generated in the system, utilizing its residual heat to further preheat the raw liquid. Through the aforementioned three-stage preheating setup, the waste heat resources within the system are fully utilized, progressively increasing the temperature of the raw liquid. This ensures that the raw liquid reaches a near-boiling point evaporation temperature before entering the evaporation and concentration system 20, significantly reducing the system's need for external fresh steam during normal operation. It is understandable that the arrangement of the distilled water preheater 113, non-condensable gas preheater 114, and fresh steam preheater 115 can be adjusted appropriately based on actual operating conditions and heat energy sources. The number of preheaters can also be increased or decreased according to the needs of waste heat utilization.

[0038] As a further definition of the falling film evaporation unit in the evaporation and concentration system 20, the falling film evaporation unit is a double-effect falling film evaporation unit, comprising a single-effect falling film evaporator and a double-effect falling film evaporator connected in series. The single-effect falling film evaporator includes a single-effect falling film heat exchanger 21 and a single-effect falling film separator 22, and the double-effect falling film evaporator includes a double-effect falling film heat exchanger 24 and a double-effect falling film separator 25. The steam compressor is a concentrated steam compressor 51. The inlet of the concentrated steam compressor 51 is connected to the secondary steam outlet of the double-effect falling film separator 25, and the outlet is connected to the heat source inlet of the single-effect falling film heat exchanger 21. A concentration scrubbing tower 52 is also provided between the double-effect falling film separator 25 and the concentrated steam compressor 51. The evaporation and concentration system 20 also includes a falling film transfer pump 27, used to transport the concentrated liquid that has reached the required concentration in the double-effect falling film evaporator to the thermal crystallization system 30. During operation, the material flows down the tube wall in a film form from the top in the first-effect falling film heat exchanger 21, exchanging heat with the heating steam in the shell side. Some water evaporates, increasing the material concentration. The first-effect falling film separator 22 separates the secondary steam (approximately 101°C) generated by evaporation from the concentrate. The secondary steam is sent to the shell side of the second-effect falling film heat exchanger 24 as a heating source, while the concentrate is partially circulated back to the first-effect falling film heat exchanger 21 by the first-effect falling film circulation pump 23 for further evaporation and concentration, and partially sent to the second-effect falling film heat exchanger 24 for further evaporation and concentration. The second-effect falling film separator 25 operates under a slight negative pressure of approximately 70.1 kPa, generating secondary steam at approximately 90°C. After being washed by the concentration scrubbing tower 52, the secondary steam enters the concentration steam compressor 51, where it is compressed and heated to approximately 110°C and 143.3 kPa before being sent to the shell side of the first-effect falling film heat exchanger 21 as an evaporation heat source, replacing the fresh steam. The aforementioned configuration of a double-effect falling film evaporator combined with a steam compressor allows the secondary steam generated in the first effect to serve as a heat source for the second effect, and the secondary steam generated in the second effect, after compression, to serve as a heat source for the first effect. This achieves cascaded utilization and closed-loop circulation of steam thermal energy, significantly reducing the energy consumption of evaporation and concentration. The concentration scrubbing tower 52 is equipped with a concentration scrubbing circulation pump 53, used for circulating spray scrubbing to remove droplets and foam entrained in the secondary steam, preventing impurities from entering the concentration steam compressor 51 and affecting its normal operation. A concentration sludge pump 54 is connected to the inlet pipe of the concentration steam compressor 51 to remove sludge from the pipe. The sludge is discharged through a pipeline into a distillation water tank 61 for recycling, further improving water resource utilization. It is understood that, depending on the processing scale and energy consumption requirements, the falling film evaporator unit can also be configured as a single-effect, triple-effect, or multi-effect unit, and the steam compressor can be a centrifugal, Roots, or screw compressor.

[0039] As a further definition of the forced circulation heating unit and supporting discharge separation equipment in the thermal crystallization system 30, the forced circulation heating unit of the thermal crystallization system 30 includes at least one stage of forced circulation pump and forced circulation heat exchanger. The inlet of the forced circulation pump is connected to the liquid outlet of the gas-liquid separator 31, and the outlet is connected to the feed inlet of the forced circulation heat exchanger. The discharge outlet of the forced circulation heat exchanger is connected to the gas-liquid separator 31. The thermal crystallization system 30 also includes an ammonium salt discharge pump 36 and an ammonium salt centrifuge 37. The inlet of the ammonium salt discharge pump 36 is connected to the crystal slurry outlet of the gas-liquid separator 31, and the inlet of the ammonium salt centrifuge 37 is connected to the outlet of the ammonium salt discharge pump 36. The clear mother liquor outlet of the ammonium salt centrifuge 37 constitutes the clear mother liquor outlet of the ammonium salt in the thermal crystallization system 30. A crystallization scrubbing tower 56 is also provided between the gas-liquid separator 31 and the crystallization steam compressor 55. During operation, the gas-liquid separator 31 operates under a slight negative pressure of approximately 70.1 kPa. The concentrated liquid, after forced circulation heating, returns to the gas-liquid separator 31 for flash evaporation. Moisture is vaporized and carried away, resulting in a supersaturated solution. Ammonium sulfate crystals precipitate and deposit at the bottom of the gas-liquid separator 31. The crystallization scrubbing tower 56 is used to wash away droplets and foam entrained in the secondary steam, and is equipped with a crystallization scrubbing circulation pump 57 for circulating spray washing. The crystallization steam compressor 55 compresses and heats the purified secondary steam to approximately 110°C and 143.3 kPa, then sends it to the shell side of the forced circulation heat exchanger as a heating source, achieving closed-loop recycling of the secondary steam. The ammonium salt discharge pump 36 sends the crystal slurry containing ammonium sulfate crystals to the ammonium salt centrifuge 37 for centrifugal separation, obtaining industrial-grade ammonium sulfate salt. A crystallization slurry pump 58 can be connected to the inlet pipe of the crystallization steam compressor 55 to remove accumulated slurry.

[0040] As a further limitation on the discharge separation equipment in the flash cooling crystallization system 40, the flash cooling crystallization system 40 also includes a phosphate discharge pump 44 and a phosphate centrifuge 45. The inlet of the phosphate discharge pump 44 is connected to the crystal slurry outlet of the flash crystallizer 41, and the inlet of the phosphate centrifuge 45 is connected to the outlet of the phosphate discharge pump 44. The clear mother liquor outlet of the phosphate centrifuge 45 constitutes the clear mother liquor outlet of the flash cooling crystallization system 40. During operation, the flash vacuum system 43 evacuates the flash crystallizer 41, maintaining a low-pressure state inside. The water in the solution flashes and vaporizes under low pressure, carrying away a large amount of heat, and the solution temperature drops rapidly to about 45°C. At the same time, some water evaporates, causing the solubility of monoammonium phosphate to decrease and crystallize out. The secondary steam generated by flash evaporation enters the flash condenser 42 and is condensed. The condensate can be recycled. The phosphate discharge pump 44 sends the crystal slurry containing monoammonium phosphate crystals to the phosphate centrifuge 45 for centrifugal separation to obtain industrial-grade monoammonium phosphate salt. By employing vacuum flash evaporation to cool and crystallize the solution, the water-cooled heat exchanger in traditional cooling crystallization processes is completely eliminated, eradicating the problem of scale buildup and clogging in the cooler from the equipment structure. The flash crystallizer 41 is equipped with a flash propeller 410 to agitate the crystal slurry, preventing crystallization particles from depositing at the bottom and clogging the outlet, maintaining material flow, and further ensuring continuous and stable operation of the equipment. It is understood that the flash condenser 42 can be a shell-and-tube, plate, or direct-contact condenser, and the flash vacuum system 43 can employ vacuum equipment such as a water ring vacuum pump, a Roots vacuum pump, or a steam jet pump.

[0041] In some embodiments, the thermal crystallization system 30 further includes an ammonium salt mother liquor tank 38 and an ammonium salt mother liquor pump 39. The inlet of the ammonium salt mother liquor tank 38 is connected to the mother liquor outlet of the ammonium salt centrifuge 37, and the outlet of the ammonium salt mother liquor pump 39 is connected to the return port of the gas-liquid separator 31. During operation, the saturated ammonium salt mother liquor discharged after centrifugation by the ammonium salt centrifuge 37 enters the ammonium salt mother liquor tank 38 and is sent back to the gas-liquid separator 31 by the ammonium salt mother liquor pump 39 to continue participating in the circulating thermal crystallization, so that the uncrystallized ammonium sulfate re-enters the thermal crystallization process, thereby improving the recovery rate of ammonium sulfate.

[0042] Furthermore, the flash cooling crystallization system 40 also includes a phosphate mother liquor tank 46 and a phosphate mother liquor pump 47. The inlet of the phosphate mother liquor tank 46 is connected to the mother liquor outlet of the phosphate centrifuge 45, and the outlet of the phosphate mother liquor pump 47 is connected to the return port of the flash crystallizer 41. During operation, the low-temperature saturated phosphate mother liquor discharged after centrifugation by the phosphate centrifuge 45 enters the phosphate mother liquor tank 46 and is sent back to the flash crystallizer 41 by the phosphate mother liquor pump 47 for continued circulation and crystallization. This allows the uncrystallized monoammonium phosphate to re-enter the cooling crystallization process, improving the recovery rate of monoammonium phosphate. At the same time, the purified phosphate mother liquor is transported to the gas-liquid separator 31 of the regenerative crystallization system 30 via the phosphate mother liquor pump 49 and the return pipeline to continue participating in the circulating thermal crystallization, forming the only liquid passage from the flash cooling crystallization system 40 to the thermal crystallization system 30.

[0043] As a further optimization of the forced circulation heating unit in the thermal crystallization system 30, the forced circulation pump and forced circulation heat exchanger of the thermal crystallization system 30 are configured in two stages, including a first-stage forced circulation heat exchanger 32 and a first-stage forced circulation pump 33 connected in parallel, and a second-stage forced circulation heat exchanger 34 and a second-stage forced circulation pump 35. The bottom outlet of the gas-liquid separator 31 is connected to the inlet of the first-stage forced circulation pump 33 and the second-stage forced circulation pump 35, respectively. The outlet of the first-stage forced circulation pump 33 is connected to the inlet of the first-stage forced circulation heat exchanger 32, and the outlet of the first-stage forced circulation heat exchanger 32 is connected to the gas-liquid separator 31. The outlet of the second-stage forced circulation pump 35 is connected to the inlet of the second-stage forced circulation heat exchanger 34, and the outlet of the second-stage forced circulation heat exchanger 34 is connected to the gas-liquid separator 31. By configuring the two stages in parallel, the heat exchange area and circulation flow rate are increased, allowing the concentrate to obtain more heat per unit time, resulting in higher flash evaporation efficiency, more complete ammonium sulfate crystallization, and improved thermal crystallization efficiency and production capacity. The crystallization steam compressor 55 sends the compressed high-temperature, high-pressure steam to the shell side of the primary forced circulation heat exchanger 32 and the secondary forced circulation heat exchanger 34 as a heating source. It is understood that, depending on the production capacity requirements, the forced circulation heating unit can be configured as a single-stage, two-stage, or more-stage parallel system. Each stage can use the same heat exchange area or different heat exchange areas to adapt to different heat load distributions.

[0044] Furthermore, the device of the present invention also includes a distilled water tank 61 and a distilled water pump 62. The distilled water tank 61 collects the condensate discharged from the shell side of the steam compressor in the evaporation and concentration system 20 and the thermal crystallization system 30. The distilled water pump 62 sends the distilled water from the distilled water tank 61 into the raw liquid preheating system 10 to exchange heat with the raw liquid before discharge. During system operation, the high-temperature and high-pressure steam output from the concentration steam compressor 51 and the crystallization steam compressor 55 heats the material in the shell side of their respective heat exchangers and then condenses into high-temperature distilled water. This part of the distilled water has a high temperature and calorific value. If it is discharged directly, it will waste heat and water resources. After being collected uniformly by the distilled water tank 61, it is sent by the distilled water pump 62 into the raw liquid preheating system 10 to exchange heat with the raw liquid. The waste heat is used to preheat the raw liquid. The distilled water after heat exchange and cooling is then discharged from the system, which not only recovers waste heat and reduces system energy consumption, but also realizes the cascade utilization of water resources.

[0045] In this embodiment, during normal continuous operation, all the evaporation heat sources for the evaporation concentration system 20 and the thermal crystallization system 30 are provided by the concentration steam compressor 51 and the crystallization steam compressor 55, respectively, with virtually no consumption of external fresh steam. The materials within the system are continuously circulated in a closed loop. Ammonium sulfate and monoammonium phosphate in the lithium iron phosphate wash water and mother liquor are recovered as industrial-grade crystalline salts, without generating any impurity salt emissions. Compared to existing technologies, this equipment connects evaporation concentration, thermal crystallization, and flash cooling crystallization in a specific sequence, forming a closed-loop material flow through a return pipeline, resulting in a compact structure. Flash cooling replaces traditional water-cooled crystallization, eliminating the industry problem of scale buildup and blockage in the cooler. The elimination of impurity salt evaporation and insoluble matter discharge units significantly reduces investment and maintenance costs. The return of ammonium salt mother liquor and ammonium phosphate mother liquor to their respective systems, along with the recovery of waste heat from distilled water, maximizes the closed-loop utilization of materials and thermal energy. The production of two types of industrial-grade crystalline salts achieves zero wastewater discharge and full resource recovery.

[0046] This invention also provides a closed-loop desalination treatment method for lithium iron phosphate wastewater, which utilizes the closed-loop desalination treatment equipment for lithium iron phosphate wastewater described in the above embodiments. The main components of the wastewater are ammonium sulfate and monoammonium phosphate, with a concentration of approximately 10%. Using the method of this embodiment, ammonium sulfate and monoammonium phosphate in the wastewater can be recovered as industrial-grade crystalline salts, achieving zero discharge and full resource recovery of the wastewater.

[0047] The closed-loop salt separation treatment method for lithium iron phosphate wastewater in this embodiment includes the following steps: S1, the lithium iron phosphate wastewater is fed into the raw liquid preheating system 10 and preheated to the evaporation temperature; S2, the preheated material is fed into the evaporation concentration system 20 for evaporation concentration to obtain a saturated solution; S3, the saturated solution is fed into the thermal crystallization system 30 for forced circulation thermal crystallization to precipitate ammonium sulfate crystals, and ammonium sulfate crystals and ammonium salt clear mother liquor are obtained by centrifugation; S4, the ammonium salt clear mother liquor obtained in step S3 is sent to the flash cooling crystallization system 40, and monoammonium phosphate crystals are precipitated by vacuum flash cooling, and monoammonium phosphate crystals and ammonium phosphate clear mother liquor are obtained by centrifugation; wherein, the material sent into the flash cooling crystallization system 40 consists of ammonium salt clear mother liquor, and no other materials are added for dilution; S5, all the ammonium phosphate clear mother liquor obtained in step S4 is sent back to the thermal crystallization system 30 for circulation thermal crystallization.

[0048] The above five steps are performed sequentially according to the flow order of the materials, forming a continuous wastewater treatment and salt separation process. Specifically, step S1 preheats the lithium iron phosphate wash water and mother liquor to the evaporation temperature, creating conditions for subsequent evaporation and concentration; step S2 concentrates the preheated material using falling film evaporation to obtain a nearly saturated concentrate; step S3 uses forced circulation heating and flash evaporation to cause ammonium sulfate to crystallize due to supersaturation, and centrifuges to obtain ammonium sulfate crystals and ammonium salt mother liquor; step S4 directly sends the ammonium salt mother liquor separated after thermal crystallization to the flash cooling crystallization system 40, where vacuum flash cooling causes monoammonium phosphate to crystallize due to reduced solubility, and centrifuges to obtain monoammonium phosphate crystals and ammonium phosphate mother liquor; step S5 returns all the ammonium phosphate mother liquor separated after flash cooling crystallization to the thermal crystallization system 30 for continued recycling. Since the material fed into the flash cooling crystallization system 40 in step S4 consists of ammonium salt mother liquor, no other materials are added for dilution. Compared with the existing technology, which requires mixing and diluting the mother liquor with the original material before cooling and crystallization, this method is simpler to operate, has a smaller material throughput, and avoids interference from impurities that may be introduced due to the introduction of the original material, which is beneficial to improving the purity of the monoammonium phosphate crystallization product.

[0049] Meanwhile, the materials continuously circulate within a closed loop throughout the entire process. Step S5 transfers all the ammonium phosphate mother liquor to the regenerative crystallization system 30, ensuring that no impurities are generated during system operation that need to be discharged. Therefore, there is no need to include impurity evaporation steps or insoluble matter discharge steps in the process flow, making the process flow simpler and more compact, and significantly reducing equipment investment and operating costs. In addition, step S4 uses vacuum flash evaporation to cool the ammonium phosphate mother liquor and crystallize monoammonium phosphate, completely eliminating the water-cooling heat exchange step in the traditional cooling crystallization process. This eliminates the clogging problem caused by scaling on the heat exchange wall, ensuring long-term continuous and stable operation of the process.

[0050] As a further limitation on the evaporation and concentration method in step S2, the evaporation and concentration system 20 adopts a double-effect falling film evaporation system. Specifically, step S2 includes: passing the preheated material into a first-effect falling film heat exchanger 21 for falling film evaporation and concentration, followed by gas-liquid separation in a first-effect falling film separator 22. The secondary steam (temperature approximately 101°C) generated by the first-effect falling film separator 22 serves as the heat source for the second-effect falling film heat exchanger 24. Part of the concentrated liquid separated by the first-effect falling film separator 22 is recycled back to the first-effect falling film heat exchanger 21 for further evaporation and concentration, while part is sent to the second-effect falling film heat exchanger 24 for further evaporation and concentration. The second-effect falling film separator 25 operates under a slight negative pressure of approximately 70.1 kPa, generating secondary steam at a temperature of approximately 90°C. After being washed by the concentration scrubbing tower 52, the secondary steam enters the concentration steam compressor 51, where it is compressed and heated to 110°C and 143.3 kPa before serving as the heat source for the first-effect falling film heat exchanger 21. The saturated solution that has reached the required concentration in the second-effect falling film separator 25 is transported to the thermal crystallization system 30 by the falling film transfer pump 27.

[0051] The aforementioned dual-effect falling film evaporation combined with steam compression allows the secondary steam generated in the first effect to serve as a heat source for the second effect, and the secondary steam generated in the second effect, after compression, to serve as a heat source for the first effect. This achieves cascaded utilization and closed-loop circulation of steam thermal energy, significantly reducing energy consumption in the evaporation and concentration steps. It is understandable that, depending on the processing scale and energy consumption requirements, the evaporation and concentration system 20 can also employ single-effect, triple-effect, or more-effect falling film evaporation. The concentration steam compressor 51 can be a centrifugal, Roots, or screw compressor. The concentration scrubbing tower 52 can be equipped with a concentration washing circulation pump 53 for circulating spray washing. Liquid accumulated on the inlet pipe of the concentration steam compressor 51 can be discharged into the distillation water tank 61 via a concentration liquid pump 54 for recycling, further improving water resource utilization.

[0052] As a further limitation on the thermal crystallization operating conditions in step S3, step S3 includes: the gas-liquid separator 31 operates under a slight negative pressure of approximately 70.1 kPa; the concentrated liquid is returned to the gas-liquid separator 31 for flash evaporation after forced circulation heating, the water vaporizes and is carried away, the solution becomes supersaturated, and ammonium sulfate crystals precipitate and deposit at the bottom of the gas-liquid separator 31; the secondary steam generated by the gas-liquid separator 31 is at a temperature of approximately 90°C, and after being washed by the crystallization scrubbing tower 56, it enters the crystallization steam compressor 55, is compressed and heated to 110°C and 143.3 kPa, and then sent to the forced circulation heat exchanger as a heat source, realizing the closed-loop recycling of secondary steam; the crystal slurry containing ammonium sulfate crystals is sent by the ammonium salt discharge pump 36 to the ammonium salt centrifuge 37 for centrifugal separation to obtain industrial-grade ammonium sulfate salt and ammonium salt clear mother liquor. The crystallization scrubbing tower 56 can be equipped with a crystallization washing circulation pump 57 for circulating spray washing, and the accumulated liquid on the inlet pipe of the crystallization steam compressor 55 can be discharged by the crystallization sludge pump 58. By maintaining a slight negative pressure operating condition within the gas-liquid separator 31, the boiling point of the solution is lowered, which facilitates evaporation and crystallization at a lower temperature, reducing the risk of decomposition of heat-sensitive substances. At the same time, the secondary steam is compressed and reused by the crystallization steam compressor 55, significantly reducing the energy consumption of the thermal crystallization step.

[0053] As a further limitation on the flash cooling crystallization operation conditions in step S4, step S4 includes: the flash vacuum system 43 evacuates the flash crystallizer 41 to maintain a low-pressure state inside. Water in the solution flashes and vaporizes under low pressure, carrying away a large amount of heat, causing the solution temperature to drop rapidly to approximately 45°C. Simultaneously, some water evaporates, leading to a decrease in the solubility of monoammonium phosphate and crystallization. The secondary steam generated during flash evaporation enters the flash condenser 42 and is condensed; the condensate can be recycled. The crystal slurry containing monoammonium phosphate crystals is sent by the ammonium phosphate discharge pump 44 to the ammonium phosphate centrifuge 45 for centrifugal separation, obtaining industrial-grade monoammonium phosphate salt and clear mother liquor. The flash temperature of 45°C is an optimized crystallization temperature selected based on the solubility curve of monoammonium phosphate. At this temperature, the solubility of monoammonium phosphate is low, the crystallization rate is high, and excessively low temperatures prevent the precipitation of other impurities in the solution, which would affect product purity. During normal continuous operation, all the heat sources for evaporation in the evaporation concentration system 20 and the thermal crystallization system 30 are provided by the concentration steam compressor 51 and the crystallization steam compressor 55, respectively, with virtually no consumption of external fresh steam, resulting in a significant reduction in operating energy consumption. It is understood that the flash condenser 42 can be a shell-and-tube, plate, or direct-contact condenser, and the flash vacuum system 43 can employ vacuum pumping equipment such as a water ring vacuum pump, a Roots vacuum pump, or a steam jet pump; the specific selection can be determined based on the processing scale and site conditions.

[0054] To further improve the recovery rate of ammonium sulfate, the saturated ammonium salt mother liquor discharged after centrifugation by ammonium salt centrifuge 37 in step S3 enters the ammonium salt mother liquor tank 38, and is then sent back to the gas-liquid separator 31 by ammonium salt mother liquor pump 39 to continue participating in the circulating hot crystallization, allowing the uncrystallized ammonium sulfate to re-enter the hot crystallization process. To further improve the recovery rate of monoammonium phosphate, the low-temperature saturated monoammonium phosphate mother liquor discharged after centrifugation by ammonium phosphate centrifuge 45 in step S4 enters the ammonium phosphate mother liquor tank 46, and is then sent back to the flash crystallizer 41 by ammonium phosphate mother liquor pump 47 to continue circulating crystallization, allowing the uncrystallized monoammonium phosphate to re-enter the cooling crystallization process. Simultaneously, the ammonium phosphate clear mother liquor discharged from the ammonium phosphate centrifuge 45 in step S4 is transported to the gas-liquid separator 31 of the reheat crystallization system 30 via the reflux pipeline by ammonium phosphate clear mother liquor pump 49, continuing to participate in the circulating hot crystallization. This constitutes the only liquid pathway from the flash cooling crystallization system 40 to the hot crystallization system 30, achieving closed-loop material circulation and zero wastewater discharge.

[0055] As a further optimization of the system's waste heat recovery and utilization, the high-temperature and high-pressure steam output from the evaporation and concentration system 20 and the crystallization system 30, from the concentration steam compressor 51 and the crystallization steam compressor 55, heats the material in the shell side of each heat exchanger and then condenses into high-temperature distilled water. This part of the distilled water is collected by the distilled water tank 61 and then sent to the raw liquid preheating system 10 through the distilled water pump 62 to exchange heat with the raw liquid. The waste heat is used to preheat the raw liquid. The distilled water after heat exchange and cooling is then discharged from the system. This not only recovers waste heat and reduces system energy consumption, but also realizes the cascade utilization of water resources.

[0056] Compared with existing technologies, the method in this embodiment sequentially evaporates and concentrates lithium iron phosphate wastewater, performs thermal crystallization, and flash cooling crystallization. The mother liquor separated after flash cooling crystallization is entirely recycled back to the thermal crystallization system for continued circulation, forming a complete closed-loop salt separation process. This eliminates the need for steps involving the evaporation of impurities and the removal of insoluble substances, achieving efficient separation and separate recovery of ammonium sulfate and monoammonium phosphate. The process flow is simpler and more compact, significantly reducing equipment investment and operating costs. Simultaneously, the flash cooling crystallization step uses vacuum flash evaporation to cool the ammonium salt mother liquor and precipitate monoammonium phosphate. Furthermore, the material fed into the flash cooling crystallization system consists entirely of the ammonium salt mother liquor, without additional... Adding other materials for dilution simplifies the process, reduces material throughput, and fundamentally avoids the scaling and clogging problems caused by water-cooled heat exchangers in traditional cooling crystallization processes, ensuring long-term continuous and stable operation. Thermal crystallization and flash cooling crystallization produce ammonium sulfate crystals and monoammonium phosphate crystals respectively, achieving resource recovery of these two salts from wastewater without generating any impurities, resulting in significant economic and environmental benefits. Furthermore, the secondary steam generated in the evaporation and concentration steps and the thermal crystallization steps is compressed by a steam compressor and recycled as the evaporation heat source for each step. During normal continuous operation, virtually no external fresh steam is consumed, significantly reducing operating energy consumption.

[0057] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A closed-loop desalination treatment device for lithium iron phosphate wastewater, characterized in that, include: Raw material preheating system; An evaporation and concentration system, wherein the inlet of the evaporation and concentration system is connected to the outlet of the raw liquid preheating system, and the evaporation and concentration system includes a falling film evaporation unit and a steam compressor connected to the secondary steam outlet of the falling film evaporation unit; A thermal crystallization system, wherein the inlet of the thermal crystallization system is connected to the outlet of the concentrated liquid of the evaporation and concentration system, and the thermal crystallization system includes a gas-liquid separator with a forced circulation heating unit inside, and a steam compressor connected to the secondary steam outlet of the gas-liquid separator. A flash cooling crystallization system, wherein the feed inlet of the flash cooling crystallization system is connected to the ammonium salt clear mother liquor outlet of the hot crystallization system, and the flash cooling crystallization system includes a flash crystallizer, a condensing device connected to the steam outlet of the flash crystallizer, and a vacuuming device; The flash cooling crystallization system is provided with a phosphate ammonium salt clear mother liquor outlet, which is connected to the reflux inlet of the hot crystallization system through a reflux pipeline. The outlet of the ammonium phosphate mother liquor, via the reflux pipeline, forms the only liquid passage to the thermal crystallization system.

2. The closed-loop desalination treatment equipment for lithium iron phosphate wastewater according to claim 1, characterized in that, The raw liquid preheating system includes a raw liquid tank, a raw liquid pump, a distilled water preheater, a non-condensable gas preheater, and a fresh steam preheater connected sequentially by pipelines. The distilled water preheater is connected to a distilled water tank via a distilled water pump. The distilled water tank is also connected to the fresh steam preheater via a pipeline to collect the distilled water generated by steam condensation in the evaporation and concentration system and the thermal crystallization system, and to pump the water into the distilled water preheater to exchange heat with the original liquid.

3. The closed-loop desalination treatment equipment for lithium iron phosphate wastewater according to claim 1, characterized in that, The falling film evaporation unit is a double-effect falling film evaporation unit, comprising a first-effect falling film evaporator and a second-effect falling film evaporator connected in series. The first-effect falling film evaporator includes a first-effect falling film heat exchanger and a first-effect falling film separator. The second-effect falling film evaporator includes a second-effect falling film heat exchanger and a second-effect falling film separator. The steam compressor is a concentrated steam compressor, the inlet of which is connected to the secondary steam outlet of the second-effect falling film separator, and the outlet of which is connected to the heat source inlet of the first-effect falling film heat exchanger. A concentrated gas scrubbing tower is also provided between the double-effect falling film separator and the concentrated steam compressor; The evaporation and concentration system also includes a falling film transfer pump, which is used to transport the concentrated liquid that has reached the required concentration in the double-effect falling film evaporator to the thermal crystallization system.

4. The closed-loop desalination treatment equipment for lithium iron phosphate wastewater according to claim 1, characterized in that, The forced circulation heating unit of the thermal crystallization system includes: At least one stage of forced circulation pump and forced circulation heat exchanger, wherein the inlet of the forced circulation pump is connected to the liquid outlet of the gas-liquid separator, the outlet is connected to the feed inlet of the forced circulation heat exchanger, and the discharge outlet of the forced circulation heat exchanger is connected to the gas-liquid separator; An ammonium salt discharge pump, the inlet of which is connected to the crystal slurry outlet of the gas-liquid separator; An ammonium salt centrifuge, the inlet of which is connected to the outlet of the ammonium salt discharge pump, and the clear mother liquor outlet of the ammonium salt centrifuge constitutes the clear mother liquor outlet of the ammonium salt in the thermal crystallization system; A crystallization scrubbing tower is also provided between the gas-liquid separator and the steam compressor.

5. The closed-loop desalination treatment equipment for lithium iron phosphate wastewater according to claim 1, characterized in that, The flash cooling crystallization system also includes: A phosphate ammonium salt discharge pump, the inlet of which is connected to the crystal slurry outlet of the flash crystallizer; The inlet of the ammonium phosphate centrifuge is connected to the outlet of the ammonium phosphate discharge pump, and the clear mother liquor outlet of the ammonium phosphate centrifuge constitutes the clear mother liquor outlet of the flash cooling crystallization system.

6. The closed-loop desalination treatment equipment for lithium iron phosphate wastewater according to claim 4, characterized in that, The thermal crystallization system also includes an ammonium salt mother liquor tank and an ammonium salt mother liquor pump. The inlet of the ammonium salt mother liquor tank is connected to the mother liquor outlet of the ammonium salt centrifuge, and the outlet of the ammonium salt mother liquor pump is connected to the return port of the gas-liquid separator. The flash cooling crystallization system also includes a phosphate ammonium salt mother liquor tank and a phosphate ammonium salt mother liquor pump. The inlet of the phosphate ammonium salt mother liquor tank is connected to the mother liquor outlet of the phosphate ammonium salt centrifuge, and the outlet of the phosphate ammonium salt mother liquor pump is connected to the return port of the flash crystallizer.

7. A processing method, employing the equipment as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Pass the lithium iron phosphate wastewater into the raw liquid preheating system and preheat it to the evaporation temperature; S2. The preheated material is fed into the evaporation and concentration system for evaporation and concentration to obtain a saturated solution; S3. The saturated solution is introduced into the thermal crystallization system for forced circulating thermal crystallization to precipitate ammonium sulfate crystals. The ammonium sulfate crystals and ammonium salt mother liquor are obtained by centrifugation. S4. The ammonium salt mother liquor obtained in step S3 is fed into the flash cooling crystallization system. The monoammonium phosphate crystals are precipitated by vacuum flash cooling. The monoammonium phosphate crystals and ammonium phosphate mother liquor are obtained by centrifugation. The material fed into the flash cooling crystallization system is composed of the ammonium salt mother liquor. S5. All the phosphate ammonium salt mother liquor obtained in step S4 is transported back to the thermal crystallization system for cyclic thermal crystallization.

8. The method according to claim 7, characterized in that, In step S2, the evaporation and concentration system is a double-effect falling film evaporation system. The secondary steam generated by the first-effect falling film separator is used as the heat source of the second-effect falling film heat exchanger. The secondary steam generated by the second-effect falling film separator is pressurized and heated to 110°C and 143.3 kPa by the concentrated steam compressor and then used as the heat source of the first-effect falling film heat exchanger.

9. The method according to claim 7, characterized in that, In step S3, the gas-liquid separator maintains a slight negative pressure of 70.1 kPa, and the generated secondary steam has a temperature of 90°C. After being pressurized and heated to 110°C and 143.3 kPa by the crystallization steam compressor, it is sent to the forced circulation heat exchanger as a heat source.

10. The method according to claim 7, characterized in that, In step S4, the flash temperature for flash cooling crystallization is 45°C; During normal continuous operation, the entire heat source for evaporation of the evaporation concentration system and the thermal crystallization system is provided by their respective steam compressors, without consuming external fresh steam.

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