Resource recovery system and method for wastewater containing sodium citrate and sodium sulfate

Abstract

The invention provides a resource recovery system and method for wastewater containing sodium citrate and sodium sulfate, and the system comprises a buffer mixing unit which is used for collecting wastewater and multiple paths of reflux liquid; the first nanofiltration assembly is used for preliminarily separating the mixed solution into a first concentrated solution enriched with sodium citrate and a first permeate enriched with sodium sulfate; the sodium citrate recovery branch is used for carrying out secondary concentration and evaporative crystallization on the first concentrated solution through a second nanofiltration assembly and a first crystallization unit so as to recover sodium citrate crystals or a high-concentration sodium citrate solution; the sodium sulfate recovery branch is used for carrying out concentration and freezing crystallization on the first permeate through a desalination concentration assembly and a second crystallization unit so as to recover sodium sulfate or mirabilite; and permeate liquid of the second nanofiltration assembly, mother liquid of the first crystallization unit and mother liquid of the second crystallization unit flow back to the buffer mixing unit. According to the invention, high-efficiency separation and high-purity recovery of two salt resources are realized, and the energy consumption is obviously reduced.

Description

Technical Field

[0001] This invention relates to the field of industrial wastewater treatment and resource recovery technology, specifically to a resource recovery system and method for wastewater containing sodium citrate and sodium sulfate. Background Technology

[0002] In industrial processes such as food processing, pharmaceutical manufacturing, and the production of sodium-ion battery cathode materials, mixed wastewater containing sodium citrate and sodium sulfate is frequently generated. Currently, treatment technologies for this type of wastewater are significantly inadequate. One common method is single-stage evaporation concentration, which enriches salts by evaporating water. However, this method is energy-intensive and prone to co-crystallization during evaporation, resulting in low purity and high impurity levels in the recovered salt product, making it difficult to directly reuse in production, and the recovery rate is typically low. Another method is biochemical treatment, which primarily aims to degrade organic matter in the wastewater. However, when the sodium sulfate concentration in the wastewater is high, it severely inhibits microbial activity, leading to low chemical oxygen demand (COD) removal efficiency. More importantly, biochemical treatment cannot recover valuable salt resources from the wastewater; the high-salinity effluent after treatment still requires further disposal, which not only increases the overall treatment cost but also introduces potential environmental risks. Therefore, existing technologies struggle to achieve the separation and high-purity recovery of both sodium citrate and sodium sulfate resources from wastewater while remaining economical and efficient. Summary of the Invention

[0003] In view of the deficiencies in the prior art, the purpose of this invention is to provide a resource recovery system and method for wastewater containing sodium citrate and sodium sulfate.

[0004] A resource recovery system for wastewater containing sodium citrate and sodium sulfate, provided by the present invention, comprises: A buffer mixing unit is used to collect the wastewater to be treated and the return liquid, and output the mixed liquid; A first nanofiltration unit, the inlet of which is connected to the outlet of the buffer mixing unit, is used to separate the mixture to obtain a first concentrate enriched with sodium citrate and a first permeate enriched with sodium sulfate. A sodium citrate recovery branch is connected to the first concentrate outlet of the first nanofiltration component; the sodium citrate recovery branch includes: a second nanofiltration component for secondary concentration of the first concentrate to obtain a second concentrate and a second permeate, wherein the second permeate is returned to the buffer mixing unit; the second concentrate is a sodium citrate-rich solution that can be directly reused or sold as a product. A sodium sulfate recovery branch is connected to the first permeate outlet of the first nanofiltration unit; the sodium sulfate recovery branch includes: a desalination and concentration unit for treating the first permeate to obtain recycled water and a third concentrate rich in sodium sulfate.

[0005] Preferably, a third nanofiltration component is provided between the first permeate outlet of the first nanofiltration component and the inlet of the desalination and concentration component; The third nanofiltration unit is used to process the first permeate to obtain a fourth concentrate and a fourth permeate enriched with sodium citrate. The fourth concentrate is returned to the buffer mixing unit, and the fourth permeate is transported to the inlet of the desalination and concentration component.

[0006] Preferably, the first permeate outlet of the first nanofiltration component is directly connected to the inlet of the desalination and concentration component.

[0007] Preferably, at least one of the first nanofiltration component, the second nanofiltration component, and the third nanofiltration component employs an organic nanofiltration membrane.

[0008] Preferably, the second concentrate obtained from the second nanofiltration component enters a first crystallization unit for evaporation and crystallization to recover sodium citrate and obtain a first mother liquor, wherein the first mother liquor is refluxed to the buffer mixing unit.

[0009] Preferably, the third concentrated liquid containing sodium sulfate obtained from a desalination and concentration component of the sodium sulfate recovery branch enters a second crystallization unit for evaporation and crystallization of the third concentrated liquid to recover sodium sulfate or sodium sulfate and obtain a second mother liquor, wherein the second mother liquor is refluxed to the buffer mixing unit.

[0010] Preferably, the second crystallization unit is configured to lower the temperature of the third concentrate to 0-5°C for freeze crystallization.

[0011] Preferably, the system further includes a wastewater pretreatment unit located upstream of the buffer mixing unit, the wastewater pretreatment unit being used for solid-liquid separation and / or pH adjustment of the wastewater.

[0012] Preferably, the system further includes a drying unit connected to the solid outlet of the second crystallization unit, the drying unit being used to dry the recovered hydrous sodium sulfate crystals into anhydrous sodium sulfate.

[0013] This invention also provides a resource recovery method for wastewater containing sodium citrate and sodium sulfate, based on the aforementioned resource recovery system for wastewater containing sodium citrate and sodium sulfate, comprising the following steps: Mixing and homogenization step: Wastewater containing sodium citrate and sodium sulfate is mixed with the second permeate, the first mother liquor and the second mother liquor from subsequent steps in a buffer mixing unit to obtain a mixed liquid; Preliminary separation step: The mixture is fed into the first nanofiltration unit for separation to obtain a first concentrated solution enriched with sodium citrate and a first permeate enriched with sodium sulfate; Sodium citrate recovery steps: The first concentrate is sent to the second nanofiltration unit for secondary concentration to obtain the second concentrate and the second permeate; the second concentrate is sent to the first crystallization unit for evaporation and crystallization to recover sodium citrate and obtain the first mother liquor; Sodium sulfate recovery steps: The first permeate is sent to a desalination and concentration unit for treatment to obtain recycled water and a third concentrate rich in sodium sulfate; the third concentrate is sent to a second crystallization unit for freeze crystallization to recover sodium sulfate and obtain the second mother liquor.

[0014] Preferably, after the preliminary separation step and before the desalting and concentration treatment in the sodium sulfate recovery step, the method further includes: The first permeate is fed into the third nanofiltration unit for processing to obtain a fourth concentrate and a fourth permeate enriched with sodium citrate. The fourth concentrate is refluxed to the mixing and homogenization step; The fourth permeate is used for desalting and concentration in the sodium sulfate recovery step.

[0015] Preferably, the freezing crystallization in the sodium sulfate recovery step is controlled at a crystallization temperature of 0-5°C.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention has a high resource recovery rate and good product purity. Through the precise coupling of graded membrane separation and dual crystallization path, and the setting of a closed-loop system with multiple mother liquor and permeate recirculation to the buffer mixing unit, material loss is minimized. This allows the recovery rate of sodium citrate and sodium sulfate to reach more than 95%, and the product purity to reach more than 99% and 98% respectively, meeting the industrial reuse standards.

[0017] 2. The present invention significantly reduces energy consumption by using membrane concentration technology to replace large-scale evaporation, evaporating only the highly concentrated sodium citrate solution, and using a lower energy consumption freeze crystallization method for sodium sulfate, resulting in a total energy consumption reduction of more than 60% compared to the traditional total evaporation method.

[0018] 3. The system of the present invention operates stably. The buffer mixing unit plays a key role in buffering and homogenization, which can effectively absorb fluctuations in the quality and quantity of the influent, ensure the stable operation of the subsequent precision membrane separation system, and help extend the service life of the membrane module.

[0019] 4. This invention is environmentally friendly, achieving near-zero emissions. No chemical precipitants need to be added throughout the process, avoiding secondary pollution. At the same time, a large amount of high-quality reclaimed water is generated through the desalination and concentration components, with a wastewater reuse rate of over 70%, significantly reducing wastewater discharge. Attached Figure Description

[0020] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram of the process flow of a resource recovery system for wastewater containing sodium citrate and sodium sulfate according to an embodiment of the present invention. Figure 2 This is a schematic diagram of the structure of a freeze crystallization unit provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the process flow of a resource recovery system for wastewater containing sodium citrate and sodium sulfate, provided in another embodiment of the present invention.

[0021] The diagram shows: 1. Wastewater pretreatment unit; 2. First membrane separation unit; 3. Second membrane separation unit; 4. Third membrane separation unit; 5. Fourth membrane separation unit; 6. Freeze crystallization unit; 61. Plate heat exchanger; 62. Low-temperature crystallization tank; 621. Agitator; 622. Temperature sensor; 63. Centrifuge; 7. Drying unit; 8. Evaporation crystallization unit; 9. Mixing storage tank. Detailed Implementation

[0022] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0023] Example 1 This embodiment provides a resource recovery system and method for treating wastewater containing sodium citrate and sodium sulfate, aiming to maximize the recovery of these two salts from the wastewater and obtain high-purity products. (Refer to...) Figure 1 It shows a schematic diagram of the process flow of the resource recycling system in this embodiment.

[0024] The system mainly includes a wastewater pretreatment unit 1 as the upstream inlet, a mixing storage tank 9 as the core for material circulation and buffering, and two parallel processing branches for recovering sodium citrate and sodium sulfate, respectively. Specifically, the system includes a first nanofiltration unit, a second nanofiltration unit, a third nanofiltration unit, a desalination and concentration unit, a freeze crystallization unit 6, a drying unit 7, and an evaporation and crystallization unit 8. These units are interconnected via pipes, valves, and pumps (not shown in the figure), forming a closed-loop system. The first nanofiltration unit is the first membrane separation unit 2, the second nanofiltration unit is the second membrane separation unit 3, the third nanofiltration unit is the third membrane separation unit 4, and the desalination and concentration unit is the fourth membrane separation unit 5.

[0025] In one embodiment of the present invention, the raw wastewater to be treated originates from a sodium-ion battery cathode material production workshop. Its main pollutants are sodium citrate and sodium sulfate. The typical water quality is as follows: sodium citrate concentration is about 50 g / L, sodium sulfate concentration is about 20 g / L, and it also contains a small amount of suspended solids, with a pH value of about 6.5.

[0026] The specific workflow of this system is described below.

[0027] The raw wastewater is first pumped into wastewater pretreatment unit 1. This unit removes any suspended solids, colloids, and other impurities that may be present in the wastewater and adjusts the pH value to protect the subsequent membrane modules from physical clogging, scratches, and chemical corrosion, thereby ensuring the stable operation of the membrane system. Specifically, wastewater pretreatment unit 1 may include a multi-media filter, an activated carbon filter, and a precision filter. The multi-media filter removes larger suspended particles, the activated carbon filter adsorbs some organic matter and color from the wastewater, and the precision filter (e.g., with a pore size of 5 micrometers) acts as a security filter for the reverse osmosis and nanofiltration systems, further intercepting fine particles. In addition, this unit includes a pH adjustment tank, which precisely adds acid (such as sulfuric acid) or alkali (such as sodium hydroxide) via a metering pump to adjust the pH value of the wastewater to approximately 7.2. Understandably, a neutral pH environment is beneficial for improving the separation selectivity of the subsequent nanofiltration membrane for citrate and sulfate ions and can effectively slow down membrane hydrolysis.

[0028] After pretreatment, clean wastewater with a suitable pH is transported to mixing tank 9. Mixing tank 9 is designed to achieve the recycling of materials within the system and the homogenization of water quality. It not only receives external influent from wastewater pretreatment unit 1 but also collects return liquid from multiple units within the system. For example... Figure 1As shown, at least four streams of reflux enter the mixing tank 9: permeate from the second nanofiltration unit (second permeate), concentrate from the third nanofiltration unit (fourth concentrate), mother liquor from the evaporation crystallization unit 8 (first mother liquor), and mother liquor from the freeze crystallization unit 6 (second mother liquor). Since these refluxes contain incompletely separated or crystallized sodium citrate and sodium sulfate, collecting them in the mixing tank 9 and mixing them with fresh wastewater effectively improves the overall recovery rate of both salts. Furthermore, the mixing tank 9 has a large volume, which can buffer fluctuations in the flow rate or concentration of the upstream water, thereby providing a relatively stable feed with consistent composition and flow rate to the downstream first nanofiltration unit, thus ensuring the smooth operation of the entire system. A stirring device can be installed inside the tank to ensure uniform liquid mixing.

[0029] The mixture in mixing tank 9 is pumped into the first nanofiltration module via a high-pressure pump for preliminary separation. The first nanofiltration module is a key step in achieving the separation of the two salts. In this embodiment, the first nanofiltration module uses a nanofiltration membrane with a molecular weight cutoff of 200-300 Daltons, such as a charged-modified polyamide composite nanofiltration membrane. This type of membrane typically has a negatively charged surface and its pore size is between that of reverse osmosis and ultrafiltration membranes. Based on the synergistic effect of the Donnan effect and size sieving effect, this nanofiltration membrane has a high rejection rate for high-valence anions. In the technical solution of this invention, the first nanofiltration module mainly utilizes the selectivity difference of the nanofiltration membrane for ions of different valence states. Citrate, as a trivalent anion, is rejected to a much higher degree than divalent sulfate ions. Under an operating pressure of approximately 2.5 MPa, most of the sodium citrate is retained on the concentrate side of the membrane, forming a first concentrated solution enriched with sodium citrate; while most of the sodium sulfate permeates through the membrane with water molecules, forming a first permeate enriched with sodium sulfate.

[0030] Subsequently, the system splits into two branches to process the two liquids separately.

[0031] In the sodium citrate recovery branch, the first concentrate from the first nanofiltration unit enters the second nanofiltration unit for further concentration. The second nanofiltration unit uses a nanofiltration membrane similar to or the same as the first nanofiltration unit, aiming to further increase the concentration of sodium citrate with lower energy consumption, thereby reducing the amount of water evaporated for subsequent evaporation and crystallization steps and improving the purity of sodium citrate. Under an operating pressure of approximately 2.8 MPa, the sodium citrate is further concentrated to obtain a second concentrate with a concentration as high as 190 g / L. The second permeate generated during this process, containing a small amount of unretained sodium citrate and sodium sulfate, is directly recycled back to the mixing storage tank 9, achieving a closed-loop resource recycling.

[0032] The resulting high-concentration second concentrate is then fed into evaporation crystallization unit 8. This unit is the core equipment for recovering solid sodium citrate. In this embodiment, evaporation crystallization unit 8 consists of a falling film evaporator and a forced circulation crystallizer connected in series. The second concentrate first enters the falling film evaporator for pre-concentration, using steam heating or an MVR evaporator to evaporate the water. Subsequently, the higher-concentration solution enters the forced circulation crystallizer. Inside the crystallizer, by maintaining a temperature of approximately 90°C and a vacuum of -0.06 MPa, the solution reaches a supersaturated state, and sodium citrate crystals begin to precipitate and grow. A forced circulation pump drives the solution to circulate between the heating chamber and the crystallization chamber to ensure uniform temperature and effectively prevent scaling on the inner wall of the crystallizer. The generated crystal slurry is subjected to solid-liquid separation by a centrifuge to obtain a high-purity sodium citrate crystal product (with a purity of over 99% as tested), as well as the separated first mother liquor. To avoid resource waste, the saturated sodium citrate and some sodium sulfate still contained in the first mother liquor are all pumped back to the mixing storage tank 9 for re-entry into the separation cycle.

[0033] In the sodium sulfate recovery branch, the first permeate from the first nanofiltration unit first enters the third nanofiltration unit 4. Since the first nanofiltration unit does not retain 100% of the sodium citrate, the first permeate still contains a small amount of valuable sodium citrate. The third nanofiltration unit is designed to recover this residual sodium citrate. The third nanofiltration unit also uses a nanofiltration membrane and operates at a pressure of approximately 1.6 MPa. Its concentrate (i.e., the fourth concentrate) enriches this portion of sodium citrate and is then returned to the mixing tank 9. Its permeate (i.e., the fourth permeate) further reduces the sodium citrate content and becomes the feed liquid for the next unit.

[0034] After treatment by the third nanofiltration unit, the fourth permeate, mainly composed of sodium sulfate solution, is sent to the desalination and concentration unit. This unit is typically a reverse osmosis membrane unit, employing a high-pressure resistant reverse osmosis membrane. At an operating pressure of approximately 2.1 MPa, the reverse osmosis membrane can efficiently retain almost all solutes, including sodium sulfate, while allowing water molecules to permeate. Therefore, the product water from this unit is high-quality pure water, which can be directly reused in the production workshop, achieving water resource recycling with a reuse rate exceeding 70%. The concentrate side yields a third concentrate with a significantly increased sodium sulfate concentration, reaching up to 98 g / L.

[0035] The high-concentration third concentrate, with a sodium sulfate concentration exceeding 9%, is sent to cryogenic crystallization unit 6 to recover sodium sulfate. (Refer to...) Figure 2The diagram illustrates the internal structure of the cryogenic crystallization unit 6. This unit utilizes the characteristic that the solubility of sodium sulfate decreases sharply at low temperatures for crystallization. The specific process is as follows: the third concentrate from the desalination and concentration unit first enters the plate heat exchanger 61, where it exchanges heat with the refrigerant or low-temperature mother liquor from the low-temperature crystallization tank to achieve pre-cooling, thereby saving energy. The pre-cooled liquid then enters the low-temperature crystallization tank 62, which is equipped with a stirrer 621 and a temperature sensor 622. Inside the tank, the temperature of the liquid is precisely controlled and maintained at 2°C through an external refrigeration jacket or internal refrigeration coils. The temperature sensor 622 monitors the temperature inside the tank in real time and feeds the signal back to the refrigeration system for precise temperature control; the stirrer 621 rotates slowly to promote uniform solution temperature and guide crystal growth in suspension, thus avoiding scaling on the tank walls and obtaining crystals with uniform particle size. Under low-temperature conditions of 0-5°C, sodium sulfate crystallizes out in large quantities as sodium sulfate decahydrate (commonly known as Glauber's salt), forming a solid-liquid mixed crystal slurry.

[0036] The crystal slurry is discharged from the bottom of the low-temperature crystallization tank 62 and sent to the centrifuge 63 for solid-liquid separation. The separated solid phase is wet Glauber's salt crystals, while the liquid phase is the separated second mother liquor. To maximize resource utilization, the saturated sodium sulfate and a small amount of sodium citrate still contained in the second mother liquor are also pumped back to the mixing storage tank 9 for the next cycle of processing.

[0037] Finally, the wet sodium sulfate crystals obtained from the freeze crystallization unit 6 are sent to the drying unit 7. The drying unit 7 can employ equipment such as a hot air circulating drying oven or a fluidized bed dryer. At a temperature of approximately 85°C, sodium sulfate decahydrate loses its water of crystallization, transforming into anhydrous sodium sulfate (commonly known as Glauber's salt). The dried Glauber's salt product, after testing, shows a purity of over 98%, meeting industrial reuse standards.

[0038] Through the above process flow, this embodiment can recover sodium citrate and sodium sulfate from wastewater in the form of high-purity crystals using a low-energy-consumption process combination, thereby achieving resource utilization and near-zero discharge of wastewater, with significant economic and environmental benefits.

[0039] Example 2 As an alternative implementation, this embodiment provides a simplified resource recycling system designed to reduce equipment investment and operational complexity. It is suitable for scenarios where the required sodium citrate recovery rate is not extreme, or where there is a significant difference in the concentration of sodium citrate and sodium sulfate in the original wastewater.

[0040] The system structure of this embodiment is basically the same as that of Embodiment 1, and can be referred to accordingly. Figure 1The core difference in the overall framework shown is the omission of the third nanofiltration unit in the sodium sulfate recovery branch. In this simplified system, the first permeate (rich in sodium sulfate) from the first nanofiltration unit 2 no longer undergoes a dedicated nanofiltration step to recover residual sodium citrate, but is instead directly fed to the inlet of the desalination and concentration unit.

[0041] The specific process adjustments are as follows: After being treated by the wastewater pretreatment unit 1 and the mixing tank 9, the wastewater enters the first nanofiltration component for preliminary separation to obtain the first concentrate and the first permeate. This step is exactly the same as in Example 1.

[0042] The process of the sodium citrate recovery branch is also consistent with that of Example 1: the first concentrate enters the second nanofiltration unit and the evaporation crystallization unit 8 in sequence to recover the sodium citrate product, and the second permeate and the first mother liquor generated are both returned to the mixing storage tank 9.

[0043] The key change lies in the sodium sulfate recovery branch. The first permeate from the first nanofiltration unit, containing most of the sodium sulfate and a small amount of unretained sodium citrate, is directly pumped into the desalination and concentration unit (reverse osmosis membrane unit). Due to the high retention capacity of the reverse osmosis membrane, water molecules permeate through the membrane to become recycled water, while the sodium sulfate and this portion of sodium citrate are retained on the concentrate side, forming the third concentrate.

[0044] Subsequently, this third concentrate containing a small amount of sodium citrate impurities was fed into the cryo-crystallization unit 6. At a low temperature of 0-5°C, sodium sulfate preferentially crystallized to precipitate as sodium sulfate decahydrate. The presence of sodium citrate in the feed solution may slightly affect the sodium sulfate crystallization process; for example, a lower temperature or longer crystallization time may be required to achieve the same precipitation rate, and the purity of the final sodium sulfate crystals may decrease slightly. In the tests of this embodiment, the same wastewater as in Example 1 was treated, and the purity of the obtained sodium sulfate product was approximately 97%, with a total recovery rate of approximately 92%. However, because some sodium citrate ultimately entered the sodium sulfate product or was recycled with the second mother liquor, the overall recovery rate of sodium citrate also slightly decreased to approximately 93%.

[0045] Understandably, while there is a slight sacrifice in product purity and recovery rate, the advantage of this embodiment lies in eliminating a complete nanofiltration membrane separation system (i.e., the third nanofiltration module and its associated pumps, piping, and control system). This reduces the overall system's equipment cost, footprint, and energy consumption, and simplifies operation and maintenance. Therefore, for users with limited investment budgets or slightly higher requirements for product purity, this embodiment provides a more economical technical option.

[0046] Example 3 This embodiment provides a resource recovery device and method for wastewater containing sodium citrate and sodium sulfate, relating to the field of industrial wastewater treatment and resource recovery technology. Specifically, it relates to a resource recovery device and method for wastewater containing sodium citrate and sodium sulfate based on graded membrane separation-dual crystallization coupling, applicable to the treatment of such wastewater generated in food processing, pharmaceutical preparation, and sodium-ion battery cathode material production.

[0047] Mixed wastewater containing sodium citrate and sodium sulfate is typical wastewater in food processing (such as beverage flavoring), pharmaceutical preparation (such as antibiotic production), and the preparation of sodium-ion battery cathode materials (sodium citrate as a complexing agent). The concentration of sodium citrate in this type of wastewater is usually 3-50 g / L and the concentration of sodium sulfate is 3-25 g / L. Direct discharge not only wastes high-value-added sodium citrate and commonly used industrial sodium sulfate, but also leads to eutrophication of water bodies and soil salinization. Existing treatment technologies have significant drawbacks: a) Single evaporation concentration method: requires a large amount of heat energy (triple-effect evaporation consumes 0.35~0.4 tons of steam / ton of water), and sodium citrate and sodium sulfate easily form co-crystallization, resulting in a product purity of less than 60%, making direct reuse difficult and the resource recovery rate less than 60%; b) Biochemical treatment method: high concentrations of sodium sulfate (≥10g / L) inhibit microbial activity, with a COD removal rate of only 40%-60%, and salt resources cannot be recovered, leaving the remaining saline wastewater requiring further treatment. Therefore, there is an urgent need for a wastewater resource recovery and treatment scheme that can achieve efficient salt separation and high resource recovery rate.

[0048] To address the shortcomings of existing technologies, this embodiment provides a resource recovery device and method for wastewater containing sodium citrate and sodium sulfate. Through a synergistic design of "pretreatment-mixing storage tank buffer-stage membrane separation-double crystallization", high-purity recovery of sodium citrate and sodium sulfate is achieved separately, while simultaneously achieving wastewater reuse that meets standards, reducing energy consumption and environmental risks.

[0049] like Figure 3 As shown, this embodiment provides a resource recovery device for wastewater containing sodium citrate and sodium sulfate, including a wastewater pretreatment unit 1, a mixing storage tank 9, a first membrane separation unit 2, a second membrane separation unit 3, a third membrane separation unit 4, a fourth membrane separation unit 5, an evaporation crystallization unit 8, a freeze crystallization unit 6, and a drying unit 7.

[0050] The mixing tank 9 is connected to the outlet of the wastewater pretreatment unit 1, the product water of the second membrane separation unit 3, the concentrate of the third membrane separation unit 4, the mother liquor outlet of the freeze crystallization unit 6, and the mother liquor outlet of the evaporation crystallization unit 8, respectively. The input of the first membrane separation unit 2 is connected to the mixing tank 9, and its output is split into two paths connected to the second membrane separation unit 3 and the third membrane separation unit 4. The output of the second membrane separation unit 3 is split into two paths connected to the evaporation crystallization unit 8 and the mixing tank 9. The output of the third membrane separation unit 4 is split into two paths connected to the fourth membrane separation unit 5 and the mixing tank 9. The output of the fourth membrane separation unit 5 is split into two paths connected to the freeze crystallization unit 6 and the recycled water outlet. The output of the evaporation crystallization unit 8 is connected to the mixing tank 9 and the sodium citrate product outlet, and the output of the freeze crystallization unit 6 is connected to the mixing tank 9 and the sodium sulfate product outlet. The first membrane separation unit 2, the second membrane separation unit 3, and the third membrane separation unit 4 are charged modified polyamide nanofiltration membranes, organic nanofiltration membranes made of piperazine amide, or molecular sieve inorganic nanofiltration membranes with alumina, zirconium oxide, and titanium dioxide as the main components, with a molecular weight cutoff of 200-300 Da. The fourth membrane separation unit 5 is a high-pressure aromatic polyamide reverse osmosis membrane with a desalination rate ≥99%.

[0051] The resource recycling device in this embodiment includes a core unit that is connected to a delivery pump via pipelines to form a closed-loop reflux system. The specific connections are as follows: Wastewater pretreatment unit 1 is connected to mixing tank 9, which is used to buffer pretreated wastewater and return liquid from each unit.

[0052] The mixing tank 9 is connected to the first membrane separation unit 2, and the first membrane separation unit 2 is connected to the first branch. The first branch includes a second membrane separation unit 3, a third membrane separation unit 4, and an evaporation and crystallization unit 8. The second membrane separation unit 3 is connected to the third membrane separation unit 4, the third membrane separation unit 4 is connected to the evaporation and crystallization unit 8, and the evaporation and crystallization unit 8 is connected to the sodium citrate product outlet.

[0053] The first membrane separation unit 2 is connected to the second branch. The second branch includes a fourth membrane separation unit 5 and a freeze crystallization unit. The fourth membrane separation unit 5 is connected to the freeze crystallization unit, and the freeze crystallization unit is connected to the Glauber's salt product outlet.

[0054] The water production end of the second membrane separation unit 3, the water production end of the third membrane separation unit 4, the mother liquor outlet of the freeze crystallization unit 6, and the mother liquor outlet of the evaporation crystallization unit 8 are all connected to the mixing storage tank 9, and the water production end of the fourth membrane separation unit 5 is the outlet for qualified recycled water.

[0055] Wastewater pretreatment unit 1 consists of a bar screen filter (pore size 50-100μm), a precision filter (pore size 5-10μm), and a pH adjustment tank (with built-in variable frequency stirrer, online pH monitor, and automatic dosing device) connected in series. It is used to remove suspended solids, colloids, and other membrane fouling substances from the wastewater (retention efficiency ≥99%), adjust the wastewater pH to 4.0-8.0 to provide suitable operating conditions for subsequent membrane separation, and then transport the effluent to the mixing storage tank 9. The concentration of sodium citrate in wastewater pretreatment unit 1 is typically 3-50 g / L, and the concentration of sodium sulfate is typically 3-25 g / L.

[0056] The mixing tank 9 is made of 304 stainless steel and has a volume 1.2-1.5 times the processing capacity. It has a built-in liquid level sensor and a paddle agitator (speed 20-100r / min). It is used to collect the effluent from the wastewater pretreatment unit 1, the product water from the second membrane separation unit 3, the product water from the third membrane separation unit 4, the return concentrate from the fourth membrane separation unit 5, and the frozen crystallization mother liquor. This achieves water quality homogenization and flow buffering, and avoids the decrease in separation efficiency of the membrane separation unit due to fluctuations in the influent concentration.

[0057] The graded membrane separation system includes a first separation membrane unit 2, a second separation membrane unit 3, a third separation membrane unit 4, and a fourth separation membrane unit 5.

[0058] The first separation membrane unit 2 uses a charged modified polyamide nanofiltration membrane, an organic nanofiltration membrane made of polypiperazine amide, or an inorganic nanofiltration membrane made of molecular sieve with alumina, zirconium oxide, and titanium dioxide as the main components. The molecular weight cutoff is 200-300 Da, the operating pressure is 2-3 MPa, and the temperature is 25-35℃. It is used for the preliminary separation of sodium citrate and sodium sulfate. The sodium citrate rejection rate is 50%-80% (enriched in the concentrate), and the sodium sulfate permeation rate is ≥20%~50% (enters the product water). The concentrate enters the second membrane separation unit 3, and the product water enters the third membrane separation unit 4.

[0059] The membrane material of the second membrane separation unit 3 is the same as that of the first separation membrane unit 2, and the operating pressure is 2-4 MPa. Its function is to deeply enrich the concentrate of the first separation membrane unit 2, increase the sodium citrate concentration to 180-220 g / L (to meet the requirements of evaporation and crystallization), and the mass ratio of sodium citrate to sodium sulfate is greater than 9. The permeate is returned to the mixing storage tank 9, and the concentrate is sent to the evaporation and crystallization unit 8.

[0060] The membrane material of the third membrane separation unit 4 is the same as that of the first membrane separation unit 2, and the operating pressure is 1-3 MPa. Its function is to concentrate the permeate from the first membrane separation unit 2, enrich sodium citrate, and ensure that the mass ratio of sodium citrate to sodium sulfate in the concentrate is greater than 2. The concentrate is returned to the mixing storage tank 9, and the permeate is sent to the fourth membrane separation unit 5.

[0061] The fourth membrane separation unit 5 uses a high-pressure resistant reverse osmosis membrane with a desalination rate of ≥99% and an operating pressure of 2-5 MPa. It is used to desalinate the permeate from the third membrane separation unit 4, and the permeate is reused after meeting the standards. The sodium sulfate concentration in the concentrate is greater than 10%, and it is sent to the freeze crystallization unit.

[0062] The crystallization and drying unit includes a freeze crystallization unit 6, an evaporation crystallization unit 8, and a drying unit 7.

[0063] The cryogenic crystallization unit 6 includes a plate heat exchanger, a low-temperature crystallizer (with built-in temperature sensor and anchor stirrer), and a horizontal centrifuge; it is used to cool the concentrate from the fourth membrane separation unit 5 to 0℃-5℃, so that sodium sulfate crystallizes into sodium sulfate (Na2SO4). 10H2O), centrifuged to obtain sodium sulfate (purity ≥98%), and the mother liquor was refluxed into the mixing storage tank 9.

[0064] The evaporation and crystallization unit 8 includes a falling film evaporator (evaporation temperature 80-100℃, vacuum degree -0.06~-0.09MPa), a forced circulation crystallizer, and a centrifugal separator; its function is to concentrate the concentrated water from the second membrane separation unit 3 at low temperature, precipitate sodium citrate crystals, and centrifuge to obtain crude product with a sodium citrate purity greater than 98%; the mother liquor is returned to the mixing storage tank 9.

[0065] Drying unit 7 is a hot air circulating dryer with a drying temperature of 80-90℃ and an air velocity of 1.2-1.5m / s; it is used to remove the crystal water of Glauber's salt to obtain a sodium sulfate product with a purity of ≥98%.

[0066] This embodiment also provides a resource recovery method for wastewater containing sodium citrate and sodium sulfate, which includes at least the following steps: Step 1: Use an organic or inorganic nanofiltration membrane to initially separate sodium citrate and sodium sulfate.

[0067] Step 2: Using an organic or inorganic nanofiltration membrane, further concentrate the sodium citrate in the concentrated water from Step 1. The concentrated water can be reused as a sodium citrate solution or purified by evaporation and crystallization.

[0068] Step 3: Using an organic or inorganic nanofiltration membrane, further concentrate the sodium citrate in the permeate from Step 1. The concentrated water is returned to Step 1, and the permeate enters the next process.

[0069] Step 4: Use a high-pressure resistant reverse osmosis membrane to desalinate the permeate from Step 3. The permeate meets the standards for reuse, and the concentrate enters the freeze crystallization process.

[0070] Taking the treatment of wastewater containing sodium citrate and sodium sulfate from a sodium-ion battery cathode material factory as an example, the influent parameters are: sodium citrate 50g / L, sodium sulfate 20g / L, suspended solids 72mg / L, pH 4.8, and treatment capacity 15m³ / h.

[0071] Pretreatment: 99% of suspended solids are removed by bar screen and precision filter. NaOH is added to adjust the pH to 7.2. The effluent is then sent to the mixing storage tank 9.

[0072] First membrane separation unit 2: operating pressure 2.5MPa, temperature 32℃, concentrated water sodium citrate 120g / L, sodium sulfate 23g / L, product water sodium citrate 20g / L, sodium sulfate 15g / L.

[0073] Second membrane separation unit 3: operating pressure 2.8MPa, concentrated water sodium citrate 190g / L, sodium sulfate 24g / L, product water reflux mixing storage tank 9.

[0074] Third membrane separation unit 4: operating pressure 1.6MPa, concentrated water sodium citrate 200g / L, sodium sulfate 21g / L, product water reflux mixing storage tank 9.

[0075] Fourth membrane separation unit 5: operating pressure 2.1MPa, product water conductivity 92μS / cm (reuse rate 73%), concentrated water sodium sulfate 98g / L, 12% concentrated water reflux mixing storage tank 9.

[0076] Freeze-crystallization: Cool to 4℃, centrifuge to obtain sodium sulfate with a purity of 97%, mother liquor is returned to mixing storage tank 9, sodium sulfate recovery rate is 92%.

[0077] Drying Unit 7: Drying temperature 85℃, wind speed 1.3m / s, sodium sulfate purity after drying 98%, recovery rate 98%.

[0078] Evaporation and crystallization unit 8: Falling film evaporation temperature 90℃, vacuum degree -0.06MPa, after crystallization, centrifugation is used to obtain crude sodium citrate.

[0079] The apparatus and method of this embodiment have a high resource recovery rate. Through the synergistic effect of the mixing tank 9 and the reflux of the mother liquor from multi-stage membrane separation, freeze crystallization, and evaporation crystallization, the recovery rate of sodium citrate is ≥95%, the recovery rate of sodium sulfate is ≥95%, the purity of sodium citrate product is ≥99%, and the purity of sodium sulfate product is ≥98%, all of which meet industrial-grade standards.

[0080] The apparatus and method of this embodiment have low energy consumption. They use staged membrane separation to replace traditional total evaporation, and the amount of water evaporated is only 5% to 10% of the original wastewater volume. By combining evaporation and freeze crystallization (0-5℃), the total energy consumption is reduced by 60% to 70% compared with the traditional evaporation method. The apparatus and method of this embodiment operate stably. The homogenization effect of the mixing tank 9 avoids the impact of influent fluctuations on membrane separation, and extends the service life of the membrane module to 3-4 years, which is more than 50% better than conventional membrane separation processes.

[0081] The apparatus and method of this embodiment are environmentally friendly and produce no secondary pollution. No chemical precipitants need to be added throughout the process, no sludge is generated, and the wastewater reuse rate is ≥70%, achieving the dual goals of "resource recycling + zero wastewater discharge".

[0082] This invention achieves efficient separation and high-purity recovery of two types of salt resources by coupling graded membrane separation with dual crystallization paths and combining them with a multi-path reflux design, significantly reducing energy consumption.

[0083] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship 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 limitations on this invention.

[0084] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features of the present invention can be arbitrarily combined with each other.

Claims

1. A resource recovery system for wastewater containing sodium citrate and sodium sulfate, characterized in that, include: A buffer mixing unit is used to collect the wastewater to be treated and the return liquid, and output the mixed liquid; A first nanofiltration unit, the inlet of which is connected to the outlet of the buffer mixing unit, is used to separate the mixture to obtain a first concentrate enriched with sodium citrate and a first permeate enriched with sodium sulfate. A sodium citrate recovery branch is connected to the first concentrate outlet of the first nanofiltration component; the sodium citrate recovery branch includes: a second nanofiltration component for secondary concentration of the first concentrate to obtain a second concentrate and a second permeate, wherein the second permeate is returned to the buffer mixing unit; A sodium sulfate recovery branch is connected to the first permeate outlet of the first nanofiltration unit; the sodium sulfate recovery branch includes: a desalination and concentration unit for treating the first permeate to obtain recycled water and a third concentrate rich in sodium sulfate.

2. The resource recovery system for wastewater containing sodium citrate and sodium sulfate according to claim 1, characterized in that, A third nanofiltration unit is also provided between the first permeate outlet of the first nanofiltration unit and the inlet of the desalination and concentration unit; The third nanofiltration unit is used to process the first permeate to obtain a fourth concentrate and a fourth permeate enriched with sodium citrate. The fourth concentrate is returned to the buffer mixing unit, and the fourth permeate is transported to the inlet of the desalination and concentration component.

3. The resource recovery system for wastewater containing sodium citrate and sodium sulfate according to any one of claims 1 or 2, characterized in that, The second concentrate obtained from the second nanofiltration component enters a first crystallization unit for evaporation and crystallization to recover sodium citrate and obtain a first mother liquor, wherein the first mother liquor is returned to the buffer mixing unit.

4. The resource recovery system for wastewater containing sodium citrate and sodium sulfate according to any one of claims 1 or 2, characterized in that, The third concentrated liquid containing sodium sulfate obtained from a desalination and concentration component of the sodium sulfate recovery branch enters a second crystallization unit for crystallizing the third concentrated liquid to recover sodium sulfate or sodium sulfate and obtain a second mother liquor, wherein the second mother liquor is returned to the buffer mixing unit.

5. The resource recovery system for wastewater containing sodium citrate and sodium sulfate according to any one of claims 1 or 2, characterized in that, The second crystallization unit is configured to lower the temperature of the third concentrate to 0-5°C for freeze crystallization.

6. The resource recovery system for wastewater containing sodium citrate and sodium sulfate according to any one of claims 1 or 2, characterized in that, It also includes a wastewater pretreatment unit located upstream of the buffer mixing unit, the wastewater pretreatment unit being used for solid-liquid separation and / or pH adjustment of the wastewater.

7. The resource recovery system for wastewater containing sodium citrate and sodium sulfate according to any one of claims 1 or 2, characterized in that, It also includes a drying unit connected to the solid outlet of the second crystallization unit, the drying unit being used to dry the recovered hydrous sodium sulfate crystals into anhydrous sodium sulfate.

8. A method for resource recovery from wastewater containing sodium citrate and sodium sulfate, characterized in that, The resource recovery system based on any one of claims 1 to 7 for wastewater containing sodium citrate and sodium sulfate includes the following steps: Mixing and homogenization step: Wastewater containing sodium citrate and sodium sulfate is mixed with the second permeate, the first mother liquor and the second mother liquor from subsequent steps in a buffer mixing unit to obtain a mixed liquid; Preliminary separation step: The mixture is fed into the first nanofiltration unit for separation to obtain a first concentrated solution enriched with sodium citrate and a first permeate enriched with sodium sulfate; Sodium citrate recovery steps: The first concentrate is sent to the second nanofiltration unit for secondary concentration to obtain the second concentrate and the second permeate; the second concentrate is sent to the first crystallization unit for evaporation and crystallization to recover sodium citrate and obtain the first mother liquor; Sodium sulfate recovery steps: The first permeate is sent to a desalination and concentration unit for treatment to obtain recycled water and a third concentrate rich in sodium sulfate; the third concentrate is sent to a second crystallization unit for freeze crystallization to recover sodium sulfate and obtain the second mother liquor.

9. The resource recovery method for wastewater containing sodium citrate and sodium sulfate according to claim 8, characterized in that, The procedure includes, after the preliminary separation step and before the desalting and concentration treatment in the sodium sulfate recovery step: The first permeate is fed into the third nanofiltration unit for processing to obtain a fourth concentrate and a fourth permeate enriched with sodium citrate. The fourth concentrate is refluxed to the mixing and homogenization step; The fourth permeate is used for desalting and concentration in the sodium sulfate recovery step.

10. The resource recovery method for wastewater containing sodium citrate and sodium sulfate according to any one of claims 8 or 9, characterized in that, The freezing crystallization in the sodium sulfate recovery step is controlled at a crystallization temperature of 0-5℃.

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