A method for sulfate purification by step-cooling separation and directional crystallization

Abstract

This invention discloses a method for sulfate purification through step-cooling separation and directional crystallization, belonging to the field of industrial high-salinity wastewater treatment technology. In this invention, by using ultrafiltration pre-filtration for impurity removal and nanofiltration for ion cutting, the pathway for organic impurities to enter the crystal lattice is cut off. Through a multi-step method and 32.4℃ salt fusion large-particle recrystallization technology, the product not only overcomes the problems of easy agglomeration and difficult dehydration of decahydrate sodium sulfate, but is also purified into large-particle anhydrous sodium sulfate and refined sodium chloride with a purity >99%, transforming hazardous waste into industrial raw materials.

Description

Technical Field

[0001] This invention belongs to the field of industrial high-salinity wastewater treatment technology, and particularly relates to a sulfate purification method using step-cooling separation and directional crystallization. Background Technology

[0002] Globally, with the rapid expansion of the coal chemical, fine chemical, textile printing and dyeing industries, and wet desulfurization processes in coal-fired power plants, zero discharge of high-salinity wastewater has become a core bottleneck restricting the green development of these industries. This wastewater mainly contains high concentrations of sulfate and chloride ions, along with trace amounts of heavy metal ions and recalcitrant macromolecular organic matter.

[0003] In traditional wastewater end-of-pipe treatment, multi-effect evaporation or mechanical steam recompression is usually used for full evaporation. However, since all water is removed by relying on the latent heat of phase change, the thermodynamic backflow is severe, resulting in high operating costs. Direct evaporation produces a complex mixture of salts containing organic matter and heavy metals. Under current environmental regulations, this mixture is strictly defined as hazardous waste, and outsourcing its disposal is costly.

[0004] Traditional crystallization processes often result in mixed salt co-precipitation, and large organic molecules act as crystal growth inhibitors, causing crystals to turn yellow, become small and deformed, making them unusable as industrial raw materials. There is room for improvement. Summary of the Invention

[0005] The purpose of this invention is to address the problem that in wastewater end-of-pipe treatment, multi-effect evaporation or mechanical steam recompression is commonly used for total evaporation and drying, but the process relies entirely on the latent heat of phase change for water removal, resulting in severe thermodynamic backflow and high operating costs. Therefore, this invention proposes a step-cooling separation and directional crystallization method for sulfate purification.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for purifying sulfate by step-cooling separation and directional crystallization includes the following steps performed sequentially: S1. Membrane-based multidimensional coupling salt separation: saline wastewater is intercepted by ultrafiltration to remove large organic molecules, then concentrated by reverse osmosis, and then sent to a nanofiltration membrane system for ion-level cutting to obtain nanofiltration concentrate rich in sodium sulfate and nanofiltration permeate rich in sodium chloride. S2. One-step evaporation crystallization: Nanofiltration concentrate is fed into a forced circulation evaporation crystallizer. Under a set pressure, the temperature at the evaporation terminal is strictly controlled to be no more than 9°C above the corresponding boiling point of the solution. A constant supersaturation is generated in the local microenvironment, and water removal and anhydrous sodium sulfate precipitation are achieved simultaneously. S3. Multi-step fractionation and melt directional crystallization: The sodium sulfate mother liquor containing the risk of co-saturation is first frozen and cooled to precipitate sodium sulfate decahydrate crystals; after centrifugation, the sodium sulfate decahydrate crystals are put into the melting reactor and heated to above 32.4°C to cause the crystals to undergo autolysis phase transition. In a self-solution, the cooling process is controlled by step cooling based on the Mullin-Nyvlt nonlinear trajectory to induce directional epitaxial growth of anhydrous sodium sulfate and exclude interfacial impurities. Finally, high-purity anhydrous sodium sulfate is obtained by dehydration and drying.

[0007] As a further description of the above technical solution: In step S1, the ultrafiltration system must have a retention rate of >90% for high molecular weight organic matter and colloids in the wastewater in order to eliminate the interference of organic matter on the subsequent crystallization morphology. The nanofiltration membrane has a negatively charged surface, and by utilizing physical steric hindrance and the electrochemical Downan effect, the SO42- rejection rate reaches over 95%.

[0008] As a further description of the above technical solution: The one-step evaporation crystallization process in step S2 also includes a reverse chemical cleaning mechanism; When the system detects the formation of sulfuric acid and sodium sulfate double salt due to concentration or temperature fluctuations, it diverts unconcentrated fresh solution to rinse the crystals, breaking the double salt lattice and carrying the impurities back to the mother liquor. At the end of the process, the crystals are rinsed with a very low concentration of dilute alkaline solution to adjust the pH value to neutral or slightly alkaline.

[0009] As a further description of the above technical solution: The Mullin-Nyvlt nonlinear trajectory cooling control in S3 specifically refers to: In the early stage of crystallization after autolysis phase transformation, an extremely slow cooling rate is maintained to control the supersaturation at a very low level in the metastable region in order to suppress secondary nucleation. As the crystal surface area increases, the cooling rate is gradually accelerated to match the consumption of crystal growth.

[0010] As a further description of the above technical solution: In the directional crystallization process of step S3, by controlling the local temperature gradient and the high-velocity forced circulation field in the reactor, impurities are physically repelled to the boundary layer at the solid-liquid interface and carried back to the mother liquor through turbulent diffusion, thus preventing the components from becoming too cold and crystal defects from forming.

[0011] As a further description of the above technical solution: The nanofiltration permeate and the residual sodium chloride mother liquor after freezing and crystallization in step S3 are sent to a separate evaporator crystallizer. By continuously inputting heat energy, the water is forced to evaporate, and high-purity sodium chloride crystals are precipitated.

[0012] As a further description of the above technical solution: A sulfate purification system with step-cooling separation and directional crystallization includes an ultrafiltration unit, a reverse osmosis unit, a nanofiltration unit, an OSLO or DTB type forced circulation evaporator crystallizer, a freeze crystallizer, and a melt crystallization reactor with a precision temperature control sleeve, connected in sequence by pipelines.

[0013] As a further description of the above technical solution: The main body of the forced circulation evaporator crystallizer, heat exchange tube bundle, and circulation pump impeller are made of super duplex stainless steel or titanium alloy to resist pitting corrosion and stress corrosion cracking of high temperature and high concentration brine.

[0014] As a further description of the above technical solution: The system integrates an artificial intelligence-based model predictive control platform; An online refractometer and a focused beam reflectance meter are deployed in the crystallization circuit to acquire data on mother liquor concentration and crystal particle size distribution in real time using in-situ scanning.

[0015] As a further description of the above technical solution: The MPC platform receives real-time data based on the NaCl-Na2SO4 phase diagram model and adjusts the steam regulating valve opening, cooling water flow rate, and seed crystal addition frequency in a closed-loop manner at the millisecond level.

[0016] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. In this invention, by using ultrafiltration pre-purification and nanofiltration ion cutting, the path of organic impurities entering the crystal lattice is completely cut off. Through multi-step method and 32.4℃ salt melting large particle recrystallization technology, the product not only overcomes the problems of easy agglomeration and difficult dehydration of decahydrate sodium sulfate, but also purifies it into large particle anhydrous sodium sulfate and refined sodium chloride with a purity of >99%, transforming hazardous waste into industrial raw materials.

[0017] 2. In this invention, a one-step synergistic process is adopted to combine evaporation and dehydration with sodium sulfate precipitation into a single thermodynamic process, completely eliminating the cumbersome traditional process of high-temperature concentration, drastic cooling and reheating.

[0018] 3. In this invention, by combining FBRM in-situ sensing with MPC model predictive control algorithm, the operating conditions are anchored in the optimal metastable zone under the condition of drastic fluctuations in industrial water quality, ensuring the consistency of product particle size distribution between batches and eliminating system scaling and product deterioration. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of a sulfate purification method using step-cooling separation and directional crystallization proposed in this invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Please see Figure 1 The present invention provides a technical solution: Example 1: A method for purifying sulfate by step-cooling separation and directional crystallization, characterized by comprising the following steps performed sequentially: S1. Membrane-based multidimensional coupling salt separation: saline wastewater is intercepted by ultrafiltration to remove large organic molecules, then concentrated by reverse osmosis, and then sent to a nanofiltration membrane system for ion-level cutting to obtain nanofiltration concentrate rich in sodium sulfate and nanofiltration permeate rich in sodium chloride. S2. One-step evaporation crystallization: Nanofiltration concentrate is fed into a forced circulation evaporation crystallizer. Under a set pressure, the temperature at the evaporation terminal is strictly controlled to be no more than 9°C above the corresponding boiling point of the solution. A constant supersaturation is generated in the local microenvironment, and water removal and anhydrous sodium sulfate precipitation are achieved simultaneously. S3. Multi-step fractionation and melt directional crystallization: The sodium sulfate mother liquor containing the risk of co-saturation is first frozen and cooled to precipitate sodium sulfate decahydrate crystals; after centrifugation, the sodium sulfate decahydrate crystals are put into the melting reactor and heated to above 32.4°C to cause the crystals to undergo autolysis phase transition. In a self-solution, the cooling process is controlled by step cooling based on the Mullin-Nyvlt nonlinear trajectory to induce directional epitaxial growth of anhydrous sodium sulfate and exclude interfacial impurities. Finally, high-purity anhydrous sodium sulfate is obtained by dehydration and drying.

[0022] In step S1, the ultrafiltration system must have a retention rate of >90% for high molecular weight organic matter and colloids in the wastewater in order to eliminate the interference of organic matter on the subsequent crystallization morphology. The nanofiltration membrane has a negative charge on its surface. By utilizing physical steric hindrance and the electrochemical Downan effect, the rejection rate of SO42- can reach more than 95%. The one-step evaporation crystallization process in step S2 also includes a reverse chemical cleaning mechanism; When the system detects the formation of sulfuric acid and sodium sulfate double salt due to concentration or temperature fluctuations, it diverts unconcentrated fresh solution to rinse the crystals, breaking the double salt lattice and carrying the impurities back to the mother liquor. At the end of the process, the crystals are rinsed with a very low concentration of dilute alkaline solution to adjust the pH value to neutral or slightly alkaline. The Mullin-Nyvlt nonlinear trajectory cooling control in S3 specifically refers to: In the early stage of crystallization after autolysis phase transformation, an extremely slow cooling rate is maintained to control the supersaturation at a very low level in the metastable region in order to suppress secondary nucleation. As the crystal surface area increases, the cooling rate is gradually accelerated to match the consumption of crystal growth; In the directional crystallization process of step S3, by controlling the local temperature gradient and the high-velocity forced circulation field in the reactor, impurities are physically repelled to the boundary layer at the solid-liquid interface and carried back to the mother liquor through turbulent diffusion, thus preventing the components from becoming too cold and crystal defects from forming. The nanofiltration permeate and the residual sodium chloride mother liquor after freezing and crystallization in step S3 are sent to an independent evaporator crystallizer. By continuously inputting heat energy, the water is forced to evaporate, and high-purity sodium chloride crystals are precipitated. A sulfate purification method system using step-cooling separation and directional crystallization includes an ultrafiltration unit, a reverse osmosis unit, a nanofiltration unit, an OSLO or DTB type forced circulation evaporator crystallizer, a freeze crystallizer, and a melt crystallization reactor with a precision temperature control sleeve connected in sequence via pipelines. The main body, heat exchange tube bundle, and circulating pump impeller of the forced circulation evaporator are made of super duplex stainless steel or titanium alloy to resist pitting corrosion and stress corrosion cracking of high temperature and high concentration brine. The system integrates an artificial intelligence-based model predictive control platform; An online refractometer and a focused beam reflectance meter are deployed in the crystallization circuit to acquire data on mother liquor concentration and crystal particle size distribution in real time using in-situ scanning. The MPC platform receives real-time data based on the NaCl-Na2SO4 phase diagram model and adjusts the steam regulating valve opening, cooling water flow rate, and seed crystal addition frequency in a closed-loop manner at the millisecond level.

[0023] A co-purification process for high-salt wastewater from desulfurization in coal-fired power plants. The high-salt wastewater from the end-of-pipe FGD of a certain coal-fired power plant has a flow rate of 50 m³ / h and contains a large amount of sulfate, chloride ions and some organic colloids.

[0024] Membrane-coupled salt separation: Wastewater first enters an ultrafiltration (UF) tubular membrane, where the organic matter and suspended solids rejection rate reaches 92% and the turbidity of the produced water is <0.1 NTU. Subsequently, the salt is concentrated to 80,000 mg / L through two-stage reverse osmosis (RO).

[0025] The concentrate enters the nanofiltration (NF) unit, and the operating pressure is controlled. By utilizing the Donnan effect, the purity of sodium sulfate on the concentrate side is increased to 96% (percentage), while most of the sodium chloride enters the permeate.

[0026] One-step evaporation: Nanofiltration concentrate enters a DTB-type forced circulation crystallizer. The system operating pressure is set, and the boiling point is controlled at 85℃. The DCS system precisely controls the temperature of the feed liquid at the heating terminal at 93℃ (temperature difference 8℃, meeting the <9℃ requirement) by adjusting the steam inlet valve. While the water flashes away, the supersaturation is maintained constant, and anhydrous sodium sulfate precipitates directly. During this process, the discharged crystals are micro-washed with dilute alkaline water at pH=8, producing sodium sulfate with a purity of 98.5%.

[0027] Example 2: Directed Large-Particle Crystallization of Complex Mixed Salts Based on Phase Diagram Regarding the waste liquid in Example 1 that was at the edge of co-saturation due to incomplete nanofiltration separation: The mixed salt mother liquor was introduced into a cryogenic crystallizer and cooled to -5°C. Taking advantage of the high temperature sensitivity of sodium sulfate, a large amount of high-purity sodium sulfate decahydrate (sodium sulfate) crystals were precipitated, while sodium chloride remained in the liquid phase.

[0028] The decahydrate sodium sulfate obtained after centrifugation (containing nearly 60% water of crystallization) was transferred to a titanium alloy melting reactor.

[0029] The reactor was slowly heated through the critical phase transition point of 32.4℃, and the crystals "autolyzed" in their own water of crystallization.

[0030] FBRM scans particle size in real time, and the MPC system initiates the step-cooling directional crystallization process: Initially, the cooling rate is controlled within 0.1℃ / min to guide the formation of primary crystal nuclei; later, as the crystal specific surface area increases, the cooling rate is increased to 0.5℃ / min. Accompanied by a forced circulation flow rate of 3.0 m / s within the OSLO fluidized bed, impurities are repelled to the fluid boundary layer.

[0031] Finally, centrifugation and drying were performed to obtain large-particle anhydrous sodium sulfate.

[0032] Experimental Example: Comparison of the Invention Process with the Traditional Total Evaporation Process Experimental Background: The same batch of high-concentration brine (total concentration of sodium sulfate and sodium chloride approximately 15%) was selected and continuously tested for 72 hours using the synergistic process of this invention (experimental group) and the traditional multi-effect full-volume evaporation process (comparative example). The processing scale was set at a target output of 4 tons / hour of salt crystals.

[0033]

[0034] Comparison of test data results: The test data fully confirms that traditional processes can only produce hazardous waste that requires high-cost disposal and consumes a lot of energy; while this invention, through the synergy of membrane separation and step-cooling directional recrystallization, not only breaks through the 99% purity barrier to achieve complete resource utilization, but also achieves energy-saving effects under the thermodynamic integration of the "one-step method", which has extremely high economic and environmental application value.

[0035] In this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance; the term "multiple" refers to two or more unless otherwise explicitly defined. The terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; "linking" can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0036] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for purifying sulfate by step-cooling separation and directional crystallization, characterized in that, It includes the following steps performed in sequence: S1. Membrane-based multidimensional coupling salt separation: saline wastewater is intercepted by ultrafiltration to remove large organic molecules, then concentrated by reverse osmosis, and then sent to a nanofiltration membrane system for ion-level cutting to obtain nanofiltration concentrate rich in sodium sulfate and nanofiltration permeate rich in sodium chloride. S2. One-step evaporation crystallization: Nanofiltration concentrate is fed into a forced circulation evaporation crystallizer. Under a set pressure, the temperature at the evaporation terminal is strictly controlled to be no more than 9°C above the corresponding boiling point of the solution. A constant supersaturation is generated in the local microenvironment, and water removal and anhydrous sodium sulfate precipitation are achieved simultaneously. S3. Multi-step fractionation and melt directional crystallization: The sodium sulfate mother liquor containing the risk of co-saturation is first frozen and cooled to precipitate sodium sulfate decahydrate crystals; after centrifugation, the sodium sulfate decahydrate crystals are put into the melting reactor and heated to above 32.4°C to cause the crystals to undergo autolysis phase transition. In a self-solution, the cooling process is controlled by step cooling based on the Mullin-Nyvlt nonlinear trajectory to induce directional epitaxial growth of anhydrous sodium sulfate and exclude interfacial impurities. Finally, high-purity anhydrous sodium sulfate is obtained by dehydration and drying.

2. The method for sulfate purification by step-cooling separation and directional crystallization according to claim 1, characterized in that, In step S1, the ultrafiltration system must have a retention rate of >90% for high molecular weight organic matter and colloids in the wastewater in order to eliminate the interference of organic matter on the subsequent crystallization morphology. The nanofiltration membrane has a negatively charged surface, utilizing physical steric hindrance and the electrochemical Downan effect to enable... The retention rate reached over 95%.

3. The method for sulfate purification by step-cooling separation and directional crystallization according to claim 1, characterized in that, The one-step evaporation crystallization process in step S2 also includes a reverse chemical cleaning mechanism; When the system detects the formation of sulfuric acid and sodium sulfate double salt due to concentration or temperature fluctuations, it diverts unconcentrated fresh solution to rinse the crystals, breaking the double salt lattice and carrying the impurities back to the mother liquor. At the end of the process, the crystals are rinsed with a very low concentration of dilute alkaline solution to adjust the pH value to neutral or slightly alkaline.

4. The method for sulfate purification by step-cooling separation and directional crystallization according to claim 1, characterized in that, The Mullin-Nyvlt nonlinear trajectory cooling control in S3 specifically refers to: In the early stage of crystallization after autolysis phase transformation, an extremely slow cooling rate is maintained to control the supersaturation at a very low level in the metastable region in order to suppress secondary nucleation. As the crystal surface area increases, the cooling rate is gradually accelerated to match the consumption of crystal growth.

5. The method for sulfate purification by step-cooling separation and directional crystallization according to claim 1, characterized in that, In the directional crystallization process of step S3, by controlling the local temperature gradient and the high-velocity forced circulation field in the reactor, impurities are physically repelled to the boundary layer at the solid-liquid interface and carried back to the mother liquor through turbulent diffusion, thus preventing the components from becoming too cold and crystal defects from forming.

6. The method for sulfate purification by step-cooling separation and directional crystallization according to claim 1, characterized in that, The nanofiltration permeate and the residual sodium chloride mother liquor after freezing and crystallization in step S3 are sent to a separate evaporator crystallizer. By continuously inputting heat energy, the water is forced to evaporate, and high-purity sodium chloride crystals are precipitated.

7. A method system for sulfate purification by step-cooling separation and directional crystallization, applied to the sulfate purification method by step-cooling separation and directional crystallization as described in any one of claims 1-6, characterized in that, It includes an ultrafiltration unit, a reverse osmosis unit, a nanofiltration unit, an OSLO or DTB type forced circulation evaporator crystallizer, a freeze crystallizer, and a melt crystallization reactor with a precision temperature control sleeve, all connected in sequence by pipelines.

8. The sulfate purification system for step-cooling separation and directional crystallization according to claim 7, characterized in that, The main body of the forced circulation evaporator crystallizer, heat exchange tube bundle, and circulation pump impeller are made of super duplex stainless steel or titanium alloy to resist pitting corrosion and stress corrosion cracking of high temperature and high concentration brine.

9. A sulfate purification system for step-cooling separation and directional crystallization according to claim 7, characterized in that, The system integrates an artificial intelligence-based model predictive control platform; An online refractometer and a focused beam reflectance meter are deployed in the crystallization circuit to acquire data on mother liquor concentration and crystal particle size distribution in real time using in-situ scanning.

10. A sulfate purification system for step-cooling separation and directional crystallization according to claim 7, characterized in that, The MPC platform receives real-time data based on the NaCl-Na2SO4 phase diagram model and adjusts the steam regulating valve opening, cooling water flow rate, and seed crystal addition frequency in a closed-loop manner at the millisecond level.

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