Recycling method of sulfuric acid method titanium dioxide post-treatment wastewater

Abstract

The invention belongs to the technical field of titanium dioxide production wastewater treatment, and particularly relates to a sulfuric acid method titanium dioxide post-treatment wastewater recycling method. According to the method, lime milk is added into wastewater for pretreatment, titanium and iron ions are removed, and titanium resources in sludge are recycled; carrying out nanofiltration grading on the pretreated wastewater, and intercepting sulfate radicals; carrying out reverse osmosis deep purification on nanofiltration produced water; and according to the requirements of each post-treatment process, reverse osmosis produced water is used for enveloping, nanofiltration produced water is used for pulping, and concentrated water is refluxed or calcium sulfate is recycled. The wastewater is thoroughly treated, and the conductivity of reverse osmosis produced water is less than or equal to 50 microseconds per centimeter; the resource utilization rate is larger than or equal to 90%, and 5-8 tons of water is saved for each ton of products Titanium resources and calcium sulfate are recycled, and economic benefits are remarkable; the method provided by the invention is stable and reliable in operation, the membrane cleaning period is prolonged to 30-45 days, the continuous operation time of the system is greater than or equal to 180 days, zero discharge of wastewater can be realized, and the method is suitable for advanced treatment and cyclic utilization of post-treatment wastewater of a sulfuric acid process titanium dioxide enterprise.

Description

Technical Field

[0001] This invention belongs to the field of titanium dioxide production wastewater treatment technology, specifically relating to a method for recycling post-treatment wastewater from sulfuric acid process titanium dioxide. Background Technology

[0002] In the sulfuric acid process for titanium dioxide production, post-treatment steps (mainly including pulping, coating, washing, and drying) generate large quantities of acidic wastewater with complex compositions and high pollutant concentrations. Typical characteristics of this type of wastewater include the presence of titanium ions (Ti). 4 ⁺, 50-200 mg / L), iron ions (Fe 3+ 30-100 mg / L), high concentration of sulfate (SO4) 2- ,1000-3000 mg / L) and small amounts of inorganic suspended matter and organic coating agent residues.

[0003] Currently, the industry generally uses the traditional process of "coagulation sedimentation + filtration + discharge to meet standards" to treat this wastewater. Heavy metal ions are neutralized and precipitated by adding alkaline substances (such as lime or liquid alkali), and the treated effluent is directly discharged. Patent CN112499813A discloses a production method for the full recycling of wastewater from the sulfuric acid process for titanium dioxide production. It employs lime neutralization for hardening → nanofiltration membrane separation of sodium sulfate → causticization / carbonation recovery. This method treats the comprehensive wastewater after lime neutralization, recovering sulfate ions (in the form of sodium sulfate and sodium carbonate), but does not recover titanium resources, and the quality of the produced water is not described in detail. Patent CN119661008A discloses a method for utilizing waste acid water, with a technical route of flocculation stratification → filtration → return to the front-end process as dilution water. This patent does not mention membrane technology, does not provide deep purification, and does not specifically address membrane fouling and scaling problems caused by heavy metals and organic matter in the post-treatment wastewater.

[0004] The existing processing methods have the following significant drawbacks: Incomplete treatment leads to significant environmental pressure: effluent often contains trace amounts of dissolved heavy metal ions such as titanium and iron, as well as high concentrations of sulfates, making it difficult to meet increasingly stringent wastewater discharge standards (especially the control requirements for total salinity), posing environmental pollution risks and compliance pressures.

[0005] Severe waste of water resources: Wastewater that has been treated but not deeply purified is discharged directly without being recycled, while the titanium dioxide post-processing process itself is a major consumer of fresh water in industry. This linear "one-pass, end-of-pipe treatment" model leads to high water consumption per unit of product for enterprises, which runs counter to the current water scarcity situation and the requirements of green and low-carbon development.

[0006] Loss of valuable resources: Titanium-containing sludge generated by coagulation and sedimentation is usually disposed of as solid waste, failing to effectively recover the titanium resources in it. This not only wastes resources but also increases the hazardous waste disposal costs for enterprises.

[0007] In response to the call for water conservation and emission reduction, some enterprises have attempted to introduce membrane separation technology to achieve wastewater reuse. However, existing reuse solutions are mostly limited to using a single reverse osmosis (RO) membrane technology to desalinate pretreated wastewater. In practice, this approach has revealed two major problems: Poor operational stability of membrane systems: High concentrations of sulfate ions in wastewater readily combine with cations such as calcium, strontium, and barium, forming stubborn sulfate scale on the membrane surface. Meanwhile, residual colloids, trace heavy metal ions, and organic matter can lead to severe membrane fouling, causing rapid decline in membrane flux, increased operating pressure, and a surge in cleaning frequency, which seriously affects the long-term stable operation of the system and significantly increases maintenance costs.

[0008] The mismatch between water quality and demand leads to inefficiency: Current processes typically reuse membrane permeate uniformly, failing to fully consider the differentiated water quality requirements of various post-treatment steps. For example, the preparation of coating agents and final product rinsing require high-purity water with extremely low conductivity, low hardness, and almost no heavy metal ions; while pulping, equipment and floor washing can accept reclaimed water with lower quality (such as moderate salinity and hardness). The current extensive reuse model of "multiple uses of water" but "mismatch between quality and demand" can not only affect the product quality of high-end processes due to substandard water quality, but also fail to achieve graded optimization of "high-quality water for high-end use and low-quality water for low-end use," resulting in the waste of high-quality permeate and low overall water resource recycling rate.

[0009] Therefore, developing an integrated process that can efficiently and deeply purify wastewater, significantly alleviate membrane fouling, ensure long-term stable operation of the system, and accurately match and supply water in a tiered manner with the "differentiated" water quality requirements of each link in the post-treatment production chain is the key technology to achieve efficient resource utilization of post-treatment wastewater in the sulfuric acid process titanium dioxide industry and solve the dual dilemma of water conservation and environmental protection for enterprises. Summary of the Invention

[0010] The technical problem to be solved by this invention is to provide a method for recycling wastewater from the post-treatment of titanium dioxide produced by the sulfuric acid process. The method involves pre-treating the wastewater with lime slurry to remove titanium and iron ions and recover titanium resources from the sludge; the pre-treated wastewater is then classified by nanofiltration to retain sulfate ions; the nanofiltration permeate is then deeply purified by reverse osmosis; and the treated water is used in different processes according to the requirements of each post-treatment step to achieve matched utilization, with a wastewater resource utilization rate of ≥90%.

[0011] The technical solution adopted is as follows: A method for recycling wastewater from the post-treatment of titanium dioxide produced by the sulfuric acid process, wherein the wastewater generated in the post-treatment process (containing Ti) 4+The content is 50-200 mg / L, Fe 3+ The content is 30-100 mg / L, SO4 2- The treatment targets are those with a content of 1000-3000 mg / L, and include the following steps: (1) Pretreatment and impurity removal: The post-treatment wastewater is introduced into the equalization tank, and the pH is adjusted to alkaline conditions by adding alkaline solution. The reaction and precipitation are carried out to obtain titanium-containing sludge and pretreatment supernatant. (2) Nanofiltration fractionation: The pretreated supernatant is separated by nanofiltration membrane to obtain nanofiltration permeate and nanofiltration concentrate; (3) Deep purification by reverse osmosis: The nanofiltration permeate is separated by a reverse osmosis membrane to obtain high-quality reverse osmosis permeate and high-salinity reverse osmosis concentrate; (4) Recycling: The titanium-containing sludge is recycled to the titanium dioxide acid hydrolysis process for reuse, and the water generated from the post-treatment is used in the production process for differentiated recycling.

[0012] Preferably, in step (1), the pH is adjusted to 8.0-9.5, and the reaction is stirred for 30-60 min; the alkaline solution is at least one of lime milk, sodium hydroxide solution or sodium carbonate solution; the stirring speed is 150-200 r / min.

[0013] Preferably, the pretreatment for impurity removal further includes dewatering the settled sludge to obtain dry sludge containing ≥25wt% TiO2.

[0014] Preferably, in step (1), the supernatant is filtered through a precision filter to remove suspended particulate matter, so that the solid content is ≤5mg / L.

[0015] Preferably, in step (2), the pretreated supernatant is pumped into the nanofiltration membrane system, and the operating pressure is controlled at 1.5-2.5 MPa and the temperature at 25-35°C to obtain nanofiltration permeate rich in monovalent ions and nanofiltration concentrate mainly composed of sulfate ions. Among them, the nanofiltration membrane has a sulfate rejection rate of ≥95% and a water molecule and a small amount of chloride ion rejection rate of ≤10%; The concentration of SO4²⁻ in nanofiltration permeate is ≤150 mg / L, and the concentration of SO4²⁻ ions in nanofiltration concentrate ranges from 5000 to 8000 mg / L.

[0016] Preferably, the nanofiltration membrane is a fouling-resistant polyamide composite membrane with a pore size of 1-5 μm.

[0017] Preferably, in step (3), a low-pressure composite reverse osmosis membrane is selected as the reverse osmosis membrane; the operating pressure is controlled at 3.0-4.0 MPa and the temperature at 25-35℃; the reverse osmosis membrane has a rejection rate of ≥99% for residual impurities, and reverse osmosis permeate is obtained.

[0018] Preferably, the high-quality reverse osmosis permeate obtained by the reverse osmosis deep purification step meets at least one of the following indicators: conductivity ≤ 50 μS / cm, total concentration of heavy metal ions ≤ 0.1 mg / L.

[0019] Preferably, the method further includes a step of further treating the nanofiltration concentrate and / or the reverse osmosis concentrate, the further treatment including at least one of evaporation crystallization, freeze crystallization, or acid hydrolysis / hydrolysis.

[0020] Preferred, differentiated recycling includes: Reverse osmosis permeate: transported to the coating process as process water for preparing the coating solution; Nanofiltration permeate: is supplied to the pulping process as pulping water; Reverse osmosis concentrate: returned to the equalization tank for reprocessing; Nanofiltration concentrate: After evaporation, concentration, cooling and crystallization, calcium sulfate crystals are recovered, and the condensate generated during the concentration process is reused in the pretreatment process.

[0021] The cleaning cycle for the nanofiltration membrane and reverse osmosis membrane used in this invention is 30-45 days, which can be achieved by cleaning with 1-2% citric acid solution and 0.5-1.0% sodium hydroxide solution, respectively.

[0022] Compared with the prior art, the significant advantages of the present invention are as follows: The method of this invention thoroughly treats wastewater, and the quality of the permeate fully meets or even exceeds the water requirements of the most sensitive processes such as membrane coating and rinsing, making large-scale, high-quality reuse possible and solving the problem that traditionally treated water cannot be reused in core processes. After treatment by the combined process, the conductivity of the reverse osmosis permeate is ≤50μS / cm and the heavy metal ion concentration is ≤0.1mg / L, fully meeting the process water requirements.

[0023] This invention offers a high water resource utilization rate, transforming the traditional "linear consumption-end-of-pipe treatment" model into a "closed-loop circulation-tiered reuse" model. This directly and significantly reduces fresh water consumption and water intake costs, with particularly outstanding benefits in areas with water scarcity or high water prices. The differentiated reuse design ensures a wastewater resource utilization rate of ≥90%, drastically reducing fresh water consumption, saving 5-8 tons of water per ton of product.

[0024] This invention provides a method for efficient resource recovery, turning waste into valuable resources and generating revenue. It not only avoids the loss of valuable titanium resources and the costs of hazardous waste disposal, but also opens up new revenue streams by recovering byproducts such as calcium sulfate, transforming environmental investment into economic benefits. Titanium resources in the sludge are recovered to the acidolysis process, and calcium sulfate is recovered from the nanofiltration concentrate, achieving a resource recovery rate of ≥95% and preventing resource waste.

[0025] The method of this invention is stable and reliable in operation. Through the combined process of "pretreatment + nanofiltration classification", it effectively alleviates membrane fouling, a core pain point restricting the application of membrane technology, from the source. It significantly reduces the cost of chemical cleaning, maintenance, energy consumption, and downtime frequency, ensuring the long-term economic feasibility and stability of the process. Pretreatment removes most impurities, reduces the risk of membrane fouling, extends the membrane cleaning cycle to 30-45 days, and allows the system to operate continuously for ≥180 days.

[0026] The method of this invention has significant environmental benefits, achieving zero wastewater discharge, completely eliminating the environmental pollution risks caused by wastewater discharge and the compliance pressure of increasingly stringent environmental regulations (especially total salt discharge limits), avoiding environmental pollution caused by wastewater discharge, meeting the requirements of green production, and laying the foundation for the green and sustainable development of enterprises.

[0027] This invention provides an economical, stable, and efficient method for achieving near-zero discharge and full resource recovery of wastewater from the post-treatment of sulfuric acid titanium dioxide. While achieving excellent environmental performance, it significantly reduces production water consumption and resource costs, and improves the economy and sustainability of the entire production system. Attached Figure Description

[0028] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0029] The accompanying drawings are for illustrative purposes only, and some prior art or common knowledge may be omitted. In order to enable those skilled in the art to better understand the technical solution of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples, but they should not be construed as limiting the present patent.

[0030] Unless otherwise specified, the experimental or testing methods described in the following examples are all conventional methods; unless otherwise specified, the reagents, materials, and instruments are all obtained from conventional commercial sources or prepared using conventional methods.

[0031] Example 1 A method for recycling wastewater from the post-treatment of titanium dioxide produced by the sulfuric acid process, wherein the wastewater generated in the post-treatment process (containing Ti) 4+ The content is 90 mg / L, Fe 3+ The content is approximately 55 mg / L, SO4 2- The treatment target is a substance with a content of approximately 1500 mg / L, and includes the following steps: (1) Pretreatment and impurity removal: The post-treatment wastewater is introduced into the equalization tank, and the pH is adjusted to alkaline conditions by adding alkaline solution. The reaction and precipitation are carried out to obtain titanium-containing sludge and pretreatment supernatant. The pH was adjusted to approximately 8.0, and the reaction was stirred for 40 minutes. The alkaline solution was lime slurry with a mass fraction of 10%, and the stirring speed was 150 r / min. 4+ Fe 3+ Hydroxide precipitate is generated; after the slurry settles in the sedimentation tank, the supernatant enters the precision filter for filtration to remove suspended particulate matter (solid content ≤5mg / L); the settled sludge is sent to the sludge dewatering machine for dewatering, and the dry sludge (containing TiO2 ≥30%) is recycled to the acid hydrolysis process for reuse.

[0032] (2) Nanofiltration fractionation: The pretreated supernatant is separated by a nanofiltration membrane system to obtain nanofiltration permeate and nanofiltration concentrate; the operating pressure is controlled at about 1.5 MPa, the temperature at about 25℃, and the recovery rate is 66%; the nanofiltration membrane has a sulfate rejection rate of ≥95% and a water molecule and a small amount of chloride ion rejection rate of ≤10%; finally, nanofiltration permeate (SO4²⁻ concentration ≤150 mg / L) and nanofiltration concentrate (SO4²⁻ concentration of 6000 mg / L) are obtained.

[0033] (3) Deep purification by reverse osmosis: The nanofiltration permeate is pumped into the reverse osmosis membrane system for separation to obtain high-quality reverse osmosis permeate and high-salinity reverse osmosis concentrate; wherein, the operating pressure is controlled at 3.0 MPa, the temperature at 25℃, and the recovery rate at 75%; the reverse osmosis membrane has a rejection rate of ≥99% for residual impurities, and reverse osmosis permeate (conductivity ≤50 μS / cm, heavy metal ions ≤0.1 mg / L) and reverse osmosis concentrate (SO4²⁻ concentration is 350 mg / L).

[0034] (4) Recycling: The titanium-containing sludge is recycled to the titanium dioxide acid hydrolysis process for reuse, and the water generated from the post-treatment is used in the production process for differentiated recycling.

[0035] Differentiated recycling includes: Reverse osmosis permeate: transported to the coating process as process water for preparing the coating solution; Nanofiltration permeate: is supplied to the pulping process as pulping water; Reverse osmosis concentrate: returned to the equalization tank for reprocessing; Nanofiltration concentrate: After evaporation, concentration, cooling and crystallization, calcium sulfate crystals are recovered (purity ≥98%), and the condensate generated during the concentration process is reused in the pretreatment process.

[0036] The system can run continuously for 185 days. The cleaning cycle for nanofiltration membranes and reverse osmosis membranes is about 40 days, and they are cleaned with 2% citric acid solution and 1.0% sodium hydroxide solution, respectively.

[0037] The wastewater recycling method for sulfuric acid process titanium dioxide post-treatment in Example 1 of the present invention is compared with the traditional treatment process, and the results are shown in Table 1.

[0038] Table 1. Comparison of wastewater treatment methods for the sulfuric acid process of titanium dioxide with traditional processes. According to Table 1, the treatment method of Example 1 of the present invention saves RMB 5.2 million in fresh water costs annually; recovers approximately 200 tons of TiO2 annually; and recovers 3,500 tons of calcium sulfate annually, generating a profit of RMB 1.05 million.

[0039] Example 2 A method for recycling wastewater from the post-treatment of titanium dioxide produced by the sulfuric acid process, wherein the wastewater generated in the post-treatment process (containing Ti) 4+ The content was 135 mg / L, Fe 3+ The content is approximately 63 mg / L, SO4 2- The treatment target (containing approximately 2300 mg / L) includes the following steps: (1) Pretreatment and impurity removal: The post-treatment wastewater is introduced into the equalization tank, and the pH is adjusted to alkaline conditions by adding alkaline solution. The reaction and precipitation are carried out to obtain titanium-containing sludge and pretreatment supernatant. The pH was adjusted to approximately 8.5, and the reaction was stirred for 60 minutes. The alkaline solution was lime slurry with a mass fraction of 15%, and the stirring speed was 200 r / min. 4+ Fe 3+ Hydroxide precipitate is generated; after the slurry settles in the sedimentation tank, the supernatant enters the precision filter for filtration to remove suspended particulate matter (solid content ≤5mg / L); the settled sludge is sent to the sludge dewatering machine for dewatering, and the dry sludge (containing 32% TiO2 by mass) is recycled to the acid hydrolysis process for reuse.

[0040] (2) Nanofiltration fractionation: The pretreated supernatant is separated by a nanofiltration membrane system to obtain nanofiltration permeate and nanofiltration concentrate; the operating pressure is controlled at about 2.0 MPa, the temperature at about 35℃, and the recovery rate is 65%; the nanofiltration membrane has a sulfate rejection rate of ≥95% and a water molecule and a small amount of chloride ion rejection rate of ≤10%; finally, nanofiltration permeate (SO4²⁻ concentration ≤150 mg / L) and nanofiltration concentrate (SO4²⁻ concentration of 7000 mg / L) are obtained.

[0041] (3) Deep purification by reverse osmosis: The nanofiltration permeate is pumped into the reverse osmosis membrane system for separation to obtain high-quality reverse osmosis permeate and high-salinity reverse osmosis concentrate; wherein, the operating pressure is controlled at 3.0 MPa, the temperature at 25℃, and the recovery rate at 75%; the reverse osmosis membrane has a rejection rate of ≥99% for the remaining impurities, and reverse osmosis permeate (conductivity ≤50 μS / cm, heavy metal ions ≤0.1 mg / L) and reverse osmosis concentrate (SO4²⁻ concentration is 420 mg / L).

[0042] (4) Recycling: The titanium-containing sludge is recycled to the titanium dioxide acid hydrolysis process for reuse, and the water generated from the post-treatment is used in the production process for differentiated recycling.

[0043] Differentiated recycling includes: Reverse osmosis permeate: transported to the coating process as process water for preparing the coating solution; Nanofiltration permeate: is supplied to the pulping process as pulping water; Reverse osmosis concentrate: returned to the equalization tank for reprocessing; Nanofiltration concentrate: After evaporation, concentration, cooling and crystallization, calcium sulfate crystals are recovered (purity ≥98%), and the condensate generated during the concentration process is reused in the pretreatment process.

[0044] The system can run continuously for 185 days. The cleaning cycle for nanofiltration membranes and reverse osmosis membranes is about 43 days, and they are cleaned with 2% citric acid solution and 1.0% sodium hydroxide solution, respectively.

[0045] Application Example 1 Taking the post-treatment wastewater from a sulfuric acid process titanium dioxide production line with an annual output of 80,000 tons as the treatment target, the wastewater quality is: Ti 4+ 120 mg / L, Fe 3+ With a water concentration of 65 mg / L and SO4²⁻2200 mg / L, and a treatment capacity of 100 m³ / h, this process is implemented as follows: Pretreatment and impurity removal: Wastewater enters the equalization tank, 12% lime milk is added to adjust the pH to 8.5, the stirring speed is 180 r / min, and the reaction is carried out for 45 min; after settling in the sedimentation tank, the supernatant is filtered through a 3μm precision filter and has a solid content of 3.2 mg / L; after sludge dewatering, the dry sludge contains 35% TiO2 and is recycled to the acid hydrolysis process; Nanofiltration staged treatment: After pretreatment, the wastewater is pumped into a fouling-resistant polyamide nanofiltration membrane system with an operating pressure of 2.0 MPa, a temperature of 30℃, and a recovery rate of 65%; the nanofiltration permeate has an SO4²⁻ of 120 mg / L, and the nanofiltration concentrate has an SO4²⁻ of 6800 mg / L. Deep purification via reverse osmosis: 60% of the nanofiltration permeate is pumped into the low-pressure reverse osmosis membrane system, operating at 3.5 MPa, 30℃, and a recovery rate of 75%; the reverse osmosis permeate has a conductivity of 38 μS / cm and a Ti content of [missing information]. 4+ 0.08 mg / L, Fe 3+ It is 0.06 mg / L; Differentiated recycling: Reverse osmosis permeate is transported to the membrane coating process (39 m³ / h); the remaining nanofiltration permeate is transported to the pulping process (35 m³ / h); reverse osmosis concentrate is returned to the equalization tank; nanofiltration concentrate is evaporated and concentrated to recover calcium sulfate crystals (98.3% purity), and the condensate is reused for pretreatment (22 m³ / h).

[0046] Implementation results: Wastewater resource utilization rate is 92%, water saving is 6.5 tons per ton of product, saving 5.2 million yuan in fresh water costs annually; sludge titanium resource recovery rate is 96%, recovering approximately 200 tons of TiO2 annually; nanofiltration concentrate recovers 3,500 tons of calcium sulfate annually, generating economic benefits of 1.05 million yuan; the system operates stably for 190 consecutive days, with a membrane cleaning cycle of 42 days.

[0047] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for recycling waste water in the post-treatment of titanium dioxide by the sulfuric acid method, characterized in that, The method comprises the following steps: (1) Pretreatment and impurity removal: introducing the post-treatment wastewater into a conditioning tank, adding alkali liquor to adjust the pH to alkaline conditions, and performing reaction and precipitation to obtain titanium-containing sludge and pretreated supernatant; (2) Nanofiltration grading treatment: performing nanofiltration membrane separation on the pretreated supernatant to obtain nanofiltration product water and nanofiltration concentrated water; (3) Reverse osmosis deep purification: performing reverse osmosis membrane separation on the nanofiltration product water to obtain high-quality reverse osmosis product water and high-salinity reverse osmosis concentrated water; (4) Recycling: recycling the titanium-containing sludge to the titanium dioxide acidolysis process for reuse, and using the water produced in the post-treatment for the production process for differential recycling.

2. The method according to claim 1, wherein the method is characterized by, In step (1), the pH is adjusted to 8.0-9.5, and the stirring reaction is performed for 30-60 min; the alkali liquor is at least one of lime milk, sodium hydroxide solution or sodium carbonate solution; the stirring speed is 150-200 r / min.

3. The method according to claim 1, wherein the method is characterized by, The pretreatment and impurity removal also comprises dewatering treatment of the precipitated sludge to obtain dry sludge containing TiO2≥25 wt%.

4. The method according to claim 1, wherein the method is characterized by, In step (1), the supernatant is filtered through a precision filter to remove suspended particulate matter, so that the solid content is ≤5 mg / L.

5. The method according to claim 1, wherein the method is characterized by, In step (2), the pretreated supernatant is pumped into a nanofiltration membrane system, the operating pressure is controlled to be 1.5-2.5 MPa, and the temperature is controlled to be 25-35°C, to obtain nanofiltration product water rich in monovalent ions and nanofiltration concentrated water mainly containing sulfate ions; wherein the nanofiltration membrane has a sulfate ion retention rate ≥95% and a water molecule and a small amount of chloride ion retention rate ≤10%; the concentration of SO4²⁻ in the nanofiltration product water is ≤150 mg / L, and the concentration of SO4²⁻ ions in the nanofiltration concentrated water is in the range of 5000-8000 mg / L.

6. The method according to claim 5, wherein the method is characterized by, The nanofiltration membrane is a pollution-resistant polyamide composite membrane with a pore size of 1-5 μm.

7. The method according to claim 1, wherein the method is characterized by, In step (3), the reverse osmosis membrane is a low-pressure composite reverse osmosis membrane; the operating pressure is controlled to be 3.0-4.0 MPa, and the temperature is controlled to be 25-35°C; the reverse osmosis membrane has a remaining impurity retention rate ≥99% to obtain reverse osmosis product water.

8. The method according to claim 1, wherein the method is characterized by, The high-quality reverse osmosis product water obtained in the reverse osmosis deep purification step meets at least one of the following indicators: electrical conductivity ≤50 μS / cm, and total heavy metal ion concentration ≤0.1 mg / L.

9. The method according to claim 1, wherein the method is characterized by, The method further comprises a step of further processing the nanofiltration concentrated water and / or the reverse osmosis concentrated water, and the further processing comprises at least one of evaporation crystallization, freeze crystallization or acidolysis / hydrolysis process.

10. The method according to claim 1, wherein the method is characterized by, The differential recycling comprises: the reverse osmosis product water is delivered to a coating process as process water for preparing coating liquid; the nanofiltration product water is delivered to a beating process as beating water; the reverse osmosis concentrated water is returned to the conditioning tank for reprocessing; the nanofiltration concentrated water is evaporated and concentrated, cooled and crystallized, calcium sulfate crystals are recovered, and the condensate produced in the concentration process is returned to the pretreatment process.

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